Why do chelating ligands improve the activity of NiMo hydrotreating ...
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Research Collection
Doctoral Thesis
Why do chelating ligands improve the activity of NiMohydrotreating catalysts?
Author(s): Cattaneo, Riccardo
Publication Date: 2000
Permanent Link: https://doi.org/10.3929/ethz-a-004056334
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ETH Library
Diss. ETH Nr. 13760
Why do chelating ligands improvethe activity of
NiMo hydrotreating catalysts?
A dissertation submitted to the
Swiss Federal Institute of Technology Zurich
for the degree of
Doctor of Technical Sciences
presented by
Riccardo Cattaneo
Dipl. Chcm. Eng. ETI I Zurich
born on 25 November 1971
from Porlezza (Italy)
Accepted on recommendation of
Prof. Dr. R. Prins, Examiner
Prof. Dr. A. Wokaim, Co-examiner
7urich. 2000
Contents
Chapter 1 Introduction 1
Hydrotreatmg 1
The catalysts 2
Chelating ligands 3
This thesis 4
Literature 6
Chapter 2: X-ray Absorption Spectroscopy 9
X-rays 9
Synchrotron beam lines 11
The photoelectric effect 14
EXAFS 16
Data extraction 21
Data analysis 25
Pros & Cons ofEXAFS 29
Literature 32
Chapter 3: How does the Structure of NiMo/SiCh CKidic Precursors
Influence the Activity of the Final Sulfided Catalysts? 35
Inti oduction 35
Expei iwenttil methoch 36
Sample preparation and characterization 36
Stilfidation and reaction 38
EXAFS measuiements 41
EXAFS anah sis 42
Results 43
Catalytic Activity).
43
Characterization ofNickel 45
i
Characterization ofMo 55
Discussion 60
Conclusions 62
Liteiatuie 63
Chapter 4: A Quick EXAFS Study of the Sulfidation of NiMo/Si02
Hydrotreating Catalysts Prepared with Chelating Ligands 67
Introduction 68
Expei imental methods 68
Sample pi epcn atwn and tests of activity 68
XIFS measin ements 68
hXAFSanahsis 70
Results 72
Molybdenum 72
Nickel 81
Discussion 88
Moh hdenum m the catalyst pi ecm soi s 88
Sulfidation of Molybdenum 89
Chelating ligands and catalytic activity 94
Conclusions 96
Liteiatuie 97
Chaptci 5 Influence of chelating ligands on sulfided TNiMo catalysts:
an EXAFS study 101
Introduction 101
Lxpenmcntal methods 102
Catalysts pi cpai atwn and catnit\ tests 102
FXiFS measw ements 103
EXIFSanahsis 103
Results 104
MoK-edgc 106
u
Ni K-edge
Discussion
Molybdenum
Nickel
Conclusions
Liteiatuie
Chaptei 6' Influence of chelating ligands on
y-AhCh-supported NiMo catalysts
Intioduction
IIDN i eaction netw ai k
/IDS i eaction mechanism
Expei imental
Catalysts pi epai alion
IIDN activity lests
HDS activity test s
N4FS measui ements
ENI PS analysis
Results
Catalytic pei foi mane e
Catalysts chai ac ici nation
Discussion
Moh bdenum
Nickel
Conclusions
Liteiatuie
Chaptei 7 Summary & Conclusions
The oxidic state
The sulfidation
Ihe sulfidic state
Reniai ks & suggestions 165
Acknowledgements 167
Curriculum Vitae 169
iv
Summary
Hydrotreating catalysts are used in the refining industry to remove S, N, O and metals
from petroleum derivatives. The aims of this process are two. On the one hand, the decrease
of SO2 and N(\ emissions during the combustion of fuels which contribute to the
phenomenon of acid rain, on the other hand, the protection of S-sensitive catalysts which
arc used in later stages of the refining process. The materials studied in this work consisted
of sulfided NiMo catalysts supported on S1O2 and y-ALOa. The addition of chelating
ligands during the preparation of these catalysts induces an increase in the activity of the
final catalysts. The used chelating ligands were nitrilotriacetic acid (NTA), ethylene
diamine (EN) and ethylene diamine tctraacetic acid (EDTA). This work aims to
characterize these catalytic materials in the oxidic precursors, during sulfidation and in the
final sulfided state. The information obtained concerning the structural features should
explain the beneficial effect of the ligands on the catalytic activity.
The structure of SiC^-supported catalyst precursors was studied by means of Raman,
UV-Vis and extended X-ray absorption fine structure (EXAFS) spectroscopy. These
techniques revealed that in the absence of chelating ligands Ni interacts strongly with the
support and nickel silicates are formed. The chelating agents form complexes with Ni and
hinder the contact between Ni and the support. Mo is mainly present as polymolybdate
units in the catalyst precursors when no ligands or small ligand amounts are employed. The
concentration of the ligands was \aried in order to observe the effects on Ni and Mo. As
soon as the amount of ligands present in the catalyst precursors is higher than the quantity
needed to complex all Ni, Mo starts to be affected by the ligands and to be present in
smaller units on the support.
The sulfidation processes of Ni and Mo were followed by means of Quick EXAFS
(QEXAFS). The presence of the ligands retards the sulfidation of Ni. The change in the
sulfidation temperature of Ni indicates that the mechanism of sulfidation of Ni changes
when chelating ligands are used. In the catalyst containing Ni and Mo the sulfidation of Mo
takes place in a narrower temperature interval than in the catalyst containing only Mo.
When the concentration of the ligands is increased, the sulfidation of Mo tends to be similar
v
to that in the catalyst prepared in the absence of Ni. According to these observations the
presence of Ni influences the sulfidation of Mo when no ligands are employed. With the
addition of chelating ligands Ni does not interact with the environment, so that Mo and Ni
arc sulfided independently.
EXAFS measurements of the sulfided catahsts suggested the presence of nickel
sulfide clusters in which Ni has either a square pyramidal or an octahedral geometry. The
formation of smaller clusters with similar geometry could be enhanced by the use of
chelating ligands. Therefore, the higher activity observed in catalysts prepared with
chelating ligands could be due to a higher dispersion of Ni. No Ni-Mo proximity could be
detected by EXAFS, what suggests that the Ni-Mo-S model proposed in the past on the
basis of EXAFS data was a wrong interpretation. The interactions between Mo and the
support, formed in the presence of small amounts of ligands, induce the formation of more
regular M0S2 crystallites. When no ligands are employed the static disorder of the M0S2
particles is more pronounced. When larger amounts of NTA and EDTA (molar ratios
NTA:Ni and EDTA:Ni > 1.5) are employed the size of the M0S2 crystallites decreases
because of the formation of Mo complexes in the catalyst precursors.
The hydrodesulfurisation (HDS) activity is strongly affected by changes in the
sulfidation mechanism and final structure of Ni, while the hydrodenitrogenation (HDN)
activity is more dependent on the structure of the M0S2 slabs.
vi
Riassunto
I catahzzatori di idroraffmazione \engono usati nell'industria petrolifera per
eliminare S, N, O e metalli dai derivati del petrolio. Questo processo ha due scopi. Da un
lato la diminuzione delle emissioni di SO2 c di NC\ durante la combustione dei carburanti
derivati dal petrolio. che contribuiscono al fenomeno delle pioggie acide, dall'altro la
protezione dei catahzzatori che vengono impiegati nei successivi trattamenti delle frazioni
del greggio, che spesso vengono disattivati dallo zollb. I catahzzatori studiati in questo
lavoro sono a base di solfuri di Ni e Mo dispersi su Si02 e Y-AI2O3. L'aggiunta di agenti
chelanti durante la preparazione di questi materiali favorisée un aumento dell'attività
catalitica. Gli agenti chelanti utilizzati sono facido nitrilotriacetico (NTA),
retilendiammina (EN) e facido etilendiamminotetracetico (EDTA). Lo scopo di questa tesi
c caratterizzare questi materiali durante Fintera preparazione, partcndo dai precursori
essiccati, per passare alia fase di solforazione e quindi ai catahzzatori veri e propri in forma
solforica. Le informazioni riguardanti la struttura dovrebbero spicgare Faumento
delfattività catalitica conseguito con l'aggiunta dei leganti.
La struttura dei precursori dispersi su silice è stata studiata tramite Raman e
spettroscopie UV-Vis e EXAFS (extended X-ray absorption fine structure). Queste tecniche
hanno rivelato che, in assenza di agenti chelanti. il Ni interagisce con il supporta formando
silicati di nichel. I leganti formano complessi con il Ni e impediscono il contatto fra Ni e
supporte. Se non vengono impiegati leganti 0 ne vengono utilizzate piccole quantità, il
molibdeno c essenzialmente présente sotto forma di polimolibdati nei precursori essiccati.
La concentrazione dei leganti è stata variata per osservare gli effetti su Ni e Mo. Non
appena la concentrazione di legante nel precursore supera la quantità necessaria a
complessare tutto il Ni. il Mo commcia a subire l'effetto dei leganti e a formare unità più
piccole.
II processo di solforazione di Ni e Mo è stato studiato tramite la teenica EXAFS
veloce (QEXAFS). La presenza dei leganti ritarda la solforazione del Ni. Lo spostamento
délia temperature di solforazione del Ni indica che il meccanismo di solforazione del Ni
cambia quando vengono utilizzati gli agenti chelanti. Nel catalizzatore contenente Ni e Mo,
vu
preparato senza leganti, la solforazione del Mo avvicne in un intervallo di temperatura più
stretto rispetto al catalizzatore contenente solo Mo. Alf alimentäre délia concentrazione dei
leganti, la solforazione del Mo diventa simile a quella nel catalizzatore preparato in assenza
di Ni. Questo comportamento dimostra che la presenza del Ni influenza la solforazione del
Mo. I leganti impediscono ogni contatto fra il Ni e Fambiente circostante, in modo che Ni e
Mo vengano solforati indipendentemente.
Le misurazioni EXAFS sui catahzzatori solforati hanno suggerito la presenza di
clusters di solfuro di nichel, nei quali il Ni ha una struttura quadrata piramidale o ottaedrica.
L'uso di agenti chelanti potrebbe favorire la formazione di clusters più piccoli ma di simile
struttura geometrica. Perciô, Felevata attività dei catahzzatori preparati con agenti chelanti
potrebbe essere dovuta a una maggiore dispersione del Ni. Nessun segno di vicinanza fra
Ni e Mo è stato rivelato dai risultati EXAFS nei catalizzatori solforati. Ciô suggerisce che il
modello Ni-Mo-S, proposto in passato sulla base di risultati EXAFS, era una
interpretazione sbagliata. Le interazioni tra le speci di Mo e il supporte, formatesi in
presenza degli agenti chelanti. porta allô sviluppo di particellc di M0S2 più regolari.
Quando, invece non vengono usati i leganti, il disordine statico del disulfuro di molibdeno
è maggiore. L'uso di grandi quantità di N fA e EDTA (rapporte molare NTA:Ni e
EDTANi > 1.5) provoca la formazione di particellc di M0S2 più piccole rispetto agli altri
casi. Questo effetto viene spiegato con la presenza di complessi di Mo nei precursori
essiccati.
L'attività di deidrosolforazione (1ÏDS) è fortemente infiuenzata da cambiamenti del
meccanismo di solforazione e dalla struttura finale del Ni, mentre Fattività di
deidronitrogenazione (IIDN) c maggiormente dipendente dalla struttura delle piastrine di
M0S2.
vin
Chapter 1
Introduction
Hydrotreating
Petroleum is one of the most important raw materials in our society. An enormous
number of products are derived from crude oil. Ehe market sectors which mainly use oil
derivatives are the fuel and the transport industries. Additionally, a large variety of
chemicals tire derived from the processing of crude oil. For these reasons the industrial
processes involved in the refinery of petroleum have been widely developed. The idea
behind oil refining is the conversion of the raw material into clean lighter and more
valuable hydrocarbons. In brief, in oil refining, all the different hydrocarbon crude oil
fractions are separated in a distillation step, according to their boiling point. Before
undergoing other treatments, such as isomerisation. alkylation and cracking, the largest
amount of products obtained from the distillation step are hydrotreatcd. Hydrotreating
refers to a variety of catalytic hydrogénation processes in which unsaturated hydrocarbons
are saturated and S. N, O and metals are remo\ed from molecules which contain these
atoms. Ehis purification process is important for two reasons. The first is the decrease of
the polluting effects of the produced substances. Lower S andN contents induce a decrease
of SO2 and NC\ emissions during the combustion of fuels. These gases contribute to the
phenomenon of acid rain. legislations concerning the S and N emissions in most
Chapter 1
industrialised countries become continuoush stricter because of the awareness of this
problem. The second reason for the importance of hydrotreating is that most catalysts that
are used in further treatments of the oil products do not tolerate sulfur and metals.
Therefore, the performance of hydrotreating catahsts must be steadily improved.
The catalysts
The most commonly used h\ drotrcating catalysts are composed of the sulfides of two
metals supported, usually, on y-ALCX The bimetallic combinations are Ni-Mo, Co-Mo or
Ni-W. Supported CoMo sulfides are excellent in the removal of sulfur
(hydrodcsulfurisation, FIDS) but are slight!} less active in the removal of N
(hydrodenitrogenation, IIDN) and the hjdrogenation of aromatics. Supported NiMo
sulfides, on the other hand, are very good FTDN and hydrogénation catalysts. The NiW
containing catalysts have the highest activity for aromatic hydrogénation at low H2S
pressures [ 1, 2]. Since the catahsts studied during this thesis project were composed of Ni
and Mo, we will refer to these two elements during this general introduction, keeping in
mind that many considerations are \ alid also for the other combinations.
The catalysts are generally produced b\ impregnation of the support with solutions
containing salts of Ni and Mo. The impregnated support is then dried at temperatures
around 120°C. Usually a further heating step to higher temperatures (400-500°C), the so
called calcination, follows the drying procedure. The so obtained catalyst precursors are
then transformed into the actual hydrotreating catalysts by sulfidation at temperatures
around 400°C in a mixture of IE and a molecule containing S. ELS, thiophene, CS2 or
dimethyldisulfide can be used for this. The properties of the final sulfidic catalysts are
dependent on the whole preparation procedure, from the pll and concentration of the
impregnation solution to the sulfidation temperature.
The support empkned during this thesis work was mainly SiCT because it has been
claimed that silica-supported catahsts prepared with chelating ligands can reach activities
superior to alumina supported ones [3. 4]. Moreover silica based catalysts should be less
prone to coke formation because of the kwver acidity of silica [3J. Since coke deposition is
2
Introduction
a major catalyst deactivator in hydrotreating processes, an active and stable silica-based
catalyst would have an important impact on the refining industry.
Chelating ligands
Subject of this thesis was the study of the effects of chelating ligands on the structure
of NiMo catalysts during the whole preparation procedure. Chelating (from the greek chelé:
forked nail, claw) ligands are organic molecules possessing two or more arms with which
they can bind a metal and form a chelate. The chelating ligands used in this work were
nitrilotri acetic acid (NTA), ethylenediamine (EN) and ethylenediamine tetraacetic acid
(EDTA). Their formulae arc depicted in Fig. 1. fhesc compounds arc added to the
impregnating solutions and therefore affect the structure of Ni and Mo during the
preparation. When chelating ligands are used in the preparation of catalysts no calcination
is carried out in order to prevent the combustion of the organic molecules.
The presence of chelating ligands during catahst preparation induces a strong
increase in the hydrotreating activity |3-7|. However, the scientific explanation for this
effect is still unclear.
HO
HO N'"
^y—OHo
NTA EDTA EN
Fig. 1. The chelating ligands used in this woik
40, A0HO, y0
.N
N H7N'NH0
O OH
O OH
j
Chapter I
This thesis
Previous work by our group showed that m a solution containing NEf and Mo6h salts
NTA preferentialh complexes Ni21 [8]. Nevertheless, what consequences this fact has on
the structure of the dned catalysts is still unexplained Chapter 3 of this thesis will deal with
this question.
Another important point is how the chelating molecules influence the sulfidation of
Ni and Mo. Medici and Prms showed that NIA could delay the sulfidation of Ni to higher
temperatures, but no effect was reported on the sulfidation of Mo [9]. In chapter 4 we will
see, in more detail, that the suggestion of Medici and Prins was correct but that also the
sulfidation of Mo is affected by the presence of chelating ligands.
In Chapter 5 the structure of the final sulfidic catahsts is discussed. This topic has
been extremely controversial and se\ ei al models ha\ e been proposed in the past. In general
Mo is considered responsible for the foimation of the active species and Ni as the promoter
agent [2, 10, 11]. There is agreement on the fact that Mo is present on the support as M0S2
crystallites. These crystallites are believed to stack on top of each other and form small
tower-like structures as depicted in Fig 2
Mo
S
Fi«. 2. Layeicd stiuctute of MoS-.
> Y tA «O D
V*\ \*.
^<!, ^ W "A
J?
V
v ~V k
O Y) $
4
Introduction
The most discussed topic concerns the structure ofNi. Since NiMo catalysts are much
more active than Mo-only catalysts, it is supposed that Ni interacts somehow with M0S2.
The most widely accepted model for this interaction suggests that the promoter is situated
on the edges of the M0S2 particles as sketched in fig. 3 [12]. An infrared absorption study
of the absorption of NO molecules on a series of sulfided C0M0/ALO3 catalysts indicated
that this model, the so-called edge-decoration model, is very likely [13]. This work
suggests that the M0S2 edges are covered b\ Co. Nevertheless the exact structure of the
promoter is not known. Extended X-ra\ absorption fine structure (EXAFS) studies
proposed a precise structure for Ni on the IM0S2 edges [5], but we will see that the
interpretation of the data was not totally correct.
Support
Fig. 3. The edge decoration model orNi-Mo-S phase
Another topic that still attracts attention concerns the assignment of the element
responsible for the catahtic acthity in these materials. Unsaturated Mo has been generally
believed to be the active site. Ehe promoting role of Ni was ascribed to an electron donating
effect towards Mo [14J. ITowever, the idea that Ni is the real catalyst is getting more and
more consensus. This idea arises from the observation that cobalt and nickel sulfides
supported on carbon ha\e a higher acti\it\ than MoS^'C [15]. On the contrary, AI2O3-
supported Co and Ni sulfides ha\e a \er\ low FIDS acthity. In the alumina-supported Ni
and Co catalysts, cobalt and nickel interact so strongh with the support that, under normal
sulfidation conditions, the metal ions are not properly sulfided. On the contrary, in carbon-
5
Chapter 1
supported catalysts interactions between the support and Ni or Co absent and they can be
sulfided. From these considerations it has been suggested that Ni is the actual catalyst. In
this model MoS? is considered a secondary support which avoids the Ni support
interactions and enhances its catalytic acth it\.
The results presented in Chapter 5 will show that the presence of chelating ligands
has an important effect on the structure of the final catalysts. This will be also confirmed in
Chapter 6 where similar conclusions will be drawn also for catalysts supported on y-ALOv
Since X-ray absorption spectroscopy was the main tool used to characterize the
investigated catalysts during this work, Chapter 2 gives an introduction to this
spectroscopic technique.
Literature
1. Stanislaus. A., and Cooper, B. FF. Cat Rev -Sei. Eng. 36(1), 75 (1994).
2. Prins, R., Hydrotreating Reactions, in "Handbook of heterogeneous
catalysis", Ertk G.. Knözinger, FI. and Weitkamp, J. Eds., VC1T, Weinheim,
1908 (1997).
3. Thompson, M. S.. 0.181.035. European Patent Application (1986).
4. van Veen, J. A. R., Gerkema. E., van der Kraan, A. M., and Knoester, A.,,/.
Cheni SocChem Commun 1987,1684(1987).
5. Louwers, S. P. A., and Prins. R.. J. Catal. 133, 94 (1992).
6. Inamura, K.. Uchikawa, K., Matsuda. S.. and Akai, Y., Appl. Surf Sei.
121/122.468(1997).
7. Hiroshima. K.. Mochizuchi. T.. Flonma, T., Shimizu, T., and Yamada, M.,
Appl Surf Sei 121/122. 433 (1997).
8. Medici, L.. and Prins, R.. J Catal 163, 28 (1996).
9. Medici. L.. and Prins. R.. J Catal 163, 38 (1996).
10. Prins, R., dc Beer. V. IT. J., and Somorjai, G. A.. Catal. Rev.-Sei. Eng. 31, 1
(1989).
11. Chianelli, R., Daage, M.. and Ledoux. M. T, Adv. Catal. 40. 177 (1994).
6
Introduction
12. Ratnasamy, P., and Sivasankcr. S.. Catal. Rev.-Sei. Eng. 22, 401 (1980).
13. Topsoe, N. Y., and Tops0e. IF. .7 Catal 84, 386 (1981).
14. Harris. S., and Chianelli, R. R„J Catal. 98, 17 (1986).
15. Duchet, J. C, van Oers, E. M., de Beer, V. H. .T., and Prins, R., J Catal 80,
386(1983).
7
Chapter 1
W^,~ "* I
Chapter 2
X-ray Absorption Spectroscopy
X-rays
In 1895 Röntgen discovered a new type of radiation which he called X-rays [1. 2].
For his work on X-ra\s he received the first Nobel Prize in physics in 1901. Röntgen's
achievements were more than pure discover}, he studied the properties of the new rays so
well that he laid the foundations not only for important methods of X-ray detection and for
radiography, but also for the application of X-ra\ absorption to analytical chemistry.
In general. X-ray photons can be produced in two ways. In the first way they
originate when electrons with high velocity are strong!) decelerated, either by a target
material or through bending their path by a magnet, and as a result of a secondary process
from interaction of electrons with atoms. When an electron is stopped or decelerated
Brcmsstrahlung is produced, which consists of an electromagnetic radiation with all sorts of
wavelengths (polychromatic radiation). In the second waw the electron hits an atom and
subsequently an electron is ejected from the atom, creating a core-hole in the atom. Then an
electron from an outer shell can relax to the core-hole and as a result an X-ray quantum is
emitted with a very specific wavelength (monochromatic radiation). As both ways of X-ray
production are the result of the interaction of electrons with matter, it can easily be seen
Chapter 2
that both processes will take place simultaneously. However, as shown in Fig. 1, while
Bremsstrahlung is emitted all the time due to deceleration, the production of
monochromatic X-rays is limited to specific electron energy values and depends on the
target material.
Hot tungstenfilament
(catodc)
5=>
Tai get mateiial
(anode)
E ->
FIC. 1. Electrons arc accelerated towards the target material. Through the impact, X-rays with different
wavelengths are produced. When the electrons have the same energy as the electronic shells of the target
material, X-rays with characteristic wavelengths are produced
Nowadays, synchrotrons are the most powerful X-ray source. In a synchrotron,
charged particles such as electrons or positrons travel under ultra high vacuum with a speed
approaching that of light. When their trajectory is bent by means of strong magnets, X-rays
are emitted tangential to the particle's orbit. Fig. 2 show's why a relativistic speed is needed.
For non-relativistic accelerated charges, the energ} llux is emitted isotropically around the
acceleration. At relativistic energies the radiation pattern becomes sharply peaked in the
direction of motion of the radiating charge. Ehe higher is the energy of the particles, the
A BElectron
orbit
L lectron
orbit
FFG. 2. Radiation emission pattern of electrons in circular motion. Case A: non-relativistic electrons.
Case B: relativistic electrons.
10
X-ray Absorption Spectroscopy
more collimated becomes the emitted radiation.
Normally, the synchrotron radiation source is a three-stage machine, comprising a
linear accelerator (linac). a booster synchrotron and a storage ring (Fig. 3). Electrons are
fired from the linear accelerator into the booster s}nchrotron, where they are accelerated to
almost the speed of light before being injected into the storage ring. Ffere the electrons
travel in a vacuum inside a tube around the circumference of the ring and remain stored in
orbit producing synchrotron radiation for 10 to 20 hours. The data presented in this work
were collected at the European Synchrotron
Radiation Facility (ESRF) in Grenoble,
France. and at the Hamburger
S}nchrotronstrahlungslabor (11ASYLAB) in
1 lamburg. Germany. ESRF is, at present, one
of the most powerful synchrotrons in the
world. Its storage ring has an 844 m
circumference where electrons travel with an
energy of 6 GeV and an intensity of 200 mA.
At HASYLAB synchrotron radiation is
emitted from positrons in the storage ring
DORIS. The beam energy is 4.45 GeV and
FIG. 3. Schematic lepresentation of a synchrotron. ^lie intensity 100 niA.
Synchrotron beam lines
Three kinds of devices are used in a S}iichrotron to generate X-rays by bending the
trajectory of the electrons: bending magnets, wigglers and undulators [3]. Bending magnets
consist of a single magnet that induces a uniform curved motion of the charged particles.
Wigglers and undulators are constituted of several magnets that transmit periodical
impulses to the electrons that therefore move in an oscillatory path through the device. The
difference between a wigglcr and an undulator is the amplitude of the oscillations imparted
on the electrons. In a wiggler the radiation from each oscillation is incoherent and can be
approximated as the sum of N separate sources where N is the number of bends. Wigglers
It
Chapter 2
allow the spectrum to be shifted considerabh to higher energy, enabling high-energy
photons to be produced at a more easily achiev able electron energy. In an undulator the size
of the oscillations is smaller and their number is much larger. Therefore, the interference
effects become important and the spectrum becomes a set of several extremely intense
peaks. In case of EXAFS measurements onh bending magnets and wigglers arc used.
Beam lines are built in correspondence with these devices. The produced X-rays are
collected in highly evacuated tubes and further collimatcd by means of very sophisticated
optics. The so obtained beam is polychromatic. Since, during an X-ray absorption
experiment, the energy of the X-rays has to be precisely scanned in a defined interval.
monochromators are used in order to keep onh the radiation with the desired wavelength.
Almost monochromatic light is obtained according to the Bragg relation:
nX-2dsinO (1)
with d the lattice spacing of the crystal used as monochromator, 9 the incident angle, n the
order of reflection, and Xthe desired wavelength. Usually. Si crystals are employed. For X-
raysfrom2to 10 keV Si (111) (d - 3.1350 A) is used. 1 he range between 10and20keVis
covered best by Si (311) crystals (d = 1.6372 A) and above about 20 keV Si (511) (d -
1.0450 Â) is appropriate. Two kinds of monochromators are generally used in X-rays
absorption dedicated beam lines: double cnstal and channel-cut monochromators [4]. In
both cases the beam is reflected twice in order to maintain a horizontal path. In the case of
double crystal monochromators the two crvstals move independently. Usually they are not
exactly parallel: the second one is slightly adjusted in order to remove harmonics (X-rays
with wavelength nz. with n^ 1). A channel-cut monochromator consists of a crystal in
which a precise channel has been incised. LTsing the same principle, the X-ray wavelength
is chosen changing the angle of the monochromator. but this time harmonics have to be
rejected in another way: usually Au or Cr mirrors are employed. By tuning their angle it is
possible to choose a cut-off energ}. after which X-ra} s are no longer reflected.
The X-rays obtained in these \\a\s are used to execute the experiments that are
carried out in an experimental hutch. The most frequently used method to quantify the
absorption of X-ra} s in heterogeneous catahsis is to measure the intensity of the incoming
12
X-ray Absorption Spectroscopy
(I0) and transmitted (lt) beams by means of ionisation chambers. Depending on the energy
range, the chambers arc filled with different gases, usually nitrogen, argon, or krypton and
mixtures thereof. Fig. 4 summarises the set-up of an X-ray absorption beam line.
Reference
Sample compound
Synchrotron ^n n —« -— —
Radiation /
Slits V f /Monocfiomator \ /
Ionisation chambers
FIG. 4. Schematic set-up of a Upical X-ray Absorption beam line
In a classical EXAFS experiment the monochromator is rotated to a certain position
and halted. Before collecting data at the corresponding X-ray wavelength, a certain dead
time has to be waited to allow a stabilisation of the mechanical equipment. After that, the
real data collection starts, that amounts to 1 s for data points close to the edge energy and
gradually increases with energ}, reaching 6 s if high k \ allies have to be reached or in case
of very diluted samples. Ehe collection of a spectrum in the classical way takes therefore a
relatively long time. 20-40 min. Since the collection of spectra in shorter times offers
interesting application possibilities, several methods have been developed to collect an
EXAFS spectrum in a much shorter time range.
The Quick EXAFS (QEXAFS) technique is very similar to classical EXAFS with the
difference that the monochromator is constanth moving and that the absorption data are
integrated over small time intervals (e.g. 0.01 s) [5. 6]. Ehe minimal time required to record
a spectrum is about 1 min. Hie shortening of the measuring time is due to the fact that no
dead time is needed between the data points. Hie drawback of QEXAFS is a slight loss of
data quality in comparison with classical EXAFS data.
An even faster wa} to collect EXAFS data consists in the use of dispersive optics,
where a polychromatic beam is produced and focussed on the sample by means of a bent
crystal (polychromator) [7. 8j (Fig. 5). The detection is carried out by means of a position
sensitive detector that is calibrated with an energ}-position correlation. In this way the
spectrum can be theoretical!} recorded in milliseconds, because the full data set is recorded
13
Chapter 2
simultaneously, but the signal must often be accumulated for several seconds in order to
obtain a good signal to noise ratio. This method requires a complex set up and a different
data treatment. Moreover onh' a limited data range can be recorded, usually 400-600 eV
ADemazriithîiition minci // _—r~:
,/ /f~-fr
//,' S- Etfe;f:"e
Wrticjl focn;in2 urn M
7->
'
/
Ta-etf !nnhi.-i^
P >:ifi< n ;er_citr*e detector
FIG. 5. Principle of dispersive energ} EXAFS (DEXAFS). Set-up of the DEXAFS beam line ID24 at ESRF
(fromhttp:/'www esrf fr/e\p facilities 1Ü24TD24 html)
after the edge, which corresponds to a k-range (s. next sections) of 10-13 Â"1.
Thephotoelectric effectAfter having described how X-rays are produced and brought to the sample, the basic
principles about the interaction of X-rays with matter will be discussed. When X-rays hit a
material two main processes can occur. Hiev can be absorbed by the atoms composing the
material or they can be scattered, and if none of these processes takes place, they pass
through the material as transmitted beam. When the monochromatic photon beam passes
through the sample, its incident intensity h will be decreased by a specific amount which is
determined by the absorption capacity of the material and the path length that the X-rays
have covered:
dl =•- jj, I0 dx (2)
14
X-ray Absorption Spectroscopy
The linear absorption coefficient/./ is a function of the photon energy. Integration of the
attenuation in intensity o\ er the total thickness of the sample x results in Lamberf s law:
I0 - lt e^E^ (3)
During an X-ray absorption experiment, Iq and the intensity of the transmitted beam I, are
measured before and after the sample, respectively. The absorbance of the sample is
therefore calculated bv:
MX=
ta|j; (4)
O
60
O
The degree of absorption is a function of the energ} of the photons. When the energy of the
X-rays that hit a sample is increased, a gradual decrease of the absorption is observed due
to the decrease of the ionisation cross-section of atoms with energy. ITowever, when the X-
ra\s ha\e enough energy to excite and
knock an electron out of its orbit a sharp
increase in absorption will be observed.
Therefore, the absorption profile over a
large range is a decreasing line with
jumps at the energies corresponding to
the binding energies of the electrons of
the various Orbitals, as depicted in Fig.
6. If this excitation energy corresponds
to the energy of an electron belonging to
an s core level, the absorption increase is
called a A-edge. if the energy
corresponds to a p type core level, the
absorption increase is called an L edge.
Iog(Energ\)
FIG. 6. Profile of the re!ati\e absorption cross-
section over a larçe energy tance
15
Chapter 2
EXAFS
It is interesting to closely observe how an absorption spectrum around the edge is
structured. In Fig. 7 the absorption spectrum at the K- edge of Mo in M0S2 is plotted. In
general, the interval about 50 eV before the edge is called the pre-edge region. The edge
region extends until about 20 eV after the edge. For an isolated atom the shape of the
absorption profile after the edge is regularly decreasing, whereas for an atom surrounded by-
neighbours a series of oscillations are present until 1000-1500 eV after the edge. The shape
of these wiggles is dependent on the kind, number and distance of neighbouring atoms. The
principle of EXAFS. which is an acronym for Extended X-ray Absorption Fine
Structure, is to extract information about the surrounding of the investigated species
through a mathematical processing of the absorption data. It is also possible to obtain
qualitative structural information
from the absorption spectrum in
the first 50 eV right after the
edge. This region is called X-ray
Absorption Near Edge Structure
^"^(XANES) 19, 10]. The physical
principles that explain the
Pre-edgen _
presence of a fine structure in
the absorption data will be given
21500 in this section, together with the
basic formulae.
When the incoming X-ray
beam has enough energy to
1 0
EXAFS
XAXES
05
1t«l 20020 20060
-0 1
19500 20000 20500 21000
X-ray energy | eV]
FIG. 7. X-rav absorption spectrum of \foS^
excite an electron of the sample to a vacant state or to the continuum, the kinetic energy of
the produced photoelectron will be:
Ep = hv-F, (5)
\6
X-ray Absorption Spectroscopy
where hv is the energy of the X-rays and E0 the energy necessary to kick out the electron
from its orbit. Flowever, the removed photoelectron can be considered as a spherical wave
expanding around the absorbing atom. Its wav elength X is defined as:
2/t
(6)
with the wav c v ector k87TTT1
l h2 J (hv-E0) M15123AyïA:_I,>s[eVj-E0LeVl (7)
where m is the electron mass and h the Planck's constant.
Once the X-rays have surmounted the edge energ}. the removed electron has enough
kinetic energy to be excited and expand towards the continuum. Flowever. when
neighbouring atoms are present, the photoelectric wave is backscattered towards the
original absorbing atom and the outgoing and backscattered waves interfere, as show in
Fig. 8. The wavelength A. of the outgoing wave is dependent on the X-ray energy and, since
during an EXAFS experiment the energ} of the X-ra} s is increased constantly, the
wavelength of the photoelectron decreases continuously. The interference between the two
waves is alternatively constructive or destructive depending on the X-ray energy. The
absorption of X-rays is close!} related to the electronic state of the atom. Therefore, when
the outgoing and backscattered waves interfere constructively the absorption coefficient
becomes larger. When the} are out of phase it becomes smaller. The nature of the
hv
,A
nv
//•nv A>
FTG. 8. X-tays excite an election of the absotbei atom, that is emitted as a cnculai photoelection-wave
Neighbouimg atoms backscattet the outcoimtig \\a\e tow aids the absotber Outgoing and backscattered
waves mtei feie
17
Chapter 2
neighbouring atoms (backscatterers) influences the backscattered wave and consequently
the absorption of X-ra} s. For this reason in case of an isolated atom no fine structure after
the edge is observed. One of the factors that led to the development of EXAFS was the
possibility to obtain precise information about the nature, the number and the distance of
neighbouring atoms. Although position and amplitude of the oscillations are connected to
the distance and number of neighbours, it is necessary to transform the data to achieve the
exact results, as will be explained in the next section.
Eo understand how the X-ra} absorption is related to the electronic aspects of this
process it is helpful to consider the formulae that describe the dependence of the absorption
coefficient from the X-ray energ} according to the dipole approximation [11]:
H- 4 N 7t2 e2 - |<T'j| 11 TV2 P(hr) (8)
Here is N the number of atoms of one kind per unit volume, co the radial frequency of the
X-ray photon with energy hco. |T',X the wave function of the photoelectron in its initial state
(the core state) and 'fl'\)\ the final wave function of the photoelectron. p(Ef) is the density of
final states, Ef the energy of the outgoing electron. FI is the photon-electron interaction
ITamiltonian.
The interference between outgoing and backscattered electron wave gives rise to the
sinusoidal variation of u versus the energ}. We can see that u depends on \fV\)\, lvF-r)| and
p(Ef). |VF,)| is fixed at a given core level; p(Ft) accounts for a monotonous contribution;
only bFf)| can therefore be responsible for the sinusoidal oscillation of the absorption
coefficient. )vFf)| is given by the sum of the outgoing wave and the backscattered wave, due
to the backscattering action of each neighbouring atom. The fine structure or EXAFS
function is defined as the normalized oscillator} part of u and is given by:
18
X-ray Absorption Spectroscopy
X(E)-bo
(9)
p0 being the smoothly varving part of p which corresponds to the absorption by an isolated
atom, that is the absorption coefficient if the phenomenon of backscattering would not
exist. The EXAFS signal so defined is normalized and contains information per absorbing
atom.
The most commonly used formula to describe the EXAFS phenomenon is derived
from the generally accepted short-range single-electron single-scattering theory [12-15].
The meaning of single and multiple scattering is shown in Fig. 9. The mentioned theory
assumes that multiple scattering has no significant effect on the EXAFS function. This is
hvA
nvB
hv
4
c
FIG. 9. The produced photoelectron can experience \aiious scatteimg patterns: A. Single scattering,
B Multiple scattering, 0, Foiwaid scatteimg.
generally valid if one considers that the length of the path in multiple scattering tends to
cancel out the involved waves. It has, nevertheless, to be noted that in case of
approximately collinear arrangement of two neighbouring atoms, multiple scattering
becomes important due to a forward scattering effect that enhances the transmission of the
wave (Fig. 9). According to the short-range single-electron single-scattering theory, % can
be expressed as a function of k as a wave function composed of an amplitude and an
oscillatory term:
Shells
X(k)- X Aj(k) sm5j(k)i
(10)
19
Chapter 2
The index j represents the different coordination shells. Each jn-shell is made up of N,
identical atoms at a distance R, from the absorber atom. Ehe amplitude A,(k) contains the
coordination number (N) and the Debye Waller factor a that accounts for thermal and static
disorder [16]:
Aj(k)=is02E1(k)e-2k:öl2e-2R./^ (11)
S0 is the amplitude reduction factor due to man} bod} effects, which takes Into account the
relaxation of the central atom and the multielectron excitation [17]. F,(k) is the
backscattering amplitude which depends on the scattering power of each neighbouring
atom. The term e"~ '"'
,where X represents the mean free path of the photoelectron, takes
into account the inelastic losses due to the finite lifetime of the excited state (the hole
lifetime and the photoelectron lifetime).
The sine argument §j(k) is:
8,(10-
2kR, + (I>,(k) (12)
One period of the sine function is completed when the ratio of double distance 2R,, between
absorber and backscatterer. and photoelectron wavelength has increased by one. This is the
meaning of the expression sin(2kR,). Fhe photoelectron suffers additional energy dependent
phase shifts in the absorber and in the backscatterer. They are summarised in the scattering
phase shift 0,(k).
In general we can sav that the EXAFS signal is determined bv the backscattering&
Ni TD n /I \
amplitude E,(k), multiplied b} the factor =; and decreased by the terms e~~ i^
,
k lvj~
e ', S0" and the sinusoidal factor which is a function of interatomic distances, and
phase shift d>,(k).
20
X-ray Absorption Spectroscopy
Data extraction
After an introduction about the physical principles of EXAFS wc will discuss how
the experimentally collected data have to be treated in order to obtain the data that will be
used for the analysis. Processing of the data presented in this work was performed by
means of the program XDAP (Version 2.2) [18]. Ihe explanation is therefore based on the
procedures used in this program.
1 3
1 1
09
07
05
03
01
-0 1
-0 3[
-
19500
A
20000 20500 21000
X-ra} energ} [eV]
21^09
B
1 3
1 1
09
07
s05
I 03
01
-0 1
-0 3
19500 20000 20500 21000 21500
X-ray energ} [eV]
FIG. 10. Preedge subtraction' \. Preedge approximation.
B. Data aftei preedge subti action
The spectra obtained during a
measurement, the so-called raw data,
are usually plotted as absorption data
versus energy. The aim of processing
these data is to subtract the
monotonically decreasing
background (p0) due to the decrease
in absorption with the energy and
isolate the real EXAFS signal arising
from the interaction of the
photoelectron with the neighbouring
atoms. Before p0 can be determined,
the pre-edge has to be subtracted
(Fig. 10). The pre-edge is
approximated by a Victorccn
empirical form [19]:
V(EV E+co (13)
where Ci. C| and cu are fitting parameters. The pre-edge is extrapolated and subtracted from
the whole data range. After the pre-edge subtraction, the edge energy has to be defined. The
inflection point (maximum in the first deriv ativ e) in the edge region is generally considered
as a systematic choice for the edge (fig. 1 1). At this point, the signal of the isolated atom p0
21
Chapter 2
1 2
1
08
06
04
02
A
0
19970 19990 20010 20030 20050
X-ray energ} [eV]
05
>
Q
0 09
0 05 -
0 01
B
has to be determined and
subtracted from the data (Fig. 12).
The parametcriscd technique used
by XDAP to carry out this step is a
smoothing spline algorithm [20].
which is a polynomial function
defined over a series of intervals.
The ends of every interval are tied
together so that the function and
some derivatives (usually the first
and the second one) are made
continuous across the ends. The
calculation of the background is
controlled with the following
expression:
-0 03
19970 19990 20010 20030 20050
X-ra} energy [eV]
FTG. 11. Edge energy determination A. Edge legion.
B. Derivative of the absorption m the edge region.
V
1
S,- ux(E,)f-W, kj2
ye1 j
<SM (14)
Fhe value of SM is defined by the
user. W is a weighting parameter,
usually 0.075. S, is the value of the background that is varied in order to satisfy the
equation (14). The two following criteria have to be fulfilled in choosing SM: no
oscillations with frequencies similar to the EXAFS spectrum should be removed and low
frequency oscillations should not be included in the data. Both conditions can be easily
checked by observing the first derivative of the background that is more sensitive to the
oscillatory behaviour than its primitive function. Checking the Fourier transformation is
also extremch useful: when a too high SM v alue is chosen, signals below 1 Â start to grow
very fast.
22
X-ray Absorption Spectroscopy
Before %(k) can be obtained, the total absorption as obtained from experiments has to
be normalized per absorber atom. The spectrum obtained from background removal
corresponds to Au. ~ \i~ u0. According to equation (9)
Ii-Mox-
Eo
1 2
1 1
1
09
0 8 '
0 500 1000
X-rav energ} [eV]
0 1
M
-0 1
13
A
B
i{
Wave vector [X ]
FIG. 12. Backgtound subtiaction A. Absoiptton data and
calculated backgtound (dotted line), B. /_ data and
dcuvative of backgtound (dotted line)
Au must be divided by u,0. Ideally
one would like to measure the
absorption coefficient in the
absence of neighbouring atoms,
without any fine structure
oscillation. It is easy to understand
1500 that in practice this is not possible.
Ihe first solution is to divide the
absorption coefficient by the just
calculated background.
Unfortunately this is not always
acceptable. The background usually
decreases with increasing energy;
however, the negative slope can
vary strongly depending on the
experimental condition (materials in
the sample, gases in the ion
chambers). This could distort the
obtained results that are based, as
will be shown later, on comparing
spectra of the sample under anal} sis and spectra of reference compounds. Therefore, the
data were normalized with:
X(h)Au
(15)
Chapter 2
where \l{ is the calculated background absorption coefficient at 50 eV above the edge
(normalization by edge jump). This is allowed as long as the same procedure is applied in
the reference spectra with which the experimental spectra are compared. When using
theoretical references (see section "Data anal} sis*') [21], one has to consider whether it is
better to normalize the data b\ the edge jump, by the calculated background or by a linearly
or exponential!} decreasing function. Normalization b\ the background can produce large
errors, but, on the other hand, it takes into account the decay in absorption with energy.
Generally, the error induced by the normalization with the edge jump is small, but the
decrease in the atomic cross-section with energ} is neglected.
Once the x data are obtained, they are expressed as a function of the wave vector k,
using equation (7) to perform the transformation of E |eV| into k [A" ]. A further treatment
is thereafter needed in order to produce the data that are used to perform the actual analysis.
Applying a Fourier Transformation (FT) to the x data was first suggested by Sabers, Lytle
and Stern, who realised that this process allowed to obtain an estimation of the distance of
the neighbouring atoms [22]. In fact, the Fourier frans formation consists in applying the
complex formula:
1 .kma\ :^i ,
FT-p(r>-j= J X(k) eUvl dk (16)
As result of this complex function one obtains a real (Re) and an imaginary (lm) part. The
magnitude of the function, or the absolute part, is obtained with the following equation:
Absolute part - y/lm2 ~ Re^ (17)
Plotting the absolute part of the transform versus the distance r\ a figure is obtained with
peaks at the distance f roughly corresponding to the interatomic distances between
absorbing and backscattering atoms, r* is about 0.2-0.5 A shorter than the actual distance
due to the phase factor m the sine term of the yjk) function, eq. (10).
24
X-ray Absorption Spectroscopy
The Fourier Transform can be taken w ith different k-weighting (multiplication by k ):
FT---pn(0=-?==1 Ikniak"x(k)ei2k,'dk (18)\J2ti k>"i"
This option can be used to distinguish between high and low Z scatterer atoms [23]. An
element with low mass (like ox} gen) will scatter mainly at lower k-values, while high mass
elements (like mohbdenum) will scatter significantly at higher k-values. In fact, the peak in
the FT corresponding to the heav} atom will grow more, relatively to that corresponding to
the light atom, when going from the k - to the k -weighted FF. Because of the non-linearity
of the phase shift as a function of k. the position of a light scatterer peak in the FF will shift
to larger distances going from k to V weighting. This effect is much less pronounced with
heavy atoms. In general a k'-weighted transform is more advisable because it minimises
chemical effects on the EXATS signal (e. g. small changes in E0 values) and makes the
oscillations more uniform by compensating for the natural decrease of the signal due to the
1/k factor present in eq. (11) and to the backscattering amplitude, which falls off
approximately as 1/k".
Data analysisThe analysis of EXAFS data is usual!} based on equations (10) to (12) that combined
give the following expression:
ShellsN.
X (k) - X —L S02 F,(k) c"2k-cV c"2Ri X{k)sin(2kR, + fl>,(k)) (19)
j k Rp
Chapter 2
The aim of the analysis is to reproduce the data obtained experimentally using this
equation, i.e. to find out the structural information of each shell around the absorber atom.
Considering the fact that an EXAFS spectrum can be composed of one to five shells, the
first step is to concentrate on the single shells, starting from the ones at the lowest
distances. For this purpose a window has to be chosen in the FT spectrum for the selected
shell (Fig. 13). Ehe analysis should deliver the coordination number N, the distance from
the absorber R and the Delwe-Waller factor o" for each single shell. Among these
2RA,parameters also the quantities So, F, e"
,and <I> arc unknown. To circumvent the
complexity of a system with such a large number of variables, the use of reference
compounds was adopted. For a given pair of absorber and backscatterer, a reference
compound is taken and its FXAFS spectrum is either measured under the same conditions
as the investigated sample, or simulated by means of specialised programs. The code Feff
(Version 7), developed at the FAiversity of Washington [24], was used in this work to
produce the spectra of reference compounds and simulate EXAFS spectra of compounds
with a structure known from crystallographic studies.
Absolute part
(magnitude)
Imaginary part
6 7 8
FIG. 13. k'-weighted Fourier transformation of MoS? I he dashed rectangle represents the window isolating
the first shell (Mo-S).
<
30
15
3
O
dJ
-15
'5
-30 \i.
3 4
RlAl
26
X-ray Absorption Spectroscopy
Since the coordination number and the distance of the chosen shell in the reference
compound are known, its wave function is expressed as:
N r
X.er (k) - 7-rH A.ei (k) sin(2kRiel + <I>„IVU) (20)kKief
In this expression the term Alcl is composed of:
A,el<k) - S02 Fiel(k) e^cf e-2R«^k) (21)
From equation (20) the redefined amplitude AIC| and the phase shift <Flcf can be calculated
and used in the equation that describes the spectrum of the investigated shell and the
function to be minimised becomes:
Xevp (10 " HT^ Ad (1C) c"2k'2A°2 sin(2k'Rshcll + <Die(fk')) (22)[v Kshell
The transferability of A1Cf and <I>1C( has been shown for compounds with the same absorber-
scatterer pair [25. 26], and even for absorbers or backscatterers which arc neighbours in the
periodic table [27. 28]. The parameters that have to be optimised arc written in bold
characters. Ao~ is the Debye-Waller factor relative to the one of the reference compound. It
is not possible to calculate direct!} <r of the sample because in the expression for the
amplitude of the reference compound A1C| the Delwe-Waller factor of the reference is
contained. It can be noticed that the calculated term of eq. (22) is a function of k' instead of
k. k" is the photoelectron wave vector corrected for the difference in inner potential
between the sample and the reference compound (AE()):
2mck^irAEo (23)
27
Chapter 2
AEo is the fourth parameter that is optimised during the minimisation process that consists
of anon-linear least squares fit procedure.
For each shell four parameters are optimised, fhe number of variables is, however,
limited by the k-range used in the x(k) data and by the R-range of the fitted window in the
FT spectrum, fo calculate the number of free parameters (degrees of freedom) the Nyquist
theorem is used |29]:
P-^^.l (24)
This series of formulae stays behind the fitting procedure. The practical development
involves the production of a reference file for each shell that has to be fitted, the guessing
of a starting value for the parameters that have to be optimised and the judgement of the fit
produced by the computer. Each of these steps has great importance. Ehe chosen reference
should be as similar as possible to the studied absorber-backscattercr pair. The choice of the
starting values for the iterative process has an influence on the results. It is therefore useful
to try different starting values in case no convergence is obtained. The result of the fits has
to be judged firstly optically, comparing the simultaneous plot of the experimental and
calculated data. This should be done for the x and for the FT data. Since N and <f arc
strongly correlated, it is possible to obtain several solutions for the same spectrum with
different N and a" values. It is. therefore, always useful to compare the data using different
weighting factors. A good fit should give similar results using k and k' as weighting
factor. It is also very important to consider the statistical errors calculated from the fit to
have a more accurate idea about the quality of the fit. This step consists in checking the
standard deviation for each parameter and consider the variance of the fit j 30].
28
X-ray Absorption Spectroscopy
Pros & Cons ofEXAFS
To conclude this chapter some comments have to be added in order to give an
overview on advantages and disadvantages of the discussed spectroscopic technique and to
add some word of advice that are connected to the interpretation of the data presented in
this thesis.
The three main features that made FXAFS such a useful characterisation technique,
particularly in heterogeneous catalysis, are the following:
• Since the edge energy is typical for even element, this technique is element
specific; it allows to concentrate on a single element even if it is mixed with other
elements in a compound.
• The information obtained from EXAFS is due to the fact that the electrons excited
by the X-tays are backscattered by the neighbouring atoms. Since the mean free
path of such electrons is usually smaller than 10 A. EXAFS provides the
structural parameters concerning the local structure around the absorber atom.
• Because of the high penetrating capacity of X-rays through matter, the results
attained with EXAFS are an average of all species of the chosen elements present
inside and on the surface of the studied material. It is therefore a bulk technique.
A further advantage which was utilized during this thesis is the possibility of time
resolved experiments. Quick FXAES and Dispersive EXAFS allow to follow how a
reaction proceeds in situ. The quality of the data is usually reduced in comparison with
classical EXAFS. but a lot of useful information can be attained.
Among the disadvantages of EXAFS there is the usually limited beam time that is
available at synchrotron sources and the high sensitiv it} to experimental procedures. Fhe
samples have to be as (macroscopicalh) homogeneous as possible: the presence of
inhomogeneities or small holes provokes big experimental errors. Another factor that has to
be avoided, in order to eliminate the presence of glitches and irregularities in the data, is the
presence of higher harmonics in the X-ray beam that hit the sample. This can be obtained
by using proper harmonic rejection s} stems.
29
Chapter 2
As far as the interpretation of the data is concerned, it is always important to keep in
mind that the parameters obtained from the fits contain some error, partially due to
experimental problems, partiall} to the insufficient statistics (low signal to noise ratio). The
experimental errors can be almost eliminated by means of a proper sample and beam line
preparation. The statistical errors are usually reduced averaging several scans of the same
sample, typically 3-5. Another aspect that has to be considered when Interpreting EXAFS
data is the underestimation of the coordination number. Ehe reason for the false estimation
is a disorder in the sample either due to thermal vibration (c. g. [31]) or to structural
disorder (e. g. |32]). Fo reduce the thermal disorder samples are usually cooled to liquid
nitrogen or liquid helium temperatures. Ehe static disorder is. on the contrary, a feature that
can not be reduced. To understand the effect of the static disorder on the value obtained for
the coordination number, one has to keep in mind that every absorber-backscatterer pair
produces a contribution that, summed with the other ones, will produce the oscillation
observed in an EXAES spectrum. Every contribution consists of a sinusoidal oscillation
that arises from the term sin(2kR \ O(k)) in equation (12). The phase shift <D(k) is
dependent only on the kind of backscatterer atom. An absorber atom with two
backscatterers of the same kind but with slightly different distances will produce two
oscillations with different periods. The amplitude of the curve obtained by the sum of these
oscillations will reach a maximum when the single oscillations overlap as depicted in Fig.
14. If the overlapping of the two sinus functions takes place in the experimentally
measurable k-range. the estimation of the coordination number will be correct. lfy on the
contrary, this maximum is outside the accessible k-range. an underestimation of the
coordination number will occur. Since for a small difference in R. the maximum will be at
high k values, this factor has to be taken into account for low Z-backscattcrers. This is due
to the fiict that for light elements the maxima of the oscillation are reached at relatively low
(< 3 A"1) and high (> 10 A"1) k-values. ITowever, in the region k <^ 3 A"1, the signal is still
influenced by edge effects and is therefore excluded from the used data, whereas in the
region k > 10 A"1 low Z-elements scatter ven weakly and the signal at high k values is low,
so that no information can be obtained from this latter region, neither.
30
X-ray Absorption Spectroscopy
B=sin(2 k 2)
-1 5
2
-2 5
25
2
05
0
-0 5
I
-1 5
-2
-2 5
10
A-B
15 '
FIG. 14. The sum of two sinus functions with different periods
Chapter 2
Literature
1. Liebhafsky. 11. A., Pfeiffer, FI. G., Winslow, E. H., and Zcmany, P. D., "X-rays,
electrons, and anah tical chemistry". John Wiley & Sons, New York ( 1972).
2. Agarwal. B. K.. "X-ray Spectroscop} ". Springer-Verlag. New York (1991).
3. Raoux, D., Introduction to synchrotron radiation and to the physics of storage rings,
/'// "Neutron and synchrotron radiation for condensed matter studies". Springer-
Verlag, Berlin. 37 (1993).
4. Freund. A., X-Ray optics for synchrotron radiation, in "Neutron and synchrotron
radiation for condensed matter studies". Springer-Verlag. Berlin, 88 (1993).
5. Frahm. R.. Physica B 158, 342 (1989).
6. Frahm. R.. Rev Sei Instriim 60(7). 2515 (1989).
7. Shido. f., and Prins. R.. Current Opinion in Solid State and Material Science 3 (4),
330(1998).
8. Fontaine, A., Interaction of X-rays with matter: X-ray absorption spectroscopy. /';;
"Neutron and synchrotron radiation for condensed matter studies", Springer-Verlag,
Berlin. 353 (1993).
9. Kosugi. N.. Theory and Anah sis of XANES. in "X-Ray Absorption Fine Structure
for Catalysts and Surfaces". Iwasawa. Y. Eds.. World Scientific. Singapore, 59
(1996).
10. Durham, P. J.. Theory of XANES. in "X-Ray Absorption", Koningsberger, D. and
Prins, R. Eds.. Wiley. New York. 53 ( 1988).
11. Bethe, FF. and Salpeter. F.. "Quantum Mechanics of One and Two Electron
Systems". Springer-Verlag, Berlin (1957).
12. Lee. P. A.. Teo. B. K.. and Simsons. A. L.. J.im Chem Sue. 99, 3856 ( 1977).
13. Fee, P. A., and Pendry, J. B„ Phys Rev B 11, 2795 (1975).
14. Stern. E. A., rim Rev B 10. 3027 ( 1 974).
15. Stern. E. A.. Savers, D. E.. and Lytic. F. \V„ Pin s Rev B IE 4836 (1975).
16. Fco, B. K., "EXAFS: Basic Principles and Data Analysis", Springer-Verlag. Berlin.
28(1986).
32
X-ray Absorption Spectroscopy
Stern. E. A., Eheory of EXAFS. in "X-Ray Absorption". Koningsberger, D. and
Prins, R. Eds., Wiley. New York. 37(1988).
Vaarkamp, M., Dring, F, Oldman. R. T. Stern, E. A., and Koningsberger, D. C,
Phys Rev. 5 50.7872(1994).
Savers, D. E.. and Bunker, B. A.. Data analysis, in "X-Ray Absorption",
Koningsberger. D. and Prins, R. Eds.. Wiley. New York, 215 (1988).
Cook, J. W. ,k, and Savers, D. E.,./ Appl Phys. 52. 5024 (1981).
Haskek D., Ravel, B., Newville, M.. and Stern, E. A., Physica B 209-209, 151
(1995).
Sayers. D. E.. Stern. E. A., and Lytle. F, W.. Phvs. Rev Lett. 27, 1204 (1971).
Vaarkamp. M.. Cat. Poday 39. 271 (1998).
Ankudinov. A. L., and Rehr, J. T, Phys Rev B 56. R1712 (1997).
Bunker. B. A., and Stern. F, A.. Phys. Rev. B 27. 1017 (1983).
Citrin. P. IT.. Eisenberger. P.. and Kincaid. B, M., Phys. Rev. Lett. 22, 3551 (1976).
Lengeler. B., J. Phys (Paris) 47, 75 (1986).
Teo. B. K.. and Lee. P. A.../. .4?;?. Client Soe. 101. 2815 (1979).
Brigham, E. O., "The Fast Fourier Fransform", Prentice Hall, FTiglcwood Cliffs,
New Jersey. (1974).
Vaarkamp. M., Linders. .1. C, and Koningsberger. D. C, Physica B 209 (1-4), 159
(1995).
Rockenberger. J., froger. L., Komowski. A.. Vossmeyer, E., Eychmüller, A.,
Feldhaus. T. and Weller, IF. J. Phys Chem B 101. 2691 ( 1997).
Shido. T., and Prins. R.. J. Phys. Chem B 102, 8426 (1998).
33
Chapter 2
i
i
Chapter 3
How does the Structure of NiMo/Si02 Oxidic
Precursors Influence the Activity of the Final
Sulfided Catalysts?
Introduction
The most stable Mo species on the silica surface, which has a point of zero charge at
pH 2, are polymolybdates [Mo-024.m(OIl)mj(A"mV [1. 2], although Raman studies suggested
the presence of interactions with the support and the formation of molybdosilicic acid |2.
3J. Nickel can interact with silica to form Ni phy llosilicates [4, 5]. This was. however,
seldom observed for Ni/SiCE samples prepared by the classical incipient wetness
impregnation method |6].
The addition of chelating ligands. such as nitrilotriacetic acid (NTA) or
ethylenediamine tetraacetic acid (EDEA). to the impregnating solution has a beneficial
effect on the catalytic activity of NiMo/SiCE catahsts [7-12]. fhe resulting catalysts are
even more active than alumina-supported catalysts. Ehe improved activity has been
ascribed to a better dispersion of nickel and molybdenum on the support in the catalyst
precursors and. in the case of NTA. to the delayed sulfidation of Ni [12]. Both factors
would enhance the formation of the so-called Ni-Mo-S type II phase, which, at present, is
considered to be the active phase in hydrotreating catalysts [9, 13, 14].
Previous work in our group concentrated on the effect of NTA on Ni and Mo in the
impregnating solutions and in the catal} st precursors |1]. It was found that NTA
Chapter 3
preferentially complexes Ni and causes a delà} in its sulfidation [1, 12]. Eo extend this
work, in this chapter the effect of other chelating ligands on NiMo/SiCE catalyst precursors
is reported, especially ethylenediamine (EN), and a more detailed structural description of
the metal ions on the support is given. The structure of the catalyst precursors is correlated
with the catalytic activity of the final sulfided catalysts in the hydrodcsulfuri/ation of
thiophene at atmospheric pressure. The explanation of the structure of the oxidic precursors
helps, therefore, to understand the factors that Influence the formation of active sites on the
support during sulfidation. In particular, the structural changes in the catalyst precursors are
revealed by means of extended X-ray absorption fine structure (EXAFS) and by UV-VIS
and Raman spectroscopy, as a function of the concentration of two chelating ligands,
cthylenencdiamine (EN) and nitrilotriacctic acid (NTA). The information attained with this
work is essential to understand how Ni and Mo behave during the sulfidation process. As
the next chapters of this thesis will show, a mechanistic explanation of the sulfidation of
NiMo/SiOo is possible only after having a clear idea about the structure of the catalyst
precursors.
Experimental methods
Sample preparation and characterization
All catalysts were prepared by pore volume impregnation with a solution containing
the metal salts and the chelating ligands. The solution was prepared by dissolving the
organic complcxing ligand in 15 ml of aqueous ammonia (25%, Flukapuriss. p.a.), adding
3.60 g M0O3 (25 mmol Fluka puriss. p.a.) and heating the solution to 80°C in order to
dissolve MoO, [1], After cooling to about 60°C. 2.18 g Ni(N(>,)v6(IEO) (7.5 mmol, Fluka
purum p.a.) were added and dissolved. At this point the solution turned from uncoloured to
the colour typical for the formed Ni complex, that varied from blue to dark violet
depending on the concentration and the kind of ligand employed. Ehe pH was then adjusted
to 8.0 with a 2 M UNO-, solution and the solution was diluted to 25 ml with deionized
water.
36
Structure of Precursors and Catalytic Activity
Several chelating ligands were tested. The investigated catalyst precursors arc listed in
Tabic 1. In all cases, the support was SiCE (C560 Chemie Uetikon), which had a particle
size of 125-250 um, a BEE surface of 565 m2/g. a BEE pore volume of 0.83 ml/g. and
which had been dried overnight at 120°C prior to impregnation. After impregnation the wet
powder was again dried overnight at 120°C with a heating rate of 2°C/min. To avoid the
destruction of the metallorganic complexes in the catalyst precursors before the sulfidation,
no calcination was carried out at higher temperatures.
TABLE 1. NiMo'SiCE catalysts prepared with different chelating ligands (molar ratio
Ni:Mo:ligand=0.3:1:0.3)
Catalyst Loading Loading nL/Ni log [Ki(NiL)j
Ni [%] Mo I%]
NiMo 1.4 7.5
NiMo-nitrilotriacctic acid (NTA) 1.3 7.2 4 ll.5
NiMo-EDEA 1.3 7.0 6 18.0
NiMo-cthy lenediamine (EN) 1.3 6.9 2 7.5
Ni-Mo-diethylenetriamine (D f) 1.3 7.0o
j 10.7
NiMo-tricthylcnctetraamme (ET) 1.3 7.2 4 14.0
NiMo-tetraethylenepentamine (TP) 1.3 6.9 5 17.8
NiMo-pentaethy lenehexamine (PIT) 1.3 7.3 6 19.3
NiMo-18-crown-6 1.3 7.3 .1
NiMo-formic acid 1.3 7.1 l 0.5
NiMo-citric acid 1.3 *7 1 4 4.2
UV-VIS reflectance measurements were carried out on a Perkin Elmer Lambda 16
spectrometer, equipped with an integration sphere that allowed measurements in the
reflection mode, using pure SiCE as a reference. Solutions were measured in 1 cm. quartz
cells. The NK concentration in the anah zed solutions was 0.03 mol/1, therefore the
impregnating solutions had to be diluted.
37
Chapter 3
The Raman measurements were performed on a Bruker Equinox 55 spectrometer
equipped with an F'E-Raman-Modul FRA 106. with a Nd:YAG laser (1064 nm). a CaF2
beamsplitter, and a Ce detector (Ü418-S) cooled by liquid nitrogen. About 10 mg of the
samples were pressed into a hole (diameter 2 mm. depth 1.5 mm) on an aluminium disk
(diameter 1 cm). The number of accumulated spectra for each sample was 4096.
Sulfidation and reaction
The sulfidation and thiophene HDS reaction occurred at atmospheric pressure in the
apparatus represented in Fig. 1. The presence of two parallel reactors allowed the
simultaneous sulfidation of a catalyst in one reactor and the catalytic testing of another one
in the other reactor, "fhe reactor contained in a quartz tube (inner diameter 13 mm. outer
diameter 16 mm, length 350 mm) equipped with a quart/ frit in the middle. A layer of
about 20 mm quartz wool was pressed over the frit. 100 mg of the catalyst precursors were
diluted and mixed with 1 g dried SiCE and the obtained powder was poured in the reactor
on the quartz wool with some additional quartz wool (20 mm) on it. Ehe quartz reactor was
heated by means of ovens consisting of an insulated cUindrical copper-bronze heating
block, into which four high power heating cartridges (220 V, 160 W each) were fitted. The
oxidic precursors were sulfided at 400°C (heating rate 6°C7min) for 2 h with a mixture of
10% ILS in FE (Messer Griesheim 3.0) that (lowed through the reactor from the beginning
of the heating process. Ehe activity of all catalysts was tested in the hydrodesulfurization of
thiophene at 400°C [12|. 'fhe feed, consisting of 3% thiophene in EL, was obtained by
bubbling PL through a series of four thiophene saturators that were cooled to 2°C. The
contact between the catalyst and the mixture FEThiophene had to be controlled very
carefully, 'fhe flow rate was always increased vet"} slowly from 0 to 75 mEs. The flow rates
of ELS and IE were regulated by means of Brooks thermal mass flow controllers. Ehe
products of the desulfurisation reaction were conducted directly to a gas Chromatograph
(Hewlett Packard FTP 5890) and analyzed on line. The temperature of the gas
Chromatograph was kept constant to 40°C and a good separation of the products was
obtained (in order of appearance 1-butène, n-butane. trans-2-butene. cis-2-butcne and
thiophene). The sample was injected in the column by means of a six-way-valve and a 125
pi loop, 'fhe column was a WCO f fused silica 0.25mmx 50 m capillary column with a CP-
38
Sti uctui e of Pi ecursors and Catalytic Activity
SIL 5CB coating A flame ionization dctectoi (FID) was used to analyze the composition of
the outcommg gas Ehe data were collected even 20 mm with a Nelson 900 Analytical
intelligent mteiface (Peikm Elmei), fiom which they were transferred to a personal
computet, wheie the data anah sis was petfoimed with the piogiam Turbochiom Veision
6 0 2 (Peikm Elmei) fhe tesults obtained E; h aftei the fust measiuement weic used foi
the determination of the kinetic paiameteis
h s:
Ile
II
NHL)t i
ne
ri\
c i, 11,
s I
HC
Btooks theimal mass
flow contiollei
f I Filter
On-oft \al\e
rc
I nie metenne; \al\e
Back pi essuie icgulatoi
Ihiee-/fout-vva\ valves
I IG 1 Plow scheme ol the tl lopiiciie hAdiodcsullutizition
1 Biooks the mill in iss llow conti olsi
2 I hcimostit (set it 2 C) with I thiophene sit ii itois
3 Oven loi icictoi
l Icmpei iluie eontiolkr
") Ois ehiomtto^nph IIP ->S)0
6 Intel (ice Pctlun I Imel )0H
/ Petsonil compute)
S Bubble llow nutet
(1 (low I lempetiluie 1 mlie it<u Ceoitiolle)
19
Chapter 3
The reaction rate was described with a first order expression in the thiophene partial
pressure pu,. The reaction rate constant was calculated assuming an ideal steady-state plug
flow reactor behaviour [J 5] with the formula:
k=irMrâJ (,)
where F0,, is the molar thiophene flow at the reactor inlet. W the catalyst weight and X the
thiophene conversion defined as:
AT1PTjA-p|
X=l-— M -——^ (2)P?h AIh+lACt
where p^ is the initial thiophene partial pressure. Aj, is the thiophene chromatographic
peak area and 2jA^ is the sum of the contributions of 1 -butène, n-butanc and eis- and
trans-2-butcne. If one takes Into account that the response factor of the FID built in the GC
is not the same for all species, the mass balance is in good agreement with this evaluation
of the conversion 116].
In order to directly compare catalysts with slightly different metal loadings, another
reaction rate was calculated based on the Mo loadine:
IE -k-|^-| (3)'nui,,
' v
where is the Mo loading in the oxide catal} st precursor.-tflsupp
40
Structure of Precursors and Catalytic Activity
EXAFS measurements
EXAFS spectra of several compounds were measured at the Ni and Mo A-edges. Ehe
data were collected at the Swiss Norwegian Beam Fine (SNBL, BM1) at the European
Synchrotron Radiation Facility (ESRF), Grenoble. France, fhe electron energy and ring
current were 6 GeV and 130-200 niA, respectiveh. At SNBL the incident X-rays are
monochromated by a Si (111) channel-cut monochromator and harmonics arc rejected by a
gold-coated mirror positioned at an angle of 7 inrad to the beam. The X-ray intensity is
monitored by ionizing chambers and the estimated resolution is 1 eV at the Ni A-edge and 2
eV at the Mo A-edge. For measurements at the Ni A-edge. the chambers for measuring In
and E were 17 and 31 cm long, and were filled with pure nitrogen and a mixture of argon
and nitrogen (Ar/AE - 40A0). respectively. Fhe chambers used for measurements at the Mo
A-edge were 17 and 62 cm long and filled with pure argon and a mixture of krypton and
nitrogen (Kr/Ni - 25/75), respectively. Some of the data were collected at the General
Purpose Italian Line for Diffraction and Absorption (GILDA, ID8) at the ESRF [17].
Ehe Ni A-edge spectra were divided into six regions: 7780-8300 eV (pre-edge). 8300-
8370 eV (edge region) and the four post-edge regions (3-6.5, 6.5-10, 10-15, 15-17 A"1)
between 8370 and 9433 eV. Ehe collection times for the data points in each scan region
were 1, 1, 2, 3. 4 and 4 s. respectively. As far as the Mo A-eclge was concerned, the
collection times for each data point in a scan were 1, 2. 2, 3 and 3 s for the intervals 19454-
19954 eV (pre-edge), 19954-20164 eV (edge region) and the three 6.5-10, 10-15, 15-21 Â"1
post-edge regions between 20164 and 21684 eV. respectiveh. Fhe distance between the
post-edge data points was determined so that the difference in their k values was smaller
than 0.05 A"1, Five scans were averaged for the Ni A-edge and three for the Mo A'-edgc.
Catalyst samples were pressed into self-supporting wafers and mounted in an in situ
EXAFS cell [18]. fhe thickness of the samples was chosen to adjust the total absorption to
u.x •=- 4 for the Ni A-edge (low Ni concentration) and the edge lump to 1 for the Mo A-edge.
Ehe sample was cooled to liquid nitrogen temperature for all measurements.
41
Chapter 3
EXAFS analysis
The program XDAP (version 2.1.5) was used to analyze and fit the data [19. 20]. The
pre-edge background was approximated by a modified Victorcen curve and the background
was subtracted using a cubic spline routine. The spectra were normalized by the edge jump.
The k'-weighted, and in some cases k1 -weighted EXAFS functions were Fourier
transformed and fitted in R-space. Ehe free parameters were interatomic distance,
coordination number. Debye-Waller factor and the correction of the edge energy. The
errors of the parameters were statistically estimated using the random errors of the observed
data. The goodness of fit was calculated for every model from the k- and R-space fit range
and the number of free parameters [21].
Reference spectra were calculated using FelT7 [22] for several cluster models that
will be discussed in more detail later. Crystallographic data were obtained from the
Inorganic Crystal Structure Database (ICSD-CRYS'llN) and the Cambridge Structure
Database (CSD).
42
Structure of Precursors and Catalytic Activity
Results
Catalytic Activity
The activity of catahsts prepared with various chelating ligands (Table 1) was
investigated. The ligands can be divided in three groups: the chelating amines, such as
ethylene diamine (EN), diethylenc triamine (DT), triethylcne tetraamine (IT), tetracthylene
pentamine (TP) and pentaethylene hexamine (PH), the combined amino-acetic acid-
containing complexes such as NTA and ED fA, the organic formic and citric acids, and
the crown ether 18-crown-6 (CT). All amines form very stable Ni complexes [23 j. In a Ni-
CE complex reported in the literature, Ni coordinates three of the six oxygen atoms of the
organic complex |24j. Ehe stability constants of the Ni complexes of formic acid and citric
acid are lower than those of the amines (Table I) [23]. Attempts to use oxalic acid were
abandoned because of the low solubility of nickel oxalate.
The results of the hydrodesulfurization measurements of thiophene at atmospheric
pressure are presented in Fig. 2. fhe Ni:Mo:ligand molar ratio was 0.3:1:0.3 in all catalyst
<4
CO
M
^4
00
p
0.12
0.1
0 08
0.06 -
CO
OO
0 04
CD
0.02 -
0
^P „O^ „V A^ n<0 (ZT M <<y *-sN>
$ XÎV ^ xs# K®^ ^ # X „A&
r>>
XP <?#
FIG. 2. Thiophene I IDS activ ity of Xi\lo SiO cataly sts prepared w ith different chelating ligands.
43
Chapter 3
precursors. Catalysts prepared with NEA and EDEA show the highest rate constants. Ehe
ni/Ni column in Table 1 gives the number of coordinating ligand atoms available per Ni
atom, taking into account all potential coordinating atoms and that the Ni/ligand ratio is 1
for all samples. The number of theoretically coordinating atoms of CE was set at three as
suggested in [24].
-àCO
M
oo
CD
00
Q
0.12 i .
*
;EDTA ,'
O,
0 1
0 0c fJ'
0.06
0 04
0.02 -
EN
NTA
ligand/Ni molar ratio
FIG. 3. Thiophene HDS activity of NiMo/Si02 catalysts prepared with different amounts ol
ethylenediamine and nitrilotriacetic acid (NEMo^O.S: l molar ratio).
The concentration of the complexing agents was varied for two catalysts in order to
follow their catalytic activity as a function of the composition of the coordination spheres
around the two metals. NfA and FN were chosen because of their different structures and
because of what is known about their complexing properties. In Fig. 3, the thiophene ITDS
rate constants of the obtained catahsts are plotted as a function of the ligand to Ni ratio. For
NEA a dramatic increase in catalytic activity is observed for the ligand to Ni ratio between
0 and 1. A maximum is reached between NTA'Ni ~ 1.5 and 2; the catalytic activity
decreases for higher NEA/Ni ratios. "The activity of catalysts prepared with EN is generally
lower but increases at first rapidly, then more gradually until EN/Ni = 4.
44
Structure of Precursors and Catalytic Activity
Characterization ofNickel
EV-VTS Spectroscopy. UV-VTS spectra of the impregnating solutions and of
several dried catalyst precursors were collected to investigate the coordination ol the NE
ions. Replacing coordinating oxygen atoms with nitrogen atoms shifts the VIS absorption
bands in the region between 500 and 700 nm. attributed to the JA2g(F) to Tig(F) transition,
to lower wavelengths [25]. Table 2 lists the wavelengths of this absorption band for
aqueous solutions of NkNCEEAfFliO) with different amounts of EN. The solutions were
prepared In the presence and absence of NEE. Ihe absorption maxima of both solutions
shift to lower wavelengths with increasing EN concentration. Those species that, according
to thermodynamics, should be present in aqueous Ni-EN solutions were calculated from the
stability constants of the Ni2' complexes using an extended version of the program Spex [1,
26. 27]. In the solution with EN/Ni ^ E 63% of Ni"' should be present as
[Ni(EN)(HA))t]2 20% as [Xi(ENTh(FI20h]2 and 17% as ENi(lEO)6]2+. Hence, in passing
from the solution without EN to the solution with FNXti =• 1, about one third of the oxygen
atoms of FLO coordinated to Ni are replaced with nitrogen atoms of EN. This explains the
large difference in the wavelength of the absorption bands of the two solutions. In the
9 i
aqueous solution with EN/Ni = 2, about 85% of Ni" should be present as
[^(ENEOEOEEE ,whereas for EN/Ni - 3. |\i(EN),|2 corresponds to 98% of all Ni2"
present in the solution.
TABLE 2. UV-VIS absorption bands of Ni-EN complexes in water and ammonia solutions
EN:Ni In H^O In ammonia solution
|nm] (8 M, pH 8.0) [nm]
0 723 608
1 618 595
2 565 562
3 545 546
45
Chapter 3
TABLE 3. UV-VIS absorption bands of the impregnating solutions (pH-8.0, Ni:Mo=0.3:l)
and of the dried catalyst precursors
9
10
9
8
7
6
5
4
3
2
1
EN:Ni Impregnating Dried catalyst
molar ratio solutions [nm] precursors [nm]
0 620 708
0.16 594 709
0 66 583 710
1.33 557 659
530 585
6.66 523 549
A
V
300
EN Ni=3 3
ENNi=1 33
EN NfO66
EN Ni=6 6
No ligand
400 500 600 700
Wavelength [nm]
^
800
FIG. 4. UV-VIS spectia of NiMoFN impiegnatmg •solutions containing diffeient amounts ot
ethylenediamine. ]Nf|,M- 0 0~î mol I. pi 1-8
46
Structure of Precursors and Catalytic Activity
1 he absorption maxima of the ammonia solutions have lower wavelengths than those
of the aqueous solutions for EN/Ni ~ 0 and 1 but are practically the same for higher EN
concentrations. This is due to the fact that when no EN is present in the solution, Ni2"1 is
present as 45% [NiCNHOe]2 -45% [Ni(NIl ATEO)^ and 10% rNi(NH3)4(FI20)2]2E When
KN is added to the solution, mixed Ni-EN-NFE,-I EO complexes are formed.
Ehe spectra of some NiMoEN impregnating solutions (Fig. 4) show that an increasing
EN concentration affects the Ni"1 environment. In the regions around 850, 500-650 and
300-400 nm, the absorption bands shift to higher energies with increasing EN
concentration. Ehe absorption maxima for the bands m the region between 500 and 650 nm
are listed in fable 3. Ammonia is present m the solutions; thus, the absorption maxima
should be compared with those of the ammonia solutions in fable 2. Ehe spectrum of the
0 07
0 06
s
0 05
0 04
0 03
0 02
0 01
ENNi=6 66
\ l
\\
ENNi=3 33
ENNi=1 33
ENN!=0 66
A"'aJ»
^.
A //
No ligand
^v,VMAAfv,Af
400 500 600 700 800
Wav elencth I nm]
FIG. 5. Reflectance UV-VIS spectra of dned \i\fo SiO cataKst ptecuisois piepaied with diffeient
amounts of ethv lenedtamine
47
Chapter 3
impregnating solution without EN has a maximum at 620 nm, whereas the Ni-NFE solution
has a maximum at 608 nm. Ehe impregnating solutions with the highest EN/Ni ratios (3.33,
6.66) show absorption bands with particularh low wavelengths. Ehe presence of Mo
explains this shift of the maxima, because it is the only parameter that is different to the
ammonia solutions. 'Fhe reflectance UV-VIS spectra of the dried catalyst precursors are
presented in Fig. 5. Mo compounds on the support absorb very strongly below 500 nm.
covering all signals arising from nickel in this region. Nevertheless, it is possible to
distinguish an absorption band for each spectrum. The absorption maxima arc listed in
Table 3. From the data and the spectra, it is clear that, for the catalyst precursor without a
ligand and for catalyst precursors with EN Ni = 0.16 and 0.66, almost no Ni-amine
complexes were present after drying. Some EN was apparently removed from the Ni2h ions
in the catal}st precursors with higher ligand concentrations (see Tabic 3), because the
wavelengths of the absorption maxima for the dried catahst precursors with EN/Ni 1.33,
3.33 and 6.66 are higher than those for Ni" complexes containing the same number of
amine ligands. Ehe values for Ni-EN complexes in aqueous solutions (Eable 2) suggest that
the dried catalyst precursors with EN/Ni - 1.33. 3.33 and 6.66 contain less than 1. fewer
than 2 and slightly fewer than 3 EN molecules per Ni"" ion. respectively.
NiXAS of NiMoEN/SiCE. To follow the change in structure of the catalyst
precursors as a function of the amount of ligand used during the preparation, we compared
fhe Fourier transformed yjkfk' functions measured at the Ni A-edge for five different
catalyst precursors: NiMo SiCE and four NiMoEN'SiCE compounds with EN/Ni = 0.16,
0.66. 3.33 and 6.66.
The Ni A-edge Fourier transformed x(k)-k' functions are shown in Fig. 6 (k-range:
3-14 À"1). Ehe results of the fits are listed in Eable 4. In the NiMo/Si02 spectrum, there are
two main peaks at 1.6 and 3 A (uncorrected for phases). The first is ascribed to the oxygen
atoms that surround the Ni" ions. The reference for oxvgen was produced with the Feff
code using a Ni-0 distance of 2.12 A. The first shell of the catalyst precursors prepared
with EN was analyzed with references for Ni-0 and Ni-N that were calculated with the
Feff code, with distances and coordination numbers taken from the crystallographic data
of the complex Ni(EN)2-2EEO [28]. Ehe first shell of the Ni2 ions of the catalyst precursors
48
Structure of Precursors and Catalytic Activity
0 12 3 4 5 6
R[A|
FIG. 6. Absolute pairs of the formet tiansforms of the \'i x-edge ySf)k EXAFS of NiMoAtCA catalyst
piecursois as a iunction ol the amount of ethylenediamine used dutmg the prepaiation (k-range 3-14 Ä ')
with EN/Ni = 0.16 was fitted using an oxygen contribution only, since EIV-VIS showed
that no EN molecule is surrounding Ni in the catah st precursor (sec Eable 3). In contrast,
for the catalyst precursors with 1 N'Ni - 0 66 and 3 33. both the nitrogen and the oxygen
contribution were used for the fitting. Nevertheless, a relativel} low Ni-N coordination
number (0.2) was obtained. For the EN'NT - 3 33 catalyst precursor, it was 3.7, whereas
fitting with oxygen gave a coordination number of 3 2. Ebese results agree with the
UV-VIS measurements that rev ealed Ni to be surrounded, on average, by fewer than one
and by two FN molecules for the catahst precursors with ENAh - 0.66 and 3.33,
respectiveh. Moreover, in the same spectrum, the features of a Ni-EN coordination become
visible. Ihe peak at 2.5 A (not phase-corrected) arises from the scattering of the carbon
atoms of EN. as was seen in a Eefi7 simulated spectrum (Fig. 7) of the complex
49
Chapter 3
Ni(EN)2-2IEO (28], from which we calculated the Ni-C reference. Ehe small peak at
2.15 A does not belong to a real distance, it is caused by an interference of the signals of
the nitrogen and oxygen atoms in the first shell. Ihe Ni-C coordination number of 2.1 for
the catalyst precursor with FN/Ni= 3.33 is. however, lower than expected; since it should
equal the Ni-N coordination number (3.7). Ehe greater distance from the central atom may
lead to an enhanced mobility of the carbon atoms and a resulting underestimation of the
coordi nation number.
TABLE 4. Structural parameters resulting from the Ni A-edge Fourier-filtered k1-weighted
EXAFS functions of the dried NiMoEN'SiCE catahst precursors prepared with different
amounts of ethvlencdiamine
Catalyst
precursor
Shell Ncomd, Ä|A] A<r
[1(E\42]
AEo [eV] Goodness R-range
of fit pA]
NiMo 0 4.4 1.99 -2.50 7.8
Si 4.5 3.35 -2.91 4.7
EN:Ni=O.I6 C) 6.8 2.00 3.19 5.3
Si 3.0 3.35 -3.85 -4.8
EN:Ni-0.66 N 0.2 1.82 -6.73 3.0
0 6.4 2.03 3.83 3.7
Si 1.5 3.37 -3.63 -7.2
EN :Nh 3.3 3 N 3.7 2.03 1.10 0.8
0 3.2 2.11 1.18 5.3
c 2.1 2.82 4.24 11.9
SI 0.8 2>.J / -3.46 _7,7
EN:Ni-6.66 N 5.5 "2.13 0.51 -1.5
c 8.0 2.92 4.17 3.1
3.50 0.60-3.60
0.82 .00-3.6
1.08 1.00-3.4
1.50 1.00-3.40
2.09 1.20-2.90
50
Structure ofPrecursors and Catalytic Activity
8
6
R|A1
FIG. 7. Fouiiet transform of the yyk) E £\\VS of»KENT 2H>0 simulated by Fcft7
In the case of the catalyst precursor with ENNi ~- 6.66. fitting with the Ni-0
reference yielded a coordination number of onh 4.1. With the Ni-N reference, in contrast, a
coordination number of 5.5 was obtained, closer to the value of 6 for the expected
octahedral geometry around nickel. Ehe degrees of freedom were not sufficient to perform
a fit using the Ni-N and the Ni-0 contributions simultaneous!}. fhe second shell was fitted
with a Ni-C contribution, but the resulting coordination number was 8, which is too large
even for a Ni(EN)3 complex. Moreover, the features of the fitting curve suggested that an
additional contribution overlaps with that of Ni-C. Several attempts (Ni-Ni. Ni-Si. Ni-Mo)
were made to identify the element responsible for the second contribution but with no
success. From the UV-VIS spectra, the average number of EN molecules around Ni in the
catalyst precursor with EN'Ni - 6.66 should be slightly smaller than 3. It is not possible to
calculate the exact number of EN molecules coordinated by Ni by means of EXAFS,
because the N and O atoms in the first shell can not be distinguished, even though it is clear
51
ChapIer 3
that nitrogen is the main contribution and the number of carbon atoms resulting from the fit
is overestimated.
As far as the second shell is concerned, comparing the spectrum of NiMo/Si02 with
the spectra of the catahst precursors with FN Ni - 0.16 and 0.66, there is a clear decrease
in the intensif} of the signal at 3.05 Ä (not phase corrected). This is assumed to be
generated by a Si shell of the support. Clause et al [29] reported on Ni silicates in their
Ni/Si02 compounds that were prepared by incipient wetness impregnation as well as by ion
exchange. In their EXAFS spectra they observed signals mainly at 2.8 A (not phase-
corrected) that they attributed to a combined contribution of Ni and Si. ITowever, they
measured all samples at room temperature. In contrast, in our spectrum, judging from the
distance from the central atom, the peak at 3.05 A is thought to arise exclusively from a Si
contribution. Since the measurements were carried out at liquid nitrogen temperature, the
contributions of the thermal vibrations are drasticalh reduced, enabling the detection of a
cleaner signal of the backscatterer atoms. The reference for the Si shell was calculated with
the Feff code using a Ni-Si distance of 3.3 A. A fit with a Ni-Mo coordination was
attempted, but the F-tcst showed that there is a 85% probability that the Ni-Si model
describes the system better than the Ni-Mo shell. In the catalyst precursor with
EN/NI = 0.16. the Ni-Si coordination number decreases from 4.5 to 3.0. The signal of the
Ni-Si shell becomes smaller, but also broader, with increasing amounts of EN, but it is still
present at EN/Ni - 3.33. until it disappears at the highest FN concentration.
52
Structure of Precursors and Catalytic Activity
NiXASofNiMoNTA/Si02. The ^-weighted Fourier transformed EXAFS
functions of NiMoN fA/Siü2 catalyst precursors with NEA/Ni = of 0.25, 0.66. 1.5. 3.0 and
6.66 (Fig. 8) show that N fA strongly affects the first and second shells around Ni. The k-
range of the presented data is 3-14 Â"1 for the catalyst precursor without a ligand as well as
for the precursors with NTA Ni = 0.25, 1.5. 3 0. 6.66. while It is 3-10 Â"1 for the catalyst
precursor with NEA/Ni = 0.66. Ehe second signal at 2.3 A (not phase corrected) arises from
the carbon atoms of NTA. The results of the fits (Table 5), with the exception of the
precursor catalyst with NTA/Ni = 1.5, clearly show that the Ni-C coordination number
increases gradually with increasing amount of NFA. For the catalyst precursor with
NTA/Ni - 0.25 the signal was too weak to be fitted. On the contrary, for NTA/Ni - 1.5. the
Ni-C coordination number obtained by the fit was too large (9.0), as in the case of the
catalyst precursor prepared with ethylenediamine with EN/Ni = 6.66 (sec above). For
. H
r-i
cd
3'
5 a. u.
NlANi=6 66
NTA Nf3 0
NTA Ni=1 5
NTA Ni=0 66
NTA Ni=0 25
No ligand
R[w]
FIG. 8. Absolute parts ot the foutier ttanstoims of the \'t x-edge y.(k) k' EXAFS of NtMo/SiO;
catalyst precursors as a function of the amount ot nitttlottiacetic acid used dining preparation de¬
range 3-14 A k except for X" fA \u - 0 66 k-range 3-10 A ')
53
Chapter 3
NTA/Ni = 0.66. the average Ni-C coordination number is only 0.9, while it increases to
almost 5 and 6 when the ratio is 3.33 and 6.66. respectively. In fact, if it is assumed that all
NTA present binds to Ni and that Ni is complexed by the three acetate arms of the chelating
agent, then the Ni-C coordination number should be 3.9 for NTA/Ni- 0.66 and 6 for
NTA/Ni =- 1. Two explanations for such a low experimentally observed Ni-C coordination
number are that the nickel ions exchanged some of the ligand bonds for interactions with
the support and are coordinated by only two ol' the three acetate arms and by the nitrogen
atom of the NTA molecules, or that a fraction of the nickel is present on the support in
another structure. When NTA/Ni - 6.66. the Ni-C coordination number is about 6,
suggesting that all Ni atoms are completely complexed by one N fA molecule.
The Ni-Si contribution at 3.05 Ä (not phase corrected) is still present for the catalyst
precursor with the lowest NTA concentration but is not detected at higher NTA
concentrations.
TABLE 5. Structural parameters resulting from the Ni A-edge Fourier-filtered k -weighted
EXAFS functions of the dried NiMoNTA/Si02 catalyst precursors prepared with different
amounts of Nl'A
Catalyst Shell Ncooni. R [Â] ArT AEo Goodness R-range
precursor IH)"3Â2] [eV] of fit [Ä]
3.50 0.60-3.60
1.69 1.00-3.60
4.33 1.00-3.00
4.1 1.00-2.70
1.62 1.00-2.70
2.75 "0.60-2.80—
54
NlMO 0 4.4 1.99 -2.50 7.8
Si 4.5 3.35 -2.91 4.7
NTA:Ni=0.250 5.9 2.02 0.53 4.0
Si 3.1 3.35 -4.19 -4.0
NTA:NE4),660 5.8 2.05 1.44 2.9
c 0.9 2.86 -13.82 9.5
NTA:Ni=-E500 6.0 2.04 1.45 1.8
c 9.0 2.83 2.03 -2.0
NTA:Nr=3.000 5.7 2.04 0.84 4.1
c 4.8 2.87 -0.78 3.6
NTA:NU6.66 0 5.1 2.05 0.70 2.8
C 5.9 2.84 1.45 8.0
Structure of Precursors and Catalytic Activity
Characterization ofMo
Laser Raman Spectroscopy. Raman spectra were recorded for dried catalyst
precursors. The spectra of Si02-supported Mo. NiMo. and NiMoEN (EN/NI - 0.16, 0.66)
are presented in Fig. 9. Catalyst precursors containing NTA and higher EN concentrations
could not be investigated b\ Raman, because the samples were destroyed by the laser beam.
Ehe spectra show common features arising from silica (bands at 485, 217 cm" ) and from
the NO/ ion (710 cm"1). The bands at 973, 955 and 616 cm" in the spectrum of the sample
containing onh Mo correspond in wave number to (Emolybdosilicic acid [3], a Eeggin type
structure consisting of 12 octahedral MoO(1 units surrounding a Si04 tetrahedron [30].
However, the high intensity of the band at 955 cm"1 and the presence of the band at 367 cm"
1and the shoulder at 238 cm"1 suggest that poh moh bdates are also present on Si02. The
mentioned Raman bands are consistent with octahedral!} coordinated polymolybdates
NiMo,
EN:Ni=0.66
NiMo
EN:Ni=0.16
NiMo onlv
Mo onh
1000 900 800 700 600 500 400 300 200
Wave number (cnf
FIG. 9. Raman spectra of SiO-supported catalyst precursois
Chapter 3
interacting weakly with the silica surface, similar in structure to the aqueous
isopolymolybdate anions M07O946" and MogCEf,4" [2, 31]. Several aqueous
isopolymolybdate anions composed of edge sharing MoOe octahedra display an intense
Mo-0 symmetric stretching vibration from 943 to 965 cnf' [32, 33]. Moreover, the weak
shoulders at 900 and 315 cm"1 evidence the presence of molybdates in this supported
material.
A band at 961 cm"1 is visible in the spectrum of the NiMo/Si02 sample, similar to the
955 cm"1 isopolymolybdate band in the Mo-only sample. Nevertheless, the signals at 905
1 ^
and 315 cm arc characteristic for MoOf" 133-36]. As a consequence, in the absence of a
ligand, Mo Is present on the support as a mixture of polymolybdate clusters and isolated
MoCEf~
molecules.
In the spectra of catalyst precursors prepared with EN a shoulder around 900 cm"1,
typical of MoOf2" was also observed. The two bands at 960 and 940 cm"1 are, nevertheless,
the most intense signals. It is difficult to sa} whether both bands arise from polymolybdate
species, or just one of them, fhe weak signal at 360 cm"1 is a further proof that such
clusters are present. Polyanions in aqueous solutions and supported on silica show a band
around 960 cm"1 [2, 31]. whereas for crystalline (NEE)6MoA02p4IFO and (NH0&Mo2O7 the
band shifted to 937 cnf' |37|. Ehis suggests that two kinds of Mo polyanions are present in
the samples prepared with EN: one which forms weak interactions with Si02, the other
which is like the crystalline compound and is not influenced by the support.
Mo XAS of NiMoEN/SiOi. Fig. 10 shows the Fourier transformed k'-weighted
EXAFS functions of NiMo'Si02 and NiMoEN SiCE catalyst precursors with EN/Ni - 0.16,
0.66, 3.33 and 6.66. Ehe results of the fits are presented in Table 6. We did not fit the Mo-0
shells, because the overlapping Mo-0 contributions at différent distances cause a reduction
in the signals, with consequent underestimation of the coordination numbers (38]. The
signal at 3 Â, due to the backscattering of the first Mo-Mo shell, was fitted with 0.9 Mo-Mo
contributions for the catahst precursor without a ligand. The Mo-Mo coordination number
increases to 1.3 for the catahst with FN Ni - 0.66. Ehe Mo-Mo contribution is more
intense and narrower for the catahst precursor prepared without a ligand than for the
56
Structure ofPrecursors and Catalytic Activity
3 a. el
^s
ai
u
tS:
\
ENNi=6 66
ENNF3 33
EN Ni=0 66
ENNi=0 16
R[A|
FTG. 10. Absolute parts of the I ouner transforms of the \lo A-edge y(k) C EXAFS of NiMo/SiCE catalyst
precursors as a function of the amount of ethylenediamine used dui mg preparation (k-range 3-17 A ')
catalyst precursors prepared with EN; the Mo-Mo coordination number, however, is
comparable. Ehis is a sign of a better defined distance distribution of the Mo-Mo shell. Ehe
signal splits into two parts in the spectrum of the catahst precursor with EN/Ni - 6.66,
suggesting that the structure of the second shell in this compound is more complex than in
the other catalyst precursors. A fit with two Mo-Mo contributions was carried out and gave
0.5 and 0.4 Mo neighbour atoms at a distance of 3.16 and 3.42 A. respectively, typical Mo-
Mo distances in Mo-CEf" clusters. A similar split was observed in the k3-weighted EXAFS
spectrum of (NFEEMo-^i simulated with the Eefi7 code. The reference for the Mo-Mo
shell was taken from the experimental Mo-Mo EXAFS contribution in MoS2.
57
Chapter 3
TABLE 6. Structural parameters resulting from the Mo K-edgt Fourier-filtered
k -weighted EZXAFS functions of the dried NiMoEN/Si02 catalyst precursors prepared with
different amounts of ethvlenediamine: nearest Mo shell
Catalyst Shell JNoohI, R[Al Act2 AE() Goodness R-range
precursor I10"3Â21 [eV] of fit [Â]
NiMo Mo 0.9 3.29 -0.81 6.0 2.39 2.70-3.30
ENfNfi-0.66 Mo 1.3 3.30 0.90 7.2 2.35 2.70-3.30
EN:Ni-3.33 Mo E0 J.JO 0.33 3.0 3.68 2.75-3.35
EN:Na6.66 Mo 0.5 3.16 -1.65 15.6 4.09 2.60-3.60
Mo 0.4 3.42 -4.20 -3.9
Mo XAS of NiMoNTA/SiCE. Ehe features of the Mo A-edge Fourier transformed
l</-weighted EXAFS functions of NiMoNEA'SiCA catalyst precursors, plotted in Fig. 11,
demonstrate how Alo is affected by the presence of the chelating ligand. To understand the
changes caused by the increasing NTA concentrations, it must be taken into account that
the formation of the [MoO^NTA)]'" complex is possible only when NTA is present at
NTA/Ni ratios higher than one. because Nl'A is much more strongly bonded by Ni21 than
by Mo61 [1|.
The Mo-Mo contribution at 3 Ä (not phase-corrected) is reduced drastically when the
NTA/Ni ratio is increased from 0.66 to 1.5. \s can be seen from the spectra of the catalyst
precursors with NEA/Ni - 0.25 and 0.66, Mo does not react to the presence of the chelating
agent, and some polymolybdate is still present on the support. In the three cases in which a
Mo-Mo contribution at 3 A is observed, the Mo-Mo coordination number is always around
1.1. In the spectra of the three catalyst precursors with NTA/Ni ratios higher than I. a new
weak signal around 2.8 A is observed. Its intensify was too low to carry out a significant fit,
however.
58
S/; ucture of Precursors and Catalytic Activity
5 a. u.
NTA Ni=6 66
NTA Nf3 00
NTANi=1 50
NTA NfO 66
NTA NfO 25
No ligand
R|A]
II. Absolute patts of the fouttet ttanstoims ot the \lo A-edge y(k) E EXAFS of NiMo'Si02
st piecuisots as a function otthe amount of mttilottiacettc acid used duttng piepaiation (k-iange 3-
)
5
Chapter 3
Discussion
The beneficial effect of chelating ligands during wet catalyst preparation on FIDS
catalytic activit} is not limited to mixed amino-acetic acid-containing complexes such as
NTA and EDTA. Amines, crown ethers and organic acids also improve activity. However,
ligands with the largest number of coordinating atoms (NTA. EDTA, PT, citric acid) have
the strongest effect. The ligand/Ni ratio was 1 in all cases. For NTA and EDTA this means
that Ni. but not Mo. is coordinated by the ligands [I ]. fhe Mo A-edge EXAFS showed that
this is also true for EN (Fig. 10). This is likely to be the case for the other amine ligands as
well, since Ni forms very stable complexes with these ligands (Table 1). These
observations indicate that better protection of the nickel ions in the catalyst precursors
enables the preparation of more active catalysts. EAr-\TS results showed that, in the
absence of ligands, the ammonia molecules in the first coordination shell of Ni arc
substituted by oxygen atoms. Ehis is due to the evaporation of NEE, during the drying
procedure, leaving behind Ni ions that are exclusive!} coordinated by oxygen atoms from
H20, OH", 0~" or SiO" [1 [. Ehis was confirmed by the Ni A-edge EXAFS data, which
clearly showed a Ni-Si interaction. In fact, the Ni-Si coordination number of 4.5 suggests
that Ni is cither adsorbed on the surface or built in the Si02 frame in a tetrahedral position,
since the Ni-0 coordination number is also close to 4,
For the dried EN-eontaining catalyst precursors. UV-VIS and EXAFS showed that
EN could be removed from the Ni"' ions as well, the EXAFS curve fittings indicated that
Nr'
has no carbon neighbour in the catalyst precursors when EN/Ni = 0.16 and 0.66. about
two carbon neighbours when EN Ni - 3.33 and probably six carbon neighbours in the dried
catalyst precursor with the highest EN amount. The reason for the decrease in EN
molecules coordinated to Ni" after the drying procedure is again the exchange of ligand
molecules for oxygen atoms of the support. Ronneviot et al. observed the same
phenomenon for Ni/Si02 samples prepared from [\T(N1 FEI"4" by i°n exchange [25].
Burattin el al used the deposition precipitation method to deposit Ni2^ on Si02 [4, 5J.
According to these authors, the Ni(II) hvdroaqua complexes close to the silica surface can
react with the silanol groups via an h} droh tic adsorption:
60
Structure of Precursors and Catalytic Activity
Si-OH+[Ni(El20)TOET)2] +± [Si-0-Ni(IEO)4(OH)]+ FI20 (4)
They suggest that a Ni phyllosilicate layer forms on the support with subsequent
stacking. We concluded from EXAFS spectroscopy that, in the absence of ligands, nickel is
present almost exclusively as nickel silicate. EN inhibits the formation of nickel silicates,
because the chelating diamine molecules prohibit hydrolytic adsorption on the support.
Nevertheless. UV-VIS spectroscopy demonstrates that a fraction of EN coordinated to Ni
is removed during drying and is substituted by oxygen atoms of the support. Ehe EN/Ni
ratio must be substantially higher than 6 to ensure that all the Ni is coordinated by three EN
molecules.
NTA behaved different]} to EN. Ni-Si interactions were observed only at the lowest
NEA concentration, whereas in catalyst precursors containing larger amounts ofN FA, Ni is
present as isolated ions on the support. Ehis can be explained by the fact that EN has only
two binding sites, whereas NFA has four and forms much more stable complexes with Ni
than does EN, protecting Ni better and hindering interactions with the support. For this
reason, the activity of catahsts prepared with N EV Is much more dependent on the ligand
concentration for NEA/Ni ratios below 1. FA en for this chelating ligand the amount needed
to keep all Ni fully coordinated by NTA is eonsiderabl} higher than the expected ligand/Ni
ratio of 1. The formation of [Ni(NTA)2|4" would also be possible with larger amounts of
NTA. Flowever, the complex w ith only one ligand molecule seems to be the species present
on the support, since in (Ni(NTA)2]4" Ni would have eight neighbouring carbon atoms,
whereas the NI-C coordination number obtained from the fit of the spectrum of the
precursor catahst with NfA'Ni = 6.66 was onh 5.9.
'Fhe addition of ligand molecules causes the complexation of the Ni ions, and the
decrease in the number of Ni support connections. We could show that, as far as silica is
concerned, the weaker the Ni-SiCE interactions in the catalyst precursor are, the higher the
catalytic activity of the eventual catahst is. In fact, the absence of links with the support
enables better mobilit} of Ni on Si02 so that, during sulfidation. it can easily move closer
to the MoS2 and form the Ni-Mo-S phase, fhe presence of the ligand. meanwhile, delays
the sulfidation ofNi and subsequent formation of NES? [ 12. 39 J.
61
Chapter 3
Raman spectroscopy indicates the presence of different Mo-species, polymolybdates
and isolated M0O4A Ewo kinds of polymolybdates seem to be present on the support, one
which is like the crystalline compound and is not influenced by the support, the other which
forms weak interactions with Si02. EXAfS revealed no evidence of a Mo-support
interaction, as was indicated by Raman spectroscopy. EN does not have a significant effect
on the structure of the Mo compounds. NTA. on the other hand, has a dramatic effect on the
structure of Mo when it is present at NTANi ratios higher than 1. Mo then forms the
complex [MoOANTA)jA Since the Ni/Mo molar ratio is 0.3/1, all Mo will be complexed
by NEA only when NTA/Ni = 4.3. We conclude from the Mo A-edge spectrum of the
catalyst precursors with NTA/Ni - 1.5 that the first effect of NEA is that polymolybdate
clusters are not formed, since a Mo-Mo contribution was not observed.
Ehe catalytic activity of catalysts prepared with NTA decreases for NTA/Ni ratios
higher than 2. At this NTA concentration, about one third of the Mo atoms should be
complexed with NTA. The further decrease in catahtic activity corresponds to the gradual
complexation of Mo, suggesting that NTA has a negative effect on the performance of the
catalysts when it interacts with Mo. In fact, the presence of NTA can delay the formation of
MoS2 crystallites, so that at the temperature at which the MoS2 crystallites are formed, Ni
has already reacted to NES2 and can not join the MoS2 edges and constitute the active sites
[391.
Conclusions
Catahtic tests in the lndrodesulfurization of thiophene at atmospheric pressure
showed that a wide variety of organic ligands have a beneficial effect on the catahtic
activity of NiMo catalysts supported on SiO?. This observation already demonstrates that
the improvement in catahtic activity is closeh connected with the effect that the used
organic molecules have on Ni. because all chosen catahsts form stable complexes with Ni
but not all of them interact with Mo. Ehis Iwpothesis is reinforced by the fact that the
activity of catalysts prepared with EN and NTA exhibited a significant improvement with
ligand/Ni ratios between 0 and 4 for FN and from 0 to 1.5 for N FA, whereas a decrease in
the acthity was observed for higher ligand concentrations. Amounts of chelating ligands
62
Structure ofPrecursors and Catalytic Activity
higher than EN/Ni - 4 and NTA/Ni = 1.5 do not to affect the structure of Ni any more, but
only that of Vfo as E/XAFS measurements showed.
The increase in activit} is explained by the elimination of Ni-support interactions
obtained b} the addition of the ligands. In the case of catalyst precursors prepared with EN.
EXAFS reveals a Ni-Si shell until EN/NI - 3.33. whereas NEA eliminates such interactions
already at NTA/Ni lower than 0.66. For these reasons the activity profile of catalysts
prepared with EN shows a slower increase in respect to the ligand concentration. Moreover,
UV-VIS spectroscopy showed that part of the FN is removed from the Ni environment
during the drying procedure.
Raman spectroscopy and Mo A-cdge EXAFS both showed that Mo is present on the
support as a mixture of MoOf" and polymolybdate clusters. Raman spectroscopy suggests,
furthermore, the presence of polymolybdate clusters interacting with the support. The
observation of two kinds of polyanions should not to be underestimated because it is
important for the interpretation of the results describing the sulfidation mechanism of the
catalysts that will be discussed in the next chapter. The presence of the chelating agents,
especially of EN. could influence and enhance the formation of one of the two
polymolybdates and therefore change the sulfidation behaviour of Mo. Fourier-filtered Mo
A-edge EXAFS spectra of the samples containing different amounts of EN and NfA
showed that EN has hardly an} effect on Mo. whereas for NTA concentrations higher than
I [MoOsINTA)]'" is formed, fhe formation of this complex has a negative effect on the
performance of the catalysts as the decrease in catahtic activity of catalysts with NTA/Ni
ratios higher than 2 showed. Ihe reason for this negative influence has to be searched in the
changes of the sulfidation behaviour of Mo and in the consequences on the structure of the
sulfided catalysts. Ihese factors will be considered, among others, in chapters 4 and 5.
Literature
1. Medici. E.. and Prins. R„ ./ Catal 163. 28 (1996).
2. Williams, C. C, Ekerdt, J. G„ Jehng. E. Hardcastle. F. D., Turck, A. M.. and
Wachs, f. R...7 Pins Chem 95. 8781 (1991),
63
Chapter 3
3. Banares, M. A.. JIu. H.. and Wachs, 1. F..,/ Catal. 155, 249 (1995).
4. Burattin. P.. Che. M.. and Louis, C. J. Phys Chem /J 101. 7060 (1997).
5. Burattin. P., Che, M., and Louis, C,./ Phys. Chem. B 102, 2722 (1998).
6. Louis, C, Cheng, Z. X.. and Che, M..,/ Phys. Chem. 97, 5703 (1993).
7. Thompson. M. S... European Patent Application, 1986.
8. van Veen, E A. R., Gerkema, E., van der Kraan, A. M., and Knocster, A., J. Chem.
Soc, Chem. Commun. 1987, 1684 (1987).
9. Eouwers, S. P. A., and Prins, R., J. Catal 133, 94 (1992).
10. Hiroshima. K.. Mochizuchi, T., Elonma. T., Shimizu, T.. and Yamada, M., Appl.
Surf Sei. 121/122. 433 (1997) and Chapter 4 of the present thesis.
11. Inamura, K., Uchikawa. K., Matsuda. S.. and Akai. Y.. Appl. Surf. Sei. 121/122, 468
(1997).
12. Medici, L., and Prins. R.../. Catal. 163. 38 (1996).
13. Candia. R., Sorenson. O.. Villadscn. E. Eopsoe. N. Y., Clausen, B. S., and Eopsoe,
FF, J. Phys Chem. 95. 123(1991).
14. Eijsboufs, S.. Appl. Catal. .1158, 53 (1997),
15. Levenspicl. O., "Chemical Reaction Engineering", .lohn Wiley and Sons. New York
(1972).
16. Dietz. W. A.. J. Gas Chrom 5, 61 (1967).
17. Pascarelli. S., Boscherini. F., D'Acapito. F., Hardy, E, Meneghini, C, and Mobilio,
S.,,Z Synchrotron Rad 3. 147(1996).
18. Kampers. F. W. IE. Maas. E. M. E. van Grondelle, E, Brinkgreve, P.. and
Koningsberger, D. C, Rev. Sei lustrum. 60. 2fc>35 (1989).
19. van Zon. J. B. A. D.. Koningsberger. D. C. van 't Blik. EI. F. E. and Sayers, D. E.. J.
Chem. Phys. 82.5742(1985).
20. Kirim, P. S., van Zon. F. B. M., Koningsberger, D. C, and Gates, B. C, J. Phys.
Chem. 94.8439(1990).
21. Lytic, F. W.. Sayers. D. E.. and Stern. E. A.. Physica B ÏS8. 701 (1989).
22. Zabinsky, S. !.. Rehr. J. E. Ankudinov, A., Albers, R. C. and Eller, M. E. Phys,
Rev. B 52, 2995(1995).
23. "Ginelin Handbook of Inorganic Chemistrv ". Springer Verlag (1989).
64
Structure ofPrecursors and Catalytic Activity
24. Larson. S. B., Simonsen, S. H., Ramsden. J. N., and Lagowski. J. E, Acta Cryst.
C45. 161 (1989).
25. Bonneviot, L., Legendre. 0., Kcrmarec, M., Olivier, D., and Che, M., ./. Colloid
Interface Sei. 134, 534 (1990).
26. Martelk A. E., and Motekaitis, R. E. "fhe Determination and Use of Stability
Constants", VCII, New York (1988).
27. Smith. R. M„ and Martelk A. E., "Critical Stability Constants", Plenum Press, New
York (1989).
28. Garcia-Granda. S.. and Gomez-Beltran. F.. Acta Cryst. C40, 1145 (1984).
29. Clause, O., Kcrmarec. M.. Bonneviot. L.. Villain. F., and Che, M., ,/. Am. Chem.
Soc. 114,4709(1992).
30. Greenwood. N. N.. and Earnshaw. A.. "Chemistry of the Elements", Pergamon
Press. New York (1986).
31. Williams, C. C, Ekerdt. J. CE. Jehng. E, Ilardcastle. F. D., and Wachs. I. E.. J. Phys.
Chem. 95.8791 (1991).
32. Griffith. W. P.. and Lesniak, P. J. B..,/. Chem Soc. A.1066 (1969).
33. Eytko. K. Ik. and Schonfeld, B. Z.. Z Xa/wforsch 30B. 471 (1975).
34. Gonzales-Vilchez. F.. and Griffith, W. P.. J Chem Soc. Dalton Trans. 1972. 1416
(1972).
35. Icziorowski. FE. and Knözinger, FF../ Phys Chem. 83, 6642 (1980).
36. Payen. E.. Grimblot. E. and Kasztelan. S.. ,7 Phvs. Chem. 91, 6642 (1987).
37. Ilardcastle. F. D.. and Wachs. I. E.. J. Raman Spec!/: 21, 683 (1990).
38. Kisfaludi. G.. Leyrer. E. Enö/inger. IE. and Prins. R., J. Catal. 130, 192 (1991).
39. Cattaneo. R.. Weber, Hi.. Shido, E„ and Prins. R., J. Catal. 191, 225 (2000),
65
Chapter 3
, //
66
Chapter 4
A Quick EXAFS Study of the Sulfidation of
NiMo/Si02 Hydrotreating Catalysts Prepared with
Chelating Ligands
Introduction
To optimize the reactivity of the CoMo and NiMo hydrotreating catalysts used in the
removal of S and N atoms from petroleum derivatives, a better understanding of the
processes involved in the production of such catahtic materials is needed. This chapter
reports a study of a fundamental step in the preparation of hydrotreating catalysts:
presulfiding. Presulfiding is carried out before catalysts are used, in hydrotreating
reactions, in order to convert the oxidic catalyst precursor in the final sulfided catalyst. The
information presented in the previous chapter about the catalyst precursors will be the basis
of the discussion about the mechanisms involved in sulfidation.
Several studies have been made of this pretreatment process using various techniques
such as temperature-programmed sulfidation [1], laser Raman spectroscopy [2]. extended
X-ray absorption fine structure (EXAFS) [3. 4]. transmission electron microscopy [5] and
surface science techniques [6J. Presulfiding consists in heating the catalyst precursor to
400°C in the presence of H2S. Because of these conditions and the dynamics of the system,
a reliable study must be carried out //? situ. In this work. Quick EXAFS was used to
characterize the various steps in the sulfidation ofMo and NT.
Chapter 4
Despite the fact that the first goal of this work was an investigation of the influence of
chelating ligands on the sulfidation temperature of Mo and Ni, during the analysis of the
data important information was obtained also on the sulfidation mechanism of Mo and
partly of Ni in absence of chelating agents. In fact, a complete understanding of the effects
of chelating ligands on the sulfidation of the employed metals is only possible after
clarifying the basic steps in the preparation of the unmodified catalysts.
Experimental methods
Sample preparation and tests ofactivity
All the catalysts were prepared on an Si02 carrier. Ehe support material (C560,
Chemie Uetikon; particle size: 125-250 um. BEE surface: 565 m2/g, BEE pore volume:
0.83 ml/g) was dried at 120°C overnight prior to impregnation. Ehe catalyst precursors
were prepared by pore volume co-impregnation of the SiCE support with MoO-, (Fluka
puriss. p. a.) andNi(NO/h'6H20 (Fluka puriss. p. a.), as described in Chapter 3.
Hie oxidic precursors were sulfided for 2 h at 400°C (heating rate 6°C/min) with a
mixture of 10% ELS in IE (Messer Griesheim 3.0). Ihe activity of all the catalysts was
tested in fhe hydrodesulfurization of thiophene at 400°C. Ehe feed (3% thiophene in H2)
was obtained by bubbling IE through a series of four thiophene saturators that were cooled
to 2°C. Ehe product stream was analyzed on line with an HP5890 gas Chromatograph. The
sulfidation and thiophene FIDS reactions occurred at atmospheric pressure in the apparatus
presented inFig.l of Chapter 3.
XAFS measurements
The Quick EXAFS and classical EXAFS measurements were carried out at the XI
(RÖMO II) beam line at HASYLAB (Hamburg. German}), where the monochromator is
equipped with three parallel mounted pairs of Si(ll 1), Si(311), and Si(511) crystals. Ehe
energy of the beam line ranges from 6 to 70 keV [7]. Ihe X-ray energy is regulated by a
continuously moving two crystal monochromator [8|. fhe first crystal was detuned with
respect to the second one to eliminate higher orders of diffraction in the transmitted beam
68
Sulfidation of NiMo/Si02 catalystsfollowed hi QEXAFS
[9], so that the outeoming X-ray had 60% of the original intensity. The Si (311) and the Si
(111) crystals were used for the Mo and Ni A-edges. respectively. The accumulation time
was about 0.2 s/stcp at the Mo A-edge and about 0.4 s'step at the Ni A-edgc. The k-rangcs
used for the analysis of the data were 3-17 A"1 for the Mo and 3-12 A"1 for the Ni A-edge
(Fig. 1). The catahst samples were pressed into self-supporting wafers and mounted in an
in situ EXAFS cell [10|.
-6
3 5
Mo X-edge
13 15 17
M
45
25
05
-1 5
-3 5
Ni ÄT-edge
55
k[À-'l
FIG. 1. x(k) Ê data foi the Mo and \i x-edges measttied at station ROMO II. HASYLAB in Quick
EXAFS mode (spectia collected attct sulfidation at 400 C)
69
Chapter 4
The thickness of the samples was chosen to adjust the edge jump to 1 for the Mo
A'-edgc and the total absorption to px= 4 for the Ni A-edge (low Ni concentration). First,
two spectra of the fresh samples in an He atmosphere were collected. The samples were
then sulfided in situ during data collection. A stream of 10% H2S/H2 flowed through the
cell while it was being heated to 400°C. Fhe heating rate was 6°C/min for the
measurements carried out at the Mo K-edgc and 3°C/min for those at the Ni A-edge. This
difference was chosen in order to increase the accumulation time at the Ni A-edge and,
thus, to obtain reasonabh good data, since the Mo loading was 7 wt %, while the Ni
loading was only 1.3 wt %. fhe catalytic performance of the catalysts was checked using
both heating rates: no difference in activit} was found. A comparison of the data for the
two heating rates was carried out and onh an improvement of the data quaht} was
observed. Moreover, an isothermal experiment at 150°C showed that transport limitations
can be neglected for the order of time of our measurement.
For the classical EXAFS measurements, the samples were cooled to room
temperature after they had reached the desired intermediate sulfidation temperature. Once
at room temperature, the H2S still present in the cell was replaced by Fie by flushing for 10
min; the cell was cooled to liquid nitrogen temperature prior to the EXAFS measurement.
EXAFS analysis
The program XDAP (version 2.2.2) was used to analyze and fit the data [11]. The
pre-edge background was approximated b} a modified Victorccn curve, and the
background was subtracted using a cubic spline routine. I he spectra were normalized by
the edge jump. Ihe k'-weighted EXAFS functions were Fourier transformed and. in the
case of EXAFS spectra measured at liquid nitrogen temperature, fitted in R space. The
quality of the fit was estimated from the value of the goodness of the fit (sv~). Fhe zp value
takes the number of free parameters into account and is used to determine whether the
addition of new parameters would make sense. It was calculated with the formula:
2^ V-'WVS XmoJJ. ~ X.\ni
,,.
Cv > L-(1)
NPTS(v-A/„i() ^ [
OeV j
70
Sulfidaiion 0/N1M0/S1O2 catalysis followed In QEXAFS
where %modei and Xe\p are the model and experimental EXAFS functions respectively, gcxp is
the error in the experimental data (assumed to be 0,01 for each data point), v is the number
of independent data points in the fit range, and NPES is the actual number of data points in
the fit range. Although the absolute value of cx has no meaning, sv enables a comparison of
the goodness of the fit in the same spectra and at the same k weighting with different
parameters, a smaller value of e.x meaning a better fit.
Apart from AloS2 (references for Mo-S. Mo-Mo), reference spectra were calculated
using the Feft7 code [12, 131. Crystallographic data were obtained from the Inorganic
Crystal Structure Database (TCSD-CRYSTTN).
71
Chapter 4
Results
Molybdenum
The 1 IDS activities of catalysts prepared with NTA and EN are shown m Fig. 2. NTA
has a more pronounced effect than EN, and a maximum in catalytic activity is observed
between molar ratios NFA.Ni - 1.5 and 2. For catahsts prepared with EN, however, the
highest activity is obtained at EN:Ni = 4. EDI A has an effect similar to that of NEA at the
concentration studied.
0 12
CO
g 0 08 è
rt 0 06
ÖO
t 004
ta
0 02
EDTA »
EN
NTA
3 4 5
Liçand/Ni molar ratio
FÏG. 2. Thiophene HDS activity of NtVto Si CE catahsts ptepaied with ethylenediamine tettaacetic acid
and different amounts of ntttilotttacettc acid and ethv lenediamme (molai ratio Ni Mo = 0 3)
Ehe Mo A-edge XANES spectra, measured during the sulfidation of the catalyst
without a ligand. are presented in l ig. 3. Mo A-edge XANES has been subject of several
studies, according to which the first three components m the near-edge spectrum of oxidic
molybdenum should be assigned to 1 s —> 4d. Is-a 5s and Is -> 5p excitations |14]. Fhe
first transition is allowed in a tetrahedral field, but is forbidden in an octahedral field. This
selection rule breaks down when the octahedral s}mmetry is distorted. The shapes of the
72
Sulfidation of Vi\fo SiO catalysts followed In QEXAFS
near-edge absorption functions for all the samples can be divided, qualitatively, into three
types, all of which correlate with the extent of reduction. In the first type (Fig. 3, 41-
I72°C), the shape is ver} similar to that of the oxide catahsts. As sulfiding progresses, the
pre-edge feature fades and the edge moves towards lower energies (Fig. 3. 209-279°C).
Finally, after extensive treatment, the near edge absorption attains a shape which is
essentially identical with that of MoS2.
A more accurate description of the sulfidation mechanism of Mo is, however,
obtained from the Fourier transformed spectra of the QEXAFS data. Fig. 4 shows the
sulfidation process of Mo in the sample without a ligand (Ni:Mo - 0.3), as monitored by a
series of Quick EXAFS scans.
.2V-»On
Om
X3a
"O<u
"cd
£o
19980 20010 20040 20070
X-ray energy [eV]
FIG. 3. Mo Ä'-edge XANFS spectta measttied dining the sulfidation of NiMo SiCE (no ligand) plotted as a
junction of the sulfidation tempeiatuie
7-}
Chaptei 4
M
ST
[T. ^
0
et
CTQ
O
I—I
<!
o
C/5
CD
CTQI—i .
O
P
o"P
00
ELSa
W Pn1n> 1—i
CTQ O>i • 3O
Pr-f.
CD
CD
O
RE\1
FIG. 4. Tonner ttanstoims of the Mo x-edge k-weighted Quick tYAFS functions meastiied
dining the sulfidation of NiMo SiO- contamina no ligand 1 he lout sulfidation legions ate shown
(Heating täte 6 C min ktanse AI 7 \')
-74
Sulfidation of X/Mo/Si02 catalysis followed by QEXAFS
While the first spectrum is that of the untreated sample, the others were obtained at
increasing temperature upon treatment with FES. Ehe numbers next to the spectra denote
the average temperature during the scans.
The sulfidation process of molybdenum can be divided into four regions. The first
region is the temperature range in which molybdenum is present in the oxidic form. Fig. 4
shows that the distorted environment of Mo remained unchanged up to 110°C. While the
signals of the Mo-0 shells are between 0.5 and 1.9 A (not phase corrected), an Alo-Mo
contribution due to the presence of polymoly bdate anions can be seen at 3 Ä (not phase
corrected). In the second region, the longest Mo-0 shell as well as the Mo-Mo shell are
eliminated, while the shorter Mo-0 shell is still present; a signal around 2 Â (not phase
corrected), due to the presence of sulfur, is detected. This points to the co-existence of Mo-
O and Mo-S shells with the Mo-0 shell corresponding to Mo=Ot groups. The Mo-S signal
at 1.8 A (not phase corrected) has two maxima. A simulation with the Fefl7 program
showed that this signal can be caused by interference between an Mo-0 shell at 1.7 to 1.8 Ä
and a sulfur shell at 2.45 to 2.5 A. Eherefore. wc consider the peak at 1.8 Â not to be due to
a new Alo species. At the end of the second region a new signal at 2.5 Â (not phase
corrected) appears at a sulfidation temperature of 225°C. Ehis signal belongs to an
intermediate product which dominates the third region and contains Mo-S and Mo-Mo
contributions only. Ehe final product of the sulfidation process, MoS2, starts to form at a
temperature of 3 15°C and corresponds to the fourth region in Fig. 4. Ehe spectra measured
at temperatures higher than 315°C correspond to MoS2. m agreement with low-temperature
EXAFS measurements. Compared to a spectrum measured at liquid nitrogen temperature,
the Mo-Mo signal at 3 A (not phase corrected) has a lower intensif}. The shoulder on the
left side of the Mo-Mo shell in the catalyst sulfided at 400°C Is not due to the presence of
an additional signal but to interference of the Mo-S and the Mo-Mo shells, as shown by
spectra simulated w ith the Eef17 program.
The occurrence of four regions during the sulfidation process was common to all our
catalysts. Flowever. the temperatures at which the transitions from one region fo the other
take place were different, fhe onh exception was the sample with a molar ratio of
NTA:Ni - 6.66. Ehe sulfidation profile of Mo in this catalyst is shown in Fig. 5.
Molybdenum starts to sulfide at room temperature, i.e.. the first region is practically absent.
75
Chapter 4
f—i
400
400eu
400 CfQy—i. »
400 OP
400 i—i
400<
400 'à400
o
400oo
400 p
365^T!
350 £2.
340o
320
CP
C6
310 3CTQ AS
295o
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245o
225 o
210
195
180
160£0
145 CD
125 O
110P
95>—1
75
45
fresh Reeionl
R|A]
F1G. S. Fourier transforms of the Mo xAdge k-weighted Quick FXAFS functions measured
during the sulfidation of NiMo(N fA) SiCK molat îatto \ 1 \ Ni ~ 6 66 Mo starts to sulfide at room
temperature. (Heating rate 6A mm, k tange 3-17 \ ')
76
Sulfidation of NiMo/SiOî catalystsfollowed by QEXAFS
The Mo-S shell at 1.8 Â and an additional signal at 2.3 Â (not phase corrected)
already appears in the spectrum measured at 45°C. Ehe signal at 2.3 Â, which was observed
only during the sulfidation of this catalyst, is present until 210°C. At temperatures above
210°C. a peak appears at 2.5 Ä, which belongs to the intermediate product of the third
region (vide supra). The third region is present up to 375°C, and MoS2 crystallites are
formed only at about 400°C.
350
wJ3
300<DM
3
3 250S-<
1)
200
3
Ö 150or-<
"5s100
CO50
I third recion
second region
_>. o w ^ CO CD T_ (O CO ^r
i
CD
c 1 o II II CD II CDCN
n CD
of
oz
II
Z
<
z
<h-
z
<
CDII
z
Z
<
O
II
zz
z
zUJ
CDII
z
h- z Z < Q z UJ z
z H- UJ UJ UJ
FIG. 6. Temperature ranges for the second and third regions of sulfidation of Mo on SiCA-supported
catalysts of different composition (molar ratio Ni:]Vlo - 0.3 for catalysts with and without ligands).
Fig. 6 shows a comparison of the sulfidation temperature ranges of the second
(coexistence of AIo-0 and Mo-S shells) and third (presence of the signal at 2.5 Ä) regions
in the investigated catalyst samples, as obtained from the Mo A-edge Quick EXAFS
spectra. In the catalyst which contains Mo only, the temperature range covered by the
second and the third regions is relativeK large. Fhe situation changes completely when
nickel is added to molybdenum, fhe addition of Ni retards the reaction of Mo with FES. so
that it starts at 125°C but ends at a slightly lower temperature. NEA has a significant effect
on the sulfidation profile onh w hen the N TA:Ni molar ratio is higher than 1. In the extreme
case, i.e. a molar ratio of N fA:Ni = 6.66, sulfidation takes place over most of the
Chapter 4
temperature range. Ihe presence of EN induces the same sulfidation behaviour of Mo.
Although in the catalyst with the molar ratio 1 N:Ni - 0.66 no significant influence can be
seen, additional amounts of EN cause a marked broadening of the sulfidation interval of
molybdenum. In addition, it is evident that, m presence of EN, the first phase of the
sulfidation ends at much lower temperature, whereas the second phase is distributed over a
larger temperature range. For catalysts with molar ratios of NFA:Ni - 3. EDTA:Ni ~ 1. and
EN:Ni ~ 2.3. the sulfidation features of Mo are v ery similar.
To obtain more information about the structure of Mo in the second and third regions,
we measured classical EXAFS spectra of some of our catahsts at liquid nitrogen
temperature, after sulfidation at intermediate temperatures. Fig. 7 shows the k1-weighted
Fourier-transformed x(k) functions of the Ni\lo/Si02 catalysts without ligands after
sulfidation at 140. 170 and 290°C. It is difficult to compare the spectra shown in Fig. 7 and
the corresponding QEXAFS spectra measured at similar sulfidation temperatures, because
the latter were measured in a temperature range of about 15°C around the temperatures
given in Figs. 2 and 3. Nevertheless, several similarities can be observed, fhe spectrum
pp
A,
S
03
3
H
4 a. u
0 12 3 4
R[Ä]
FTG. 7. Absolute parts of the Mo Aedge k -weighted I XAFS functions of the NiMo'SiCA catalyst
containing no ligand, sulfided at thtee diffeient tempeiatutes (k tange AI7 Â"1. mcasuted at liquid
nitrogen tcmpctatuie)
78
Sulfidation of ht\!o'St02 catalysis folloM ed by QEXAFS
measured at 140°C (Fig. 7) clearly shows the co-existence of Mo-0 and Mo-S shells. While
a reasonable fitting of this spectrum was not possible because of the high number of
overlapping shells and the low intensities of the signals, the results of the fits of the other
two spectra are presented in Eable 1. In both cases, the Mo-S distance is 2.49 A, which is
greater than the corresponding MoS2 distance of 2.42 A. Fhe Mo-S coordination number is
relatively low (4.6) in the sample sulfided at 170°C, whereas it is extremely high (8.2) for
the catalyst sulfided at 290°C. In both cases, the goodness of the fit has a relatively low
value, thus indicating a good fit. Similar observations (i.e. high coordination numbers of
Alo in intermediate sulfidation regions) have been reported by other groups [5] and can be
explained by the ov erlapping of Mo-0 and Mo-S shells or b} a different Mo valence.
As far as the second shell is concerned, we did not consider the possibility that it
originates from Ni. because the same signal is present in the QEXAFS spectrum of a
catalyst containing only molybdenum. 'Flic same shell is better defined in the spectrum of
TABLE E Structural parameters resulting from the Mo A-edge Fourier-filtered
IE-weighted EXAFS functions of two Si Unsupported catalysts (Figs 7 and 8) sulfided at
di fièrent temperatures
Catalyst Suif. T Shell Ncooul R Ao2 [10"3 AEo Goodness
[°C] [A] A2! [eV| of fit
NiMoA).3:l 170 Mo-S 4.6 2.49 4.06 0.90 0.13
Mo-Mo 0.4 2.82 1.09 0.42
290 Mo-S 8.2 2.49 6.02 0.57 0.39
Mo-Mo 1.4 2.76 1.17 6.53
NiMoNTA- 245Mo-S 8.0 2.49 7.87 0.06 0.27
0.3:1:0.3Mo-Mo 1.4 i jß 1.97 8.82
270 Aio-S 5.4 2.40 6.37 5.32 0.35
Mo-Mo 0 6 2.79 0.30 7.89
79
Chapter 4
R[Ä]
FIG. 8. Absolute parts of the Mo x-edge ^'-weighted LXArS functions of the "NiMo(NTA)/Si02 catalyst
with the molar ratio NTA Ni _ 1. sulfided at three different temperatures (k range: 3-J7 Â"1, measured at
liquid nitrogen temperature)
the NTA:Ni -- 1 catahst sulfided at 270°C (Fig. 8). For this reason, we compared its
backscattering phase with that of the Mo-Mo reference extracted from the spectrum of
crystalline MoS2. Since the two phases overlap almost completely as a lunction of k, we
concluded that the signal at 2.5 A (not phase corrected) arises from a Mo-Mo shell. The
Mo-Mo distances, obtained from the fits of the different spectra (Table 1), range from 2.76
to 2.82 A.
Classical EXAFS spectra of intermediates after sulfiding the NEA:Ni - 1 catalyst
sample at 200, 245. and 270°C are presented in Fig. 8. The spectrum measured at 200°C
could not be fitted because of problems with the positions of the Mo-0 shells. From the two
other spectra, we obtained Mo-S coordination numbers of 8 (245°C) and 5.4 (270°C). while
the Mo-S distance in the latter case (2.40 A) is very close to that in MoS2.
80
Sulfidation of NiMo/SiO- catalysis followed In QEXAFS
Nickel
To stud} the sulfidation of the promoter, wc measured QEXAFS spectra at the Ni K-
edge. Since fhe loading of Ni was only about E3 wt %, the collection of data with a good
signal to noise ratio was not easy. For this reason, we used a lower heating rate of 3°C/min
rather than 6°C/min as for the Mo measurements. Fig. 9 presents two series of XANES
spectra measured during the sulfidation of the catalyst without a ligand and the catalyst
with EDEA NT -" I It is cas} to distinguish sulfide and oxide environments by measuring
the Ni A-edge XANES spectra, because the} show a strong white line when atoms of the
second row. such as nitrogen. ox}gen or fluorine, are present in the first coordination
sphere of Ni Due to the non-ionic character of the Ni-S bond, this feature is absent for
8300 8350 8400 8450 8"*00 8350 8400
X-ray energy [eV] X-ray energy [eV]
Fig. 9. Ni A-edge XANFS spectia ineasiiied dunna the sulfidation ol (A) NiMoAiCE (no ligand) and
(B) NiMoEDTA SiCA (FDFA Ni~T) and plotted as a function ot the sulfidation tempetatuie
81
Chapter 4
P
COr-t-
O
pc-f
CD
B
p
CD
O
R[À]
F1G. 10. Fourier transforms of the Ni A-edge k-weighted Quick EXAFS functions measured
during the sulfidation ofNiMo SiCV containing no ligand (heating rate SXA'min, k range: 3-12 A"1).
82
Sulfidation ofXiMo SiO: catalysts follow cd by QEXAFS
sulfided systems [15]. A comparison of the two series of spectra clearly shows that Ni is
sulfided at a higher temperature in the catal} st prepared with EDEA. Also in the case of Ni
we mainly used the Fourier transformed QEXAFS spectra to draw conclusions about the
changes in structure. The XANES features show, nevertheless, that Ni is completely
sulfided after treatment at 400°C.
* i 'X -r-
Fig. 10 shows a series of Ni A-edge Fourier transformed k -weighted QEXAFS
spectra of the sulfidation of the NiMo/Si02 catal} st. Ni-0 and Ni-Si shells are found at 1.65
and 2.9 A (not phase corrected), respectiveh. in the spectrum of the fresh catalyst. There is
no significant change in the spectra up to a sulfidation temperature of 90°C. The spectrum
measured at 105°C no longer shows an Ni-Si contribution. The transition of nickel in the
oxidic form to nickel in the sulfidic form is revealed by an increase of about 0.1 A in the
distance of the first shell. A first minor shift is observed between 50 and 125°C, but
replacement of ox}gen by sulfur took place between 125 and I40°C. As far as can be
monitored by QEXAFS. the structure of the first shell remains unchanged up to 400°C.
Fig. 11 shows our Ni A-edgc QEXAFS results of the NiMo EN:Ni r 4 catalyst during
sulfidation. The spectrum of the fresh catahst shows the features of the Ni(ENE complex,
while a Ni-Si shell is not present. Ehe occurrence of Ni-support interactions is prevented by
the presence of the ligand as described in chapter 3 and in réf. 116]. A shift of the first shell
occurred during the measurement of the first spectrum after the beginning of the
sulfidation. Ni is. therefore, surrounded by sulfur after sulfidation at room temperature, and
an apparent change in the structure of the first shell is not found at higher sulfidation
temperatures.
83
Chapter 4
P
cd
oo
pj—*
Cl
O
pr-t-
CD
3
O>-i
far-+-
c
o
o
O
R [Â]
FIG. 11. Fourier transforms ofthe Nh E-edge kAwtghted Quick EXAFS functions measured during
the sulfidation ofNiMo(EN)'Si02 with the molar ratio EN Ni~ 4. (Heating rate 3A7min. k range: 3-12 Â"1).
84
Sulfidation ofNih Jo Si02 catalysts folIon cd bv QEXAFS
Fig. 12 helps to clarify the effect of NEA on Ni during sulfidation. From the spectra
of Fig. 12 it is visible how the shift of the first shell takes place at a much higher value than
in the catalyst containing no ligand (Fig. 10). The data presented in this picture evidence, in
addition, that some interesting features about the second shell can be extracted from Ni K-
edge QEXAFS. In the spectra of the catahst in the oxidic state and sulfided until 210°C,
the presence of the Ni-C shell Is confirmed by the signal at 2.3 Â (not phase corrected).
p
d
r—1
ST
C/3
pI—J
tri
H-* »
O
P(-+
CD
BA3CD<-i
CD
O
n
R[A]
FIG. 12. Fourier transforms of the Nt Aedge k -weighted Quick EXAFS functions measured
during the sulfidation of NiVloNTA SiO-. with the molar tatio NT\Ni = 3 (Heating late 3°C/mm. k
range 3-12 ÀA
85
Chapter 4
Flowever. in the following 5 spectra a shoulder at 2.5 Â is observed. A more careful
observation of the previous two figures shows that this signal is also present in the spectra
of the catalyst prepared with EN (Fig. IE 205-240°C) and in the figure of the catalyst
prepared in the absence of ligands (Fig. 10. 140-175°C). even though it is less pronounced.
Since this signal is even observed when no chelating ligands are employed, we can exclude
that it arises exclusively from a Ni-C contribution. On the other hand, the complete absence
of aNi-C shell cannot be assumed when ligands are emplo}ed. Similar signals were already
reported for partiaux sulfided NiMo/SiOi [4J and CoMo/C |17] catalysts and were fitted
w ith a Ni-Ni shell, suggesting the presence of an intermediate stage in the sulfidation of Ni.
Ehis intermediate product precedes the final product of the sulfidation that can be
recognised from the weak signal at 2.8 Â (phase uncorrected) present in the three presented
Ni A-edge QEXAFS spectra. Ehis latter signal can be studied quantitatively only by means
of classical EXAFS. as will be done in the next chapter.
400
o 350 i
o1 1
C-( 300 '
3-h->
KÎT-H 250 !
aB8 200
do
150"3Td
kP 1003en
50
1
y
l.iI
Intermediate
product
Shift of first
shell
FIG. 13. Sulfidation temperature of \t in StCA-supported catalysts of different composition the
black regions represent the temperatures at which the shift of the first shell is observed in the QFXAFS
spectra The white rectangles correspond to the temperature interval where the intermediate product
(shoulder at 2 3 4) is obsetved (molar ratio \i Mo - 0 3 for catahsts with and without ligands)
86
Sulfidation ofNiMo/Si02 catalystsfollowed In QEXAFS
Figs. 10 to 12 show that the substitution of the Ni-0 shell by a Ni-S shell takes place
in a relatively small, well-defined temperature range. Ehis step is the first one in the
sulfidation of Ni. .A second step is revealed from the obtained data and corresponds to a
rearrangement of neighbouring nickel sulfide particles either interacting with the support or
forming small nickel sulfide clusters of unknown nature. The sulfidation temperatures of
our catalysts are presented in Fig. 13. In the figure two regions are plotted for each catalyst,
one corresponding to the shift of the first shell, the other to the observation of the shoulder
at 2.5 A denoting the intermediate sulfidation product. In the catalyst containing only Ni
supported on SKX sulfidation begins at room temperature and continues in a relatively
wide range of temperature. Ehe presence of an intermediate sulfidation product in the Ni-
only catalyst can not be clearly determined, fhe addition of Mo causes a delay in the
sulfidation of Ni. In the catalyst with a molar ratio of NTA:Ni = 0.25 (not shown in Fig.
13), NEA has no effect on the sulfidation of Ni, while for NTA:Ni ^ 1, a continuous
increase in the sulfidation temperature is observed. Ehe catalyst prepared with EDTA
represents the extreme case, where nickel is sulfided only at around 350°C. The presence of
EN seems to broaden the sulfidation interval of Ni. because the N atoms in the coordination
sphere around Ni are substituted at ver\ low temperatures, whereas the intermediate
product is observed at much higher temperatures.
87
Chapter 4
Discussion
In this section we will first consider the structure of the molybdenum species present
in the oxidic catalyst precursors and propose a sulfidation mechanism based on QEXAFS
and EXAFS measurements as well as on the underlying chemistry. Ehen, the effects of the
ligands on Mo and Ni as well as on the activit} will be discussed.
Molybdenum in the catalyst precursors
Analysis of NiMo/SiOi catalyst precursors by means of Raman spectroscopy and
EXAFS showed that Alo is present on SiO: as polyanions composed with seven or eight
Mo atoms (e. g. [Mo70:i]6" or [MogOoô]4") [16. 18]. The presence of such clusters is due to
the depolymerization of Mod at the pFl used in the preparation of the catalyst precursors
119). If the structure of these compounds had been well-ordered, with equal Mo-Mo
distances, then the theoretical Mo-Mo coordination number of the shell at 3.3 Â obtained
by EXAFS would have been about 3.4. Flowever. the Mo-Mo distances in these
polymolybdates are not well-ordered [16]. Elms, the EXAFS functions for the different Mo
atoms interfere so that the averaged signal is lower than expected (see Chapter 2 "Pros &
Cons of EXAFS""). For this reason, the Mo-Mo coordination number obtained from the fits
is only about 1 (sec Chapter 3, Eable 6), and the signal at 3 Ä (not phase corrected) in the
EXAFS and QEXAFS spectra has a relatively low intensity. A comparison of the EXAFS
spectra of [AI07O24]6" or [MosO^r,]4" (simulated by Feff7 on the basis of the Mo-Mo
distances as determined by means of crystallographic data), with the spectra of our catalyst
precursors confirmed this model. Depending on the chemical nature of the bond, the Mo-0
distances in the [M07O24]6" or [MoAAd]*"anions can v ary by several tenths of an Angstrom.
The shortest Mo-0 bonds are those of the terminating Mo=0 units (about 1.7 Â), while the
Mo-0 bonds in the bridging Mo-O-Alo functions are longer. Eheir length can vary from
1.75 to 2.6 Â [20. 21]. Such a bond length distribution can be noticed for the fresh catalyst
in the multitude of Mo-0 signals between 0.5 and 1.8 A (Fig. 4).
88
Sulfidation ofNiMo/St02 catalysts followed by QEXAFS
Sulfidation ofMolybdenum
QEXAFS spectra were measured from room temperature to 400°C. It is well known
that an increase in thermal vibrations causes a decrease in the EXAFS signal, which can
lead to a false estimation of the coordination number [22]. Nevertheless, we compared the
QEXAFS spectra with spectra measured at liquid nitrogen temperature for catalysts
sulfided at 400°C for 30 min. Ehe results of the fits show that the Dcbye-Waller factor was
about one order of magnitude larger for the QEXAFS spectra. The coordination number for
the Mo-S shell was about 10% smaller, whereas the Mo-Mo coordination number was
about 20% larger. The spectra of the third and fourth regions were also fitted. A
comparison of the results of the fits for the various catalysts showed that there are no
significant differences.
The analysis of QEXAFS data will, therefore, be based on a qualitative discussion of
the presence and disappearance of various signals but not on a quantitative analysis. For
quantitative analyses, classical EXAFS spectra will be used. We did not interpret the
changes in the relative amplitudes of the different Mo-0 signals, because an increase in
temperature and small changes in the bond distances can cause relatively large variations in
the amplitudes. This is seen in the first five spectra in Fig. 4. It cannot be determined from
the features of these spectra, whether the number of terminal or bridging oxygen atoms is
changing.
The Mo-S signal can clearly be distinguished in the spectrum collected at a
sulfidation temperature of 125°C. This signal has two maxima; these should not be
interpreted as two real signals but as only one. since Feff7 simulations showed that the first
one is the result of interference between Mo-0 and Mo-S shells.
Mo-O and Mo-S shells are observed simultaneously during the second region of the
sulfidation process. To interpret these observations correctly, the experimental set-up must
be considered. Samples were pressed into wafers, of 1 to 1.5 mm thickness, and then
mounted in our EXAFS cell. This thickness induces an inhomogeneous sulfidation of the
sample, as was observed for samples in which sulfidation was interrupted during the
temperature ramp. Fhe outer layers were already sulfided, as shown by the brown colour of
the surface, while the inner part was still in a previous sulfidation state as shown by its
89
Chapter 4
yellow colour. The inner part of the sample was. therefore, sulfided later than the surface of
the sample. This phenomenon is not important for samples that are treated for long periods
of time under the same conditions; it must, however, be considered when measuring
QEXAFS, because changes are observed in periods of few minutes. The question is
whether the delay between the sulfidation of the surface and the inner part of the sample is
significant compared to the time taken to register a QEXAFS spectrum.
Because of the time delay between sulfidation of the outer and inner parts of the
wafer, assignment of the four observed sulfidation regions to four transition periods must
be done with care before concluding that four different regions are present. It is clear that in
the beginning the sample is oxidic and that it is in the sulfidic state by the end of the
sulfidation process. There is no doubt that a different transition product forms in region
three, because the peak at 2.5 A (phase uncorrected) is not observed at the initial oxidic
stage or in the final sulfidic state. Does a first transition product start to form at the end of
the first period, which is transformed into the second transition product (with the 2.5 Ä
peak) during the third period? A simpler explanation would be that only three actual
regions exist during temperature-programmed sulfidation and that the spectra measured in
the second region consist of mixtures of the oxidic and the transition regions with the peak
at 2.5 A. In that case, however, the spectra obtained from sulfidation between room
temperature and 280°C should be linear combinations of the spectrum of the fresh sample
and of the spectrum of the transition product with the signal at 2.5 Â (Fig. 4, 280°C).
To determine whether the spectra from room temperature to 280°C are linear
combinations or not. several intermediate spectra were analyzed. Fig. 14 shows a
comparison of the spectrum measured around 170°C with the best fit of a linear
combination of the oxidic and transition regions. Ihe best fit was obtained with 73% of the
oxidic region and 27% of the third region. Ibis figure shows that the Mo-0 shells are well
fitted, while a rough fit only is obtained for the Mo-S signal. Moreover, the signals at 2.5
and 3 A, present in the calculated spectrum and which arise from Mo-Mo sheiks, are absent
in the measured spectrum. Hie peak at 3 A, due to pol} molybdate species, may be absent at
170°C because of increased thermal vibrations induced by the higher temperature than that
at which the oxidic sample was measured (room temperature). Flowever, the peak at 2.5 A
is visible in spectra measured at higher temperatures (280°C) and should, therefore, also be
00
Sulfidation ofMMo/Si02 catalystsfoilowed by QEXAFS
present at 170°C if some of the molybdenum had already reacted to the transition state. The
same differences in experimental and linear fit were observed for other spectra between
room temperature and 280°C. Therefore, the spectra measured in the second region in
R [Ä]
FIG. 14. Comparison of the Mo Aedge Fourier transforms of the QEXAFS function of the
catalyst containing no ligand sulfided at 170°C (solid line) and a fit calculated with 73% of the
spectrum in the oxidic state and 27% of the spectrum of the catalyst sulfided at 280°C (dotted line).
particular cannot be explained by a simple transition of the oxidic precursor to the transition
region (with the peak at 2.5 A); the existence of another transition product must be
assumed.
The transformation of the fresh catahst precursor to the first transition product might
occur as follows. The terminall}' bonded oxygen atoms are the most exposed structural
features of the polymolybdate species and. therefore, are accessible for a reaction with FES.
They are the first to be substituted by sulfur upon reaction with FES, according to
[-Mo=0] +1ES -> {-Mo-S} -t-11:0 (2)
91
Chapter 4
Details of this reaction step arc given in [23]. Ehe replacement of terminally bonded
oxygen (0~") by the larger sulfur (S ") has structural consequences. As mentioned above,
polymolybdate anions show a variety of bonding distances, fhis is due to the irregularity of
the octahedral [MoOß] building blocks and to the way in which they arc interconnected.
When oxygen is replaced b\ sulfur, structural distortions cause the long Mo-0 bonds to
break. This process, which converts bridging Mo-O-Mo functions into terminal Mo=0
units, is quite common and can even be predicted by a simple force field based molecular
modelling calculation. If bond breaking takes place at the edges of the anions, mononuclear
molybdate ions. M0O4"\ are expelled from the M07 particle, as can be seen in our QEXAFS
spectra. As soon as the Mo-S signal emerges, the Mo-Alo shell at 3 A, which is due to the
presence of polymolybdate species, decreases in intensity (Fig. 4, 110-155°C). Hardly any
change is noticed in the short Mo-0 shells in the QEXAFS spectra. Ehe expected loss in
intensity due to the replacement of Mo-_0 by Mo=S functions is compensated by the
generation of new terminally bonded oxygen atoms from bridging atoms and the formation
of molybdate anions, which contain only short Mo-0 bonds due to considerable d7t-p7t
bonding contributions. Moh bdenum coordinates 0 and S simultaneously, and consequently
a molybdenum oxysulfide is the intermediate product of the sulfidation. Above 195°C. the
intensity of the Alo-0 and Mo-S signals changes dramatically, i.e., the Mo-0 signal
decreases and the A1o-S signal increases, .fudging from the small side band at 2.5 Â. the
third region starts to form at about 245°C.
At 260°C, all the Mo is present on the support as the intermediate product of the third
region before it reacts further to MoS:. Our EXAFS measurements (Eable 1) show that the
intermediate product has Aio-\lo distances between 2.7 and 2.8 Â. This indicates that Mo is
present in a reduced state, i.e.. either in a A5 or +4 oxidation state. While the bonding
distance of Mo(IV)-Mo(IV) lies within this interval (2.73 A in the case of the [Molv3Si3]2"
cluster anion [24]), MofV)-Alo(V) bonds are usualh slight!}' longer than 2.8 Ä (2.82 Â in
the complex anion |MoV:Si:]"" [251). fhis shows how difficult it is to describe structural
properties of the intermediate region based on EXAFS data alone. However, based on
structural and inorganic chemistix. an approximate description of the intermediate structure
can be made.
92
Sulfidation ofNiMo/St02 catalysis followed by QEXAFS
The initial reduction of molybdenum proceeds via an internal redox process between
Mo(VI) and S"" as a consequence of the oxygen-sulfur exchange explained above. Two
adjacent {-Mo64=S2"} functions convert into a {-Mo5'~(S2)2TMoD,~} unit, which corresponds
to oxidation of sulfur (2S~" -> S2"
+ 2e") and reduction of molybdenum (2Mo(Vl) + 2e" ->
2Mo(V)). A detailed discussion of this reaction step can be found in Refs. [23] and [26].
This redox process has a profound influence on the breaking of the Mo-0 bonds and,
therefore, on the generation of new Mo-=0 entitles, lipon reduction, the Mo-Mo distances
become shorter, and the interconnection of Mo centres by oxygen links is weakened, what
facilitates the formation of new reactive Mo=0 sites. Temperature and the increasingly
destabilized oxysulfide structure enable all the oxygen to be replaced by sulfur, while Mo is
further reduced to the 4 f state. This mechanistic view is in line with our QEXAFS data,
which indicate that bridging oxygen atoms are no longer present in the intermediate product
at 280°C but are eliminated before the formation of M0S2. An analogous mechanism
applies in the sulfidation of the expelled molybdate species. We expect that the O-S
exchange reaction starts at higher temperatures than in the case of the aggregated species.
Ehis would explain why the short Mo-0 shell is still present in the QEXAFS spectra while
the Mo-S shell is already quite well developed. Ehe replacement of two oxygen ligands by
sulfur, followed by the redox process, leads to an intermediate similar to {(S:)Mo(fV)0).
The same chemistry and analogous types of intermediates are involved in the
decomposition reactions of the complexes (NIEVyMoOoS: [23] and (NEE)2MoS4 [27]. This
intermediate is coordinatively unsaturated and, therefore, unstable. Stabilization is achieved
by aggregation with other intermediate species or with the multinuclear oxysulfide. The
rapid formation of Mo(IV) rather than Mo(V) is also favoured by the presence of a second
reducing agent (Hi) and the structural stability is dependent on the coordination number.
While the metal-ligand coordination number of Mo is 6 in the initial oxide (Mo(VI) -
octahedral coordination) and in the final MoS: (Mo(IA) - trigonal prismatic coordination),
Mo(V) favours lower coordination numbers. For this reason, we conclude that the Mo-Mo
distances of 2.7 to 2.8 Â in our EXAFS spectra are mainly due to the presence of Mo(IV)-
Mo(lV) bonds.
93
Chapter 4
Similar EXAFS spectra have already been reported [4, 28, 29]. In some cases, high
Mo-S coordination numbers and high Mo:S stoichiometry. together with short Mo-Mo
distances, led to the conclusion that MoSi is an intermediate during sulfidation. Based on
the structural model for amorphous MoSi of Cramer et al. [28]. it was suggested that these
MoS^-like sulfidation intermediates are Mo(V)-containing materials, in which chain-like
Mo^+ centres are interconnected by S~" and Sy" species [30]. Flowever, it has been shown
that amorphous MoS;, is a compound of Mo(IV) and, in agreement with the chemical
behaviour of Mo(IV), the arrangement of the metal centres is not chain-like [27]. We do not
exclude the possible existence of an MoSv-like material in intermediate stages of the
sulfidation process. Ehis would be in line with our measurements and would also fit the
mechanistic description of the sulfidation reaction given above. Flowever, the existence of
stable, i.e., observable, Mo-S intermediates of Mo(V) is very unlikely.
The transformation of the third MoSMike product to the final M0S2 is a relatively
fast process and takes place in the narrow temperature range from 280 to 315°C. This is so,
because the transition to MoS: does not require further reduction of the metal centres. A
discussion of the decomposition of amorphous M0S1 to microcrystalline M0S2 can be
found in Ref. [27].
Chelating ligands and catalytic activity
A comparison of the catalyst containing only Ni and Mo and the other catalysts
shows that the ligands tend to broaden the sulfidation interval of Mo (Fig. 6). Towering the
temperature at which Mo begins to be sulfided could be explained by the fact that smaller
particles, such as the complexes formed between Alo and NfA or Mo, are attacked more
easily by FES than are the larger polymoh bdate units. In contrast, the delay in terminating
the sulfidation shows that the presence of ligands influences the nature of the intermediate
product that precedes MoS:. Otherwise. MoS: would start to form at the same temperature
in all samples, if temperature dependence of the reaction from the third region to MoS: is
assumed. 'Fhe effect of the ligands on the intermediate may be related to its dispersion on
the support as well as its aggregation state. A quantitative analysis can. however, not be
carried out on the basis of the QEXAFS data.
94
Sulfidation ofNiMo/Si02 catalysts followed bv QEXAFS
The sulfidation of Ni is dramatically affected by the presence of the chelating ligands.
By means of QEXAFS we could detect three regions during the sulfidation of Ni: the
oxidic state, an intermediate region characterized by the presence of a shoulder at 2.3 Ä
(not phase corrected) and the final region whose structure still has to be clarified and will
be discussed in Chapter 5. The passage to the intermediate region is usually preceded by the
substitution, in the first coordination sphere, of 0 or N ligands by S. In the case of the
catalyst containing no ligands and those prepared with NTA and FDTA. the O-S or N-S
exchange is more or less direct!} followed b} the formation of the intermediate product. In
the presence of EN. on the contrary, the two processes take place at extremely different
temperatures. All three ligands have in common the fact that, in conclusion, they all delay
the termination of the sulfidation of Ni in comparison with the catalyst prepared without
chelating agents.
A relationship with the activity data is. therefore, certainly connected with the effect
of the organic ligands on the sulfidation of Ni. Maximum activity ( sec Fig. 2) is observed
for catalysts with NTA:Ni - 1.5 to 2, FN:Ni = 4, and EDTA:Ni = 1. Catalysts with
NTA:Ni - 3. 6.6 and EN:Ni = 6.66 have lower activities, in spite of the fact that Ni is
sulfided at higher temperatures in these catalysts, fhis observation suggests that the
beneficial influence of the delay in the sulfidation of Ni is counterbalanced by an effect of
the ligands on Mo. The fact that the highest concentration of NTA and EN
(ligand:Ni = 6.66) causes a significant decrease in activity can be explained by the
excessive broadening of the sulfidation region for Mo (Fig. 6). In the case of NEA, this
effect is due to the fact that, at such a high NEA concentration, not only Ni but also Alo is
complexed by NEA with the formation of [MoO}(NTA)[A the sulfidation of which
apparently starts at a lower temperature but ends at a higher one than for polymolybdates.
In the presence of EDTA. Ni is sulfided at a higher temperature, while the sulfidation
behaviour of Mo is similar to the catah st with NEAfNi - 3.
An indirect effect of the ligands on the sulfidation of Alo may explain why EN also
has the same effect on the FIDS activit}. All the tested catalysts preferentially form
complexes with Ni, i.e., thev hinder direct contact between Ni and the support (8). In the
absence of the ligands. the Ni~" cation in the impregnating solution (pFI 8) is attracted by
the SiO" groups that cover the surface of the support (point of zero charge of SiO: is at
95
Chapter 4
about pH 2). The interaction of Ni with silica changes the surface properties of the support
and, consequently, the way in which Mo interacts with it. This furthers the absorption of
the molybdenum entities on SiO:, as concluded from the sulfidation behaviour of Alo in the
catalyst without ligand (Fig. 6). The thiophene FIDS results (Fig. 2) reveal that the activity
of the catalysts decreases at NTA:Ni ratios higher than 2 and at EN:Ni ratios higher than 4.
This negative influence may be due to the fact that, at the mentioned concentrations, the
ligands start to form complexes with molybdenum, because only one NTA molecule and
three EN molecules can complex Ni. No mention of an Mo-EN complex was. however,
found in the literature. Therefore, the negative effect of high EN concentrations may be the
result of the absorption of excessive amounts of protonated EN on the support and a change
in the surface properties. As a consequence, the more positively charged surface could
more easily absorb Alo species of the same kind that was detected by means of Raman
spectroscopy, as reported in Chapter 3. Such pohanions interacting with the support have
likely a different sulfidation behaviour in comparison with crystalline heptamolybdate
anions that have no connection w ith the support.
EDTA is present in a relativ eh low concentration (EDTA:Ni - 1). At this
concentration Mo does not perceive the presence of the ligand, and is present as polyanion
on the support.
Conclusions
Ehe collected QEXAFS data led to an interpretation of the sulfidation processes of
Mo and Ni on a support. It was shown that the sulfidation of Mo takes place in four
regions. Two intermediate regions were observed during the sulfidation of molybdenum,
the first consisting of mohbdenum oxysulfides and the second of apure Mo(IV)-S product,
which might be similar to amorphous MoSs.
QEXAFS measurements demonstrated that also the sulfidation of Ni is a complex
reaction involving at least one intermediate product, fhe shift of the first shell in the Ni K-
edge Fourier transformed data represents the substitution of the O or N ligands with sulfur
in the first coordination sphere of Ni. Subsequently. Ni forms an intermediate product
whose nature was not clarified }et. Ihe effect of the chelating ligands on the sulfidation
96
Sulfidation ofNiMo/Si02 eatah sts followed by QEXAFS
temperature ofNi is very pronounced. All chelating agents induce a delay in the sulfidation
of Ni. Interaction between Ni and Mo is supposed to take place after the intermediate
region of the sulfidation of Ni. Ehe increase in the IIDS activity, caused by the presence of
the ligands. is therefore ascribed to the sulfidation of Ni at higher temperatures. It was
suggested that there must also be an optimum sulfidation region for Mo, which judging
from the results, is relatively narrow and should be contained in the temperature range from
80 to 300°C, and depends on the Mo species present and on the molybdenum-SiO:
interactions. An excessive broadening of this interval could cause a decrease in catalytic
activity.
Ehe use of Quick EXAFS showed the advantage of following the sulfidation process
in situ; a comparison with classical EXAFS spectra measured at liquid nitrogen temperature
showed that signals observed in QEXAFS spectra are reliable and enable qualitative
analysis in spite of the higher temperatures. Flowever. with such a time resolution, the
diffusion limitations must be considered, what can be made negligible decreasing the
heating rate of the sulfidation and increasing the collecting time for QEXAFS spectra.
Another factor that must be considered when interpreting EXAFS spectra is the overlap of
the different shells.
Literature
1. Scheffer. B.. Arnoldy. P., and Moulijn. E A., ,7 Catal 112, 516 (1988).
2. Payen. E., Kasztelan. S., IToussenba}. S.. Sz}manski. R.. and Grimblot. E. J Phys.
Chem 93,6501 (1989).
3. Topsoe, EF, Clausen. B. S., and Massoth. F. E.. "Catalysis Science and
Technology". Springer Verlag. New York. 69 If (1996).
4. Medici, L., and Prins. R.. J Catal 163. 38 (1996).
5. De Boer, AE. van Dillen. A. E. Koningsberger. D. C. and Geus. J. W., Jpn J. Appl.
Phys 32-2,460(1992).
6. Jong. A. M. D., Muijsers. J. C. Weber. Eh.. van IJzendoorn, L. E, de Beer, V. EI. E.
van Veen. E A. R.. and Niemantsverdriet, J. W., in "Transition Metal Sulphides.
97
Chapter 4
Chemistry and Catalysis", Weber, Th.. Prins, II. and van Santen, R.A. Eds.. Kluwer,
Dordrecht. 207 (1998).
7. Tröger. L.. Synchr. Rad. News 6, 11 (1997).
8. Frahm, R., Rev. Sei. lustrum. 60, 2515 ( 1989).
9. Krolzig, A., Matcrlik, G., Swars, AE» and Zegenhagen. E, Nucl. Instr. and Meth.
219,430(1984).
10. Kampers. F. W. EE, Alaas, T. AT E, van Grondelle, E, Brinkgreve, P., and
Koningsberger. D. C. Rex. Sei. Instrum. 60. 2635 (1989).
11. Vaarkamp, M., Firing. E, Oldman. R. E. Stern, E. A., and Koningsberger, D. C,
Phys. Rev. B 5i). 7872(1994).
12. Ankudinov. A, L.. and Rehr, J. E, Phys. Rev. JB56, Rl712 (1997).
13. Zabinsky, S. !.. Rehr, E E. Ankudinov. A., Albers. R. C, and Ellcr. AT E. J. Phys.
Rev. 5 52,2995(1995).
14. Chiu, N.-S.. Bauer. S. IE. and Johnson. Al. F. E.., J. Catal. 89, 226 (1984).
15. Eouwers, S. P. A.. Crajé. Al. W. E. van der Kraan. A. M., Geantet, C, and Prins, R.,
J. Catal. 144, 579 (1993).
16. Cattanco. R., Shido. T.. and Prins. R,. J. Catal. 185. 199 ( 1999).
17. Crajé, M. W. E, Louwers, S. P. A., de F3eer, V. H. E, Prins, R., and van der Kraan,
A. M., A Phys. Chem. 96, 5445 (1992).
18. Williams. C. C, Ekerdt. E G., Jehng. E. Hardcastie, F. D., Turek, A. M., and
Wachs, I. E.. J. Phys. Chem. 95. 8781 (1991).
19. Mcdici. E., and Prins. R., J. Catal. 163. 28 (1996).
20. Weakley. E. E, Polyhedron E 17 ( 1982).
21. Evans, 11. T. jr., Gatehouse, B. M., and Leverett, P., J. Chem. Soc. Dalton Trans.
1975.505(1975).
22. Rockcnberger. E, Tröger. E., Kornowski, A., Vossmcyer, T., Eychmüller, A.,
Feldhaus. E. and Weller. IE. J. Phys Chem B 10E 2691 (1997).
23. Weber, Eh., Aluijsers. .1. C, van Wolput. E Fl. Al. C, Verhagen. C. P. E, and
Niemantsverdriet. J. W.,,7. Phys. Chem. 100. 14144 (1996).
24. Müller. A., Diemann. E.. Krickemeyer, E.. Walberg, IT.-.k. Bögge, H., and
Armatage, A.. Eur J. Solid State Inorg. Chem 30. 565 (1993).
98
Sulfidation ofNiMo/SiO: catalysts followed In QEXAFS
25. Müller, A., Nolle. W.-O., and Krebs, B.. Angev. Chem 90, 286 (1978).
26. Muijsers, J. C, Weber, Th., van Ilardeveld, R. AT, Zandbergen, H. W., and
Niemantsvcrdriet. J. W..,/ Catal. 157, 698 (1995).
27. Weber. Th.. Muijsers. J. C, and Niemantsv erdriet, J. W., J. Phys. Chem. 99, 9144
(1995).
28. Cramer. S. P., Tiang, K. S., Jacobson, A. E. Chang. C. H., and Chianelli, R. R..
Inorg Chem 23. 1215(1984).
29. Eeliveld. R. G.. van Dillen, A. E, Gcus. J. W.. and Koningsberger, D. C. J. Catal.
171, 115(1997).
30. De Boer, M.. van Dillen, A. E, Koningsberger. D. C, and Geus. J. W.. J. Phys.
Chem 98.7862(1994).
99
Chaplei 4
100
Chapter 5
Influence of chelating ligands on sulfided NiMo
catalysts: an EXAFS study
Introduction
Ehe sulfidic state of supported NiMo (CoAIo) catalysts has been investigated by
means of a large number of techniques, among which the most powerful ones are
Mössbaucr emission spectroscop}' (AIES) [1-3]. X-ray photoelectron spectroscopy (XPS)
[4-61, electron microscopy [7-91. and extended X-ray absorption fine structure (EXAFS)
[10-14]. In this chapter we employ the latter technique in order to study sulfided
NiMo/SiO: catalysts. EXAFS spectroscopy has the great advantages of being a bulk,
element specific technique that can be applied to dispersed and amorphous materials.
Ehercfore, this technique is very suitable for the study of this bimetallic system, where the
loading of the studied elements ranges from 1 to 10 vvt%.
Ehe parameters obtained from the anah sis of EXAFS data allow us to understand the
composition of the coordination spheres around the absorber atom. Nevertheless, the
obtainable information is dependent on the quality of the collected data. Ehe quality of
EXAFS data is influenced b} various factors, the most important ones being the
concentration aucl the edge energy of the studied material. Ehe two elements studied in our
system have very different characteristics, while the loading of Alo is 7.1 wt%, that of Ni is
1.3 wt%. The energy of the Alo A-edge is 20000 eV and that of the Ni A-edge is 8333 eV.
Chapter 5
These parameters induce a significant difference in the data quality for the two metal
sulfides, the quality of the Alo A-edge data being substantially better.
In this chapter the findings are reported of an EXAFS study at the Mo and Ni A-edge
of SKE-supported NiMo catalysts prepared with nitrilotriacetic acid (NTA) and
ethylenediamine tetraacetic acid (EDEA). Ihe structure of these catalysts is compared with
that of catalysts prepared in the absence of ligands. Ehe conclusions drawn in the previous
two chapters are taken into account throughout this chapter in order to understand the
causes of the differences observed in the structure of the sulfided catalysts.
Experimental methods
Catalysts preparation and activity tests
Ehe catalysts precursors were prepared b\ incipient wetness impregnation of SiO:
with an aqueous solution containing Ni(NO;):. MoO^ and the chelating ligands as described
in Chapter 3. Ehe oxidic precursors were sulfided for 2 h at 400°C (heating rate 6°C/min)
with a mixture of 10% HiS in IE (Messer Griesheim 3.0). The activity of all catalysts was
tested in the hydrodesulfuri/ation of thiophene at 400°C. The feed (3% thiophene in IE)
was obtained by bubbling IE through a series of four thiophene saturators that were cooled
to 2°C. Ehe product stream was analyzed on line with an 1IP5890 gas Chromatograph. The
sulfidation and thiophene HDS reactions occurred at atmospheric pressure in the apparatus
presented in Fig. 1 of Chapter 3.
EXAFS measurements
The EXAFS spectra reported were collected at the Swiss Norwegian Beam Fine
(SNBE) at the European Synchrotron Radiation Eacilitx (ESRF), Grenoble France. Ehe set
up of the beam line is described in Chapter 3 and ref. [15]. The catalyst precursors were
pressed in self-supporting wafers, mounted in a sealed EXAFS cell [16[ and sulfided in the
chemistry laborator} of the ESRF. The thickness of the samples was chosen to adjust the
total absorption to px - 4 for the Ni A-edge (low Ni concentration) and the edge jump to 1
for the Mo A-edge.
102
Sulfided NiMo/Si02 catalysts
An H2S/H2 (10/90) mixture was flown through the cell at a rate of 60 ml/min. The
unreacted hydrogen sulfide was abated by leading the gas exiting from the cell through two
washing bottles containing a basic FeCE solution. Ehe cell was heated to 400°C with a
heating rate of 6°C and was kept at this temperature for 30 min. After decreasing the
temperature to 40°C, the cell was flushed for 10 min and filled with He to an overpressure
of about 0.5 bar. Ehe cell containing the treated sample was then transported to the beam
line and placed in the hutch. Ehe sample was cooled with liquid nitrogen by means of a
dewar, whose structure allowed the cooling liquid to come in direct contact with the cell.
As far as the Alo A-edgc was concerned, the collection times for each data point in a
scan were 1, 2. 2, 3 and 3 s for the intervals 19454-19954 cV (pre-edge). 19954-20164 eV
(edge region) and the three 6.5-10. 10-15. 15-21 A"1 post-edge regions between 20164 and
21684 cV, respectively. Ehe Ni A-edge spectra were divided into six regions: 7900-8300
eV (pre-edge). 8300-8370 e\T (edge region) and the four post-edge regions (3-6.5, 6.5-10.
10-15, 15-17 A"1) between 8370 and 9433 eV. fhe collection times for the data points in
each scan region were 1, 1.2. 3. 4 and 4 s, respectively. Ehe distance between the post-edge
data points was determined so that the difference in their k values was smaller than
0.05 Ä"1. Five scans were averaged for the Ni A-edge and three for the Mo A-edge.
EXAFS analysis
Ehe program XDAP (version 2.2.2) was used to analyze and fit the data 117]. The pre¬
edge background was approximated by a modified Victoreen curve and the background was
subtracted using a cubic spline routine, "fhe spectra were normalized by the edge jump. The
k'-weighted and k1 -weighted EXAFS functions were Fourier transformed and fitted in R-
space. The free parameters were interatomic distance, coordination number, Dcbye-Waller
factor and the correction of the edge energy. The errors of the parameters were statistically
estimated using the random errors of the observed data. The goodness of fit was calculated
for every model from the k- and R-space fit range and the number of free parameters as
described in Chapter 4. Reference spectra lor the Alo Ä'-edge were obtained from the
measured spectrum of AloS: whereas for the Ni A-edge they were calculated using Feff7
[18].
103
Chapter 5
Results
Ehe catalysis studied in this chapter were chosen in order to explain which features of
these materials in the sulfided state influence their catalytic performance. A comparison
was made between catahsts showing the lowest (without ligands. NTA:Ni ~ 6.66, see
Fig. 1) and the highest (NEA. FDTA:Ni= 1-2) IIDS rates. In Fig. 2 the quality of the data
at the Mo and Ni A-edges for the catalyst prepared without ligands can be compared. The
0 14
0 12 y^ ^^\
^Aa *i-
"o 0 08 4B
£ 0 06
p
« 0 04
To
0 02 j
0
0 12 3 4 5 6 7
Ligand/Ni molar ratio
FIG. I. IIDS activity profile of SKA-suppotted catahsts prepated with different amounts oEN FA (\) and
FDTA () (molat tatto Ni Mo-0 3 1)
Mo A-edge data show an extremeh good signal to noise ratio, whereas the quality of the Ni
A-edge data is less good. Elus difference is due partly to the higher loading of Mo (7.1
wt%) in comparison to that of Ni (1 3 wt%) and partly to the lower edge energy of the Ni
A-edge. since at this energy the X-ra} absorption, and thus the corresponding noise,
produced by the other elements present in the sample is still relatively high. The higher
number of averaged scans for measurements at the NI A-edge (5 versus 3) allowed only a
limited improvement of the data quality, fhe k-ranges used for the Fourier transformations
were 3-19 and 3-15 Â"1 for the Alo and the Ni A-edge data, respectiv eh.
104
Sulfided NiA to/Si02 catalysts
¥
-16
9 12
kl A"1]
15
11 13 153 5 7 9
k|A-'|
FIG. 2. k'-weighted %(k) data of the catahst prepaied without ligands measiited at the Mo (above) and Ni
(below) Aedges
105
Chapter 5
Mo K-edge
Fig. 3 shows the Mo A-edge k3-weighted Fourier transformed EXAFS spectrum of
the catalyst prepared without ligands. Ehe first two signals, at 2 and 2.8 Â (phase
uncorrected) correspond to the nearest Alo-S and Mo-Mo shells, respectively. Fhe weak
signal at about 3.5 A (phase uncorrected) is produced by the second nearest Mo-S shell,
18 -,-,
-18 '
R[A]
FTG. 3. Fourier tiansformatton of the Mo À'-edge E-weighted 1 XAFS function of fhe catalysts prepared
without ligands
whereas the two signals around 5 and 6 A (phase uncorrected) are the result of a
superposition of the multiple scattering signal arising from the nearest Mo~Alo shell and of
further Alo-S and Alo-A1o shells [9J. During the fitting procedures, significant differences
were observed onh' in the parameters concerning the nearest Mo-Mo shell. Fig. 4 shows a
detailed view of this shell for the inv estigated catal} sis. From this figure one can notice that
the amplitudes of the various signals are different. 1 low ever, it is not possible to understand
106
Sulfided N1M0/S1O2 catalysts
44
H
16
14
12
10
8
6
4
2 3
— no ligands
NTA Ni=1 5
- NTA Ni=6 66
-EDTANI=1
EDTA'Ni=E5
EDTA Ni=2
^"
2 5
R[Â]
2 9 3 1
FTG. 4, Closer view of the Mo-Mo shell in the Mo Aedge Fourier transformed EXAFS functions of the
various sulfided catalvsts
TABEE 1. Parameters obtained from the fit of the Alo ÄAedge k'-weighted EXAFS
spectra of the sulfided NiAlo/Si02 catalysts (AR: 1.00-3.25 A. Ak: 3-19 Ä"1)
Catalyst Shell CN R Aa2 AE° Goodness
[Â] [1<E4A2] [eV] of fit
No ligands Mo-S 5.7 2.42 5.0 2.9 0.75
Mo-Alo 3.6 3.15 6.7 3.8
NTA:Ni-1.5 Alo-S 5.8 2.42 7.2 0.66
Mo-Mo 3.4 3.15 5.7 4.5
NTA:Ni=6.66 Alo-S 5.8 2.42 8.1 3.9 0.72
Mo-AIo j.J 3.15 3.3 5.2
EDTA:Ni=l Alo-S 5.9 2.42 5.4 3.3 0.71
Mo-Alo 3.7 3.15 5.5 4.3
EDTA:NaE5 Alo-S 5.8 2.42 4.8 3.8 0.76
Mo-Mo 3.6 3.15 5.2 4.8
EDTA:Ni=2 Mo-S 5.9 241 6.3 4.2 0.76
Mo-AIo 3.4 3.15 5.1 4.8
107
Chapter 5
the reason for these differences without fitting the data because the intensify of the signals
is a lunction of the coordination number and of the Debye Waller factor. Since the two
parameters are correlated, a fitting procedure is needed to discriminate the contributions of
the two parameters. Table 1 shows the results of the fits for the first two shells.
As far as the first shell is concerned, no significant change can be noticed in the
structural parameters between catalysts prepared with and without chelating ligands. On the
contrary, the parameters of the second shell show some interesting features. Increasing
amounts of NT/A and EDEA tend to lower the coordination number and the Debye Waller
factor of the Mo-AIo shell. Ehe effect of NEA is already observable for a molar ratio
NTAfNi - 1.5. whereas for EDTA a molar ratio of EDTA:Ni -~ 2 is needed to reduce the
Mo~Alo coordination number. From the Alo-Mo coordination number it is possible to
deduce the size of the MoSi crystallites [19], Mo-Mo coordination numbers of 3.2 to 3.7
suggest the presence of particles with a diameter varying approximately from 20 to 30 A,
what corresponds to MoSi slabs composed of40 and 90 Mo atoms, respectively.
Ni K-edge
The Ni A-edge Fourier transformed EXAE S spectra of the catalysts prepared without
ligands, and with different amounts of NTA and EDEA sulfided at 400°C are shown in
Figs. 5 and 6. The first shell was fitted with a Ni-S contribution, whereas the low intensity
and the proximity of the signals at 2.3 and 2.7 A (phase uncorrected) required a careful
checking of the results of the fits. Alan} fitting attempts were needed to study these two
signals.
In previous studies these two signals were fitted with various combinations of shells
[10, 12]. The larger k-range of the data obtained in our experiments made the results
presented here more reliable, however. Ehercfore, we fitted the two shells combining in
pairs the backseattercrs S. Ni and Alo and trying all possible sequences of shells. We
excluded contributions from the support because similar signals were obtained by Louwers
and Prins with carbon-supported NiAio catalysts, for which it can be assumed that the
carbon atoms from the support do not interact with Ni 110].
108
Sulfided XiMo/SiO2 catalysis
R[A|
FIG. 5. Fouitet ttansfotmation of the Xt A-edge L -weishted LXAFS function ol the catalysts piepated
with diffeient amounts of NTA
R[A]
FIG. 6. } outlet üanstotmation ot the Ni À. edge k -weighted LXAFS function of the catalysts piepared
with diftetent amounts ol FDl A
109
( 'haptcr 5
The main criterion adopted to accept a combination of shells was that the fit would
converge for the k1- and ^-weighted spectra. Moreover, the value of the edge energy
correction (AE°) should be smaller than 15 eV. Ewo models satisfied these two restrictions.
Ehe first model consisted of three Ni-S shells, fhe parameters resulting from the fits of the
spectra with these shells are presented in Eable 2.
TABLE 2. Parameters obtained from the fit of the k3-weighted Ni K-edge EXAFS spectra
of the NiMo/SiO: sulfided catal} sts with three Ni-S shells (AR: 0.90-3.15 A, Ak: 3-15 A"1)
Catalyst Shell CN R A<rz AE° Goodness
[Al [103Â2| |eV] of fit
No ligands Ni-S 3.6 2.19 0.55 2.0 0.30
Ni-S 0.7 2.75 -3.75 -3.2
Ni-S 0.6 3.11 -4.08 -7.8
NTA:NE-E5 Ni-S 3.6 2.19 0.39 2.4 0.31
Ni-S 0.7 2.75 -2.84 -0.9
Ni-S 0.5 3.12 -4.83 -8.5
NTA :NEA). 66 Ni-S 3.7 2.19 0.88 1.5 0.34
Ni-S 0.5 2.74 -6.34 -1.7
Ni-S 0.5 3.11 -5.99 -8.5
EDTA:Ni=l Ni-S 3.6 2.20 0.18 1.1 0.63
Ni-S 0.5 2.73 -4.96 3.3
Ni-S 0.3 3.09 -7.23 -0.3
EDTA:Ni=E5 Ni-S 3.7 2.19 0.63 1.9 0.31
Ni-S 0.6 2.76 -4.49 -3.8
Ni-S 0.5 3.11 -5.00 -8.4
Ehe second model was composed of two Ni-S shells and one Ni-Mo shell (Table 3).
The values of the goodness of the fit and AE° are slightly better for the first model. In
addition, the difference in the coordination numbers between the fits of the k1- and the ki-
weighted spectra varies from 0 to 0.15 for the first model and from 0.1 to 0.25 for the
110
Sulfided NiMoJSi02 catalysts
second one. All these factors suggest, therefore, that the model composed of three
subsequent Ni-S contributions has to be preferred.
As far as the differences between the various catalysts are concerned, one must be
very careful when comparing the coordination numbers obtained from the fits of the two
models, because the standard deviations van from 5% for the first shell to 40% for the
third one. It is. therefore, not possible to compare the catalysts on the basis of the
coordination numbers.
TABLE 3. Parameters obtained from the fit of the k""-weighted Ni A-edge EXAFS spectra
of the NiMo/SiO: sulfided catalysts with two Ni-S shells and one Ni-Mo shell (AR: 0.90-
3.15Â. Ak:3-15 A1)
Catalyst Shell CN R Act2 AE° Goodness
[A| [103A2] leVJ of fit
No ligands Ni-S 3.6 2.19 0.55 1.9 0.31
Ni-S 0.4 2.74 -4.85 0.1
Ni-Alo 0.7 3.07 0.17 10.6
NEA:NE-E5 Ni-S 3.6 2.19 0.42 2.3 0.36
Ni-S 0.4 2.73 -4.31 3.0
Ni-Mo 0.6 3.09 -0.34 9.0
NEA:Ni-6.66 Ni-S 3.7 2.19 0.92 1.4 0.42
Ni-S 0.3 i 70 -7.62 2.9
Ni-Alo 0.6 3.07 -0.91 9.1
EDTA:Ni--l Ni-S 3.6 2.19 0.22 1.0 0,68
Ni-S 0.3 2.71 -6.66 7.4
Ni-Alo 0.3 3.05 -3.11 17.5
EDEA:Nh-E5 Ni-S 3.7 2.19 0.63 1.8 0.32
Ni-S 0.4 2.74 -5.63 -0.7
Ni-Mo 0.5 3.07 -1.06 10.2
111
Chapter 5
Figs. 7 to 10 show the results of the fit for the catalyst prepared with EDEA (molar
ratio EDTA:NET .5) usine the two models.
13
k [A-
03
22*
03
R[A]
FTG. 7. Compaiison between measured and calculated spectra k'-wetghted %(k) (above) and Fomier
Iransfotmed (below) spectia of the catalyst containing LDIA with the molar ratio EDTA Ni - 1 5. The fit
was obtained usine three Ni-S contributions
112
SulfidedNiMo/Si02 catalysts
2T
10
— measured
fit I
k[A
measured
fit
R[\]
FIG. 8 Compaiison between measuied and calculated spectia EAveighted xOA (above) and 1 ounei
tiansfotmed (below) spectia of the catahst contamina: EDTA wnh the molat tatio EDTANi- 1 S fhe fit
was obtained usin« three Ni-S conti lbutions
in
ci 5
2'
k[A
_4
22
-0 1
-0 2
-0 3
RjÂ]
HG. 9. Compaiison between measuied and calculated spectia k-weighted %{k) (above) and Foiniei
tiansfoimed (below) spectia of the catalyst containing f D1A with the molai tatto EDTANi = i 5 The fit
was obtained using two Ni-S and one Ni-Mo conti ibuttons
Sulfided N1M0/S1O2 catah sis
22
Ph
R[A]
FTG. 10. Compaiison between measuied and calculated spectia lAweighted y(k) (above) and Fouitei
tiansfoimed (below) spectia oi the catahst containing FDl A with the molar ratio EDTANi = 1 s The fit
was obtained using two Ni-S and one Ni-Mo conti ibutions
IIS
Chapter 5
Discussion
In this section the structure of Mo and Ni in the sulfided catalysts will be discussed
taking into account the findings concerning the catalysts precursors discussed in Chapter 3
and the sulfidation mechanisms treated in Chapter 4. fhe role of chelating ligands through
the whole preparation procedure will be considered,
Molybdenum
Molybdenum is present on SiCE in the oxidic precursors as a mixture of polyanions
such as M07CE4A and molvbdatc units (AIoöaA Ihe chelating ligands NTA and EDEA
form complexes with Mo onh when the molar ratio ligand:Ni is larger than i [15, and
Chapter 3, 20]. Ehe presence of chelating ligands induces a broadening of the sulfidation
interval of Alo, i.e. the sulfidation starts at lower and is completed at higher temperature in
comparison with the catalyst prepared in absence of chelating agents. The consequences of
the different sulfidation behaviour can be noticed in the final sulfided catalysts, whose
structure is discussed here. The data of Table 1 show that the addition of chelating ligands
induces a reduction of the AI0S2 particle size and an increase in the structural order, as
deduced from the values of the coordination number and of the Debye Waller factor,
respectively. It can be noticed (see Table 1) that these effects are observable for ligand:Ni
molar ratios larger than 1. which suggests that as long as only Ni is complexed by the
organic molecules. Alo does not perceive the presence of the chelating ligands, either in the
oxidic state, nor in the sulfided state.
Ehe IIDS activity profile of the investigated catahsts (Fig. I) shows a decrease for
NTA:Ni ratios larger than 2. fhe explanation of this decrease in activity must be connected
with the observed changes in the structure of the M0S2 particles, since no change In the
structure of Ni is expected and observed for large ligand concentrations. The fact that a
lower activity is observed for smaller AI0S2 crystallites (Table 2) is in apparent contrast
with the idea that for smaller particles a higher number of active sites should be present. A
reduction of the particle dimensions should raise the fraction of Alo atoms at the edges of
the M0S2 crystallites. According to the C0AI0S model, these atoms are responsible for the
116
Sulfided NiMo/Si02 catalysts
formation of the active sites [1. 21]. If it is assumed that this model is correct, the fact that
we find a lower activitv when the particles are smaller, would mean that cither the
arrangement of the Ni atoms on the edges of the MoS2 crystallites is hindered by some
factor, or that the ratio of Alo atoms exposed at the MoS: edges and Ni atoms becomes
unfavourable for the catahtic activity.
The Mo-Mo coordination number of the catalyst prepared in the absence of ligands is
3 6. According to Shido and Prins [19], a coordination number of 3.6 corresponds to a
M0S2 particle diameter of about 30 A. A crystallite with such a dimension is composed
approximately of 90 Mo atoms, as sketched in Fig. 11. One third of them are situated at the
edges. For the catalyst prepared with the highest amount of NFA (NTA:Ni -= 6.66) the
Mo-Mo coordination number is 3.2, which indicates that the AI0S2 particles have a
diameter of ca. 20 Â. and half of the atoms are situated at the edges. In our catalyst the
31 A
C-A^ -V V^
1
1 1 !
1 1 ' i
1'
î
-^^ %,• *S,v. V,\ i 1 f
la A k A >.
y^-
v.-* N>^ ^~-* x^.
1 ! ! ! Ï
- 'A. ^ -
^ ^^- N v^ X-
\ ] 1 \
f A A A XyA, l~A ^
y ^"" v-1 Ï 111 1
lit'
~"~Y~Y y"AXv v X,- "s~- X,- l^
t ! 1 t 1
ANv v\^ .. \ v-*, - "w -
v.
IS ! 1 !
1^
II!1!S s
"V\ y Va- 1 1 ] 1
1 1 i 1
j | | 1
' 1 \
S ^
X - A,- N/K
1 ! !
' 1
f y '""
-A,-A- A.
I9Ä
l l 1 ' l
v -yA/laAAa
ii
A
Fig. 11. M0S2 paiticles with diffeient diameteis.
Ni:Mo ratio is 0.3:1, which means that there are slightly more than 3 Mo atoms for each Ni
atom. We assume that with the help of the ligands all Nt becomes situated on the M0S2
edges. It is clear from these approximate calculations that for the larger MoS: particles
(diameter of ca. 30 A) the number of edge Alo atoms and that of Ni atoms present in the
catalysts are very similar. On the contrary, m the smaller particles the number of edge Mo
117
Chapter 5
atoms is significantly larger than the number of Ni atoms. The increase of the fraction of
free Mo atoms on the MoS: edges could be the reason for the decrease in activity observed
for higher N FA concentrations.
The explanation of this effect could be twofold: either the Ni atoms arranged on the
AI0S2 edges are too far apart to carry out the hydrodesulfurisation reaction properly, or the
presence of MoS: vacancies has a negative effect on the active sites. It has been proposed
that the promoting of Ni or Co is due to an increase of the electron density on Mo, which
would lead to more anion vacancies and consequently to a higher activity [22]. In the
presence of a high number of Mo atoms exposed at the edges, the electron donating effect
of Ni would be distributed over a larger edge area and the promoting effect would be
spread.
To understand the changes in the structural features of the sulfided catalysts, the
sulfidation mechanism of Alo has to be considered. We showed that before M0S2 is formed,
Mo is present on the support as a MoSs-like material [23, and Chapter 4]. This material is
transformed to M0S2 and the aggregation to larger crystallites takes place from the end of
the third intermediate sulfidation region (AloSMike material) until 400°C. As observed by
fitting the QEXAFS data, no significant change in the Alo-Alo signal was noticed after
reaching a sulfidation temperature of 400°C, i.e.. even though the temperature wras kept at
400°C for 30 min. small changes were observed only in the first few minutes. This suggests
that the size of the MoS: particles is dependent on the length of the temperature interval
ranging from the formation of the first MoS: units to 400°C. In the case of the catalyst
prepared without ligands. AloS: starts to form at 295°C (see Fig. 6 in Chapter 4), whereas
in the catalyst containing NTA with the molar ratio NTA:Ni =" 6.66, the first MoS: units are
detected at 360°C. Ehese observations are understandable when considering that Alo is
present in the catalyst prepared without ligands as polyanions, where groups of
molybdenum atoms are already collected in units of 7-8 atoms, whereas, when the amount
of NEA is large enough to bind also all Alo (at least for NfA:Ni = 4.3), Mo is present on
the support as isolated [MoOnNTA)p complexes. Therefore it is more likely to obtain
smaller particles when employing larger amounts of ligands.
Another phenomenon could explain why differences are observed in the structure of
Mo in the sulfided catalysts. The conclusions drawn from the QEXAFS results suggested
lis
Sulfided NiMo/Si02 catalysis
that the sulfidations of Ni and of Mo are mutually influenced. The reason for this effect is
still unclear but the fact that Ni is sulfided at higher temperatures prevents the formation of
segregated clusters of nickel sulfide and should increase the number of interactions between
Ni and M0S2. The presence of a higher number of Ni-MoS: interactions could be the reason
for a smaller M0S2 particle size. Ni could prevent further enlargement of the particles
because of its structure (see Eable 2 and 3). which is different from the trigonal prismatic
geometry of Mo in AI0S2. A similar hypothesis was proposed from 'FEM results by Ledoux
el al who suggested that the role of Co in C0AI0/AI2O3 catalysts is to stabilize small M0S2
patches and to avoid sintering of M0S2, thus keeping the M0S2 dispersion high [24]. Other
authors arrived at the same conclusion for unsupported CoAlo and NiMo compounds [25,
261.
Nickel
Nickel is present as silicate in the catalyst precursors prepared in the absence of
ligands. whereas the formation of complexes with the chelating ligands isolates the metal
ion from the support. The difference in structure in the oxidic precursors leads also to
marked differences during the sulfidation process. Increasing amounts of NTA. EDTA and
EN delay the end of the sulfidation of Ni. Ehe goal of the EXAFS investigation presented
here was to explain the consequences of the different sulfidation behaviour on the structure
of the final sulfided catalysts. Indeed, the EXAFS technique gave useful information. We
calculated that Ni is surrounded by four sulfur ligands at a distance of 2.2 A, which either
corresponds to a tetrahedral or to a square planar geometry.
The nickel-sulfur system is rather complex and the combination of the two elements
can give rise to a large number of compounds. The most common minerals composed of
these two elements are hcazlewoodite (NES:), millerite (NTS), polydymite (NES4) and
vacsite (NTS:) [27]. These four minerals are present in nature and are established mineral
species. Other binary nickel sulfides have been reported, such as NißSs, NES6, NESg [28],
and Ni|7Si8 |29J. The crystallographic data about NES:. NTS, NE,St, NiS2 and Ni^Sis can
be found in the literature. Among all these compounds, the only one in which Ni has a
tetrahedral coordination and a Ni-S distance of 2.21 A is Ni3Si. This material, however.
119
Chapter 5
decomposes at 200°C to NiS when treated with FES and hydrogen. In principle it is,
therefore, impossible that it is present at our sulfidation conditions (400°C). EXAFS
measurements are carried out at liquid nitrogen temperature, so that the presence of M3S4
is, in principle, possible. It is. however, very unlikely, since it means that it should be
reformed when the sample is cooled after the sulfidation. However, fits of the QEXAFS
spectra measured at 400°C gave coordination numbers around 3.5 and a Ni-S distance of
2.19 Ä. demonstrating that the observed phase is the same as the one observed in the
EXAFS spectra measured at liquid nitrogen temperature. Ehe other inorganic Ni-S
compound with a tetrahedral structure is heazlewoodite (NES:), in which a Ni-S distance is
reported at 2.28 A and a Ni-Ni distance at 2.51 A. which is not observed in our spectra.
A survey of organometallic compounds gave us a more complete picture of the
possible structures of Ni in a sulfur environment. In general, the Ni-S distance in
compounds where Ni has a tetrahedral coordination ranges from 2.25 to 2.35 A [30],
whereas it amounts to 2.14-2.20 A in case of a square planar structure [31, 32]. Ehe
distance obtained by EXAFS (2.19 A) thus suggests a square planar geometry. In
accordance with this suggestion. Ni is sterically protected by the S ligands in case of a
tetrahedral structure and the absorption of a substrate during catalytic reaction would be
hindered. On the contrary, the square planar geometry leaves the surface free for interaction
with substrate molecules.
'fhe nature of the second and third shells might clarify the structure of the
investigated material. Louwers and Prins suggested that these two shells arc due to Ni-Mo
and Ni-Ni contributions [10]. Ehe data range that they used for the Fourier transformation
was 3-11 A"1. Fig. 12 shows how different the spectrum of the same sample can be when
using different k-ranges. As a consequence, also the interpretation of the data can change
dramatically. Thanks to the more powerful X-ray source and long collecting times, we
could collect data that were usable in the range 3-15 A"1. We tried to fit the data with the
shell combinations proposed by formers and Prins but the fits did not satisfy the adopted
criteria: the fits converged for the IE-weighted spectra but never for the k'-weighted data.
We obtained two possible structures of Ni in the sulfided state. The first model, consisting
of three Ni-S shells docs not show any Ni-Mo proximity. If this model were valid, it would
mean that the EXAFS results do not allow to say anything about the position of the Ni
120
Sulfided NiA4o/Si02 catalysts
R [A]
FIG. 12. Comparison between Fourier transformed data obtained using different k-range.
atoms on the MoS: edges in these catalysts. Ihe second model, in which Ni is surrounded
by two Ni-S shells at 2.2 and 2.75 A and a Ni-Alo contribution at 3.07 A, would be in
agreement with the proposed structure of the active phase, in which Ni decorates the edges
of the MoS: crystallites [33].
Ehe inability to discern which is the correct model and the inaccuracy of the
coordination number of the second and third shell, show some limitations in the use of
EXAFS. Nevertheless, both models have many similarities. In both models Ni is
surrounded by 3.6-3.7 S atoms at 2.19 A and about 0.4-0.7 S atoms at 2.75 A. They differ
in the composition of the third shell, being S for the first and Alo for the second model. 'Fhe
parameters of the first Ni-S shell suggest a square planar geometry. As far as the Ni-S shell
at 2.75 A is concerned, a similar Co-S distance was found by Crajé et al. in Co/C sulfided
at 400°C [ 34]. Their explanation for this observation was a shrinkage of the second nearest
Co-S distance belonging to CoçSg. which usually is 3.47 A. Such a reduction seems,
however, excessive.
121
Chapter 5
No other inorganic Ni-S compounds reported in the literature show a similar Ni-S
distance. Ehe only compound which has a Ni-S distance around 2.75 A is trimeric
bis(dithiobenzoato)-nickel(H). whose structure is depicted in Fig. 13 [35].
v_ T~ A v
Fig. 13. Structure of trimeric bis(dithiobenzoato)-nickel(ll) (from [35])
In this compound every monomer is composed of a Ni atom positioned in the centre
of four sulfur atoms of two dithiobenzoate anions with a Ni-S distance of 2.22 A. The two
external Ni are arranged in a square pyramidal geometry, whereas the central one has either
a octahedral geometry or a square planar structure with significant Jahn Teller distorsions.
The three units are interconnected by means of Ni-S bridges having a bond length of 2.78
and 3.11 A (Fig. 14). These two latter distances are different because the external Ni atoms
are slightly displaced towards the central monomer. Ihe resulting Ni-S distance is, thus,
reduced to 2.78 A.
Because of the strong resemblance between the distances observed in our data and in
this compound, we simulated the spectrum of the trimer by means of the Feff code. For the
simulation an overall Debye Waller factor of 0.004 A" was used. The phenyl rings were
excluded from the simulation, only the carbon atoms as indicated in Fig. 14 were retained.
In Fig. 15 the x(k)lA data of the simulation are plotted together with the data of the catalyst
containing N'EA with the molar ratio NFA:Ni=- 1.5. A horizontal shift can be observed
between the two curves, which is more marked around 13 A"1. Ehis shift is likely due to the
122
Sulfided NiMo/Si02 catalysis
3,107
\ 2.776
Fig. 14. Distances in trimeric bis(dithiobenzoato)-ntckel(lI).
fact that the distances of the first and second shell in our compounds are shorter in
comparison to the corresponding distances in the trimer. A shorter distance produces a
longer period of the EXAFS oscillations as can be understood from equation (12) in
Chapter 2. Apart for this discrepancy, the two functions are practically identical. Fig. 16
shows the Fourier transformed spectra of the same compounds. Also in this plot a shift of
the imaginary part can be noticed. A comparison between the k - and the IE-weighted
Fourier transforms for the two spectra showed that the second and third shells behaved in
the same way for the two weighting factors. Vaarkamp showed that it is possible to
distinguish between Alo and S backseattercrs comparing the k - and the k3- weighted
spectra [361. Since Mo is a heavier backscatterer. the signal belonging to a Ni-Mo shell
should be amplified by a larger extent than a Ni-S signal, when increasing the power of the
123
Chapter 5
10
22
NTA:Ni=E5
Ni-trimer
k IÄ-1]
Fig. 15. Comparison between the Ni A-edge %(k) E data of the measured NiMoNTA/Si02 catalyst (molar
ratio NTA:NI-1.5) and the simulated spectrum of trimeric bis(Dithiobenzoato)-nickeI(II).
13
22
-6 5
— NTA:Ni=E5
" " "
Ni-trimer
Fig. 16. Ni K-edge Fourier transformed spectra of the cataly st containing N FA with the NfA:Ni molar
ratio 6.66 and the simulated spectrum of trimeric bis(Dithiobenzoato)-nickel(TT).
124
Sulfided NiMo/Si02 catalysis
weighting factor. The amplitudes of the second and third shells for the simulated spectrum
and the spectra of our catalysts were similar not only when using a k'-weighting but also
when using a lA-weighting, Therefore, both signals of the catalyst spectra consist of Ni-S
contributions. If one of the signals had been a Ni-Alo contribution there would have been a
larger amplification passing from the k1- to the IE-weighted spectrum. This consideration
excludes the second model that we proposed which involved the presence of Mo in the
third shell.
The main difference between the two spectra of Fig. 16 consists in the signal at 3.4 A
(phase uncorrected), which has a relatively large intensity for the simulated spectrum,
whereas it is very weak for the spectrum of the catalyst. Ehis signal is due to the Ni-Ni
contribution at 3.75 A. which is the distance between two Ni atoms belonging to two
different monomers. The fit of the signal at 3.3 A (phase uncorrected) in the spectra of the
catalysts was converging only in the k'-weighted spectra, but not when using ^-weighting.
We attribute this inability to the weak intensity of the signal. The weak amplitude of the
Ni-Ni signal suggests that not all Ni in the catalyst is present as a small cluster. However,
the explanation of this feature could be twofold. One interpretation is that part of it is
present as isolated units. In fact, we noticed that the addition of isolated square planar Ni
complexes gave a marked decrease in the amplitude of the Ni-Ni signal in the simulated
spectrum. Another interpretation is that Ni forms larger sulfided clusters, in which more
than one Ni-Ni distance is present. The superposition of the signal of Ni-Ni shells at
different distances could lead to the weakening and eventually to the disappearance of the
signal in the EXAFS spectra. A possible structure of the clusters could be similar to the
well-known PdCE structure, in which chains of square planar PdCf units are ordered
parallel with a distance of 3.8 A to each other (37). Another difference between the two
spectra of Fig. 16 is the ratio between the Ni-S signals at 2.3 and 2.8 A (phase uncorrected).
In the simulated spectrum of the trimer the first signal has a significantly larger intensity
than the second one. whereas in the catalyst spectrum the two signals have almost the same
125
Chapter 5
intensity. An accurate analysis of the composition of the Ni-S signal at 2.3 A (phase
uncorrected) showed that two contributions overlap and produce the observed peak, one
being the Ni-S contribution at 2.77 A, the other the Ni-C contribution at 2.69 A. A
simulated spectrum in which the Ni-S contribution at 2.78 A was omitted showed that a
good fit can not be obtained without the Ni-S contribution. Proof for the presence of Ni-C
is difficult to obtain because carbon is a weak backscatterer and even the presence of a high
number of carbon neighbours has a minor effect on the intensity of the signal. Thus, the
elimination of the Ni-C contributions from the simulated spectra gave only a slight decrease
of the signal at 2.3 A (phase uncorrected). Changes in the intensity of the signal at 2.3 A
can, therefore, be induced by a change in either the Ni-S or the Ni-C coordination number.
From these observations a new picture is obtained of the structure of Ni in SiO:
catalysts. The correspondence between the structure of trimeric bis(dithiobenzoato)-
nickel(ll) and the data obtained from our EXAFS spectra suggests the presence of small
sulfided Ni(II) clusters in our catalysts. This would explain the simultaneous presence of
two Ni-S distances, at 2.77 and 3.11 A. Aloreover. the weak signal at 3.3 Â (phase
uncorrected) suggests that a Ni-Ni shell is even visible in our spectra.
The question that arises at this point is whether Ni can be present as a thiocarbamate
complex in the sulfided state. We first consider the catalysts prepared with chelating
ligands. It has been proved that Ni in the catalyst precursors is complexed by NTA and
EDTA [15, 20]. Fhe behaviour of the organic ligands under sulfidation conditions has not
been investigated, yet. Flowever. it is likely that the keto groups in the acetate arms of the
ligands may be substituted by FES and that thiocarboxylic groups are formed, which can
replace the other ligands around Ni and form dithiocarbamate-Ni complexes as proposed in
the mechanism in Fig. 1 8.
In the catalyst prepared without ligands. the presence of carboxylic groups can be
excluded since no organic compound was used during the preparation of this catalyst.
Therefore, the formation of metallorganic complexes in the catalyst prepared without
ligands can not be considered. It is more likely that larger clusters are formed of the kind
proposed in Fig. 17. Nevertheless, no prediction can be made about the size of the cluslers.
126
Sulfided NiAlo/Si02 catalysts
K
S—H
H.XS 0
,0H2
N- -CH;
M
H.
HO-
,©
,0G
N-
N
-CH2
S H
-CHo
M
S. S"
—-CH?
HS JS
© \f°H2o-rY
N- -CHc
HH.
NE
.©
,0H
-CH,
Fig. 18. Proposed mechanism for the formation of the dithiocarbamate group from the carboxylic groups
of the chelating ligands.
According to this interpretation the role of the chelating ligands is to avoid formation
of larger Nl-S clusters and favour the separation of the metal sulfide in smaller units.
The EXAFS data did not allow us to investigate the presence and structure of
metallorganic complexes on the catalysts. Ehe only sign that could indicate the presence of
0 at 2.7 A from Ni is the increase of the intensity of the signal at 2.3 A (not phase-
corrected) for the spectrum of the catalyst containing NEA with the molar ratio
NEA:Ni = 6.66 (Fig. 5). In the same spectrum it is possible to observe that the signal at
3.3 A is significantly lower in comparison to the spectra of the other catalysts. As far as the
relationship between Ni and AloS: is concerned, it is impossible to tell from our data
whether an interaction is present between the Ni complexes and the M0S2 crystallites.
127
Chapter 5
Conclusions
The use of EXAFS for the study of supported sulfided NiAlo catalysts demonstrated
that this technique is a powerful tool for the study of this amorphous and dispersed catalytic
system. We could show that molybdenum does not perceive any significant effect from the
use of chelating ligands during the preparation of the catalyst precursors when low amounts
of ligands are employed. From this we deduced that the beneficial effect of chelating
ligands is due to their effect on Ni, since higher ligand amounts influence the structure of
Mo and induce a decrease in the FIDS catalytic performances. A change in size and
structural order of the MoS: crystallites was detected for higher NEA and EDEA
concentrations. It would have been extremely difficult to prove differences in particle size
of less than 10 A on the same catalytic system by means of other techniques. From these
observations it is possible to say that the lower HDS activity rate observed for high ligands
concentrations must be ascribed to the changes in the structure of Mo in the sulfided
catalysts. No Mo-support interaction was detected in the studied catalysts.
Even though the quality of the Ni A-edge data was lower than the Mo A-edge data, it
was possible to propose a structure for Ni in the sulfided catalysts. Ehe relatively short
distance of the nearest Ni-S shell (2.19 A) suggests that Ni is present in a square planar
geometry rather than in tetrahedral coordination. Moreover, from the Ni-S distances (2.77
and 3.11 A) and the signal at 3.3 A (phase uncorrected), that we attributed to a Ni-Ni shell,
it was possible to propose that Ni forms a lay ered structure.
The mechanism of sulfidation of the ligands was discussed, 'fhe presence of the
chelating ligands during the sulfidation could enable the formation of a thiocarbamate
complex composed of a small number of Ni atoms. Therefore, the role of the ligands is to
improve the dispersion of Ni by forming complexes that stay apart from the larger nickel
sulfide clusters.
To improve the signal to noise ratio of the Ni A-edge EXAFS data, the Ni loading
should be raised proportionally with the Alo loading trying to stay below the monolayer
coverage of the support. Ehe support could be changed to carbon, on which the fraction of
Ni present as active phase is relatively high in comparison with other supports. In this way
the danger of having a mixture of different Ni compounds on the support would decrease
128
Sulfided NiA4o/Si02 catalysts
significantly. Moreover, measuring EXAFS in fluorescence mode, instead of transmission
mode, could improve considerably the quality of the data because the noise produced by the
other elements present in the samples would be excluded and the Ni signal could be
isolated, fhe scarce availability of appropriate detection systems makes, however, the
collection of EXAFS data in fluorescence mode still impracticable.
Literature
1. Fopsoe, FE, Clausen. B. S.. Candia, R., Wivel. C, and Morup, S,, J. Catal. 68, 433
(1981).
2. van der Kraan, A. AT. Crajé, M. W. E, Gerkema, E.. Ramselaar, W. L. T. AE, and
de Beer. V. El. L.Appl Catal. 39. L7 ( 1988).
3. Crajé. AE W. E, de Beer. A. FF E. and van der Kraan. A. M.. Appl. Calai 70. L7
(1991).
4. Weber. Eh., Muijsers. J, C. and Niemantsverdriet, E W., J. Phys. Chem 99, 9144
(1995).
5. Muijsers. E C. Weber, fh.. van Ilardeveld. R. AI., Zandbergen, EI. W7., and
Niemantsverdriet. EW..J Catal 157.698(1995).
6. Weber. Th.. Aluijsers. E C, van Wolput. J. FI. M. C, Vcrhagen. C. P. E, and
Niemantsverdriet.E W.,,7 Phys Chem 100. 14144(1996).
7. Eouwers, S. P. A.. Crajé. Al. W. E. van der Kraan. A. AE, Geantet. C, and Prins, R.,
,7 Catal 144, 579(1993).
8. Stockmann. R. M., Zandbergen. FI. W.. van Eangeveld, A. D., and Moulijn. J. A..
J.Mol Catal. 102, 147(1995).
9. Calais. C, Matsubayashi. N., Geantet, C., Yoshinuira. A7., Shimada, FF, Nishijima.
A.. Lacroix, M., and Breysse. AE. ,7 Catal. 174. 130 (1998).
10. Eouwers. S. P. A., and Prins. R.. J Calai 133. 94 (1992).
11. de Boer. M., van Dillen. A. E. Koningsberger. D. C. and Gcus, J. W., J. Phys
Chem 98,7862(1994).
12. Aledici, L.. and Prins. R.. J Catal 163. 38(1996).
129
Chapter 5
13. Topsoe, IT., Clausen, B. S., and Massoth. F. E., "Catalysis Science and
Eechnology", Springer Verlag, New York, 175 (1996).
14. Eeliveld, R. G., van Dillen. A. E, Geus. J. W.. and Koningsberger, D. C,,/ Catal.
165.184(1997).
15. Cattaneo, R.. Shido. T.. and Prins, R.. d Catal. 185, 199 (1999).
16. Kampers. F. W. FF. Alaas, E. AE E. van Grondellc, E, Brinkgreve, P., and
Koningsberger, D. C, Rev. Sei. Instrum. 60, 2635 (1989).
17. Vaarkamp. AE. Dring, !.. Oldman, R. E. Stern, E. A., and Koningsberger, D. C,
Phys. Rev. «50,7872(1994).
18. Zabinsky. S. E, Rehr. J. E. Ankudinov. A., Albers. R. C. and Eller, AT E. Phys.
Rev. S 52. 2995 (1995).
19. Shido, T.. and Prins, R.. J. Phys. Chem. B 102. 8426 (1998).
20. Aledici. L., and Prins. R.. ,7 Catal. 163. 28 (1996).
21. Ratnasamy. P., and Sivasanker, S., Catal Rev.-Sei Eng. 22. 401 (1980).
22. Harris. S., and Chianelli. R. R., J. Catal. 98. 17 (1986).
23. Cattaneo. R.. Weber. Th.. Shido, T.. and Prins. IE, .7. Catal. 191, 225 (2000).
24. Eedoux. Al. E. Maire. G.. ITantzer. S., and Alichaux. O.. in "Proc. 9th Int. Congr.
Catal.". 'fernan, AE and Philips, M.E Eds.. Chem. Institute of Canada, Ottawa, 74
(1988).
25. Pratt. K. C. and Sanders, J. V., in "Proc. 7th Int. Conf. Catal.". Seiyama, T. and
Tanabe, K. Eds., Kodansha'Elsevier. fokyo/Amsterdam, 1420 (1981).
26. Vrinat, M. L., and Alourgues, E. d„ Appl Catal. 5, 43 (1983).
27. Kullerud, G.. and Yund, R. A.. J. Petrology 3, 126 (1962).
28. Honig. J. AE. and Spalek. E. Chem. Mater. 10. 2910 ( 1998).
29. Collin. G., Chavant. C. and Comes. R.. Acta Crystallogr. Sect. B: Struct. Sei. 39,
289(1983).
30. Rheingold. A. L.. Beall. K. S.. Riggs. P. E. and Groh. S. E., Acta Crystallogr., Seel.
C 49, 542(1993).
31. Coucouvanis, D.. Patil, P. R.. Kanatzidis. AE G.. Dctering. B., and Baenziger, N. C.
fnorg. Chem. 24,24(1985).
130
Sulfided NiMo/Si02 catalysts
32. Kushch, N., Faulmann, C, Cassoux. P.. \ralade. L.. Alalfant, E, Legros, J.-P.,
Bowlas, C. Errami. A.. Kobayashi, A., and Kobayashi. IE, Mol. Cryst. Liq. Cryst.
Sei. Techno!., Sect. A 284, 247 (1996).
33. Candi a, R., Sorenson, O., Villadscn. E. Eopsoe. N. Y., Clausen, B. S.. and Topsce.
H.,.7. Phys. Chem 95. 123 (1991).
34. Crajé. AI. W. E. Eouwers. S. P. A., de Beer, V. H. E, Prins, R., and van der Kraan,
A. AE,,/. Phys. Chem. 96, 5445 (1992).
35. Bonamico, M., Dessy, G.. Fares, AE. and Scaramuzza, L., ,/. Chem. Soe. Dalton
Trans. .2250(1975).
36. Vaarkamp, AE. Cat. Today 39, 271 ( 1998).
37. Wells. A. F.. Zeit. Kristall. 100, 189 (1938).
131
Chaptei 5
132
Chapter 6
Influence of chelating ligands on the HDN and HDS
behaviour of y-Al203-supported NiMo catalysts
Introduction
After the detailed study of the effects of chelating ligands on SiO:-supported catalysts
presented in the previous three chapters, we concentrate in this chapter on alumina-
supported catalysts, fhis support is the one mainly used In industrial applications because
of the stronger interactions between the metal sulfides and the support, which induces a
higher stability of the catalysts at elevated temperatures [1, 2].
In this chapter first the catalytic performances in hydrodcnitrogenation (HDN) and
hydrodesulfurisation (IIDS) are compared. Ehen the QEXAFS results measured during the
sulfidation of the catalysts are presented, that allow to study the sulfidation process of Mo
and Ni in the presence and absence of chelating ligands. To complete the characterisation
part, the FXAES data of the sulfided catalysts arc discussed in order to understand the
structural features that influence the activity of the studied materials.
HDN reaction network
Hydrodcnitrogenation (HDN) is an important step in hydrotreating. Several authors
have studied the activity and structure of the catalysts [3] as well as the mechanisms and the
kinetics of the E1DN reaction [4. 5]. We have choosen toluidine as a model compound
because in its network the most important reactions occur which take place In a HDN
network, folmdine (TOT) can be hydrogenated to methy Icyclohexylamine (MCHA) or can
Chapter 6
react to toluene (T). Finder our conditions. E can not react further, as was shown in a study
of its selectivity in our laboratory [6. 7]. AICHA then reacts via elimination to
methylcyclohexene (MCHE) that can react further via hydrogénation of the double bond to
the final product methyleyclohexane (MCEI). AICHA can also react directly to MCEI via
nncleophilic substitution of the amine group by a sulfhydril group |8]. The reaction network
is shown in Fig. 1.
Fig. 1. O-tohndine HDN reaction network
IIDS reaction mechanism
'fhe field of hydrodesulfurization has been the subject of numerous reviews |9-11 ].
Thiophene (TP) is often used to test the HDS catalytic activity of catalysts [12, 13]. Its
network (Fig. 2) is relatively uncomplicated. Vt elevated IE pressures the major reaction
path in the IIDS of thiophene is via hydrogénation of thiophene to tetrahydrothiophene
[14]. fhis intermediate can react to butadiene through two successive ß-EI eliminations. At
high EE pressure, butadiene reacts quickly further to 1-butène, 2-butene (eis and trans) and
eventually to butane.
Ehe HDS mechanism of thiophene at one atmosphere pressure is still under debate
after several decades of investigation. Based on the presence of butadiene (BDE) and the
absence of fetrahydrothiophene (EHE) m the reaction products obtained in atmospheric
pressure studies, several authors have proposed that the path through BDEi (a. b pathways
shown in Fig.l) is the major route [15. 16]. A direct pathway (pathway f shown in Fig. 2)
for the hydrodesulfurization of IP directly to butène (BE) has been suggested on the basis
of the absence of Ulf [17]. Ehe reaction intermediates are supposedly retained on the
134
IIDN-HDS ofNiAdo'AfO^ catalysts
surface sites during reaction. Another study with tetrahydrothiophene over MoS: at low
pressure found substantial quantities of thiophene, as well as butadiene and suggested two
reactions paths (e. b and c. a. b) for HDS of this compound [18]. It has been revealed that
hydrogenated S-intermediates are present, suggesting parallel paths [19] or even that
prehydrogenation may be necessary before C-S bond cleavage occurs [20].
THE
B
BDE
Fig.2. Network of the HDS of thiophene
Experimental
Catalysts preparation
The catalysts used in this work contained about 7 wt% Mo and 2.5 wt% Ni and were
prepared by pore volume coimprcgnation of y-AKE (CONDEA. pore volume: 0.5 cmVg,
specific area: 100 nr'g) with an aqueous solution (pll 9.5) of (TNFH)6A1o70:a4H:0
(Aldrich) and Ni(NOY'6H:0 (Aldrich) with 25% ammonia in the presence or absence of
the chelating ligands nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid
(EDTA). The support was dried at 120°C for 12 h prior to impregnation. The impregnated
powder was dried in air at ambient temperature for 4 h and dried in an oven at 120°C for 15
h. Calcination was carried out only for the catalyst prepared without ligands at 500°C for
4h. For all other catalysts calcination was not carried out in order to avoid the
decomposition of the complexes in the cataly st precursors. Ehe investigated catalysts are
listed in Eable 1.
135
Chapter 6
TABEE 1. List of catalysts used in this work
Catalyst Loading (wt%) Ligand :Ni
Ni Mo (molar ratio)
NiMo calcined 6.8 2.6 0.0
NiMo not calcined 6.9 2.5 0.0
NiAloNFA 6.0 2.2 0.5
NiMoNTA 5.8 2.2 1.0
NiAloNFA 6.0 2.2 1.5
NiMoEDFA 6.0 2.2 1.0
HDN activity tests
70 mg of the oxidic precursor were diluted with 8 g SiC to achieve plug-flow
conditions in the continuous flow fixed bed reactor. The catalyst was sulfided in situ with a
mixture of 10 % IES in H: at 400°C and 1.0 AlPa for 2 h. After sulfidation, the pressure
was increased to 5.0 MPa. and the liciuid reactant was fed to the reactor by means of a high
pressure syringe pump (ISCO 500D). All reactions were performed at 370°C.
Dimethyldisulfidc (E)A1E)S) was added to the liciuid feed to generate FES in the reaction
stream. Toluidine was used as model compound to study the HDN reaction and
cyclohexene was added to the feed to study the hydrogénation reaction. The composition of
the gas phase reactant for the catalytic tests was 7 kPa of toluidine, 4 kPa of cyclohexene,
4800 kPa of H2. 134 kPa of octane. 20 kPa heptane (reference) and 17.5 kPa of H2S.
"fhe reaction products were analysed by on-line gas chromatography with a Variait
3800 GC instrument equipped with a 30 m DB-5 fused silica capillary column (J & W
Scientific, 0.32 mm Ed.. 0.25 pm film thickness), a flame ionisation detector (FID) and a
pulsed flame photometric detector (PFPD). Space time was defined as x = wc / niCCd. where
wc denotes the catalyst weight and ntccj the total molar flow fed to the reactor. Space time
(t) was changed by varying the liquid and gaseous reactant flow rates, while their relative
ratios remained constant.
136
HDN-HDS ofNiAfo'AI-,0, catalysts
HDS activity tests
100 mg of the oxidic precursor mixed with 1 g of SiC were sulfided for 2 h at 400°C
(heating rale. (ACTnin) with a mixture of 10% H:S in ET: (Messer Griesheim) that flowed
through the reactor from the beginning of the heating process. The activity of all the
catalysts was tested in the hydrodesulfurization of thiophene at 370°C. The feed (3%
thiophene in TE) was obtained by bubbling IT: through a series of four thiophene saturators
that were cooled to 2°C. Ehe product stream was analyzed online with an HP5890 gas
Chromatograph. The sulfidation of the oxidic precursors and the thiophene HDS reactions
occurred at atmospheric pressure in the apparatus described in Fig. 1 of Chapter 3.
X4FS measurements
The Quick EXAFS measurements were carried out at the XI (RÖA10 II) beam line at
EIASYEAB (Flamburg. Germany), whose set-up is described in Chapter 4. Si (311) and the
Si (111) crystals were used in the monochromator for the Mo and Ni A'-edges, respectively.
Ehe accumulation time was about 0.2 sAtep at the Alo A-edge and about 0.4 s/step at the Ni
Ä'-edge. Ehe k-ranges used for the analysis of the data were 3-17 A"1 for the Mo and 3-12
A"1 for the Ni A-edge. Ehe EXAFS spectra were collected at the Swiss Norwegian Beam
Line at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Ehe
beam line is described in Chapter 3. "Fhe k-range used for the analysis of the data was 3-
19.5 A"1 for the Mo A-edge and 3-16 A"1 for the Ni A-edge.
For both kinds of measurements the catalyst samples were pressed into self-
supporting wafers and mounted in an in situ EXAFS cell [21]. Ehe thickness of the samples
was chosen to adjust the edge jump to 1 for the Alo A-edge and the total absorption to
px - 4 for the Ni A-edge (lower Ni concentration).
For the QEXAFS measurements first two spectra of the fresh samples in an He
atmosphere were collected, 'fhe samples were then sulfided m situ during data collection. A
stream of 10% H:S/1E flowed through the cell while it was being heated to 400°C. Ehe
heating rate was 3°C/min.
For the classical EXAFS measurements, the samples were cooled to room
temperature after they had been sulfided at 400°C for 30 min. Once at room temperature,
137
Chapter 6
the ILS still present in the cell was replaced by He by flushing for 10 min. Ehe cell was
then cooled to liquid nitrogen temperature prior to the EXAFS measurement.
EXAFS analysis
Ehe prognim XDAP (version 2.2.2) was used to analyze and fit the data [22] as described in
Chapters 3 and 4. Apart from AloS: (references for Alo-S, Alo-Mo), reference spectra were
calculated using the Feff7 code [23. 24].
138
HDN-HDS of NtA4oC\I20, catalysts
Results
Catalytic performance
HDN activity tests
Ehe HDN of o-toluidine (IOL) shows that toluene (T) and methylevelohcxylamine
(MCHA) are primary products. Fhe selectiv ity of toluene as a function of conversion is
constant, which proves that f is produced in parallel to the other products [6]; the path from
EOE to E corresponds for all catalysts tested to about 5-10% of the total conversion. The
produced AlCITA reacts so quickly that its concentration can not be detected, or it is
100 — — — — — — —
1
80I i i |
['
.2 60 -
i—| I I'
o I l i
>
o 40 ;o
20
oLJ _l—L _L_L I—I 1—1__ I—1_Calcined Dried NTA Ni=0 5 NTA Ni=1 NTA Ni=1 5 EDTANi=15
Fig. 3. Activities of'NiMoAEO- catalysts in the HDN of o-toluidine
observed in trace amounts only when the HDN of EOE alone is carried out [8].
Fig. 3 summarizes the results of the IIDN of fOL with the different catalysts
investigated in this work. No significant difference is observed in the HDN activity
between the calcined and the dried catalysts prepared in the absence of chelating ligands.
Ehis comparison is important because calcined catalysts arc usually employed in industrial
applications, fhe addition of NEA induces an increase in catalytic activity and a maximum
139
Chapter 6
is obtained for the catalyst with the molar ratio N fA:Ni = 1, which is about 55% more
active than the catalyst prepared without ligands. For higher and lower NTA concentrations
lower activities are obtained. Ehe addition of EDEA does not have the same effect as NTA.
Only a slight increase in EIDN activity is observed for the catalyst containing EDTA with
the molar concentration EDTA:Ni = 1.
Fig. 4 shows the conversion of CITE with the different catalysts. In this case the
behaviour of the catalysts is completely different in comparison to the FOL reaction.
Increasing the NTA loading provokes an increase in the conversion of CHE. Ehe calcined
catalyst shows a higher activity than the dried catalyst prepared without ligands. The dried
catalyst prepared without chelating ligands shows a slightly higher activity than the catalyst
containing the smallest NTA amount.
100 ,-—---
80
> |
o 40 [
20 !
I
0 _L__J ^l 1 I 1 I 1 L__J _,—I 1_
Calcined Dried NTANi=0 5 NTANiA NTA Ni=1 5 EDTA Ni=1
Fig. 4. Conversion of cyclohevene on AECAsiipported "NiMo catahsts prepared with and without chelating
ligands.
77775* activity tests
'Fhe I IDS catalytic performance was tested in the hydrodesulfurisation of thiophene at
atmospheric pressure. Ehe results of the tests are shown in Fig, 5. 'Ehe activity of the
calcined catalyst is significantly higher than that of the dried catalyst prepared in the
absence of ligands. Ehe effect of the addition of NEA is dependent on the used amount of
the chelating ligand. For the molar ratios NlA:Ni - 0.5 and 1 a decrease in activitv is
140
HDN-IIDS ofNiMo/AM, catalysts
observed, whereas for the molar ratio NEANT =-1.5 the activity becomes comparable to
that of the calcined cataly st.
0 12 >
0.1
w" i 1
| 0 08
o) I I!
I 0 06 | I I
g
? o 04 ;
0 02
0J—L_ I 1 1—I J—L J—I—,—I 1
calcined dried NTANi=0 5 NTANi=1 NTANi=15 EDTANi=1
Fig. 5. fhtophene HDS acttv it\ täte fot NiMoAECE catah sts ptepai ed in the presence and absence of
chelating ligands
Although no important catalytic improv ement is observed when chelating agents are
employed, it is interesting to compare Fig. 5 with the data obtained for the
hydrodenitrogenation of! OL (Fig. 3) and the hvdrogenation of CHE (Fig. 4). The catalyst
showing the lowest HDS activity is the most active in the HDN reaction. Similarly, it can
be noticed that the catalysts showing the highest FIDS activity are the least active in the
IIDN of FOL. A comparison between the HDS and the hvdrogenation reaction shows that
the two plots (Figs. 4 and 5) have a similar profile. Ihe catalysts containing NTA with the
molar ratio NTA:Ni = 0.5 and 1 arc the least active ones, while the calcined and those with
the molar ratios NTA:Ni = 1.5 and FDTANi - 1 are the most active.
The various reactions investigated in this work follow different pathways. From the
data presented in Figs. 3 to 5 it is possible to see that a change in the structure of the
catalyst enhances some reactions and hinders others. From the QEXAFS and classical
EXAFS data we wanted to learn what features in the sulfidation behaviour and in the
141
Chapter 6
structure of the final catalysts induce the observed differences in catalytic performance for
the various reactions.
Catalysts characterisation
Molybdenum
Mo A'-edge Quick EXAFS. In Fig. 6 the QEXAFS spectra measured during the
sulfidation of the calcined NiMo/AEO^ catalyst are plotted against the sulfidation
temperature. In the spectrum of the fresh catalyst one recognizes the Mo-0 contributions
w
o
CD
B
a>>-i
CD
O
R IÂ]
Fig. 6. Mo A-edge QtXAFS spectra collected dining the sulfidation ofthe calcined NiMo catalyst
142
HDN-HDS ofNiMo/AfO, catalysis
between 0.8 and 2 À (phase uncorrected) and a Mo-Mo shell at 3 A (phase uncorrected).
fhis latter shell suggests the presence of poly moly bdates in the catalyst precursor. The
nature of the signal at 2.2 A (phase uncorrected) is not clear.
Mo starts to perceive the effects of the sulfidation in the spectrum measured at 50°C,
in which the amplitude of the Mo-0 signal has decreased in comparison to the spectrum of
the fresh catalyst. In the spectrum measured around 105°C a Alo-S signal starts to grow at a
distance of 2 A (phase uncorrected) from the absorber atom. Around the same temperature
22
2h
400
380
350
330
in
Spu
O
285
260
230
0r-t-
3A3CD
205
180 0>
1 55
145
o
n
1 25
95
70
50
fresh
2
R[A]
Fig. 7. Mo /v-edge QtXAFS spectia collected dining the sulfidation of theNiMoNTA/AECh catalyst with
fhe molar tatio N fA Ni-1
143
Chapter 6
another signal is visible in the spectra at 2.5 A (phase uncorrected). Ehe same signal was
observed in SiOo-supported NiMo catalysts and was attributed to a Mo-Mo shell present in
an intermediate product of the sulfidation of molybdenum [25, and Chapter 4]. 'Ehe Mo-S
and the Mo-AIo signals grow with increasing sulfidation temperature, but in the spectrum
recorded around 255°C the Alo-Mo shell starts to shift towards larger distance. At this
temperature MoS: starts to form and two Mo-Mo signals overlap: one belonging to the
AloSrlikc material, which is an intermediate product of the sulfidation, the other produced
by the Mo-Mo shell of MoS: at 3.16 A. It is interesting to see that the Mo-0 signal is still
clearly present in the spectra collected at 400°C.
The sulfidation of Alo in the NiAloNTA/ALOi catalyst with the molar ratio
NTA:Ni -- 1 is represented in Fig. 7. The QEXAFS spectra show that the sulfidation of Mo
In this catalyst is very similar to that in the calcined catalyst. Nevertheless, for the spectra
collected between 205 and 260°C a decrease can be observed in the Mo-S signal at 2 A
(phase uncorrected). After fitting the QEXAFS spectra with a Mo-S and a Mo-Mo shell we
could see that in the mentioned temperature range the Mo-S coordination number first
decreases and then increases acain.
100 150 200 250 300 350 400
r ^q
Fig. 8. Mo-S cooidmation number obtained ttom the fit of the QEXAFS spectia as a function of the
sulfidation tempetature tot the alumina suppotted catahsts
CT)1
o
HDN-HDS of NiMo/Ah03 catalysts
A similar behaviour was observed for all dried catalysts. Fig. 8 shows the values
obtained by fitting the QFXAFS spectra for the Alo-S coordination number as a function of
the sulfidation temperature for the various catalysts. A maximum is present for every
catalyst, after which a decrease is observed, that corresponds to the decrease of the
amplitude of the Mo-S signal noticed in the QEXAFS spectra. The maximum is reached at
different temperatures for the various catalysts. Ihe catalyst for which the maximum is
reached at the lowest temperature (215°C) is the one prepared using NTA with the molar
ratio NTA:Ni - 1. Ehe other maxima are observed at 240°C for the catalyst containing
NEA with the molar ratio N fA:Ni = 1.5.at 255°C for the one prepared without ligands
and at 250-260°C for the catalyst prepared with EDEA. A further clarifying plot is shown
in Fig. 0, where the Alo-S and Alo-Mo distances are plotted as a function of the sulfidation
temperature. From this figure it is visible that, in the same temperature regions where the
maxima for the Mo-S coordination number were observed, a shift of the Mo-S shell (from
2.49 to 2.42 A) and of the Mo-Mo shell (from 2.79 to 3.19 A) are detected. The shift of the
Alo-S shell is minor but significant, whereas the change in the Mo-Mo distance is very
3.3
3.2
3.1
3
< 2.9 -
Sp 2.8
DO
aû 2.7
2.6
2.5
2.4
23
Mo-S distance
%^auA^)A^Aj;i
No liganda NTA:Ni=1
+ NTA:Ni=1.5
o EDTA:Ni=1
x Calcined
^a Ay ®>AAfHQA^y_AffiA^
100 150 200 250 300 350 400
Sulfidation temperature [°C|
Fig. 9. Mo-Mo and Mo-S distances obtained horn the QCNAFS spectia as a function of the sulfidation
temperature
145
Chapter 6
marked. Because of some experimental problems the spectra of the sulfidation of the
catalyst containing no ligands were not measured in the region between 270 and 350°C.
Nevertheless, it is visible that the sequence observed in Fig. 8 is kept here. In the catalyst
containing NEA with the molar ratio NTA:Ni = 1 the shift of the distances takes place at
the lowest temperature, whereas the catalyst prepared with EDTA is the last to undergo this
step.
In fact, the distances 2.42 Â for the Alo-S shell and 3.19 A for the Mo-Mo shell are
very close to the corresponding distances in AloS:. which are 2.42 and 3.16 A, respectively.
We could, therefore, find a precise method to detect when AloS: starts to be present in our
catalyst during the sulfidation process. The next step consists in understanding what
consequences this shift of the temperature of formation of MoS: has on the structure of the
final catalysts. Classical EXAFS data will allow us to investigate the structural features of
the sulfided catalysts.
As far as the beginning of the sulfidation is concerned no clear difference could be
noticed between the spectra of the different catalysts, fhe data presented in Chapter 4
evidenced that in the sulfidation of Alo in silica-supported NiAlo catalysts two intermediate
products were detected, consisting of an oxysulfide and of a M0S3 like material, which was
recognized from the signal at 2.5 A. On alumina, on the contrary, the MoSrlike material
starts to be formed at lower temperatures and there is no clear evidence of the presence of
the oxysulfide intermediate. However, its presence can not be excluded.
Mo JÇ-edgeEXAFS. For three catalysts, classical FXAFS spectra were collected
after 30 min sulfidation at 400°C. fhe quality of the data can be viewed in Fig. 10, where
the k -weighted %(k) data of the dried catalyst containing no ligand are plotted. The Fourier
transformed data are collected in I ig. 11, in which a comparison of the various spectra
evidences a clear difference in the amplitude of the Alo-Mo shells at 2.9 A (not phase
corrected). Ehe results of the fits, presented in "fable 2. revealed that the observed
difference is mainly due to dissimilar Debye Waller factors for the Mo-Mo shells. In the
catalyst prepared without ligands. the Debye Waller factor has the highest value, suggesting
a higher static disorder and a lower crystallinity of the AloS: particles. The presence of
NTA, on the contrary, seems to improve the order of the MoS: slabs. For the catalyst
146
IIDN-HDS ofNiMo/Al203 catalysis
containing NTA with the molar ratio NTA:Ni -1 the Debye Waller factor of the Mo-Mo
shell has the lowest value fhcre is a clear correlation between the value of the Debye
Waller factor and the sequence of the temperature of formation of M0S2 (see Fig. 8). The
lower is the formation temperature of MoS:. the higher is the static order in the final MoS:
22
-16
k | A"1]
Fig. 10. Mo A-edge k'-weighted fXAFS function of the dtied N1M0 catalyst containing no ligands
sulfided at 400°C
yj^
~no ligand
NTA-Ni=1
NTANi=1 5
R [A[
Fig. IE Mo A'-cdgc Foutiet tiansformed EXAFS spectra of the sulfided catalysts prepared in the presence
and absence ofNTA
147
Chapter 6
particles. The same trend was observed for the SKE-supported catalysts presented in
Chapter 4 and 5 but the effect on the Debye Waller factor was not so marked as here.
As far as the first shell is concerned, it is noteworthy that the Mo-S coordination
number of the catalyst made with NTA with the molar ratio NTA:Ni = 1 amounts to 6.0,
whereas for the other catalysts, and in general (see Chapter 5). values below 6 are obtained.
This observation suggests that all the edges of the MoS: crystallites are saturated with S.
Table 2. Parameters obtained from the fit of the spectra of the sulphided NiMo/AbCE
catalysts.
Catalyst Shell CN R A(T AE° Goodness
[À] [1(F4A2] [cV] of fit
No ligands Alo-S 5.8 2.42 5.0 3.1 0.79
Mo-Mo 3.6 3.16 8.2 3.7
N fA:Nr-l Alo-S 6.0 2.42 5.0 3.8 0.79
Alo-Mo 3.6 3.16 4.1 4.7
NEA:Ni=E5 Alo-S 5.9 2.42 4.6 4.1 0.60
Mo-Mo 3.7 3.16 5.4 4.8
Nickel
Ni A-edge QEXAFS. Ehe sulfidation of Ni in the catalysts was investigated by
means of QEXAFS. Fig. 12 shows the example of the sulfidation of Ni in the
NiMoNTA/AEO, catalyst (\TA:Ni - I) as followed by QEXAFS. Ehe spectrum of the
catalyst precursor (fresh) shows two main signals. Ehe first, at 1.8 A (phase uncorrected), is
produced by the Ni-0 shells, whereas the second one. at 2.2 A (phase uncorrected) is due to
the presence of the carbon atoms belonging to NEA around Ni [13. and Chapter 3]. A
careful observation of the first signal allows us to detect when the oxygen atoms around Ni
are replaced by sulfur. The Ni-S distance Is about 0.2-0.3 A larger than the Ni-0 distance.
148
1IDN-HDS ofNiMo/AfOi catalysts
22
400
390
365t/3
A!!->
330Pr-t-
o
2900r+
o
p
250pr-1-
C
205 CD
r~,
0
o
160 1 J
135
100
60
fresh
Fig. 12. Ni A-edge QFXA1 S spectra collected during the sulfidation of the NtMoNTA/AEO catalyst with
the molar tatio M 1A NiA
A more precise picture about the sulfidation of Ni in the different catalysts can be
achieved by monitoring the XANES spectra. As can be seen in Fig. 13 the shape of the
spectra changes during the sulfidation process. By means of a linear fitting it is possible to
estimate the fraction of sulfided Ni in the catalyst. Ehe spectra during sulfidation are
considered a linear combination of the spectrum of the fresh catalyst and ofthat collected
149
Chapter 6
after sulfidation at 400°C. The spectrum of the dried catalyst containing no ligands was
used as reference for the sulfided state for the calcined catalyst. The results of this
procedure are plotted in Tig 14. As far as the dried catalysts are concerned it is visible that
the presence ofNTA shifts the sulfidation of Ni at lower temperatures. No difference can
be noticed between the two NTA containing catalysts. On the contrary, EDTA delays the
sulfidation of Ni. Ihe behaviour of the calcined catalyst is different. In this catalyst even
8350 8400
X-ray energy [eV|
Fig. 13. Ni K-edge XANES spectia collected dining the sulfidation ot the dtied NiMo catahst containing
no ligands
150
8300
HDN-HDS ofN,Mo/Al203 catalysts
though the initial sulfidation rate is fast, the fraction of sulfided Ni after sulfidation at
400°C for 30 min is only about 70%. This is due to the fact that under the calcination
condition part of Ni was incorporated in the support. This fraction of nickel was not
sulfided. fhe EXAFS signal is therefore, an average of the sulfided Ni present on the
surface of the support, and of the unsulfided Ni segregated in y-AEOv
0 100 200 300 400
Sulfidation temperature [°C]
Fig. 14 Degtee of sulfidation ofNi on the alumina-suppoited catahsts as obtained from the XANES spectia
Ni Ä-edge EXAFS. Ihe k1-\\eighted x(k) data of the catalyst containing no
ligands plotted in Tig. 15 allow to have an idea about the quality of the Ni X-edge data
obtained from our measurements. Ehe data were Fourier transformed using the k-range 3-
16 A"1. Ehe spectra of the Fourier transformed data are plotted in Fig. 16. They are
composed of three main signals. According to the interpretation proposed in Chapter 5 all
three signals are produced by Ni-S contributions. A comparison between the k1- and k'-
wcighted spectra with a Ni-S and a Ni-Alo signal simulated with the Feff code clearly
ISj
Chaptei 6
22"
k[\']
Fig. IS. Ni A-cdge 12-weighted FXAFS function of the di tied catalyst containing no ligands
11
55
no ligand
NTA:Ni=1
NTANi=1.5
22><;
>—i
(A
-5 5
-11
0 £2S2^
R[Ä]
Flg. 16. Ni A-edge Fouiiet tiansfoimed EXAf S spectia of the sulfided catahsts ptepaied m the piesence and
absence ofNIA
H2
HDN-HDS of NiMo/AhOi catalysts
Table 3. Structural parameters obtained from the fits of the Ni A-edge EXAFS
spectra of the alumina supported catalysts prepared in the presence and absence ofNTA
Catalyst Shell CN R Ac/ AE° Goodness
[Â] [io3Â2i [eV] of fit
No ligands Ni-S 3.4 2.20 1.24 1.4 0.17
Ni-S 1.2 2.74 2.08 -0.6
Ni-S 0,7 3.06 -0.19f\ /"¥
NTA:NEd Ni-S 3.5 2.19 1.05 2.4 0.20
Ni-S 1.2 2.74 3.21 -0.1
Ni-S 0.6 3.09 -3.53 -4.9
NTA:NiM.5 Ni-S 3.4 2.20 1.01 1.1 0.17
Ni-S 1.6 2.74 4.13 -0.1
Ni-S 0.9 3.05 0.24 0.1
confirmed that also the last two signals are Ni-S contributions, fhe results of the fits arc
given in Eable 3. fhe coordination number of the first shell has a relatively low value for
all catalysts, which suggests that some Ni could have a coordination number of the first
shell lower than 4.
To interpret the results concerning the second and third shell the structure proposed in
the previous chapter should be considered. In the trimer bis(dithiobenzoato) Ni(II) complex
reported by Bonamico et at the coordination number of the second and third Ni-S shell
should amount to 0.66 because there are two Ni-S bridges with a ditance 2.78 A and two
with a distance 3.11 A (Chapter 5. Fig. 14) [26). In the present data of the alumina-
supported catalysts, on the contrary, the coordination number of the Ni-S shell at 2.74 A is
1.2 for the first two cataly sts and 1.6 for the last one.
In our alumina-supported catalysts the Ni loading is relatively high (2.5%) in
comparison to the A1o loading (7°o). fhe resulting NEAlo molar ratio is about 0.6. The
higher Ni loading was chosen in order to improve the quality of the XAFS data. On the
other hand, at this high concentration only a fraction of Ni is present in the catalyst as
153
Chapter 6
active phase. The remaining nickel is disturbing the EXAFS signal because what is visible
in the spectra is an average of all Ni species present in the catalyst. According to the data
presented in the previous chapter, in which the Ni loading was lower (1.3%), the active
species should consist of small Ni clusters, fhe inactive phase could consist of larger
clusters in which Ni has a similar geometry. Ehe presence of this compound is suggested by
the coordination number of the first Ni-S shell close to 4 and would explain the high
coordination number obtained for the second and third Ni-S shell.
Ehe third Ni-S shell belongs to the Ni atoms sandwiched between two other
monomers. Assuming that the clusters can be composed of more than three units, the
coordination number of the last Ni-S shell could increase and reach 2 for extremely large
clusters. In our catalysts we obtained values ranging from 0.7 to 0.9 suggesting that the
clusters arc composed of a relatively small number of units. In the spectrum of the catalyst
containing NEA with the molar ratio NTA:Ni = 1 the third shell is expanded towards larger
distances. This feature is explained by the large Debye Waller factor of the shell. The Ni-S
bond length distribution must be relatively large.
The presence of small Ni clusters would involve the detection of a Ni-Ni shell at
around 3.3 Â (phase uncorrected) by means ol'EXAFS. A weak signal at this distance can
be observed for the catalyst containing NEA with the molar ratio NEA:Ni - 1 but not for
the other two materials. The absence of a Ni-Ni shell can, however, be explained by a large
bond length distribution which induces the disappearance of the signal in the EXAFS
spectra.
154
HDN-1IDS of NiAh/ihO, catalysts
Discussion
In this section we will first consider the mechanism of sulfidation of Alo and the way
it is affected by the ligands. Ehen we will concentrate on the sulfidation of Ni. In both cases
we will explain how the structural features influence the catalytic performance.
Molybdenum
fhe use of QEXAFS enabled us to distinguish three regions in the sulfidation
mechanism of Mo: the oxidic state, an intermediate MoSs-like material and the final MoS:.
Ehis sequence differs from the one observed for SiO:-supported catalysts discussed in
Cahpter 4. On silica two intermediate regions were detected, a Mo-oxysulfide and the
MoSs-like product. On alumina the oxysulfide intermediate can not be clearly
distinguished. Because of the high pH of the impregnation solution (9.5) Alo is present on
the AEOj-supported catalyst precursors mainly as AI0O42" units, even though a fraction of it
is present as polyanions as the signal at 3 A suggests. On the contrary, polymolybdates are
the main species on SiO: [E3]. The smaller molybdate molecules are easier to sulfide
because no Mo-O-Alo bridging bonds have to be cleaved. Therefore, the passage from
MoOf" to the MoS^-like material proceeds in a much faster way and can not be detected by
QEXAFS.
In addition, the behaviour of the Mo-S and Mo-Mo shells evidences the passage from
the intermediate product to AloS:. Fhis feature is very important because it allows us to
detect precisely when AloS: starts to he formed and the temperature of formation is not
assigned only by an optical analysis but also by monitoring the distance of the two shells.
Ehe comparison between the formation of AloS: in the various catalysts showed that the
presence of N fA and ETEfA has opposite effects on the sulfidation of Mo (Figs. 8 and 9).
In catalysts prepared with NFA AloS: is formed at lower temperatures, whereas EDTA
retards the formation of AloS:. At the concentrations NEA:Ni = 1 and EDTAfiNi = 1 Mo is
not complexed by the ligands. because they preferentially form complexes with Ni.
Therefore, the shift in the formation temperature of MoS: must be rather due to an
interaction of Alo with Ni or with the support, fhe sequence for the sulfidation of Ni is, in
fact, the same as for the formation of AloS: as can be seen in Fig. 14. The kind of
155
Chapter 6
interactions affecting the sulfidation of Mo are difficult to predict and the data at our
disposal do not allow us to understand the chemical nature of this mutual influence.
Nevertheless, now we want to discuss which consequences a shift of the formation
temperature of Mo has on the final MoS: slabs, fhe answer to this point can be found in the
Mo A-edge EXAFS data. Ehe results of the fits showed that the catalyst in which AloS: is
formed at the lowest temperatures (NTA:NI - 1) is the one with the highest Alo-S
coordination number and the lowest Mo-Mo Debye Waller factor. The high coordination
number indicates a complete saturation of the MoS: edges, whereas the second parameter
points to a high order of the AloS: slabs. A large regularity of the Mo-Mo distances and a
narrow particle size distribution can induce a decrease in the Debye Waller factor.
A comparison of the XAFS data with the results of the IIDN reaction (Fig. 3) shows a
clear correlation between the order of the AloS: particles and the HDN catalytic
performance. The catalyst NiAloNTA/AfO, with the molar ratio NEA:Ni ~ 1 is the most
active. The inverse correlation is valid for the cyclohexene hydrogénation. The explanation
oE these activity profiles must be searched in the mechanism of the HDN reaction (Fig. 1).
The rate-determining step of the HDN of o-toluidine is the hvdrogenation of the phenyl ring
which leads to the formation of AICHA. A1CEIA then reacts quickly to MCIIE and MCH.
EEydtogenation of olefins (CHE) over sulfided Alo catalysts takes place relatively easily,
occurring already at 1 atni pressure of by drogen. Ehe reactivity generally decreases with
increasing olefin chain length and number of substituent groups adjacent to the double bond
[27 [. Contrary to olefins, hvdrogenation of aromatics requires high pressures of hydrogen to
effect the saturation. Ehis is partially due to the low reactivity of the aromatic structure
induced by resonance stabilization of the conjugated system. Ehe highest conversion of
EOE is obtained with the catalyst which is least active in the hvdrogenation of CHE. Ehe
same trend is shown by most of the tested catalysts. Ehis observation indicates that
different sites are Involved in the two reactions. Catalysts active for olefin hydrogénation
are not always capable of catalyzing aromatic hvdrogenation [28]. Moreover, benzene
(aromatic) and cyclohexene (olefin) hvdrogenation occur at different sites on a sulfided
catalyst [29]. ITowever. the dissimilar activities for the two reactions may be due to
different degrees of adsorption for the two compounds. The CHE can form a cr-bond with
Mo and Ni, while the aromatic ring is more likelv to be rt-bonded to the metal sulfides and.
156
HDN-jHDS ofNtAlo/Al203 catalysts
thus, less strongly adsorbed. TOL should adsorb parallel to the edge plane of MoS:, in
p6-adsorption mode. Considering the size of the aromatic ring, such adsorption needs more
than one Mo site [30]. A more crystalline structure of the active phase should favour the
planar adsorption and thus the hydrogénation of toluidine. The EXAFS results show that
the catalyst with the highest crystallinity is the one containing NTA with the molar ratio
NTA:Ni = 1 followed by the other NTA cataly sts. fhe dried catalyst containing no ligands
shows the most amorphous structure and, at the same time, the lowest HDN activity. 'Fhe
adsorption of the aromatic ring is enhanced by the regularity of the edges of MoS:. On the
contrary, the irregular Mo-Mo distances in the catalyst prepared without ligands make the
adsorption of the ring more unlikely.
This consideration does not affect the HDS of thiophene, because it is assumed that
thiophene adsorbs on the catalyst by means of the S atom present in the heterocyclic ring.
Therefore, as for the hydrogénation of cyclohexene. the regularity of the catalyst surface
does not play such an important role.
Nickel
From the Ni A'-edge QEXAFS measurements we could see that the sulfidation of Ni
in the various catalysts proceeds at different rates. In the catalyst containing NEA, Ni is
sulfided at much lower temperatures than in the other catalysts. From the QEXAFS spectra
of the fresh catalysts it is clearly visible that in the catalyst containing NTA and EDEA Ni
is complexed by the ligands because the Ni-C shell is detected at around 2.4 A (phase
uncorrected). In the catalyst prepared without ligands, on the contrary. Ni is believed to
interact with the support. Ehe difference in the sulfidation behaviour between the NTA- and
the EDEA- containing catalysts can be explained with the higher stability of the EDTA
complexes. ITowever. the sulfidation mechanism could be the same for both kinds of
catalysts, fhe most probable reaction pathway is a substitution of the keto group of the
acetate arms of the ligands by l ES. The resulting S" group would substitute for the acetate
oxygen group of the ligand coordinated by Ni. which would also be replaced by HAS. In
this way a thioacetate would be formed which could complex Ni and form a complex in
which Ni is situated in a plane formed by four S atoms as proposed in Chapter 5, either in a
square pyramidal or in a octahedral geometry, fhis reaction can take place only in the
157
Chapter 6
presence of the chelating ligands, and it is likely enhanced in catalysts where the sulfidation
of Ni occurs at higher temperatures. When Ni is sulfided at lower temperatures FES can
replace the ligand without reacting with it. For this reason we think that in the EDTA
containing catalyst the fraction of Ni which forms the thioacetate complex is higher than in
the other catalyst. The formation of this complex has the main result of achieving a higher
dispersion of Ni on the support. In the catalyst containing no ligands Ni is supposed to be
present in larger clusters. The fact that no Ni-Ni EXAFS signal can be detected by EXAFS
could be due to the fact that Ni-Ni shells with different distances are present, whose signals
interfere and cancel each other.
We believe that the formation of small Ni clusters has a higher importance in the
HDS reaction of thiophene and in the hydrogénation reaction of cyclohexene than in the
HDN of o-toluidine. The fact that a difference in IIDS activity is observed between the
catalysts containing NIA with the molar ratios NTA:Ni = 1 and 1.5 suggests that not only
the sulfidation temperature of Ni is an important factor for the formation of the thioacctate-
Ni complex. An increase of the amount of ligand in the catalyst precursor can also have a
beneficial influence on it. However, a too large amount of ligands would start to interact
with Mo and therefore have a negative effect on the HDS performance as the data of the
previous chapters showed.
Conclusions
The data presented in this chapter allowed us to extend the knowledge about the
effect of chelating ligands obtained from the study of SiO:-supported NiMo catalysts to
y-AEOrsupported catalysts. Aloreover. we compared the effect of the chelating agents on
two hydrotreating reactions, the hydrodenitrogenation and the hydrodesulfurisation.
Ehe IIDN catalytic performance is strongly enhanced by the use of NfA. Ehe effect
of this ligand is believed to be a decrease of the temperature of formation of MoS: during
the sulfidation process, which enables the development of more regular MoS: slabs, onto
which the hydrogénation of o-toluidine is enhanced.
On the contrary, we observed that, for the tested ligand concentrations, the
improvement in activity obtained in the HDS of thiophene is limited. Nevertheless, also for
158
HDN-HDS o/AWoAAA catalysts
this reaction a trend was observed in the performance of the catalysts. The improvement in
HDS activity was ascribed to the presence of NTA and EDEA. Ehe active phase for the
FIDS reaction is believed to be composed of a thioacetate complex, which enables the
formation of small Ni clusters. Its presence allows a better dispersion of Ni in the final
catalysts. Ehe high Ni loading did not allow us to detect this phase clearly by means of Ni
A-edge EXAFS.
'fhe different trend in the HDN and HDS reactions was explained with the fact that
for the HDN reaction the adsorption and hydrogénation of o-toluidine on MoS: is the rate
determining step, and this step is enhanced by the presence of more regular MoS:
crystallites. On the contrary, for the HDS of thiophene the presence of a higher Ni
dispersion plays a more important role.
Literature
1. van Veen, J. A. FE, Gerkema, E.. van der Kraan, A. M., and Knocster, A.,./
Chem Soc, Chem Commun. 1684(1987).
2. Thompson. Al. S., European Patent Application 0.181.035 (1986).
3. Clausen, B. S.. Eopsoe, H.. Canctia, R... Villadsen. E. Ecngeler, B., Als-
Niclsen. E. and Christenscn. F.. ,7 Pins Chem. 85, 3868 (1981).
4. Perot. G.. Catal Today 10, 447 (1991).
5. Satterficld. C. NA. and Yang. S. IE. hid Eng. Chem. Process Des Dev. 23,
11 (1984).
6. Rota, F.. and Prins. IE, Stud Surf Sei Catal 127, 319 (1999).
7. Rota. F.. and Prins. R., Topic m Catal 11/12, 327 (2000).
8. Rota, F.. and Prins. R.,./. Mol Catal (to be published).
9. Prins. R.. de Beer. V. EI. E. and Somorjai. G. A.. Catal Rev.-Sci. Eng. 31. 1
(1989).
10. Chianelli. R„ Daage. AE. and Eedoux. M. E. Adv. Catal. 40, 177 (1994).
11. Eopsoe, IE. Clausen. B. S.. and Alassoth, F. E., "Catalysis Science and
Technology". Springer Verlag, New York. (1996).
12. Startsev. A. N.. Catal, Rev -Sei Eng 37, 353 (1995).
159
Cattaneo, R., Shido, T., and Prins. R.. J. Catal. 185, 199 (1999).
Schulz. H.. Schon, M., and Rahman. N. AE. Stud. Surf. Sei Calai. 27, 201
(1986).
Hargreaves. A. E.. and Ross. J. R. H.. J. Catal. 56, 363 (1979).
McCarthy. K. F.. and Schrader. G. L.. ,7 Catal 117, 246 (1987).
Startev, A. N.. Burmistrov. Y. A., and Yermakov, Y. h., Appl. Catal. 45, 191
(1988).
Blake. M. R., Eyre, M., Moyes. R. B., and Wells, P. B., Bull. Soc. Belg. 90,
1293(1981).
Daly. F. P..,/ Catal. 51, 221 (1981).
Geneste. P.. Amblard, P.. Bonnet. AT, and Graffin, P., J. Catal 61, 1015
(1980).
Kampers. F. W. H., Maas, E. AE E. van Grondelle, E, Brinkgreve, P., and
Koningsberger, D. C, Rev Sei Instrum 60. 2635 (1989).
Vaarkamp. M., Dring, F, Oldman. R. E. Stern, E. A., and Koningsberger, D.
C.Phys Rev Z? 50, 7872 (1994).
Zabinsky, S. F. Rchr, J. E, Ankudinov. A.. Albers. R. C, and Eller, M. E. ,7
Phys Rev B 52, 2995 (1995).
Ankudinov. A. E.. and Rehr. E E. Phys Rev. B56, R1712 (1997).
Cattaneo, R.. Weber. Th., Shido. f.. and Prins, R., J. Catal. 191. 225 (2000).
Bonamico, M.. Dessy. G., Fares. V.. and Scaramuzza, L.. ,7 Chem. Soc.
Dalton Tram .2250(1975).
Uchytil.E. and Jakubickova. E..J Catal 64. 143 (1980).
Krishnamurthy. S.. Panvelker. S., and Shah. Y. X.AlChEJ 27. 994 (1981).
Voorhoeve. R. E FF. and Stuiver. J. C. AE.,/ Catal 23, 228 (1971).
Drew, AE G. B., EAjmondson, S, E, Forsyth. G, A., Flobson, R. .T., and
Mitchell. P. C. IF. Catal Today 2. 663 (1988).
Chapter 7
Summary & Conclusions
Ehe aim of this dissertation was a structural characterisation of supported NiMo
hydrotreating catalysts during the whole preparation, in the absence and presence of
chelating ligands. In this final chapter, we want to give a brief overview of the obtained
results, starting from the structure of the catalyst precursors, considering then the
sulfidation process and the final catalysts.
The oxidic state
From the combined EXAFS. Raman. EV-Vis study of the Siü:-supported catalyst
precursors we can conclude that in the absence of ligands Ni interacts strongly with the
support, 'fhe chelating ligands. on the contrary, avoid the interaction between the NE'
ions
and the support and induce a dispersion of the Ni-chelate complex over the support.
As far as Mo is concerned, we could conclude that in the absence of chelating agents
Ato is predominantly present on the support as polyanion units. Ehese polymolybdates are
present even when low ligand concentrations are used. In fact, the first amounts of ligands
bind preferentially Ni and Alo does not perceive any effect of them. Only when all Ni is
complexed by the organic molecules. Alo starts to form complexes with the chelating
agents. However, the formation of Mo-chelates was observed only when using NTA and
Chapter 7
EDTA. With these two ligands polymolybdate anions were no longer present on the
support.
The sulfidation
With this background it is possible to understand the behaviour of Ni and Alo during
the sulfidation process. Ihe QEXAFS studies of Si02- and AEOrsupported catalysts
clearly showed that the temperature of sulfidation ofNi in the presence of chelating ligands
is different from that in the catalyst prepared without ligands. For silica-supported catalysts
the presence of NEA and EDEA induced a higher sulfidation temperature of Ni. EN also
has a delaying effect on the sulfidation of Ni. although this influence is less detectable by
means of QEXAFS. On alumina-supported catalysts NFA and EDEA have opposite effects
on the sulfidation of Ni. In comparison to the catalyst prepared without ligands, Ni is
sulfided at lower temperatures in NLA-containing catalysts, while in the presence of EDTA
it is sulfided at higher temperatures. Hie sulfidation temperature of Ni in catalysts
containing the same ligand amounts is similar on both supports, 'fhis suggests that in the
absence of ligands NI is sulfided at higher temperatures on AEO^ than on SKA.
Combining these considerations with IIDS activity data, it becomes clear that the
catalytic activity increases with increasing sulfidation temperature of Ni. Nevertheless, this
is not always valid. On AEOi the NTA-containing catalysts had an activity comparable to
and even higher than the dried catalyst containing no ligands. even though Ni was sulfided
at lower temperatures in the NTA-based catalysts. These considerations lead to the
conclusion that the presence of chelating ligands has a beneficial effect on the HDS activity
not only because of a delay in the sulfidation temperature of Ni. We think that the
retardation of the sulfidation of Ni is only a sign that indicates that the mechanism of
sulfidation of Ni changes when using chelating agents. Probably the ligands are directly
involved in the sulfidation mechanism of Ni. A study of the mechanism of sulfidation of
the ligands could clarify this question. A'T-NMR investigation of the system would be an
interesting way to shed light on this process. We think that the chelating property of the
ligands is essential for the formation of a more active structure of Ni. In fact, the formation
of thiocarbamate complexes my be aided by the fact that uncoordinated chelating arms can
162
Summary and Conclusions
be sulfided more easily than the carboxylic arms coordinated to NE Ehe formed
thiocarboxylic groups could then replace the other oxidic ligands coordinated to Ni because
of the high affinity between Ni and S.
By monitoring the sulfidation of the catalysts by QEXAFS. it has become clear that
also Mo perceives some effects from the presence of the chelating ligands during the
sulfidation. The sulfidation interval of Alo becomes wider with increasing amounts of NEA
and EN, i.e. the sulfidation starts at lower and ends at higher temperatures. Differently from
the other ligands, a relatively small amount of EDfA is needed to strongly broaden the
sulfidation range of Alo. However, these observations do not mean that the changes in the
sulfidation of Mo are due to a direct interaction between Mo and the chelating ligands. It is
much more likely that these changes are due to the formation of the Ni-chelates in the
catalyst precursors. As we saw in the previous section the ligands protect Ni from any
interaction with the surrounding environment. Fhis effect has clear consequences also on
Alo. Ehe sulfidation intervals of Mo show-n in Fig. 6 of Chapter 4 suggest that the more Ni
is complexed by the ligands. the more the sulfidation of Mo resembles that of the catalyst
containing only Mo (Alo'SiO:). The presence of the ligands enables an independent
sulfidation of Alo and Ni. This means that, in the absence of ligands, the sulfidation of Mo
is affected by the presence of Ni. We can not say with certainty whether this influence
arises from a direct contact between the two metal ions or from the interactions ofNi or Mo
with the support. However, the explanation could be that the removal of Ni from the
support enhances stronger Mo-support interactions. This would explain why the sulfidation
of Mo takes such a large temperature range in the cataly st containing no Ni and in those
containing the chelating agents. Assuming that in these cases Alo interacts with the support,
the fraction of Mo exposed to the external atmosphere can be sulfided at lower
temperatures, whereas the part of Mo interacting with the support is only sulfided at
elevated temperatures.
In fact, there should be a correlation between the width of the sulfidation range of Mo
and the degree of removal of Ni from the support. The degree of detachment of Ni from the
support should be proportional to the number of coordination sites occupied by the ligands.
For this reason an EDTA:Ni molar ratio of only 1 is needed to make the sulfidation range
of Mo as wide as the Alo/SiO: cataly st. ED fA has six ligand coordination positions, so that
163
Chapter 7
only one molecule is needed to occupy all 6 coordination sites of Ni~A On the contrary, the
ENENi ratio must be larger than 3 because FN has only 2 binding amino groups so that the
chelate [Ni(ENL,]~ must be formed to avoid contact between Ni and the support. The
sulfidation range of Mo becomes wider with increasing NTA concentration, which suggests
that more than one NIA molecule is needed to completely detach Ni from SKA.
The sulfidic state
Hie relatively high quality of the Ni A-edge EXAFS data allowed us to shed some
more light on the structure of Ni in the sulfided catalysts. We could determine that the
second and third shells visible in the Ni A-edge spectra are two Ni-S shells and not Ni-Ni or
Ni-Mo shells as was erroneously interpreted in the past. Ehis observation has a great impact
on the clarification of the structure of Ni. It shows that the postulated proximity between Ni
and Mo in the sulfidic state can not be detected by FXAFS. The correspondence of the fits
with the structure of a trimeric Ni complex reported in the literature suggests that Ni is
present in the final cataly its in a layered structure.
The chelating ligands are supposed to favour the formation of smaller units and,
therefore, to increase the dispersion of Ni. However, the absence of a Ni-Ni shell from the
EXAFS spectra does not allow us to confirm this hypothesis.
We could observe that the structure of Alo in the final sulfided catalysts is influenced
by the sulfidation process, and. therefore, by the presence of the chelating ligands. Ehe
static order of the AloS: particles increases with increasing ligand concentrations. Ehis is
valid as long as only Ni is complexed by the chelating agents In the catalyst precursors.
Apparently, the presence of stronger Mo-support interactions in the catalyst precursor
favours the formation of more crystalline AloS: particles. However, for larger N'FA and
EDTA concentration the si/e of the MoS: cry stallites decreases. This is due to the fact that
at high ligand concentrations also Alo is complexed by NfA and EDEA in the catalyst
precursors, which induces a higher dispersion of Alo in the final catalysts. Ehe increase in
static order of MoS? has no apparent effect on the HDS activity but is extremely beneficial
in HDN. The decrease in particle size, on the contrary, has a negative influence on the FIDS
catalytic activity. The reason for this is still unexplained.
164
Summary and Conclusions
Remarks & suggestions
The variation of the ligand concentrations allowed us to obtain a detailed picture of
Ni and Mo during the whole preparation procedure. We tried to concentrate especially on
the study of Ni. The understanding of its structure has been generally considered somehow
of secondary importance because Ni is considered the promoter in this catalytic system.
Moreover, the low concentration of Ni in hydrotreating catalysts makes the understanding
of its structure even more difficult. Our results show, however, that more attention should
be devoted to the structure of Ni. Concerning the question whether Ni or Mo is the actual
catalyst, we have no clear answer. The presence of chelating ligands primary affects Ni and
has onlv an indirect influence on Mo. Therefore, we think that, indeed. Ni could be the
actual catalyst.
The QEXAFS technique was applied successfully to the study of hydrotreating
catalysts. Wc showed that this method can deliver a large amount of information and is
much more powerful than the temperature programmed sulfidation techniques used in the
past. Its applicability has been confirmed also in the study ofNiW/AfO-, catalysts in recent
experiments of our group (Ph. D. projects M. Sun. A. van der Vlies).
We think that the exact sulfidation mechanisms of Ni in the presence and absence of
chelating ligands could clarify the formation of the active sites in hydrotreating catalysts,
'fo this purpose fluorescence EXAFS should be used to characterize the final catalysts and
catalysts sulfided at intermediate temperatures. However, good data can be obtained from
fluorescence EXAFS only if an appropriate multi-element detector is used. Only a limited
number of such detectors is obtainable in the synchrotron facilities and they are partially
still under development. '"T-NA1R could he a successful tool to understand the mechanism
of sulfidation of the chelating ligands.
Thiocarboxylic ligands or thiocarbamate-Ni complexes should be tested for the
synthesis of hydrotreating catalysts. Catalysts could be prepared in which a trimeric Ni
complex similar to the one presented in Chapter 5, is deposited on the surface of sulfided
supported AloS:.
165
Chapter 7
166
Summary and Conclusions
Acknowledgements
This work would not have been possible without the help of many people that I
would like to sincerely thank.
In particular 1 am grateful to Prof. Prins for all his advises and his art of transforming
a rough manuscript into interesting text.
I am thankful to Prof. Wokaun for agreeing to be the co-examiner.
I am also in debt with Dr. Takafumi Shido. he introduced me to EXAFS spectroscopy
in a very kind and subtle way.
In spite of the hard work I had a really funny time with all the colleagues that shared
with me day- and night-shifts in Grenoble and Hamburg. Fhanks to all of you.
In addition I want to thank all the people that helped us solving many problems:
Mr. Urs Krebs of the workshop of the technical chemistry laboratory at ETFl for his nice
ideas about the EXAFS cells; Wouter van Beek. Elermann Emerich of the Swiss Norwegian
beam line at ESRF and Mathias Hermann of the RÖA10 II station at HASYLAB for their
technical help and their kindness; Larc Tröger of EIASYEAB for his patience and his
enthusiasm.
1 am grateful to Christoph Stinner and to Patrizia Fabrizioli for the Raman and
EW-Vis measurements. 1 appreciated a lot the scientific collaboration of Dr. Thomas Weber
and Fabio Rota.
1 considered myself very lucky to have met all the nice people that have worked in
the group of Prof. Prins in the past 4 years, fhe atmosphere in the group made the work
much more pleasant.
Thanks to the friends with whom I have spent v ery funny lunches and evenings.
Thanks to Alonica and to my family for their proximity.
Thanks to mv bike.
167
Chapter 7
\
168
Summary and Conclusions
Curriculum Vitae
Riccardo Cattaneo
Date of birth: 25 November 1971
Nationality: Italian
Education
1977- 1982:
1982- 1986:
1986-1990:
1990- 1995:
1995
1996- 2000
Primary school, Porlezza (CO) - Italy
Secondary school, Lugano - TI
High school in Lugano - TI: mattira type B
ETFI Zürich, Diploma in Chemical Engineering
Diploma work in Birmigham (EJK) under the supervision of
Prof. Bourne in the field of surface science
Ph. D. Thesis in the Taboratorv of Technical Chemistry at ETH
Zürich in the group of Prof. Prins
169