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Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Adsorption Calorimetry:
Basics and Applications
in Heterogeneous Catalysis
Sabine WrabetzFritz-Haber-Institut of the Max Planck Society
Department of Inorganic Chemistry, 14195 Berlin, Germany,
Beijing - November 11, 2010
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Introduction
Calor (Latin): heat, warmth
Metron (Greek): measure
Johan C. Wilcke (1732-1796)
Antoine L. Lavoisier (1743-1794)
and Marie P. Lavoisier
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Introduction
▒ direct method to determine number, strength and energy distribution of the adsorption sitesAdsorption Isothermal Micrcalorimetry
▒ allows measurement of the differential heats (ads. enthalpy) evolved when known amounts of gas probe molecules are adsorbed on the catalyst surface
▒ the evolved heat is related to the energy of the bonds formed between the adsorbed species and the adsorbent and hence to the nature of the bonds and to the chemical reactivity of the surface
▒ adsorption steps, surface reaction processes, and desorption steps
▒ the energetics of these surface physical-chemical events play an important role in the determination of the catalytic properties of the surface
▒ key to the effective use of adsorptive microcalorimetry is the careful choice ofprobe molecules and the adsorption temperature
▒ The data obtained are of substantial importance for comparing theoretical and experimental hypotheses about reaction pathways.
Analyzing of the catalytic data with respect to the surface processes occurring on the catalyst material during adsorptive microcalorimetry.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
▒ careful choice of probe molecules and the adsorption temperature
▒ metal oxide catalysts provides acid/base properties
strong basic NH3 at RT
weak basic CO at 77 K
acidic CO2 at RT
Brønsted acid sites:
transfer of H+
from OH to
Adsorbate
Lewis acid sites:
coordination to an electron-
deficient metal atom
cusMen+
oxygen vacancy:
O2-
OH-metall cations
under
participation of
oxygen: MexO-y
Motivation: surface sites
▒ use of probe molecules such as educt, intermediate, product or moleculesclosely related to the reactants is an elegant method to study the surfacesites relevant for catalytic reaction Tadsorption < Treaction study of the pure ads. processesTadsorption = Treaction study of the surface chemical events during the reaction
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Equipmentat the Fritz-Haber-Institute
Calorimeter
Sample cell Reference cell
Adsorptive
Microcalorimetry
Volumetric system(probe molecule dosing system)
Sample cell
Isolation
Electronics
Sample cell chamber
Heating
Heat flow
Calorimetric Element
Thermocouple
Body sample holderL = 70 mm
= 15 mm
catalyst
MS 70 Tian-Calvet calorimeterof SETARAM combined with acustom-designed high vacuumand gas dosing apparatus.Karge, H.G. etal., J. Phys. Chem. 98, 1994, 8053.
Calorimetric Element
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
The Calorimetric Element
The sample cell is placed into a
calorimeter element
The cell is surrounded by a thermopile
made of more than 400 conductive
thermocouples in series
Thermopile has 2 functions:
1. heat transfer
2. signal generation
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Heat and Heat Flow
The heat produced by the
adsorption/reaction of a dosed probe
molecule with the catalyst surface
is consumed by 2 processes
1. Increase of the temperature of the
sample cell
2. Once there is a temperature gradient
between cell and surrounding block,
heat flow through the thermopile
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Reference Cell
Setup according to
Tian and Calvet
The calorimetric block consists of
a sample cell and a reference
cell.
The reference cell compensate
external temperature fluctuations
and it provides a good stability of
the baseline.
Measurement of the temperature
difference Δ θ
Reference Sample
Cell Cell
The heat-flow detector gives a
signal “U” which is propotional to
the heat transferred per time unit.
Δ θ
N. C. Cardona-Martinez and J.A. Dumesic, Advances in Catalysis 38 150-243.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Physisorption and Chemisorption
Physisorption Chemisorption
Type of interaction and
heat of adsorption
(negative enthalpy of
adsorption)
van der Waals forces
10 – 20 kJ/mol
noble gases, CH4, N2
Dipole-dipole
20 – 50 kJ/mol
water on oxides
chemical bonding,
electron transfer
80 – 500 kJ/mol
CO on metals
dissociative adsorption
(O2, H2 on Pt, H2O on
oxides)
Reversibility reversible reversible or irreversible
Speed fast can be slow
(e.g. activated
adsorption)
Coverage multilayers possible monolayer only
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Physisorption and Chemisorption
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Power Balance of Tian-Calvet Calorimeter & Adsorption Heat Signal
The power P [W] necessary to heat the cell by d is
proportional to the heat capacity C [J/K] of the cell dt
dCP
The heat flow [power] is proportional to the
temperature gradient between cell and block and to
the thermal conductivity G [W/K]
GG blockcell )(
Gdt
dCPtotal
gU
Ug
G
dt
dU
g
CPtotal
Total thermal power of cell
The electrical signal is proportional to the temperature
difference; (proportionality factor g=f (number and type of the thermocouple))
The relation between power and electrical signal is then
G [W/K] is constant and if C [J/K] can be considered
constant, then C/G is a constant with units of time G
C
The Tian equation shows that the power is not proportional to
the temperature difference, the power is delayed with respect to
the signal U produced by the celldt
dUU
g
GPtotal
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Evolved Heat & Differential Heats of Adsorption
The integral under the curve is proportional to the evolved
heat
AfdtUg
GQ int
A: area under curve [Vs]
f: calibration factor [J/(Vs)]
The calorimeter can be calibrated
by using an Ohm resistance which
produces a certain amount of heat.
Q = U*I*t
f = (U*I*t) / Aohm resistance [Ws/Vs]
ATdiff nQq ,int )/(Differential heats of adsorption as a
function of coverage can be determined
If heat is released in the cell for a limited period of time,
e.g. through adsorption, then a electrical signal U with an
exponential decrease is obtained .
Calculation of Adsorbed Amount
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Calibration of the volumetric system
Vcal = 31.8 ml
= 134.4 ml
Wall adsorption isotherm reflects number of molecules in thegas phase and on the inner walls.
Cal
DosDosCal
DosCalCalDos V
pp
ppV
...)()()()( 4
,
3
,
2
,,,, iSCiSCiSCiSCigwSC pdpcpbpan
Amount of adsorbed molecules
Determination of the dosed amount
The number of molecules adsorbed
in the (i+1)th step is then
The total number of ads. molecules
in a given steps is
Volumetric-Barometric System
vacuum
probe molecule
sample cell + catalyst
pressure gauge dosing system
DOSING VOLUME VDos
CELL VOLUME
CALIBRATION
VOLUME Vcal
pressure gauge sample cell
vacuum
reference cell
T = constant
nads
p
p
RT
Vppn
DosaftDosbefDos
i
)( ,,
int,
1,,,,1int,1, igwSCigwSCiiads nnnn
1,,,1,, iadsitotadsitotads nnn
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
registered Raw Data
0 2 4 20 22 24 26 28 30 32 343
4
5
6
7
8
9
10
0.0
0.5
1.0
1.5
2.0
Pre
ss
ure
in
do
sin
g v
olu
me
/ h
Pa
Zeit / h
Pre
ss
ure
in
sa
mp
le c
ell
/ h
Pa
20 21 22 23 24 25 26 27 28 29 300.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Th
erm
os
ign
al / V
Time / h
Eq
uil
ibri
um
pre
ss
ure
/ h
Pa
Raw data
equilibrium pressure
Raw data
equilibrium pressure and thermosignal (Qint.)
DOSING
VOLUME
CELL VOLUME
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Adsorption isotherm qdiff strength of surface sites qdiff = f (nads) distribution of surface sites
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
obtained results
Adsorption Isotherm
nads = f ( pequ.)
nads (total) : overall adsorbed amount under an equilibrium pressure of 95 mbar
nads (irrev.) : chemisorbed amount
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Adsorption Isotherm of propane on 10%V/SBA15 catalyst
active in oxidation of propane
0,00
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0 10 20 30 40 50 60
equilibrium pressure [mbar]
am
ou
nt
of
ad
so
rbe
d p
rop
an
e [
mm
ol
/ g
]
T activation
Analysis
higher order Langmuir model,
Nads - coverage with certain equilibrium pressure
Nmono - monolayer coverage
p - equilibrium pressuren - adsorption order
K/A - adsorption equilibrium constant
R2 - correlation coefficient; goodness of fit
S - stoichiometry
Specific surface area propane = N mono * S * cross-section area propane,T * Avogadro constant
; R2
Freundlich model
The enthalpy of adsorption ΔaH (qdiff )
per site is constant with coverage Θ
The enthalpy of adsorption ΔaH (qdiff )
per site decreases with coverage Θ
10%V/SBA15
dehydration
temperature
Nmono
µmol *g-1
n R2
Spropanem2*g-1
BETSN2
m2*g-1
373 K 0.9 (2) 1.20 (2) 0.99983 226 (10) 329 (4)
573 K 1.3 (4) 1.22 (2) 0.99982 304 (10)
673 K 1.2 (3) 1.22 (2) 0.99905 290 (10)
R2=0.99983
R2=0.99853
R2=0.99973
higher order Langmuir
0 10 20 30 40 50 60
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14 Higher order Langmuir model
1st order Langmuir model
Freundlich model
N / m
mo
l*g
px,i
/ mbar
n = 1 non-dissociative ads.
n > 1 dissociative ads. ;
activated ads.
evaluation criteria
Analysis of the adsorption isotherm
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
obtained results
Differential heat versus the nads
Differential heat
Adsorbed volume
Enthalpy of condensation
strongLewis
acid sitesBrønstedacid sites
Homogeneous acid strength
Heterogeneousacid site
Interaction between adsorbed molecules
A. Auroux, Lecture Oct. 23,2009
Classical calorimetric curveDifferential heat
Adsorbed volume
Enthalpy of condensation
strongLewis
acid sitesBrønstedacid sites
Homogeneous acid strength
Heterogeneousacid site
Interaction between adsorbed molecules
A. Auroux, Lecture Oct. 23,2009
Differential heat
Adsorbed volume
Enthalpy of condensation
strongLewis
acid sitesBrønstedacid sites
Homogeneous acid strength
Heterogeneousacid site
Interaction between adsorbed molecules
A. Auroux, Lecture Oct. 23,2009
Classical calorimetric curve
Classical Calorimetric Curve
Initial differential heat
Slope: heterogeneously distributed andenergetically different adsorption
sites
Completionof monolayer
Plateau: homogeneously distributed andenergetically uniform adsorption sites
Amount of adsorbed propane on phase-pure MoVTeNb#6059 / mmol/g
Diffe
rential h
ea
t o
f a
dso
rbe
d
pro
pa
ne
/ k
J/m
ol
0.0 0.1 0.2 0.3 5 10 15 20 25 30
0
100
200
300
400
500
600
700
800
Dif
fere
nti
al
heat
of
ox
yg
en
ad
s. [
kJ/
mo
l]
amount of ads. oxygen [ mol/g]
Dif
fere
nti
al h
ea
t o
f a
ds
orb
ed
o
xyg
en
ato
m / k
J/m
ol
High diff. heats & heat oscillation :dynamic ads. processchemisorption-oxidation-reaction
Amount of ads. O2 on Pd/N-CNF at Treact. / µmol/g
most active
less active
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
q int (ads.) > q int (des.) partially irreversible ads.;activated ads. process
q int (ads.) < q int (des.) instability of the catalyst in the presence of probe molecule
q int (ads.) = q int (des.) reversible ads. process
+ qintegral (ads.) = 7892 mJ
- qintegral (des.)= 6560 mJ
Integral heat signal of
adsorption and desorptionstepwise adsorption of propane on MoVTeNb oxide at 313K
0 2000 4000 6000 8000 10000 12000 14000
0.430
0.435
0.440
0.445
0.450
0.455
0.460
SZ
the
rmo
sig
na
l / V
time / s
Background of the thermo signal
during the stepwise adsorptionstepwise n-butane ads. on sulf. ZrO2 at 313 K
Background deviates from the base-line
Adsorption process is accompanied by
secondary processes
e.g. during n-butane ads. a partial
isomerization of n-butane to isobutane
in the calorimeter cell was observed
Evaluation criteriaof the calorimetric experiment
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
0xxexpAyy 0
200 300 400 500 600 700 800 9000.2
0.4
0.6
0.8
1.0
Th
erm
osi
gn
al [
V]
Response time / 10.3 h
C
Shape of the integral heat signalO2 adsorption on 2%Pd/N-CNF473K at 353K=Treaction
0 10 20 30 40
0.30
0.35
0.40
Th
erm
osi
gn
al [
V]
Response time / 0.4 h
K(t)= 377 s
A
2 μmol/g oxygen
Evaluation criteriaof the calorimetric experiment
300 310 320 330 340 350 3600.2
0.4
0.6
0.8
1.0
Th
erm
osi
gn
al [
V]
Response time / 0.7 h
K(t)= 998 s
B
14 μmol/g oxygen 0.039 μmol/g oxygen
quasi pure dissociative
oxygen chemisorption
on Pdnads.↑
oxygen chemisorption
combined by
secondary processes
Determination of the time constant of the integral heat signal
8.8 104 8.85 104 8.9 104 8.95 104 9 104 9.05 104 9.1 104
Time (s)
1.24
1.26
1.28
1.3
1.32
1.34
1.36
Sig
nal T
herm
ocolu
mns (
V)
R=1kΩ →27.45 mJ
→ (calorimeter+cell) ~265 s
8000 9000 1 104 1.1 104 1.2 104 1.3 104 1.4 104
Time (s)
0.52
0.54
0.56
0.58
0.6
0.62
Sig
nal T
herm
ocolu
mns (
V)
gamma Al2O3, Methanol-Ads, 40grd, 3. Ads.-Step
(A+B*exp(-(x-C)/D),A=0.5438~,B=0.0249~,C=9237.80~,D=722.458~))
5.9 µmol/g propylene
adsorbed on MoVTeNb
at 313 K
= 287 s ~ cal+cell
pure ads. process
4.8 µmol/g
methanol adsorbed
on Al2O3 at 313 K
= 722 s > cal+cell
ads. + reaction
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
1. Introduction & motivation
2. Adsorptive microcalorimetric setup
3. Power balance of Tian-Calvet calorimeter &
Evolved adsorption heat &
Differential heats of adsorption
4. Volumetric-Barometric System
calibration & measurement of adsorbed amount
5. Obtained physical quantities & evaluation criteria of
the calorimetric results
6. Applications of microcalorimetry in heterogeneous
catalysis
Contents
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Applications
Probe
molecule
Catalyst / Activation / Catalytic activity T adsorption
[K]
q initial+
[kJ/mol]
n-butane 0.5wt%Pt/H-Mordenite / H2 reduced at 648 K /less active 313 350
n-butane 0.5wt%Pt/H-Mordenite / dehydr. at 723 K / active 313 42
n-butane 3.5wt% VOx-Al2O3 / H2 reduced at 773 K /active 313 63
n-butane Θ Al2O3 / H2 reduced at 773 K /non-active 313 35
propane 10 wt% VxOy-SBA15 / dehydrated at 373 K / active
573 K
637K
313 45
80
160
propane 3 wt% VxOy-SBA15 / dehydrated at 373 K / active 313 45
propane SBA15 / dehydrated at 373 K / non-active 313 32
propane hydrothermal synth. – M1 /dehydr. at 423 K /active, 58* 313 71
propane precipitation – M1 /dehydrated at 423 K /active , 43* 313 56
propane SHWVT ** – M1 / dehydrated at 423 K / active, 5* 313 64
CO2
CNFox functionalized by NH3 at 873 K
673 K
473 K
313
150
50
90
CO2 Fe-CNT / dehydrated at 373 K
FeIO-XT 24PS-CT / dehydrated at 373 K
313 272
191
O2
2wt%Pd#/N-CNT873K/ dehydrated at 353 K
2wt%Pd#/N-CNT473K/ dehydrated at 353 K
2wt%Pd/CNT/ H2 reduction at 423 K
353
353
313
500-700
500
175
O2 *** precipitation – M1 / H2 activated at 653 K
precipitation – M1 / propane activated at 653 K
473 218
257
O2 *** 8wt%VxOy-SBA15 / dehydrated at 673 K 473 248
Selected calorimetric measurements on supported metal oxide and mixed metal oxides .
+ We have adopted the calorimetric sign criterion (positive energetic quantity for an exothermic process).* partial oxidation of propane (POP); Selectivity to acrylic acid [mol-%]** superheated-water vapor treatment (SHWVT)*** cooperation with Instituto de Quimica Fisica “Rocasolano", CSIC, Madrid (Spain)# CNFox functionalized by NH3 at 873K or 473K, Pd catalysts are obtained via sol-immobilization on the CNFs
in which Na2PdCl4, NaBH4 and polyvinylalcohol (PVA) were used . Hence, Pd particles are partially covered by PVA.## Vapor Growth Commercial Carbon Nanofiber; oxygen-containing nanocarbon was obtained by treating with HNO3 at 373 K.
The method was broadly
employed in several projects of
our department and yielded a
surprising spread of energetic
data for the same molecule on
different surfaces .
In addition, we observed
significant differences of the
energetic data for the same
molecule on slightly modified
surfaces.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
0.0 0.1 0.2 0.3 0.4 0.5
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
am
ou
nt
of
ad
so
rbe
d C
O
/ m
mo
l*g
euilibrium pressure of CO / mbar
Model: higher order Langmuir model
qdiff = 200kJ/mol = constant versus nads
R2
= 0.88731
Nmono = 0.02631 (1st
run) - 0.00583(2nd
run)
K = 1662.35201 --> irreversibility
n = 1.15679
CO adsorption on Pt/Al2O3 at 40°C
Differential heat of adsorbed CO Adsorption isotherm of CO
n ads. (irreversible)
= 20.4 µmol/g
Pt-CO = 1:1 = 206 kJ/mol Pt2-CO = 94 kJ/mol[
J.Therm. Anal. Cal., 82, (2005) 105
Saturation
concentration
Saturation concentration
SCO = Nmono * S * cross-section-area CO,T * Avogadro constant
SCO = 20.4*10-6mol/g * 1* 16.2*10-20m * 6.022*1023mol-1 = 1.99 m2/g
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
n-butane (educt) adsorption on Pt/H-Mordenite catalyst at 40°C
Aim: structure-activity relationship study of Pt/HM for n-butane isomerization
0 1 2 30
100
200
1600
1800
2000
Time on stream [h]
Ra
te o
f is
om
eri
sa
tio
n [
mo
l/g
.h]
fully H2-reduced Pt/HMcatalyst activated in H2 at 350 oC for 2 h
n-butane isomerisation15 kPa n-butane in H2 at 300 oC
not completely H2-reduced Pt/HMcatalyst activated in H2 at 300 oC for 1 h
0.0 0.1 0.2 0.3 0.4 0.5
40
60
80
300
400
D
iff.
he
at
of
ad
so
rbe
d n
-bu
tan
e [
kJ
/mo
l]Equilibrium pressure of n-butane [mmol/g]
2300 2200 2100 2000 1900 1800
Wavenumber / cm-1
Pt0
Al3+
oct.
Ptx-CO
Ptx+
0.0 0.1 0.2 0.3 0.4 0.5
40
60
80
300
400
D
iff.
he
at
of
ad
so
rbe
d n
-bu
tan
e [
kJ
/mo
l]Equilibrium pressure of n-butane [mmol/g]
2300 2200 2100 2000 1900 1800
Wavenumber / cm-1
Pt0
Al3+
oct.
Ptx-CO
Ptx+
Differential heat of ads. n-butane at 40 C
IR spectra of ads. CO at RT
at 0.7 mbar
200 mol/g
450 mol/g
The active states of Pt/HM are characterized by:
a small amount of strong Lewis acid sites (Al3+oct., extraframework alumina)
well dispersed metallic platinum particles (1-3 nm)
higher number of n-butane adsorption sites
weak interaction of the surface acid sites with n-butane.The weaker interaction with the alkane is apparently favorable for the catalytic performance; perhaps because of facile product desorption.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
20
30
40
50
60
70
80
0.00 0.01 0.01 0.02 0.02 0.03 0.03 0.04 0.04
Dif
fere
nti
al h
eat
[k
J/m
ol]
Amount of neo-pentane or iso-butane sorbed [mmol/g]
Fe/sulf.ZrO2 is active in isomerization of n-butane to isobutane at low temp. (100°C)
The scattering of the diff.
heats for iso-butane at low
coverage indicates the
presence of exothermic and
endothermic processes during
the adsorption.
Question: Does iso-butane (product) reacts with surface species
of Fe/sulf. ZrO2 at low temperature ?
Differential heat of adsorbed iso-butane & neo-pentane at 40 C
3
3
3CH
CH
CH
CH
ads. + reaction
Answer: Yes, iso-butane (product) reacts with surface species of
Fe/sulf. ZrO2 at low temperature.
pure ads.
The neo-pentane heat profile
follow the classical calorimetric
heat profile - heat decreases
with increasing the coverage
3
3
3
CH
C CH
CH3
CH
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
pure-phase MoVTeNb catalyst active indirect oxidation of propane to acrylic acid (aa)
Sabine Wrabetz, Yury V. Kolen’ko, Jutta Kröhnert, Lenard Csepei, Olaf Timpe, Wei Zhang, Annette Trunschke, and Robert Schlögl,
Characterization of MoVTeNb catalyst in their as prepared and active state by adsorption microcalorimetry and -FTIR spectroscopy ; in preparation 2010.
Aim: study of the post-reaction state of the surface “used catalyst” in
comparison with the prepared state of the surface “fresh catalyst”
Differential heat of propane adsorption at 40°C
phase-pure M1, Saa = 53%Spropane=11.4m2/g
oxidized M1, Saa = 37%Spropane=10.3m2/g
MoV oxide, Saa = 1.8%
Spropane=13.4m2/g
Saa → homogeneity → strength of interaction between educt and surface sites → density of propane ads. sites energetically uniform ( which is apparently favorable for catalytic performance;
ads. sites perhaps because of facile intermediate desorption)
the prepared state of the surface is different from the post-reaction state of the surface
dynamic surface during reaction
Amount of propane adsorption / mmol/m2
80
70
60
50
40
30
20
10Diffe
rential heat
/
kJ/m
ol
0 0.001 0.002 0.003 0.004 0.005 0 0.001 0.002 0.003 0.004 0.005 0 0.001 0.002 0.003 0.004 0.005
Amount of propane adsorption / mmol/m2Amount of propane adsorption / mmol/m2
MoV
Mo
Nb
Te
O
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
quasi in-situ microcalorimetry
Aim: Study of the educt and product interaction with the surface of sulf. ZrO2
if the surface at the state of highest activity in the n-butane isomerization
n-butane &
iso-butane ads.
at 313 K
after degassing at 378K
0 20 40 60 80 100 120 140
0
5
10
15
20
25
30
35
40
45
Rate
of
Isom
erization (
mol h
-1 g
-1)
Time on Stream (min)
n-butane isomerisation at 378Kin the flow-type calorimetric sample cell
0
10
20
30
40
50
60
70
80
0,000 0,005 0,010 0,015 0,020 0,025D
iffe
ren
tia
l h
ea
t [k
J/m
ol]
Amount of adsorbed n-butane & iso-butane [mmol/g]
K n-butane= kads./kdesorp. = 0.162
K iso-butane= kads. /kdesorp. = 0.011
The state of maximum activity of sulf. ZrO2 is characterized by:
A stronger interaction with n-butane (~70kJ/mol, educt) than with iso-butane (~45kJ/mol, product).
The weak interaction with iso-butane and the equilibrium constant (Kiso-butane=0.011) indicate
an increasing easiness of desorption of iso-butane from the surface sites.
stoppedat
TOS=120min
Differential heat at 40 C
Wrabetz, Sabine; Yang, Xiaobo; Tzolova-Müller, Genka; Schlögl, Robert; Jentoft, Friederike C., J. of Catalysis 269 /2 (2010) 351 - 359.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
CNF surface chemistry plays a crucial
role in Metal/CNF interaction
Fe/N-CNF873K is the best catalyst in
electrocatalytic conversion of CO2 .
Pd/N-CNF873K is the best catalyst in
oxidation of benzyl alcohol to
benzaldehyde
Microcalorimetric titration of basic sites on N-CNFs by CO2 adsorption at 40°C
Different oxygen and nitrogen species in CNFs
Oxidation of VGCNF by HNO3 at 373K for 2h and
amination using NH3 at 473/673/873 K.
0 2 4 6 8
20
40
60
80
100
120
140
160
a)
0 2 4 6
b)
0 2 4 6
c)
0 1 2 3
d)
partially irreversible
adsorbed
Differential heat of adsorbed CO2
N-CNF873K N-CNF473K N-CNF673K VGCNF
Arrigo, Rosa; Haevecker, Michael; Wrabetz, Sabine; Blume, Raoul; Lerch, Martin; McGregor, James; Parrott, Edward; Zeitler, J; Gladden,
Lynn; Knop-Gericke, Axel; Schlogl, Robert; Su, Dangsheng, Journal of the American Chemical Society 132/28 (2010) 9616 - 9630.
depending on the Tamination the N-CNF
surface provides energetically different
basic sites which are heterogeneously
distributed
Amount of adsorbed CO2 [ mol/g]
Diffe
rential h
ea
t o
f a
dso
rbe
d C
O2
[kJ/m
ol]
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Conclusion
very sensitive and hence a selective surface characterization methodbut very time consuming ( ~1 week for 1 experiment)
The nature of the reactive surface sites can be studied by
measuring the thermal effects during the reaction.
The knowledge about the energetics of the surface chemical
events helps better to understand the catalytic properties of the
surface and hence the catalytic reaction characteristics.
Microcalorimetry alone or combined with to other techniques, is applied
for the characterization of catalysts, supports and adsorbents, and to
the study of catalytic reactions (adsorption-desorption phenomena).
The calorimetric data obtained are of substantial importance for comparing
theoretical and experimental hypotheses about reaction pathways.
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
A. Auroux “Thermal Methods: Calorimetry, Differential Thermal Analysis, and
Thermogravimetry” in “Catalyst characterization: physical techniques for solid
materials”, Eds. B. Imelik, J.C. Vedrine, Plenum Pr., New York 1994
E. Calvet, H. Prat, H.A. Skinner “Recent progress in microcalorimetry”,
Pergamon Pr., Oxford1963
B. E. Handy, S.B. Sharma, B.E. Spiewak and J.A. Dumesic, “A Tian-Calvet
heat-flux microcalorimeter for measurement of differential heats of adsorption”,
Meas. Sci. Technol. 4 (1993) 1350-1356.
N. C. Cardona-Martinez and J.A. Dumesic, “Application of Adsorption
Microcalorimetry to the Study of heterogeneous Catalysis”, Advances in
Catalysis 38 150-243.
Z. Knor, “Static Volumetric Methods for Determination of Adsorbed Amount of
Gases on Clean Solid Surface”, Catalysis Reviews 1 (1) (1968) 257-313.
S. Černý and V. Ponec, „ Determination of Heat of Adsorption on Clean Solid
Surfaces”, Catalysis Reviews 2 (1) (1969) 249-322.
Literature
Sabine Wrabetz, Electronic Structure and Adsorption / Metals, Dept. AC, Fritz Haber Institute of the MPG, Berlin, Germany
Acknowledgement of financial support
Thanks the European Union for finacial support of the project
“Integrated Design of Nanostructured Catalytic Materials for a
Sustainable Development” .
Financial support for “Brückenschläge zwischen idealen und realen
Systemen in der Heterogenen Katalyse“ through DFG priority
program 1091 (JE 267/1-3) is gratefully acknowledged.
The SFB-project “Struktur, Dynamik und Reaktivität von
Übergangsmetalloxid- Aggregaten“ was sponsered by DFG.
Author thanks the DFG for providing an Emmy Noether
fellowship to the project leader “propane oxidation over
V/SBA15” Prof. C. Hess
The project “Pt-doped H-mordenite is used as a solid acid
catalyst for the isomerization of light alkanes” was financially
supported by BMBF grant 03C0307E.