Effect of Ceria nanoshapes in ethanol steam reforming over … · 2016-12-04 · POLITECNICO DI...

POLITECNICO DI MILANO

Scuola di Ingegneria Industriale e

dell’informazione

Corso di Laurea Magistrale in Ingegneria

Chimica

Effect of Ceria nanoshapes in ethanol steam reforming over

RhPd/CeO2 catalysts

RELATORE: Prof. Lietti Luca

CORRELATORE: Prof. Llorca Jordi

Tesi di Laurea Magistrale in Ingegneria Chimica di:

Brambilla Matteo, matricola 818822

A.A. 2014-2015

i

Milano, 18/12/2015

ii

TABLE OF CONTENTS

1 HYDROGEN PRODUCTION BY BIOETHANOL .......................................................................... 1

1.1 Hydrogen as energy carrier ........................................................................................... 1

1.2 Bioethanol for a sustainable process ............................................................................ 3

1.3 Ethanol Steam Reforming for hydrogen production ..................................................... 6

1.3.1 Catalytic Pathways ................................................................................................ 8

1.4 Catalysts for ESR: Why RhPd/CeO2? ............................................................................ 10

1.4.1 Metallic active part .............................................................................................. 12

1.4.2 Supports .............................................................................................................. 14

2 EXPERIMENTAL WORK ........................................................................................................ 18

2.1 Preparation and characterization of catalysts ............................................................ 18

2.1.1 Preparation of the ceria supports ....................................................................... 18

2.1.2 Impregnation of metallic nanoparticles on ceria supports ................................. 23

2.1.3 Characterization of ceria supports ...................................................................... 24

2.1.4 Characterization of impregnated catalysts ......................................................... 26

2.2 Set up of the reaction system ..................................................................................... 27

2.2.1 Gas Chromatograph ............................................................................................ 30

2.3 Catalytic tests .............................................................................................................. 31

2.3.1 Temperature-ramping tests ................................................................................ 31

2.3.2 Fixed-temperature tests ...................................................................................... 34

2.4 Characterization of exhausted catalysts ..................................................................... 35

3 RESULTS AND DISCUSSION .................................................................................................. 36

3.1 Characterization of the support .................................................................................. 36

3.2 Characterization of impregnated catalysts ................................................................. 39

3.3 Catalytic tests .............................................................................................................. 46

3.3.1 Temperature-ramping tests ................................................................................ 46

3.3.2 Fixed-temperature tests ...................................................................................... 68

3.4 Characterization of exhausted catalysts ..................................................................... 71

4 CONCLUSIONS ..................................................................................................................... 83

4.1 OUTLOOK ..................................................................................................................... 84

iii

LIST OF FIGURES

Figure 1.1 Fuels world consumption (millions of tonnes oil equivalent) [2]................................. 2

Figure 1.2 Scheme of bioethanol production process [14]. .......................................................... 4

Figure 1.3 Different feedstocks for bioethanol production [15]. .................................................. 5

Figure 1.4 Catalytic pathways of ethanol steam reforming [26]................................................. 10

Figure 2.1 Hydrothermal reactor. ............................................................................................... 19

Figure 2.2 Centrifuge machine. ................................................................................................... 21

Figure 2.3 Ceria nanocubes (C), nanorods (R) and polycrystals (P). ........................................... 22

Figure 2.5 Impregnated catalyst. ................................................................................................ 24

Figure 2.4 Scheme of SEM functioning (C=cathode, A=anode, L=lens, S&D=sample and

detector, M=monitor). ................................................................................................................ 25

Figure 2.6 Scheme of HRTEM functioning. .................................................................................. 27

Figure 2.7 Representative diagram of the reaction system. ....................................................... 28

Figure 2.8 Reaction system. ........................................................................................................ 30

Figure 3.1 SEM image of ceria nanocubes with a magnification of 125 K X. .............................. 36

Figure 3.2 SEM image of ceria nanocubes with a magnification of 200 K X. .............................. 37

Figure 3.3 SEM image of ceria nanorods with a magnification of 125 K X. ................................ 38

Figure 3.4 SEM image of ceria polycrystals with a magnification of 250 K X. ............................. 39

Figure 3.5 TEM image of ceria nanocubes with RhPd nanoparticles on them. .......................... 40

Figure 3.6 TEM image of ceria nanocube showing atoms and distances between them. .......... 41

Figure 3.7 TEM image of ceria nanocube (b) with RhPd nanoparticle (a) showing atoms and

distances between them. ............................................................................................................ 41

Figure 3.8 TEM image of ceria nanorods with RhPd nanoparticles on them. ............................. 42

Figure 3.9 TEM image of a ceria nanorod with RhPd nanoparticle showing atoms and distances

between them. ............................................................................................................................ 43

Figure 3.10 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms and


Figure 3.11 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms. ............ 45

Figure 3.12 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms and


Figure 3.13 Hydrogen yield over impregnated catalysts during temperature-ramping tests. ... 47

iv

Figure 3.14 Ethanol conversion over RhPd/ceria nanocubes during temperature-ramping tests.

..................................................................................................................................................... 50

Figure 3.15 Ethanol conversion over RhPd/ceria polycrystals during temperature-ramping

tests. ............................................................................................................................................ 50

Figure 3.16 Ethanol conversion over RhPd/ceria nanorods during temperature-ramping tests.

..................................................................................................................................................... 51

Figure 3.17 Ethanol conversion over impregnated catalysts during the first cycle of the

temperature-ramping tests. ........................................................................................................ 52

Figure 3.18 Ethanol conversion over impregnated catalysts during the second cycle of the


Figure 3.19 ESR products selectivity over RhPd/ceria nanocubes during the first cycle of the


Figure 3.20 ESR products selectivity over RhPd/ceria polycrystals during the first cycle of the


Figure 3.21 ESR products selectivity over RhPd/ceria nanorods during the first cycle of the


Figure 3.22 ESR products selectivity over RhPd/ceria nanocubes during the second cycle of the


Figure 3.23 ESR products selectivity over RhPd/ceria polycrystals during the second cycle of the


Figure 3.24 ESR products selectivity over RhPd/ceria nanorods during the second cycle of the


Figure 3.25 Hydrogen yield over ceria supports during temperature-ramping tests. ................ 60

Figure 3.26 Ethanol conversion over ceria nanocubes during temperature-ramping tests. ...... 61

Figure 3.27 Ethanol conversion over ceria polycrystals during temperature-ramping tests. .... 61

Figure 3.28 Ethanol conversion over ceria nanorods during temperature-ramping tests. ........ 62

Figure 3.29 Ethanol conversion over ceria supports during the first cycle of the temperature-

ramping tests. .............................................................................................................................. 63

Figure 3.30 Ethanol conversion over ceria supports during the second cycle of the


Figure 3.31 ESR products selectivity over ceria nanocubes during the first cycle of the


v

Figure 3.32 ESR products selectivity over ceria polycrystals during the first cycle of the


Figure 3.33 ESR products selectivity over ceria nanorods during the first cycle of the


Figure 3.34 ESR products selectivity over ceria nanocubes during the second cycle of the


Figure 3.35 ESR products selectivity over ceria polycrystals during the second cycle of the


Figure 3.36 ESR products selectivity over ceria nanorods during the second cycle of the


Figure 3.37 Ethanol conversion and ESR products selectivity at 823 K. ..................................... 69

Figure 3.38 Hydrogen yield at 823 K. .......................................................................................... 70

Figure 3.39 SEM image of RhPd/ceria nanocubes after the temperature-ramping test with a

magnification of 50 K X. .............................................................................................................. 71

Figure 3.40 SEM image of RhPd/ceria nanocubes after temperature-ramping test with a

magnification of 200 K X. ............................................................................................................ 72

Figure 3.41 SEM image of RhPd/ceria polycrystals after temperature-ramping test with a


Figure 3.42 SEM image of RhPd/ceria polycrystals after temperature-ramping test with a


Figure 3.43 SEM image of RhPd/ceria nanorods after temperature-ramping test with a


Figure 3.44 SEM image of RhPd/ceria nanorods after temperature-ramping test with a


Figure 3.45 SEM image of ceria nanocubes after fixed-temperature test at 823 K with a


Figure 3.46 SEM image of ceria nanocubes after fixed-temperature test at 823 K with a


Figure 3.47 SEM image of ceria polycrystals after fixed-temperature test at 823 K with a


Figure 3.48 SEM image of ceria polycrystals after fixed-temperature test at 823 K with a


vi

Figure 3.49 SEM image of ceria nanorods after the fixed-temperature test at 823 K with a


Figure 3.50 SEM image of ceria nanorods after the fixed-temperature test at 823 K with a


vii

LIST OF TABLES

Table 2.1 Preparation conditions of the different supports ....................................................... 22

Table 3.1 Carbon balance over impregnated catalysts at 723 K calculated with Eq. 2.10. ........ 59

Table 3.2 Carbon balance over bare CeO2 at 723 K calculated with Eq. 2.10. ............................ 68

viii

ESTRATTO IN LINGUA ITALIANA

L’obiettivo di questo studio è la valutazione dell’attività di catalizzatori, formati da

supporti di ceria nano-strutturata impregnati con nanoparticelle di una lega rodio-

palladio, nella reazione di Ethanol Steam Reforming (ESR) per la produzione di

idrogeno puro.

Nonostante le tecnologie atte all’utilizzo di H2 come fonte energetica debbano essere

migliorate, questo processo rappresenta un’interessante alternativa ai combustibili

fossili, le cui riserve stanno rapidamente diminuendo. Infatti, l’idrogeno ha un alto

potere calorifico e può essere ottenuto a partire da bioetanolo attraverso un processo

ecosostenibile in grado di fornire, teoricamente, un’elevata quantità di idrogeno.

Il lavoro sperimentale si è svolto in diversi passaggi: dapprima una ricerca bibliografica,

per trovare in letteratura studi riguardanti la reazione d’interesse, il comportamento

dei catalizzatori utilizzati nel presente lavoro e le differenze fra le tre nano-strutture

del supporto; in seguito, la preparazione e la caratterizzazione dei catalizzatori, con lo

scopo di vedere l’effettiva presenza delle nano-strutture desiderate; infine svariati test

catalitici, seguiti dalla caratterizzazione dei catalizzatori esausti, per valutarne il

comportamento.

Un risultato importante del lavoro svolto è la minore attività dei catalizzatori impiegati

rispetto ai catalizzatori ottenuti per co-precipitazione di sali di Pd e Rh sul supporto,

utilizzati in un altro studio. Ciò è dovuto ad una maggiore dimensione delle particelle

metalliche, quindi a un minor numero di siti attivi. Questo fatto influisce anche sulla

selettività verso H2, che è minore sotto i 900 K.

Il trend di attività delle tre diverse nano-strutture di ceria si è dimostrato essere:

policristalli > nanotubi > nanocubi mentre nel precedente lavoro era l’inverso. In

questo caso la spiegazione è da attribuire alle dimensioni delle particelle del supporto

in quanto i policristalli, essendo i più piccoli, erano caratterizzati da una maggiore area

superficiale e quindi da una maggiore attività catalitica.

Un ulteriore risultato è che cambiamenti morfologici dei nanocubi a causa dello stress

termico sono avvenuti sopra i 550°C (823 K).

ix

PREFACE

The main goal of this study is the evaluation of the activity of catalysts composed by

rhodium-palladium alloy nanoparticles supported over nanoshaped cerium oxide

during the ethanol steam reforming reaction for the production of pure hydrogen.

Although the techniques regarding hydrogen as energy source still need to be

improved, it is for sure an interesting alternative to fossil fuels since it has a high

calorific power and it can be obtained starting from bioethanol. In this way, its

production can be environmentally sustainable and it is possible to limit the

dependence on fossil fuels, whose reserves are becoming less and less. Among all the

processes, the ethanol steam reforming is very promising since it guarantees,

theoretically, the highest amount of hydrogen.

The present work was carried out within the research group of Prof. Jordi Llorca at the

“Institute of Energy Technologies” belonging to the Universitat Politècnica de

Catalunya placed in Barcelona.

The possibility of supporting a thesis work abroad is due to the Erasmus Plus

programme that I joined during the second semester of the academic year 2014-2015

thanks to an agreement between Politecnico di Milano and Universitat Politècnica de

Catalunya.

The whole work was divided in the following steps:

Bibliographic research in order to find in literature previous studies about the

different catalysts used to perform the reaction of interest, the efficacy of

RhPd/CeO2 catalysts and the differences between the three different ceria

nanoshapes (nanocubes, nanorods and polycrystals) analysed in this work.

Preparation of the catalysts according to methods found in literature. This step

is composed by the preparation of the three nanoshaped supports and the

impregnation of preformed rhodium-palladium nanoparticles on them.

x

Characterization of catalysts in order to see the effective presence of the

different nanoshapes.

Catalytic tests performed at atmospheric pressure in a reaction system placed

inside a fume-hood.

Characterization post-reaction of the exhaust catalysts aimed to the evaluation

of their behaviour during the catalytic tests.

The present thesis is structured in different parts, namely:

Chapter 1: Hydrogen Production by Bioethanol. In this section a general

overview of the topic dealt would be given. Information regarding hydrogen,

ethanol (bioethanol), cerium dioxide, catalysts employed and ethanol steam

reforming are based on results and information of previous works found in

literature.

Chapter 2: Experimental Work. This section explains the experimental methods

employed to prepare the three different supports, the impregnation technique

necessary to deposit the metallic nanoparticles on them, the executed

characterization of the samples, the preparation and optimization of the

reaction system to perform the catalytic tests and, finally, the equations used

to analyse the experimental data.

Chapter 3: Results and Discussion. In this section all the results will be discussed

in order to explain in detail the processes involved during the reaction. Both

the data and the results of characterizations will be examined to make

hypothesis on the chemical and physical aspects.

Chapter 4: Conclusions. In this section conclusions based on the results

obtained will be given.

1

1 HYDROGEN PRODUCTION BY BIOETHANOL

The topic of this work is the study of the behaviour of catalysts composed by Rh-Pd

alloy nanoparticles supported on CeO2 during the production of hydrogen by ethanol

steam reforming. The reaction has to be conducted with a selective catalyst to

minimize the formation of undesirable compounds like CO and CH4. In a previous work

researchers from the Universitat Politècnica de Catalunya (UPC) studied this matter

focusing on catalysts formed by the co-precipitation of PdCl3 and RhCl3 on nanoshaped

CeO2 such as nanocubes, nanorods and octahedral polycrystals respectively

characterized by {1 0 0}, {1 1 0} and {1 1 1} crystallographic planes. According to its

stability and considering the operating temperature, the nanocube shape turned out

to be the most active followed by nanorods [1].

1.1 Hydrogen as energy carrier

The most important part of the energy produced by humans derives from fossil fuel

sources like carbon, oil and natural gas. From an energetic point of view these fuels are

very competitive since they allow the production of a great amount of energy in

comparison to that consumed to extract them. In fact their EROEI index (Energy

Returned On Energy Invested), which shows the convenience of each energetic source,

is very high.

Unfortunately fossil fuels present also some disadvantages such as non-renewability

and environmental pollution.

The reserves we are using nowadays have been formed over millions of years ago

thanks to the decomposition of buried dead organisms like vegetables or animals. A

major global problem is that this process is very slow and humans are depleting fossil

fuels reserves very quickly (Figure 1.1).

2

Figure 1.1 Fuels world consumption (millions of tonnes oil equivalent) [2].

Moreover, fossil fuels are converted into energy by combustion processes that

generate pollutants such as carbon monoxide (CO), which is a poison, carbon dioxide

(CO2), which is associated with global warming due to its greenhouse effect, nitrogen

oxides (NO, NO2 and N2O), that are greenhouse gases and increase environmental

acidity, sulphur oxides (SO2 and SO3), which are acid substances and cause respiratory

problems, and unconverted hydrocarbons, which mainly form solid particulate [3].

For these reasons the trend is the research of alternative energy sources,

environmentally safe and renewable to allow their exploitation. Hydroelectric, solar,

wind, geothermal and sea energies can be used where possible but efficiencies are low

and much more progress on renewable energies must be done. Incinerators are not

convincing too since their use do not solve the problem of the emission of pollutants.

Concerning to hydrogen, it is a very interesting chemical element, especially for energy

production. It has a very high calorific power related to its mass ratio, even higher than

those of fossil fuels. Its combustion generate only water as reaction product so it has a

minimal environmental impact [3]. Moreover, hydrogen can also be used to feed fuel

cells (especially proton exchange membrane fuel cells, PEMFCs) both for automotive

propulsion and for small portable devices[4][5][6][7]. Hydrogen can be catalytically

produced from the ethanol steam reforming, with the advantage that ethanol is not

toxic and can be stored on board [4][6]. Furthermore, it produces more or less the

3

same volume of hydrogen than other substrates [7]. As we can notice these

characteristics are very promising. However, a clarification has to be done. Hydrogen is

not an energy source but an energy carrier since it cannot be found in nature as free

gas (H2) and it must be produced from other sources that contain it. The most used

feedstocks are fossil fuels, intermediate chemical compounds, alternative sources such

as biomass and biofuels, and water by electrolytic process [3]. The use of fossil fuels

and intermediate chemical compounds do not solve the problem of non-renewability.

Concerning to electrolytic processes, they require a large amount of electric energy so

they are not used for industrial applications. The use of alternative sources containing

hydrogen seems to be the best solution and bioethanol is one of them. Ethanol steam

reforming is very convenient since it allows to obtain six moles of hydrogen for each

mole of ethanol converted.

Summarizing, hydrogen is a promising energy carrier considering that it has a very high

calorific power, it burns producing water and it can be obtained by alternative sources

like bioethanol. All these benefits overshadow some disadvantages, which are present

because of the nature of the molecule. In fact, hydrogen is the smallest chemical

element and a gas and it presents a very high diffusion coefficient and a very low

density. The former problem is important in case of lack from a tank since it quickly

dissipates in the air; the latter problem makes the storage very difficult so it has to be

compressed, liquefied or dissolved in metal hybrids [7].

Once generated by catalytic reforming, the resultant hydrogen cannot be directly used

on its final applications because it has to be purified. The most important byproducts

that must be separated are CO, CO2, CH4 and H2O.

1.2 Bioethanol for a sustainable process

Bioethanol is a volatile, flammable and colourless liquid with a pleasant and

characteristic odour. It belongs to the category of biofuels.

Brazil was the first country to produce wholesale bioethanol as automotive fuel due to

the first oil crisis in 1973. Since then its production has continued to grow and has also

affected other countries such as United States, which at present are the best

4

bioethanol producers followed by Brazil. They together cover about the 86% of world’s

total production [8].

The production of this type of fuels can be achieved directly from plants or indirectly

from agricultural, industrial and domestic wastes [9]. All these sources are generically

named biomass. The use of biomass does not theoretically change the total amount of

CO2 in the atmosphere because its formation through photosynthesis and its

combustion produce the same quantity. For this reason biomass is considered CO2-free

[10].

One of the greatest problems regarding the use of biomass is related to how obtain

them. Obviously there would not be concerns if the sources were only wastes but to

allow a wide use is required to grow them in appropriate fields. As expected, this fact

generated controversy in a world where the farmlands are less and less numerous. To

grow biomass precludes the development of cities, increases deforestation and raises

the price of those foods which enter in competition with the biomass themselves for

the employment of arable lands [11]. These are the main reasons why biomass is not

widely used yet and why new feedstocks are sought.

Bioethanol is obtained by enzyme digestion of hemicellulose and cellulose contained in

biomass, such as sugar or starch crops (corn, sugarcane, wheat, maize, sugar beet and

sweet sorghum) [11][12][13], followed by the fermentation of deriving sugars (Figure

1.2).

Figure 1.2 Scheme of bioethanol production process [14].

The industrial feedstock is composed by the outer coverings and the starchy

endosperm of each grain or seed. It mainly consists of starch, cellulose, hemicellulose,

5

proteins and lignin but only hemicellulose and cellulose can be converted into

bioethanol. The former is a heteropolymer containing sugar residues like hexoses,

methylpentoses, pentoses and uronic acid. The branched nature makes it easy to

hydrolyse to its constituent sugars, which give bioethanol by fermentation (first

generation bioethanol). There is also another way to produce bioethanol based on the

use of ligno-cellulosic feedstock (second generation bioethanol). The main difference

consists in the hydrolysis step which is now more complicated since cellulose is made

up of long chains of glucose so a more complex set of enzymes is required to break the

long chains [12][11]. For this reason the second generation bioethanol is more

expansive than first generation one. However, over than 50% of available biomass is

ligno-cellulosic so the second generation ethanol allows to widen exploitable

feedstocks [13]. Recently, a new way of producing ethanol (third generation

bioethanol) has been proposed starting from algae; however, such technologies are

still at an early-stage development. The three generations and their feedstocks are

shown in Figure 1.3.

Figure 1.3 Different feedstocks for bioethanol production [15].

Once produced, bioethanol can be used in many different ways. Mainly it can be added

to gasoline to increase its octane number, it can replace conventional ethanol for the

production of chemicals and it can be converted in hydrogen by steam reforming. This

6

last application is achievable because ethanol has a high hydrogen/carbon ratio of 3

and so it has a high hydrogen content per unit volume in the liquid phase.

To obtain pure ethanol to use as gasoline additive, water contained in the aqueous

bioethanol solution resulting from fermentation process has to be distilled. In fact

bioethanol is a mixture of ethanol and water with molar ratio 1:13 (about 12% wt. of

ethanol) [13]. This distillation is energy-intensive and contributes for the great part of

the total cost of bioethanol production [16]. However, bioethanol is a fuel which

allows to get more energy than the one used for its production from biomass. In fact

there is an energy gain of about 34% (53% under a best case scenario) [17]. As the

elimination of water from bioethanol is very expensive from an energetic point of

view, the global energy balance would be improved by processes that use both

bioethanol and water as reactants. It follows that ethanol steam reforming represents

a better use of bioethanol than adding it to gasoline [9].

1.3 Ethanol Steam Reforming for hydrogen production

Hydrogen can be produced starting from ethanol in three reactive, catalytic ways:

steam reforming, partial oxidation and autothermal reforming [18]. The highest

hydrogen production can be achieved by ethanol steam reforming, which is the

reaction considered in the present work. It is an endothermic reaction that involves

ethanol and water for the production of hydrogen and carbon dioxide:

CH3CH2OH + 3 H2O → 6 H2 + 2 CO2 ΔH0298 = 174 kJ/mol (Eq. 1.1)

Theoretically, this reaction guarantees a large amount of hydrogen since six moles are

obtained for each mole of bioethanol converted [19][20]. Moreover, though the

stoichiometric steam/ethanol molar ratio (S/E) is 3, higher ratios can be used allowing

so to feed directly the bioethanol produced by biomass fermentation which has a S/E

ratio of 13 without distilling it and allowing, in this way, to save a lot of energy.

Considering the growing necessary of alternative energetic sources, the excellent

qualities of hydrogen as fuel, its environmental sustainability and the possibility to

obtain it without the use of fossil fuels, and considering also the high theoretical yield,

this reaction seems to be a very promising way to produce clean energy.

7

Like every endothermic reaction, it is necessary to give a certain amount of heat to the

system in order to enhance it. Temperature can reach up to 800°C [18]. For this reason

is usually employed a tubular reactor in which the reaction zone is heated by radiation

due to flames burning in the shell covering the tubes. This high quantity of energy

needed to run the reaction and to generate the necessary steam is one of the

drawbacks of the process. Another disadvantage is that hydrogen yield is limited by

thermodynamic equilibrium but production can be improved by increasing the S/E

ratio [18]. Nevertheless, to achieve this result further steam needs to be generated

and further energy has so to be spent.

As shown in the work by Vasudeva et al. [21], arising the water/ethanol feed ratio and

reducing temperature from 1200 to 800 K, the carbon monoxide yield decreases and

the hydrogen one increases. Almost stoichiometric hydrogen yields are obtained with

S/E > 20. So it is clear that operating conditions should include low temperature and

high steam/ethanol feed ratios. Another important result proved in the

aforementioned work [21] is that hydrogen yield increases until it reaches the

stoichiometric value (six moles of hydrogen for each mole of ethanol converted)

asymptotically. Reducing the operative temperature and arising the steam/ethanol

ratio allows therefore to approach the theoretical yield of hydrogen. Regarding the

soot production, it appears that the values are quite low but the maximum quantities

are reached at low temperature and low steam/ethanol feed ratio [18][21]. So there

should not be problems working with high feed ratios. Low pressures can contribute to

avoid soot production and an operative pressure of 1 atm is appropriate [18].

During the course of the reaction carbon monoxide formation is possible as it is a

byproduct. This compound is strongly undesirable because it is a poison either as

environmental pollutant either for the PEMFCs. The charge that has to be fed to these

fuel cells should have a CO content below a few tens of parts per million (ppm) [7].

This is possible using a purification step or not producing CO as byproduct. In this case

water-gas shift reaction can be employed to generate further hydrogen [19][20]:

CO + H2O H2 + CO2 ΔH0298 = -40 kJ/mol (Eq. 1.2)

8

Ethanol, especially at low temperature, may decompose into acetaldehyde, ethylene,

methane and carbon oxides. The first may decompose, in turn, into methane and CO

and the second is a precursor of coke formation [10]. All the reactions that give

methane are exothermic since they are favoured at low temperatures so they may

occur before the beginning of the ethanol steam reforming. Furthermore, they have

the effect of a decrease of hydrogen production. Then, the formation of methane must

be avoided because is counterproductive [22]. Moreover, the produced hydrogen may

give hydrogenated byproducts, which also cause a decrease of the amount of the

hydrogen obtained. Hydrogenation reactions are exothermic too and could be

prevented by choosing a selective catalyst.

1.3.1 Catalytic Pathways

The ethanol steam reforming is a complex reaction which involves a lot of different

catalytic pathways depending on the kind of catalyst and the operative conditions. The

first step is the absorption of ethanol on the catalyst [23], which can undergo

dehydration to ethylene, dehydrogenation to acetaldehyde, acetone formation [24]

and decomposition to methane, carbon monoxide and hydrogen.

CH3CH2OH → C2H4 + H2O ΔH0298 = 45 kJ/mol (Eq. 1.3)

CH3CH2OH → CH3CHO + H2 ΔH0298 = 68,4 kJ/mol (Eq. 1.4)

2CH3CH2OH → CH3COCH3 + CO + 3H2 ΔH0298 = 141,7 kJ/mol (Eq. 1.5)

CH3CH2OH → H2 + CO + CH4 ΔH0298 = 49 kJ/mol (Eq. 1.6)

At this point, the products formed in the first step can further react with their own

pathways. Acetaldehyde and methane can undergo steam reforming and

decomposition [23][25], while acetone can undergo steam reforming [25]:

CH3CHO + H2O → 3H2 + 2CO ΔH0298 = 187,1 kJ/mol (Eq. 1.7)

CH3CHO → CH4 + CO ΔH0298 = -18,8 kJ/mol (Eq. 1.8)

2 CH3CHO → CH3COCH3 + CO + H2 ΔH0298 = 5,3 kJ/mol (Eq. 1.9)

9

CH4 + H2O → 3H2 + CO ΔH0298 = -206 kJ/mol (Eq. 1.10)

CH4 → 2H2 + C ΔH0298 = 75,6 kJ/mol (Eq. 1.11)

CH4 + 2H2O → CO2 + 4H2 ΔH0298 = 164,4 kJ/mol (Eq. 1.12)

CH3COCH3 + 5 H2O → 8 H2 + 3 CO2 ΔH0298 = 246 kJ/mol (Eq. 1.13)

The formation of ethylene is usually followed by polymerization and formation of coke

[23]. This is absolutely undesired since coke can deactivate the catalysts by deposition

and covering the active sites.

C2H4 → polymers → 2C + 2H2 ΔH0298 = -52,1 kJ/mol (Eq. 1.14)

The carbon monoxide formed in the steps showed can in turn react through the water-

gas shift reaction (WGS, Eq. 1.2) and the Boudouard reaction [23]:

2CO → CO2 + C ΔH0298 = -171,5 kJ/mol (Eq. 1.15)

Beside ethanol steam reforming, high temperature can favour another reaction

involving ethanol and water. This reaction is not desired because it leads also to the

formation of CO [23]:

CH3CH2OH + H2O → 2 CO + 4 H2 ΔH0298 = 256 kJ/mol (Eq. 1.16)

10

Figure 1.4 Catalytic pathways of ethanol steam reforming [26]

1.4 Catalysts for ESR: Why RhPd/CeO2?

The Pd-Rh/CeO2 catalyst has been chosen as it has been demonstrated to have all the

requirements for the good performance of the reaction, namely: dissociation of the

ethanol carbon-carbon bond, capacity to keep low the carbon monoxide concentration

and stability under catalytic operative conditions. The simultaneous presence both of

palladium and rhodium allows the catalyst to be appropriate for this purpose.

Rhodium is able to break the ethanol C-C bond and palladium enhances the water gas

shift reaction and the hydrogen-hydrogen recombination reaction [1][27][28]. In this

way high conversions of ethanol and carbon monoxide are possible and the reaction

products are the desired ones.

The use of an appropriate support is essential since it has been proved that the

interaction established between the metal and the support itself (called Strong Metal-

Support Interaction, SMSI) prevents metal sintering and reduces coke formation.

Among the various supports, ceria is preferred due to its redox and oxygen storage

properties. Over it, the reaction proceeds following three main steps which follow one

another with the increase of temperature. They are: ethanol decomposition, water-gas

11

shift reaction (WGS) and methane steam reforming (MSR) [29][30]. The better catalytic

activity exhibited by rhodium and palladium supported on ceria nanocubes and

nanorods is related to the relative strength of the bimetallic-oxide support interaction

and to the enhanced capability of metal reorganization over {1 0 0} and {1 1 0}

crystallographic planes of ceria respect to {1 1 1} planes [1].

The deactivation processes such as the coke deposition, which occur during the

catalytic tests, can be completely reverted by re-oxidation of the catalyst [30].

However, this step is quite delicate because it has been found out that structural

changes in the support can occur only at temperatures above 500°C [31].

The nanoshape dependent catalytic activity of ceria has also been studied for other

reactions such as NO reduction [32], CO oxidation [33], WGS [34], soot combustion

[35] and hydrogen oxidation [36]. Except in the last case, the catalytic activity of

nanorods has been proved to be higher than that of the other nanoshapes because of

a better capacity to anchor, stabilize and disperse the metal and because of a better

strong metal-support interaction. For hydrogen oxidation, the order of reactivity is

similar to the reported one for ethanol steam reforming, so nanocubes are more

reactive than nanorods and both are better than polycrystals.

The morphology dependence of ceria-based catalyst has been studied in methane dry

reforming over Ni/CeO2 [37], methanol steam reforming over Au/CeO2 [38], ethanol

oxidative steam reforming over Rh/CeO2/Al2O3 [39] and ethanol steam reforming over

Co/CeO2 [40]. In the first two cases nanorods proved to be better than nanocubes and

polycrystals for the same reasons written above. Regarding ethanol reforming, the

behaviour of nanocubes and nanorods seemed to be similar, showing a decreasing

activity with time due to an alteration of ceria morphology.

Ethanol steam reforming has also been tested with catalysts whose active phase was

different from Pd-Rh. Co3O4 [41], Ru [42], Ni [43] and Pt-Ni [44] showed a different

behaviour from each other. Cobalt oxide proved itself to be very active allowing to

achieve high ethanol conversions and high hydrogen yields; Ru presented sintering

after prolonged high temperature aging and presented a small amount of coke; Ni

12

capacities resulted improved by adding oxygen to the feed; Pt-Ni turned out to be a

stable catalyst but conversion and yield were not so high.

Considering the rhodium-palladium alloy not so many studies have been already

conducted. However, according to Vedyagin et al. [45], the stability, especially to

reduction, of the alloyed Pd–Rh catalysts was found to exceed that of the single-metal

and mechanically mixed samples with identical metal concentrations. This could so be

an interesting starting point and now we are going to consider catalysts whose metallic

part is composed with a Pd-Rh alloy particles supported on the same nanoshaped

ceria, in order to compare activity, hydrogen yield and resistance to deactivation. The

composition and the size of these Pd-Rh alloy particles are already established and

constant so that the different behaviour of ceria nanoshapes can be compared to the

previous results.

1.4.1 Metallic active part

1.4.1.1 Noble Metals-based Catalysts

Many catalysts have been studied in order to find the best one. In most cases the

emerged problem was the deactivation due to coke formation and its deposition over

the catalyst. For this reason rhodium and ruthenium turned out to be the best

transition metals since they are able to break the C-C bond without excessive coke

formation [29].

With the progress of studies, rhodium demonstrated to be better than ruthenium

because the latter induces dehydration of ethanol by forming ethylene so to lead the

formation of coke by polymerization [42]. Moreover, ruthenium capacity to produce

hydrogen is comparable to the rhodium, but only for high quantities of metal. Since Ru

is more expensive than Rh, this is an evident disadvantage.

Other transition metals showed different abilities. For example platinum

demonstrated to have a strong oxidation activity, to promote the water-gas shift

reaction and to be selective towards hydrogen [46]; palladium has a good

dehydrogenation activity [1][27][28]; iridium turned out to be responsible for C-C bond

rupture in acetaldehyde with the formation of acetone [47]; silver, on the contrary,

showed a poor activity [42].

13

1.4.1.2 Nickel-based Catalysts

Nickel-based catalysts have been widely used in literature because of the high C-C

bond rupture capacity. A general observed result is that the selectivity towards

hydrogen increases with temperature, water/ethanol molar ratio and nickel loading

[48][49]. However, differently from noble metals, one of the most important

drawbacks of nickel is its tendency to sinter under reaction conditions [43][49].

Moreover, nickel showed other problematic behaviours, especially when deposited

over alumina. The acid sites of this support favour the dehydration of ethanol to

ethylene with the consequent formation of coke. Adding this phenomenon to the

metal sintering due to high temperatures, the result is a substantial deactivation of the

catalyst. To avoid these inconveniences, different promotors have been taken into

account and the best one turned out to be copper since it is active in the water-gas

shift reaction to produce hydrogen and avoid the growth of nickel particles by sintering

[50][51].

1.4.1.3 Cobalt-based Catalysts

Cobalt catalysts are less active for ethanol steam reforming than noble metals-based

catalysts but they are very selective towards hydrogen and carbon dioxide, since they

enhance the reforming at low temperature so that the WGS reaction occurs

simultaneously. In this way, the CO concentration is kept low and methane is not an

intermediate product allowing to achieve high hydrogen yields [26]. Cobalt has been

tested especially over Al2O3 and MgO but the catalyst turned out to be quickly

deactivated because of coke deposition [52][53]. Among the two supports MgO proved

to be a little bit more stable than alumina guaranteeing a better resistance to

deactivation. Other studies proved that cobalt suffers a phase transition at high

temperature besides coke deposition which is the best drawback of this kind of

catalysts [54][55]. Moreover, it is important to underline that the greatest part of the

studies performed considered diluted conditions instead of realistic ethanol-water

mixtures, which present a more relevant impact on coke formation. In order to

minimize this problem, researchers explored the possibility of adding promoters to the

14

metal. Na+ proved to be one of the best promoters since it is able to block the acid

sites of the support on which ethylene, and consequently coke, can be formed [56].

1.4.1.4 Bimetallic-based Catalysts

Among the different strategies, the best solution is the use of bimetallic catalysts since

they always proved better activity and versatility due to the synergic effect of more

than one metal. These bimetallic catalysts have not been studied as much as the

monometallic ones but the results are significant too. Rh-Pd/CeO2 catalysts are

characterised by both rhodium and palladium capacities. So while rhodium is able to

break the ethanol C-C bond even below 400 K, palladium enhances the

dehydrogenation and WGS reactions by favouring the hydrogen recombination

[1][27][28]. The possibility to work at lower temperatures is very interesting since

there is minor necessity of energy to run ESR. According to other bimetallic catalysts

we can find in literature few examples of Rh-Pt and Pt-Pd supported on ceria [57].

Dopants could also be employed to improve the catalytic performance. Potassium for

example is a very good promoter because it can make the catalyst more stable and

active [58]. Tin and lanthanum can be also used to achieve the same result [59].

1.4.2 Supports

A lot of supports have been tested for ethanol steam reforming. In monometallic

catalysts Al2O3, Al2O3-CeO2, CeO2, ZrO2, CeO2-ZrO2 were taken in consideration and the

ceria-zirconia mixed oxide exhibited the best performance proving to be the most

active. In fact zirconia improves ceria redox properties and reduces its basicity [60].

Moreover, a better thermal resistance was found and the metal dispersion on the

support resulted improved [61].

Concerning bimetallic catalysts, the previous works studied CeO2, Al2O3, ZrO2, SiO2,

Y2O3, MgO, TiO2 as supports. Zirconia showed a large oxygen storage capacity, a high

oxygen mobility and a steam activation ability [61]. Alumina, as known, has a very high

chemical and mechanical resistance but its acid nature promotes ethanol dehydration

into ethylene which easily gives coke [62]. The best support turned out to be ceria

according to its strong metal-support interaction which prevents sintering and reduces

the formation of carbonaceous species avoiding deactivation [29][30]. Moreover, ceria

15

participates to the reaction improving catalytic activity since it is active for the reaction

too. The metal-support interaction plays a pivotal role in the reorganization of metal

atoms on the catalyst support under the operative conditions and this effect has a

strong influence on the catalytic performance [63].

The ceria crystallographic structure could be formed by nanorods, nanocubes and

polycrystals which are different kind of nanoshapes characterised respectively by {1 1

0}, {1 0 0} and {1 1 1} crystallographic planes. The commercial ceria usually findable on

the market is composed by polycrystals. Nevertheless, nanocubes and nanorods could

be synthetized in laboratories. Theoretically it is also possible to switch from a

nanoshape to another by adding adequate ions into the hydrothermal treatment,

which is necessary to prepare nanoshaped ceria [64]. In fact, nanorods could be

converted into nanocubes by the addition of an appropriate amount of NO₃⁻ ions into

the hydrothermal treatment. Br⁻, I⁻, and SO₄2⁻ ions have roles similar to Cl⁻ ions, which

lead to the formation of nanorods. Moreover, the introduction of BrO₃⁻ ions can bring

to the generation of irregular nanoparticles.

The catalytic activity of ceria is due to the formation of oxygen vacancies related to the

high oxygen storage capacity and oxygen mobility of the material. These vacancies

create structural imperfections, defects, which increase the exposed surface giving

more catalytic material to the reaction. The result is an improvement of the catalytic

activity because the more the vacancies, the higher the activity [65]. The oxygen atoms

that generate the vacancies are called “lattice oxygen” since they belong to the crystal

lattice of the material. As a result, it is evident that the catalytic activity of the different

ceria nanoshapes depends on the formation velocity of the vacancies. According to

Weng et al. [66] the required energy to form these imperfections is different for each

form and that required on the {1 1 1} surface is higher than those necessary on {1 0 0}

and {1 1 0} ones. For this reason polycrystals are less reactive than nanocubes and

nanorods. Moreover, it has been demonstrated that the catalyst preparation method

may affect the concentration of larger oxygen vacancies [67]. A direct relationship

between the concentration of the larger size oxygen vacancies clusters and ceria

reactivity has been proved too.

16

It must be said that the catalysts dealt with until this time have been used for

laboratory tests with pure ethanol and model ethanol/water mixtures. The direct use

of raw bioethanol is still a problem because it has not been already found a catalyst

resistant enough to the impurities contained in it. In fact bioethanol, as every natural

resource, contains a great variety of both inorganic and organic impurities, which

involve a lot of functional groups that are too dangerous for the catalyst under the

reaction conditions. For this reason the catalysts tend to deactivate themselves since

they are not enough stable [68][69][70].

1.4.2.1 Cerium Dioxide

The cerium(IV) oxide, better known as cerium dioxide or ceria, is an oxide of the rare

earth metal cerium. It is a solid powder characterised by a pale yellow-white ivory

colour and its molecular formula is CeO2. Like every chemical substance, it is classified

by a CAS number which allows to identify it: 1306-38-3.

Cerium dioxide has a cubic face centred crystallographic structure also known as

fluorite structure. At high temperatures this structure can be reduced to a non-

stoichiometric form characterised by a dark blue-black colour. This form shows both

ionic and electronic conduction with ionic being the most significant at temperature

above 500°C.

Concerning its chemical and physical properties, it is a simple compound, absolutely

not dangerous. It is not flammable or explosive, it has a very high melting point (>

2400°C) and its vapour pressure is very low, so that cerium dioxide volatility is

negligible. It presents a very low solubility in water while it is moderately soluble in

strong mineral acids. Furthermore, ceria is slightly hygroscopic and absorbs moisture

and carbon dioxide from atmosphere. For this reason it should be stored in dry

conditions in order to avoid this possibility.

Cerium dioxide is regularly classified according to the REACH regulation and it is

considered a safe substance both for human health and environment. For this reason it

is not marked with labels indicating pictograms, signal words or precautionary

statements like the CLP regulation imposes.

17

Ceria is widely used nowadays for many applications. It is mainly employed as catalyst

or catalyst support but it can be also used as high precision polishing agent for glass

products, as raw material for the production of glasses and ceramics and as additive

for varnishes and paints. In the doped form ceria is an interesting material for

electrolytes in solid oxide fuel cells (SOFCs) because of its quite high ionic conductivity

of oxygen between 500 and 800°C. Cerium dioxide has the capacity to absorb

ultraviolet radiation so it should be used like replacement of zinc oxide and titanium

dioxide in sunscreen since it has lower photocatalytic activity. However, as already

said, the main application for ceria is catalysis. It can be used in catalytic converters in

automotive applications thanks to its ability to reduce NOx and to oxidize carbon

monoxide to carbon dioxide. It is widely used as co-catalyst or catalyst support in many

reactions such as ethanol steam reforming (ESR), water gas shift (WGS), Fischer-

Tropsch reaction and selective oxidation. Its catalytic activity depends on oxygen

vacancies since the higher the structural defect concentration, the higher the activity.

18

2 EXPERIMENTAL WORK

2.1 Preparation and characterization of catalysts

As already discussed, the catalyst is composed by rhodium-palladium alloy

nanoparticles deposited on nanoshaped ceria supports. These supports are prepared

in laboratory using a stainless steel batch autothermal reactor which is filled with the

precursor suspension. The operative conditions are different, depending on the

desired nanoshape. The metallic nanoparticles are then deposited on the support by

incipient wetness impregnation.

2.1.1 Preparation of the ceria supports

Nanorods and nanocubes are very similar to prepare. The main differences are the

molarity of the precursor solutions and the temperature of the reactions. Nanorods

are obtained when the OH- concentration in the precursor suspension is between 6-9

M while nanocubes require a 6 M OH- solution [71]. Moreover, the suspension had to

be loaded in a reactor where the reaction is conducted at 100°C or at 180°C to

synthetize nanorods and nanocubes, respectively [71]. It is very important to keep

constant these temperature values during the synthesis, since an intermediate value of

temperature could provide a mixture of nanorods and nanocubes, which is absolutely

undesirable taking into account that the goal of this work is a comparison of the

catalytic performances between the different nanoshapes. According to Divins et al.

[1], nanorods are synthetized in a 7,9 M NaOH solution and 0,05 M Ce(NO3)3. Initially,

6,08 g of Ce(NO3)3·6H2O are dissolved in 35 ml of deionized water and 88,133 g of

NaOH are dissolved in 245 ml of the same type of water. Then, both the solutions are

mixed using an electrospray, which generates an aerosol of very small liquid droplets

of the first solution that is added in the second one in order to favour the dispersion

and nucleation of nanostructures. To achieve an optimal mixing and prevent

aggregation of the generated colloidal particles, the second solution is vigorously

agitated by a magnetic stirrer (300-400 rpm), which allows an appropriate uniformity

during the mixing. After the addition of the aerosol, the resultant suspension is kept

under agitation for 30 minutes, obtaining 280 ml with the desired molarity of NaOH.

19

The quantities are chosen according to the maximum volume of liquid that can be

loaded in the available reactor (Figure 2.1). The precursor suspension is then loaded to

it and the hydrothermal reaction was conducted for 24 h.

Figure 2.1 Hydrothermal reactor.

Nanocubes are obtained following the same process but, in this case, the precursor

suspension contains a final molarity of 6 M NaOH and 0,05 M Ce(NO3)3. Consequently,

6,08 g of Ce(NO3)3·6H2O are added to 35 ml of deionized water and 67,23 g of NaOH

are added to 245 ml of H2O. Their mixture, following the aforementioned

methodology, gives the desired precursor suspension.

The preparation of nanocubes and nanorods gave satisfactory results since an

acceptable amount of powder was produced for both nanoshapes.

Regarding polycrystalline ceria nanoparticles, five experiments were performed in

order to obtain a satisfactory result. According to Mai et al. [71] a 0,01 M NaOH final

molarity in the precursor suspension was required. Therefore, a solution obtained

dissolving 6,083 g of Ce(NO3)3·6H2O in 35 ml of deionized water was added in a

solution obtained dissolving 0,114 g of NaOH in 245 ml of deionized water. The

solutions were mixed following the same steps performed for the synthesis of

nanocubes and nanorods. However, the final amount of solid powder was very

exiguous. In the second test, the idea was to start from a more concentrated sodium

hydroxide solution, keeping the same final molarity but reducing the total volume of

the solution. So, 6,077 g of Ce(NO3)3·6H2O were dissolved in 35 ml of deionized water

20

and 3,018 g of NaOH were dissolved in 50 ml of H2O in order to obtain a solution of

1,55 M NaOH. Then, 1 ml of this last NaOH solution was diluted to a final volume of

120 ml in water and the resultant solution was mixed with the cerium nitrate solution,

obtaining the desired final molarity of 0,01 M NaOH. Also in this case the amount of

solid was scarce. In the third experiment we modified the protocol according to Wu et

al. [72] where Na3PO4·12H2O was used instead of NaOH and where the reaction time

was 15 h instead of 24 h. Since this compound was not available, we used

NaH2PO4·2H2O monobasic. Compared to the article, the quantities were multiplied per

7 so to reach the usual value of 280 ml as final volume. The phosphate solution was

obtained dissolving 0,011 g of NaH2PO4·2H2O monobasic in 260 ml of deionized water.

Measuring the pH of the NaH2PO4 solution it was found that it was between 6 and 7

when it should have been 12 to favour the precipitation of the solid. So, the phosphate

solution was adjusted to a pH 12 with 1 M NaOH. Then, 3,044 g of Ce(NO3)3·6H2O in 20

ml of deionized water were added to the phosphate solution using the electrospray

previously mentioned. During the stirring we added 7,5 ml of 1 M and 2 M sodium

hydroxide solutions (pH=14) to adjust the pH to the desired value. This experiment

provided, finally, a satisfactory quantity of solid. In the fourth experiment we tried to

make a new attempt using again NaOH but ensuring to reach the necessary pH to

achieve precipitation of colloidal ceria. An exploratory test was performed preparing a

number of solutions mixing different concentrations of NaOH in deionized water and

Ce(NO3)3, observing that the required minimum molarity of NaOH to achieve

precipitation was ca. 0,2 M. According to this result we dissolved, as usual, 6,066 g of

Ce(NO3)3·6H2O in 35 ml of deionized water and 1,964 g of NaOH in 245 ml of deionized

water. Then we conducted the reaction at 180°C for 15 h. The final amount of powder

was acceptable but a SEM analysis showed that these polycrystals were not perfect

octahedra and this is the reason which led to the fifth trial. For this experiment we

mixed the solutions obtained dissolving 6,072 g of Ce(NO3)3·6H2O in 35 ml of deionized

water and 1,988 g of NaOH in 245 ml of deionized water. Then we conducted the

reaction at 150°C for 24 h, in the usual hydrothermal reactor, so as to provide more

time for the completion of the reaction in order to obtain polycrystals. However, the

21

SEM analysis of the calcined powder resulting from the fifth trial gave exactly the same

result of the previous attempts.

The theoretical yield of CeO2 powder expected after calcination is assessable in this

way:

6,076 g Ce(NO3)3·6H2O · 1 𝑚𝑜𝑙 Ce(𝑁𝑂3)3·6𝐻2O

𝑃𝑀 Ce(𝑁𝑂3)3·6𝐻2O ·

1 𝑚𝑜𝑙 𝐶𝑒𝑂2

1 𝑚𝑜𝑙 Ce(𝑁𝑂3)3·6𝐻2O ·

𝑃𝑀 𝐶𝑒𝑂2

1 𝑚𝑜𝑙 𝐶𝑒𝑂2 =

g CeO2 g CeO2 = 2,41 g (Eq. 2.1)

However, since it is logical to expect loss of solid during the various steps of

preparation, especially during the washing steps, the final amount of CeO2 is less than

the theoretical value. Neglecting the first two attempts to obtain polycrystals, the final

amount of nanorods is 2,372 g, the nanocubes one is 1,673 g, the third experiment

with polycrystals has given 1,167 g of solid, the fourth 2,265 g and the last one 2,125 g.

After the hydrothermal reaction to synthetize nanocubes, nanorods or polycrystals, a

solution containing a white precipitate at the bottom was obtained. The next step was

to wash the precipitate using an ultrasound bath and a centrifuge machine (Figure 2.2)

to remove all the sodium hydroxide present in the solution, washing thrice with

deionized water and twice with ethanol. Every centrifugation was conducted at 15°C,

6000 rpm and for 10-15 minutes depending on achieving a good separation.

Figure 2.2 Centrifuge machine.

In between each centrifugation step, the suspension was immersed in an ultrasound

bath with the aim of aid the washing steps and break up particle aggregates. The solid

22

recovered after the washing process was characterised by a pale-yellow colour and

finally it was dried overnight at 55°C to eliminate all the moisture. At this point the

solid presented agglomeration so that it was necessary to grind it with an agate mortar

in order to obtain a fine powder and, consequently, a higher exposal surface. The

following step was the calcination of this solid powder in a furnace. This process was

conducted at 450°C for 4 h and the result was a pale-yellow powder (Figure 2.3).

Figure 2.3 Ceria nanocubes (C), nanorods (R) and polycrystals (P).

The table 1 summarizes the parameters related to the experiments performed to

obtain the different supports:

Nanoshape M[mol/L] T[°C] t[h] g Ce(NO3)3·6H2O g NaOH

nanorods 7,875 100 24 6,08 in 35 mL 88,133 in 245 mL

nanocubes 6 180 24 6,08 in 35 mL 67,23 in 245 mL

polycrystals 1 0,01 180 24 6,083 in 35 mL 0,114 in 245 mL

polycrystals 2 0,01 180 24 6,077 in 35 mL 3,018 g in 50 mL of which 1 mL in 119 mL

polycrystals 3 0,0003 170 15 3,044 in 20 mL 0,011 in 260 mL (NaH2PO4·2H2O

monobasic) polycrystals 4 0,2 180 15 6,066 in 35 mL 1,964 in 245 mL

polycrystals 5 0,2 150 24 6,072 in 35 mL 1,988 in 245 mL

Table 2.1 Preparation conditions of the different supports

23

2.1.2 Impregnation of metallic nanoparticles on ceria supports

The final step in the preparation of the catalysts required the deposition of the

metallic, active nanoparticles on the synthetized supports. As previously said, these

nanoparticles are composed by a preformed rhodium-palladium alloy and the method

used to deposit them on the supports is the incipient wetness impregnation. The

metallic nanoparticles were provided from the research group of Dr. Imma Angurell

(Inorganic Chemistry Dept., University of Barcelona) to us in vials containing dark

brown solutions, which are made dispersing 20 mg of Rh-Pd alloy nanoparticles (molar

ratio 1:1) in 6 ml of toluene.

The catalysts have been prepared with a metal loading of the 3% in weight. To do so,

taking into account the available quantity of metallic nanoparticles, we weighted 400

mg of nanocubes, nanorods and polycrystals (we chose the polycrystals produced in

the fourth test) to consume 12 mg of metal for each nanoshape. In fact, 12 mg of Rh-

Pd nanoparticles correspond to 3,6 ml of suspension, so 10,8 ml were used.

The three different nanoshapes were impregnated at the same time in order to

dispose exactly the same amount of nanoparticles on each support, with the aim of

making an accurate comparison during the catalytic tests.

The incipient wetness impregnation was performed using a micropipette that is able to

transfer 50 μl of liquid and to draw off it on the support as small droplets. This step

had to be repeated 72 times to reach the desired amount of 3,6 ml for each sample. It

is very important to release the solution in form of droplets to control the real

absorption of the liquid in the solid powder. For this reason the impregnation was

conducted adding small fractions of 50 μl at each time. When the powder was wet,

because it could not absorb all the solvent, the sample had to be heated up for several

minutes at ca. 75°C in order to evaporate the excess of toluene. Once dried, the

powder could be impregnated again. Since the density of nanorods is lower than

nanocubes and polycrystals, the suspension was easily absorbed and the drying steps

were required only for nanocubes and polycrystals.

24

After impregnation, the solid powder was dried again at 80°C for 1 h to remove all

traces of toluene and calcined at 300°C for 6 h to fix the metallic nanoparticles on the

surface of nanoceria.

Due to possible formation of agglomerates during the impregnation step of the

powder, the samples must be ground with an agate mortar in order to obtain again a

fine powder and to homogenise it to have an equal distribution of the metallic

nanoparticles. The final result was a grey solid powder (Figure 2.5).

Figure 2.4 Impregnated catalyst.

2.1.3 Characterization of ceria supports

The effective presence of the desired nanoshapes was analysed by Scanning Electron

Microscope (SEM) characterization. This microscope is able to provide images of the

external surfaces of the supports with a good resolution. Thus, it is possible to see

their morphology and to recognize the different kinds of nanoshapes.

The SEM microscope can host samples which are supported on a very thin sample-

holder suitable for the vacuum chamber. In order to prepare a sample, a very small

amount of solid powder is added in an 1,5 ml “Eppendorf tube” that is afterwards filled

with ethanol. The tube is then placed in an ultrasonic bath to obtain a homogeneous

suspension. A droplet of this suspension was placed on a silicon wafer with a

micropipette. The sample was dried at atmospheric conditions during 1-2 minutes

(ethanol evaporates very quickly) in order to form a monolayer of nanoparticles. The

silicon wafer was stuck to the sample-holder with a silver ink glue, which is electrical

conductor. In this way the electron beam can pass through the sample without charge

it, otherwise the resulting image would not be sharp.

25

2.1.3.1 Scanning Electron Microscope (SEM)

This microscope is able to provide images of a sample scanning it with a focused beam

of electrons (0,5 – 30 kV), which interact with the atoms of the sample. These atoms

are excited by the beam and they emit other electrons, called secondary electrons,

which are collected by a detector in order to obtain the information about the

morphology of the sample and, consequently, the microscopic image. The number of

secondary electrons detected depends on the angle of incidence between the scanning

beam and the surface of the sample. The more the secondary electrons, the clearer

the image obtained.

The electron beam is very narrow, so SEM micrographs have a large depth of field. This

allows to produce a three-dimensional image useful for a better knowledge of the

sample’s structure.

Most of the SEMs present a detector for back-scattered electrons too. These electrons

belong to the main beam but they are reflected due to the interaction with the atoms

of the sample. Since heavy elements characterized by a high atomic number reflect

more electrons than the light ones, they are better reproduced. A contrast between a

clearer zone and a not well identified one can provide an idea of the chemical

composition of the sample. A scheme of a SEM functioning is shown in Figure 2.4.

Figure 2.5 Scheme of SEM functioning (C=cathode, A=anode, L=lens, S&D=sample and detector, M=monitor).

SEMs allow to reach very good resolution images and samples can be analysed until 1-

5 nm. The higher resolution is achieved with a low intensity of the electron beam (<1

kV).

26

The scanning electron microscope is cheap and it is a non-destructive technology so it

is widely used especially for a preliminary study of the surface of a sample. However,

the maximum resolution is not comparable with the TEM’s one which allow a more

careful study of the sample.

2.1.4 Characterization of impregnated catalysts

A high resolution transmission electron microscope (HRTEM) JEOL JEM 2010F

equipped with a field emission source at an accelerating voltage of 200 kV, has been

used to analyse the quality of the different synthetized catalysts. The high resolution of

this microscope allows to examine every single metallic nanoparticle deposited on the

supports. The point-to-point resolution achieved was 0,19 nm and the resolution

between lines was 0,14 nm.

The preparation of the sample for the analysis is very similar to the one made for SEM

microscope. The difference is that the droplet of the suspension of the powder in

ethanol is placed on holey carbon-coated grids instead of a silicon wafer.

2.1.4.1 High Resolution Transmission Electron Microscope (HRTEM)

This microscope allows to study the morphology and the properties of a material on

the atomic scale reaching resolutions comparable to one Ångstrom. The instrument is

able to analyse every type of material but the sample must not release gases since the

HRTEM works in high-vacuum atmosphere.

The image is produced by high-energy electrons transmitted on a fluorescent screen or

a photographic film. Electrons are preferred to photons since they can move on

wavelengths (3,7x10-3 nm) smaller than those of light. This allows a very high

resolution magnification.

Electrons sent by an electron gun reach the sample passing through condenser lenses.

Then the beam pass through an objective lens, which provides an intermediate image,

and meet the projector lens which gives the final picture on the monitor. Figure 2.6

shows the functioning scheme of a HRTEM.

27

Figure 2.6 Scheme of HRTEM functioning.

2.2 Set up of the reaction system

Before starting the catalytic tests, it is important to set up properly the reaction system

in order to run the catalytic tests easily and safely. For this reason the experimental

setup is placed inside a fume hood. The reaction system is composed as follow in

Figure 2.7.

28

Figure 2.7 Representative diagram of the reaction system.

The reagents are contained into two bubblers, one for ethanol and one for water,

which are disposed in two water baths to maintain them at the desired temperature.

The bubbler containing ethanol is kept at 35°C and the one containing water between

58 and 59°C. In this way the vapour pressures of the two compounds are 100 mmHg

and about 140,7 mmHg, respectively.

The bubblers are fed with argon, which creates bubbles in the liquids and allows to

obtain the desired flows needed for the reaction. According to Divins et al. [1], the

steam/ethanol molar ratio must be 6 so, choosing an ethanol volumetric flow of 1,5

ml/min, the water one should be 9 ml/min. To get these flows 9,9 ml/min of argon are

29

needed for the ethanol bubbler and 39,6 ml/min are required for the water one. In this

way, the total flow is 60 ml/min. In order to have the desired reagents’ flows from the

beginning, a saturation step of the bubblers with argon is necessary before reaction.

Reagents’ flows are sent to a three-way valve, which remains open during the catalytic

tests while it is closed during the reduction step and during the saturation of the

liquids inside the bubblers. In this case the outlet gases are expelled to the atmosphere

and, therefore, they are aspirated by the fume hood.

The connection between the three-way valve and the reactor is also coupled to

hydrogen and nitrogen gas sources, which provide these gasses for the reduction step

of the catalyst. According again to Divins et al. [1], this step must be performed with a

flow of 50 ml/min composed by 5 ml/min of hydrogen and 45 ml/min of nitrogen (10%

H2/N2).

The reactor used is a stainless steel tubular reactor which is located in an oven in order

to provide the required temperature for the reaction. The catalyst is placed in the

reactor with a handmade paper funnel and disposed between two layers of quartz

wool so to make a stable and homogeneous catalytic bed. The quartz wool is preferred

to glass wool because of the high temperature reached during the reaction. In fact, the

former wool is characterized by a better thermal resistance than the latter one.

The reactor outlet is connected to a condenser, which has the function to collect

eventual liquid droplets still present in the gas flow, because we have to remove

liquids before analysing the reaction products in the gas chromatograph. The final

gaseous outflow is finally sent to a gas chromatograph in order to analyse and quantify

every compound contained in it.

Every connection pipe is heated thanks to a heating stripe wrapped around them. In

order to prevent heat losses and any risk for health, an insulating band, made of glass

fibres, covers the heating stripe. The experimental setup is shown in Figure 2.8.

30

Figure 2.8 Reaction system.

Once prepared the reaction system, it is important to regulate the instrumentation

such as the temperature of the water baths and the mass flow controllers to get the

desired gas. The former are easy to regulate since a simple measure with thermometer

is enough to reach the required temperature but the latter are more complex because

the real values of gas flows have to be checked for each mass flow controller

measuring the time, with a chronometer, of the displacement of a soap bubble in a

graduate cylinder, in which a particular gas is flowing.

2.2.1 Gas Chromatograph

The gas chromatograph used to perform the catalytic tests is a micro GC (Agilent

3000A) equipped with MS 5A, Plot U and Stabilwax capillary columns and TCD

detectors. The heart of the instrument is the GC unit which is composed by the

injector, the capillary columns and by thermal conductivity detectors (TCD). Particles

and liquids are not allowed to be injected in this particular GC model, and two

membrane filters are placed before the injector to remove particles and liquid droplets

from our gas simple.

To start the analysis, a valve is open in order to allow the gas flow entering the loop

that will be injected to the capillary columns. The three columns of the gas

chromatograph are characterized by the capacity to detect different compounds. In

fact, the first column (Plot U) is able to detect, among the compounds of interest for

31

this work, carbon dioxide, ethane, ethylene and acetylene; the second one (Stabilwax)

detects water, ethanol, acetone and acetaldehyde; the third one (MS 5A) detects

instead hydrogen, methane and carbon monoxide. The gas chromatograph is able to

work because different volatile molecules have unique partitioning characteristics

between the columns’ substrate and the carrier gas. These differences allow

component separation and following detection. The detection happens because the

separated gases pass over different hot filaments where the varying thermal

conductivity of sample molecules causes a change in the electrical resistance of the

sample filaments when compared to the reference or carrier filaments. For every

compound detected there is a peak on the baseline of the chromatogram. The position

of the peaks, i.e. the retention time, indicates the chemical compound and the area of

these peaks gives the amount of the relative compounds. The exact amount of each

substance is automatically calculated by integrating the area under the peak and

interpolating this value in the corresponding calibration curve, which was previously

measured and recorded in the GC software.

The results given by the equipment for each analysis are percent composition values of

each compound related to the total gas flow. The time between each injection is ca. 5

minutes.

2.3 Catalytic tests

2.3.1 Temperature-ramping tests

The catalytic evaluation of the ethanol steam reforming is performed in a range of

temperature between 600-1050 K and atmospheric pressure.

After loading the catalyst into the reactor, an activation treatment is necessary before

the reaction starts. It is carried out at 573 K for one hour, flowing 5 ml/min of

hydrogen and 45 ml/min of nitrogen (10% H2/N2). The appropriate value of

temperature is reached applying a temperature ramp starting from room temperature

and increasing it with a speed of 5 K per minute.

After the reduction step, the oven temperature is brought to 600 K keeping flowing

nitrogen in the reactor and applying again a 5 K/min temperature ramp. At the same

32

time the thermal baths containing the bubblers with the reagents are heated up to the

desired temperature (35°C for ethanol and about 58,5 for water), the heating stripe to

keep hot the gas pipelines of the reaction system is turned on, and the bubblers

containing the reagents are saturated with argon in order to obtain the desired flows

for the reaction. It must be noted that the initial flow is composed by 60 ml/min of

which 1,5 ml/min are ethanol and 9 ml/min are water, according to the fixed

ethanol/steam molar ratio of 6. The thermal bath containing the bubbler with ethanol

is kept at 35°C in order to have a vapour pressure of 100 mmHg. Consequently, the

required argon flow to feed the reactor with the desired amount of ethanol is

calculated by:

FArEtOH = 𝐹𝐸𝑡𝑂𝐻 (760− 𝑃𝑣

𝐸𝑡𝑂𝐻)

𝑃𝑣𝐸𝑡𝑂𝐻 (Eq. 2.2)

and the result is 9,9 ml/min. In parallel, the quantity of argon destined to the bubbler

with water is evaluated as:

FArH2O = Ftot – FEtOH – FH2O - FArEtOH (Eq. 2.3)

and it is 39,6 ml/min. At this point it is possible to calculate the vapour pressure of the

water contained in the bubbler as:

PvH2O = 760− 𝐹𝐻2𝑂

(𝐹𝐻2𝑂+ 𝐹𝐴𝑟𝐻2𝑂)

= 140,74 mmHg (Eq. 2.4)

The temperature of the thermal bath with the bubbler containing water is evaluated

as:

TH2O = 1,0658+ √1,06582 −4 ·0,0561(11,454− 𝑃𝑣

𝐻2𝑂)

2 ·0,0561 = 58,44°C (Eq. 2.5)

which is the solving formula of the second degree equation:

0,0561· TH2O2 + 1,0658· TH2O + 11,454 – PvH2O (Eq. 2.6)

33

coming from the plot of data found in handbooks [73][74].

Once the temperature is 600 K, nitrogen is disconnected, the three-way valve is

opened to allow to the reagents to pass through the reactor and the analysis with the

gas chromatograph is started. The 600 K are maintained for 30 minutes and then a

new ramp is performed in order to bring temperature to 1050 K with a speed of 1 K

per minute. The final temperature of 1050 K is also maintained for 30 minutes to

stabilize the results given by the gas chromatograph.

In this work, the analysis of the obtained results is based on the evaluation of the

ethanol conversion during the reaction, the hydrogen yield and the selectivity towards

hydrogen production. Each calculation is made considering molar flows, so, the first

step in the analysis is the estimate of the molar flows for every compound coming out

from the reactor. To do this the following equivalence is used:

1 mol = 24,4 L (Eq. 2.7)

which is valid for every gas in the hypothesis of ideal gas at 298 K and 1 atm. This

assumption is acceptable since the gas chromatograph works at atmospheric pressure

and room temperature. Consequently, the molar flows turn out to be:

Fi = % 𝑣𝑎𝑙𝑢𝑒

100 ·

𝐹𝑡𝑜𝑡𝑜𝑢𝑡

24400 (Eq. 2.8)

where 𝐹𝑡𝑜𝑡𝑜𝑢𝑡 is the total outflow expressed in ml/min. This value is estimated as 60

ml/min, a theoretical value assuming the moles variation negligible during the

reaction. Concerning the amount of ethanol entering the reactor, it is possible to

convert the theoretical value of 1,5 ml/min using again the same equivalence.

The conversion of ethanol during the reaction is evaluated using the conventional

formula:

χEtOH = 𝐹𝐸𝑡𝑂𝐻

𝑖𝑛 − 𝐹𝐸𝑡𝑂𝐻𝑜𝑢𝑡

𝐹𝐸𝑡𝑂𝐻𝑖𝑛 (Eq. 2.9)

The formation of coke, which is one of the main downside of the process, is evaluated

with a carbon balance:

34

𝐶𝑜𝑢𝑡

𝐶𝑖𝑛=

(𝐹𝐶𝑂𝑜𝑢𝑡+𝐹𝐶𝑂2

𝑜𝑢𝑡+𝐹𝐶𝐻4𝑜𝑢𝑡 +2·𝐹𝐶𝐻3𝐶𝐻𝑂

𝑜𝑢𝑡 + 3·𝐹𝐶𝐻3𝐶𝑂𝐶𝐻3𝑜𝑢𝑡 +2·𝐹𝐶2𝐻4

𝑜𝑢𝑡 +2·𝐹𝐸𝑡𝑂𝐻𝑜𝑢𝑡 )

2·𝐹𝐸𝑡𝑂𝐻𝑖𝑛 (Eq. 2.10)

Eq. 2.10 allows to compare the amount of carbon atoms coming out from the reactor

respect the initial ones given by the ethanol entering the reactor. The result is a

number included between 0 and 1 and its complementary indicates the quantity of

carbon atoms that leaded to the coke. The absence of coke is in correspondence of a

unitary value. Every compound is also multiplied by its number of carbon atoms.

The six compounds reported in brackets in Eq.2.10, except ethanol, are the main

products that contain carbon atoms. Other byproducts such as methanol, ethane,

acetylene and formic acid are detected in negligible quantities and they are so

neglected.

The hydrogen yield is calculated taking into account the stoichiometry of the ethanol

steam reforming reaction. According to it, six moles of hydrogen are obtained from

one mole of ethanol and the yield can be estimated as:

YH2 = 𝑛𝐻2

6 · 𝑛𝐸𝑡𝑂𝐻𝑖𝑛 (Eq. 2.11)

The selectivity is the ratio of the amount of the compound of interest to all those

reaction products outflowing from the reactor. The reaction products considered are

carbon monoxide, carbon dioxide, acetaldehyde, acetone, ethylene, methane and,

obviously, hydrogen. Considering the selectivity towards hydrogen, it can be calculated

as follows:

σH2 = 𝑛𝐻2

(𝑛𝐻2+ 𝑛𝐶𝑂+ 𝑛𝐶𝑂2+ 𝑛𝐶𝐻4+ 𝑛𝐶𝐻3𝐶𝐻𝑂+ 𝑛𝐶2𝐻4+ 𝑛𝐶𝐻3𝐶𝑂𝐶𝐻3) (Eq. 2.12)

2.3.2 Fixed-temperature tests

Once analysed the data recovered by temperature-ramping tests, it was clear that

further experiments were necessary in order to explain better the behaviour shown by

the catalysts tested.

The new idea was to increase the amount of catalyst loaded into the reactor, aiming to

improve activity, and to evaluate each catalyst at a constant temperature of 823 K in

35

order to compare their performance, and trying to avoid the formation of coke, which

is more significant at lower temperatures. Moreover, reagents were fed to the reactor

mixed with a known amount of nitrogen gas, which was used as internal standard. In

fact, knowing exactly the amount of nitrogen flowing through the reactor and detected

by the gas chromatograph, it makes possible to estimate in a more accurate way all the

molar flows of every compound from the percentages provided by the gas

chromatographic analysis. To do this a proportion was applied (as example hydrogen is

considered):

% H2 : X = % N2 : 𝐹𝑁2𝑜𝑢𝑡 X =

%𝐻2 · 𝐹𝑁2𝑜𝑢𝑡

%𝑁2 (Eq. 2.13)

The amount of catalyst chosen to run experiments was due to the remaining

quantities. The original idea was to double the 50 mg used previously but, since only

96 mg of nanocubes were still available, 96 mg had been used for every catalyst.

The reduction step was conducted exactly like in the previous tests and, after that, a

ramp of temperature of 5 K per minute was used to reach the desired value of 823 K.

At this point the temperature value was kept constant and the analysis with the gas

chromatograph is started. The goal was to achieve stabilization of the concentrations

detected and, so, a small number of runs was required.

The following analysis of the data was made as already explained in the previous

section.

2.4 Characterization of used catalysts

After testing the performance of catalysts, it is important to characterize the used

catalysts in order to observe eventual changes in their nanoshapes during the reaction

and to evaluate the coke deposition. To do so, the scanning electron microscope has

been used again.

The preparation of the samples suitable for the SEM was conducted as already

explained in the section 2.1.2.

36

3 RESULTS AND DISCUSSION

3.1 Characterization of the support

In order to verify the success of all the experiments performed we analysed the

samples with a scanning electron microscope (SEM).

Figure 3.1 and Figure 3.2 show how ceria nanocubes appear after their synthesis. We

can see that they present very different sizes. In fact, it is possible to see particles with

a side of 10 nm and others that reach 150 nm. However they are well defined and

there are no doubts about their purity since each nanoparticle seems to be cubic and,

so, it is possible to exclude the presence of different shapes.

Figure 3.1 SEM image of ceria nanocubes with a magnification of 125 K X.

37

Figure 3.2 SEM image of ceria nanocubes with a magnification of 200 K X.

Regarding nanorods, Figure 3.3 shows clearly that they are pure and very long (ca. 600-

700 nm).

38

Figure 3.3 SEM image of ceria nanorods with a magnification of 125 K X.

The synthetized polycrystals are very small (< 10 nm), as illustrated in Figure 3.4, even

smaller than nanocubes, and presenting undefined shape. They are pure and there are

not nanorods or nanocubes among them. However, it is impossible to observe the real

shape since they are too small for the SEM. Probably they are generic polyhedra and

different geometries could be present. Consequently the particles could present not

only the {1 1 1} crystallographic planes but also other ones.

39

Figure 3.4 SEM image of ceria polycrystals with a magnification of 250 K X.

3.2 Characterization of impregnated catalysts

In order to see the size and the dispersion of the metallic nanoparticles deposited on

the supports a HRTEM analysis was performed. As expected, the size of these particles

is constant (ca. 5 nm) and their dispersion on each nanoshaped surface is very high.

The fact of having everywhere the same size allows to compare the three different

nanoshapes’ activity while the high dispersion guarantees better resistances to

sintering and coke deposition, and a higher activity.

Moreover, this microscope is able to show also every atom individually, thanks to its

high resolution, and it is possible to evaluate the distance between them and

consequently the kind of crystallographic planes that characterize the structures of the

ceria and the metallic nanoparticles.

As shown in Figure 3.5, nanocubes have (2 0 0) crystallographic planes since the

distance between every plane of atoms is ca. 2,7 Å. It is possible to state this according

to the tables belonging to the “International Centre for Diffraction Data”, which report

the distance between each crystallographic plane for all the chemical elements or

compounds. The greatest part of the materials present (1 0 0) crystallographic planes

40

when the nanoshape involved is the one formed by nanocubes. Ceria is different

because this disposition is not possible. However nanocubes show a (2 0 0) disposition

which belongs the same to the {1 0 0} family. Consequently it is possible to say that

ceria nanocubes present {1 0 0} crystallographic planes. In the same picture it is easy to

see the metallic nanoparticles deposited on the support. As already said their

dimension is constant and comparable to ca. 5 nm.

Figure 3.5 TEM image of ceria nanocubes with RhPd nanoparticles on them.

Figure 3.6 shows a more accurate detail of a nanocube. It is possible to see every atom

and the distance between them is clearer. The symbology used, [1 0 0], indicates that

the nanocube is seen along the 1 0 0 crystallographic direction. In fact the frontal face

is characterized by (1 0 0) crystallographic planes, the lateral face by (0 1 0) ones and

the upper face by (0 0 1) ones. They all belong to the {1 0 0} family.

41

Figure 3.6 TEM image of ceria nanocube showing atoms and distances between them.

Figure 3.7 illustrates a RhPd nanoparticle, which is supported on a ceria nanocube, and

its atomic disposition. The distances between atoms depends on the metal and the {1

1 1} crystallographic planes are 2 or 2,2 Å.

Figure 3.7 TEM image of ceria nanocube (b) with RhPd nanoparticle (a) showing atoms and distances between

them.

The deposition of metallic nanoparticles over nanorods is shown in Figure 3.8, while in

the following one (Figure 3.9) it is possible to see the distance between nanorods’

atomic planes. In this case not only a family of crystallographic planes is present since

42

nanorods present both {1 1 0} and {1 1 1} ones. In fact, the distances between atomic

planes are different: 1,9 and 3,1 Å respectively.

Figure 3.8 TEM image of ceria nanorods with RhPd nanoparticles on them.

43

Figure 3.9 TEM image of a ceria nanorod with RhPd nanoparticle showing atoms and distances between them.

Figure 3.10 shows the deposition of metallic nanoparticles over polycrystals. As it is

possible to see, the obtained polycrystals have not a defined shape but they are

formed by particles different from each other. Consequently, it is logical to expect the

presence of different crystallographic planes even if the main family is the {1 1 1}.

Moreover, these polycrystals are clearly smaller than nanocubes (ca. 5-10 nm).

44

Figure 3.10 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms and distances between them.

Figure 3.11 and Figure 3.12 provide other views of the deposition of RhPd

nanoparticles over polycrystals. Also in this case the metallic nanoparticles are

characterized by (2 0 0) and (1 1 1) crystallographic planes while ceria present the {1 1

1} ones.

45

Figure 3.11 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms.

46

Figure 3.12 TEM image of ceria polycrystals with RhPd nanoparticles showing atoms and distances between them.

3.3 Catalytic tests

3.3.1 Temperature-ramping tests

3.3.1.1 RhPd/CeO2 (impregnated catalysts)

The hydrogen yield obtained over the impregnated nanoshaped catalysts during the

catalytic cycles is presented in Figure 3.13.

47

Figure 3.13 Hydrogen yield over impregnated catalysts during temperature-ramping tests.

In the graphic above, C, P and R represent the three different nanoshapes

(C=nanocubes, P=polycrystals and R=nanorods) during the first catalytic cycle, while

C2, P2 and R2 indicate the three nanoshapes during the second catalytic cycle. The

data are referred to the temperature-ramping tests but only some values are reported

in order to better compare the results to the ones obtained in the reference work [1],

where a similar graphic was used.

It is evident that the yield increases considerably with temperature, in accordance

with the endothermic behaviour of the ethanol steam reforming reaction. While

nanorods do not show significant differences in the final hydrogen yield between the

first and the second catalytic cycle (it is possible to state this by comparing the

RhPd/CeO2-R and RhPd/CeO2-R2 data), nanocubes and polycrystalline nanoparticles

turn out to be better at 1050 K during the second cycle. Regarding the yield obtained

at low temperatures (below 850 K), for all the three nanoshapes the trend is the same

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

Rh

Pd

/Ce

O2

-C

Rh

Pd

/Ce

O2

-P

Rh

Pd

/Ce

O2

-R

Rh

Pd

/Ce

O2

-C

2

Rh

Pd

/Ce

O2

-P

2

Rh

Pd

/Ce

O2

-R

2

Y

Hydrogen Yield over RhPd-CeO2

650

750

850

950

1050

T [K]

48

and the first cycle proves to be better than the second one. This result was absolutely

expected since it is reasonable that the catalytic activity of the catalysts is decreased,

after the first cycle, due to structural changes, coke formation and sintering. It is also

possible to see different catalytic activities, for each nanoshape, from one cycle to the

other. During the first one, polycrystals are the most active without any doubt while

nanocubes and nanorods are comparable; during the second cycle, especially above

850 K, nanocubes and polycrystals show very similar activities and they are definitely

better than nanorods, which even present a lower activity than the one showed during

the first cycle. The unexpected similarity between nanocubes and polycrystals during

the second cycle could be explained with a change in the structure of ceria nanocubes

due to the increase of temperature. In fact, nanocubes can undergo thermal stress

and, consequently, the shape results deformed and blunt turning to polycrystalline

one, going from {1 0 0} crystallographic planes to {1 1 1} ones. Consequently, the

catalytic activity shown in the second cycle is comparable to the first one. During the

first cycle, it is possible to see that the hydrogen yield over nanocubes increases

slightly from 750 to 850 K so, probably, morphological changes begin to take place

around 850 K. The different behaviour shown by the different nanoshapes underlines

the importance of the morphological structure of the support, which affects the

catalytic activity. In fact, the deposited rhodium-palladium alloy nanoparticles have the

same size for each support and they were impregnated at the same concentration for

each support. Consequently, the dissimilar catalytic behaviours are given by the

interaction of Rh-Pd nanoparticles with ceria nanoparticles, which are different for

each shape. Comparing the obtained results of the present study with the previous

one by Divins et al. [1], taken as main reference, in our case nanorods and nanocubes

proved to be less active than polycrystals, but Divins et al. reported that the catalytic

activity followed the trend nanocubes > nanorods > polycrystals. This unexpected

result could be ascribed to different reasons. First of all, the size of the polycrystalline

nanoparticles is very small (5-10 nm, Figures 3.10-3.12), certainly smaller than the

polycrystals studied in the previous work. This characteristic is reflected in a higher

surface area and, consequently, in a better catalytic activity. Another reason that could

49

have affected the catalytic activity is the synthesis method, since in the present work

ceria is obtained adding a cerium nitrate solution to a sodium hydroxide solution using

an electrospray (this could also explain the very small size of the particles) while in the

previous one it was obtained by a normal precipitation of a homogenous acidic

solution of cerium chloride with a base. Finally, a further hypothesis is that the size of

Rh-Pd nanoparticles differ depending on the support shape in the previous study (Rh-

Pd were prepared from precursor salts), whereas in this work all the Rh-Pd

nanoparticles exhibit the same geometry and size because the metallic nanoparticles

were preformed prior to impregnation. Continuing the comparison between the

present work and the reference article [1], it is evident that the hydrogen yield

obtained with preformed rhodium-palladium alloy nanoparticles impregnated on ceria

supports is much lower than the one obtained with bimetallic rhodium-palladium

nanoparticles from precursor salts supported on CeO2. The reason for this could be

explained considering the size of the metallic particles deposited on the support. In

fact, the preformed Rh-Pd alloy nanoparticles used in this work are bigger (ca. 5 nm,

Figures 3.5, 3.7, 3.12) and, consequently, the number of Rh-Pd nanoparticles in our

samples is lower since the impregnated concentrations (in mass percent) and the total

weight of metal are the same. Therefore, the contact points between Rh-Pd and ceria

nanoparticles, which represent the active parts of the catalysts, is lower too.

Turning now to consider the ethanol conversion registered on the different catalysts

(Figures 3.14, 3.15, 3.16), it is evident that a total conversion is reached in all the

catalytic cycles. However, nanocubes are less active than the other nanoshapes since

the ethanol conversion values are always lower and the total conversion is reached

only at the end of the catalytic test when the temperature is extremely high (1050 K).

50

Figure 3.14 Ethanol conversion over RhPd/ceria nanocubes during temperature-ramping tests.

Figure 3.15 Ethanol conversion over RhPd/ceria polycrystals during temperature-ramping tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over RhPd/CeO2-C

cycle 1

cycle 2

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T[K]

Ethanol Conversion over RhPd/CeO2-P

cycle 1

cycle 2

51

Figure 3.16 Ethanol conversion over RhPd/ceria nanorods during temperature-ramping tests.

Polycrystals and nanorods have a very similar catalytic behaviour since the first cycle

converts more ethanol below 850 K and then, above 850 K, both conversions reach the

unit value (total conversion). This result was perfectly expected and makes sense

because after the first cycle the catalyst lost, partially, its catalytic activity since it is

slightly covered by coke formed during the reaction and it has also lost the original well

defined nanoshape. The coke deposition was clearly evident by the colour of the

exhausted catalysts since it changed, during the reaction, from a pale yellow to an

intense black. The abrupt increase of the conversion is due to the onset of the ethanol

steam reforming reaction because of the high temperature reached. Nanocubes’ first

cycle is also better than the second one below 850 K but the main difference between

nanocubes and the other two nanoshapes is related to what happens above 850 K. In

fact, it is possible to notice that the second cycle turns out to be better than the first

one. While the trend of the second cycle is more or less comparable to those of

nanorods and polycrystals, the trend of the first cycle is very different since the

ethanol conversion decreases between 800 K and 900 K and, then, it increases again

until the unit value. This fact could be explained again with a change in ceria’s

crystallographic planes from {1 0 0} to {1 1 1}. It seems, therefore, that nanocubes

suffer thermal stress more than polycrystals and nanorods.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over RhPd/CeO2-R

cycle 1

cycle 2

52

Considering the Figure 3.17 and Figure 3.18 in which the ethanol conversions over the

different nanoshaped catalysts and during each catalytic cycle are compared, it is

evident that in both cycles nanorods reach the total conversion before and they

provide higher conversion from the beginning.

Figure 3.17 Ethanol conversion over impregnated catalysts during the first cycle of the temperature-ramping tests.

Figure 3.18 Ethanol conversion over impregnated catalysts during the second cycle of the temperature-ramping

tests.

0

0,2

0,4

0,6

0,8

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over RhPd/CeO2 1st cycle

nanocubs

polycrystallines

nanorods

0

0,2

0,4

0,6

0,8

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over RhPd/CeO2 2nd cycle

nanocubs

polycrystallines

nanorods

53

From Figures 3.17 and 3.18 it is clearer the different catalytic behaviour of

impregnated nanocubes during the first cycle while the trends representative of the

second cycle are very similar.

Figures 3.19, 3.20 and 3.21 show the selectivity towards hydrogen (H2), carbon

monoxide (CO), carbon dioxide (CO2), methane (CH4), acetaldehyde (CH3CHO), acetone

(CH3COCH3) and ethylene (C2H4) over the three different nanoshaped catalysts during

the first catalytic cycle.

Figure 3.19 ESR products selectivity over RhPd/ceria nanocubes during the first cycle of the temperature-ramping

tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over RhPd/CeO2-C 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

54

Figure 3.20 ESR products selectivity over RhPd/ceria polycrystals during the first cycle of the temperature-ramping

tests.

Figure 3.21 ESR products selectivity over RhPd/ceria nanorods during the first cycle of the temperature-ramping

tests.

Below 750 K the selectivity towards CO2 is very low while methane, hydrogen and

carbon monoxide are produced in higher quantities. This is explainable considering the

reaction of decomposition of ethanol to those three compounds (Eq. 1.6), which is a

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over RhPd/CeO2-P 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over RhPd/CeO2-R 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

55

mildly endothermic reaction and it happens mainly at the beginning. Since this

reaction generates one mole for each product, the initial amount of methane, carbon

monoxide and hydrogen should be theoretically 33,3% and the experimental values

are not so far. The hydrogen production keeps rising up until 800 K since it is

continually produced by the decomposition of ethanol. Regarding carbon monoxide, as

soon as it is produced, it is consumed because of the water-gas shift reaction (Eq. 1.2),

which is mildly exothermic and it occurs below 800 K. In fact, the carbon monoxide

selectivity decreases until this value of temperature. In parallel, the carbon dioxide

production increases. Methane is consumed by its steam reforming (Eq. 1.12) that

concurs to increase the hydrogen production. This reaction is endothermic and this is

the reason why methane does not increase anymore. Over 800 K it is possible to notice

that hydrogen and carbon dioxide present a soft decrease while CO begins to rise up

again. In fact, the inverse of the water gas-shift reaction prevails now according to its

endothermicity. However, the influence of this reaction is not so much considerable

since the onset of the ethanol steam reforming reaction (Eq. 1.1) changes immediately

the thermodynamic equilibrium. In fact, starting from ca. 900 K, hydrogen rises again

because the amount of gas produced with the ethanol and methane steam reforming

is more than the one consumed by the reverse water gas-shift. Even carbon dioxide

stops decreasing since the ESR reaction gives 2 moles of CO2 beside 6 of hydrogen.

In every experiment performed during this study, quantities of acetaldehyde, acetone

and ethylene were always found. In fact, beside the reaction of interest, other

undesired reactions may occur in our experimental conditions and they cannot be

avoided. Acetaldehyde comes from the dehydrogenation of ethanol (Eq. 1.4). This

reaction is a little bit more endothermic than the ethanol decomposition (Eq. 1.6) and

this fact explains why the produced acetaldehyde is less than methane, carbon

monoxide and hydrogen at the beginning of the catalytic test. However, also the

endothermicity of this reaction is not high and it occurs only below 900 K. After this

temperature the production of acetaldehyde decreases very quickly also because this

compound can lead to further reactions such as its steam reforming and its

decompositions (Eq. 1.7, Eq. 1.8 and Eq. 1.9). The first one is endothermic and it

56

concurs to the decrease of acetaldehyde and to the increase of hydrogen and carbon

monoxide; the second one is mildly exothermic and, consequently, its importance is

not so remarkable since it consumes acetaldehyde at low temperature when the

amount is very low; the last one explains the presence of small quantities of acetone at

the beginning since it is mildly endothermic and consumes acetaldehyde leading to the

formation of acetone. However, with the increase of temperature, the steam

reforming of acetaldehyde is the most relevant according to its endothermicity. This is

very important to avoid further productions of undesired compounds such as methane

and acetone and to generate higher quantities of hydrogen.

As already anticipated, acetone is formed by acetaldehyde decomposition at low

temperature (Eq. 1.9). However, the most relevant amount of acetone is due to an

undesired reaction that involves the decomposition of ethanol (Eq. 1.5). This reaction

is endothermic and it takes place below 800 K, rising the concentration of acetone to

its maximum value. Above 800 K acetone decreases until it reaches zero because of

the onset of the steam reforming of acetone (Eq. 1.13), which is more endothermic

and it occurs at high temperature. Moreover, this reaction concurs considerably to the

increase of hydrogen production since for each mole of acetone converted, 8 moles of

hydrogen are produced.

Concerning ethylene, its presence is due to the dehydration of ethanol (Eq. 1.3).

However, the selectivity towards it is low for all the nanoshapes and this was expected

since the presence of RhPd metallic nanoparticles minimizes the formation of ethylene

in favour of the hydrogen one.

Globally, the main reaction scheme can be summarized in three principal reactions:

ethanol decomposition (Eq. 1.6) with the formation of hydrogen, carbon monoxide and

methane, water-gas shift reaction (Eq. 1.2), which consumes CO increasing the

production of hydrogen and generating carbon dioxide, and methane steam reforming

(Eq. 1.12) that consumes methane and increases the amount of hydrogen too.

From the results illustrated in Figures 3.19, 3.20 and 3.21, it can be concluded that

nanocubes are less selective to hydrogen than polycrystals and nanorods. In fact,

nanocubes approach the 60% only at 1050 K, while the other two nanoshapes

57

approach that value already at 800 K and they reach also 65% at 1050 K. The lower

selectivity to hydrogen over nanocubes is also reflected in a higher selectivity to

undesired compounds such as acetaldehyde and carbon monoxide. Nanorods and

polycrystals have similar trends but the second ones are a little bit more selective to

hydrogen confirming to be the best nanoshape.

The Figures 3.22, 3.23, 3.24 show the selectivity trends over the three different

catalysts after the second catalytic cycle.

Figure 3.22 ESR products selectivity over RhPd/ceria nanocubes during the second cycle of the temperature-

ramping tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 700 900 1100

σ

T [K]

Selectivity over RhPd/CeO2-C 2nd cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

58

Figure 3.23 ESR products selectivity over RhPd/ceria polycrystals during the second cycle of the temperature-

ramping tests.

Figure 3.24 ESR products selectivity over RhPd/ceria nanorods during the second cycle of the temperature-ramping

tests.

Compared to the first catalytic cycle, it is possible to see that the selectivity towards

hydrogen rises up slower, it is lower at the beginning and it is similar for all the three

catalysts. The global decrease of the selectivity towards hydrogen and the

homogenization of the performances given by the different nanoshapes could be

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 700 900 1100

σ

T [K]

Selectivity over RhPd/CeO2-P 2nd

cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

500 700 900 1100

σ

T [K]

Selectivity over RhPd/CeO2-R 2nd cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

59

explained through the structural changes involved during the first catalytic cycle. In

fact, nanocubes could have been smoothed and turned into polycrystals, polycrystals

could have sintered forming bigger polyhedra and nanorods could got deformed and

bent. The original crystallographic planes would have been lost and the observed

result is a worsening of the catalytic performances, compared to the first cycle.

Another proof of this deterioration is the high amount of acetaldehyde, which is

substantially higher than the first catalytic cycle. However, it must be said that not all

the acetaldehyde shown in the graphics could be generated by the reactions of the

second cycle but a part could derive from the fact that some droplets were still

present, after the first catalytic cycle, on the membrane filter that prevents liquids

from entering the gas chromatograph. In this case acetaldehyde decreases very quickly

because it is important also the decomposition of acetaldehyde itself (Eq. 1.8) since

the amount of compound at the beginning is considerable. This reaction generates

methane as product and this could explain why it has a new increase in

correspondence of the decrease of acetaldehyde.

Once catalysts were removed from the reactor, they were black because of the coke

formed during the process. Coke is produced in different ways, such as the

decomposition of methane (Eq. 1.11) and the Boudouard reaction (Eq. 1.15), but it is

mainly due to the polymerization of ethylene (Eq. 1.14). The carbon balance formula

(Eq. 2.10) allows us to evaluate the amount of ethanol converted to products. The

Table 3.1, reported below, compares, as example the values obtained at 450°C (723 K)

and during the first catalytic cycle. As it is evident the formation of coke is important,

especially for nanorods. In fact, the ethanol conversion over this shape is higher than

those over nanocubes and polycrystals. Consequently, the amount of coke and other

products is higher.

Catalyst Cout/Cin (Eq. 2.10)

RhPd/CeO2-C 1st cycle 73,8%

RhPd/CeO2-P 1st cycle 70,7%

RhPd/CeO2-R 1st cycle 47,2%

Table 3.1 Carbon balance over impregnated catalysts at 723 K calculated with Eq. 2.10.

60

3.3.1.2 CeO2 (supports)

Figure 3.25 shows the hydrogen yield obtained over the three different ceria

nanoshapes during both the catalytic cycles.

Figure 3.25 Hydrogen yield over ceria supports during temperature-ramping tests.

Contrary to what happens for the impregnated catalysts, the differences between the

three nanoshapes are less noticeable. However, in both catalytic cycles nanorods

prove to be worse than the other nanoshapes, which are more or less comparable with

the nanocubes a little bit more active than the polycrystals during the first cycle. These

results agree with those obtained by Divins et al. [1] where the same small differences

between the three bare supports were noticed. The second cycle keeps the similarity

between the nanoshapes but the values are a little bit lower than those of the first

cycle. Since the deposition of metallic nanoparticles leads to obtain different activities,

it means that the influence of the structure is important for impregnated catalysts

where it can affect the interactions between the metal and the support. The bare

supports show a lower hydrogen yield than impregnated catalysts under 800 K, as

expected. The importance of the metallic nanoparticles lies in being able to enhance

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

Ce

O2

-C

Ce

O2

-P

Ce

O2

-R

Ce

O2

-C2

Ce

O2

-P2

Ce

O2

-R2

Y

Hydrogen Yield over CeO2

650

750

850

950

T [K]

61

the hydrogen production at low temperatures when the ethanol steam reforming is

not favoured yet.

Figures 3.26, 3.27, 3.28 show the ethanol conversion obtained over the three different

supports.

Figure 3.26 Ethanol conversion over ceria nanocubes during temperature-ramping tests.

Figure 3.27 Ethanol conversion over ceria polycrystals during temperature-ramping tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over CeO2 - C

cycle 1

cycle 2

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over CeO2 - P

cycle 1

cycle 2

62

Figure 3.28 Ethanol conversion over ceria nanorods during temperature-ramping tests.

It is evident for each support that the first cycle is better than the second one but both

reach the total conversion, exactly as happened over the impregnated catalysts. In this

case the behaviour of nanocubes is very similar to those of polycrystals and nanorods.

This is an expected result since the bare supports should act in the same way.

Consequently it is possible to say that the deposition of preformed metallic

nanoparticles over ceria nanocubes affects negatively the catalyst since the similarities

just noticed comparing the supports disappear and RhPd/CeO2-nanocubes prove to be

the worst impregnated catalyst.

The similarities between the three supports are also evident in Figures 3.29 and 3.30

where the ethanol conversions during each catalytic cycle are compared.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over CeO2 - R

cycle 1

cycle 2

63

Figure 3.29 Ethanol conversion over ceria supports during the first cycle of the temperature-ramping tests.

Figure 3.30 Ethanol conversion over ceria supports during the second cycle of the temperature-ramping tests.

Also for the supports nanorods provide a higher ethanol conversion in both cycles.

Figures 3.31, 3.32, 3.33 show the selectivity towards the main seven products over the

ceria supports during the first catalytic cycle.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over CeO2 1st cycle

cubes

polycrystals

rods

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

500 600 700 800 900 1000 1100

χ

T [K]

Ethanol Conversion over CeO2 2nd cycle

cubs

polycrystals

rods

64

Figure 3.31 ESR products selectivity over ceria nanocubes during the first cycle of the temperature-ramping tests.

Figure 3.32 ESR products selectivity over ceria polycrystals during the first cycle of the temperature-ramping tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-C 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-P 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

65

Figure 3.33 ESR products selectivity over ceria nanorods during the first cycle of the temperature-ramping tests.

It is immediately evident also from these graphics that the three nanoshapes are

characterized by a very similar behaviour. However, important differences can be

noticed respect to the impregnated catalysts. First of all it is clear that the bare

supports are not able to minimize the formation of undesired compounds like acetone

and ethylene, which are present in very high quantities. This phenomenon underlines

the importance of the deposition of the metallic nanoparticles since they can enhance

the selective production of hydrogen keeping low the formation of these two

compounds. Moreover, the decrease of ethylene is accompanied by an increase of

acetaldehyde that is another undesired product. Probably, around 800 K, the

decomposition of ethanol to acetaldehyde (Eq. 1.4) is more favoured than the

decomposition to ethylene (Eq. 1.3) according to its higher endothermicity. The

consumption of acetaldehyde, due to its steam reforming (Eq. 1.7), seems to be

particularly favoured above 950 K since acetaldehyde decreases and carbon monoxide

increases considerably.

According to the second catalytic cycle, Figures 3.34, 3.35, 3.36 show, once again, how

similar is the behaviour of the three bare supports.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-R 1st cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

66

Figure 3.34 ESR products selectivity over ceria nanocubes during the second cycle of the temperature-ramping

tests.

Figure 3.35 ESR products selectivity over ceria polycrystals during the second cycle of the temperature-ramping

tests.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-C 2nd cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-P 2nd cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

67

Figure 3.36 ESR products selectivity over ceria nanorods during the second cycle of the temperature-ramping tests.

Compared to the first cycle, the production of ethylene is less favoured probably

because the surface of the catalysts is already covered by coke and so it is more

difficult for ethanol to decompose over it. On the other hand, the production of

acetaldehyde is higher and it is evident that the amount is considerable at the

beginning. However, as already said for the impregnated catalysts, this fact can be also

ascribed to the presence of droplets on the filter of the gas chromatograph. Another

observation is that the selectivity to hydrogen is lower. Considering as example 900 K

the difference is more or less the 10%.

As it is known, ethylene is a coke precursor and, consequently, it is not surprising that

the catalysts were black after the reaction. Also in this case the carbon balance

provides an important information about the coke formed during the process. The

values shown in Table 3.2 are higher respect the ones obtained for the impregnated

catalysts because the conversion is 15%-20% lower and, consequently, the amount of

coke and other products is lower too. The production of coke over nanorods is more

significant like for the impregnated catalysts since the ethanol conversion is higher.

Also this case, the considered values are referred to 450°C (723 K):

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

500 600 700 800 900 1000 1100

σ

T [K]

Selectivity over CeO2-R 2nd cycle

H2

CO

CO2

CH4

CH3CHO

CH3COCH3

C2H4

68

Catalyst Cout/Cin (Eq. 2.10)

CeO2-C 1st cycle 85,7%

CeO2-P 1st cycle 80,6%

CeO2-R 1st cycle 51,8%

Table 3.2 Carbon balance over bare CeO2 at 723 K calculated with Eq. 2.10.

3.3.2 Fixed-temperature tests

The temperature-ramping catalytic tests provided unexpected results since the

polycrystals-based catalyst turned out to be the best one while nanocubes, which were

supposed to be the best according to Divins et al. [1], proved to be the worst. In order

to explain the result, several hypothesis have been made, but it was clear that new

tests were necessary to provide an explanation for this unexpected result.

Since the formation of ethylene, and consequently of the coke, is favoured at low

temperature (below 800 K) the new idea was to perform the test starting from a

higher temperature which was kept constant in order to stabilize the flow resulting

from the reaction and the concentration of the products. The chosen value was 823 K

(550°C). Moreover, it is known that structural changes of the support’s nanoparticles

happen between 800 and 900 K so the characterization of the catalysts tested at 823 K

could provide better information about this phenomenon. To increase the ethanol

conversion and the catalytic activity, the amount of catalysts has been almost doubled

from 50 to 96 mg.

A control experiment, blank test, was conducted to see the effect of the temperature

only and the effectiveness of the employed catalysts.

Figure 3.37 shows the ethanol conversion values obtained and the selectivity to each

product for each catalyst and after the blank test.

69

Figure 3.37 Ethanol conversion and ESR products selectivity at 823 K.

The results confirm those obtained with the temperature-ramping tests. Among the

impregnated catalysts, polycrystals prove to be the best and nanocubes the worst.

Nanorods present a catalytic behaviour very similar to the observed one for

polycrystals but the values of ethanol conversion and selectivity to hydrogen are a

little bit lower. The three supports are very similar and they reach total conversion.

However, nanorods are less selective to hydrogen while the amount of ethylene is

higher. Nanocubes and polycrystals are almost identical. As expected, the blank test

provides a much lower conversion since the catalyst is not present inside the reactor

and the only reaction happened seems to be the dehydrogenation of ethanol to

acetaldehyde and hydrogen (Eq. 1.4) since their quantities are definitely higher than

those of the other products and since they are produced almost with the same

percentage, in accordance to the stoichiometry of the reaction. Consequently, it is

evident the importance of the catalyst that is able to enhance selectively the desired

reactions instead of the undesired ones, keeping low the concentrations of

byproducts, and to increase the conversion of ethanol.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

Rh

Pd

/Ce

O2

-C

Rh

Pd

/Ce

O2

-P

Rh

Pd

/Ce

O2

-R

Ce

O2

/-C

Ce

O2

-P

Ce

O2

-R

bla

nk

Ethanol Conversion

Ethanol Conversion and Selectivity a 823 K

C2H4

CH3COCH3

CH3CHO

CH4

CO2

CO

H2

70

The hydrogen yield is calculated according to Eq. 2.11 and it is presented in Figure

3.38.

Figure 3.38 Hydrogen yield at 823 K.

Also in this case the trend is absolutely confirmed, and impregnated polycrystals

provide a higher hydrogen yield than impregnated nanorods and impregnated

nanocubes. The yield over RhPd/CeO2 nanocubes in particular is very low and it can be

compared with those obtained employing the three supports, which give similar

values. However, polycrystals are a little bit better than the other two nanoshapes.

Error bars are reported on this graphic to show the fluctuation of the values during the

stabilization of the analysis performed with the gas chromatograph. Since the

hydrogen yields obtained over the supports are lower than those obtained over

impregnated catalysts, the error bars are shorter and the fluctuation not so evident.

The evaluation of the ethanol conversion performed with the carbon balance (Eq. 2.10)

provided, for each catalyst, values different from those obtained with the conventional

formula (Eq. 2.9) for less than the 5%. Consequently the formation of coke during the

tests at 823 K was not important.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

Yield

Hydrogen Yield at 823 K

RhPd/CeO2-C

RhPd/CeO2-P

RhPd/CeO2-R

CeO2-C

CeO2-P

CeO2-R

Blank

71

3.4 Characterization of used catalysts

The used catalysts have been analysed with the same scanning electron microscope

employed for the characterization of the supports. The analyzed samples are the three

impregnated catalysts after the temperature-ramping catalytic tests and all the six

catalysts tested at 823 K.

The images are provided with two values of magnification: 50 K X for a general

overview and 200 K X for a closer one. However, it would be impossible to see the

metallic nanoparticles since they are too small for the SEM.

Figure 3.39 shows the impregnate nanocubes-based catalyst after the two cycles of the

temperature-ramping test. It is evident that the catalyst appears surrounded by an

intricate mesh of carbon fibres. It is also possible to see that not only dispersed

nanocubes are present but there are some agglomerates probably due to the sintering

happened at high temperatures. Consequently, the exposed surface is less than the

original one.

Figure 3.39 SEM image of RhPd/ceria nanocubes after the temperature-ramping test with a magnification of 50 K X.

72

In Figure 3.40 the magnification has been increased in order to see the nanocubes

closer. The result is quite clear: the biggest particles are still cubic but their vertices are

blunt and deformed while the smallest ones are more similar to undefined

polycrystals. It is so possible to say that a structural change happened and, now, it is

easier to understand the reason why the nanocubes behaved like polycrystals during

the second catalytic cycle.

Figure 3.40 SEM image of RhPd/ceria nanocubes after temperature-ramping test with a magnification of 200 K X.

Figure 3.41 and Figure 3.42 show the impregnated catalyst, with polycrystalline ceria

as support, as it appears after the temperature-ramping test. As already seen for the

previous sample, this catalyst is covered by a lot of coke and it is composed both by big

agglomerates and by dispersed nanoparticles. Analysing the sample closer it is possible

to see that the polycrystals are very small and somewhere they are fused one into the

other, because of the sintering, forming bigger particles. Compared to the images

showing the fresh polycrystals in the Chapter 2, these particles are clearer and sharper

because of the presence of coke, which is a conductor material and it is so able to

73

avoid that the sample gets charged by the electrons beam. In this way better pictures

could be taken.

Figure 3.41 SEM image of RhPd/ceria polycrystals after temperature-ramping test with a magnification of 50 K X.

74

Figure 3.42 SEM image of RhPd/ceria polycrystals after temperature-ramping test with a magnification of 200 K X.

In Figure 3.43 it is possible to see how the nanorods look like after the temperature-

ramping test. They suffered sintering due to the high temperature and they are

surrounded by carbon fibres exactly like nanocubes and polycrystals.

75

Figure 3.43 SEM image of RhPd/ceria nanorods after temperature-ramping test with a magnification of 50 K X.

Anyway, the most interesting observation is deductible from Figure 3.44. In fact, it is

possible to see that some nanorods are a shorter than the original ones (< 500-600

nm) and they are mixed with very small nanoparticles, probably polyhedra. So the

nanorods got broken generating those small particles and getting shorter. The shape is

not lost but the extremities are rounded and, somewhere, it is possible to see bent

nanorods.

76

Figure 3.44 SEM image of RhPd/ceria nanorods after temperature-ramping test with a magnification of 200 K X.

Regarding the used catalysts coming from the fixed-temperature tests, there are not

visible differences between impregnated and non-impregnated catalysts. For this

reason each nanoshape will be analysed once.

How it is possible to see in Figure 3.45, the nanocubes- based catalysts did not produce

a remarkable amount of coke at 823 K. in fact, the sample is not surrounded by a

carbon mesh like those that worked in the temperature-ramping tests. Moreover, the

nanoparticles seem to be less aggregate so the sintering was less important and the

loss of exposal surface is lower.

77

Figure 3.45 SEM image of ceria nanocubes after fixed-temperature test at 823 K with a magnification of 50 K X.

Figure 3.46 provides a closer view of the sample allowing us to see that the nanocubes

are well defined and sharp so that structural change did not occur. These results are

absolutely satisfactory and it is possible to state that the choice of 823 K was correct

since the catalysts worked in a perfect condition avoiding coke formation, sintering

(partially) and structural changes.

78

Figure 3.46 SEM image of ceria nanocubes after fixed-temperature test at 823 K with a magnification of 200 K X.

Also for polycrystals it is possible to see the absence of coke (Figure 3.47). Sintering is a

bit more important compared to nanocubes but it is less remarkable than the one of

the catalysts employed in the temperature-ramping test. In fact, some agglomerates

are present among the dispersed particles. Polycrystals suffer sintering more than

nanocubes and nanorods because of the smaller size of the nanoparticles and because

the tendency to sinter of the crystallographic planes {1 1 1} is higher than that of the {1

0 0} and {1 1 0} ones.

79

Figure 3.47 SEM image of ceria polycrystals after fixed-temperature test at 823 K with a magnification of 50 K X.

In Figure 3.48, it is possible to see that the polycrystals are characterized by a very

small size, even smaller than the one of the catalysts used in the temperature-ramping

test. This was an expected result since sintering is favoured at high temperatures.

80

Figure 3.48 SEM image of ceria polycrystals after fixed-temperature test at 823 K with a magnification of 200 K X.

Figure 3.49 and Figure 3.50 show how nanorods appear after the catalytic tests at 823

K. As already anticipated they suffer sintering less than polycrystals according to the

disposition of their crystallographic planes. Moreover, coke is not present exactly like

for the other two nanoshapes. A closer view allow us to say that these nanorods are

longer than the one employed in the temperature-ramping tests. This was foreseeable

since thermal stresses are favoured at higher temperatures. However, it is possible to

see the presence of some small nanoparticle among the nanorods, so a small structural

change occurred.

81

Figure 3.49 SEM image of ceria nanorods after the fixed-temperature test at 823 K with a magnification of 50 K X.

Figure 3.50 SEM image of ceria nanorods after the fixed-temperature test at 823 K with a magnification of 200 K X.

82

Every observation makes clearer the analysis of the experimental data and it is evident

that the results of the characterization of the used catalysts match perfectly with the

results obtained by the treatment of the data.

83

4 CONCLUSIONS

Summarizing very shortly, the first step was the preparation of the three different

nanoshapes: nanocubes, polycrystals and nanorods. The following step was the

impregnation of the preformed metallic nanoparticles on the supports in order to

obtain three different impregnated catalysts. These three catalysts and the three bare

supports have been characterized and employed to run an ethanol steam reforming

reaction both during temperature-ramping tests and during fixed-temperature tests.

Finally, they have been characterized again to see structural changes and coke

deposition that may be occurred during the reaction. The main goals were the

evaluation of the different catalytic activities between each nanoshape and between

impregnated catalysts and bare supports. The results have also been compared to the

ones obtained by Divins et al. [1]. The conclusions are numerous.

First of all, it is evident that the RhPd/CeO2 catalysts employed in this study present a

lower catalytic activity than the ones tested in the reference work [1]. This result is due

to the size of the metallic nanoparticles. In the present work, preformed RhPd

nanoparticles were impregnated on the supports and their dimensions are bigger.

Since the mass percentage and the total weight of metal is identical to [1], the contact

points between metal and supports, which represent the active sites of the catalysts,

are less; thus, the catalytic activity is lower.

Another conclusion, which is very connected with the previous one, is that the

catalysts employed in this study are less selective towards hydrogen below 900 K

favouring higher formation of undesired compounds such as acetaldehyde, acetone

and ethylene.

Compared to the reference work [1], another important difference emerges. The

catalytic activity trend shown by the employed catalysts was RhPd/CeO2-polycrystals >

RhPd/CeO2-nanorods > RhPd/CeO2-nanocubes, while in the previous study it was the

opposite: RhPd/CeO2-nanocubes > RhPd/CeO2-nanorods > RhPd/CeO2-polycrystals.

Surely, this trend is affected by the dimensions of ceria nanoparticles since polycrystals

are very small (< 10 nm) and nanocubes are clearly bigger (they can reach also 150

nm). Consequently, the exposed surface is higher for polycrystals and the catalytic

84

activity has increased. Another explanation could be the preparation method of the

catalysts, which is different in the two studies. It should be recalled that the size of

RhPd nanoparticles in [1] varied depending on the planes exposed of CeO2, which

would have a strong effect on the catalytic activity.

During the second catalytic cycle of the temperature-ramping tests, impregnated

nanocubes and nanorods showed a similar behaviour. This fact is due to the

morphological change that occurs at high temperature. Nanocubes are blunt and

turned into polycrystals and the performed SEM analysis (Figure 3.40) shows this

phenomenon. The ethanol conversion over RhPd/CeO2-nanocubes (Figure 3.14)

reaches the unit value only at 1050 K, so, nanocubes suffer thermal stress more than

the other two nanoshapes.

One of the important conclusions derivable from the fixed-temperature tests is that

morphological changes of the catalysts do not occur below 550°C (823 K) since the

SEM analysis of the exhausted catalysts still shows defined nanoshapes (Figures 3.46,

3.48, 3.50). Moreover, the coke formation was avoided since ethylene is produced only

at lower temperature.

4.1 OUTLOOK

I would like to suggest some further studies to the research group that hosted me at

UPC to perform this Master’s thesis.

In order to approach the industrial conditions, it would be interesting to work with a

less diluted feed increasing in this way the concentrations of the reagents.

Moreover, since the polycrystals obtained in this study are very small, it could be

interesting to increase the amount of metal, trying to improve the catalytic activity.

The fixed-temperature tests could be performed also at temperatures higher than

550°C to understand when structural changes in the ceria morphology happen.

Finally, a very important feature of a catalyst is its life; for this reason, performing

stability fixed-temperature tests would allow to evaluate the resistance of the

catalysts.

85

ACKNOWLEDGEMENTS

The present Master’s thesis is the final step of an important period of my life, full of

difficulties and sacrifice. For this reason I would like to thank those people who stayed

close to me all along my academic path because without them, probably, this end

would not be so rewarding. First of all I express my deep gratitude to Professor Jordi

Llorca, who accepted me in his group and helped me since the beginning with his

precious advices. Furthermore, I would like to thank my supervisor, Lluís Soler Turu, for

answering patiently all my questions, for explaining me everything in a very exhaustive

way and for being always available and helpful, also outside the academic context. A

special thank goes to Professor Luca Lietti, my Italian relator, who provided important

advices despite the distance. How not to mention my colleagues of the INTE group?

Thank you all for being like a family for me in these months at UPC. I do not forget my

Italian friends, both the eternal ones and the ones I met at university because they

always supported me during these years and because they were a source of relief from

difficulties. The deepest gratitude is for my entire and wonderful family, from my

parents to my little sister, from my grandparents to my aunts and cousins. They had

the most delicate role since they managed to drive me through every difficulty. They

always believed in me and for their support and faith I will be grateful forever. My last

thought goes to my beloved girlfriend, who faced side by side with me every trouble

we met during this hard climb. This was an extraordinary period of my life and I always

knew it because it started meeting you.

86

BIBLIOGRAPHY

[1] N. J. Divins, A. Casanovas, W. Xu, S. D. Senanayake, D. Wiater, A. Trovarelli, and J. Llorca, “The influence of nano-architectured CeOx supports in RhPd/CeO2 for the catalytic ethanol steam reforming reaction,” Catal. Today, Jan. 2015.

[2] “World consumption.” [Online]. Available: https://www.bp.com/content/dam/bp/pdf/Energy-economics/statistical-review-2015/bp-statistical-review-of-world-energy-2015-full-report.pdf. [Accessed: 25-Feb-2015].

[3] D. Das, “Hydrogen production by biological processes: a survey of literature,” Int. J. Hydrogen Energy, vol. 26, no. 1, pp. 13–28, Jan. 2001.

[4] R. Koch, E. López, N. J. Divins, M. Allué, A. Jossen, J. Riera, and J. Llorca, “Ethanol catalytic membrane reformer for direct PEM FC feeding,” Int. J. Hydrogen Energy, vol. 38, no. 14, pp. 5605–5615, May 2013.

[5] N. J. Divins, E. López, Á. Rodríguez, D. Vega, and J. Llorca, “Bio-ethanol steam reforming and autothermal reforming in 3-μm channels coated with RhPd/CeO2 for hydrogen generation,” Chem. Eng. Process. Process Intensif., vol. 64, pp. 31–37, Feb. 2013.

[6] E. López, N. J. Divins, and J. Llorca, “Hydrogen production from ethanol over Pd–Rh/CeO2 with a metallic membrane reactor,” Catal. Today, vol. 193, no. 1, pp. 145–150, Oct. 2012.

[7] L. F. Brown, “A comparative study of fuels for on-board hydrogen production for fuel-cell-powered automobiles,” Int. J. Hydrogen Energy, vol. 26, no. 4, pp. 381–397, Apr. 2001.

[8] “http://ethanolrfa.org/pages/World-Fuel-Ethanol-Production,” Renewable Fuels Association, 2012. .

[9] G. A. Deluga, J. R. Salge, L. D. Schmidt, and X. E. Verykios, “Renewable hydrogen from ethanol by autothermal reforming.,” Science, vol. 303, no. 5660, pp. 993–7, Feb. 2004.

[10] M. Benito, J. L. Sanz, R. Isabel, R. Padilla, R. Arjona, and L. Daza, “Bio-ethanol steam reforming: Insights on the mechanism for hydrogen production,” J. Power Sources, vol. 151, no. 1–2, pp. 11–17, Oct. 2005.

87

[11] G. Juodeikiene, L. Basinskiene, D. Vidmantiene, T. Makaravicius, E. Bartkiene, and H. Schols, “The use of β-xylanase for increasing the efficiency of biocatalytic conversion of crop residues to bioethanol,” Catal. Today, vol. 167, no. 1, pp. 113–121, Jun. 2011.

[12] R. Luque, L. Herrero-Davila, J. M. Campelo, J. H. Clark, J. M. Hidalgo, D. Luna, J. M. Marinas, and A. A. Romero, “Biofuels: a technological perspective,” Energy Environ. Sci., vol. 1, no. 5, p. 542, 2008.

[13] M. Ni, D. Y. C. Leung, and M. K. H. Leung, “A review on reforming bio-ethanol for hydrogen production,” Int. J. Hydrogen Energy, vol. 32, no. 15, pp. 3238–3247, Oct. 2007.

[14] “Bioethanol Production Process.” [Online]. Available: http://www.bioenergyconsult.com/ethanol-production-via-biochemical-route/. [Accessed: 26-Feb-2015].

[15] “Bio-ethanol Production.” [Online]. Available: http://free-stock-illustration.com/bioethanol+production+process. [Accessed: 26-Feb-2015].

[16] V. Subramani and C. Song, “Advances in Catalysis and Process for Hydrogen Production from Ethanol Reforming,” catalysis, vol. 20, pp. 65–106, 2007.

[17] H. Shapouri, J. A. Duffield, and M. Wang, “The Energy Balance of Corn Ethanol Revisited,” Trans. Am. Soc. Agric. Eng., vol. 46, no. 4, pp. 959–968, 2003.

[18] W. Wang and Y. Q. Wang, “Thermodynamic analysis of steam reforming of ethanol for hydrogen generation,” Int. J. Energy Res., vol. 32, no. 15, pp. 1432–1443, Dec. 2008.

[19] A. Denis, W. Grzegorczyk, W. Gac, and A. Machocki, “Steam reforming of ethanol over Ni/support catalysts for generation of hydrogen for fuel cell applications,” Catal. Today, vol. 137, no. 2–4, pp. 453–459, Sep. 2008.

[20] B. Banach, A. Machocki, P. Rybak, A. Denis, W. Grzegorczyk, and W. Gac, “Selective production of hydrogen by steam reforming of bio-ethanol,” Catal. Today, vol. 176, no. 1, pp. 28–35, Nov. 2011.

[21] K. VASUDEVA, “Steam reforming of ethanol for hydrogen production: Thermodynamic analysis,” Int. J. Hydrogen Energy, vol. 21, no. 1, pp. 13–18, Jan. 1996.

[22] N. Bion, D. Duprez, and F. Epron, “Design of nanocatalysts for green hydrogen production from bioethanol.,” ChemSusChem, vol. 5, no. 1, pp. 76–84, Jan. 2012.

88

[23] N. J. Divins, “Catalytic hydrogen production over RhPd/CeO2 catalysts and CO purification over Au/TiO2 catalysts,” Universitàt Politecnica de Catalunya, 2015.

[24] J. Vicente, J. Ereña, C. Montero, M. J. Azkoiti, J. Bilbao, and A. G. Gayubo, “Reaction pathway for ethanol steam reforming on a Ni/SiO2 catalyst including coke formation,” Int. J. Hydrogen Energy, vol. 39, no. 33, pp. 18820–18834, Nov. 2014.

[25] J. Y. Z. Chiou, J.-Y. Siang, S.-Y. Yang, K.-F. Ho, C.-L. Lee, C.-T. Yeh, and C.-B. Wang, “Pathways of ethanol steam reforming over ceria-supported catalysts,” Int. J. Hydrogen Energy, vol. 37, no. 18, pp. 13667–13673, Sep. 2012.

[26] J. Llorca, V. C. Corberàn, N. J. Divins, R. O. Fraile, and E. Taboada, “Hydrogen from Bioethanol,” in Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety, Elsevier, 2013, pp. 139–145.

[27] P. SHENG, W. CHIU, A. YEE, S. MORRISON, and H. IDRISS, “Hydrogen production from ethanol over bimetallic Rh-M/CeO2 (M=Pd or Pt),” Catal. Today, vol. 129, no. 3–4, pp. 313–321, Dec. 2007.

[28] M. Scott, M. Goeffroy, W. Chiu, M. A. Blackford, and H. Idriss, “Hydrogen Production from Ethanol over Rh–Pd/CeO2 Catalysts,” Top. Catal., vol. 51, no. 1–4, pp. 13–21, Oct. 2008.

[29] H. Idriss, M. Scott, J. Llorca, S. C. Chan, W. Chiu, P. Y. Sheng, A. Yee, M. A. Blackford, S. J. Pas, A. J. Hill, F. M. Alamgir, R. Rettew, C. Petersburg, S. D. Senanayake, and M. A. Barteau, “A phenomenological study of the metal-oxide interface: the role of catalysis in hydrogen production from renewable resources.,” ChemSusChem, vol. 1, no. 11, pp. 905–910, 2008.

[30] E. López, N. J. Divins, A. Anzola, S. Schbib, D. Borio, and J. Llorca, “Ethanol steam reforming for hydrogen generation over structured catalysts,” Int. J. Hydrogen Energy, vol. 38, no. 11, pp. 4418–4428, Apr. 2013.

[31] S. Bernal, J. J. Calvino, M. A. Cauqui, J. M. Gatica, C. Larese, J. A. Pérez Omil, and J. M. Pintado, “Some recent results on metal/support interaction effects in NM/CeO2 (NM: noble metal) catalysts,” Catal. Today, vol. 50, no. 2, pp. 175–206, 1999.

[32] L. Liu, Z. Yao, Y. Deng, F. Gao, B. Liu, and L. Dong, “Morphology and Crystal-Plane Effects of Nanoscale Ceria on the Activity of CuO/CeO2 for NO Reduction by CO,” ChemCatChem, vol. 3, no. 6, pp. 978–989, Jun. 2011.

[33] X.-S. Huang, H. Sun, L.-C. Wang, Y.-M. Liu, K.-N. Fan, and Y. Cao, “Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-

89

temperature CO oxidation,” Appl. Catal. B Environ., vol. 90, no. 1–2, pp. 224–232, Jul. 2009.

[34] N. Yi, R. Si, H. Saltsburg, and M. Flytzani-Stephanopoulos, “Active gold species on cerium oxide nanoshapes for methanol steam reforming and the water gas shift reactions,” Energy Environ. Sci., vol. 3, no. 6, p. 831, 2010.

[35] E. Aneggi, D. Wiater, C. de Leitenburg, J. Llorca, and A. Trovarelli, “Shape-Dependent Activity of Ceria in Soot Combustion,” ACS Catal., vol. 4, no. 1, pp. 172–181, Jan. 2014.

[36] T. Désaunay, G. Bonura, V. Chiodo, S. Freni, J.-P. Couzinié, J. Bourgon, A. Ringuedé, F. Labat, C. Adamo, and M. Cassir, “Surface-dependent oxidation of H2 on CeO2 surfaces,” J. Catal., vol. 297, pp. 193–201, Jan. 2013.

[37] X. Du, D. Zhang, L. Shi, R. Gao, and J. Zhang, “Morphology Dependence of Catalytic Properties of Ni/CeO 2 Nanostructures for Carbon Dioxide Reforming of Methane,” J. Phys. Chem. C, vol. 116, no. 18, pp. 10009–10016, May 2012.

[38] N. Yi, R. Si, H. Saltsburg, and M. Flytzani-Stephanopoulos, “Steam reforming of methanol over ceria and gold-ceria nanoshapes,” Appl. Catal. B Environ., vol. 95, no. 1–2, pp. 87–92, Mar. 2010.

[39] W.-I. Hsiao, Y.-S. Lin, Y.-C. Chen, and C.-S. Lee, “The effect of the morphology of nanocrystalline CeO2 on ethanol reforming,” Chem. Phys. Lett., vol. 441, no. 4–6, pp. 294–299, Jun. 2007.

[40] I. I. Soykal, B. Bayram, H. Sohn, P. Gawade, J. T. Miller, and U. S. Ozkan, “Ethanol steam reforming over Co/CeO2 catalysts: Investigation of the effect of ceria morphology,” Appl. Catal. A Gen., vol. 449, pp. 47–58, Dec. 2012.

[41] H. WANG, J. YE, Y. LIU, Y. LI, and Y. QIN, “Steam reforming of ethanol over Co3O4/CeO2 catalysts prepared by different methods,” Catal. Today, vol. 129, no. 3–4, pp. 305–312, Dec. 2007.

[42] I. A. C. Ramos, T. Montini, B. Lorenzut, H. Troiani, F. C. Gennari, M. Graziani, and P. Fornasiero, “Hydrogen production from ethanol steam reforming on M/CeO2/YSZ (M=Ru, Pd, Ag) nanocomposites,” Catal. Today, vol. 180, no. 1, pp. 96–104, Jan. 2012.

[43] F. FRUSTERI, S. FRENI, V. CHIODO, S. DONATO, G. BONURA, and S. CAVALLARO, “Steam and auto-thermal reforming of bio-ethanol over MgO and CeO2CeO2 Ni supported catalysts,” Int. J. Hydrogen Energy, vol. 31, no. 15, pp. 2193–2199, Dec. 2006.

90

[44] T. S. Moraes, R. C. R. Neto, M. C. Ribeiro, L. V. Mattos, M. Kourtelesis, S. Ladas, X. Verykios, and F. Bellot Noronha, “The study of the performance of PtNi/CeO2–nanocube catalysts for low temperature steam reforming of ethanol,” Catal. Today, vol. 242, no. PA, pp. 35–49, Mar. 2015.

[45] A. A. Vedyagin, A. M. Volodin, V. O. Stoyanovskii, R. M. Kenzhin, E. M. Slavinskaya, I. V. Mishakov, P. E. Plyusnin, and Y. V. Shubin, “Stabilization of active sites in alloyed Pd–Rh catalysts on γ-Al2O3 support,” Catal. Today, vol. 238, pp. 80–86, Dec. 2014.

[46] M. Dömök, A. Oszkó, K. Baán, I. Sarusi, and A. Erdőhelyi, “Reforming of ethanol on Pt/Al2O3-ZrO2 catalyst,” Appl. Catal. A Gen., vol. 383, no. 1–2, pp. 33–42, Jul. 2010.

[47] J.-Y. Siang, C.-C. Lee, C.-H. Wang, W.-T. Wang, C.-Y. Deng, C.-T. Yeh, and C.-B. Wang, “Hydrogen production from steam reforming of ethanol using a ceria-supported iridium catalyst: Effect of different ceria supports,” Int. J. Hydrogen Energy, vol. 35, no. 8, pp. 3456–3462, Apr. 2010.

[48] M. Li, X. Wang, S. Li, S. Wang, and X. Ma, “Hydrogen production from ethanol steam reforming over nickel based catalyst derived from Ni/Mg/Al hydrotalcite-like compounds,” Int. J. Hydrogen Energy, vol. 35, no. 13, pp. 6699–6708, Jul. 2010.

[49] Y. YANG, J. MA, and F. WU, “Production of hydrogen by steam reforming of ethanol over a Ni/ZnO catalyst,” Int. J. Hydrogen Energy, vol. 31, no. 7, pp. 877–882, Jun. 2006.

[50] A. Bshish, Z. Yaakob, B. Narayanan, R. Ramakrishnan, and A. Ebshish, “Steam-reforming of ethanol for hydrogen production,” Chem. Pap., vol. 65, no. 3, pp. 251–266, Jan. 2011.

[51] M. H. Youn, J. G. Seo, P. Kim, J. J. Kim, H.-I. Lee, and I. K. Song, “Hydrogen production by auto-thermal reforming of ethanol over Ni/γ-Al2O3 catalysts: Effect of second metal addition,” J. Power Sources, vol. 162, no. 2, pp. 1270–1274, Nov. 2006.

[52] S. Cavallaro, N. Mondello, and S. Freni, “Hydrogen produced from ethanol for internal reforming molten carbonate fuel cell,” J. Power Sources, vol. 102, no. 1–2, pp. 198–204, Dec. 2001.

[53] J. Llorca, “Efficient Production of Hydrogen over Supported Cobalt Catalysts from Ethanol Steam Reforming,” J. Catal., vol. 209, no. 2, pp. 306–317, Jul. 2002.

91

[54] H. Song, L. Zhang, and U. S. Ozkan, “Investigation of the Reaction Network in Ethanol Steam Reforming over Supported Cobalt Catalysts,” Ind. Eng. Chem. Res., vol. 49, no. 19, pp. 8984–8989, Oct. 2010.

[55] P. Bichon, G. Haugom, H. J. Venvik, A. Holmen, and E. A. Blekkan, “Steam Reforming of Ethanol Over Supported Co and Ni Catalysts,” Top. Catal., vol. 49, no. 1–2, pp. 38–45, Apr. 2008.

[56] J. Llorca, “Effect of sodium addition on the performance of Co–ZnO-based catalysts for hydrogen production from bioethanol,” J. Catal., vol. 222, no. 2, pp. 470–480, Mar. 2004.

[57] H. Idriss, “Ethanol Reactions over the Surfaces of Noble Metal/Cerium Oxide Catalysts,” Platin. Met. Rev., vol. 48, no. 3, pp. 105–115, Jul. 2004.

[58] M. Dömök, K. Baán, T. Kecskés, and A. Erdőhelyi, “Promoting Mechanism of Potassium in the Reforming of Ethanol on Pt/Al2O3 Catalyst,” Catal. Letters, vol. 126, no. 1–2, pp. 49–57, Sep. 2008.

[59] S. M. de Lima, A. M. da Silva, G. Jacobs, B. H. Davis, L. V. Mattos, and F. B. Noronha, “New approaches to improving catalyst stability over Pt/ceria during ethanol steam reforming: Sn addition and CO2 co-feeding,” Appl. Catal. B Environ., vol. 96, no. 3–4, pp. 387–398, Jun. 2010.

[60] J. . Breen, R. Burch, and H. . Coleman, “Metal-catalysed steam reforming of ethanol in the production of hydrogen for fuel cell applications,” Appl. Catal. B Environ., vol. 39, no. 1, pp. 65–74, Nov. 2002.

[61] A. Birot, F. Epron, C. Descorme, and D. Duprez, “Ethanol steam reforming over Rh/CexZr1−xO2 catalysts: Impact of the CO–CO2–CH4 interconversion reactions on the H2 production,” Appl. Catal. B Environ., vol. 79, no. 1, pp. 17–25, Feb. 2008.

[62] S. Cavallaro, “Ethanol Steam Reforming on Rh/Al2O3 Catalysts,” Energy and Fuels, vol. 14, no. 6, pp. 1195–1199, 2000.

[63] N. J. Divins, I. Angurell, C. Escudero, V. Pérez-Dieste, and J. Llorca, “Nanomaterials. Influence of the support on surface rearrangements of bimetallic nanoparticles in real catalysts.,” Science, vol. 346, no. 6209, pp. 620–3, Oct. 2014.

[64] Q. Wu, F. Zhang, P. Xiao, H. Tao, X. Wang, Z. Hu, and Y. Lü, “Great Influence of Anions for Controllable Synthesis of CeO 2 Nanostructures: From Nanorods to Nanocubes,” J. Phys. Chem. C, vol. 112, no. 44, pp. 17076–17080, Nov. 2008.

92

[65] M. Abid, V. Paul-Boncour, and R. Touroude, “Pt/CeO2 catalysts in crotonaldehyde hydrogenation: Selectivity, metal particle size and SMSI states,” Appl. Catal. A Gen., vol. 297, no. 1, pp. 48–59, Jan. 2006.

[66] X. Weng, J. K. Cockcroft, G. Hyett, M. Vickers, P. Boldrin, C. C. Tang, S. P. Thompson, J. E. Parker, J. C. Knowles, I. Rehman, I. Parkin, J. R. G. Evans, and J. A. Darr, “High-throughput continuous hydrothermal synthesis of an entire nanoceramic phase diagram.,” J. Comb. Chem., vol. 11, no. 5, pp. 829–34, Jan. 2009.

[67] X. Liu, K. Zhou, L. Wang, B. Wang, and Y. Li, “Oxygen vacancy clusters promoting reducibility and activity of ceria nanorods.,” J. Am. Chem. Soc., vol. 131, no. 9, pp. 3140–1, Mar. 2009.

[68] A. Simson, B. Farrauto, and M. Castaldi, “Steam reforming of ethanol/gasoline mixtures: Deactivation, regeneration and stable performance,” in 11AIChE - 2011 AIChE Annual Meeting, Conference Proceedings, 2011.

[69] A. Le Valant, F. Can, N. Bion, D. Duprez, and F. Epron, “Hydrogen production from raw bioethanol steam reforming: Optimization of catalyst composition with improved stability against various impurities,” Int. J. Hydrogen Energy, vol. 35, no. 10, pp. 5015–5020, May 2010.

[70] A. Le Valant, A. Garron, N. Bion, F. Epron, and D. Duprez, “Hydrogen production from raw bioethanol over Rh/MgAl2O4 catalyst,” Catal. Today, vol. 138, no. 3–4, pp. 169–174, Nov. 2008.

[71] H.-X. Mai, L.-D. Sun, Y.-W. Zhang, R. Si, W. Feng, H.-P. Zhang, H.-C. Liu, and C.-H. Yan, “Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes.,” J. Phys. Chem. B, vol. 109, no. 51, pp. 24380–5, Dec. 2005.

[72] Z. Wu, M. Li, J. Howe, H. M. Meyer, and S. H. Overbury, “Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption.,” Langmuir, vol. 26, no. 21, pp. 16595–606, Nov. 2010.

[73] D. R. Lide, CRC Handbook of Chemistry and Phisics, 85th ed. CRC Press, 2005.

[74] D. W. Green and R. H. Perry, Perry’s Chemical Engineers' Handbook, 8th ed. Mc Graw-Hill, 2008.

Effect of Ceria nanoshapes in ethanol steam reforming over … · 2016-12-04 · POLITECNICO DI...

Documents

Transcript of Effect of Ceria nanoshapes in ethanol steam reforming over … · 2016-12-04 · POLITECNICO DI...