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Title Methanation of Carbon Dioxide

Permalink https://escholarship.org/uc/item/3nd6n502

Author Goodman, Daniel Jacob

Publication Date 2013

Peer reviewed|Thesis/dissertation

eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA

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Methanation of Carbon Dioxide

A thesis submitted in partial satisfaction of the requirements for the degree Master of Science

in Chemical Engineering

Daniel Jacob Goodman

2013

ABSTRACT OF THE THESIS

Methanation of Carbon Dioxide

Daniel Jacob Goodman

Master of Science in Chemical Engineering

University of California, Los Angeles, 2013

Professor Selim M. Senkan, Chair

The emission of greenhouse gases into the atmosphere has been linked to global warming.

Carbon dioxide’s (CO2) one of the most abundant greenhouse gases. Natural gas, mainly methane, is the cleanest fossil fuel for electricity production helping meet the United States ever growing energy needs. The methanation of CO2 has the potential to address both of these problems if a catalyst can be developed that meets the activity, economic and environmental requirements to industrialize the process. Producing methane using carbon dioxide as a reactant would have the dual effect of keeping CO2 from entering earth’s atmosphere by consuming it to produce natural gas which in turn can produce electricity to meet growing power needs. This thesis aims to introduce the current methanation of carbon dioxide research, provide guidance into what needs to be considered before entering this field and how one might go about it.

The thesis of Daniel Jacob Goodman is approved.

Yunfeng Lu

Yi Tang

Selim M. Senkan, Committee Chair

University of California, Los Angeles

2013

iii

DEDICATION

I would like to dedicate this thesis and the work to produce it to the following people, for without them it would not have been completed. Thanks to my advisor, Dr. Selim Senkan, for his patience, belief in my abilities and guidance in completing this degree. As well as my family and friends for their support throughout my time in graduate school and wishing the best for me even if it took longer than intended and was not what I initially set out to do. Lastly, a great thanks to my Girls for their love, understanding, and help getting back on track toward finishing this degree.

TABLE OF CONTENTS

Abstract of the Thesis……………………………………………………………………….…….ii

Committee Page…………………………………………………………..………………………iii

Dedication Page…………………………………………………………..………………………iv

Introduction………………………………………………………………………………………..1

Thermodynamics of the Methanation of Carbon Dioxide………………………………………...3

Methanation Catalyst.....…………………………………………………………………………..8

Experimental………..……………………………………………………………………………17

Mechanism……………………………………………………………………………………….21

Conclusions………………………………………………………………………………………26

References………………………………………………………………………………………..27

INTRODUCTION

The United States emits 6 billion tons of carbon dioxide (CO2) per year into earth’s atmosphere.

Greenhouse gases are so named because of their ability to absorb and emit infrared radiation.

Water vapor and CO2 are the most common greenhouse gases in earth’s atmosphere. Recent studies indicate a high probability of a link between anthropogenic greenhouse gas emissions and observed effects on global warming, precipitation patterns, ocean acidification, and weather patterns [1].

Petroleum use for transportation accounts for about 1/3 of the total annual U.S. emissions of

CO2. Fossil fuel use for electricity generation accounts for more than another 1/3. For electricity generation coal emits twice as much CO2 as natural gas which emits 20-40 times what nuclear or renewable methods do [1]. Nature gas is the cleanest of the fossil fuels, based on greenhouse gas emissions. 86% of the natural gas consumed in the U.S. is produced domestically and most of the remainder from Canada. This domestic production removes dependence on foreign sources and the international market price fluctuations that come with them. Steady prices in natural gas would allow for the construction of cleaner natural gas combined cycle plants, possibly with carbon capture and storage (CCS) technology, to replace existing coal plants helping reduce emitted CO2 [1].

There are 3 main strategies for reducing CO2 emission: reduce the amount of CO2 produced, storage of CO2, and usage of CO2. Hydrogenation of CO2 is an attractive C1 building block for making organic chemicals, materials, and carbohydrates (i.e. foods) if considering reducing emissions by usage of CO2. CO2 as a chemical feedstock in current industrial processes is 1 limited: synthesis of urea and its derivatives, salicylic acid, and carbonates. This limitation is due to the thermodynamic stability of CO2, which requires high energy substances to transform it into other chemicals [2]. The hydrogenation of CO2 into more useful fuels or chemicals uses hydrogen as the required high energy material for transformation. The products of CO2 hydrogenation currently being researched include carbon monoxide, methane, methanol, ethanol and higher alcohol, hydrocarbons, dimethyl ether, formic acid, formates and formamides. Some of these products can be fuels in internal combustion engines, raw materials and intermediates in many chemical industries, easily liquefied allowing for easy storage and transportation, and in general are more desirable than CO2. Effecting the scale up of CO2 hydrogenation to industrial levels include lack of satisfactory catalysts (with desirable cost, activity, selectivity, stability, recovery, reuse, and handling), efficient and economic reactor design and the availability of hydrogen which has issues with its production, storage and transportation [2].

Methane is the main component of natural gas [3]. If a natural gas plant with carbon capture and storage technology were utilized for producing electricity using methane/natural gas produced from CO2 all 3 strategies for reducing CO2 emissions would be implemented. This is why there is great interest in producing methane from CO2. Catalytic hydrogenation of CO2 to methane,

CO2(g) + 4 H2(g)  CH4(g) + 2 H2O(g), also called the Sabatier reaction (named after chemist Paul

Sabatier in 1902 who observed the reaction over a Nickel catalyst) [4] is a topic most recently reviewed by Wang et al [5], who also reviewed the hydrogenation of carbon dioxide into other products [2].

This paper is meant as an introduction into the research being conducted on CO2 methanation:

The thermodynamics of the CO2 methanation reaction; types of catalysts being explored, what factors need to be considered when choosing which to use (i.e. type of metal(s), support, metal loading, and preparation method), and a table of CO2 catalysts investigated in the literature with their relevant conditions and results; The Sabatier reaction mechanism and how to design a reactor system to test your catalyst; Characterization techniques to use to better understand and evaluate your catalyst’s performance; and some conclusions and recommendations developed from reviewing the current state of methanation technology.

THERMODYNAMICS OF THE METHANATION OF CARBON DIOXIDE

Thermodynamic equilibrium calculations of chemical systems can give answers to important questions, such as the type of thermodynamically stable reaction products produced along with their selectivity and yield, if a reaction proceeds endothermically or exothermically, the impact of reaction parameters like temperature, pressure and reactant ratios [6]. Comparing calculations with experimental results allows for the identification of kinetic hindrances, i.e. thermodynamically allowed but somehow suppressed chemical reactions, providing guidance in catalyst development and process control of methanation.

CO2, and its Carbon-Oxygen double bonds, is a stable molecule. Both terms of the Gibbs free energy disadvantage the conversion of CO2 into other products. At atmospheric pressure and 298

K, the ΔH is about 293 kJ/mol CO2 for the dissociation of CO2 into CO and O2. The addition of the higher Gibbs free energy co-reactant H2 in the reversible and exothermic methanation

o reaction makes the conversion of CO2 thermodynamically easier with a ΔH of -167 kJ/mol [7]. 3

Gao et al. conducted systematic thermodynamic analysis of carbon oxide (CO and/or CO2) methanation using the total Gibbs free energy minimization method which is without any hindrances caused by kinetics, transport phenomena, or hydrodynamics. Table 1 lists possible reactions involved in the methanation of carbon oxides. Their calculations were based on gaseous compounds containing H2, O2, N2, CO, CO2, CH4, H2O, C2H4, and solid carbon. Note that all the reactions may simultaneously happen and since 3 of the reactions produce CO2 it is difficult to completely convert it. The van’t Hoff equation was used to calculate equilibrium constants (K), which are plotted versus temperature in Figure 1. It can be seen that the exothermic CO2 methanation (R2) is suppressed as temperature increases and that it plays an important role in the methanation reaction system because of its high equilibrium constant in the

200-500 oC range [6].

Table 1. Possible reactions involved in the methanation of carbon oxides [6]

Figure 1. The calculated K values of the reactions involved in methanation. [6]

Figure 2 shows the typical product fraction for CO2 methanation determined by Gibbs minimization at equilibrium for a stoichiometric 4:1 H2:CO2 feed at 1 atm. At low temperatures

o CH4 and H2O are the main products. As the temperature is increased above 450 C, the formation of CO by-product by the reverse water-gas shift reaction (R5) occurs and then dominates above

o 550 C. Increasing the temperature is unfavorable since CO2 methanation is strongly exothermic,

o and operating below 300 C is necessary for high CH4 yield at 1 atm.

Figure 2. Product fraction of CO2 methanation at equilibrium. [6]

Gao et al. also investigated the effect of pressure and temperature, Figure 3, and different H2/CO2 ratios, Figure 4, on CO2 methanation, i.e. CO2 conversion, CH4 selectivity and yield. From

Figure 3 it can be seen that CO2 conversion is inversely proportional to temperature and directly proportional to pressure. The switch to a direct proportionality with termperature at 1 atm beyond 600 oC is attributed to the reverse water gas-shift reaction dominating the system and consuming CO2. The authors note that CO2 is more difficult to methanize than CO at the same temperature and pressure even though CH4 selectivity is better in CO2 methanation. As seen in

Figure 4, higher H2/CO2 ratios lead to higher CO2 conversion and CH4 selectivity.

Figure 3. Effects of pressure and temperature on CO2 methanation: (a) CO2 conversion, (b) CH4 selectivity, and (c) CH4 yield. [6]

Figure 4. Effect of different H2/CO2 ratios on CO2 methanation: (a) CO2 conversion, (b) CH4 selectivity, (c) CH4 yield, and (d) carbon yield. [6]

As shown by the Gibbs minimization method the methanation of CO2 is thermodynamically favorable and low temperature, high pressure and proper H2:CO2 ratio (equal to or more than 4) are required to optimize the process [6].

METHANATION CATALYST

The reduction of the fully oxidized carbon, CO2, to methane is an eight-electron process with significant kinetic limitations, which requires a catalyst to achieve acceptable rates and selectivities for potential industrial use [8]. For a catalyst to be of use in industry it needs to meet certain cost, activity, selectivity, stability, recovery, reuse, and handling requirements.

Metal

The catalysts being investigated for CO2 methanation are generally made up of Group VIII, IX,

X and XI transition metals. Nickel and Ruthenium based catalysts produce almost exclusively methane, while less reactive metal constituents Pd, Pt, Rh, Mo, Re and Au catalyze simultaneously CH4, CH3OH and CO (by reverse water-gas shift reaction). Cu and Ag catalyze mainly CH3OH [9]. Nickel based catalysts are the most common studied because of their high activity and low price, but sintering at reaction conditions diminishes their industrial viability

[10]. Ruthenium has been shown to be the most active metal for methanation, but its high cost makes it less attractive as an industrial catalyst [11].

Multi-metallic catalysts that seek to utilize the functionality of different metals in a concerted manner are also of interest. Park et al. investigated a Pd-Mg on Silica catalyst based on

Palladium’s ability to dissociate molecular hydrogen, which can then react with surface species

8 formed by the reaction of CO2 on a Mg-containing oxide [8]. While Pd/SiO2 is active for CO2 reduction to CO and Mg/SiO2 (without Pd) is relatively inactive, when combined they attained selectivities to methane greater than 95% with 59% CO2 conversion for this system.

The metal loading of a catalyst can affect dispersion, pore volume and pore size and blockage of micropores caused by the agglomeration of species leading to a reduced surface area. Although the number of active sites increases with increasing metal loading, the dispersion of metal will decrease with increasing loading [12]. As metal loading is increased a significant decrease of the

BET surface area is usually observed, usually due to formation of large oxide aggregates, which limits the access to the internal surface of the catalyst [13].

Support

The type of support used for heterogeneous catalysts is another important factor to consider. The interaction between the metal and support plays a very active role on catalyst performance and are termed “metal-support effects” [12]. The support plays an important role on the active site dispersion, activity and stability [14]. Typical supports include Silica (SiO2) [8], Aluminum

Oxide (Al2O3) [15], Lanthanum Oxide (La2O3) [16], and composite supports (ZrO3-AlO3) [17].

Chang et al. used an amorphous silica rice husk ash-alumina support (RHA-Al2O3) with Nickel for a catalyst because they found the support to exhibit high selectivity for CO2 hydrogenation

[12].

Zhang et al. used Mobile Composition Matter (MCM) based mesoporous silica nanoparticle supports for Nickel based catalysts. MCM supports exhibit high specific surface area, regular pore structures and adjustable heteroatom contents. When nickel was incorporated into MCM-41

9 supports they were found to be novel, thermally stable and highly active for CO2 methanation, on par with the best Ru/SiO2 catalysts [14].

Table 2 lists CO2 catalysts investigated in the literature with relevant conditions and results.

Another summary of the results of methanation of CO2 on various catalysts can be found elsewhere [40, 9].

Table 2. CO2 catalysts investigated in the literature with their relevant conditions and results.

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

4.29% Ni/RHA-Al2O3 IE 500 4:1 34 56 19 18

4.09% Ni/ SiO2-gel IE 500 4:1 25 45 11 18

15% Ni/RHA-Al2O3 IWI 500 4:1 63 90 58 12

15% Ni/ SiO2-Al2O3 IWI 600 4:1 63 29 19 12

NiFeAl-(NH4)2CO3 CP 220 4:1 58.5 99.5 58.2 15

NiFeAl-Na2CO3 CP 220 4:1 55.7 99.5 55.4 15

NiFeAl-NH4OH CP 220 4:1 54.5 99.4 54.2 15

NiFeAl-NaOH CP 220 4:1 49.1 99.6 48.9 15

69.1% Ni5Al-R CP 400 4:1 92.4 99 -- 47

LaNi4Al AM 400 4:1 91.5 95 -- 48

25% Ni/Al2O3 CP 235 9:1 99 99.7 -- 49

15% Ni/Al2O3 I 250 2:1 14.5 97 -- 50

15% Ni-5%Mo/Al2O3 I 250 2:1 17.2 97 -- 50

15% Ni/Al2O3 I 300 4:1 45 99 -- 51

15% Ni-2%CeO2/Al2O3 I 300 4:1 71 99 -- 51

40.4% Ni/Al2O3 SG 220 4:1 61.5 99.2 -- 52

10% Ni/Al2O3 I 400 4:1 5 99 -- 53

9.8% Ni/Al2O3 I 516 11:1 98 100 -- 36

10% Ni/Al2O3 I 350 3.5:1 69 92 -- 58

15% Ni/Al2O3 I 350 3.5:1 71 100 -- 58

20% Ni/Al2O3 I 350 3.5:1 76 100 -- 58

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

25% Ni/Al2O3 I 350 3.5:1 74 99 -- 58

10% Ni/La2O3 I 208 4:1 4.5 100 -- 16

10% Ni/La2O3 I 230 4:1 13.4 100 -- 16

10% Ni/La2O3 I 252 4:1 33.0 100 -- 16

10% Ni/La2O3 I 280 4:1 76.6 100 -- 16

10% Ni/La2O3 I 300 4:1 90.0 100 -- 16

10% Ni/La2O3 I 320 4:1 97.1 100 -- 16

10% Ni/La2O3 I 380 4:1 100 100 -- 16

10% Ni/γ- Al2O3 I 380 4:1 6.9 88.9 -- 16

12 % Ni/ZA-IP I 360 3.5:1 70 -- 69 17

12 % Ni/ZA-IMP I 360 3.5:1 42 -- 43 17

12 % Ni/ZA-CP I 360 3.5:1 50 -- 48 17

12 % Ni/ZA-0 I 360 3.5:1 42 -- 43 17

12 % Ni/ZA-3 I 360 3.5:1 71 -- 70 17

12 % Ni/ZA-9 I 360 3.5:1 63 -- 57 17

12 % Ni/ZA-15 I 360 3.5:1 60 -- 65 17

8% Ni/zeolite IE 600 10:1 100 -- 80 37

3% Ni-MCM-41 C16 573 18:7 5.6 100 -- 41

Raney Ni-42 L 300 4:1 65 95 -- 42

Raney Ni-50 L 300 4:1 85 100 -- 42

Raney Ni-60 L 300 4:1 87 100 -- 42

5% Ni-Ce0.72Zr0.28O2 pSG 350 4:1 38.4 94.7 -- 44

10% Ni-Ce0.72Zr0.28O2 pSG 350 4:1 75 99.1 -- 44

15% Ni-Ce0.72Zr0.28O2 pSG 350 4:1 71.4 98.7 -- 44

5% Ni-Ce0.5Zr0.5O2 pSG 350 4:1 65.9 98.2 -- 73

5% Ni-Ce0.14Zr0.86O2 pSG 350 4:1 52.4 97.8 -- 73

5%Ni 0.5%Rh- pSG 350 4:1 61.2 97.5 -- 73

Ce0.72Zr0.28O2

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

Ni20(Zr0.9Ce0.1)Ox S 350 4:1 75 100 -- 43

Ni30(Zr0.9Ce0.1)Ox S 350 4:1 78 100 -- 43

Ni40(Zr0.9Ce0.1)Ox S 350 4:1 81 100 -- 43

Ni20(Zr0.9Sm0.1)Ox S 350 4:1 73 100 -- 43

Ni30(Zr0.9Sm0.1)Ox S 350 4:1 78 100 -- 43

Ni40(Zr0.9Sm0.1)Ox S 350 4:1 76 100 -- 43

Ni30Zr2O2-sono S 350 4:1 80 100 -- 43

5% Ni-SiO2 I 350 4:1 27.6 85.5 -- 54

5% Ni-CZ pSG 350 4:1 67.9 98.4 -- 54

5% Ni-CZ I 350 4:1 25.4 84.7 -- 54

5Ni-CZ4 I 420 4:1 40 86 -- 55

5Ni-CZ I 420 4:1 56 90 -- 55

5Ni-C4Z I 420 4:1 75 91 -- 55

5%NiUSY I 400 4:1 24.7 61.4 -- 56

4%Ni3%CeUSY I 400 4:1 37.7 72.1 -- 56

4%Ni7%CeUSY I 400 4:1 44.3 75.5 -- 56

5%Ni5%CeUSY I 400 4:1 55.0 86.2 -- 56

10%NiUSY I 400 4:1 47.9 78.8 -- 56

8%Ni7%CeUSY I 400 4:1 51.7 85.6 -- 56

14%NiUSY I 400 4:1 65.5 94.2 -- 56

14%Ni7%CeUSY I 400 4:1 68.3 95.1 -- 56

35Ni5Fe0.0RuAX SG 220 4:1 63.4 99.5 63.1 63

35Ni5Fe0.2RuAX SG 220 4:1 63.8 99.4 63.4 63

35Ni5Fe0.4RuAX SG 220 4:1 65.2 99.2 64.7 63

35Ni5Fe0.6RuAX SG 220 4:1 68.2 98.9 67.4 63

35Ni5Fe0.8RuAX SG 220 4:1 64.7 99.2 64.2 63

35Ni5Fe1.0RuAX SG 220 4:1 64.3 98.9 63.6 63

PH(Ni/SiO2) I+GDP 250 2:1 90.0 100 -- 64

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

10% Ni/CeO2 I 350 4:1 90 100 -- 68

10% Ni/γ-Al2O3 I 450 4:1 82 99 -- 68

10% Ni/TiO2 I 450 4:1 76 98 -- 68

10% Ni/MgO I 450 4:1 70 97 -- 68

15% Ni/SiC I 350 4:1 83 100 -- 69

Ni-La/SiC I 350 4:1 76 100 -- 69

70Ni-30(Zr0.833Sm0.167) R 350 -- 86 100 -- 72

50Ni-50(Zr0.833Sm0.167) R 350 -- 95 100 -- 72

Ni-40Mm AM 300 4:1 91 100 -- 76

Ni-40Zr MS 300 4:1 89 100 -- 76

Ni-40Sm AM 300 4:1 86 100 -- 76

0.8% Ru/TiO2 SD 160 4:1 -- -- 100 45

0.76% Ru/TiO2 G 250 4:1 -- -- 100 45

0.75% Ru/TiO2 IWI 385 4:1 -- -- 100 45

Ce0.99Ru0.01O2 C 500 4:1 16 90 -- 33

Ce0.98Ru0.02O2 C 500 4:1 24 95 -- 33

Ce0.97Ru0.03O2 C 480 4:1 51 99 -- 33

Ce0.96Ru0.04O2 C 450 4:1 55 99 -- 33

Ce0.95Ru0.05O2 C 450 4:1 55 99 -- 33

Ce0.95Co0.05O2 C 500 4:1 49 0 -- 33

Ce0.95Ni0.05O2 C 500 4:1 50 52 -- 33

Ce0.98Pd0.02O2 C 500 4:1 50 0 -- 33

Ru-Microlith M 360 4:1 83.5 100 -- 39

Ru-Microlith M 350 5.5:1 93 100 -- 39

Rh-Microlith M 395 4:1 92 100 -- 39

Rh-Microlith M 225 5.5:1 99.8 100 -- 39

RuMnNi(5:35:60)/Al2O3 I 400 4:1 99.74 72.36 -- 61

RuMnCu(10:30:60)/Al2O3 IWI 220 4:1 100 70 -- 62

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

Pd/SiO2 rME 450 4:1 40.8 10.4 4.3 8

Pd/SiO2 I 450 4:1 40.6 6.5 2.6 8

Mg/SiO2 rME 450 4:1 0.8 10.3 0.1 8

Pd-Mg/SiO2 rME 450 4:1 59.2 95.3 56.4 8

Mg/Pd/SiO2 I 450 4:1 40.0 76.2 30.4 8

Pd-Fe/SiO2 rME 450 4:1 44.7 2.8 1.3 8

Pd-Ni/SiO2 rME 450 4:1 50.5 89.0 44.9 8

Ni/SiO2 rME 450 4:1 36.8 81.8 30.1 8

Pd-Li/SiO2 rME 450 4:1 42.6 88.5 37.7 8

Pd/Ru/Ni(2:8:90)/Al2O3 I 200 4:1 43.60 6.82 -- 67

Pd/Ru/Ni(2:8:90)/Al2O3 I 400 4:1 52.95 39.73 -- 67

Au25Zr75 A 220 3:1 13.5 5 -- 38

20 Cu/TiO2/YSZ/Au S 380 -- 40 40 -- 46

Rh/Ni(30:70)/Al2O3 I 200 4:1 43 3.8 -- 70

Rh/Ni(30:70)/Al2O3 I 400 4:1 90.1 70.75 -- 70

2% Rh/TiO2 IWI 270 1:1 7.89 72.7 -- 74

2% Rh/TiO2-CO purified IWI 270 1:1 19.2 93.3 -- 74

2% Rh-2.5%Fe/TiO2 IWI 270 1:1 9.16 57.2 -- 74

2.5% Fe/TiO2 IWI 270 1:1 2.65 11.6 -- 74

LaNiO3 Cit 300 4:1 11.8 31.8 -- 75

LaNiO3-400C Cit 300 4:1 55.4 98.7 -- 75

LaNiO3-500C Cit 300 4:1 77.7 99.4 -- 75

LaNiO3-600C Cit 300 4:1 71.7 99.3 -- 75

LaNiO3-700C Cit 300 4:1 59.7 99.2 -- 75

5% Ni/La2O2CO3 Cit 300 4:1 40.3 88.9 -- 75

Pt/κ-βAl2O3/Au DC 400 1:1 1.0 31.5 -- 60

Pt/κ-βAl2O3/Au DC 400 2:1 11.5 38 -- 60

Pt/Co(10:90)/Al2O3 I 400 4:1 70.10 67.80 47 66

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

Co/KIT-6 EI 300 4.6:1 51.0 98.9 -- 57

Co/meso-SiO2 EI 280 4.6:1 40.0 94.1 -- 57

14% Co/CNF IWI 250 2:1 19.2 94.2 -- 59

5% Co/SiO2 IWI 260 25:1 0.8 -- 0.35 65

10% Co/SiO2 IWI 260 25:1 1.0 -- 0.60 65

15% Co/SiO2 IWI 260 25:1 1.25 -- 0.80 65

20% Co/SiO2 IWI 260 25:1 1.40 -- 0.81 65

5% Co/Al2O3 IWI 260 25:1 1.1 -- 0.75 65

10% Co/Al2O3 IWI 260 25:1 1.75 -- 1.50 65

15% Co/Al2O3 IWI 260 25:1 2.6 -- 2.25 65

20% Co/Al2O3 IWI 260 25:1 3.5 -- 3.0 65

5% Co/MgO IWI 260 25:1 1.4 -- 1.2 65

10% Co/MgO IWI 260 25:1 3.35 -- 3.0 65

15% Co/MgO IWI 260 25:1 3.85 -- 3.70 65

20% Co/MgO IWI 260 25:1 3.25 -- 2.85 65

5% Co/TiO2 IWI 260 25:1 1.3 -- 0.80 65

10% Co/TiO2 IWI 260 25:1 1.65 -- 1.30 65

15% Co/TiO2 IWI 260 25:1 2.0 -- 1.75 65

20% Co/TiO2 IWI 260 25:1 1.80 -- 1.65 65

5% Co/Nb2O5 IWI 260 25:1 0.95 -- 0.30 65

10% Co/Nb2O5 IWI 260 25:1 0.80 -- 0.35 65

15% Co/Nb2O5 IWI 260 25:1 0.80 -- 0.35 65

20% Co/Nb2O5 IWI 260 25:1 0.85 -- 0.50 65

5% Co/CeO2 IWI 260 25:1 0.75 -- 0.45 65

10% Co/CeO2 IWI 260 25:1 0.95 -- 0.65 65

15% Co/CeO2 IWI 260 25:1 3.60 -- 3.5 65

20% Co/CeO2 IWI 260 25:1 6.50 -- 6.35 65

5% Co/ZrO2 IWI 260 25:1 1.4 -- 1.25 65

a o b b Catalyst Prep. Temp, C Feed Ratio, CO2 Conversion , CH4 Selectivity , CH4 Ref.

b H2:CO2 % % Yield , %

10% Co/ZrO2 IWI 260 25:1 2.1 -- 1.80 65

15% Co/ZrO2 IWI 260 25:1 2.25 -- 2.1 65

20% Co/ Co/ZrO2 IWI 260 25:1 3.0 -- 2.75 65

20% Co-SSP IWI 220 10:1 27 89.5 -- 71

20% Co-MCM IWI 220 10:1 28 91.4 -- 71

20% Co-TiSSP IWI 220 10:1 16 92.1 -- 71

20% Co-TiMCM IWI 220 10:1 34 94.9 -- 71 a: IE: ion exchange, IWI: incipient wetness impregnation, CP: co-precipitation, AM: arc melting, I: impregnation, SG: sol-gel, C16: C16 Ni-MCM-41 (16 carbon chain length surfactant), L: leaching, pSG: pseudo sol-gel, S: sonication, I+GDP: impregnation + glow discharge plasma, R: reduction, MS: melt spinning, SD: sputter deposition, G: Gratzel method, C: combustion, M: microlith, rME: reverse microemulsion, A: alloy, Cit: citrate, DC: dip coating, EI: excess impregnation b: Note: results were obtained under different experimental conditions.

EXPERIMENTAL

Preparation Method

An important consideration when designing a catalyst is its preparation method. How the metal is combined with the support can affect the metal dispersion, crystal structure, and allowable metal loading.

Incipient wetness impregnation is the simplest preparation method, allowing for easier control of loading and no liquid waste generation, but is not the best for attaining good dispersion of metal on support [12].

Catalysts prepared by ion exchange yield finely dispersed and evenly distributed metal crystallites, but are limited to catalysts with low loading [18].

For high metal loadings a precipitation-deposition technique should be used in place of impregnation of a support which is not practical [19]. The precipitation agent selected has been shown to greatly influence the catalyst performance and should be carefully considered and investigated when using this preparation method [15].

The method of reverse microemulsion can be utilized to achieve a highly dispersed catalyst when intimate mixing of the heterospecies is required and phase separation needs to be minimized [8].

Based on the expansive array of metals, supports, loading and preparation methods being considered currently a high throughput combinatorial catalysis approach to discovery and optimization of CO2 catalysts seems ideal to expiating the advancement of this technology to industrial standards.

Reactor

Catalyst evaluations can be performed using the Senkan lab computer controlled array channel microreactor system described previously [20] in which up to 80 catalysts can be screened in parallel. In the array microreactors, reactant gases flow over the flat surfaces of catalyst powder which are individually isolated within reactor channels; the flow regime is similar to that of a monolithic reactor [21]. All experiments should be performed under atmospheric pressure and at a gas hourly space velocity (GHSV) of 20,000 h-1, to represent differential reactor conditions.

GHSV can be calculated using the gas flow rate and the volume of the catalyst used in each channel. Prior to activity measurements, all catalysts should be reduced in hydrogen at temperatures above those used for the reaction (i.e. 500 °C). Initial screening experiments should be performed within the temperature range 25-450°C, using a feed gas, supplied by pre-filled high-pressure bottles of H2, CO2 and inert carrier gas (i.e. Ar, He, N2), with a composition of carbon dioxide and hydrogen at a ratio of CO2:H2 = 1:4 and balance inert carrier [12]. Reactor effluent gases will be analyzed by withdrawing the products using a heated capillary sampling probe followed by on-line gas chromatography (Varian CP-4900 Micro GC with thermal conductivity detector, Porapak Q (10m) and Molecular sieve 13X (10m) columns) and mass spectrometry (MS, SRS RGA-200, Sunnyvale, CA).

Should the array channel microreactor be unavailable for these experiments a micro fixed-bed reactor (4-6 mm i.d. quartz tubing) can be constructed [22]. The feed gas control and effluent gas analysis systems described above can be incorporated into or reproduced for this new reactor, while a tubular furnace can be used to control the reactor temperature.

Marwood et al. note that the methanation reaction rate is inhibited by water [23]. A water trap can be placed at the entrance to the reactor to control the partial pressure of water in the reactor.

A water trap for maintaining a constant water partial pressure would consist of 20 mL glass U- tube filled with 1mm glass beads, which is filled with a predetermined volume of water (i.e. 0.5 mL H2O to maintain 8.1 mbar partial pressure) before each experiment and kept at 4°C [24].

If catalyst deactivation caused by excess temperature generated by the exothermic nature of the

Sabatier reaction needs to be avoided the use of a microchannel reactor can be considered. These reactors allow for precise temperature control along the length of the reactor [3].

Characterization

Characterization of a catalyst is essential when trying to better understand and evaluate your catalyst’s properties and performance. Below is a list of techniques being used and how they can help you understand the microscopic and macroscopic properties of your catalyst and provide guidance on how to move forward in your research.

Inductively coupled plasma-atomic emission spectrometer + Elemental analyzer (Elemental analysis) can be used for determining metallic impurities. Contaminants can affect catalyst activity and the chemical reaction [12].

Temperature programmed reduction (TPR) can be used to characterize the effect of metal loading on the fraction and dispersion of catalyst species, and the reduction behavior of catalyst

[12].

Temperature programmed desorption (TPD) can be used to determine the metal dispersion by identifying the amount of surface metal versus total metal atoms in the catalyst and thereby the metal surface sites surface area [25].

X-ray photoelectron spectroscopy (XPS) can be used for understanding the role of the metals interactions with active components of the catalyst support [12].

X-ray diffraction (XRD) can be used to determine the crystal structure of the components of the catalyst and how metal loading affects dispersion and crystal growth (size, structure, crystallinity, crystal form) [12]. XRD techniques combined with TPR can be used to confirm metal-support interactions [22].

Scanning election micrographs (SEM) can be used to study the surface structure of the catalyst and support and how calcination and other catalyst preparation techniques affect it [12].

A transmission electron microscope (TEM) can be used to determine the catalyst morphology, its particle size distribution, and average particle size. Elemental distribution can be determined by energy dispersive X-ray spectroscopy (EDS) with a system coupled to the TEM [8].

The specific surface area (BET), pore volume, and pore size of a catalyst can be analyzed by a

Nitrogen adsorption-desorption analyzer. This information can help determine how varied metal loading can affect dispersion, pore volume and pore size and blockage of micropores caused by the agglomeration of species leading to a reduced surface area [12].

Thermogravimetric analysis can be conducted to determine the loss in weight taking place in the decomposition process [25].

MECHANISM

Insight into the mechanism of a chemical reaction is necessary to optimize the reaction process and improve performance. Much work has gone into establishing the mechanism of CO2 methanation, but to date no consensus on the kinetics and mechanism exists in the field. The split in opinion has to do with the nature of the intermediate compound involved in the rate- determining step and the process of methane formation. There are two main categories for the proposed mechanism of CO2 methanation. The first and more agreed upon path involves converting CO2 to CO which is converted to CH4 following the same mechanism as CO methanation, which itself does not have a consensus mechanism either. The other proposed mechanism directly hydrogenates CO2 to CH4 without the formation of a CO intermediate.

Mechanism 1: CO intermediate

The CO intermediate pathway for CO2 methanation involves the conversion of the gaseous CO2 into CO, which is subsequently hydrogenated to methane, CH4, following the CO methanation mechanism. While CO as the direct intermediate for methanation is consistent for this pathway the procedure for how CO is formed from gaseous CO2 proposed can differ. There is also disagreement on the CO methanation mechanism and its intermediates. The varied CO intermediate mechanisms proposed are discussed below.

Using transient measurement techniques to measure gas phase components time evolution and diffuse-reflectance infra-red (DRIFT) spectroscopy to indentify species on the surface of a 2%

Ru/TiO2 catalyst Marwood et al. propose the mechanism reproduced here in Figure 5. Gaseous

CO2 reacts with a surface hydroxyl group to a form surface-bound hydrogen carbonate species,

- HCO3 , which is reduced with metal adsorbed hydrogen, HM, to an interfacial formate species, 21

HCOO-. The formate diffuses in an equilibrium reaction between the interface and the support, where it can be identified by DRIFT spectroscopy, but transient experiments indicate the support formate is not active in the methanation reaction. The interfacial formate decomposes to adsorbed CO species while regenerating the surface hydroxyl group. Infrared spectroscopy was unable distinguish the steps of adsorbed CO hydrogenation and they are thus presented in a non- elementary lumped form involving 6 metal adsorbed hydrogens [23].

2HM

- (HCO3 )S

H2O CO2

- - (HCOO )I (HCOO )S

- (OH )S

(CO)M

+ 6 HM

CH4 + H2O

S: the support; M: the metal; I: the metal-support interface

Figure 5. CO2 methanation reaction mechanism proposed by Marwood et al. Reproduced from

[23].

Beuls et al. recently conducted methanation experiments at low temperatures on a Rh/γ-Al2O3 catalyst. DRIFT results showed identical adsorbed species for CO2 and CO methanation, but no formate was detected in either case indicating CO2 and CO methanation followed the same 22 mechanism, but one different from the formate pathway Marwood proposed. XPS showed the catalyst being oxidized following exposure to CO2, which is believed to be caused by the adsorbed oxygen species generated by the dissociation of gaseous CO2 , CO2(g) CO(a) and O(a), which also generates the CO intermediate for methanation [26].

Atom Superposition and Electron Delocalization-Molecular Orbital (ASED-MO) theory was applied to a Ni (111) surface by Choe et al. for a surface science approach to determining a reaction mechanism where CO2 is converted to CO and then to surface C before hydrogenation.

The elementary reaction steps proposed are reproduced in Figure 6 and consist of two mechanisms: carbon formation (steps 1-3) and carbon methanation (steps 4-6) [27]. The dissociation of CO2 to form the CO intermediate, step 1, occurred with an intermediate complex composed of atomic oxygen and π-bonded CO and a calculated activation energy of 1.27 eV

[28].

CO2ads  COads + Oads Step 1

COads  Cads + Oads Step 2

2COads  Cads + CO2gas Step 3

Cads + Hads  CHads Step 4

CHads + Hads  CH2ads Step 5

CH2ads + 2Hads  CH4gas Step 6

Figure 6. CO2 methanation reaction mechanism proposed by Choe et al. Reproduced from [27].

Steps 2-6 concern the CO methanation mechanism. There are two pathways in CO dissociation.

Dissociation of CO into C and O, step 2, is rate-determining based on a calculated activation

23 energy of 2.97 eV, highest of any of the elementary steps. This CO dissociation pathway was originally proposed by Fitzharris et al. [29], and has the support of thermodynamics [6]. The alternative adsorbed carbon formation path is the CO disproportion into surface carbon and gaseous CO2, step 3, proposed by Martin [30].

There is support for a CO methanation mechanism that does not require the formation of adsorbed carbon, known as the associative pathway. Employing DRIFTS spectroscopy techniques over a 5% Ru/TiO2 catalyst Panagiotopoulou et al. argue that CO2 methanation occurs via intermediate formation of adsorbed CO species produced via the reverse water-gas shift

(RWGS) reaction at the metal-support interface where the (TiO2)Ru-CO species interacts with adsorbed hydrogen atoms to form a partially oxygenated carbonyl species and, eventually, methane in the gas phase without surface carbon being involved in the hydrogenation. Transient mass spectroscopy techniques indicated that the interaction of the reaction mixture with the catalyst surface does not result in oxidation of Ru crystallites. This indicates that the dissociation of CO2 into CO and O, as proposed by Beuls on Rh/γ-Al2O3, does not take place in this Ru/TiO2 system and thus the associative pathway dominates in CO2 methanation [31].

Mechanism 2: Direct Hydrogenation (No CO intermediate)

The direct hydrogenation of CO2 to CH4 without the formation of a CO intermediate is the other proposed mechanism for CO2 methanation. By means of conventional ultrahigh vacuum surface science techniques, combined with density functional theory calculations and corroborated by high pressure reactivity tests on Ni (110) Vesselli et al. support the direct hydrogenation mechanism. They noted that at low temperature, CO2 is negatively charged and is chemically bonded mainly via the carbon atom. The molecule receives electronic charge from the metal,

24 bends, and binds to the surface with the carbon atom in a “V” configuration: the resulting energy barrier (0.43 eV) for its hydrogenation is relatively small and, most importantly, smaller than that for dissociation into CO + O and that for CO2 desorption, both of which were found to have an activation barrier of 0.60 eV. When adsorbed H approaches CO2, the H-CO2 complex flips and binds to the surface with the two oxygen atoms and H binds to the carbon atom, thus yielding a formate intermediate. The presence of hydrogen prevents the formation of CO [32].

Sharma et al. concluded that CO2 methanation does not take place through a CO intermediate.

For CO2 methanation their Ce0.95Ru0.05O2 catalyst yields 55% CO2 conversion with 99% selectivity for CH4. Though when performing temperature-programmed reaction (TPR) of CO with H2, on the catalyst essentially no activity for CO methanation is seen. Since this catalyst methanates CO2 but not CO a mechanism with a CO intermediate is rejected. Additionally IR adsorption in the region between 2700 and 3100 cm-1 where formate bands are expected were not seen, so this catalyst methanates CO2 without formate or CO intermediates [33].

Another factor to consider when investigating CO2 methanation mechanisms is the impurities in the feed gas. As noted thermodynamically the reverse water-gas shift reaction tends to dominate over carbon oxide methanation as temperature is increased. This can cause CO2 in the feed to be converted to CO. This is significant because Schlid et al. observed that while CO2/H2 feed mixtures rapidly form surface formate and gas phase CH4 during catalytic hydrogenation on

Pd/ZrO2 catalysts, the main reaction product is methanol, CH3OH, when a CO/H2 feed is used

[34]. The presence of CO in the feed mixture, due to CO2 conversion by RWGS or feed impurities, can alter what product is formed by the catalyst.

From this summary of proposed mechanisms for just a few of the CO2 methanation catalysts the difficulty of settling on a consensus mechanism is evident. A change in any or all of the possible variables (i.e. metal(s), support, feed composition, approach (experimental vs. theoretical), reaction conditions, preparation method) can lead to compelling evidence for a different CO2 methanation mechanism, or lead to a catalyst that yields a different product. Even this variability doesn’t always hold true. Schlid et al. observed that two differently prepared Ni/ZrO2 catalysts exhibit an almost identical catalytic behavior; both are good CO2 methanation catalysts and reveal about equal activation energies, indicating that the reaction mechanism is probably the same and that the active sites are similar on both catalysts. This result is remarkable in the light of the vastly different structural properties of both systems. The performance of the catalysts for

CO2 methanation does not appear to be significantly influenced by the chemical and structural properties of the zirconia matrix [35].

CONCLUSIONS

The high throughput combinatorial catalysis approach to discovery and optimization of catalysts used in the Senkan lab, with the help of the computer controlled 80 array channel array microreactor system, is well suited for investigating catalysts for the methanation of CO2 and advancing them to performance levels desired by industry for scale up. This is based on the expansive array of metals, supports, loading and preparation methods being considered currently for the methanation of CO2. The factors to consider for designing, testing and characterizing a methanation catalyst have been introduced in this thesis with the hope that it will expedite the determination of an acceptable industrial catalyst and allow the pursuit of a solution to an issue of environmental, political and economic significance.

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