The effect of carbon monoxide on carbon dioxide methanation over nickel oxide catalysts

ABSTRACT This research was to investigate the effect of carbon monoxide CO on carbon dioxide CO2 methanation over a variety of NiO catalysts including different promoters MgO, CeO2, Pt,

NICKEL OXIDE CATALYSTS

SPECIALIZATION: PETROCHEMICAL ENGINEERING CODE: 60520330

GRADUATION THESIS

HO CHI MINH CITY, 2019

PHÒNG DẦU KHÍ VÀ XÚC TÁC, VIỆN CÔNG NGHỆ HÓA HỌC

VIỆN HÀN LÂM KHOA HỌC VÀ CÔNG NGHỆ VIỆT NAM

Cán bộ hướng dẫn khoa học:

1 GS TSKH Lưu Cẩm Lộc Chữ ký:……… Cán bộ chấm nhận xét 1: Nguyễn Mạnh Huấn

Chữ ký:……… Cán bộ chấm nhận xét 2: Nguyễn Hữu Lương

Chữ ký:……… Luận văn thạc sĩ được bảo vệ tại Trường Đại học Bách Khoa, ĐHQG Tp.HCM ngày 18 tháng 7 năm 2019

Thành phần Hội đồng đánh giá luận văn thạc sĩ:

1 GS TSKH Lưu Cẩm Lộc

2 PGS TS Nguyễn Quang Long

3 TS Nguyễn Mạnh Huấn

4 TS Nguyễn Hữu Lương

5 TS Nguyễn Thành Duy Quang

Xác nhận của Chủ tịch Hội đồng đánh giá luận văn và Trưởng khoa quản lý chuyên ngành sau khi luận văn đã được chỉnh sửa (nếu có)

NHIỆM VỤ LUẬN VĂN THẠC SĨ

Ngày, tháng, năm sinh: 05/09/1992 Nơi sinh: Quảng Ngãi

Chuyên ngành: Kỹ thuật Hóa dầu Mã số: 60520330

I TÊN ĐỀ TÀI:

Ảnh hưởng của CO đến quá trình methane hóa CO2 trên hệ xúc tác NiO mang trên các chất mang khác nhau

II NHIỆM VỤ VÀ NỘI DUNG

- Điều chế các xúc tác NiO có và không có biến tính CeO2, MgO, CaO, Pt và Urea mang trên các chất mang khác nhau (Al2O3, SBA-15, MSN) bằng phương pháp tẩm

- Nghiên cứu các tính chất lý hóa của các xúc tác: BET, XRD, SEM, TEM,

H2-TPR, CO2-TPD

- Khảo sát hoạt tính xúc tác trong phản ứng hydro hóa CO2 thành CH4 trên sơ

đồ dòng vi lượng ở vùng nhiệt độ 225-400 oC, áp suất 1 atm với tỷ lệ (19 % mol CO2 + 1% mol CO)/H2 và (19% mol CO2 + 1% mol N2)/H2, tổng lưu lượng dòng là 3 L.h-1

Từ đó làm sáng tỏ 2 vấn đề:

- Ảnh hưởng của CO đến quá trình methane hóa CO2

- Mối quan hệ giữa thành phần, bản chất của chất mang, tính chất và hoạt tính của các hệ xúc tác NiO trong phản ứng methane hóa khi có sự hiện diện của

CO

III NGÀY GIAO NHIỆM VỤ (Ngày ký Quyết định giao đề tài): 20/08/2018

IV NGÀY HOÀN THÀNH NHIỆM VỤ: 18/07/2019

V CÁN BỘ HƯỚNG DẪN: GS TSKH Lưu Cẩm Lộc

Tp.HCM, ngày tháng năm 2109

CÁN BỘ HƯỚNG DẪN CHỦ NHIỆM BỘ MÔN ĐÀO TẠO

GS TSKH Lưu Cẩm Lộc TS Đào Thị Kim Thoa

TRƯỞNG KHOA

GS TS Phan Thanh Sơn Nam

ACKNOWLEDGEMENT

Firstly, I would like to express my special gratitude to my advisor, Prof Dr Sc Luu Cam Loc, who instructed me throughout the process of conducting and writing this thesis Without her assistance and supports in every step of the process, this work would not have been accomplished

Secondly, I desire to offer my sincere appreciation to all those working at the Department of Petro-chemistry & Catalysis, Institute of Chemical Engineering for guiding and inspiring me on both specialized knowledge and great motivation Special thanks are due to PhD Nguyen Tri for counseling and supervising me over the period

of experimental duration

In addition, I thank all lecturers in Division of Petroleum Processing Engineering, Faculty of Chemical Engineering, University of Technology – VNU-HCM for providing invaluable knowledge, insightful comments and positive encouragement

Last but not least, I crave for expressing myself grateful from the bottom of my heart towards my family, who always unconditionally support, believe and stimulate

me in my all decisions Under the auspices of my family, I could attain extraordinary achievements

Sincerely

Do Pham Noa Uy

ABSTRACT

This research was to investigate the effect of carbon monoxide (CO) on carbon dioxide (CO2) methanation over a variety of NiO catalysts including different promoters (MgO, CeO2, Pt, CaO, Urea) and catalytic supports (γ-Al2O3, SBA-15, MSN) The catalysts were prepared by using wet impregnation method and characterized by X-Ray Diffraction (XRD), Temperature Programmed Reduction (TPR), Temperature Programmed Desorption (TPD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area method CO2 methanation was performed with two different gaseous mixtures, (19% mol CO2 + 1% mol CO)/H2 and (19% mol CO2 + 1% mol N2)/H2 that are fed at the total flowrate of 3 L.h-1, from 225 oC to 400 oC at 1 atm

Nano-structure promoted NiO catalysts on porous alumina, santa barbara amorphous and mesostructured silica nanoparticles were successfully prepared by the impregnation method The particle size of all samples was found to distribute in a range of 15-45 nm and the catalysts’ surface area varied in range of 79  95 m2.g-1,

176  215 m2.g-1 and 127  272 m2.g-1 respectively for catalysts supported on γ-Al2O3, SBA-15 and MSN Additives increased the thermal stability of catalysts, enhanced the dispersion and reducibility of NiO and improved the support’s basicity that led to improve the activity and selectivity of hydrogenation of carbon oxides into CH4 Besides positive impacts, Urea, in contrast, prevented the adsorption of CO, CO2 and

H2 on the support, leading to decrease the effectivity of CO2 methanation

CO had a promoted effect on the hydrogenation of CO2 In term of catalysts supported on γ-Al2O3, conversion of CO and CO2 in hydrogenation reaction can be reached 100% at 250-275 oC and 80-90% at 400 oC respectively Addition of CO in feedstock enhanced the efficacy of CO2 methanation; CO2 conversion increased by 3-25%, CH4 selectivity always achieved 95-100% As for catalysts supported on SBA-15 and MSN, CO was completely converted at 275-300 oC and 80-90% CO2 were converted at 400 oC Presence of CO in reactants increased CO2 conversion by 4-8% and CH4 selectivity reached 90-100%

TÓM TẮT LUẬN VĂN

Trong khuôn khổ luận văn “Ảnh hưởng của CO đến quá trình methane hóa CO2

trên hệ xúc tác NiO mang trên các chất mang khác nhau”, các xúc tác NiO biến tính

MgO, CeO2, Pt, CaO và Urea mang trên các chất mang γ-Al2O3, MSN, SBA-15 được

điều chế bằng phương pháp tẩm ướt, và được nghiên cứu đặc trưng lý – hóa: thành

phần pha (XRD), kích thước lỗ xốp (BET), hình thái bề mặt (TEM, SEM), đặc tính

của các oxide kim loại bằng phương pháp khử xúc tác với H2 theo chương trình nhiệt

độ (TPR), xác định tâm hoạt tính bằng phương pháp giải hấp CO2 theo chương trình

nhiệt độ (CO2-TPD) Quá trình methane hóa được tiến hành với hai hỗn hợp khí: (19%

mol CO2 + 1% mol CO)/H2 và (19% mol CO2 + 1% mol N2)/H2, tổng lưu lượng dòng

phản ứng là 3 L.h-1, nhiệt độ phản ứng từ 225 oC đến 400 oC ở áp suất 1 atm

Kết quả cho thấy, kích thước hạt NiO của xúc tác mang trên γ-Al2O3, SBA-15

và MSN dao động trong khoảng 15-45 nm Xúc tác mang trên chất mang γ-Al2O3 có

bề mặt riêng thay đổi trong khoảng 79-95 m2.g-1, đại lượng này của các xúc tác mang

trên chất mang SBA-15 và MSN lần lượt là 176-215 m2.g-1 và 127-272 m2.g-1 Việc

thêm chất xúc tiến vào xúc tác làm thay đổi các đặc trưng lý hóa, tăng độ bền nhiệt,

tăng sự phân tán và khả năng khử của NiO và cải thiện tính kiềm của chất mang, nhờ

đó làm tăng hoạt tính xúc tác, độ chọn lọc methane trong quá trình hydro hóa CO2

Bên cạnh các tác động tích cực của chất xúc tiến, Urea làm giảm sự hấp phụ CO, CO2

và H2 trên bề mặt chất mang do quá trình phân hủy và bay hơi của Urea ở nhiệt độ cao,

dẫn đến làm giảm hiệu quả của quá trình chuyển hóa CO2 thành CH4

CO đóng vai trò như một chất xúc tiến khi được thêm vào hỗn hợp phản ứng

Đối với các xúc tác mang trên γ-Al2O3, trong phản ứng hydro hóa độ chuyển hóa CO

có thể đạt được 100% ở 250-275 oC và độ chuyển hóa CO2 là 80-90% ở 400 oC Sự có

mặt của CO trong hỗn hợp phản ứng làm tăng 3-25% độ chuyển hóa CO2, độ chọn lọc

CH4 đạt được 95-100% Đối với các xúc tác mang trên chất mang SBA-15 và MSN,

CO cũng có hiệu quả tương tự CO chuyển hóa hoàn toàn ở nhiệt độ 275-300 oC,

80-90% CO2 được chuyển hóa tại 400 oC Sự hiện diện của CO trong hỗn hợp phản ứng

làm tăng 4-8% độ chuyển hóa CO2 và độ chọn lọc CH4 luôn cao hơn 90%

COMMITMENT

Promise me that this thesis was accomplished by my own research The results

in the thesis are not used in any publications

Ho Chi Minh City,………, 2019

Do Pham Noa Uy

TABLE OF CONTENT

NHIỆM VỤ LUẬN VĂN THẠC SĨ i

ACKNOWLEDGEMENT ii

ABSTRACT iii

TÓM TẮT LUẬN VĂN iv

COMMITMENT v

TABLE OF CONTENT vi

LIST OF FIGURES ix

LIST OF TABLES xi

LIST OF ABBREVIATIONS xii

1 INTRODUCTION 1

2 LITERATUREREVIEW 3

2.1 Carbon dioxide in the Earth’s atmosphere 3

2.2 Carbon monoxide in the Earth’s atmosphere 5

2.3 Dual benefits from converting CO and CO2 into fuel gas 6

2.3.1 Environmental benefits 6

2.3.2 Economic benefits 8

2.4 Carbon oxides methanation 9

2.4.1 Fundamentals 11

2.4.2 Mechanism 13

2.4.2.1 CO methanation 13

2.4.2.2 CO2 methanation 16

2.4.3 A consistent mechanism for CO and CO2 methanation 19

2.5 Catalysts for methanation 21

2.5.1 Active compound 21

2.5.1.1 Rhodium 21

2.5.1.2 Ruthenium 22

2.5.1.3 Palladium 24

2.5.1.4 Nickel-Based Catalysts 24

2.5.2 Support 25

2.5.2.1 Alumina-Supported Nickel 25

2.5.2.2 Silica-Supported Nickel 27

2.5.3 Promoters 30

3 EXPERIMENTALTECHNIQUE 32

3.1 Preparation of SBA-15 catalytic support 32

3.1.1 Instruments 32

3.1.2 Chemicals 32

3.1.3 Procedure of SBA-15 catalytic support synthesis 32

3.2 Preparation of MSN catalytic support 34

3.2.1 Instruments 34

3.2.2 Chemicals 34

3.2.3 Procedure of MSN catalytic support synthesis 34

3.3 Preparation of promoted Nickel oxide catalysts 36

3.3.1 Preparation of promoted Nickel oxide catalysts using wet impregnation method 36

3.3.1.1 Preparation of non-promoted and promoted NiO catalyst supported on γ-Al2O3 37

3.3.1.2 Preparation of non-promoted and promoted NiO catalyst supported on SBA-15 37

3.3.1.3 Preparation of non-promoted and promoted NiO catalyst supported on MSN 37 3.4 Characterization technique 38

3.4.1 X-ray diffraction 38

3.4.2 Brunauer-Emmett-Teller (BET) method 40

3.4.3 Scanning Electron Microscopy 41

3.4.4 Transmission Electron Microscopy 41

3.4.5 Temperature Programmed Reduction 42

3.4.6 Temperature Programmed Desorption 43

3.5 Catalyst activity testing system 44

3.5.1 Schematic diagram 44

3.5.2 Reaction conditions 45

3.6 Procedure for conducting reaction 45

3.6.1 Catalytic reduction 45

3.6.2 Conducting CO2 methanation 45

3.6.3 Calculate CO, CO2 conversion and CH4 selectivity 46

4 RESULTSANDDISCUSSION 47

4.1 Explanation of catalyst’s selection 47

4.2 Catalyst characterization 51

4.2.1 X-ray Diffraction (XRD) 51

4.2.2 SEM results 56

4.2.3 TEM results 59

4.2.4 BET specific surface area 62

4.2.5 H2 Temperature Programmed Reduction (H2-TPR) 63

4.2.6 CO2 Temperature Programmed Desorption 67

4.3 The effect of CO on CO2 methanation 69

4.3.1 The effect of CO on CO2 methanation over catalysts supported on γ-Al2O3 69

4.3.2 The effect of CO on CO2 methanation over catalysts supported on SBA-15 and MSN 75

5 CONCLUSIONANDRECOMMENDATION 81

5.1 Conclusion 81

5.2 Recommendation 82

REFERENCES 83

APPENDIX 98

PUBLICATIONS 106

LIST OF FIGURES

Figure 2.1 The Keeling curve of atmospheric CO2 concentrations measured at Mauna

Loa Observatory 3

Figure 2.2 CO2 in Earth's atmosphere if half of global-warming emissions are not absorbed (NASA computer simulation) 4

Figure 2.3 A generic energy cycle using captured or sequestered CO2 and sustainable or renewable hydrogen to yield carbon-neutral or renewable carbonaceous fuels 7

Figure 2.4 Exemplary biomass/coal-to-SNG plant setup with CO methanation 10

Figure 2.5 Exemplary PtG plant setup with CO2 methanation 10

Figure 2.6 Pressure and temperature influence on the equilibrium composition of CO methanation and water–gas shift reaction (left: 300 oC, right: 1 bar) Educt gas composition: yCO = 0.25, 2 H y = 0.75 12

Figure 2.7 Pressure and temperature influence on equilibrium composition of CO2 methanation and water–gas shift reaction (left: 300 oC, right: 1 bar) Educt gas composition: 2 H y = 0.8, 2 CO y = 0.2 13

Figure 2.8 Mechanistic schemes for the catalytic hydrogenation of carbon monoxide The symbols * and # represent ordinary (type 1) and 5-fold coordinated reaction sites (type 2) [79], respectively 14

Figure 3.1 SBA-15 synthesis procedure diagram 33

Figure 3.2 MSN synthesis procedure diagram 35

Figure 3.3 Nickel oxide and promoters impregnation procedure diagram 36

Figure 3.4 Temperature programmed reduction profile for a metal oxide 43

Figure 3.5 CO2 methanation reaction system 44

Figure 4.1 The CO2 conversion in CO2 hydrogenation over CeNiAl catalysts 47

Figure 4.2 The CO2 conversion in CO2 hydrogenation over Pt11CaNiAl catalysts 48

Figure 4.3 The CO2 conversion in CO2 hydrogenation over NiMSN catalysts 49

Figure 4.4 The CO2 conversion in CO2 hydrogenation over Ce50NiSBA15 catalysts 50 Figure 4.5 The CO2 conversion in CO2 hydrogenation over Urea50NiMSN catalysts 51 Figure 4.6 X-ray diffraction spectra of (a) NiAl, (b) 3MgNiAl, (c) 6CeNiAl, (d) 0.1Pt11CaNiAl 52

Figure 4.7 NiO crystallite sizes of catalysts supported on γ-Al2O3 53

Figure 4.8 X-ray diffraction spectra of (a) 50NiSBA15, (b) 4Ce50NiSBA15, (c) 50NiMSN-Urea, (d) 50NiMSN 54

Figure 4.9 NiO crystallite sizes of catalysts supported on SBA-15 and MSN 55

Figure 4.10 SEM images of catalysts supported on -Al2O3 57

Figure 4.11 SEM images of catalysts supported on SBA-15 and MSN 58

Figure 4.12 TEM images of catalysts supported on -Al2O3 59

Figure 4.13 TEM images of catalysts supported on SBA-15 and MSN 61

Figure 4.14 H2-TPR diagrams of catalysts supported on γ-Al2O3 63

Figure 4.15 H2-TPR diagrams of catalysts supported on SBA-15 and MSN 65

Figure 4.16 CO2-TPD diagrams of catalysts supported on -Al2O3 67

Figure 4.17 CO2-TPD diagrams of catalysts supported on SBA-15 and MSN 68

Figure 4.18 CO2 conversion in hydrogenation of carbon oxides mixture (solid lines) and of single CO2 (dashed lines) over catalysts supported on γ-Al2O3 71

Figure 4.19 CO conversion in hydrogenation of CO + CO2 mixture on catalysts supported on γ-Al2O3 72

Figure 4.20 CH4 selectivity in CO2 hydrogenation over catalysts supported on γ-Al2O3 74

Figure 4.21 CO2 conversion in hydrogenation of carbon oxides mixture (solid lines) and of single CO2 (dashed lines) over catalysts supported on SBA-15 and MSN 76

Figure 4.22 CO conversion in hydrogenation of CO + CO2 mixture on catalysts supported on SBA-15 and MSN 78

Figure 4.23 CH4 selectivity in CO2 hydrogenation over catalysts supported on SBA-15 and MSN 79

LIST OF TABLES

Table 2.1 Natural gas reserve and CO2 concentration in several gas fields in Vietnam 8

Table 3.1 The studied catalysts 38 Table 3.2 Reaction parameters 45 Table 4.1 NiO crystallite sizes of all the catalysts at 2θ = 43.3o 56

Table 4.2 Specific surface area, pore volume and pore diameter of catalysts supported

Table 4.5 CO, CO2 conversion and CH4 selectivity in the hydrogenation of CO2 over catalysts supported on SBA-15 and MSN with (I) and without (II) the addition of CO

in reactants 75

LIST OF ABBREVIATIONS

XRD X-ray diffraction analysis

BET Brunauer-Emmett-Teller surface area analysis TPR Temperature programmed reduction

SEM Scanning electron microscopy

TEM Transmission electron microscopy

SMSI Strong metal to support interaction

WHSV Weight hourly space velocity

1 INTRODUCTION

High consumption of fossil fuels worldwide resulted in rising carbon dioxide (CO2) concentration in the atmosphere, leading to global warming and climate shift One of the most effective ways of dealing with the issue is hydrogenation of CO2 into synthetic fuel CH4 [1] This reaction was catalyzed by noble and transition metals [2-5] such as rhodium (Rh), ruthenium (Ru), palladium (Pd), iridium (Ir), cobalt (Co), gold (Au), iron (Fe) and nickel (Ni) Among these metals, Ni was seen as the best one, thanks to high catalytic activity and reasonably manufacturing cost compared to noble metals [5-7] Metals and promoters were usually supported on γ-Al2O3, santa barbara amorphous (SBA-15) and mesostructured silica nanoparticles (MSN) While γ-Al2O3

is beneficial for CO2 hydrogenation due to its high selectivity toward methane and relatively low cost [8-10], SBA-15 and MSN are appealing to scientists because their pore structures as well as fundamental properties such as catalytic, conductive, adsorbed and magnetic can be adjustable [11-19] Overall, NiO/γ-Al2O3, NiO/SBA-15 and NiO/MSN were considered as potential catalysts for methanation To further increase the activity of catalysts in hydrogenation of CO2, a variety of promoters was used

Alkaline or alkaline earth metal oxides were used to weaken the catalyst acidity; precious metals were added to increase the reduction of NiO For instance, MgO was used to alkalize and increase the thermal stability of catalyst and the NiO dispersion [20], that led to improve the activity and selectivity of CO2 hydrogenation into CH4 CaO was reported to be able to stabilize NiO catalyst, increase CO2adsorption [21], preventing the decomposition of CH4 to produce coke [22], led to the increase in activity of Ni catalyst for methanation of CO [23] In addition to increasing the dispersion of NiO, CeO2 also showed the ability to improve the reduction of Ni2+into high active Nio for hydrogenation of carbon oxides [24] Pt was also an additive of interest for NiO catalysts Pt had the ability to form Pt-Ni alloy to increase the dispersion of the active phase and reduce the size of metal particles [25], improving the catalyst stability [26], enhancing H2 adsorption, that made the activity of NiO catalyst in dry reforming of methane to be raised [27] Plus, Urea is a promising promoter because it was formed by a carbonyl (CO) functional group and two

Thus, this research aims to these targets:

- Understand the effect of CO on CO2 methanation

- Explain the relation between physico-chemical properties and catalytic activity

of Nickel oxide catalysts on carbon dioxide methanation with the presence of carbon monoxide

This research was conducted at Department of Petro-chemistry & Catalysis, Institute of Chemical Technology, Vietnam Academy of Science and Technology

2 LITERATURE REVIEW

2.1 Carbon dioxide in the Earth’s atmosphere

Figure 2.1 The Keeling curve of atmospheric CO2 concentrations measured at Mauna

Loa Observatory Carbon dioxide in Earth's atmosphere is a trace gas The current concentration

of CO2 is about 0.04% (410 ppm) by volume (or 622 parts per million by mass) [34] having risen from pre-industrial levels of 280 ppm

Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay Concentrations also vary on a regional basis, most strongly near the ground

with much smaller variations aloft In urban areas concentrations are generally higher and indoors they can reach 10 times background levels [35]

The concentration of carbon dioxide has risen due to human activities Combustion of fossil fuels and deforestation has caused the atmospheric concentration

of carbon dioxide to increase by about 43% since the beginning of the age of industrialization Most carbon dioxide from human activities is released from burning coal and other fossil fuels Other human activities, including deforestation, biomass burning, and cement production also produce carbon dioxide Human activities emit about 29 billion tons of carbon dioxide per year, while volcanoes emit between 0.2 and 0.3 billion tons [36, 37] Human activities have caused CO2 to increase above levels not seen in hundreds of thousands of years Currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere and is not absorbed

by vegetation and the oceans

Figure 2.2 CO2 in Earth's atmosphere if half of global-warming emissions are not

absorbed (NASA computer simulation) While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm-1, as opposed to light emission from the much hotter sun which is most

intense in the visible region Absorption of infrared light at the vibrational frequencies

of atmospheric carbon dioxide traps energy near the surface, warming the surface and the lower atmosphere Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption Increases in atmospheric concentrations of CO2 and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined and because it has a long atmospheric lifetime (hundreds to thousands of years)

Not only do increasing carbon dioxide concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide This produces a positive feedback for changes induced by other processes such as orbital cycles [38] Five hundred million years ago the carbon dioxide concentration was 20 times greater than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago [39]

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals After sunrise the gas is dispersed by convection [40] High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986 [41]

2.2 Carbon monoxide in the Earth’s atmosphere

Carbon monoxide (CO) is present in small amounts (about 80 ppb) in the Earth's atmosphere About half of the carbon monoxide in Earth's atmosphere is from the burning of fossil fuels and biomass [42] Most of the rest of carbon monoxide comes from chemical reactions with organic compounds emitted by human activities

and plants Small amounts are also emitted from the ocean, and from geological activity because carbon monoxide occurs dissolved in molten volcanic rock at high pressures in the Earth's mantle [43] Because natural sources of carbon monoxide are so variable from year to year, it is difficult to accurately measure natural emissions

2.3 Dual benefits from converting CO and CO 2 into fuel gas

Fossil fuels pose a fundamental dilemma for our human society On the one hand, their importance cannot be overstated - the combustion of coal, oil and natural gas supply close to 90 percent of our current energy needs and makes much of what

we do possible On the other hand, their widespread use comes at a cost the gases emitted during the burning of fossil fuels are strongly implicated as the main drivers of climate change

There are several reasons why fossil fuels remain so popular Firstly, they are accessible in one form or another in almost all regions of the world Secondly, humankind has learned how to use them effectively to provide energy for a myriad of applications at every scale Thirdly, they are without equal as fuels for transportation, they are portable and contain a considerable amount of stored chemical energy The overwhelming energy source for transportationderives from oil stocks Considering all

of this, it becomes obvious thatour energy supply for the foreseeable future will surely

be based on fossil-derivedhydrocarbon fuels, with the unavoidable production of CO2 2.3.1 Environmental benefits

The only way to stabilize the Earth’s climate is to stabilize the concentration of greenhouse gases in the atmosphere A successful long-term local and global strategy

must surely be able to stabilize the atmospheric CO2 levels by substitution of fossil fuels by renewable energy sources It is often written that the potential of renewable energy sources is higher - by several orders of magnitude - than any estimated world energydemand Unfortunately, the vast majority of large renewable energy sources arealmost always located far away from the main consumption areas One possibilityis the production of electricity to join an electric grid If this is not a realisticpossibility, the renewable energy must be harvested in the form of energy carriers As well as conventional energy storage for sustainable electricity, the generation of chemical energy carriers is another attractive alternative, with hydrogen, gas and liquid carbonaceous carriers as the primary candidates.

Figure 2.3 A generic energy cycle using captured or sequestered CO2 and sustainable

or renewable hydrogen to yield carbon-neutral or renewable carbonaceous fuels The great value of gas and liquid carbonaceous fuels (e.g methane, petrol, diesel and others) lies both in their intrinsic (high) chemical energy content and in the ease with which they are stored and transported using existing infrastructure It is of course possible to reduce CO2 directly with hydrogen (hydrogenation), or potentially electricity, to synthesize carbonaceous fuels However, such an approach would not

impact positively on the global carbon balance since hydrogen and electricity as produced today are largely derived from fossil fuels, which, themselves, produce large amounts of CO2

If, however, renewable sources could be used as the energy vector to transform

CO2 into fuels, one has a most attractive route to providing carbonaceous fuels that would not contribute to net CO2 emissions (figure 2.3)

2.3.2 Economic benefits

Converting CO and CO2 into fuel gas especially methane is able to meet the demands of energy which has been rapidly increased

Table 2.1 Natural gas reserve and CO2 concentration in several gas fields in Vietnam

CO 2

concentration (mol %)

Reserve (million m 3 )

In Vietnam, CO2 has found in many natural gas fields with high concentration and large reserve as shown in table 2.1 [44] CO2 methanation is a promising solution

to generate surplus energy Besides, not only does the hydrogenation of CO2 enhance heating value but it resolves several issues related to CO2 in gas processing such as corrosion and CO2 concretion as well

2.4 Carbon oxides methanation

Methane is an energy carrier of significant importance to the industry, energy, and transportation sectors worldwide Its existing distribution infrastructure in many countries makes it a constitutive element of modern economies The major share of industrially used methane comes from fossil natural gas resources However, the debate of the finiteness of fossil resources and climate change caused the research expenditures relating to catalytic and biological methane production from carbon oxide-rich gases (methanation) to increase over the last years Biological methanation proceeds at low temperatures (<70 oC) in stirred tank reactors or trickle-bed reactors (e.g [45-48]) In contrast, catalytic methanation is operated at temperatures above 250

oC, predominantly in fixed bed reactors Research into catalytic methanation processes focuses on two options, CO methanation and CO2 methanation

CO methanation (Eq (1)) is an exothermic process using carbon monoxide and hydrogen as educts for the catalytic production of methane and water [49] Educt gases mainly come from coal or biomass gasification at synthetic fuel production plants (figure 2.4) [50, 51]

Figure 2.4 Exemplary biomass/coal-to-SNG plant setup with CO methanation

Figure 2.5 Exemplary PtG plant setup with CO2 methanation

CO2 methanation processes (Eq (2)) use carbon dioxide and hydrogen If electrolysis hydrogen is supplied as an educt (PtG – Power-to-Gas), CO2 methanation allows for the chemical storage of electricity (figure 2.5) [52-54]

Current research efforts relating to CO and CO2 methanation primarily focus on the optimization of processes developed in the 1970s and 1980s and pick up on a multitude of findings from that time However, new methods (e.g micro reactor manufacturing [55-57]) and material properties (e.g advanced heat transfer fluids [58]) unleash a potential for the adaptation and optimization of state-of-the-art technologies from the last century to meet new requirements of a more and more decentralized energy system To accompany and facilitate new methanation developments, this work is aimed at giving researchers a comprehensive overview of methanation research in the last century and of new developments against the background of a changing energy system The overview focuses on methanation fundamentals, mechanisms and catalysts

2.4.1 Fundamentals

Methanation processes aim to produce methane from hydrogen and carbon oxides The conversion of carbon monoxide is referred to as CO methanation (Eq (1)), the conversion of carbon dioxide as CO2 methanation, respectively (Eq (2))

of carbon monoxide releases 206 kJ heat per mole (Eq (1)), the conversion of carbon dioxide releases 164 kJ per mole (Eq (2)) For each 1 m3 methane (STP) produced per hour this corresponds to 2.3 and 1.8 kW heat, respectively In addition, the reaction is characterized by a significant volume contraction of the reacting gases, which is stronger for CO methanation (volume reduction by 50%) than for CO2 methanation (volume reduction by 40%)

CO2 methanation (Eq (2)) is a linear combination of CO methanation and reverse water–gas shift reaction (Eq (3)), which always accompanies the CO methanation reaction using nickel catalysts in practical operation [59] However, conversion of CO2 is inhibited, if the CO concentration exceeds a certain threshold [60]

CO H COH O g 41 kJ.mol-1 (at 298 K) (3)

The equilibrium of both reactions is influenced by pressure and temperature Detailed studies regarding the pressure and temperature dependence of methanation and water–gas shift reaction can be found in literature (e.g [61, 62]) Basically, these studies rely on thermodynamic equilibrium models available from commercial process simulation software, such as Aspen Plus or Chemcad

In thermodynamic equilibrium, high pressures favor the production of methane High temperatures, by contrast, limit methane formation The influence of pressure and temperature on the chemical equilibrium of CO methanation and water–gas shift reaction is shown in figure 2.6 by way of example The equilibrium composition was calculated based on a simplified equilibrium model described in [51] with equilibrium constants for CO methanation and water–gas shift reaction taken from [63]

Figure 2.6 Pressure and temperature influence on the equilibrium composition of CO

methanation and water–gas shift reaction (left: 300 oC, right: 1 bar) Educt gas

composition: yCO = 0.25,

2

H

y = 0.75 The influence of pressure and temperature on the equilibrium composition of

CO2 methanation and water–gas shift reaction is presented in figure 2.7

Figure 2.7 Pressure and temperature influence on equilibrium composition of CO2methanation and water–gas shift reaction (left: 300 oC, right: 1 bar) Educt gas

2.4.2.1 CO methanation

In the past, several reviews (e.g [64-67]) and studies (e.g [68-72]) were made with respect to CO methanation using different catalytic materials In addition, some mechanistic studies of the hydrogenation of CO on Ni were published (e.g [73-78]) A selection of mechanisms suggested in literature is given in figure 2.8

Van Meerten et al [73] studied kinetics and the mechanisms of methanation of

CO on a 5% Ni/SiO2 catalyst in a differential flow microreactor at 187T567 oC,

Figure 2.8 Mechanistic schemes for the catalytic hydrogenation of carbon monoxide

The symbols * and # represent ordinary (type 1) and 5-fold coordinated reaction sites

(type 2) [79], respectively

Van Meerten et al assumed that CO and H2 compete for the adsorption on the same sites, since their reaction orders mutually interact Only less than 1% of the Ni surface is expected to be active for the rate-determining step (RDS) of this suggested mechanism Hayes et al [76] used a tubular microreactor with a sample mass of up to

1 g at a pressure of 1.3 bar and temperatures of 270T380 oC for dynamic response studies of alumina-supported Ni catalysts prepared by impregnation and co-precipitation Furthermore, scanning electron microscopy, X-ray photoelectron spectroscopy, and computer-enhanced multiple reflectance infrared spectroscopy were applied to investigate the adsorption and desorption of CO, CO2, CH4, and H2 on the catalysts prepared The effects of different promoters (Ag, K, La, Pt, Ru) were studied

as well The dynamic response studies of Hayes et al suggested that the surface reaction of C and H is the RDS, while CO and H2 compete for dissociative adsorption (figure 2.8)

Sehested et al [79] investigated the methanation activity and adsorption of CO and H2 on a supported Ni catalyst and micrometer-sized Ni threads with a mass of up

to 0.4 g at low CO partial pressures (pCO < 25 mbar) in single-pass and recirculation reactors at a total pressure of 1.4 bar (mostly H2) and temperatures of 225T400 oC Two different reactive sites were suggested: Type 2 sites (depicted in figure 2.8) are connected to 5-fold coordinated sites that are energetically favorable for C adsorption only, while type 1 sites are related to step edges (depicted in figure 2.8), where all other species can adsorb It was found that CO and H2 compete for adsorption at low

CO pressures, while the surface is nearly completely covered by CO at higher CO pressures The suggested mechanism (see figure 2.8) includes non-dissociative adsorption of CO and dissociative adsorption of H2 at type 1 sites The dissociation of

CO at type 2 sites, which might amount to 5% of all surface nickel atoms only [79, 80], is defined as the RDS This is supported by experimental findings of Dalmon et al [74] All hydrogenation reactions are expected to proceed with nonadsorbed hydrogen (Eley–Rideal mechanism) Sehested et al comment that the hydrogenation mechanism might be different at lower H2/CO ratios, where more than one RDS may be active Since adsorbed CO is inactive relative to the active surface carbon, negative reaction

orders with respect to pCO result at moderate to high CO partial pressures and were observed by van Meerten, Dalmon, and Sehested et al [73, 74, 79]

Luu Cam Loc et al [81] also proposed reaction scheme for the methanation of

CO on nickel catalysts, as studied by isotopic and nonstationary methods (Z is an active surface site):

2.4.2.2 CO2 methanation

Methanation is a catalytic reaction Depending on the nature of the metal serving as the catalyst, either CO or CH4 is the main product of the reaction [82] Methanation of CO2, Eq (2), is a fairly simple reaction However, its mechanism is difficult to establish and some controversial opinions on the intermediates involved have been presented The reaction pathways of CO2 methanation are divided into two main categories The first one proposes the conversion of CO2 to CO via the reverse water gas shift reaction, and its subsequent reaction to methane through the same pathway as CO methanation [83-85] The second pathway proposes direct CO2methanation [3, 86] Nowadays, it is generally accepted for most catalysts that in CO2methanation, CO is the main intermediate It should be noted that even in case of CO methanation, an agreement on the reaction kinetics and mechanism has not been met

Most of the studies listed in this work focus on CO2 hydrogenation at H2 : CO2 ratios close to 4 : 1, which is the ratio most often applied in syngas hydrogenation Atmosphere with high excess of hydrogen, with the H2 : CO2 ratio of up to 100 : 1, which is typical for selective methanation reactions, may react following different mechanisms than those described in this work According to the first proposed mechanism of CO2 methanation through the main intermediate product CO, the Sabatier reaction (Eq (2)) is a combination of the reverse water gas shift reaction (Eq (3)) and CO methanation (Eq (1))

This means that after the CO2 adsorption and dissociation on the catalyst surface, CO2 methanation proceeds through the same route as CO methanation It should be noted that water as a side product can have negative effect on the methanation reaction Borgschulte, et al [85] stated in their work on CO2 methanation over nickel catalysts supported on zirconia that CO formed by the reverse water gas shift reaction is an important intermediate However, water removal from the reaction centres is critical to increase the reaction yield of CH4 and to minimize the release of

CO as a side product In their work, Weatherbee and Bartholomew [60] stated that the first step of CO2 hydrogenation is dissociative adsorption to hydrogen atoms, COad and oxygen atoms Adsorbed CO can either dissociate to carbon and oxygen atoms (Eq (4)) or desorb

An important factor to be established when describing methanation is the determining step Based on the above described reaction mechanism, the carbine species hydrogenation to CH2 was initially assumed as the rate-determining step [87] However, this assumption was abandoned in later works and different pathways have been proposed Generally, it is accepted that COad is produced via the reverse water gas shift reaction (Eq (3)) involving either a redox mechanism [88] or the formation and decomposition of formate species [89] Alternatively, dissociative adsorption of

rate-CO2 is also proposed [84] In their study on methanation on nickel catalysts, Coenen,

et al [90] proposed that the rate determining step can be either CO dissociation to surface carbon (Eq (4)) or CHO dissociation In case of CO dissociation, the reactions proceed according to Eqs (5– 9) CHO dissociation hypothesis introduces two new reactions: Eq (13) and consequent CHO interaction with hydrogen (Eq (14))

Reaction temperatures, pressure, used catalyst and the particle size of the catalyst influence the methanation reaction mechanism Taking this under consideration, dissociation of the CHO intermediate can be the rate-determining step for temperatures just below 850 K [91]

Even when the CO2 methanation occurs via CO formation, it does not necessarily mean that CO formation should proceed through the reverse water gas shift reaction Jacquemin, et al [33] performed CO2 methanation using a rhodium based catalyst, specifically rhodium supported on γ-Al2O3 (Rh/γ-Al2O3) It was observed that the first step in CO2 methanation is its dissociative adsorption (Eq (15)) to form COadand Oad on the surface of the catalyst

Formation of COad on the surface of the catalyst was proven by in situ DRIFT (Diffuse Reflectance Infrared Fourier Transformation) measurements The present bands were associated with oxidized Rh which reacted rapidly with hydrogen to form methane The most accepted pathway is COad dissociation to Cad according to Eq (4) Oxidization of rhodium occurring during the reaction confirmed that CO2 is dissociated on the surface of the catalyst and that the catalyst is oxidized by the Oad

species In a related work, Beuls, et al [92] performed CO2 methanation using an Rh/γ-Al2O3 catalyst and DRIFT measurements It was confirmed that CO2 dissociation

is responsible for the oxidation of Rh Additionally, their results support the mechanism proposed by Jacquemin, et al [33] Reaction intermediates in CO2methanation were investigated in a different study using ruthenium supported on alumina (Ru/γ-Al2O3) as catalysts [84] The intermediates were investigated by steady state isotopic transient kinetic analysis coupled with DRIFT experiments Due to the non-reducible support material of the Ru catalyst, the redox mechanism was excluded Formate mechanism was considered as highly unlikely to be the dominant rate determining reaction in this case The dominant formate mechanism would require a rapid decrease of the formate related bands after DRIFT analysis, which is in contrast with the experimental results Instead, during DRIFT analysis the formate related bands grow, indicating that the decomposition of formate species is too slow compared

to the COad exchange rate As a result it was proposed that on a Ru catalyst, CO2methanation proceeds via dissociative adsorption (Eq (3)) forming COad and Oad, which is the rate determining reaction of the process In a different study [31], steady-state transient measurements coupled with IR spectroscopy were performed using ruthenium supported on titania (Ru/TiO2) as the catalyst

2.4.3 A consistent mechanism for CO and CO2 methanation

The review of numerous literature sources reveals that mechanisms suggested for CO and CO2 methanation do not include sorbed oxygenated species (e.g CHO, HCOH, HCOO-) as reaction intermediates contrary to suggestions by some authors (e.g [31, 93-95]) The mechanism proceeding via the formation of surface carbon seems to be more likely, since this species forms typically on industrial methanation catalysts and reacts very slowly with hydrogen [68, 75, 96] Oxygenated intermediates might be important in the formation of higher hydrocarbons in Fischer–Tropsch synthesis [68] As discussed above, it is likely that CO2 hydrogenation proceeds by

CO2 adsorption and dissociation and subsequent hydrogenation of adsorbed CO [68,

77, 97] CO2 adsorption takes place preferably on the metal-support interface (if present), while CO2 dissociation takes place on the active metal surface [98] CO2conversion can be enhanced by a supporting material that fosters high CO2 coverages

(e.g Al2O3) or promoters on the catalyst surface, which improve the conversion of CO (e.g La, K, Na, V) [98] It can be decreased by an additive that suppresses CO2dissociation (e.g Cl) [98] Furthermore, small amounts of CO suppress the methanation of CO2, since the Ni surface is preferably covered by CO rather than by

CO2 [99] CO methanation (see figure 2.8) starts with CO adsorption on step edges of the active metal surface [79] Adsorbed CO, CO2, H2, H2O, and CH4 species seem to

be in equilibrium with the surrounding gas phase and to be located on these edges The active metal and the catalyst support, together with temperature and pressure, determine the equilibrium concentrations of these species on the catalyst surface The dissociation of CO into adsorbed C and O seems to take place on 5-fold coordinated sites of the active metal [79], where carbon atoms can form several bonds 5-fold coordinated sites are rare in comparison to step edges and amount to a few percent of the active metal surface only Their ratio may be increased by decreasing the crystallite size of the active metal or applying a combination of materials that improve formation (e.g Ni/AlVOx, Ru–Ni/Al2O3) High CO conversions at temperatures below 220 oC were reported [100] and might result from an increase in the amount of 5-fold coordinated sites Adsorbed C and O undergo stepwise hydrogenation reactions forming sorbed CH and OH species or inclusion reactions forming sorbed CH2 and desorbed H2O species [75], with gas-phase hydrogen in Eley–Rideal rather than in Langmuir–Hinshelwood surface reactions [79, 96] A hydrogenation mechanism that involves Eley–Rideal surface reactions is assumed especially under conditions in commercial methanation facilities (high partial pressures of H2 and CO), where the Ni surface lacks adsorbed hydrogen atoms The rate-determining steps in CO and CO2methanation are the dissociation of adsorbed CO and the hydrogenation reaction of surface C depending on reaction conditions [79, 96] Subsequent hydrogenation reactions of CHi species (1i3) are faster than the first formation of a C–H bond Unfortunately, it is not clear which of the two suggested RDS is rate-limiting under a certain condition CO dissociation might be the RDS at T < 300 oC, while the hydrogenation of C might become the RDS at higher temperatures according to Weatherbee and Bartholomew [60] This contradicts Klose et al who excluded the

dissociation of CO to be the RDS at T < 284 oC [75] On the other hand, Sehested et al suggested CO dissociation to be the RDS between 270 and 400 oC [79]

2.5 Catalysts for methanation

2.5.1 Active compound

2.5.1.1 Rhodium

Rh is one of the most investigated metals for the CO2 methanation reaction Particular emphasis has so far been placed on the hypotheses of a mechanism that allows methane to be formed on the surface of the catalyst, especially in the presence

of alumina as a support The steps leading to methane could be: (i) chemisorption of carbon dioxide; (ii) dissociation of carbon dioxide into CO and O adsorbed on the surface; (iii) reaction of dissociated species with hydrogen [33] The oxidation state of the metal may also play an important role in the evolution of the reaction, since CO2oxidizes the catalyst Moreover, the production of methane depends upon the temperature, pressure, presence, and absence of promoters Obviously, when varying the Rh content different metal particle sizes are formed, and at low temperatures (130–

150 ◦C) the activity of larger particle sizes of Rh was found to be higher than that of smaller ones Furthermore, the addition of Ba and K on the Al2O3 support allows significant differences in the catalytic behavior in the temperature range 300–700 oC

CH4 was preferentially formed below 500 oC on Ba-containing and pure Rh/Al2O3while, at higher temperatures, significant amounts of CO were formed Only CO was observed with the K-containing catalyst [33, 92, 101, 102]

The presence of O2 could have a positive effect on the methanation of CO2 and

CO Belus et al highlighted that oxygen, in low proportion, improves the catalytic performance of the Rh/γ-Al2O3 catalyst, but if the amount of O2 is too high, a negative effect is observed [92]

A synergistic effect was recently observed by mechanically mixing Rh/γ–Al2O3and Ni/activated carbon (Ni/AC) It appears that higher production of CH4, with respect to the single catalysts, is not due to a chemical interaction since no formation

of new structures occurs when the catalysts are mixed Authors hypothesize that

Rh/γ-Al2O3 is able to efficiently adsorb CO2, whereas Ni/AC adsorbs a high amount of H2

but a small amount of CO2, resulting in high CO2 conversion and methane formation [103]

TiO2 has been also extensively investigated as a support for Rh in the CO2methanation reaction, especially at low temperatures Rh/TiO2 is one of the most active catalysts for the reaction, but the high cost of the metal prevents its widespread utilization at industrial scale Such a high activity is thought to be attributed to a metal–support interaction that facilitates the breaking of the C=O bond, resulting in an increase of the catalytic activity Recently, the Density Functional Theory method (DFT) has been employed to study the CO2 methanation reaction on an Rh1/TiO2 (101) model [104] The results show the co-adsorption properties of the Rh1/TiO2 catalyst surface By the proposed mechanism, CO2 and H2 are co-adsorbed on the Rh atom and, subsequently, can react with each other to form CO Further adsorption of H2 is inhibited on the CO adsorbed on Rh; therefore, the consecutive CO hydrogenation does not occur The proposed mechanism explains why, experimentally, a high selectivity of Rh1/TiO2 to CO is observed, as a consequence of the frontier orbital charge density symmetry matching principle [105]

2.5.1.2 Ruthenium

Ru is one of the most active methanation catalysts Its catalytic activity and selectivity to CH4 are, however, largely dependent on the dispersion of the metallic phase (at high dispersion the apparent activation energy reaches a minimum), on the type of the support, and an addition of modifiers/promoters that can more or less chemically interact with the metal Ru catalysts have been supported on a number of oxide materials, such as Al2O3, TiO2, SiO2, MgO, MgAl2O4, C, and Ce0.8Zr0.2O2 [106, 107] The activity of pre-reduced 3%Ru/Al2O3 catalyst (CO2 conversion, CH4 and CO yields), as a function of the reaction temperature, is reported in [108] The best catalytic performance occurs at 673 K, with the highest CO2 conversion, CH4 yield, and limited CO yield

In particular, in order to lower the reaction temperature in CO2 methanation reaction, Ru/TiO2 catalysts have been used since the 1980s Akamaru et al [109] reported that they failed to reproduce the results of Gratzel et al [110, 111] using Ru/TiO2 catalysts prepared under the same experimental conditions The main reason

was the difficulty of controlling the preparation conditions in a wet process Therefore,

a dry technique, called polyhedral barrel sputtering, was developed; this technique, according to [112], is able to disperse metal nanoparticles on the support in clean conditions and control their size and deposition density Using a Ru/TiO2 catalyst prepared with this technique, an onset temperature of 60 oC for CH4 generation was observed that increases upon increasing the size of metal particles Recently, Xu et al have demonstrated that the Ru particle size is not the main reason for the different behavior of Ru/rutile (TiO2) catalyst [113] They reported that the increasing of hydroxyl groups on the TiO2 layer strongly increases the CO2 dissociation The suggested mechanism assumes that the adsorbed CO2 reacts with the hydroxyl groups, producing CO via formate species intermediates So, the pre-treatment temperature of the support should not be higher than 800 oC, because over this temperature value condensation of the hydroxyl groups occurs and the support does not play its important role

Results of DFT analysis attribute the high activity of the catalyst to the difference between the Ru structure and the Ru surface, and to the weak charge transfer from adsorbed species to Ru atoms It is recognized that CeO2 plays an important role in promoting CO2 methanation and enhancing CH4 formation On account of its basicity, CO2 is adsorbed on CeO2 and reduced due to the oxygen vacancies on CeO2, resulting in high CO2 conversion at T ≤ 350 oC The addition of 30% CeO2 to 2 wt% Ru/Al2O3 leads to an enlarged specific surface area of the catalyst Furthermore, the reaction intermediates (formates, carbonates) react with H2faster on this catalyst than over Ru/Al2O3 [114] Ceria is also a very good methanation catalyst when doped with 0.05 wt% Ru Electron microscopy and XRD analyses suggest that Ru is incorporated into the ceria lattice At T = 450 oC, this catalyst converts 55% CO2 with 99% selectivity to CH4 The reaction takes place on the reduced Ce0.95Ru0.05O2; the role of Ru is to lower the reduction temperature with respect to pure ceria [3]

Ru supported on TiO2–Al2O3 exhibits a much higher (3.1 times) activity than that on supported Al2O3 This result was attributed to the smaller averaged particle size

of Ru supported on TiO2–Al2O3 (2.8 nm) versus the 4.3 nm measured on Al2O3,

resulting from the interaction between the metal and the rutile TiO2, which hinders the aggregation of Ru particles [115]

Very recently, trimetallic catalysts drew attention to the reaction of the Baker group [116-118], which highlights how the adsorption strength of CO2 is controlled by the Lewis basicity of the catalyst, the d-band center of the metal surface, and the charge transfer from the metal surface to the chemisorbed CO2 Pd, Rh, and Ru supported on Mn/Cu–Ce–Al2O3 catalysts were also investigated and Ru/Mn/Cu–Al2O3was found to be the most promising catalyst (10.9 wt% Ru loading, calcination temperature 1035 oC) Baker et al claim the suitability of their catalysts for industrial application

2.5.1.3 Palladium

A good catalytic performance has also been observed with Pd-based catalysts

Pd is able to dissociate molecular hydrogen [119] and makes available hydrogen atoms for the subsequent transfer and reaction with activated surface carbonate species formed by the reaction of CO2 on a Mg-containing oxide [120-122], with the aim of providing a pathway to minimize CO formation by using metal oxides that inhibit CO desorption Intermixed Pd and Mg sites are obtained by using the reverse microemulsion synthesis route; 95% selectivity to CH4 and 59% CO2 conversion have been measured at 450 oC [123]

More recently, shape-controlled Pd nanoparticles embedded in mesoporous silica have been tested in the reaction; their performance has been compared with a Pd/SiO2 catalyst prepared by wet impregnation The encapsulation was demonstrated

to have a better stability towards sintering Moreover, different activities and selectivities for the CO2 methanation were demonstrated by the different exposed facets (100, 111) of the metal [124]

2.5.1.4 Nickel-Based Catalysts

Supported Ni catalysts are the most widely investigated materials for CO2methanation due to their high efficiency in CH4 production and low cost [125-128] A lot of supports have been investigated for Ni catalysts since, as is well known, the catalytic performance strongly depends upon the nature and properties of the support Its influence can be generally linked to physico-chemical peculiarities: (i) varying the

dispersion of the active phase; (ii) modifying the reducibility of the oxide precursors

by tuning the interaction between the active phase and the support

In this part of the thesis, we will examine the role of the most investigated supports for the hydrogen reaction with CO2 to CH4 (Al2O3 and SiO2) with particular emphasis on the role of promoters/modifiers in their catalytic performance in the reaction

2.5.2 Support

2.5.2.1 Alumina-Supported Nickel

The Ni/Al2O3 catalyst shows a high catalytic activity, although it suffers from severe carbon deposition or poor stability due to the high reaction temperature used [129, 130] Therefore, the aim throughout the years was to develop catalysts able to show both high activity and resistance to carbonaceous deposits in the reaction Rahamani et al [10] have prepared, by impregnation, a series of Ni catalysts supported

on mesoporous nanocrystalline γ-Al2O3, having high surface area and different Ni contents The influence of calcination temperature was also investigated on CO2conversion and CH4 selectivity The catalyst with 20 wt% Ni shows higher activity and stability between 200 oC and 350 oC According to He et al [130], the combination of highly dispersed Ni particles with a strong basic support is thought to be responsible for the high performance of the Ni–Al hydrotalcite-derived catalyst Such basic sites are originated from the formation of the Ni–O–Al structure; 100% CH4 selectivity and 82% CO2 conversion are reached at 350 oC

In [131] a study on the methane yield as a function of different Ni-based compositions and reaction temperatures (250–500 oC) is reported The data suggest that good performance is obtained at the expense of a CO intermediate on the corners

of nanoparticles interacting with alumina, likely via an oxygenate mechanism

In the light of a fluctuating supply of renewable hydrogen, various IR studies, under both stationary and transient conditions, have been done in order to get insight into the reaction mechanism Since Ni is very prone to oxidation, studies have been carried out under “operando” conditions [132-134]

The results reported in [134] (81% CO2 conversion, 80% CH4 yield at 400 oC with a 23 wt% of Ni/CaO/Al2O3 catalyst), however, strongly suggest the importance of

conducting experiments under realistic (i.e., transient) conditions To ameliorate the performance of Ni/Al2O3 catalysts via substantial modifications of some specific (structural, electronic) properties, the addition of several promoters has been also attempted Among these, CeO2 has often been employed on account of some positive effects, such as: (i) improvement of the thermal stability of Al2O3; (ii) promotion of the dispersion of the metal onto the support; (iii) change of the properties of the metal due to strong metal–support interaction (SMSI) [130, 135] Moreover, CeO2 is a well-known oxygen storage material, able to store and release in a reversible manner large amounts of oxygen depending on the experimental conditions adopted The results show that the presence of CeO2 has significant influence on CO2 conversion (for Ni/Al2O3, from 350 oC to 400 oC, 45% vs 71% with a CeO2 content of 2 wt%) CH4selectivity, independently from the CeO2 content, is higher than 99% Stability was measured for a reaction time up to 120 h [136] In [137], the effect of different promoters (CeO2, MnO2, IrO2, La2O3) on the catalytic performance of Ni supported on

Al2O3 mesoporous nanoparticles was investigated Both CO2 conversion and CH4selectivity were demonstrated to be affected by the different promoters The catalyst promoted by ceria exhibits the highest activity; also in this case, the catalyst with 2 wt% ceria was the most active and selective to CH4 The best results were obtained at

350 oC Few works have been published on Ni supported on zeolites for CO2methanation [97, 138] Recently, Graca et al [139] prepared, by ion exchange and impregnation, Ni–Ce catalysts supported on a partially exchanged HNaUSY zeolite

Since temperatures >800 oC are necessary to reduce the exchanged Ni species, those present on the catalyst prepared by Graca et al [139] are not in the metal form during the reaction; the activity observed was attributed to the zeolite support Moreover, addition of ceria causes an improvement in both CO2 conversion and CH4selectivity [139] Thus, the catalyst properties result from the synergistic effect between the metal active sites and the promoters The “operando” IR spectroscopy technique was also used to get a better insight into the mechanism of the reaction According to [140], dissociated hydrogen reacts with carbonates and/or physisorbed

CO2, giving rise firstly to formation of monodentate formates, then carbonyls, and finally to CH4 Recently, Rivero-Mendoza et al reported La–Ni/γ-Al2O3 catalytic