Prof Philippe Guibert

Guillaume Legros

Alexis Matynia

Evelyne Mignon

Anne Marchal

Renaud Jalain

Jean-François Krawczynski

Jean-Marie Citerne

Jérôme Bonnety

Hugo Dutilleul

Christian Ollivon

Dominique Busquet

Keywords: Reformage à sec du méthane, Gaz de synthèse, Catalyseurs au nickel, ZrO2 synthétique, Solution solide Dry reforming of methane, Syngas, Nickel catalysts, Synthetic ZrO2, Solid solution

In the end, I want to thank my friends

Carbon dioxide reforming of methane also called dry reforming of methane (DRM) is of significant importance because of at least the following reasons. Firstly, since CO2 and CH4 were both greenhouse gases, the utilization of CO2 and CH4 could provide a way to reduce the greenhouse effect. Secondly, CO2 and CH4 were also produced in the pyrolysis of biomass, then the utilization of CO2 and CH4 could also make more valuable the pyrolysis gases, enhancing the whole process more practical. Thirdly, because the main components of the marsh gas from digestion were CO2 and CH4, the utilization of CO2 and CH4 could provide a way to make the biogas value-added.

However, if pyrolysis gases and biogas were directly used as fuel, the purification of the gases to remove CO2 was generally needed, since CO2 could not contribute to combustion, providing heat. Gases with a high content of CH4 could also be used in fuel cells for electricity generation, and synthesizing organic compounds such as CHFCl2, CCl4, CH3OH, and HCOOH. On the other hand, in addition to the use of CO2 in beverage additives, CO2 could be used in supercritical extraction agents and synthesizing polyketide and polycarbonate. Whereas the DRM reaction utilizes directly both CO2 and CH4 simultaneously, which reduces the step of separation. Hence the DRM reaction is an effective way for the protection of the environment and the effective utilization of energy resources, thereby providing ways for sustainable development.

It is well known that the C-H bond in CH4 is difficult to be activated, while CO2 is the upmost oxidized state of carbon, which is also very stable. The co-activation of both C-H bond in CH4 and C-O bond in CO2 faced challenging difficulties. Besides, because of the thermodynamics limitation, DRM reaction was usually performed at high temperature (~800 °C). On that conditions, other reactions would influence the performance of the catalysts for DRM reaction (Equation 1), among which were methane cracking (Equation 2), the reverse water-gas shift (RWGS) reaction (Equation 3), and the disproportionation of CO (Equation 4). CH4+CO2→2CO+2H2 (Equation 1) CH4→C+ 2H2 (Equation 2) H2+CO2→H2O+CO (Equation 3) 2CO→CO2+C (Equation 4) Many researches already focused on Pt, Ir and Rh for DRM reaction, because noble metal exhibited high activity, stability and selectivity for DRM reaction even at so high temperature. However, noble metals are very expensive, so more and more researches investigate on Ni-based catalysts for dry reforming of methane. Because Ni-based catalysts with activity and selectivity comparable to those of noble metals, but with lower cost, have been extensively employed as promising catalysts for reforming of methane. While, high-temperature operation usually caused metal Ni sintering, which would lead to catalyst deactivation. Another challenge is that carbon could form on nickel metal, thereby leading to the deactivation of catalyst. So, research dose much investigation to solve those problems. Promoters, such as La, Ce, La, Zr and so on, can inhibit the sintering of nickel metal, restrain the formation of carbon and enhance the ability to remove coke. The supports possess basic sites, the periclase-like structure and mesoporous structure would enhance the activity for DRM reaction. Those special material supports would exert effect on nickel particle size and dispersion of nickel, thereby improve the property of the catalysts. Some special catalysts like core-shell catalysts could prevent the accumulation of carbon, thereby enhancing the stability of catalyst. Although many efforts, such as the addition of promoter, the special supports, mesoporous structure and Core-shell structure, have been made in solving the problems, there are many challenges to achieve industry for Ni-based catalysts. ZrO2 had been widely used for reforming catalysts, because of the high oxygen mobilization, enhancement of metal dispersion, and the proper interaction between nickel and ZrO2.

Besides, ZrO2 could increase the basic site, which could enhance the adsorption of CO2, thereby enhance the ability for carbon deposition elimination, as a consequence, and promote the performance of catalyst. While if it possesses too strong basic site, it could promote carbon deposition. So, we should adjust the interaction between nickel and zirconium by synthesis method and the promoter. My Ph.D. research concentrates on the Ni-based catalyst with new ZrO2 support for dry reforming of methane. It also investigates the effect of synthesis method and mesoporous structure ZrO2 adjusted by Al, Mg, Y and Mn on the performance of catalyst for dry reforming of methane.

The thesis is thus divided into 7 chapters. Chapters 1-3 include the general introduction, research background and experiments of catalytic activity and characterizations respectively. The fourth, fifth and sixth chapters introduce the main experimental results, including a series of Ni-Zr catalysts, synthetic Ni-Zr catalysts modified by Mg, Y, Al and Mn, and the relationship between structure and activity. The seventh chapter is a summary of the experimental results

The first chapter is a general introduction to this thesis. The general contents of various chapters and the research objectives are introduced.

The second chapter presents a literature review on the research background. At firstly, as carbon dioxide emissions increase, as well as the severe greenhouse effect, the utilization of carbon dioxide is becoming more and more important for our environment.

The source of carbon dioxide is introduced. Secondly, the three aspects of the capture, fixation and utilization of carbon dioxide are described as the second part. Thirdly, synthetic fuels are introduced, which is developing potential in CO2 resources utilization. The definition of synthetic fuels, synthetic routes and problems that need to be solved in the development of synthetic fuels are also discussed. The fourth part presents the introduction of the dry reforming of methane, including three aspects of experiment methods, mechanism and catalysts.

The third chapter mainly introduces the experimental part. In the first part an introduction of the catalytic test is presented, including the procedure of activity test based on temperature effects, and the stability test, as well as the calculation methods of both CH4, CO2 conversion and H2/CO ratio. The second part describes the different characterizations (BET, XRD, H2-TPR, CO2-TPD, TGA, TEM, XPS and Raman).

The four chapter focuses on the comparison of the catalytic performance between a series of Ni-Zr catalysts prepared by one-step urea hydrolysis method and Ni/ZrO2 catalyst prepared by traditional ethanol impregnation method. The catalysts were tested in dry reforming of methane (DRM) at 750 °C with a mixed flow of CH4:CO2:Ar=10:10:80 and GHSV= 24,000 h -1 . The materials were characterized by means of N2 adsorption-desorption, X-ray diffraction, temperature-programmed reduction by H2, temperature-programmed desorption of CO2, X-ray photoelectron spectroscopy, and thermogravimetry analysis coupled by mass spectrometry. In this work, the NiO-ZrO2 solid solution could obtain by new method with strong interaction between nickel and zirconium, which leads to the high surface area, small nickel particle size and more content of weak and medium-strength basic sites. Besides, more adsorption oxygen species and just slight sintering occur on 10Ni-Zr and 15Ni-Zr catalysts during the reaction. Therefore, 10Ni-Zr and 15Ni-Zr exhibited higher activity and stability for dry reforming of methane.

In chapter fifth, the results of Ni-Zr catalysts modified by Y, Mg, Al and Mn are presented. All the catalysts are prepared by one-step urea hydrolysis method. In the first part, Y is introduced to Ni-Zr catalyst. And found that yttrium-doped NiO-ZrOm catalyst was found to be novel for carbon resistance in the CO2 reforming of methane.

The second part is the Mg promoter. The NiO-ZrO2 catalyst modified by Mg results in a decrease in the activity, selectivity, and stability of NiO-MgO-ZrO2 catalyst for DRM.

The promoter of Al is the third part. An enhancement in the sintering resistance and high resistance to loss of nickel during a long-term catalytic test was found on Al promoted catalyst. Mn is the last part. Mn could promote the stability of ZrO2 support during the reaction process.

The chapter sixth compared the effect of the promoters on the catalytic performance of Ni-Zr catalysts for dry reforming of methane. The calcined, reduced or after reaction catalysts were characterized by H2-TPR, CO2-TPD, XRD, XPS, TGA, Raman and TEM.

Meanwhile, the specific surface area, Ni 0 crystallite size, Ni reducibility, basicity, surface species and carbon deposition are studied to understand the relationship between structure and activity, selectivity and stability of Ni-Zr based catalysts.

Finally, I conclude this thesis by a summary of all experiments and the perspectives for further experiments on dry reforming of methane. The main results of the experimental chapters (chapter 4, chapter 5 and chapter 6) are summarized, which could contribute to the further study of Ni-Zr catalysts and synthetic ZrO2 supported catalysts. Besides, based on doctoral research, some suggestions are provided for further research on dry reforming of methane.

Objectives of Ph.D. work

The aims of this Ph.D. thesis to:

 Investigate a series of Ni-Zr catalysts and Ni/ZrO2 catalysts to obtain the optimal nickel content of Ni-Zr catalyst, at the same time, investigate the influence of different content nickel metal on performance of catalyst.

 Investigate Ni-Zr catalysts modified by Al, Mg, Y and Mn on performance of dry reforming of methane. as well as understand their physicochemical properties by a variety of characterization, such as H2-TPR, CO2-TPD, TG-MS, XPS, XRD, TEM, IR.

 Investigate the influence of the different promoters on the physicochemical properties of ZrO2 materials to support Ni-based catalysts for dry reforming of methane.

Thus, this research topic could help to understand how to enhance the properties of ZrOx materials, and then obtain the optimal catalyst for dry reforming of methane.

Chapter II

Literature review on the utilization of carbon dioxide

2. Literature review on the utilization of carbon dioxide

Source of carbon dioxide

With the development of the economic world, the demand for energy is increasing, such as, crude oil, natural gas and coal. However, those carbon resources would decrease with our utilization, simultaneously, release the CO2. Thus, there is an urgent need to find a new renewable carbon resource. Except for the biomass and pyrolysis gases as the perfect gas for renewable resources, CO2 could be also created renewable carbon resources [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF]. Besides, the content of CO2 in the air has increased rapidly since industrial time, because of the increasing of combustion of fossil fuels. Still in 2000, the amount of CO2 in the atmosphere increased to 720 GT/year (1 GT = 1*10 9 tons), and the detailed data showed in Figure 2-1. Interestingly, the carbon dioxide produced from the ocean can reach to 100 GT for one year. The amounts of biological respiration are the same as the photosynthesis land and water, which is approximately 60 GT.

Except for the carbon dioxide produced by humans, plants and animals, about 500 GT comes from human activity, which mainly comes from fossil fuels. Large amounts of carbon dioxide are released into the atmosphere, leading to an increase in the content of CO2 year-by-year. warming potential (GWP) of methane is about 28-36 times higher than that of CO2, which also causes a serious Earth's radiative imbalance [START_REF] Abdulrasheed | A review on catalyst development for dry reforming of methane to syngas: Recent advances[END_REF][START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF]. Thus, there are so many CO2 resources we can use, especially the carbon dioxide produced by fossil fuels. Although that carbon dioxide comprises only about 0.034 % by volume in air, it can lead to the acidity (pH 5.7) of precipitations from the troposphere, as well as the natural acid rain, which could result in a series of social problems, such as corrosion of buildings. Besides, CO2 is greenhouse gas, the same as methane, which is mainly responsible for global warming and environmental problems [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF]. It is pointed out that the global greenhouse gas emission sources in 2004 of which approximately 77 % are represented by CO2 emissions, as shown in Figure 2-3. [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF]. 56.6 % comes from carbon dioxide derived from the consumption of fossil fuels. In addition, the demand for energy increases year-by-year, as shown in Figure 234. Most of them are oil, coal and gas. The utilization of those energies (oil, coal and gas) could increase CO2 emission. So, there is a desperation to decrease the emission of CO2 and creating a renewable carbon resource. The reduction of fossil fuel consumption is the main way to decrease the emission of CO2. However, we can't leave for the utilization of fossil fuel in the 21 st . Therefore, in order to meet the amount of energy consumption, an alternative resource should be created. Thus, to reduce the amount of carbon dioxide in the atmosphere, the utilization of CO2 plays a very important role in the environment and energy.

Figure 234The annual demand for energy.

The utilization of carbon dioxide

Capture of carbon dioxide

It is widely accepted that the majority reduction of CO2 in the atmosphere is now mainly due to the capture of carbon dioxide and storage (CCS). On one hand, the content of CO2 in the atmosphere is very high. On the other hand, the huge emission of CO2 from the fossil fuel would continue until 2030 and beyond. Therefore, the reduction of CO2 in the atmosphere by the capture of carbon dioxide plays a very important role in the aspect of global warming [START_REF] Mumford | Review of solvent based carbondioxide capture technologies[END_REF].

There are two ways to capture CO2. The most feasible solution is to compress the CO2 in the flue gas of the power plant and inject it into a closed underground environment, which is convenient for the storage of CO2 without causing leakage and has a small impact on the surrounding environment. This method is suitable for burying carbon dioxide in the exhausted oil and natural gas wells, coal, shale, and saline formations for a long time. The need for long burials and special terrain has hindered widespread development [START_REF] Benson | Carbon dioxide capture and storage: An overview with emphasis on capture and storage in deep geological formations[END_REF]. Another way is capturing the gaseous CO2 by some adsorption material, such as the capture of CO2 from power plant emissions. In general, carbon dioxide is absorbed by special materials, which are difficult to reuse and possess low adsorption capacity. It is generally known that the selectivity of CO2 is the crucial performance parameter for the materials of CO2 capture. High selectivity is fundamental, such that the CO2 could be removed from the flue gas completely. However, the affinity between materials and CO2 should be optimized. Because a too strong interaction would lead to higher energy for desorption of CO2, while too weak interaction would result in too weak adsorption of CO2. Furthermore, the high stability under the conditions of capture and regeneration also should be considered. Thus, some special materials, such as aqueous alkanolamine absorbents, solid porous adsorbent materials and metal-organic frameworks are widely concerned for CO2 capture, due to the suitable interaction [START_REF] Rochelle | Amine scrubbing for CO2 capture[END_REF][START_REF] Cejka | Zeolites and catalysis: synthesis, reactions and applications[END_REF][START_REF] Bao | Adsorption of CO2 and CH4 on a magnesium-based metal organic framework[END_REF][START_REF] Z.R. Yazaydın | Screening of metal-organic frameworks for carbon dioxide capture from flue gas using a combined experimental and modeling approach[END_REF].

Many researchers have focused on the investigation of CO2 capture in metal-organic frameworks (MOF) [START_REF] Bao | Adsorption of CO2 and CH4 on a magnesium-based metal organic framework[END_REF][START_REF] Z.R. Yazaydın | Screening of metal-organic frameworks for carbon dioxide capture from flue gas using a combined experimental and modeling approach[END_REF][START_REF] Britt | Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites[END_REF]. Because except for above affinity, metal-organic frameworks possess their potential applications in a variety of aspects, including gas storage, drug delivery, heterogeneous catalysis, molecular separations, extraordinary surface areas, potential scalability to industrial scale, as well as finely tunable pore surface properties. Such as Zn4O(BDC)3 (MOF-5), as shown in Figure 2-6 [START_REF] Furukawa | Ultrahigh porosity in metal-organic frameworks[END_REF][START_REF] Ko | Enhanced carbon dioxide adsorption on post-synthetically modified metal-organic frameworks[END_REF].

Thus, metal-organic frameworks show the good activity of the material for CO2 capture.

While metal-organic frameworks show low chemical and thermal stability, which is generally lower than that of zeolites and other porous inorganic solids. Because the coordination bonds of metal and ligand components are relatively weak [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF]. McDonald [START_REF] Mcdonald | Capture of carbon dioxide from air and flue gas in the alkylamine-appended metalorganic framework mmen-Mg2 (dobpdc)[END_REF] prepared metal-organic frameworks mmen-Mg2 (dobpdc) materials to capture CO2. Dynamic gas adsorption/desorption cycling experiments indicated that the mmen-Mg2 (dobpdc) materials could be regenerated at the condition of repeated exposures to simulated air and flue gas mixtures. And the cycling capacities is 1.05 mmol/g (about 4.4 wt %) after 60 min of exposure to flowing carbon dioxide (390 ppm)

in simulated air at 25 °C, and 2.52 mmol/g (about 9.9 wt %) after 15 min of exposure to flowing carbon dioxide (15%) in nitrogen at 40 °C. The purity of the carbon dioxide removed from dry air and flue gas in these processes reached to 96% and 98%, respectively. In brief, the good performance ≪ characteristics ≫ of mmen-Mg2

(dobpdc) materials manifested that compared with both amine-grafted aqueous and silicas amine solutions, it would become to be an exceptional new adsorbent for carbon dioxide capture. Huang et al. [START_REF] Huang | Two-dimensional covalent organic frameworks for carbon dioxide capture through channel-wall functionalization[END_REF] provided a new method by channel-wall functionalization for preparing a new adsorbent two-dimensional covalent organic frameworks (2D COFs) to capture CO2, which exhibited the outstanding ability for CO2 capture. They thought that the present channel-wall engineering method would be of vital importance to exploring high-performance 2D COFs in the field of gas storage and separation.

A lot of CO2 is adsorbed and separated from the atmosphere, which is the first step, and the second one is the utilization of carbon dioxide, e.g. fixation or conversion.

Fixation of carbon dioxide

Carbon dioxide is a very widely available and cheaper carbon resource as important chemical intermediates, compounding to polymers, pharmaceuticals, and agrochemicals [START_REF] Halmann | Chemical Fixation of Carbon Dioxide Methods for Recycling CO2 into Useful Products[END_REF]. Besides, fixation CO2 can be transformed to carbonate species, which are important for various applications, such as aprotic solvents, additives in fuel, monomers in polymeric structures, raw materials in various reactions, and electrolytes in secondary batteries [START_REF] Huang | Recent advances in dialkyl carbonates synthesis and applications[END_REF]. Therefore, the fixation of carbon dioxide is the way as a carbon feedstock to build organic structures with high atom economy [START_REF] Bhanage | Transformation and utilization of carbon dioxide[END_REF].

Actually, the main way of CO2 fixation is fixed by photosynthesis in plants, microorganisms and algae. And the other way is the alternative autotrophic CO2 fixation pathways [START_REF] Berg | Ecological aspects of the distribution of different autotrophic CO2 fixation pathways[END_REF][START_REF] Fuchs | Alternative pathways of carbon dioxide fixation: insights into the early evolution of life[END_REF]. For the latter, there are many methods for fixation and utilization of carbon dioxide, such as, traditional catalytic, photocatalytic, electrochemical, photochemical, and photoelectrochemical. Most of them are catalytic systems, which have been investigated to catalyze the fixation of carbon dioxide in the past decades, for example, metal oxides, metal-salen complexes, N-heterocyclic olefins, Nheterocyclic carbenes, Schiff bases and Ionic liquids [START_REF] Martín | Recent advances in the catalytic preparation of cyclic organic carbonates[END_REF][START_REF] Aresta | Nb (V) compounds as epoxides carboxylation catalysts: the role of the solvent[END_REF][START_REF] Zhou | N-Heterocyclic carbene functionalized MCM-41 as an efficient catalyst for chemical fixation of carbon dioxide[END_REF][START_REF] Wang | CO2, COS and CS2 adducts of N-heterocyclic olefins and their application as organocatalysts for carbon dioxide fixation[END_REF][START_REF] Shen | Chemical Fixation of Carbon Dioxide Co-Catalyzed by a Combination of Schiff Bases or Phenols and Organic Bases[END_REF]. Among them, the Ionic liquids (ILs) are a new outstanding research field. The utilization of ILs for the CO2 fixation into value-added chemicals is considered a promising technology, due to the high thermal and chemical stability, high solubility, easy recyclability, nonflammability, low vapor pressure, functional variability, and biodegradability [START_REF] Wang | Imidazolatsysteme zur CO2-Abscheidung und photochemischen Reduktion[END_REF][START_REF] Fiorani | Sustainable conversion of carbon dioxide: the advent of organocatalysis[END_REF][START_REF] Wang | Imidazolium ionic liquids, imidazolylidene heterocyclic carbenes, and zeolitic imidazolate frameworks for CO2 capture and photochemical reduction[END_REF].

Dai et al. [START_REF] Chen | Multi-molar absorption of CO2 by the activation of carboxylate groups in amino acid ionic liquids[END_REF] found that amino-acid-based ILs act as bifunctional absorbing materials of CO2 through the amine group and carboxylate group. It is pointed out that nontoxic, recyclable, and biodegradable ILs are more popular in the research of fixation CO2.

Saptal et. al [START_REF] Saptal | Bifunctional ionic liquids derived from biorenewable sources as sustainable catalysts for fixation of carbon dioxide[END_REF] prepared a new multimolar Ionic Liquids (ILs) with a quaternary alkyl ammonium cation and an amine anio to fix the carbon dioxide. Those synthesized ILs not only exhibited highly catalytic performance, but also were the biodegradable sources with the Non-toxic and recyclable properties.

General application of carbon dioxide

CO2 is also commonly used in the food, physics and agriculture industry. At first, CO2 as food additives in beverages. CO2 could be used for sparkling wine, soft drinks and beverages, and the amount depends on the requirements of normal production. For example, making soft drink, increased the amount of carbon dioxide by pressurizing, so that the carbonic acid of calcium carbonate reacts with water. Generally speaking, it is produced by fermentation of sugar, washed, purified, dried, cooled, and then compressed into a liquid. CO2 is primarily used in carbonated drinks, such as carbonation agents, packaging gas, refrigerant, leavening agent, pH regulator and processing aid [START_REF] Aresta | Catalysis for the valorization of exhaust carbon: from CO2 to chemicals, materials, and fuels[END_REF][START_REF] Li | MnOx-Promoted[END_REF].

Carbon dioxide, which would be a gas under standard conditions, forms a solid when cooled, called dry ice (Frozen solid CO2), which is applied for refrigeration of foodstuffs, meat products and other frozen foods. Dry ice has also found application in many technical processes where low temperature is required.

CO2 is the most used supercritical fluid. When the temperature and pressure are higher than the critical temperature (31°C) and pressure (73.8 atm) of CO2, the properties of CO2 will change. The density of CO2 is close to liquid and its viscosity is close to gas.

This CO2 is called the supercritical state CO2. There are several advantages to the use of supercritical fluid CO2 as an extraction agent. Firstly, the solubility of many substances in carbon dioxide changes with pressure, so selective extraction can be carried out. Secondly, the CO2 is very stable, thus the properties would not change during the process of extraction. Thirdly, CO2 is less toxic and harmful to the environment. Therefore, CO2 supercritical fluid has been used in the food industry, cosmetic industry, chemical industry and medical industry currently [START_REF] Bhusnure | Importance of supercritical fluid extraction techniques in pharmaceutical industry: A Review[END_REF].

The use of CO2 in agriculture is focused on application of carbon dioxide in photosynthesis systems by chlorophyll in plants. The biological technologies of CO2 in man-made active photosynthesis systems to produce food biomass and biofuels have been obtained in the laboratory [START_REF] Quadrelli | Carbon dioxide recycling: emerging large-scale technologies with industrial potential[END_REF]. Carbon-based fossil fuels, such as natural gas, oil, and coal supplied the main of the world's energy. However, the sources of fossil energy are nonrenewable, and they are used out finally. So, renewable energy sources have gained an increased attention.

Synthetic fuels

There is various way to synthesize fuels, most of them are synthesized by syngas. The syngas is a mixture of gas with CO and H2. As shown in Figure 2-7, the syngas can synthesize ethanol, isobuthane, hydrogen, methanol, ammonia and so on. Co and Rh usually used to make ethanol. While Cu has been identified as the most active metal to synthesize methanol [START_REF] Santos | Biomass-derived syngas production via gasification process and its catalytic conversion into fuels by Fischer Tropsch synthesis: A review[END_REF]. Another important utilization of syngas is the Fischer-Tropsch. This will be explained in detail in the next section.

Fischer-Tropsch (FT) synthesis

The Fischer-Tropsch (FT) synthesis is an extremely important route to utilization of the carbon monoxide hydrogenation to produce fuels, like higher hydrocarbons and oxygenates compounds. The Fischer Tropsch synthesis started in the 20 th century and went through three periods [START_REF] Stranges | A history of the fischer-tropsch synthesis in Germany 1926-45[END_REF]: Cobalt-based catalysts were applied in most modern FT synthesis, especially for gasto-liquids reaction (GTL) [START_REF] Van Steen | Comparing a cobalt-based catalyst with iron-based catalysts for the Fischer-Tropsch XTL-process operating at high conversion[END_REF][START_REF] Van Deelen | Cobalt nanocrystals on carbon nanotubes in the Fischer-Tropsch synthesis: Impact of support oxidation[END_REF]. Deelen et al. [START_REF] Van Deelen | Cobalt nanocrystals on carbon nanotubes in the Fischer-Tropsch synthesis: Impact of support oxidation[END_REF] found cobalt nanocrystals supported on a carbon nanotube (CNT) exhibited high stability for FT synthesis even a reaction of 100 h. Because, the cobalt is relatively expensive, Fe is an alternative active metal [START_REF] Panzone | Power-to-Liquid catalytic CO2 valorization into fuels and chemicals: focus on the Fischer-Tropsch route[END_REF]. However, some challenges still have to be faced. The Co-based catalyst suffered from the deactivation, due to the aggregation of active metal. The control of product selectivity depends on catalytic property and operational conditions, which is the main challenge of FT synthesis research [START_REF] Mehariya | Fischer-Tropsch synthesis of syngas to liquid hydrocarbons[END_REF].

other methods

In addition to Fischer-Tropsch (FT) synthesis, biomass offers the possibility to create a renewable synthetic fuel. Lignocellulosic biomass is the renewable resource, who has gained much attention. Because the production of biofuels has substantially less harmful effects on the environment when compared to the fossil non-renewable process.

The following is the main routes of producing fuels from the Lignocellulosic biomass:

(1) gasification to produce syngas, (2) liquefaction and pyrolysis to produce bio-oils,

(3) hydrolysis to form aqueous phase sugars or lignin, which can be further processed via fermentation or reforming, and (4) transesterification of triglycerides to produce biodiesel [START_REF] Yung | Review of catalytic conditioning of biomass-derived syngas[END_REF][START_REF] Devi | A review of the primary measures for tar elimination in biomass gasification processes[END_REF]. These four sustainable routes in the production of liquid biofuel by low-cost feedstock may reach the industrial level in the further.

Mixed alcohol synthesis (MAS) is another way to produce fuels [START_REF] Fang | A short review of heterogeneous catalytic process for mixed alcohols synthesis via syngas[END_REF]. MAS is a process that alcohol produces from coal or natural gas derived from syngas (Equation 4). The obtained mixed alcohols can be used as fuels, fuel additives for octane, and intermediates for value-added chemicals such as medicine, cosmetics and polyester [START_REF] Mahdavi | Synthesis of C1-C6 alcohols over copper/cobalt catalysts: investigation of the influence of preparative procedures on the activity and selectivity of Cu-Co2O3/ZnO, Al2O3 catalyst[END_REF]. This process realizes the efficient conversion of natural gas and clean utilization of coal resources. This process is a highly exothermal reaction (e.g. 2 CO(g) + 4 H2 (g) → C2H5OH (g) + H2O (g), ∆H298 = -253.6 kJ/mol of ethanol, ∆G298 = -221.1 kJ/mol of ethanol), which essentially needs the combination of CO insertion functions and carbon-chain growth over the catalysts. Thus, the key to the development of mixed alcohol synthesis is the selective control of alcohol. Usually, considering the commercial production of mixed alcohols, it is most important to achieve higher C 2+ alcohol selectivity and comparable alcohol yield. Those two side reactions composed of hydrocarbon formation (Equation 5) and water-gas-shift reaction equilibrium (Equation 6) should be avoided in this process [START_REF] Fang | A short review of heterogeneous catalytic process for mixed alcohols synthesis via syngas[END_REF].

2n H2 + n CO → CnH2n+1OH + (n-1) H2O (Equation 4) (2n+1) H2 + nCO → CnH2n+2 + n H2O (Equation 5)

H2O + CO → CO2 + H2 (Equation 6)

Dry reforming of methane

Experiment condition

Dry reforming of methane (DRM) is an effective way to use two greenhouse gas (methane and carbon dioxide) at the same time to produce syngas (hydrogen and carbon monoxide), which is not only waste utilization, but also beneficial to the environment.

As the global warming problem is more serious, more and more researchers turned to investigate dry reforming of methane, due to the utilization of two greenhouse gas (methane and carbon dioxide). So, DRM reaction is very meaningful for environmental sustainability, especially for the shortage of energy. Besides, DRM reaction (CH4 + CO2 → 2CO + 2H2) produces the two gas with CO/H2 ratio of 1, which could produce methanol by F-T synthesis because of the very suitable CO/H2 ratio [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF]. However, a large number of parallel side reactions, such as the CO disproportionation, CH4 decomposition, reverse water gas shift and so on (Table 2-1 ), take place during the DRM reaction [START_REF] Nikoo | Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation[END_REF]. To avoid the side reactions, several methods are conducted on the dry reforming of methane, such as the traditional catalytic method, Plasma catalytic method, Photocatalytic method and Microwave-assisted catalytic method.

CH 4 → C + 2H 2 +74.9 2CO → C + CO 2 -172.4 CO + H 2 → C + H 2 O -131.3 CO 2 + 2H 2 → C + 2H 2 O -90
Oxidative coupling of methane

CO 2 + 2CH 4 → C 2 H 6 + H 2 O + CO +106 2CO 2 + 2CH 4 → C 2 H 4 + 2H 2 O + 2CO +284 dehydrogenation C 2 H 6 → C 2 H 4 + H 2 +136
Hydrogenation of CO and CO2

CH 3 OCH 3 + CO 2 → 3CO + 3H 2 +258.4 14 C 2 H 6 + 2H 2 O → 2CO + 5H 2 +350 15 C 2 H 6 + 2CO 2 → 4CO + 3H 2 +430 16 C 2 H 4 + 2CO 2 → 4CO + 2H 2 +290 17 C 2 H 4 + 2H 2 O → 2CO + 4H 2 +210 Dehydration 18 2CH 3 OH → CH 3 OCH 3 + 2H 2 O -37 Hydration 19 CH 3 OCH 3 + 3H 2 O → 2CO 2 + 6H 2 +136 20 CH 3 OCH 3 + H 2 O → 2CO + 4H 2 +204.8 Methanation 21 CO 2 + 4H 2 → CH 4 + 2H 2 O -165 22 CO 2 + 3H 2 → CH 4 + H 2 O -206.2

Traditional catalytic method

Most researchers investigated the traditional catalytic method for dry reforming of methane. The traditional catalytic method achieves the reaction temperature by heating, which is very convenient to operate. Because it needs a very high temperature, so that the energy consumption is too large, especially for industrial applications. Under traditional method, DRM reaction need high temperature (about 800 o C), because DRM reaction (CH4 + CO2 → 2CO + 2H2, ∆H298K= +247 kJ/mol) is a strong endothermic reaction [START_REF] Gili | Surface carbon as a reactive intermediate in dry reforming of methane to syngas on a 5% Ni/MnO catalyst[END_REF]. Besides, the gas volume of the forwarding reaction increases. Therefore, increasing reaction temperature and decreasing pressure is beneficial to the reaction.

Besides, traditional DRM also require efficient and selective catalysts to promote the forward reaction, however, the catalyst suffered from the deactivation by carbon deposition and/or sintering of active metal [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Li | Sintering resistant Ni nanoparticles exclusively confined within SiO2 nanotubes for CH4 dry reforming[END_REF].

Plasma catalytic method

In recent years, many researches pay attention to the plasma catalytic method for dry reforming of methane. X. Tu [START_REF] Tu | Plasma-catalytic dry reforming of methane in an atmospheric dielectric barrier discharge: Understanding the synergistic effect at low temperature[END_REF] investigated the influence of plasma-catalyst on activity for low temperature DRM reaction. They found that Ni/γ-Al2O3 catalyst showed high activity at 50 W conditions that correspond to 270 °C. Yabe [START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF] found that 1 wt.%Ni/10 mol%La-ZrO2 catalyst in 6.9 w electric field equivalently at 150 °C showed high DRM activity with about 77.2% CH4 conversion and 87.6% CO2 conversion. So, there are several advantages to the plasma catalytic method. At firstly, it can conduct at low-temperature (lower 300 o C). Secondly, the synergistic effect of the low-temperature plasma and solid catalyst could strengthen both strong interaction of plasma-catalyst, thereby improving the activity of catalysts. Lastly, the catalyst could be restructured in the electric field, thereby enhancing the activity of the catalysts.

However, there are also several disadvantages to plasma catalytic method. At firstly, after reaction at high degree plasma conditions, the structure of catalyst would change, thus it needs special catalysts. Secondly, the stability of catalyst is not very good, due to the structure of catalysts would change under plasma conditions. Lastly, it needs the special equipment [START_REF] Zhu | Structure and reactivity of plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane[END_REF][START_REF] Hua | Preparation of Ni/MgO catalyst for CO2 reforming of methane by dielectric-barrier discharge plasma[END_REF][START_REF] Li | CO2 reforming of CH4 by atmospheric pressure glow discharge plasma: a high conversion ability[END_REF].

Photocatalytic method

Only a few literatures investigate DRM through Photocatalytic method [START_REF] Shimura | Photocatalytic steam reforming of methane over sodium tantalate[END_REF][START_REF] Yuliati | Photocatalytic conversion of methane and carbon dioxide over gallium oxide[END_REF]. Y.

Kohno et al. [START_REF] Yuliati | Photocatalytic conversion of methane and carbon dioxide over gallium oxide[END_REF] investigate that photocatalytic dry reforming of methane over ZrO2

and MgO, and found that it could occur around room temperature. D. Shi et al. [START_REF] Shi | Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS-TiO2/SiO2 catalyst[END_REF] reported that the photocatalytic CO2 reforming of methane on Cu/CdS-TiO2/SiO2 catalyst at 393 K and they can obtain the main product of acetone. According to the literature, the use of photoenergy with photocatalyst would break the thermodynamic barrier of endothermic reaction, thus the reaction could occur at lower temperature.

While unfortunately H2 was scarcely obtained from methane, because other products such as H2O, surface formate species and so on, were observed instead. So, the selectivity of catalyst is difficult to control.

Microwave-assisted catalytic method

Microwave-assisted catalytic method leads to a higher performance of the dry reforming reaction. B. Fidalgo et al. [START_REF] Fidalgo | Microwave-assisted dry reforming of methane[END_REF] found that the conversion of CO2 and methane under microwave heating is higher than those under electric furnace heating. Because the catalyst could be heated by the energy of microwave directly, while for traditional heating, the temperature of catalyst is higher than that of the surrounding atmosphere [START_REF] Meredith | Engineers' handbook of industrial microwave heating[END_REF]. However, the CH4 decomposition reaction and the CO2 gasification reaction would be improved under the microwave conditions.

Mechanisms purposed for DRM

It is well known that the C-H bond in CH4 is difficult to be activated, while CO2 is the most stable state of carbon, which is also very stable. The co-activation of both C-H bond in CH4 and C-O bond in CO2 faced challenging difficulties [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF]. Besides, because of the thermodynamics limitation, DRM reaction is usually performed at high temperature (~800 °C). Much progress and significant achievements have been reported to understand the mechanism of DRM reaction [START_REF] Gili | Surface carbon as a reactive intermediate in dry reforming of methane to syngas on a 5% Ni/MnO catalyst[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF][START_REF] Lustemberg | Room Temperature Activation of Methane and Dry Reforming with CO2 on Ni-CeO2(111) Surfaces: Effect of Ce 3+ Sites and Metal-Support Interactions on C-H bond Cleavage[END_REF][START_REF] Yao | Low-temperature CO2 reforming of methane on Zr-promoted Ni/SiO2 catalyst[END_REF][START_REF] Jiang | Insight into the reaction mechanism of CO2 activation for CH4 reforming over NiO-MgO: A combination of DRIFTS and DFT study[END_REF][START_REF] Liang | Low-temperature activation of methane on the IrO2 (110) surface[END_REF][START_REF] Guo | DRIFTS Study on Adsorption and Activation of CH4 and CO2 over Ni/SiO2 Catalyst with Various Ni Particle Sizes[END_REF].

Generally, CO2 is adsorbed on surface of catalyst and forms bicarbonate, monodentate and/or bidentate carbonate species. The structure of those three types of carbonate species is shown as shown in Figure 2-8 [START_REF] Huang | Highly stable mesoporous NiO-Y2O3-Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion[END_REF]. Some studies showed that bidentate carbonates could promote coke elimination [START_REF] Huang | Highly stable mesoporous NiO-Y2O3-Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion[END_REF][START_REF] Li | Ordered mesoporous Ni/La2O3 catalysts with interfacial synergism towards CO2 activation in dry reforming of methane[END_REF][START_REF] Li | On the role of Ce in CO2 adsorption and activation over lanthanum species[END_REF]. While Zhang et al. [START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF] Guo et al. [START_REF] Guo | DRIFTS Study on Adsorption and Activation of CH4 and CO2 over Ni/SiO2 Catalyst with Various Ni Particle Sizes[END_REF] found that depending on the dissociation of CH4, the formation of monodentate carbonate species might be as follows:

CH4 + * → CHx + (4 -x)/2H2 H2 + 2* → 2H* CO2 * + H* → CO + OH* CO2 * + OH* → CO3 * + H*
where * denotes active sites on the Ni catalyst.

Bachiller-Baeza et al. [START_REF] Bachiller-Baeza | Interaction of carbon dioxide with the surface of zirconia polymorphs[END_REF] and Takano et al. [START_REF] Huang | Highly stable mesoporous NiO-Y2O3-Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion[END_REF] reported that the bidentate carbonate can be formed on the presence of oxygen vacancies in ZrO2 as follows:

where vδ denotes oxygen vacancies in ZrO2.

It is pointed out that the mechanism of dry reforming of methane is similar to various catalysts. There are some differences between different catalysts. For example, Jiang et al. [START_REF] Jiang | Insight into the reaction mechanism of CO2 activation for CH4 reforming over NiO-MgO: A combination of DRIFTS and DFT study[END_REF] reported the reaction mechanism over NiO-MgO. At first, CH4 dissociates on the Ni 0 site to give out C and H species. CO2 reacts with the surface C species to form CO. The adsorbed CO2 also reacts with H and then decomposes to give CO. Therefore, the reaction mechanism was proposed as follows:

CH4,g → CHx,s + (4-x)Hs (1)

CHx,s → Cs + xHs (2) Cs + CO2,g → COg (3) CO2,g → CO2,s (4)
CO2,s + Hs → COOHs (5a)

COOHs → COg + OHs (5b) (s: adsorbed states, g: gas phase).

Wang et al. [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF] reported that CH4 could be dissociated and adsorbed as CH3 and H

DRM catalysts

Generally, many parallel reactions (Table 2-1) occur during the DRM process, which seriously influences the activity, selectivity and stability of the target products (H2 and CO) [START_REF] Usman | Dry reforming of methane: Influence of process parameters-A review[END_REF]. Therefore, it is the key point of industrialization to explore an efficient and stable catalyst for DRM. Thus, studies on active metal [START_REF] Yentekakis | Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane[END_REF][START_REF] Wu | Lattice Strained Ni-Co alloy as a High-Performance Catalyst for Catalytic Dry Reforming of Methane[END_REF][START_REF] Horlyck | Elucidating the impact of Ni and Co loading on the selectivity of bimetallic NiCo catalysts for dry reforming of methane[END_REF][START_REF] Bian | Highly carbon-resistant Ni-Co/SiO2 catalysts derived from phyllosilicates for dry reforming of methane[END_REF][START_REF] Singh | Sonochemical synthesis of Pt, Ru doped TiO2 for methane reforming[END_REF][START_REF] Sajjadi | Sol-gel synthesis of Ni-Co/Al2O3-MgO-ZrO2 nanocatalyst used in hydrogen production via reforming of CH4/CO2 greenhouse gases[END_REF][START_REF] Dai | Hollow zeolite encapsulated Ni-Pt bimetals for sintering and coking resistant dry reforming of methane[END_REF][START_REF] Souza | Reforming of methane with carbon dioxide over Pt/ZrO2/Al2O3 catalysts[END_REF], catalyst synthesis [2, 69, 84-87], as well as choice of precursors and supports [88][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Steib | Structural response of Ni/ZrO2 to feed modulations during CH4 reforming reactions[END_REF][START_REF] Pechimuthu | Deactivation Studies over Ni-K/CeO2-Al2O3 Catalyst for Dry Reforming of Methane[END_REF][START_REF] Lustemberg | Room-Temperature Activation of Methane and Dry Re-forming with CO2 on Ni-CeO2(111) Surfaces: Effect of Ce 3+ Sites and Metal-Support Interactions on C-H Bond Cleavage[END_REF], promotion effects [START_REF] Yao | Low-temperature CO2 reforming of methane on Zr-promoted Ni/SiO2 catalyst[END_REF][START_REF] Świrk | Ce-and Y-Modified Double-Layered Hydroxides as Catalysts for Dry Reforming of Methane: On the Effect of Yttrium Promotion[END_REF][START_REF] Świrk | Effect of low loading of yttrium on Ni-based layered double hydroxides in CO2 reforming of CH4, Reaction kinetics[END_REF][START_REF] Yao | Synthesis Gas Production via Dry Reforming of Methane over Manganese Promoted Nickel/Cerium-Zirconium Oxide Catalyst[END_REF][START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF][START_REF] Taherian | A comparative study of ZrO2, Y2O3 and Sm2O3 promoted Ni/SBA-15 catalysts for evaluation of CO2/methane reforming performance[END_REF] and mechanism of carbon deposition [START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF][START_REF] Li | MnOx-Promoted[END_REF][START_REF] Gili | Surface carbon as a reactive intermediate in dry reforming of methane to syngas on a 5% Ni/MnO catalyst[END_REF][START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF][START_REF] Han | Highly coke-resistant Ni nanoparticle catalysts with minimal sintering in dry reforming of methane[END_REF] are main strategies for improving DRM catalysts in recent years (  Carbon intermediates during CO2 reforming of methane over NiCaOZrO2 catalyst [START_REF] Wang | Carbon intermediates during CO2 reforming of methane over Ni-CaO-ZrO2 catalysts: A temperature-programmed surface reaction study[END_REF]  High coke-resistance MgAl2O4 islands decorated catalyst [START_REF] Son | High coke-resistance MgAl2O4 islands decorated catalyst with minimizing sintering in carbon dioxide reforming of methane[END_REF]  Coke tolerance of Ni/Al2O3 nanosheet catalyst [START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF] The noble metal-based catalysts exhibited high activity, stability and selectivity almost without carbon deposition, such as Rh, Ru, and Pt [START_REF] Singh | Sonochemical synthesis of Pt, Ru doped TiO2 for methane reforming[END_REF][START_REF] Özkara-Aydınoğlu | The effect of impregnation strategy on methane dry reforming activity of Ce promoted Pt/ZrO2[END_REF][START_REF] Wang | Carbon dioxide reforming of methane to synthesis gas over supported rhodium catalysts: the effect of support[END_REF][START_REF] Moral | Syngas production by means of biogas catalytic partial oxidation and dry reforming using Rh-based catalysts[END_REF][START_REF] Fernández | Insights into dynamic surface processes occurring in Rh supported on Zr-grafted γ-Al2O3 during dry reforming of methane[END_REF]. However, nickelbased catalysts would be a more suitable choice for the DRM, because of the comparable activity, stability and selectivity to noble metal catalysts [START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Movasati | Dry reforming of methane over CeO2-ZnAl2O4 supported Ni and Ni-Co nano-catalysts[END_REF][START_REF] Asencios | Synthesis of NiO-MgO-ZrO2 catalysts and their performance in reforming of model biogas[END_REF].

Considering the cost, Ni-based catalyst is the alternative active metal for dry reforming of methane. However, the fatal drawback of the Ni-based catalyst is the rapid deactivation deriving from carbon deposition and sintering of Ni species [START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF][START_REF] Guo | CO2 reforming of methane over Mn promoted Ni/Al2O3 catalyst treated by N2 glow discharge plasma[END_REF].

Several methods have been studied in the resistance of coke and sintering [START_REF] Bian | Highly carbon-resistant Ni-Co/SiO2 catalysts derived from phyllosilicates for dry reforming of methane[END_REF][START_REF] Wang | CO2 reforming with methane over small-sized Ni@SiO2 catalysts with unique features of sintering-free and low carbon[END_REF][START_REF] Zhang | A sintering and carbon-resistant Ni-SBA-15 catalyst prepared by solid-state grinding method for dry reforming of methane[END_REF][START_REF] Son | High coke-resistance MgAl2O4 islands decorated catalyst with minimizing sintering in carbon dioxide reforming of methane[END_REF]. The carbon deposition on the active sites can via CH4 decomposition (Equation 2), CO disproportionation (Equation 4) and the reverse carbon gasification reaction (Equation 7) [START_REF] Gili | Surface carbon as a reactive intermediate in dry reforming of methane to syngas on a 5% Ni/MnO catalyst[END_REF][START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF]. Thermodynamically, the equation (2) tends to be at high temperature and low pressure, while the other two equations ( 4) and ( 7) are favored at low temperature and high pressure. Herein, DRM reaction is processed under high temperature with an atmosphere, thus the coke usually comes from the equations (2). 4)

𝐶𝐶𝐻𝐻 4 → 𝐶𝐶 + 2𝐻𝐻 2 , ΔH°298K = 75 kJ•mol -1 (Equation 2) 2𝐶𝐶𝐶𝐶 → 𝐶𝐶 + 𝐶𝐶𝐶𝐶 2 , ΔH°298K = -172 kJ•mol -1 (Equation
𝐶𝐶𝐶𝐶 + 𝐻𝐻 2 → 𝐶𝐶 + 𝐻𝐻 2 𝐶𝐶, ΔH°298K = -131 kJ•mol -1 (Equation 7)
Generally, suitable supports and promoters improved the catalytic performance by adjusting the interaction between nickel and the support. Besides, the temperature of calcination, the order of materials loading on support, the reduction temperature, and the nickel particle size also altered the performance of the catalysts [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF][START_REF] Van Haasterecht | Effect of initial nickel particle size on stability of nickel catalysts for aqueous phase reforming[END_REF].

It is suggested that by investigating the interaction with the supports, promoters with active metal, as well as their structural adjustment, the development of DRM catalysts to reach industrial level is feasible [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF].

Effect of promoters

Promoters could enhance the activity of nickel-based catalysts via several mechanisms.

Firstly, promoters could increase the basic site, which could promote the adsorption and/or activation of CO2. Debek et al. [START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Dębek | Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts atlow temperature CO2 methane reforming[END_REF] [START_REF] Huang | Highly stable mesoporous NiO-Y2O3-Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. Thus, the Ypromoted catalyst exhibited higher catalytic performance for DRM. Sajjadi et al [START_REF] Sajjadi | Sol-gel synthesis of Ni-Co/Al2O3-MgO-ZrO2 nanocatalyst used in hydrogen production via reforming of CH4/CO2 greenhouse gases[END_REF] prepared Ni-Co/Al2O3-MgO-ZrO2 nanocatalysts were synthesized via sol-gel method with various amounts of Mg (5, 10 and 25 wt %). According to the results of TEM, the particle size distribution became narrower with the increase of Mg content. Thirdly, promoters could increase the reduction of nickel oxide. Liu et al. [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF] studied the effect of La promoter for 550 and 600 °C DRM reaction. They found that the addition of lanthanum would enhance the reduction of the nickel leading to Ni 0 species existing in the La-promoted catalyst. The H2/CO increased with time-on-stream, because of the improvement of direct methane decomposition reaction, corresponded to the presence of Ni 0 species. The conversion of CH4 and CO2 on HN2 catalyst (Ni0.215La0.012Mg0.535Al0.238) containing 2 wt.% La for DRM reaction at 550 °C was 33% and 36%, respectively, which was higher than non-promoted catalyst. Debek et al. [START_REF] Dębek | Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF] also studied the Ce-promoted Ni-containing hydrotalcite derived catalyst for DRM reaction and found that Ce promoter could increase the reduction of the nickel particle.

The H2-TPR results showed that the reduction peak of Ni-phase shifted to low temperature in the presence of promoter Ce, because the Ce 3+ /Ce 4+ redox pairs could transfer electron, which promoted the reduction of nickel oxides in the nearby site [START_REF] Grossi | Sulfonated polystyrene: A catalyst with acid and superabsorbent properties for the esterification of fatty acids[END_REF].

Albarazi et al. [START_REF] Albarazi | Hydrogen and syngas production by methane dry reforming on SBA-15 supported nickel catalysts: On the effect of promotion by Ce0.75Zr0.25O2 mixed oxide[END_REF] studied the effect of Ce0.75Zr0.25O2 mixed oxide on the performance for DRM reaction. The H2-TPR results showed that Ce0.75Zr0.25O2 mixed oxide could increase the content of nickel that could be reduced below 600 °C, thus enhance the reduction of the nickel particles.

Lastly, promoters could promote the formation of oxygen vacancies. The formation of oxygen vacancies (in samples containing ZrO2 and Y2O3) seemed to promote removal of the coke deposited on the nickel surface during the reforming process [START_REF] Youn | Hydrogen production by autothermal reforming of ethanol over nickel catalyst supported on mesoporous yttriastabilized zirconia[END_REF][START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF].

Asencios et al. [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF] studied the NiO/Y2O3/ZrO2 catalyst synthesized by the polymerization method in a single step. The formation of oxygen vacancies on Ypromoted catalyst, due to the generation of Y2O3-ZrO2 solid solution, have a positive influence on the elimination of the coke deposited on the Ni 0 surface. Youn et al. [START_REF] Youn | Hydrogen production by autothermal reforming of ethanol over nickel catalyst supported on mesoporous yttriastabilized zirconia[END_REF] prepared mesoporous yttria-stabilized zirconia support by a sol-gel method and observed that oxygen vacancies was increased by the addition of yttria. The results of XRD showed that graphitic carbon was observed on non-promoted catalyst, while the peak of graphitic carbon became weak on Y-promoted catalyst. It implied that yttria also played an important role in enhancing the resistance against carbon deposition. The properties of ZrO2 can be tailored by the introduction of a cation with valence lower than 4+, e.g. Y 3+ [START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. Bellido et al. [START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF] found the surface oxygen vacancies are stabilized by the increasing Y2O3 load. Meanwhile the formation of ZrO2-Y2O3 solid solution could increase the formation of oxygen vacancies in the synthesized support.

Effect of supports

Generally, the interaction between Ni and support plays a very important role in the catalytic performance of catalyst for DRM reaction. Bradford et al. [START_REF] Bradford | Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity[END_REF] Lemonidou et al. [START_REF] Lemonidou | Carbon dioxide reforming of methane over 5 wt.% Ni/CaO-Al2O3 catalyst[END_REF] reported that the 5 wt.% Ni/CaO-Al2O3 catalyst exhibited high stability, and the conversion of CH4 did not decrease even reaction for 50 h. The strong interaction between the CaO-Al2O3 support and metal could increase the stability of the catalyst. While the strong interaction between nickel and CeO2 support did not enhance the activity of Ni/CeO2 catalyst for DRM. Because, Ni/CeO2 catalyst promoted the reverse water-gas shift reaction during the process of DRM reaction, resulting in the decrease of hydrogen selectivity [START_REF] Kumar | Nickel-based ceria, zirconia, and ceria-zirconia catalytic systems for low-temperature carbon dioxide reforming of methane[END_REF]. However, on Ni/SiO2 and Ni/C catalysts, filamentous carbon is formed under reaction conditions leading to the deactivation of the catalysts [START_REF] Bradford | Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity[END_REF], because of the weaker interaction of the nickel and support SiO2 [START_REF] Yao | Low-temperature CO2 reforming of methane on Zr-promoted Ni/SiO2 catalyst[END_REF][START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF][START_REF] Bradford | Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity[END_REF]. The above reports indicated that the nature and strength of the interaction of nickel with the supports would alter the performance of the catalysts.

Besides, the supports with special structure could promote the catalytic performance of catalysts for DRM reaction. For example, mesoporous material could limit the sintering of nickel species during the reaction, thus improve the performance of the catalysts [START_REF] Bachiller-Baeza | Transient studies of low-temperature dry reforming of methane over Ni-CaO/ZrO2-La2O3[END_REF][START_REF] Gálvez | Enhanced catalytic stability through nonconventional synthesis of Ni/SBA-15 for methane dry reforming at low temperatures[END_REF][START_REF] Kaydouh | Effect of the order of Ni and Ce addition in SBA-15 on the activity in dry reforming of methane[END_REF][START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF]. Sokolov et al. [START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF] discussed the influence of the morphology of the La2O3-ZrO2 support on catalytic stability. Three types of La2O3-ZrO2 supports nonstructured (LaZr-ns), mesostructured (LaZr-meso) and macropore skeleton (LaZr-macro) were prepared to supported nickel. Ni/LaZr-meso catalyst showed the highest stability in 180 h with almost no inactivation, due to the pore confinement effect and high dispersion of NiOx species Ni/LaZr-meso catalyst. Hydrotalcite derivative with the periclase-like structure has been widely studied in DRM reaction [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Dębek | Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Dbek | Ni-Al hydrotalcite-like material as the catalyst precursors for the dry reforming of methane at low temperature[END_REF]. Debek et al. [START_REF] Dbek | Ni-Al hydrotalcite-like material as the catalyst precursors for the dry reforming of methane at low temperature[END_REF] showed that the Ni-containing hydrotalcite-derived materials (HT-NiAl) showed high catalytic performance for DRM. The average conversions of CH4 and CO2 were 48% and 54% at 550 °C, respectively. Hydrotalcite derivative materials were beneficial to the activity for DRM reaction, which was attributed to the mixed oxide with the periclase-like structure. Besides, the interaction between nickel species and hydrotalcite derivative supports could restrain the sintering of nickel. Simultaneously, prevent the formation of inactive phases, such as NiAl2O4 or NiO-MgO solid solution [START_REF] Dębek | A Short Review on the Catalytic Activity of Hydrotalcite-Derived Materials for Dry Reforming of Methane[END_REF].

CexZr1-xO2 support has defect structure due to the formation of the CeO2-ZrO2 solid solution and the high oxygen mobility, which led to celerity oxidoreduction ability by releasing and getting oxygen rapidly [START_REF] Kumar | Nickel-based ceria, zirconia, and ceria-zirconia catalytic systems for low-temperature carbon dioxide reforming of methane[END_REF][START_REF] Pengpanich | Catalytic oxidation of methane over CeO2-ZrO2 mixed oxide solid solution catalysts prepared via urea hydrolysis[END_REF][START_REF] Trovarelli | Catalysis by Ceria and Related Materials[END_REF][START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF]. 5% Ni/Ce0.6Zr0.2O2 catalyst showed remarkable stability for DRM reaction, due to the cubic CexZr1-xO2 support offered mobile oxygen species to nickel particles, reported by Kumar [START_REF] Kumar | Nickel-based ceria, zirconia, and ceria-zirconia catalytic systems for low-temperature carbon dioxide reforming of methane[END_REF]. TPO experiment implied that Ni/CexZr1-xO2 catalyst inhibited carbon deposition. CexZr1-xO2 solid solution could enhance reducibility, confirmed by H2-TPR. Those special material supports would exert effect on nickel particle size and dispersion of nickel, thereby improve the property of the catalysts for DRM reaction.

The carbon deposition limitation

Carbon deposition is one of the main issues for the deactivation of Ni-based catalysts for DRM reaction. Thus, many researchers investigated how to decrease the coke or restrain the formation of coke. Generally, there are several methods to decrease the carbon deposition [START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF][START_REF] Xu | NiO-MgO nanoparticles confined inside SiO2 frameworks to achieve highly catalytic performance for CO2 reforming of methane[END_REF][START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts atlow temperature CO2 methane reforming[END_REF][START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF][START_REF] Dębek | Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition[END_REF][START_REF] Li | Studies on stability and coking resistance of Ni/BaTiO3-Al2O3 catalysts for lower temperature dry reforming of methane (LTDRM)[END_REF].

Firstly, the enhancement of the ability for carbon deposition elimination could decrease the formed coke during the DRM reaction. Liu [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF] reported that La promoters could increase the formation of medium strength and strong basic sites, thereby enhancing the ability of CO2 adsorption and further remove carbon deposition. Besides, the presence of La may favor the formation of lanthanum oxycarbonates, able to gasify the amorphous carbon deposits. Thus, the content of coke on La-promoted catalyst was Thirdly, optimum process parameters could decrease the carbon deposition. Maneerung et al. [START_REF] Maneerung | LaNiO3 perovskite catalyst precursor for rapid decomposition of methane: Influence of temperature and presence of H2 in feed stream[END_REF] discovered that the introduction of hydrogen into feed gas enhanced the stability of catalysts, where hydrogen could react with the excess carbon, thereby reduced the rate of catalytic deactivation. Yabe [START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF] found that Ni/La-ZrO2 catalyst showed lower carbon deposition in the electric field than traditional conditions for DRM reactions. The content of coke was 0.15% and above 3.81% in the electric field and no electric conditions, respectively. The catalyst could be restructured in the electric field, thereby decreasing the carbon deposition of catalysts.

ZrO2 materials

ZrO2 had been widely used for reforming catalysts, because of the high oxygen mobilization, enhancement of metal dispersion, and the proper interaction between nickel and ZrO2 [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF][START_REF] Jing | Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor[END_REF][START_REF] Lercher | Design of stable catalysts for methane-carbon dioxide reforming[END_REF]. Jing et al. [START_REF] Jing | Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor[END_REF] [START_REF] Lercher | Design of stable catalysts for methane-carbon dioxide reforming[END_REF] and found that the rate of carbon formation on the support is proportional to the concentration of Lewis acid sites.

Besides, the CO2-TPD results showed that CO2 easier reactive with coke on Pt/ZrO2 than that on Pt/Al2O3 catalyst. Wei et al. [START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF] studied the influence of the preparation method on performance of Ni/ZrO2 catalyst at 1037 K for DRM reaction. Ni/ZrO2-A is prepared by impregnation with 18 nm particle of ZrO2, Ni/ZrO2-B is prepared by impregnation with 6 nm particle of ZrO2, and Ni/ZrO2-C is prepared by co-precipitation with 7 nm particle of ZrO2, Ni/ZrO2-B catalyst showed highly active and stable even reaction for 600 h, while other two catalysts showed the same initial activity as Ni/ZrO2-B but deactivate rapidly. The extremely stable activity of Sample B catalyst may be induced by a higher concentration of oxygen defects or higher concentrations of basic sites for CO2 activation at the support surface derived from the ultra-fine ZrO2-B. Lou et al. [START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF] designed a Ni/ZrO2 with 1.1 nm nickel particle diameter, which exhibited outstanding stability in DRM reaction with very little carbon deposition after reaction.

Because small nickel particles could result in more interface of Ni-O-Zr, which could promote the removal of carbon.

In this work, I focused on the Ni-based with new ZrO2 catalysts prepared by a one-step synthesis method. I also Investigated the effect of synthesis method and the promoter of Al, Mg, Y and Mn on the performance of catalyst for dry reforming of methane. In order to understand the relationship between structure and activity of ZrO2-based catalyst.

Chapter III Experiment 3. Experiment

Activity test of dry reforming of methane

Activity test

The dry reforming activity test was carried out with a rising temperature from 550 °C to 800 °C (interval of each 50 °C in 30 min) in a fixed-bed flow microreactor (inner diameter 10 mm). A thermocouple (Type K) was placed near to the catalytic bed to monitor the reaction temperature for each test. The total reactant gas flow rate was 100 mL/min with a molar composition of CH4/CO2/Ar = 1/1/8. The total gas hourly space velocity (GHSV) values were 24,000 h -1 or 48,000 h -1 . For time on stream tests, the same reactant gas was introduced through the catalytic bed held at target temperature for time on stream. Prior to DRM reaction, the catalysts were reduced at 700 or 750 °C for 1 h in the stream of 5 vol% H2/Ar. Those steps were shown in Figure 3-1. The outgases were detected by a gas micro chromatography (Agilent Varian GC490, Agilent, Les Ulis, France), equipped with a TCD detector to analyze the composition of products.

computational formula

The conversion of CH4 (X CH 4 ) and CO2 (X CO 2 ), and H2/CO molar ratio were calculated according to the following equations:

X CH 4 = n CH 4 ,in -n CH 4 ,out n CH 4 ,in × 100% (Equations 8) X CO 2 = n CO 2 ,in -n CO 2 ,out n CO 2 ,in × 100% (Equations 9) H 2 CO ⁄ = n H 2 ,out n CO,out ⁄ ( Equations

Characterization of the catalyst

Temperature-Programmed Reduction (TPR)

Temperature-programmed reduction by H2 (H2-TPR) is a widely used technique for the characterization of metal oxides, mixed metal oxides, and metal oxides dispersed on a support. The TPR method yields quantitative information on the reducibility of the oxide's surface, as well as the heterogeneity of the reducible surface. Generally, the reducing gas is a mixture flow with 3-17 % hydrogen diluted in argon or nitrogen [START_REF] Chorkendorff | Concepts of modern catalysis and kinetics[END_REF]. The test for the reducibility of catalysts was measured on a BELCAT-M (BEL Japan, BEL Europe GmbH, Krefeld, Germany) instrument. The mass of catalyst was about 60 mg.

Temperature-Programmed Desorption (TPD)

Temperature programmed desorption of CO2 (TPD) technique allows to study the interaction of CO2 gas reaction with solid surfaces, due to its natural tendency to get adsorbed (strong-pair acceptor) on the most basic sites (electron-pair donor) on the surface, thereby, is a powerful tool for both the evaluation of active sites on catalyst surfaces and the understanding of the mechanisms of catalytic reactions including adsorption, surface reaction and desorption [171,[START_REF] Ishii | Temperature Programmed Desorption[END_REF].

The measurement of CO2-TPD was carried out on the same equipment, and the scheme of set-up for CO2-TPD was shown in Figure 3-3 (B). The catalysts were reduced at 700 °C under a 5% H2/Ar flow for 60 min. CO2 was adsorbed at 80 °C for 60 min under a mixture flow of 10% CO2/He. Afterward, cleaned the weakly adsorbed CO2 by helium flow for 30 min. Last, the catalysts were heated to 900 °C (10 °C/min) under a He flow.

The process of CO2-TPD is presented in Figure 34. The data collected from 80 to 900 °C.

X-Ray Diffraction (XRD)

The X-Ray Diffraction technique is very important and most frequently characterization applied in the structure of catalysts. It is an indispensable method of material investigation, characterization and quality control. Where d h*k*l is the lattice spacing of (h*k*l) crystal planes, n is the diffraction series, λ is the incident wavelength. 3-1. Besides, the crystallite sizes of Ni 0 play a key role in the activity of Ni-based catalysts for dry reforming of methane. Thus, the crystal particle size on catalysts before and after reaction was calculated using the Scherrer Equation (Equation 12) to understand the relationship between activity and crystallite sizes of Ni 0 [START_REF] Monshi | Modified Scherrer equation to estimate more accurately nano-crystallite size using XRD[END_REF]. Where τ is the mean size; K is a dimensionless shape factor, with a value close to unity about 0.9, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), θ is the Bragg diffraction angle.

τ = 𝐾𝐾 𝜆𝜆 𝛽𝛽 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
(Equation 12)

The XRD patterns were recorded by a DX-1000 CSC diffractometer with a Cu Kα radiation source (λ=0.15406 nm). The anode current and voltage were 25 mA and 40 kV, respectively. All the XRD diffractograms were registered at 25°C. The diffraction patterns were recorded in a range of 10° < 2θ < 80°, with a scan step size of 0.03° and a scan speed of 0.3 s step -1 .

Brunauer-Emmett-Teller (BET)

The surface area, mean pore size, and pore volume were obtained by the N2 adsorption-desorption method using a Belsorp Mini II apparatus (BEL Japan) instrument. Before measurement, the sample was degassed under vacuum condition at 200 °C for 2 h. Then the test was carried out under liquid nitrogen temperature (-196 °C).

The surface area of the sample was calculated according to the Brunauer-Emmett-Teller (BET) equation (Equation 13) [START_REF] Brunauer | The use of low temperature van der Waals adsorption isotherms in determining the surface areas of various adsorbents[END_REF]. p and p0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption. v is the adsorbed gas quantity.

vm is the monolayer adsorbed gas quantity. And c is the BET constant. The pore volume and pore diameter were calculated by the Barrett-Joyner-Halenda (BJH) equation (Equation 14) [START_REF] Rasneur | Isothermbgraphe: automatic appartus for surface area mesurement and pore size analysis of porous materials or fine powders from nitrogen adsorption isotherm[END_REF]. 𝑉𝑉 𝑎𝑎𝑎𝑎𝑐𝑐 (𝑥𝑥 𝑘𝑘 ) is the total adsorption amount at 𝑥𝑥 𝑘𝑘 = Pk/P0. 𝑟𝑟 𝑖𝑖 is the radius of capillary condensation. 𝑟𝑟 𝑘𝑘 is the Kelvin radius of capillary condensation.

∆𝑉𝑉 𝑖𝑖 is the total adsorption amount at 𝑟𝑟 𝑖𝑖 (𝑟𝑟 𝑖𝑖 ≤ 𝑟𝑟 𝑘𝑘 ). ∆𝑆𝑆 𝑗𝑗 is the total adsorption amount at 𝑟𝑟 𝑗𝑗 �𝑟𝑟 𝑗𝑗 ≥ 𝑟𝑟 𝑘𝑘 �. 𝑡𝑡 𝑗𝑗 is the thickness of the liquid film on the wall of the tube.

1 𝑣𝑣{� 𝑃𝑃0 𝑃𝑃 �-1} = 𝑐𝑐-1 𝑣𝑣 𝑚𝑚 𝑐𝑐 � 𝑃𝑃0 𝑝𝑝 � + 1 𝑣𝑣 𝑚𝑚 𝑐𝑐
(Equation 13)

𝑉𝑉 𝑎𝑎𝑎𝑎𝑐𝑐 (𝑥𝑥 𝑘𝑘 ) = � ∆𝑉𝑉 𝑖𝑖 (𝑟𝑟 𝑖𝑖 ≤ 𝑟𝑟 𝑘𝑘 (𝑥𝑥 𝑘𝑘 )) 𝑘𝑘 𝑖𝑖=1 + � ∆𝑆𝑆 𝑗𝑗 𝑡𝑡 𝑗𝑗 �𝑟𝑟 𝑗𝑗 ≥ 𝑟𝑟 𝑘𝑘 (𝑥𝑥 𝑘𝑘 )� 𝑛𝑛 𝑗𝑗=𝑘𝑘+1
(Equation 14)

The majority of physisorption isotherms might be divided into six types [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF], as is shown in Figure 3

Thermal Gravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) is a technique for measuring the relationship between the quality of a sample and temperature (or time). The thermogravimetric curve (TGA curve) is carried out by the heat balance with time on stream, as shown in shows the scheme of power compensation DSC instrument [START_REF] Barton | The application of differential scanning calorimetry (DSC) to the study of epoxy resin curing reactions, Epoxy resins and composites I[END_REF].

Herein, used to characterize the carbon deposition of used catalysts. The sample (10 mg) was treated under air atmosphere with a flow rate of 30 mL/min -1 at room temperature until the scales balanced. Then, the temperature increased from room temperature to 800 °C with a heating rate of 5 °C/min -1 . The data was recorded (the weight of the sample) from room temperature to 800 °C.

Transmission Electron Microscope (TEM)

Transmission electron microscopy (TEM) is a characterization method that could provide the information in nanoscale. The size distribution, and catalyst morphology also could be determined by the TEM characterization [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. Electron diffraction is the main contrast mechanism of sample imaging in TEM, which usually uses to study large particle crystals with defects [START_REF] Wei | The structure and analysis of complex reaction systems[END_REF]. According to the schematic diagram of diffraction (Figure 3456789), the lattice spacing d can be calculated by the equation (Equation 15). L is the length of the camera. r is the distance from the diffraction point to the transmission point. λ is the electron wavelength. Besides, single, polycrystalline, and amorphous shapes can be distinguished by diffraction patterns, as shown in Figure 3-8 (B) [START_REF] Rauch | Automatic crystal orientation and phase mapping in TEM by precession diffraction[END_REF].

In this work, the morphology of catalysts was obtained by on an FEI Tecnai G2 20 Twin instrument with an acceleration voltage of 200 kV.

Raman Spectroscopy (Raman)

Raman spectroscopy has been used in the field of catalysis since the 1970s. Brown, Makovsky and Rhee reported the MoO3/λ-Al2O3, CoO-MoO3/λ-Al2O3 catalysts were characterized by Raman [START_REF] Brown | Raman spectra of supported molybdena catalysts: I. Oxide catalysts[END_REF]. Later, Raman was widely used to characterize catalysts, such as transition metal oxide, molecular sieve and carbon [START_REF] Iliev | Raman spectroscopy of ordered double perovskite La2CoMnO6 thin films[END_REF][START_REF] Xiong | Direct spectroscopic evidence for vanadium species in V-MCM-41 molecular sieve characterized by UV resonance Raman spectroscopy[END_REF]. Herein, Raman characterization was carried out for the in-depth study of carbon species deposited on the used catalysts. For example, on used Ni-Si/ZrO2 and Ni-Zr/SiO2 catalyst (Figure 3-10), two distinct peaks at about 1328 cm -1 (D band) and 1585 cm -1 (G band) were observed [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. D band corresponded to the disorder-induced band, ascribed to the structural imperfections on defective carbon materials. G band corresponded to graphitic carbon, ascribed to the in-plane C-C stretching vibrations of sp 2 atoms in coke [START_REF] Liu | Methane reforming with carbon dioxide over a Ni/ZiO2-SiO2 catalyst: Influence of pretreatment gas atmospheres[END_REF][START_REF] Mcfarlane | The application of inelastic neutron scattering to investigate the steam reforming of methane over an alumina-supported nickel catalyst[END_REF][START_REF] Bai | Coking behaviors and kinetics on HZSM-5/SAPO-34 catalysts for conversion of ethanol to propylene[END_REF]. Raman spectroscopy experiments were conducted on an objective (X50LWD) with a Laser of 532.17

X-ray photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is very widely applied in the field of surface analysis. XPS is based on the photoelectric effect of electrons (photoelectrons) generated when high-energy photons (usually in the keV range) collide with a material.

The photoelectron kinetic energy, Ek, which is the measured quantity in the experiment, is given by Einstein's law (Equation 16) [START_REF] Venezia | X-ray photoelectron spectroscopy (XPS) for catalysts characterization[END_REF]. 16)

Ek = hv-Eb (Equation
where hν is the energy of the incident radiation and Eb the binding energy of the electron at a particular level. According to the Eb, we will know the element, like Eb (C 1s ) = 284.5 eV, Eb (O 1s ) = 531 eV, and Eb (Al 2p) = 73 eV [START_REF] Briggs | Surface analysis by Auger and X-ray photoelectron spectroscopy[END_REF]. Except for the qualitative information, we can know the quantitative information about the element. Quantitative information arises from the element specificity of the binding energies and the relation between the intensity of the photoelectron peaks and the element concentration [START_REF] Venezia | X-ray photoelectron spectroscopy (XPS) for catalysts characterization[END_REF].

The database of XPS is listed following: https://xpssimplified.com/periodictable.php https://srdata.nist.gov/xps/selEnergyType.aspx

In this work, the XPS spectra were recorded on an AXIS Ultra DLD (KRATOS) spectrometer instrument with monochromated Al radiation. All of the data (the electron binding energy) were referenced to the C1s peak at 284.5 eV [START_REF] Xin | Production of high-grade diesel from palmitic acid over activated carbon-supported nickel phosphide catalysts[END_REF]. For quantitative analysis of the surface, composition was determined by the peak areas and the elementspecific Scofield factors.

Chapter IV

One-step synthesis of highly active and stable Ni-ZrO x for dry reforming of methane 4. One-step synthesis of highly active and stable Ni-ZrOx for dry reforming of methane

Introduction

Methane and carbon dioxide could be obtained from natural gas, shale gas, marsh gas, and biomass pyrolysis. CO2 reforming of methane (DRM) could not only utilize the CO2 and CH4, which are the main greenhouse gases but also produce synthesis gas (a mixture of CO and H2). The latter can be used in the synthesis of high-value chemicals [START_REF] Buelens | Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier's principle[END_REF][START_REF] Bhattar | 110th Anniversary: Dry Reforming of Methane over Ni-and Sr-Substituted Lanthanum Zirconate Pyrochlore Catalysts: Effect of Ni Loading[END_REF]. Furthermore, the syngas with a H2/CO ratio close to unity is attractive for the synthesis of hydrocarbon fuels, e.g. in Fischer-Tropsch synthesis over iron-based catalysts [START_REF] Wu | Lattice Strained Ni-Co alloy as a High-Performance Catalyst for Catalytic Dry Reforming of Methane[END_REF][START_REF] Yao | Synthesis Gas Production via Dry Reforming of Methane over Manganese Promoted Nickel/Cerium-Zirconium Oxide Catalyst[END_REF][START_REF] Bae | Eco-friendly prepared iron-ore-based catalysts for Fischer-Tropsch synthesis[END_REF]. However, side reactions could occur during the DRM reaction, such as the reverse water-gas shift (RWGS) reaction, methane cracking, and CO disproportionation, resulting in the deactivation of catalyst and/or the lower H2/CO molar ratio. Thus, many researches have paid much attention to enhance both high catalytic activity and stability of used catalysts. Although numerous efforts have been given to these issues, several problems like low stability, coke deposition, and metal sintering are still main obstacles towards the large-scale application of DRM catalysts [START_REF] Xu | NiO-MgO nanoparticles confined inside SiO2 frameworks to achieve highly catalytic performance for CO2 reforming of methane[END_REF][START_REF] Han | Highly coke-resistant Ni nanoparticle catalysts with minimal sintering in dry reforming of methane[END_REF][START_REF] Ojeda-Niño | Role of Pr on Ni-Mg-Al Mixed Oxides Synthesized by Microwave-Assisted Self-Combustion for Dry Reforming of Methane[END_REF].

ZrO2 support has been widely used in the preparation of catalysts, such as Pt/ZrO2 [START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF][START_REF] Özkara-Aydınoğlu | The effect of impregnation strategy on methane dry reforming activity of Ce promoted Pt/ZrO2[END_REF][START_REF] Mattos | Partial oxidation and CO2 reforming of methane on Pt/Al2O3, Pt/ZrO2, and Pt/Ce-ZrO2 catalysts[END_REF], Ru/ZrO2 [START_REF] Jakobsen | Steam and CO2 reforming of methane over a Ru/ZrO2 catalyst[END_REF][START_REF] Matsui | Reaction mechanisms of carbon dioxide reforming of methane with Ru-loaded lanthanum oxide catalyst[END_REF], Co/ZrO2 [START_REF] Abasaeed | Catalytic performance of CeO2 and ZrO2 supported Co catalysts for hydrogen production via dry reforming of methane[END_REF][START_REF] Gonzalez-Delacruz | In Situ XAS Study of Synergic Effects on Ni-Co/ZrO2 Methane Reforming Catalysts[END_REF] and Ni/ZrO2 [START_REF] Li | MnOx-Promoted[END_REF][START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF][START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF],

for DRM reaction. Among them, Ni-based catalysts are known to be one of the most promising materials for CO2 reforming of methane, due to their good catalytic performance, low cost, and high availability as compared to noble-metal based catalysts.

Therefore, Ni modified ZrO2 catalyst is a viable alternative for DRM, as it is possible to obtain: (i) high dispersion of the active sites, (ii) medium-strength metal interaction between Ni and ZrO2 matrix which suppress the sintering of metallic nickel, (iii) high concentration of medium-strength basic sites. Apart from that, Ni/ZrO2 catalyst also exhibits a high concentration of surface oxygen species, which are favorable for CO2 adsorption and decomposition [START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF][START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Gonzalez-Delacruz | Modifying the Size of Nickel Metallic Particles by H2/CO Treatment in Ni/ZrO2 Methane Dry Reforming Catalysts[END_REF][START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF][START_REF] Jing | Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor[END_REF]. Jing et al. [START_REF] Jing | Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor[END_REF] [102] synthesized Ni/ZrO2 catalysts by a deposition-precipitation method with different calcination atmosphere (H2 , N2 and O2) and reported that the stability of Ni/ZrO2 catalyst under the calcination ambience of H2 was improved by the higher content of adsorbed oxygen species, which were beneficial in carbon removal. Moreover, Lou et al. [START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF] prepared Ni/ZrO2 catalyst by the impregnation method and found smaller nickel particle sizes which led to the increased interface between metal and support, promoting the carbon accessibility towards oxygen and hindering the carbon accumulation.

Sokolov et al. [START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF] reported that a one-step synthesis of mesoporous La2O3-ZrO2

supported Ni catalysts resulted in good stability, due to the high dispersion of nickel particles and the confinement effect of pore structure.

Considering the advantages of material prepared by the one-step method and increasing

Synthesis of a series of Ni-Zr catalysts and

Ni/ZrO2 catalyst.

Synthesis of Ni-Zr catalysts

The Ni-Zr materials were prepared by the urea hydrolysis method reported everywhere [START_REF] Liu | A facile route to synthesize mesoporous zirconia with ultra high thermal stability[END_REF][START_REF] Pengpanich | Catalytic oxidation of methane over CeO2-ZrO2 mixed oxide solid solution catalysts prepared via urea hydrolysis[END_REF]. In a typical process, 5 g ZrO(NO3)2•2H2O (Aldrich), 7 g urea (Sigma-Aldrich) and x g Ni(NO3)2•8H2O (x=0.57, 1.14, and 1.71) were dissolved in 375 mL of deionized water, then, 7 g of Pluronic P123 (Aldrich) amphiphilic block copolymer was added into the solution above. After stirring at 95 °C for 48 h, the mother solution was aged at 100 °C for 24 h. Afterward, the material was isolated by filtration, washed thoroughly with deionized water, and placed at an ambient atmosphere. Finally, the solid material was calcined in flowing air at 800 °C for 5 h with a heating rate of 1 °C/min. The obtained materials were denoted as 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalyst, regarding Ni% loading, respectively.

Synthesis of 10Ni/ZrO2 catalyst

The ZrO2 support was prepared by the same method used above. The synthesized support was employed to prepare 10Ni/ZrO2 catalyst by the impregnation method reported everywhere [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. Particularly, 0.991 g of Ni(NO3)2 was dissolved in ethanol (Sigma, 99%), then, 2 g of synthesized support was added into this solution and mixed well, afterward, impregnation was carried out in the ambient atmosphere for 24 hours.

After that, the material was placed in a dryer at 80 °C to evaporate ethanol, and the second step of drying was carried out at 110 °C for 4 hours immediately. Finally, the material was calcined at 800 °C for 5 h with a heating rate of 1 °C/min. The sample was denoted as 10Ni/ZrO2.

The formation of NiO-ZrO2 solid solution 4.3.1 The formation of NiO-ZrO2 solid solution confirmed by H2-TPR

The results of H2-TPR profiles were presented in Figure 4-1. A broad reduction peak was found in the temperature range from 350 to 600 °C on the 10Ni/ZrO2 catalyst. The main peak at low temperature (at about 404 °C) is ascribed to bulk nickel oxide on the surface of catalyst because it shows very weak interaction between nickel and zirconium [START_REF] Roh | Highly active and stable Ni/Ce-ZrO2 catalyst for H2 production from methane[END_REF][START_REF] Anchieta | Effect of ionic liquid in Ni/ZrO2 catalysts applied to syngas production by methane tri-reforming[END_REF]. The other peak at ca. 500 °C is assigned to the reduction of NiO species in the pores of ZrO2, which needs a higher temperature to be reduced, due to the pore confinement effect and strong interaction between nickel and zirconia [START_REF] Li | Steam reforming of acetic acid over Ni/ZrO2 catalysts: Effects of nickel loading and particle size on product distribution and coke formation[END_REF][START_REF] Xu | Highly active and coking resistant Ni/CeO2-ZrO2 catalyst for partial oxidation of methane[END_REF]. In comparison to the 10Ni/ZrO2 catalyst, the reduction peaks on the 10Ni-Zr catalyst shifted to higher temperature, indicating that the interaction between nickel and zirconia on the 10Ni-Zr catalyst was higher than that on 10Ni/ZrO2 catalyst. Also, two broad peaks (423 and 660 °C) can be observed. The peak at 423 °C can be assigned to the NiO on the surface of catalyst, and the maximum reduction peak at about 660 °C can be ascribed to NiO species that is inset in the skeleton of ZrO2, which is the NiO-ZrO2 solid solution according to the literature [START_REF] Iriondo | Effect of ZrO2 addition on Ni/Al2O3 catalyst to produce H2 from glycerol[END_REF][START_REF] Seo | Hydrogen production by steam reforming of liquefied natural gas (LNG) over Ni/Al2O3-ZrO2 xerogel catalysts: effect of calcination temperature of Al2O3-ZrO2 xerogel supports[END_REF]. Besides, the reduction peak on Ni-Zr catalysts shifts to higher temperature with the increase of nickel content. The reduction peak on 15Ni-Zr catalyst shifts to 450 and 690 °C, respectively. The later ascribed to the increase of NiO-ZrO2 solid solution, resulting in a stronger interaction between zirconia and nickel oxide. The NiO in the NiO-ZrO2 solid solution becomes more difficult to be reduced when NiO content is increased. There are two reasons for this. Firstly, the content of NiO-ZrO solid solution increases with the increase of nickel content. Considering the second reduction peak starts at the same temperature for 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts. Therefore, the reduction peak shifts to high temperature with the increase of NiO-ZrO2 solid solution. Secondly, the higher the nickel content, the more stable the formed NiO-ZrO2 solid solution. Because of the temperature of crystallization increases with the increase of NiO content (from 405 °C for 0 mol% NiO to 600 °C for 30 mol% NiO) [START_REF] Štefanić | The influence of thermal treatment on the phase development of ZrO2-NiO precursors[END_REF]. Chen et al. [START_REF] Chen | Growth kinetics of sintered NiO/ZrO2 (5 mol.% Y2O3) composites[END_REF] also found that the extra nickel NiO species located intergranular and acted as pinning particles. Those the intergranular NiO particles surrounded by ZrO2 possess strong interaction between Ni and ZrO2. Both the intergranular and intragranular NiO particles could enhance again the interaction, led to the more stable the NiO-ZrO2 solid solution. This stable nickel oxide could enhance the stability of the catalyst because the strong interaction would limit the nickel sintering during the DRM reaction [START_REF] Dai | Hollow zeolite encapsulated Ni-Pt bimetals for sintering and coking resistant dry reforming of methane[END_REF][START_REF] Wang | Facile one-pot synthesis of highly dispersed Ni nanoparticles embedded in HMS for dry reforming of methane[END_REF]. Å on the 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts, respectively. The radius of Zr 4+ ion is larger than that of Ni 2+ ion [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. Therefore, the Ni 2+ ion probably inserts into the crystal lattice of ZrO2 and replaces some Zr 4+ ions at the lattice site to form NiO-ZrO2 solid solution, resulting in the reduction of the lattice parameter. Štefanić et al [START_REF] Štefanić | The influence of thermal treatment on the phase development of ZrO2-NiO precursors[END_REF]. reported that the maximum solubility of Ni 2+ ions in the ZrO2 lattice in metastable crystallization products prepared by the incorporation method was 5 mol%. The lattice parameters of ZrO2 decreased with the increase in the content of Ni 2+

when the content of Ni 2+ was less than 5 mol%. While the further increase in the content of Ni 2+ could not affect the lattice parameters, but increased stabilization of the t-ZrO2type phase, due to the strong interaction between NiO and ZrO2. Actually, according to Table 4-1, the d-spacing of ZrO2 was decreased with the increase of nickel content, indicating that the solubility of Ni into ZrO2 was not saturated in this system. There were some nickel species on the surface instead of in the ZrO2 lattice. Because part of NiO was dissolved under 800 °C calcination. Interestingly, more amount of Ni 2+ ion embeds into the skeleton of ZrO2 with the increase of nickel content. Those NiO-ZrO2 solid solutions in the 10Ni-Zr and 15Ni-Zr catalysts contribute to a higher nickel oxide dispersion and a stronger interaction between NiO and ZrO2 [START_REF] Chen | Effect of preparation methods on structure and performance of Ni/Ce0.75Zr0.25O2 catalysts for CH4-CO2 reforming[END_REF]. The physical property of catalysts was evaluated by nitrogen adsorption-desorption isotherms, the specific surface area via the BET method, and the pore volume and pore diameter by means of the BJH method. The results were shown in Table 4-2 and Figure 4-3. All the shapes of the isotherms were assigned to the type IV isotherm with the H3 type of adsorption hysteresis [START_REF] Sing | Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF]. Compared with the 10Ni/ZrO2, the adsorption hysteresis on the 10Ni-Zr catalyst could be observed more evident, which corresponded with the results of pore size distribution. The pore diameter on 10Ni/ZrO2 and 10Ni-Zr catalysts almost distributed at about 2 nm. While, according to the pore size distribution, the later exhibited larger pore volume. Except for the pore volume and pore diameter, the particle size also affects the surface area, because the large particle size of Ni species can lead to a lower surface area [START_REF] Rezaei | CO2-CH4 Reforming over Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia with High Surface Area[END_REF]. The NiO particle size on 10Ni/ZrO2 catalyst was about 15 nm, while NiO crystal peaks could not be observed mostly on the 10Ni-Zr catalyst, therefore, 10Ni/ZrO2 catalyst exhibits lower surface area of 72 m 2 /g, whereas surface area increases obviously to 113 m 2 /g for the 10Ni-Zr catalyst. XRD profiles of the catalysts before and after reaction are reported in Figure 4-4 (A) and (B). After reduction, from Scherrer Equation, the nickel particle size formed (11 nm) on the 10Ni-Zr catalyst is smaller than that on 10Ni/ZrO2 (20 nm), which shows the 10Ni-Zr catalyst prepared by one-step synthesis method could enhance the dispersion of nickel species. 5Ni-Zr catalyst exhibits the lowest surface area and the smallest pore volume and diameter, which might go against the distribution of nickel metal species after reduction, thereby resulting in the sintering of nickel on 5Ni-Zr catalyst. The small size of particles contributes to the increase of the activity for the DRM reaction because it is well known that the nickel particle size can affect the activity of the catalyst [START_REF] Baudouin | Particle size effect in the low temperature reforming of methane by carbon dioxide on silica-supported Ni nanoparticles[END_REF]. Meanwhile, according to the results of H2-TPD, the Ni dispersion of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalyst is 1.3 %, 3.2 %, 4.3 % and 2.6 %, respectively. Obviously, 10Ni-Zr catalyst exhibits the highest dispersion, while the 10Ni/ZrO2 shows the lowest dispersion. A better dispersion of nickel would contribute to the catalytic performance of catalyst, due to the more accessible nickel metallic sites form on surface of catalyst [START_REF] Huang | Highly stable mesoporous NiO-Y2O3-Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. The higher dispersion of nickel species might promote the performance of catalysts for dry reforming of methane. for Bragg angles and d-spacing of ZrO2 on 10Ni/ZrO2 catalyst, respectively. While the Bragg angles and d-spacing of ZrO2 on 5Ni-Zr catalyst after reduction and reaction were similar to those on 10Ni/ZrO2 catalyst, indicating that 5Ni-Zr shifted to traditional catalyst after reduction, like 10Ni/ZrO2 catalyst. Therefore, the nickel particle on 5Ni-Zr catalyst was easy to sinter after reduction, as well as to be oxidized after reaction.

Whereas the Bragg angles of ZrO2 on 10Ni-Zr catalyst decreased, and the d-spacing increased after reduction. While after reaction, the Bragg angles increased from 30.32 to 30.48 o , meanwhile the d-spacing decreased from 2.9480 to 2.9332, which manifested that part of nickel can be released from the skeleton of ZrO2 under the reduction condition, and can enter into the structure of ZrO2 under the reaction conditions. Similar phenomenon could be observed on 15Ni-Zr catalyst. Those phenomena indicated that NiO-ZrO2 solution could be kept on both 10Ni-Zr and 15Ni-Zr catalyst during the reaction, thereby restraining the sintering and oxidation of nickel metal. Moreover, the Bragg angles of ZrO2 on 10Ni-Zr catalyst were lower than that on 15Ni-Zr catalyst after reduction, while it was higher than that on 15Ni-Zr catalyst after reaction. It means more nickel could insert into the structure of ZrO2 on 10Ni-Zr catalyst during the reaction.

The basicity of the catalysts

The CO2 temperature-programed desorption profiles, which are commonly used to describe the basicity of catalysts, are shown in Figure 45. First, we can observe that the number of basic sites on one step Ni-Zr catalysts slightly increases, as a list in Table 4-4. The total basicity of 10Ni/ZrO2 and 10Ni-Zr catalysts is about 65 and 73 µmol CO2 per gram of catalyst respectively. Besides, it can be also noted that desorption peaks on the 10Ni-Zr catalyst slightly shift to lower temperature, compared with the 10Ni/ZrO2 catalyst, which indicates that the introduction of nickel into ZrO2 structure can decrease the strength of basicity. In order to further investigate the CO2 desorption on catalysts, the curve is fitted to three peaks according to the literature [START_REF] Naeem | Syngas production from dry reforming of methane over nano Ni polyol catalysts[END_REF][START_REF] Koo | Combined H2O and CO2 reforming of CH4 over nano-sized Ni/MgO-Al2O3 catalysts for synthesis gas production for gas to liquid (GTL): Effect of Mg/Al mixed ratio on coke formation[END_REF][START_REF] Choudhary | Oxidative coupling of methane over alkaline earth oxides deposited on commercial support precoated with rare earth oxides[END_REF], and the results are also shown in Table 4-4. The peak at low temperature area is the weak basic site, which is mainly consisted of weak Brønsted basic sites such as surface OH groups, the peak at intermediate temperature area (medium-strength basic sites) consists of Lewis base sites such as unsaturated (cus) O 2-and Zr 4+ -O 2-centers, and the peak at high temperature area (strong basic sites) corresponds the CO2 adsorption of the cus Zr 4+ centers with strong Lewis acidity [START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF][START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. Takano et al. found that the CO2 desorption occurred below 250 °C during H2/CO2 TPR experiment. Thus they presumed that the CO2 adsorbed on the weak basic sites was less reactive with H2, due to the weak adsorption of CO2 [START_REF] Takano | Highly active Ni/Y-doped ZrO2 catalysts for CO2 methanation[END_REF]. Therefore, the weak basic sites might restrain the reverse water-gas shift reaction (RWGS). Besides, CO2 adsorbed on the medium-strength basic sites (cus-O 2-and Zr 4+ -O 2- centers) to form monodentate and bidentate carbonates, respectively [START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF][START_REF] Viinikainen | Acidic and basic surface sites of zirconia-based biomass gasification gas clean-up catalysts[END_REF][START_REF] Takano | Highly active Ni/Y-doped ZrO2 catalysts for CO2 methanation[END_REF]. Some studies showed that bidentate carbonates could promote coke elimination [START_REF] Li | Ordered mesoporous Ni/La2O3 catalysts with interfacial synergism towards CO2 activation in dry reforming of methane[END_REF][START_REF] Li | On the role of Ce in CO2 adsorption and activation over lanthanum species[END_REF][START_REF] Takano | Highly active Ni/Y-doped ZrO2 catalysts for CO2 methanation[END_REF]. On the other hand, Zhang et al. [START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF] thought that monodentate carbonates on Ni-ZrO2 catalyst were the main active intermediates for CO2 dissociation, while polydentate carbonate species could lead to a reduction of the activity for CO2 dissociation. These polydentate carbonate species were derived from the CO2 adsorption of strong basic site. Therefore, weak and medium-strength basic sites could promote the performance of catalyst for DRM reaction. The weak and medium-strength basic sites on 10Ni-Zr were higher than those on 10Ni/ZrO2 catalyst. And the content of weak basic sites increases with the increase of Ni content for series of Ni-Zr catalysts.

Structure and morphology of the catalysts

36 L•h -1 •g -1 CH4:CO2:N2=1:1:1 ~95 ~97 - S c [219]
Ni/ ZrO2 5 800

- CH4:CO2=1:1 ~11 ~10 - - [220] Ni/ZrO2 12 757 24 L•h -1 •g -1 CH4:CO2 =1:1 ~87 - 1.2 S [221]
Ni/ZrO2 10 750

24 L•h -1 •g -1 CH4:CO2 =1:1 83.8 84.2 1.18 S [165]
NiO-ZrO2 10 750

64.3 L•h -1 •g -1
CH4:CO2 =1.5:1 ~35 ~57 0.84 ~1.8 [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF] Ni/ZrO2 26 750

150 L•h -1 •g -1 CH4:CO2:He=1:1:8 ~55 - - 1.7 [108] Ni/ZrO2 8 750 120 L•h -1 •g -1 CH4:CO2:He=3:2:5 ~50 ~79 0.93 ~1-2 [223]
Ni-ZrO2 20 700

12,000 h -1 CH4:CO2:N2=1:1:3 ~86 73 -~0.29 [START_REF] Liu | CO2 reforming of CH4 over stabilized mesoporous Ni-CaO-ZrO2 composites[END_REF] Ni/ZrO2 3 700

24 L•h -1 •g -1 CH4:CO2 =1:1 ~70 ~80 ~0.99 4 [102]
Ni/ZrO2 2 700 37.4 L•h -1 •g -1 65.9 67.9 -- [START_REF] Irusta | Hydrogen Production Using Ni-Rh on ZrO2 as Potential Low-Temperature Catalysts for Membrane Reactors[END_REF] CH4:CO2:He=33:3 3:34

Ni/ZrO2 5 700

37.6 L•h -1 •g -1 CH4:CO2 =1:1 ~95 - - 4 [149]
Ni/ZrO2 5 700 The ratio of H2/CO is very close to 1, about 0.95. Besides, the CH4 and CO2 conversion on 10Ni-Zr catalyst could respectively reach to 92.9 % and 93.1 % even at 700 °C. The H2/CO ratio is higher than that at 750 °C, which was closer to 1, about 0.97. hours, finally stabilize at about 0.8 (see in Figure 4567). A similar phenomenon can be observed on the 5Ni-Zr catalyst that CH4 and CO2 conversions decrease from respective 50.7 and 60.0 % to 24.5 and 29.0 % within the 5 h period. In brief, 5Ni-Zr catalyst exhibits the lowest activity and the lowest H2/CO ratio. The CO2 conversion is slightly higher than CH4 conversion on both 10Ni/ZrO2 and 5Ni-Zr catalysts, while the lower H2/CO, especially for the 5Ni-Zr catalyst, even lower than 0.5, indicates that it promotes the reverse water-gas shift reaction (RWGS) on both 10Ni/ZrO2 and 5Ni-Zr catalysts.

150 L•h -1 •g -
10Ni-Zr and 15Ni-Zr catalysts exhibit high stability without deactivation upon 5 h time-on-stream under the same situation. Simultaneously, the H2/CO ratio on the 10Ni-Zr catalyst oscillates between 0.9 and 0.96. Under DRM condition, CO2 is a source of oxygen, while the CO and H2 are the reductants. Therefore, the nickel metal may be oxidized when CO2 adsorption and/or activation on the surface of catalyst. While the NiO species might be reduced by the production of H2 and CO, and thereby the re-dispersion of nickel particles [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Świrk | Ce-and Y-Modified Double-Layered Hydroxides as Catalysts for Dry Reforming of Methane: On the Effect of Yttrium Promotion[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF][START_REF] Li | Sustainable Ru-doped Ni catalyst derived from hydrotalcite in propane reforming[END_REF]. Besides, according to the XRD results, the nickel 10Ni-Zr and 15Ni-Zr catalysts could be released from the NiO-ZrO2 solid solution under reduction condition and enter into the NiO-ZrO2 solid solution under the reaction condition. Meanwhile, the shift on 10Ni-Zr catalyst was more than that on 15Ni-Zr catalyst. Those shifts might affect the adsorption and activation of CO2. Therefore, 10Ni-Zr catalyst exhibited the oscillations of the CO2 conversion, as well as the H2/CO molar ratio, because the CO2 conversion could influence the H2/CO molar ratio. For the 15Ni-Zr catalyst, the shift was too weak to arise the activation and/or adsorption of CO2.

The performance of 10Ni-Zr catalyst at different temperatures

As shown in Figure 45678, except for the same high stability at 700 °C DRM reaction, the 10Ni-Zr catalyst exhibits slightly higher selectivity than that at 750 °C, resulting in the slightly higher H2/CO ratio, stabilizing at 0.97 (see in Figure 4-8 B), which is almost unity. Besides, the conversion of CO2 is more stable at 700 °C than that at 750 °C, resulting in a more stable H2/CO ratio, which implies that the conversion of CO2 can affect the H2/CO ratio. This phenomenon suggests that lower temperature can adjust the H2/CO ratio. No carbon deposition could be detected in 10Ni/ZrO2 and 5Ni-Zr catalyst, while only low amounts of carbon of about 0.9 % and 0.5 % on 10Ni-Zr and 15Ni-Zr catalysts are measured, respectively. Besides, the little increase peak in the sample weight curve can be observed on all the catalysts, as well as the endothermic peak on DSC curve, due to the oxidation of the nickel metal. It implies that there is nickel metal on all of the catalysts after reaction. Thus, the coke formation is not the main reason for the deactivation of 10Ni/ZrO2 and 5Ni-Zr catalysts.

In addition, according to the sample weight curve, the decrease of weight on 10Ni-Zr and 15Ni-Zr catalysts starts at about 500 °C. A similar observation was reported that the coke might be the graphitic carbon, which was difficult to be removed [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. In order to investigate the sintering of nickel metal on catalyst, the XRD experiment was conducted for testing catalysts after reaction, as shown in Figure 4-10 (B) and Table 4-6. Compared with before reaction, serious sintering of nickel metal can be observed on the 10Ni/ZrO2 catalyst, and the nickel metal particle size increases from 20 nm to 31 nm, causing to the deactivation of 10Ni/ZrO2 catalyst. It is worth noting that NiO peak arises on 10Ni/ZrO2 catalyst with the particle size of 15 nm. This NiO particle can lead to a decrease in the activity in DRM, resulting in the lower stability [START_REF] Baudouin | Particle size effect in the low temperature reforming of methane by carbon dioxide on silica-supported Ni nanoparticles[END_REF]. For the 5Ni-Zr catalyst, the nickel metal particle size decreased from 31 to 26 nm under reaction condition, because part of nickel metal was re-oxidized to NiO during the reaction.

Evolution of Ni particle size

Thus, the NiO crystal size of 16 nm on the 5Ni-Zr catalyst causes the decreased catalytic performance of the catalyst. While slightly sintering can be observed on 10Ni-Zr and 15Ni-Zr catalysts since the nickel metal size increases from 11 nm to 15 and 13 nm for 10Ni-Zr and 15Ni-Zr catalysts, respectively. Moreover, no NiO peak was detected on both 10Ni-Zr and 15Ni-Zr catalysts. Because of the formation of the NiO-ZrO2 solid solution, the structure of nickel particle is more stable, and the sintering of nickel is then minimized, further leads to stability of catalyst for dry reforming of methane. Thus, the increase in the NiO-ZrO2 solid solution is more likely linked to an enhancement of the ability to avoid sintering. XPS experiments are conducted in order to obtain the information of Nickel and Carbon oxidation states and chemical environment on the surface of catalyst. As shown in Figure 4-11 (A), it is worth noting that after reaction, Ni 0 species can be observed on all the catalysts. The surface content of Ni 0 on 10Ni/ZrO2 and 5Ni-Zr catalysts are 9.6 % and 17 % (Table 4567), respectively, while 10Ni-Zr and 15Ni-Zr catalysts possess higher surface content of Ni metal, about 24.8 %, and 34.1 %, respectively. NiO crystal phase can be observed evidently on 10Ni/ZrO2 and 5Ni-Zr catalysts, while no NiO peak are detected on 10Ni-Zr and 15Ni-Zr catalysts according to the XRD results. According the the XPS results, there are two reasons for the NiO species on the surface of 10Ni-Zr and 15Ni-Zr catalysts after reaction. At first, part of Ni 0 species might be oxidized when it exposes in air. Second, the oxide materials such as CO2 and H2O might be adsorbed on Ni 0 site [START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF]. However, 10Ni-Zr and 15Ni-Zr catalysts show higher content of Ni 0 species on the surface. Because surface content of Ni 0 is related to the activity of catalyst for DRM reaction [START_REF] Wei | Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts[END_REF], thus 10Ni-Zr and 15Ni-Zr catalysts exhibit higher activity for DRM reaction. Besides, the two resolvable peaks at a binding energy of about 854.1 and 856.3 eV on 10Ni/ZrO2 catalyst are assigned to NiO with weak and strong interaction with ZrO2, respectively. Whereas, those two resolvable peaks shift to higher values of about 855. The O 1s profile is shown in Figure 4-11 (C). There are three kinds of oxide species that exist on all the catalysts: lattice oxygen, carbonate species, and hydroxyl species.

Surface analyses

Carbonate species mainly come from the adsorption of CO2, while the hydroxyl species come from the adsorption of water or hydrogen atom [START_REF] Sutthiumporn | ): Roles of lattice oxygen on C-H activation and carbon suppression[END_REF]. Interestingly, 10Ni-Zr and 15Ni-Zr catalysts possess similar content of surface oxide species (carbonate species and hydroxyl species) of about 45 % and 44 % (Table 4-8), which are higher than those on 5Ni-Zr (28 %) and 10Ni/ZrO2 (25 %) catalysts. Because adsorption oxide species can promote the adsorption and activation of CO2, as well as dissociation of CH4, further promote the removal of carbon deposition and enhance the activity [START_REF] Oemar | Pd-Ni catalyst over spherical nanostructured Y2O3 support for oxy-CO2 reforming of methane: role of surface oxygen mobility[END_REF]. After DRM reaction, more carbonate species and hydroxyl species form on 10Ni-Zr and 15N-Zr catalysts, contributing to higher activity and stability. While more lattice oxide species exist on 5Ni-Zr and 10Ni/ZrO2 catalysts, manifesting less adsorption oxide species can be observed on the catalysts, resulting in lower activity and stability. A similar phenomenon can be observed, as shown in Figure 4-11 (D), that is, similar content of C-O and C=O species on 10Ni-Zr (56 %) and 15Ni-Zr (51 %) catalysts, which is higher than those on 5Ni-Zr (28 %) and 10Ni/ZrO2 (30 %) catalysts.

Discussion

The 10Ni-Zr catalyst prepared from direct synthesis exhibits better catalytic performance for dry reforming of methane, compared with traditional impregnation 10Ni/ZrO2 catalyst. The conversion of CO2 and CH4 can reach c.a. 96 % on 10Ni-Zr catalyst even after 5 hours reaction. while on 10Ni/ZrO2 catalyst, a deactivation is observed with time on stream. The activity and the stability of the 10Ni-Zr catalyst can be explained by the presence of NiO-ZrO2 solid solution, which increases the surface area, and also by small nickel particle size (Ni°) (11 nm). The NiO-ZrO2 solid solution with those special properties can contribute to the activity for dry reforming of methane [START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Baudouin | Particle size effect in the low temperature reforming of methane by carbon dioxide on silica-supported Ni nanoparticles[END_REF][START_REF] Miyamoto | Effect of basicity of metal doped ZrO2 supports on hydrogen production reactions[END_REF]. The strong interaction in NiO-ZrO2 solid solution on the 10Ni-Zr catalyst can limit the sintering of nickel particles under the reaction conditions, contributing to the stability of the 10Ni-Zr catalyst. Besides, adsorption oxide species can promote the adsorption and activation of CO2, as well as dissociation of CH4, and further promote the removal of carbon deposition and then the enhancement of the activity in DRM as described in the literature [START_REF] Oemar | Pd-Ni catalyst over spherical nanostructured Y2O3 support for oxy-CO2 reforming of methane: role of surface oxygen mobility[END_REF]. Besides, more content of weak basic sites and medium-strength basic sites exists on the 10Ni-Zr catalyst, thus exhibits the highest H2/CO ratio, about 0.95, which is very close to 1. Because CO2 preferential adsorbed on weak basic sites and at the same time formed active carbonate species that less react with H2 and more react with methane [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Takano | Highly active Ni/Y-doped ZrO2 catalysts for CO2 methanation[END_REF]. Whereas, 10Ni/ZrO2 catalyst, prepared by impregnation, presents bigger nickel particle (20 nm), higher content of strong basic sites and lower surface area, leading then to lower activity and selectivity for DRM reaction. Besides, severe sintering takes place on 10Ni/ZrO2 catalyst, due to the weak interaction between NiO and ZrO2 support. More lattice oxygen species form on 10Ni/ZrO2 catalyst after reaction, leading to less adsorption oxygen species. Therefore, the 10Ni/ZrO2 catalyst shows the bad performance of the catalyst for the dry reforming of methane. Besides, the big nickel particle size of 31 nm forms on the 5Ni-Zr catalyst, as well as the lower surface area, leading lower activity and lower selectivity for DRM reaction. Whereas, the same nickel particle size of 11 nm forms on 10Ni-Zr and 15Ni-Zr catalysts, as well as similar surface area and content of adsorption oxygen species after reaction. Therefore 10Ni-Zr and 15Ni-Zr catalysts exhibit similar activity for DRM reaction. Considering the cost, the 10Ni-Zr catalyst is the optimal for DRM reaction.

Summary

10Ni-Zr catalyst exhibits the higher performance of catalyst for CO2 reforming of methane, compared with 5Ni-Zr and 10Ni/ZrO2 catalysts. Because 10Ni-Zr catalyst possesses more NiO-ZrO2 solid solution with strong interaction between nickel and zirconium, which leads to the high surface area, small nickel particle size and more content of weak and medium-strength basic sites. Besides, more adsorption oxygen species and just slight sintering occur on 10Ni-Zr and catalyst during the reaction.

Therefore, the conversion of CO2 and methane at 750 °C on 10Ni-Zr catalyst is over 96 % even reaction for 5 hours. The H2/CO ratio on 10Ni-Zr catalyst is very close to 1.

While big nickel particle size and low surface area lead to lower activity and selectivity on 5Ni-Zr and 10Ni/ZrO2 catalysts for DRM reaction. The weak interaction between nickel and ZrO2 on the 10Ni/ZrO2 catalyst can cause severe sintering, thereby leading to deactivation.

Chapter

Introduction

In general, noble metal catalysts (Pt, Ir, Rh) exhibit good performance for dry reforming of methane [START_REF] Singha | Low temperature dry reforming of methane over Pd-CeO2 nanocatalyst[END_REF][START_REF] Liang | Low-temperature activation of methane on the IrO2(110) surface[END_REF][START_REF] Polo-Garzon | Dry reforming of methane on Rh-doped pyrochlore catalysts: A steady-state isotopic transient kinetic study[END_REF]. Considering the high cost of noble metals for the industrial scale, many efforts have focused on the Ni-based catalysts, because nickel metal has a high potential for industrial application in DRM [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Hassan | Low temperature dry reforming of methane on rhodium and cobalt based catalysts: active phase stabilization by confinement in mesoporous SBA-15[END_REF][START_REF] García-Diéguez | Nanostructured Pt-and Ni-based catalysts for CO2-reforming of methane[END_REF][START_REF] Guo | Dry reforming of methane over nickel catalysts supported on magnesium aluminate spinels[END_REF]. However, it was well known that Ni-based catalysts suffered deactivation caused by carbon deposition and/or sintering. Therefore, the development of nickel-based catalysts, with the significantly promoted Ni-sintering resistance and coke-tolerance, has been extensively investigated in recent years [START_REF] Li | Sintering resistant Ni nanoparticles exclusively confined within SiO2 nanotubes for CH4 dry reforming[END_REF][START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF][START_REF] Christensen | Effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming[END_REF]. According to the literature [START_REF] Kathiraser | Inverse NiAl2O4 on LaAlO3-Al2O3: unique catalytic structure for stable CO2 reforming of methane[END_REF][START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF], larger Ni particles on the surface of the catalyst led to catalytic selectivity towards methane cracking (Equation (2)), thereby causing carbon deposition. Another approach of the coke deposition was the disproportionation of CO (Equation ( 4)).

CH4→2H2+C

(Equation 2) 4)

2CO→CO2+C (Equation
It is found that the adjustment of the catalyst by promoters is an efficient approach to restrain the formation of carbon [START_REF] Taherian | A comparative study of ZrO2, Y2O3 and Sm2O3 promoted Ni/SBA-15 catalysts for evaluation of CO2/methane reforming performance[END_REF][START_REF] Li | Y2O3-promoted NiO/SBA-15 catalysts highly active for CO2/CH4 reforming[END_REF][START_REF] Li | Alumina supported Ni and Co catalysts modified by Y2O3 via different impregnation strategies: Comparative analysis on structural properties and catalytic performance in methane reforming with CO2[END_REF]. Thus, the modification of the raw catalyst with rare earth elements has been reported to enhance both the activity of the Except for the active metal and promoter, the support also plays an important role in the catalytic test for DRM. The ZrO2 is considered to be a promising support for dry reforming of methane, because of the special property, e.g. the reducing and oxidizing abilities, the unexceptionable thermal and chemical stability, and the high oxygen mobilization [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF][START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Li | Y2O3-promoted NiO/SBA-15 catalysts highly active for CO2/CH4 reforming[END_REF]. These properties of ZrO2 can be improved by adding a cation with valence lower than 4+, e.g. Y 3+ [START_REF] Li | Y2O3-promoted NiO/SBA-15 catalysts highly active for CO2/CH4 reforming[END_REF][START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF], which results in the formation of solid solution, together with oxygen vacancies. Furthermore those oxygen vacancies play a key role in gasifying carbon deposits during the activation of CO2 and O2 [START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF].

Bellido et al. [START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF] prepared a series of Y2O3-ZrO2 catalysts with different yttrium content (4 to 12 mol%). 5Ni8YZ (mole fraction of Y=8 mol%) catalyst showed the best In order to understand the ability of carbon-resistant on the Y-modified ZrO2 catalyst, the NiO-ZrOm and NiO-ZrOm-YOn catalysts were prepared by one step urea hydrolysis method, and the performance of which was discussed here in detail. This latter catalyst, which was not the most active catalyst in DRM exhibited, however, a high resistance against carbon deposition. We investigate the ability of carbon resistance on NiO-ZrOm-YOn catalyst, further to understand the relationship between structure and carbonresistance on Y-modified catalyst. stirring for 48 h. After that, the mixture was aged at 100 °C for 24 h. Then the slurry was filtered, washed with a little of distilled water and dried at room temperature. The catalysts were calcined in air at 800 °C for 5 h with an increasing rate of 1 °C/min. The obtained materials were denoted as NiO-ZrOm-YOn (Ywt%=10 %). In order to understand the function of yttrium, NiO-ZrOm without yttrium was prepared by the above method and denoted as NiO-ZrOm.

Preparation of the

The physicochemical properties of NiO-ZrOm-YOn catalyst

The BET results of Y-doped and Y-free NiO-ZrOm catalysts

The physical properties of Y-doped and Y-free NiO-ZrOm material were investigated by the N2 adsorption-desorption method (Table 5-1). Both of the samples display the same narrow pore size of about 2 nm. The NiO-ZrOm catalyst shows a surface area of 113 m 2 /g and a pore volume of 2 nm; while, the NiO-ZrOm-YOn catalyst exhibits the lower specific surface area (79 m 2 /g ) and the smaller pore volume (0.1 cm 3 /g), which may decrease the activity of the catalyst, because the high surface area can enhance the activity of catalyst [START_REF] Rezaei | CO2-CH4 Reforming over Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia with High Surface Area[END_REF]. The H2-TPR profile of Y-doped and Y-free NiO-ZrOm catalysts is presented in Fig.

5-1 (A).

Two reduction peaks at about 450 and 650 °C, denominated α and β, respectively, are observed on both catalysts. The first peak (α) corresponds to the reduction of the NiO species of weak and strong interaction with Y and/or Zr [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF]. This first peak shifts to low temperature on NiO-ZrOm-YOn catalyst, as compared to NiO-ZrOm catalyst. This shift may be attributed to the increase of oxygen vacancies by the addition of the yttrium promoter, because oxygen vacancies can promote the reduction of NiO by weakening the Ni-O bond [START_REF] Asencios | Partial oxidation of methane on NiO-MgO-ZrO2 catalysts[END_REF][START_REF] Dow | Yttria-stabilized zirconia supported copper oxide catalyst: I. Effect of oxygen vacancy of support on copper oxide reduction[END_REF]. β peak on NiO-ZrOm catalyst is related to the reduction of the solid solution of NiO-ZrO2 and/or ZrO2, while on NiO-ZrOm-YOn catalyst, β peak may be assigned to the reduction of solid solution (NiO-ZrO2 and/or NiO-Y2O3), ZrO2 and/or surface-capping oxygen ions of the Y2O3-ZrO2 solid solution [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF][START_REF] Maia | Catalytic features of Ni supported on CeO2-ZrO2 solid solution in the steam reforming of glycerol for syngas production[END_REF]. On the contrary, the β peak shifts to a higher temperature for NiO-ZrOm-YOn catalyst. A similar phenomenon about the shift was found by Asencios et al. [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF] on NiO-Y2O3-ZrO2 catalysts. The yttrium addition could promote the reduction of surface-capping oxygen ions of NiO-ZrOm-YOn catalyst, leading to the formation of new surface oxygen vacancies at about 700 °C [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. Therefore, the β peak shifts to about 700 °C. Except for the shift of peak, the total amount of H2 consumption on NiO-ZrOm catalyst (0.62 mmol H2/g) is higher than that on NiO-ZrOm-YOn catalyst (0.37 mmol H2/g), presented in Table 5-2. The latter result is consistent with the theoretical value of 0.39 mmol H2/g, while the H2 consumption on NiO-ZrOm catalyst is higher than the theoretical value of 0.47 mmol H2/g. This phenomenon proves that the ZrO2 is also reduced by hydrogen. Besides, for α peak, the amount of H2 consumption on NiO-ZrOm-YOn catalyst decreases by adding the yttrium, which indicates that part of this free NiO would be inserted into the structure of ZrO2 and/or Y2O3 to form NiO-ZrO2 and/or NiO-Y2O3 solid solution. Thus, yttrium can promote nickel embedding into the structure of ZrO2 and/or Y2O3. For β peak, the amount of H2 consumption on NiO-ZrOm catalyst is higher than that obtained on NiO-ZrOm-YOn catalyst. In order to understand the reduction of ZrO2, the Zr 3d peak is resolved into two peaks, which is shown in Fig. 5-1 (B). The peaks at about 181.4 and 182.3 eV are ascribed to Zr 3+ and Zr 4+ , respectively [START_REF] Imparato | Unraveling the Charge State of Oxygen Vacancies in ZrO2-x on the Basis of Synergistic Computational and Experimental Evidence[END_REF][START_REF] Ingo | Origin of Darkening in 8 wt% Yttria-Zirconia Plasma-Sprayed Thermal Barrier Coatings[END_REF]. It is very obvious to be observed that most Zr 3+ formed on both catalysts, indicating that the most Zr 4+ is reduced to Zr 3+ after reduction. From Table 1, the content of Zr 3+ on NiO-ZrOm catalyst (90 %) is higher than that on NiO-ZrOm-YOn catalyst (84 %), which manifests that more Zr 4+ is reduced to Zr 3+ on NiO-ZrOm catalyst, which is conformed to H2-TPR results (Figure 5-1 (A) and Table 5-2), that is, NiO-ZrOm catalyst consumes more H2 during the reduction.

These phenomena indicate that the formation of ZrO2 defected by Ni 2+ on NiO-ZrOm catalyst is very easy to be reduced from Zr 4+ to Zr 3+ in the presence of hydrogen. While the ZrO2 defected by Y 3+ is very stable, which is very hard to be reduced. Therefore, the introduction of the Y 3+ into ZrO2 lattice can also stabilize the crystal structure, and create new oxygen vacancies.

Basicity of Y-doped and Y-free NiO-ZrOm catalysts

Table 5- The peaks on NiO-ZrOm-YOn catalyst shift to low temperature, as compared to NiO-ZrOm catalyst. The total number of basic sites increases from 73 to 100 μmol CO2/g on NiO-ZrOm catalyst before and after introducing of yttrium (see in Table 5-3), which indicates that the yttrium can enhance the total number of basic sites. As already described elsewhere [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF][START_REF] Dębek | Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition[END_REF], there are three types of basic sites (weak, mediumstrength and strong). The content of weak peak on NiO-ZrOm-YOn catalyst (36.9 %) is higher than that on NiO-ZrOm catalyst (15.5 %). While the content of strong peak decreases to 13.0 % on NiO-ZrOm-YOn catalyst. Besides, the position of weak and medium-strength peaks on NiO-ZrOm-YOn catalyst are at about 150 and 240 °C, which is lower than those on NiO-ZrOm catalyst (185 and 252 °C), respectively. This phenomenon manifests that yttrium can promote the formation of weak basic sites on NiO-ZrOm-YOn catalyst. Thus, one can note that the addition of yttrium modifies both the distribution and the number of basic sites. A similar phenomenon can be observed in our group previous works [START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. According to the literature [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Dębek | Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition[END_REF], the weak and medium-strength basic sites can promote the formation of activation carbonate species, thereby enhancing the ability to remove the carbon deposition. While, too strong basic sites lead to too strong CO2 adsorption, thereby promoting more carbon deposition. Therefore, the weak and medium basic sites present on NiO-ZrOm-YOn catalyst can enhance the ability to eliminate coke. 5-4). The peak at about 44.5 o can be attributed to the metallic nickel [START_REF] Wang | The bi-functional mechanism of CH4 dry reforming over a Ni-CaO-ZrO2 catalyst: further evidence via the identification of the active sites and kinetic studies[END_REF]. The Ni 0 size on NiO-ZrOm catalyst is about 12 nm after reduction, and it increases to 24 nm after reaction. While for NiO-ZrOm-YOn catalyst, it decreases from 16 nm to 10 nm after reaction for 8 hours, which indicates the re-dispersion of Ni 0 . Similar phenomenon had been reported by other researches [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Świrk | Ce-and Y-Modified Double-Layered Hydroxides as Catalysts for Dry Reforming of Methane: On the Effect of Yttrium Promotion[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. Nakayam et al. [START_REF] Li | Sustainable Ru-doped Ni catalyst derived from hydrotalcite in propane reforming[END_REF] found the redispersion of Ni under an alternating condition between H2 reduction and oxidation atmosphere. Under dry reforming of methane condition, CO2 is a source of oxygen.

When CO2 adsorption and activation on the surface of catalyst, the nickel metal may be oxidized. The production of H2 and CO as reduction atmosphere, may contribute to the reduction of the NiO, and thereby the re-dispersion of nickel particles. On one hand, the Ni 0 size is related to the sintering of nickel. Severe sintering takes place on NiO-ZrOm catalyst, due to the lower interaction between Ni and Zr, which is confirmed by the results of H2-TPR. Furthermore, the addition of yttrium can limit the sintering of nickel during the reaction. On the other hand, It is well known that the large nickel particle size maybe favor for selective reactions that lead to carbon deposition [START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. As a consequence, NiO-ZrOm-YOn can limit the carbon deposition. Besides, NiO-ZrOm-YOn exhibits more basic sites, which could enhance the ability of adsorption of CO2, thereby promoting the removal of carbon deposition. both sintering and carbon deposition could contribute to the deactivation of catalyst, thereby reducing the stability of the catalyst.

The performance of Y-doped and Y-free NiO-ZrOm catalysts

The catalytic performance of the catalysts was investigated at 700 °C for 8 hours (Figure 5-4). The conversion of methane on both catalysts decreases within 1 hour, and finally stabilizes at 67 % and 85 % for NiO-ZrOm-YOn and NiO-ZrOm catalyst, respectively. Except for the higher methane conversion on NiO-ZrOm catalyst, the CO2 conversion and the ratio of H2/CO are higher than those on NiO-ZrOm-YOn catalyst.

The CO2 conversion on the NiO-ZrOm catalyst is about 89 % with the H2/CO ratio of 0.95, which is very close to one. When adding yttrium into NiO-ZrOm catalyst, the CO2 conversion decreases within 60 min and stabilizes at 70 % for 8 hours on stream. At the same time, the H2/CO ratio decreases to about 0.85. Because NiO-ZrOm-YOn catalyst exhibits the lower specific surface area, the smaller pore volume, and the bigger metallic nickel particle size. Those properties could lead to the lower performance of NiO-ZrOm-YOn catalyst. However, the catalyst activity remained stable during time on stream.

Carbon formation evidenced by Raman Spectroscopy

Fig. 56shows the results of Raman experiment. Two peaks can be found on NiO-ZrOm catalyst and no peak can be observed on NiO-ZrOm-YOn catalyst, which manifests that no or less coke form on NiO-ZrOm-YOn catalyst. The peak at about 1328 cm -1 is attributed to the structural imperfections on not-organized carbon materials, namely disorder-induced band (D band), and another peak at about 1585 cm -1 is corresponded to the in-plane C-C stretching vibrations of sp 2 atoms in coke, namely graphitic carbon (G band) [START_REF] Kim | Coke tolerance of Ni/Al2O3 nanosheet catalyst for dry reforming of methane[END_REF][START_REF] Bai | Coking behaviors and kinetics on HZSM-5/SAPO-34 catalysts for conversion of ethanol to propylene[END_REF][START_REF] Liu | Methane reforming with carbon dioxide over a Ni/ZiO2-SiO2 catalyst: Influence of pretreatment gas atmospheres[END_REF]. The intensity of peak is named I, while the ratio of IG/ID is about 1.7 (Table 5-5), indicating that more graphitic carbon forms on NiO-ZrOm catalyst, which is conformed to the results of XPS, more C-C species on NiO-ZrOm catalyst. All those phenomena show that the carbon deposition on NiO-ZrOm catalyst is higher than that on NiO-ZrOm-YOn catalyst.

Figure 56The Raman profiles of used NiO-ZrOm and NiO-ZrOm-YOn catalysts.

On the study of the morphology of carbon studied by TEM

The morphology structure of carbon on the catalysts after reaction is determined by the transmission electron microscopy experiment (Fig. 567). After reaction, the nickel particle size decreases on NiO-ZrOm by adding the yttrium promoter (Fig. 567(A) and (C)). The nickel particle size formed on NiO-ZrOm-YOn is about 10-15 nm (Fig.

5-7 (D)

), while about 15-20 nm formed on NiO-ZrOm (Fig. 5-7 (B)), which is corresponding to the results of XRD. It can be noted that large Ni particle (over 20 nm) can be observed on NiO-ZrOm catalyst. According to the results of H2-TPR, the interaction between Ni and ZrOm on NiO-ZrOm catalyst is lower than that on NiO-ZrOm-YOn catalyst, leading to nickel sintering. Therefore, bigger particles of Ni 0 are present on NiO-ZrOm catalyst. These large nickel particles maybe favor selective reactions that tend to form carbon deposition [START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF], thereby resulting in formation of carbon deposition. Thus, a lot of carbon deposition can be observed on NiO-ZrOm catalyst in the form of carbon nano tube (

Summary

NiO-ZrOm and NiO-ZrOm-YOn catalysts were prepared by urea hydrolysis method and were characterized by BET, TPR-H2, CO2-TPD, XRD, TEM, and XPS. After adding yttrium, the strong interaction between Ni and ZrOm formed on NiO-ZrOm-YOn catalyst, resulting in the small size of nickel particle distributed on catalyst after reaction. While for NiO-ZrOm catalyst, large particles formed on catalyst after reaction, contributing to the deposition of carbon. Besides, more amount of weak and mediumstrong basic sites formed on NiO-ZrOm-YOn catalyst, which could enhance the ability of removal of carbon. Except for the advantages, the NiO-ZrOm-YOn catalyst showed lower specific surface area and the smaller pore volume. Thus, from the results of the activity test at 700 °C for 8 hours, NiO-ZrOm catalyst exhibited higher methane and CO2 conversion, while NiO-ZrOm-YOn catalyst exhibited highly carbon-resistant for dry reforming of methane at 700 o C. Therefore, yttrium can modify the interaction between Ni and ZrOm, in enhancing the dispersion of nickel particles during the reaction, and promote the formation of more weak and medium-strength basic sites.

Syngas production by dry methane reforming over Mg doped

NiO-ZrO2 catalysts

Introduction

The literature data showed that nickel-based catalysts presented high activity in dry reforming of methane reaction [START_REF] Rezaei | CO2-CH4 Reforming over Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia with High Surface Area[END_REF][START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF][START_REF] Koo | Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process[END_REF]. However, Ni-based catalysts easily deactivated, mainly due to the sintering of nickel and/or carbon deposition [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. The former could be modified by adjusting the interaction between nickel and support [START_REF] Wang | Durable Ni/MgO catalysts for CO2 reforming of methane: activity and metal-support interaction[END_REF].

Wang et al. [START_REF] Wang | Durable Ni/MgO catalysts for CO2 reforming of methane: activity and metal-support interaction[END_REF] investigated the effect of calcination temperatures and Ni loadings on the stability of Ni/MgO catalyst for DRM dry reforming of methane. They found that the formation of NiO-MgO solid solution could enhance the interaction between Ni and MgO, thereby the promotion of stability of Ni/MgO catalyst. Because the interaction between Ni and MgO on NiO-MgO solid solution was so strong that could resist the sintering of nickel [START_REF] Koo | Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process[END_REF]. In addition, it was demonstrated that the nickel particle size was not only related to the sintering of nickel but also affected the formation of carbon deposition. In our previous results, larger particles of nickel on the support surface may be responsible for the carbon deposition, due to the catalytic selectivity towards carbon forming reactions [START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. Except for the nickel particle size, the basicity of catalyst also has an influence on carbon deposition. Koo et al. [START_REF] Wang | Kinetic modeling and transient DRIFTS-MS studies of CO2 methanation over Ru/Al2O3 catalysts[END_REF] studied the effect of MgO content on performance of MgO-promoted Ni/Al2O3 catalyst for CO2 reforming of methane and carbon deposition after reaction. The authors found that MgO could enhance the basic strength over the surface of catalyst, thereby increase the adsorption of CO2, which could contribute to suppressing carbon deposition. As the consequence, the catalyst containing 20 % MgO exhibited the highest activity and stability for steam and carbon dioxide reforming of methane. It was shown, in the last years, that the addition of alkali oxide into support could modify the support, eg. CaO, MgO. The later has attracted extensive research as a promoter for dry reforming of methane [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF][START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF][START_REF] Koo | Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process[END_REF].

In order to understand the effect of MgO on performance of NiO-ZrO2 catalyst for dry reforming of methane, NiO-ZrO2 and NiO-MgO-ZrO2 catalysts were prepared and characterized by the means of H2-TPR, TG, CO2-TPD, and TEM. The results are explained here in detail.

Preparation of the NiO-MgO-ZrO2 catalyst

The two catalysts were prepared by a one-step synthesis method [START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF]. The loadings of nickel and magnesium were both fixed at 10 wt%. A certain mass of zirconium (IV) oxynitrate hydrate (Aldrich), magnesium acetate tetrahydrate (Alfa Aesar), nickel(II) nitrate hexahydrate, Pluronic P123 (Aldrich), urea (Sigma-Aldrich) was dissolved in distilled water. The obtained solution was heated to 95 °C with stirring constantly for 48 h, after that, transferred into the oven (100 °C) for 24 hours, then filtrated, washed and dried at room temperature. At last, the precursor was calcined at 800 °C for 5 hours under airflow with an increasing rate of 1 °C/min. The obtained sample was denoted NiO-MgO-ZrO2. The free Mg catalyst prepared by the same method without Mg, was named NiO-ZrO2.

Characterization of NiO-MgO-ZrO2 and NiO-ZrO2

The reducibility of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts

The reducibility of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts was studied by H2-TPR, and the results are shown in Figure 5678. Two distinct peaks could be observed on both catalysts, the one at the low-temperature area and the other one at high-temperature area. According to the literature [START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF][START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF][START_REF] Titus | Dry reforming of methane with carbon dioxide over NiO-MgO-ZrO2[END_REF], the peak at low temperature corresponded to the weak interaction between Ni and Zr, and the peak at high temperature was attributed to the NiO-ZrO2 solid solution. These two peaks shifted to higher temperature area on NiO-MgO-ZrO2 catalyst, compared to NiO-ZrO2 catalyst, which proved that the interaction between Ni and Zr could be enhanced by introducing the Mg. This stronger interaction might be attributed to the NiO-MgO solid solution, which could restrain the sintering of nickel [START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF][START_REF] Koo | Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process[END_REF]. Three kinds of basic sites were identified on both catalysts: weak, medium-strength and strong basic sites [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. The desorption peaks on NiO-MgO-ZrO2 catalyst shifted to low temperature. Moreover, the total basicity of NiO-MgO-ZrO2 catalyst decreased from 73μmol CO2/g to 69 μmol CO2/g by introducing of Mg (Table 56). However, the weak basic sites increased to 37.5 % on NiO-MgO-ZrO2 catalyst. This phenomenon showed that Mg could promote the creation of weaker basic sites, thereby weaken the strength of basicity. This too weak basic sites might weaken the adsorption of CO2, which might result in the carbon deposition for dry reforming of methane [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF]. catalyst might result in the low performance for dry reforming of methane [START_REF] Rezaei | CO2-CH4 Reforming over Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia with High Surface Area[END_REF]. 5-8 shows the crystallite size and content of Ni 0 species. It is worth noting that the crystallite size of Ni 0 species increases from 12 nm to 20 nm by introducing of Mg into the NiO-ZrO2 catalyst after reduction. There is a different observation on both catalysts after reaction, it decreases to 17 nm over NiO-MgO-ZrO2 catalyst, while it increases to 24 nm on NiO-ZrO2 catalyst. The content of Ni 0 species on NiO-ZrO2 catalyst is higher than those on NiO-MgO-ZrO2 catalyst all the time. However, the variation in content of Ni 0 species is different. It decreases from 34 % to 28 % on NiO-ZrO2 catalyst after reaction, while it keeps at 22 % on NiO-MgO-ZrO2 catalyst all the time. Generally, the content and crystallite size of Ni 0 species could influence the catalytic performance of catalyst for DRM reaction [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. Thus those different changes might affect the activity and/or stability for DRM reaction. Figure 5-8 reported the results of the activity test. NiO-ZrO2 catalyst exhibited better performance for dry reforming of methane. The methane and CO2 conversion on NiO-ZrO2 catalyst were slightly higher than those on NiO-MgO-ZrO2 catalyst. Because NiO-ZrO2 catalyst showed better physical properties, eg. higher surface area, bigger pore volume, and larger pore diameter. Besides, smaller Ni 0 crystallite size (12 nm) and lower Ni 2+ content (66 %) could be observed on NiO-ZrO2 catalyst after reduction (Table 5678), which contributed to the activity of catalyst for dry reforming of methane [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. It was worth noting that the conversion of methane on NiO-ZrO2 catalyst decreased a little bit within 50 min, and increased to 87 %. While CO2 conversion increased a little bit within 50 min and stabilized at 90 %. Those results indicated that the stability of NiO-MgO-ZrO2 catalyst was slightly higher than NiO-ZrO2 catalyst, because the formation of NiO-MgO solid solution could enhance the interaction between Ni and support, which could restrain the sintering of nickel. Therefore, severe sintering happened on NiO-ZrO2 catalyst. The crystallite size of Ni 0 on NiO-ZrO2 catalyst increased from 12 nm to 24 nm after reaction (Table 5678), while it decreased from 20 nm to 17 nm on NiO-MgO-ZrO2 catalyst.

The crystalline Size and content of Ni

The performance of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts

The carbon deposition on NiO-MgO-ZrO2 and NiO-ZrO2 catalysts

The content of carbon formed during DRM test was determined by TG experiment. The content of coke on NiO-MgO-ZrO2 catalyst was about 16.3 % (Table 5-8), which was clearly higher than that obtained on NiO-ZrO2 catalyst (3.7 %).

Syngas production via CO2 reforming of methane over

Aluminium promoted NiO-ZrO2-Al2O3 catalyst: On the role of aluminum species

Introduction

Since nickel shows as excellent catalytic activity as noble ones ( Pt, Ru, and Rh), Nibased catalysts have been widely reported on the DRM reaction [START_REF] Singh | Sonochemical synthesis of Pt, Ru doped TiO2 for methane reforming[END_REF][START_REF] Moral | Syngas production by means of biogas catalytic partial oxidation and dry reforming using Rh-based catalysts[END_REF][START_REF] Pompeo | Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2[END_REF][START_REF] Polo-Garzon | Dry reforming of methane on Rh-doped pyrochlore catalysts: A steady-state isotopic transient kinetic study[END_REF].

Pompeo et al. [START_REF] Pompeo | Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2[END_REF] compared the catalytic performance of Ni and Pt supported on α-Al2O3, ZrO2 and α-Al2O3-ZrO2 catalysts for dry reforming of methane. They found nickel was slightly more active than platinum. Moreover, the nickel-based catalysts served as the optimal alternatives considering economic viability [START_REF] Albarazi | Hydrogen and syngas production by methane dry reforming on SBA-15 supported nickel catalysts: On the effect of promotion by Ce0.75Zr0.25O2 mixed oxide[END_REF]. However, the deactivation of Ni-based catalysts due to sintering and/or carbon deposit needs to be addressed in order to achieve facile scaling-up in industrial process. Thus tremendous efforts have been focused on the development of promoters or novel support materials to address these two problems, further contributing to the design of catalysts with higher stability [START_REF] Wang | CO2 reforming with methane over small-sized Ni@SiO2 catalysts with unique features of sintering-free and low carbon[END_REF][START_REF] Han | Highly coke-resistant Ni nanoparticle catalysts with minimal sintering in dry reforming of methane[END_REF].

It has been found that aluminum (Al) as a promoter and/or support could enhance the catalytic performance mainly because Al could improve the reducibility and dispersity of nickel. Liu et al. [88] investigated the effect of La, Al and Mn promoter on Fe modified natural clay supported Ni catalysts for dry reforming of methane, and confirmed that Al promoted catalyst exhibited the highest catalytic performance in dry reforming of methane. Talkhoncheh et al. [START_REF] Talkhoncheh | Syngas production via dry reforming of methane over Ni-based nanocatalyst over various supports of clinoptilolite, ceria and alumina[END_REF] studied the effect of Ni-based nanocatalysts supported on clinoptilolite, ceria, and alumina on the catalytic performance for DRM reactions. Ni/Al2O3 nanocatalysts exhibited the best performance, due to even dispersion of nickel, homogenous distributions and high specific surface area. On the other hand, although various supports have been employed as the support, such as CeO2, ZrO2 , SiO2, La2O3 and Al2O3 [START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF][START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF][START_REF] Verykios | Catalytic dry reforming of natural gas for the production of chemicals and hydrogen[END_REF]. ZrO2 exhibits favorable properties such as thermal stability, oxygen storage capacity, enhancement of the metallic dispersion, and the ability of CO2 adsorption. Therefore, many researchers have focused on the utilization of ZrO2 support for DRM [START_REF] Sajjadi | Sol-gel synthesis of Ni-Co/Al2O3-MgO-ZrO2 nanocatalyst used in hydrogen production via reforming of CH4/CO2 greenhouse gases[END_REF][START_REF] Barroso-Quiroga | Catalytic activity and effect of modifiers on Ni-based catalysts for the dry reforming of methane[END_REF][START_REF] Kumar | Nickel-based ceria, zirconia, and ceria-zirconia catalytic systems for low-temperature carbon dioxide reforming of methane[END_REF]. Pompeo et al. [START_REF] Pompeo | Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2[END_REF] found that ZrO2 as a basic support could promote the adsorption of CO2, thereby inhibiting the carbon deposition. Mesoporous La2O3-ZrO2 could promote the stability of the catalyst due to the high dispersion of nickel species and the pore confinement effect [START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF]. Ni supported on SBA-15 and mesoporous ZrO2 also exhibit a better catalytic performance owing to the special pore confinement. Liu et al. [START_REF] Liu | A facile route to synthesize mesoporous zirconia with ultra high thermal stability[END_REF] found mesoporous ZrO2 with a specific surface area of 198 m 2 g -1 could be obtained even calcination at 700 o C via a sol-gel approach treated by NaOH or KOH solution. Miao et al. [START_REF] Miao | One-pot synthesis of Al2O3 modified mesoporous ZrO2 with excellent thermal stability and controllable crystalline phase[END_REF] found M-ZrAl-10 with the Al content of 10 % exhibited better physical properties even treated at a high temperature of 800 o C with a high specific surface area of 56 m 2 /g, a large pore volume of 0.13 cm 3 /g and narrow pore size distribution of 8.2 nm. On one hand, the properties of ZrO2 support could be modified by the introduction of alumina species. On the other hand, ZrO2 could enhance the performance of the catalyst with Al2O3 support for dry reforming of methane. Therdthianwong et al. [START_REF] Therdthianwong | Synthesis gas production from dry reforming of methane over Ni/Al2O3 stabilized by ZrO2[END_REF][START_REF] Therdthianwong | Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition[END_REF] demonstrated that Ni/Al2O3 catalyst modified by ZrO2 exhibited higher stability for dry reforming of methane. Because ZrO2 could promote the elimination of carbon deposition by the dissociation of CO2 to form oxygen intermediates. Souza et al. [START_REF] Souza | Reforming of methane with carbon dioxide over Pt/ZrO2/Al2O3 catalysts[END_REF] also found Pt/ZrO2/Al2O3 showed higher stability than Pt/Al2O3 for dry reforming of methane due to the Pt-Zr n+ interfacial sites, which could suppress the carbon deposition.

The performance of the catalyst could be affected by the spinel NiAl2O4 formed on the catalyst of Ni supported on Al2O3 during the reduction. While zirconia could avoid the formation of spinel NiAl2O4, which could enhance the performance of the catalyst. Li et al. [START_REF] Li | Study on CO2 reforming of methane to syngas over Al2O3-ZrO2 supported Ni catalysts prepared via a direct sol-gel process[END_REF] showed that the catalyst of nickel supported on Al2O3-ZrO2 catalyst prepared via a direct sol-gel process exhibited excellent activity and stability for dry reforming of methane compared with Ni/Al2O3 catalyst. Because of the small metallic Ni particles with the properties of highly and uniformly dispersed on Al2O3-ZrO2 support. Besides, the interaction between nickel and Al2O3-ZrO2 support could avoid the formation of spinel NiAl2O4.

In the present work, the enhancement of catalytic performance by a combination of zirconia and alumina is reported. Thus, a series of 10NiO-xAl2O3-(90-x)ZrO2 (x=0, 10, 20, 45, 90) catalysts were prepared by a one-step synthesis method and characterized by BET, XRD, XPS, H2-TPR, ICP, Raman and physical chemisorption then tested in dry reforming of methane as a function of temperature and in steady-state in isothermal conditions at 700 °C. A correlation between the structure and the reactivity of promoted and non-promoted catalyst was proposed here in order to highlight the positive effects of aluminum promotion.

Synthesis of a series of 10NiO-xAl2O3-(90-x)ZrO2 Catalysts

At the First, the dissolution of P123, Ni(NO3)2•6H2O, ZrO(NO3)2•xH2O, CH4N2O, and C9H21AlO3 in distilled water. The molar ratio of Ni, Al, and Zr was 10, x, 90-x (x=0, 10, 20, 45, 90), as listed in table 5-9, respectively. Second, the obtained suspension was heated to 95 °C under vigorous stirring for two days. Nest, the suspension was subsequently placed at 100 °C for one day. After filter, washing, and dry at 25 °C, the obtained precursor was calcined at 800 °C for 300 min. Lats, white powder were denoted as NiO-ZrO2, NiO-10Al2O3-ZrO2, NiO-20Al2O3-ZrO2, and NiO-45Al2O3-ZrO2, NiO-Al2O3, respectively. on NiO-10Al2O3-ZrO2 were about 91 % and 92 %, respectively, which were slightly higher than those on NiO-ZrO2 catalyst. Besides, NiO-10Al2O3-ZrO2 catalyst exhibited a slightly higher H2/CO ratio of about 0.96 even up to reaction for 8 hours.

Those phenomena indicated that NiO-10Al2O3-ZrO2 catalyst was notably active towards methane dry reforming even upon 8 h time-on-stream. In addition, the CO2 conversion was higher than CH4 conversion on NiO-ZrO2 catalyst, with a lower H2/CO ratio, indicating that side reaction reverse water-gas shift (RWGS) occurred on NiO-ZrO2 catalyst. While for NiO-10Al2O3-ZrO2 catalyst, the CO2 conversion was lower than CH4 conversion, which indicated that the NiO-10Al2O3-ZrO2 catalyst could inhibit the reverse water-gas shift (RWGS) reaction, thereby enhancing the selectivity of the catalyst. Combined with XPS results, the graphic consisted of C-C species, while disorder carbon materials were composed of C-O and C=O species. of the stability test, the reverse water-gas shift (RWGS) reaction occurred on NiO-ZrO2 catalyst. A part of CO2 reacted with hydrogen rather than carbon, thus this carbon may be transformed into an inactive carbon gradually. And due to the accumulation, further forming a type of coke very difficult to be removed, namely graphite nano-tubes of carbon [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. On the other hand, the RWGS reaction was inhibited on NiO-10Al2O3-ZrO2 catalyst. Therefore, amorphous carbon that could be easily eliminated was formed on NiO-10Al2O3-ZrO2 catalyst. This is in agreement with the fact that the CO2 conversion was lower than CH4 conversion on NiO-10Al2O3-ZrO2 catalyst, which indicated that the rate of carbon deposition was higher than that of carbon elimination, thus leading to more amorphous carbon on NiO-10Al2O3-ZrO2 catalyst. NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts, respectively. The pore volume (VP) and the specific surface area (SBET) were listed in Table 5-10. Large pore volume (0.25 cm 3 /g) and pore diameter (6 nm) were observed on NiO-10Al2O3-ZrO2 catalyst. Thus, the physical properties (pore volume and pore diameter) could be promoted by the introduction of aluminum. This large pore volume and pore diameter may increase the proportion of metal nickel dispersed in the porous structure of the catalysts.

Carbon formation on

The morphology of carbon deposition

Physicochemical features of NiO-ZrO2 and

Table 5-10 The results of the BET experiment for NiO-ZrO2 and NiO-Al2O3-ZrO2 catalysts, the pore diameter (DP) and the pore volume (VP) determined by BJH method, and the specific surface area (SBET) was determined by the BET method. Catalyst DP, nm SBET, m 2 /g VP, cm 3 /g NiO-ZrO2 The crystallite sizes on NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts were determined by XRD, as shown in Figure 5-16. NiO peak could not be observed on both catalysts after reduction, indicating that all the Ni species were reduced. Ni metal peaks were obvious on both catalysts before and after reaction, and the crystallite size of Ni metal was shown in Table 5-12. Before reaction, those two catalysts exhibited the same Ni 0 crystallite size (12 nm). According to the H2-TPR results, NiO-ZrO2 catalyst exhibited higher content of α peak (the NiO is well known to be easily sintered) and lower content of β peak (the NiO was hard to sinter), which led to very severe sintering. Therefore, the Ni metal particle size increased to 24 nm on NiO-ZrO2 catalyst after reaction, which corresponded to the nickel particle size determined by the TEM experiment, concentrated at 15-20 nm. On the contrary, it decreased to 11 nm on NiO-10Al2O3-ZrO2 catalyst after reaction, and according to the TEM results, the nickel particle size mainly distributed at 10-15 nm. Because lower content of α peak and higher content of β peak formed on NiO-10Al2O3-ZrO2 catalyst, the Ni species might be better to stay pristine in the porosity, as compared with NiO-ZrO2 catalyst. There were several reasons to explain the decrease of Ni metal particle size. Firstly, the Ni metal can be redispersed during reaction. Secondly, part of metallic Ni (Ni 0 ) could be oxidized to NiO or NiCOx by CO, and CO2 could be adsorbed on Ni 0 . Thirdly, part of Ni 0 could be covered by carbon present on the catalysts [START_REF] Świrk | Ce-and Y-Modified Double-Layered Hydroxides as Catalysts for Dry Reforming of Methane: On the Effect of Yttrium Promotion[END_REF][START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF][START_REF] Li | Sustainable Ru-doped Ni catalyst derived from hydrotalcite in propane reforming[END_REF].

Obviously, the structure of the nickel particle on NiO-10Al2O3-ZrO2 catalyst was more stable, contributing to the stability of the catalyst. Therefore, NiO-10Al2O3-ZrO2 catalyst exhibited higher stability for the dry reforming of methane. Besides, the large nickel metal particle size could affect the selectivity of catalyst towards carbon deposition [START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. Hence, the carbon deposited on NiO-ZrO2 catalyst was difficult to be removed because of its graphite form.

The surface content of Zr and Ni on NiO-10Al2O3-ZrO2 and NiO-ZrO2 catalysts

From XPS, the Zr 3d profiles of NiO-10Al2O3-ZrO2 and NiO-ZrO2 catalysts before and after reduction were studied and the results were presented in Figure 567891011121314151617. The peak at 182.3 and 181.4 eV were assigned to Zr 4+ and Zr 3+ , respectively [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Imparato | Unraveling the Charge State of Oxygen Vacancies in ZrO2-x on the Basis of Synergistic Computational and Experimental Evidence[END_REF]. The proportion of Zr 4+ and Zr 3+ was listed in Table 5-13. The proportion of Zr 4+ on NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts were respective 11 and 14 %, which indicated that part of Zr 4+ could be reduced to Zr 3+ after reduction on both catalysts and more Zr 4+ could be reduced on NiO-ZrO2 catalyst, corresponding with the results of H2-TPR.

After reaction, part of Zr 3+ species was reduced to lower valence, eg Zr 2+ and Zr 1+ [START_REF] Ma | Non-equilibrium oxidation states of zirconium during early stages of metal oxidation[END_REF][START_REF] Liu | Electronic and chemical properties of Ce0.8Zr0.2O2 (111) surfaces: photoemission, XANES, density-functional, and NO2 adsorption studies[END_REF]. Then, NiO-ZrO2 catalyst exhibited higher content of lower valence zirconium, especially containing Zr 1+ species of 35 %, while no Zr 1+ species could be observed on NiO-10Al2O3-ZrO2 catalyst, manifesting that the structure of ZrO2 might be more stable by the modification of Al. The loss of surface nickel species after DRM reaction for 8 h. b: The loss of surface nickel metal species after DRM reaction for 8 h.

We also followed the Ni species at the surface by XPS. The peak at about 852.8 eV was attributed to Ni 0 species, while another peak was assigned to NiO species [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF]. After reaction (DRM, 700 °C, 8 h), the NiO peak shifted slightly to lower binding energy on NiO-10Al2O3-ZrO2. It suggested that the electrons could transfer to Ni species during the reaction, which might contribute to the catalytic performance of the catalyst. While the NiO peak shifted slightly to higher binding energy on NiO-ZrO2 catalyst, which indicated that the ability of nickel species to donate electrons on NiO-10Al2O3-ZrO2 catalyst was stronger than the one on NiO-ZrO2 catalyst. Except for the electron transfer, the nickel content also varied visibly.

Before reaction, the content of surface nickel species on NiO-ZrO2 catalyst was about 5.6 wt%, while after reaction for 8 h, it decreased by 24 %. A larger amount of reduction could be observed in the content of surface Ni 0 species. It decreased by 42 % after reaction. Thus, the activity on NiO-ZrO2 catalyst also decreased. However, nickel species were relatively stable on NiO-10Al2O3-ZrO2 catalyst after reaction, the surface nickel content was 5.8 wt% at the beginning, while it just decreased by 3 % after reaction for 8 h. Meanwhile, the initial content of surface Ni 0 species was about 1.0 wt%, and increased by 10 % on NiO-10Al2O3-ZrO2 catalyst after reaction for 8 h. The content of nickel species at the surface of NiO-10Al2O3-ZrO2 catalyst was more stable than that on NiO-ZrO2 catalyst, which might contribute to the stability of the catalyst. Therefore, NiO-10Al2O3-ZrO2 catalyst exhibited higher stability for the dry reforming of methane. After reduction, a large amount of NiO species could be detected, which may be explained by the following three reasons. First of all, Ni 0 species on the surface of the catalyst could adsorb the oxygen in the air; secondly, the Ni 0 species may be reoxidized when the reduced catalyst exposed to air; and the last, it could be due to the strong interaction between metal and support [START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF][START_REF] Saw | Bimetallic Ni-Cu catalyst supported on CeO2 for high-temperature water-gas shift reaction: Methane suppression via enhanced CO adsorption[END_REF].

Discussion

The aluminum could be embedded into the skeleton of zirconia during the synthesis of NiO-10Al2O3-ZrO2 catalyst, which could then modify the structure of zirconia, resulting in the increase of the pore volume (0.25 cm 3 /g) and pore diameter (6 nm). The larger pore volume can accommodate higher content of nickel species and the larger pore diameter can restrict grain growth, due to the confinement effect, thereby enhancing the dispersion and interaction. This pore confinement effect and the strong interaction enhanced by the introduction of aluminum could restrain the sintering and the loss of nickel during the DRM reaction. Meanwhile, the introduction of Al could enhance the stability of the zirconia species, which might promote the stability of the content and the grain size of Ni 0 . Therefore, the content and particle size of nickel metal can maintain the original state during the DRM reaction, thereby promoting the stability of the catalyst.

summary

In this study, we clearly showed that Al, as promoter could enhance the activity, the selectivity and the stability of NiO-ZrO2 catalyst for dry reforming of methane. The activity tests were conducted from 550 °C to 800 °C, and the NiO-10Al2O3-ZrO2 catalyst exhibited the highest catalytic performance for dry reforming of methane at lower temperature.

Furthermore, the stability tests, carried out at 700 °C for 8 hours, showed that the NiO-10Al2O3-ZrO2 catalyst presented higher activity, selectivity and stability than those on NiO-ZrO2 catalyst. This high activity was directly linked to the higher content at surface of nickel particles and the smaller particle size, which was attributed to the larger pore volume and pore diameter on NiO-10Al2O3-ZrO2 catalyst.

Therefore, NiO-10Al2O3-ZrO2 catalyst exhibited higher stability for DRM reaction. In fact, after 8h runs, on NiO-10Al2O3-ZrO2 catalyst, formed easier to be eliminated coke as a form of amorphous and disordered carbon (C-O and C=O species). Also, on NiO-ZrO2 catalyst, a severe metal sintering occurred and more nano-tube carbon (graphite) that was very difficult to be removed formed.

Dry reforming of methane over Ni-ZrOx catalysts doped by

Manganese: On the effect of the stability of Zirconia structure

Introduction

As already presented in the previous chapter. ZrO2 has received much attention as promoter and/or support in dry reforming of methane, due to its high carbon resistance [START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Fernández | Insights into dynamic surface processes occurring in Rh supported on Zr-grafted γ-Al2O3 during dry reforming of methane[END_REF][START_REF] Nagaoka | Carbon deposition during carbon dioxide reforming of methane-comparison between Pt/Al2O3 and Pt/ZrO2[END_REF][START_REF] Alirezaei | Syngas production in chemical looping reforming process over ZrO2 promoted Mn-based catalyst[END_REF]. Katsutoshi et al. [START_REF] Nagaoka | Carbon deposition during carbon dioxide reforming of methane-comparison between Pt/Al2O3 and Pt/ZrO2[END_REF] compared Pt/ZrO2 and Pt/Al2O3 catalysts for the carbon deposition over DRM reaction. They found that more carbon deposited on Pt/Al2O3 catalyst, which would cover on Pt, thereby leading to the deactivation of Ni/Al2O3 catalyst. While coke is hardly formed on Pt/ZrO2 catalyst. Besides, the structural properties, like thermal and mechanical stability, particle size, and crystal phase are very important for reforming reactions [START_REF] Zhang | Influence of zirconia phase on the performance of Ni/ZrO2 for carbon dioxide reforming of methane[END_REF][START_REF] Özkara-Aydınoğlu | Carbon dioxide reforming of methane over Co-X/ZrO2 catalysts (X= La, Ce[END_REF][START_REF] Alirezaei | Syngas production in chemical looping reforming process over ZrO2 promoted Mn-based catalyst[END_REF][START_REF] Laosiripojana | Effect of high surface area CeO2 and Ce-ZrO2 supports over Ni catalyst on CH4 reforming with H2O in the presence of O2, H2, and CO2[END_REF][START_REF] Xu | Size limit of support particles in an oxide-supported metal catalyst: Nanocomposite Ni/ZrO2 for utilization of natural gas[END_REF]. Xu et al [START_REF] Xu | Size limit of support particles in an oxide-supported metal catalyst: Nanocomposite Ni/ZrO2 for utilization of natural gas[END_REF] reported that the Ni/ZrO2 catalyst with the smaller size of ZrO2 exhibited higher stability for dry reforming of methane than the one with a bigger particle size of ZrO2 (>25 nm). The Ni/am-ZrO2 catalyst (amorphous) exhibited the highest activity and stability for DRM reaction than Ni/t-ZrO2 (tetragonal) and Ni/m-ZrO2 (Monoclinic) catalysts, reported by Zhang et al. [START_REF] Zhang | Influence of zirconia phase on the performance of Ni/ZrO2 for carbon dioxide reforming of methane[END_REF].

It is well known that manganese (Mn) based catalysts possess a high oxygen storage capacity and limit agglomeration at high reduction temperatures [START_REF] Alirezaei | Syngas production in chemical looping reforming process over ZrO2 promoted Mn-based catalyst[END_REF][START_REF] Forutan | Expert representation chemical looping reforming: A comparative study of Fe, Mn, Co and Cu as oxygen carriers supported on Al2O3[END_REF]. Wang et al. [START_REF] Wang | Manganese promoting effects on the Co-Ce-Zr-Ox nano catalysts for methane dry reforming with carbon dioxide to hydrogen and carbon monoxide[END_REF] reported that the improved catalytic performance of Co-Ce-Zr-Ox catalyst modified by Mn was mainly attributed to the increase of oxygen mobility and the surface oxygen species. Because of larger amount of mobile oxygen, the coke on the surface could be easily oxidized to COx on Mn-promoted Co-Ce-Zr-Ox catalyst. Yao et al. [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF] and Seok et al. [START_REF] Seok | Mn-promoted Ni/Al2O3 catalysts for stable carbon dioxide reforming of methane[END_REF] showed that the Mn promoter could enhance the dispersion of Ni and less carbon deposition through resisting the aggregation of nickel metal. Özkara-Aydınoğlu et al. [START_REF] Özkara-Aydınoğlu | Carbon dioxide reforming of methane over Co-X/ZrO2 catalysts (X= La, Ce[END_REF] noticed that the addition of Mn could reduce carbon deposition with a decrease in catalytic activity. Similar phenomenon has been reported by Luna et al. [START_REF] Luna | Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst[END_REF] and Liu et al. [88].

On the contrary to that is presented in the literature. In this study, we showed an increase with time on stream. In order to understand the positive effect of Mn on the activity for DRM, we developed here Ni-ZrOx and Ni-Mn-ZrOx catalysts prepared by a one-step synthesis method. The relationships between the catalytic performance and surface properties of Ni-ZrOx and Ni-Mn-ZrOx catalyst were discussed as function of time on stream

Preparation of the Ni-Zr catalyst modified by Mn

Ni-ZrOx and Ni-Mn-ZrOx materials were prepared by the urea hydrolysis method (one-step synthesis) according to [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF]. For Ni-ZrOx material, 7.06 g urea (Sigma-Aldrich), 1.14 g nickel (II) nitrate hexahydrate (Emsure), 7.96 g of Pluronic P123

(Sigma-Aldrich) amphiphilic block copolymer and 5.03 g zirconium(IV) oxynitrate hydrate (Sigma-Aldrich) were dissolved in distilled water. The synthesis mixture for Ni-Mn-ZrOx material contained the same amounts of reactants plus 0.93 g Manganese (II) nitrate tetrahydrate (Sigma-Aldrich) (corresponding to a loading of 10 wt %). In both cases, a light blue precipitate was formed at 95 °C with constant stirring for 48 h.

Then, the obtained precipitate was aged at 100 °C for 24 h. After that, the slurry was isolated by filtration, washed by a small amount of distilled water and then placed at room temperature. Finally, these two precipitates were calcined in flowing air at 800 °C for 5 h (heating rate 1 °C/min). The obtained materials were denoted as Ni-ZrOx and Ni-Mn-ZrOx catalysts. it stabilized at about 87.8 % after 300 min. The H2/CO ratio showed a similar trend. It decreased from 0.96 to 0.95 within 100 min and then stabilized at 0.95. The stability of CO2 conversion was relatively higher. On the whole, it slowly increased to 91.0 %.

The basic of catalysts

The effect of Ni-Zr catalyst modified by Mn on performance for reforming of methane

While for Ni-Mn-ZrOx catalyst, the CH4 conversion slowly increased to 97.1 % with time on stream and almost stabilized after reaction 300 min. Similar phenomena could be observed on CO2 conversion and H2/CO ratio. After 480 min, the CO2 conversion and H2/CO ratio on Ni-Mn-ZrOx catalyst increased to 93.5 % and 0.96, respectively.

In order to understand this activation during the time on stream, catalysts after reduction, and after reaction for 100 min, 300 min and 480 min time on stream were characterized.

The results are presented in the section below. phase [START_REF] Wang | The bi-functional mechanism of CH4 dry reforming over a Ni-CaO-ZrO2 catalyst: further evidence via the identification of the active sites and kinetic studies[END_REF]. The tetragonal phase ZrO2 was evidenced in all the catalysts. No monoclinic ZrO2 and NiO phases were detected in these catalysts. Significantly, the Ni 0 reflection plane (2θ=44.5 º) on Ni-ZrOx catalyst shifted to bigger Bragg angles after reaction for 100 min, while it shifted to smaller Bragg angles after reaction for 300 min.

The maximum shift distance for Ni 0 was about 0.22 º during the reaction. For Ni-Mn-ZrOx catalyst, the swing became gentler. The shift is slightly lower (0.07 º). Except for the Bragg angles, the d-spacing of Ni 0 plane on Ni-ZrOx catalyst shrunk from 2.04 Å to 2.03 Å, while d-spacing of ZrO2 (2θ=30.4 º) reflection plane increased from 2.94 Å to 2.95 Å. In our previous work [START_REF] Wang | One-step synthesis of highly active and stable Ni-ZrOx for dry reforming of methane[END_REF], it is about 2.96 Å on Ni/ZrO2 catalyst that prepared by traditional impregnation method without NiO-ZrO2 solid solution. Because the radius of Ni 2+ ion (0.78 Å) is smaller than that of Zr 4+ ion (0.84 Å), thus the lattice parameter of ZrO2 would shrink after the substitute of same of zirconium sites by nickel.

On the contrary, the expansion of ZrO2 manifested that some nickel was set free from the skeleton of ZrO2 phase [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. Thus, For Ni-Mn-ZrOx catalyst, the d-spacing of Ni 0 plane kept to 2.03 Å. Those phenomena indicated that the structure of nickel species was more stable on the Mn-promoted catalyst [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF][START_REF] Seok | Mn-promoted Ni/Al2O3 catalysts for stable carbon dioxide reforming of methane[END_REF][START_REF] Seok | The role of MnO in Ni/MnO-Al2O3 catalysts for carbon dioxide reforming of methane[END_REF]. Yao et al. [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF] reported the addition of Mn could enhance the interaction between Ni and support by the formation of NiMn2O4. Besides, some manganese oxide species MnOx can cover on nickel particles, thereby inhibiting the sintering of nickel species, reported by Seok et al. [START_REF] Seok | The role of MnO in Ni/MnO-Al2O3 catalysts for carbon dioxide reforming of methane[END_REF]. Both of those two effects could limit the aggregation and migration of metal Ni. The crystallite sizes of both catalysts calculated from the Scherrer equation was presented in Table 5-16. the Ni metal particle size on Ni-ZrOx catalyst increased from 12 to 22 nm after reaction for 100 min, manifesting severe sintering on Ni-ZrOx catalyst in the first 100 minutes of the reaction, and then leading to the decrease of activity in DRM within reaction for 100 min. After 480 min reaction, the particle size of Ni 0 slowly increased to 24 nm, which corresponded with the results of activity test-stabilized from 100 minutes. On the contrary, for Ni-Mn-ZrOx catalyst, the particle size of Ni 0 decreased from 29 to 25 nm after reaction for 300 min and to 24 nm after 480 min, corresponding to an increase in the DRM activity up to 97 % after reaction 300 min.

The redispersion of Ni 0 species might occur in the condition of continuous alternating reduction and oxidation [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Świrk | Ce-and Y-Modified Double-Layered Hydroxides as Catalysts for Dry Reforming of Methane: On the Effect of Yttrium Promotion[END_REF][START_REF] Świrk | Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming[END_REF]. This redispersion indicated that the particle size of Ni 0 was related to the activity in DRM. Li et al. [START_REF] Li | CO2 reforming of methane over modified Ni/ZrO2 catalysts[END_REF] found Mn could improve the dispersion of nickel and enhance the interaction between nickel and support during the dry reforming of methane, and further improve the catalytic performance of Mnmodified catalyst. It was worth pointing out that the graphite peak could be observed after reaction for 300 min with the graphite crystal size of 20 nm. While after reaction for 480 min, it decreased to 14 nm. Mn could avoid carbon deposition or lead to its suppression as reported by Yao et al. [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF] and Seok et al. [START_REF] Seok | Mn-promoted Ni/Al2O3 catalysts for stable carbon dioxide reforming of methane[END_REF], due to the formation of smaller Ni particle size on Mn-modified catalyst. Small Ni particles showed better resistance against coke formation [START_REF] Kim | Effect of metal particle size on coking during CO2 reforming of CH4 over Ni-alumina aerogel catalysts[END_REF]. The particle size greater than 20 nm on Ni-Mn-ZrOx catalyst led to catalytic selectivity towards methane cracking (Equation ( 1)), and caused carbon deposition [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Kathiraser | Inverse NiAl2O4 on LaAlO3-Al2O3: unique catalytic structure for stable CO2 reforming of methane[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. the original states over Ni-Mn-ZrOx catalyst. This shift manifested that the Zr species could be reduced during the reaction. According to the literature, the Zr3d peak is resolved into several peaks [START_REF] Liu | Electronic and chemical properties of Ce0.8Zr0.2O2 (111) surfaces: photoemission, XANES, density-functional, and NO2 adsorption studies[END_REF][START_REF] Li | CO2 activation on ultrathin ZrO2 film by H2O co-adsorption: In situ NAP-XPS and IRAS studies[END_REF]. The peaks at about 182.3 eV, 181.4 eV, 180.5 eV and 179.3 eV were attributed to the Zr 4+ , Zr 3+ , Zr 2+ and Zr 1+ , respectively [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Ma | Non-equilibrium oxidation states of zirconium during early stages of metal oxidation[END_REF][START_REF] Song | Oxidation of amorphous Ni-Zr alloys studied by XPS, UPS, ISS and XRD[END_REF]. At the beginning of the reaction, the percentage of Zr 4+ and Zr 3+ on Ni-ZrOx catalyst was about 12 and 88 %, respectively. About 73 % of surface Zr species were reduced to Zr 2+ on Ni-Zr catalyst after reaction for 100 min. It was pointed out that the Zr 1+ species could be observed after reaction for 480 min. This indicated that the Zr species on Ni-ZrOx catalyst were reduced during the reaction. Generally, the Zr cations in ZrO2 are very difficult to be reduced [START_REF] Ma | Non-equilibrium oxidation states of zirconium during early stages of metal oxidation[END_REF][START_REF] Gionco | Paramagnetic defects in polycrystalline zirconia: an EPR and DFT study[END_REF]. Herein, the Zr 4+ species were gradually reduced to Zr 1+ , indicating that this material has high solubility of oxygen, like TiO2 [START_REF] Lu | Oxidation of a polycrystalline titanium surface by oxygen and water[END_REF]. However, unlike the redox cycle between Ti 2+ and Ti 3+ , the Zr 4+ species were being reduced all the time, which might affect the stability of ZrO2 structure.

While for Ni-Mn-ZrOx catalyst, the initial percentage of Zr 4+ and Zr 3+ was about 15.7 % and 84.3 %, respectively. The surface Zr species were not reduced after reaction. The percentage of Zr 4+ and Zr 3+ species after 300 min reaction was about 20 % and 80 %, respectively. And after 480 min reaction, it was found about 22 % and 78 % for Zr 4+ and Zr 3+ species. In conclusion, Ni-Mn-ZrOx catalyst exhibited a more stable structure for ZrO2 species.

carbon deposition

The carbon deposition of both catalysts after for 480 min was analyzed by TGA, Raman and TEM. The results were presented in Figure 5-23. The carbon deposition on Ni-Mn-ZrOx catalyst (5.3 %) was higher than that on Ni-ZrOx catalyst (3.7 %). Generally, the presence of Zr could promote the adsorption and/or activation of CO2, thereby decreasing the carbon deposition in DRM process [START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF]. Meanwhile, the addition of Mn could result in less carbon deposition since the small nickel particles [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF][START_REF] Seok | Mn-promoted Ni/Al2O3 catalysts for stable carbon dioxide reforming of methane[END_REF].

However, the existence of both Mn and Zr results in higher carbon deposition, which might be attributed to the large nickel particles. Because [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. From the TGA results, the carbon on both catalysts might be the same type, because it could be removed at the same temperature. In order to understand the nature of carbon deposition, Raman experiments were carried out, as shown in Figure 5-23 (B). Two peaks could be observed on both catalyst after reaction. D band and G band were ascribed to the disorder and graphite of carbon materials, respectively [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Bai | Coking behaviors and kinetics on HZSM-5/SAPO-34 catalysts for conversion of ethanol to propylene[END_REF][START_REF] Mcfarlane | The application of inelastic neutron scattering to investigate the steam reforming of methane over an alumina-supported nickel catalyst[END_REF]. The ID and IG represented the intensity of D band and G band, respectively. The ratio of IG/ID was related to the crystalline degree of the carbon materials. According to our previous work, it was about 1.7 on Ni-ZrOx catalyst [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF]. While it was 2.0 on Ni-Mn-ZrOx catalyst, indicating that the coke exhibited a higher crystalline degree. The morphology of coke was characterized by TEM experiment, as presented in Figure 5-23 (C) and (D). It observed that the carbon nanotubes formed on both catalysts after reaction for 480 min.

And more carbon nanotubes existed on Ni-Mn-ZrOx catalysts, which was agreement with the results of TAG and Raman. Besides, similar distribution of nickel particles could be observed on both catalysts, almost at 15-20 nm, which corresponded to the XRD results.

Summary

The DRM activity test was carried out at 700 °C for 8 hours, with a GSHV of 48,000 h -1 . In this work, we clearly showed on Ni/ZrO2 catalysts that the presence of Mn enhanced the percentage of surface metallic nickel and led to a redistribution of the Ni 0 species during the reaction, leading to a decrease in nickel particle size, which might be assigned to the more stable structure of ZrO2 species. Furthermore, the catalytic activity increased with time on stream. Whereas for Ni-Zr catalyst, because Zr species were reduced to Zr 2+ with time on stream, the content of surface Ni° decreased and the Ni° particle size increased, exhibiting a decrease of DRM activity in the first 100 min reaction. Thus, the structure of ZrO2 might play a key role in active sites for dry reforming of methane, enhancing the performance of ZrO2 catalyst. support. The stronger interaction might inhibit the sintering of catalysts. Jang [START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF] found that the formation of MgO-NiO solid solution could enhance the interaction between Ni and Mg, thereby inhibition of catalyst sintering under the condition of the DRM reaction, and further promote the stability of Mg-modified catalyst. It was worth noting that a broad peak formed on Ni-Zr-Mn catalyst, which might result from the multistep reduction of Mn oxides (MnO2→Mn2O3→Mn3O4→Mn 2+ ) [88]. On the whole, the reduction peak on Ni-Zr-Mn catalyst shifted to low temperature area, as compared with the Ni-Zr catalyst. This phenomenon manifested that the Mn promoter weakened the interaction between the nickel and the support, which might result in the sintering of nickel on Ni-Zr-Mn catalyst. While almost no changes observed on Y-promoted catalyst. The physical properties of catalyst were evaluated from the N2 adsorption isotherms acquired at -196 °C. The special surface area, the pore diameter and the pore volume on Ni-Zr catalysts listed in Table 6-2. Obviously, the Ni-Zr catalyst exhibited the highest specific surface area of about 113 m 2 /g. Generally, higher specific surface area could contribute to the distribution of nickel species, which might increase the dispersion of nickel [START_REF] Rezaei | CO2-CH4 Reforming over Nickel Catalysts Supported on Mesoporous Nanocrystalline Zirconia with High Surface Area[END_REF][START_REF] Talkhoncheh | Syngas production via dry reforming of methane over Ni-based nanocatalyst over various supports of clinoptilolite, ceria and alumina[END_REF]. Talkhoncheh et al. [START_REF] Talkhoncheh | Syngas production via dry reforming of methane over Ni-based nanocatalyst over various supports of clinoptilolite, ceria and alumina[END_REF] found that Ni/Al2O3 nanocatalyst exhibited the highest conversions (93% and 96% for CH4 and CO2 at 850 °C) and yields (90% and 93% for H2 and CO), as compared with nickel supported on clinoptilolite or CeO2 support. Ni/Al2O3 nanocatalyst showed the best performance due to its high specific surface area, homogenous distributions and good dispersion of Ni. The author thought that the high specific surface area could not only improve the NiO dispersion, but also increase the availability of reactant molecules and further improve the activity of the catalyst. Besides, the Ni-Zr-Al catalyst illustrated the highest pore diameter (6.1 nm) and pore volume (0.25 cm 3 /g). It implied that the promoter Al could modify the pore structure of ZrO2. While the introduction of other promoters (Mn, Y or Mg) led a decrease of the special surface area, the pore diameter and the pore volume on Ni-Zr catalysts. Those opposite effects might lead to a decrease of catalytic performance of catalyst for DRM. 1, 2 and 3 are the weak basic site, medium-strength basic sites and strong basic sites, which are mainly consisted of weak Brønsted basic sites such as surface OH groups, Lewis base sites such as unsaturated (cus) O 2-and Zr 4+ -O 2-centers, and the CO2 adsorption of the cus Zr 4+ centers with strong Lewis acidity, respectively [START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF][START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. The position and content of peaks are listed in Table 6- Ni-Zr and Ni-Zr-Al catalysts, the lower Ni particle sizes might contribute to its higher catalytic performance. Similar results have also been reported by He et al [START_REF] He | Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 and Ni-Al2O3/SiO2 catalysts with different precursors[END_REF]. While it increases to 29, 20 and 16 on Ni-Zr-Mn, Ni-Zr-Y and Ni-Zr-Mg catalysts, respectively.

Chapter VI

Texture properties of a series of promoted Ni-Zr catalysts

Those relatively large sizes of crystal nickel metal can lead to carbon deposition [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. respectively by the introduction of Mn. At the same time, the ratio of H2/CO increases from 0.6 to 1 [START_REF] Wang | Manganese promoting effects on the Co-Ce-Zr-Ox nano catalysts for methane dry reforming with carbon dioxide to hydrogen and carbon monoxide[END_REF]. On the contrary, although the good Ni dispersion, a slight decrease of activity observed on Cu-promoted catalyst. The conversion of CH4 decreases from 100 % to 90 % at 750 o C DRM [START_REF] Dong | Effect of Transition Metals (Cu, Co and Fe) on the Autothermal Reforming of Methane over Ni/Ce0.2Zr0.1Al0.7Oδ Catalyst[END_REF]. Chemical promoters provide new active sites or enhance the chemical property relating to the reactivity of the catalysts such as basicity or redox property [START_REF] Jang | A review on dry reforming of methane in aspect of catalytic properties[END_REF]. The introduction of MgO could enhance the reducibility of Ni species and the interaction between Ni and support [START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF]. Besides, MgO [START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF][START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF] and CaO [START_REF] Liu | CO2 reforming of CH4 over stabilized mesoporous Ni-CaO-ZrO2 composites[END_REF] could increase basic property of catalysts, which improved the chemisorption of CO2 and promoted the gasification of deposited coke on the catalysts, and further enhance the stability of the catalysts. Mg-promoted Ni-Ce0.8Zr0.2O2 catalyst exhibits higher stability for 800 o C DRM without deactivation even after reaction for 40 h, while the activity of non-promoted Ni-Ce0.8Zr0.2O2 catalyst decreases over 40 h on stream with the rate of 0.43 %•h -1 [START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF]. In addition, the formation of oxygen vacancies promotes removal of the coke deposited on the nickel surface, and further enhance the stability of catalysts, reported by Asencios [START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. The author also observed that the CH4 conversion on Y-promoted catalyst decreased with the rate of 1.3 %•h -1 at calcium oxide phase easily carbonated by CO2 acts as a continuous supplier of gaseous CO and active oxygen species which react with the surface carbon arising from the methane cracking. Thus, the non-promoted Ni/ZrO2-La2O3 catalyst is deactivated within 6 h reaction, while the conversion of CH4 on Ca-modified Ni/ZrO2-La2O3 catalyst decreases with the rate of 5.2 %•h -1 over 14 h DRM reaction [START_REF] Bachiller-Baeza | Transient studies of low-temperature dry reforming of methane over Ni-CaO/ZrO2-La2O3[END_REF]. The results of activity tests are presented in Figure 6-1, including both conversions of CH4 and CO2, and H2/CO molar ratio for non-promoted/promoted Ni-Zr catalysts, as a function of reaction temperature. CH4 and CO2 conversions increase with reaction temperatures, together with H2/CO molar ratio overall catalysts. When the reaction temperature reaches 700 o C, the conversion of CH4 and CO2 for non-promoted and Alpromoted catalysts do not show noticeable difference between them, suggesting that the activity at elevated temperature for these two catalysts displays a more significant dependence on reaction temperature than the catalyst composition. This can be understood considering the strong endothermic nature of the reaction. In the whole temperature process, Ni-Zr-Al catalyst exhibited the highest activity and selectivity for dry reforming of methane. The higher activity observed on Al promoted catalysts can be attributed to its high surface content of nickel species, Ni dispersion and basic sites.

XPS analysis of a series of promoted Ni-Zr catalysts

Liu et al. [88] and He et al. [START_REF] He | Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 and Ni-Al2O3/SiO2 catalysts with different precursors[END_REF] also reported the introduction of Al results in a low Ni particle sizes, thereby enhance the activity for DRM. While the introduction of Mg, Y and Mn resulted in a decrease in both CO2 and CH4 conversion. Jang [START_REF] Jang | Metal oxide (MgO, CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases[END_REF] found Mg promoted the activity for DRM reaction over Ni-MgO-Ce0.8Zr0.2O2. Because the presence of MgO could enhance in strong resistance against Ni sintering, due to the basic property of MgO and an intimate interaction between Ni and MgO. Herein, the low basic sites on Ni-Zr-Mg catalyst results in a decrease of activity for DRM. The lowest catalytic performance for DRM on Y-promoted catalysts might be ascribed to the lowest surface content of nickel species since the content of the surface Ni species play an important role in the catalytic activity [START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF]. Except for the surface Ni species and basic sites, the lower specific surface area and larger particle size of nickel metal over Mn, Y and Mg-promoted catalysts also lead to a decrease of activity.

The same trend in the selectivity can be observed on all the catalysts, as is presented in Figure 6-4 (C). All the catalysts showed increasing indicative of selectivity along with the increase of reaction temperature from 550 to 800 °C. The Ni-Zr-Al catalyst is the best catalyst, always delivering higher ratio of H2/CO than the others in the whole reaction temperature studied. For instance, at a low reaction temperature of 550 °C, the Ni-Zr-Al catalyst delivered a H2/CO ratio of 0.87, slightly higher than the others (0.83 for Ni-Zr, 0.82 for Ni-Zr-Mg, 0.78 for Ni-Zr-Mn and 0.73 for Ni-Zr-Y). At a high temperature of 800 °C, a very high H2/CO ratio of 0.972 could be achieved over Ni-Zr-Al catalyst. Same H2/CO ratio of 0.968 was obtained on both Ni-Zr and Ni-Zr-Mg catalysts. Ni-Zr-Y catalyst also exhibited the lowest of the H2/CO ratio, about 0.90. Side reactions such as the reverse water-shift reaction (RWGS) and the disproportionation of CO reaction could results in the lower than unity H2/CO ratio [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF][START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. The disproportionation of CO reaction (2CO→CO2+C, ∆H = -172.4 kJ/mol) is a highly exothermal reaction [START_REF] Nikoo | Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation[END_REF]. Thermodynamically, the reaction is favored at low temperature and high pressure. Besides, the RWGS reaction is thermodynamically favorable and rapid to reach equilibrium under the reaction temperature between 500 and 700 °C [START_REF] Souza | Reforming of methane with carbon dioxide over Pt/ZrO2/Al2O3 catalysts[END_REF][START_REF] Rostrupnielsen | CO2-reforming of methane over transition metals[END_REF]. Thus, both RWGS and disproportionation of CO could give rise to the lower H2/CO ratio at 550 °C. While at 800 °C, the side reaction is mainly RWGS reaction. It implies that the Y, Mn and Mg promote Ni-Zr catalysts show higher activity for the reverse water-shift reaction. In contrast, Al-promoted catalyst exhibits lower activity for the RWGS reaction.

Isotherm stability test on the Ni-Zr catalysts

Besides activity and selectivity, stability is also a very important practical consideration for a heterogeneous catalyst. Figure 6-5 presents the 8 h runs experiments for nonpromoted and promoted catalysts at 700 o C. For non-promoted Ni-Zr catalyst, the CH4 conversion decreases within 60 min, as well as the ratio of H2/CO. Meanwhile, the CO2 conversion is higher than that of CH4 and the H2/CO ratio is lower than unity during the whole process, indicating the occurrence of side reactions, mainly of RWGS [START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF][START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF]. For Ni-Zr-Al catalyst, higher stability is observed in comparison to Ni-Zr catalyst.

Chai et al [START_REF] Chai | Monolithic Ni-MOx/Ni-foam (M= Al, Zr or Y) catalysts with enhanced heat/mass transfer for energy-efficient catalytic oxy-methane reforming[END_REF]. reported that the Ni-Al2O3/Ni-foam catalyst exhibited an on-stream time of 85 h till the reaction terminated at a CH4 conversion of ~70%, almost 7 times longer than Ni-ZrO2/Ni-foam catalyst, since the former had a more stable pore-structure during the reaction condition. It is worth noting that the activity of both CO2 and CH4 conversion, and ratio of H2/CO on Ni-Zr-Mn catalyst increases over the first 300 minutes. And it exceeds the activity of Al-promoted catalyst after reaction for 300 min.

Li et al. [START_REF] Li | CO2 reforming of methane over modified Ni/ZrO2 catalysts[END_REF] found Mn could improve the dispersion of nickel and enhance the interaction between nickel and support during the dry reforming of methane. Thus, the catalytic performance of Mn-modified catalyst increases with time on stream. Besides, the CH4 conversion during the reaction is higher than the CO2 conversion, which suggests that the introduction of Al and Mn is conducive to the activation of CH4, similar results were reported by Li et al. [START_REF] Li | Effect of the addition of Ce and Zr on the structure and performances of Ni-Mo/CeZr-MgAl (O) catalysts for CH4-CO2 reforming[END_REF]. However, Y and Mg promoted catalysts exhibits lower stability for DRM. The CO2 and CH4 conversion are slowly decreasing, as well as the ratio of H2/CO. Besides, the CO2 conversion is higher than CH4 conversion. Meanwhile, the ratio of H2/CO shows a decreasing trend, suggesting that the reverse water-shift reaction is promoted [START_REF] Li | Effect of the addition of Ce and Zr on the structure and performances of Ni-Mo/CeZr-MgAl (O) catalysts for CH4-CO2 reforming[END_REF]. sizes increased from 12 nm to 24 nm. While for promoted catalysts, the crystallite Ni 0 sizes on used catalysts are slightly smaller than those on the reduced catalysts, since might be the Ni redispersion upon continuous reduction and oxidation of nickel [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Nakayama | Dispersion of nickel particles supported on alumina and silica in oxygen and hydrogen[END_REF]. Similar phenomenon was reported by Świrk [START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF]. It implies that the promoted catalysts could suppress the sintering of nickel metal during the DRM reaction.

The physicochemical features of Ni-Zr catalysts after runs 6.5.1 XRD analysis of a series of used Ni-Zr catalysts

Interestingly, the crystallite Ni 0 sizes on Mn-modified catalyst decreased from 29 nm to 24 nm, at the same time, the activity increased. It means the Ni redispersion might promote the activity for DRM by existence of Mn, which is in agreement with the results reported by Li et al. [START_REF] Li | CO2 reforming of methane over modified Ni/ZrO2 catalysts[END_REF]. While for Al-promoted catalyst, the slight decrease of the crystallite Ni 0 sizes did not affect the activity for DRM. The crystallite Ni 0 sizes on Y and Mg modified catalysts decrease from 16 and 20 nm to 10 and 17 nm, respectively. On the contrary, the activity is decreasing during the 8 h DRM. Those phenomena manifests that the effect of particle size is different for different catalysts. [START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF].

Besides, the low basic sites also affect the carbon deposition [88,[START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF]. For Ni-Zr-Mg catalyst, both large nickel and low basic sites result in the carbon deposition. It has been also found that the addition of these basic oxides reduces not only the formation of coke but also the rate of the reforming reaction [START_REF] Osaki | Role of potassium in carbon-free CO2 reforming of methane on K-promoted Ni/Al2O3 catalysts[END_REF], therefore, the Ni-Zr-Mg exhibits higher carbon deposition and lower activity for DRM. While for Al-promoted catalyst, the initial carbon removal temperatures (the initial decrease of sample weight) shifts to low temperature as compared to the non-promoted Ni-Zr catalyst. It means the carbon deposition on Al-promoted catalyst is easier to be removed. Thus, the coke about 5.3 % is not effect on activity of Al-modified catalyst for DRM. More amount of weak and medium-strong basic sites formed on catalyst by the addition of Y, therefore enhanced the ability for carbon removal. Since the weak and medium-strength basic sites could promote the adsorption and/or activation of CO2, and further reduce the carbon deposition [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Dębek | Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition[END_REF]. promoted catalyst, the disordered carbon is the one that can be removed very easily [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF].

Summary

Al, Mn, Y and Mg-promoted Ni catalysts supported on ZrO2 were synthesized by onestep synthesis method and tested in DRM at temperatures between 550 and 800 °C.

Based on the DRM results, Al-modified catalysts promote the catalytic performance whereas Mn, Y and Mg-modified catalysts inhibited it. For the stability test, the addition of Mn resulted in the increase of the catalytic activity during the 300 min DRM. The existence of Al exhibited the highest stability for 8 h DRM.

In addition, according to the characterization results, such as BET, XRD, H2-TPR, CO2-TPR, XPS, TEM and Raman, a relationship between the catalytic performance and the texture properties, Ni 0 dispersion, and CO2 basicity is evidenced. The addition of Al leads to an increase of the pore diameter, the CO2 basicity and the surface nickel content and the Ni 0 dispersion during the reaction. Thus, an enhancement of the activity, stability and selectivity for DRM is observed. Meanwhile, the formation of carbon easy to be removed on Al-promoted catalyst is also observed and does not impact the activity for DRM. The presence of Mn leads to the redistribution of nickel particles during the reaction process, thus increase in the performance of catalyst for DRM. On the contrary, In particular, the pore structure is much more important in the DRM reaction because of the high reaction temperature of DRM (> 750 °C) and larger carbon source compared to the conventional reforming reaction. Thus, a dedicated study on pore structure modification would be useful.

 Design a more stable ZrO2 support. The structure of ZrO2 plays an important role in the activity of catalyst. Thus, a more stable ZrO2 results in higher stability for dry reforming of methane. This point would also be addressed in a future study.

Table des illustrations

Table des tableaux

  and Prof. Changwei HU. They gave me the incentive and support to perform this research. Prof. Patrick DA COSTA and Prof. Changwei HU have profound knowledge, rigorous experimental attitude, and unique insights into the development of the research fields. This rigorous scientific attitude and spirit are very helpful for my future research work. Herein, I would like to express my sincere respect and sincere thanks to Prof. Patrick DA COSTA and Prof. Changwei HU. I would like to thank my reviewers Prof. FECHETE Ioana and Prof. LIU Haichao. Meanwhile, I would like to thank my dissertation committee members, Prof. GUIBERT Philippe, Prof. JIANG Bo, thank all of you for willing to participate in my defense. Under your comments and suggestions, my doctoral dissertation has been greatly improved. I am very grateful to the research group that gives me a lot of help both in research and life. Chao SUN, Hailong ZHANG, Mira IBRAHIM, Katarzyna Świrk, Bo WANG, Maria MIKHAIL, Yutao LI, Armando IZQUIERDO and my colleagues in China Qing ZHAO, Yannan WANG, Li LI, Chaojun CUI. I need to thank all of you for helping me so that I can complete my doctoral research. I would like to thank all professors, researchers, administrators, engineers and technicians in Institut Jean le Rond d'Alembert. Prof. Stéphane ZALESKI, Prof.

Figure 2 - 1

 21 Figure 2-1 The detail of CO2 emissions for one year

Figure 2 - 2

 22 Figure 2-2 The annual mean carbon dioxide (A) and carbon dioxide growth rates (B) at Mauna Loa observatory [2].

Figure 2 - 3

 23 Figure 2-3 Global greenhouse gas emission sources in 2004 of which approximately 77% are represented by CO2 emissions

  Figure 2-5 presents the basic schemes showing the types of CO2 capture by special materials.

Figure 2 - 5

 25 Figure 2-5 The basic schemes showing the types of CO2 capture

Figure 2 - 6 A

 26 Figure 2-6 A portion of the crystal structure of Zn4O(BDC)3 (MOF-5). Blue tetrahedra represent ZnO4 units, while gray and red spheres represent C and O atoms, respectively; H atoms are omitted for clarity.

Figure 2 - 7

 27 Figure 2-7 The ethanol, hydrogen, isobuthane, methanol and ammonia synthesized by syngas (adapted from Ref. [38])

  (i) 1910-1926: Development of the coal liquefaction by means of hydrogenation, and the Fischer-Tropsch (FT) synthesis from; (ii) 1927-1945: Scientific works and industrialization of the coal-to-petroleum process and FT synthesis; (iii) 1930-1990: Consolidation and expansion of the synthetic-fuel technology fromGermany to other countries Now, researches focus on the efficient large-scale production of synthetic fuels from FT synthesis. And those fuels are attractive, clean, carbon-neutral, and sustainable energy sources and also can replace non-renewable fuels in a wide range of applications.

Figure 2 - 8

 28 Figure 2-8 The adsorption carbonate species on metal sites (M).

Figure 3 - 1

 31 Figure 3-1 Programs used during dry reforming of methane: (A) Multi-Steady experiment at different temperatures and (B) Isothermal experiments.

10 )Figure 3

 103 Figure 3-2 (A) Scheme and (B) physical profile of dry reforming of methane set-up.

Figure 3 -Figure 3 - 3

 333 Figure 3-3 Programs (A), scheme (B) and set-up (C) used the temperature-programmed reduction of H2 (H2-TPR).

Figure 3 - 4

 34 Figure 3-4 The programs for temperature-programmed desorption of CO2 (CO2-TPD)

Figure 3 - 5

 35 illustrates schematically how diffraction of X-rays by crystal planes allows one to derive lattice spacings by using the Bragg relation. X-ray diffraction occurs in the elastic scattering of X-ray photons by atoms in a periodic lattice. The scattered monochromatic X-rays that are in phase give constructive interference[START_REF] Leofanti | Catalyst characterization: characterization techniques[END_REF].

Figure 3 - 5 X

 35 Figure 3-5 X-rays scattered by atoms in an ordered lattice interfere constructively in directions

Figure 3 . 6

 36 Figure 3.6 Crystallographic parameters for ZrO2, according to the standard ICSD # 88-1007

- 7 .Figure 3 - 7

 737 Figure 3-7 The types of physisorption isotherms (A) and (B) hysteresis loops

Figure 3 - 8 (

 38 Figure 3-8 (A). Differential scanning calorimetry analysis (DSC) is a technique in which the difference in energy inputs into a substance and a reference material is measured as a function of temperature. There are two modes, power compensation DSC and heat flux DSC depending on the method of measurement used.Figure 3-8 (B)

Figure 3 -

 3 8 (B)

Figure 3 - 8

 38 Figure 3-8 The scheme of set-up for TGA (A) and TGA-DSC (B).

Figure 3 - 9

 39 Figure 3-9 Schematic diagram of diffraction (A) and diffraction of various crystals.

  nm, a Grating of 600 gr/mm, a Filter of D1and a Hole of 200 μm. The data was collected from the wavenumber values of 40-4000 cm - 1 .

Figure 3 -

 3 Figure 3-10 Raman result of carbon deposited on Ni-Zr/SiO2 catalyst after reforming at 400 o C (a) and 450 o C (b) and Ni-Si/ZrO2 catalyst after reforming at 400 o C (c) and 450 o C (d).

  found that Ni/SiO2 catalysts modified with ZrO2 had better Ni dispersion, improving DRM activity and resistance of nickel sintering. Gonzalez-Delacruz et al.[START_REF] Gonzalez-Delacruz | Modifying the Size of Nickel Metallic Particles by H2/CO Treatment in Ni/ZrO2 Methane Dry Reforming Catalysts[END_REF] also observed that Ni/ZrO2 catalyst had better dispersion of nickel metallic phase upon modification with CO, due to the formation of Ni(CO)4 complexes, which deposited on the zirconia surface by colliding. This process contributed to the re-dispersion of nickel without nickel loss, due to the interaction between Ni and Zr, thereby made the catalyst exhibit excellent stability. According to Pompeo et al.[START_REF] Pompeo | Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2[END_REF], ZrO2 played an important role in promoting adsorption of CO2, thereby enhancing the C removal. Besides, the surface oxygen species on Ni/ZrO2 influenced the acidity and basicity, affecting the adsorption and activation of CO2. Despite these advantages, many studies have been focused on the enhancement of long-term stability by applying different types of Ni/ZrO2 syntheses[START_REF] Lou | Design of stable Ni/ZrO2 catalysts for dry reforming of methane[END_REF][START_REF] Zhang | Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide[END_REF][START_REF] Sokolov | Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst[END_REF][START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF]. Wei et al.[START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF] prepared Ni/ZrO2 by impregnation method with different Zr(OH)4 particles size, and co-precipitation of ultra-fine nickel-zirconium hydroxide method. It revealed that the smaller ZrO2 particles size contributed to the formation of more basic sites and oxygen vacancies for CO2 activation. Zhang et al.

  the interface between Ni and Zr, in this work, we prepared Ni-ZrOx catalysts with different Ni loadings as described in the synthesis of Ni-Zr and Ni/ZrO2 catalysts part, and investigated their catalytic behavior in dry reforming of methane. The applied characterization techniques allowed us to further investigate the Ni-Zr interactions in dry reforming of methane, showing a new understanding of this catalytic system.

Figure 4 - 1

 41 Figure 4-1 The H2-TPR profiles of a series of Ni-Zr and 10Ni/ZrO2 catalysts, and ZrO2.

Figure 4 -

 4 Figure 4-2 The XRD profiles of catalysts: (a) 10Ni/ZrO2, (b) 5Ni-Zr, (c) 10 Ni-Zr and (d) 15 Ni-Zr.

Figure 4 -

 4 Figure 4-3 (A) N2 adsorption-desorption isotherms and (B) the pore size distribution of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts.

  a: The results of Bragg angles (B-a) and d-spacing (d-s) of ZrO2 on catalysts, determined by XRD. b: The Ni 0 and NiO crystallite sizes on catalysts, determined by XRD. c: The Ni dispersion (DNi) of catalysts, determined by H2-TPD.

Figure 4 - 4

 44 Figure 4-4 XRD profiles of (a) 10Ni/ZrO2, (b) 5Ni-Zr, (c) 10Ni-Zr and (d) 15Ni-Zr catalysts after reduction at 750 °C.

Figure 4 - 5

 45 Figure 4-5 The CO2-TPD profiles of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts.

Figure 4 -

 4 Figure 4-6 presented the TEM results. Similar morphology formed on all the catalysts, and it looked like an elliptic grain. It can be observed that the 10Ni/ZrO2 and 5Ni-Zr catalysts have quite large sizes, especially on 5Ni-Zr catalyst with the randomlydistributed particles, between 10 and 40 nm. While the particle size decreased on 10Ni-Zr and 15Ni-Zr catalysts, and the range significantly changed to 8-12 nm. It implied a significant enlargement in their sizes on 10Ni/ZrO2 and 5Ni-Zr catalysts compared to 10Ni-Zr and 15Ni-Zr catalysts, in agreement with the XRD results.

Figure 4 - 6

 46 Figure 4-6 The TEM profiles of (A) 10Ni/ZrO2, (B) 5Ni-Zr, (C) 10Ni-Zr, and (D) 15Ni-Zr catalysts after reduction.

4. 5

 5 Catalytic performance in DRM 4.5.1 Zirconia support nickel catalysts for dry reforming of methane

4. 5 . 2

 52 The performance of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts. The performance in CO2 reforming of methane (activity and stability) of 10Ni/ZrO2 and the series of Ni-Zr catalysts was conducted in isothermal conditions at 750, as shown in Figure 4-7. A loss of 26.3 % in CH4 conversion is observed on the 10Ni/ZrO2 catalyst for 5 h time on stream. The CH4 and CO2 conversions of the 10Ni/ZrO2 catalyst decrease in the first hour (Figure 4-7 A and B). The H2/CO ratio decreases within 3

Figure 4 - 7

 47 Figure 4-7 The (A) CH4 conversion, (B) CO2 conversion and (C) H2/CO ratio of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalyst at 750 °C.

Figure 4 - 8

 48 Figure 4-8 The (A) CH4 and CO2 conversion and (B) the ratio of H2/CO of 10Ni-Zr catalyst at 700 and 750 °C time-on-stream.

4. 6 Structure of used catalysts 4 . 6 . 1

 6461 Figure 4-9 TG-DSC-MS profiles of (A) 10Ni/ZrO2, (B) 5Ni-Zr, (C) 10Ni-Zr and (D) 15Ni-Zr catalysts after reaction at 750 °C.

Figure 4 -

 4 Figure 4-10 XRD profiles of (a) 10Ni/ZrO2, (b) 5Ni-Zr, (c) 10Ni-Zr and (d) 15Ni-Zr catalysts after reaction at 750 °C in an isothermal run for 5 h.

  5 eV and 857.8 eV on the 10Ni-Zr catalyst, indicating that the interaction between Ni and Zr is very strong, due to the NiO-ZrO solid solution, corresponding with the XRD and H2-TPR results. The content of Zr 3d5/2 on 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts is similar to Figure 4-11 (B).

Figure 4 -

 4 Figure 4-11 XPS profiles of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts after reaction at 750 °C.

V 5 . 1

 51 Modified mesoporousNi-Zr catalysts by the promotion of Al, Mn, Y and Mg: on the change of surface properties of the catalysts 5. Modified mesoporous Ni-Zr catalysts by the promotion of Al, Mn, Y and Mg: on the change of surface properties of the catalysts Promotion of Ni-Zr catalyst by Y addition 5.1.

  catalyst and inhibition of carbon formation, such as La and Y. The later has attracted huge interest in reforming catalyst[START_REF] Yabe | Low-temperature dry reforming of methane to produce syngas in an electric field over La-doped Ni/ZrO2 catalysts[END_REF][START_REF] Li | Y2O3-promoted NiO/SBA-15 catalysts highly active for CO2/CH4 reforming[END_REF][START_REF] Bellido | Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF][START_REF] Sutthiumporn | Promotional effect of alkaline earth over Ni-La2O3 catalyst for CO2 reforming of CH4: role of surface oxygen species on H2 production and carbon suppression[END_REF]. Taherian et al.[START_REF] Taherian | A comparative study of ZrO2, Y2O3 and Sm2O3 promoted Ni/SBA-15 catalysts for evaluation of CO2/methane reforming performance[END_REF] found that comparing with mesoporous Ni/SBA-15, Ni/SBA-15 modified by Y2O3 can promote the control of the nickel particle size and thus enhance the dispersion of nickel.Moreover, Ni-Y/KIT-6 catalyst exhibited better dispersion of nano-size Ni particle inside the pores of the support as compared to Ni/KIT-6 material, leading to lower carbon deposition in the initial stability test at 700 °C[START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. Li et al.[START_REF] Li | Alumina supported Ni and Co catalysts modified by Y2O3 via different impregnation strategies: Comparative analysis on structural properties and catalytic performance in methane reforming with CO2[END_REF] investigated the effect of different types of impregnation for yttrium introduced on performance of Ni/Y/Al2O3 catalyst, and observed that compared with Y-free catalyst, the activity and stability of Y-modified catalyst increased, because of smaller nickel metallic particle size, more basic sites, the less amount of carbon deposit, and the small degree of graphitization. Furthermore, Li et al.[START_REF] Li | Methane reforming with CO2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol-gel process[END_REF] also observed highly carbon-resistance for Ni-SBA-15 catalyst modified by yttrium promoter, because of high dispersion of the nano-particles of yttrium and nickel. Moreover, more oxygen vacancies were created by introducing of yttrium.

  activity and stability for DRM. Because Y 3+ increased the formation of oxygen vacancies in YZ support. Those modification properties of ZrO2 could improve the performance of catalyst. Asencios et al.[START_REF] Asencios | Oxidative reforming of model biogas over NiO-Y2O3-ZrO2 catalysts[END_REF] investigated the effect of solid solution on performance of NiO-Y2O3-ZrO2 catalyst and found that the formation of NiO-Y2O3 and Y2O3-ZrO2 solid solution could enhance the activity of catalyst for oxidative reforming of model biogas. Moreover, the formation of oxygen vacancies could promote the removal of carbon deposition. Świrk et al.[START_REF] Świrk | Effect of low loading of yttrium on Ni-based layered double hydroxides in CO2 reforming of CH4, Reaction kinetics[END_REF][START_REF] Świrk | Dry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF] also found that the formation of Y2O3-ZrO2 solid solution could improve the stability of catalyst modified by yttrium.

Figure 5 -

 5 Figure 5-1 (A) the H2-TPR profiles of NiO-ZrOm-YOn and NiO-ZrOm calcined catalysts, and (B) the Zr 3d profiles from XPS measurements of NiO-ZrOm-YOn and NiO-ZrOm catalysts after reduction.

3

 3 Basicity measured by the content of CO2 desorption on CO2desorption (CO2 TPD) experiment was conducted to determine the basicity of the Y-doped and Y-free NiO-ZrOm catalysts (Figure 5-2).

Figure 5 - 5 . 1 . 3 . 4 Fig. 5 -

 551345 Figure 5-2 The CO2-TPD profiles of NiO-ZrOm-YOn and NiO-ZrOm catalysts.

Figure 5 - 3

 53 Figure 5-3 The XRD profiles of (a) NiO-ZrOm and (c) NiO-ZrOm-YOn catalysts after reduction at 700 °C for 1 hour and after reaction at 700 °C for 8 hours, and (b) NiO-ZrOm and (d) NiO-ZrOm-YOn catalysts after reaction at 700 °C for 8 hours.

Figure 5 - 4 5 . 1 . 5 5 . 1 . 5 . 1

 545155151 Figure 5-4 The conversion of CH4 (A) and CO2 (B), and the H2/CO ratio (C) of NiO-ZrOm-YOn and NiO-ZrOm catalysts at 700 o C with the mixed flow of CH4:CO2:Ar=10:10:80, GSHV=48,000 h -1 .

Figure 5 - 5

 55 Figure 5-5 The C 1s profiles of used NiO-ZrOm and NiO-ZrOm-YOn catalysts.

  Fig 5-7 (A)), and no carbon deposition is observed on NiO-ZrOm-YOn catalyst (Fig 5-7 (C)), which indicates that no or little carbon formes on NiO-ZrOm-YOn catalyst. These carbon nanotubes are graphitic carbon, which is in agreement with the results of Raman experiment. This phenomenon shows that yttrium can suppress the sintering of nickel particles, and also inhibits the formation of carbon during the DRM reaction.

Figure 5 - 7

 57 Figure 5-7 TEM images of used catalysts (A) NiO-ZrOm and (C) NiO-ZrOm-YOn and particle sizes of catalysts (B) NiO-ZrOm and (D) NiO-ZrOm-YOn.

Figure 5 - 8

 58 Figure 5-8 The H2-TPR profiles of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts

Figure 5 -

 5 Figure 5-9 showed the CO2-TPD patterns of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts.

Figure 5 - 9

 59 Figure 5-9 The CO2-TPD profiles of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts

Figure 5 - 8

 58 Figure 5-8 The conversion of (A) CH4 and (B) CO2, and (C) the H2/CO ratio on NiO-MgO-ZrO2 and NiO-ZrO2 catalysts at 700 o C with the mixed flow of CH4:CO2:Ar=10:10:80, GSHV=48000 h -1

  Figure 5-9 presented the TEM images and the particle size of spent catalysts. A lot of carbon nanotube could be observed easily on NiO-MgO-ZrO2 catalyst, while a little carbon nanotube could be observed on NiO-ZrO2 catalyst, which corresponded to the results of TG. Those phenomena indicated that Mg could promote the formation of carbon, due to the formation of a large amount of very weak basic sites on NiO-MgO-ZrO2 catalyst. Those weak basic sites exhibited very weak adsorption of CO2, resulting in lower CO2 conversion. The particle size of nickel after reaction on NiO-MgO-ZrO2 catalyst was about 15-17 nm (Fig. 5-9 (B)), while it was 15-20 nm on NiO-ZrO2 catalyst (Fig. 5-9 (D)), which conformed to the results of XRD. Those phenomena manifested that Mg could restrain the sintering of nickel by formation of NiO-MgO solid solution during the reaction process.

Figure 5 - 9

 59 Figure 5-9 TEM images of used catalysts (A) NiO-MgO-ZrO2 and (C) NiO-ZrO2 catalysts and particle sizes of catalysts (B) NiO-MgO-ZrO2 and (D) NiO-ZrO2 catalysts.

5. 3 . 3 Catalytic performance in dry reforming of methane 5 . 3 . 3 . 1 Figure 5 -Figure 5 -

 33533155 Figure 5-10 (A) CH4 conversion as function of temperature, (B) CO2 conversion as function of temperature, and (C) H2/CO ratio as function of temperature for all the catalysts, CH4:CO2:Ar=10:10:80, GSHV=48000 h -1 .

5 . 3 . 4 . 1

 5341 NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts The content and the type of carbon depositionThe carbon deposition of both catalysts was followed by TGA experiments and were shown in Figure5-12. The carbon formation on NiO-10Al2O3-ZrO2 catalyst (5.3 %) (Figure5-12 (B)) was slightly higher than that on NiO-ZrO2 catalyst (3.7 %) (Figure5-12 (A)), which can be ascribed to the slight reduction of CO2 conversion observed for NiO-10Al2O3-ZrO2 catalyst during 8 hours time-on-stream. From the DSC curve, one peak corresponding to the maximum of carbon consumption at c.a. 560 °C for NiO-10Al2O3-ZrO2 catalyst was observed. Whereas a shift to higher temperature (630°C) was observed for NiO-ZrO2 catalyst. It is worth noting that the initial carbon removal temperatures (the initial decrease of sample weight) on NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts are 550 and 420 o C, respectively. Thus, the formed carbon was easier to be removed on NiO-10Al2O3-ZrO2 catalyst, which contributes to the stability of the catalyst. According to the different initial carbon removal temperature, two types of carbon were named as C1 and C2 for 550 and 420 o C, respectively.

Figure 5 -

 5 Figure 5-12 TGA profiles of (A) NiO-ZrO2 and (B) NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours. (C) The C 1s profiles from XPS spectroscopy and (D) Raman profiles of NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours

Figure 5 -

 5 Figure 5-13 The morphology of ((A) and (C)) NiO-ZrO2, and ((B) and (D)) NiO-10Al2O3-ZrO2 catalysts after reaction, determined by TEM.

Figure 5 -

 5 Figure 5-14 Isotherm (A) and pore size distribution (B) of NiO-ZrO2 and NiO-Al2O3-ZrO2 catalysts.

Figure 5 -

 5 Figure 5-14 presented the adsorption/desorption isotherms. The adsorption isotherms for NiO-10Al2O3-ZrO2 and NiO-ZrO2 catalysts were type IV, with an H2 hysteresis loop according to the literature [263], typical of mesoporous materials. The pore size distribution (Figure 5-14 (B)) indicated the most probable pore size of 3 and 6 nm for

  Figure 5-16 XRD profiles of (a) NiO-ZrO2 and (c) NiO-10Al2O3-ZrO2 catalysts after reduction at 700 °C for 1 hour, and (b) NiO-ZrO2 and (d) NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours.

Figure 5 -

 5 17 (B) presented the Ni 2p profiles of NiO-10Al2O3-ZrO2 and NiO-ZrO2 catalysts before and after reaction.

Figure 5 -

 5 Figure 5-17 (A) Zr 3d and Ni 2p profiles of (a) NiO-10Al2O3-ZrO2 and (c) NiO-ZrO2 catalysts after reduction at 700 °C for 1 hour and (b) NiO-10Al2O3-ZrO2 and (d) NiO-ZrO2 catalysts after reaction at 700 °C for 8 hours.

Figure 5 -

 5 Figure5-19 shows the CO2-TPD curves of the reduced catalysts. In agreement with the literature[START_REF] Wang | One-step synthesis of highly active and stable Ni-ZrOx for dry reforming of methane[END_REF], the three desorption peaks on Ni-ZrOx catalyst centered at 159, 249 and 395 °C were attributed respectively for weak, medium and strong basic sites, which corresponded to surface OH -groups, zirconium-oxygen ion pairs, and isolated O 2- anions, respectively[START_REF] Cannilla | Highly effective MnCeOx catalysts for biodiesel production by transesterification of vegetable oils with methanol[END_REF][START_REF] Cannilla | Catalytic Behaviour of MnZrOx System for Heterogenous Biodiesel Production[END_REF]. Whereas, the desorption peaks on Ni-Mn-ZrOx catalyst shifted to low temperature. The three peaks shifted to 140, 232, and 375 °C for weak, medium, and strong basic sites, respectively. According to Table 5-14, Mn promoted the formation of weak basic sites and decreased the strong basic sites. Besides, the total basic sites increased from 73 to 92 μmol CO2/g by the introduction of Mn.

Figure 5 -

 5 Figure 5-19 The CO2-TPD profiles of Ni-ZrOx and Ni-Mn-ZrOx catalysts.

Figure 5 -

 5 Figure 5-20 The CH4 and CO2 conversion, and the H2/CO ratio with time on stream over Mn-promoted and non-promoted catalysts.

5. 4 . 5 5 . 4 . 5 . 1 Figure 5 -

 4554515 Figure 5-21 The XRD profiles of (A) Ni-ZrOx and (B) Ni-Mn-ZrOx catalyst. (a) reduced and after reaction for 100 min (b), 300 min (c) and 480 min (d).

Figure 5 -

 5 Figure 5-21 presented the XRD results after different time on stream. The reflection peaks at 30.5, 35.3, 51.0, 60.5 and 74.9 º can be attributed to the tetragonal phase of ZrO2. Another peak about at 44.5 º corresponded to the most intense peak of the Ni 0

Figure 5 -

 5 22 (A) and (B) presentedthe results of Ni 2p3/2 profiles for non-promoted and promoted catalysts. According to the literature[START_REF] Liu | Elucidating the interaction between Ni and CeOx in ethanol steam reforming catalysts: A perspective of recent studies over model and powder systems[END_REF][START_REF] Wang | Cu and Zn) supported on attapulgite as catalysts for hydrogen production from glycerol steam reforming[END_REF], the obtained curve of Ni 2p spectra was deconvoluted in three peaks. The peaks centered at about 852.3 eV, 855 eV, and 861 eV are assigned to Ni 0 species, Ni 2+, and satellite, respectively[START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-Si/ZrO2 catalyst[END_REF][START_REF] Németh | Impregnated Ni/ZrO2 and Pt/ZrO2 catalysts in dry reforming of methane: Activity tests in excess methane and mechanistic studies with labeled 13 CO2[END_REF]. And the percentage of Ni 0 species from XPS analysis was listed in Table5-17. The percentage of surface Ni 0 species on Ni-ZrOx and Ni-Mn-ZrOx catalysts at the initiation of the reaction was about 24.4 and 20.0 % respectively. This is in agreement with the fact that Ni-ZrOx catalyst exhibited higher initial activity than the one on Ni-Mn-ZrOx catalyst. It was worth noting that the percentage of NiO species was higher that of Ni 0 species at the initiation of both catalysts. The higher content of NiO species might be oxidized by air/CO2 or CO2/CO adsorb on Ni 0 , which is explained elsewhere[START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF][START_REF] Yao | The influence of reduction temperature on the performance of ZrOx/Ni-MnOx/SiO2 catalyst for low-temperature CO2 reforming of methane[END_REF]. The percentage of surface Ni 0 species on two catalysts saw an opposite trend with time on stream. It decreased to 17.4 % after reaction for 100 min and to 13.3 % after reaction for 480 min on Ni-ZrOx catalyst.On the contrary, for Ni-Mn-ZrOx catalyst, it increased to 40.0 % after 100 min reaction, to 23.2 after 300 min reaction and finally kept at about 22.6 % after reaction for 480 min. Those observations were consistent with the XRD and activity test results.

Figure 5 -

 5 Figure 5-22 The Ni 2p3/2 and Zr 3d profiles of Ni-ZrOx ((A) and (C)) and Ni-Mn-ZrOx ((B) and (D)) catalysts. (a) reduced and after reaction for 100 min (b), 300 min (c) and 480 min (d).

Figure 5 -

 5 Figure 5-22 (C) and (D) presented the Zr 3d results. The Zr 3d binding energies on Ni-ZrOx catalyst shifted to lower values with the time on stream. While it kept on almost

Figure 5 -

 5 Figure 5-23 the TGA (A) and Raman (B) profiles of Ni-ZrOx and Ni-Mn-ZrOx catalysts. And the TEM profiles of Ni-ZrOx (C) and Ni-Mn-ZrOx (D) catalysts.

Figure 6 -Figure 6 - 1 .

 661 Figure 6-1 the H2-TPR profiles of Ni-Zr catalysts with different metal promoters.

3 .

 3 The position of peaks on the Ni-Zr catalyst is located at 180, 252 and 380 °C, respectively. Most of peaks on promoted catalysts shifts to low temperature. Especially, the existence of Al decreases the centers of peak 1, 2 and 3 by 36, 30 and 39 °C, respectively. The addition of Mn and Y mainly shifts the weak and medium-strength basic sites. Mg-promoted catalyst slightly changes the weak and strong basicity, and the variations are about -20 and -10 °C. Besides, the content of each peak is different on non-promoted and promoted catalysts. For Ni-Zr catalyst, it is about 15.5, 64.4 and 20.1 % for weak, medium-strength and strong basic sites, respectively. The content of weak basic sites on all the promoted catalyst increases to equal to or greater than 30 % compared to Ni-Zr catalyst. The introduction of Mn, Y and Mg lead to a decrease in the amount of CO2 on strong basic sites compared to nonpromoted catalysts. While it slightly increases on Ni-Zr-Al catalyst, which might give rise to coke. Because the weak and medium-strength basic sites could promote the adsorption and/or activation of CO2, and further reduce the carbon deposition, as well as the deactivation of catalyst.[START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO2 methane reforming[END_REF][START_REF] Dębek | Catalytic activity of hydrotalcite-derived catalysts in the dry reforming of methane: on the effect of Ce promotion and feed gas composition[END_REF].

Figure 6 - 3

 63 Figure 6-3 XRD profiles of (a) Ni-Zr, (b) Ni-Zr-Al, (c) Ni-Zr-Mn, (d) Ni-Zr-Mg and (e) Ni-Zr-Y catalysts, and (f) ZrO2 (A), after reduction (B) at 700 °C for 1 hour.

Figure 6 - 4

 64 Figure 6-4 The average conversion of CH4 (A) and CO2 (B) and the rate of H2/CO (C) of Ni-Zr-M catalyst (M=Al, Mg, Y, Mn).

Figure 6 - 5

 65 Figure 6-5 The conversion of CH4 and CO2, and the rate of H2/CO of Ni-Zr-M (M=Al, Mg, Y, Mn) catalyst at 700 o C with the mixed flow of CH4:CO2:Ar=10:10:80, GSHV=48000 h -1 .

Figure 6 -

 6 Figure 6-6 shows XRD patterns of the spent catalysts after 8 h DRM activity tests. The crystallite sizes of the metallic Ni 0 and ZrO2 are summarized in Table 6-4. Very serious sintering occurred on non-promoted catalyst after DRM tests, and the crystallite Ni 0

Figure 6 - 6

 66 Figure 6-6 XRD profiles of (a) Ni-Zr, (b) Ni-Zr-Al, (c) Ni-Zr-Mn, (d) Ni-Zr-Mg and (e) Ni-Zr-Y catalysts after reaction at 700 °C for 8 hours.

6. 5 . 2 Figure 6 - 7 Figure 6 - 8 Figure 6 -

 5267686 Figure 6-7 The TEM profiles of used catalysts (A) Ni-Zr, (B) NI-Zr-Al, (C) Ni-Zr-Mn, (D) Ni-Zr-Mg and (E) Ni-Zr-Y.

6. 5 . 4 Figure 6 - 9

 5469 Figure 6-9 Raman profiles of used catalysts.

  the addition of Mg and Y results in the large Ni 0 particle size, lower nickel content at the surface, and pore physical structure (specific surface area). Thus, lower performance on Mg and Y-modified catalysts are obtained. Besides, the lowest amount of basic sites and large particle size on Mg-promoted catalyst results in the highest amount of carbon deposition. The promotion by Y affects the adsorbed CO2 on each type of basic sites, and the almost suppress the strong basic sites, which inhibits the carbon deposition during the reaction.In the first part of this work, a series of Ni-Zr catalysts with different Ni loadings were prepared by a novel one-step urea hydrolysis method, and compared to 10Ni/ZrO2 catalyst prepared by ethanol impregnation method. The catalysts were tested in dry reforming of methane (DRM) at 750 °C with a mixed flow of CH4:CO2:Ar=10:10:80 and GHSV= 24,000 h-1. The materials were characterized by BET, XRD, XPS, H2-TPR, CO2-TPD, TEM, TGA and Raman. 10Ni-Zr and 15Ni-Zr catalysts prepared by the one-step method exhibited higher activity, long-run stability, and selectivity of H2/CO compared to 10Ni/ZrO2 catalysts. This could be ascribed to the smaller Ni crystallite size, increased specific surface area and enhanced weak and mediumstrength basic sites over reduced 10Ni-Zr and 15Ni-Zr catalysts. The 10Ni-Zr and 15Ni-Zr catalysts showed a suppression of metallic nickel sintering, due to the formation of NiO-ZrO2 solid solution, leading to the stronger interaction between Ni and ZrO2 matrix. Moreover, stability enhancement for these catalysts was possibly linked with a higher amount of adsorbed oxygen species.In the second part, modified the Ni-Zr catalysts were synthesized in order to improve catalytic activity and stability. Y, Mg, Al and Mn were respectively used as promoters on Ni-ZrO2 catalysts. Yttrium-doped NiO-ZrOm catalyst was found to be novel for carbon resistance in the CO2 reforming of methane. The addition of Y to NiO-ZrOm catalyst resulted in higher interaction between Ni and ZrO2, higher distribution of weak and medium basic sites, and smaller Ni crystallite size, as compared to the Y-free NiO-ZrOm catalyst after reaction. The DRM catalytic tests were conducted at 700 °C for 8 h, leading to a significant decrease of activity and selectivity for the yttrium-doped NiO-ZrOm catalyst, due to the larger initial particle size of nickel species and lower surface nickel content. The carbon deposition after the DRM reaction on yttrium-doped NiO-ZrOm catalyst (1.0 %) was lower than on yttrium-free NiO-ZrOm catalyst (3.7 %), which indicated that yttrium could promote the inhibition of carbon deposition during the DRM process.NiO-MgO-ZrO2 catalysts were also studied. The activity was carried out at 700 °C in a fixed-bed micro-reactor under CH4:CO2:Ar=1:1:8. It was shown that the introduction of Mg led to an unexpected decrease in the activity when compared to non-promoted catalyst. It was also shown that the surface area, pore-volume, pore diameter, and weak basicity decreased when the Mg was introduced into NiO-ZrO2 catalyst. All these properties can cause a decrease in the activity, selectivity, and stability of NiO-MgO-ZrO2 catalyst for DRM.Introduction of aluminum was used to promote NiO-ZrO2 catalysts. The Al-doped catalyst exhibited higher catalytic performance in comparison with Al-free catalyst. The enhancement of catalytic activity in dry reforming could be associated with the alterations in surface properties due to Al promotion. First, the introduction of Al changes the structure of ZrO2, leading to an increase in its pore volume and pore diameter. Moreover, the promotion of ZrO2 supported Ni catalysts with Al led to a better distribution of Ni species in the NiO-10Al2O3-ZrO2 catalysts. Besides, it was found that in the presence of aluminum, higher content of surface Ni species were present on NiO-ZrO2 catalyst. Finally, an enhanced ability for the sintering resistance and high suppression to loss of nickel during a long-term catalytic test was also found on Al promoted catalyst.Finally, Mn was used as dopant for NiZrO2 catalyst. This catalyst studied versus time of stream during the DRM reaction. The activity increased on Ni-Mn-ZrOx with 300 min on stream, while it decreased on Ni-ZrOx within 100 min. Because an increase of surface nickel content and a decrease of Ni particle size on Ni-Mn-ZrOx catalyst have been observed by applying in XRD and XPS after reaction for 100 min, 300 min and  Further characterization techniques should be applied. For example, to trace the process of reaction through in-situ experiment. Such as in-situ XPS, in-situ XRD and in-situ TEM could be performed  Pore structure offers the excellent resistance against sintering and carbon formation.

Figure 2 - 1 9 Figure 2 - 2 Figure 2 - 3 Figure 2 - 4 12 Figure 2 - 5 13 Figure 2 - 6 A 14 Figure 2 - 7

 219222324122513261427 Figure 2-1 The detail of CO2 emissions for one year………………………………….9 Figure 2-2 The annual mean carbon dioxide (A) and carbon dioxide growth rates (B) at Mauna Loa observatory………………………………………………………………10 Figure 2-3 Global greenhouse gas emission sources in 2004 of which approximately 77% are represented by CO2 emissions………………………………………………11 Figure 2-4 The annual demand for energy…………………………………………....12 Figure 2-5 The basic schemes showing the types of CO2 capture…………………....13 Figure 2-6 A portion of the crystal structure of Zn4O(BDC)3 (MOF-5). Blue tetrahedra represent ZnO4 units, while gray and red spheres represent C and O atoms, respectively; H atoms are omitted for clarity……………………………………….........................14 Figure 2-7 The ethanol, hydrogen, isobuthane, methanol and ammonia synthesized by syngas………………………………………………………………………………...17

Figure 2 - 8 Figure 3 - 1 42 Figure 3 - 43 Figure 3 - 3 44 Figure 3 - 4 Figure 3 - 5 X 47 Figure 3 . 6 1007………………………………………………………………………………….. 48 Figure 3 - 7 51 Figure 3 - 8 52 Figure 3 - 9 54 Figure 3 -Figure 4 - 1 64 Figure 4 - 2 65 Figure 4 - 3 ( 67 Figure 4 - 4 Figure 4 - 5 Figure 4 - 6 Figure 4 - 7 77 Figure 4 - 8 78 Figure 4 - 9 Figure 4 - 80 Figure 4 -

 28314234333443435473648375138523954341644265436744454647774878494804 Figure 2-8 The adsorption carbonate species on metal site (M)………………………25 Figure 3-1 Programs used during dry reforming of methane: (A) Multi-Steady experiment at different temperatures and (B) Isothermal experiments……………...42Figure 3-2 (A) Scheme and (B) physical profile of dry reforming of methane set-up……………………………………………………………………………………..43 Figure 3-3 Programs (A), scheme (B) and set-up (C) used the temperature-programmed reduction of H2 (H2-TPR) …………………………………………………………….44 Figure 3-4 The programs for temperature-programmed desorption of CO2 (CO2-TPD) …………………………………………………………………………………46 Figure 3-5 X-rays scattered by atoms in an ordered lattice interfere constructively in directions……………………………………………………………………………..47 Figure 3.6 Crystallographic parameters for ZrO2, according to the standard ICSD # 88-1007…………………………………………………………………………………..48 Figure 3-7 The types of physisorption isotherms (A) and (B) hysteresis loops……...51 Figure 3-8 The scheme of set-up for TGA (A) and TGA-DSC (B) ………………....52

Figure 5 - 1 (

 51 Figure 5-1 (A) the H2-TPR profiles of NiO-ZrOm-YOn and NiO-ZrOm calcined catalysts, and (B) the Zr 3d profiles from XPS measurements of NiO-ZrOm-YOn and NiO-ZrOm catalysts after reduction………………………………………………………………95 Figure 5-2 The CO2-TPD profiles of NiO-ZrOm-YOn and NiO-ZrOm catalysts……..97

Figure 5 - 3 Figure 5 - 4 99 Figure 5 - 5 Figure 5 - 6 101 Figure 5 - 7 Figure 5 - 8 107 Figure 5 - 9 Figure 5 - 8

 535499555610157581075958 Figure 5-3 The XRD profiles of (a) NiO-ZrOm and (c) NiO-ZrOm-YOn catalysts after reduction at 700 °C for 1 hour and after reaction at 700 °C for 8 hours, and (b) NiO-ZrOm and (d) NiO-ZrOm-YOn catalysts after reaction at 700 °C for 8 hours…………98 Figure 5-4 The conversion of CH4 (A) and CO2 (B), and the H2/CO ratio (C) of NiO-ZrOm-YOn and NiO-ZrOm catalysts at 700 o C with the mixed flow of CH4:CO2:Ar=10:10:80, GSHV=48,000 h -1 …………………………………………..99 Figure 5-5 The C 1s profiles of used NiO-ZrOm and NiO-ZrOm-YOn catalysts……100 Figure 5-6 The Raman profiles of used NiO-ZrOm and NiO-ZrOm-YOn catalyst….101 Figure 5-7 TEM images of used catalysts (A) NiO-ZrOm and (C) NiO-ZrOm-YOn and particle sizes of catalysts (B) NiO-ZrOm and (D) NiO-ZrOm-YOn…………………102 Figure 5-8 The H2-TPR profiles of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts……...107 Figure 5-9 The CO2-TPD profiles of NiO-MgO-ZrO2 and NiO-ZrO2 catalysts……108 Figure 5-8 The conversion of (A) CH4 and (B) CO2, and (C) the H2/CO ratio on NiO-MgO-ZrO2 and NiO-ZrO2 catalysts at 700 o C with the mixed flow of CH4:CO2:Ar=10:10:80, GSHV=48000 h -1 ………………………………………….110

Figure 5 - 9 111 Figure 5 -Figure 5 - 119 Figure 5 - 121 Figure 5 -

 591115511951215 Figure 5-9 TEM images of used catalysts (A) NiO-MgO-ZrO2 and (C) NiO-ZrO2 catalysts and particle sizes of catalysts (B) NiO-MgO-ZrO2 and (D) NiO-ZrO2 catalysts……………………………………………………………………………..111Figure 5-10 (A) CH4 conversion as function of temperature, (B) CO2 conversion as function of temperature, and (C) H2/CO ratio as function of temperature for all the catalysts, CH4:CO2:Ar=10:10:80, GSHV=48000 h -1 ………………………………118 Figure 5-11 (A) CH4 conversion , (B) CO2 conversion and (C) H2/CO ratio in isothermal conditions (700°C) on NiO-10Al2O3-ZrO2 in the presence of CH4:CO2:Ar=10:10:80, GSHV=48,000 h -1 …………………………………………………………………..119 Figure 5-12 TGA profiles of (A) NiO-ZrO2 and (B) NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours. (C) The C 1s profiles from XPS spectroscopy and (D) Raman profiles of NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours…………………………………………………………………………..121 Figure 5-13 The morphology of ((A) and (C)) NiO-ZrO2, and ((B) and (D)) NiO-10Al2O3-ZrO2 catalysts after reaction, determined by TEM………………………..122

Figure 5 - 124 Figure 5 - 125 Figure 5 - 127 Figure 5 - 130 Figure 5 - 135 Figure 5 - 136 Figure 5 -Figure 5 -Figure 5 - 141 Figure 5 - 144 Figure 6 - 152 Figure 6 - 2 Figure 6 - 3 156 Figure 6 - 4 Figure 6 - 5 Figure 6 - 6 166 Figure 6 - 7 167 Figure 6 - 8 Figure 6 - 9

 512451255127513051355136555141514461526263156646566166671676869 Figure 5-14 Isotherm (A) and pore size distribution (B) of NiO-ZrO2 and NiO-Al2O3-ZrO2 catalysts……………………………………………………………………….124 Figure 5-15 The H2-TPR profiles of NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts..125 Figure 5-16 XRD profiles of (a) NiO-ZrO2 and (c) NiO-10Al2O3-ZrO2 catalysts after reduction at 700 °C for 1 hour, and (b) NiO-ZrO2 and (d) NiO-10Al2O3-ZrO2 catalysts after reaction at 700 °C for 8 hours………………………………………………….127Figure 5-17 (A) Zr 3d and Ni 2p profiles of (a) NiO-10Al2O3-ZrO2 and (c) NiO-ZrO2 catalysts after reduction at 700 °C for 1 hour and (b) NiO-10Al2O3-ZrO2 and (d) NiO-ZrO2 catalysts after reaction at 700 °C for 8 hours………………………………….130 Figure 5-18 The H2-TPR profiles of Ni-ZrOx and Ni-Mn-ZrOx catalysts………....135 Figure 5-19 The CO2-TPD profiles of Ni-ZrOx and Ni-Mn-ZrOx catalysts……….136 Figure 5-20 The CH4 and CO2 conversion, and the H2/CO ratio with time on stream over Mn-promoted and non-promoted catalysts………………………………….…137 Figure 5-21 The XRD profiles of (A) Ni-ZrOx and (B) Ni-Mn-ZrOx catalyst. (a) reduced and after reaction for 100 min (b), 300 min (c) and 480 min (d)…………..138 Figure 5-22 The Ni 2p3/2 and Zr 3d profiles of Ni-ZrOx ((A) and (C)) and Ni-Mn-ZrOx ((B) and (D)) catalysts. (a) reduced and after reaction for 100 min (b), 300 min (c) and 480 min (d)………………………………………………………………………......141 Figure 5-23 the TGA (A) and Raman (B) profiles of Ni-ZrOx and Ni-Mn-ZrOx catalysts. And the TEM profiles of Ni-ZrOx (C) and Ni-Mn-ZrOx (D) catalysts….144 Figure 6-1 the H2-TPR profiles of Ni-Zr catalysts with different metal promoters...152 Figure 6-2 The CO2-TPD profiles of Ni-Zr catalysts with different metal promoters……………………………………………………………………………154 Figure 6-3 XRD profiles of (a) Ni-Zr, (b) Ni-Zr-Al, (c) Ni-Zr-Mn, (d) Ni-Zr-Mg and (e) Ni-Zr-Y catalysts, and (f) ZrO2 (A), after reduction (B) at 700 °C for 1 hour……...156 Figure 6-4 The average conversion of CH4 (A) and CO2 (B) and the rate of H2/CO (C) of Ni-Zr-M catalyst (M=Al, Mg, Y, Mn)……………………………………………162 Figure 6-5 The conversion of CH4 and CO2, and the rate of H2/CO of Ni-Zr-M (M=Al, Mg, Y, Mn) catalyst at 700 o C with the mix flow of CH4:CO2:Ar=10:10:80, GSHV=48000 h -1 ……………………………………………………………………165

Table 2 -

 2 1 Possible parallel reactions during dry reforming of methane

	Reaction		ΔH°298K
	Number	Reaction	KJ/mol
		Dry reforming
		CO 2 + CH 4 → CO + H 2	+247
		Reverse water gas shift
		CO 2 + H 2 → 2CO + H 2 O	+41
		Carbon forming reactions

  species on Ni-Si/ZrO2 catalyst, whereas CO2 could be adsorbed and dissociated to form adsorbed CO and O species. Part of adsorbed H species could be combined by adsorbed CO2 to form formate, while part of adsorbed O species could react with adsorbed CH3 species, thereby promoted the dissociation of methane, which might be as follows:

I: CH4 ⇋ CH3(ads)+H(ads) CO2(ads) ⇋ CO(ads)+O(ads) 2Hads ⇋ H2 COads ⇋ COg II: CH4 ⇋ CH3(ads)+H(ads) CO2(ads)+ H(ads) ⇋ COOHads ⇋ COads+OHads (like RWGS) 2OHads ⇋ H2+H2O COads ⇋ COg

Table 2

 2

-2).

Table 2

 2

	-2 A brief description and comparative summary for catalysts of dry reforming
	of methane.
	Method	Species/Approach				Reference
	active	 Rh nanoparticles for CO2 reforming of methane [76]
	metal
		 Lattice Strained Ni-Co alloy as a high-	[77]
		performance
		 Elucidating the impact of Ni and Co loading on	[78]
		the selectivity of bimetallic NiCo catalysts
		 Pd-CeO2 nanocatalyst for Low temperature dry	[100]
		reforming of methane
		 Pt, Ru doped TiO2 for methane reforming	[80]
		 Rh and Co based catalysts for dry reforming of	[101]
		methane
	catalyst	 Stable Ni-Mo nanocatalysts on single-	[2]
	synthesis	crystalline MgO
		 Different calcination atmospheres (N2, O2 and	[102]
		H2) for Ni/ZrO2 catalyst
		 Sol-gel synthesis of Ni-Co/Al2O3-MgO-ZrO2	[81]
		nanocatalyst
		 Small-sized Ni@SiO2 catalyst with shell-	[84]
		nuclear structure
		 Sequential	impregnation	vs.	sol-gel	[103]
		synthesized Ni/Al2O3-CeO2 nanocatalysts
		 A sintering and carbon-resistant Ni-SBA-15	[104]
		catalyst prepared by solid-state grinding
		method
		 Facile one-pot synthesis of highly dispersed Ni	[86]
		nanoparticles embedded in HMS

Table 3 -

 3 [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF] The Peak list for Pattern ZrO2, according to the ICSD standard # 88-1007

		2-Theta d(Å)	I(v)	( h k l)	Theta	1/(2d)	2pi/d
	1	30.271	2.9501	100.0	( 1 0 1)	15.136	0.1695	2.1298
	2	34.798	2.5760	9.2	( 0 0 2)	17.399	0.1941	2.4391
	3	35.243	2.5445	15.3	( 1 1 0)	17.622	0.1965	2.4694
	4	43.153	2.0946	1.3	( 1 0 2)	21.577	0.2387	2.9997
	5	50.366	1.8103	54.9	( 1 1 2)	25.183	0.2762	3.4709
	6	50.697	1.7992	29.8	( 2 0 0)	25.349	0.2779	3.4922
	7	53.934	1.6986	0.2	( 2 0 1)	26.967	0.2944	3.6990
	8	59.603	1.5499	22.3	( 1 0 3)	29.801	0.3226	4.0540
	9	60.194	1.5361	43.4	( 2 1 1)	30.097	0.3255	4.0905
	10 62.961	1.4750	11.4	( 2 0 2)	31.481	0.3390	4.2597
	11 68.719	1.3648	0.4	( 2 1 2)	34.359	0.3663	4.6037
	12 73.460	1.2880	4.1	( 0 0 4)	36.730	0.3882	4.8783
	13 74.523	1.2722	9.7	( 2 2 0)	37.262	0.3930	4.9387
	14 78.869	1.2127	0.5	( 1 0 4)	39.435	0.4123	5.1813
	15 81.987	1.1743	20.3	( 2 1 3)	40.993	0.4258	5.3508
	16 82.502	1.1682	10.4	( 3 0 1)	41.251	0.4280	5.3784
	17 84.180	1.1492	7.4	( 1 1 4)	42.090	0.4351	5.4676
	18 84.951	1.1407	6.2	( 2 2 2)	42.476	0.4383	5.5082
	19 85.208	1.1379	8.8	( 3 1 0)	42.604	0.4394	5.5217

Table 4

 4 This result corresponds with the results of H2-TPR. Besides, the d-spacing of ZrO2 (2θ=30.14) on the 10Ni/ZrO2 catalyst is about 2.9627 Å, while it is about 2.9576, 2.9402 and 2.9352

	-1. The peaks of tetragonal and monoclinic zirconia are
	identified on 10Ni/ZrO2, while only tetragonal ZrO2 forms on series of Ni-Zr catalysts
	[207, 211], which indicates that the addition of nickel into ZrO2 can modify the
	structure of ZrO2. It is worth noting that the addition of nickel to zirconia causes a shift
	of ZrO2 peaks (2θ=30.14 o for 10Ni/ZrO2) to higher Bragg angles (2θ=30.19, 30.38 and
	30.43 o for 5NI-Zr, 10Ni-Zr and 15NI-Zr, respectively), meanwhile, NiO crystal peaks
	on 10Ni/ZrO2 is very evident, but it becomes weak and wide on the series of Ni-Zr

catalysts, which indicates that NiO could be dispersed well or part of nickel enters the lattice of ZrO2 to form the solid solutions (NiO-ZrO2)

[START_REF] Wang | Template-free preparation of bimetallic mesoporous Ni-Co-CaO-ZrO2 catalysts and their synergetic effect in dry reforming of methane[END_REF][START_REF] Asencios | Synthesis of NiO-MgO-ZrO2 catalysts and their performance in reforming of model biogas[END_REF][START_REF] Chen | Effect of preparation methods on structure and performance of Ni/Ce0.75Zr0.25O2 catalysts for CH4-CO2 reforming[END_REF]

.

Table 4

 4

	-1 The XRD and H2-TPD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr
	catalysts.
	Catalyst	Bragg angles / o	d-spacing /Å
	10Ni/ZrO2	30.14	2.9627
	5Ni-Zr	30.19	2.9576
	10Ni-Zr	30.38	2.9402
	15Ni-Zr	30.43	2.9352

Table 4 -

 4 2 The BET results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts.

	Catalyst	Pore diameter nm	Surface area m 2 /g Pore volume cm 3 /g
	10Ni/ZrO2	2	72	0.19
	5Ni-Zr	2	66	0.17
	10Ni-Zr	2	113	0.20
	15Ni-Zr	2	103	0.18
	4.4.

2 The particle size of nickel species

Table 4

 4

	-3 The XRD and H2-TPD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr
	catalysts.
		Reduced	Post-Reaction Reduced	Post-Reaction	DNi
	Catalyst	B-a/ o d-s/Å B-a/ o d-s/Å Ni 0 /nm Ni 0 /nm NiO/nm	(%)
	10Ni/ZrO2 30.28 a 2.9521 30.26 2.9534	20 b	31	15	1.3 c
	5Ni-Zr	30.24 2.9556 30.27 2.9522	31	26	16	3.2
	10Ni-Zr	30.32 2.9480 30.48 2.9332	11	15	-	4.3
	15Ni-Zr	30.38 2.9421 30.44 2.9363	11	13	-	2.6

Table 4 -

 4 [START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF] The Peak position and deconvolution of the CO2-TPD profiles obtained for the 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts.

				CO2 desorbed (%)			Total
				medium-			basicity
		Weak		strength		Strong	(µmol
	Catalyst	Position Content Position Content Position Content	CO2/g)
	10Ni/ZrO2	180	14.7	273	56.8	409	28.5	65
	5Ni-Zr	180	16.7	260	60.5	392	22.8	77
	10Ni-Zr	180	20.8	260	59.4	385	19.8	73
	15Ni-Zr	180	27.8	260	53.6	399	18.6	67

Table 4

 4

	-5 the performance of the 10Ni/ZrO2 catalysts at various temperatures for DRM
	reaction.
				Reactant	Conversion		Lost in
		Ni a	T b	GSHV	(%)	Ratio	activity
	Catalysts	%	°C	Gas flow	CH4 CO2 H2/CO	%.h -1	Ref.
	Ni/ZrO2	8.1 850

Table 4 -

 4 exhibited the highest CH4 and CO2 conversion, and the best H2/CO ratio about 83.8 %, 84.2 % and 1.2, respectively. And at 700 °C, the highest CH4 and CO2 conversion, and the best H2/CO ratio on one of 10Ni/ZrO2 catalysts from the work of

			1
			CH4:CO2 =1:1	59.3 63.7 0.92	0.1	[213]
			240 L•h -1 •g -1
	Ni/ZrO2	10 600	CH4:CO2:He=1:1:2 34	33	-	0.08	[226]
			40.7 L•h -1 •g -1
	Ni/ZrO2	10 550	CH4:CO2=1:1	10.4 30.6 -	1.10	[140]
			15.6 L•h -1 •g -1					This
	10Ni/ZrO2 10 750	CH4:CO2:Ar=1:1:8 74.5 79.4 0.86	5.32	work
			15.6 L•h -1 •g -1					This
	10Ni-Zr	10 750	CH4:CO2:Ar=1:1:8 96	96.7 0.95	S	work
			15.6 L•h -1 •g -1					This
	10Ni-Zr	10 700	CH4:CO2:Ar=1:1:8 92.9 93.1 0.97	S	work
	a: The content of nickel
	b: The reaction temperature
	c: Calculation by methane
	d: After reaction for 5 h
	e: After reaction for 8 h
	~: About, read by the figure
	S: Stable
	D: Deactivation
	-: No data

[START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF]

summarizes the studies of different 10Ni/ZrO2 catalysts in dry reforming of methane. As presented in Table

4

-5, 10Ni/ZrO2 catalyst from the work of Wei

[START_REF] Wei | Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of methane[END_REF]

, at 750 °C

Table 4 -

 4 [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF] The XRD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts.

		Reduced	Post-Reaction
	Catalyst	Ni 0 /nm	Ni 0 /nm	NiO/nm
	10Ni/ZrO2	20	31	15
	5Ni-Zr	31	26	16
	10Ni-Zr	11	15	-
	15Ni-Zr	11	13	-

Table 4 -

 4 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF] The surface content of Zr 3d and Ni 2p over 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts after reaction at 750 °C for 5 hours.

		The surface elemental content	Ni 2p (%) a
		Ni 2p	Zr 3d
		(%)	(%)	Ni 0	Ni 2+
	10Ni/ZrO2	13.2	50.4	9.6	90.4
	5Ni-Zr	3.6	55.9	17.5	82.5
	10Ni-Zr	4.8	50.9	24.3	75.7
	15Ni-Zr	6.4	53.7	34.8	65.2

a: The content of Ni 0 and Ni 2+ species in nickel species, calculated by Figure 4-11 (A).

Table 4 -

 4 [START_REF] Mumford | Review of solvent based carbondioxide capture technologies[END_REF] The surface content of C 1s and O 1s over 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts after reaction at 750 °C for 5 hours.

			C 1s (%) a			O 1s (%) b
					C-O			CO3 2-
					+			+
		C-C	C-O C=O	C=O	O 2-	CO3 2-OH -	OH -
	10Ni/ZrO2 69.5 18.9	11.6 30.5	75.2	18.5	6.3	24.8
	5Ni-Zr	71.9 17.2	10.9 28.1	72.3	21.9	5.8	27.7
	10Ni-Zr	44.1 38.8	17.1 55.9	54.5	34.9 10.6 45.5
	15Ni-Zr	48.9 41.2	9.9	51.1	56.0	30.8 13.2	44

a: The content of C-C, C-O and C=O species in C species, calculated by Figure 4-11 (C). b: The content of O 2-, CO3 2-and OH -species in O species, calculated by Figure 4-11 (D)

NiO-ZrOm-YOn catalyst

5.03 g zirconium(IV) oxynitrate hydrate (Aldrich), 1.14 g nickel(II) nitrate hexahydrate (Emsure) 7.06 g urea (Sigma-Aldrich), 0.86 g yttrium(III) nitrate hexahydrate (Aldrich) (corresponding to a loading of 10 wt.%) and 7.96 g of Pluronic P123 (Aldrich) amphiphilic block copolymer were dissolved in 375 mL of distilled water. The mixed liquor was heated from room temperature to 95 °C with constant

Table 5 -

 5 [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF] The results of the BET experiment for NiO-ZrOm and NiO-ZrOm-YOn calcined catalysts.

		Pore diameter	Specific surface area	Pore volume
	Catalyst	nm	m 2 /g	cm 3 /g
	NiO-ZrOm-YOn	2	79	0.1
	NiO-ZrOm	2	113	0.2
	5.1.3.2 The reducibility of Y-doped and Y-free NiO-ZrOm catalysts

Table 5 -

 5 2 The H2 consumption of calcined catalysts determined by H2-TPR. The content of Zr 4+ and Zr 3+ on both catalysts after reduction determined by XPS. And the content of Ni on both catalysts determined by ICP.

	Catalyst	H2 consumption, mmol H2/g Zr 3d (%) Total Theory a α β Zr 4+ Zr 3+	Ni (wt %)
	NiO-ZrOm-YOn	0.37	0.39	0.10 0.27 16	84	10
	NiO-ZrOm	0.62	0.47	0.13 0.49 10	90	12
	a : Calculated by ICP and the consumption of reduction of pure NiO

Table 5 -

 5 [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF] The content of CO2 desorption on the CO2-TPD experiment over NiO-MgO-ZrO2 and NiO-ZrO2 catalysts.

				CO2 desorbed (%)			Total
		Peak 1	Peak 2	Peak 3	basicity
		weak		medium-strength		strong	(μmol
	Catalyst	Position Content Position Content Position Content	CO2/g)
	NiO-ZrO2	185	15.5	252	64.4	380	20.1	73
	NiO-MgO-ZrO2	165	37.5	256	53.9	370	16.1	69

Table 5 -

 5

		pore diameter	specific surface area	pore volume
	Catalyst	nm	m 2 /g	cm 3 /g
	NiO-ZrO2	1.9	113	0.20
	NiO-MgO-ZrO2	1.6	90	0.10
	Table 5-7 exhibited the results of BET experiment. NiO-ZrO2 showed higher value of
	surface area, pore-volume, and pore diameter than those on NiO-MgO-ZrO2 catalyst,
	which indicated that the introduction of Mg did not modify the main structure of NiO-

[START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF]

The specific surface area determined by BET method; the pore volume and the pore diameter determined by BJH method on both catalysts.

ZrO2 catalyst, on the contrary, those low physical properties on NiO-MgO-ZrO2

0 over NiO-MgO-ZrO2 and NiO-ZrO2 catalystsTable 5

 5

	-8 Crystallite sizes of Ni 0 and content of Ni 0 over NiO-MgO-ZrO2 and NiO-
	ZrO2 catalysts
		Ni 0 Crystalline Size (nm) a	Ni 0 content (%) b	Coke
	Catalyst	Reduction	Reaction	Reduction	Reaction	%
	NiO-ZrO2	12	24	34	28	3.7
	NiO-MgO-ZrO2	20	17	22	22	16.3
	a: Crystallite sizes of Ni 0 determined by XRD
	b: The content of Ni 0 determined by XPS
	Table

Table 5 -

 5

	9 The dosage of Al, Ni and Zr for prepare a series of 10NiO-xAl2O3-(90-
	x)ZrO2
	Catalyst	Al %	Zr %	Ni %
	NiO-ZrO2	-	90	10
	NiO-10Al2O3-ZrO2	10	80	10
	NiO-20Al2O3-ZrO2	20	70	10
	NiO-45Al2O3-ZrO2	45	45	10
	NiO-Al2O3	90	0	10

Table 5 -

 5 [START_REF] Bao | Adsorption of CO2 and CH4 on a magnesium-based metal organic framework[END_REF] The ZrO2 and Ni 0 crystallite size on both catalysts before and after DRM reaction, determined by XRD. The particle size of Ni after DRM reaction, determined by TEM.

		ZrO2 crystallite	Ni 0 crystallite sizes
		sizes (nm)	(nm)		particle size of
	Catalyst	initial	8 h	initial	8 h	Ni a (nm)
	NiO-ZrO2	7	7	12	24	15-20
	NiO-10Al2O3-ZrO2	7	7	12	11	10-15
	a: measured on catalyst after DRM reaction for 8 h.

Table 5 -

 5 [START_REF] Z.R. Yazaydın | Screening of metal-organic frameworks for carbon dioxide capture from flue gas using a combined experimental and modeling approach[END_REF] The content of Zr and Ni species on both catalysts before and after reaction at 700 °C for 8 hours, determined by XPS.

				Zr proportion (%)		Ni content (wt %)
		initial		8 h		initial	8 h
	Catalyst	Zr 4+ Zr 3+ Zr 4+ Zr 3+ Zr 2+ Zr 1+ Ni Ni 0 Ni a	Ni 0 b
	NiO-ZrO2	11	89	17 25	23	35 5.6 1.9 -21 % -42 %
	NiO-10Al2O3-ZrO2 14	86	22 34	44	-	5.8 1.0 -3 % +10 %
	a:

Table 5 -

 5 [START_REF] Britt | Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites[END_REF] The deconvolution of the CO2-TPD profiles obtained for the Ni-ZrOx and

	Ni-Mn-ZrOx catalysts
		CO2 desorbed (μmol CO2 /g)	Total basicity
		Weak medium-strength Strong	(μmol CO2/g)
	Ni-ZrOx	21	59	20	73
	Ni-Mn-ZrOx	34	48	10	92

Table 5 -

 5 [START_REF] Furukawa | Ultrahigh porosity in metal-organic frameworks[END_REF] Stability results showing initial activity and after 100 min, 300 min and 480 min tests.

		CH4 Conversion (%)		CO2 Conversion (%)			H2/CO ratio
		initial	100	300	480	initial	100	300	480	initial	100	300	480
	Ni-ZrOx	89	84	86	88	87	90	89	91	0.96	0.95	0.95	0.95
	Ni-Mn-ZrOx	85	86	96	97	85	85	92	94	0.93	0.95	0.96	0.96

Table 5 -

 5 [START_REF] Ko | Enhanced carbon dioxide adsorption on post-synthetically modified metal-organic frameworks[END_REF] The ZrO2 and Ni 0 crystal size of Ni-ZrOx and Ni-Mn-ZrOx catalysts.

			Ni 0 (nm)			C (nm)
			100	300	480	100	300	480
		initial	min	min	min	min	min	min
	Ni-ZrOx	12	22	18	24	-	-
	Ni-Mn-ZrOx	29	18	25	24	-	20	14
	5.4.5.2 X-rays photoelectron spectroscopy (XPS) analysis
	XPS experiment is conducted for both catalysts at different reaction times in order to
	analyze the Ni and Zr at the surface of the catalysts.

Table 5 -

 5 [START_REF] Mcdonald | Capture of carbon dioxide from air and flue gas in the alkylamine-appended metalorganic framework mmen-Mg2 (dobpdc)[END_REF] The percentage of zirconium and nickel in different valence states on Ni-ZrOx and Ni-Mn-ZrOx with time on stream, determined by XPS.

	catalysts	Time	Zr species (%)			Ni species (%)
		(min)	Zr 4+	Zr 3+	Zr 2+	Zr 1+	Ni 0	NiO
	Ni-ZrOx	initial 12	88	-	-	24.4	75.6
		100	10.5	15.9	73.6	-	17.4	82.6
		300	-	9.1	90.9	-	12.9	87.1
		480	17.1	25.4	23.4	34.1	13.3	86.7
	Ni-Mn-ZrOx	initial 15.7	84.3	-	-	20	80
		100	15.3	84.7	-	-	40	60
		300	20	80	-	-	23.2	76.8
		480	22.1	77.9	-	-	22.6	77.4

Discussion on the promotion of Nickel Zirconia based catalysts 6. Discussion on the promotion of Nickel Zirconia based catalysts 6.1 Introduction

	6.2
	Generally, promoters could enhance the activity of nickel-based catalyst via following
	mechanisms: i) promoters could increase the basic site, which could promote the
	adsorption and/or activation of CO2; ii) promoter could enhance the dispersion of nickel
	particles; iii) promoters could increase the reduction of nickel oxide; iiii) promoters
	could promote the formation of oxygen vacancies [65, 96, 135, 137, 146, 230]. Jang et
	al. [122] prepared Ni-Ce0.8Zr0.2O2 catalysts modified by Metal oxide (MgO, CaO, and
	La2O3). The MgO promoted catalyst exhibited the highest activity and stability for
	DRM reaction, because of strong basicity of MgO, small Ni metal crystallite size, and
	an intimate interaction between Ni and MgO. Pompeo [290] investigated the influence
	of supports α-Al2O3, ZrO2 and α-Al2O3-ZrO2 on performance of catalysts. Found that
	Ni/α-Al2O3-ZrO2 showed higher stable for 700 o C DRM reaction, due to the lower coke
	formation on Ni/α-Al2O3-ZrO2 catalyst. Because the ZrO2 could decrease the carbon
	deposition and increase the CO2 adsorption. Świrk et al. [117] reported Ni-based
	double-layered hydroxides modified by Yttrium. 1.5 wt% Y-promoted catalyst
	exhibited the highest activity, selectivity and stability for 700 o C DRM reaction. The
	conversion of CH4 and CO2, and the ratio of H2/CO were 78.6 %, 74.4 % and 0.925,
	respectively. After reaction for 10 h, they were 77.8 %, 73.3 % and 0.92 for the
	conversion of CH4 and CO2, and the ratio of H2/CO , respectively. While for other two
	catalyst, the initial conversions of CH4 on 0.6 wt% Y-promoted and non-promoted
	catalysts were 78.1 % and 75.7 %, respectively. After reaction for 10 h, the conversion
	of CH4 was 74.3 % and 72.1 % on 0.6 wt% Y-promoted and non-promoted catalysts,
	respetively. Because higher specific surface area, smaller Ni crystallite size, and higher
	distribution of weak and medium basic sites formed on 1.5 wt% Y-promoted catalyst,
	as compared to Y-free catalyst. Kumar et al. [149] found that the 5% Ni/Ce0.6Zr0.2O2

The preparation of a series of promoted Ni-Zr catalysts

  Mg and Mn was listed in Table6-1. The obtained suspension was heated from room temperature to 95 °C under vigorous stirring for 48 h. And then this suspension was subsequently aged at 100 °C for 24 h. Next, this suspension was filtered and washed with a little distilled water, and dried at room temperature. Finally, the obtained precursor was calcined at 800 °C for 5 h with an increasing rate of 1 °C/min under airflow. The obtained catalysts were denoted as Ni-Zr, Ni-Zr-Al, Ni-Zr-Mn, Ni-Zr-Y and Ni-Zr-Mg respectively.

	A certain amount of pluronic P123 amphiphilic block copolymer, nickel (II) nitrate
	hexahydrate, zirconium (IV) oxynitrate hydrate, urea, and aluminum isopropoxide (or
	Manganese (II) nitrate tetrahydrate or yttrium (III) nitrate hexahydrate or magnesium
	acetate tetrahydrate) were dissolved in distilled water. The molar ratio of Ni, Al, Y, Zr,

Table 6 -

 6 1 The dosage of Ni, Al, Y, Zr, Mg and Mn on a series of promoted Ni-Zr materials

	Catalyst	Ni % Zr % Al % Mn % Y % Mg %
	Ni-Zr	10	80	-	-	-	-
	Ni-Zr-Al	10	80	10	-	-	-
	Ni-Zr-Mn	10	80	-	10	-	-
	Ni-Zr-Y	10	80	-	-	10	-
	Ni-Zr-Mg	10	80	-	-	-	10

The physicochemical features on a series of promoted Ni-Zr catalysts 6.3.1 The reducibility of a series of promoted Ni-Zr catalysts

Table 6 -

 6 2 The results of BET experiment for Ni-Zr catalysts with different metal promoters.

	Catalyst	Dp nm	Vp cm 3 /g	SBET m 2 /g
	Ni-Zr	1.9 a	0.20 b	113 c
	Ni-Zr-Al	6.1	0.25	82
	Ni-Zr-Mn	1.6	0.12	103
	Ni-Zr-Mg	1.6	0.10	90
	Ni-Zr-Y	1.9	0.09	79
	a: The pore diameter (DP) determined by BJH method
	b: The pore volume (VP) determined by BJH method
	c: The specific surface area (SBET) determined by the BET method

Table 6 -

 6 3 the CO2-TPD data of Ni-Zr catalysts with different metal promoters

				CO2 desorbed (%)			Total
		Peak 1	Peak 2	Peak 3	basicity
		weak		medium-strength		strong	μmol
	Catalysts	Position Content Position Content Position Content	CO2/g
	Ni-Zr	185	15.5	252	64.4	380	20.1	73
	Ni-Zr-Al	149	31.6	222	40.6	341	22.5	102
	Ni-Zr-Mn	141	30.0	228	51.9	389	10.6	92
	Ni-Zr-Mg	165	37.5	256	53.9	370	16.1	69
	Ni-Zr-Y	150	36.9	240	55.4	380	13.0	100

4 XRD analysis of a series of promoted Ni-Zr catalysts

  The XRD patterns of calcined and reduced promoted Ni-Zr based catalysts are shown in Figure6-3. The formation NiO-ZrO2 solid solution on Ni-Zr catalyst results in a shift to higher Bragg angles in ZrO2 peaks, like a peak at 2θ = 30.65 o , which shifts from 2θ = 30.14 o , as is explained in detail in Chapter 4.3.2. This movement is getting smaller on all of the promoted catalyst, especially Al-promoted catalyst. It just shifts by 0.1 o . It means that those promoters decrease the formation of NiO-ZrO2 solid solution. While after reduction, this peak on all the catalysts is located at 30.40 o .

	20	30	40	50	60	70	80

Table 6 -

 6 4 Crystalline sizes of Ni-Zr, Ni-Zr-Al, Ni-Zr-Mn, Ni-Zr-Mg and Ni-Zr-Y catalysts, after reduction at 700 °C for 1 hour and after reaction at 700 °C for 8 hours

Table 6 -

 6 [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF] presents the surface content of element on a series of promoted Ni-Zr catalysts after reduction. For the Ni-Zr catalyst, the content of Zr, Ni and O is about 22.5 %, 5.6 and 71.9 %, respectively. For promoted Ni-Zr catalysts, the content of Zr species decreases slightly. One observation can be note that the surface nickel content on Y and It implies that parts of nickel and promoters exist inside the catalysts, which might form the solid solutions, such as NiO-ZrO2, Y2O3-ZrO2[START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF]. The solid solutions are explained in detail in Chapter 4.3. Meanwhile, the surface content of Mg species is 13.5 %, which is above 10 %. This phenomenon indicates that the most of Mg species form on the surface, resulting in a decrease of surface content of nickel species.

	Mg promoted catalysts decreases to 3.4 % and 5.0 %, respectively, compared to the Ni-
	Zr catalyst. Yao et al. [120] reported that the relatively high content of surface Ni
	species might contribute to the activity of ZrOx/Ni-MnOx/SiO2 catalyst reduced at
	550 °C for DRM. Therefore, the decrease of surface content of nickel species might
	result in a lower catalytic performance for DRM. Especially, Y-modified catalysts
	exhibit the lowest surface content of nickel species, which is reduced by 1.2 %. The
	existence of Mn results in almost no change in the surface content of nickel species. It

just decreases by 0.1 %. It's worth noting that the addition of Al increases slightly the surface content of nickel species, which might enhance the activity for DRM. The dosages of nickel and promoters are 10 % (table 6-1), while the surface content of nickel

The catalytic performance on a series of promoted Ni-Zr catalysts 6.4.1 The activity test on a series of promoted Ni-Zr catalysts in literaturesTable 6 -

 6 [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF] summarized the catalytic performance of ZrO2 supported nickel catalysts modified by various promoters for DRM reaction. Generally, promoters are classified into two types: textural (structural) and chemical (electronic)[START_REF] Fan | Catalytic technology for carbon dioxide reforming of methane to synthesis gas[END_REF]. Textural promoters enhance the textural properties of the catalysts. Mn[START_REF] Wang | Manganese promoting effects on the Co-Ce-Zr-Ox nano catalysts for methane dry reforming with carbon dioxide to hydrogen and carbon monoxide[END_REF] and Cu[START_REF] Dong | Effect of Transition Metals (Cu, Co and Fe) on the Autothermal Reforming of Methane over Ni/Ce0.2Zr0.1Al0.7Oδ Catalyst[END_REF] promoted the dispersion of the nano-sized CoOx crystallites and NiO species, respectively. The conversion of CO2 and CH4 at 700 o C increase from 36 % and 25 % to 74 and 70 %,

Table 6 -

 6 6 the property of the different catalysts at various temperature

	Catalysts	T ( o C) Reduction/ Reaction	Reactant (ml.g -1 .h -1 )	Conversion % CH4 CO2	Ratio H2/CO	Time Lost a Ref. h %•h -1
	8%Ni/Zr		120000	~50		~79	0.93			[118]
	8%Ni/LaZr	800/750	CH4:CO2:H	~52		~78	0.91	28	~1-2
	8%Ni/CeZr		e=3:2:5	~55		~67	0.97
	15%Ni-CeO2 15%Ni-ZrO2 15%Ni-Ce-ZrO2 10%Ni/CeO2 10%Ni/ZrO2	700/800	100800 CH4:CO2:N 2=1:1.04:1	nd nd	>97		0.97	100 95 2	S S 0.042	[297]
	10%Ni/Ce-ZrO2			80					32.5
	Ni/a-Al2O3 Ni/CeO2 Ni/La2O3 Ni/ZrO2	327/550	12320 CH4:CO2=1 :1	8.4 11.7 4.4 10.4		34.7 29.7 24.3 30.6		7	4.08 [140] D D 1.10
	Ni-CaO-ZrO2		96000	~30		~52	~0.53			[298]
	Ni-CaO-ZrO2-NaOH	700/700	CH4:CO2=1	~72		~85	~0.91	>35	S
	Ni/CaO-ZrO2		:1	~60		~73	~0.80
	0.2Ni-0.2CaO-ZrO2			~87.5				S	[224]
	0.2Ni-0.5CaO-ZrO2		12000	~86		80			~0.15
	0.2Ni-ZrO2	700/700	CH4:CO2:N	~85.5	73			~0.29
	0.1Ni-0.2CaO-ZrO2		2=1:1:3	~87		81			~0.14
	0.5Ni-0.2CaO-ZrO2			~86		73			~30
	Ce-Zr-Ni0.49 Ce-Zr-Ni0.49-Co0.29 Ce-Zr-Ni0.49-Fe0.23 Ce-Zr-Ni0.49-Rh0.03	750/750	30000 CH4:CO2:N 2=1:1:8			~80 ~80 ~50 ~90		25 17 2 150	0.8 2.53 29.5 0.13	[299]
			40000							[300]
	NiCeO2ZrO2MgAl2O4	670/670	CH4:CO2=1	~54				5	0.15
			:1
	Ni/Ce0.2Zr0.1Al0.7O			~100		4.8			[294]
	Ni/Fe0.05Ce0.2Zr0.1Al0.65O Ni/Co0.05Ce0.2Zr0.1Al0.65O	750/750		~88 ~90			3.5 3.8
	Ni/Cu0.05Ce0.2Zr0.1Al0.65O			~90			4.8
	10%Ni/ZrO2		240000	34		33			0.08 [226]
	10%Ni/ZrO2-0.4Mg	500/600	CH4:CO2:H e=1:1:2	27		34		5	S
	Ni-Mo/MgAl			~94		~95.5 ~0.95		S	[301]
	Ni-Mo/5%CeZr-MgAl		60000	~96		~96	~0.93		~1.48
	Ni-Mo/10%CeZr-MgAl	800/800	CH4:CO2=1	~97		~96	~0.95	24	~2.10
	Ni-Mo/15%CeZr-MgAl		:1	~96		~94.5 ~0.94		~1.43
	Ni-Mo/20%CeZr-MgAl			~94		~94.5 ~0.95		~0.04

2 The activity test on a series of promoted Ni-Zr catalysts

	20
	550 0	600	650	700	750	800

Table 6 -

 6 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF]. It could be found that the introduction of Mg, Mn and Al promotes, the carbon deposition for Ni-Zr catalysts increases to 9.5 % and 5.3 %, (B), all the DSC curve, all the peaks indicate this is the endothermic reaction, which is corresponded to the removal of coke. One can be observed a slight deactivation (Ni-Zr-Mg) mainly due to carbon deposition (16.3 %) that is the highest content on used catalyst. Generally, large nickel particle size maybe lead to carbon deposition

respectively. While Y-promoted catalyst (3.6 %) exhibits a slightly lower amount of carbon deposition than those obtained by Ni-Zr catalyst (3.7 %). From the Figure

6-8

Table 6 -

 6 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF] The amount of carbon deposition on used catalysts.

	Catalyst	Coke (%)	Particle size (nm) a	IG/ID b
	Ni-Zr	3.7	15-20	1.7
	Ni-Zr-Al	5.3	10-15	1.3
	Ni-Zr-Mn	9.5	15-20	2.0
	Ni-Zr-Mg	16.3	15-17	1.8
	Ni-Zr-Y	3.6	10-15	-
	a: The particle size of nickel species, determined by TEM

Table 6 -

 6 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF]. The crystalline degree of the carbon on non-promoted, and Al, Mn and Mg-promoted catalysts is 1.7, 1.3, 2.0 and 1.8 respectively. It is worthy pointing that the lowest of IG/ID on Al-modified catalyst means more much disorder of carbon. Considering the easy removal of carbon formed on Al-

Table 2 -

 2 1 Possible parallel reactions during dry reforming of methane……………..21 Table 2-2 A brief description and comparative summary for catalysts of dry reforming of methane……………………………………………………………………………27

Table 3 -

 3 [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF] The Peak list for Pattern ZrO2, according to the ICSD standard # 88-

	1007………………………………………………………………………………….49

Table 4 -

 4 [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF] The XRD and H2-TPD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts………………………………………………………………………………65

Table 4 -

 4 2 The BET results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts….[START_REF] Guo | DRIFTS Study on Adsorption and Activation of CH4 and CO2 over Ni/SiO2 Catalyst with Various Ni Particle Sizes[END_REF]

Table 4 -

 4 [START_REF] Abdulrasheed | A review on catalyst development for dry reforming of methane to syngas: Recent advances[END_REF] The XRD and H2-TPD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts………………………………………………………………………………68

Table 4 -

 4 [START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF] The Peak position and deconvolution of the CO2-TPD profiles obtained for the 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts…………………………….[START_REF] Zhang | Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix[END_REF]

Table 4 -

 4 5 the performance of the 10Ni/ZrO2 catalysts at various temperatures for DRM reaction…………………………………………………………………………...…..74

Table 4 -

 4 [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF] The XRD results of 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr and 15Ni-Zr catalysts…81

Table 4 -

 4 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF] The surface content of Zr 3d and Ni 2p over 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts after reaction at 750 °C for 5 hours…………………………..[START_REF] Sajjadi | Sol-gel synthesis of Ni-Co/Al2O3-MgO-ZrO2 nanocatalyst used in hydrogen production via reforming of CH4/CO2 greenhouse gases[END_REF]

Table 4 -

 4 [START_REF] Mumford | Review of solvent based carbondioxide capture technologies[END_REF] The surface content of C 1s and O 1s over 10Ni/ZrO2, 5Ni-Zr, 10Ni-Zr, and 15Ni-Zr catalysts after reaction at 750 °C for 5 hours……………………………….[START_REF] Wang | CO2 reforming with methane over small-sized Ni@SiO2 catalysts with unique features of sintering-free and low carbon[END_REF]

Table 5 -

 5 [START_REF] Liu | Using carbon dioxide as a building block in organic synthesis[END_REF] The results of the BET experiment for NiO-ZrOm and NiO-ZrOm-YOn calcined catalysts……………………………………………………………………..[START_REF] Świrk | Effect of low loading of yttrium on Ni-based layered double hydroxides in CO2 reforming of CH4, Reaction kinetics[END_REF]

Table 5 -

 5 2 The H2 consumption of calcined catalysts determined by H2-TPR. The content of Zr 4+ and Zr 3+ on both catalysts after reduction determined by XPS. And the content of Ni on both catalysts determined by ICP…………………………………..94

Table 5 -

 5 3 Basicity measured by the content of CO2 desorption on CO2-temperature program………………………………………………………………………………96

Table 5 -

 5 [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF] The particle size and carbon deposition on NiO-ZrOm-YOn and NiO-ZrOm catalysts……………………………………………………………………………..100

Table 5 -

 5 [START_REF] Kennedy | Chemical fixation of carbon dioxide methods for recycling CO2 into useful products: MM Halmann[END_REF] The content of CO2 desorption on the CO2-TPD experiment over NiO-MgO-ZrO2 and NiO-ZrO2 catalysts…………………………………………………….....107

Table 5 -

 5 [START_REF] Sumida | Carbon dioxide capture in metal-organic frameworks[END_REF] The specific surface area determined by BET method; the pore volume and the pore diameter determined by BJH method on both catalysts…………………...108

Table 5 -

 5 8 Crystallite sizes of Ni 0 and content of Ni 0 over NiO-MgO-ZrO2 and NiO-

	ZrO2 catalysts……………………………………………………………………….109

Table 5 -

 5 [START_REF] Benson | Carbon dioxide capture and storage: An overview with emphasis on capture and storage in deep geological formations[END_REF] The dosage of Al, Ni and Zr for prepare a series of 10NiO-xAl2O3-(90-

	x)ZrO2………………………………………………………………………………117

Table 5 -

 5 [START_REF] Rochelle | Amine scrubbing for CO2 capture[END_REF] The results of the BET experiment for NiO-ZrO2 and NiO-Al2O3-ZrO2 catalysts, the pore diameter (DP) and the pore volume (VP) determined by BJH method, and the specific surface area (SBET) was determined by the BET method………....[START_REF] Akri | Atomically dispersed nickel as coke-resistant active sites for methane dry reforming[END_REF]

Table 5 -

 5 [START_REF] Cejka | Zeolites and catalysis: synthesis, reactions and applications[END_REF] The consumption of H2 and the proportion of three peaks on the H2-TPR experiment…………………………………………………………………………..[START_REF] Son | High coke-resistance MgAl2O4 islands decorated catalyst with minimizing sintering in carbon dioxide reforming of methane[END_REF]

Table 5 -

 5 [START_REF] Bao | Adsorption of CO2 and CH4 on a magnesium-based metal organic framework[END_REF] The ZrO2 and Ni 0 crystallite sizes on both catalysts before and after DRM reaction, determined by XRD. The particle size of Ni after DRM reaction, determined by TEM……………………………………………………………………………...[START_REF] Wang | Carbon dioxide reforming of methane to synthesis gas over supported rhodium catalysts: the effect of support[END_REF]

Table 5 -

 5 [START_REF] Z.R. Yazaydın | Screening of metal-organic frameworks for carbon dioxide capture from flue gas using a combined experimental and modeling approach[END_REF] The content of Zr and Ni species on both catalysts before and after reaction at 700 °C for 8 hours, determined by XPS……………………………………..…...129

Table 5 -

 5 [START_REF] Britt | Highly efficient separation of carbon dioxide by a metal-organic framework replete with open metal sites[END_REF] The deconvolution of the CO2-TPD profiles obtained for the Ni-ZrOx and Ni-Mn-ZrOx catalysts………………………………………………………………136

Table 5 -

 5 15 Stability results showing initial activity and after 100 min, 300 min and 480 min tests………………………………………………………………………...…...138

Table 5 -

 5 [START_REF] Ko | Enhanced carbon dioxide adsorption on post-synthetically modified metal-organic frameworks[END_REF] The ZrO2 and Ni 0 crystal size of Ni-ZrOx and Ni-Mn-ZrOx catalysts...140

Table 5 -

 5 [START_REF] Mcdonald | Capture of carbon dioxide from air and flue gas in the alkylamine-appended metalorganic framework mmen-Mg2 (dobpdc)[END_REF] The percentage of zirconium and nickel in different valence states on Ni-ZrOx and Ni-Mn-ZrOx with time on stream, determined by XPS……………….…..[START_REF] Zhao | Sc promoted and aerogel confined Ni catalysts for coking-resistant dry reforming of methane[END_REF]

Table 6 -

 6 1 The dosage of Ni, Al, Y, Zr, Mg and Mn on a a series of promoted Ni-Zr materials…………………………………………………………………………….151 Table 6-2 The results of BET experiment for Ni-Zr catalysts with different metal promoters……………………………………………………………………………153

Table 6 -

 6 3 the CO2-TPD data of Ni-Zr catalysts with different metal promoters…....155

Table 6 -

 6 4 Crystalline sizes of Ni-Zr, Ni-Zr-Al, Ni-Zr-Mn, Ni-Zr-Mg and Ni-Zr-Y catalysts, after reduction at 700 °C for 1 hour and after reaction at 700 °C for 8 hours………………………………………………………………………………...157

Table 6 -

 6 [START_REF] Wang | Low-temperature catalytic CO2 dry reforming of methane on Ni-based catalysts: a review[END_REF] The content of element on the surface of non-promoted and promoted catalysts, determined by XPS……………………………………………………….[START_REF] Trovarelli | Catalysis by Ceria and Related Materials[END_REF]

Table 6 -

 6 6 the property of the different catalysts at various temperature……………160Table 6-7 The amount of carbon deposition on used catalysts……………………...169 Table annex 1-2 List symbols and explanations…………………………………….220 Table annex 1-1 List abbreviation and explanations………………………………..210

Acknowledgments

I Acknowledgments I sincerely thank China and the China Scholarship Council for my scholarship in the Sorbonne Universités, France. First and foremost, I would like to thank my supervisors, Prof. Patrick DA COSTA

Summary

The dry reforming of methane has been carried out at 700 o C with a very high GHSV of 48000 h -1 over NiO-MgO-ZrO2 and NiO-ZrO2 catalysts prepared by one-step synthesis. The NiO-MgO-ZrO2 catalyst showed slightly lower activity and selectivity of catalyst, due to the lower surface area, smaller pore volume, smaller pore diameter, and bigger particle size of nickel after reduction. Besides, a lot of carbon deposited on NiO-MgO-ZrO2 catalyst after reaction owing to the formation of a large amount of very weak basic sites. On the other hand, NiO-MgO-ZrO2 catalyst exhibited a higher sintering resistance, due to the strong interaction between Ni and Zr on such catalyst.

The NiO-ZrO2 catalyst exhibited slightly higher activity and selectivity for DRM reaction, however, a severe sintering happened on NiO-ZrO2 catalyst due to the weaker interaction between Ni and Zr. catalyst, which was an indication for a stronger interaction between Ni and support on the promoted catalyst. This stronger metal-support interaction could inhibit the sintering of Ni particles, hence enhancing the stability of the catalyst [START_REF] Corthals | Influence of composition of MgAl2O4 supported NiCeO2ZrO2 catalysts on coke formation and catalyst stability for dry reforming of methane[END_REF]. The total H2 consumption on NiO-10Al2O3-ZrO2 catalyst was about 479 µmol H2/g (Table 5-11), which was higher than the theoretical value (434 µmol H2/g), indicating that this 9 % ((479-433)/479*100 %=9 %) of H2 consumption was assigned to the reduction of Zr 4+ to Zr 3+ , as proved by Zr 3d profiles. While more amount of Zr 4+ was reduced to Zr 3+ on NiO-ZrO2 catalyst (about 25 %), manifesting that the structure of ZrO2 might be more stable by adding the Al into NiO-ZrO2 catalyst. However, three peaks could be distinguished (denominated α, β and γ) on both catalysts. The first peak (α) at 400-500 o C was related to the reduction of NiO species with weak interaction between zirconia, which might be easy to sinter. The second peak (β) at 500-600 o C corresponded to the reduction of NiO species inside mesoporous with strong interaction between zirconium, which could maintain the original state even after reduction and reaction, due to the pore confinement effect. The third peak (γ) at 600-800 o C was assigned to the NiO

The reducibility of NiO-ZrO2 and NiO-10Al2O3-ZrO2 catalysts

Characterization of Ni-Zr catalyst modified by Mn

The reducibility of catalysts

Figure 5-18 presented the H2-TPR results of calcined samples. According to our previous works [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF], the peak at about 400 °C on Ni-ZrOx catalyst was attributed to the reduction of NiO species with weak interaction between Ni and Zr. The main reduction peak (600 °C) was attributed to the reduction of NiO species with strong interaction between Ni and Zr, and the ZrO2 species. The last peak at 850 °C was also assigned to the reduction of ZrO2 species. The introduction of Mn promoter could affect the reduction properties of Ni-Mn-ZrOx catalyst. Whereas, A new weak peak appeared between 200-400 °C, which was assigned to the reduction of MnO2 to Mn2O3 [88,[START_REF] Shi | Effect of a second metal (Y, K, Ca, Mn or Cu) addition on the carbon dioxide reforming of methane over nanostructured palladium catalysts[END_REF].

A wide peak between 400-700 °C came from the reduction of NiO species, Mn2O3 and ZrO2 [START_REF] Wang | Highly Carbon-Resistant Y Doped NiO-ZrOm Catalysts for Dry Reforming of Methane[END_REF]88,[START_REF] Shi | Effect of a second metal (Y, K, Ca, Mn or Cu) addition on the carbon dioxide reforming of methane over nanostructured palladium catalysts[END_REF]. Thus, the reduction peaks shifted to lower temperature on Mnpromoted catalyst. Similar observation was reported [START_REF] Lu | Comparative study on the promotion effect of Mn and Zr on the stability of Ni/SiO2 catalyst for CO2 reforming of methane[END_REF][START_REF] Guo | CO2 reforming of methane over Mn promoted Ni/Al2O3 catalyst treated by N2 glow discharge plasma[END_REF][START_REF] Wang | Manganese promoting effects on the Co-Ce-Zr-Ox nano catalysts for methane dry reforming with carbon dioxide to hydrogen and carbon monoxide[END_REF]. These shifts implied that the interaction between Ni and support on Ni-Mn-ZrOx catalyst was relatively weaker than that on Ni-ZrOx catalyst, which might result in the sintering of nickel under reduction condition. Ni/ZrO2 at 800 °C were about 72 %, 82 %, 45 % and 13 %, respectively. The author found the interaction between the surface oxygen vacancies and nickel species on the Y2O3-ZrO2 support enhanced by the increasing Y2O3 load, which could improve the CH4 and CO2 conversion in the dry reforming reaction. Besides, the surface oxygen vacancies could promote the elimination of coke, because the surface oxygen vacancies could induce oxygen radicals from the CO2 to react with carbon deposition.

Herein, we prepared Ni-Zr materials by promoted metal (Y, Mg, Al and Mn) to apply to the dry reforming of methane. To understand the relationship between the modified structure and the promoter metal, H2-TPR, CO2-TPD, TEM, XPS, Raman and TGA was conducted. Meanwhile, we compared the catalytic performance of catalysts modified by the several metal. Similar results over magnesia-zirconia oxides were reported by Aramendía et al [START_REF] Aramendıa | Magnesium-containing mixed oxides as basic catalysts: base characterization by carbon dioxide TPD-MS and test reactions[END_REF].

Basicity of a series of promoted Ni-Zr catalysts

It is known that the addition of basic promoters favors CO2 adsorption and its dissociation, which contributes to gasify the carbonaceous deposits and to decrease the deactivation by coke formation [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] García | Effect of MgO addition on the basicity of Ni/ZrO2 and on its catalytic activity in carbon dioxide reforming of methane[END_REF]. Therefore, the decrease of basic sites on Ni-Zr-Mg catalyst might cause the carbon deposition.

We had explained in the chapter 4.4.3. The peak was dissociated into three peaks. Peak particle size formed on Ni-ZrOx catalyst. Simultaneously, the Zr species on Ni-ZrOx reduced to Zr 1+ after reaction for 480 min. Whereas, the Zr species did not change on Ni-Mn-ZrOx catalyst. This stable ZrO2 structure on Mn-promoted catalyst might promote the redistribution of nickel and the resistance to the loss of nickel species during the reaction, thereby promoted the activity for DRM.

Perspectives

In producing syngas, which offers environmental benefits, dry reforming of methane (DRM) could promote the installation of the future carbon tax. This reaction has been already extensively studied. Since nickel shows as excellent catalytic activity as noble ones (Pt, Ru, and Rh), Ni-based catalysts have been widely reported on the DRM reaction. While the catalytic performance of Ni-based catalyst is influenced by supports, promoters, catalyst preparation, reaction conditions and so on. Generally, these factors affect the dispersion, basicity, particle size, oxygen storage capacity, interaction between components, porosity and surface area, which are known for playing a significant role in the catalytic performance for dry reforming of methane. Based on this doctoral work, there are some problems that still need to be addressed in the process of industrialization. As a result, a few promising perspectives are listed as follows  Enhancement of interaction between the active metal and support from the formation of a solid solution eg. NiO-ZrO2, NiO-MgO. Some solid solution components help to incorporate the active metals into supports or promoters, resulting in the high dispersion and small size of active metals and further improving the catalytic performance of catalysts.

 Investigation of the detailed kinetic and mechanistic study. The study of the reaction mechanism plays an important guiding role in the development and optimization of the catalyst for dry reforming of methane. The conversion of CO2 X CH [START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF] The conversion of CH4

Annexes

Annex. 1 Abbreviation and symbols