Design And Preparation Of Nickel Based Catalyst For Carbon Dioxide Reforming Of Methane To Synthesis Gas

With the increase of population and the rapid consumption of fossil fuels,human beings are facing a serious energy shortage crisis.In addition,global warming caused by the greenhouse effect has become one of the most pressing environmental problems in recent years.In order to achieve sustainable development,it is necessary to explore new technologies to tackle the problems of energy shortage and environmental pollution.Carbon dioxide reforming of methane,also known as dry reforming of methane,is considered a feasible solution to the above problems.It can convert the greenhouse gases(CH4 and CO2)into synthesis gas(H2 and CO)for the production of clean energy.Compared with the industrialized steam reforming of methane to syngas technology,this technology utilizes CO2 instead of steam as the feedstock,and obtains synthesis gas with a relatively lower H2/CO ratio.This change can reduce process costs and optimize downstream production steps.In addition,this technology can effectively reduce the emissions of two greenhouse gases,contributing to environmental protection.It is also of great significance to the realization of "carbon neutrality".However,the industrial application of this technology is still limited by catalysts.Ni-based catalysts are considered to be the most promising reforming catalysts.They have high catalytic activity in CO2 reforming of methane,but easily deactivated rapidly due to carbon deposition.Therefore,how to improve the carbon deposition resistance of the Ni-based catalysts has been the focus of research.In recent years,with the deepening of the research on the structure of Ni-based catalysts,scientists have found that some key structural parameters,such as the size of Ni particles,surface basic sites and the redox characteristics of the catalysts,have important effects on the performance of Ni-based catalysts.It gives us a direction for further improvement.In this thesis,the key structures of Ni-based catalysts are designed and studied.On one hand,in order to control the size of Ni particles,two kinds of highly dispersed catalysts with ultra-small Ni particles were prepared by "cation-anion double hydrolysis(CADH)method" and "in-situ growth reduction method".On the other hand,from the research direction of "structure-activity relationship",three typical conventional methods,subsequent coating,co-impregnation and sequential impregnation,were employed to investigate the effect of introduction methodologies for CeO2-promoted Ni/SiO2 catalysts.A variety of characterization methods are used to analyze the structure,physicochemical properties and reaction mechanism of the catalyst.The main contents are as follows:1.NaAlO2,Ni(NO3)2·6H2O and Mg(NO3)2·6H2O are the precursors of Al,Ni and Mg,respectively.Through the hydrolysis of anionic AlO2-and cationic Ni2+,Mg2+,the Ni/Mg-Al-O catalyst was successfully prepared and used for CO2 reforming of methane reaction.In comparison with the Ni/MgO-Al2O3 catalyst prepared by the typical co-impregnation method,the Ni/Mg-Al-O catalyst has higher activity and better long-term stability.Structural characterizations revealed that the average nanoparticle size of Ni on the Ni/Mg-Al-O catalyst is only 2.1 nm,which has higher metal dispersion and intrinsic activity than the Ni/MgO-Al2O3 catalyst,thereby increasing its initial activity.The ultra-small nickel nanoparticles and abundant alkaline sites of the Ni/Mg-Al-O catalyst further improve the anti-coke property,which was improving the stability of the catalyst.In addition,in-situ DRIFTS characterization was used to investigate the reaction mechanism of the catalysts.2.Two-dimensional(2D)transition metal hydroxide(TMH)nanosheets are only a few nanometers thick and easily decompose into corresponding metal oxides under high temperature conditions.Since metal particles play an active role in the reforming reaction,it can take advantage of its high-temperature decomposition characteristics to grow Ni(OH)2 in-situ on the y-Al2O3 nanoplate support and heat it to 500? to in-situ reduction by hydrogen.Then a high dispersion Ni-based catalyst alumina nanoplate(labeled as Ni@?-Al2O3 nanoplate)catalyst with ultra-small Ni particle size can be obtained.Compared with the Ni/y-Al2O3 nanoplate catalyst prepared by the impregnation method,the dispersion of the Ni@y-Al2O3 nanoplate catalyst is increased by 1.6 times,and the intrinsic activity(TOF at 700 ?)is improved 2.1 times.The carbon deposition of the two catalysts was quantitatively analyzed by TG characterization,and the results showed that the carbon deposition of the Ni@y-Al2O3 nanoplate catalyst was significantly reduced.Therefore,controlling the size of the nickel particle size is an effective way to improve catalyst activity and resistance to carbon deposition.3.CeO2 is widely used in the CO2 reforming of methane reaction because it has good oxygen migration ability,which is helpful to gasification of carbon deposition and thus improves the stability of catalysts.This study is devoted to investigate the effect of different introduction methods of CeO2 as a promoter on the performance of the catalysts.Therefore,SiO2 microspheres are selected as the model support to reduce the influence of the support.The typical introduction methods of CeO2 additives are subsequent coating,co-impregnation and sequential impregnation.The catalysts prepared by the three methods are labeled as CeO2@Ni/SiO2,Ni-CeO2/SiO2 and CeO2-Ni/SiO2,respectively.The initial activity of the three catalysts in the CO2 reforming of methane at 600-750? was investigated.The results show that the performance of CeO2@Ni/SiO2 and Ni-CeO2/SiO2 catalyst are significantly better than that of the CeO2-Ni/SiO2 catalyst.In the long-term stability tests,the CeO2@Ni/SiO2 catalyst is more stable than the Ni-CeO2/SiO2 catalyst.The characterization studies found that the CeO2@Ni/SiO2 catalyst has a smaller Ni particle size,which helps to improve the initial activity of the catalyst.In addition,CeO2@Ni/SiO2 catalyst has superior coke resistance due to its significant redox characteristics and strong surface basicity,which improves the stability of the catalyst.