Synthesis and Characterization of Ni-based Catalysts and Applications in Dry Reforming of Methane

Author:Sheng Ke Fa

Supervisor:ge jian ping

Database:Doctor

Degree Year:2019

Download:35

Pages:140

Size:8887K

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Energy is not only the basic support for a country’s economic construction and social development,but also an important guarantee for the sustainable development and reproduction of human society.The centuries-old prosperity of human society has benefited from the widespread use of coal,oil,natural gas and other energy sources,and at the same time it has caused irreversible damage to the environment on which humans depend.However,with the continued use of non-renewable resources in social development,resource depletion is not alarmist.The carbon dioxide reforming methane(DRM)reaction is one of the hot topics in this view.This is mainly because the reaction directly uses the greenhouse gases CH4 and CO2 as raw materials,and simultaneously produces a high H2/CO ratio syngas,not only in terms of environmental protection,it is also of great significance in terms of resource utilization and economic development.However,the DRM reaction is a highly endothermic process accompanied by a large number of side reactions.Although substrate molecules can be efficiently converted by metal catalysts(Ru,Rh,Pt,Pd,Ir,Ni,Co,etc.),the catalyst will be sintering and carbon deposition inevitably during the reaction,especially the Ni-based catalyst which has been extensively studied.In order to improve the practical application effect of Ni-based catalysis in DRM reaction,it is necessary to synthesize a more efficient Ni-based catalyst and study the corresponding DRM catalytic reaction mechanism.In this paper,the following work was carried out in three aspects:the application of Nix Coy alloy catalyst in DRM reaction and the catalytic mechanism,the influence of different loading methods on Ni-based catalysts,and the influence of metal-support interaction on catalytic activity.1.NixCoy/H-ZrO2 catalyst composed of mesoporous ZrO2 hollow spheres and highly dispersed NixCoy nanoparticles was synthesized by template method and impregnation process.Based on the relevant research results,a synergistic reaction mechanism was proposed to explain the reason why the Ni0.8Co0.2/H-ZrO2 catalyst has better catalytic performance.In the DRM reaction,Ni and Co are the catalytic active centers of CH4cracking and CO2 reduction,respectively.The residual carbon species on the surface of metal Ni can quickly combine with the active oxygen species on the surface of metal Co to ensure the regeneration of the metal surface,thus continuing the follow-up reaction.The reason why the Ni0.8Co0.2/H-ZrO2 catalyst has the highest activity and stability is the strong metal-support interaction and the efficient mass transfer efficiency of the reactants and substrate molecules.More importantly,the proper Ni/Co ratio balances the rate of CH4 cracking and CO2 reduction,avoiding a large accumulation of carbon species on the surface of the alloy or excessive oxidation of the catalyst,ensuring efficient and stable conversion of the DRM reaction.2.Ni-ACS-Dop,Ni-ACS-Iex and Ni-ACS-Im nanocatalysts were prepared by doping,ion exchange and impregnation methods in different stages of ACS nanosheet synthesis.Although Ni is loaded in different ways,three Ni-ACS catalysts have similar chemical compositions and microstructures.Ni nanoparticles are highly dispersed on ultra-thin ACS nanosheets with large specific surface area and good thermal stability.However,the electronic states of three catalysts are significantly different.Ni-ACS-Iex catalyst with"surface Ni"as the main form shows the highest catalytic activity in DRM reaction due to the high electron density on the surface of metal Ni.The Ni-ACS-Dop catalyst with"lattice Ni"as the main form exhibites electron-deficient properties and lowest catalytic activity.Different from the construction of nanostructures or the introduction of chemical components in the catalyst system,this work controls the chemical environment of the metal precursor by different loading methods,and obtains a metal catalyst with adjustable surface electronic states,which provides a new strategy to synthesize highly active catalyst.3.Strong metal-carrier interaction(SMSI)is considered to be one of the most important concepts in heterogeneous catalysts,which is important for regulating the catalytic performance of the catalyst and controlling the microstructure of the supported metal.Here,we synthesized a series of Ni-Al2O3 catalysts with adjustable metal-support interaction by different calcination temperature.The related research results confirmed that different strength interaction was formed between the Ni NPs and Al2O3matrix.As the calcination temperature gradually increased from 500,700 to 900°C,the catalyst component gradually changed from Ni and Al2O3 to nickel-aluminum spinel structure andγ-Al2O3,resulting in a sharp decrease in the number of active sites of the catalyst.The catalytic results show that the calcination temperature is too high or too low to be detrimental to the activity and stability of the catalyst.Only when the calcination temperature is 700°C,the Ni-Al2O3 catalys with medium-strength metal-support interactiont can enhance the activity and stability of the catalyst.