Mechanisms Of Rare Earth Oxides Affecting The Performance Of Ni-based Catalysts For Methane Carbon Dioxide Reforming

The coking and Ni sintering are the bottlenecks for the industrialization of the carbon dioxide reforming of methane(CDR)over Ni-based catalysts.Thus,in this thesis,we focus on the development of the high-performance Ni-based catalyst for the pressurized CDR by introducing different rare earth metal oxides as promoters and regulating the porous properties of the catalyst,in which the Ni size and its distribution,the interactions between Ni and the support,and the catalyst structure can be optimized.The specific research content and main conclusions are summarized as follows.1.The Gibbs free energy minimization method was used to analyze the equilibrium of the pressurized CDR reaction.The effect of temperature,pressure,CH4/CO2 molar ratio,and the dilution with the inert gas on the equilibrium composition and the conversion of methane were quantitatively calculated for CDR either with carbon or without carbon in the reaction system.2.From the perspective of regulating metal Ni particle size,distribution and metal-support interaction,the Ni-SiO2 and Ni-RExOy-SiO2(RE=La,Sm,Pr,Nd,Ce)catalysts were prepared via the combustion-decomposition method by using nickel nitrate and ethyl orthosilicate as precursors of Ni and SiO2,respectively.The Ni-SiO2 and Ni-RExOy-SiO2 catalysts were comparatively investigated for the pressurized CDR under the conditions of P=1.0 MPa,T=750?,CH4/CO2=1.0,and GHSV=53,200 mL g-1 h-1.Results indicate that the initial CH4 conversion of all the tested catalysts was approaching the equilibrium value at the reaction conditions,slightly increasing in order of Ni-7CeO2-SiO2<Ni-SiO2?Ni-7Nd2O3-SiO2<Ni-7Pr6O11-SiO2<Ni-7La2O3-SiO2<Ni-7Sm2O3-SiO2.However,the stability of all the tested catalysts is very different,following the order of Ni-7La2O3-SiO2<Ni-SiO2<Ni-7Nd2O3-SiO2<Ni-7Pr6011-SiO2<Ni-7Sm2O3-SiO2<Ni-7CeO2-SiO2.Moreover,the CH4 conversion of Ni-7CeO2-SiO2 in 1 h and 50 h is 46%and 42.9%,respectively,indicating better stability.The calcined,reduced,and/or spent catalysts were comprehensively characterized by the characterization techniques.Results show the main factors determining the optimal stability of Ni-7CeO2-SiO2 were smaller size of Ni particles(5.0 nm)and the moderate Ni-support interaction.The increased graphitization of the deposited coke and sintering of Ni were the main reasons for the deactivation of Ni-7CeO2-SiO2 during the 100 h stability test and regeneration experiment.3.A series of Ni-OMA and Ni-yCe0.5Zr0.5O2-OMA catalysts with ordered mesoporous structure were synthesized via one-pot EISA method.The Ni-OMA and Ni-yCe0.5Zr0.5O2-OMA catalysts was comparatively evaluated for the CDR under the conditions of P=0.1?1.0 MPa,T=750?,CH4/CO2=1.0,GHSV=60,000?100,000 mL g-1 h-1.Results indicate the catalytic performance of Ni-yCe0.5Zr0.5O2-OMA is better than that of Ni-OMA under atmospheric pressure.Meanwhile,Ni-9Ce0.5Zr0.5O2-OMA exhibited the best catalytic activity and stability under both atmospheric pressure and high pressure(P=1.0 MPa).The best catalytic performances were closely related to the oxygen storage capacity and excellent thermal stability of Ce0.5Zr0.5O2 solid solution,which could effectively inhibit the carbon deposition and sintering of Ni.