Preparation And Catalytic Performance Of Nickel-based Zirconia Catalysts For Carbon Dioxide Methanation

Climate change and energy crisis are two major challenges facing the sustainable development of contemporary human society.The extensive combustion of carbon-rich fossil fuels has increased atmospheric CO₂concentration dramatically,which leads to a series of environmental problems.So conversion of CO₂ as carbon resource has become a hot research topic.CO₂methanation technology not only reduce the concentration of carbon dioxide in the atmosphere,carbon may also be implemented recycling of resources,and is considered to be an effective means of CO₂ chemical utilization.Catalyst is the core of this technology research.Compared with noble metal-based?Rh,Ru and Pd?catalysts,Ni-based catalysts are widely used due to its low cost,pretty excellent catalytic activity and CH₄ selectivity.However,the strong exothermic reaction in the process of CO₂ methanation could cause the sintering of catalyst,resulting in poor catalytic stability.Therefore,the key to CO₂ methanation is to develop a novel Ni-based catalyst with high activity and improved stability.In this paper,from the viewpoint of improving the low-temperature catalytic activity and stability,Ni/ZrO₂ catalysts with different morphologies and structures were prepared and their catalytic performance in CO₂methanation was studied.The main findings are as follows:?1?Zirconia nano-supports with different morphologies was prepared by hydrothermal method using zirconium nitrate and ammonium fluoride?NH₄F?as Zr precursor and structure-directing agent,respectively.The amount of NH₄F was found have a great influence on the morphology of zirconia,and ZrO₂-1.5nanosheets was synthesized by appropriate amount of NH₄F?1.5 is the molar ratio of NH₄F/Zr?.The nanosheet morphology was formed by the ordered accumulation of a large number of nanoparticles with a thickness of 20-40 nm and a specific surface area of 17.72 m²·g^-1.?2?Compared with the traditional ZrO₂ nanoparticulates supported Ni catalyst?Ni/ZrO₂-0?,the nanosheet structure of ZrO₂-1.5 can promote the dispersity of Ni metal,reduce Ni crystalline size and enhance the metal-support interaction.Thus,Ni/ZrO₂-1.5 exhibited higher low-temperature activity:The CO₂ conversion and CH₄ selectivity were 94.3%and 95.4%at 360°C,respectively.?3?Based on the conventional impregnation method,Ni-En/ZrO₂-1.5catalyst was synthesized by complexation reaction of ethylenediamine and nickel nitrate precursor.The optimum En dosage was 15 wt.%.Compared with Ni/ZrO₂-1.5 catalyst,Ni-En/ZrO₂-1.5 exhibits higher Ni dispersion and smaller Ni crystalline size,which is beneficial to expose more active sites and thus promote hydrogen adsorption and dissociation.Moreover,more surface strong basic sites promote CO₂ adsorption and accelerate the progress of the CO₂methanation reaction.In addition,the presence of oxygen vacancies on the ZrO₂surface facilitates the activation of CO₂.?4?A group of Ni-Fe-Zr ternary ordered mesoporous catalysts with varied Fe content were synthesized by one-pot solvent evaporation-induced self-assembly?EISA?method using zirconium n-butoxide and triblock copolymer F127 as Zr precursor and template,respectively.The catalyst samples all have a large specific surface area(¹20-150 m²·g^-1),uniformly ordered mesopores,and highly dispersed Ni.?5?OMNixZrFe catalysts were carried out CO₂ methanation performance evaluation.The results showed that the addition of Fe electron promoter could improve the reducibility of Ni and increase the number of active sites,thereby increasing H₂ adsorption capacity.OMNi3ZrFe?Zr/Fe molar ratio is 3?exhibited the best CO₂ methanation activity,with CO₂ conversion and CH₄selectivity are 96.2%and 94.5%respectively at 360°C.In addition,the excellent?confined effect?of the optimized ordered mesoporous structure could restain Ni agglomeration and sintering,OMNi3ZrFe catalyst can maintain high stability in 80 h long-term stability test at 450°C.