Preparation Of Ni Catalysts Supported On Different Rare Earth Metal Oxides For Methane Dry Reforming To Produce Hydrogen

The use of fossil fuels produces a large amount of CO₂,which causes global greenhouse effect and triggers a series of environmental problems such as glacier melting,sea level rise and extreme climate.Methane dry reforming（DRM）can efficiently convert two greenhouse gases,CH₄ and CO₂,into syngas having a H₂/CO ratio around 1:1,which can be directly used as raw materials for Fischer-Tropsch reaction.Therefore,DRM reaction can not only alleviate energy crisis,but also help to improve the living environment of human being.As a transition metal,Ni has been widely used in DRM reaction due to its high initial activity,low price and abundant resources.However,its easy coking and Ni grain aggregation at high temperature during the reaction can lead to rapid decline of catalytic performance and even deactivation.Therefore,the preparation of Ni-based catalysts with good performance and excellent coking-resistance is critical to accomplish large-scale industrialization of DRM reaction.To solve these problems,different rare earth metal oxide supports(La₂O₃,CeO₂,Pr₆O₁₁,Sm₂O₃ and Y₂O₃)have been fabricated to load Ni to prepare catalysts for DRM reaction.Moreover,the effect of phase structures,surface oxygen species and alkaline sites of the rare earth oxide supports on the reaction performance have been explored.Part Ⅰ:By using a glycine combustion method,rare earth oxide supports,La₂O₃,CeO₂,Pr₆O₁₁,Sm₂O₃,Y₂O₃,were prepared.With a wet impregnation method,2%（wt）Ni catalysts were prepared on the rare earth oxide supports for DRM reaction.Low Ni loading can usually make Ni disperse more evenly on the surfaces of the supports,thus comparing better the interaction between Ni and the supports,and understanding deeper the reasons leading to the reaction performance difference.XRD and N₂-BET results show that Y₂O₃ support has the largest specific surface area,and the smallest Ni grain size.H₂-TPR and XPS results show that among all the catalysts,both 2Ni/Y₂O₃and 2Ni/Sm₂O₃ have relatively stronger Ni-support interaction than other catalysts,thus having relatively higher initial activity.O₂-TPD,XPS O1s and CO₂-TPD results have confirmed that 2Ni/Y₂O₃ catalyst possesses the most abundant surface active oxygen species and alkaline sites,thus it displays the best reaction stability and coking-resistance.Part Ⅱ:In Part I,it was found that 2Ni/Y₂O₃ catalyst shows the best DRM reaction performance,but its stability and coking-resistance should be further improved.Therefore,in this part,Ni loaded on Ce-,Zr-and Al-modified Y₂O₃ were prepared by coprecipitation method for DRM reaction.XRD and Raman results show that the three kinds of cations can be doped into the lattice of cubic Y₂O₃ to form solid solutions,thus reducing the crystallinity and increasing the specific surface area of the supports.H₂-TPR results testify that the Ni-support interaction for the modified catalyst is stronger than that on the unmodified 5Ni/Y₂O₃ catalyst.XRD,TEM and H₂-TPD results demonstrate that for the modified catalysts,Ni dispersion increases,hence the reactivity and stability of the catalysts are improved.O₂-TPD and XPS O1s results have confirmed that the modified catalysts possess more abundant active surface oxygen species,such asO₂^{δ-（0<δ<1）}andO₂^-,hence the coking-resistance is significantly improved.In situ DRIFTS results have confirmed that the addition of the secondary metals can improve the DRM activity of Ni/Y₂O₃ catalysts by accelerating the conversion of formate intermediate species.Part Ⅲ:Three kinds of rare earth Y₂O₃ supports with different morphologies and structures were prepared by precipitation,glycine combustion and colloidal template methods,and then loaded 5%（wt）Ni for DRM reaction.SEM results show that Y₂O₃-3DOM with a three-dimensional ordered macrosporous structure has been successfully prepared by the colloidal template method.XRD and N₂-BET results prove that the Y₂O₃-3DOM support has the smallest grain size and the largest specific surface area,thus it can get the highest Ni dispersion（H₂-TPD）.H₂-TPR and XPS results testify that5Ni/Y₂O₃-3DOM catalyst has the strongest Ni-support interaction,which can better anchor Ni grains on its surface.Therefore,this catalyst shows the highest activity and stability in the 50 hours’DRM stability test.CO₂-TPD andO₂-TPD results have confirmed that 5Ni/Y₂O₃-3DOM,the 3DOM catalyst prepared by colloidal template method,has the best ability to adsorb and activate CO₂ molecules in all the catalysts,and possesses the largest amount of active surface oxygen species,thus it has the best coking-resistance ability.