Preparation Of CeO₂ Supported Ni-based Catalyst And Its Application In Dry Reforming Of Methane

In recent years,large amount of CO₂ emission have caused global warming,while the clean and efficient utilization of natural gas is still an urgent problem to be solved.Dry reforming of methane（DRM）consumes CO₂ and CH₄simultaneously to produce H₂ and CO mixture with high added value,providing a feasible way to reduce CO₂ emission and achieve the rational utilization of CH₄.Nickel based catalysts were widely used in DRM reactions due to their low cost and high catalytic activity.However,Ni based catalysts tend to be inactivated due to carbon deposition and metal sintering,so it is very urgent to develop Ni based catalysts with coke resistance,sinter resistance and high catalytic activity for DRM reactions at high temperature.In this paper,CeO₂ was used as the support and the Ni/CeO₂ catalyst was prepared by polyol-mediated reductive growth method（polyol method）and applied to DRM reaction.The effects of preparation method,oxygen vacancy content of support and Ni-Co bimetallic system on the catalytic performance of Ni/CeO₂ catalyst were investigated.XRD,Raman,H₂-TPR,CO₂-TPD,XPS,N₂-adsorption and desorption were used to characterize the catalysts.The specific research contents are as follows:（1）The Ni/CeO₂ catalyst was prepared by polyol method,and the influence of the preparation method on the catalytic activity of the catalyst for DRM reaction was studied.The results showed that the polyol method made the Ni particles with smaller particle sizes highly disperse on the Ni/CeO₂ catalyst.While the enhanced interaction between the Ni species and CeO₂ support was more conducive to the reduction of the Ni species and the exposure of the alkaline sites.According to the activity test results,the optimal Ni loading of the catalyst was10 wt%and the optimal gas hourly space velocity（GHSV）was 3.0×10⁴m L·g^-1·h^-1.In the range of 700～900℃,the CO₂ conversion,CH₄ conversion and R_H2/CO of Ni/CeO₂ catalyst prepared by polyol method were all higher than that of catalyst prepared by impregnation method.Although the catalysts were deactivated in the stability experiments,the R_H2/CO of Ni/CeO₂ prepared by the polyol method was maintained at 0.81,showing a higher selectivity for H₂.（2）Ni/CeO₂ catalysts with different oxygen vacancy contents were prepared by polyol method,and the role of oxygen vacancy contents in the DRM reaction was studied.After reaction at 800℃for 50 h,the CO₂ and CH₄ conversion of Ni/CeO₂-rod with maximum oxygen vacancy content were 89%and 50%,respectively,and R_H2/CO was 0.92.The results showed that the catalytic stability of Ni/CeO₂-rod was attributed to the high coke elimination rate caused by abundant oxygen vacancy and the high Ni dispersion maintained by the strong metal-support interaction.However,the Ni/CeO₂-cube and Ni/CeO₂-bulk catalysts showed severe inactivation after 30 h reaction.The main factor for inactivation of Ni/CeO₂-cube with moderate oxygen vacancy content is agglomeration of Ni particles rather than carbon deposition.The inactivation of Ni/CeO₂-bulk with the lowest oxygen vacancy content was attributed to its highest carbon content and the highest degree of graphitization,indicating that the low oxygen vacancy content limited its ability to eliminate carbon deposition.（3）The Ni-Co bimetal catalyst supported by CeO₂ was prepared by polyol method and its catalytic DRM reaction was studied.The results showed that the amount of strong alkaline sites exposed by catalyst 5Ni-5Co/CeO₂-rod was more dominant,and the smaller size of Ni-Co alloy particles was conducive to the improvement of metal dispersion and the enhancement of metal-support interaction.At 700～900℃,the CO₂ conversion of 5Ni-5Co/CeO₂-rod increased from 92%to 100%,the CH₄ conversion increased from 50%to 71%,and the conversion rate of R_H2/CO was about 0.94.Compared with Ni and Co single metal catalysts,5Ni-5Co/CeO₂-rod showed the better catalytic stability and faster elimination rate of activated carbon species while maintaining higher catalytic stability,which reflected the synergistic effect of Ni-Co bimetals in DRM reactions.