The Influence Of Surface Active Oxygen On The Catalytic Performance Of Dry Reforming Of Methane Over Ni/Ce_xZr_1-xO₂ Catalyst

Dry reforming of methane（DRM）has attracted much attention because it can utilize two greenhouse gases of CO₂ and CH₄ simultaneously to produce the high value-added syngas（H₂ and CO）.Ni-based catalysts are widely used in dry reforming of methane due to their low cost and excellent catalytic performance.However,carbon deposition（coke）and sintering have always been the main problems.It was reported that the active oxygen species of cerium-zirconium support can eliminate the coke by oxidizing the deposited carbon to CO.Besides,the strong interaction between cerium-zirconium support and Ni can inhibit the sintering of Ni particles at high temperature,both of the two advantages can improve the catalytic performance of Ni-based catalysts.However,it is still unclear which type of surface active oxygen species is more favorable for carbon removal and whether there is any disadvantage of surface active oxygen on the catalytic performance of dry reforming of methane reaction.In this project,Ce_xZr_1-xO₂ supports with different types and concentrations of surface active oxygen species were prepared by changing the cerium/zirconium ratios,and a series of Ni/Ce_xZr_1-xO₂ catalysts were synthesized by strong electrostatic adsorption method.The effect of different surface active oxygen species on the catalytic performance and reaction mechanism for dry reforming of methane was investigated.The detailed research contents are as follows:1.Ce_xZr₁-_xO₂ support with different surface active oxygen was prepared by co-precipitation method.The H₂-TPR results show that the surface active oxygen content is the highest when Ce:Zr=5:5,and the superficial surface active oxygen reach the highest at Ce:Zr=7:3.Then different amount of Ni was loaded on Ce_xZr_1-xO₂ support by strong electrostatic adsorption method.With the combination of XRD,HAADF-STEM and XPS,it is found that Ni atoms were highly dispersed in the form of Ni²⁺on the surface of Ni/Ce_xZr_1-xO₂catalyst.2.The catalytic performance of Ni/Ce_xZr_1-xO₂ catalysts for dry reforming of methane was investigated from 400 to 600℃at the atmosphere of[CH₄]=[CO₂]=1%.The results show that Ni/Ce_0.7Zr_0.3O₂ exhibited higher catalytic activity,as Ce_0.7Zr_0.3O₂ support has the highest content of superficial surface active oxygen,which was more favorable to the elimination of carbon deposition during reaction.Kinetic studies show that the DRM catalytic activity was determined by the content of superficial surface active oxygen.The reaction order tests show that the decomposition of CH₄ was the rate-determining step during DRM reaction,indicating that the superficial surface active oxygen provides clean Ni sites for CH₄decomposition,which therefore can controll the overall catalytic activity.When the0.5 wt.%loading group catalysts were evaluated at 600℃in 20%reactants（[CH₄]=[CO₂]=20%）for long-term test,it was found that 0.5Ni/Ce O₂ exhibited better catalytic performance than 0.5Ni/Ce_0.7Zr_0.3O₂ for dry reforming of methane,but0.5Ni/Ce O₂produced more graphitic carbon.However,when the Ni loading amount increased to 1.0 wt.%,1.0Ni/Ce O₂ showed the same catalytic performance as1.0Ni/Ce_0.7Zr_0.3O₂,but 1.0Ni/Ce O₂ catalyst still has more graphitic carbon than1.0Ni/Ce_0.7Zr_0.3O₂ sample,indicating that the amount of graphitic carbon would not be proportional to the catalytic activity of the catalyst,while the formation of graphitic carbon has an indispensable relation to the size and chemical states of Ni particles.3.In order to explore the active sites of Ni/Ce_xZr₁-_xO₂ catalyst,the structure of post-reaction catalysts after different reaction conditions was characterized.The combined characterization of HAADF-STEM and EDS demonstrated the existence of large Ni particles on the surface of post-reaction catalystswith size ranging from 2 to12 nm.Moreover,Ni species on the reaction-spent 0.5Ni/Ce_0.7Zr_0.3O₂ samples with inferior activity has higher dispersion,indicating that the highly dispersed Ni species produces less graphitic carbon,it was not favorable to the dry reforming of methane.In situ DRIFTS studies under dry reforming of methane conditions show that Ni particles mainly exist in the form of metallic Ni⁰,while the interfacial Ni atoms linking the metallic NPs and Ce_xZr_1-xO₂ support exist as oxidized Ni⁺,they work collectively to finish the whole DRM reaction.That is,CH₄ decompose to C and H₂on the surface of metallic Ni⁰ NPs,the generated carbon would be removed by the superficial surface active oxygen（C+[O]=CO）,CO₂ would subsequently adsorb on the interfacial Ni⁺atoms and the leftover oxygen vacancies for activation and the production of new CO molecules.However,the carbon migration on small Ni NPs,for instance the Ni clusters smaller than 2 nm,would be much more difficult,and the cluster tends to be covered easily by the deposited carbon and eventually deactivated.That’s the reason that 0.5Ni/Ce_0.7Zr_0.3O₂has a higher dispersion and produces less graphitic carbon,but possesses inferior DRM catalytic performance.