Comparative Density Functional Theory Study Of Mechanism Of C-H/C=O Activation On Rh Modified Ni-based Catalyst In The CO₂/CH₄ Reforming

Due to the influence of the greenhouse effect,various conversion and utilization technologies of greenhouse gas CO₂ have been studied by many scholars in recent years.Dry reforming of methane（DRM）can not only utilize CH₄ and CO₂ at the same time but also produce syngas（H₂:CO volume ratio≤1）,which is one of the main technical routes of the natural gas chemical industry.Non-noble metal Ni catalyst is suitable for DRM reaction because of its good catalytic activity.However,the main factor affecting its industrial application is that it is easy to produce a lot of carbon deposition and sintering in the reaction.Therefore,in this paper,the performance and stability of the catalyst were further strengthened and improved by adding noble metal Rh,and the catalyst surface model configuration with good catalytic activity and anti-carbon deposition performance was obtained.Based on density functional theory（DFT）,the activation mechanism of the C-H/C=O bond in CO₂/CH₄ reforming over Rh doped Ni-based catalysts was studied in detail.The direct dissociation of CH₄ and the dissociation assisted by O*over pure Ni catalyst and Rh-Ni bimetallic catalysts were systematically studied.Firstly,the adsorption mechanism of intermediates on the catalysts were studied.Then,the mechanism and reasons of Rh modified Ni catalyst in improving catalytic activity and anti-carbon deposition ability under different ratios and doping methods were explored.According to the calculation results,the correlation between the charge transfer and the activation energy of the reactions were determined.The main achievements are as follows:（1）The activation mechanism of C-H bond in CO₂/CH₄ reforming reaction on Ni（111）,Rh/Ni（111）and Rh₆Ni（111）surfaces were revealed,the adsorption mechanism of intermediate products was determined,and the factors affecting the catalytic activity of the catalysts were elucidated.The order of adsorption strength of Ni（111）>Rh/Ni（111）>Rh₆Ni（111）was obtained.It was confirmed that Rh doping can reduce the absolute value of adsorption energy of CH_x*on Ni-based catalyst surface,indicating that RH doping can promote the desorption of CH_x*on Ni-based catalyst surface.It is found that the first and fourth steps of dehydrogenation are the key steps to affect the reaction rate.Ni（111）is the most favorable surface for the dehydrogenation of CH₄*and Rh₆Ni（111）is the most favorable surface for the（2）dehydrogenation of CH₃*and CH₂*and the inhibition of the dissociation of CH*.Therefore,appropriate Rh doping can promote the activation of C-H,enhance the catalytic activity of the catalyst and inhibit the formation of carbon deposition.（3）The mechanism of C=O formation in CO₂/CH₄ reforming reaction on Ni（111）,Rh/Ni（111）and Rh₆Ni（111）surfaces was revealed.The adsorption mechanism of intermediates was determined and the main reaction path was obtained.It is found that the main reaction paths are CH₄*→CH₃*→CH₂*→CH*→C*（or CH*→CHO*→CO*）on Ni（111）and CH₄*→CH₃*→CH₂*→CH*→CHO*→CO*on Rh/Ni（111）and Rh₆Ni（111）.It is clarified that Rh doping can promote the oxidation of CH*and inhibit the decomposition of CH*so as to reduce the formation of C*and improve the anti carbon deposition performance of the catalyst.At the same time,the activation energies of the main reactions are reduced,and the catalytic activity of the catalyst is improved.（4）The reaction pathways of CH/C conversion over Rh modified Ni-based catalysts with different ways and ratios were obtained,and mechanism of the anti-carbon deposition performance of Ni-based catalysts were revealed.It is found that the oxidation of C*in DRM reaction is more disadvantageous than the formation of C2,which means that to inhibit the formation of carbon deposition,it is more important to reduce the content of C*on the catalyst.CH*→CHO*→CO*and CH*→C*→C2 may occur simultaneously on Ni（111）surface.When Rh atoms（Rh/Ni and Rh₆Ni）are doped,CH*→CHO*→CO*is the more favorable reaction path and has better carbon deposition resistance.The main reaction path on the Rh@Ni（111）is CH*→C*→C2.The adsorption of Rh atoms improves the catalytic activity of the catalyst but may be accompanied by more carbon deposition.（5）The relationship between charge transfer and activation energy of Rh-Ni catalyst in C*conversion was found.It is found that for the reaction C*+C*→C2,the charge transfer between reactants and products has a linear negative correlation with the activation energy of the reaction;in the reaction C*+O*→CO*,the charge transfer between reactants and products has a linear positive correlation with the energy barrier of the reverse reaction.