A DFT Study On Co₂ Methanation On Redoped Ni?111? Surface

Catalytic conversion of CO₂ is an effective method to mitigate the excessive carbon dioxide emission and it has attracted much attention.Carbon dioxide methanation is considered as a key reaction,making it a carbon source for high value chemicals.Herein,Re-doped Ni?111??Re@Ni?111??surface was used as a model to investigate the effect of Re on the C-O bond scission and on the selectivity of CO₂methanation on a Ni-based catalyst.Density functional throry?DFT?calculation was used to elucidate the reaction mechanism for CO₂ methanation.In order to study the mechanism of CO₂ methanation,we first determined the most stable adsorption structures and energies of the reaction intermediates involved in CO₂methanation on Re@Ni?111?and Ni?111?surfaces.The results indicate that Re improves the adsorption and activition of CO₂.The adsorption of the O-containing species on Re@Ni?111?are all more stable than those on the Ni?111?surface.Then,three pathways,including CO₂ direct dissociation into CO*followed by CO*methanation,CO₂ reduction through the HCOO*and COOH*intermediates,were analyzed based on the results from DFT calculations.In CO₂ direct dissociation pathway,doped-Re improves the production of CH₄ by lowering the activation barrier of CO₂*and CHO*dissociation.The HCOO*pathway could also contribute to methane formation on Re@Ni?111?through HCOO*dissociation to CHO*.In COOH*pathway,COOH*prefers to dissociate to CO*and Re promotes the subsequent methanation of CO*.The main role of rhenium on Re@Ni?111?is facilitating the C-O bond scission in adsorbed CO₂*and oxygen-containing intermediates.Microkinetics analysis was used to analysis the coverage of surface species.The results showed that CO*and H*were the most abundant species on the Ni?111?surface whereas appreciable amount of O adatoms were present on Re@Ni?111?in addition to CO*and H*,with the O adatoms on the Re sites.The O adatoms can be removed by increasing H₂ pressure.On both surfaces,increasing H₂ partial pressure resulted in an increase in H*coverage but decreased CO*coverage.With the surface coverage at steady state,the product selectivity can be calculated.The results showed that the presence of Re greatly increases the selectivity toward CH₄.The strong affinity of Re toward O makes Re@Ni?111?more effective for C-O bond scission and thereby enhances methane selectivity.The results in this study provide a better understanding about the effect of doping Re on CO₂ methanation on a Ni-based catalyst,providing new ideas for the experimental workers in searching the high efficient catalysts.