Study On Experiment And Mechanism Of CO₂ Methanation Over Nickel-Based Catalysts

CO₂ is not only a major greenhouse gas,but also a abundant,cheap and easily available carbon resource.Conversion of CO₂ to energy chemicals is one of the sustainable development routes to solve energy shortage and environmental problems.Catalytic hydrogenation of CO₂ to methane is a promising technology for CO₂ conversion and utilization.The key is to prepare economical and efficient catalysts.Catalysts using nickel as active metal have been widely studied,but they are easy to agglomerate at high temperature and the reaction mechanism is not clear.Based on this,this paper mainly studies from the following aspects:SSZ-13 molecular sieve has good pore structure and excellent hydrothermal stability.In this paper,Ni/SSZ-13 catalysts with Ni content of 5,10,15,20 wt%were prepared by hydrothermal impregnation method.The physical and chemical properties of the catalysts were characterized by BET,XRD,H₂-TPR and TEM.The effects of Ni content,reaction temperature and flow rate on CO₂ methanation performance of Ni/SSZ-13 catalysts were investigated in a fixed-bed reactor.The results showed that micropores were the main type of pore structure of SSZ-13 zeolite and Ni/SSZ-13 catalysts.The Ni species were uniformly dispersed in the catalyst,and the metal Ni was the main active component in the reaction.The excellent pore structure of the support inhibited the agglomeration of Ni nanoparticles.Ni/SSZ-13 catalysts had a higher activation temperature and showed the best reactivity at450°C.At the same temperature,the higher the Ni content was,the better the catalytic activity of CO₂ methanation was.Ni/SSZ-13 catalysts had good high temperature stability and exhibits higher catalytic activity at lower flow rate.Based on the calculation method of density functional theory,the mechanism of CO₂methanation reaction on Ni/SSZ-13 catalyst was investigated at the molecular level.The adsorption behavior,activation mechanism,metal-support interaction and reaction pathway of intermediates in the reaction process were described in detail.The results showed that the Ni atoms on the surface of Ni/SSZ-13 catalyst could provide a suitable active site for the adsorption and dissociation of H₂.Bridge site at the top of Ni₆ cluster was the best adsorption site for CO₂ molecules.The intermediate species HCOO*,HOCO*and CO*were obtained by the activation of CO₂*（*represented the adsorption state）through C-terminal hydrogenation,O-terminal hydrogenation or direct cleavage of the C-O bond.The methanation pathways of CO₂ were calculated according to the three intermediate species.The optimal CO₂ reaction pathway on Ni₆/SSZ-13（111）surface was as follows:CO₂*→CO*→C*→CH*→CH₂*→CH₃*→CH₄*.The rate-limiting step of this reaction pathway was CO*+O*+2H*→CO*+H₂O*,and the required activation energy barrier was95.51 kcal/mol.Although Ni/SSZ-13 catalysts exhibited excellent high temperature activity and catalytic stability,its reactivity at low temperature was not outstanding.A large number of studies had shown that Ni/Ce O₂ catalysts were a kind of CO₂ methanation catalyst with good activity at low temperature,but the activation mechanism was still controversial.Therefore,the electronic properties and gas-solid reaction mechanism of Ni/Ce O₂ catalyst surface were studied by DFT calculation method.The results showed that there was a strong metal-support interaction between Ni clusters and the surface of Ce O₂（111）.CO₂ molecules could be stably adsorbed on the top of Ni₄ cluster on Ni/Ce O₂ catalysts.In the formate pathway and the C-O bond direct cleavage pathway,the activation energy barriers required by the rate-limiting steps were obviously higher than that of the RWGS+CO-hydro pathway,which didn’t have a competitive advantage in reactions.Therefore,the optimal pathway on Ni/Ce O₂ catalysts was CO₂*→HOCO*→CO*→HCO*→H₂CO*→CH₂*→CH₃*→CH₄*.The rate-limiting step of this pathway was the cleavage of C-O bond in H₂CO*species under H*auxiliary conditions.