Study On CO₂ Methanation Mechanism On CeO₂-based Solid Solution Supported Ni And Ru Catalysts

The conversion of CO2 as a carbon source into high value-added chemicals or liquid fuels is of great significance to the carbon cycle.The CO2 catalytic hydrogenation to methane is one of the effective ways to utilize C1 resources.In this thesis,we have combined the experiments and density functional theory(DFT)calculations to investigate the mechanism of CO2 methanation for two sets of catalysts,which are the Ni catalysts supporting on CeO2(Ni/CeO2)and Sn-doped CeO2(Ni/Ce0.9Sn0.1Ox),and the Ru catalysts supporting on CeO2(Ru/CeO2)and Cr-doped CeO2(Ru/Ce0.9Cr0.1Ox).The CO2 methanation mechanism,and the roles of active metal,oxide support and interface,and the Sn or Cr doping effects on the reaction have been elucidated by using in-situ FTIR and DFT methods.The main results are as follows:(1)In-situ FTIR results demonstrate that CO2 methanation and CO2 adsorption on Ni/CeO2 catalysts follows the formate(HCOO*)pathway,without CO species detected.The CO2 adsorption results clearly show that the bicarbonate is produced from CO2 interaction with OH.Three reaction paths have been performed.The formate path is most favorable due to the lowest energy barrier.The rate determining step is HCOO*hydrogenation to H2COO*,with the activation energy barrier 1.08 eV.The results of DFT and in-situ FTIR calculation results indicate that the CO2 methanation mechanism on the Ni/CeO2 catalyst follows the HCOO*pathway,that is CO2+OH?CO3H*?CO2*?HCOO*?H2COO*?H2COO*?H2CO*?H2C*?CH3*?CH4*.(2)It is different from the selective generation of methane by Ni/CeO2 catalyst,that the Ni/Ce0.9Sn0.1Ox catalyst selectively generates CO.H2-TPR results show that stronger interaction occurs between Ni and O-Sn that between Ni and O-Ce species.As a result,after Ni supporting on Ce0.9Sn0.1Ox solid solution,Sn species in the support prefer to migrate to the surface layer to interact with Ni,so that the amount of Sn is higher than that in Ce0.9Sn0.1Ox support,and the amount of oxygen vacancies is lower than that in Ni/CeO2,as testified by XPS results.STEM-mapping results indicate that indeed Ni prefer to move to Sn in the Ni/Ce0.9Sn0.1Ox catalyst,with the formation of Ni-O-Sn interface.It is believed that the different selectivity on Ni/CeO2,Ni/Ce0.9Sn0.1Ox and Ni/0.1SnO2/0.9CeO2 catalysts can be attributed to the different interaction interfaces.On the Ni/Ce0.9Sn0.1Ox catalyst,CO2-TPD results showed that less amount of medium-strength basic sites are present so that the amount of adsorbed species are lower than the cases on Ni/CeO2,as testified by In-situ FTIR experimental results.DFT results indicates the change of the interface between Ni and the support results in the change of the adsorption structure and adsorption site for CO2,thus inducing the change of the selectivity.(3)In-situ FTIR results demonstrate that CO2 methanation on the Ru/CeO2 catalyst follows a double-path mechanism:the formate(HCOO*)pathway and CO*pathway.The temperature of formate pathway is lower than the CO*pathway,and the active site of CO*hydrogenation is Ru site.(4)In comparison with Ru/CeO2,it is easier to form oxygen vacancies on the Ru/Ce0.9Cr0.1Ox catalyst,thus the catalysts possess more vacancies,as proved by Raman,DFT and XPS results.H2-TPR results indicate that the Cr addition promotes the H spillover effect.In-situ FTIR results showed that the amount of surface bicarbonate and formate are higher on the Ru/Ce0.9Cr0.1Ox catalyst,compared with that on the Ru/CeO2 catalyst.In summary,more vacancies and thus more CO2 adsorption activation site result in the easier CO2 adsorption activation,which is accounting for the improved the catalytic activity on the Ru/Ce0.9Cr0.1Ox catalyst.