Study On CO₂ Methanation Mechanisms Over Ce-modified Ni/Y₂O₃ And Ru₁/CeO₂ Catalyst

CO₂ methanation is an effective way to capture and store CO₂ as renewable energy,which is of great importance in the fields of environmental protection and energy.Investigating CO₂ methanation mechanism is conducive to deeply understanding of the reaction active site,reaction path and support effect,and is also critical to design better catalysts with high reaction performance.In this paper,density functional theory（DFT）calculations and experiments were used to study reaction performance and mechanism for CO₂ methanation on Y₂O₃ and Ce-modified Y₂O₃ supported Ni catalysts,and DFT calculations were adopted to investigate the reaction mechanism for CO₂ methanation on CeO₂ supported Ru single atom catalysts.Ni/Y₂O₃ and Ru₁/CeO₂ interface models were used to simulate the catalyst,and the mechanism of H₂ dissociation adsorption and CO₂ adsorption activation in CO₂methanation reaction was explored.The active sites of the catalysts for the reaction were determined.The reaction paths of methane formation were proposed.The influence of metal-support interaction to CO₂ methanation reaction performance were explored from the level of reaction mechanism.The main conclusions include:（1）DFT calculations show that CO₂ were adsorbed and activated on the Ni-Y₂O₃interface,and CO₂ were preferred to be hydrogenated to generate HCOO*;DFT calculations combined with in-situ FTIR results show that the formation of methane on Ni/Y₂O₃ catalyst surface was through the formate path with HCOOH*as an intermediate:CO₃H*→CO₂*→HCOO*→HCOOH*→HCO*→H₂CO*→CH₂*→CH₃*→CH₄*.The rate determining step was the hydrogenation of HCOO*to HCOOH*,and the reaction energy barrier was 1.08 e V.（2）Compared with the Ni/Y₂O₃ catalyst,the Ce-modified Ni/Y_0.8Ce_0.2O_x catalyst possessed better activity for CO₂ methanation.The results of H₂-TPR,XPS and H₂-TPD show that the modification of Ce weakened the metal-support interaction in the Ni/Y_0.8Ce_0.2O_x catalyst.The weaker interaction in the Ni/Y_0.8Ce_0.2O_x catalyst resulted in more nickel species which were weakly interacted with the support.DFT calculation results show that Ni species with weak interaction with the support had stronger ability to dissociate and adsorb hydrogen,which is conducive to the increase of hydrogen concentration on the catalyst surface.The results of DFT calculation and experiments show that the strong interaction limited the ability of Ni/Y₂O₃ catalyst to dissociate and adsorb hydrogen.The reason why Ni/Y_0.8Ce_0.2O_x catalyst had better activity was that there are more weakly interacting Ni sites on the modified catalyst,thus favoring the dissociation and adsorption of hydrogen and then increasing the concentration of hydrogen atoms on the surface.The higher hydrogen concentration reduced the energy barrier of the rate determining step and promoted the hydrogenation of HCOO*.（3）On the basis of DFT calculations,Ru sites were the active site for the dissociation and adsorption of H₂,the activation of CO₂ and the hydrogenation of intermediate to methane on the Ru₁/CeO₂ catalyst.CeO₂ support stably anchored Ru atom,and the hydroxyl group on its surface was conducive to the adsorption of CO₂.Compared with Ni-based catalyst,Ru directly activated CO₂ to disintegrate it into CO,so the formation of methane followed the CO*path:CO₂*→CO*→HCO*→H₂CO*→H₂COH*→CH₂*→CH₃*→CH₄*.The determining step was the process of H₂COH*dissociation to H₂C*with the reaction energy barrier of 1.1 e V.（4）DFT calculation results show that Ti,Zr and Pr doping into CeO₂ support improved the interaction strength between Ru and the support,thus favoring the anchoring of the Ru atom and improving the structural stability of the catalyst.The strong interaction also led to the decrease of the charge density of Ru atoms,which weakened the ability of Ru to activate C-O bonds,thus reducing the activity of the Ru₁ catalyst for CO₂ methanation.