First-principle Investigation On The Initial Reduction Of CO₂ On CeO₂（111） Surfaces

With the development of the modern industrial society,the excess burn of coal,oil and natural gas and the emission of automobile exhaust cause to continuously increasing of the greenhouse gase–CO₂ concentrations in the atmosphere,which lead to global warming.It is significant for the sustainable development of energy and environment to efficient recycling utilize CO₂.Catalytic conversion of CO₂to liquid fuels or valuable hydrocarbon by using of the heterogeneous catalysts has gradually drawn the attention of people in recent years.Owing to the inertness of CO₂ molecule and the complexity of the reaction process,pure metals used for this task usually have low catalytic activity.Therefore,here we present ceria（CeO₂）as the CO₂ activation of catalyst,which rely on its famous capability of storing and releasing oxygen.The theory of DFT+U is used to study the initial reduction mechanisms of CO₂,the product selectivity and the impact of O-defect and dopant from the micro level.The adsorption of CO₂,reductive dissociation to CO and reductive hydrogenation towards COOH or HCOO are explored in detail.The results provide theory clues for reducted transformation of CO₂ molecules and the design of the new efficient catalyst.First-principle calculations are performed to explore the initial reduction of CO₂ on perfect and O-defective CeO₂（111）surfaces via direct dissociation and hydrogenation,to elucidate the product selectivity towards CO,COOH,or HCOO.Three reaction pathways of energy competition is validated including reductive dissociation of CO₂ to CO,reductive hydrogenation of CO₂ towards COOH and HCOO.Results are shown as follows:（1）adsorbed CO₂ is activated on the perfect surface,forming a carbonate species by combining with surface oxygen.CO₂ prefers a bent configuration with the C atom of CO₂ occupying the oxygen vacancy site on O-defctive surface.（2）Reductive hydrogenation CO₂+H?COOH*is more competitive than CO₂+H?HCOO*on both perfect and O-defective CeO₂（111）surfaces.Comparatively,CO₂ hydrogenation towards COOH is slightly more favorable on the perfect surface,whereas reductive dissociation of CO₂ is predominant on the O-defective CeO₂（111）surface.（3）Electronic localization function,charge density difference,and density of states are utilized to analyze the effect of charge accumulation and redistribution on adsorption and reductive dissociation of CO₂ caused by the presence of O vacancy.Electronic structure analyses confirm that the O vacancy leads to highly delocalized electron distribution of the adjacent O and Ce atoms,facilitates the charge accumulation to strengthen the CO₂adsorption,and therefore promotes direct reductive dissociation of CO₂ on the O-defective CeO₂（111）surface.On this basis,the influence of dopant on the initial reduction of CO₂ is further studied.A Ce ions on the second layer of surface is replaced a Pd,Ru,and Cu metal atoms,respectively.The effects of dopant on the product selectivity,catalytic activity and initial reduction mechanism are calculated by DFT+U.The results are shown as follows:（1）Pd atom doping can substantially lower the formation energy of O vacancy,whereas the effect of the activation barrier of CO₂ molecules is not obvious.The energy barriers of CO₂ dissociation and hydrogenation are high.（2）CO₂ hydrogenation towards COOH is slightly more favorable on the Ru doped surface.CO₂ molecules can be easily adsorbed on the surface of the Cu doped and form similar carbonate CO₃^2-structure by combining with the surface O atom.The barrier of CO₂ hydrogenation towards COOH and reductive dissociation of CO₂ are great reduced on the Cu doped surface.（3）The Energy barriers and contribution factors are analysised and find that the structure deformation energy of initial state（IS）to final state（TS）is increased due to Pd atom,enhance the interaction between CO₂ and H atom.On the Ru/CeO₂（111）and Cu/CeO₂（111）surfaces,the structure deformation energy is decreased,so the barrier of CO₂ hydrogenation towards COOH is lower.