Theoretical Investigation On The Catalytic Oxidation Of Ethylene And CO Over Defect Stabilized Single Au And Pt Atoms On Graphene

Precious metals are widely used as catalysts in chemical industry,especially in conversion of chemicals,energy transfer and etc.For supported precious metal catalysts,the reaction centers are uncoordinated metal atoms exposed on edges and corners and the overall catalytic performance depends more on the morphology than the size of the precious metal nanostructures.Therefore,down-sizing the precious metal structures to single atom would be efficient to maximize the utilization of precious metals and lower the production cost.With a suitable support,the interaction at the metal-support interaction can not only stabilize the single precious metal atoms but also tune their electronic structure to exhibit superior catalytic performance and selectivity.Graphene is a typical 2-dimentional material with outstanding electronic and mechanical properties,and is widely used as catalyst support.There are various types of defects on graphene samples synthesised by reduction of graphene oxide.These defects are capable to anchor precious metal atoms.To rationalize the impact of interfacial interaction on the electronic structure and catalytic performance and selectivity of precious metal atoms and to deliver the single atom catalysts made of precious metals to chemical processes,such as olefin oxidation,fuel cells,etc.,we investigated the electronic structure and catalytic performance in ethylene and CO oxidation of Au and Pt atoms stabilized by defects and Bdoped defects on graphene by first-principles based calculations.The key contents of this thesis are as the following:First,we investigated the electronic structure and the mechanism of ethylene oxidation over defect stabilized single Au on graphene by first principles based calculations.The results show that the interactions between Au atoms and graphene vacancies not only suppress the diffusion and ripening of Au atoms,but also regulate Au-d orbital energy levels for the activation of the adsorbed O₂ and ethylene for the formation and dissociation of the peroxide intermediates.The epoxidation reaction process includes:?1?the formation of a peroxide intermediate by reaction of coadsorbed ethylene and O₂,?2?the dissociation of the peroxide intermediate to form ethylene oxide and adsorbed O atom,?3?the reduction of Au by the direct oxidation of gaseous ethylene with the adsorped O atom with formation of another ethylene oxide.The calculated energy barriers for the formation and dissociation of peroxide intermediate and the regeneration of Au sites were 0.30 eV,0.84 eV and 0.18 eV,respectively,which were significantly lower than the formation energy barriers of acetaldehyde.Further structural analysis indicated that due to the limited coordinate sites on the isolated single atoms,Au could not catalyze the hydride transfer for the formation of aldehyde.This differentiates the energy barriers among various competing path ways for ethylene oxidation,making epoxide the majority product.Then,we investigated the mechanism of CO oxidation on Pt atoms stabilized by defects on B-doped graphene.The Pt-sp and Pt-d states interact strongly with the B-sp states and the resulting binding energy is-5.64 eV,which make Pt atoms diffusion and aggregation hard to take place in conventional environment.This interaction also alters the distribution of the Pt atomic states around the Fermi Level resulting in different reactivity of these isolated atoms.Under oxygen-rich conditions,the co-adsorption of CO and O₂ is plausible as the initial species.The CO oxidation proceeds with the formation and the dissociation of peroxide intermediate from co-adsorbed CO and O₂,and reduction of Pt site by the reaction with gaseous CO,and the corresponding energy barriers were calculated as 0.40 eV,0.30 e V and 0.13 eV,respectively.The potential excellent CO oxidation of PtB3 can be attributed to the better matching of Pt atomic states with those of adsorbed reaction species induced by the Pt-B interactions.Finally,we investigated the dissociation of HOOH intermediate in oxygen reduction and the in-situ oxidation of CO over Pt atoms stabilized by vacancy defects on graphene that is confirmed to efficient as electrode materials for fuel cells,to address the potential CO poisoning over single atom electrocatalysts.We showed that Pt atoms can be stabilized by defects on graphene and the calculated binding energy is as high as-7.07 eV.The interfacial interaction also shifts the Pt atomic states to promote the dissociation of HOOH intermediate to form coadsorbed OH,which will further react with HOOH leaving coadsorbed H₂O,OH and OOH.In existence of CO,it takes the place of H₂O.Then,OOH will interact with coadsorbed CO to form a peroxide intermediate coadsorbed with a H₂O.The H₂O will catalyze the dissociation of peroxide intermediate and form a CO₂ and 2 coadsorbed OH.The calculated barriers along this path are 0.10 eV,0.11 eV,0.52 eV and 0.10 eV,respectively,indicating that Pt atoms stabilized by graphene are not only efficient for oxygen reduction,but are also tolerant to CO.