Theoretical Studies On Oxide-supported Single-Atom Catalysts

Recently,significant attention in heterogeneous catalysis is called on single-atom catalysts.Single-atom catalysts exhibit not only high utilization,but also unique catalytic properties in various reactions,including CO oxidation,water-gas shift,selective oxidation of alcohols,selective hydrogenation of dialkenes or alkynes,and so on.However,there are still some challenges in single-atom catalysis,including the agglomeration of isolated atoms in the reaction process,the low loading of isolated atoms,the comparasion between isolated atoms and nanoclusters,and the local structure of single atoms under specific reaction conditions.In the present work,we systematically studied the support effect,the crystal plane support,the role as well as the local environment of single atoms using density functional theory.By investigating different redox properties of the metal dioxides?MO₂,M=Ti,Zr,Ce,Hf,Th?,different charge state of single-atom gold on various supports as well as the different interaction between the adsorbed species and the Au₁/support SAC,it is found that the nominal quantum primogenic effect is the nature of support effect.Although Ti,Zr,Ce,Hf and Th atoms all have four valence electrons,Ti O₂ and Ce O₂ show diverse characters with Zr O₂,Hf O₂ and Th O₂,since quantum primogenic effect causes the radial contraction and low orbital energies of Ti-3d and Ce-4f orbitals.Further study of the single-atom Pt catalysts dispersed on Ce O₂?111?,?110?and?100?surfaces suggests that the single-atom Pt substitution of a lattice Ce ion is thermodynamically stable on each ceria surface.It is further demonstrated that on all the surfaces the single-atom Pt catalysts can catalyze CO oxidation and the order follows electron acceptor/donor during the oxygen subtracting and O₂ replenishing,leading to the high activity of Pt₁@Ce O₂?111?and Pt₁@Ce O₂?100?SACs for CO oxidation.This single-Pt active center also averts the formation of poisoning carbonate species.To further investigate the role of single atoms,the water adsorption and dissociation on Au₁/Ce O₂ single-atom catalyst are studied.It is found that the positively charged gold single atoms on ceria surface not only provide the activation site for water adsorption but also facilitate water dissociation by weakening the intramolecular O-H bond.By investigating the optimal structure of the single Rh atoms on Ti O₂ during different reaction conditions,it is demonstrated that Rh single atoms can adapt their local coordination and reactivity in response to variation in environmental conditions.In oxidative conditions,Rh prefers to substitute the 6-coordinated Ti atoms in the surface plane of the oxide.In contrast,under a pressure of H₂ or CO,it prefers to be supported on-top on the oxide.Further studies show that single-atom catalysts are not at all static but can switch from inactive to active structure under RWGS reaction conditions.These results provide a new understanding of the oxide-supported single atom catalysts,as well as theoretical support for the design of single atom catalysts with high stability,activity and selectivity.