Theoretical Studies On The Oxidation Of Carbon Monoxide By Gold Clusters Supported On The Zinc Oxide Surface

Since Haruta found the nano-sized gold（Au）supported by transition metal oxides showing outstanding catalytic performance for low-temperature oxidation of carbon monoxide（CO）,tremendous experimental and theoretical studies have been designated to investigate the details in the process of CO oxidation by Au catalysts.The main points are the active sites and the catalytic mechanisms.To understand the origin of the Au catalytic activity,several factors are considered including the Au particle size and the morphological shape,the coordination numbers of Au atoms,the charge and valence states,and the metal-support interactions.Strong metal-support interaction（SMSI）is a common phenomenon for the group VIII noble metal catalysts（Pt,Pd,etc）supported by reducible oxides（TiO₂,CeO₂,etc）.Wurtzite zinc oxide（ZnO）as irreducible support has a wide band gap and hardly renders SMSI with noble metal catalysts.However,Mou et al.developed a technique to control the oxidative（O-）and reductive（R-）SMSIs of Au/ZnO by the O₂ and H₂ pretreatments,respectively.The significant effects of O-and R-SMSIs have been demonstrated on the catalytic activity for CO oxidation.In this dissertation,we attempt to investigate the origins of the O-and R-SMSIs of Au/ZnO and the microscopic catalytic mechanism of the CO oxidation by first-principle density functional theory（DFT）calculations,which are expected to show insights into the rational design of gold catalyst for CO oxidation.DFT studies are performed to study the origin of oxidative and reductive strong metal-support interactions（O-and R-SMSIs）of Au/ZnO modeled by Au cluster supported on the O-and Zn-termination ZnO（101）surfaces（O-ZnO（101）and R-ZnO（101））,respectively.We find the interaction of Au/O-ZnO（101）is stronger than that of the Au/R-ZnO（101）indicating the O-SMSI favors the stability of Au cluster.Bader charge and electron density difference analyses show that localized charge transfer is directed from Au to O-ZnO（101）surface leading to the positively charged Au,which is reversed from R-ZnO（101）to Au with negative charges.Density of state analysis shows the chemical bonds of the Au/ZnO interface characterized by the hybridization of Au-5d and O-2p states.Due to the different dipole fields of the two ZnO（101）surfaces,the O-2p of O-ZnO（101）has higher energy level than that of R-ZnO（101）,which drags the Au-5d close to the Fermi level（E=0 eV）favoring the catalytic activity of O-ZnO（101）.Both short-ranged charge transfer and long-ranged dipole field favor the catalytic activity of Au/ZnO,which is confirmed by the adsorption of CO and O₂.For Au/O-ZnO system,CO and O₂ inclines to adsorb on the positively charged Au atoms with adsorption energy of 1.44 and 0.64 eV,respectively.For Au/R-ZnO system,CO and O₂ prefer adsorbing on the bridge site of sub-and top-layer Au atoms with smaller adsorption energy（0.84 and 0.54 eV）.Noteworthy,the O₂ molecule adsorbed on the R-ZnO surface is highly stable（2.86 eV）,which indicates the preferred adsorption site is the ZnO surface rather than the gold catalyst.The coadsorption of CO and O₂ is investigated based on the favorable CO/O₂ adsorptions.The CO and O₂ favorably adsorb on interfacial Au atoms for O-SMSI with adsorption energy of 2.33 eV,which is lower than that（3.54 eV）of the R-SMSI with CO and O₂ adsorbing on top-Au and Zn sites,respectively.According to the Sabatier principle,the highest catalytic activity is obtained for the system with medium adsorption strength,while weaker or stronger adsorptions disfavor the catalytic activity.Finally,the catalytic mechanisms of the CO oxidation on Au/ZnO are investigated including the Langmuir-Hinshelwood（L-H）and Mars-van Krevelen（M-vK）mechanisms.L-H mechanism is initiated from the coadsorptions of CO and O₂ with two sequential reactions（CO+O₂?CO₂+O;CO+O?CO₂）.On the one hand,R-SMSI favors the CO oxidation thermodynamically.Due to the highly stable adsorption of O₂ on the surface of R-ZnO（101）,the reaction energy profile of R-SMSI is lower than that of the O-SMSI.The thermodynamic stability of the CO₃ product results in the higher exothermic reaction heat of R-SMSI（3.85 eV）than that of O-SMSI（2.01 eV）.On the other hand,O-SMSI is kinetically favored to the R-SMSI because the rate-determining barrier（0.78 eV）of O-SMSI is lower than that（1.73 eV）of R-SMSI.M-vK mechanism refers to the CO reacting with the lattice oxygen(O_lattice)(CO+O_lattice?CO₂),from which the surface O vacancy is recovered by a second reaction:CO+O₂?CO₂+O_lattice.The activities of the lattice oxygen(O_lattice)atoms at the Au/ZnO interface are dependent on their coordination numbers and orientations.Three3-coordinated(O_3c,O_3c?,and O_3c??)and one 2-coordinated(O_2c)O_latticeattice are investigated,of which the O_3c??shows the highest activity（energy barriers are 0.32 and 0.48 eV）for the CO oxidation due to the feasible achievement of the single-atom active site.Different from the Au/CeO₂,the easily occurred CO₂ desorptions do not determine the kinetics of CO oxidation.Thus,the activity of the lattice oxygen for CO oxidation by M-vK mechanism cannot be measured by the oxygen vacancy formation energy.Rather,the reaction feasibility is closely related with the Bader charge of carbon in the transition state.Comparing the L-H and M-vK mechanisms,the CO oxidation mainly occurs by M-vK mechanism for O-SMSI,while the R-SMSI favors thermodynamics of CO oxidation.