The Study Of The Oxidation Of Small Molecules By Heteronuclear Metal Oxide Clusters

The key to design the environmentally friendly catalyst with high activity and high selectivity lies in well understanding the detailed structure-property relationship about the active sites on surfaces.However,it is technically difficult to characterize the chemical bonding and reactivity of the catalyst at molecular level by traditional methods due to its complexity in condensed phase.Atomic clusters establish a bridge between isolated atoms and the bulk materials and have long been proposed as the ideal models to uncover the mechanistic details in its related condensed phase systems.In this dissertation,gas phase studies of the reaction of heteronuclear metal oxides clusters toward small molecules,such as H₂ and CO,have been performed by means of both experimental and computational methods.The studies can improve our understanding on the the behavior of multiple components,the detailed mechanism and structure-property relationship of heterogeneous catalysis at a strictly molecular level,which may provide important clues and theoretical basis in designing efficient supported catalysts.The summary is as follows:（1）Consecutive oxidation of three H₂ molecules by a gold-vanadium oxide cluster cation AuVO₄⁺Time-of-flight mass spectrometry experiments demonstrated that laser ablation generated and mass selected gold-vanadium heteronuclear oxide cluster AuVO₄⁺can oxidize three H₂ molecules consecutively in an ion trap reactor.Quantum chemistry calculations were employed to reveal the elementary steps involved in the consecutive H₂ oxidation.The positively charged gold in AuVO₄⁺functions as the active site to capture H₂ and split the H-H bond in collaboration with the terminal lattice oxygen O^2-,during the process of which the superoxide species O₂^·-in Au VO₄⁺is activated and then dissociated to supply enough oxygen sites for the subsequent H₂ oxidation.After the oxidation of three H₂ molecules,the Au-O bond in Au VO₄⁺is converted to Au-V bond in product AuVO⁺that is inert towards H₂.In contast,cluster AuNbO₄⁺that is structurally related to AuVO₄⁺can oxdize only one H₂ molecule.The origin of the different reactivities between two clusters was explored and the importance of different central metal in H₂ oxidation was emphasized.（2）Noble-metal free single atom catalysts Cu Al₄O^-_7-9 for CO oxidation by O₂We demonstrate that the single copper atom doped clusters Cu Al₄O^-_7-9 can catalyze CO oxidation by O₂.The CuAl₄O^-_7-9 clusters represent the first group of experimentally identified noble-metal free single atom catalysts for such prototypical reaction.The reactions were characterized by mass spectrometry and density functional theory calculations.The CuAl₄O₉CO^-is much more reactive than CuAl₄O₉^-in the reaction with CO to generate CO₂.One adsorbed CO is crucial to stabilize Cu of Cu Al₄O₉^-around+I oxidation state and promote the oxidation of another CO.The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level.The remarkable difference between Cu catalysis and noble metal catalysis was discussed.