Theoretical Study On Three Miportant Reactions Catalyzed By Nano-Gold Catalysts

Catalysis by nano-gold has attracted significant attention in the lastdecades, since Haruta discovered that the gold nanoparticles loaded in thetransitional metal oxides has a very high catalytic activity in COoxidation reaction, which is uniquely enhanced in humid atmosphere.This discovery changes the traditional concept that gold does not have thecatalytic activity, so a research fever is raised all over the world on thenano-gold clusters. The studies on the chemical and physical properties ofgold are based and have very significant scientific meanings. It is full ofapplication values for conducting related experimental and theoreticalstudies.In this dissertation, We utilize the density functional theory to studyadsorption and reaction of CO, NO, O₂ and other gas molecules in the gold clusters and gold surface. By quantum chemical calculations tosystematically study their structure and catalytic properties, we shedlights on the relevant information of microstructure, and discussesintrinsic relations between its structure and catalytic properties, revealsthe nature of catalytic activity, and identify the key factors that control itscatalytic activity. This should be helpful for the design of the newefficient nano-gold catalysts.The valuable results in this dissertation can be summarized asfollows:1. The research history and current Progress of gold-based catalystshave been briefly reviewed. A number of important chemical reactions atlow temperature catalyzed by gold-based catalysts were reported.Different explanations have been proposed to account for the apparentlyhigh catalytic activity of gold-based catalysts. Moreover, the theory ofquantum chemistry and the calculation methods of this paper aresummarized. The contents of these reports were the basis and backgroundof our studies and offer us with useful and reliable quantum methods.2. A detailed density functional theory (DFT) investigation revealedthree possible mechanisms(the redox mechanism, the carboxylmechanism, the formate mechanism) of the Water-gas shift reaction onAu(111). The adsorption behavior of adsorbed species on the surface(H2O, CO, OH, O, H, CO₂, COOH, HCOO) is studied to get the best active adsorption centers. By analyzing the activation energy of the 14elementary reactions in three mechanisms, we can conclude that WGSR inthe Au (111) in accordance with the mechanism of carboxyl andoxidation-reduction is more likely, while the formate mechanism areunlikely due to the high formation barrier. Our calculations also indicatethat carboxyl mechanism is more probable compared with the redoxmechanism and the most feasible reaction pathway is3. The mechanism of CO oxidation catalyzed by gold clusters Au10was elucidated by first-principle Density-functional theory (DFT). All theAdsorbate (CO,O₂,O,CO₂,CO+O₂,CO₂+O,CO+O,CO+O+O) havebeen calculated to obtain their preferred adsorption sites. Two reactionmechanisms of the CO oxidation were considered: Langmuir-Hinshelwood (containing two pathway L1 and L2) and Eley-Rideal(containing two pathway E1 and E2). We have characterized the fourreaction pathway and The absorption activity . The calculation show thatthe reaction CO+O₂â†'CO₂+O probably proceed through LH mechanism,Comparing pathway L1 with L2, pathway L1 is more feasible becausereaction could proceed through L1 pathway of LH mechanisms withlower activation barriers of 33.9 and 56.4 kJ·mol^-1. The most feasiblereaction pathway is O₂(gas)+CO(gas)â†'O₂(ads)+CO(gas)â†'O₂(ads)+CO(ads)â†'OCOO(ads)â†'O(ads)+ CO₂(ads); then CO+Oâ†'CO₂ reaction with the activation barriers of 6.9 and 4.3 kJ·mol^-1 and exothermic by 352.1kJ·mol^-1, Which suggests that the remaining O atom could be easilyreacted off by another comolecule to formate the second CO₂.4. The NO + CO reaction mechanism on the Au(111) surface werestudied by the density functional theory (DFT). The main elementarysteps are taken into account, namely: N₂ production. By simulating thetransitional state of each reaction step, we collect the data of reactionenergy barrier and reaction enthalpy etc. Our calculations indicate that thereaction NO + CO probably proceed through dimer mechanism, and themost feasible reaction pathway is 2a 2b 2 2 . 2NOâ†'(NO)â†'(NO)â†'N Oâ†'NThe determine step is N₂Oâ†'N₂+O with barriers of 49.5 kJ·mol^-1.