Theoretical Studies Of Gold Nanocatalysts Supported On Reducible Metal Oxides

Gold has been widely investigated as catalysts ever since nano-sized goldparticles supported on reducible metal oxides (such as CeO2, TiO2, Fe2O3etc.)were found by Haruta, Hutchings, and others to be particularly effective for avariety of important catalytic reactions. However, controversies still exist on thefollowing issues regarding the role of the reducible supports: the interactionbetween supports and gold particles, the charge state of supported Aunanoparticles and its role in the reaction process, and the active site ofsupported Au nanoparticles. In this work, we chose to investigate twoprototypical examples as Au20/TiO2and Au20/CeO2in the context of COoxidation reaction using density functional theory (DFT). Both staticelectronic structure calculations and ab initio molecular dynamic (AIMD)simulations were performed to clarify the role of reducible metal oxides and toelucidate how finite-temperature dynamics of metal particles andsubstrate/metal particle charge transfer contribute to the catalytic activity of Aunanoclusters supported on reducible oxides.We first studied surface chemical properties of bare reduced TiO2surface. Theavailable charge of the surface is found to control the charge state and the maximumcoverage of adsorbed O2speces on TiO2surface. With the in-depth understanding ofchemical properties of reduced TiO2surface, we further studied the CO oxidationmechanism on TiO2supported Au nanocatayst. It is found that the charge state of theAu particle is negative in a reducing chemical environment, whereas in the presence ofoxidizing species co-adsorbed to the oxide surface the cluster obtains a net positivecharge. In the context of the well-known CO oxidation reaction, charge transferfacilitates the plasticization of Au20, which allows for a strong adsorbate inducedsurface reconstruction upon addition of CO that leads to the formation of mobile Au-COspecies on the surface. The charging/discharging of the cluster during the catalytic cycleof CO oxidation enhances and controls the amount of O2adsorbed at oxidesurface/cluster interface, thus strongly influencing the energetics of all redox steps inthe catalytic conversions. Similarly, it is shown that the localized4f electrons are highly mobile even at roomtemperature and can be captured by adsorbed O2species and hydroxyl species (OH*).As a result, we also propose a proton-mediated Mars-van Krevelen mechanism for COoxidation and suggest that the surface hydroxyl may play an inhibiting role in COoxidation. After Au20is deposited on CeO2surface, both Au20cluster and the surfaceare polarized, which leads to the formation of excess4f electronic states and positivelycharged Au atoms at the perimeter sites. On one hand, the polarization of the CeO2surface favors the O2activation at the Au-Ce3+interface; on the other hand, theformation of positively charged atoms would enhance the binding of CO molecules. Inaddition, the interaction between Au20and CeO2surface also leads to the formation oflow-coordinate Au sites that also provides active sites for O2activation.In conclusion, the charge interaction between reducible oxide and Au nanoclusteraffects the strucutral performance and charge state of Au cluster. This further stronglyaffects the adsorption and activation of reactant species and the energetics of the wholecatalytic cycle.