Basic Research on the Mechanism of Catalytic Activity of Metal/Oxide Nano-Hetero Interfaces

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In order to elucidate the mechanism of catalytic activity of metal/oxide interfaces such as gold catalysts, we have applied the three methods ; electron microscopy observation, scanning probe microscopy (SPM) observation, and ab initio calculation, closely combined to each other. For Au/TiO_2 interfaces, the SPM observation has shown that the reduced Ti-rich TiO_2 surfaces have stronger attractive interactions with Au atoms or clusters, which is consistent with ab initio calculations indicating that Ti-rich or O-rich TiO_2 surfaces can make stronger adhesion with Au layers than stoichiometric surfaces. By ab initio calculations, we have made comparison between the simulated scanning tunneling microscopy (STM) image and the experimental STM image for the Au adsorption on the reduced Ti-rich surface, and we have also examined Pt/TiO_2 and Ag/TiO_2 systems for comparison. By applying the electron holography through the electron microscope to Au/TiO_2 catalysts, we observed that the mean inner potential in Au clusters suddenly increases for the cluster sizes less than 2-4 nm. The dependence of this phenomenon on the methods of sample preparation has been examined, and it seems that the interface dipole associated with O-rich interfaces is important for the potential increase. In this way, it can be said that the interface stoichiometry is the key for the catalytic activity. About the Au/CeO_2 system with remarkable catalytic activity, we have found dynamic and reversible shape changes of Au clusters deposited on CeO_2 during the electron microscopy observation, in accordance with easy formation and ordering of O-vacancies by electron beams. During this phenomenon, we have observed that the interface layer itself is tightly attached to the CeO_2 surface, where the interface is supposed to be off-stoichiometric. In any case, the dynamical shape changes of deposited metal clusters according to the atmosphere should be closely concerned with the catalytic activity.