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

为了阐明金属/氧化物界面（如金催化剂）催化活性的机理，我们应用了三种方法；电子显微镜观察、扫描探针显微镜（SPM）观察和从头计算三者紧密结合。对于Au/TiO_2界面，SPM观察表明，还原富ti TiO_2表面与Au原子或团簇具有更强的吸引相互作用，这与从头计算结果一致，表明富ti或富o TiO_2表面与Au层的附着力比化学测量表面强。通过从头计算，将模拟扫描隧道显微镜（STM）图像与实验STM图像进行了比较，并对Pt/TiO_2和Ag/TiO_2体系进行了比较。利用电子全息技术对Au/TiO_2催化剂进行了研究，发现当Au团簇尺寸小于2 ~ 4 nm时，Au团簇的平均内电位突然增大。研究了这种现象对样品制备方法的依赖性，似乎与富o界面相关的界面偶极子对电位的增加很重要。由此可见，界面化学计量是决定催化活性的关键。对于具有显著催化活性的Au/CeO_2体系，电镜观察发现沉积在CeO_2上的Au团簇具有动态可逆的形状变化，且易于在电子束作用下形成和排列o空位。在这一现象中，我们观察到界面层本身紧密地附着在CeO_2表面，在那里界面应该是非化学计量的。在任何情况下，沉积金属团簇的动态形状随大气的变化都应与催化活性密切相关。

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.