Gold (Au) is a chemically stable metal, however, Au nano-clusters supported on metal oxides, such as titanium dioxide (TiO2) show high catalytic activity for oxidation of carbon monoxide (CO) at low temperature [1]. The origin of the activity is still unclear. In order to elucidate the origin, in this work, relationships between electronic structure and catalytic activity of Au/TiO2 have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS), density functional theory (DFT), photoelectron spectroscopy (PES) and catalytic activity measurements.
By STM/STS and DFT, we found the excess charges on TiO2(110) induced by oxygen vacancies distribute on 5-coordinated Ti atoms with delocalization [2], although the localization of the excess charge at 6-coordinated Ti under oxygen vacancies have been generally believed. This is the first direct observation of the spatial distribution of the excess charge in atomic scale..
Since electronic structure of Au nano-clusters on TiO2 had not been well understood, by using PES and STM, we firstly observed that the Au nano-clusters on defects (steps and oxygen vacancies etc.) of TiO2(110) are negatively charging due to the charge transfer from TiO2(110) to Au [3]. Also, we studied relationship between electronic structure and catalytic activity of Au/TiO2(110). Our results of reactivity measurements and PES showed that the catalytic activities of Au/TiO2(110) correlate with the charge transfer from TiO2 to Au [4]. This indicates that negatively charged Au nano-clusters have crucial roles in the catalytic reaction. We believe that our finding is important to understand the origin of the catalytic activity of Au/TiO2.
[1] M. Haruta et al., Chem. Lett. 2, 405 (1987). [2] T. Minato et al., submitted. [3] T. Minato et al., Surf. Sci. 566, 1012 (2004). [4] T. Minato et al., in preparation.
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