Nanoscale characterization of Pd/TiO2 catalysts and Ag/TiO2 catalysts by electron holography

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0900-O09-31.1

Nanoscale characterization of Pd/TiO2 catalysts and Ag/TiO2 catalysts by electron holography S. Ichikawa 1, T. Akita 2, K. Okazaki 2, K. Tanaka 2, and M. Kohyama 2 1. Organization for the Promotion of Research on Nanoscience and Nanotechnology, Osaka University, Toyonaka, OSAKA, 560-0043 Japan. 2. Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, 563-8577 Japan. ABSTRACT Catalytic properties of noble metal catalysts are often caused by their nanostructures and the interaction between nano particles and oxides supports. We examine the mean inner potential of Pd particles in Pd/TiO2 catalyst and Ag particles in Ag/TiO2 catalysts using electron holography and HRTEM. And we compare the results of the Pd/TiO2 and Ag/TiO2 systems with our previous results of the Au/TiO2 and Pt/TiO2 systems. In the case of the Pd/TiO2 catalysts, when the size of Pd particle is smaller than 5nm, the mean inner potential of Pd changes to be higher than the calculated value of the mean inner potential which is thought to be higher than the experimental values of bulk Pd by 3-5V. The mean inner potential of Pd becomes to increase gradually as the size of the Pd particle decreases. The rate of the increment in the mean inner potential of Pd is lower than that of Au/TiO2 catalysts and Pt/TiO2 catalysts. On the other hand, in the case of Ag/TiO2 catalysts, the mean inner potential of Ag does not increase though the size of the Ag particle decreases to under 2nm. INTRODUCTION Nano particles of noble metals are often used as a catalyst. Catalytic properties of noble metal catalysts are often caused by their nanostructures and the interaction between nano particles and oxides supports. Au/TiO2 catalysts show the size dependence of the catalytic activities owing to the change of the local electronic structure of Au particles and the strong interaction at the interface between Au particles and TiO2 supports [1]. Not only in Au/TiO2 system, but also in Pd/TiO2 system, the phenomena of strong metal-support interaction (SMSI) [2] have been investigated by scanning tunneling microscope (STM) [3, 4], electron paramagnetic resonance (EPR) [5] and so on. We measured the mean inner potential of Au particles supported on TiO2 using electron holography and high resolution electron microscopy (HREM), and found that the mean inner potential of Au depend largely on the size of the Au particles [6]. When the size is lager than 5nm, the mean inner potential is the same as the reported values of bulk Au (experimental: 21-23V, calculated: 25-30V). When the size is smaller than 5nm, it begins to increase over 30V, and it begins to increase suddenly over 40V at the size smaller than 2nm. It indicates that the electronic structure of the Au particle varies from that of the bulk state as the size of the Au reduces. In this paper, we report not only the behavior of the mean inner potential of the Pd particles supported on TiO2 but also that of Ag/TiO2 catalysts which is known to