Analytical TEM Observation of Gold Nano-Particles on Cerium Oxide
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0900-O06-37.1
Analytical TEM observation of gold nano-particles on cerium oxide Tomoki Akita1, Koji Tanaka1, Masanori Kohyama1, and Masatake Haruta2 1
Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka1-8-31, Ikeda 563-8577, Osaka, Japan 2 Materials Chemistry Course, Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji 192-0397, Tokyo, Japan ABSTRACT The structure of Au nano-particles supported on CeO2 was investigated by using an analytical transmission electron microscope (TEM). The shape change of Au particles was observed during TEM observation, such as shrinking down to a mono-atomic layer on the CeO2 substrate. The electron beam irradiation experiment revealed that the shape change of Au particles is concerning with the oxidation state or the density of oxygen vacancies of CeO2 substrate where the rapid desorption and adsorption of oxygen occurs. It was also found that the reduction by strong electron beam irradiation and subsequent oxidation generate the decoration of Au particles by cerium oxide. INTRODUCTION It was reported that Au/CeO2 catalysts show high catalytic activity for low-temperature water gas shift reaction and CO oxidation [1-3]. Recently, it was also reported that atomically dispersed small Au clusters are active species for low-temperature water gas shift reaction [4]. However, the structure of Au particles on CeO2 was not investigated directly since it is difficult to observe small Au clusters on CeO2 by a conventional transmission electron microscopy (TEM) due to the disturbance of the complicated diffraction and phase contrast by small CeO2 crystals [5]. The Au/CeO2 catalysts prepared by deposition precipitation method was observed using an analytical transmission electron microscope in our previous work [5,6]. Annular dark field scanning transmission electron microscopy (ADF-STEM) revealed that Au particles smaller than 2 nm in diameter were highly dispersed on the CeO2 support [5]. TEM observations were also performed on a model catalyst sample that was prepared using powder of larger CeO2 grains. During the observations, we found novel structural changes of Au particles on CeO2 crystals. The shape of Au changes and the Au particles diffuse with remaining Au mono-layers on CeO2 [6]. In the present study, we investigate the detailed behavior of such structural changes of Au particles and CeO2. We use the model structures prepared by using a poly-crystalline CeO2 substrate. The changes of Au/CeO2 systems by electron beam reduction were investigated by analytical TEM.
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EXPERIMENTAL The Au/CeO2 model structure was prepared by vacuum deposition using CeO2 polycrystalline substrate. The tablet of CeO2 powder (Kojundo Chemicals Lab. Co., purity 99.9%) was heated in air at 1773-1973K for 12 hours. The tablet of CeO2 was cut in a disk with 3mm in diameter. The thin CeO2 substrate was obtained by mechanical polishing and Ar ion milling. The thin CeO2 s
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