Oxygen-Induced Reversible Structural Change of Supported Fine Silver Particles Observed by In Situ Optical Absorption an
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SILVER OXYGEN-INDUCED REVERSIBLE STRUCTURAL CHANGE OF SUPPORTED FINE PARTICLES OBSERVED BY IN SITU OPTICAL ABSORPTION AND X-RAY DIFFRACTION AKIHISA YANASE*, HIROSHI KOMIYAMA* AND KAZUNOBU TANAKA** *The University of Tokyo, Dept. of Chemical Engineering, Hongo, Bunkyo-ku, Tokyo 113, Japan **Electrotechnical Laboratory, Umezono, Tsukuba-shi, Ibaraki 305, Japan ABSTRACT In situ measurements of optical absorption and x-ray diffraction for small supported silver particles were performed under defined atmospheres at temperatures below 723 K. We found, for the first time, reversible changes both in optical absorption spectrum and lattice parameter of silver particles against the cyclic exchange of ambient gases between 5% H2 /He and 1% 0 2 /He. The present data indicate that the adsorption of oxygen results in a reduction in surface stress of silver particles, leading to a change in the shape of particles. INTRODUCTION The system of small metal particles has a big advantage for studying surface phenomena because of both a large surface area of metal and a high degree of freedom in structure [1,2]. In particular, it is of interest to investigate the interaction of chemisorbed species with metal surfaces by observing an adsorbate-induced structural change of small supported metal particles since the structure can be determined by the balance between adsorbate-metal and metal-support interactions [3,4]Small silver particles exhibit an anomalous optical absorption due to the excitation of collective oscillation of free electrons, which depends significantly on the particle shape [5-7]. Thus, the morphology of small silver particles can be discussed by measuring the optical absorption spectra. Here we present new experimental results of optical absorption of supported fine silver particles measured in situ and in real time against the cyclic exchange of ambient gases at temperatures below 673 K. From the comparison between measured spectra and calculations using Maxwell-Garnett theory, we show an occurrence of an oxygen-induced reversible change in particle shape between spherical in 5% H /He and oblate-like in 1% 02 /He. In situ x-ray diffraction (XRD) measuremenis were also made in a temperature range of 298 to 723 K, in which a gas-induced reversible change in lattice parameter of silver particles was found [8]. The data indicate that the adsorption of oxygen results in a reduction in the surface stress of silver particles. EXPERIMENTAL Small supported silver particles were prepared and subjected to optical and structural measurements. A deposited layer ( 3 jm) of pure silica spheres of 40-50 nm in diameter, which was formed on both sides of a quartz glass plate by dip-coatings followed by calcination, was used as the supporting material for optical experiments. On the other hand, a 1.1 mmthick Vycor porous glass plate (Corning 7930) was employed for x-ray diffraction experiments. For both samples, silver particles were prepared chemically by impregnation with aqueous solutions of AgNO 3 and reduction in hydrogen stream at
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