Design of Catalysts with Artificially Controllable Functions Using Surface Acoustic Waves and Resonance Oscillations Gen
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Mat. Res. Soc. Symp. Proc. Vol. 497 © 1998 Materials Research Society
deposited on the substrate[10-14]. More recently, with a single crystal of z-cut lithium niobate, the TE mode of RO gives rise to anomalous enhancement of the catalytic activity for the same reaction on Pd[15]. From a viewpoint of approach to new catalyst materials, it is of importance to investigate the effects of the RE and TE modes on different kinds of the catalytic reactions over various catalysts. This is also useful for a better understanding of a surface acosutic wave(SAW)- and a RO-induced catalyst activation mechanism. The present paper deals with the RO effects on ethanol oxidation over Ni and Ag catalysts and on CO oxidation on Pt in order to reveal the different effects of the TE and RE mode. The results are discussed in comparsion with the SAW effects. Changes in surface properties with the RE and TE mode were investigated by measuring surface potential. The same measurement was also applied to a surface on which Rayleigh SAW propagates. The previous study on ethanol oxidation over Pd and Ni catalysts has shown that the SAW effects are larger when catalysts surfaces are covered with strongly adsorbed oxygen or oxide layers[3,4]. In this regard, the effects of RO on metal oxide catalysts are interesting, and W0 3 was chosen as a catalyst in the present work. In order to obtain information about RO effects on selectivitry, dehydrogenation and dehydration of ethanol was investigated. EXPERIMENT A poled ferroelectric polycrystalline Pbo. 95Sr0. 05 Zr0 .53 Ti0 .47 03 (referred to here as PSZT) and a poled ferroelectric single crystal of z-cut LiNbO3 ( referred to as z-LN) were employed as a substrate. These crystals have a polarization axis normal to surface, thus exposing a positively polarized surface at one plane and a negatively polarized surface at the other. PSZT was in the form of a disc(25 mm in diameter and 0.22 mm in thickness) and showed resonance line of the RE mode at a frequency of 80(first), 209(second), 327(third) kHz and the TE mode at 10, 30 and 50 MHz. A substrate of z-LN has a rectangle with 1.0 mm in thickness, and its TE mode appeared at a frequency of 3.4(First), 10.5(second) and 17.4(third) MHz. In the present work, the first resonance frequencies were used for catalyst activation, unless otherwise specified. Both planes of a PSZT disk were covered with Ag paste electrodes for input of high frequency electric power. The Ag paste electrodes were inactive for the catalytic reactions under the present reaction conditions. A Pt film was deposited on PSZT by an evaporation method with electron beam heating of a pure Pt metal in high vacuum(denoted here as Pt/PSZT). For z-LN, the front and back planes were covered with a catalytically active Ni and Ag thin film which functions as both electrodes and catalysts. The films were deposited at a thickness of 100 nm by resistance-heating of the respective metals in vacuum(referred here to as Ni/z-LN and Ag/z-LN, respectively). High frequency electric power was generated from
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