Artificial Control of Catalytic Activity of Thin Film Catalysts by Surface Acoustic Waves

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The artificial control of catalytic activities in heterogeneous catalysis is one of the most important topics. Surface acoustic waves(SAWs) generated on ferroelectric crystals by electric power have been drawn attention. In the present study, different types of the SAWs were employed: one is a Rayleigh SAW(R-SAW) with the vibration vertical to the crystal surface, and the other is a shear-horizontal leaky SAW(SH-LSAW) with vibration parallel to the surface. A 128ﾟ-rotated Y cut LiNbO_3 and a 36ﾟ-rotated Y cut LiTaO_3 single crystal were used. The input and output interdigital transducer(IDT) electrodes were photolithographically fabricated on each end of the crystal surface, Metals(Cu,Pd) and oxides(NiO, TiO_2) were deposited as a thin film catalyst by evaporation in the middle of two IDT electrodes. The catalytic activity of these thin film catalysts for ehtanol oxidation to produce acetaldehyde at 343-363 K increased by a factor of 3-4 with R-SAW-on at 1W. The R-SAW caused de- crease … More s in the activation energy of the reaction from 55 to 39 KJ mol^<-1> for the Cu catalyst and from 42 KJ mol^<-1> to 29 KJ mo1^<-1> for the Pd catalyst. With a 10 nm-thick Pd film, the activity increase by the SH-LSAW was close to 3 for the reaction at 343 K. These results indicate that the effects upon the increase of the catalytic activity are similar between the SH-LSAW and the R-SAW. A Cu-doped ZnS thin film was deposited on the propagation path of the crystal. The propagation of these SAWs gave rise to luminescence, which indicated the ability of the SAWs to generate high electric fields which were estinated to be 9x10^3 V cm^<-1> for the R-SAW and 1x10^4 V cm^<-1> for the SH-LSAW. This indicates the contribution of the high electric field to the catalyst activation. From the present study, it is concluded that both the R-SAW and the SH-LSAW have the intrinsic effects on the increase of catalytic activity, and the application of SAWs leads to the development of catalysts whose catalytic functions are artificially controlled. Less