Application of Resonance Oscillation to the Design of Heterogeneous Catalysts with Artificially Controllable Functions for Chemical Reactions

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Resonance oscillation (RO) of bulk acoustic waves generated by piezoelectric effects on poled ferroelectric crystals by rf electric power causes the dynamic distortion of thin metal film surfaces deposited. In C_2H_5OH and CO oxidation and C_2H_5OH decomposition on Ag and Pd catalysts, thickness-extension mode RO (TERO) was found to have a capability of enhancing not only catalytic activity, but also reaction selectivity. In contrast, thickness-shear mode resonance oscillation (TSRO) showed no activation effects. Laser Doppler and photoemission spectroscopic measurements showed that TERO caused large lattice displacement vertical to the surface and to induce work function shifts of the metal catalysts, whereas no such changes occurred with TSRO. The TERO effects on catalyst activation depended on the polarization direction of ferroelectric substrates and resonance frequency, indicating that large vertical lattice displacement and the density of sites causing lattice displacement are responsible for strong catalyst activation. The TERO effects were intrinsically different in ethanol decomposition between Ag and AgAu alloy catalysts. TERO-induced activation occurred only for ethylene production on a Ag metal, whereas TERO effects had strong dependence on alloy compositions. The activation for ethylene production was nearly independent of alloy composition, but that for acetaldehyde production exhibited a maximum at around 80% surface Au content where the vertical lattice displacement became the smallest. Thus, it was proposed that the TERO effects on acetaldehyde production over the AgAu binary alloy catalysts increased in the case where Ag-Au atom interactions became strong. The RO effects on heterogeneous catalysis by metals are new phenomena and are concluded to be quite useful for the development of a heterogeneous catalyst with artificially controllable functions for chemical reactions.