Effects of acoustic waves on activation of thin film Pd and Ni catalysts for ethanol and CO oxidation

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The effects of acoustic wave excitation of surface acoustic waves (SAWs) and resonance oscillations (ROs) on catalytic and adsorptive properties of metal thin films have been studied. A 5–200 nm thick Pd thin film was deposited as a catalyst on the Rayleigh SAW propagation path of a 128°-rotated y-cut LiNbO3 single crystal. For ethanol oxidation, the introduction of a 10 MHz SAW increased acetaldehyde production. The activity enhancement varied, depending not only on the surface states (reduced and oxidized Pd surfaces), but also on the thickness of the Pd films: larger activity enhancement occurred for a thick Pd film with an oxidized surface. For CO oxidation, the oxidized Pd had slightly greater effect than reduced Pd, but catalyst activation with SAW-on was small over a whole range of film thickness for both the reduced and the oxidized Pd. The high-resolution IR reflection absorption spectra of CO adsorbed on a 40 nm Pd thin film deposited on the propagation path of a shear horizontal leaky SAW (36°-rotated y-cut LiTaO3) exhibited a large peak, assigned to bridging CO, at 1982 cm-1 and a small on-top CO peak at 2073 cm-1. With SAW-on, the intensity and position of both the peaks remained nearly unchanged, but a shoulder appeared on the bridging CO peak at ca. 1930 cm-1. A laser Doppler measurement showed that a vertical lattice displacement due to the Rayleigh SAW increased with increasing Pd film thickness, up to 50 nm, and was nearly constant over the range 50–200 nm, in which the magnitude of lattice displacement was larger for reduced than for oxidized Pd. SAW-induced catalyst activation is discussed on the basis of reaction mechanisms and the adsorbed states of the reactants. From the dependence of lattice displacement on the thickness and surface states of the Pd films, a model in which the SAW interacts with the charged carriers in the thin films and affects the adsorbed states of the reactants is proposed. Furthermore, it was demonstrated that RO generated on a Pd thin film deposited on z-cut LiNbO3 had a remarkable effect on its catalytic activity for ethanol oxidation.