Abstract
The effects of surface acoustic waves (SAWs) and resonance oscillations generated on ferroelectric substrates, which have been studied in an attempt to design heterogeneous catalysts with artificially controllable functions, are summarized. A thin film catalyst was deposited on the propagation path of Rayleigh and shear horizontal leaky SAWs which were generated on ferroelectric LiNbO3 and LiTaO3 single crystals. The SAWs caused considerable increases in the activity of ethanol oxidation on Ag, Pd and Ni catalysts and CO oxidation on a Pd catalyst. The enhancement of the catalytic activity is associated with the electromechanical coupling coefficients of the ferroelectrics employed and with the surface states of the catalysts. While the SAWs were on, the activation energy and reaction orders varied significantly for the reactions on the oxidized catalytic surfaces, whereas much smaller changes occurred for the reduced surfaces. The effects of SAWs are discussed on the basis of lattice displacement at surface, the generation of electric fields, and interactions between the carriers in the catalysts and ultrasonic waves. For the resonance oscillations, a disc of polycrystalline Sr-doped lead zirconate titanate was employed as a substrate. For ethanol oxidation on Pd, the activity increased 140-460 fold at a resonance frequency, which is related to a large lattice distortion at the same frequency. Both SAWs and resonance oscillations are concluded to be useful for the development of catalysts with artificially controllable functions.
