Design of heterogeneous catalysts with artificially controllable functions using effects of acoustic wave excitation

The effects of surface acoustic waves (SAWs) and resonance oscillation (RO) of bulk acoustic waves on catalytic activity and selectivity were studied in an attempt to design a heterogeneous catalyst which has artificially controllable functions for chemical reactions. The propagation of Rayleigh SAW and shear horizontal leaky SAW through thin Pd catalyst films deposited on poled ferroelectric LiNbO₃ and LiTaO₃ crystals caused considerable activity increases in ethanol oxidation: the enhancement was much larger for the oxidized than the reduced Pd. Similar effects were observed for a Ni catalyst. The RO effects on catalyst activation for ethanol oxidation were associated with the polarized (positive and negative) surfaces and the vibration modes of the ferroelectric substrates. The thickness extension mode resonance oscillation (TERO) of z-cut LiNbO3 caused different changes in the activation energy and reaction order between the positive and negative plane of the LiNbO3 substrate. Different catalyst activation was induced between TERO and the radial extension mode resonance oscillation (RERO) of a Pb_0.95Sr_0.05Zr_0.53Ti0.47O₃ ferroelectric substrate. For ethanol decomposition on a Ag catalyst, the TERO of z-cut LiNbO₃ increased ethylene production without significant enhancement of acetaldehyde production, thus demonstrating that TERO has the ability to change the selectivity in the catalytic reaction. Large dynamic lattice displacement, surface potential changes, and work function shifts were observed with TERO. A mechanism of acoustic wave excitation effect on the catalyst activation is discussed.