Effects of acoustic waves-induced dynamic lattice distortion on catalytic and adsorptive properties of metal, alloy and metal oxide surfaces

Resonance oscillation (RO) of bulk acoustic waves and surface acoustic wave (SAW) that were generated by piezoelectric effects on poled ferroelectric crystals by rf electric power enabled to cause the dynamic distortion of thin metal and metal oxide film surfaces deposited. The present paper reviewed the advances in the effects of RO and SAW on catalysis and surface properties. In C2H5OH and CO oxidation and C2H5OH decomposition on Ag and Pd surfaces, the thickness extension mode RO (TERO) had 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 were responsible for strong catalyst activation. The SAW changed the reaction orders and lowered the activation energy for CO and C2H5OH oxidation on Pd and Ni catalysts. In C2H5OH decomposition over AgAu catalysts, the TERO effects had strong dependence on alloy compositions and became strongest at the specific alloy composition. In infrared absorption spectroscopy (IRAS) study, the SAW decreased the peak intensity of CO adsorbed on Cu(111), whereas it increased the CO peak assigned to high-index planes and step sites. Photoelectron emission microscope study showed that the SAW increased the work function of the smooth metal surfaces and decreased that for rough metal surfaces. The effects of TERO and SAW were related to the work function changes, and it was proposed that the vertical lattice displacement causes spatial changes in coordinatively-unsaturated surface atoms located at the high-index planes and step sites. The effects of SAW on the adsorptive properties of a thin NiO film, measured by conductivity changes, showed that the SAW accelerated both adsorption and desorption that were associated with alternating changes in the charge distributions in the metal oxide by dynamic lattice displacement. The acoustic wave effects being a new concept in heterogeneous catalysis are encouraging for the development of a physical method to artificially change the electronic states and hence their chemical properties of catalyst surfaces.