EFFECT OF SURFACE ACOUSTIC-WAVES ON CATALYTIC ACTIVITY OF REDUCED AND OXIDIZED NICKEL THIN-FILMS DEPOSITED ON A LITAO3 SUBSTRATE

The effect of a surface acoustic wave (SAW) of a shear-horizontal type upon ethanol oxidation over reduced and oxidized Ni catalysts deposited on the propagation path of the SAW was studied. When the SAW was applied, the catalytic activity of the oxidized Ni surface increased by a factor of 6, which was 3.5-fold larger than that for the reduced Ni surface. For the oxidized Ni surface, the SAW caused an anomalous change in the dependence of the activity on the partial pressure of oxygen, P-O: the activity increased with increasing P-O, attained a maximum at 4 kPa, and then decreased. For the reaction in the range of P-O higher than 4 kPa, the reaction order with respect to P-O varied from 0.5 with SAW-off to -0.5 with SAW-on, whereas the order remained unchanged in the range of P-O lower than 4 kPa. The change in the reaction order with SAW-on is explained in terms of the stronger adsorption of oxygen and weaker adsorption of ethanol. For the reduced Ni surface, no significant change in the reaction order with respect to P-O occurred with SAW-on. XPS and the loss of SAW propagation showed that the oxidized Ni catalyst comprised the dual structure of a NiO layer-covered Ni metal, different from a metallic surface of the reduced Ni catalyst. The SAW effect on the catalytic reaction is discussed in terms of possible mechanisms, and it is proposed that the marked SAW effect on the NiO-covered Ni catalyst is related to semiconducting properties of the oxide surface, which has an electric field and acoustoelectric effect and possibly a unique vibrational mode caused by interactions with the SAW.