Effects of Resonance Oscillation on Activation of Thin film Catalysts Deposited on a Ferroelectric Substrate

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In an attempt to design a heterogeneous catalyst with artificially controllable functions, resonance oscillation phenomena generated on a ferroelectric crystal by applying radio frequency electric power were applied. A poled ferroelectric single crystal of lithium niobate (LiNbO_3) was used as a substrate, on which either Pd film or Al was deposited. These metal films were used not only as a catalyst but as an electrode to introduce rf power. In the catalytic ethanol oxidation on a 40 nm thick Pd film deposited on LiNbO_3, resonance oscillation at a frequency of 3.4 MHz at a power of 3 W caused an immediate increase in acetaldehyde production by a factor of 1900, and the high activity was maintained until the power was turned off. The application of non-resonance frequency caused no significant activity enhancement. The activation energy of the reaction was 156 kJ mol^<-1> with power-off, whereas it decreased dramatically to 12 kJ mol^<-1> with power-on. For a sample using a catalytically inactive Al thin film, little increase in catalytic activity was observed. Surface potential measurements showed that the resonance frequency affected the distribution of electrons at surfaces, whereas no change occurred when non-resonance frequency was applied to the catalyst. A laser Doppler method was employed to measure lattice displacement. On the basis of these results, the effects of resonance oscillation which lead to the anomalous enhancement of catalytic activity are considered to be associated with the lattice displacement of catalyst surface and changes in the surface potential.