Static and Dynamic Effects of Ferroelectric Piezoelectrics upon Adsorptive Catalytic Properties of Combined Thin Metal and Semiconducting Oxide Films.

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A poled ferroelectric piezoelectrics exposes the positive and negative polar suraces and permits the generation of surface acoustic wave(SAW). In the present work, a poled single crystal of lithium niobate(LinbO_3) was employed as support on which catalytically active phases such as semiconducting oxides(TiO_2,SnO) or metal(Pd) were deposited as thin layers. The static effects of polarization-direction of the support as well as the dynamic effects due to SAW(20 MHz) upon the adsorptive and catalytic properties of the combined catalytically active phases were studied. The changes in surface conductivity due to the chemisorption of various gases such as H_2,CO,CO_2 and C_2H_4 on TiO_2/LiNbO_3 and SnO/LiNbO_3 were markedly greater than for the positive polar support than for the negative polar one, and the differences between the polar supports became larger with increasing ionization potential of the adsorbed gases. These polar direction-dependences were accounted for in terms of band bending caused in the semiconducting oxides by contact with LinbO_3 surface. For the Pddeposited SAW catalysts, the intensity of SAW was found to be sensitive to the adsorption of small amounts of O_2 and H_2. The SAW effects were examined for different kinds of catalytic reactions such as oxidation (CO+O_2, H_2+o_2), decomposition (CH_3OH, CH_3 OH+O_2) and hydrogenation (C_2H_4+H_2), and it was observed that the dynamic SAW effects were prominent for the reactions involving polar molecules.