Activation of Thin Film Catalysts Deposited on Ferroelectrics by Surface Acoustic Waves

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In an attempt to develop smart catalysts with functions of activity and selectivity control during catalytic reactions, a device catalyst was prepared using IDT electrode-fabricated ferroelectric substrates which generate surface acoustic waves (SAWs), since the waves are able to give dynamic lattice displacement to thin film catalysis deposited on the path of SAW propagation. Ferroelectric substrates employed were a 128ﾟ-rotated y-cut LiNbO_3 for the generation of Rayleigh SAW (R-SAW) and a 36ﾟ- and a 41ﾟ- rotated y-cut LiTaO_3 for shear horizontal leaky SAW (SH-LSAW) single crystals having different electromechanical coupling coefficients. Between two IDTs, Pd, Ag and Cu catalysis were deposited by vacuum deposition as thin films of 10-50 nm and were subjected to either oxidation or reduction.The SAW effects upon the activity enhancement are strongly dependent on the sueface conditions of the catalyst. For the oxidation of ethanol at 373K on a 30nm Pd catalyst, the R-SAW caused a larger activity increase by a factor of 6 for an oxidized Pb catalyst than for a reduced Pb catalyst. For the oxidized Pb catalyst, activation energy decreased from 81 kJ mol^<-1> to 64kJ mol^<-1> with SAW-on. The XPS study showed that the surface of the oxidized Pb catalyst was covered with adsorbed oxygen, and the character of Pb-O at surface is considered to bring about the higher SAW effects. A difference was also observed between metallic Ni and NiO-covered Ni : activation efficiency by SAW was 1.6 for the former which increased to 6 for the latter. The oxygen partial pressure dependence in the reaction on Ni remained unchanged with SAW-on, whereas it exhibited a maximum for NiO-covered Ni. The activation mechanism by the SAWs is discussed on the basis of the effects such as changes in potential due to lattice distortion, the generation of local electric field, ultrasound-electron interaction and different vibrational modes of the lattice.