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

为了开发具有催化反应活性和选择性控制功能的智能催化剂，利用表面声波(SAW)对沉积在SAW传播路径上的薄膜催化剂产生动态晶格位移的特性，利用IDT电极制备的铁电衬底制备了一种装置催化剂。所采用的铁电衬底是用于产生瑞利声表面波(R-SAW)的12 8ﾟ旋转y切LiNbO_3和用于剪切水平漏波(SH-LSAW)的不同机电耦合系数的36ﾟ和41ﾟ旋转y切LiTaO_3单晶。在两种IDT之间，用真空沉积的方法将Pd、Ag和Cu催化剂沉积成10-50 nm的薄膜，并进行氧化或还原。对于373K下乙醇在30 nm Pd催化剂上的氧化反应，氧化铅催化剂的活性比还原铅催化剂的活性提高了6倍。氧化后的铅催化剂的活化能由81kJ mol1-1降至64kJ mol1-1。XPS研究表明，氧化后的铅催化剂表面覆盖了大量的吸附氧，表面的铅-氧的性质导致了较高的表面声表面波效应。金属镍和NiO包覆镍的SAW活化效率也不同，前者的活化效率为1.6，后者的活化效率为6。反应中对Ni的氧分压依赖关系随时间变化不大，而NiO包覆的Ni对反应的氧分压依赖关系最大。从晶格扭曲引起的电势变化、局域电场的产生、超声-电子相互作用以及晶格的不同振动模式等方面讨论了声表面波的激活机理。

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.