Composite electrochemical catalysis utilizing mixed potential at the heterogeneous contact

本研究旨在开发一种新型的复合电化学催化剂，该催化剂可以利用具有离子-电子混合导电性的多孔氧化物电极、固体电解质和金属电极之间的非均相界面处的气体物种的异常反应性。在微观尺度上，催化被看作是具有非对称电极结构的微尺度电池的混合，它们利用所谓的混合电位，这与非均相界面上的空间电荷效应密切相关。在项目的前期，我们考察了Nd_<2-X>Ce_xCuO_4、Nd掺杂CeO_2和Au粉末的混合物对NO/NO_2 Red-Ox反应的催化活性。与单组分催化剂相比，复合催化剂对该反应的催化活性有明显提高。NiO、YSZ、Au复合粉体对NOx Red-Ox的催化反应也有一定的增强作用。为了研究增强催化剂的作用机理， 关于我们 研究了复合材料、具有三个电极的电化学电池的反应。结果表明，该反应的速率决定步骤是NO_2和O_2在多孔氧化物上的竞争吸附和电子转移反应的混合过程，以及在Au电极上的简单电子转移反应。由模型推导出两种反应的交换电流密度。应当指出的是，由移动的离子和吸附的气体物种在非均相接触界面的界面处建立的空间电荷的弛豫是选择性增强催化反应的基本机理。因此，氧化物的电子能带结构和电子输运性质在混合电位的形成中起着重要作用，这是理解复合电催化剂异常反应性的关键之一。基于这些观点，调查的可能的候选人的氧化物电极进行根据电子和离子的传输性能和电子能带结构的详细调查，通过光电子能谱。由于NOx的直接还原由于非常高的活化能而难以进行，因此提出了利用氧化物电极中的质子或溶解水进行间接还原反应。对氧化物质子导体也进行了详细的讨论。少

The present research project was aimed to develop a novel composite electrochemical catalysis which can utilize anomalous reactivity of gas species at the heterogeneous interface among porous oxide electrode with ion-electron mixed conductivity, solid electrolyte, and metallic electrode. In a microscopic scale, the catalysis is regarded as the mixture of microscaled cells with asymmetric electrode configuration that utilize so-called mixed potential, which is closely related to the space charge effect at the heterogeneous interface. In the early stage of the project, the mixture of Nd_<2-X>Ce_XCuO_4, Nd-doped CeO_2, and Au powder were examined for the catalytic activity to the NO/NO_2 Red-Ox reaction. The composite catalysis showed enhanced activity for the reaction compared to those of single component ones. It is also shown that the composite of NiO, YSZ, Au powder also showed the enhanced catalytic reaction for NOx Red-Ox. In order to examine the mechantism of the enhanced catalytic … More reaction of composites, electrochemical cells with three electrodes were examined. We found that the rate determinant steps are the mixed process of the competent absorption of NO_2 and O_2 on porous oxide and electron transfer reaction, and the simple electron transfer reaction on the Au electrode. The exchange current density of both reactions are deduced from the model. It should be noted that the relaxation of space charge, built-up at the interface of heterogeneous contact interface by both mobile ion and the adsorbed gas species is the essential mechanism for the selective enhancement of the catalytic reaction. Therefore the electronic band structure and electronic transport properties of oxides are estimated as playing important role in the formation of the mixed potential, that is one of the key to understand the anomalous reactivity of composite electrochemical catalysis. Based on these view points, the survey of possible candidates for the oxide electrode is conducted in accordance with the detailed investigation of electronic and ionic transport properties and electronic band structure by means of the photoelectron spectroscopy. As the direct reduction of NOx is difficult because of the very high activation energy, it is proposed the utilization of the proton or dissolved water in oxide electrode for the indirect reduction reaction. The detailed discussion on the oxide protonic conductors are also conducted. Less