Composite electrochemical catalysis utilizing mixed potential at the heterogeneous contact

本课题旨在开发一种新型复合电化学催化剂，利用具有离子-电子混合电导率的多孔氧化物电极、固体电解质和金属电极之间的非均相界面上气体的异常反应性。在微观尺度上，催化作用被认为是具有不对称电极结构的微尺度电池的混合，利用所谓的混合电位，这与异质界面上的空间电荷效应密切相关。在项目前期，考察了Nd_<2-X>Ce_XCuO_4、nd掺杂CeO_2和Au粉的混合物对NO/NO_2 Red-Ox反应的催化活性。复合催化剂的催化活性明显高于单组分催化剂。此外，NiO、YSZ、Au粉的复合材料对NOx Red-Ox的催化反应也有增强。为了研究复合材料增强催化…More反应的机理，对三电极电化学电池进行了研究。研究发现，决定速率的步骤是NO_2和O_2在多孔氧化物上的有效吸收和电子转移反应的混合过程，以及Au电极上的简单电子转移反应。根据模型推导出两种反应的交换电流密度。需要指出的是，在非均相接触界面上，移动离子和吸附气体共同形成的空间电荷弛豫是催化反应选择性增强的重要机制。因此，认为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，是理解复合电化学催化异常反应性的关键之一。在此基础上，利用光电子能谱对氧化电极的电子、离子输运性质和电子能带结构进行了详细的研究，对氧化电极的候选材料进行了考察。由于NOx的活化能很高，直接还原很困难，因此提出利用质子或氧化物电极中的溶解水进行间接还原反应。并对氧化物质子导体进行了详细的讨论。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