Catalysis mediated by oxygen storage: HCl Oxidation Reaction over CeO2-x(111)-based Model Catalysts

一般来说，多相催化被认为是一种纯粹的表面现象，在催化剂表面，产物被转化为所需的产物。然而，这种观点对于混合导电（电子和离子导电）的可还原氧化物来说过于狭隘，因为不仅电子与反应物交换，而且来自大部分催化剂材料的氧离子也可以直接参与催化反应。将固态离子与多相催化联系起来的第一种方法是使用氧储存容量（OSC）的概念。事实上，固态离子和非均相催化之间的关系比OSC所能捕捉到的更为复杂。氧缺陷的化学性质影响催化剂的电子结构、活性位点的化学性质等。在本项目中，催化与固态缺陷化学之间的密切关系将以一个技术上重要的多相催化反应为例，即通过CeO2催化剂催化氧化从工业上普遍存在的副产物HCl中回收氯。迪肯工艺允许工业过程在氯循环方面是封闭的。我们计划在合适的单晶CeO2（111）模型体系上进行表面化学和固态离子学实验。本文将结合基于密度泛函理论方法（DFT）的从头算理论（ab-initio theory），对反应机理进行深入的研究，重点研究氧缺陷在体和表面的作用。我们将开发一种CeO2（111）薄膜电化学电池，它可以通过电化学泵送来改变氧缺陷浓度，而不依赖于实际的反应条件。通过特殊设计的间歇式反应器，我们将能够研究本体和表面氧缺陷对催化HCl氧化动力学的影响，并使用基于operando同步加速器的方法确定表面结构和Ce3+浓度。特别是，可以识别催化活性相，并研究其在反应条件下的稳定性。

In general, heterogeneous catalysis is regarded as a pure surface phenomenon where the educts are converted to the desired product at the catalyst surface. However, this view is too narrow for mixed-conducting (electron and ion conduction) reducible oxides, since not only electrons are exchanged with the reactants, but also oxygen ions from the bulk of the catalyst material can participate directly in the catalytic reaction. A first approach to correlate solid state ionics with heterogeneous catalysis uses the concept of oxygen storage capacity (OSC). In fact, the relationships between solid state ionics and heterogeneous catalysis are more complex than what can be captured by OSC. The oxygen defect chemistry influences the electronic structure of the catalyst, the chemical nature of the active sites, and much more. In this project, the close relationship between catalysis and solid state defect chemistry will be exemplified with a technically important heterogeneous catalysed reaction, namely the recovery of chlorine from the industrially omnipresent by-product HCl via catalytic oxidation over CeO2 catalysts. The Deacon process allows industrial processes to be closed with regard to chlorine cycle. We plan to conduct experiments on surface chemistry and solid state ionics on suitable single-crystalline CeO2(111)-based model systems. In close interaction with ab-initio theory based on the density functional theory method (DFT), the reaction mechanism will be elucidated with special focus on the role of oxygen defects in the bulk and on the surface. We will develop a CeO2(111) thin film electrochemical cell, which allows to vary the oxygen defect concentration by electrochemical pumping, independent of the actual reaction conditions. With a specially designed batch reactor we will be able to investigate the influence of oxygen defects in the bulk and at the surface on the kinetics of the catalysed HCl oxidation and to determine the surface structure and Ce3+ concentration with operando synchrotron-based methods. In particular, the catalytically active phase can be identified and its stability under reaction conditions can be investigated.