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

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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.