Heterogeneous Oxidation Catalysis in the Liquid Phase

B10: Probing the Oxide Interface Structure under Reaction Conditions by Operando Surface X-Ray Diffraction

Prof. Dr. Olaf Magnussen

The central goal of this project is to determine the structure of Co3O4 and (FexCo1-x)O4 oxide interfaces in liquids under conditions of catalytic oxidation reactions, especially the oxidation of alcohols (e.g., ethylene glycol). Of key interest is whether the oxide catalyst is stable under reaction conditions or transforms structurally in the near-surface region, as found in previous studies under oxygen evolution conditions. This will be clarified by operando surface X-ray diffraction (SXRD) studies of thin epitaxial oxide films with defined surface orientation and morphology. Measurements of alcohol oxidation will be performed (i) for electrocatalysis in alkaline electrolyte at different temperatures (20-80 °C) and (ii) for heterogeneous oxidation catalysis in O2-saturated water and acetonitrile under moderate conditions (80-150 °C, 1-5 bar), which will provide a bridge between liquid phase thermal catalysis and electrocatalysis. For both cases, we will investigate in systematic studies whether reversible or irreversible surface transformation occurs and how its onset and extent depend on critical reaction parameters (potential, temperature, pressure, reactant concentration). We will further determine how the oxide film structure, its morphology, and its Fe content influence these surface structural changes. These data will be correlated with the catalytic properties, which should allow to clarify the role of such structural transformations in the oxidation reaction as well as provide insights into catalyst degradation mechanisms.

(Figure: Operando SXRD. (A) Schematic principle, illustrating how the diffracted X-ray intensity is measured as a function of scattering vector with a 2D detector. (B) Electrochemical cell for operando SXRD studies. (C) Potential-induced reversible changes in the intensity of a Co3O4 Bragg peak, showing the employed measurements principle for fast operando studies of epitaxial films).