A11: Modulation of Liquid Phase Thermal Oxidation Catalysis using Optical Stimuli

Prof. Dr. Bastian Mei

This project is based on the detailed understanding developed within the first funding period of the CRC and will focus on the liquid-phase oxidation of 2-propanol, cyclohexane, cyclohex­ene, and ethylene glycol with Co_xFe_3-xO₄ spinels of different shape allowing to disclose the impact of charge-carrier enhancement. The goal is to obtain kinetic information and to develop in-depth knowledge on the specific influence of the surface charge carrier concentration on the C-(O)H bond activation, the product selectivity and the decomposition of organic hydroperoxides within the temperature range used in thermo-catalytic reactions. Kinetic studies will be accompanied by spectroscopic investigations using ATR-FTIR spectroscopy thereby allowing to further elaborate on the reaction mechanism. Moreover, the activity of individual facets will be investigated on faceted metal oxide particles. Using electron traps (“internal cathodes”) allows to enhance the transfer of electrons to charge separated states at the unmodified facet. The outcome of the proposed research will be fully integrated into the progress on identification of re­action mechanisms and active sites in liquid-phase oxidation catalysis and performed in strong collaboration with projects A01, A02, A09 investigating the catalytic properties of the metal oxides and the projects B03, C01, C03, and C05 that are essential to provide well-defined materials and elaborate on their surface characteristic and their surface charge carrier population.

The project aims at bridging the world of electrocatalytic oxidation and heterogeneous aqueous-phase (aerobic) oxidation on well-defined Co_xFe_3-xO₄ spinels and will further develop and strengthen the role of thermal contributions in liquid-phase catalysis. The working hypothesis of A11 is that the formation of non-equilibrium surface charge carriers by photon excitation will lead to separation of redox reactions on the metal oxide surface, which will allow to deduce the common active sites, elementary steps and reaction mechanisms in thermal and electrocatalytic conversion under mild reaction conditions. This hypothesis will be studied for selected oxidation reactions. The working principle of such non-equilibrium charge carrier-enhanced oxidation is depicted in Figure 1 showing that upon photon excitation non-equilibrium charge carriers will drive redox reactions at the surface of semiconducting metal oxide nanoparticles. The operation of charge carrier-enhanced thermal oxidation is straightforward and allows for a systematic comparison of the same (model) catalyst in a selection of different oxidation reactions while closely mimicking the operation under thermal conditions. Thus, the project will provide valuable information to correlated mechanistic understanding, catalyst deactivation and formation of active sites in the course of catalytic reactions obtained in A01 and A02.

(Figure: Schematic repre-sentation of the research rationale: The interface between a metal oxide nanoparticle (here SrTiO₃) and an electrolyte is depicted schematically. The facet-dependent accumulation of charge carriers at the surface after photon excitation is driven by an internal electric field, which results in a facet-dependent reactivity.)