Prof. Dr. Bastian Mei
In the current second funding period (FP2), A11 aims to answer the following fundamental research question: How does the formation of non-equilibrium charge carriers on the surface of metal oxide particles, which arise from narrow-band light excitation, impact proton-coupled electron transfer steps, and consequently, the activity and selectivity of heterogeneously catalyzed liquid-phase oxidation reactions? This question is currently being explored for C-H bond activation of cyclohexane and ethylene glycol oxidation using the various Co3O4 materials (non-doped variations) available within the CRC. Clear evidence for light-induced modulation of conversion and selectivity has been obtained. Given the early stage of the project (start date 7/2024), various open questions remain to be addressed in more detail in the forthcoming months, thereby solidifying the CRC internal collaborations with both experimental and theoretical groups.
In FP3, the project will concentrate on charge carrier density modulation by pulsed and/or intensity modulated pulsed irradiation (controlled periodic illumination (CPI)) using the various samples of semiconducting nature provided by the C projects. Initially, undoped materials such as Co3O4 and LaCoO3 will be screened for light-modulated activity enhancement. Subsequently, (un)doped samples will be studied in order to achieve an in-depth understanding of CPI effects. The general hypothesis of A11 guiding the work of FP3 is that pulsed and intensity modulated irradiation directly impacts the dynamics of the (frustrated) phase transition (FPT) of the catalyst surface. Changes in the dynamics (of the FPT) are assumed to have implications for conversion and product selectivity, particularly with regard to the mitigation of overoxidation and catalyst stability. Furthermore, pulsed irradiation is regarded to have an impact on transport phenomena, which may result in modifications in product selectivity. Consequently, thermal oxidation catalysis coupled with additional stimuli provided by CPI control and advanced characterization are considered to provide an in-depth understanding of active sites. The knowledge gained from batch reactor studies will be transferred to flow processes, and the approach of charge carrier-enhanced oxidation will be extended to electrochemical experiments. Catalytic insights will be substantiated using a variety of methods, including luminescence (C01), X-ray-based characterization techniques (e.g., C07(N)), and electron paramagnetic spectroscopy (B06). In agreement with the overarching conceptual framework of the CRC, the approach of A11 using CPI-modulated thermal oxidation catalysis will facilitate a more comprehensive understanding of the fundamental steps involved in liquid phase oxidation and the identification of active site motifs.
Figure: left: Photothermal cyclohexane oxidation at 110 °C, 5 bar O2 pressure, 6 h. The reaction was performed with and without 450 nm LED illumination using Co3O4; right: schematic electronic structure of Co3O4 adapted from reference
A11: Modulation of Liquid Phase Thermal Oxidation Catalysis using Optical Stimuli
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 CoxFe3-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 CoxFe3-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.
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Last update: Jul 28, 2026
