Prof. Dr. Heiko Wende
Crucial for optimizing Co-based spinel and perovskite catalysts in heterogeneous liquid phase oxidation reactions, this project utilizes advanced techniques to analyze structural and spin-resolved electronic properties. The focus of the project are element specific investigations of magnetic properties (i.e., spin state, local spin and orbital moments, element specific hysteresis) and the electronic structure (e.g., valency of cobalt and dopant atoms such as Mn and V in as-prepared and spent catalysts) along (frustrated) phase transformations during catalysis. X-ray absorption spectroscopy (XAS) and especially X-ray magnetic circular dichroism (XMCD) measurements will probe e.g., 3d states and magnetic properties of the transition metals, complemented by positron annihilation spectroscopy (PALS) for oxygen vacancy detection. We integrate this with operando Mössbauer spectroscopy on 57Fe-enriched samples under electrochemical conditions and high magnetic field/low temperature Mössbauer spectroscopy. This multi-technique approach, correlated with DFT calculations performed in the theory projects, establishes fundamental structure-property relationships, enabling rational catalyst design.
Figure: (a) Mössbauer spectra recorded at 4.3 K and an applied field of 10 T parallel to the 
-ray propagation direction of Co2FeO4 calcined at 400 °C, 800 °C, and 900 °C. Experimental data (black dots) is shown together with a theoretical data fit (red line) composed of several superimposed subspectra, representing Fe atoms residing on B- (green) and A-sites of the spinel lattice, with latter being assigned to contributions from Fe3+ ions in Fe-rich (blue) and Co-rich (violet) environments. (b) Demonstration of importance of high magnetic field measurements: experimental data (black dots) shown together with a theoretical data fit (red line) composed of several superimposed subspectra (same color code as before). The contributions of the Fe3+ ions in Fe-rich (blue) and Co-rich (violet) environments can be clearly separated due to the stronger splitting (dotted arrow) induced by the 10 T field (bottom). At 5 T (top) these contributions are strongly superimposed and only visible as one broad contribution (magenta.) [See related publication.]
B02: Element-Specific Electronic, Magnetic and Local Geometric Structure Effects in Mixed Oxide Catalysts
Our main goal for the second funding period is the extension of our local, element-specific methodology to catalysts in the liquid phase. The aim is to improve the catalysts’ performance while also encompassing the shift to different material systems that have gained importance within the CRC during the first funding period.
To achieve this transition from dry powder samples to liquid phase systems, we will construct a cell to enable in operando Mössbauer measurements of the Fe-containing catalyst material. Building on the results of our preliminary experiments, we aim to optimize the construction of this cell to achieve maximum efficiency in terms of reducing the time required to record spectra, maximizing the signal to noise ratio, and ensuring minimum non-resonant absorption and stray signals thanks to the use of mesh counter electrodes. Our other main goal consists of the in-depth characterization of catalyst materials from research area C with our new 10 T magnet cryostat, permitting a much more precise and detailed determination of the distribution of ions across crystallographic sites (degree of inversion for spinel systems). This is of particular importance for Co-rich systems that are currently of high interest for catalysis, with dramatic changes in underlying magnetic properties being used as a probe for the improvement of catalytic activity. Our ability to produce well-ordered Co-oxide reference systems with our pulsed laser deposition (PLD) system will provide additional possibilities to achieve a more detailed understanding of the local geometric and structural properties, enabling further optimization when selecting appropriate synthesis methods and material compositions for an increased catalytic efficiency. These films, along with selected samples produced within the CRC, are also to be characterized in the soft X-ray regime, utilizing X-ray magnetic circular dichroism (XMCD) to perform element-specific measurements even on Fe-free systems, which is of notable interest for Co-oxide catalysts as well as V-doped Co-oxides.
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Last update: Aug 17, 2026
