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Complex oxides like Co3-xFexO4 spinel nanoparticles are promising heterogenous catalysts. However, the identification of quantitative structure-activity relationships of high specific surface area materials like nanoparticles is challenging. Furthermore, the distribution of cations on tetrahedrally and octahedrally coordinated sites in the spinel crystal adds an additional layer of structural complexity.
We present a Reverse Monte Carlo study using rmcxas$^{[1]}$ where a mutual structural model of Co3-xFexO4 spinel is refined simultaneously by EXAFS datasets form the Fe and Co K-edges. Moreover, the refined model differentiates between tetrahedrally and octahedrally coordinated cation absorber sites to enable the identification of the cation distribution.
We find that the Co3-xFexO4 nanoparticles consist of a spinel structure where tetrahedrally and octahedrally cation sites are occupied by both elements and the overall cationic distribution is strongly influenced by the overall elemental composition. Frequencies of the cation pairs Co$\mathrm{_{oct}}$-Co$\mathrm{_{oct}}$ and Co$\mathrm{_{oct}}$Co$\mathrm{_{tet}}$ are extracted from the refined structural model and directly correlate to the catalytic performance in selective oxidation of 2 propanol.$^{[2]}$
[1] Winterer, M. Reverse Monte Carlo Analysis of Extended X-Ray Absorption Fine Structure Spectra of monoclinic and Amorphous Zirconia. J. Appl. Phys. 2000, 88, 5635−5644.
[2] Geiss, J.; Falk, T.; Ognjanovic, S.; Anke, S.; Peng, B.; Muhler, M.; Winterer, M. Atom Pair Frequencies as a Quantitative Structure-Activity Relationship for Catalytic 2-Propanol Oxidation over Nanocrystalline Cobalt-Iron-Spinel. J. Phys. Chem. C 2022, 126, 10346−10358.
