X‐ray absorption fine structure characterization of a multicomponent spinel catalyst

Demand for high-performing, inexpensive catalysts has motivated the development of mixed 3d transition metal architectures. Here, x-ray spectroscopies are uniquely positioned to inform intensive searches of compositional spaces by elucidating structure-function relationships. However, thorough analyses of complex systems are often non-trivial. In this work, one such example was pursued, a chemical series of spinel-based emissions catalysts of the general formula Mn0.1CuxCo2.9-xO4 for 0.1 <= x <= 0.8. The local electronic and atomic structures of these materials were characterized by x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) analyses, respectively. EXAFS analyses required models to be constructed that captured the relevant structural changes across the chemical series. Building these models required insights from XANES, x-ray diffraction (XRD), and inductively coupled plasma atomic emission spectroscopy analyses. XANES measurements at the Co K-edge, when complemented with ab initio multiple scattering calculations, suggested that the fraction of Co atoms in the octahedral sites of the spinel increased as the concentration of substituted Cu increased. Similarly, XANES measurements at the Mn K-edge suggested that the occupancy of the first coordination shell varied throughout the series. Finally, the XRD results revealed an impurity, a CuO phase, formed at higher copper concentrations. This is consistent with the results of the principal component analysis performed on the Cu K-edge XANES spectra. These hypotheses were incorporated into the EXAFS fitting models and are consistent with the subsequent quantitative analyses.