Combining X-ray Diffraction and X-ray Absorption Spectroscopy to Unveil Zn Local Environment in Zn-Doped ZrO2Catalysts

Zinc-doped zirconia catalytic properties, promising toward CO2and CO hydrogenation, are commonly ascribed to the synergic interaction between Zn and Zr, yet an atomic-scale perspective on how this synergy is structurally realized remains elusive. In this work, to address this ongoing challenge, we deeply investigated the structure of three Zn-doped ZrO2catalysts by combining powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). PXRD showed the complete formation of a tetragonal solid solution, undistinguishable by Rietveld refinement from the cubic polymorph. Fit of extended X-ray absorption fine structure (EXAFS) spectra at the Zr and Zn K-edges unveiled the presence of hexagonal/cubic ZnO nanoclusters embedded and chemically bonded to the tetragonal ZrO2matrix. The concentration of Zn dopant was evaluated via both PXRD and EXAFS analysis.In situEXAFS study of the catalyst during activation further confirmed the presence of a chemical interaction at ZnO/ZrO2interface, most probably the active site toward CO2hydrogenation. The ZnO cluster radius was found to be in the 11-13 Å range, using the Greegor and Lytle spherical model. Taken together, the results demonstrate how the combination of X-ray techniques probing both long-range and local structural properties could unlock an unprecedented level of understanding in mixed metal oxide catalysts.