Mixed-oxide formation during preparation of alumina-supported zirconia: an EXAFS and DFT study

Abstract

Alumina-supported zirconia catalysts, containing from 2.1 to 20.1 wt% ZrO₂ and prepared by multiple incipient wetness impregnation using benzene solutions of zirconium acetylacetonate, were characterised using the EXAFS technique. The results indicated that the supported phase consists of Zr⁴⁺ species that do not have Zr as second neighbours, hexacoordinated to oxide anions, suggesting that these ions occupy octahedral positions in the defective spinel structure of the γ-alumina support. In order to test this hypothesis, an idealised unit cell was constructed with the formula ZrAl₄O₈, with spinel structure. Atomic positions in this cell were optimised using theoretical calculations based on DFT. The smallest energies corresponded to structures where the Zr⁴⁺ ions occupy octahedral sites in the spinel structure. Simulations of the EXAFS spectra using this structure provided excellent agreement between the theoretical and experimental spectra.