Insight in kinetics from pre-edge features using time resolved in situ XAS

The kinetics of reduction of a 10 wt %Fe₂O₃-MgAl₂O₄ spinel were investigated using XRD and time resolved Fe-K QXANES. The Rietveld refinement of the XRD pattern showed the replacement of Al with Fe in the spinel structure and the formation of MgFeAlO_x. The XANES pre-edge feature was employed to study the reduction kinetics during H₂-TPR (Temperature Programmed Reduction) up to 730°C. About 55% of the Fe³⁺ in MgFeAlO_x was reduced to Fe²⁺. A shrinking core model, which takes into account both solid-state diffusion via an oxygen diffusion coefficient, and gas-solid reaction through a reaction rate coefficient, was applied. The activation energy for chemical reaction showed a linear dependence on the conversion, increasing from 104 to 126 kJ/mol over the course of material reduction. The good accordance between the shrinking core model description and the experimental data indicates that XANES pre-edge features can be used to correlate changes in material structure and reaction kinetics.