An in situ study using anomalous wide-angle X-ray scattering and X-ray absorption spectroscopy of the catalytic system ZnAl2O4 supported on alumina

A combined approach based on X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous wide-angle X-ray scattering (AWAXS) has been used to obtain information on the cation distribution, the electronic state of the metal atoms, and the size of metal oxide clusters for a highly dispersed spinel phase, here the supported system ZnAl2O4/Al2O3. Through numerical simulations of the modulations of the X-ray absorption coefficient and ab initio calculations of the differential diffraction intensities, it has been shown that the tetrahedral sites of the spinel normally occupied by zinc atoms are partially filled. A model of the zinc distribution inside the alumina particles is proposed, in which the zinc atoms are mostly located at the surface of the alumina particle with a concentration gradient between the surface and the particle center. In situ studies reveal a significant increase of the cell parameter as well as a dramatic increase of the Debye-Waller factor associated with the vibrations of the zinc-zinc pairs for a spinel-like environment of zinc when such a phase is submitted to gas-solid reactions such as those employed for automotive exhaust control. An explanation of these two effects is given on the basis of the incorporation of a light atom (such as O) coming from the reactive gases into the solid via the defective properties of the ZnAl2O4/Al2O3 spinel surface.