Reactivity of solids studied by in situ XAS and XRD

High-temperature oxidation processes of metals are important solid-state reactions from a fundamental point of view and also concerning technical applications. The kinetics of these solid-state reactions can be studied in situ by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) at elevated temperatures and defined oxygen partial pressures. Oxidation of thin cobalt foils in air was studied by means of Quick X-ray Absorption Spectroscopy (QEXAFS) at the Co–K edge. By deconvolution of the spectra, the fractions of Co, CoO and Co3O4 were determined as a function of time. The overall oxidation reaction can be described by a three-step mechanism in which CoO is formed intermediately. By in situ XRD, the growth kinetics of both oxides and the depth-resolved texture of the growing oxides were determined simultaneously. At the beginning of the oxidation process, a nearly statistical distribution of oxide grain orientations is found, whereas the evolution of a distinct texture is observed during further growth of the oxide scale.