TY - JOUR A1 - Böse, Olaf A1 - Unger, Wolfgang A1 - Kemnitz, E. A1 - Schroeder, S. L. M. T1 - Active sites on an oxide catalyst for F/Cl-exchange reactions: X-ray spectroscopy of fluorinated y-Al2O3 JF - Physical chemistry, chemical physics N2 - The dismutation of CHClF2 to CHF3 and CHCl3 was used to probe the effect of γ-Al2O3 fluorination on fluorine-against-chlorine (Cl/F) exchange reactions. X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES) and X-ray absorption near edge structure (XANES) spectroscopy were employed to probe the modifications at the surface of γ-Al2O3 accompanying the initial stages of fluorination. The results suggest that fluorine uptake by γ-Al2O3 starts initially at the surface of the oxide particles, with the formation of a fluorine species that is characterised by a single bond and a high effective charge q. Catalytic Cl/F-exchange is only observed after accumulation of higher fluorine concentrations, for which insertion of fluorine into the sub-surface region of the oxide crystallites has commenced. Quantitative XPS shows that the fluorination level must exceed approximately 10 atom% of the anions to achieve catalytic activity. The spectroscopic evidence suggests that the formation of aluminium oxofluorides is sufficient to provide catalytic activity. The presence of an AlF3 phase is not a condition for the observation of catalytic activity, even though defect-rich AlF3 is ultimately the endpoint of the fluorination process and is itself a Cl/F exchange catalyst. PY - 2002 DO - https://doi.org/10.1039/b110792e SN - 1463-9076 SN - 1463-9084 VL - 4 IS - 12 SP - 2824 EP - 2832 PB - The Royal Soc. of Chemistry CY - Cambridge AN - OPUS4-6940 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -