Active sites on an oxidecatalyst for F/Cl-exchange reactions: X-ray spectroscopy of fluorinated γ-Al2O3

Abstract

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.