Electronic structure of Lewis acid sites on high surface area aluminium fluorides: a combined XPS and ab initio investigation

Abstract

High surface area (HS) AlF₃ samples have been examined by X-ray photoelectron spectroscopy (XPS). The experimentally observed binding energy (BE) shifts were analysed by reference to core level BEs obtained from ab initio total energy calculations on a range of different, clean and hydroxylated α- and β-AlF₃ surfaces. Examination of the two components visible in the Al 2p emission indicates that surface Al³⁺ sites can, depending on the local geometric structure, contribute to both a high BE peak at 77.0 eV and a low BE peak at 76.1 eV. Consequently, the areas under the peaks do not quantitatively correlate with surface area or Lewis acidity. However, a significant correlation between the number of surface Al centres with dangling F or OH groups and the appearance of an Al 2p emission component at a BE lower than in the α-AlF₃ bulk is predicted. The experimental F 1s emission data indicate that dangling F species are essentially absent. Examination of the O 1s emission suggests that HS AlF₃ handled at room temperature under any practical laboratory conditions, including glovebox environments, probably contains intrinsically a significant amount of OH groups and adsorbed water, which results in the covering of AlF₃ surfaces by dangling or bridging OH groups. These Brønsted acid species must be removed by treatment at higher temperature before HS AlF₃ reagents can fully develop their Lewis acidity.