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

Abstract

High surface area (HS) A1F3 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 alpha- and beta-A1F3 surfaces. Examination of the two components visible in the A1 2p emission indicates that surface A13+ 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 A1 centres with dangling F or OH groups and the appearance of an A1 2p emission component at a BE lower than in the alpha-A1F3 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 A1F3 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 A1F3 surfaces by dangling or bridging OH groups. These Bronsted acid species must be removed by treatment at higher temperature before HS A1F3 reagents can fully develop their Lewis acidity.

Surface plasma pretreatment for enhanced diamond nucleation on AlN

“…Finally, after CF 4 plasma pretreatment, the surface of AlN converts into AlF 3 as derived from the Al2p 3=2 peak position of 76.6 eV (Fig. 3(d)), which agrees well with previous reports 18 and reflects the highest oxidation state observed for our plasma pretreated samples. The attenuation of the N1s photoelectrons (Fig.…”

“…The ionic bond Al-F is so strong that electrons are practically attached to F. 25,26 Here, the presumption of the F presence at the surface has to be made. 18,24 In that case, electrons are attracted from the bulk to the surface at F sites. The consequent negative charge at F attracts the ND seeds and a very strong hydrogen bond forms between F and R-COOH.…”

Aluminum Fluoride‐Supported Platinum and Palladium as Highly Efficient Catalysts of n‐Pentane Hydroisomerization

“…[46] XPS studies of Makarowicz et al showed that handling of freshly fluorinated HS-AlF 3 under a protecting Ar atmosphere and in a glovebox exposes the samples to higher humidity levels, so that Lewis sites are probably readily covered with OH or H 2 O surface species. [47] However, the authors argue that these Brønsted acid species should be removed by treatment at higher temperature before HS-AlF 3 reagents can fully develop their Lewis acidity.…”

Aluminium(III) fluoride originating from decomposition of hydrazinium fluoroaluminate(III) under oxidative conditions: Syntheses, X-ray photoelectron spectroscopy and some catalytic reactions

“…XP spectra of HS-AlF 3 samples prepared by the non-aqueous sol-gel route have previously been reported in some detail [7,8] with the aim of understanding the origins of characteristically high acidity (both Lewis and Brønsted) of these materials. Although simple correlations between BE values or the areas under BE peaks are not possible [8] it is clear that variations in local geometric structures are responsible for the strong Lewis acid sites, as they result in better exposure of potential reagent species to the surface acid sites.…”

“…A short account of this route has recently appeared and the nanoscopic nature of some of the products demonstrated by electron microscopy, particularly TEM [6]. The Lewis acid properties of the AlF 3 materials prepared in the present work have been examined by pyridine adsorption monitored by FTIR spectroscopy [6] and now through X-ray photoelectron spectroscopic (XPS) analysis in order to permit comparisons with analogous data from sol-gel prepared HS-AlF 3 [7,8]. Finally, the behaviour of the materials as possible catalysts in two reactions, isomerisation of 1,1,2-trichlorotrifluoroethane and the room temperature dehydrochlorination of tert-butyl chloride is examined.…”