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

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.

F K-edge X-ray absorption near-edge structure (XANES) of AlF3 polymorphs: combining ab initio calculations with Walsh correlation diagrams.

The X-ray absorption near-edge structures (XANES) at the F K-edge of alpha-AlF(3), beta-AlF(3) and a tetragonal AlF(3) phase are analysed by a combination of ab initio calculations with the FEFF8 code and a phenomenological discussion of local molecular orbital (MO) symmetries at the absorbing fluorine atoms. By means of a Walsh correlation diagram it is shown that the two intense absorption bands observed at the F K-edges of the AlF(3) polymorphs can be interpreted as transitions to anti-bonding MOs in [Al-F-Al]-units that have C(2v) and D(infinity h) point group symmetries. The energies of both anti-bonding orbitals are very insensitive to the angle between the Al-F bonds, which explains the close similarity of the XANES signatures from the three polymorphs. The FEFF8 analysis shows that the increased broadening of the XANES structure for beta-AlF(3) and the tetragonal AlF(3) phase is due to the superposition of the individual absorption spectra from the crystallographically distinct F species. The interpretation in terms of local MOs provides for the first time a "chemically intuitive" approach to investigations of solid fluorides by XANES spectroscopy and provides a simple conceptual framework for the discussion of the electronic structure in AlF(3) materials.

An in situ and operando X-ray absorption spectroscopy setup for measuring sub-monolayer model and powder catalysts.

A new spectroscopic cell has been designed for studying model catalysts using in situ or operando X-ray absorption spectroscopy. The setup allows gas treatment and can be used between 100 and 870 K. Pressures from 10(-3) Pa up to 300 kPa can be applied. Measurements on model systems in this particular pressure range are a valuable extension of the commonly used UHV characterization techniques. Using this setup, we were able to analyze the Au L3 EXAFS of a silica wafer covered with sub-monolayer concentrations of gold (0.05 ML). By modifying the sample holder, powder catalysts can also be analyzed under plug-flow conditions. As an example, the reduction of a Au/SiO2 powder catalyst prepared from HAuCl4 was followed.