Barry G. Searle

An experimental and theoretical investigation of the electronic structure of CdO

Photoemission spectra of polycrystalline CdO have been measured under excitation with monochromatic x-rays and with variable energy UV radiation from a synchrotron source. Experimental data are compared with density of states profiles calculated by periodic Hartree-Fock and density functional methods. Both calculations suggest that there is significant mixing between Cd 4d and O 2p states, but the Hartree-Fock calculations better reproduce the experimental O 2p to Cd 4d separation and the O 2p bandshape. Constant initial state profiles measured with variable photon energy in the range between 30 eV and 100 eV provide experimental evidence for O 2p-Cd 4d mixing.

An ab initio study of α-Al2O3(0001): the effects of exchange and correlation functionals

Abstract The structure of the α-Al 2 O 3 (0001) surface has been the subject of a number of investigations which have yielded results that are both qualitatively and quantitatively different. Previous theoretical studies differ both in terms of the numerical approximations employed and the underlying treatment of electronic exchange and correlation. We have re-examined the surface structure using a variety of exchange–correlation functionals within the same numerical scheme. We find a suprisingly large variation in the predicted surface structure with the choice of functional. The computed geometries are consistent with all previous theoretical predictions and we have thus managed to unify these apparently disparate results. Nevertheless, the predicted structure is at variance with that deduced from experimental studies, particularly with respect to the contraction of the first-to-second interlayer distance. Possible reasons for this discrepancy are discussed.

Parallel implementation of the ab initio CRYSTAL program: electronic structure calculations for periodic systems

CRYSTAL is an ab initio electronic structure program, based on the linear combination of atomic orbitals, for periodic systems. This paper concerns the ability of CRYSTAL to exploit massively parallel computer hardware. A brief review of the theory, numerical implementations and parallel solutions will be given and some of the functionalities and capabilities highlighted. Some features that are unique to CRYSTAL will be described and development plans outlined.

Ab initio studies of aluminium fluoride surfaces

Solid aluminium fluorides have great potential for use in a range of reactions that are catalysed by strong Lewis acids. However, very little is known about the detailed atomic scale structure of their surfaces. We present new results for the surface structure of β-AlF3 based on first principles simulation and compare these with our earlier work on α-AlF3. On the basis of these simulations we can explain the observed reactivity of the aluminium fluoride materials. We can also use these results to postulate a mechanism for the observed high reactivity shown by amorphous, ‘high-surface area’ AlF3.

Identification of possible Lewis acid sites on the β-AlF3(100) surface: an ab initio total energy study

Strong Lewis acid catalysts are widely used in a variety of industrial processes including Cl/F exchange reactions. Aluminium fluorides (AlF3) have great potential for use in such reactions. Despite the importance of the surface in the catalytic process little is known about the detailed atomic scale structure of AlF3 surfaces. In the current study we employ state of the art total energy calculations based on hybrid-exchange density functional theory to predict the composition and structure of the (100) surface of beta-AlF3 for the first time. We examine six possible terminations of the beta-AlF3 (100) surface and demonstrate that there are two relatively low energy terminations that result in the formation of under co-ordinated Al3+ ions at the surface. Such under co-ordinated ions are expected to be strong Lewis acid sites. This is the first ab initio determination of the atomic scale structure of such sites on the surface of beta-AlF3.

Characterization of Lewis acid sites on the (100) surface of β-AlF3: Ab initio calculations of NH3 adsorption

The current study employs hybrid-exchange density functional theory to show that the Lewis base, NH(3), binds to the beta-AlF(3) (100) surface with a binding energy (BE) of up to -1.96 eV per molecule. This is characteristic of a strong Lewis acid. The binding of NH(3) to the surface is predominately due to electrostatic interactions. There is only a small charge transfer from the NH(3) molecule to the surface. The BE as a function of coverage is computed and used to develop a lattice Monte Carlo model which is used to predict the temperature programed desorption (TPD) spectrum. Comparison with experimental TPD studies of NH(3) from beta-AlF(3) strongly suggests that these structural models and binding mechanisms are good approximations to those that occur on real AlF(3) surfaces.

Reactivity of the β-AlF3(100) surface: defects, fluorine mobility and catalysis of the CCl2F2 dismutation reaction

Hybrid exchange density functional theory is used to model defects on the beta-AlF(3) (100) surface. The stability of the surface with respect to the diffusion of surface F ions is investigated. It is shown that under typical reaction conditions (600 K) the surface is not kinetically hindered from reaching thermodynamic equilibrium. A reaction mechanism for the catalysis of 2CCl(2)F(2)--> CClF(3) + CCl(3)F is proposed. The mechanism and corresponding reaction barriers are calculated using a double-ended transition state search method. It is predicted that the processes that determine the overall reaction rate occur at defect sites.

First principles characterisation of aluminium trifluoride catalysts

The recently discovered high surface area AlF3 catalyst is characterised with respect to surface composition and structure using calculations based on density functional theory. Under typical reaction conditions the surfaces are found to expose five fold coordinated Al reaction centres and to preferentially adsorb water. The acidic centres are probed using NH3 adsorption which binds strongly indicating strong Lewis acidity. The predicted temperature probed desorption spectrum has features from competing surfaces and features due to strong intermolecular interactions, which are used to interpret the observed spectrum.

A study of the soft X-ray photoreduction of [NiIV(S2CNEt2)3][BF4] and soft X-ray photoisomerisation of [PPh4][NiII(S2COEt)3] at the Ni L-edge†

A detailed study of the soft X-ray photochemistry occurring in some nickel dithiocarbamate and xanthate complexes has shown that soft X-ray induced photoreduction of [NiIV(S2CNEt2)3][BF4] results in a NiII square-planar species and the rate is dependent on photon flux, photon energy and ligand set, but is essentially independent of temperature. Also, it has been demonstrated that [PPh4][NiII(S2COEt)3] is photoisomerised to a NiII square-planar species on exposure to soft X-rays. The implication for experiments on third generation synchrotron sources is discussed briefly.