Richard J. Oldman

Oxygen adsorption and dissociation on yttria stabilized zirconia surfaces

We have modelled the nature of oxygen species on the surfaces of yttrium stabilized zirconia (YSZ) in relation to its application as a catalyst for partial oxidation of methane. Quantum mechanical DFT-GGA calculations have been carried out to characterize the interaction between oxygen molecules and the perfect ZrO2 and defective YSZ (111) planar and stepped surfaces. In general, the results suggest that the creation of oxygen vacancies by yttrium doping provides an active site for oxygen adsorption. Depending on the topography and defect dispersion, various oxygen intermediates have been identified and analyzed by their density of states. The low-coordinated Zr cations on the YSZ surfaces can attract strongly reduced oxygen species and the most stable adsorption state of oxygen achieves a higher bond saturation of the neighbouring Zr site. In addition, the probable reaction pathways have been predicted for oxygen dissociation on the plane and stepped (111) surfaces of YSZ.

Combined experimental and computational modelling studies of the solubility of nickel in strontium titanate

A combination of X-ray techniques and atomistic computational modelling has been used to study the solubility of Ni in SrTiO3 in relation to the application of this material for the catalytic partial oxidation of methane. The experiments have demonstrated that low temperature, hydrothermal synthesis is successful in preparing monophase, crystalline material with up to 5 atom % Ni doped in the octahedrally coordinated Ti4+ site of the SrTiO3 lattice. Computational modelling indicates limited solubility and identifies the most energy favoured Ni state as Ni2+ as opposed to Ni3+. However, modelling also suggests that thermodynamically Ni should occupy the Sr2+ site. This apparent contradiction is resolved by considering the kinetic effects operating during hydrothermal synthesis. An in situ EXAFS study of the thermal behaviour of Ni doped SrTiO3 in air and H2/He shows that NiO phase segregates and reduces to give a high dispersion of Ni metal particles on the SrTiO3 oxide.

Zirconium dioxide topological surfaces with low coordination sites

Density functional theory has been applied to model the plane and topologically complex surfaces for pure and yttrium stabilized cubic ZrO2. For pure ZrO2, the dominance of the (111) plane surface found in previous studies has been confirmed. Based on this surface, a series of stepped, cornered and kinked surfaces has been constructed and optimized. We observe a trend relating a decrease in stability to the decrease in the oxygen coordination number of surface Zr ions which occurs with increasing surface topological complexity. For yttrium stabilized zirconia (YSZ), the segregation of yttrium to the (111) surface predicted previously has been confirmed. As for pure ZrO2, the most stable stepped surface topology has been identified and shown to be only slightly less stable than the planar surface.

Sulfur K-edge X-ray absorption spectroscopy study of the reaction of zinc oxide with hydrogen sulfide

We report in situ sulfur K-edge XAS data which show that a small number of very reactive sites are involved in the reaction of zinc oxide with hydrogen sulfide. Such sites react before the majority of the bulk oxide and are capable of forming zinc sulfide rapidly even at ambient temperatures. In situ sulfur K-edge EXAFS also provides evidence for a long sulfur–zinc interaction at 2.71 A during the formation of bulk islands of zinc sulfide on the oxide. Powder diffraction and X-ray photoelectron spectroscopy on ex situ treated hydrogen sulfide absorbents confirm the formation of a twin phase material of crystalline zinc oxide and less crystalline zinc sulfide, both wurtzite polymorph, as suggested by EXAFS.

INVESTIGATING THE ORIENTATIONS OF LANGMUIR BLODGETT FILMS OF COPPER TETRA-T-BUTYL PHTHALOCYANINE USING POLARISED, ULTRA-SOFT XANES SPECTROSCOPY

Abstract Phthalocyanines have long been of special interest as essentially organic molecular conductors with properties that can be modulated by varying the central atoms. Soluble phthalocyanines have recently been developed enabling the production of Langmuir-Blodgett films, of interest to the electronics industry. Data are presented for the determination of the orientation of a Langmuir-Blodgett film of copper tetra-t-butyl phthalocyanine using XANES spectroscopy and showing that the molecules in the films are oriented normal to the substrate surface.

Investigation of the local structure around iron dispersed in vinyl chloride–vinylidene chloride (VC–VdC) copolymer coatings on mild steel using glancing-angle X-ray absorption spectroscopy

The chemistry occurring in vinyl chloride–vinylidene chloride polymer coatings on mild steel has been investigated by probing the local environment around Fe ions using glancing-angle X-ray absorption spectroscopy. Measurements made as a function of penetration depth reveal that close to the air interface the Fe species are present as octahedrally co-ordinated FeIII and have a local structure similar to that observed in disordered γ-FeOOH. Although this structure is found to be independent of the pH of the coating formulation, Fe ion transport from the metal substrate is apparently enhanced at acidic pH formulations. Deeper into the coating the Fe has a structure typical of a mixed tetrahedral–octahedral environment such as Fe3O4.

Application of X-ray absorption spectroscopy to the structural characterisation of monodispersed benzotriazole coatings on partly oxidised copper thin films

X-Ray absorption spectroscopy using synchrotron radiation has been used to probe the structure of Copper-benzotriazole complexes and that of the benzotriazole involved in surface films following adsorption onto partly oxidised Cu thin films. Data recorded at the Cu-K edge and, using 4- and 5-bromo-substituted benzotriazole, at the Br-K edge are consistent with the CuI atoms coordinated to three or four benzotriazole molecules through the N(2) position on the triazole ring. Examination of the thin films using glancing-angle geometry suggests the adsorbed film to be only a few monolayers in thickness with the benzotriazole molecules coordinated to CuI ions in the oxidised Cu surface forming a hydrogen-bonded surface complex similar, in part, to that proposed by Fang et al. (Surf. Sci., 1986, 176, 476). At the interface the surface reaction appears to involve a topotactic arrangement with the (100) plane of Cu2O where the in-plane Cu–Cu interatomic distances closely match that expected for the surface complex. The overall limitations of the glancing-angle technique to the characterisation of this kind of interfacial system are discussed.