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Toward atomic-based understanding of some reactive and non-reactive surfaces

Assaf, Niveen (2018) Toward atomic-based understanding of some reactive and non-reactive surfaces. PhD thesis, Murdoch University.

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The thesis is composed of two broad themed sections with the underlying aim of understanding on a precise atomic basis, the electronic and structural factors governing the reactive and non-reactive surfaces of two metal oxides belonging to the same group in the periodic table; boron (B) and aluminium (Al). Using accurate density functional theory (DFT) computations, we first elucidate the initial reaction steps of the surface oxidation of elemental boron into its respective oxide; boron trioxide (B₂O₃). The highly exoergic reaction obtained for the dissociative adsorption of molecular oxygen over the boron surface coincides with the widely used boron oxidation reaction as secondary energy source in rockets. The relatively large activation energy for the O-O dissociation step marks the non-spontaneity of elemental boron oxidation at room temperature. Having established routes for the formation of B₂O₃-like precursors, we then investigate the relative stability of four low-index surfaces of the low-pressure B₂O₃ phase; namely the B₂O₃-I configuration. We demonstrate that none of the investigated low-index surfaces have dangling bonds, which reasonably relates to the experimentally observed low reactivity of this compound. The most stable surface terminations of B₂O₃ orientations entail tetrahedral BO₄ units. Such termination incurs a lower surface energy than orientations that consist of only triangular BO₃ units. Electronic and structural factors provide atomic-base elucidation of the observed inertness of B₂O₃.

Combined experimental techniques (i.e. diffuse reflectance infrared spectroscopy) and DFT simulation are used to answer some of the most intriguing questions pertinent to factors underpinning the well-documented catalytic inhibition by B₂O₃ and its hygroscopic behaviour. We investigate the adsorption and dissociation mechanisms of two hydrogen chalcogenides, namely water (H₂O) and hydrogen sulfide (H₂S) molecules over B₂O₃-I (101) surfaces. We show that the diboron trioxide surface exhibits high physiochemical reactivity towards water molecules. The Lewis acid properties of B₂O₃-I lead to the formation of a molecular adsorption state (rather than dissociative adsorption) of the H₂S molecule via the acceptance of an electron pair into the low-energy orbital of the boron valence shell. While acting as water scavenger to generate dissociated radicals, B₂O₃ exhibits an inhibitor characteristic towards the dissociation of H₂S molecules, representing an ideal reactor wall coating in such systems.

Alumina have been widely utilised as independent catalysts or as support materials for other catalysts. From an environmental perspective, alumina nanoclusters dispersed on surfaces of particulate matter PM₁₂ generate from various combustion processes play a critical role in the synthesis of environmental persistent free radicals (EPFR). Of particular importance are phenoxy-type EPFR that often acts as building blocks for the formation of notorious pollutants. Herein, we provide a comprehensive thermo-mechanistic account of alumina-surface mediated formation of phenoxy-type EPFR on different structural alumina models encompassing the following surfaces: dehydrated alumina surface, fully hydrate alumina surface, surfaces with different hydration coverage, and silicon-alumina doped surface. We show that fission of the phenol’s hydroxyl bond over dehydrated alumina systematically incurs lower energy barriers in reference to the hydrate surfaces. The catalytic activity of the alumina surface in producing the phenoxy/phenolate species reversibly correlates with the degree of hydroxyl coverage. Furthermore, we clarify the effect doping on the catalytic activity of alumina. The activation energy barrier required to form phenoxy moiety on Si-substituted Al₂O₃(0001) surface is ~40% lower than that of analogous barriers encountered over undoped dehydrate alumina surface. Overall, all considered models of alumina configurations are shown to produce adsorbed phenolate; however, desorption of the latter into the gas phase requires a rather sizable energy. Thus, the fate of absorbed phenolate is most likely to be dictated by decomposition affording carboneous layer of self-decomposition into other stable molecules.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Altarawneh, Mohammednoor, Dlugogorski, Bogdan and Radny, Marian
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