Browsing by Author "Lang, C I"

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    Adsorption of oxygen molecules on platinum surfaces modified with subsurface atoms of vanadium : a DFT study
    (2014) Matengaifa, R; Lang, C I; Sithole, H
    The aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.
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    High temperature study on thin aluminium coatings deposited onto thick platinum substrates
    (Elsevier, 2009) Topić, M; Pineda-Vargas, C A; Bucher, R; du Plessis, H E; Breedt, B; Pischedda, V; Nxumalo, S; Lang, C I
    Intermetallics formation in the Pt–Al binary system at temperatures in the range of 20–600 °C have been investigated by the study of thin aluminium coatings deposited on thick platinum substrates. In order to characterise the changes caused by elevated temperature several techniques such asmicroscopy, X-ray diffraction, nuclear microprobe techniques and microhardness testing were used. In addition to changes in morphology, the high temperature X-ray diffraction (XRD) analysis showed that the initial phase formed when Pt and Al reacted was Pt2Al3. The formation of Pt2Al3 phase started at 250 °C and it is followed by the PtAl and Pt3Al phases. Furthermore, the Pt–Al intermetallics increased the surface hardness; the process of surface hardening by formation of intermetallics without compromising the purity of pure platinum is of great importance for jewellery and other applications where the properties such as hardness and scratch resistance are crucial.
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    The Pt₈V ordering transformation in Pt 11 at. % V
    (2006) Nxumalo, Silethelwe; Lang, C I
    The Pt₈V ordering transformation in Pt 11 qt.% V alloys has been studied. The study included determining the thermodynamic stability of Pt₈V, the kinetics of Pt₈V ordering transformation and the strengthening due to the presence of the ordered phase. Transmission electron microscopy, using election diffraction and dark and bright field imaging, and X-ray diffraction were used for structural characterisation. Electrical resistivty was used to investigate the kinetics of Pt₈V ordered phase. Microhardness measurements were used to investigate the Pt₈V ordered phase and its effects on hardness.