Adsorption of oxygen molecules on platinum surfaces modified with subsurface atoms of vanadium : a DFT study

Master Thesis


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University of Cape Town

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.

Includes bibliographical references.