In-situ surface x-ray scattering measurements of the electrode/electrolyte interface have been made along with complementary electrochemical and ultra-high vacuum measurements. Using these techniques, two main systems 'have been studied; bimetallic surfaces which have shown enhanced activity for the oxygen reduction reaction (ORR) and single-crystal surfaces to probe the effect of temperature on the interfacial structure and surface reactions. The Pt3Ni(111) electrode surface has been found to be the most highly catalytically active surface ever detected for the ORR, 1/202 + 2H+ + 2e- = H20. , In-situ surface x-ray scattering measurements of this surface revealed that the Pt concentration profile oscillates over three atomic layers, whereby the oute'rmost atomic layer was found to be pure Pt, followed by a Ni-rich second layer and a slightly Pt-rich third layer, before reaching the bulk occupation value. It is the concentration profile which causes the surface' electronic structure of Pt3Ni(111) to be significantly different from the atomically identical surface of pure Pt(lll). The surface electronic structure of Pt3Ni(111) acts to optimise conditions for the adsorption of reactant species involved in the ORR. Density-functional theory calculations have indicated that 670-atom octahedral Pt3Ni nanoparticles, where each surface plane is [111] orientated, would be thermodynamically stable and have approximately the same Pt concentration profile as the Pt3Ni(111) single crystal, which, if synthesised, would prove to be an improved catalyst for practical application in fuel cells. The solution/substrate temperature plays an important role in determining the molecular adsorbate structure of CO (COad ) formed on the Pt(111) surface. At electrode potentials approaching hydrogen evolution, 0.05 V (versus the reversible hydrogen electrode), the (2x2) and vII9 CO-structures coexist at high temperature (rv45°C). This is due to a significant negative shift in the onset of OH adsorption and subsequent partial oxidation of COad through the LangmuirHinshelwood reaction (COad +OHad --+ CO2 +H+ +e-). The second investigation into the role of temperature in electrochemical systems has been made on the surface reconstruction of the AU(100) and Au(111) electrodes. Surface reconstruction involves the rearrangement of surface atoms and can be directly monitored by in-situ surface x-ray scattering measurements. The AU(100) reconstruction is greatly enhanced at high temperature, rv45°C, however, the Au(lll) reconstruction is unaffected by temperature changes. The apparent discrepancy is due to the initial step by which the respective reconstructions form. As observed in the CO/Pt(l11) system, the onset of OH adsorption is shifted negatively with increased temperature. The negative shift increases the nucleation sites of hex-strings, the initial step in forming the Au(100) reconstruction, which causes an enhancement of the reconstruction.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485868 |
| Date | January 2007 |
| Creators | Fowler, Ben |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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