Single crystal studies of platinum alloys for oxygen reduction electrodes

In this thesis the discovery, characterization and testing of new catalysts for the oxygen reduction reaction (ORR) is investigated. Experiments on sputter cleaned, polycrystalline Pt₅Y and Pt₅Gd crystals have shown that these alloys are excelent candidates for catalysts for the ORR. To mimic the behavior of bulk single crystals, deposition of yttrium and gadolinium on a clean, annealed Pt(111) crystal were investigated in UHV. Pt_xY and Pt_xGd alloys terminated with a single atomic layer of platinum were formed after annealing to 500 °C in UHV. These alloys will be referred to as Y/Pt(111) and Gd/Pt(111) in the following. The prepared alloys were investigate using Low Energy Electron Diffraction (LEED), Xray Photoelectron Spectroscopy (XPS), Ion Scattering Spectroscopy (ISS) and temperature Programmed Desorption (TPD). The LEED pattern indicated that the Y/Pt(111) sample had formed a 1.89×1.89 structure, and the Gd/Pt(111) sample has formed a 1.90×1.90 structure compared to pure platinum. From the XPS measurements, it is most likely that alloys with the Pt₅Y and Pt₅Gd stoichiometry have been formed. The reactivity of the surfaces were probed using TPD. These measurements showed sharp desorption peaks with a large shift towards lower temperatures. The change in desorption temperature was ∆T = −180°C for the Y/Pt(111) sample and ∆T = −200°C for the Gd/Pt(111) sample. The ORR activity was measured showing a large enhancement for both alloys. Angle resolved XPS performed on the samples after the electrochemical experiments showed overlayers with a thickness of approximately 15 Å had been formed. It was possible to investigate the structure of these overlayers using Gracing Incident X-ray Diffraction, GIXRD. These experiments were performed at the Stanford Synchrotron Radiation Lightsource. The GIXRD was performed on the samples as prepared, and after electrochemical cycling between 0.05 V and 1.0 V VS. RHE. and between 0.05 V and 1.2 V VS. RHE. Diffraction experiments carried out after the cycling to 1.0 V showed that an overlayer with crystalline order had been formed on the Y/Pt(111) sample and the Gd/Pt(111) samples. These overlayers were slightly compressed compared to pure platinum and had a 6-fold symmetry. After cycling to 1.2 V VS. RHE. the correlation length of the overlayer on the Gd/Pt(111) sample had decreased significantly, and the overlayer on the Y/Pt(111) had disappeared completely. This indicates that the crystalline ordered structures corrode after this electrochemical treatment.