Surface Studies of Spontaneously Deposited Ruthenium on Pt(hk) Electrodes

摘要

Recently, there has been an increased interest in the surface morphology of platinum-based catalysts, especially in reference to fuel-cell based research. In this area, a wide array of research is currently being conducted, as revealed by recent review articles. ~(1,2) Of urgent interest is the development and characterization of "poison-tolerant" catalysts. In the Direct Methanol Fuel Cell (DMFC), for example, there are a number of aspects keeping the technology from being highly efficient, one of which is catalytic inefficiency. Current theory suggests that platinum, ruthenium, and osmium constitute the most promising catalysts for the DMFC; ~(3-5) therefore, our research has focused on first the individual Pt/Ru and Pt/Os systems, ~6 in an attempt to understand the enhancement effects of Ru and Os on the Pt catalyst.Depositing controlled amounts of ruthenium onto well-defined surfaces (e.g., Pt(hkl) electrodes), allows the study of surface structure effects in Pt/Ru methanol oxidation electro-catalysis. The deposits are spontaneously formed by placing the electrode in a hydrated RuCl_3 solution, where RuO~(2+) species adsorb onto the platinum surface. After a brief voltammetric treatment, the species are reduced to form strongly adhered ruthenium islands; these islands are incredibly stable in the voltammetric region below platinum oxidation, which makes them ideal surfaces to explore by scanning tunneling microscopy (STM). It was determined previously that the Pt(111) surface covered by ruthenium yields the highest activity toward methanol electro-oxidation. ~7 Herrero et al. determined that on Pt(111), spontaneously deposited Ru islands form three-dimensional structures, even at low Ru coverage. ~8 Here, we explored the deposition process using ex-situ STM on Pt(111), Pt(100) and Pt(110). The ruthenium was deposited onto the surfaces using the electroless method, and Ru coverage values ere obtained on the electrodes at various deposition times. These Ru coverage values give us an insight into the surface dynamics of the Ru island formation, especially with regard to surface site preference.