Determination of the platinum and ruthenium surface areas in platinum-ruthenium alloy electrocatalysts by underpotential deposition of copper. I. Unsupported catalysts

Underpotential deposition (upd) of copper has been used to characterize platinum, ruthenium, and platinum-ruthenium high-surface-area unsupported (powder black) electrocatalysts. The surface areas thus obtained compare favorably with those determined by the more conventional electrochemical methods of monolayer CO and hydrogen oxidation. The differing adsorption energies for Cu on either Pt or Ru allow the peaks for upd copper deposited on alloy Pt-Ru to be resolved into their constituent components. Thus, in addition to the surface area, the surface composition of the Pt-Ru electrocatalyst can be determined. This approach distinguishes between bare ruthenium (i.e., metallic) and oxidized ruthenium sites as the upd copper does not deposit on the latter. The ruthenium surface area is found to remain high up to 0.45 V (vs RHE) and then to fall linearly with potential. Polarization at high potentials [1.45 V (vs RHE)] leaves a material in which metallic ruthenium cannot be recovered by electrochemical reduction. This is caused by oxidation of the ruthenium to a state that either dissolves in the aqueous phase and is lost or produces a form of oxidized ruthenium that is in a state that cannot be electrochemically reduced.