Electrooxidation of H2, CO, and H2/CO Mixtures on a Well-Characterized Pt70Mo30Bulk Alloy Electrode

Abstract

The electrochemical oxidation of hydrogen (H 2 ), carbon monoxide (CO), and their mixtures (500 ppm−2%) on a well-characterized Pt 70 Mo 30 bulk alloy was examined using the rotating disk electrode technique in 0.5 M H 2 SO 4 at 333 K. The electrodes were transferred to and from a UHV chamber, where surface analyses were conducted using a combination of low-energy ion scattering (LEIS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The surface composition of this alloy after sputter-etching and annealing in UHV was slightly enriched in Pt to a composition of Pt 77 Mo 23 . The kinetics of H 2 oxidation are not measurably affected by the presence of the Mo in the surface. The shapes of the polarization curves for the oxidation of the H 2 /CO mixtures are qualitatively similar to those for the Pt 50 Ru 50 alloy examined previously: a high current−potential slope (ca. 0.5 V/dec) at low overpotential followed by a transition to a highly active state where the current approaches the diffusion-limiting current; the potential where the transition to the active state occurs decreases with decreasing CO concentration and decreases with increasing temperature; the current in the low-overpotential region is roughly inverse half-order in the CO partial pressure. The current densities in the low-overpotential region are comparable to those on Pt 50 Ru 50 alloy and about a factor of 50 times those on a pure Pt surface. While the anodic stripping of irreversibly adsorbed CO is very different from that on the Pt 50 Ru 50 alloy, with most of the CO ads being oxidized only above 0.6 V (RHE), there is direct evidence for the oxidation of small amounts of CO ads in the low-overpotential region of 0.05−0.5 V. The mechanism of action of Mo in the Pt surface in enhancing H 2 oxidation in the presence of CO thus appears to be very similar to that of Ru: a reduction in the steady-state coverage of CO ads on the Pt sites by oxidative removal, freeing Pt sites for H 2 oxidation.