The electro-oxidation of H-2 and H-2/CO mixtures on carbon-supported PtxMoy alloy catalysts

Abstract

Electro-oxidation kinetics of H-2 and H-2/CO mixture were studied on bimetallic Pt-Mo catalysts supported on a high-surface-area carbon black. The Pt:Mo atomic ratios in the catalysts were 3:1 and 4:1. Characterization of these catalysts by X-ray diffraction indicated the existence of a face-centered cubic metallic phase with an average particle size of ca. 4 nm. Because the lattice constants for the Pt-Mo solid solutions are so close to those of pure Pt, the composition of the nanocrystalline phase, could not be determined. The kinetic results with the supported catalysts were compared quantitatively with results from bulk alloy electrodes having well-characterized surface compositions varying from 15 to 33 atom % Mo. The kinetic properties of the supported catalysts were comparable to those of bulk alloys having somewhat higher Mo concentrations than the atomic ratios in the catalysts. This suggests that either the surface segregation phenomena in the alloy nanocrystals are different from those in the bulk or that the alloying by Pt is incomplete, and the alloy nanocrystals are rich in Mo relative to the atomic ratios in the catalysts. We prefer the latter interpretation. These Pt-Mo alloy catalysts are predicted to have significantly better "CO tolerance" in polymer electrolyte membrane fuel cells than Pt-Ru alloy catalyts, consistent with previous predictions based on studies of bulk alloy electrodes.