Structure and activity of Pt-Co alloys as oxygen reduction electrocatalysts

Carbon supported Pt-Co (3:1 atom ratio) catalysts were prepared in both acid and alkaline aqueous media, followed by heat treatments to promote alloy formation. Both preparations began with a commercial 10% Pt on carbon catalyst with Pt particle sizes of 15 to 30 A. Significantly greater alloying was observed in the acid media prepared catalyst. X-ray diffraction studies of the acid prepared catalyst demonstrated lattice parameters tending away from Pt (3.937 A) and toward that for Pt/sub 3/Co (3.831 A), greatly increased particle sizes, and significant ordering evidenced by the presence of superlattice reflections. In all cases, the base media prepared catalysts were alloyed to a lesser extent, were of moderately increased particle size and gave no indication of alloy ordering. Activity testing under phosphoric acid fuel cell conditions demonstrated that the most highly alloyed catalysts had the greatest activity. Loss of cobalt in the phosphoric acid environment was the lowest in catalysts which were the most alloyed