Anodic catalysts for polymer electrolyte fuel cells:: the catalytic activity of Pt/C, Ru/C and Pt-Ru/C in oxidation of CO by O2

The catalytic activity for oxidation of CO by O-2 was investigated on commercial Pt/C, Pt-Ru/C (Pt/Ru atomic ratio = 20, 3, 1, 1/3) and Ru/C. All samples contained 20 wt.% metal. Assuming equal surface and bulk composition, the number of surface Pt and Ru atoms was calculated from the average size of the supported metal particle as determined by TEM. On Pt-Ru/C alloys, the turnover frequency per Ru atom, N-Ru/molecules s(-1) Ru-atom(-1), was independent of chemical composition. This finding suggests that the active site in these alloys is Ru. In the temperature range 300-400 K, the turnover frequency per active metal atom was 50-300 times higher on Pt-Ru/C than on Pt/C. The turnover frequency was 400 times higher on Ru/C than on Pt/C at 313 K and 90 times higher at 353 K. Addition of water vapor to the reactant mixture left the catalytic activity of Ru/C unchanged but slightly increased the activity of Pt/C. On both catalysts the activation energy and reaction orders were nearly the same as in dry atmosphere. Conversely, the addition of water markedly decreased the activation energy for Pt-Ru(1 : 1)/C alloy (from 19 to 11 kcal mol(-1)). These findings suggest that fuel cells equipped with Pt-Ru/C anodes perform better than cells with Pt/C anodes. They do so because Ru effectively oxidizes the carbon monoxide present as an impurity in the H-2-reformed fuel. (C)2000 Elsevier Science B.V. All rights reserved.