Current-time behavior of smooth and porous PtRu surfaces for methanol oxidation

Smooth ultrahigh vacuum-cleaned PtRu alloys used as model catalysts for met hanol oxidation, present a continuous loss of activity under potentiostatic conditions. After a potential step, e.g., from 50 to 500 mV vs. reference hydrogen electrode, chronoamperometric curves first show a steep decrease o ver 5-10 min followed by a slower decrease over several hours. The latter d ecay exhibits a time(-1) behavior for different catalyst compositions, with higher slopes for the catalysts with less activity. The first current deca y can be repeatedly observed by restarting the experiment after setting the potential back to the initial value, thus indicating a certain degree of r eversibility. The slow decrease in activity, however, cannot be recovered b y this means. But stepping the potential to higher anodic values, e.g., up to 1.2 V, the original surface activity can be obtained again. Optimized po rous PtRu surfaces, on the other hand, do not show any comparable decrease after a respective potential step. The possible origin of the different beh avior of smooth and porous surfaces is discussed. (C) 2001 The Electrochemi cal Society.