Structure Sensitivity in Electrocatalytic Properties of Pt: II . Oxygen Reduction on Low Index Single Crystals and the Role of Steps

Abstract

Single crystal surfaces were used to simulate the different geometric sites that may occur on small (∼1 nm) Pt clusters of arbitrary shapes and to determine the structure sensitivity of oxygen molecule reduction in acid electrolyte. The static electrode configuration for the single crystals restricted the kinetic measurements to the high potential (0.9–1.0V) region where the reaction occurs on an oxygenated Pt surface. In dilute or there was no measurable difference in the oxygen reduction activity of Pt ,, or stepped surfaces. Post‐test analyses of the electrode surfaces by LEED and AES showed that these active surfaces were highly ordered and contained no sublattice or subsurface oxygen (<0.1 atomic percent). Triangular sweep potential cycling to anodic limits above 1V, or anodic prepolarization to 2V with potentiostatic reduction of the anodically formed oxide, did not enhance the activity. It was concluded that there are no specifically active sites for oxygen reduction on an oxygenated Pt surface, rather the Pt surface is uniformly active. Cyclic voltammetry did show that at low coverage oxygen reduction intermediates are adsorbed more strongly (irreversibly) at steps than on atomically flat surfaces. At high coverage, repulsive interaction of adsorbed intermediates apparently diminishes the effect of structure on the adsorption energies, so that no structure sensitivity is observed in the potential region under kinetic control.