Role of Crystalline Defects in Electrocatalysis:  Mechanism and Kinetics of CO Adlayer Oxidation on Stepped Platinum Electrodes

The kinetics of the electrochemical oxidation of a CO adlayer on Ptn(111)×(111) electrodes in 0.5 M H2SO4 has been studied using chronoamperometry. The objective is to elucidate the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approximation for the Langmuir−Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Ptn(111)×(111) surfaces under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation, determined by a fitting of the experimental data with the mean-field model, is found to be proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidation. An apparent intrinsic rate constant is determined. The potential dependence of the apparent rate constants is found to be structure insensitive with a Tafel slope of ca. 80 mV/dec, suggesting the presence of a slow chemical step in an ECE reaction mechanism.