Modeling CO Oxidation on Pt(111) Electrodes

CO oxidation curves on the Pt(111) electrode in the presence and the absence of CO are simulated using a mean field Langmuir−Hinselwood mechanism, in which the rate-determining step is an electrochemical reaction between adsorbed CO and adsorbed OH. The OH adsorption process has been modeled using a Frumkin isotherm that reproduces the experimental OH adsorption behavior on the clean Pt(111) electrode. From the results of the simulation, the rate constants of the different steps in the mechanism are determined. Although the model reproduces quite well the main characteristics of the chronoamperometric and voltammetric curves, some deviations are observed due to factors that cannot be included in the model. These factors are the existence of a nonhomogeneous distribution of defects on the surface of a real Pt(111) electrode, the CO−CO and OH−CO interactions in the adsorbed adlayer, and the nonhomogeneous flux of the CO from the bulk to the electrode surface. Using the model, the effective Tafel slope that would have been obtained experimentally is calculated and compared to the literature values in order to understand the wide range of different values reported. Those differences can be easily justified maintaining the same mechanism, but with a different OH adsorption behavior.