Electrooxidation of CO and H2/CO mixtures on a carbon-supported Pt catalyst—a kinetic and mechanistic study by differential electrochemical mass spectrometry

Abstract

We report the results of a differential electrochemical mass spectrometry study on the electrooxidation of CO and H₂/CO mixtures on a high surface area carbon-supported Pt/Vulcan catalyst. The measurements were performed in a thin-layer flow-cell, allowing the quantitative determination of the individual reaction rates under controlled mass transport conditions as a function of the electrode potential, parallel to the Faraday current. CO_ad monolayer oxidation (stripping) experiments in a 0.5 M H₂SO₄ solution saturated with H₂ or with 2% CO in H₂ show that (i) the hydrogen oxidation reaction (HOR) in CO-free, H₂-saturated solution starts already at potentials right at the onset of CO oxidation (0.15 V_RHE), as evidenced by the simultaneous increase in Faraday current, the increase in the CO₂ signal, and the decrease in H₂ content in the solution, and that (ii) in CO containing solution the onset of the HOR is shifted to more positive potentials due to continuous CO readsorption. In the former case 90% of the mass transport limited current is reached already at a potential where only ∽5% of the saturated CO adlayer is oxidized, in the latter case the limiting HOR current is reached only in the potential regime of the main CO stripping peak. Furthermore, in the second case CO oxidation in the pre-wave regime is enhanced, while the HOR is suppressed in this potential regime compared to CO stripping in CO-free H₂-saturated electrolyte. As a result the ignition of the HOR in the H₂/CO(2%) saturated electrolyte occurs only together with the main CO oxidation peak. The data are direct proof that the HOR proceeds in an almost closed CO adlayer, most likely in small (fluctuating) holes of the adlayer and has to compete with CO readsorption. Kinetic measurements at constant electrode potential (0.04 V) in H₂/CO(2%)-saturated solution show that CO adsorption from an H₂/CO mixture follows a precursor-type adsorption kinetics. For the HOR the data show substantial deviations from both a (1 − θ_CO) and (1 − θ_CO)² dependence of the HOR rate on the CO coverage, pointing to a more complex mechanism than a simple site-blocking mechanism.