Influence of chemisorbed carbon monoxide on hydrogen evolution at smooth platinum electrodes in sulfuric acid solution

Cathodic current—potential curves were measured potentiostatically on two smooth platinum electrodes of different shape in 0.5 M H₂SO₄ at room temperature in the presence of chemisorbed carbon monoxide. Coverages of carbon monoxide between 0 and 0.6 lead to a relatively small decrease of the current density at constant hydrogen overvoltage. Mass transport processes remain largely rate-controlling. A rapid decrease of current follows, with a further increase of the carbon monoxide coverage. A transition from mass transport to kinetic control takes place. The rapid decrease is followed by a slower decrease at coverages above 0.85. This leveling-off results from hydrogen evolution on top of the chemisorbed layer of carbon monoxide. The retarding effect of chemisorbed carbon monoxide on hydrogen evolution is compared with the similar effect on hydrogen dissolution for the same electrodes.