Mass transport effects in CO adsorption and continuous CO electrooxidation over regular arrays of Pt nanostructures on planar glassy carbon supports

The interplay between mass transport and the kinetics of CO adsorption/CO electrooxidation was studied on nanostructured electrodes, which consist of regular arrays of catalytically active cylindrical Pt nanodisks supported on a planar glassy carbon (GC) substrate, and are fabricated via Hole-mask Colloidal Lithography. CO adsorption and oxidation were measured under controlled transport conditions in a thin-layer flow cell interfaced to a mass spectrometer. The temporal evolution of the relative COad coverage, the effective sticking coefficient and the dependence of the adsorption rate on the COad coverage were evaluated for CO adsorption at 0.06 V (vs. reversible hydrogen electrode) at systematically varied CO concentrations and Pt nanodisk coverages. Continuous CO oxidation was studied at different electrode potentials under identical mass transport condition. While qualitatively, the characteristics of the adsorption/reaction kinetics do not depend on the transport conditions, the absolute rates of CO adsorption and CO (bulk) oxidation, normalized to the Pt coverage, increase strongly with decreasing Pt coverage on the nanostructured Pt/GC electrodes. This is explained by a gradual transition from one-dimensional planar diffusion (concentration gradients planar to the surface–extended Pt surface) to three-dimensional hemispherical diffusion (nanostructured surfaces).