Adsorption kinetics involving precursor states and promoters

Abstract

If the sticking coefficient for adsorption on a perfect clean single-crystal surface is low, the adsorption process on a real surface may primarily occur via trapping of molecules into precursor states, random walks in these states, and then chemisorption at e.g. step sites, defects or other adsorption promoters. This scheme of adsorption is analyzed in detail, and general expressions describing the coverage dependence of the sticking coefficients are derived for different cases. The latter include intrinsic and/or extrinsic precursors, combined with step sites, randomly distributed promoters, and self-promoted adsorption. Some of the results are briefly used to interpret or discuss experimental data for sticking of O 2 on a partly potassium-covered graphite surface, sticking of CO on Pt(111), and D 2 on Cu(100), respectively.