We first consider selectivity as a function of catalyst coverage or loading on the electrode support. This effect corresponds to catalyst roughening at the particle level and has been systematically investigated by the group of Behm for different electrocatalytic reactions on Pt electrodes. Figure shows digitized data from two such experimental studies for the (bottom). Both of these reactions involve volatile intermediates that may give rise to early, partially-converted products following the "desorption-re-adsorption-reaction" mechanism.