Effects of surface impurities on surface diffusion of CO on Ni„110..

A small amount of coadsorbed impurity species could significantly alter the surface diffusion of CO on Ni~110!. Three impurity species, sulfur, oxygen, and potassium are studied here. The former two are known as ‘‘poisons’’ and the latter as a ‘‘promoter’’ for CO hydrogenation on Ni. All three are found to impede CO diffusion drastically. The apparent diffusion activation energy ED increases from the clean-surface value of 2–3 kcal/mol, to a saturation value of 7–8 kcal/mol at sufficiently high impurity coverages. The impeding effect decreases from S to O to K. Mechanisms responsible for the effect are discussed in detail. With S and O, the impurity-covered step-controlled diffusion appears to be the dominant mechanism. With K, the nearestneighbor attractive interaction between CO and K seems to be most important. @S0163-1829~97!01944-9#