EFFECTS OF SURFACE IMPURITIES ON SURFACE-DIFFUSION OF CO ON NI(110)

A small amount of coadsorbed impurity species could significantly alte r the surface diffusion of CO on Ni(110). Three impurity species, sulf ur, oxygen, and potassium are studied here. The former two are known a s ''poisons'' and the latter as a ''promoter'' for CO hydrogenation on Ni. All three are found to impede CO diffusion drastically. The appar ent diffusion activation energy E-D increases from the clean-surface V alue of 2-3 kcal/mol, to a saturation value of 7-8 kcal/mol at suffici ently 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 nearest-neighbor attractive interaction between CO and K seems to be most important.