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 E-D 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 nearest-neighbor attractive interaction between CO and K seems to be most important.