It was noted that a small amount of co-adsorbed impurity (S, O, K) markedly changed the diffusivity of CO on Ni (110). The first 2 impurities were well-known poisons, and the third was a promoter of CO hydrogenation on Ni. All 3 impurities greatly impeded CO diffusion. The apparent diffusion activation energy increased from the clean-surface value (2 to 3kcal/mol), to a saturation value of 7 to 8kcal/mol at sufficiently high impurity coverages. The impeding effect decreased in the order: S → O → K. In the case of S and O, impurity-covered step-controlled diffusion appeared to be the predominant mechanism. In the case of K, the nearest-neighbor attractive interaction between CO and K seemed to be the most important factor.

Effects of Surface Impurities on Surface Diffusion of CO on Ni (110) X.Xiao, Y.Xie, C.Jakobsen, Y.R.Shen: Physical Review B, 1997, 56[19], 12529-38