Macroscopic CO diffusion on the (110) surface of Ni (figure 12) was found to be markedly impeded by surface contamination by only a few percent of a monolayer of S (figure 13). The effect was even greater if the sample was annealed at high temperatures. The results were explained by proposing that S-covered steps led to a much higher energy barrier to CO diffusion.

X.D.Xiao, Y.Xie, C.Jakobsen, H.Galloway, M.Salmeron, Y.R.Shen: Physical Review Letters, 1995, 74[19], 3860-3