The diffusion of CO on Ru(001) was measured by using laser-induced thermal desorption techniques. The surface diffusion coefficients exhibited a marked dependence upon the CO coverage. At coverages of less than 0.33 of a monolayer, the surface diffusion coefficient at 290K was approximately constant, at a value below 10-8cm2/s. As the CO coverage increased from 0.33 to 0.58 of a monolayer, the surface diffusion coefficient at 290K increased sharply from 10-8 to 10-6cm2/s. The surface diffusion coefficients at 250K also exhibited a similar increase as a function of the CO coverage. At various temperatures and CO coverages, the surface diffusion coefficients obeyed an Arrhenius behavior. At a coverage of 0.27 of a monolayer, the relationship was:

D (cm2/s) = 0.38 exp[-11(kcal/mol)/RT]

for temperatures ranging from 300 to 370K. At coverages greater than 0.45 of a monolayer, the pre-exponential factor and the activation energy decreased with increasing CO coverage at temperatures of between 210 and 290K. At a coverage of 0.45 of a monolayer, the Arrhenius relationship was:

D (cm2/s) = 0.28 exp[-8.0(kcal/mol)/RT]

At a coverage of 0.58 of a monolayer, the Arrhenius relationship was:

D (cm2/s) = 0.06 exp[-6.2(kcal/mol)/RT]

The coverage dependence of CO surface diffusion on Ru(001) was consistent with strong repulsive nearest-neighbor CO interactions. A repulsive CO-CO pair-wise interaction energy of -1.4kcal/mol was obtained by using a diffusivity versus coverage model which was based on the quasi-chemical approximation.

A.A.Deckert, J.L.Brand, M.V.Arena, S.M.George: Surface Science, 1989, 208[3], 441-62