The diffusion of CO on Ru(S)[15(001) x 2(100)] was studied by using laser-induced thermal desorption techniques. It was found that diffusion on the stepped Ru(001) surface was highly anisotropic. At 290K, and at a coverage of 0.27 of a monolayer, the diffusion coefficient for the direction parallel to the step edges was about 100 times greater than the coefficient for diffusion perpendicular to the step edges. The diffusion coefficients for diffusion along the terraces and over the steps were deduced from the measured diffusivities parallel and perpendicular to the step edges. In the case of diffusion along the terrace, the activation barrier was 9.4kcal/mol; with a pre-exponential factor of 3.6cm2/s. The kinetic parameters for diffusion over a step were 18.1kcal/mol and 1400cm2/s. The coverage dependence of the surface diffusion was also measured on the stepped Ru(001) surface (table 22). As the coverage was increased from 0.04 to 0.34 of a monolayer, the surface diffusion coefficient at 350K increased by a factor of 60. This coverage dependence was identical to the previously observed coverage dependence for CO diffusion on Ru(001) and was attributed to repulsive pair-wise interactions in the CO adlayer. The marked anisotropy on Ru(S)[15(001) x 2(100)], and the identical coverage dependences on Ru(001) and Ru(S)[15(001) x 2(100)], indicated that the steps did not trap CO, but instead acted as repulsive barriers to CO diffusion.

E.D.Westre, D.E.Brown, J.Kutzner, S.M.George: Surface Science, 1994, 302[3], 280-94

Table 22

Diffusion of CO on Ru(001) and Ru(S)[15(001) x 2(100)] Surfaces