ANISOTROPY AND COVERAGE DEPENDENCE OF CO SURFACE-DIFFUSION ON RU(S)-[15(001) X 2(100)]

The surface diffusion of CO on Ru(S)-[15(001) x 2(100)] was studied using laser-induced thermal desorption (LITD) techniques. CO diffusion on the stepped Ru(001) surface was highly anisotropic. At 290 K and theta(CO) = 0.27 ML, the diffusion coefficient in the direction parallel to the step edges (D(parallel-to) was approximately 100 times greater than the diffusion coefficient perpendicular to the step edges (D(perpendicular-to)). Surface diffusion coefficients for diffusion along the terraces (D(t)) and over the steps (D(s)) were obtained from the measured values of D(parallel-to) and D(perpendicular-to). For diffusion along the terrace, the surface diffusion barrier was E(t) = 9.4 +/- 0.5 kcal/mol and the diffusion preexponential was D0,t = 3.6 +/- 0.8 cm2/s. Kinetic parameters for diffusion over a step were E(s) = 18.1 +/- 0.8 kcal/mol and D0,s = 1.4 x 10(3+/-0.3) cm 2/s. The coverage dependence of the CO surface diffusion was also measured on the stepped Ru(001) surface. As the CO coverage increased from theta(CO) = 0.04 to 0.34 ML, the surface diffusion coefficient at 350 K 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 ascribed to repulsive pairwise interactions in the CO adlayer. The strong 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)] indicate that the steps do not trap CO, but rather act as a repulsive barrier to CO diffusion.