The surface diffusion of H on Ru(S)-[15(001) x 2(100)] was studied by using laser-induced thermal desorption techniques. It was found that the H diffusion on this stepped (001) surface exhibited a marked anisotropy. At 230K, the diffusivity parallel to the step edges was some 50 times greater than the diffusivity perpendicular to the step edges. The H surface diffusion coefficients across a terrace and over a step were deduced from the measured values of diffusion parallel to the step edges and diffusion perpendicular to the step edges, at a H coverage of 0.60 of a monolayer (table 157). Diffusion along a terrace could be described by:

D (cm2/s) = 3.9 x 10-4exp[-3.2(kcal/mol)/RT]

For diffusion over a step, the equivalent expression was:

D (cm2/s) = 3.8 x 10-3exp[-5.9(kcal/mol)/RT]

It was noted that, although the H surface mobility was very anisotropic, the surface diffusion coefficient was coverage-independent. This suggested that H adatoms were not trapped by the step edges. The marked anisotropy of the diffusion coefficient, and the coverage-independent diffusion, indicated that the step edges on Ru(S)-[15(001) x 2(100)] acted as a repulsive barrier to H diffusion. The close agreement between the surface diffusion parameters for H diffusion on (001), and H diffusion on the terraces of Ru(S)-[15(001) x 2(100)], demonstrated that surface steps had not dominated earlier laser-induced thermal desorption measurements on (001).

M.V.Arena, E.D.Westre, D.E.Brown, J.Kutzner, S.M.George: Surface Science, 1995, 325, 151-62

Table 157

Diffusion Parameters for H on Ru Surfaces