SURFACE-DIFFUSION OF HYDROGEN ON A STEPPED RU(001) SURFACE

The surface diffusion of hydrogen on Ru(S)-[15(001) x 2(100)] was studied using laser-induced thermal desorption (LITD) techniques. Hydrogen diffusion on this stepped Ru(001) surface displayed a dramatic anisotropy. At 230 K, the diffusion coefficient parallel to the steps edges (D-parallel to) was approximately 50 times greater than the diffusion coefficient perpendicular to the step edges (D-perpendicular to). Hydrogen surface diffusion coefficients across a terrace (D-t) and over a step (D-s) were obtained from the measured values of D-parallel to and D-perpendicular to at theta(H) = 0.60 ML. The activation energy for diffusion along a terrace was E(t) = 3.2 +/- 0.3 kcal/mol and the corresponding diffusion preexponential was D-o,D-1 = 3.9 x 10(-4+/-0.1) cm(2)/s. For diffusion over a step, the kinetic parameters were E(s) = 5.9 +/- 0.6 kcal/mol and D-o,D-s = 3.8 x 10(-3+/-0.2) cm(2)/s. Although the hydrogen surface mobility was highly anisotropic, the surface diffusion coefficient was observed to be coverage independent. The coverage independent diffusion suggests that hydrogen adatoms are not trapped by the step edges. The dramatic anisotropy of the diffusion coefficient and the coverage independent diffusion indicate that the step edges on Ru(S)-[15(001) x 2(100)] act as a repulsive barrier to hydrogen diffusion. The good agreement between the surface diffusion kinetic parameters for hydrogen diffusion on Ru(001) and hydrogen diffusion on the terraces of Ru(S)-[15(001) x 2(100)] demonstrates that surface steps did not dominate the earlier LITD measurements on Ru(001).