COVERAGE DEPENDENCE OF N-BUTANE SURFACE-DIFFUSION ON A STEPPED RU(001) SURFACE

The surface diffusion of n-butane on a stepped ruthenium surface, Ru(S)-[15(001) x 2(100)], was studied using laser-induced thermal desorption (LITD) techniques. These experiments were performed to understand the role of steps in surface mobility and to determine the effect of steps on LITD surface diffusion measurements. In contrast to earlier results on Ru(001), the LITD measurements revealed that the mobility of n-butane on Ru(S)-[15(001) x 2(100)] was strongly coverage-dependent. The surface diffusion coefficient increased by over four orders of magnitude as the coverage increased from theta-0.05-theta(sat) to theta-theta(sat). The coverage dependence was consistent with Monte Carlo simulations that modeled the steps as trapping sites. The surface diffusion coefficient was also measured as a function of temperature at theta = theta(sat) on Ru(S)-[15(001) x 2(100)]. An Arrhenius analysis of these measurements yielded a surface diffusion activation barrier of E(diff) - 4.0 +/- 0.4 kcal/mol and a diffusion pre-exponential of D(a) = 7.0 x 10(1:01) cm2/s. These kinetics parameters were similar to the previously observed values of E(dif) = 3.5 + 0.3 kcal/mol and D(o) = 1.0 x 1.0(1.02) cm2/s on Ru(001). This similarity indicates that the effect of steps that act as trap sites can be nearly removed by the titration of the trap sites at higher coverage. Consequently, the effect of steps on LJTD surface diffusion measurements on single-crystal surfaces is minimal at high coverage.