Using molecular dynamics studies and static potential-energy minimization, the mechanisms by which n-alkanes (ethane through n-decane) diffused on a model Pt(111) surface in the low-coverage limit of a single adsorbed molecule were resolved. The simulations reproduced all of the experimental trends seen for the adsorption and diffusion of C3 to C6 on Pt(111) and Ru(001). The short alkanes (C2 to C8) behaved as rigid rods and their motion involved coupled translation and rotation in the surface plane. For this series, there was a linear increase in the diffusion barrier with the molecular chain length. The compliance of the motion to the assumptions of a nearest-neighbor hopping model was analyzed. Although hopping motion could be observed for all of the molecules at sufficiently low temperatures, the hopping involved a significant fraction of long jumps. As the temperature increased, the adsorption became essentially delocalized. Despite the extensive deviations of the motion from the assumptions of a nearest-neighbor hopping model, the static diffusion-energy barriers, arising from the minimum-energy path for hops between nearest-neighbor binding sites, agreed well with those obtained from the tracer-diffusion coefficients for butane, hexane and octane. For these molecules, multiple-site hops and long flights appeared to influence the values of the pre-exponential factors, which were too large. Neither the diffusion barrier nor the pre-exponential factor for ethane agreed well with theoretical estimates. These discrepances were attributed to the smallness of the static diffusion barrier and/or the existence of an unique dynamic behavior for this molecule. Due to the increased difficulty of in-plane rotation and increased mismatch between the geometries of the molecule and the surface, the diffusion barrier for n-decane fell below that for n-hexane. The characteristic mechanism of motion for n-decane involved significant C-C-C bond-angle bending.

A Molecular-Dynamics Simulation Study of the Adsorption and Diffusion Dynamics of Short n-Alkanes on Pt(111). Huang, D., Chen, Y., Fichthorn, K.A.: The Journal of Chemical Physics, 1994, 101[12], 11021-30