SURFACE-DIFFUSION AND DESORPTION OF PENTANE ISOMERS ON RU(001)

The surface diffusion and desorption of n-pentane, isopentane, cyclopentane, and neopentane were examined on Ru(001). These investigations used the techniques of laser-induced thermal desorption and temperature-programmed desorption. The surface diffusion activation energies varied from Edif = 3.0 ± 0.3 kcal/mol for neopentane to Edif = 4.4 ± 0.2 kcal/mol for n-pentane. The desorption activation energies ranged from Edes = 10.7 ± 0.2 kcal/mol for neopentane to Edes = 13.8 ± 0.9 kcal/mol for n-pentane. The surface corrugation ratio, defined as Ω ≡ Edif/Edes, was remarkably constant at Ω ≈ 0.30 for all the pentane isomers. In agreement with earlier results for n-alkanes on Ru(001), this constancy indicates a linear scaling between the diffusion activation energy and the desorption activation energy. These results suggest that the pentane isomers have similar binding configurations and move on the Ru(001) surface in a concerted process. The magnitudes of the surface diffusion and desorption activation energies were observed to scale inversely with the degree of branching of the pentane isomer. The Wiener index, W(G), was employed to characterize the degree of branching. In addition, a simple physisorption model using a Lennard-Jones (6-12) pair potential was utilized to predict surface binding energies and configurations. This model was also used to quantify the scaling between the activation energies and the degree of branching. © 1990 American Chemical Society.