The adsorption dynamics of small alkanes on (111) surfaces of platinum group metals

摘要

The adsorption dynamics of methane, ethane, propane, and n-butane on Ni(111) as well as methane and w-butane on Pd(111) have been investigated using super-sonic molecular beam techniques, and stochastic trajectory simulations. For each alkane the initial adsorption probability was measured as a function of incident energy and incident angle, and compared with corresponding values for other alkanes on Pd(111) and Pt(111). In general, at a fixed incident energy and angle, the trapping probability is highest on Pd(111), followed by Pt(111) and Ni(111), a result which deviates from the prediction of simple mass matching arguments [Surf. Sci. 187 (1987) 67]. The lower binding energy of the alkanes and the higher lattice force constant for Ni(111) compared with Pt(111) and Pd(111) give rise to the lower trapping probability. Three-dimensional stochastic trajectory simulations for alkane trapping on the three metals clearly indicate that incoming molecules lose considerable energy to Pd lattice vibrations, resulting in a high trapping probability. On the other hand, the stiffer Ni lattice prevents the excitation of surface phonons, consistent with experimental results. A simple scaling principle based only on surface masses and Debye temperatures was applied to scale the trapping probabilities of methane, ethane, and propane on Pd(111) and Ni(111) from that on Pt(111), and quantitative agreement was obtained within about 30%.