The adsorption dynamics of small alkanes on ( [formula omitted]) surfaces of platinum group metals

Abstract

The adsorption dynamics of methane, ethane, propane, and n-butane on Ni(1 1 1) as well as methane and n-butane on Pd(1 1 1) 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(1 1 1) and Pt(1 1 1). In general, at a fixed incident energy and angle, the trapping probability is highest on Pd(1 1 1), followed by Pt(1 1 1) and Ni(1 1 1), 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(1 1 1) compared with Pt(1 1 1) and Pd(1 1 1) 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(1 1 1) and Ni(1 1 1) from that on Pt(1 1 1), and quantitative agreement was obtained within about 30%.