AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuM

Invited Paper SS1-TuM5
The Dynamics of Alkane Adsorption on Pt(111), Pd(111) and Ni(111): Prediction from One Metal to Another?

Tuesday, November 5, 2002, 9:40 am, Room C-108

Session: Hydrocarbon Catalysis
Presenter: R.J. Madix, Stanford University
Authors: C.-L. Kao, Stanford University
J.F. Weaver, University of Florida
R.J. Madix, Stanford University
Correspondent: Click to Email
The adsorption and reaction of alkanes on metal surfaces is fundamental to hydrocarbon catalysis. In order to adsorb the kinetic energy of the incident molecule must be dissipated in the gas-surface collision. To develop a predictive capability for the adsorption probabilities of alkanes on surfaces we have combined molecular beam methods with molecular dynamics simulations. The objective is to determine simple empirical potential parameters that govern the alkane-metal interaction from experimental measurements for a single alkane-surface combination and to use these parameters to predict trapping probabilities for other alkanes on other surfaces. A single set of potential parameters, determined from measurements of the trapping probabilities of ethane on Pt(111) can be used to predict the trapping probabilities of C3-C5 alkanes on Pt(111) and ethane and propane on Pt(110). More recently, these predictions have been extended to C1 - C5 alkane trapping on Pd(111). Palladium and platinum have similar lattice constants and Debye temperatures, differing primarily in their atomic mass, and thereby offer a good first order test of the predictive capability of the molecular dynamics simulations. Indeed, the trapping probabilities for Pd(111) are accurately predicted. Generally, the trapping probabilities for a given alkane are higher on Pd(111) due to the lower mass of the palladium atoms. The simulations show the importance of both the excitation of lattice vibrations and cartwheel rotational motion in affecting trapping. Predictions for Ni(111) are potentially more challenging, since the Ni-Ni force constants are much higher and both the mass and lattice parameter differ significantly from those of platinum. The theory correctly predicts that the adsorption probabilities on Ni(111) are lower than those for both Pt(111) and Pd(111). In general trapping probabilities are predicted to within about 50%.