AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS3-WeM

Paper SS3-WeM2
Probing the Extrinsic Precursor State: Methane Trapping on Ethylidyne-covered Pt(111)

Wednesday, October 4, 2000, 8:40 am, Room 210

Session: Gas/Surface Dynamics
Presenter: A. Carlsson, Stanford University
Authors: A. Carlsson, Stanford University
R.J. Madix, Stanford University
Correspondent: Click to Email
Under steady state conditions in catalytic reactions surfaces may be covered with stable species that affect the adsorption kinetics. It has been well documented that the adsorption probability of many molecules is facilitated by a species which can migrate across the adsorbed layer in search of a binding site. This species is referred to as an extrinsic precursor state. In order to obtain a more general understanding of this precursor state and its effect on adsorption, we have studied the kinetics of methane trapping on ethylidyne-covered Pt(111) with supersonic molecular beam techniques at surface temperatures spanning the range in which methane desorbs from the saturated monolayer and the second adsorbed layer. Because the surface is nearly saturated with ethylidyne, in order to adsorb methane must first occupy an extrinsic precursor state and then migrate to vacant sites within the ethylidyne structure to adsorb. In accordance with a precursor model for adsorption, the dependence of the adsorption probability on surface temperature gives the difference in activation energies for desorption and migration from the extrinsic precursor and the ratio of pre-exponential factors. The temperature dependence of the steady state coverage was used to determine the activation energy for conversion of the adsorbed state back into the extrinsic precursor to be 5.7 kJ/mol, compared to the desorption energy from the extrinsic precursor of about 9 kJ/mol.