The Extrinsic Precursor Kinetics of Methane Adsorption onto Ethylidyne-Covered Pt(111)

The kinetics of methane trapping on ethylidyne-covered Pt(111) were investigated using supersonic molecular beam techniques at surface temperatures between the temperature at which methane desorbs from the monolayer and from a second layer. The inability of the Langmuir, Kisliuk, or modified Kisliuk , models to fit the temperature dependence of the adsorption probability prompted the derivation of a model in which adsorption is forced through an extrinsic precursor. The difference in activation energies for desorption from the precursor state and migration of the extrinsic precursor, E d‘ − E m‘, is 5.4 kJ/mol, and the ratio of preexponential factors, / , is 2 × 104. The difference in activation energies suggests that the extrinsic precursor makes about 105 site hops near the desorption temperature in search of a binding site. The high degree of corrugation in the gas−surface potential may be responsible for the relatively high barrier to migration in the extrinsic precursor.