| Heterogeneous catalysis, the process by which a chemical reaction's facilitated by a solid surface, accounts for 90% of all chemicals produced; engineering catalytic reactions requires a fundamental understanding of chemical interactions on surfaces. During heterogeneous catalysis, reactants from the gas phase adsorb onto a surface, react, and subsequently desorb as products. Under reaction conditions, the catalytic surface may be partially or fully covered with adsorbates or contaminants which influence the adsorption of reactants onto the surface. Molecules which are transiently trapped on top of an adsorbed layer and migrate to a binding site are said to be in an extrinsic precursor state. In this dissertation, the dynamics and kinetics of adsorption of gas phase molecules through the extrinsic precursor on catalytic surfaces are described in detail.; The adsorption of alkanes and rare gases on alkane, alkylidyne, and rare-gas overlayers on Pt(111) was studied using molecular beam techniques, Auger electron spectroscopy, low energy electron diffraction, and temperature-programmed desorption in an ultra-high vacuum environment. Trapping of incident molecules on top of an adsorbed layer is facilitated by corrugation in the gas-surface potential, which serves to redirect perpendicular momentum parallel to the surface. In general, the corrugation of the gas-surface potential increases with adsorbate coverage. Trapping is facilitated to greater degrees on adsorbates which are less rigidly bound to the surface and can accommodate energy from the incident molecule. Once trapped into the extrinsic precursor state, molecules may either desorb or migrate to a binding site; a temperature dependence in the net adsorption probability arises from the competition between desorption and migration to a binding site. The activation energy for desorption from the extrinsic precursor is dependent on the precursor molecule and not on the adsorbates. In contrast, the activation energy for migration to a binding site is dependent on the adsorbate structure. The difference in activation energies for desorption and migration allow calculation of precursor lifetimes and predict that the extrinsic precursor state is relevant to reactions under industrial conditions. |
