Adsorption and Desorption Behavior of n-Butane and Isobutane on Pt(111) and Sn/Pt(111) Surface Alloys

The adsorption/desorption behavior of n-butane and isobutane on Pt(111) and the p(2×2) Sn/Pt(111) and (√3×√3)R30° Sn/Pt(111) surface alloys has been examined using a combination of adsorption kinetics measurements utilizing a collimated molecular beam and temperature programmed desorption (TPD) mass spectroscopy. Initial sticking probabilities for both molecules on Pt(111) and the surface alloys at temperatures below the monolayer desorption threshold are essentially unity (S₀ ≥0.95). The monolayer saturation coverages of n-butane and isobutane were also independent of the amount of Sn in the surface layer. The desorption activation energies measured by TPD for the monolayer states of both n-butane and isobutane progressively decrease by 5-8 kJ/mol compared to Pt(111) as the surface concentration of Sn increases from 0.25 to 0.33 atom fraction in the respective surface alloys. The decrease in the desorption activation energy scales linearly with the Sn concentration. No thermal decomposition of either molecule on any surface occurred during TPD measurements. Molecular interactions probed by adsorption and desorption of saturated C₄ hydrocarbons are not influenced as strongly by the presence of Sn in the Pt(111) surface as previously observed for unsaturated molecules, such as ethylene and isobutylene. The rate constants for adsorption and desorption of n-butane derived from these ultrahigh vacuum studies are used to help explain the kinetics determined in recent, moderate pressure (50-200 Torr) studies of the hydrogenolysis of n-butane over these Sn/Pt(111) surface alloys.