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(2X2) Sn/Pt(111) and (root 3x root 3)R30 degrees Sn/Pt(111) surface alloys has been examined using a combination of adsorption ki netics measurements utilizing a collimated molecular beam and temperat ure programmed desorption (TPD) mass spectroscopy. Initial sticking pr obabilities for both molecules on Pt(111) and the surface alloys at te mperatures below the monolayer desorption threshold are essentially un ity (S-o greater than or equal to 0.95). The monolayer saturation cove rages 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 progres sively decrease by 5-8 kJ/mol compared to Pt(111) as the surface conce ntration of Sn increases from 0.25 to 0.33 atom fraction in the respec tive 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. Mole cular interactions probed by adsorption and desorption of saturated C- 4 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 a nd desorption of n-butane derived from these ultrahigh vacuum studies are used to help explain the kinetics determined in recent, moderate p ressure (50-200 Torr) studies of the hydrogenolysis of n-butane over t hese Sn/Pt(111) surface alloys.