Adsorption of methanol, ethanol and water on well-characterized Pt-Sn surface alloys

Adsorption and desorption of methanol (CH3OH), ethanol (C2H5OH) and water on Pt(111) and two, ordered, Pt-Sn alloys has been studied primarily using temperature-programmed desorption (TPD) mass spectroscopy. The two alloys studied were the p(2 x 2)Sn/Pt(111) and (root 3 x root 3)R30 degrees Sn/Pt(111) surface alloys prepared by vapor deposition of Sn on Pt(111), with theta(Sn)=0.25 and 0.33, respectively. All three molecules are weakly bonded and reversibly adsorbed under UHV conditions on all three surfaces, molecularly desorbing during TPD without any decomposition. The two Pt-Sn surface alloys were found to chemisorb both methanol and ethanol slightly more weakly than on the Pt(111)surface, The desorption activation energies measured by TPD, and hence the adsorption energies, of both methanol and ethanol progressively decrease as the surface concentration of Sn increases, compared with Pt(111). The decreased binding energy leads one to expect a lower reactivity for these alcohols on the two alloys. The sticking coefficients and the monolayer coverages of these alcohols on the two alloys were identical to that on Pt(111) at 100 K, independent of the amount of Sn present in the surface layer. Alloying Sn in Pt(111) also slightly weakens the adsorption energy of water. Water clusters are formed even at low coverages on all three surfaces, eventually forming a water bilayer prior to the formation of a condensed ice phase. These results are relevant to a molecular-level explanation for the reactivity of Sn-promoted Pt surfaces that have been used in the electro-oxidation of simple organic molecules. (C) 1998 Elsevier Science B.V.