ADSORPTION OF METHANOL, ETHANOL AND WATER ON WELL-CHARACTERIZED PT-SNSURFACE ALLOYS

Adsorption and desorption of methanol (CH3OH), ethanol (C2H5OH) and wa ter on Pt(111) and two, ordered, Pt-Sn alloys has been studied primari ly using temperature-programmed desorption (TPD) mass spectroscopy. Th e 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 m olecules are weakly bonded and reversibly adsorbed under UHV condition s on all three surfaces, molecularly desorbing during TPD without any decomposition. The two Pt-Sn surface alloys were found to chemisorb bo th methanol and ethanol slightly more weakly than on the Pt(111)surfac e, The desorption activation energies measured by TPD, and hence the a dsorption energies, of both methanol and ethanol progressively decreas e as the surface concentration of Sn increases, compared with Pt(111). The decreased binding energy leads one to expect a lower reactivity f or these alcohols on the two alloys. The sticking coefficients and the monolayer coverages of these alcohols on the two alloys were identica l 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 th e adsorption energy of water. Water clusters are formed even at low co verages on all three surfaces, eventually forming a water bilayer prio r to the formation of a condensed ice phase. These results are relevan t to a molecular-level explanation for the reactivity of Sn-promoted P t surfaces that have been used in the electro-oxidation of simple orga nic molecules. (C) 1998 Elsevier Science B.V.