| Chemisorption and reaction of CH3OH (methanol), C2H 5OH (ethanol), and H2O (water) on Pt(111) and Sn/Pt(111) alloys, and CO (carbon monoxide), NO (nitric oxide), and C2D 2 (acetylene) on Pt(100) and Sn/Pt(100) have been studied under ultrahigh vacuum conditions using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS). Small organic molecules like CH3OH and C2H5OH are potential fuels for low-temperature hydrocarbon fuel cells and it is important to understand the role of tin as a promoter in electrooxidation of these molecules. Also, the catalytic reactions of CO, NO and C2H2 are of considerable interest for improving of automotive exhaust-gas catalytic converters and other heterogeneous catalysts. Ordered Pt-Sn alloys can be prepared by vapor deposition of Sn on Pt surfaces. A (2 x 2) structure (theta Sn = 0.25) and a (√3x√/3)R30° structure are formed on Pt(111), and a c(2 x 2) and (3√2x√2)R45° structures with theta Sn = 0.5 and 0.67, respectively, are formed on Pt(100). CH3OH, C2H5OH and H2O are all weakly bound and reversibly adsorbed on Pt(111) and both of the Sn/Pt(111) alloys under UHV conditions. Alloying Sn into the Pt(111) surface weakens the adsorption of these molecules from that on Pt(111) and leads to a lower reactivity as the surface concentration of Sn increases. TPD measurements reveal a reduction in the saturation coverage and chemisorption bond energy for CO, NO and C2HL chemisorption, on the two Sn/Pt(100)alloys compared to that on Pt(100). CO chemisorption is completely reversible on these two Sn/Pt(100) alloys. However, NO is partially reduced to form N2O on these alloys, so that N2O along with NO and O2 desorption was observed. We propose that dinitrosyl species, i.e., two NO molecules bound to one Pt atom, are intermediates in N2O formation from adsorbed NO on these Sn/Pt(100) surface alloys. Irreversible dissociative adsorption of acetylene was strongly suppressed (∼80--90%) on both of the two Sn/Pt(100) alloys. About 15% of the adsorbed acetylene monolayer was converted to benzene and desorbed during TPD on the (3√2x√2)R45°Sn/Pt(100) alloy, but no benzene was desorbed from the c(2x2) alloy. |
