13) Scong, H. S.; Kim, U. S.; Kim, Y.; Seff, K. J. Phys. Chem., submitted (14) Swng, H. S.; Kim, Y.; Seff, K. J. Phys. Chem. 1991,95,9919-9924. (15) Sun, T.; Heo, N. H.; Seff, K., unpublished work. (16) Stamovlasis, D.; Wilson, J. R.; Seff, K., unpublished work.The interactions of cyclohexene with a Pt( 1 11) surface have been studied using a combination of bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). BPTDS is a technique which utilizes vapor-deposited Bi (at a surface temperature of -100 K) to passivate a previously prepared adlayer against intraadsorbate bond-breaking reactions. After such passivation, the surface is heated and adsorbed intermediates desorb intact for mass spectral identification, provided they have stable gas-phase analogues. Here we show that BPTDS is a useful technique for monitoring the coverages of adsorbed intermediates produced during the dehydrogenation of cyclohexene on Pt(ll1). At 95 K, cyclohexene adsorbs molecularly in a di-a fashion. By 200 K (E, z 13.7 kcal/mol) this species converts to another form of di-a, molecularly adsorbed cyclohexene, which itself converts to r-allyl c-C6H9, (plus adsorbed hydrogen) at 200-240 K (E, z 14.4 kcal/mol). At about the same temperature, part of the cyclohexene decomposes, producing small amounts of adsorbed benzene and cyclohexadiene. At about 340 K, the C-CgHg,, species converts to adsorbed benzene (E, = 20.8 kcal/mol) and the adsorbed hydrogen desorbs as H2 The C-CgHg, intermediate is identified in BPTDS by its deuteration to c-CsH9D gas at -190 K when coadsorbed with deuterium. No significant H-D exchange occurs between coadsorbed deuterium and molecularly adsorbed hydrocarbons when probed by BPTDS.
The interactions of 1,3-and l,4-cyclohexadiene with Pt(ll1) have been studied with bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). Both species adsorb molecularly on Pt( 11 1) at 100 K. Upon warming, both species dehydrogenate completely to produce adsorbed benzene. The reaction occurs at about 230-260 K with an activation energy of 14 f 2 kcalfmol. The adsorbed benzene thus produced either dehydrogenates, ultimately to graphitic carbon, or desorbs, both occurring above 450 K. When cyclohexadiene is coadsorbed with hydrogen adatoms, no significant amounts of hydrogenated products (<2%) are observed in normal TDS or in BPTDS.
The adsorption and reactions of tetraethoxysilane (TEOS) vapor on clean and water-predosed rutile TiO2-(1 10) have been studied using temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). The molecule sticks with a probability near unity at temperatures of 130-300 K, and at low coverage, TEOS dissociates upon heating. At higher coverages, some desorbs. On the waterand hydroxyl-free surface, the dissociation reaction occurs rapidly between 200 and 350 K, with the initial products being Si(OEt)z,, plus 2Et0, (Et = C2H5). This is a new mechanism for silane coupling to oxide surfaces which requires neither hydroxyl groups nor surface defects. The E t 0 ligands, whether attached to Ti or Si atoms, decompose at -650 K via j3-hydrogen elimination to yield ethylene gas and surface-bound hydrogen, which rapidly attaches to another E t 0 ligand, yielding ethanol gas. By 700 K, the net products evolved are equal amounts of ethylene and ethanol gas (two molecules of each per dissociated TEOS molecule), while Si02 remains on the surface. With predosed D20, the initial reaction also leads to Si(OEt)z,, + 2Et0,, but the EtO, thus produced now reacts with the D20, and/or OD, to give an EtOD peak in the TPD spectrum at -350 K. The amount of TEOS which dissociates is nearly doubled when D20 is present in high coverages. This is attributed to the fact that the D2O enhances elimination of EtO, (as EtOD), thus creating more free sites to accommodate dissociation products. The Si(OEt)l,, species is much less reactive with water than EtO, and will remain on the surface to 600 K in TPD, irrespective of water predose.
The adsorption of ethylene on bare and cesiated Pt(111) surfaces has been investigated by means of thermal desorption spectroscopy and ultraviolet photoemission spectroscopy, at temperatures ranging between 37 and 800 K. On the bare Pt(111) surface, a new π-bonded C2H4 species has been observed at T<52 K. It is believed to be nearly undistorted with respect to the gas phase molecule. At T>52 K, it transforms into di-σ C2H4 and then into ethylidyne (∼300 K). Conversely, cesium preadsoption gives rise to reversibly adsorbed π-bonded ethylene, meanwhile it inhibits the usual reaction path (di-σ species leading to ethylidyne). Furthermore, valence-level binding energy shifts induced by cesium adatoms have been shown to be consistent with the theoretical picture of the electrostatic potential surrounding adsorbed alkali elements.
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