title: Adsorption of [D2]methanol on Ru(001)--O surfaces: the influence of preadsorbed oxygen on the methoxide geometry.
abstract: The influence of oxygen precoverage on the bonding geometry of methoxide on Ru(001) was studied using the isotopically labeled molecule CHD2OH by reflection-absorption infrared spectroscopy (RAIRS). This molecule is an excellent model because the vibrational spectra of CHD2O- may be unambiguously correlated with the adsorption configuration. For Ru(001)--O layers with an effective oxygen coverage (theta0) between 0.25 and 0.6 ML (ML=monolayer), the influence of the oxygen precoverage was shown to vary with the initial methanol exposure. For an extremely low dose of [D2]methanol (0.01 L; L=Langmuir, 1 L=10(-6) torr s), at 90 K, no oxygen-coverage effects were detected on the geometry of [D2]methoxide: it adsorbs in an upright orientation (pseudo-C(3v) local symmetry), just as on clean Ru(001). An increase in the methanol exposure to 0.1 L, at the same temperature, results in the formation of a disordered layer of tilted methoxide: for theta(O)=0.25 ML, C(s)/C1 and intrinsic C1 configurations are present on the surface, whereas for theta(O)> or =0.5 ML, only the former species were identified. The thermal activation of these tilted layers to 105 K results in a lower coverage of upright methoxide for any oxygen precoverage, coadsorbed with decomposition products, as confirmed by the detection of adsorbed formaldehyde and, on the denser oxygen layer (theta(O)=0.6 ML), formate. The influence of the oxygen precoverage becomes determinant when annealing a [D2]methanol multilayer to 105 K: for theta(O)=0.25 ML, the RAIR spectrum correlates with a disordered layer of tilted methoxide and formaldehyde, whereas for theta(O)=0.6 ML upright methoxide, formate, and carbon monoxide were identified. On clean Ru(001), for methanol exposures > or =0.1 L, the C(3v) methoxide configuration was never attained upon thermal activation.
authors.affiliation: Centro de Química-Física Molecular, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal.
authors.forename: Ricardo B
authors.initials: RB
authors.lastname: Barros
authors.forename: Ana Rosa
authors.initials: AR
authors.lastname: Garcia
authors.forename: Laura M
authors.initials: LM
authors.lastname: Ilharco
mesh_terms: D000327:Adsorption; D003903:Deuterium / Q000737:chemistry*; D000432:Methanol / Q000031:analogs & derivatives / Q000737:chemistry*; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D012680:Sensitivity and Specificity; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013499:Surface Properties
chemical_list: D003903:Deuterium; D012238:Rhodium; D010100:Oxygen; D000432:Methanol