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.

Effective Rate Constants for the Surface Reaction between Solid Methanol and Deuterium Atoms at 10 K

“…Bands at 1700−1600 cm -1 in CH 3 OH, CH 2 DOH, and CH 3 OD are noise. b Bands were assigned on the basis of literature values. − c Values are normalized to the CO streching mode in CH 3 OH at 1043.6 cm -1 . d Band positions and integrated area absorbance (relative integrated band strength) were obtained by the least-square method (peak deconvolution).…”

Methanol oxidation on Ru(0001) for direct methanol fuel cells: analysis of the competitive reaction mechanism

“…[20][21][22][23][24] To achieve rational modi-cation on the anode catalysts, many efforts have been devoted to understanding the CH 3 OH oxidation mechanism on Ru catalysts experimentally. Unfortunately, although these studies were performed by using the same infrared absorption spectroscopy, conicting evidence on adsorption and the reaction mechanism of CH 3 OH oxidation have been observed on clean 16,17 and O-precovered 25,26 Ru(0001) surfaces. For the adsorption of reaction intermediates, Barros et al 17 found upright and tilted methoxy (CH 3 O) and formaldehyde (CH 2 O); while Gazdzicki et al 19 proposed an upright conguration of CH 3 O.…”