An FTIR study of the bonding of methoxy on Ni(100): Effects of coadsorbed sulfur, carbon monoxide and hydrogen

The vibrational spectra of CH3O(a) CD3O(a), CDH1O(a) and CD2HO(a) on Ni(100) are analyzed and interpreted in terms of resonances between fundamental modes and either combinations or overtones, Analysis of the symmetry of the modes observed suggests that methoxy binds normal to the surface with C-s symmetry, at least at low coverages. Two distinct vibrational bands emerge in the vibrational spectrum of methoxy in the nu(CO) region as the coverage increases which are attributed to bonding in four-fold hollow sites and bridging sites. These bands exhibit blue shifts of about 25 cm(-1) with increasing coverage up to the saturation coverage. The vibrational bands in the nu(CH) region appear concomitantly at all coverages and shift down 12 cm(-1) as the coverage is increased. These shifts are attributed to changes in the metal-oxygen bond which are reflected in changes in the strength of the C-O and C-H bonds. Affects on the bonding also appear to occur with the coadsorption of hydrogen or CO with methoxy. Coadsorption of 0.36 ML hydrogen with 0.04 ML methoxy induces blue shifts of 15 and 7 cm(-1) for the nu(CO) bands at 949 and 984 cm(-1), respectively. Adsorbing 0.43 ML of CO with 0.04 ML methoxy (and 0.04 ML hydrogen) causes a red shift of 20 and 12 cm(-1) for these bands. A drastic drop in mode intensities for methoxy when CO is coadsorbed suggests that the methoxy tilts away from the surface normal. Pre-adsorbing sulfur on the Ni(100) surface reduces the amount of methoxy formed from methanol, but the nu(CO) methoxy bands are unshifted in frequencies relative to their position for the same methoxy coverage on the clean surface.