The Decomposition Pathways of Methanol on Clean Ru(0001), Studied by Reflection−Absorption Infrared Spectroscopy (RAIRS)

Abstract

The adsorption and decomposition of methanol on clean Ru(0001) were investigated by reflection−absorption infrared spectroscopy (RAIRS). At low temperature (90 K) and coverage (0.1 L), it was confirmed that methanol adsorbs dissociatively as methoxide (CH3O−). No experimental evidence was obtained of an alternative decomposition for high coverage. Different bonding sites and geometries, depending on temperature and coverage, were proposed for methoxide and correlated with the corresponding CO stretching wavenumbers. Methoxide may either undergo complete dehydrogenation into CO(ads) and H(ads), if annealed in small temperature steps (in the range between 110 and 320 K), or partial dehydrogenation into very stable η2-formaldehyde, by a one-step thermal activation (from 130 K to at least 190 K), in the presence of previously formed products (CO and atomic species). At high temperatures (≥190 K), methanol undergoes O−H, C−H, and C−O bond scission, leaving surface fragments undetectable by RAIRS. However, in a sequential dosing, the fragments from the first methanol molecules that hit the surface seem to have a passivating effect on Ru(0001). Subsequent doses undergo only partial dehydrogenation, yielding η2-formaldehyde, which was isolated on the surface in two bonding configurations: bridging [μ2-η2(C,O)-H2CO] and chelating [μ1-η2(C,O)-H2CO], characterized by the νCO mode at 1262 and 1277 cm-1, respectively. This assignment was confirmed by adsorbing CD3OH. The bridging form is favored at lower coverage. Formaldehyde prepared by sequential dosing is stable on the surface up to at least 290 K, although some dehydrogenates to CO(ads) above 190 K.