A RAIRS study of the methanol decomposition on oxygen precovered Ru(0001)

The effect of a low oxygen precoverage on the Ru(0001) surface towards the adsorption and decomposition of methanol was investigated by reflection–absorption infrared spectroscopy (RAIRS). At 90 K, a very low dose of methanol (0.1 L) dehydrogenates very efficiently on the Ru(0001)–(2×2)-O surface to methoxide (CH3O–), slightly tilted. For exposures above 0.5 L, a multilayer is formed, which desorbs by annealing to 130 K. At this temperature, three stages of the methoxide decomposition were identified: methoxide in a precursor configuration (lying down on the surface, as only the νasCH3 mode could be detected), a small amount of formyl [μ3–η2(C,O)–HCO] (resulting from partial dehydrogenation) and, as predominant species, on-top carbon monoxide (from total dehydrogenation). The two former stages were not detected on clean Ru(0001), where at 130 K the main species is still slightly tilted methoxide. A more extensive dehydrogenation is therefore promoted on the modified surface. Formyl apparently desorbs without decomposing into CO(ads), above 150 K. At 190 K, for very low methanol exposures (0.05 L), the oxygen covered Ru(0001) is highly selective towards dehydrogenation, since only CO(ads) is observed, approaching saturation coverage for 2 L of methanol. For exposures between 0.1 and 2 L, a small fraction undergoes partial dehydrogenation to η2-formaldehyde. The surface poisoning by CO(ads) is clear when further exposure to methanol results only in the formation of methoxide and OH(ads). Although methoxide recombination starts above 190 K, it is stable on this surface, since a considerable fraction is still present at 250 K.