Effect of oxygen precoverage on the reactivity of methanol on Ru(001) surfaces

The thermal decomposition of methanol on oxygen modified Ru(001) surfaces was investigated by reflection-absorption infrared spectroscopy (RAIRS), under UHV conditions. The stability of the different intermediates was interpreted in terms of the oxygen coverage (theta(O) ranging from 0.25 to 0.75 ML), as well as of the methanol dose. For a low methanol exposure at 90 K, dissociative adsorption into methoxide (CH3O-) is favored by increasing oxygen coverage up to 0.6 ML, becoming almost inhibited for theta(O) = 0.75 ML. The reactivity of methoxide is also enhanced by increasing theta(O) from 0.25 to 0.6 ML, as the oxidation temperature drops from 130 to 100 K. Regardless of the oxygen coverage, the oxidation of methoxide yields the intermediate formaldehyde (H2CO), whose stability decreases with increasing theta(O). Formate (HCOO-) was only identified on surfaces with theta(O) greater than or equal to 0.5 ML, in the temperature range 100 K less than or equal to T < 130 K, with maximum yield for theta(O) approximate to 0.6 ML. This was the first spectroscopic observation of the in termediate formate in the decomposition of methoxide on Ru(001) surfaces, proving the participation of preadsorbed oxygen in the reaction. A more stable intermediate, formyl (HCO), which decomposes above 190 K, was only detected on surfaces with 00 = 0.25 and 0.6 ML, probably being formed on the domain boundaries present on these surfaces, namely Ru(001)-(2 x 2)-O/clean Ru(001) and Ru(001)-(2 x 1)-O/Ru(001)-(2 x 2)-30, respectively. For a high methanol exposure (methoxide at saturation coverage), methoxide is stabilized by neighboring interactions, and the oxidation process does not occur below 130 K, even for high oxygen coverages (theta(O) = 0.6 ML). All of the intermediates other than formate were detected. Either this decomposition channel (the deep oxidation of methanol) is inhibited for high methoxide coverage or any formate produced is not stable on the surface at such high temperatures.