The influence of surface oxidation on the reactions of methanol on Fe(110)

Abstract

The thermal decomposition of methanol occurs via adsorbed methoxy on clean and oxidized Fe(110). Methoxy is detected following adsorption of CH 3OH at 100 K using X-ray photoelectron spectroscopy. On clean Fe(110), methoxy dehydrogenates at ∼400 K, yielding gaseous H 2 and CO. The hydroxyl hydrogens leave the surface as H 2 at slightly lower temperature, 375 K, based on isotopic labeling experiments. Carbon-oxygen bond scission competes with CO evolution at 400 K, accounting for ca 25% of the CO yield on clean Fe(110). Surface oxygen inhibits OH and CH bond dissociation in methanol leading to a decrease in the total amount of reaction and an increase in the amount of methanol desorbed from the surface with increasing oxygen coverage. At coverages greater than one monolayer (ML), there is no detectable reaction of methanol. In addition, the temperature required for CH bond activation in methoxy increases to 475 K for an initial oxygen coverage of 0.75 ML. Surprisingly, the ratio of CO bond dissociation to retention increases as a function of initial oxygen coverage. The increase in C—O bond dissociation is tentatively attributed to defects in mixed phases of FeO(111) and Fe(110)-c(3 × 1)-O, based on comparison with STM data taken from the literature. The potential implications of the different reactivity of methanol as a function of oxygen coverage with regard to the function of lubricant additives is discussed.