THE INFLUENCE OF SURFACE OXIDATION ON THE REACTIONS OF METHANOL ON FE(110)

The thermal decomposition of methanol occurs via adsorbed methoxy on c lean and oxidized Fe(110). Methoxy is detected following adsorption of CH3OH at 100 K using X-ray photoelectron spectroscopy. On clean Fe(11 0), methoxy dehydrogenates ar similar to 400 K, yielding gaseous H-2 a nd CO. The hydroxyl hydrogens leave the surface as H-2 at slightly low er temperature. 375 K, based on isotopic labeling experiments. Carbon- oxygen bond scission competes with CO evolution at 400 K, accounting f or ca 25% of th, CO yield on clean Fe(110). Surface oxygen inhibits O- H and C-H bond dissociation in methanol leading to a decrease in the t otal amount of reaction and an increase in the amount of methanol deso rbed from the surface with increasing oxygen coverage. At coverages gr eater than one monolayer (ML), there is no detectable reaction of meth anol. 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 increas es as a function of initial oxygen coverage. The increase ill C-O bond dissociation is tentatively attributed to defects in mixed phases of FeO(lll) and Fr(110)-c(3 x 1)-O, based on comparison with STM data tak en from the literature. The potential implications of the different re activity of methanol as a function of oxygen coverage with regard to t he function of lubricant additives is discussed. (C) 1997 Elsevier Sci ence B.V.