METHANOL DECOMPOSITION ON OXYGEN PRECOVERED AND ATOMICALLY CLEAN PD(111) SINGLE-CRYSTAL SURFACES

The adsorption and decomposition of methanol on a clean and oxygen precovered Pd(111) single crystal surface has been studied by means of static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS). Besides Pd(x)+(x = 1-3) the SSIM spectra contain a variety of ions, including Pd(m)O+ (m = 1,2), PdOCH3+, PdCH3+ and Pd(n)CO+ (n = 1-3). The intensities of these species are dependent on the oxygen precoverage and follow different trends in relaxation type measurements (uptake-depletion) or when the surface is heated in a temperature programmed manner (TPSSIMS). Preadsorbed oxygen (1 L O2 = 1.3 x 10(-4) Pa . s) acts as a trapping agent for hydrogen atoms released during methanol decomposition. The occurrence of PdOCH3+ and PdCH3+ ions suggests the formation of adsorbed methoxy and methyl after both O-H and O-C bond activation in CH3OH. This is supported by the C 1s XP spectra obtained at 310 K. Within the range of temperatures applied (300-500 K, during TPSSIMS) the formation of CO(ad) occurs exclusively through decomposition of the CH3O(ad). Substantial amounts of this intermediate build up after accumulation of CO(ad). While CH3O(ad) undergoes reversible thermal desorption (after recombination with H(ad)) CH3,ad is little inclined to any reaction with coadsorbed species below 500 K. A suppression of methyl formation by the presence of adsorbed oxygen is not found.