Methanol on O/Mo( 110): specific site for methyl radical formation

The adsorption and decomposition of methanol on the variously oxidized Mo(110) surface has been studied using temperature programmed desorption spectroscopy, Auger electron spectroscopy, low energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). Distinct degrees of oxidation are distinguished. That giving rise to a two-dimensional surface oxide (up to ~0.5 monolayers) is identified by LEED patterns and the presence of two features below 700cm ~ in the HREEL spectra. Upon further high-temperature (>1000K) oxidation a three-dimensional oxide is formed, marked by the appearance in the HREEL spectra of a feature at ~ 1000 cm -1 associated with the M o = O moiety. The two surfaces can also be distinguished by their respective Auger (O/Mo) ratios. They are also distinct [from each other and from clean Mo(110) surface] in the decomposition processes of adsorbed methanol. Though both produce methoxy intermediates, the three-dimensional oxide surface is much less reactive with desorption of all products complete below ~490 K. On the two-dimensional oxide surface the methoxy species is stable to much higher temperatures and decomposes to produce hydrogen and methyl radicals, as previously reported. This decomposition leaves behind an oxygen which is found to form Mo=O, as in the three-dimensional oxide, identified by its vibrational frequency. The observed Auger (O/Mo) ratio increases following adsorption of methanol on the two-dimensional oxide surface and annealing to above 700 K. Thus the availability of a specific site (on top) is seen to open up the methyl radical reaction pathway. © 1997 Elsevier Science B.V.