The adsorption and surface reactions of allyl iodide on the Mo2C/Mo(100) surface

The adsorption and surface reactions of allyl iodide on Mo2C/Mo(100) surface have been investigated by thermal desorption spectroscopy, X-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy (HREELS) in the 100-1200 K temperature range. Allyl iodide adsorbed dissociatively and molecularly on the clean Mo2C/Mo(100) surface at 100 K and desorbed in two TPD peaks at 160 and 180 K, characteristic of condensed and chemisorbed layers, respectively. The rupture of C-I bond in the molecularly adsorbed C3H5I started at 100 K, and was completed at 220 K. C3H5 fragments formed underwent hydrogenation to propene and to propane. Propane desorbed with a T-p = 280-300 K and propene with 400-420 K. The desorption of iodine occurred at high temperature with a T-p similar to 950 K. Analysis of HREELS spectra of the annealed layer suggests the formation of eta(1)-allyl and eta(3)-allyl groups between 100 and 220 K and the presence of di-sigma bonded propene at 250-410 K. The latter species underwent dehydrogenation to propylidyne on the one hand, which was identified at 300-400 K on the surface, and was hydrogenated to propyl groups on the other hand. Propylidyne decomposes at higher temperatures leaving carbon on the surface and releasing hydrogen into the gas phase with T-p similar to 580 K. When the adsorption temperature was 300 K, the accumulation of propylidyne species was facilitated. Illumination of the adsorbed layer at 100 K enhanced the dissociation of allyl iodide. Allyl species formed in higher concentration coupled into hexadiene (Tp = 172 K) and hexane (Tp = 230 and 325 K). (C) 2002 Elsevier Science B.V. All rights reserved.