The adsorption and surface reactions of allyl iodide on the Mo2C/Mo(1 0 0) surface

The adsorption and surface reactions of allyl iodide on Mo2C/Mo(1 0 0) 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(1 0 0) 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 Tp=280–300 K and propene with 400–420 K. The desorption of iodine occurred at high temperature with a Tp∼950 K. Analysis of HREELS spectra of the annealed layer suggests the formation of η1-allyl and η3-allyl groups between 100 and 220 K and the presence of di-σ 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 Tp∼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).