Generation and reactions of CH2 and C2H5 species on Mo2C/Mo(111) surface

The adsorption and dissociation of CH2I2 and C2H5I on Mo2C/Mo(111) surface have been investigated with the purpose of producing adsorbed CH2 and C2H5 species. Methods used include high resolution electron energy loss, X-ray photoelectron, Auger electron, and temperature programmed desorption spectroscopies. Independently of the coverage, CH2I2 adsorbs molecularly at 90-100 K. The dissociation of an adsorbed layer starts around 180-190 K. The primary products of thermal dissociation are adsorbed CH2 and I. The species CH2 undergoes self-hydrogenation to CH4 at T-p = 300 K and dimerization into C2H4 at and above 222-280 K. Ethylene formed desorbs above 400 K. C2H5I also adsorbs molecularly on Mo2C at 90-100 K and dissociates to C2H5 and I above 150 K. The reaction of C2H5 on Mo2C/Mo(111) surface yielding C2H6 and C2H4 proceeds at a much lower temperature, above 180 K, than that of CH2. Neither the cleavage of the C-C bond nor the coupling of C-2 compounds occurred to detectable extent under the reaction conditions. The ethylene formed in the reactions of both CHx species exhibited the same features as observed following C2H4 adsorption on Mo2C: the stable di-sigma-bonded ethylene is transformed into ethylidyne at higher temperature. The results are discussed in relevance to the conversion of methane into benzene on Mo2C deposited on ZSM-5. (C) 1999 Academic Press.