Reactions of gas phase H atoms with ethylene, acetylene and ethane adsorbed on Ni(111)

The products of the reaction of the most energetic form of hydrogen, gas phase H atoms, with ethylene, acetylene and ethane adsorbed on a Ni(1 1 1) surface at 60 K are probed. Adsorbed ethylidyne (CCH3) is identified by high resolution electron energy loss spectroscopy to be the major product (30% yield) in all three cases. Adsorbed acetylene is a minor product (3% yield) and arises as a consequence of a dynamic equilibrium between CCH3 and C2H2 in the presence of gas phase H atoms. The observation of the same product for the reaction of H atoms with all three hydrocarbons implies that CCH3 is the most stable C-2 species in the presence of coadsorbed hydrogen. The rates of CCH3 production are measured as a function of the time of exposure of H atoms to each hydrocarbon. A simple kinetic model treating each reaction as a pseudo-first order reaction in the hydrocarbon coverage is fit to these data. A mechanism for the formation of CCH3 via a CHCH2 intermediate common to all three reactants is proposed to describe this model, The observed instability of the CH2CH3 species relative to C2H4 plays a role in the formulation of this mechanism as does the observed stability of CHCH2 species in the presence of coadsorbed hydrogen. The CH2CH3 and the CHCH2 species are produced by the translational activation of ethane and the dissociative ionization of ethane and ethylene, respectively. In addition, the binding energy and the vibrational spectrum of ethane adsorbed on Ni(1 1 1) are determined and exceptionally high resolution vibrational spectra of adsorbed ethylene and acetylene are presented. (C) 2001 Elsevier Science B.V. All rights reserved.