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 C₂ 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.

The reactions of hydrogen atoms adsorbed on a Ni(111) surface (surface-bound H) and hydrogen atoms just below the surface (bulk H) with coadsorbed acetylene are probed under ultrahigh vacuum conditions. Bulk H is observed to react with acetylene upon emerging onto the surface at 180 K. Gas-phase hydrogenation products, ethylene and ethane, are produced as well as an adsorbed species, ethylidyne. Ethylidyne is identified by high-resolution electron energy loss spectroscopy. Surface-bound H reacts with adsorbed acetylene above 250 K to produce a single product, adsorbed ethylidyne. No gas-phase hydrogenation products, such as ethylene or ethane, are observed. The reaction of surface-bound H is extremely slow, with a rate constant determined from measurements of the initial reaction rate to be in the range of 10^-5−10^-3 (ML s)^-1 for a temperature range of 250−280 K. The activation energy for the rate-determining step, which is shown to be the addition of the first surface-bound H to acetylene to form an adsorbed vinyl species, increases from 9 to 17 kcal/mol as the total coverage decreases from 0.92 to 0.74 ML. The reaction rate cannot be described by a simple first-order dependence on the coverage of either reactant, indicating the presence of strong interactions between reactants. Measurements of the equilibrium constant reveal strong interactions between the reactant surface H and the product ethylidyne, possibly resulting in island formation. Mechanisms for the formation of ethylidyne by the reactions of both surface-bound and bulk H are proposed, as well as mechanisms for the formation of ethylene and ethane by bulk H. The different product distributions resulting from the reaction of acetylene with the two forms of hydrogen are discussed in terms of the large energy difference between bulk and surface-bound H.

The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identified by high-resolution electron energy loss spectroscopy to be ethylidyne (C−CH₃). This study is the first direct spectroscopic observation of a C−CH₃ species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C−CH₃, ¹³C−¹³CH₃, C−CD₃, and ¹³C−¹³CD₃, are reported, and a complete and consistent vibrational assignment of their fundamental modes is presented. Based on this assignment, a force field is derived from the measured vibrational frequencies using a normal-modes analysis and is found to be in good agreement with that deduced from IR spectra of an ethylidyne species in an organometallic complex. Inspection of the eigenvectors of the normal-mode displacements reveals that substantial mixing of harmonic bond motions is the origin of the unusual upshift in frequency of the C−C stretching mode upon deuteration. A quantitative determination of the relative dynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C−C stretching and symmetric CH₃ deformation modes upon deuteration, phenomena common to all C−CD₃ spectra, also arise from extensive mixing of bond motions. A detailed analysis of the spectra strongly suggests a C_3v or C₃ local environment for ethylidyne and a 3-fold hollow adsorption site.

We report that both surface-bound H atoms and bulk H atoms, upon moving out from the bulk of a Ni single crystal to its surface of a (111) orientation, are reactive with adsorbed C₂H₂, but the two kinds of H atoms have unique product distributions. Both bulk H and surface-bound H react with C₂H₂ to produce adsorbed ethylidyne, CCH₃, while only bulk H hydrogenates C₂H₂ to gas-phase ethylene and ethane, the products of interest in acetylene hydrogenation catalysis for the purification of ethylene streams. Their distinct reactivities arise from both their different directions of approach to the Ï€ orbitals of the unsaturated hydrocarbon and their substantially different energetics. These observations demonstrate that H embedded in the metal catalyst is a reactant in alkyne hydrogenation and is not solely a source of surface-bound H which then reacts with acetylene, as proposed from correlations between the hydrogenation activity of Raney Ni and Pd catalysts and the amount of H absorbed in these catalysts. The reactivities of these two kinds of H atoms are clearly distinguished in this experiment because of the capability to synthesize either bulk H or surface-bound H cleanly in an ultrahigh vacuum environment.