Catalytic hydrogenation of acetylene on Ni(111) by surface-bound H and bulk H

The reactions of hydrogen atoms adsorbed on a Ni(111) surface (surface-boun d H) and hydrogen atoms just below the surface (bulk H) with coadsorbed ace tylene are probed under ultrahigh vacuum conditions. Bulk H is observed to react with acetylene upon emerging onto the surface at 180 K. Gas-phase hyd rogenation products, ethylene and ethane, are produced as well as an adsorb ed species, ethylidyne. Ethylidyne is identified by high-resolution electro n 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 react ion of surface-bound H is extremely slow, with a rate constant determined f rom 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 ener gy 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, incre ases from 9 to 17 kcal/mol as the total coverage decreases from 0.92 to 0.7 4 ML. The reaction rate cannot be described by a simple first-order depende nce on the coverage of either reactant, indicating the presence of strong i nteractions between reactants. Measurements of the equilibrium constant rev eal strong interactions between the reactant surface H and the product ethy lidyne, possibly resulting in island formation. Mechanisms for the formatio n of ethylidyne by the reactions of both surface-bound and bulk H are propo sed, as well as mechanisms for the formation of ethylene and ethane by bulk H. The different product distributions resulting from the reaction of acet ylene with the two forms of hydrogen are discussed in terms of the large en ergy difference between bulk and surface-bound H.