Hydrogenation of acetylene and ethylene using Pd foil as a model catalyst: Bridging the gap between UHV and high pressure

The kinetics of acetylene and ethylene hydrogenation when catalyzed by a clean palladium foil have been measured carefully as a function of reactant pressure and sample temperature. The experimentally measured pressure dependences can be rationalized by proposing that the catalysts surface is covered by a strongly bound and relatively unreactive carbonaceous layer which consists of vinylidene species when acetylene is used as the reactant and ethylidyne with ethylene. It is shown using infrared spectroscopy that carbon monoxide can adsorb onto the surface in spite of the presence of the carbonaceous layer. It is proposed therefore that the hydrocarbon reactants can also adsorb onto the surface while it is covered by this layer. Benzene is suggested to form by reaction between adsorbed acetylene and a vinylidene species via a ;Finally, the hydrocarbon pressure dependences, which are generally either about zero or negative, are quantitatively explained by proposing that the amount of hydrogen that can be accommodated on the hydrocarbon-covered palladium surface decreases with the addition of hydrocarbon. That is, hydrogen adsorption is suppressed by hydrocarbons. A possible origin for this effect is that the hydrocarbon layer may be compressed by the adsorption of additional hydrocarbon inhibiting the adsorption of hydrogen.