An investigation of the reaction pathway for ethylene hydrogenation on Pd(111)

The hydrogenation of ethylene on Pd(111) is probed using a combination of t emperature-programmed desorption (TPD) and reflection-absorption infrared s pectroscopy (RAIRS). Ethylene adsorbs on clean Pd(111) in a di-sigma config uration but converts to pi -bonded species when the surface is presaturated by hydrogen. Ethane is formed with an activation energy of 3.0 +/- 0.3 kca l/mol only when Pd(111) is pre-covered by hydrogen and not when ethylene an d hydrogen are co-dosed, indicating that ethylene blocks hydrogen adsorptio n. Experiments performed by grafting ethyl species onto the surface by reac tion with ethyl iodide indicate that ethyl species hydrogenate much more ra pidly than the overall rate of ethylene hydrogenation, demonstrating that t he addition of the first hydrogen atom to adsorbed ethylene to form an ethy l species is the rate-limiting step in the hydrogenation reaction. The adso rption geometry of ethyl iodide is found to depend on dosing conditions. Wh en adsorbed at low exposures at 80 K, the mirror symmetry plane of ethyl io dide is oriented close to parallel to the surface. At higher exposures, it adopts a geometry in which the symmetry plane is closer to perpendicular to the surface.