AN INVESTIGATION OF ETHYLENE HYDROGENATION CATALYZED BY METALLIC MOLYBDENUM USING AN ISOLATABLE HIGH-PRESSURE REACTOR - IDENTIFICATION OF THE REACTION SITE AND THE ROLE OF CARBONACEOUS DEPOSITS

Ethylene hydrogenation catalyzed by a model Mo(100) single crystal catalyst is investigated using an isolatable high-pressure reactor. The observed reaction kinetics are in agreement with those measured for supported transition metal hydrogenation catalysts showing a zero-order dependence in ethylene and a first-order dependence in hydrogen partial pressures. The reaction activation energy is 8.1 ± 0.5 kcal/mol for a Mo(100) single-crystal model catalyst and for a foil and mass spectral analysis of the products formed following substitution of deuterium for hydrogen shows that a maximum of two deuterium atoms are incorporated into the ethane that is produced. Auger analysis of the surface following reaction reveals the presence of large amounts of carbon (up to ∼7 monolayers). This carbonaceous deposit decomposes on heating the surface to above 1000 K to desorb C1 species. The experimental data suggest that either hydrogen or ethylene can permeate this layer so that hydrogen transfer takes place either at the metal surface or on top of the layer. Blocking the fourfold sites on Mo(100) by oxygen or carbon has only a marginal effect on the overall reaction kinetics, and does not substantially alter the amount of carbon deposited onto the surface during reaction. This result implies that the enhanced catalytic activity observed on metal carbide catalysts is not only due to site blocking, but also arises because of chemical modification of the catalyst by compound formation. © 1991.