Kinetic Regimes in Ethylene Hydrogenation over Transition-Metal Surfaces

A first-principles microkinetic model has been developed and applied to ethylene hydrogenation over close-packed transition-metal surfaces of Ru, Rh, Pd, Os, Ir, and Pt. The model is based on density functional theory calculations, which have been used to determine the activation energies of the elementary steps of the reaction according to the Horuiti-Polanyi mechanism. A sensitivity analysis of the activity with respect to the kinetic parameters reveals distinctly different kinetic regimes across the periodic table. For Ru and Ir, the activity is controlled by the activation energy for ethylene to ethyl hydrogenation, whereas the other metals also have a sensitivity to the second hydrogenation step. The analysis shows, furthermore, that the activity could be enhanced considerably with minor reductions of the hydrogenation barriers.