TRENDS IN PREEXPONENTIAL FACTORS AND ACTIVATION-ENERGIES IN DEHYDROGENATION AND DISSOCIATION OF ADSORBED SPECIES

The kinetics of the elementary reaction involving the dissociation of an adsorbed molecule into two adsorbed fragments have been studied over the past decade in considerable detail for a number of different adsorbed molecules on a variety of single-crystalline transition metal surfaces. We point out here that the observed preexponential factors for such reactions are generally one-to-three orders-of-magnitude lower than the preexponential factors for the desorption of the same molecules. This result is explained within transition state theory as being due to the conversion of highly excited low energy motions (e.g., rotations) in the adsorbed molecule to unexcited vibrational modes in the transition state. We further show that if this dissociation reaction involves C-H bond cleavage in an adsorbed hydrocarbon, then its activation energy follows understandable trends with respect to: (1) the hybridization of the carbon atom involved, (2) the atomic-scale roughness of the surface, and (3) the nature of the transition metal.