Dissociative adsorption of alkanes on clean and sulfur-modified nickel surfaces

Recent studies of the dissociative adsorption of methane on clean Ni(111), Ni(100), Ni(110), and sulfur-modified Ni(100), as well as ethane, propane, and n-butane on Ni(100) have been carried out under the high incident flux conditions of 1.00 Torr methane, 0.10 Torr ethane, 0.01 Torr propane, and 0.001 Torr n-butane, respectively. It has been found that the activation energies for these processes range from 3.1±1.0 to 13.3±1.5 kcal mol^–1. A comparison with the results of corresponding molecular beam studies suggests that the effects of vibrational energy on sticking probabilities must be accounted for and the sticking probabilities of molecules with very low normal kinetic energies must be accurately known when attempting to model high pressure processes using molecular beam techniques. While dissociation of ethane, propane, and n-butane on Ni(100) is believed to proceed primarily via a trapped molecular precursor, the results on sulfur-modified Ni(100) surface indicate that the direct channel to methane dissociation likely dominates and the contribution from the trapped molecular precursor mechanism is likely relatively small, with the sulfur atoms poisoning this reaction by a simple site blocking mechanism.