Theoretical and kinetic assessment of the mechanism of ethane hydrogenolysis on metal surfaces saturated with chemisorbed hydrogen

Ethane hydrogenolysis involves C–C bond rupture in unsaturated species in quasi-equilibrium with gaseous reactants and H 2 on metal clusters, because C–C bonds weaken as C-atoms replace hydrogen with exposed metal atoms from catalyst surfaces. The nature and reactivity of such adsorbed species are probed here using kinetic data and density functional theory (DFT) for the case of Ir surfaces, but with conclusions that appear to be general to hydrogenolysis on noble metals. On surfaces saturated with chemisorbed H-atoms (H Ã), theory and experiments indicate that C–C cleavage occurs predominantly via an a,b-bound à CHCH à species that forms via sequential dehydrogenation of adsorbed ethane; all other intermediates cleave C–C bonds at much lower rates (>10 7-fold). Measured activation energies (213 kJ mol À1) and free energies (130 kJ mol À1) reflect the combined values for quasi-equilibrated steps that desorb H à , adsorb C 2 H 6 , form C 2-intermediates by dehydrogenation, and form the transition state from à CHCH à species. DFT-derived activation energies (218 kJ mol À1) and free energies estimated from these values and statistical mechanics treatments of reaction and activation entropies (137 kJ mol À1) are in excellent agreement with measured values. The removal of four H-atoms in forming the kinetically-relevant à CHCH à intermediates, taken together with measured effects of H 2 pressure on hydrogenolysis rates, show that 2–3 H à must be removed to bind this intermediate and the transition state, as expected from the structure of the proposed adsorbed species and H à adsorption stoichiometries on Ir surface atoms that vary slightly with surface coordination on the non-uniform surfaces of metal clusters. Theory and experiments combine here to provide mechanistic insights inaccessible to direct observation and provide compelling evidence for reaction pathways long considered to be plausible for hydrogenolysis on noble metals. The extent of unsaturation in the single relevant intermediate and its C–C cleavage rates will depend on the identity of the metal, but the elementary steps and their kinetic relevance appear to be a general feature of metal-catalyzed hydrogenolysis. Hydrogenolysis of C–C bonds on metals is the reaction of choice for decreasing the chain length of acyclic molecules and for opening rings in cyclic hydrocarbons [1–12]. It is also responsible, however, for yield losses during hydrocarbon reforming and isomerization [13,14], making mechanistic models of hydrogenolysis reactivity and selectivity useful in practice [15]. C–C bond rupture has been widely used …