Mechanism of C2H4 dehydrogenation to C2H2 on the Ni(111) surface

The thermally induced decomposition of ethylene, C2H4, to acetylene, C2H2, on the Ni(lll) surface has been treated theoretically: potential-energy curves were computed within a modified extended-Huckel framework and reaction rates estimated via a master equation approach. As a preliminary step, adsorbate geometries and binding energies were optimized for one and two molecules of each hydrocarbon species chemisorbed in various sites on rigid bulk cluster models of the Ni substrate. A fragmentation pathway is proposed, eliciting the concerted tunnelling of intermolecular H's from a transient bimolecular complex comprised of two rotated C2H4's, which rationalizes the observed second-order decomposition kinetics.