Mechanism of C(2)H(4) dehydrogenation to C(2)H(2) on the Ni(111) surface.
Date
1995
Authors
Lam, Mimi Elizabeth.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
The thermal-induced decomposition of ethylene, C$\sb2$H$\sb4$, to acetylene, C$\sb2$H$\sb2$, on the Ni(111) surface has been treated theoretically within both a semiempirical, modified extended-Huckel framework, the atom-superposition and electron-delocalization molecular-orbital (ASED-MO) method, supplemented with selected density-functional theoretical (DFT) calculations, and a non-equilibrium, statistical mechanical, master-equation formalism. As a preliminary step, adsorbate geometries and binding energies were optimized for one and two molecules of each hydrocarbon species adsorbed in various sites on rigid cluster models of the Ni substrate: on the largest, four-layered, eighty-atom cluster, C$\sb2$H$\sb4$ favours the di-$\sigma$ bonding configuration, while C$\sb2$H$\sb2$ prefers the di-$\sigma$/$\pi$, or triangular binding site. A fragmentation pathway, entailing the concerted tunnelling of interior intermolecular H's from a transient, bimolecular complex comprised of two rotated C$\sb2$H$\sb4$'s, is proposed which rationalizes the observed second-order kinetics.
In the dehydrogenation mechanism advanced, each di-$\sigma$-bonded C$\sb2$H$\sb4$ rotates, at elevated temperatures, relative to the Ni substrate, bringing the two interior H's on adjacent molecules closer to each other and a corresponding surface Ni atom; the resultant H-H and metal-H interactions weaken the respective C-H bonds, which lengthen and eventually rupture. The intermolecular H's tunnel, in a process fast compared with the rate of thermal activation, through the potential barrier arising along the shortening of their internuclear distance to form the decoupled dehydrogenation products, C$\sb2$H$\sb4$ fragments and an intermediate, chemisorption precursor, physisorbed H$\sb2$. The C$\sb2$H$\sb2$'s rotate to the fourfold bridging or $\mu$ site, oriented along the (112) direction, from which they are slightly perturbed by the subsequent dissociation of the metastable, molecular H$\sb2$ to atomic H's chemisorbed in the threefold sites of the Ni lattice.
Thesis (Ph.D.)--Dalhousie University (Canada), 1995.
In the dehydrogenation mechanism advanced, each di-$\sigma$-bonded C$\sb2$H$\sb4$ rotates, at elevated temperatures, relative to the Ni substrate, bringing the two interior H's on adjacent molecules closer to each other and a corresponding surface Ni atom; the resultant H-H and metal-H interactions weaken the respective C-H bonds, which lengthen and eventually rupture. The intermolecular H's tunnel, in a process fast compared with the rate of thermal activation, through the potential barrier arising along the shortening of their internuclear distance to form the decoupled dehydrogenation products, C$\sb2$H$\sb4$ fragments and an intermediate, chemisorption precursor, physisorbed H$\sb2$. The C$\sb2$H$\sb2$'s rotate to the fourfold bridging or $\mu$ site, oriented along the (112) direction, from which they are slightly perturbed by the subsequent dissociation of the metastable, molecular H$\sb2$ to atomic H's chemisorbed in the threefold sites of the Ni lattice.
Thesis (Ph.D.)--Dalhousie University (Canada), 1995.
Keywords
Chemistry, Physical.