We use a quasiempirical valence bond (QVB) scheme [E.A. Carter, Chem. Phys. Lett. 169 (1990) 218] for calculating the heats of formation of adsorbed species on surfaces to provide reliable estimates of the relative stabilities of several of the surface intermediates and adsorbate-surface bond strengths that are important in the selective dehydrogenation of cyclohexane to benzene over Pt surfaces. We estimate heats of adsorption and formation for adsorbed cyclohexyl (c-C6H11), a cycloallylic intermediate (c-C6H9), cyclohexadiene (c-C6H8), cyclohexadienyl (c-C6H7), phenyl (c-C6H5), and benzyne (c-C6H4) on Pt surfaces. Estimates of these needed formation energies are then combined with the experimentally measured adsorption energies of cyclohexane (c-C6H12), cyclohexene (c-C6H10), and benzene (C6H6), to provide heats of reaction and an equilibrium thermodynamic description of a selective dehydrogenation mechanism that involves the step-wise, sequential removal of one H atom at a time from cyclohexane to form benzene. In addition, several further decomposition products of benzene are considered as precursors to undesirable carbon-forming reactions. In agreement with experimental observations, a cycloallylic species (c-C6H9) is shown to be an important stable intermediate in cyclohexane dehydrogenation that could also be involved in the catalytic rate-limiting step. Carbon-carbon bond cleavage and other possible surface reaction pathways are not considered herein. Addition of measured or estimated values for the activation barriers involved in the reaction on Pt(111) can now give a fairly complete description of the energetics of this prototypical hydrocarbon conversion reaction on Pt(111) surfaces.
Bibliographical noteFunding Information:
BEK acknowledges the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division for support of this research. EAC is grateful for support for this work from a Union Carbide Innovation Recognition Award, a Camille and Henry Dreyfus Foundation Teacher Scholar Award, and a Alfred P. Sloan Foundation Research Award. It is a pleasure to acknowledge valuable discussions with Prof. G.B. Ellison.