Thermochemistry of the selective dehydrogenation of cyclohexane to benzene on Pt surfaces
نویسندگان
چکیده
Ž . w Ž . x 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 cyclohexŽ . ane to benzene over Pt surfaces. We estimate heats of adsorption and formation for adsorbed cyclohexyl c-C H , a 6 11 Ž . Ž . Ž . Ž . cycloallylic intermediate c-C H , cyclohexadiene c-C H , cyclohexadienyl c-C H , phenyl c-C H , and benzyne 6 9 6 8 6 7 6 5 Ž . c-C H on Pt surfaces. Estimates of these needed formation energies are then combined with the experimentally measured 6 4 Ž . Ž . Ž . adsorption energies of cyclohexane c-C H , cyclohexene c-C H , and benzene C H , to provide heats of reaction 6 12 6 10 6 6 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 Ž . observations, a cycloallylic species c-C H is shown to be an important stable intermediate in cyclohexane dehydrogena6 9 tion 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 Ž . the reaction on Pt 111 can now give a fairly complete description of the energetics of this prototypical hydrocarbon Ž . conversion reaction on Pt 111 surfaces. q 1998 Elsevier Science B.V.
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