Kinetic Study of the Initial Stages of Dehydrogenation of Cyclohexane on the Pt(111) Surface

Article abstract of DOI:10.1021/j100183a068

The initial dehydrogenation in the conversion of cyclohexane to benzene has been studied by laser-induced thermal desorption (LITD) combined with Fourier transform mass spectrometry (FTMS).Our previous work has shown that the initial dehydrogenation of cyclohexane occurs at ca. 180 K while benzene is not formed at a substantial rate until ca. 280 K.A stable surface intermediate exists in the temperature range 180-280 K.We have studied the kinetics of the dehydrogenation of cyclohexane to form this intermediate in the temperature range 175-215 K.LITD combined withFTMS was used to measure the surface concentration of cyclohexane as a function of time.From the kinetic data we find that the initial dehydrogenation is first order in cyclohexane coverage for all cyclohexane starting coverages studied.The activation energy at low coverage (0.05 ML) is 9.5 +/- 0.5 kcal/mol and increases to 13.5 +/- 0.9 kcal/mol at ca. 0.30 ML.Over this same coverage range the rate constant preexponential factors increase from 2.2 * 1E9 s^-1 at 0.05 ML coverage to 7.4 * 1E12 s^-1 at 0.30 ML coverage.We discuss the coverage-dependent rate in terms of a simple site-blocking model and pseudo-first-order kinetics, recognizing that the total hydrocarbon coverage on the surface does not change as the reaction proceeds.At higher coverages (0.50 ML), the desorption of cyclohexane contributes significantly to the observed rate.At these high coverages, the rate constant measured by LITD is the sum of the desorption rate and the reaction rate constant.The kinetics analysis at this coverage is complicated by the changing surface coverage due to desorption of cyclohexane.Our data also show that the reaction rate is notsubstantially inhibited by the surface hydrogen which is a product of the reaction.