Kinetic Evidence of Most Abundant Surface Intermediates Variation over Ptn and Ptp: Few-Atom Pt Ensembles Enable Efficient Catalytic Cyclohexane Dehydrogenation for Hydrogen Production-II

C–H bond activation is very important in upgrading of alkanes or aromatics into value-added products, such as alkenes or even oxygenates, etc., during natural gas utilization and hydrogen transportation via chemical strategies. Because of the low polarization ability and high bond energies of the C–H bonds within the hydrocarbons, the selective C–H bond activation suffers from low efficiency for a long time and requests systematic work on understanding the reaction mechanisms, in which the kinetic studies are highly desired and not well understood in a comprehensive manner. Herein, a universal kinetic model has been established to explain the entire reaction process of C–H bond activation for cyclohexane dehydrogenation (CDH) over various structures of Pt (cluster: Ptn and particle: Ptp) decorated nanodiamond@graphene (Ptx/ND@G) nanocomposite with specific emphasis on the elementary steps, the rate-determining step(s) (RDS), and the most abundant surface intermediates (MASIs). With the combination of kinetic and thermodynamic measurements, it was found that cyclohexane dehydrogenation shows different reaction mechanisms over Ptn catalysts, compared to Ptp catalysts. And C6H12 dehydrogenation rates showed nearly first-order (Ptn: rDH ∼ [C6H12]∼0.6; Ptp: rDH ∼ [C6H12]∼1.0) dependence on the C6H12, while the orders of H2 are obviously different over the two catalysts, in which the addition of H2 shows negligible effect (nearly zero-order) once a trace amount of H2 is introduced, and then obviously promotes (nearly second-order) dehydrogenation over Ptn (Ptn: rDH ∼ [H2]∼0–2), while the dehydrogenation rate over Ptp is prominently inhibited by adding H2 to the catalytic system under similar reaction conditions (Ptp: rDH ∼ [H2]∼ –1.0–0), thus indicating that Ptn and Ptp catalysts have diverse MASIs, which is further supported by the developed universal kinetic model of dehydrogenation. That perfectly explains the relationship between the dehydrogenation events and the coverage of surface hydrogen species. The predicted reaction process following a Langmuir–Hinshelwood model that matches the experimental configurations very well, suggesting that the first C–H bond rupture of C6H12 is probably the RDS for both Ptn and Ptp catalysts, while the MASIs varied from C6H10* to H* explained the diversities of the H2 dependencies between Ptn and Ptp catalysts systematically. This kinetic case study as well as the established universal model could be easily extended to some other systems related to C–H bond rupture and attracts the attention on exploring the correlations between nanostructure and the reaction performance.