Toluene Hydrogenation Catalyzed by Pt Nanoparticles: Kinetically Relevant Steps, Binding Ensembles, and Temperature Effects on Turnover Rates.

Metal surfaces mediate the hydrogenation of aromatic rings in arenes, but the diversity of hydrocarbon intermediates formed has hindered conclusive identification of the predominant surface species and kinetically relevant steps. Here, kinetic data and density functional theory calculations are combined to elucidate the mechanism of toluene hydrogenation on Pt nanoparticles at temperatures where turnover rates decrease with increasing temperature (> 490 K), a behavior typical of arene hydrogenations. Concurrent measurements of site-time-yields to methylcyclohexane (MCH), the final product, and the pressures of methylcyclohexene isomers (MCHE), intermediates, reveal that partial toluene hydrogenation to MCHE isomers reaches equilibrium, with the subsequent hydrogenation of MCHE limiting MCH formation rates. MCHE concentrations decrease with increasing temperature due to their exothermic formation, resulting in a concomitant decrease in hydrogenation rates. The transitions states that mediate MCHE hydrogenation are found to form exothermically from gaseous MCHE and H2 reactants (-20 ± 20 kJ mol-1), leading to lower hydrogenation rates as temperatures increase. Kinetic barriers exist, despite the negative enthalpic barrier, due to the entropic penalties for binding and reacting MCHE isomers (-210 ± 40 J mol-1 K-1). These hydrogenation processes occur on surfaces saturated by H-adatoms and by bound methylenebenzene, which forms by abstracting benzylic H from toluene. These species bind to different Pt atom ensembles (1 and 6 Pt atoms for H and methylenebenzene, respectively), which differ from those which mediate hydrogenation (5.3 ± 0.4 Pt atoms). Multi-site kinetic models, developed using lattice statistics to rigorously account for these site requirements, are essential for interpreting reactant pressure and temperature effects. These findings underscore the need for multi-site descriptions of metal-catalyzed reactions, particularly when surface species and transition states differ in sizes, orientations, and numbers of surface contacts.