(677d) Catalytic Hydrogenation of Arenes on Densely-Covered Pt Nanoparticles: Kinetically-Relevant Steps and Unusual Temperature Effects on Turnover Rates

The potential of benzene and alkyl-substituted arenes as vehicles for H₂ storage via hydrogenation and subsequent release in dehydrogenation processes has restored interest in hydrogenation catalysis on metal surfaces. This work addresses unresolved mechanistic questions for these reactions surrounding the negative rate dependence on temperature and increasing kinetic orders in H₂ and arenes at higher temperatures. Turnover rates for methylcyclohexane (MCH) formation from toluene-H₂ reactants were measured on SiO₂-supported Pt nanoparticles. Turnover rates for MCH formation decreased exponentially with temperature above 490 K (Figure 1a); these rates showed a dependence on H₂ pressure that exceeded a second-order trend and a near one-half order dependence on toluene pressure. 1-Methylcyclohexene (1MCHE) and 4-methylcyclohexene (4MCHE) isomers were also observed at low but detectable pressures (<1 Pa). Their approach to equilibrium (η) was >0.5 at temperatures >490 K (Figure 1b), consistent with their formation from toluene-H₂ reactants in a sequence of quasi-equilibrated steps. Plane-wave density-functional theory calculations (performed on Pt(111) slabs) were used to determine the changes in free energies required to form surface species present as intermediates in ring-hydrogenation routes, thus informing coverages of Pt-nanoparticle surfaces under reaction conditions. A mechanism-based kinetic model consistent with computational and experimental observations reveals that rates at higher temperatures become limited by reactions that convert MCHE isomers to MCH. The chemical processes that form transition states for these kinetically relevant steps from toluene-H₂ reactants on crowded Pt surfaces occur exothermically, resulting in a negative apparent activation energy that underpins the negative rate dependences on temperature. These insights encourage bifunctional catalysts that leverage cascade reactions involving equilibrated MCHE isomers and catalyst developments that increase rates of kinetically relevant MCHE hydrogenation steps at higher temperatures.