Elementary steps and bifunctional scavenging pathways in methylcyclohexane dehydrogenation reactions on dispersed Pt nanoparticles

Abstract

Dehydrogenation-hydrogenation cycles of methylcyclohexane (MCH) and toluene (TOL) provide a feasible strategy for hydrogen release and storage. This study describes the kinetic relevance of MCH dehydrogenation on Pt and addresses the support effect on turnover rates, which involves kinetically-competent Lewis acid-base (LAB) pairs on the supports (e.g., Al₂O₃, TiO₂, and ZrO₂) that affect the bound species on Pt surfaces with varying structures and reactivities. The less competent species in producing TOL on Pt surfaces are present at significant coverages and can desorb and are scavenged by LAB sites on the support, thus increasing the number and size of Pt surface atom ensembles for the more competent ones to form TOL. The support effect on MCH dehydrogenation turnover rates diminishes upon propionic acid titration, which occupies LAB sites irreversibly. H₂-D₂ exchange and CO titration experiments under simultaneous MCH dehydrogenation confirm the significant coverage of the less competent species derived from MCH on Pt surfaces. For all investigated Pt catalysts, MCH dehydrogenation turnover rates depend linearly on pressures of methylcyclohexene (MCHE) as detectable intermediates that are in equilibrium with MCH and H₂, indicating that the kinetically-relevant step occurs after MCHE formation and involves C–H activation in bound MCHE-derived species on Pt surfaces that are sparsely covered by reactive intermediates. The implications and practical significance of this work are not restricted to the subject reaction, and can be extended to other bifunctional catalytic systems.