Elementary Steps,\nSite Requirements, and Support Effects\nin Methylcyclohexane Dehydrogenation Reactions on Dispersed Pd Nanoparticles

Hydrogenation-dehydrogenation cycles enable the efficient\nstorage,\ntransport, and release of hydrogen via chemical means. Practical kinetic,\nthermodynamic, and H-density considerations make cyclic hydrocarbons\nthe preferred organic hydrogen carriers. This study addresses the\nmechanism of methylcyclohexane (MCH) dehydrogenation to toluene (TOL),\nthrough methylcyclohexene (MCHE) intermediates on Pd nanoparticles\n(2–11 nm diameter) dispersed on Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, MgO, and CeO<sub>2</sub>. Turnovers occur on Pd\nsurfaces densely covered with MCH-derived intermediates differing\nin isomeric structure and reactivity via sequential C–H activation\nelementary events, irrespective of nanoparticle size or support. The\nkinetically relevant step shifts from the second to the first H-abstraction\nstep in MCH as temperature increases (from 453 to 553 K). The reactivity\nof Pd nanoparticle surfaces is insensitive to their size but supports\nwith more competent Lewis acid–base (LAB) pairs lead to higher\nrates and stronger rate enhancements (relative to SiO<sub>2</sub>)\nwith decreasing temperatures, which reflect the lower coverages of\nless reactive intermediates when supports can scavenge desorbable\nspecies. These dense adlayers retain interstices within which dehydrogenation\nturnovers occur, but no longer expose the most distinctive low-coordination\natoms prevalent on small nanoparticles, leading to the observed structure\ninsensitivity of turnover rates. The prevalence of such adlayers leads\nto surfaces without the saturation hydrogen coverages expected for\nPd surfaces devoid of such organic species. These mechanistic insights\nare consistent with (i) the elimination of support effect by titration\nof LAB pairs; (ii) initial rate transients that are inhibited by competent\nsupports; (iii) the relative reactivity of metal-free supports for\ndehydrogenation of MCHE and methylcyclohexadienes (but not MCH); and\n(iv) measured kinetic effects of MCH, MCHE, and H<sub>2</sub> on turnover\nrates. The support effects provide strategies for maximizing the exposure\nof bare atom ensembles during dehydrogenation reactions. Its conceptual\nimpact and practical significance are not restricted to the subject\nreaction in this study.