(258d) Hydrogen Carriers in Spillover-Mediated Hydrogenation Catalysis
Rate enhancement of hydrogenation reactions on supported metal catalysts has been reported in the literature when γ-alumina instead of silica is used as the catalyst support material . Interpretation of this phenomenon is disputed: some claim that hydrogen atoms spillover from the metal function to the support to provide additional reactive hydrogen atoms , while others believe that different concentrations of non-reactive carbonaceous deposits (impurities) on the catalyst supports contribute to the discrepancies in the hydrogenation rates .
The possibility of hydrogenating organic molecules using spillover hydrogen produced by encapsulated metal accessible only to H2 has been used, here, as a strategy to corroborate the existence of atomic hydrogen spillover. Pt clusters encapsulated within FAU were ineffective for spillover-mediated hydrogenation of TIBP, an organic molecule whose kinetic diameter is larger than FAU aperture. Toluene, small organic molecule which can access metal clusters within FAU cages, however, can act as shuttles to transport hydrogen from confined Pt clusters to large TIBP for its hydrogenation, leading to a rate enhancement commensurate to that measured on unconfined Pt clusters supported on mesoporous silica diluted with γ-alumina. The data suggest that partially hydrogenated toluene intermediates, present in the gas-phase only at very low concentrations, can transfer active hydrogen for spillover routes on insulating supports to hydrogenate aromatics that are unable to directly access Pt clusters encapsulated within zeolites.
To address selective removal of non-reactive residues by various supports, we also carried out toluene hydrogenation and CO oxidation reactions, in alternation, on a given catalyst (Pt/SiO2 or Pt/Al2O3) to ascertain whether enhancements were only limited to reactions involving hydrogen, thereby implicating the “spillover” effect and casting doubt on the role of impurities on these observed trends. The turnover rates for toluene hydrogenation and CO oxidation remained same throughout, for up to two hydrogenation-oxidation cycles on both catalysts, with hydrogenation rates on Pt/Al2O3 showing an enhancement over Pt/SiO2 as expected. We conclude from the unattenuated hydrogenation rates, despite intervening CO oxidation reactions, that measured rate differences of hydrogenation reactions on metal catalysts with different insulating supports are unlikely a result of impurities on the support.
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