Catalysis by Confinement: Enthalpic Stabilization of NO Oxidation Transition States by Micropororous and Mesoporous Siliceous Materials

Mesoporous silica and purely siliceous zeolites with voids of molecular dimensions (MFI, CHA, BEA) catalyze NO oxidation by O-2 at near ambient temperatures (263-473 K) with reaction orders in NO and O-2 identical to those for homogeneous routes and with negative apparent activation energies. These findings reflect the stabilization of termolecular transition states by physisorption on surfaces or by confinement within voids in processes mediated by van der Waals forces and without the involvement of specific binding sites. Such interactions lead to the enthalpic stabilization of transition states relative to the gaseous reactants; such enthalpic benefits compensate concomitant entropy losses upon confinement because of the preeminent role of enthalpy in Gibbs free energies at low temperatures. These data and their mechanistic interpretation provide clear evidence for the mediation of molecular transformations by confinement without specific chemical binding at active sites.