Enthalpic Stabilization of NO Oxidation Transition States by Micropororous Silicates

Zeolite catalysts play essential roles in the chemical and petrochemical industries. They exhibit high surface areas, interconnected channels of molecular dimensions, and the ability to select reactants and transition states based on their size. In particular, acid and metal-exchanged zeolites catalyze NO oxidation even at ambient tem-peratures, in sharp contrast with the higher temperatures required on supported metal catalysts. Here, we examine the kinetic effects of reactant concentrations and the activation barriers for NO oxidation on amorphous mesopo-rous silica and on microporous pure silica forms of zeolites MFI, beta (BEA), and chabazite (CHA). These data show the remarkable reactivity of microporous structures of pure silicate frameworks, even in the absence of speci-fic sites for chemical binding of reactants or transition states. These data, taken together with the measured nega-tive activation barriers, lead us to conclude that reactivity is induced by mere physical confinement through van der Waals forces, which provide significant enthalpic stabilization for the relevant trimolecular transition states.