Identification of Intrinsic Active Sites for the Selective Catalytic Reduction of Nitric Oxide on Metal-Free Carbon Catalysts via Selective Passivation

Abstract

Although carbon catalysts have been commercialized for the low-temperature selective catalytic reduction (SCR) of NO with NH3 for decades, the nature of the active sites remains unclear. Herein we design a proof-of-concept study to directly evidence that the nucleophilic ketonic carbonyl groups are the intrinsic SCR active sites on metal-free carbon catalysts through ex situ and in situ selective passivation strategies. The turnover frequency of the ketonic carbonyl group is provided, and the structure–reactivity relationship is established for the first time. Density functional theory calculations combined with in situ spectroscopy reveal that the standard carbon-catalyzed SCR reaction involves a redox cycle of ketonic carbonyl/phenol (C═O/C–OH) pairs, during which the activation of NH3 is the rate-limiting step. This work identifies the intrinsic SCR active sites and advances the metal-free carbon catalysis. As a further step, the carbon catalysts demonstrate analogous mechanistic features with vanadyl catalysts, opening up the possibility to parallel the SCR mechanisms on metal-free carbon catalysts to those on transition-metal catalysts on the atomic scale.