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

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.