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 NH 3 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 NH 3 is the ratelimiting 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.

Potential Risk of NH3 Slip Arisen from Catalytic Inactive Site in Selective Catalytic Reduction of NOx with Metal-Free Carbon Catalysts

“…In this study, we found that the carbon catalysts with similar contents of ketonic carbonyl and phenol groups but different contents of carboxyl groups exhibited similar SCR activities under the same reaction conditions (Figures , , and ), indicating that the carboxyl groups may not contribute to the SCR activity. This result is in accordance with the outcome of our previous study, in which the electrophilic carboxyl groups were clarified to act as spectators in SCR reactions through selective passivation and quasi- in situ XPS . These results clearly indicated that carboxyl groups served as inactive sites in the SCR reaction.…”

“…Currently, SCR technology along with carbon catalysts have been large-scale applied in steel sintering through a reaction–regeneration process which consists of a catalytic reactor and a regenerator (Figure ). It is well established that the surface of a carbon catalyst is dominated by plenty of oxygen-containing groups, and our recent study identified that the ketonic carbonyl group is the SCR active site . Under this sense, special attention should be paid to the SCR catalytic inactive sites.…”

“…The regeneration process, in which the used carbon catalysts are thermally treated under N 2 , is critical for preserving long-term operations and decreasing operation costs. However, our recent study revealed that the intrinsic catalytic active sites in the SCR reaction are not the total oxygen-containing groups present on carbon surfaces but only the nucleophilic ketonic carbonyl groups . It is important to note that the electrophilic oxygen groups dominate the surface of carbon in many cases, − but are not involved in SCR reactions.…”

“…However, our recent study revealed that the intrinsic catalytic active sites in the SCR reaction are not the total oxygen-containing groups present on carbon surfaces but only the nucleophilic ketonic carbonyl groups. 21 It is important to note that the electrophilic oxygen groups dominate the surface of carbon in many cases, 22−24 but are not involved in SCR reactions. However, due to their strong adsorption on NH 3 , the catalytic reactor would be overloaded with ammonia, causing it to leak out in the regenerator as well.…”

Discovering Surface Structure and the Mechanism of Graphene Oxide-Triggered CeO2–WO3/TiO2 Catalysts for NO Abatement with NH3

“…Finally, the NH 4 + detected in the catalytic system reacts with OH* to complete the overall pathway, accompanied with the generation of NH 3 and H 2 O. Therein, the first N–H bond breakage is the rate-determining step. − For the E–R mechanism, the absorbed NH 3 * first dissociates the N–H bond with an energy barrier of 1.23 eV and then reacts with NO 2 to form NH 2 NO 2 *, which undergoes a multistep bond breakage to generate N 2 . Specifically, this process does not require N–N coupling in contrast to the L–H mechanism.…”