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.

A superior desulfurization catalyst (AC-Cop) was synthesized by blending copper ore and coals through a one-step carbonization–activation process. The sulfur capacities of the modified catalysts were enhanced significantly and that of AC-Cop with 1 wt % of copper ore was 174.8 mg/g, which was 50% higher than that of blank activated coke. Their excellent desulfurization performance could be attributed to the introduction of much more active sites on the surface of AC-Cop, i.e., CuO, Fe2O3, Fe3O4, and CO groups, which promoted the adsorption and oxidation of SO2. In addition, the used AC-Cop could be easily regenerated by thermal treatment, with a slow decrease in desulfurization performance during four desulfurization–regeneration cycles. This was ascribed to the gradual accumulation of the desulfurization products, i.e., Fe2(SO4)3 and CuSO4 and the decreased contents of CO groups. The results suggested that such copper ore-modified activated coke could be a kind of efficient, regenerable, and inexpensive desulfurization catalyst.

Carbon-based catalysts have been extensively used for flue gas desulfurization (FGD) and have exerted great importance in controlling SO2 emissions over the past decades. However, many fundamental details about the nature of the active sites and desulfurization mechanism still remain unclear. Here, we reported the experimental and theoretical identifications of active sites in FGD on carbon catalysts. Temperature-programmed decomposition allowed us to modulate the number of oxygen functional groups on carbon catalysts and to establish its correlation with desulfurization activity. Selective passivation further demonstrated that the ketonic carbonyl (CO) groups are the intrinsic active sites for FGD reaction. Combined with transient response experiments, quasi-in situ X-ray photoelectron spectroscopy, and density functional theory simulations, it was revealed that desulfurization reaction on carbon catalysts mainly proceeded via the Langmuir–Hinshelwood mechanism, during which the nucleophilic ketonic CO groups served as active sites for chemically absorbing SO2 and their adjacent sp2-hybridized carbon atoms dissociatively activated O2. It also turned out that the formation of H2SO4 is the reaction barrier step. The output of this study should not only advance the understanding of desulfurization at the atomic scale but also provide a general guideline for the rational design of efficient carbon catalysts for FGD.