Oxygen-doped graphitic carbon nitride (O−CN) was fabricated via a facile thermal polymerization method using urea and oxalic acid dihydrate as the graphitic carbon nitride precursor and oxygen source, respectively. Experimental and theoretical results revealed that oxygen doping preferentially occurred on the two-coordinated nitrogen positions, which create the formation of low and high electron density areas resulting in the electronic structure modulation of O−CN. As a result, the resultant O−CN exhibits enhanced catalytic activity and excellent long-term stability for peroxymonosulfate (PMS) activation toward the degradation of organic pollutants. The O−CN with modulated electronic structure enables PMS oxidation over the electron-deficient C atoms for the generation of singlet oxygen ( 1 O 2 ) and PMS reduction around the electron-rich O dopants for the formation of hydroxyl radical ( • OH) and sulfate radical (SO 4•− ), in which 1 O 2 is the major reactive oxygen species, contributing to the selective reactivity of the O−CN/PMS system. Our findings not only propose a novel PMS activation mechanism in terms of simultaneous PMS oxidation and reduction for the production of nonradical and radical species but also provide a valuable insight for the development of efficient metal-free catalysts through nonmetal doping toward the persulfatebased environmental cleanup.

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A nonradical oxidation process via metal-free peroxymonosulfate (PMS) activation has recently attracted considerable attention for organic pollutant degradation; however, the origin of singlet oxygen ( 1 O 2 ) generation still remains controversial. In this study, nitrogen-doped carbon nanosheets (NCN-900) derived from graphitic carbon nitride were developed for activation of PMS and elucidation of 1 O 2 production. With a large specific surface area (1218.7 m 2 g −1 ) and high nitrogen content (14.5 at %), NCN-900 exhibits superior catalytic activity in PMS activation, as evidenced by complete degradation of bisphenol A within 2 min using 0.1 g L −1 NCN-900 and 2 mM PMS. Moreover, the reaction rate constant fitted by pseudofirst-order kinetics for NCN-900 reaches an impressive value of 3.1 min −1 . Electron paramagnetic resonance measurements and quenching tests verified 1 O 2 as the primary reactive oxygen species in the NCN-900/PMS system. Based on X-ray photoelectron spectroscopy analysis and theoretical calculations, an unexpected generation pathway of 1 O 2 involving PMS oxidation over the electron-deficient carbon atoms neighboring graphitic N in NCN-900 was unraveled. Besides, the NCN-900/PMS system is also applicable for remediation of actual industrial wastewater. This work highlights the important role of electron-deficient carbon atoms in 1 O 2 generation from PMS oxidation and furnishes theoretical support for further relevant studies.

Single-atom catalysts (SACs) have emerged as efficient materials in the elimination of aqueous organic contaminants; however, the origin of high activity of SACs still remains elusive. Herein, we identify an 8.1-fold catalytic specific activity (reaction rate constant normalized to catalyst’s specific surface area and dosage) enhancement that can be fulfilled with a single-atom iron catalyst (SA-Fe-NC) prepared via a cascade anchoring method compared to the iron nanoparticle-loaded catalyst, resulting in one of the most active currently known catalysts in peroxymonosulfate (PMS) conversion for organic pollutant oxidation. Experimental data and theoretical results unraveled that the high-activity origin of the SA-Fe-NC stems from the Fe–pyridinic N4 moiety, which dramatically increases active sites by not only creating the electron-rich Fe single atom as the catalytic site but also producing electron-poor carbon atoms neighboring pyridinic N as binding sites for PMS activation including synchronous PMS reduction and oxidation together with dissolved oxygen reduction. Moreover, the SA-Fe-NC exhibits excellent stability and applicability to realistic industrial wastewater remediation. This work offers a novel yet reasonable interpretation for why a small amount of iron in the SA-Fe-NC can deliver extremely superior specific activity in PMS activation and develops a promising catalytic oxidation system toward actual environmental cleanup.