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[Display omitted] • Dramatic activity enhancement was observed over Fe-doped g-C 3 N 4. • Two-photoelectron reduction pathway was verified in Fe/g-C 3 N 4 Vis-O 3 system. • The different roles of single Fe atoms and nanoparticles was explored. • Fe(II)N x sites promoted the adsorption and reduction of O 2 rather than O 3. The identification of active sites and the establishment of structure-reactivity relationship at atomic scale is important for designing efficient catalysts and understanding the reaction mechanism. Here, single atom catalysts (SACs) of Fe anchored on g-C 3 N 4 were synthesized. The catalysts have larger surface area, more photo-generated electrons with stronger reduction ability and faster charge carrier migration than g-C 3 N 4 , thereby exhibited dramatically enhanced activity and stability in electron reduction of ozone (O 3) under visible light. However, lower catalytic performance was observed when Fe nanoparticles coexisted, which regulated more photoelectrons to reduce O 2 to OH via two or three-electron reduction pathways rather than one-electron reduction of O 3. Mössbauer spectrum and X-ray adsorption spectroscopy indicated that the presence of Fe nanoparticles led to higher proportion of Fe(II)N x , which played more significant role in the adsorption and reduction of O 2. The insight on the contribution of different Fe sites with well-defined local coordination structure is beneficial for rational design of efficient catalysts in photocatalytic ozonation. [ABSTRACT FROM AUTHOR]

Porous g-C 3 N 4 (PGCN) has attracted enormous attention due to its accessible nanoporous framework benefiting photocatalytic reactions. Here, we reported a one-pot template-free method to fabricate honeycomb-like PGCN by simply mixing ammonia chloride with the precursor of g-C 3 N 4 before calcination. The resulting PGCN exhibited obviously improved photocatalytic activity for p -hydroxybenzoic acid (PHBA) degradation under visible light due to its high surface area and enlarged band gap, but PHBA can hardly be mineralized in this process. Hence, for the first time, Vis/PGCN was coupled with ozone in this paper and the results showed that PGCN could trigger a vigorous synergy between photocatalysis and ozonation. Vis/O 3 /PGCN led to almost complete mineralization of PHBA with an ozone dosage of 1.5 mg/min, and the process could be further accelerated by increasing the ozone dosage. Such a remarkable mineralization enhancement was mainly attributed to the systematically promoted generation of non-selective hydroxyl radicals ( OH). The high CB level of PGCN benefited electron capture by ozone molecules, thus significantly enhanced charge separation and the decay of ozone into abundant OH. OH could vigorously react with PHBA and its ozone-recalcitrant intermediates such as the identified carboxylic acids, finally leading to thorough mineralization. Electrospray ionization–mass spectrometry was adopted to detect the evolution of degradation intermediates in ozonation and Vis/O 3 /PGCN, and the mineralization procedure from the original PHBA to CO 2 and H 2 O was comprehensively proposed. This study contributes to the integration of sunlight/PGCN with ozone as an efficient metal-free advanced oxidation process for water treatment. [ABSTRACT FROM AUTHOR]