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Transition-metal and nitrogen co-doped carbon-based catalysts receive much attention in peroxymonosulfate (PMS) activation. However, the contribution of doped metal and the kinetic mechanisms of the emerging nonradical pathways especially enabled by reactive singlet oxygen species remain unclear. Herein, we report Fe-N-co-doped carbon-based catalysts (FeCNx) for peroxymonosulfate activation. The catalysts achieve fast removal of a broad spectrum of organic pollutants in a wide pH range without the common Fe leaching. Both catalyst surface-activated PMS complex (catalyst-PMS*), surface-bound radicals, and singlet oxygen are identified. The new mechanism is enabled by the Fe-Nx single-atomic-site that enhances the adsorption of PMS and facilitates the formation of catalyst-PMS*. The formation route of singlet oxygen species is elucidated for the first time for Fe, N-co-doped carbon-based catalysts with density functional theory simulation. This study reveals the intrinsic role of Fe-Nx site in catalysts and provides new insights into the origin and kinetics of non-radical pathways for peroxymonosulfate activation.

Single-atom catalysts have drawn increasing attention in advanced oxidation due to their unique structure and significant promise in heterogeneous catalysis. Herein, single-atom iron anchored on nitrogen-doped carbon (SAFe-N-C) was synthesized with iron phthalocyanine (FePc) and metal-organic framework (ZIF-8). With thermal treatment and acid leaching, atomically-dispersed Fe-Nx sites were successfully formed on the surface of N-C support derived from ZIF-8. The prepared catalyst was demonstrated to activate peroxydisulfate (PDS) for chloramphenicol (CAP) degradation. Compared with N-C, the SAFe-N-C shows 6 times enhanced removal efficiency (from 15.3% to 93.1%) for CAP. Moreover, the catalyst shows high catalytic activity in a wide pH range of 5 to 9 and good resistance to inorganic anions, in which a singlet oxygen-dominated process is found. This study reveals the role of single-atom site in singlet oxygen evolution and offers a new catalytic approach for selective removal of organic pollutants in complex water matrix.

The contribution of Cu(III) produced during heterogeneous peroxydisulfate (PDS) activation to pollutant removal is largely unknown. Herein, a composite catalyst is prepared with Cu-based metal organic framework (Cu-MOF) derived Cu nanoparticles decorated in a three-dimensional reduced graphene oxide (3D RGO) network. The 3D RGO network overcomes the aggregation of nanosized zero-valent copper and reduces the copper consumption during the PDS activation reaction. The Cu/RGO catalyst exhibits high catalytic activity for 2,4-dichlorophenol (2,4-DCP) degradation in a wide pH range of 3-9, with a low Cu dosage that is only 0.075 times that of previous reports with zero-valent copper. Moreover, a high mineralization ratio (69.2 %) of 2,4-DCP is achieved within 30 min, and the Cu/RGO catalyst shows high reactivity toward aromatic compounds with hydroxyl and chlorinated groups. Unlike normal sulfate radical-based advanced oxidation, alcohols show negligible impacts on the reaction, suggesting that Cu(III), rather than SO4- and OH, dominates the degradation process. We believe that PDS activation by 3D Cu/RGO, with Cu(III) as the main active species, provides new insights in selective organic pollutant removal in wastewater treatment.

Inability to absorb visible light and fast recombination of photogenerated electrons and holes in TiO2 catalysts prevent their wide applications in photocatalysis. Herein, we report a novel strategy to in-situ synthesize Ti3+-doped TiO2 coupling with homo-dispersed NiO and β-cyclodextrin (CD-Ni/TiO2). By using nickel stearate as precursor, ethyl acetoacetate and CD as chelating agents, porous CD-Ni/TiO2 nanospheres constructed by CD, ultrafine NiO (~2 nm), and Ti3+-doped TiO2 (~5 nm) are produced. The porous nanosphere structure enhances the surface utilization and electron-hole separation through intimate contact of NiO and TiO2. The doped Ti3+ extends the photo-response of TiO2 to visible region; while the CD structure with hydrophobic cavity endows CD-Ni/TiO2 with outstanding capability to entrap hydrophobic molecules. The CD-Ni/TiO2 with decreased band gap (2.70 eV) presents high activity and stability in visible-light photodegradation of hydrophobic pollutants. This study presents a simple strategy in TiO2 doping and activity enhancement through designing inorganic–organic composite photocatalyst for hydrophobic pollutant degradation under visible light.

An efficient and inexpensive catalyst for oxygen reduction reaction (ORR), polyaniline (PANI) and β-MnO