Fe(II) regeneration is decisive for highly efficient H2O2-based Fenton-like processes, but the role of cobalt-containing reactive sites in promoting Fe(II) regeneration was overlooked. Herein, a single atom Co-N-C catalyst was employed in Fe(II)/H2O2 system to promote the degradation of diverse organic contaminants. The EPR and quenching experiments indicated Co-N-C significantly enhanced the generation of superoxide species, and accelerated hydroxyl radical generation for pollutant degradation. The electrochemical and surface composition analyses demonstrated the enhanced H2O2 activation and Fe(III)/Fe(II) recycling on the catalyst. Furthermore, in-situ Raman characterization with shell-isolated gold nanoparticles was employed to visualize the interfacial reactive intermediates and their time-resolved interaction. The accumulation of interfacial CoOOH* was confirmed when Co-N-C activated H2O2 alone, but it rapidly transformed into FeOOH* upon Fe(II) addition. Besides, the temporal variation of OOH* intermediates and the relative intensity of Co(III)-O and Co(IV)=O peaks depicted the dynamic interaction of reactive intermediates along the H2O2 consumption. With this basis, we proposed a mechanism of interfacial OOH* mediated Fe(II) regeneration, which overcame the kinetical limitation of Fe(II)/H2O2 system. Therefore, this study provided a primary effort to elucidate the overlooked role of interfacial CoOOH* in the Fenton-like processes, which may inspire the design of more efficient catalysts.

In order to achieve the sustainable production of hydrogen fuel, the heterogeneous photocatalysts with inimitable structure are always desired for the high-efficiency and stable H2 production reaction from water splitting owing to the perfect structure-activity relationship. In this study, a novel metal-free one-dimensional/twodimensional (1D/2D) heterojunction is constructed by a series of dissolution and diffusion, recrystallization and in-situ growth, and self-assembly process of 1-pyrene carboxaldehyde (PCHO) nanoribbons on the surface of polymeric carbon nitride (PCN) nanosheets. The obtained 1D/2D heterojunction can realize fast carrier transport along 1D nanoribbons to prevent the recombination of photogenerated carriers at the interface. Furthermore, the intrinsic Z-scheme reaction mechanism within the heterostructure also effectively inhibits electron-hole recombination and isolates the reduction and oxidation sites of the photocatalytic reaction. As a result, the dramatically enhanced photocatalytic hydrogen evolution (PHE) activity is achieved over the 1%-PCHO/PCN sample, the optimal PHE rate of which is approximately 5.4 times that of pure PCN. This work provides the deep insight into the design and exploitation of metal-free heterojunction with unique structure applied in the photocatalytic energy conversion field.

Fenton reaction is particularly effective for mineralizing stubborn organic pollutants due to the high oxidation potential of •OH with the green end product of H2O. However, exogenous addition of hydrogen peroxide (H2O2) is usually accompanied by the high operation risks and economic costs along with the storage and transportation of H2O2. Besides, inefficient activation of H2O2 due to the rapid accumulation of Fe3+ and slow regeneration of Fe2+ constrains the efficiency. Hence, heterogeneous catalysts that overcome those limitations by in-situ regenerating and activating H2O2 with high efficiency in the Fenton-like processes are promising alternatives to the conventional Fenton reactions. In this review, we give an overall summarization on the single-atom catalysts (SACs) attempted in the Fenton-like processes recently, and discuss their potential roles in the wastewater contaminant alleviation. Firstly, a brief introduction is made on the basic principles of H2O2 generation and activation in the Fenton-like processes, and the coordination environment of SACs is emphasized. Then, a detailed discussion on how the type of central metal sites and their coordination environment influence the catalytic performance from three perspectives: H2O2 activation, selective H2O2 generation via 2e− oxygen reduction reaction, and bifunctional SACs for electro-Fenton. After that, the recent advances on SACs catalyzed Fenton-like processes in the contaminant removal are introduced, and the kinetic perspective, influence of water quality, the stability and long-term performance, as well as the mass transfer and reactor scale up are discussed, respectively. Finally, the major challenges for the applications in real wastewater treatment are outlooked.