Single–atom catalysts based on Fenton-like/peroxymonosulfate system for water purification: design and synthesis principle, performance regulation and catalytic mechanism

Liping Hao ^a, Chao Guo ^ab, Zhenyu Hu ^ab, Rui Guo *^ab, Xuanwen Liu *^ab, Chunming Liu ^a and Ye Tian ^c
aSchool of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
^bSchool of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
^cThe First Hospital of Qinhuangdao 066099, China

First published on 10th August 2022

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1. Introduction

The seminal study of creat strong oxidants using Fe(II) and H₂O₂ to degrade tartaric acid by Fenton H.J.H in 1894 opened the door of novel oxidation processes.¹ Subsequently, the Fenton reaction has become one of the few organic chemical reactions named after a person. However, it was not until 1964 that Eisenhaner, a Canadian scholar, first applied the Fenton reagent in water treatment. The homogeneous catalyst involved in the reaction exhibited the advantages of high efficiency, low cost and wide applicability.² However, the insignificant separation and recovery of iron species, the narrow optimal pH range (pH ∼ 3) and the production of ferric hydroxide sludge, which is regarded as secondary pollution, seriously limited its practical application value.^3–5 Accordingly, Fenton-like technology using heterogeneous catalysts such as metal oxides and metal sulfides is a successful alternative strategy to overcome the main defects of the traditional Fenton reaction.⁶ However, due to the limited utilization of atoms that can contact and participate in the reaction on the surface of these catalysts, their catalytic efficiency is not satisfactory.^7–9 Therefore, reducing the particle size of the catalyst plays an indispensable role in improving the ability of its surface atomic sites to participate in the reaction and coordinating the reaction activity and homogeneous/heterogeneous reaction kinetics.

In the last decade, isolated monatomic metal sites formed by downsizing nanoparticles to the atomic level and anchoring them on support materials have been defined as single-atom catalysts (SACs) (Fig. 1).¹⁰ The term SACs was first proposed in 2011, regarding single Pt atom anchored on FeO_x as the active site for CO oxidation, its catalytic effect was 3-times higher than that of the corresponding Pt bulk.¹¹ Since then, the oxidation or reduction process with single atoms as the reaction center has become a research hotspot of catalysis. Importantly, SACs have linked heterogeneous and homogeneous catalysts, successfully bridging the gap between them.¹² Moreover, owing to the mononuclear property of their reaction sites, SACs show the highest atomic utilization efficiency and the strong covalent bonds between the isolated metal atoms and anchored substrate endow SACs with prominent structural robustness and chemical stability compared to their nanoparticle counterparts. To date, multiple types of SACs based on noble/non-noble metals (e.g., Pt, Ag, Au, Fe, Cu, Co, Mn and Cr) dispersed and fixed on various metal-free organic/semiconductor substrates (carbon supports, g-C₃N₄, TiO₂, FeO_x, Ti₂AlN, Ti₃AlC₂, VO₂, MoS₂, etc.) have been reported.^13–15 These SACs with a unique coordination environment and electronic characteristics have been applied in various catalytic reactions, such as the hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR), CO oxidation, hydrogenation reactions, and CO₂ reduction reaction (CRR).^16–20 However, the bottleneck limiting the applicability and adaptability of SACs in practical is related to the accurate regulation of their atomic configuration and the large-scale batch synthesis of SACs. This is mainly because metal atoms with high surface free energy tend to spontaneously migrate and gather into clusters or even particles.²¹ Thus, the isolation and anchoring of single metal atoms in precursors is of vital importance in the design and synthesis of SACs.

Advanced oxidation processes (AOPs) using SACs, especially Fenton-like catalytic systems are still in their infancy.²² Also, to date, most of the designed and synthesized Fenton-like SACs are still limited to carbon-based SACs and g-C₃N₄-based SACs, with relatively few studies on semiconductor-based SACs. Fenton-like catalysts can activate the oxide species (e.g., hydrogen peroxide (H₂O₂), ozone (O₃), and persulfate (PSs)) adsorbed on their surface to release active free radicals or non-radicals to act on target pollutants and degrade them into small molecular compounds or CO₂ and H₂O.^23–25 Specifically, Fenton-like SACs/PS oxidation systems are powerful catalytic systems for the decomposition and mineralization of toxic organic macromolecules because they generate reactive oxygen species (ROS), such as hydroxyl radical (HO˙) and sulfate (SO₄˙⁻).^26–29 Benefitting from the rapid development of atomic-resolution characterization technologies, it was possible to observe isolated atoms directly. Moreover, the free radical capture experiment and theoretical density functional theory (DFT) calculation also provide a vital contribution to the verification and elucidation of their catalytic mechanism. Thus, Fenton-like SACs have broken through the catalytic activity and kinetic limitations of traditional catalysts and become the most potential candidate catalysts in the field of environmental remediation. However, the reports on the application of Fenton-like SACs in the field of environmental remediation are still rare.

In this report, we aim to elaborate the concept of SACs, the design and synthesis principles, catalytic performance regulation and mechanism of Fenton-like SACs. The main method for the preparation of Fenton-like SACs using a pure carbon substrate, heteroatom-doped carbon matrix and semiconductor support to disperse and anchor a variety of isolated metal atoms is mainly discussed. The designed and synthesized catalysts have a unique coordination environment, geometric configuration and electronic characteristics, which significantly affect the reaction activity, kinetics and cycle stability of the catalytic system. This is due to the following reasons: (i) each metal atom is endowed with unique electronic properties (oxidation state or spin state), and thus a change in the metal atoms coordinated on the surface of the substrate will result in significant differences in the catalytic effect and the decomposition path of the target substance; (ii) the synthesized Fenton-like SACs also perfectly inherit the characteristics of the carefully selected support materials, such as the optimized electron transfer characteristics of carbon-based SACs, the photocatalytic or electrocatalytic properties given by semiconductor-based SACs, and the porous structure retained by MOF-based, improving their specific surface area to enhance mass transfer; (iii) the coordination environment (including heteroatoms such as O, S, Se and N) of a single metal atom reaction center can also adjust the charge characteristics and reaction activity of metal sites, and may also increase the loading content of metal atoms. Finally, the catalytic mechanism of free radical and non-radical approaches applied to sewage treatment is explained from the perspective of experimental verification and theoretical calculation in Fenton-like SAC/PMS systems for the decomposition of target pollutants.

2. Concept

One of the original purposes of introducing SACs was to achieve a breakthrough improvement in atomic utilization after downsizing nanomaterials, that is, the surface/interface of nano-catalysts involved in the reaction are endowed with a larger effective surface area and higher density of reaction centers.^30–32 Based on the definition by Gate and Thomas, with a decrease in the metal size, entities with a diameter of 2 to 8 nm are used to describe nanoparticles, whereas molecular aggregates between 2 to 30 atoms are considered clusters. According to the definition of SACs, the active metal in the form of a single atom is anchored and dispersed on the surface of a carrier through covalent or ionic interaction with adjacent atoms, and there is no interaction between each isolated metal atom.^33–35 However, the coordination environment of isolated metal species may not be completely consistent. When they are completely consistent, single-atom catalysis is also known as unit point catalysis. It is worth noting that the active center of SACs does not mean that the existing state of a single metal species is zero valence. On the contrary, due to the coordination effects such as electron transfer between the isolated metal species and other species on the carrier surface, they often show a certain charge. The concept of “single-site” and “isolated metal” catalyst indicate that each reaction center is characterized by a metal species. In some catalytic systems, the synergy between the isolated metal atoms and adjacent anchored atoms is an indispensable way to regulate their catalytic activity. To date, the number of articles related to SACs has increased annually. Among them, a variety of transition metal atoms and noble metal atoms has been successfully developed in the research on Fenton-like SACs (Fig. 2). Also, they have been proven to be popular and potential catalysts in the field of environmental remediation, especially wastewater purification.

3. Single-atom catalyst design principle

In recent years, the methods for the synthesis of Fenton-like SACs have developed unprecedentedly, and a variety of physical and chemical preparation methods has been reported, such as direct pyrolysis, hydrothermal/solvothermal treatment, chemical vapor deposition and high-energy ball milling.^36–39 These synthetic routes can be roughly divided into “bottom-up” and “top-down” approached using mononuclear metal complexes and bulk or nanoparticles metals as the initial precursors, respectively. The conditions in the “bottom-up” methods need to be accurately controlled, and the metal loading of the prepared catalyst is usually substantially lower than that of the starting precursor because it is difficult to disperse and stably form a single metal atom in the key post-treatment process of removing mononuclear metal complexes.^40,41 The “top-down” route usually accelerates the thermal movement of metal atoms under sufficient energy (high temperature), reaches the fracture of metal–metal bonds existing in metal nanoparticles or bulk metals, and makes a strong interaction between the escaped isolated metal species and the anchor point for the preparation of stable and completely dispersed SACs.^42–45

Based on the above discussion, the classification of the design and synthesis strategies for Fenton-like catalysts herein is based on the difference in their active center and support/carrier, which is preliminarily divided into three types, such as C-coordinated metal (M–C), heteroatom (e.g., N, P, O, and S)-introduced carbon substrate-fixated metal, and semiconductor-dispersed metal atom. The preparation process of Fenton-like catalysts with various active centers in the above-mentioned classification and the selection of precursor raw materials are explained in detail in the subsequent sections. These Fenton-like SACs synthesized by different design routes and methods show different geometric structures, coordination environments and electronic characteristics, which will result in significant differences in oxidation activity and kinetics in heterogeneous reactions, especially the Fenton-like systems highlighted in this review.

3.1 C-coordinated metal

At present, the “bottom-up” strategy is widely used in the preparation of Fenton-like SACs, where the single metal species existing in metal precursors are captured, reduced and anchored by coordinated in a defect-rich support. As is known, the isolated metal atoms are usually thermodynamically unstable and tend to move spontaneously to form clusters or even nanoparticles to improve their stability. Accordingly, the synthesis strategy of carbon material extraction is helpful to prevent the agglomeration of single metal elements caused by high surface energy. So far, a variety of carbon materials has been developed to promote the capture and fixation of noble or non-noble metal elements. For example, carbon materials rich in oxygen and nitrogen functional groups, such as carboxyl, anhydride, phenolic hydroxyl, carbonyl and other oxygen-containing functional groups, and nitrogen-containing functional groups, such as amide, imide, lactam, pyrrole and pyrimidine, have been widely developed and used in the preparation of Fenton-like SACs.^46–49 In addition to the capture of metal atoms by oxygen functional groups, the unsaturated coordination edges of graphene (such as serrated edges, armchair row edges, and single carbon vacancies or double vacancies) also play a significant role in the dispersion and anchoring of metal atoms.^50,51 The preparation method of carbon extraction and coordination of metal atoms is simple, easy to implement and has the advantage of economic cost. However, the only disadvantage to prevent its mass production is the unsatisfactory catalytic efficiency caused by the low metal atom loading.

3.2 Heteroatom-doped carbon material-fixed metal

The design and synthesis strategy of anchoring metal atoms and dispersing them in a heteroatom-doped carbon matrix by electrostatic interaction can realize a satisfactory loading content and stable chemical environment in atom-dispersed metal Fenton-like catalysts. This is mainly due to the greater number of unsaturated lone pair electrons and structural defects provided by the introduction of heteroatoms, which make the capture of metal precursors more efficient and a significant increase in metal atom loading possible.⁵² Among the variety of heteroatom dopants, the nitrogen atom has become the most widely studied and mature electronegative non-metallic atom for extracting and fixing metal atoms in a carbon matrix. Moreover, the simplest and most efficient methods for preparing Fenton-like SACs based on nitrogen-doped carbon substrates coordinated with isolated metal atoms include the one-step sintering method, impregnation and copolymerization synthesis strategy, and hydrothermal or solvothermal method.^53–55 Importantly, various rich-nitrogen carbon precursor materials (e.g., organic small molecule compounds, metal–organic frameworks (MOFs), polymers, and biomass) have also been successfully developed and applied to prepare Fenton-like SACs for environmental remediation, especially the water purification system highlighted herein (Fig. 3).

3.3 M-semiconductor

4. Performance regulation strategy

Generally, the strong coordination and anchoring interactions between isolated transition metals in Fenton-like SACs and adjacent atoms in the support/carrier are covalent bonds and ionic bonds. Also, with the improvement of characterization methods in recent years, research has focused on the excellent performance and catalytic mechanism of each active metal site in the oxidation process. In addition to developing and improving the existing synthetic processes for the preparation of highly atomically dispersed monatomic active center Fenton-like catalysts to maximize the atomic utilization, another indispensable strategy for the performance regulation of catalytic oxidation systems lies in the optimization of the inherent characteristics of these catalysts. Therefore, an increasing number of studies tend to deeply develop Fenton-like SACs, which can accurately regulate the reaction center and coordination environment of various transition metal species, thus endowing them with good oxidation activity, stability and selectivity in the process of catalytic oxidation and meeting the practical application requirements in industrial Fenton-like reactions (Table 1).

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4.1 Transition metal species

To date, studies on Fenton-like SACs based on various transition metal atoms, such as the widely studied Fe-, Co-, Mn-, and Ni-based heterogeneous catalysts, have proven that different anchored active metal atoms may lead to different Fenton-like oxidation activity and catalytic mechanisms. Various transition metal species coordinate with the same heteroatoms to form Fenton-like SACs with the same coordination environment (such as electronic structure and geometric configuration), which can more clearly explain the activity and mechanism of different transition metal active sites through experimental, characterization and simulation evidence.^96–98 For example, a series of SA-TM/PN-g-C₃N₄ catalysts with transition metal atoms (e.g., Cr, Mn, Fe, Co, and Cu) anchored on a PN-g-C₃N₄ support was prepared via the wet chemical method and top-down pyrolysis process in an inert atmosphere.⁶⁴ By maintaining the same coordination environment and geometric structure of the catalyst, the relationship between different coordination metal atoms and the oxidation performance of the catalyst was discussed. The deep insight into the catalytic mechanism was closely related to the development of model construction and DFT theoretical calculation. The DFT calculation of the active center and relative adsorption energy showed that the catalytic activity of the SA-TM/PN-g-C₃N₄ catalysts with the same coordination configuration followed the order of Cr > Fe > Cu ≈ Co > Mn, which was due to the difference between the dispersed and fixed monatomic metals.

Moreover, the intrinsic electronic properties (oxidation state or spin state) of transition metal species can be regulated at the atomic level in the metal atom-nitrogen coordination active center.¹⁰⁰ Importantly, the spatial electron cloud density of the d orbitals of transition group elements is closely related to their intrinsic properties. Generally, the e_g orbital of transition metals participates in σ-bonding with oxygen-containing species by overlapping with the O_2p orbitals, thus affecting the bonding strength and electron transfer rate between the metal element center and oxidants.¹⁰¹ The study of the spin states of transition metals can be further conducted to understand the mechanism of metal element-regulated PMS activation and crucial for the design and synthesis of N-coordinated metal (M–N) Fenton-like SACs for environmental applications. For example, Miao and co-workers developed M–N-CNT (M = Co, Fe, Mn, or Ni) Fenton-like SACs via pyrolysis and acid etching, focused on the relationship between the transition metal spin state and catalytic activity, and provided a deep understanding and clarification of the mechanism of metal element-regulated PMS activation (Fig. 8a).⁹⁹ The Curie–Weiss law¹⁰² can be expressed as follows:

	(1)

	(2)

where C and μ_eff are the Curie constant and the effective magnetic moment, respectively. μ_eff can be calculated through the linear fitting of χ⁻¹ − T. μ_eff can also be obtained from the spin states (S) of the 3d transition metals because its intrinsic origin is the spin splitting of d orbitals (e_g and t_2g) not fully filled with electrons.¹⁰³

	(3)

g = 2	(4)

VHS + VLS = 1	(5)

According to this formula, although the spin states of transition metals are complex and diverse, the S value is always positively correlated with the μ_eff value. Also, the experimental μ_eff and S value follow the order of Co–N-CNTs (μ_eff = 4.37μ_B, S = 2 or 3/2) > Ni–N-CNTs (μ_eff = 2.24μ_B, S = 1) > Fe–N-CNTs (μ_eff = 1.75μ_B, S = 1/2) > Mn–N-CNTs (μ_eff = 1.69μ_B, S = 1/2) (Fig. 8c). Based on the space electron configuration theory of the d orbitals in transition metals, they correspond to the interaction of oxygen-containing adsorbates on the metal central unit in the form of a large magnetic moment and high spin state and facilitate the rapid transfer of electrons with spin orientation. Therefore, the PMS adsorption, intermediate oxidation potential and electron transfer in the catalytic process are significantly improved. Meanwhile, the linear regression curves of the S value and adsorption energy (E_ads) and E_ads and μ_eff were established, respectively (Fig. 8e). According to the linear correlation coefficient, it was further clarified that the intrinsic factor of PMS activation is E_ads, whereas the external factors controlling its activation are the magnetic moment and spin state of transition metal in the M-PMS system. Therefore, due to the spatial configuration and spin state of the d orbital of the single-metal atom M in M–N-CNTs, PMS (or O–O group) prefers to adsorb on the M–N site, generating M-PMS* intermediates with a higher oxidation potential and accelerating the electron transfer rate, resulting in the fast activation of PMS and decomposition of SMX via the non-radical process (Fig. 8f).

Interestingly, the synergy between the active metal atoms and the introduced second promoting metal atoms not only can increase the number of new Fenton-like reaction centers, but also improve the activity and stability of metal species, playing a greater role in continuous or intermittent cyclic reactions. For example, Xu's team selected Fe and Mn bimetallic species with low biological toxicity and rich geological reserves and electrostatic self-assembly complexes of Prussian blue analogues as precursors to prepare nitrogen-doped graphene-coated FeMn bimetallic catalysts (FeMn@N–C) through high-temperature pyrolysis to activate PMS and produce active free radicals in a Fenton-like process to achieve the mineralization of chlorothiazide (CTD).¹⁰⁴ Taking FeMn@N–C as an example, this work focused on the synergistic effect of Fe and Mn in the Fenton-like oxidation process. Between them, the thermodynamically favorable electron transfer from Fe(II) to Mn(III) reduces the redox potential barrier between different valence states of the same metal species and accelerates the cyclic regeneration of high-valence and low-valence metal active centers. The DFT theoretical analysis showed that the redox cycle of multivalent FeMn bimetallic Fenton-like catalysts can accelerate the formation of free radicals and ensure the stability of iron under continuous PMS activation. As is known, the chemical rate constant of the reduction of the high valence state to the low valence state of transition metal species is very low, which has become the bottleneck in the transformation of Fenton-like catalysis to practical industrial application. Therefore, the strategy of the synergistic effect of multiple transition metal species can realize the rapid cycling and regeneration between redox valence states, realizing a breakthrough in the decisive step of Fenton oxidation to a certain extent.

In conclusion, the precise regulation of transition metal species can significantly optimize the oxidation activity and stability of Fenton-like SACs. (i) Different transition metal elements anchored in the same coordination environment may lead to different catalytic activity and mineralization mechanisms in Fenton-like SACs. (ii) The unique electronic properties (oxidation state or spin state) of each metal species may also affect the coordination and anchoring mode of coordination atoms provided by ligands, thus forming different geometric structures and affecting the oxidation activity. (iii) The introduction of new transition metal species may significantly optimize the oxidation kinetics of Fenton-like SACs owing to the synergistic effect between different metal species. Noteworthily, the results by the Duan and Miao research groups reflect that the activity order of a single metal center is not consistent, which may be due to the different coordination active sites.^99,104 In addition, it also shows that the coordination of nitrogen-doped carbon atoms with transition metal species will also have a great impact on the catalytic performance. Therefore, the coordination environment may also play a key role in regulating the electronic structure and oxidation–reduction activity of Fenton-like catalysts. A detailed description of this aspect is reflected in the next section.

4.2 Coordination environment

The coordination environment (coordination species or bond number) between the active transition metal species and their neighboring atoms is of vital important in the catalytic activity and stability of Fenton-like SACs.¹⁰⁵ The oxidation activity and catalytic performance of the so-called heterogeneous monatomic catalysts cannot be completed independently by the separated and anchored transition metal species. Instead, Fenton-like catalytic reactions are completed on the active units formed by coordination between isolated metal atoms and surrounding electronegative heteroatoms (e.g., C, O, N, S and B).^106–110 At present, the widely studied active centers (e.g., M–Cx, M–Nx and M–N–C) are composed of transition metal species interacting with heteroatoms in ligands in the form of covalent or ionic bonds.^111,112 Importantly, the existence of coordination atoms, especially the electrostatic interaction between anchored non-metallic atoms and transition metal elements, prevents the agglomeration trend of isolated metal elements, thus improving the chemical and structural stability of Fenton-like SACs. Moreover, the electronic properties and geometry of transition metal reaction centers are mainly determined by the local coordination chemistry, which is owing to the coupling effect of the charge density in the center of the metal element regulated by the anchor donor.^113–115 Therefore, it is necessary to explore the coordination environment around highly dispersed transition metal catalytic centers, laying a foundation for further research on the design and mechanism of Fenton-like SACs in the future.

Taking the widely studied M–Cx reaction centers as an example, most of them capture and fix transition metal atoms with carbon materials rich in oxygen functional groups (e.g., –COOH and –OH), such as graphene oxide (GO) and carbon nanotubes (CNT).^116–119 The coordination of the electronegative C atoms plays the role of fixing and isolating metal atoms. This not only realizes the stability of the chemical structure of Fenton-like SACs and the individual transition metal elements and makes the catalyst more durable and significantly improves the atomic utilization rate, but also improves the overall conductivity of the catalyst, achieving the purpose of rapid electron transfer in Fenton-like catalysis. For example, Li's group designed a Co-based Fenton-like SAC (SACo-NG) with a metal cation–π structure, which was manufactured through enhanced hydrothermal-sintering method with isolated Co atoms highly dispersed on the nanospheric C-based graphene-like structures.¹²⁰ This Co cation–π (Co²⁺–N–Cπ) structure has strong polarity due to the existence of rich/poor electronic micro-regions and its π → cation interaction accelerates the interfacial electron transfer cycle, which exists between the pollutants adsorbed on the catalyst surface (due to the π–π stacking between the catalyst aromatic ring and the pollutant aromatic ring) and the electron contribution of the pollutants in the electron deficient center. The orbital interaction (metal cation-π structure) between transition metal species and coordination atoms can successfully regulate the transfer of interfacial electrons, which is an excellent case in the field of rapid and efficient mineralization of pollutants.

Doping other heteroatoms (such as B, O, S, N and P) can significantly improve the electrochemical properties of the graphene carbon layer and produce more structural defects. These defects not only become the new active center of Fenton-like oxidation, but also increase the loading content of metal elements due to their coordination unsaturation. For example, Li et al. reported the synthesis of a Cu-based Fenton-like catalyst (Cu-SA/NGO) by dispersing a single Cu atom in N-doped graphene synthesized via the pyrolysis and freeze-drying of melamine, cyanuric acid and graphene oxide precursors, with a relatively high Cu loading of 5.8 wt% (which was the highest value reported thus far).¹²¹ In addition, the N-doped types such as graphitic N, pyrrolic N and pyridinic N will also have a significant impact on the catalytic performance. For example, He and co-workers used nitrogen-doped porous carbon as a ligand anchor to prepare a single Mn atom catalyst (Mn-ISAs@CN) with outstanding catalytic performance.¹²² A unique perspective for the design of Fenton-like SACs was the dual reaction site strategy to achieve an efficient catalytic performance. Specifically, N-coordinated single Mn element (MnN₄) is a highly catalytic active center for PMS activation, while the adjacent pyrrolic N site is the adsorption unit of the target organic molecule. The interaction between the two significantly shortens the migration distance between the free radicals and adsorbed organic molecules. Differently, the high-resolution N 1s XPS analysis of the g-C₃N₄-anchored single iron atom catalyst (SA Fe-g-C₃N₄) by Hu et al. confirmed the linear correlation between the pyridine N content (wt%) and iron content (wt%) (R² = 0.97), that is, strong linear relationship between the pyridinic N and Fe loading content.¹²³ The adsorbate on the catalyst surface can be removed by easy heat treatment, and the catalytic activity of the catalyst can be well restored.

Moreover, in this Fenton-like catalyst, in which the carbon material introduced by the electronegative non-metallic heteroatom N is coordinated and anchored with a 3d metal atom, the electron-rich N dopant can be used as the Lewis basic site, which makes it easier to fix and disperse the metal atom (Lewis acid) and forms a strong M–N (M refers to Fe, Co and Mn) reaction activation unit (M–N–C) in the carbon matrix.¹²⁴ Although there are relatively few studies on the coordination of S, P and O compared with C and N, the existence of these coordination atoms will also affect the chemical structure or local electronic properties of Fenton-like catalysts. As proposed in the literature, electron-absorbing oxidized S species can weaken the adsorption of intermediates on transition metal species. Du and co-workers successfully developed Fenton-like SACs, which can be extended to various heteroatom doping and coordination in a simple and low-cost way.⁸² Specifically, biomass-derived biosorbents containing high quaternary ammonium groups can be applied to the adsorption of sulfate or phosphate to realize the co-doping of S or P and N. The scalability and practical industrial application of the synthesis technology lay a foundation for further research.

Overall, the coordination environment around the transition metal species plays an important role in regulating the stability, catalytic activity and selectivity of Fenton-like SACs, which is mainly reflected in the following aspects. (i) The interaction between coordination atoms and isolated single metal elements can improve the stability of the catalytic unit coordination chemical structure and prevent the migration and agglomeration of metal atoms in the Fenton-like catalytic process. (ii) The introduced coordination heteroatoms can produce more unsaturated hanging bonds, thus increasing the position of capturing and anchoring metal species and improving their loading content. (iii) The existence of coordination atoms will also improve the local electronic properties of the Fenton-like SAC catalytic sites. Although numerous reports in the literature have investigated the coordination environment of Fenton-like catalysts, there are relatively few studies on the number of coordination chemical bonds compared with the reports on different coordination heteroatoms. Simultaneously, in the coordination configuration of the same coordination atom, different coordination bond numbers will also significantly affect the stability and electronic structure of the catalytic site. For example, compared with the M–C₄ site with a stable carbon double vacancy coordination, the unstable M–C₃ site is coordinated by transition metal species with a single carbon vacancy.¹²⁵ To date, most of the active sites in ligand-anchored Fenton-like catalysts are M–Cx, M–Nx and M–N–C. In contrast, few M–Sx, M–Px and M–Bx active centers have been reported, and thus further experiments, characterization and simulation data are still needed to deeply elucidate the structure and activity of these reaction units.

4.3 Support/carrier

As mentioned above, selecting appropriate supports is an effective strategy to overcome the difficulty of the migration and aggregation of isolated single metal species with high surface energy in the synthesis of Fenton-like SACs. The rigid coordination (such as coordination, electrostatic adsorption and ionic bond) between the single metal reaction sites and rigid supports ensures the durability of the catalyst, inhibits the agglomeration of inactive metal atoms and solves the bottleneck of poor cyclic stability in the Fenton-like oxidation process, even if the catalytic process occurs under extremely bad reaction conditions. In addition, the high specific surface area of the carrier can maximize the exposure of high-density monatomic active sites fixed on the substrate and enhance the mass transfer. Heterogeneous Fenton-like SACs with 0D, 1D, 2D and 3D surface morphologies have been developed to improve the catalytic activity.

The properties of the support (such as conductivity and semiconductor properties) used to stabilize the catalytic unit may also endow the catalyst with high oxidation activity and determine the application direction of the catalyst in the fields of chemistry, photocatalysis and electrocatalysis. At present, the most widely studied carbon-based substrate materials can optimize the conductivity of catalysts, which can accelerate the transfer of electrons from the bulk catalyst excited by light, electricity, heat or radiation to the reaction sites on the catalyst surface and participate in the catalytic reaction.^126–128 In addition, the use of carbon-based supports is also an effective strategy to solve the secondary pollution caused by metal ion leaching in heterogeneous Fenton-like SACs. For example, Gao et al. used in situ CVD technology to synthesize Fenton-like SACs with a single iron atom anchored on three-dimensional (3D) N-doped carbon nanosheets (Fe/NC) on a ferrocene-loaded CaO hard template (carbon source and nitrogen source from pyridine).¹²⁹ Experimental and theoretical calculations showed that electrons can be rapidly transferred from the C to Fe species through the C–N–Fe bond (formed by the combination of a carbon substrate and catalytic site Fe–N₄), which ensures that Fe species always maintain a low valence state in the redox process. Meanwhile, the presence of reducing metal species can also significantly inhibit the oxidation of carbon-based substrates.

The choice of semiconductor-based substrate also enables Fenton-like SACs materials to achieve catalytic oxidation, while also having the properties of semiconductors, and determines their application in the fields of light, electricity and chemical catalysis. For example, Wang et al. reported a coupling system in which the nanotube-assembled 3D hierarchical H₂-reduced Mn-doped CeO₂ micro-flowers (re-Mn–CeO₂ NMs) were synthesized as Fenton-like photocatalysts for the activation of PMS.¹³⁰ The obtained re-Mn–CeO₂ NMs can achieve more than 95% organic matter degradation in the re-7Mn-CeO₂ NM/PMS/Vis system within only 10 min, which was 1.1-times and 2.0-times higher than that of Fenton-like reaction and photocatalysis alone, respectively. It was confirmed that the synergistic effect of the Fenton-like reaction and photocatalysis in the coupling system has a great application prospect in environmental remediation.

In summary, the construction of strong metal–support interaction is very important to improve the activity and stability of bulk Fenton-like SACs. The effective selection of the support is significant to improve the oxidation activity and stability of Fenton-like SACs and achieve their practical application in environmental remediation. (i) A support with high specific surface area can maximize the exposure of active centers anchored on its surface. (ii) The selection of a carbon-based support can improve the conductivity of the bulk Fenton-like SACs, thus accelerating the electron transfer and optimizing the catalytic oxidation performance. (iii) Semiconductor-based materials such as g-C₃N₄, TiO₂, Fe₂O₃, WO₃ and BiVO₄ as anchored single-atom reaction units can give new performances to the whole Fenton-like catalytic system. For example, the coupling and synergy of photocatalysis/electrocatalysis and Fenton-like catalysis reported in the literature can improve the catalytic effect exponentially.

5. Mechanism

Fenton-like SACs with strong metal ligand interaction constructed by the coordination of isolated metal atoms on carefully selected carriers in the form of covalent/ionic bonds have attracted extensive attention of researchers because of their maximum atomic utilization, excellent catalytic activity, and easy recycling.^13,51,54,65 However, with the change in the metal-atom catalytic sites designed in the Fenton-like process (such as the type and loading of isolated metal species, as well as the surrounding electronic configuration, chemical structure, coordination environment and bond number), the catalytic oxidation rate will also be enhanced or inhibited. Therefore, the rapid development of various advanced instruments and theoretical calculations makes it clearer and more meaningful to visually define the catalytic sites of a single metal atom and simulate the catalytic mechanism.⁵⁰ This can deeply clarify the valence state and coordination environment of the reaction unit in the Fenton-like oxidation process, as well as its interaction with oxidants or target pollutants, electron transfer, peroxide activation, and the generation and evolution of different reactive oxygen species (ROS).⁹⁰ In this section, the catalytic mechanism of Fenton-like SACs involving the adsorption and activation of peroxides (H₂O₂, PMS, and PDS) to generate radicals (e.g., HO˙, SO₄˙⁻, and O₂˙⁻) and nonradical (e.g., direct electron transfer, singlet oxygen, and high-valent metal-oxo species) to realize the mineralization of target pollutants in organic wastewater is described in detail. This can provide a theoretical basis for future catalyst design and ensure the high kinetic mineralization of target pollutants in environmental remediation.

5.1 Radical pathways

The radical-dominated catalytic mechanism has been widely studied and recognized in Fenton or Fenton-like systems. In particular, reactive oxygen species (ROS) (e.g., HO˙, SO₄˙⁻, and O₂˙⁻) have been identified as effective AOPs for mineralizing refractory organic compounds.^22,127 The generation of the highly active HO˙ depends on the activation of H₂O₂ adsorbed on the surface reaction sites of Fenton-like catalysts, while SO₄˙⁻ is mainly generated in two main persulfate precursor oxidants excited by light, heat, electricity, ultrasound or transition metals, namely persulfate (PS), PMS or PDS/Fenton-like systems.¹³¹ Among them, PMS (also known as Oxone) is a triple salt composed of 2KHSO₅, KHSO₄, and K₂SO₄.¹³² The type of oxidant selected in the reaction system plays an indispensable role in the completion of the oxidative degradation function of the designed Fenton-like catalyst. Therefore, excluding all the external conditions of the catalytic reaction, such as catalyst type, target degradation products, and oxidation conditions (such as pH, temperature, catalyst dosage, and reaction time), by changing only the type of oxidant can enable researchers to obtain the best oxidant species from an intuitive experimental point of view. For example, Liu and colleagues used Fe-encapsulated B/N-doped carbon nanotubes (Fe@C-NB) as a catalyst and compared the differences in catalytic oxidation efficiency caused by different oxidants (H₂O₂, PMS and PDS) in the treatment of p-arsanilic acid (p-ASA), and the experimental results were ranked as PMS > PDS > H₂O₂.¹³³ Obviously, in Fenton-like oxidation, PMS exhibits excellent oxidation sensitivity compared with the existing commercial oxidants PDS and H₂O₂ because PMS molecules (HO₃S–O–O–H; I_O–O = 1.326 Å) have a longer asymmetric structure of the superoxide O–O bond (PDS: HO₃S–O–O–SO₃H; I_O–O = 1.322 Å).¹³² Therefore, PMS, which is mild and easy to store and transport, is a promising wastewater oxidation repair agent. Comparing and listing the advantages of the active oxygen radical SO₄˙⁻ generated by the activation of PMS/Fenton-like catalyst systems with HO˙: (a) SO₄˙⁻ shows a high oxidation potential (SO₄˙⁻, 2.5–3.1 V vs. NHE HO˙, 1.8–2.7 V vs. NHE); (b) SO₄˙⁻ exhibits better flexibility to a wide pH range (SO₄˙⁻, 3–9 vs. HO˙, 2–4); (c) SO₄˙⁻ has a longer half-life (SO₄˙⁻, 30–40 μs vs. HO˙, 20 ns); and (d) SO₄˙⁻ is more selective than HO˙ towards electron-rich organic micropollutants.^63,81,86 Therefore, these excellent characteristics of SO₄˙⁻ make it possible to act for a longer duration to rapidly degrade toxic organic pollutants, especially persistent organic pollutants, into smaller biodegradable molecules, even H₂O and CO₂.

Radical quenching experiments can determine the types of active radical species generated during the activation of PMS. Specifically, methanol (MeOH) is a radical scavenger for HO˙ and SO₄˙⁻, while tert-butanol (TBA) and isopropyl alcohol (IPA) are used to prove the ˙OH contribution to the reaction.^6,9,22,28 Electron paramagnetic resonance (EPR) is a sensitive and convenient technique that uses spin-trapping agents to further verify the ROS species participating in the Fenton-like reaction process.^55,63 Taking the Fe@MoS₂/PMS system as an example, the active species that play a major role in the Fenton-like oxidative degradation of atrazine (ATZ) were characterized and confirmed by EPR and radical quenching experiments.¹³⁴ As shown in Fig. 9a, the spin capture EPR experiment using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent showed that both HO˙ and SO₄˙⁻ play a role in the MoS₂/PMS and Fe@MoS₂/PMS systems, which are marked by pink circles and brown triangles, belonging to the DMPO-HO˙ adducts (four peaks) and DMPO-SO₄˙⁻ adducts (six peaks), respectively. Subsequently, a series of quenching experiments in the presence of 100 mM ethanol (EtOH) or TBA showed that they could inhibit the degradation of ATZ (40 μM) by 91.8% and 36.9%, respectively (Fig. 9b). The difference in the inhibition degradation data between the two groups was explained based on the second-order rate constant of the reaction between EtOH with the radicals (k_HO˙, _EtOH = 1.7–2.2 × 10⁹ M⁻¹ s⁻¹, _{, EtOH} = 1.6–7.7 × 10⁷ M⁻¹ s⁻¹) and TBA with radicals (k_{HO˙, TBA} = 4.2–7.6 × 10⁸ M⁻¹ s⁻¹, _{, TBA} = 4–9.1 × 10⁵ M⁻¹ s⁻¹), clearly showing that EtOH can effectively quench HO˙ and SO₄˙⁻ radicals. Moreover, although it was considered to be more effective in the quenching HO˙ radical, TBA can only quench about 30% of sulfate, which is mainly due to the instability of the DMPO-SO₄˙⁻ adduct and easy conversion to DMPO-HO˙ due to the nucleophilic substitution reaction with OH⁻/H₂O. This may be why it was observed that the DMPO-HO˙ signal was larger than the DMPO-SO₄˙⁻ signal, similar to the phenomenon in previous studies. Therefore, based on the above-mentioned experimental results, it can be further confirmed that HO˙ and SO₄˙⁻ play a significant role in the decomposition of target pollutants into small molecules or even CO₂ and H₂O.

SO4˙− + H2O → SO42− + HO˙ k < 2 × 103 M−1 s−1	(6)

Previous studies have shown that the heterogeneous transition metal sulfide MoS₂ activates MoS₂/PMS through the oxidation of Mo(IV) to Mo(V) and further to Mo(VI), and then decomposes to the high-activity SO₄˙⁻.¹³⁵ According to this degradation experiment, the PMS activation pathway was proposed in Fig. 9c. Specifically, the oxidation reaction process involves three steps, as follows: (i) the Mo atoms on the edge and basal surface contact with PMS and adsorb to form Mo-PMS ligands, which can be verified according to the low adsorption energy of PMS on Mo and the increased length of the peroxy bonds of PMS after the formation of the Mo-PMS ligand in DFT calculations and (ii) the Mo(VI)/Mo(V)/PMS system generates HO˙ and SO₄˙⁻. According to the XPS characterization of Fe@MoS₂ before and after the catalytic experiment, a new small peak of Mo(VI) appeared after the cyclic experiment, which may be due to the electron exchange between Mo(IV) and PMS. Additionally, (iii) the generated HO˙ and SO₄˙⁻ radicals play an indispensable role in the treatment of ATZ to achieve its decomposition and mineralization. Importantly, the loaded Fe atoms may also contribute to the activation of PMS and can be reduced by Mo(IV), and the Fe atoms not only activate the adjacent Mo atoms, but also the Mo atoms not adjacent to Fe, thus adsorbing and activating the PMS molecules. This result was consistent with the EPR experimental results, that is, the intensity of the radicals, especially HO˙ in the Fe@MoS₂/PMS system was much higher than that in the MoS₂/PMS system. It was further confirmed that the Fe atom anchored on the MoS₂ support plays an important role in the Fenton-like process. The Fe atom with high atomic utilization can activate the oxidant and produce more reactive oxygen species to degrade ATZ, thus effectively improving the catalytic activity of Fenton-like reactions.

The reaction Gibbs free energy (ΔG) is a thermodynamic function introduced in chemical thermodynamics to determine the direction of reaction process. The ΔG of the Au/VO₂/PDS Fenton-like system was calculated, as shown in Fig. 9d.⁹⁰ It can be concluded that the adsorption and decomposition of persulfate were both spontaneous in the reaction process because the formation of S₂O₈²⁻* and SO₄˙⁻* was an exothermic reaction. Moreover, due to the anchoring of isolated Au atoms, it was more conducive for electrons to transfer to the PDS adsorbed on the surface. Therefore, in the persulfate decomposition step, the reaction decomposition energy of Au/VO₂ was 1.83 eV, which was much less than that of the other two catalysts (5.30 eV for Au (cluster)/VO₂(Au(C)/VO₂) and 7.41 eV for VO₂(B)). More importantly, the rate-limiting step, that is, the desorption reaction of SO₄˙⁻*, is also significantly accelerated, which was reflected by the fact that the desorption energy of Au/VO₂ was much lower than that of Au(C)/VO₂ and VO₂. Therefore, the acceleration of the rate-limiting step can greatly improve the desorption capacity of the persulfate radical, which is beneficial for the subsequent oxidative decomposition reaction between ROS and organic macromolecules and acceleration of the whole sewage purification process.

In situ characterization is an important means to study the catalytic mechanism. Here, the adsorption, activation and evolution of PMS on the catalyst surface are clearly illustrated by in situ Raman spectroscopy. The adsorption of PMS on the catalyst surface is a prerequisite for starting all oxidation reactions, and thus it is particularly important to study the initial state (Fig. 9e). The in situ Raman spectrum of the pure PMS system has three main peaks at 878 cm⁻¹, 973 cm⁻¹ and 1052 cm⁻¹, representing the stretching vibration of the O–O bond and the symmetrical stretching vibration of SO₄²⁻ and SO₃ in HSO₅⁻, respectively.¹³⁶ In the NG/PMS and NG/PMS/BPA systems, only the peak representing O–O bond was different from the pure PMS solution and shifted to a lower wavenumber. It is worth noting that in SACo-NG/PMS solution, the peak of the O–O bond splits at 866 cm⁻¹ and 882 cm⁻¹, indicating that two different species in PMS adsorbed on the surface of SACo-NGs react with the O and Co sites, respectively.¹²⁰ The peak intensity corresponding to HSO₅⁻ decreased significantly, indicating the consumption of PMS. After the addition of BPA to the SACo-NGs/PMS/BPA system, the three characteristic peaks and splitting peaks of PMS became very weak, indicating that PMS reacted with the Co site and decomposed. Interestingly, the presence of BPA in solution accelerates the oxidation of PMS, which was reflected by the emergence of a new peak corresponding to some peroxy species, such as SO₅˙⁻, at 803 cm⁻¹.

5.2 Nonradical pathways

Non-radical oxidation has been recognized in many catalytic PMS oxidations, where the formation of singlet oxygen (¹O₂) and mediated electron transfer are the typical mechanisms.¹³⁷ The oxidation degradation pathway dominated by nonradicals has wide applicability and adaptability for practical use in environmental remediation because it plays a significance role in real wastewater systems with anions such as humic acid (HA), Cl⁻, HCO₃⁻, H₂PO₄⁻, NO₃⁻ and SO₄²⁻ compared with the traditional active radicals.^82,134

6. Summary and perspective

As reported in the literature, single-atom metal catalysts have many advantages, such as outstanding catalytic effect, great atomic utilization efficiency, unique coordination environment and excellent chemical stability. Thus, they have been recognized as ideal candidates with research value to inherit the advantages of traditional heterogeneous and homogeneous catalysts in many important applications. SACs have been developed and successfully applied in the field of sewage purification, especially Fenton-like single- or double-transition metal atomic reaction center catalysts in advanced oxidation processes, which have shown unprecedented chemical reaction activity and cycle stability. According to the Fenton-like SACs/PMS system reported in this review, the Fenton-like SACs realized by metal species such as Fe, Co, Cu, Mn, Mo, Cr, Ni, Ag, Pt, and Au alone or in combination as the redox reaction center have received special attention. Moreover, the active centers of isolated metal atoms synthesized via the “bottom-up” approach are usually coordinated and dispersed on carefully selected carriers in the form of covalent bonds or van der Waals forces. The strong interaction between the monatomic metal and carrier has the following advantages: (i) making charge transfer easier; (ii) preventing metal atoms from aggregating into clusters or particles; and (iii) compared with multi-metal sites, the single-metal reaction unit also has enhanced resistance to harmful side reactions. Notably, SACs possess unique electronic structures and energy level orbitals (different from traditional metal nanoparticles), which can maximize the improvement of metal dispersion and atomic utilization efficiency. Therefore, SACs are the most promising materials to realize the rational utilization of metal resources and atomic economy. Accordingly, developing reasonable strategies for the synthesis of high-quality Fenton-like SACs is significant for their practical application.

In this review, we summarized and showed the recent research progress of Fenton-like SACs used in advanced oxidation process, including the design and synthesis of metal atom reaction sites, the catalytic activity and stability, and the generation and mechanism of active species in the reaction process. Based on this, a basic criterion for Fenton-like SACs in which isolated metal atoms act as active unit and participate in the reaction is provided to explore the relationship between the synthesis principle of catalysts, the coordination configuration of active centers, chemical kinetic constants and catalytic mechanism. Among them, the coordination environment is closely related to the carrier of anchoring and dispersing metal atoms. Herein, we classified them into three main types, as follows: C-coordinated metal (M–C), heteroatom (e.g., N, S, P, and B)-doped carbon material-fixed metal, and semiconductor-dispersed metal atoms. Similarly, the performance of the support itself will also affect the overall performance of Fenton-like SACs. The elaboration of the catalytic mechanism mainly relies on radical capture experiments, electron spin resonance spectroscopy and DFT theoretical calculation. According to the properties of the dominant active species, the reaction mechanism can be divided into two categories, i.e., radical and nonradical pathways (singlet oxygen, electron-transfer regime, and high-valent metal-oxo species). Although much work and attention have been devoted to the research on Fenton-like SACs, there are still considerable challenges to be solved and opportunities for this powerful Fenton-like SACs/PMS system to truly achieve a wide range of practical applications.

(I) Applicability of Fenton-like SACs preparation: nowadays, the most widely used processes for the synthesis of Fenton-like SACs are the hydrothermal method and high-temperature pyrolysis under the protection of an inert atmosphere (N₂ or Ar). However, these preparation methods are not feasible for “real-world” applications. On one hand, the thermal movement of metal atoms is more intense at high temperature, and thus the probability of their migration and aggregation into clusters or nanoparticles is greater. This is because the surface energy of isolated metal atoms is higher, and they tend to agglomerate to enhance their stability. On the other hand, they are not applicable to meet the demand of mass production, especially the preparation requirements of Fenton-like SACs above the gram level. Furthermore, the production energy consumption and complex preparation process have become “stumbling blocks” that hinder the practical application of these catalysts. At present, the development of synthetic methods for Fenton-like SACs, especially simple methods that can be employed on an industrial scale, is still lacking. Therefore, significant time and energy need to be invested to explore methods that can realize the large-scale preparation of catalysts (g level or even kg level). To solve the problem of the agglomeration of metal atoms with high surface energy, the usual solution is to reduce the loading of metal atoms, which can realize the preparation of isolated and evenly dispersed catalysts. However, a low loading of metal atoms corresponds to a low density of active sites that can participate in the redox process, which is the main reason for their poor catalytic activity and slow chemical reaction kinetics. In addition, low active centers will also bring potential side reactions in the reaction process, that is, the reaction intermediate of the catalytic process will be accumulated due to the low loading of monatomic sites. Therefore, the low loading of metal atoms is the bottleneck limiting the practical application of catalysts, while a high loading of metal atoms tends to migrate and agglomerate into nanoclusters or even nanoparticles. In summary, it is urgent to develop Fenton-like SACs with stable high-loaded metal atom sites for industrial application. In addition, the design of the single-atom coordination environment in Fenton-like SAC/PMS systems is mostly limited to the M–Nx and M–Nx–C forms, while there are few studies on capturing and anchoring metal atoms using other heteroatoms, such as S, O, P and B. The change in the coordination environment by introducing new heteroatoms may also provide a novel way to regulate the electronic structure of the active sites, resulting in unexpected effects on the catalytic activity.

(II) Surface morphology regulation of Fenton-like SACs: as is known, the number of active sites that can participate in the catalytic process and the intrinsic reaction activity of Fenton-like SACs are the two main factors closely related to the performance of these catalysts. The accessible active reaction center and the prevention of metal atom agglomeration are directly determined and affected by the surface morphology of the catalyst. At present, there are many methods to control the surface morphology of catalysts, such as the template method (soft template method and hard template method). Various morphologies, such as 1D, 2D, 3D and porous morphologies, have been successfully developed and applied to the Fenton-like oxidation process in advanced oxidation processes. Taking platinum-based alloy nanowires as an example, the rapid development of one-dimensional materials can be attributed to their inherent anisotropic structure, excellent specific surface area and outstanding electron conduction ability compared with zero-dimensional nanoparticles. The advantage of 2D catalyst materials is that they can alleviate the aggregation of metal species in the process of high-temperature pyrolysis because the migration distance between metal species is far enough. In the case of porous materials, MOFs are a representative class because of their large surface area and controllable porosity. The most widely studied MOFs in Fenton-like systems belong to the sub-class of zeolitic imidazolate frameworks (ZIFs), which are rich in pore channels and coordination-anchored nitrogen atoms. Their unique porous structure is conducive to the exposure of active sites, and also ensures rapid mass transfer, making the target reaction and the accessibility between reaction units. However, although these materials have been widely studied and achieved considerable catalytic activity, their catalytic performance is still not enough to achieve their industrial-scale application. This is because the pore structure in the catalyst is dominated by micropores, which cannot make full use of its active sites. Therefore, there is an urgent need to design materials with macroporous channel structures, enabling the internal reaction unit of the catalyst to effectively participate in the redox reaction and be effectively utilized. For example, a medium/large pore structure with an average pore size of greater than 2 nm can effectively improve the mass transfer capacity and the utilization of internal active centers. In conclusion, the regulation of the morphology of catalyst materials, especially making full use of the advantages of various morphologies and developing mesoporous materials with accessible internal active centers, is of great significance to improve the stability of catalysts.

(III) Local structure identification and theoretical simulation of Fenton-like SACs: thus far, the characterization and confirmation of the coordination environment of the designed and synthesized monatomic active centers and the valence states of metal atoms still depend on extended X-ray absorption fine structure (EXAFS) measurements, X-ray absorption fine structure spectra (XAFS), and near edge X-ray absorption fine structure (NEXAFS, S L-edge). The XAS of these synchrotron radiation sources can indeed detect and successfully characterize the coordination environment of single metal atoms. However, this technique also has some limitations, that is, it has certain bulk sensitivity and can only provide average information on the bulk phase. Furthermore, for real catalysts, their XAS characterization may be disturbed by the coordination configuration of their different metal atoms, and thus this characterization method results in some distortion to the detected bulk catalyst. Therefore, there is an urgent need to develop more precise characterization methods for dynamic information and complementary verification. In addition, DFT theoretical calculation is an indispensable technical tool to obtain the chemical structure of Fenton-like SACs, exploring the catalytic active center playing a leading role and better describing the properties and reaction mechanism of SACs. Moreover, DFT theoretical calculation can be employed to obtain the reaction free energy of each basic step of a specific reaction, calculate the adsorption energy between the catalyst surface active site and the activated PMS oxidant or intermediate in the Fenton-like reaction process, and establish the relationship between structure and activity at the atomic level. However, not all the catalytic mechanisms obtained by DFT correspond to the experimental conclusions. Therefore, in the process of discussing and verifying the catalytic mechanism, it is necessary to build a more precise theoretical calculation model to obtain experimental evidence between the reaction unit and the kinetic structure in the process of catalytic activation.

(IV) Simulation of catalytic mechanism: in Fenton-like SACs/PMS advanced oxidation systems, the discussion on the catalytic mechanism mainly focuses on: (i) the coordination configuration and electronic structure of the reaction center involved in the adsorption and activation of PMS (transition metal species-electronegative non-metallic element coordination active centers, metal free sites around isolated metal atoms, and various defects); (ii) the reaction unit will spontaneously induce multiple oxidation paths, leading to the generation of radicals (ROS), non-radicals and electrons for the decomposition and mineralization of refractory organics; (iii) the main contribution or synergy of different active species to the decomposition process of organic matter. At present, the catalytic centers involved in the reaction and the active species activated in the degradation pathway are mainly characterized by EXAFS, XAFS, electron paramagnetic resonance (EPR) and free radical capture experiments. In addition, in situ technology is an important means to characterize the mechanism, such as in situ Raman testing technology and in situ Fourier infrared testing technology. However, the application of these in situ technologies is relatively limited. A more in-depth simulation and proof of the above-mentioned characterization come from the support of DFT theoretical calculation, which can calculate the adsorption energy and kinetic energy barrier of active species, intermediates and final products in the reaction center. Unfortunately, due to the limitation of algorithm resources of traditional DFT theory, it cannot calculate and screen a large number of models in a short time. Therefore, the rapid development of computing systems, software and hardware is necessary to establish the structure-performance relationship among the characteristics of catalyst active centers, the formation path of active species and catalytic performance. These advanced calculation methods can enable us to quickly simulate and predict the catalytic decomposition pathway and performance under near real reaction conditions, and provide new insights to understand the evolution of active species and the kinetic and thermodynamic characteristics of organic oxidation on Fenton-like SACs from the molecular to macro level.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 5197105852071072), the Fundamental Research Funds for the Central Universities (No. N182312007 and N2023001).

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