Cobalt Single Atoms Embedded in Nitrogen-Doped Graphene for Selective Oxidation of Benzyl Alcohol by Activated Peroxymonosulfate

Pomelo peel biomass derived highly active advanced-oxidation-process catalyst: Complete elimination of organic pollutants

The advanced oxidation process (AOPs) is playing an important role in the elimination of hazardous organic pollutants, but the development of inexpensive and highly active advanced catalysts is facing challenges. In this study, a low-cost and readily available agricultural waste resource pomelo peel-flesh (PPF) biomass was used as the basic raw material, and the uniformly dispersed small cobalt nanoparticles were effectively anchored in the biochar derived from pomelo peel-flesh (BDPPF) by impregnation adsorption/complexation combined with heat treatment. Co/BDPPF (BDPPF embedded with Co) can effectively activate peroxymonosulfate (PMS) to SO4·-, ·OH and 1O2 reactive oxygen species, and achieve nearly 100% degradation of tetracycline persistent organic pollutant. Co/BDPPF can not only degrade tetracycline efficiently in complex water environment, but also degrade most organic pollutants universally, and has long-term stability, which solves the problem of poor universality and stability of heterogeneous catalysts to a certain extent. Importantly, Co/BDPPF derived from waste biomass was also innovatively designed as the core of an integrated continuous purification device to achieve continuous purification of organic wastewater. In this study, agricultural waste resources were selected as biomass raw materials to achieve efficient capture of Co2+, and finally developed advanced AOPs catalyst with excellent performance to achieve the purification of organic wastewater. It also provides a promising solution for the preparation of simple, low-cost, large-scale production of AOPs catalysts that can be put into actual production.

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

Single Atom Catalyst in Persulfate Oxidation Reaction: From Atom Species to Substance

With maximum utilization of active metal sites, more and more researchers have reported using single atom catalysts (SACs) to activate persulfate (PS) for organic pollutants removal. In SACs, single metal atoms (Fe, Co, Cu, Mn, etc.) and different substrates (porous carbon, biochar, graphene oxide, carbon nitride, MOF, MoS2 , and others) are the basic structural. Metal single atoms, substances, and connected chemical bonds all have a great influence on the electronic structures that directly affect the activation process of PS and degradation efficiency to organic pollutants. However, there are few relevant reviews about the interaction between metal single atoms and substances during PS activation process. In this review, the SACs with different metal species and substrates are summarized to investigate the metal-support interaction and evaluate their effects on PS oxidation reaction process. Furthermore, how metal atoms and substrates affect the reactive species and degradation pathways are also discussed. Finally, the challenges and prospects of SACs in PS-AOPs are proposed.

Single atom manganese catalyst boosting selective oxidation of alcohols with activated peroxymonosulfate

Selective oxidations are important reactions in organic synthesis for fine chemical industry and conventional methods are expensive and produce a lot of toxic wastes. Herein, we demonstrate a facile and environmentally benign technique for liquid phase selective oxidation based on graphene-supported Mn single-atom-catalyst (SAMn-G) for efficient peroxymonosulfate (PMS) activation. The active Mn component in the developed SAMn-G catalyst reached single-atomic dispersion on graphene substrate via the coordination of individual Mn atoms with the doped N from the graphene framework. SAMn-G activated PMS via a nonradical-dominated pathway, which could convert aromatic alcohols into aldehydes or ketones at a mild temperature. The SAMn-G catalyst exhibited superior conversion and aldehyde selectivity in alcohol oxidation in comparison with their counterpart catalysts possessing either homogeneous Mn ions or oxide particles. The high activation efficiency of SAMn-G is due to the synergistic effect between Mn atoms and graphene substrate, as well as the dominated reaction pathway from nonradical oxidation, which is more selective than these free radicals to oxidize the alcohols. Concerted experimental evidence indicates that the non-radical oxidation process was highly possible to follow the electron transfer mechanism by PMS/organic adsorption on the surface of the catalyst. This study provides a fundamental understanding of PMS activation mediated by single atom catalyst for organic synthesis and the achieved insights can also help the catalyst design for other liquid phase selective oxidation processes.

3D variable Co species carbon foam enhanced reactive oxygen species generation and ensured long-term stability for water purification

Cobalt (Co) and oxides are the most common catalysts for activating peroxymonosulfate (PMS). However, practical applications of Co-based PMS-advanced oxidation processes are difficult to realize the degradation of the targeted pollutants due to poor yield of reactive oxygen species (ROS) and inaccessible active sites. Here, we designed 3D oxygen vacancy-rich (V_o-rich) variable Co species@carbon foam (Co_xO_y@CF) via coupling solvent-free and pyrolysis strategies for degrading tetracycline by PMS activation. The kinetic rate of optimized (Co@CoO) Co_xO_y@CF-1.0 (1.0 presented the molar ratio of Co²⁺ and 2-methylimidazole) enhanced by an order of magnitude compared to that of ZIFs derivatives (ZIFs-500) (0.073 vs 0.155 min^-1) due to the special structure. The flow-through unit maintained over 90% removal within 12 h, which was far better than that of ZIFs-500/PMS system. We used electrochemical analysis, quenching experiment, in-situ FTIR and Raman spectra to further investigate the possible mechanism of the 3D Co_xO_y@CF-1.0/PMS system. 3D Co_xO_y@CF-1.0 stimulated the production of the metastable catalyst-PMS* complex obtained O₂^- as intermediates accompanied by the redox cycling of Co²⁺/Co³⁺, which created the dominant ROS (more ¹O₂) in the presence of V_o, which was completely different for ZIFs-500/PMS with coordinated and dominant radical and non-radical pathways. This study could large-scale generate variable cobalt-based catalysts for enhanced ROS generation, leading the new insight for boosting practical applications.

Constructing a Photocatalyst for Selective Oxidation of Benzyl Alcohol to Benzaldehyde by Photo-Fenton-like Catalysis

A photoactive metal-organic framework (MOF), [K(H₂O)][Cu(DPNDI)][Cu(DPNDI)(CH₃CN)(H₂O)] [Cu_1.5(DPNDI)_1.5H_1.5P₂W₁₈O₆₂]·2H₂O (Cu(Ι)W-DPNDI), was prepared by combining a functional photosensitizer N, N'-bis(4-pyridylmethyl)naphthalene diimide (DPNDI), copper(I) ions, and an oxidation catalyst [P₂W₁₈O₆₂]^6- into a single framework via a hydrothermal process. Cu(Ι)W-DPNDI exhibited a stable structure, strong light absorption capacity, a suitable band gap, and photoelectric properties, which provided favorable conditions for photocatalysis. In the confined space, the well-aligned Cu(I) ions and POM polyanions played a synergetic effect in the electron-transfer process and reactive oxygen species generation. By coupling photocatalysis and heterogeneous Fenton-like catalysis, Cu(Ι)W-DPNDI displayed high efficiency for the selective oxidation of aromatic alcohols, with up to >99% selectivity and 75% yield.

Enhanced activation of persulfate improves the selective oxidation of alcohols catalyzed by earth-abundant metal oxides embedded on porous N-doped carbon derived from chitosan

Metal clusters oxide were embedded in an N-doped carbon and used as catalysts for the activation of peroxydisulfate or peroxymonosulfate in the selective oxidation of benzyl alcohol. Quenching tests were done to investigate the reaction mechanism.

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

Novel single-atom catalysts (SACs) have become the frontier materials in the field of environmental remediation, especially wastewater purification because of their nearly 100% ultra-high atomic utilization and excellent properties. SACs can be used in Fenton-like catalytic reactions to activate various peroxides (such as hydrogen peroxide (H₂O₂), ozone (O₃), and persulfate (PSs)) to release active radicals and non-radicals, acting on target pollutants, and realize their decomposition and mineralization. Among them, peroxymonosulfate (PMS) in PS systems has gradually become an important oxidant in Fenton-like processes due to its asymmetric molecular structure and characteristics of easy storage and transportation. Focusing on the numerous proposed strategies for the synthesis and performance regulation of Fenton-like SACs, it has been confirmed that the coordination of isolated metal atoms and the support/carrier enhances the structural robustness and chemical stability of these catalysts and optimizes their catalytic activity and kinetics. Moreover, the tunability of the coordination environment and electronic properties of SACs can improve their other catalytic properties, such as cycle stability and selectivity. Thus, to systematically explain the relationship between the active center, catalyst performance and the corresponding potential catalytic mechanism, herein, we focus on the representative scientific work on the preparation strategy, catalytic application and performance regulation of Fenton-like SACs. Specifically, we review the typical Fenton-like SAC reaction processes and catalytic mechanisms for the degradation of refractory organic compounds in advanced oxidation processes (AOPs). Finally, the future development and challenges of Fenton-like SACs are presented.

Unveiling the Origins of Selective Oxidation in Single-Atom Catalysis via Co-N4-C Intensified Radical and Nonradical Pathways

Single-atom catalysts (SACs)-based peroxymonosulfate (PMS) systems are highly selective to the type of organic pollutants while the mechanisms remain ambiguous. In this work, we carried out experimental and theoretical investigations to reveal the origins of selectivity of radical and nonradical pathways in a designated Co-N₄-C/PMS system. Two typical pollutants [bisphenol A (BPA) and metronidazole (MNZ)] with different molecular structures were employed for comparison. We found that radical oxidation (SO₄^•- and HO^•) and nonradical electron-transfer pathway (ETP) co-existed in the Co-N₄-C/PMS system. Pollutants (e.g., MNZ) with a high redox potential were degraded primarily by free radicals rather than ETP, while the oxidization of low-redox pollutants (e.g., BPA) was dominated by ETP at the surface region of Co-N₄-C which overwhelmed the contributions of radicals in the homogeneous phase. Intriguingly, the contributions of radical and nonradical pathways could be manipulated by the PMS loading, which simultaneously increased the radical population and elevated the oxidation potential of Co-N₄-C-PMS* complexes in ETP. Findings from this work will unravel the mysterious selective behavior of the SACs/PMS systems in the oxidation of different micropollutants.

Electron delocalization triggers nonradical Fenton-like catalysis over spinel oxides

Nonradical Fenton-like catalysis offers opportunities to overcome the low efficiency and secondary pollution limitations of existing advanced oxidation decontamination technologies, but realizing this on transition metal spinel oxide catalysts remains challenging due to insufficient understanding of their catalytic mechanisms. Here, we explore the origins of catalytic selectivity of Fe-Mn spinel oxide and identify electron delocalization of the surface metal active site as the key driver of its nonradical catalysis. Through fine-tuning the crystal geometry to trigger Fe-Mn superexchange interaction at the spinel octahedra, ZnFeMnO₄ with high-degree electron delocalization of the Mn-O unit was created to enable near 100% nonradical activation of peroxymonosulfate (PMS) at unprecedented utilization efficiency. The resulting surface-bound PMS* complex can efficiently oxidize electron-rich pollutants with extraordinary degradation activity, selectivity, and good environmental robustness to favor water decontamination applications. Our work provides a molecule-level understanding of the catalytic selectivity and bimetallic interactions of Fe-Mn spinel oxides, which may guide the design of low-cost spinel oxides for more selective and efficient decontamination applications.

Heterogeneous Activation of Peroxymonosulfate by a Spinel CoAl2O4 Catalyst for the Degradation of Organic Pollutants

Bimetallic catalysts have significantly contributed to the chemical community, especially in environmental science. In this work, a CoAl2O4 spinel bimetal oxide was synthesized by a facile co-precipitation method and used for the degradation of organic pollutants through peroxymonosulfate (PMS) activation. Compared with Co3O4, the as-prepared CoAl2O4 possesses a higher specific surface area and a larger pore volume, which contributes to its becoming increasingly conducive to the degradation of organic pollutants. Under optimal conditions (calcination temperature: 500 °C, catalyst: 0.1 g/L, and PMS: 0.1 g/L), the as-prepared CoAl2O4 catalyst could degrade over 99% of rhodamine B (RhB) at a degradation rate of 0.048 min−1, which is 2.18 times faster than Co3O4 (0.022 min−1). The presence of Cl− could enhance RhB degradation in the CoAl2O4/PMS system, while HCO3− and CO32− inhibit RhB degradation. Furthermore, the considerable reusability and universality of CoAl2O4 were testified. Through quenching tests, 1O2 and SO4•− were identified as the primary reactive species in RhB degradation. The toxicity evaluation verified that the degraded solution exhibited lower biological toxicity than the initial RhB solution. This study provides new prospects in the design of cost-effective and stable cobalt-based catalysts and promotes the application of PMS-based advanced oxidation processes for refractory wastewater treatment.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Single-Atom Cobalt Incorporated in a 2D Graphene Oxide Membrane for Catalytic Pollutant Degradation

We introduce a new graphene oxide (GO)-based membrane architecture that hosts cobalt catalysts within its nanoscale pore walls. Such an architecture would not be possible with catalysts in nanoscale, the current benchmark, since they would block the pores or alter the pore structure. Therefore, we developed a new synthesis procedure to load cobalt in an atomically dispersed fashion, the theoretical limit in material downsizing. The use of vitamin C as a mild reducing agent was critical to load Co as dispersed atoms (Co₁), preserving the well-stacked 2D structure of GO layers. With the addition of peroxymonosulfate (PMS), the Co₁-GO membrane efficiently degraded 1,4-dioxane, a small, neutral pollutant that passes through nanopores in single-pass treatment. The observed 1,4-dioxane degradation kinetics were much faster (>640 times) than the kinetics in suspension and the highest among reported persulfate-based 1,4-dioxane destruction. The capability of the membrane to reject large organic molecules alleviated their effects on radical scavenging. Furthermore, the advanced oxidation also mitigated membrane fouling. The findings of this study present a critical advance toward developing catalytic membranes with which two distinctive and complementary processes, membrane filtration and advanced oxidation, can be combined into a single-step treatment.

Descriptors and graphical construction for in silico design of efficient and selective single atom catalysts for the eNRR

Outline a screening protocol that uses density functional theory calculations to simultaneously optimize with respect to multiple criteria, thereby successfully identifying catalysts that are highly selective and also result in low overpotentials for ammonia production through eNRR.

Mechanistic inference on the roles of oxygenic functional groups that activate peroxymonosulfates in graphene for advanced oxidation

Developing carbon-based catalysts for advanced oxidation processes, owing to their abundant reserves, metal-free properties as well as great resistance to acids and alkalis, presents an enticing prospect for environmental remediation.

Single atom catalysis for electrocatalytic ammonia synthesis

This review points out major challenges and outlook of NH3 synthesis via SACs. Summarizing the deficiencies of existing research can help researchers to continuously innovate and improve, and explore new research approaches.

Wastewater Remediation Technologies Using Macroscopic Graphene-Based Materials: A Perspective

Three-dimensional (3D) graphene-based macrostructures are being developed to combat the issues associated with two-dimensional (2D) graphene materials in practical applications. The 3D macrostructures (3DMs), for example, membranes, fibres, sponges, beads, and mats, can be formed by the self-assembly of 2D graphene-based precursors with exceptional surface area and unique chemistry. With rational design, the 3D macrostructures can then possess outstanding properties and exclusive structures. Thanks to various advantages, these macrostructures are competing in a variety of applications with promising performances unlike the traditional activated carbons, biochars and hydrochars, which have less flexibilities for modifications towards versatile applications. However, despite having such a wide range of applications, 3DMs remain applicable on laboratory scale due to the associated factors like cost and extensive research. This perspective provides an overview of available graphene-based macrostructures and their diverse synthesis protocols. In the synthesis, hydrothermal route, chemical vapor deposition (CVD), wet spinning, 3D printing, vacuum filtration, spray drying and emulsion methods are highlighted. In addition, the physio-chemical properties of these macrostructures are discussed with the relationship among the porosity, surface area and the bulk density. The perspective also highlights the versatile potentials of different 3DMs in wastewater remediation by adsorption, desalination, and catalytic oxidation, etc. Following the concluding remarks, future outlooks on commercial applications of 3DMs are also provided.