Geometric and electronic perspectives on dual-atom catalysts for advanced oxidation processes

Abstract

With the escalating global challenges of energy scarcity and environmental pollution, the development of efficient and sustainable catalytic technologies has become imperative. Dual-atom catalysts (DACs) have garnered considerable interest, particularly in various catalytic processes, demonstrating exceptional promise in enhancing reaction efficiency and selectivity. Unlike prior reviews that primarily emphasized a specific or single reaction process, this review provides a systematic and comprehensive analysis of DACs across diverse oxidation chemistry, including ozone oxidation, Fenton-like reactions, photo/electro/piezo-catalysis, and enzyme-mimetic oxidation. It begins with a concise overview of the discovery, development, and evolution of DACs, together with an in-depth investigation of diverse synthesis strategies and state-of-the-art characterization techniques. Moreover, the remarkable improvement in the performance of DACs in catalytic processes is discussed on the basis of how their geometric microstructure and electronic configuration, including charge transfer, coordination environment, and spin state, influence catalytic kinetics and thermodynamics, exploring the relationships between the structural geometry, electronic interactions, and catalysis mechanisms of DACs. By integrating these multidimensional insights, this review expands conventional paradigms in the development of DACs and identifies innovative pathways for linking their microstructure and catalysis mechanism. Finally, we emphasize critical research gaps and emerging opportunities for DACs that warrant further exploration and attention. This review would provide valuable guidance and foundation in the rapidly evolving field of DACs.

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Bofan Zhang

Bofan Zhang, Ph.D., is currently affiliated with Jiangsu University. Her research focuses on the development of nanocatalysts and their mechanistic roles in advanced oxidation catalytic systems, enhanced oxidation and reduction technologies for emerging micropollutant removal, and the construction of nano-biohybrid systems for environmental and energy applications. To date, she has published over 20 SCI-indexed papers in prestigious journals such as Environmental Science & Technology, Advanced Functional Materials, Applied Catalysis B: Environmental and Energy, and Chemical Engineering Journal. She also serves as a Youth Editorial Board Member for Carbon Neutralization.

Yang-Chun Yong

Yang-Chun Yong, Professor and Ph.D. Supervisor, is currently the Deputy Director of the Institute of Energy Research at Jiangsu University. He is a recipient of the Fok Ying Tung Education Foundation Award from the Ministry of Education and a Jiangsu Provincial Distinguished Young Scholar. He has been selected for multiple prestigious talent programs, including Jiangsu Province's “Double Innovation Talent” and “Double Innovation Team” initiatives, as well as for the “Young and Middle-aged Academic Leaders” and “Six Talent Peaks” programs. He serves as a review expert for the National Key R&D Program, National High-Level Talent Programs, and the National Natural Science Foundation of China. To date, he has published over 100 SCI papers in leading journals such as Angewandte Chemie International Edition, ACS Nano, Nature Communications, and ACS Catalysis, with an h-index of 43 and a highest single-paper citation exceeding 600. He has contributed six book chapters (published by RSC, Springer, and Wiley), edited three special issues for international academic journals, and holds over 30 invention patents.

Liang Zhang

Liang Zhang, Ph.D., is currently affiliated with Jiangsu University of Science and Technology, with his research focusing on the design of iron-based Fenton catalytic materials and the development of in situ Mössbauer spectroscopy. He obtained his Ph.D. from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in 2023. He has published multiple high-impact papers in top international journals, including Accounts of Materials Research, Advanced Functional Materials, Applied Catalysis B: Environmental, Chemical Engineering Journal, ACS Applied Materials & Interfaces, Separation and Purification Technology, and Journal of Colloid and Interface Science.

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1. Introduction

With the increase in global environmental pollution and demand for sustainable energy sources, developing effective strategies for energy transformation and pollution remediation has emerged as an essential objective.^1,2 Advanced oxidation processes, which generate reactive oxygen species, have emerged as a promising technology for the efficient decomposition of organic pollutants and energy conversion, offering a viable solution to the growing problem of water contamination and energy crisis.^3,4 Oxidation chemistry is primarily classified into homogeneous and heterogeneous catalytic systems, with the former known for its reactivity and selectivity, but its practical application is limited by its high costs and concerns related to strict reaction conditions, poor yield and poor selectivity.^5,6 In contrast, heterogeneous catalysts, which can perfectly overcome the shortcomings of homogenous systems, present a more cost-effective and environmentally sustainable alternative, thereby holding greater promise for real-world applications.^7,8 However, despite the promise of heterogeneous catalysis, the pursuit of more robust and cost-efficient materials remains critical for improving the efficiency and stability.

Since Zhang et al.⁹ first demonstrated the use of a single Pt atom anchored on FeOx for CO oxidation in 2011, single-atom catalysts (SACs) have attracted significant attention owing to their unique geometric and electronic properties. Acting as a bridge between heterogeneous and homogeneous catalysis, SACs combine the uniform, well-defined active sites of homogeneous catalysts with the stability and ease of separation characteristic of heterogeneous systems. Their high catalytic activity stems from their unsaturated coordination, uniform active sites, quantum scale effects, and strong interactions with metal supports, making them crucial in advanced oxidation processes. However, SACs suffer from low site density, simple electronic and geometric structures, and a tendency to aggregate, limiting their application in multi-step reactions.¹⁰ Thus, to overcome these challenges, dual-atom catalysts (DACs) have emerged.^11–13 Inspired by methane monooxygenase, which utilizes dual metal atoms as active sites,^14–16 DACs inherit the advantages of SACs while introducing enhanced site density, broader active site diversity, and stronger synergistic effects. The interactions between the two atomic sites, such as electronic coupling, co-adsorption, and synergistic catalysis, not only optimize the adsorption of the intermediates, but also enable more efficient multi-step and cascade reactions.¹⁷ Precise control of the atomic ratio, structure, and spatial arrangement of DACs further improves their catalytic performance, making them promising next-generation catalysts. Recent studies have demonstrated their superior reactivity. For instance, Lu et al.¹⁸ reported that dual Pt atoms on graphene exhibited a catalytic rate 17-times higher than that of a single Pt atom in ammonia borane hydrolysis dehydrogenation, while the Co–Fe DAC developed by Quan et al.¹⁹ efficiently generated and activated hydrogen peroxide through a two-electron oxygen reduction reaction. These advancements highlight the growing prominence of DACs, positioning them to potentially surpass SACs and drive the next era of atomic catalysis.

With the emergence of DACs as an innovative technology, they have undergone rapid advancements and garnered increasing attention and discussion in numerous research studies and review articles. In prior reviews, Zhao et al.²⁰ focused primarily on the development and applications of DACs in electrocatalysis, while Shen et al.²¹ reviewed the fabrication and evaluation of both homonuclear and heteronuclear DACs built using carbon and carbon-nitride materials, specifically in electrocatalytic Fenton-like reactions. However, despite these existing reviews, most of the research has been confined to a narrow scope, mainly focusing on electrocatalytic applications. Thus, to date, a thorough and unified review that not only explores the catalytic mechanisms of DACs in depth but also examines the impact of dual-atom structural configurations and electronic structures across a broader range of catalytic reactions is lacking.

Thus, this review aims to bridge this gap by presenting a thorough and multidimensional analysis of DACs in advanced oxidation processes, a pivotal area in addressing both environmental and energy challenges. Although previous works typically focused on single catalytic types or isolated reactions, this review provides a broader approach by exploring the diverse roles of DACs across a range of AOPs, including ozone oxidation, Fenton-like reaction, photo-/electron-/piezo-catalysis, enzyme-like catalysis for organic decomposition, sterilization, gas purification and chemical transformation applications. Furthermore, from the perspective of catalytic oxidation kinetics and thermodynamics, the geometric microstructure and electronic configuration of DACs, together with their intrinsic relationships and impacts on catalytic kinetics and thermodynamics are extensively analyzed and summarized, offering deeper insight into how dual-atoms modulate the catalytic mechanisms at the atomic scale. Through this comprehensive approach, the significant catalytic advantages and innovative potential of DACs are highlighted, paving the way for novel strategies and views in catalyst design and optimization.

2. Overview of DACs: emergence, advantages and classification

2.1 The emergence, evolution and development of DACs

The origins of DACs can be traced back to the 1970s with the discovery of methane monooxygenases, a class of metalloenzymes known for their exceptional efficiency in methane activation. This breakthrough laid the foundation for the exploration of dual-site catalysts composed of two metal atoms. Fig. 1 depicts the timeline of the progress and milestones in the development of dual-atom catalysts. The rapid advancement of surface chemistry in the 1990s spurred significant progress in DAC research. Techniques such as X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) allowed researchers to reveal the surface structure and chemical properties of DACs, as well as introduce fundamental concepts such as the “synergistic effect”. In 2011, Zhang et al.⁹ utilized the co-precipitation method to prepare Pt₁FeO_x SAC for CO oxidation, marking the emergence of SACs as a major research frontier in catalysis. Additionally, the introduction of single-atom Fe as an atomic dopant in graphene vacancies notably enhanced the reactivity of the graphene matrix. However, the mobility of single atoms on the support surface compromised its stability. Thus, to address this, Warner et al.²² innovatively substituted single Fe atoms with Fe atom pairs, thereby creating Fe-pair-doped graphene with improved stability. This development catalyzed the development of DACs, with numerous studies further advancing the technology. For example, Li et al.²³ employed a host–guest strategy to anchor Fe–Co bimetallic sites on nitrogen-doped porous carbon, yielding a catalyst with enhanced oxygen reduction reaction activity under acidic conditions. Similarly, Zhou et al.²⁴ successfully synthesized an Mo₂ electrocatalyst on a C₂N monolayer, which significantly boosted nitrogen fixation reaction efficiency, demonstrating a free energy variation of 0.41 eV and energy threshold of 0.51 eV. These discoveries heralded a new era in DAC research.

Fig. 1 Development of dual atom catalysts, encompassing their emergence, evolution and exploration of their catalysis mechanism over an extended periods.^9,22–29 Copyright 2011, Springer Nature; Copyright 2014, the American Chemical Society; Copyright 2017, the American Chemical Society; Copyright 2018, the Royal Society of Chemistry; Copyright 2019, Lei Zhang et al.; Copyright 2023, Min Zhou et al.; Copyright 2023, the American Chemical Society; Copyright 2024, Elsevier; Copyright 2022 the American Chemical Society.

With the rapid development of DACs, the directional and precise synthesis of specific binuclear site structures has become increasingly feasible. The inherent catalytic advantages of DACs over single-atom catalysts have gradually emerged, extending their applicability across a wide range of catalytic reactions. For instance, Sun et al.²⁵ precisely synthesized a Pt–Ru bimetal catalyst using a two-step atomic layer deposition (ALD) strategy. The electronic interaction between the bimetal atoms was identified as pivotal to the hydrogen evolution activity. Concurrently, Bu et al.³⁰ proposed a pre-anchored metal pair strategy to synthesize an Fe–Co DAC, which demonstrated exceptional thermal and chemical stability in the oxygen evolution reaction. In addition to these achievements, other DACs with similar properties, such as heteronuclear precious metal and transition metal pairs, were also successfully developed. Notably, Liu et al.²⁶ precisely synthesized an La–Ni DAC composed of a rare earth metal atom and transition metal atom using an electron self-assembly driving strategy. Benefitting from the advantages of this strategy, the La–Ni DAC exhibited a good synergistic effect and excellent catalytic performance. These advances reflect the growing diversity of synthesis methods for DACs, which have now been extensively utilized across a broad range of industrial catalytic processes, underscoring the potential of DACs to revolutionize the field of catalysis.

Researchers have accomplished significant breakthroughs in catalyst development, from exploring synthesis methods to pursuing exceptional catalytic performance and uncovering precise catalytic mechanisms.³¹ This progress has driven in-depth studies on the unique underlying catalytic mechanisms for the outstanding performance of these catalysts, with a focus on establishing the relationship between structure and activity to better understand their catalytic behavior. For instance, Lin et al.²⁷ synthesized a Zn₁Co₁ DAC via high-temperature pyrolysis, which showed marked advantages in the turnover frequency compared to Zn or Co SAC. This was explained by the powerful electronic coupling between the Zn and Co atoms and the dynamic interaction between the Zn site and the Co site. Liu et al.²⁶ successfully synthesized a bimetallic Ir–Fe DAC using a wet-chemistry method combined with pyrolysis. Compared to single-metal catalysts, this bimetallic structure retained the catalytic advantages of both metals, balancing the strengths of Ir–O and O–O bonds to achieve a dual-functional catalytic effect. At this stage, researchers have focused more on the intrinsic mechanisms of DACs, as follows: (1) atomic site regulation: introducing different types of dual-atom combinations, varying atom densities or adjusting the distance between atoms can optimize their catalytic performance. (2) Dual-site synergistic effects: the two sites exhibit different catalytic behaviors, targeting specific steps in the catalytic reaction, thereby optimizing particular functions. (3) Dual-functional effects: unlike synergistic effects, dual-functional effects involve the two sites performing different catalytic functions, enabling a single catalyst to perform two distinct catalytic functions.

2.2 Unlocking the essential advantages of DACs

2.2.1 Synergistic effect. In DACs, the synergistic effect arises from the complementary roles of their two metal centers. One site may activate specific reactants or intermediates, while the other supports the reaction pathway, thus enhancing the overall reaction efficiency. This synergism improves the reaction selectivity and catalytic stability, and addresses the limitations of single-atom catalysts.^32–34 Additionally, when two distinct metal atoms collaborate, their catalytic performance often exceeds the sum of their individual contributions. This is typically due to the electronic interactions between the metal sites, where one atom activates species, and the other promotes subsequent steps, boosting the efficiency. The interaction between the two metal sites allows better control over the reaction pathway, enhancing the selectivity and reducing by-product formation. For instance, Huang et al.³⁵ designed a bimetallic DAC, Ir₁Mo₁/TiO₂, which efficiently hydrogenated 4-nitrostyrene (4-NS) to 4-vinylaniline (Fig. 2a). DFT calculations showed that the Ir sites activated H₂, while the Mo sites adsorbed 4-NS, enhancing the hydrogenation rate. Similarly, Song et al.³⁶ developed a Cu–Au DAC for photocatalytic CO₂ conversion, where the Cu sites generated *CO and Au sites acted as coupling centers, efficiently converting *CO. The synergistic effect of Cu and Au enhanced both the reaction rate and catalyst stability.

Fig. 2 Essential advantages of dual atom catalysts: (a) synergistic effect.³⁵ Copyright 2021, the American Chemical Society. (b) Electronic effect.³⁷ Copyright 2022, Wiley-VCH. (c) Spacing enhancement effect.³⁸ Copyright, 2024, Ying Wang et al. (d) Structural flexibility.³⁹ Copyright 2020, the American Chemical Society.

2.2.2 Electronic effect. The electronic effect in DACs occurs when two metal atoms with different electronegativities interact, facilitating electron transfer between the two sites. This interaction leads to the redistribution of electron density, which reconfigures the electronic structure of the catalyst surface.^37,40 Hence, the capacity of the catalyst to govern reaction intermediates is heightened, resulting in an enhanced overall catalytic performance. This electronic synergy can optimize the activation energy of the key intermediates, facilitate the reaction pathways, and increase the selectivity and stability of the catalyst (Fig. 2b). For example, Chen et al.⁴¹ developed a Zn–Mn dual-site catalyst for the electrochemical reduction of CO₂. The Mn site transferred electrons to the Zn center due to the difference in electronegativity between the two sites, establishing an electronic interaction between them. This electronic effect was optimized to ensure the stability of the catalyst structure and enhance the intrinsic activity for CO₂ electroreduction. Additionally, Gong et al.⁴² fabricated a Zn–Co DAC coordinated on N-doped carbon (Zn/Co–N–C) through pyrolysis and acid treatment of the Zn and Co precursors. DFT calculations revealed the strong electronic interaction between the Zn and Co atoms, which significantly lowered the energy barriers for both *COOH adsorption and *CO desorption. This energetic optimization, resulting from the electronic effect, facilitated CO₂ electroreduction.

2.2.3 Spacing enhancement effect. The spacing enhancement effect refers to the influence of the precise arrangement of metal atoms at adjacent sites, which can significantly improve the efficiency of catalytic reactions. When two metal atoms are positioned sufficiently close to each other, they can modify the adsorption properties of the reactants, activate chemical bonds, or even alter the pathways of intermediate reactions.⁴³ This effect is particularly crucial in DACs, where the precise distance between the active metal sites can fine-tune their catalytic behavior and improve the reaction selectivity (Fig. 2c). For instance, Lu et al.⁴⁴ achieved regulation of the atomic spacing between Pt₁ and Ni₁ in heteronuclear dual atoms by exploiting the steric hindrance effect of the metal–organic framework precursors. In the Pt₁Ni₁/C₃N₄ catalyst, the optimal distance between Pt₁ and Ni₁ allowed the bridging OH group to stabilize the H* and OH* intermediates formed during water dissociation. This stable intermediate formation lowered the activation energy for water dissociation, which is the rate-determining step in ammonia borane hydrolysis. In another example, Lou et al.⁴⁵ improved the efficiency of selective hydrogenation of dimethyl oxalate (DMO) to ethanol by precisely controlling the atomic spacing between the active centers of Rh dual atoms. The 3.5 Å distance between Rh centers matched the 3.1 Å spacing between the two O atoms in the C=O bonds of dimethyl oxalate. This geometric alignment generated a unique spatial confinement effect, promoting the adsorption and activation of C=O bonds, and thereby facilitating the selective hydrogenation of DMO to ethanol.

2.2.4 Structural flexibility. The structural flexibility of DACs is key to optimizing their performance, given that it enables precise control of parameters such as element type, coordination number, spatial arrangement, and support type.^46,47 By fine-tuning these factors, diverse DAC structures can be engineered to explore the structure–performance relationship and identify DACs with enhanced catalytic properties (Fig. 2d). For instance, Huang et al.⁴⁸ introduced a strategy that transitions nanoparticulate forms to isolated atoms and DAC configurations. This approach could create 22 types of s-, p-, and d-block metal DACs, including Al₂, Ca₂, Cr₂, Mn₂, Fe₂, Ni₂, Cu₂, Zn₂, Ru₂, Sb₂, Ce₂, Bi₂, and Co₂, marking a breakthrough in DAC structural regulation and multifunctionality. Moreover, W. Elam et al.⁴⁹ developed an ALD-based method for synthesizing supported bimetallic catalysts. By selectively depositing a second metal on the first metal nanoparticles, this technique prevents the formation of new nuclei and single-metal nanoparticles. Regulating the deposition sequence and reaction cycles allows precise control of the size, composition, and structure of bimetallic nanoparticles. Importantly, the interaction between dual-atom sites goes beyond additive effects, improving the catalytic performance by optimizing the geometry and electronic structure of DACs.⁵⁰ Additionally, the tunability of dual-atom structures enhances the scalability and precision of their synthesis, facilitating large-scale production and industrial applications. By tailoring the atomic arrangement, coordination environment, and electronic properties, the catalytic performance can be optimized for specific reactions, offering a powerful platform for designing next-generation catalysts.

2.3 Classification of DACs and categorization of supports

2.3.1 Classification of DACs. Generally, DACs can be classified into two main categories based on atomic species, bimetallic DACs and non-metal DACs. Among the bimetallic DACs, three subcategories are distinguished according to the bonding arrangements between the adjacent dual atoms.

(1) Direct M1–M2 bonding DACs

Dual atoms are directly bonded and anchored on the support through chemical bonds in the form of dimers. This coordination results in the shortest atomic spacing between dual atoms, enhancing the synergistic effect in catalytic reactions. The atomic spacing is inversely correlated with the electronic interaction, meaning that the closer the atomic spacing, the stronger the electronic interaction. This increased interaction facilitates the redistribution of electrons between atoms, significantly benefiting the adsorption of the intermediates (Fig. 3a). For instance, Qin et al.⁵¹ selectively introduced abundant in-plane epoxy functional groups on the surface of graphene to serve as anchor sites for Pt atoms. By combining this approach with ALD, they precisely prepared Pt/graphene catalysts, including single-atom, dual-atom, and cluster forms. Their experiments showed that the Pt dual-atoms exhibited higher catalytic activity than the Pt single-atoms and clusters due to their unique active structure (C–Pt–Pt–O). Similarly, Zhao et al.⁵² successfully fabricated a carbon-nitride-supported Pt–Ag DAC, achieving efficient photocatalytic water splitting to produce H₂ through strong Pt–Ag bond interactions. The direct metal–metal bond cooperation endowed these DACs with unique catalytic properties. However, although this precise architecture leads to exceptional catalytic performance in certain reactions, it also compromises structural flexibility. As a result, these DACs tend to exhibit the optimal performance only in specific catalytic environments, which limits their broader application.

Fig. 3 Different configurations of DACs.

(2) Indirectly bridged M1–X–M2 DACs

Instead of directly linking two metal atoms, indirectly bridged DACs are connected via one or two heteroatoms, which serve as a bridge. This heteroatom-bridged dual-atom configuration introduces greater structural versatility, expanding the range of catalytic environments in which the catalyst can function. The absence of a direct bond between the metal atoms reduces the risk of atomic aggregation, thereby enhancing the structural stability. Furthermore, the heteroatom bridge creates an electronic pathway between the two metal atoms, effectively modulating their electronic properties and optimizing the adsorption behavior of the intermediates during catalytic reactions (Fig. 3b). For instance, Wu et al.⁵³ developed a hollow carbon-supported N-bridged Cu–Zn DAC for ORR. The N-bridges between Cu and Zn atoms facilitated synergistic interactions, enhanced the O* binding, and lowered the ORR energy barrier. Similarly, Liu et al.⁵⁴ designed a bifunctional catalyst, RuMo/TiO₂, which was capable of catalyzing both polyester hydrolysis and carboxylic acid hydrogenation. Characterization revealed that the O-bridged Ru and Mo dual-atom sites formed an active structure of Ru–O–Mo, which effectively suppressed side reactions during alcohol hydrodeoxidation, maintaining high catalytic efficiency. Despite the heightened structural flexibility offered by heteroatom bridges, their incorporation also increases the complexity of the catalyst, posing challenges in the precise control of the positioning of atoms during the synthesis of DACs. Moreover, predicting and manipulating the behavior of these structured DACs in catalytic processes remains a difficult task, particularly when attempting to capture and direct specific reaction pathways.

(3) Coplanar independent M1–X–Y–M2 DACs

Diverging from the modes observed in the preceding categories, the third atomic configuration involves anchoring dual-atom units separately on a support, with each atom forming individual catalytic sites.⁵⁴ This configuration is conducive to fine-tuning of the individual sites, facilitated by the interactions between the support and the metal atoms, which contribute to a stable and balanced catalytic platform. This makes the structure well-suited for more demanding catalytic environments. In catalytic reactions, this type of DAC features two distinct active sites, with one typically responsible for adsorption and the other acting as an electron modulator, thereby enhancing the catalytic selectivity (Fig. 3c). For instance, Zhao et al.⁵⁵ reported an isolated dual-atom Ni–Fe catalyst immobilized on carbon nitride, which exhibited a synergistic effect due to the long-term interaction between the two sites. This interaction mitigated the reaction barriers for the formation and desorption of the intermediates, thereby enhancing the electrocatalytic process. Similarly, Li et al.⁵⁶ established isolated Fe and Co sites on nitrogen-doped carbons, demonstrating an exceptional catalytic alkaline performance. However, despite the above-mentioned merits, the random anchoring of sites on the support surface endows an element of uncertainty in catalytic behavior, complicating the regulation and prediction of the catalytic pathways. Moreover, the precise anchoring of two coplanar, separated sites within the support plane is actually a challenging engineering task, requiring high synthetic precision to achieve materials with high stability and recyclability.

(4) Metal–nonmetal composed DACs

In contrast to bimetallic DACs, which consist of two metal atoms, non-metal DACs are composed of one metal and one non-metal atom. Although bimetallic DACs exhibit an enhance catalytic performance through synergistic metal interactions, non-metal DACs leverage the unique properties of non-metal atoms to more effectively modulate the geometric and electronic environment of the metal site. One key advantage of non-metal DACs over bimetallic DACs is their ability to fine-tune the metal electronic structure without the risk of atomic aggregation seen in pure metal systems.⁵⁷ The significant electronic difference between metal and non-metal atoms optimizes the reaction energy of the intermediates, reducing the energy barrier of the rate-determining steps and resulting in ultra-low overpotentials. This makes non-metal DACs ideal for electrochemical reactions such as oxygen reduction and hydrogen evolution. For example, Huang et al.⁵⁸ screened 11 high-performance electrocatalysts from a pool of 130 non-metal DACs, showing that non-metal atoms enhanced the charge communication with the metal sites and improved the catalytic performance. Similarly, Zhai et al.⁵⁷ incorporated Se into Fe–NC materials through ball milling, demonstrating that Se provided new active sites, modulated the charge distribution and spin state of Fe, and enhanced the catalytic activity. Replacing noble metals with non-metal atoms shows potential for addressing resource shortages, reducing costs, and expanding catalyst applications. However, the application of metal–nonmetal DACs is still mainly limited to the electrochemical field, indicating that further development is needed.

2.3.2 Categorization supporting materials of DACs. Supporting materials play a crucial role in dispersing, stabilizing, and tuning DACs by enhancing their dispersion, strengthening the catalyst–support bonds, and fine-tuning their properties through catalyst–support interactions. Thus, selecting the appropriate support is the key to optimizing the catalytic performance of single/dual-atom catalysts. Broadly, DAC supports can be classified into five types including carbon-based, metal oxide-based, transition metal dichalcogenides-based, MXene-based, and alloy-based supports⁵⁹ (Fig. 4). Each category has unique properties that significantly influence the structure and activity of DACs, which will be discussed in the following sections.

Fig. 4 Categorization of supporting materials for dual atom catalysts, including carbon-based, alloy-based, metal–oxide based, TMD-based and MXene-based supports.^60–64 Copyright 2023, the American Chemical Society; Copyright 2019, the American Chemical Society; Copyright 2024, Lin He et al.; Copyright 2024, Wiley-VCH; Copyright 2008, Springer Nature.

(1) Carbon-based supporting materials

Carbon-based supports can be classified into various forms, including graphite carbon nitride (e.g., g-C₃N₄), metal-containing carbon supports, porous carbon nanosheets, and carbon nanotubes.^65,66 These supports offer several key advantages for the development of DACs, as follows: ① carbon-based supports possess excellent electronic conductivity, enabling the efficient electronic structure optimization of metal atoms through metal–support interactions. ② Their high surface area and porosity offer ample mass transfer channels, while surface defects facilitate the anchoring of metal atoms, thereby ensuring structural stability and enhancing the reaction efficiency. ③ The incorporation of heteroatoms (such as nitrogen, sulfur, and phosphorus) on the support surface further modifies its properties, strengthening the interaction between the support and metal atoms. For instance, Xie et al.⁶⁷ constructed precise N-coordinated Ni and Fe dual atoms on N-doped carbon substrates ((Ni, Fe)–N–C) through the high-temperature oxygen-limited pyrolysis of a ZIF-8 (zeolitic imidazolate framework-8) derivative containing NiFe dual metal atoms, showing exceptional efficiency in the solar-driven reduction of CO₂ and H₂O into chemical fuels.

(2) Metal oxide-based supporting materials

Metal oxide-based supports are widely employed as single-atom catalyst supports, encompassing a diverse range of materials such as Al₂O₃, Fe₂O₃, TiO₂, and ZnO. Each metal oxide support possesses distinct characteristics and application scenarios.^68–70 Generally, metal oxide-based supports demonstrate remarkable potential in the absorption and stabilization of metal species through surface ligand bridging and surface defect stabilization.⁷¹ Moreover, anchoring single atoms on the defects of metal oxide supports can prevent the migration and aggregation of single metal atoms. For instance, TiO₂, widely used in various applications, is well-known for its exceptional photocatalytic activity. It can employ ultraviolet light excitation to generate electron–hole pairs, thereby promoting chemical reactions. Li et al.⁶⁰ converted a series of precious metals into single atoms on Al₂O₃ spontaneously by using bimetallic nanocrystals as a facilitator, in which the Pd₁/AlCo₂O₄–Al₂O₃ samples could be employed as high-quality catalysts for the oxidation of CO and propane. Similarly, Song et al.⁷² realized that the uniform dispersion of Cu and Zn elements on the surface of the Al₂O₃ support benefits from its high specific surface area and robust pore structure. Also, the Al₂O₃ support is conductive to improving the thermal stability of the catalyst in CO₂ hydrogenation to methanol.

(3) TMD-based supporting materials

Transition metal dichalcogenides (TMDs) are compounds with an MX₂ structure, where M is a transition metal (e.g., Mo and W) and X is a chalcogen (e.g., S and Se). TMDs exhibit various crystal phases, including 2H (semiconductor) and 1T/1T′ (semi-metallic), with distinct properties that make them effective in electrochemical catalysis and energy storage.⁷³ The 2D structure of TMDs offers high electron and hole mobilities, making them ideal for the fabrication of electronic devices. Their basal and edge sites provide opportunities for chemical modification and doping, enabling fine-tuning of their physicochemical characteristics. In atomic catalysis, supports create bonding sites that stabilize metal atoms, enhancing the electron flow and optimizing the active site adsorption capability. For instance, Zhou et al.⁷⁴ used 2D MoS₂ to support Pt and Ru atoms on basal vacancies. This catalyst exhibited an excellent catalysis performance by controlling the bonding structure and valency of Pt and Ru on metal vacancies. Furthermore, Liu et al.⁶¹ loaded single metal atoms (e.g., Mo and W) on 2D materials via low-temperature plasma treatment. Mo single-atom/single-layer MoS₂ showed a small Tafel slope and ultra-low overpotential, demonstrating excellent hydrogen adsorption/desorption kinetics for efficient water electrolysis.

(4) MXene-based supports

MXenes represent a novel class of 2D materials with a graphene-like structure, including transition metal carbides, carbonitrides, and nitrides. Their structural formula is M_n+1X_nT_x, where M denotes early transition metals (e.g., Sc, Ti, Zr, and V), X represents C or N, and T_x indicates surface functional groups. These emerging 2D materials inherit key properties such as electronic conductivity, energy storage, optical behavior, and mechanical strength, while also featuring versatile surface functional groups,^75,76 making them ideal for application in catalysis, energy storage, tribology, etc. The tunable surface groups of MXenes provide great potential for developing atomic catalysts with customized catalytic properties. For instance, Rogach et al.⁷⁷ introduced a surface-modification strategy by first adsorbing L-tryptophan on MXene, and then fabricating a CoNi–Ti₃C₂T_x electrocatalyst by establishing N–Co/Ni–O bonds. This improved Co–Ni adsorption on Ti₃C₂T_x supports enhanced the catalytic kinetics and stability, demonstrating the promise of MXene-based supports in durable and efficient catalysis.

(5) Alloy-based supporting materials

Alloys, composed of two or more metals or metals and non-metals, exhibit metallic properties and excellent thermal stability, making them suitable for application under harsh catalytic conditions. High-entropy alloys (HEAs), made of five or more principal elements in specific proportions, have recently gained attention in catalysis.^78,79 HEAs are characterized by high entropy, resulting in complex elemental arrangements. Compared to traditional alloys, HEAs generally offer superior phase stability, mechanical properties, and thermal resistance. The structural stability of HEAs makes them efficient substrates for single atoms, while their multi-element composition enables tuning of the geometric and electronic properties of single-atom sites, enhancing catalytic activity. For instance, Yao et al.⁸⁰ employed non-precious HEA (Fe, Co, Ni, Cu) metals to stabilize the noble Pt metal, creating a new HEA@Pt catalyst that significantly outperformed pure Pt in water splitting. Subsequently, Guo et al.⁶² reported a single-atom Mo-doped PdPtNiCuZn HEA nanosheet (Mo₁–PdPtNiCuZn), where Mo was incorporated under tensile strain, enhancing the catalytic performance for methanol oxidation by reducing the Pt–Pt aggregation at low Pt concentrations.

3. Strategies for the synthesis of DACs

3.1 High-temperature pyrolysis

High-temperature pyrolysis is a common method for synthesizing atomic catalysts, involving the reduction of metal precursors under oxygen-limited conditions at high temperatures. This promotes the dispersion of metal precursors on the support surface, creating atomic-level metal sites. Huang et al.⁴⁸ prepared a Co-HPS precursor using a double-solvent impregnation method. CoNP/HCS-900 was produced by thermally treating the Co-HPS precursor, and CoNP/N-HCS was obtained by adding melamine (Fig. 5a), which decomposed into nitrogen atoms that coordinate with Co to form a Co–N_x structure. This method enabled a transition from nanoparticle to single-atom to dual-atom catalysts, which could be extended to 22 different DACs. Zhang et al.⁸¹ prepared Ni₂NC by dissolving nickel(II) acetate tetrahydrate and citric acid in ethanol, immersing a carbon substrate, drying, grinding with dicyandiamide, and heating at 800 °C under an Ar stream.

Fig. 5 Common strategies for the synthesis of dual atom catalysts: (a) pyrolysis.⁴⁸ Copyright 2023, Xingkun Wang et al. (b) Wet chemical method.⁸² Copyright 2023, Xun Sun et al. (c) Atomic layer deposition.⁴⁴ Copyright 2022, Wiley-VCH. (d) Ball milling.⁸³ Copyright 2021, Wiley-VCH. (e) Co-precipitation.⁸⁴ Copyright 2018, Elsevier. (f) Microwave-assisted solvent thermal methods.⁸⁵ Copyright 2023, Elsevier. (g) Photo-induced synthesis.⁸⁶ Copyright 2024, Springer Nature.

Wet chemistry methods prepare precursors by immersing the support in a solution with the metal precursor, allowing its adsorption on the support surface. After removing the excess solution and post-treatment such as pyrolysis, ligands are eliminated, and DACs are formed. This method is simple and cost-effective but may lead to the formation of large metal particles and their poor dispersion, affecting the catalytic activity and stability. For instance, Zhang et al.⁸² anchored Fe–Co into the cavity of carbon spheres via a two-stage solvent soaking procedure. Dopamine hydrochloride self-polymerized on an SiO₂ template, adsorbing Co atoms, which were then dispersed in an n-hexane–FeNO₃ solution (Fig. 5b). After calcining at 900 °C in NH₃ for 2 h, Fe–Co DACs were formed. Further, Wang et al.⁸⁷ synthesized a Pt₂ dual-atom material using mpg-C₃N₄ via a wet-chemical method. Subsequently, mpg-C₃N₄ and the platinum dimer were mixed in DMF, rinsed, and heated in an N₂ environment at 300 °C to remove ligands, resulting in Pt₂/mpg-C₃N₄.

3.2 Atomic layer deposition

Atomic layer deposition (ALD) is a gas-phase technique for thin film growth, emerging as a method for synthesizing heterogeneous catalysts. Similar to chemical vapor deposition (CVD), ALD alternates precursor exposure to enable controlled molecular reactions on the substrate surface. By adjusting the cycles and precursor choice, ALD provides atomic-level control of the catalyst structures, offering a “bottom-up” approach for synthesizing heteronuclear (DACs). Sun et al.⁸⁸ prepared Ni–W dimer site-modified Pt nanoparticles with nearly fully exposed Pt surfaces using the ALD process. In this method, commercial PtC was used as the substrate, with bis(ethylcyclopentadienyl)nickel(II) and tungsten hexacarbonyl as precursors for depositing Ni and W. The synergistic Ni–W dimer structure enhanced the activity and stability of Pt electrocatalysts for ORR. Additionally, Lu et al.⁴⁴ performed one cycle of Pt ALD on C₃N₄ at 200 °C to create a Pt₁/C₃N₄ SAC. Subsequently, they fabricated a Pt₁Ni₁/C₃N₄ catalyst by alternatingly exposing NiCp₂ and O₃ at 200 °C for ALD on NiOx (Fig. 5c). To form the Pt₁ + Ni₁ heteronuclear dual-atom catalyst, ALD was carried out by sequentially exposing the C₃N₄ support to MeCpPtMe₃, NiCp₂, and O₃ at 200 °C, with O₃ removing the ligands from the MeCpPtMe₃ and NiCp₂ precursors.

3.3 Ball-milling

Mechanical ball milling is a high-energy technique that facilitates mechanochemical reactions between metals and substrates, such as oxides, carbon-based materials, and metal–organic frameworks. High-speed collisions and friction between the milling balls and materials generate sufficient energy to embed or anchor metal atoms onto the substrate surface or pores, creating atomic catalysts. This method is valued for its simplicity, environmental benefits, uniform metal dispersion, and enhanced catalyst activity and stability. Song et al.⁸³ used ball milling to fabricate S,N co-doped carbon (SNC)-supported Co single-atom catalysts for the homocoupling of primary amines to imines (Fig. 5d). Co powders and SNC material were combined in the milling process to obtain Co–SNC with a Co–N₄–S structure. This method provides a promising approach for designing dual-site single-atom catalysts with improved performance. Similarly, He et al.⁸⁹ fabricated Pd₁Ba₁/Al₂O₃ DACs using a two-step ball milling method. BaCl₂·2H₂O and Al₂O₃ were milled together, then calcined to prepare Ba₁/Al₂O₃ SACs. The mixture of Ba₁/Al₂O₃ SACs and Pd(acac)₂ was milled and calcined again to obtain Pd₁Ba₁/Al₂O₃ DACs, which showed an enhanced hydrogenation performance compared to traditional SACs.

3.4 Co-precipitation method

The co-precipitation method is a widely employed technique for synthesizing atomic catalysts. Its underlying principle involves using metal ions as precursors, which are combined with a precipitating agent to form a uniform dispersion of metal atoms in solution.⁹⁰ This method enables the preparation of various atomic catalysts by selecting different metal precursors and precipitating agents. Due to its simplicity and ease of control, it is well-suited for large-scale production. Yan et al.⁹¹ described the synthesis of Pd₁/Cs SMA catalysts by first mixing styrene-maleic anhydride copolymer (SMA) with deionized water and cooling it in an ice bath. Simultaneously, CsNO₃ and appropriate amounts of Pd(NO₃)₂ were dissolved in deionized water, and the obtained solution was added dropwise to an ice-cold SMA solution with stirring for aging. Finally, the Pd₁/Cs SMA catalyst was fabricated after centrifugal washing and freeze-drying. Moreover, Zhong et al.⁸⁴ utilized the co-precipitation method to prepare Ag/hollandite-type SACs (Fig. 5e). In this case, MnO₂ served as a unique support with a special channel structure, while MnSO₄, KMnO₄, and AgNO₃ were used as raw materials and catalyst precursors, respectively.

3.5 Microwave-assisted solvent thermal method

The microwave-assisted solvent thermal method involves the use of microwave radiation to heat metal precursor solutions. By controlling the metal ion concentration and adjusting the conditions, metal atoms are dispersed as single atoms on the catalyst support. The process includes dissolving precursors, heating, precipitating, dispersing atoms, and solid loading. This method offers rapid heating and uniform reactions, enabling precise control of the structure and performance of catalysts, optimizing catalytic reactions. Zhao et al.⁸⁵ developed a microwave-aided solvothermal sulfidation method to fabricate a Cu1N₃/Mo₁S₂/CN DAC. Cu single-atom sites were first anchored on CN by photo-deposition (Fig. 5f), and then Mo₁S₂ was added using the microwave method. The high-frequency alternating electric field promoted localized surface plasmon resonance, creating a super-hot spot around Cu, which facilitated the attachment of the second metal site. Additionally, Sun et al.⁹² synthesized a dual-atom Fe catalyst by self-assembling iron phthalocyanine into a face-to-face arrangement, and then irradiating with microwaves to form a nanorod structure. The Fe sites, 4.92 Å apart, enhanced the interaction with O atoms, promoting OH⁻ reduction and improving activity and selectivity.

3.6 Photonic deposition strategy

Photo-induced synthesis is a light-driven strategy based on a ‘navigation and positioning’ approach. Under light irradiation, specific metal sites (M1) accumulate photo-generated electrons, which position a second metal ion (M2) around M1, enabling the precise synthesis of dual atoms. This method allows the fabrication of DACs with high atomic efficiency and synergistic catalytic effects, offering enhanced catalytic performances. For instance, Lu et al.⁸⁶ reported an NPS for the scalable synthesis of heteronuclear DACs on polymeric carbon nitride (PCN). Zn–PCN SACs were first synthesized by calcining urea and Zn(OOCCH₃)₂, creating Zn nucleation sites (Fig. 5g). These sites captured Ru^III under photoexcitation, forming a heteronuclear ZnRu–PCN DAC. Liu et al.⁹³ employed a photo-induced neighbor-deposition strategy to synthesize an Ir₁Pd₁–In₂O₃ DAC. Initially, an Ir₁–In₂O₃ SAC was prepared via the co-precipitation method, and Pd atoms were selectively deposited around the Ir sites using photo-deposition. The resulting Ir₁Pd₁–In₂O₃ DAC exhibited significantly enhanced methanol selectivity, benefiting from the synergistic effect between dual atoms.

4. Techniques for the characterization of DACs

4.1 Super-resolution imaging technology

Super-resolution imaging techniques surpass the diffraction limit of optical microscopy, enabling nanoscale or sub-nanometer observation of DAC structures.⁹⁴ This allows direct visualization of the atomic distribution, single and dual atom arrangements, and catalytic changes without sample damage. Transmission electron microscopy (TEM), leveraging electron wave-particle duality, images sample interiors.⁹⁵ Although it does not directly reveal the atomic distribution, it rules out the presence of non-atomic structures such as alloys, clusters, and nanoparticles. Annular dark-field TEM (ADF-TEM) captures scattered electrons via a dark-field detector, highlighting crystal structures, lattice defects, and atomically dispersed sites.⁹⁶ It produces high-contrast images, emphasizing lattice positions. High-angle annular dark-field scanning TEM (HAADF-STEM) achieves sub-angstrom resolution, directly imaging atomic structures using electron probes and aberration correction,³⁶ making it essential for the characterization of DACs. However, despite their effectiveness, these techniques struggle to distinguish atom types and chemical environments. Coupling them with energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) enables detailed elemental analysis. For instance, Pan et al.⁹⁷ used TEM and HRTEM to confirm the absence of clusters or nanoparticles in MoCo DAC/C. HAADF-STEM revealed bright dual-atom spots on the carbon support (Fig. 6a), validating the synthesis of the catalyst. Moreover, Zhang et al.⁹⁸ employed HAADF-STEM to observe the Fe and Mn distribution, confirming the presence of metal atom pairs with bright dual spots on the carbon layer. EELS further verified the presence of Fe and Mn in Fe, Mn/N–C, with a 0.25 ± 0.02 nm inter-metal distance. Parkinson et al.⁹⁹ reported constant-current STM images of Pt₂/Fe₃O₄ at 78 K, showing two Pt atoms as a double protrusion between Fe₃O₄ rows (Fig. 6b). Constant-height ncAFM images also identified bright protrusions above each Pt₂ dimer.

Fig. 6 Characterization of DACs: super-resolution imaging (a and b).^97,99 Copyright 2024, the American Chemistry Society; Copyright 2022, Matthias Meier et al. X-ray absorption fine structure (c and d).¹⁰⁰ Copyright 2023, Shuo Zhang et al. Mössbauer spectra (e and f).^101,102 Copyright 2022, Wiley-VCH; Copyright 2023, Wiley-VCH.

4.2 X-ray absorption fine structure

X-ray absorption spectroscopy (XAS) is a versatile technique used to probe the atomic structure and electronic properties of DACs. It consists of X-ray absorption fine structure (EXAFS) and X-ray near edge structure (XANES) spectroscopy, both critical for analyzing coordination environments, oxidation states, and electronic structures. EXAFS specializes in local atomic arrangements and interatomic distances, while XANES reveals oxidation states and electronic structures of the central atoms.¹⁰³ These features make XAS indispensable for studying DACs, enabling direct, in situ examination of catalytic environments without long-range order. XAS is vital for understanding the active sites in DACs, aiding catalyst design. For instance, Hu et al.¹⁰⁴ carried out XANES and EXAFS to study the coordination environment of Fe and Co in an FeCo–NCH catalyst. The oxidation states of Fe and Co were close to +3 and +2, with Fe–N and Co–N coordination numbers of 5 and 4, confirming the presence of atomically dispersed sites, respectively. In contrast, Lu et al.¹⁰⁰ reported Fe–Cu bonding in Fe/Cu–nitrogen-doped graphene. The FT-EXAFS curves showed peaks for Fe–N (1.97 Å) and Cu–N (2.05 Å) coordination, and a second peak for Fe–Cu (2.25 Å) (Fig. 6c). Shahbazian-Yassar et al.¹⁰⁵ conducted XAS and DFT calculations to analyze Cu₂@C₃N₄, revealing Cu–O–Cu bridging with bond lengths of 1.76 and 1.79 Å, and bonding angles of 82° for Cu_α and 110° for Cu_β.

4.3 Mössbauer spectroscopy

Mössbauer spectroscopy, based on the Mössbauer effect and Doppler shift, is an effective technique for studying hyperfine interactions through gamma-ray absorption spectra.¹⁰⁶ It is essential for analyzing isotopic systems such as ⁵⁷Fe, ¹¹⁹Sn, and ¹⁵¹Eu, providing insights into the oxidation states, coordination, electron density, and magnetic properties, making it ideal for characterizing atomically dispersed catalytic sites.¹⁰⁷ Mössbauer spectra offer valuable information on the electronic and structural properties of materials at the atomic scale, particularly for bimetallic sites and DACs. For example, Chen et al.¹⁰⁸ performed ⁵⁷Fe Mössbauer spectroscopy, HAADF-STEM, and XAS to analyze Fe–Te DASs/NCFs. The Mössbauer spectrum showed a doublet corresponding to square-planar FeII–N coordination and a minor singlet from Fe–Te bonds, confirming the presence of Fe and Te dual metal sites. Shi et al.¹⁰¹ utilized ⁵⁷Fe Mössbauer spectroscopy to examine the FeN₄ sites in Mn-cooperated catalysts (Fig. 6e). The absence of γ-Fe and Fe₃C/α-Fe confirmed the absence of iron crystal formation, while the incorporation of Mn increased the Fe^IIN₄-low spin site density, as indicated by the increase in the doublet D1 area (50% → 75.6%).

Besides ⁵⁷Fe Mössbauer spectroscopy, Abdel-Mageed et al.¹⁰² conducted ¹⁹⁷Au Mössbauer spectroscopy to demonstrate the formation of CuAu dual metal sites. Unlike Au foil, the singlet with positive isomer shifts (3.89, 3.63, and 3.62 mm s⁻¹) in Cu₉₃Au₇/ZnO catalysts excluded the presence of monometallic Au nanoparticles, supporting the formation of CuAu nanoalloys (Fig. 6f). Besides iron and gold, ¹¹⁹Sn Mössbauer spectroscopy is valuable for studying atomically dispersed tin sites, revealing information on chemical bonds, oxidation states, spin configurations, symmetry, and local magnetic fields. However, although Mössbauer spectroscopy is powerful for studying DACs, its use is limited by the narrow range of detectable metal elements, presenting a technical challenge.

4.4 In situ characterization techniques

In situ characterization techniques, such as FTIR, XAS, and Raman spectroscopy, enable the monitoring of structural and electronic changes in catalysts during reactions without sample degradation. These methods preserve the catalyst integrity, preventing alterations or activity loss.^109–112 Unlike traditional approaches, in situ techniques provide real-time insights into the catalytic mechanisms. Advances in TEM have revolutionized nanoscale characterization, enabling the direct observation of catalyst properties under the reaction conditions.¹¹³ Conventional techniques, limited to pre- or post-reaction analyses, provide indirect insights into structural transformations.¹¹⁴ Alternatively, in situ TEM captures real-time chemical dynamics at the atomic scale with high spatial, energy, and temporal resolution. Combined with environmental TEM (ETEM), it enables precise monitoring of catalytic site transformations. For example, Tilley et al.¹¹⁵ used in situ ETEM to observe the transformation of Pt from clusters to single atoms under hydrogen. Pt particles diffused on the surface of Ru, shrinking from 5.8 nm to 3.8 nm and forming single-atom sites (Fig. 7a), which reduced the surface energy of Ru and favored Pt–Ru bond formation. In situ TEM is crucial for studying the surface composition, active sites, and reaction mechanisms in DACs.

Fig. 7 In situ characterization: (a) TEM.¹¹⁵ Copyright 2022, Agus R. Poerwoprajitno et al., under exclusive licence to Springer Nature. (b) TOF-SIMS.¹¹⁶ Copyright 2018, Shubo Tian et al. (c) XRD.⁹⁶ Copyright 2022, Jiangwei Chang et al., under exclusive licence to Springer Nature. (d) DRIFTS.¹¹⁷ Copyright 2022, Wiley-VCH. (e) XAS techniques.¹¹⁸ Copyright 2023, Kanglei Pang et al., Elsevier.

In situ X-ray diffraction (XRD) is a powerful tool for studying structural and phase transformations in metals and polymers, providing insights into their crystal structure, lattice parameters, and defects.¹¹⁹ For instance, Guo et al.¹²⁰ utilized in situ XRD to track the phase evolution in Ti–BPDC–Pt at 400 °C, showing no TiO₂ or Pt peaks before pyrolysis but distinct peaks after 8 h. The decrease in peak intensity at 3 h indicated the collapse of the framework. Similarly, Lu et al.⁹⁶ observed M–N–C site formation during the synthesis of a UHDM–N–C SAC (Fig. 7c), capturing urea polymerization, g-C₃N₄ conversion, and metal sulfide decomposition. At 700 °C, g-C₃N₄ decomposed into N-doped carbon, stabilizing metal atoms as single-atom sites.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is valuable for investigating the molecular vibrations on the surface of DACs during catalytic reactions. It captures real-time chemical and structural transformations, which are essential for studying reaction mechanisms. By correlating infrared absorption with molecular vibrations, it provides detailed species composition and structural information. For instance, Zhang et al.¹¹⁷ used DRIFTS to study Pt–Pd dual-atom sites in NO and CO adsorption. Pt–Pd SAC-2 showed distinct peaks at 2177, 2123, and 2070 cm⁻¹, indicating a dual-site structure and confirming the dominant role of Pd in NO adsorption (Fig. 7d).

In situ X-ray absorption spectroscopy (XAS) effectively studies atomic-scale dynamic changes. For instance, Yuan et al.¹¹⁸ employed in situ XAS to examine selenium (Se) catalytic sites during HzOR, finding that the X-ray edge shift (ΔE = 1.4 eV) indicated the stability of the Se site under the reaction conditions (Fig. 7e). Combining in situ XAS with theoretical calculations, Yao et al.¹²¹ revealed how Se polarizes charge around Cu–N4, enhancing the ORR activity under alkaline conditions. However, despite its high energy resolution and surface sensitivity, in situ XAS has long data collection times, limiting real-time tracking during fast reactions. By dynamically monitoring processes and identifying surface species, in situ techniques provide critical insights into catalytic mechanisms, aiding the design and optimization of DACs.

4.5 Other advanced techniques

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a highly sensitive surface analysis technique that can be employed to characterize chemical composition, elemental distribution, and surface morphology. In TOF-SIMS, a primary ion beam bombards the sample surface, ejecting secondary ions that are analyzed based on their mass-to-charge ratio (m/z). This method provides high spatial resolution for studying the surface and near-surface properties of materials. For example, Wang et al.¹¹⁶ employed TOF-SIMS alongside HAAD-STEM and XAS to analyze Fe₂/mesoporous carbon nitride-C₃N₄ (Fe₂/mpg-C₃N₄). The TOF-SIMS spectrum revealed a fragment signal at m/z 111.88, corresponding to Fe^II from the precursor, confirming the presence of highly dispersed dual Fe–Fe moieties in Fe₂/mpg-C₃N₄ (Fig. 7b).

Low-temperature Fourier transform infrared spectroscopy (LT-FTIR) involves infrared spectroscopy at low temperatures, which suppresses thermal reactions and kinetic effects, enhancing the observation of absorption peaks that reveal structural and dynamic properties. LT-FTIR is particularly effective for studying phase transitions and adsorption. Xiong et al.¹²² conducted LT-FTIR to compare Fe₂–N–C and Fe₁–N–C during O₂ adsorption. The spectra of Fe₂–N–C, dominated by peroxo-like adsorption, were markedly different from the superoxo-like vibrations of Fe₁–N–C, indirectly verifying the increased coordination and shorter atomic distances in Fe₂ compared to the atomically dispersed Fe₁.

X-ray photoelectron spectroscopy (XPS) is widely used to investigate the oxidation states and atomic environments in catalysts. Pérez-Ramírez et al.¹²³ synthesized Pd-based catalysts with varying nuclearity (Pd_x, x = 1, 2, 3) on exfoliated carbon nitride (ECN) and performed XPS to examine the oxidation states of Pd. Two positively charged Pd species were identified, Pd^IV (338.5 eV) and Pd^II (336.9 eV), with the Pd^II : Pd^IV ratio increasing from 0.56 (Pd₁/ECN) to 0.93 (Pd₃/ECN), and no bulk Pd signal at 335.0 eV. This confirmed the atomic dispersion of Pd species on the carbon nitride support and the formation of Pd-based single-atom catalysts. Born–Oppenheimer molecular dynamics simulations further confirmed these XPS shifts, supporting the distinct coordination environments of the Pd atoms. Although these techniques do not directly reveal the bimetallic structure, they provide strong indirect evidence for the existence of bimetallic sites, offering valuable insights into the atomic-level structure and catalytic properties of DACs.

5. Application and mechanism of DACs in AOPs

To address the depletion of energy sources and environmental damage, developing clean, sustainable technologies for energy conversion and pollutant removal is essential. DACs have gained significant attention for their superior structural and catalytic properties. In advanced oxidation processes, they excel in reactions such as ozone oxidation, Fenton-like processes, photocatalysis, electrochemical oxidation, sonocatalysis, and enzyme-like catalytic oxidation^50,124,125 (Fig. 8). Their enhanced catalytic performance results from the synergistic interactions between their metal atoms and strong bonding with the support material, improving the efficiency and opening new avenues for catalyst design and mechanism exploration.¹²⁶ This section reviews the recent advances in DACs for advanced oxidation reactions, highlighting their unique advantages, scientific significance, and crucial role in advancing catalytic oxidation technologies.

Fig. 8 Classifications of AOPs: ozone oxidation.¹²⁷ Copyright 2024, Wei Qu et al. Published by PNAS. Fenton-like catalysis.²¹ Copyright 2024, Ran Zhao et al., Elsevier. Electrocatalysis.¹²⁸ Copyright 2025, Wiley-VCH. Photocatalysis.¹² Copyright 2025, the American Chemical Society. Piezocatalysis.¹²⁹ Copyright 2024, the American Chemical Society. Biomimetic catalysis.¹³⁰ Copyright 2024, Wiley-VCH.

5.1 Application of DACs in AOPs

5.1.1 Overview of AOPs: classification and principles. AOPs are advanced systems that utilize highly reactive species to efficiently mineralize and eliminate organic pollutants. With rapid reaction kinetics and high oxidation efficiency, AOPs minimize secondary pollution, making them a promising alternative for environmental remediation.¹³¹ Besides, AOPs show significant potential in energy conversion, pollutant transformation, and chemical synthesis. In energy conversion, AOPs aid in breaking down complex organic molecules, improving the efficiency of biofuel production. In chemical synthesis, they facilitate oxidation reactions, enabling the production of high-value oxidized compounds and other valuable chemicals with enhanced selectivity and efficiency.

(1) Ozone oxidation

Catalytic ozonation, a key reaction in AOPs, is a promising method for treating persistent organic pollutants. Ozone, a strong oxidizer with a “V” shape and an O–O bond length of about 1.27 Å (Fig. 9a), drives this process. Heterogeneous catalytic ozonation occurs via two main pathways,¹³² as follows: ① direct pathway: ozone molecules oxidize pollutants directly, owing to the high oxidation potential of ozone (2.07 eV), through electrophilic nucleophilic substitution and dipolar addition. Substitution occurs with organic pollutants containing –OH, –CH₃, or –NH₂ groups on an aromatic ring, which increases the electron density on the ortho and para carbons, facilitating their interaction with ozone (Fig. 9b). Dipolar addition happens due to the dipole moment of ozone (0.55 D), enabling it to react with unsaturated organic molecules.¹³³ ② Indirect pathway: ozone is activated to generate ROSs such as ˙OH and HO₃˙, which degrade pollutants via radical chain reactions. The direct pathway, although more selective, is slower and less complete. The low solubility of ozone in water and its instability upon exposure to heat or light further hinder its efficiency. DACs, with high metal utilization, excellent catalytic activity, and stability, show great promise for ozone activation in pollutant removal. DACs enhance catalytic ozonation by promoting the adsorption and activation of ozone and selective pollutant degradation. Their dual active sites lower the decomposition energy via orbital coupling, while accelerating ROS generation through valence cycling and Lewis site enrichment. For instance, Wang et al.¹³⁴ showed that an Mn–Ce DAC promoted ozone activation, generating ˙OH, O₂˙⁻, ¹O₂, and *O_ad, enhancing wastewater treatment efficiency (Fig. 9c). This synergy optimizes pollutant degradation, leading to complete mineralization and removal. The chemical equations for indirect heterogeneous catalytic ozonation are as follows:

Mⁿ⁺ + O₃ + H₂O → Mⁿ⁺¹ + ˙OH + O₂ + OH (1)

Mⁿ⁺ + O₃ + H⁺ → Mⁿ⁺¹ + HO₃˙ (2)

HO₃˙ → ˙OH + O₂ (3)

O₃ + OH⁻ → HO₂⁻ + O₂ (4)

HO₂⁻ + H⁺ → H₂O₂ (5)

HO₂⁻ + O₃ → O₃˙⁻ + HO₂˙ (6)

HO₂˙ → O₂˙⁻ + H⁺ (7)

O₂˙⁻ + ˙OH → ¹O₂ + OH⁻ (8)

Fig. 9 Overview of the classification and principles of different AOP reactions: (a–c) ozone oxidation.^134–136 Copyright 2024, the American Chemical Society; Copyright 2016, the American Chemical Society; Copyright 2020, Elsevier. (d–f) Photo-/electro-/piezo-catalysis.^124,137 Copyright 2024, Lili Zhang et al. and Copyright 2021, Yanfeng Wang et al. (g and h) Fenton-like reaction.¹³⁸ Copyright 2024, the American Chemical Society. (i) Biomimetic catalysis.¹³⁰ Copyright 2024, Wiley-VCH.

Future research on DACs in catalytic ozonation should focus on optimizing their role in ozone activation and ROS generation. Refining the coordination environment and dual-site synergy can enhance ozone decomposition and pollutant removal. Mechanistic insights from in situ spectroscopy and computational modeling will be key to improving the reaction efficiency and selectivity. Additionally, ensuring long-term stability in ozone-rich environments and developing scalable synthesis methods are vital for large-scale environmental applications.

(2) Fenton/Fenton-like reaction

In 1894, H. J. H. Fenton proposed the Fenton reaction, wherein iron and hydrogen peroxide react to produce highly reactive hydroxyl radicals.¹³⁹ The classical Fenton reaction can be described as follows:

Fe^II + H₂O₂ → Fe^III + HO˙ + OH⁻ (k = 63–76 M⁻¹ s⁻¹) (9)

Fe^III + H₂O₂ → Fe^II + HO₂˙ + H⁺ (k = 0.001–0.01 M⁻¹ s⁻¹) (10)

Fe^II + H₂O₂ → Fe^IVO²⁺ + H₂O (11)

Traditional homogeneous Fenton reactions require strict pH conditions (optimal pH < 4) and often produce insoluble iron sludge due to the formation of Fe^III, which reacts with substances in water. Additionally, this process suffers from high hydrogen peroxide consumption and inefficient Fe^III/Fe^II cycling. Recently, heterogeneous Fenton-like reactions have gained popularity in environmental remediation due to their ability to overcome these limitations.^21,140 Their advantages include broader pH range, reduced oxidant consumption, recoverable solid catalysts, and fewer by-products. Common oxidants in Fenton-like reactions include H₂O₂, persulfate (PMS: peroxymonosulfate, PDS: peroxydisulfide), HOCl, periodate (PI), and peracetic acid (PAA). The key physicochemical parameters of these oxidants including redox potentials, O–O bond lengths, pK_a values, and lifetimes are systematically summarized in Table 1.^141,142 These oxidants generate ROSs by breaking O–O bonds, providing similar oxidative properties for organic pollutant degradation. The characteristics of these oxidants and ROSs, including reduction potentials, O–O bond dissociation energies, pK_a values, and ROS redox potentials, lifetimes, pK_a, and diffusion coefficients, are outlined in Table 2. Due to their unique dual-metal active centers, DACs activate oxidants to generate ROSs through the synergistic interaction between adjacent metal atoms, enhancing the catalytic efficiency. Additionally, the tunable geometric and electronic structures of DACs allow the directional regulation of ROSs in Fenton-like systems, improving the adaptability in treating complex environmental pollutants. For instance, Wang et al.¹⁴³ demonstrated that compared to Fe–SAC (60.5% non-radical, 30% ˙OH, and 9.5% SO₄˙⁻) and Co–SAC (51% non-radical, 42% ˙OH, and 7% SO₄˙⁻), the Fe–Co DAC achieved nearly 100% electron transfer efficiency. This improvement significantly accelerated the removal of bisphenol A, increasing the reaction rate by 5 to 10 times. Future research should optimize dual-metal sites and electronic structures for better ROS generation. Expanding their role in activating persulfate and peracetic acid will enhance their applicability. In situ characterization can refine the reaction conditions, while improving stability and reusability is key for industrial use.

Mⁿ⁺ + H₂O₂ → Mⁿ⁺¹ + ˙OH (12)

Mⁿ⁺ + HSO₅⁻ → Mⁿ⁺¹ + OH⁻ + SO₄˙⁻ (13)

Mⁿ⁺ + HSO₅⁻ → Mⁿ⁺¹ + OH⁻ + SO₅˙⁻ (14)

Mⁿ⁺ + S₂O₈⁻ → Mⁿ⁺¹ + SO₄˙⁻ + SO₄²⁻ (15)

Mⁿ⁺ + CH₃C(O)OOH → CH₃C(O)O˙ + OH⁻ + Mⁿ⁺¹ (16)

Mⁿ⁺¹ + CH₃C(O)OOH → CH₃C(O)OO˙ + H⁺ + Mⁿ (17)

Mⁿ⁺ + IO₄⁻ + 2H⁺ → IO₃˙ + H₂O + Mⁿ⁺¹ (18)

Mⁿ⁺ + IO₄⁻ + H₂O → IO₃⁻ + O₂˙⁻ + Mⁿ⁺¹ (19)

Table 1 The standard reduction potentials, O–O bond dissociation energies, pK_a, and possible lifetime in ground water of common oxidants^144–146

Properties	H2O2	PMS	PDS	O3	PAA
Standard potential (E0)	1.80 VNHE	1.82 VNHE	2.08 VNHE	1.24–2.08 VNHE	1.96 VNHE
O–O bond dissociation energy	213 kJ mol^−1	377 kJ mol^−1	92 kJ mol^−1	364 kJ mol^−1	159 kJ mol^−1
pKa	10.5, 20	9.3	−3.5	—	8.2
Lifetime	Hours to days	Hours to days	>5 months	<1 h	2–12 days

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Table 2 The redox potentials, lifetimes, pK_a, and diffusion coefficients of ROSs^147,148

Reactive species	Redox potential (eV)	Lifetime (s)	pKa	Diffusion coefficient (cm^2 s^−1)
˙OH	E0(˙OH/OH^−) = 2.5–3.1	1.0 × 10^−9	11.9	2.3 × 10^−5
SO4˙^−	E0(SO4˙^−/SO4^2−) = 1.8–2.7	(3–4) × 10^−5	<0	1.49 × 10^−5
^1O2	E0(^1O2/O2˙^−) = 0.81	2.0 × 10^−6	N.A.	2.61 × 10^−5
Fe^IV=O	E0(Fe^IV/Fe^III) = 1.8–2.0	N.A.	N.A.	N.A.

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(3) Photocatalysis

Photocatalysis is advanced oxidation technology that uses a photocatalyst to generate highly oxidative reactive species (such as ˙OH and O₂˙⁻) under light irradiation, effectively degrading organic pollutants. First observed by Honda and Fujishima in the 1970s,¹⁴⁹ photocatalysis occurs when a photocatalyst absorbs photons with energy equal to or greater than its bandgap, causing electrons to move from its valence band to the conduction band, creating electron–hole pairs (e⁻/h⁺). The holes (h⁺) directly oxidize pollutants or react with water to form ˙OH radicals, which are powerful oxidants. The electrons (e⁻) react with dissolved oxygen, producing superoxide O₂˙⁻ radicals, also strong oxidants.¹⁵⁰ The photocatalyst band structure governs its ability to absorb specific wavelengths and induce electron excitation (Fig. 9d). Notably, DACs can act as photocatalysts owing to their high oxidation efficiency, and light can also facilitate the synthesis of DACs. DACs significantly enhance the performance of photocatalysis by offering high oxidation efficiency through precise control of their electronic structure. By positioning dual metal sites at the nanoscale, DACs facilitate enhanced electron–hole pair generation and efficient charge separation under light irradiation, improving the generation of ROS. For instance, Lu et al.⁸⁶ proposed a “navigation and positioning” strategy for the precise synthesis of heteronuclear M₁M₂ DACs on PCN. The M₁–PCN nucleation site, created by calcining urea and M₁ metal salts, allows photoexcited electrons to position the second metal ion (M₂) near M₁ under light irradiation, enabling the precise synthesis of DAC. The future of DACs in photocatalysis depends on designing light-responsive catalysts with optimized dual-metal sites for targeted pollutant degradation. Further research should focus on advanced synthesis methods for precise metal positioning and electronic tuning. Exploring the dual-metal synergy under different light conditions can further enhance the activity, stability, and scalability of DACs for environmental and energy applications.

(4) Electrocatalysis

Electrocatalytic oxidation is an advanced technique using electrochemical methods to drive catalytic oxidation through electrode action in an electric field. Following Faraday's law, the current magnitude is related to substance conversion in redox reactions (n = Q/F, where n is the moles of converted substance, Q is charge, and F is Faraday's constant, 96 485 C mol⁻¹).¹⁵¹ The mechanisms involved in electrocatalytic oxidation include electron transfer, reactant adsorption/desorption, and reaction pathway selectivity, influencing the efficiency and product formation.¹⁵² Electrocatalytic oxidation can be direct or indirect. ① Direct oxidation occurs when pollutants are oxidized on the electrode surface via electron transfer, with no intermediates. ② Indirect oxidation generates strong oxidative intermediates at high anode potentials (≥2.0 V vs. SHE), which oxidize pollutants.¹⁵³ Electrochemical Baeyer–Villiger oxidation, for instance, produces 99% caprolactone, overcoming the limitations of water as an oxygen source.¹⁵⁴ Electrochemical oxidation treats organic wastewater, ammonia nitrogen, and cyanides, generating hydroxyl radicals¹⁵⁵ to degrade pollutants into non-toxic substances such as CO₂ and carbonates. Ammonia nitrogen removal involves its direct oxidation to nitrogen gas or indirect electro-oxidation, where oxidative species degrade ammonia.¹⁵⁶ Multi-single-atom structured catalysts, such as dual-atom site catalysts, show promise in electrochemical oxidation. In electrocatalytic processes, the dual-metal coordination within DACs facilitates the precise control of the reaction pathways, enabling the selective oxidation of ammonia nitrogen, with high faradaic efficiency and selectivity. The synergistic interaction between the adjacent metal atoms in DACs significantly improves the catalyst stability and reaction kinetics, making them highly effective for a range of electrochemical oxidation processes, from pollutant degradation to energy conversion. For instance, Zhang et al.¹⁵⁷ developed an NiCu dual-atom site catalyst that efficiently oxidizes ammonia nitrogen in wastewater, converting it to N₂. The rapid charge transfer and mass diffusion provide excellent stability, ideal for direct electrochemical oxidation. The Ni and Cu electronic coordination enhances N–N bond formation in hydrazine-like intermediates, enabling selective oxidation to N₂, with 97.87% N₂ selectivity and 86.6% faradaic efficiency. Future research on DACs in electrocatalysis should optimize dual-metal configurations for enhanced charge transfer and reaction selectivity. Tailoring their electronic and geometric properties can improve their activity for specific reactions, such as nitrogen removal. Understanding atomic-level interactions will be crucial for boosting the stability and efficiency of DACs, advancing robust electrocatalysts for large-scale environmental and energy applications.

(5) Piezocatalysis

Piezoelectric catalysis has attracted growing attention for environmental remediation and energy conversion. In this process, ultrasound is used to induce the piezoelectric effect.¹⁵⁸ Ultrasonic oxidation relies on the cavitation generated by ultrasonic waves in a liquid, producing reactive species for catalytic oxidation. As ultrasonic waves propagate, tiny bubbles form and collapse, generating high temperatures and pressures (thousands of degrees Celsius, hundreds of atmospheres), which induce the generation of reactive species¹⁵⁹ (Fig. 9f). Although effective for organic oxidation, the efficiency of ultrasonic oxidation is limited by its low ROS production. Thus, its combination with other oxidation technologies enhances its performance. DACs integrated into piezoelectric materials optimize the charge distribution and electron transfer, boosting the generation of ROS, such as ˙OH and ¹O₂. This synergy enhances the catalytic oxidation, while the tunability of the electronic properties of DACs enables efficient piezoelectric charge activation, improving the reaction efficiency and selectivity. Fang et al.¹⁶⁰ demonstrated that coupling electrochemical treatment with ultrasonic radiation significantly improved the mass transfer and radical generation, achieving 100% removal of PFOA and 63.5% defluorination, outperforming single-treatment systems. Ultrasound-based piezoelectric catalysis has shown potential in water treatment, environmental remediation, and organic synthesis. When ultrasound is applied to piezoelectric materials, it generates piezoelectric charges that create a potential difference and electric field on the catalyst surface. This enhances the reactant adsorption, increases the activation energy, and accelerates the reaction rate. The electric field also promotes electron transfer from the catalyst to the reactants, improving the selectivity. For instance, Wang et al.¹⁶¹ synthesized piezoelectric g-C₃N₄ nanosheets (Au–Fe–g-C₃N₄) that under ultrasonic activation, enabled self-sustained H₂O₂-mediated Fenton-like cascade activation. Under hypoxic conditions, the piezoelectric effect generates H₂O₂ from H₂O. The Au sites reduced the bandgap of the material, enhancing piezoelectric charge activation to produce ¹O₂. The conduction band electrons also promoted Fe site cycling, generating more ˙OH. This process ensures the effective separation of electrons and holes, improving the reaction kinetics and providing a platform for new nanosystems with synergistic Fenton dynamics. The piezocatalytic process is summarized as follows:¹⁶²

(1) Piezo generation of electron and holes:

(2) Production of ROSs:

e⁻ + O₂ → O₂˙⁻ (21)

h⁺ + H₂O → ˙OH (22)

4˙OH → ¹O₂ + 2H₂O (23)

˙O₂⁻ + ˙OH → ¹O₂ + OH⁻ (24)

(3) Removal of organic compounds and heavy metals:

ROSs + pollutants → intermediate molecules/CO₂ + H₂O (25)

Metal^m+ + ne⁻ → Metal^m−n (26)

Further exploration of the relationship between the geometric configuration of the dual-metal centers and their interaction with the electric field generated by ultrasonic waves can provide valuable insights into optimizing the reaction kinetics and selectivity. Additionally, the development of DACs that can operate efficiently under varying ultrasonic frequencies and intensities, while maintaining their stability and catalytic performance, will be a major step toward scaling up piezoelectric catalytic systems for real-world applications.

(6) Biomimetic catalysis

Artificial enzymes, especially advanced types, are crucial in biomimetic catalysis, addressing the limitations of natural enzymes, such as their instability and sensitivity to temperature, pH, and chemicals. Nanozymes, a class of artificial nanomaterials, mimic the activity of natural enzymes. Through biomimetic design, they use the unique surface structures and electronic properties of nanomaterials to replicate the active sites of natural enzymes, enabling specific chemical reactions.¹⁶³ Nanozyme technology offers high efficiency, stability, and cost-effectiveness, making it suitable for biocatalysis, environmental remediation, and medical diagnostics. The common types of nanozymes include peroxidase-like, oxidase-like, and superoxide dismutase (SOD)-like nanozymes. Peroxidase-like nanozymes catalyze reactions such as the decomposition of H₂O₂ to generate ˙OH, which then participates in oxidation reactions. They are widely used in environmental pollutant degradation and biological detection. Oxidase-like nanozymes use oxygen to oxidize organic compounds and are commonly used in biosensors for the detection of oxygen. SOD-like nanozymes catalyze the disproportionation of O₂˙⁻ to produce oxygen and hydrogen peroxide, which are crucial for antioxidant therapy and free radical clearance. Through the precise design of dual-metal active sites, DACs can replicate the function of natural enzymes, overcoming limitations such as instability and sensitivity to environmental factors such as temperature and pH. DACs are particularly useful in enhancing the functions of nanozymes, such as peroxidase-like, oxidase-like, and superoxide dismutase (SOD)-like activities, enabling advanced applications in environmental remediation, biomedical diagnostics, and therapeutic strategies. For instance, Lin et al.¹³⁰ developed an MoCu DAzyme, a multi-enzyme mimic with multiple catalytic sites, simulating natural peroxidases (POD), oxidases (OXD), and glutathione oxidases (GSHOx) (Fig. 9i). This design enhances ROS production and accumulation, improving ROS-mediated catalytic reactions. By optimizing the electronic structure and coordination environment of DACs, it is possible to increase their catalytic efficiency and stability under various operating conditions. Additionally, exploring the cooperative effects between metal centers in DACs can lead to new classes of multifunctional biomimetic catalysts with applications ranging from antioxidant therapies to pollutant degradation, further expanding their utility in biocatalysis.

5.1.2 Contribution of DACs in AOP applications. (1) Organic pollutant conversion and sterilization

AOPs have demonstrated impressive oxidative efficiency in degrading water and soil pollutants, primarily through the generation of highly reactive ROSs, which rapidly break down organic pollutants. ROSs swiftly disrupt the molecular structures of these pollutants (Fig. 9h), transforming them into harmless intermediates or completely mineralizing them into carbon dioxide and water.^141,164,165 Research shows that AOPs can achieve removal rates over 90%, particularly for high-concentration pollutants such as pharmaceutical residues and dyes. In the case of antibiotics, AOPs not only eliminate drugs but also suppress the spread of resistance genes, significantly impacting public health. AOP research is expanding to tackle emerging pollutants, such as microplastics and antibiotics, and their resistance genes.¹⁴³ These contaminants challenge traditional methods, but AOPs show great potential for environmental remediation. In AOPs, pollutants undergo mineralization, and ROSs can also trigger coupling or polymerization reactions, especially with aromatic compounds or complex organic molecules. For example, ROSs, particularly hydroxyl radicals react with aromatic rings, causing ring cleavage or formation of reactive intermediates such as phenols and quinones. These intermediates are highly reactive and prone to self-polymerization, forming dimers or larger molecules. Radical coupling reactions are key in this process, given that ROS-generated radicals interact to create larger molecular structures. When the intermediates are not fully mineralized, radical coupling can lead to polymerization, affecting the degradation efficiency and subsequent pathways.¹⁶⁶

The following outlines the potential coupling and polymerization pathways induced by radicals.

• Self-polymerization reactions: Hydroxyl radicals reacting with aromatic compounds can form intermediates that may undergo self-polymerization, leading to macromolecules such as polymers. This reaction is particularly noticeable at higher pollutant concentrations, potentially generating complex polymeric products.

• Radical coupling: In AOPs, when two or more radicals collide, they may covalently bond to form larger molecules. For example, a phenyl radical generated by ROSs may couple with another radical, forming more complex organic structures.

• Chain reactions: Radical generation and polymerization often result from chain reactions. Radicals produced in the initial reactions continually react with other molecules, generating new radicals and triggering further polymerization.

• Generation of by-products: Polymers formed through radical coupling may interact with other organic pollutants, impacting the degradation process. Some of these polymers may be resistant to further breakdown, ultimately reducing the AOP efficiency.

The organic oxidation in the radical-based AOP-induced coupling and polymerization pathways is illustrated as follows¹³⁸ (Fig. 9f):

SO₄˙⁻ + C₆H₅OH → C₆H₅O˙ + SO₄²⁻ + H⁺ (27)

OH˙ + C₆H₅OH → C₆H₅O˙ (28)

2R˙ → R₂ (29)

R˙ + R₂ → R₂˙ + R (30)

R˙ + R → R₂˙… → R_n−1˙… (31)

R˙ + R_n−1˙ → R_n (32)

(R represents C₆H₅OH, n represents the degree of polymerization).

Mineralization:

Pollutant + Peroxides → CO₂↑ (33)

Coupling:

Polymerization:

Proposed polymerization processes in Poly-PS-AOPs can be described as follows:¹⁶⁷

AOPs are highly effective in water disinfection and microbial inactivation, neutralizing bacteria, viruses, and other microorganisms. ROSs target microbial lipid membranes, disrupting the cell integrity and affecting DNA and proteins, causing cell death through the leakage of cellular contents. Ozone, with its strong oxidizing properties, rapidly reacts with microorganisms, inactivating bacteria and viruses by destroying their cell wall and directly damaging their genetic material.¹⁶⁸ AOPs are especially effective against chlorine-resistant pathogens, reducing the bacterial counts to undetectable levels quickly. Their broad-spectrum bactericidal effects strongly inhibit various bacteria, viruses, and fungi. Additionally, by-products are mainly water and oxygen, reducing environmental burdens and chemical residues. Thus, AOPs show great potential for water disinfection and microbial inactivation due to their efficiency and eco-friendly characteristics.

(2) Waste gas purification

AOPs are crucial for treating exhaust gases, especially in removing volatile organic compounds (VOCs) and harmful gases. They degrade pollutants by generating strong oxidants such as hydroxyl radicals and ozone, effectively targeting VOCs such as benzene, toluene, and aldehydes. These compounds are difficult to degrade using traditional methods due to their low molecular weight and high volatility, but AOPs convert them into harmless substances. AOPs also remove odorous components such as hydrogen sulfide and ammonia and excel in gas disinfection, reducing pathogens in industrial environments. Their versatility is evident in their ability to handle varying gas flow rates and concentrations. Combined with technologies such as adsorption and membrane separation, AOPs further enhance the treatment efficiency. Thus, AOPs offer a powerful solution for reducing harmful gases in industrial and residential emissions. For example, Deng et al.¹⁶⁹ developed a dual-metal catalyst with palladium (Pd) and vanadium (V) on cerium dioxide (CeO₂) to synergistically remove VOCs and nitrogen oxides (NO_x). The interaction between Pd and V optimized the redox activity, improving the reactant adsorption and preventing the excessive oxidation of nitrites, enabling simultaneous VOC oxidation and NO_x reduction.

(3) Direct conversion of chemicals

AOPs are increasingly valued in organic synthesis owing to their efficiency, selectivity, and environmental advantages. By generating highly reactive ROSs, AOPs create optimal conditions for organic transformations. One application is using hydroxyl radicals to oxidize alcohols, aldehydes, ketones, and amines to carboxylic acids or ketones, improving the reaction selectivity and yield compared to traditional oxidants.¹⁷⁰ For example, AOPs enable efficient fatty acid synthesis from fatty alcohols under mild conditions, minimizing side reactions. In phenol oxidation, AOPs with H₂O₂ and hydroxyl radicals convert phenol to quinones such as p-benzoquinone (C₆H₅OH + H₂O₂ → C₆H₄O₂ + H₂O) with high selectivity. AOPs also promote cyclization reactions, such as synthesizing nitrogen-containing heterocycles, by facilitating ROS-driven cyclization. They also enable selective functional group transformations, such as converting alcohols to aldehydes or ketones.¹⁷¹ For instance, propanol is selectively oxidized to acetone: CH₃CH₂CH₂OH + H₂O₂ → CH₃C(=O)CH₃ + H₂O. Additionally, AOPs can facilitate the dehydrogenation of alcohols.¹⁷² For instance, phenylethanol can be transformed into styrene, an important precursor in the synthesis of polymers such as polystyrene, as follows: C₆H₅C(OH)H₂ + H₂O₂ → C₆H₅C=CH₂ + 2H₂O.¹⁷³ Methane conversion is a notable catalytic reaction, given that methane is a valuable resource for producing chemicals such as methanol and formic acid. Efficient catalysts for methane conversion under mild conditions are a key research focus. Lou et al.¹⁷⁴ developed a dual single-atom catalyst (Cu and Ag on ZSM-5) using H₂O₂ as an oxidant, achieving a methanol yield of 20 115 μmol g_cat⁻¹ at 70 °C in 30 min with 81% selectivity. The bimetallic sites synergistically enhance OH species production, while reducing metal leaching, improving the methanol selectivity and stability. Finally, the applications of AOPs, including organic pollutant degradation, sterilization, waste gas purification, and direct chemical conversion, are systematically summarized in Fig. 10.

Fig. 10 Application fields of AOPs, including organic pollutant conversion, sterilization, waste gas purification and direct chemical conversion.

5.2 Geometric microstructure of DACs in AOPs: influence on catalytic kinetics and thermodynamics

DACs build on the advantages of SACs, while fully utilizing the synergistic effects between the two metal atoms at the active site. By exploring a diverse range of metal atoms, DACs can form dual-atom sites through various combinations, greatly enhancing their geometric structural diversity. This flexibility is beneficial for the precise design and screening of catalysts. The geometric configuration of DACs, particularly the arrangement of the dual atoms, significantly impacts the kinetics and thermodynamics of catalytic reactions. The most common configuration is the directly bonded dual atoms, followed by non-bonded, bridged, and bilayer DACs.

5.2.1 Impact of direct/indirect bonding dual-atom configurations in AOP catalysis. The most common geometric type consists of two directly connected dual-atom sites. For instance, Wang et al.⁸⁷ synthesized a dual-atom Pt₂ catalyst directly linked and supported on C₃N₄ using a wet-chemical method. This configuration significantly lowered the energy barrier for styrene epoxidation, improving the catalytic activity. Compared to single-atom catalysts, this dual-atom design offers a marked performance enhancement. In addition to directly connected DACs, bridging DACs with heteroatomic bridges (e.g., nitrogen, oxygen, and sulfur) further diversify the coordination microstructure and exhibit a promising catalytic performance. These configurations enable fine-tuning of the electronic and geometric properties, optimizing the reaction pathways and catalytic efficiency. For example, Shahbazian-Yassar et al.¹⁰⁵ developed a Cu₂@C₃N₄ catalyst, where dual copper–oxygen centers are anchored on C₃N₄ via Cu–N bonds. This bridging oxygen structure, together with the cooperative effect of the dual copper sites, enhances H₂O₂ activation and achieves high selectivity in CH₄ oxidation to methyl ester (>98%) (Fig. 11a). During H₂O₂ activation, the dual copper sites perform distinct roles, where the first H₂O₂ dissociates, generating *H adsorbed on the bridging oxygen, while the ˙OOH radical migrates to Cu_α, forming *OOH species. The second H₂O₂ dissociates, resulting in *OH adsorbing on Cu_β, where the ˙OH radical recombines with *H on the bridging oxygen to form H₂O. Upon CH₄ introduction, C–H bond cleavage generates CH₃ radicals, which adsorb on Cu_α or Cu_β, recombining with *OOH and *OH to form *CH₃OOH (0.52 eV) and *CH₃OH (0.72 eV).

Fig. 11 Impact of geometric microstructure of DACs on catalytic kinetics and thermodynamics: (a) two copper sites bridging with oxygen exhibited symphonic mechanisms and promoted the production of value-added chemicals.¹⁰⁵ Copyright 2022, Pengfei Xie et al. (b) Axially coordinated dual Co–Mn improved over 150 times for pollutant removal.¹⁷⁵ Copyright 2024, Elsevier. (c) Fe–Ni diatoms coupled with Pt to improve methanol oxidation capacity.¹⁷⁶ Copyright 2023, Wiley-VCH. (d) Distance-dependent interaction of Ir and Co in achieving efficient synergistic catalysis.⁴³ Copyright 2024, the American Chemical Society. (e) Neighboring M₂-SAzyme (M = Fe, Ir, and Pt) featured double activity and ∼6 times catalase/peroxidase selectivity than the pristine material.¹⁷⁷ Copyright 2023, Wiley-VCH.

5.2.2 Effect of axial connected bonding dual-atom configurations in AOP catalysis. Currently, DACs mostly exhibit a planar coordination, with bimetallic sites and supports within the same plane. However, Han et al.¹⁷⁸ found that axial coordination can introduce new electronic and chemical properties, significantly improving the catalytic performance over the planar coordination. Zhou et al.¹⁷⁵ later reported two geometric structures with axially coordinated and planar dual-atom Co–Mn sites, revealing notable differences in their kinetics and stability during PMS activation for the degradation of levofloxacin (Fig. 11b). The axial coordination catalyst outperformed the planar coordination catalyst, given that it reduced the PMS adsorption energy and activation barriers, with the axial ligands acting as “electron bridges” to facilitate a superoxide-mediated chain reaction, achieving 100% ¹O₂ generation. DACs allow tunable geometries and provide additional active sites by modulating the coordination of adjacent metal atoms, resulting from orbital overlap. For instance, Cui et al.¹⁷⁹ developed a Cu–Co DAC with a uniform CuCoN₆(OH) structure, enhancing water activation and enabling efficient silane conversion into silanol with up to 98% yield. In the case of CuCo–DAC, water activation is the rate-determining step, whereas for Co–SAC and Cu–SAC, hydrogen generation limits the reaction. The activation energy for silane oxidation over CuCo–DAC is much lower than for single-metal catalysts, demonstrating the optimal thermodynamics, kinetics, and catalytic pathways.

5.2.3 Impact of interatomic distance dependent dual-atom configurations in AOPs catalysis. The atomic distance plays a key role in determining the coordination environment of active metal atoms, influencing the catalytic pathways. These distances include the metal-coordinating atom bond lengths, interatomic site distances, and metal–metal spacings. The bond length between metal (M) and nitrogen (N) varies due to the electronegativity, size, and strain effects. In Fe/Co DACs,¹⁸⁰ the Fe–N bond length increased while the Co–N bond length decreased, enhancing the covalency. This change led to distinct reaction pathways, ROS profiles, and stability during PMS activation. Furthermore, optimizing the DAC assembly by adjusting the inter-site distances induces orbital quantum effects, creating highly unsaturated coordination between metal pairs. This improves the reactant adsorption, facilitates bond cleavage, and lowers energy barriers, enhancing the catalytic activity. For instance, Yao et al.⁴³ found a volcano-shaped relationship between the spatial separation of catalytic sites and mass activity (Fig. 11d). At an optimal distance of 7.9 Å, dynamic equilibrium between OH generation and consumption was achieved, leading to efficient catalysis. Deviations from this distance caused intermediate accumulation and reduced efficiency in formic acid oxidation. Additionally, Li et al.¹⁸¹ demonstrated that atomic spacing in Fenton-like reactions significantly affects the catalytic activity. When dCu1–Cu1 is 2.6 Å, PDS adopts a vertical configuration on the Cu2–N6 site, while at 5.3 Å, it switches to a horizontal adsorption model, improving the energy barrier and enhancing the activation and turnover frequency.

5.2.4 Role of relay catalysis-dependent dual-atom configurations in AOP catalysis. As research on DACs deepens, it is clear that their geometric structures play a crucial role in catalytic activity through synergy. A key mechanism behind this synergy is relay catalysis, where multiple active centers sequentially participate in the reaction, forming a network of interactions that enhance catalytic transformations. One active site first activates the reactant or intermediate, which is then transferred to another site for further reaction. This sequential interaction optimizes the selectivity and efficiency by capitalizing on the strengths of each site, enabling precise control of the catalytic process. For instance, Li et al.¹⁸² discovered that Mn–Co dual-atom sites exhibit a unique relay mode in PMS activation for the degradation of bisphenol A. The Mn site initially interacts with the oxygen atom in PMS to form a PMS* intermediate, while the Co site accelerates the production of ¹O₂ through the further activation of PMS* (PMS* → OH* → O* → ¹O₂). This demonstrates the synergistic enhancement of the relay process at the atomic level and provides insights into the cooperative design of multi-metal systems in advanced oxidation. Additionally, Liu et al.¹⁷⁶ found that during the anodic oxidation of methanol, the ˙OH generated at the Fe–Ni sites is transferred to the Pt site for methanol oxidation (Fig. 11c). This transfer prevents the excessive oxidation of the Pt site by ˙OH and helps remove the reaction intermediates, maintaining the stability and durability of the catalyst.

5.2.5 Nanoenzyme-inspired dual-atom configurations on AOP catalysis. Although geometric configurations provide a foundation for understanding DACs, bimetallic single-atom nanoenzymes present a distinct catalytic architecture similar to enzymatic catalysis. Enzyme-mimetic catalysis utilizes the specific geometric and electronic environments of biological enzyme active sites to facilitate efficient substrate transformation and oxidation. This approach enhances the reaction rates and selectivity, mirroring natural enzymatic processes. For instance, Lin et al.¹³⁰ developed a copper dual-atom nanoenzyme exhibiting multifunctional activities, such as POD, OXD, and GSHOx, significantly outperforming a single Mo-based nanoenzyme. The copper atoms enhance the adsorption of H₂O₂, lowering the energy barrier for the formation of reaction intermediates and ˙OH. This advancement opens the door for multi-enzyme mimetic catalytic therapies based on oxidative stress amplification. Notably, the use of amorphous nitrogen-doped carbon supports promotes near-infrared photothermal conversion, improving the generation of ROS and phototherapy efficacy. Subsequently, Ji et al.¹⁷⁷ varied the interatomic distances to create iron-based nanoenzymes with different atomic spacings. They found that adjusting the distance between iron atoms influenced the catalytic pathway, shifting it from ROS generation to ROS elimination (Fig. 11e). This change, attributed to the proximity of dual iron sites, lowered the activation energy for H₂O₂ reactions, inhibiting O–O bond cleavage. The mechanism involves a synergistic hydrogen-bonding effect that enhances H₂O₂ adsorption and decomposition, improving the biomimetic catalytic performance. This approach also enabled the successful synthesis of Ir₂- and Pt₂-based single-atom nanoenzymes, with kilogram-scale yields.

5.3 Electronic configuration of DACs and their impact on catalytic kinetics and thermodynamics

The frontier orbital theory suggests that catalytic processes depend on electron transfer between the metal center and reaction intermediates. The d-orbital occupancy of the metal sites directly affects the kinetics and thermodynamics of reactions, serving as a crucial descriptor in the catalytic process. Investigating the structure–activity relationship between the electronic configuration of the catalytic sites and reaction dynamics can reveal the role of the electronic configuration in regulating reactions in advanced oxidation systems. This understanding not only deepens insight into catalysis but also provides a theoretical foundation for catalyst design and optimization. In heteronuclear DACs, differences in electron distribution between the metal atoms and their synergistic effects allow precise tuning of the catalytic activity, selectivity, stability, and pathways.

5.3.1 Intrinsic electronic coupling in DAC catalysis. Direct electronic interactions between dual atoms, especially metallic species in catalytic frameworks, significantly influence the electron transfer rates, electron flow, distribution around the metal centers, d-band orbitals, and spin states. These alterations are key in tuning the catalytic activity, selectivity, and reaction pathways in AOPs. Electron transfer is fundamental in dual-atom catalysis, enabling changes in the oxidation–reduction states of metal centers. For instance, the Fe/Ni bimetallic catalyst developed by Wang et al.¹⁸³ showed greatly enhanced catalytic kinetics during PI activation and facilitated the oxidative degradation of pollutants through a non-radical pathway. The reduction potential of Ni^III/Ni^II (2.3 V) and Fe^III/Fe^II (0.77 V) shows that Fe can effectively reduce Ni^III to Ni^II, improving the PI utilization and overcoming the rate-limiting steps in traditional systems. Additionally, the Fe/Ni–N_x structure on carbon substrate optimizes the electron transfer due to charge redistribution, boosting the catalytic activity and stability compared to single-metal sites (Fig. 12a).

Fig. 12 Electronic configuration of DACs and their impact on the catalytic kinetics and thermodynamics: (a) FeNi–NC exhibited efficient electron transport conductivity and non-radical oxidation pathways.¹⁸³ Copyright 2024, Elsevier. (b) Long-range interactions between Co and Cu lowered the reaction energy barrier and improved productivity.¹⁸⁴ Copyright 2024, Wiley-VCH. (c) Orbital coupling-modulated iron dinuclear site for enhanced catalytic ozonation activity.¹²⁷ Copyright 2024, Wei Qu., Published by PNAS.

In homonuclear coordination dual-site catalysts, a synergistic effect exists between adjacent metal sites. However, the relationship between their electronic structures (e.g., spin density, d-band center, and charge delocalization) and catalytic activity needs further exploration. Unlike the electron transfer and redox reactions between dual sites, dual-atom sites exhibit varying electron transfer mechanisms with oxidants. For example, He et al.¹²⁷ synthesized Fe1–N3 units linked by Fe–Fe bonds (Fe₂N₆ configuration) using orbital coupling. They found that strong coupling between the Fe d-orbitals reduced the oxidation state of the Fe site and lowered the antibonding energy in the Fe–O bond (Fig. 12c), facilitating O–O bond cleavage in O3, enhancing the binding of *O and *OO, and promoting ROS generation, thus improving the catalytic ozonation of CH₃SH.

DACs with long-range interactions boost the catalytic efficiency via electron transfer, coordination, or structural deformation. These dual-atom sites can influence the overall catalyst performance over longer distances. In this context, Zuo et al.¹⁸⁴ developed a cobalt–copper DAC achieving a 98% FDCA yield and production rate of 298.8 mmol FDCA per g metal per h under mild conditions. In the CoN₄/CuN₃ configuration, the cobalt and copper d-band centers are closer to the Fermi level, reducing the antibonding state occupancy. The incorporation of copper redistributes the charge, enhancing the electron transfer between the metal centers, improving the binding strength with adsorbed species, and lowering the energy barriers for adsorption, activation, and FDCA generation (Fig. 12b). This insight highlights the potential of DACs in advancing catalytic processes with tailored electronic environments and optimized structures.

The traditional methods for the synthesis of dual-atom sites in AOPs are often cumbersome and expensive. Thus, Wang et al.¹⁸⁵ introduced an innovative non-covalent assembly strategy to fabricate various DACs, including Ir–Fe, Pt–Fe, Pt–Co, and Pt–Ni. This method uses two types of non-covalent interactions, π–π stacking and electrostatic attraction. Metal coordination complexes with opposite charges attract each other and are loaded onto a carbon substrate. Then, thermal activation transforms these assemblies into dual-atom sites, creating a straightforward, cost-effective process with high efficiency. The PtFeN₆ DAC, formed in this way, demonstrates an enhanced catalytic performance due to the proximity of the Pt sites, which lower the d-band center of the iron atoms. This modification increases the electron spin polarization at the Fe site and reduces the electron pairing energy, allowing more electrons to occupy the antibonding states. These changes facilitate oxygen activation at the iron site and the desorption of intermediates, boosting the reaction rates and turnover frequency (Fig. 12e). The dual atoms interact to modulate the electronic density and spin states of the d-band of the metal, providing insights into the structure–activity relationship in catalysis.

In contrast to traditional bimetallic DACs, Mu et al.¹⁸⁶ employed a non-metallic site as the second active site, integrating Fe with boron (B) within a nitrogen-doped carbon (N–C) support. The electronegativity difference between B and N causes nitrogen to attract electrons from boron, inducing a positive charge at the B site. Additionally, the electronegativity difference between B and Fe gives the Fe/B DACs a pronounced dipole moment, enhancing the internal electron transfer in the O–O bond. This metal-nonmetal geometric and electronic synergy facilitates O–O bond cleavage in PMS to generate ¹O₂. This mechanism enables the selective oxidation of pollutants and promotes the organic synthesis of benzaldehyde. Unlike traditional redox cycling mechanisms, this catalytic pathway is driven by the structural characteristics of the catalyst, offering a novel non-redox strategy for substrate conversion with significant implications for advanced catalysis.

5.3.2 Mediated electronic interaction in DAC catalysis. The interaction between electron donors and acceptors, mediated by reactants, represents an indirect electronic interaction. In this process, electron transfer occurs not just between atoms but is also influenced by the reactants, altering the catalytic selectivity and activity. For instance, Wu et al.¹⁸⁷ developed Fe–M dual single-atom sites, where M denotes first-row transition metals prone to oxidation. The Fe–Cu pair provides dual active centers for forming the Fe⋯O–O⋯Cu intermediate, unlike traditional single-atom sites, which yield *OOH intermediates (Fig. 12d). This arrangement increases the hydrogen peroxide adsorption energy (−0.37 eV compared to FeN4 at −0.14 eV and CuN4 at −0.098 eV), enhancing O–O bond cleavage and maximizing ˙OH generation (94%). The spatially separated electron donor (Cu) and acceptor (Fe) optimize the d-orbital density and bonding orbital characteristics and enable rapid charge transfer, reducing the energy barriers in the catalytic steps. Furthermore, the Fe–Cu sites promote redox cycling (Cu^I + Fe^III → Cu^II + Fe^II), enhancing the hydrogen peroxide decomposition efficiency.

Maximizing the electronic configuration advantages of DACs, with their distinct electronic properties, is a key focus in Fenton-like reactions. Wang et al.¹³⁴ carefully adjusted the coordination structures of metals to create the Mn–(N_x–C)–Ce dual-site catalyst. In O₃ activation, the Mn–N₄ site generates ˙OH and *OH_ad, while the Ce–N₄ site favors the production of other ROSs, including O₂˙⁻, ¹O₂, and *Oad. This multiple ROS generation results from the optimized charge regulation in the Mn–(N_x–C)–Ce structure. The Ce–N₄–C configuration donates electrons to ozone, while the Mn–N₄–C reaction with H₂O is more favorable than Ce–N₄–C. The charge regulation enhances electron transfer within the catalyst. Additionally, the Ce site reduces electron transfer from Mn–OH to nitrogen, improving Mn–N₄–OH adsorption on ozone. This weakens the O–H bond in Mn–N₄–OH, promoting ˙OH generation. Ozone can also co-adsorb with Mn–OH and Ce–O, facilitating the generation of ˙OH, O₂˙⁻, and ¹O₂ at both sites.

Moreover, Lee et al.¹⁸⁸ employed first-principles screening and revealed that the synergistic interaction at the NiCo–SAD atomic interface can upshift the d-band center, promoting efficient water dissociation and optimal proton adsorption, thereby accelerating the kinetics of both acidic and alkaline hydrogen evolution reactions. This highlights the critical role of the electronic and geometric configurations in enhancing the catalytic performance, providing valuable insights for designing highly efficient catalysts. Notably, Bui et al.¹⁸⁹ investigated the effectiveness of 55 DACs supported on graphitic carbon nitride (gCN) for the hydrogen evolution reaction under both acidic and alkaline conditions. Using DFT calculations, they introduced an electronic descriptor (φ) based on the d-electron count (N_d) and electronegativity (ETM), establishing a quantitative relationship between the φ values (4.0 to 4.6) and the lowest kinetic barriers. This suggests that DACs with optimized electronic configurations enable a more efficient performance, providing a structure–activity relationship for catalytic pathways. This work offers a solid approach for developing high-performance alkaline electrocatalysts, highlighting the importance of the electronic configuration in efficient catalysis.

Metal sites are highly dispersed within supports, forming stable coordination bonds with surrounding atoms and overcoming high specific surface energy challenges. Recently, nitrogen-doped carbon substrates have emerged as the preferred matrices for single-atom metal sites due to their excellent thermal and chemical stability, which prevents metal atom aggregation. The interaction between the metal and support is crucial in tuning the geometric and electronic properties of the carbon substrate, significantly influencing the surface environment and catalytic efficiency.

For instance, Hu et al.¹⁹⁰ synthesized a nitrogen-rich Fe–Ni bimetallic catalyst on graphite carbon nitride (g-C₃N₄) with abundant nitrogen vacancies (Nv). The presence of Nv enhances the electronic density around the Fe–Ni pair, improving the electron transfer and promoting hybridization between the O 2p orbitals and Fe 3d and Ni 3d orbitals. This interaction facilitates persulfate adsorption and O–O bond cleavage, increasing the degradation efficiency. Studies show that Nv shifts the d_xz, d_yz, and d_z² states of Fe to higher energy levels near the Fermi level. This shift allows more electrons to transfer from Fe to persulfate, changing the degradation mechanism from a mixed radical/non-radical pathway to a single non-radical pathway.

The presence of Nv in carbon supports significantly impacts the catalytic kinetics and pathways in Fenton-like reactions, as well as the selective conversion of methane into high-value products. For instance, Zhao et al.¹⁹¹ constructed a nitrogen-rich, C3Nx-confined heteronuclear Fe–Cu DAC with specific coordination structures (Fe–N3–Cu1 and Cu–N1–C2–Fe1). This environment redistributes the charge within the Fe1/Cu1–C3Nx catalyst, increasing the charge on copper by 0.20e and decreasing it on iron by 0.15e. This alteration enhances the charge transfer to H₂O₂, reducing the activation energy barrier to 3.89 eV, improving the activation, and fostering efficient ROS generation and methane oxidation.

Besides conventional carbon supports, metal–support interactions, including electronic orbital hybridization and charge transfer dynamics, are crucial in catalytic reactions. The choice of the support substrate significantly affects the metal site intermediate states, energy barriers, and product selectivity. For example, Wang et al.¹⁹² synthesized a 2D heterostructure with Ru–Co dual-atom sites anchored on ultra-thin NiO (Ru–Co DAS/NiO). This heterointerface enhances the atomic rearrangements and charge redistribution between the Ru–Co sites and NiO. The narrowing of the Co 3d orbital near the Fermi level enables it to act as an electron reservoir for Ni, improving the adsorption of CO(NH₂)₂ and promoting urea oxidation over nitrogen reduction. This catalyst also exhibited remarkable durability for 330 h. Subsequently, Chen et al.¹⁹³ synthesized an Ni–Co–2H-MoS₂ DAC using molybdenum disulfide as the support. Unlike carbon-based supports, the dual-valence transition metal sites interact with molybdenum, enabling dynamic redox cycling between Co^III/Co^II and Mo^IV/Mo^V/Mo^VI. This interaction overcomes the rate-limiting barriers in Fenton-like reactions and enhances the stability of the catalytic system.

(d) Spatially separated electron donor–acceptor DACs enhanced Fenton-like catalysis.¹⁸⁷ Copyright 2024, Wiley-VCH. (e) Lowered electron pairing energy and modulated d-band center and spin states in DACs promoted the reaction process.¹⁸⁵ Copyright 2024, Wiley-VCH.

6. Challenges and future outlook

DACs, an extension of SACs, have garnered significant attention due to their exceptional intrinsic properties. They have made notable progress in environmental and energy catalysis and are considered promising candidates for advanced oxidation processes. This review provided a comprehensive overview of DACs, covering their origin, evolution, advantages, types, and supports. It also summarized common preparation methods, including pyrolysis, atomic layer deposition, ball milling, co-precipitation, microwave-assisted solvothermal methods, and photo-induced synthesis. Key characterization techniques such as TEM, XAFS, Mössbauer spectroscopy, and in situ measurements were also discussed. Additionally, this review explored advanced oxidation reactions and analyzed the underlying mechanisms for the superior performance of DACs in catalytic systems, focusing on the optimization of the catalytic kinetics and thermodynamics through the regulation of reaction mechanisms via geometric and electronic configurations. Finally, several future prospects for DAC development are outlined.

6.1 Uniform structure synthesis

The synthesis of DACs with uniform structures is essential for improving their catalytic performance and understanding the dual-atom site mechanisms. However, controlling the coordination of dual-atom sites during the synthesis of DACs remains challenging. The introduction of single-atom sites complicates the study of the dual-atom catalytic mechanisms and interferes with establishing reliable structure–performance relationships. Therefore, further research is needed to develop efficient methods for the synthesis of uniform and stable DACs. To address these challenges, several advanced synthetic strategies are proposed.

(1) Confinement engineering: Utilizing supports with well-defined nanostructures, such as metal–organic frameworks, covalent organic frameworks, and mesoporous carbon, can restrict atomic migration and stabilize the dual-atom configurations. The design of defect-rich supports, including nitrogen-doped carbon or vacancy-engineered oxides, provides anchoring sites that promote uniform DAC formation, while preventing site aggregation.

(2) Stepwise atomic deposition: Techniques such as atomic layer deposition and sequential metal precursor adsorption allow the precise layer-by-layer construction of DACs, ensuring strict control of their atomic distribution. These approaches minimize the undesirable single-atom incorporation, while enabling tunable coordination environments.

(3) In situ synthesis and dynamic regulation: Real-time monitoring during synthesis, enabled by operando spectroscopy and environmental transmission electron microscopy, provides insights into the dual-atom formation pathways. By dynamically adjusting the reaction conditions, such as precursor concentration, temperature, and gas-phase environment, it is possible to steer the synthesis toward the formation of well-defined DAC structures.

6.2 Complex catalytic mechanisms and advanced characterization

The catalytic mechanisms of DACs are more complex than that of SACs due to their dual active sites, which enable cooperative interactions between the two atoms. However, the complexity of these mechanisms, together with the evolving coordination environment and potential changes in reaction pathways, makes it difficult to fully understand their precise role. Advanced characterization techniques are crucial to unveiling the catalytic pathways, but current methods face three key limitations.

(1) Temporal resolution limitations: DAC-catalyzed reactions in AOPs involve highly reactive transient species with extremely short lifetimes. Conventional spectroscopic and microscopic techniques lack sufficient temporal resolution in real-time. Addressing this challenge requires the development of high-speed in situ characterization methods, such as time-resolved X-ray absorption spectroscopy (XAS), ultrafast spectroscopy, and synchrotron-based techniques with femtosecond time resolution.

(2) Interference from liquid-phase molecules: At the solid–liquid interface, solvent molecules and reactive intermediates can obscure surface signals, complicating the observation of catalytic transformations. This is especially problematic for techniques that are sensitive to surface chemical states. Solutions include frozen-state spectroscopy to preserve the intermediates and specialized in situ electrochemical or flow-cell reactors to minimize interference, allowing the more accurate characterization of intermediates and surface interactions.

(3) Comprehensive multi-technique characterization: Integrating complementary methods is essential. For example, combining synchrotron-based X-ray techniques with high-resolution electron microscopy and vibrational spectroscopies (operando IR) offers a comprehensive view of the structural dynamics and reaction pathways. Additionally, emerging techniques such as atomic-scale EELS and operando Mössbauer spectroscopy provide valuable insights into the oxidation state changes and metal–ligand interactions in DACs.

6.3 Machine learning and high-throughput screening

Although high-throughput screening and machine learning have been applied in SAC research, their use in DACs is emerging. Machine learning, particularly deep learning, can predict the catalytic activity of DACs by analyzing large datasets and identifying correlations between atomic configurations and performance. High-throughput screening enables the rapid testing of DACs under various conditions, helping identify promising candidates. However, challenges remain in the prediction accuracy, given that the complexity of DACs and limited training datasets can affect the model reliability.

(1) Limited and noisy training data: The complexity of DAC structures and the scarcity of high-quality experimental data hinder the accuracy of ML predictions. The integration of automated high-throughput experimentation, self-learning ML models, and transfer learning techniques can help mitigate this limitation by continuously refining predictive models based on newly acquired data.

(2) Complexity of active-site modelling: Conventional descriptor-based ML models struggle to capture these complexities. Advanced approaches, such as first-principles-based ML models, quantum chemistry-informed neural networks, and generative adversarial networks (GANs), can enhance the prediction accuracy by incorporating electronic structure calculations and DFT simulations into the learning process.

(3) Experimental validation and model generalization: The predictive power of ML models must be validated through experimental synthesis and catalytic performance testing. However, discrepancies between computationally predicted and experimentally realized structures often arise due to synthesis limitations, dynamic site evolution, and support effects. Thus, to address this, closed-loop frameworks integrating ML-based predictions, robotic synthesis platforms, and automated characterization systems should be developed, allowing iterative model refinement through experimental feedback.

6.4 Synthesis challenges and large-scale production

The scalable synthesis of DACs is challenging due to the need for precise atomic distribution and coordination, together with cost-effectiveness and reproducibility. Unlike SACs, DACs require strict spatial arrangement of dual-atom sites to prevent aggregation or phase segregation. Achieving uniform and stable DACs on a large scale demands real-time monitoring and adjustment of the synthesis conditions, which is difficult in industrial settings. Key hurdles must be addressed to bridge the gap between lab-scale synthesis and commercial production.

(1) Scalability of precision synthesis: Current techniques, such as ALD, wet-chemical methods, and pyrolysis-based strategies, offer atomic-level control but face scalability issues due to their high operational costs, stringent reaction conditions, and complex precursor requirements. Scalable alternatives, such as continuous-flow synthesis, aerosol-assisted deposition, and plasma-enhanced fabrication, need to be explored to achieve high-throughput production without sacrificing atomic precision.

(2) Reproducibility and process optimization: The synthesis of DACs often suffers from batch-to-batch variability, making it challenging to achieve a consistent catalytic performance. Implementing in situ synthesis monitoring techniques, such as operando spectroscopy and machine-learning-assisted process control, can enhance the reproducibility by dynamically adjusting the reaction parameters in real time. The development of automated synthesis platforms integrating high-throughput experimentation with artificial intelligence (AI)-driven optimization can further streamline the fabrication of DACs.

(3) Cost reduction without compromising structural integrity: The high cost of precursors, energy-intensive processes, and complex synthesis routes currently hinder the large-scale adoption of DACs. Thus, the rational design of cost-effective precursors, solvent-free synthesis methods, and low-temperature processing techniques can help reduce production costs. Additionally, leveraging abundant and inexpensive supports, such as biomass-derived carbon frameworks or earth-abundant metal precursors, can provide an economically viable pathway.

(4) Structural stability during scale-up: The atomic-scale coordination of dual-metal sites is sensitive to the synthesis conditions, and scaling up DAC production may introduce defects or uneven metal distribution, reducing the catalytic performance. Post-synthetic treatments, such as plasma activation and defect engineering, can maintain the structural integrity during large-scale production. Future DAC synthesis will require interdisciplinary collaboration and data-driven approaches, using AI and high-throughput screening to optimize parameters. Achieving the scalable, cost-effective, and stable synthesis of DACs is crucial for their widespread use in environmental and energy catalysis.

6.5 Environmental considerations

DACs generate reactive species in AOPs that interact with organic pollutants, potentially increasing the environmental toxicity. Thus, understanding these interactions is key to enhancing the catalytic efficiency and ensuring the sustainability of dual-atom oxidation technologies. Additionally, leveraging structural characteristics can redirect reactive species toward polymerization reactions, supporting carbon emission reduction and carbon-neutral goals. However, practical limitations exist when applying DACs in real-world environmental settings.

(1) Stability under harsh operating conditions: DACs must maintain long-term stability under various conditions, including high temperatures, acidic/alkaline environments, and oxidative/reductive atmospheres. Prolonged exposure may cause structural degradation, metal site leaching, or loss of catalytic activity. Thus, strategies such as encapsulation, alloying with stabilizing elements, and optimizing coordination can improve their durability. Theoretical modeling and accelerated degradation studies can offer insights into stability mechanisms and guide the development of more resilient DACs.

(2) Cost-effective controllable synthesis: The synthesis of DACs involves costly, complex procedures, which limit the scalability for their industrial use. These methods require precise atomic control, high-purity substrates, and advanced techniques such as ALD, which are hard to scale. Thus, to overcome this, cost-effective, scalable strategies such as one-pot synthesis, self-assembly, and inexpensive substrates are needed. Optimizing the reaction conditions and using computational modeling and machine learning for design can improve the production efficiency, making DACs viable for large-scale applications in energy conversion, environmental remediation, and chemical synthesis.

(3) Navigating environmental complexity in DAC catalysis: Although DACs show high efficiency in pollutant degradation, their interactions in complex environmental matrices, comprising various contaminants, organic matter, and ions, are not well understood. These factors can affect their catalytic performance, altering the reaction kinetics, selectivity, and stability in real-world applications. Coexisting pollutants may cause competitive adsorption, surface passivation, or unexpected synergistic effects, complicating the application of DACs. Thus, systematic studies are needed to understand the interplay between DACs and environmental components. Advanced in situ techniques and computational modeling can offer insights, aiding the design of more adaptable and robust DACs for efficient, sustainable remediation.

Data availability

The data that supports the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52300045), the China Postdoctoral Science Foundation (2023M741424), the Jiangsu Province Natural Science Fund (BK20241025) and the Start-up Fund for Introduced Scholar of Jiangsu University (5501370025).

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