Abrading bulk metal into single atoms

Linking atomic to mesoscopic scales in multilevel structural tailoring of single-atom catalysts for peroxide activation

A key challenge in designing single-atom catalysts (SACs) with multiple and synergistic functions is to optimize their structure across different scales, as each scale determines specific material properties. We advance the concept of a comprehensive optimization of SACs across different levels of scale, from atomic, microscopic to mesoscopic scales, based on interfacial kinetics control on the coupled metal-dissolution/polymer-growth process in SAC synthesis. This approach enables us to manipulate the multilevel interior morphologies of SACs, such as highly porous, hollow, and double-shelled structures, as well as the exterior morphologies inherited from the metal oxide precursors. The atomic environment around the metal centers can be flexibly adjusted during the dynamic metal-oxide consumption and metal-polymer formation. We show the versatility of this approach using mono- or bi-metallic oxides to access SACs with rich microporosity, tunable mesoscopic structures and atomic coordinating compositions of oxygen and nitrogen in the first coordination-shell. The structures at each level collectively optimize the electronic and geometric structure of the exposed single-atom sites and lower the surface *O formation barriers for efficient and selective peroxidase-type reaction. The unique spatial geometric configuration of the edge-hosted active centers further improves substrate accessibility and substrate-to-catalyst hydrogen overflow due to tunable structural heterogeneity at mesoscopic scales. This strategy opens up new possibilities for engineering more multilevel structures and offers a unique and comprehensive perspective on the design principles of SACs.

Regulating the Critical Intermediates of Dual-Atom Catalysts for CO2 Electroreduction

Electrocatalysis is a very attractive way to achieve a sustainable carbon cycle by converting CO2 into organic fuels and feedstocks. Therefore, it is crucial to design advanced electrocatalysts by understanding the reaction mechanism of electrochemical CO2 reduction reaction (eCO2RR) with multiple electron transfers. Among electrocatalysts, dual-atom catalysts (DACs) are promising candidates due to their distinct electronic structures and extremely high atomic utilization efficiency. Herein, the eCO2RR mechanism and the identification of intermediates using advanced characterization techniques, with a particular focus on regulating the critical intermediates are systematically summarized. Further, the insightful understanding of the functionality of DACs originates from the variable metrics of electronic structures including orbital structure, charge distribution, and electron spin state, which influences the active sites and critical intermediates in eCO2RR processes. Based on the intrinsic relationship between variable metrics and critical intermediates, the optimized strategies of DACs are summarized containing the participation of synergistic atoms, engineering of the atomic coordination environment, regulation of the diversity of central metal atoms, and modulation of metal-support interaction. Finally, the challenges and future opportunities of atomically dispersed catalysts for eCO2RR processes are discussed.

Asymmetric Coordination of Bimetallic Fe-Co Single-Atom Pairs toward Enhanced Bifunctional Activity for Rechargeable Zinc-Air Batteries

The advancement of rechargeable zinc-air batteries (RZABs) faces challenges from the pronounced polarization and sluggish kinetics of oxygen reduction and evolution reactions (ORR and OER). Single-atom catalysts offer an effective solution, yet their insufficient or singular catalytic activity hinders their development. In this work, a dual single-atom catalyst, FeCo-SAs, was fabricated, featuring atomically dispersed N3-Fe-Co-N4 sites on N-doped graphene nanosheets for bifunctional activity. Introducing Co into Fe single-atoms and secondary pyrolysis altered Fe coordination with N, creating an asymmetric environment that promoted charge transfer and increased the density of states near the Fermi level. This catalyst achieved a narrow potential gap of 0.616 V, with a half-wave potential of 0.884 V for ORR (vs the reversible hydrogen electrode) and a low OER overpotential of 270 mV at 10 mA cm-2. Owing to the superior activity of FeCo-SAs, RZABs exhibited a peak power density of 203.36 mW cm-2 and an extended cycle life of over 550 h, exceeding the commercial Pt/C + IrO2 catalyst. Furthermore, flexible RZABs with FeCo-SAs demonstrated the promising future of bimetallic pairs in wearable energy storage devices.

Electrochemical Knocking-Down of Zn Metal Clusters into Single Atoms

Single Atoms Catalysts (SACs) have emerged as a class of highly promising heterogeneous catalysts, where the traditional bottom-up synthesis approaches often encounter considerable challenges in relation to aggregation issues and poor stability. Consequently, achieving densely dispersed atomic species in a reliable and efficient manner remains a key focus in the field. Herein, we report a new facile electrochemical knock-down strategy for the formation of SACs, whereby the metal Zn clusters are transformed into single atoms. While a defect-rich substrate plays a pivotal role in capturing and stabilizing isolated Zn atoms, the feasibility of this novel strategy is demonstrated through a comprehensive investigation, combining experimental and theoretical studies. Furthermore, when studied in exploring for potential applications, the material prepared shows a remarkable improvement of 58.21% for the Li+ storage and delivers a capacity over 300 Wh kg-1 after 500 cycles upon the transformation of Zn clusters into single atoms.

Probing Coordination Number of Single-Atom Catalysts by d-Band Center-Regulated Luminescence

It is essential to probe the coordination number (CN) because it is a crucial factor to ensure the catalytic capability of single-atom catalysts (SACs). Currently, synchrotron X-ray absorption spectroscopy (XAS) is widely used to measure the CN. However, the scarcity of synchrotron X-ray source and complicated data analysis restrict its wide applications in determining the CN of SACs. In this contribution, we have developed a d-band center-regulated acetone cataluminescence (CTL) probe for a rapid screening of the CN of Pt-SACs. It is disclosed that the CN-triggered CTL is attributed to the fact that the increased CN could induce the downward shift of d-band center position, which assists the acetone adsorption and promotes the subsequent catalytic reaction. In addition, the universality of the proposed acetone-CTL probe is verified by determining the CN of Fe-SACs. This work has opened a new avenue for exploring an alternative to synchrotron XAS for the determination of CN of SACs and even conventional metal catalysts through d-band center-regulated CTL.

Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Single atomic catalysts have shown great potential in efficiently electro-converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon-based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP-NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni-N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high-performance catalysts for practical H2O2 electrosynthesis.

Atomic Pt Sites Anchored in the Interface between Grains on Vacancy-Enriched CeO2 Nanosheets: One-Step Precursor Combustion Synthesis

Atomic metal catalysts have unique electronic, structural, and catalytic properties, which are widely used in the field of catalysis. However, designing new simple synthesis methods to fabricate atomic metal catalysts is a challenge in catalytic applications. Herein, a one-step precursor combustion strategy is presented that starts directly from precursors of metal salts, using a spontaneous combustion process convert platinum nitrate to atomic Pt sites. The atomic Pt sites with low valence are anchored in the formed interface between grains on vacancy-enriched CeO2 nanosheets. The obtained Pt/CeO2-2 catalyst exhibits much higher three-way catalytic activities at low temperatures than Pt/CeO2-C catalysts prepared using the traditional impregnation method. Density functional theory calculations show that the generated lower valent Pt atoms in the CeO2 interface promote catalytic activity through reducing the energy barrier, and lead to an overall improvement of three-way catalytic activities. This facile strategy provides new insights into the study of the properties and applications of atomic noble metal catalysts.

Tailoring d-band center of high-valent metal-oxo species for pollutant removal via complete polymerization

Polymerization-driven removal of pollutants in advanced oxidation processes (AOPs) offers a sustainable way for the simultaneous achievement of contamination abatement and resource recovery, supporting a low-carbon water purification approach. However, regulating such a process remains a great challenge due to the insufficient microscopic understanding of electronic structure-dependent reaction mechanisms. Herein, this work probes the origin of catalytic pollutant polymerization using a series of transition metal (Cu, Ni, Co, and Fe) single-atom catalysts and identifies the d-band center of active site as the key driver for polymerization transfer of pollutants. The high-valent metal-oxo species, produced via peroxymonosulfate activation, are found to trigger the pollutant removal via polymerization transfer. Phenoxyl radicals, identified by the innovative spin-trapping and quenching approaches, act as the key intermediate in the polymerization reactions. More importantly, the oxidation capacity of high-valent metal-oxo species can be facilely tuned by regulating their binding strength for peroxymonosulfate through d-band center modulation. A 100% polymerization transfer ratio is achieved by lowering the d-band center. This work presents a paradigm to dynamically modulate the electronic structure of high-valent metal-oxo species and optimize pollutant removal from wastewater via polymerization.

Spatial engineering of single-atom Fe adjacent to Cu-assisted nanozymes for biomimetic O2 activation

The precise design of single-atom nanozymes (SAzymes) and understanding of their biocatalytic mechanisms hold great promise for developing ideal bio-enzyme substitutes. While considerable efforts have been directed towards mimicking partial bio-inspired structures, the integration of heterogeneous SAzymes configurations and homogeneous enzyme-like mechanism remains an enormous challenge. Here, we show a spatial engineering strategy to fabricate dual-sites SAzymes with atomic Fe active center and adjacent Cu sites. Compared to planar Fe-Cu dual-atomic sites, vertically stacked Fe-Cu geometry in FePc@2D-Cu-N-C possesses highly optimized scaffolds, favorable substrate affinity, and fast electron transfer. These characteristics of FePc@2D-Cu-N-C SAzyme induces biomimetic O2 activation through homogenous enzymatic pathway, resembling functional and mechanistic similarity to natural cytochrome c oxidase. Furthermore, it presents an appealing alternative of cytochrome P450 3A4 for drug metabolism and drug-drug interaction. These findings are expected to deepen the fundamental understanding of atomic-level design in next-generation bio-inspired nanozymes.

Current Advances on the Single-Atom Nanozyme and Its Bioapplications

Nanozymes, a class of nanomaterials mimicking the function of enzymes, have aroused much attention as the candidate in diverse fields with the arbitrarily tunable features owing to the diversity of crystalline nanostructures, composition, and surface configurations. However, the uncertainty of their active sites and the lower intrinsic deficiencies of nanomaterial-initiated catalysis compared with the natural enzymes promote the pursuing of alternatives by imitating the biological active centers. Single-atom nanozymes (SAzymes) maximize the atom utilization with the well-defined structure, providing an important bridge to investigate mechanism and the relationship between structure and catalytic activity. They have risen as the new burgeoning alternative to the natural enzyme from in vitro bioanalytical tool to in vivo therapy owing to the flexible atomic engineering structure. Here, focus is mainly on the three parts. First, a detailed overview of single-atom catalyst synthesis strategies including bottom-up and top-down approaches is given. Then, according to the structural feature of single-atom nanocatalysts, the influence factors such as central metal atom, coordination number, heteroatom doping, and the metal-support interaction are discussed and the representative biological applications (including antibacterial/antiviral performance, cancer therapy, and biosensing) are highlighted. In the end, the future perspective and challenge facing are demonstrated.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

Engineering Symmetry-Breaking Centers and d-Orbital Modulation in Triatomic Catalysts for Zinc-Air Batteries

Unraveling the configuration-activity relationship and synergistic enhancement mechanism (such as real active center, electron spin-state, and d-orbital energy level) for triatomic catalysts, as well as their intrinsically bifunctional oxygen electrocatalysis, is a great challenge. Here we present a triatomic catalyst (TAC) with a trinuclear active structure that displays extraordinary oxygen electrocatalysis for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), greatly outperforming the counterpart of single-atom and diatomic catalysts. The aqueous Zn-air battery (ZAB) equipped with a TAC-based cathode exhibits extraordinary rechargeable stability and ultrarobust cycling performance (1970 h/3940 cycles at 2 mA cm-2, 125 h/250 cycles at 10 mA cm-2 with negligible voltage decay), and the quasi-solid-state ZAB displays outstanding rechargeability and low-temperature adaptability (300 h/1800 cycles at 2 mA cm-2 at -60 °C), outperforming other state-of-the-art ZABs. The experimental and theoretical analyses reveal the symmetry-breaking CoN4 configuration under incorporation of neighboring metal atoms (Fe and Cu), which leads to d-orbital modulation, a low-shift d band center, weakened binding strength to the oxygen intermediates, and decreased energy barrier for bifunctional oxygen electrocatalysis. This rational tricoordination design as well as an in-depth mechanism analysis indicate that hetero-TACs can be promisingly applied in various electrocatalysis applications.

One-step strategy for efficient Cr(VI) removal via phytate modified zero-valent iron: Accelerated electron transfer and enhanced coordination effect

The toxic Cr(VI) from industrial wastewater pose serious threat to the human beings and eco-systems. To reduce the operation processes and enhance the removal efficiency of Cr(VI), targeted design of functionalized material is critical in practical applications. Herein, we developed a one-step strategy for simultaneous Cr(VI) reduction and total Cr capture by a novel phytate modified zero-valent iron (PA-ZVI). The reaction kinetics of Cr(VI) removal by PA-ZVI (0.2225 min-1) was 53 times higher compared to ZVI (0.0042 min-1). The Fe(0) content on the surface of PA-ZVI increased from 2.2% to 15.6% compared to ZVI. Meanwhile, Cr(VI) was liable to adsorb on the surface of PA-ZVI due to its lower adsorption energy compared with the original ZVI (-2.09 eV vs -0.85 eV). The incorporation of the phytate ligand promoted electron transfer from iron core to Cr(VI), leading to the rapid in-situ reduction of Cr(VI) adsorbed on the surface of PA-ZVI to Cr(III). PA-ZVI exhibited a satisfactory performance for Cr(VI) removal at a broad pH range (3-11) and in the presence of coexisting ions and humic acid. Moreover, the reactor with the addition of PA-ZVI achieved more than 90% Cr(VI) removal within 72 h in continuous flow experiments. The feasibility of PA-ZVI for the removal of Cr(VI) is also validated in authentic wastewater. This work provides novel ZVI materials that can effectively address decontamination challenges from Cr(VI) pollution.

Constructing Symmetry-Mismatched RuxFe3-xO4 Heterointerface-Supported Ru Clusters for Efficient Hydrogen Evolution and Oxidation Reactions

Ru-related catalysts have shown excellent performance for the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR); however, a deep understanding of Ru-active sites on a nanoscale heterogeneous support for hydrogen catalysis is still lacking. Herein, a click chemistry strategy is proposed to design Ru cluster-decorated nanometer RuxFe3-xO4 heterointerfaces (Ru/RuxFe3-xO4) as highly effective bifunctional hydrogen catalysts. It is found that introducing Ru into nanometric Fe3O4 species breaks the symmetry configuration and optimizes the active site in Ru/RuxFe3-xO4 for HER and HOR. As expected, the catalyst displays prominent alkaline HER and HOR performance with mass activity much higher than that of commercial Pt/C as well as robust stability during catalysis because of the strong interaction between the Ru cluster and the RuxFe3-xO4 support, and the optimized adsorption intermediate (Had and OHad). This work sheds light on a promsing approach to improving the electrocatalysis performance of catalysts by the breaking of atomic dimension symmetry.

Size-matched hierarchical porous carbon materials anchoring single-atom Fe-N4 sites for PMS activation: An in-depth study of key active species and catalytic mechanisms

Single-atom catalysts are considered to be one of the most promising catalysts for AOPs. However, how to design and synthesize cost-effective and highly loaded single-atom catalysts is the bottleneck limiting its development and application. In this study, we report a highly loaded single-atom iron catalyst (Fe-SAC-BC) using waste biomass as a carbon carrier to anchor Fe-N4 sites. The catalyst showed excellent catalytic performance and stability in wastewater treatment. Unlike conventional radical oxidation, the non-radical degradation process of Fe-N4 as the active site and high-valent iron-oxygen intermediates as the key active species identified by burst and probe experiments. DFT calculations and molecular dynamics simulations were applied to the catalytic mechanism of Fe-SAC-BC, in which Fe (III)-N4 is the most likely active site and Fe (IV)-OH is the most dominant active species. This study provides new strategies and understanding for the design of novel single-atom catalysts and the mechanistic probing of the non-radical pathways of AOPs.

Inductive Effect on Single-Atom Sites

Single-atom catalysts exhibit promising electrocatalytic activity, a trait that can be further enhanced through the introduction of heteroatom doping within the carbon skeleton. Nonetheless, the intricate relationship between the doping positions and activity remains incompletely elucidated. This contribution sheds light on an inductive effect of single-atom sites, showcasing that the activity of the oxygen reduction reaction (ORR) can be augmented by reducing the spatial gap between the doped heteroatom and the single-atom sites. Drawing inspiration from this inductive effect, we propose a synthesis strategy involving ligand modification aimed at precisely adjusting the distance between dopants and single-atom sites. This precise synthesis leads to optimized electrocatalytic activity for the ORR. The resultant electrocatalyst, characterized by Fe-N3P1 single-atom sites, demonstrates remarkable ORR activity, thus exhibiting great potential in zinc-air batteries and fuel cells.

Structural Self-Regulation-Promoted NO Electroreduction on Single Atoms

Simultaneously elevating loading and activity of single atoms (SAs) is desirable for SA-containing catalysts, including single-atom catalysts (SACs). However, the fast self-nucleation of SAs limits the loading, and the activity is confined by the adsorption-energy scaling relationships on monotonous SAs. Here, we theoretically design a novel type of SA-containing catalyst generated by two-step structural self-regulation. In the thermodynamic self-regulation step, divacancies in graphene spontaneously pull up SAs from transition metal supports (dv-g/TM; TM = fcc Co, hcp Co, Ni, Cu), leading to the expectably high loading of SAs. The subsequent kinetic self-regulation step involving an adsorbate-assisted and reversible vacancy migration dynamically alters coordination environments of SAs, helping circumvent the scaling relationships, and consequently, the as-designed dv-g/Ni can catalyze NO-to-NH3 conversion at a low limiting potential of -0.25 V vs RHE.

A Universal Electrochemical Synthetic Strategy for the Direct Assembly of Single-Atom Catalysts

Single-atom catalysts (SACs) have been one of the frontiers in the field of catalysis in recent years owing to their high atomic utilization and unique electronic structure. To facilitate the practical application of single-atom, it is vital to develop a sustainable, facile single-atom preparation method with mass production potential. Herein, a universal one-step electrochemical synthesis strategy is proposed, and various metal-organic framework-supported SACs (including Pt, Au, Ir, Pd, Ru, Mo, Rh, and W) are straightforwardly obtained by simply replacing the guest metal precursors. As a proof-of-concept, the electrosynthetic Pt-based catalysts exhibit outstanding activity and stability in the electrocatalytic hydrogen evolution reaction (HER). This study not only enriches the single-atom synthesis methodology, but also extends the scenario of electrochemical synthesis, opening up new avenues for the design of advanced electro-synthesized catalysts.

An emerging direction for nanozyme design: from single-atom to dual-atomic-site catalysts

Nanozymes, a new class of functional nanomaterials with enzyme-like characteristics, have recently made great achievements and have become potential substitutes for natural enzymes. In particular, single-atomic nanozymes (Sazymes) have received intense research focus on account of their versatile enzyme-like performances and well-defined spatial configurations of single-atomic sites. More recently, dual-atomic-site catalysts (DACs) containing two neighboring single-atomic sites have been explored as next-generation nanozymes, thanks to the flexibility in tuning active sites by various combinations of two single-atomic sites. This minireview outlines the research progress of DACs in their synthetic approaches and the latest characterization techniques highlighting a series of representative examples of DAC-based nanozymes. In the final remarks, we provide current challenges and perspectives for developing DAC-based nanozymes as a guide for researchers who would be interested in this exciting field.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Single-atom catalysts-based catalytic ROS clearance for efficient psoriasis treatment and relapse prevention via restoring ESR1

Psoriasis is a common inflammatory disease of especially high recurrence rate (90%) which is suffered by approximately 3% of the world population. The overexpression of reactive oxygen species (ROS) plays a critical role in psoriasis progress. Here we show that biomimetic iron single-atom catalysts (FeN4O2-SACs) with broad-spectrum ROS scavenging capability can be used for psoriasis treatment and relapse prevention via related gene restoration. FeN4O2-SACs demonstrate attractive multiple enzyme-mimicking activities based on atomically dispersed Fe active structures, which are analogous to those of natural antioxidant enzymes, iron superoxide dismutase, human erythrocyte catalase, and ascorbate peroxidase. Further, in vitro and in vivo experiments show that FeN4O2-SACs can effectively ameliorate psoriasis-like symptoms and prevent the relapse with augmented efficacy compared with the clinical drug calcipotriol. Mechanistically, estrogen receptor 1 (ESR1) is identified as the core protein upregulated in psoriasis treatment through RNA sequencing and bioinformatic analysis. Together, this study provides a proof of concept of psoriasis catalytic therapy (PCT) and multienzyme-inspired bionics (MIB).

Flash-Thermal Shock Synthesis of Single Atoms in Ambient Air

Single-atom catalysts feature interesting catalytic activity toward applications that rely on surface reactions such as electrochemical energy storage, catalysis, and gas sensors. However, conventional synthetic approaches for such catalysts require extended periods of high-temperature annealing in vacuum systems, limiting their throughput and increasing their production cost. Herein, we report an ultrafast flash-thermal shock (FTS)-induced annealing technique (temperature > 2850 °C, <10 ms duration, and ramping/cooling rates of ∼105 K/s) that operates in an ambient-air environment to prepare single-atom-stabilized N-doped graphene. Melamine is utilized as an N-doping source to provide thermodynamically favorable metal-nitrogen bonding sites, resulting in a uniform and high-density atomic distribution of single metal atoms. To demonstrate the practical utility of the single-atom-stabilized N-doped graphene produced by the FTS method, we showcased their chemiresistive gas sensing capabilities and electrocatalytic activities. Overall, the air-ambient, ultrafast, and versatile (e.g., Co, Ni, Pt, and Co-Ni dual metal) FTS method provides a general route for high-throughput, large area, and vacuum-free manufacturing of single-atom catalysts.

Dual Atom Catalysts for Energy and Environmental Applications

The pursuit of high metal utilization in heterogeneous catalysis has triggered the burgeoning interest of various atomically dispersed catalysts. Our aim in this review is to assess key recent findings in the synthesis, characterization, structure-property relationship and computational studies of dual-atom catalysts (DACs), which cover the full spectrum of applications in thermocatalysis, electrocatalysis and photocatalysis. In particular, combination of qualitative and quantitative characterization with cooperation with DFT insights, synergies and superiorities of DACs compare to counterparts, high-throughput catalyst exploration and screening with machine-learning algorithms are highlighted. Undoubtably, it would be wise to expect more fascinating developments in the field of DACs as tunable catalysts.

d-Orbital Electron Delocalization Realized by Axial Fe4 C Atomic Clusters Delivers High-Performance Fe-N-C Catalysts for Oxygen Reduction Reaction

Fe-N-C catalyst for oxygen reduction reaction (ORR) has been considered as the most promising nonprecious metal catalyst due to its comparable catalytic performance to Pt in proton exchange membrane fuel cells (PEMFCs). The active centers of Fe-pyrrolic N4 have been proven to be extremely active for ORR. However, forming a stable Fe-pyrrolic N4 structure is a huge challenge. Here, a Cyan-Fe-N-C catalyst with Fe-pyrrolic N4 as the intrinsic active center is constructed with the help of axial Fe4 C atomic clusters, which shows a half-wave potential of up to 0.836 V (vs. RHE) in the acid environment. More remarkably, it delivers a high power density of 870 and 478 mW cm-2 at 1.0 bar in H2 -O2 and H2 -Air fuel cells, respectively. According to theoretical calculation and in situ spectroscopy, the axial Fe4 C can provide strong electronic perturbation to Fe-N4 active centers, leading to the d-orbital electron delocalization of Fe and forming the Fe-pyrrolic N4 bond with high charge distribution, which stabilizes the Fe-pyrrolic N4 structure and optimizes the OH* adsorption during the catalytic process. This work proposes a new strategy to adjust the electronic structure of single-atom catalysts based on the strong interaction between single atoms and atomic clusters.

Tuning the Coordination Environment of Carbon-Based Single-Atom Catalysts via Doping with Multiple Heteroatoms and Their Applications in Electrocatalysis

Carbon-based single-atom catalysts (SACs) are considered to be a perfect platform for studying the structure-activity relationship of different reactions due to the adjustability of their coordination environment. Multi-heteroatom doping has been demonstrated as an effective strategy for tuning the coordination environment of carbon-based SACs and enhancing catalytic performance in electrochemical reactions. Herein, recently developed strategies for multi-heteroatom doping, focusing on the regulation of single-atom active sites by heteroatoms in different coordination shells, are summarized. In addition, the correlation between the coordination environment and the catalytic activity of carbon-based SACs are investigated through representative experiments and theoretical calculations for various electrochemical reactions. Finally, concerning certain shortcomings of the current strategies of doping multi-heteroatoms, some suggestions are put forward to promote the development of carbon-based SACs in the field of electrocatalysis.

Hierarchically Porous Carbons with Highly Curved Surfaces for Hosting Single Metal FeN4 Sites as Outstanding Oxygen Reduction Catalysts

Iron-nitrogen-carbon (FeNC) materials have emerged as a promising alternative to platinum-group metals for catalyzing the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells. However, their low intrinsic activity and stability are major impediments. Herein, an FeN-C electrocatalyst with dense FeN4 sites on hierarchically porous carbons with highly curved surfaces (denoted as FeN4 -hcC) is reported. The FeN4 -hcC catalyst displays exceptional ORR activity in acidic media, with a high half-wave potential of 0.85 V (versus reversible hydrogen electrode) in 0.5 m H2 SO4 . When integrated into a membrane electrode assembly, the corresponding cathode displays a high maximum peak power density of 0.592 W cm-2 and demonstrates operating durability over 30 000 cycles under harsh H2 /air conditions, outperforming previously reported Fe-NC electrocatalysts. These experimental and theoretical studies suggest that the curved carbon support fine-tunes the local coordination environment, lowers the energies of the Fe d-band centers, and inhibits the adsorption of oxygenated species, which can enhance the ORR activity and stability. This work provides new insight into the carbon nanostructure-activity correlation for ORR catalysis. It also offers a new approach to designing advanced single-metal-site catalysts for energy-conversion applications.

Intrinsic and external active sites of single-atom catalysts

Active components with suitable supports are the common paradigm for industrial catalysis, and the catalytic activity usually increases with minimizing the active component size, generating a new frontier in catalysis, single-atom catalysts (SACs). However, further improvement of SACs activity is limited by the relatively low loading of single atoms (SAs, which are heteroatoms for most SACs, i.e., external active sites) because of the highly favorable aggregation of single heteroatoms during preparation. Research interest should be shifted to investigate SACs with intrinsic SAs, which could circumvent the aggregation of external SAs and consequently increase the SAs loading while maintaining them individual to further improve the activity. In this review, SACs with external or intrinsic SAs are discussed and, at last, the perspectives and challenges for obtaining high-loading SACs with intrinsic SAs are outlined.

A General Dual-Metal Nanocrystal Dissociation Strategy to Generate Robust High-Temperature-Stable Alumina-Supported Single-Atom Catalysts

Designing new synthesis routes to fabricate highly thermally durable precious metal single-atom catalysts (SACs) is challenging in industrial applications. Herein, a general strategy is presented that starts from dual-metal nanocrystals (NCs), using bimetallic NCs as a facilitator to spontaneously convert a series of noble metals to single atoms on aluminum oxide. The metal single atoms are captured by cation defects in situ formed on the surface of the inverse spinel (AB₂O₄) structure, which process provides numerous anchoring sites, thus facilitating generation of the isolated metal atoms that contributes to the extraordinary thermodynamic stability. The Pd₁/AlCo₂O₄-Al₂O₃ shows not only improved low-temperature activity but also unprecedented (hydro)thermal stability for CO and propane oxidation under harsh aging conditions. Furthermore, our strategy exhibits a small scaling-up effect by the simple physical mixing of commercial metal oxide aggregates with Al₂O₃. The good regeneration between oxidative and reductive atmospheres of these ionic palladium species makes this catalyst system of potential interest for emissions control.

Single-Atom Gd Nanoprobes for Self-Confirmative MRI with Robust Stability

Gadolinium (Gd)-based complexes are extensively utilized as contrast agents (CAs) in magnetic resonance imaging (MRI), yet, suffer from potential safety concerns and poor tumor targeting. Herein, as a mimic of Gd complex, single-atom Gd nanoprobes with r₁ and r₂ values of 34.2 and 80.1 mM^-1 s^-1 (far higher than that of commercial Gd CAs) at 3 T are constructed, which possessed T₁ /T₂ dual-mode MRI with excellent stability and good tumor targeting ability. Specifically, single-atom Gd is anchored on nitrogen-doped carbon matrix (Gd-N_x C) through spatial-confinement method, which is further subjected to controllable chemical etching to afford fully etched bowl-shape Gd-N_x C (feGd-N_x C) with hydrophilic properties and defined coordination structure, similar to commercial Gd complex. Such nanostructures not only maximized the Gd³⁺ site exposure, but also are suitable for self-confirmative diagnosis through one probe with dual-mode MRI. Moreover, the strong electron localization and interaction between Gd and N atoms afforded feGd-N_x C excellent kinetic inertness and thermal stability (no significant Gd³⁺ leaching is observed even incubated with Cu²⁺ and Zn²⁺ for two months), providing a creative design protocol for MRI CAs.

Cu-based high-entropy two-dimensional oxide as stable and active photothermal catalyst

Cu-based nanocatalysts are the cornerstone of various industrial catalytic processes. Synergistically strengthening the catalytic stability and activity of Cu-based nanocatalysts is an ongoing challenge. Herein, the high-entropy principle is applied to modify the structure of Cu-based nanocatalysts, and a PVP templated method is invented for generally synthesizing six-eleven dissimilar elements as high-entropy two-dimensional (2D) materials. Taking 2D Cu₂Zn₁Al_0.5Ce₅Zr_0.5O_x as an example, the high-entropy structure not only enhances the sintering resistance from 400 °C to 800 °C but also improves its CO₂ hydrogenation activity to a pure CO production rate of 417.2 mmol g^-1 h^-1 at 500 °C, 4 times higher than that of reported advanced catalysts. When 2D Cu₂Zn₁Al_0.5Ce₅Zr_0.5O_x are applied to the photothermal CO₂ hydrogenation, it exhibits a record photochemical energy conversion efficiency of 36.2%, with a CO generation rate of 248.5 mmol g^-1 h^-1 and 571 L of CO yield under ambient sunlight irradiation. The high-entropy 2D materials provide a new route to simultaneously achieve catalytic stability and activity, greatly expanding the application boundaries of photothermal catalysis.

Abrading-Induced Breakdown of Ag Nanoparticles into Atomically Dispersed Ag for Enhancing Antimicrobial Performance

Silver is among the most essential antimicrobial agents. Increasing the efficacy of silver-based antimicrobial materials will reduce operating costs. Herein, we show that mechanical abrading causes atomization of Ag nanoparticles (AgNPs) into atomically dispersed Ag (AgSAs) on the surfaces of an oxide-mineral support, which eventually boosts the antibacterial efficacy considerably. This approach is straightforward, scalable, and applicable to a wide range of oxide-mineral supports; additionally, it does not require any chemical additives and operates under ambient conditions. The obtained AgSAs-loaded γ-Al₂O₃ inactivated Escherichia coli (E. coli) five times as fast as the original AgNPs-loaded γ-Al₂O₃. It can be utilized over 10 runs with minimal efficiency loss. The structural characterizations indicate that AgSAs exhibit a nominal charge of 0 and are anchored at the doubly bridging OH on the γ-Al₂O₃ surfaces. Mechanism studies demonstrate that AgSAs, like AgNPs, damage bacterial cell wall integrity, but they release Ag⁺ and superoxide substantially faster. This work not only provides a simple method for manufacturing AgSAs-based materials but also shows that AgSAs have better antibacterial properties than the AgNPs counterpart.

The optimized Fenton-like activity of Fe single-atom sites by Fe atomic clusters-mediated electronic configuration modulation

The performance optimization of isolated atomically dispersed metal active sites is critical but challenging. Here, TiO2@Fe species-N-C catalysts with Fe atomic clusters (ACs) and satellite Fe-N4 active sites were fabricated to initiate peroxymonosulfate (PMS) oxidation reaction. The AC-induced charge redistribution of single atoms (SAs) was verified, thus strengthening the interaction between SAs and PMS. In detail, the incorporation of ACs optimized the HSO5- oxidation and SO5·- desorption steps, accelerating the reaction progress. As a result, the Vis/TiFeAS/PMS system rapidly eliminated 90.81% of 45 mg/L tetracycline (TC) in 10 min. The reaction process characterization suggested that PMS as an electron donor would transfer electron to Fe species in TiFeAS, generating 1O2. Subsequently, the hVB+ can induce the generation of electron-deficient Fe species, promoting the reaction circulation. This work provides a strategy to construct catalysts with multiple atom assembly-enabled composite active sites for high-efficiency PMS-based advanced oxidation processes (AOPs).

Competitive Trapping of Single Atoms onto a Metal Carbide Surface

Controlling atomic adjustment of single-atom catalysts (SACs) can directly change its local configuration, regulate the energy barrier of intermediates, and further optimize reaction pathways. Herein, we report an atom manipulating process to synthesize Ni atoms stabilized on vanadium carbide (Ni_SA-VC) through a nanofiber-medium thermodynamically driven atomic migration strategy. Experimental and theoretical results systematically reveal the tunable migration pathway of Ni atom from Ni nanoparticles to neighboring N-doped carbon (NC) and finally to metal carbide that was obtained by regulating the competitive adsorption energies between VC and NC for capturing Ni atoms. For CO₂-to-CO electroreduction, Ni_SA-VC exhibits an industrial current density of -180 mA cm^-2 at -1.0 V vs reversible hydrogen electrode and the highest Faradaic efficiency for CO production (FE_CO) of 96.8% at -0.4 V vs RHE in a flow cell. Significant electron transfers occurring in Ni_SA-VC structures contribute to the activation of CO₂, facilitate the reaction free energy, regulate *CO desorption as the rate-determining step, and promote the activity and selectivity. This study provides an understanding on how to design powerful SACs for electrocatalysis.

Single-atom catalysts for electrochemical applications

The field of small molecule electro-activated conversion is becoming a new star in modern catalytic research toward the carbon-neutral future. The advent of single-atom catalysts (SACs) is expected to greatly accelerate the kinetics of electrocatalytic reactions such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), etc., owing to their maximum atomic efficiency, unique quantized energy level structure and strong interaction between well-defined active sites and supports. In this feature article, our group's proposed synthesis methodology applied in electrocatalysis is mainly summarized. Furthermore, we elaborate on how to achieve the stabilization of single metal atoms against migration and agglomeration during the preparation of SACs. Moreover, the electrochemical applications of SACs with a focus on the above heterogeneous reactions are presented. Finally, the prospects for the development and deficiencies of these SACs for electrocatalytic reactions are discussed.

Progress and Perspectives of Single-Atom Catalysts for Gas Sensing

Single-atom catalysts (SACs) attract extensive attention in the field of heterogeneous catalysis in recent years due to the maximum atom utilization and unique physical and chemical properties. The gas sensing is actually a heterogeneous catalysis process but the SACs are new to this area. Although SACs show huge potential in gas sensing, the SACs gas sensing area currently is still at the infancy stage. This work critically reviews the recent advances and current status of single-atom gas sensing materials. General synthesis routes, characterization methods, and sensing performance indexes are introduced. At the end, the challenges and future prospects on SACs gas sensing are presented from the authors' perspectives. This work is anticipated to provide insights and guideline for the chemical sensing community.

Highly Efficient Fabrication of Kilogram-Scale Palladium Single-Atom Catalysts for the Suzuki-Miyaura Cross-Coupling Reaction

Palladium single-atom catalysts (SACs) have caught great attention owing to their maximal atom utilization and outstanding activity for the Suzuki-Miyaura cross-coupling reaction. However, a facile manufacturing method for kilogram-scale synthesis of noble metal SACs with high productivity is still in demand. This study reports on the synthesis of SACs by direct ball milling of commercial metal oxides and nitrate precursors with a productivity of ∼100%. The as-prepared Pd₁/FeO_x SACs show high catalytic performance (the TOFs are 7844 h^-1), high stability, and general applicability for the Suzuki-Miyaura cross-coupling reaction under mild conditions. More encouragingly, kilogram-scale Pd₁/FeO_x SACs can be synthesized in one batch by this approach, endowing great potential for industrial applications. Furthermore, the preparation of Pt, Ru, and Rh SACs is also successfully carried out via the ball milling method, demonstrating favorable applicability. Our findings illustrate exciting chances presented by the highly efficient synthesis of SACs for the formation of C-C bonds.

Non-precious transition metal single-atom catalysts for the oxygen reduction reaction: progress and prospects

The massive exploitation and use of fossil resources have created many negative issues, such as energy shortage and environmental pollution. It prompts us to turn our attention to the development of new energy technologies. This review summarizes the recent research progress of non-precious transition metal single-atom catalysts (NPT-SACs) for the oxygen reduction reaction (ORR) in Zn-air batteries and fuel cells. Some commonly used preparation methods and their advantages/disadvantages have been summarized. The factors affecting the ORR performances of NPT-SACs have been focused upon, such as the substrate type, coordination environment and nanocluster effects. The loading mass of a metal atom has a direct effect on the ORR performances. Some general strategies for stabilizing metal atoms are included. This review points out some existing challenges of NPT-SACs, and also provides ideas for designing and synthesizing NPT-SACs with excellent ORR performances. The large-scale preparation and commercialization of NPT-SACs with excellent ORR properties are prospected.

The Progress and Outlook of Metal Single-Atom-Site Catalysis

Single-atom-site catalysts (SASCs) featuring maximized atom utilization and isolated active sites have progressed tremendously in recent years as a highly prosperous branch of catalysis research. Varieties of SASCs have been developed that show excellent performance in many catalytic applications. The major goal of SASC research is to establish feasible synthetic strategies for the preparation of high-performance catalysts, to achieve an in-depth understanding of the active-site structures and catalytic mechanisms, and to develop practical catalysts with industrial value. This Perspective describes the up-to-date development of SASCs and related catalysts, such as dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs), analyzes the current challenges encountered by these catalysts for industrial applications, and proposes their possible future development path.

Enhanced Antioxidant Effects of Naringenin Nanoparticles Synthesized using the High-Energy Ball Milling Method

Naringenin, one of the flavonoid components, is majorly found in and obtained from grapefruits and oranges. Naringenin also acts as a potent antioxidant, which possesses hypolipidemic as well as anti-inflammatory potential. Naringenin reduces the expressions of several inflammatory mediators, viz., NF-κB, cycloxygenase-2, and other cytokine mediators. In spite of having various biological effects, the clinical application of naringenin is restricted due to its very poor aqueous solubility. In the present study, the high-energy ball milling method was employed for the preparation of naringenin nanoparticles without using any chemical with an aim to enhance the anti-oxidant potential of naringenin. The milled naringenin nanoparticles were characterized for their physicochemical properties using scanning electron microscopy (SEM) and X-ray diffraction. Additionally, the effects of milling time and temperature were further assessed on the solubility of crude and milled naringenin samples. The antioxidant potential of milled naringenin was evaluated with various assays such as DHE, DCFDA, and cleaved caspase-3 using SH-SY5Y human neuroblastoma cells. The nanoparticle size of naringenin after milling was confirmed using SEM analysis. Crystalline peaks for milled and crude samples of naringenin also established that both the naringenin forms were in the crystalline form. The solubility of naringenin was enhanced depending on the milling time and temperature. Moreover, crude and milled naringenin were found to be cytocompatible up to doses of 120 μM each for the duration of 24 and 48 h. It was also observed that milled naringenin at the doses of 1, 2, and 5 μM significantly reduced the levels of reactive oxygen species (ROS) generated by H₂O₂ and exhibited superior ROS scavenging effects as compared to those of crude or un-milled forms of naringenin. Furthermore, milled naringenin at the doses of 1 and 2 μM inhibited H₂O₂-induced cell death, as shown by immunofluorescence staining of cleaved caspase-3 and Annexin-V PI flow cytometry analysis. Conclusively, it could be suggested that the size reduction of naringenin using high-energy ball milling techniques substantially enhanced the antioxidant potential as compared to naïve or crude naringenin, which may be attributed to its enhanced solubility due to reduced size.

Periodic One-Dimensional Single-Atom Arrays

The orderly assembly of single atoms into highly periodic aggregates at the nanoscale is an intriguing but challenging process of high-precision atomic manufacturing. Here, we discover that an in-plane film surface shrinkage can induce molecular self-assembly to arrange single atoms with unconventional distribution, contributing them to periodic one-dimensional segregation on carbon stripes (one-dimensional single-atom arrays (SAA)). This originates from the fact that metal phthalocyanine (MPc) molecules gradually aggregate and melt to form a film under a thermal drive and the help of sodium chloride templates, accompanied by surface shrinkage, self-assembly, and deep carbonization. At the nanoscale, these periodic parallel arrays are formed due to MPc molecular interactions by π-π stacking. At the atomic scale, the single atoms are stabilized by the vertical phthalocyanine-derived multilayer graphene. This can significantly modify the electronic structure of the single-atom sites on the outermost graphene (e.g., Fe-based SAA), thus optimizing the adsorption energy of oxygen intermediates and resulting in a superior oxygen reduction reaction (ORR) performance concerning disordered single atoms. Our findings provide a general route for orderly single-atom manufacturing (e.g., Fe, Co, and Cu) and an understanding of the relationship between orderly allocation and catalytic performance.

How to Make Personal Protective Equipment Spontaneously and Continuously Antimicrobial (Incorporating Oxidase-like Catalysts)

The inability of commercial personal protective equipment (PPE) to inactivate microbes in the droplets/aerosols they intercept makes used PPE a potential source of cross-contamination. To make PPE spontaneously and continuously antimicrobial, we incorporate PPE with oxidase-like catalysts, which efficiently convert O₂ into reactive oxygen species (ROS) without requiring any externally applied stimulus. Using a single-atom catalyst (SAC) nanoparticle containing atomically dispersed copper atoms as the reactive centers (Cu-SAC) and a silver-palladium bimetallic alloy nanoparticle (AgPd_0.38) as models for oxidase-like catalysts, we show that the incorporation of oxidase-like catalysts enables PPE to inactivate bacteria in the droplets/aerosols they intercept without requiring any externally applied stimulus. Notably, this approach works both for PPE that are fibrous and woven such as a commercial KN95 facial respirator and for those made of solid plastics such as an apron. This work suggests a feasible and global approach for preventing PPE from spreading infectious diseases.