Isolating Single and Few Atoms for Enhanced Catalysis

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Fabrication of Nickel Single Atoms with Ionic Liquids by Only One-Step Pyrolysis for CO2 Electroreduction

Single-atom catalysts (SACs) are appealing for carbon dioxide (CO2) electroreduction with the utmost advantages; however, their preparation is still challenging because of the complicated procedure. Here, a novel Ni-based single-atom catalyst (Ni-BB-BD) is constructed from raw materials, [BMIM]BF4, [BMIM]DCN, and NiCl2·6H2O, directly without any precursor by only one-step pyrolysis. Ni-BB-BD achieves a maximum carbon monoxide Faradaic efficiency (FECO) of 96.5% at -0.8 V vs RHE, as well as long-term stability over 16 h. High current density up to -170.6 mA cm-2 at -1.0 V vs RHE is achieved in the flow cell along with a CO selectivity of 97.7%. It is identified that [BMIM]BF4 is the nitrogen source, while [BMIM]DCN is mainly taken as the carbon source. Theoretical studies have revealed that the rich nitrogen content, especially for the uncoordinated nitrogen, plays a critical role in lowering rate-limiting barrier height. This work develops a facile and effective strategy to prepare the SACs.

Mesoporous amorphous non-noble metals as versatile substrates for high loading and uniform dispersion of Pt-group single atoms

Atomically dispersed Pt-group metals are promising as nanocatalysts because of their unique geometric structures and ultrahigh atomic utilization. However, loading isolated Pt-group metals in single-atom alloys (SAAs) with distinctive bimetallic sites is challenging. In this study, we present amorphous mesoporous Ni boride (Ni-B) as an ideal substrate to uniformly disperse Pt atoms with tunable loadings (1.7 to 12.2 wt %). The effect of the morphology, composition, and crystal phase of the Ni-B host on the growth and dispersion of Pt atoms is discussed. The resulting amorphous Pt-Ni-B mesoporous nanospheres exhibit superior electrocatalytic H2 evolution performance in acidic media. This strategy holds the potential to synthesize a diverse library of mesoporous amorphous Pt-group SAAs, by leveraging functional amorphous nanostructured 3d transition-metal borides as substrates, thereby proposing a comprehensive strategy to control atomically dispersed Pt-group metals.

Strategies for the Preparation of Single-Atom Catalysts Using Low-Dimensional Metal-Organic Frameworks

As single-atom catalysts are important energy materials, their preparation and synthesis methods have become particularly important. The unique structures of low-dimensional metal-organic frameworks and their derivatives provide various strategies for preparing single-atom catalysts. This paper summarizes various strategies for the preparation of single-atom catalysts based on low-dimensional metal-organic frameworks and their derivatives.

Platinum-Ruthenium Dual-Atomic Sites Dispersed in Nanoporous Ni0.85Se Enabling Ampere-Level Current Density Hydrogen Production

Alkaline anion-exchange-membrane water electrolyzers (AEMWEs) using earth-abundant catalysts is a promising approach for the generation of green H2. However, the AEMWEs with alkaline electrolytes suffer from poor performance at high current density compared to proton exchange membrane electrolyzers. Here, atomically dispersed Pt-Ru dual sites co-embedded in nanoporous nickel selenides (np/Pt1Ru1-Ni0.85Se) are developed by a rapid melt-quenching approach to achieve highly-efficient alkaline hydrogen evolution reaction. The np/Pt1Ru1-Ni0.85Se catalyst shows ampere-level current density with a low overpotential (46 mV at 10 mA cm-2 and 225 mV at 1000 mA cm-2), low Tafel slope (32.4 mV dec-1), and excellent long-term durability, significantly outperforming the benchmark Pt/C catalyst and other advanced large-current catalysts. The remarkable HER performance of nanoporous Pt1Ru1-Ni0.85Se is attributed to the strong intracrystal electronic metal-support interaction (IEMSI) between Pt-Se-Ru sites and Ni0.85Se support which can greatly enlarge the charge redistribution density, reduce the energy barrier of water dissociation, and optimize the potential determining step. Furthermore, the assembled alkaline AEMWE with an ultralow Pt and Ru loading realizes an industrial-level current density of 1 A cm-2 at 1.84 volts with high durability.

Noble-Metal-Free Single- and Dual-Atom Catalysts for Artificial Photosynthesis

Artificial photosynthesis enables direct solar-to-chemical energy conversion aimed at mitigating environmental pollution and producing solar fuels and chemicals in a green and sustainable approach, and efficient, robust, and low-cost photocatalysts are the heart of artificial photosynthesis systems. As an emerging new class of cocatalytic materials, single-atom catalysts (SACs) and dual-atom catalysts (DACs) have received a great deal of current attention due to their maximal atom utilization and unique photocatalytic properties, whereas noble-metal-free ones impart abundance, availability, and cost-effectiveness allowing for scalable implementation. This review outlines the fundamental principles and synthetic methods of SACs and DACs and summarizes the most recent advances in SACs (Co, Fe, Cu, Ni, Bi, Al, Sn, Er, La, Ba, etc.) and DACs (CuNi, FeCo, InCu, KNa, CoCo, CuCu, etc.) based on non-noble metals, confined on an arsenal of organic or inorganic substrates (polymeric carbon nitride, metal oxides, metal sulfides, metal-organic frameworks, carbon, etc.) acting as versatile scaffolds in solar-light-driven photocatalytic reactions, including hydrogen evolution, carbon dioxide reduction, methane conversion, organic synthesis, nitrogen fixation, hydrogen peroxide production, and environmental remediation. The review concludes with the challenges, opportunities, and future prospects of noble-metal-free SACs and DACs for artificial photosynthesis.

Structural engineering of atomic catalysts for electrocatalysis

As a burgeoning category of heterogeneous catalysts, atomic catalysts have been extensively researched in the field of electrocatalysis. To satisfy different electrocatalytic reactions, single-atom catalysts (SACs), diatomic catalysts (DACs) and triatomic catalysts (TACs) have been successfully designed and synthesized, in which microenvironment structure regulation is the core to achieve high-efficiency catalytic activity and selectivity. In this review, the effect of the geometric and electronic structure of metal active centers on catalytic performance is systematically introduced, including substrates, central metal atoms, and the coordination environment. Then theoretical understanding of atomic catalysts for electrocatalysis is innovatively discussed, including synergistic effects, defect coupled spin state change and crystal field distortion spin state change. In addition, we propose the challenges to optimize atomic catalysts for electrocatalysis applications, including controlled synthesis, increasing the density of active sites, enhancing intrinsic activity, and improving the stability. Moreover, the structure-function relationships of atomic catalysts in the CO2 reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, hydrogen evolution reaction, and oxygen evolution reaction are highlighted. To facilitate the development of high-performance atomic catalysts, several technical challenges and research orientations are put forward.

Photoinduced Precise Synthesis of Diatomic Ir1 Pd1 -In2 O3 for CO2 Hydrogenation to Methanol via Angstrom-Scale-Distance Dependent Synergistic Catalysis

The atomically dispersed metal catalysts with full atomic utilization and well-defined site structure hold great promise for various catalytic reactions. However, the single metallic site limits the comprehensive reaction performance in most reactions. Here, we demonstrated a photo-induced neighbour-deposition strategy for the precise synthesis of diatomic Ir1 Pd1 on In2 O3 applied for CO2 hydrogenation to methanol. The proximity synergism between diatomic sites enabled a striking promotion in both CO2 conversion (10.5 %) and methanol selectivity (97 %) with good stability of 100 h run. It resulted in record-breaking space-time yield to methanol (187.1 gMeOH  gmetal -1  hour-1 ). The promotional effect mainly originated from stronger CO2 adsorption on Ir site with assistance of H-spillover from Pd site, thus leading to a lower energy barrier for *HCOO pathway. It was confirmed that this synergistic effect strongly depended on the dual-site distance in an angstrom scale, which was attributed to weaker *H spillover and less electron transfer from Pd to Ir site as the Pd-to-Ir distance increased. The average dual-site distance was evaluated by our firstly proposed photoelectric model. Thus, this study introduced a pioneering strategy to precisely synthesize homonuclear/heteronuclear diatomic catalysts for facilitating the desired reaction route via diatomic synergistic catalysis.

CO-Assisted Methane Oxidation into Oxygenates over Surface Platinum-Titanium Alloyed Layers

Methane oxidation using molecular oxygen remains a grand challenge in which the obstacle is not only the activation of methane but also the reaction with oxygen, considering the mismatch of the ground spin states. Herein, we report TiO2-supported Pt nanocrystals (Pt/TiO2) with surface Pt-Ti alloyed layers that directly convert methane into oxygenates by using O2 as the oxidant with the assistance of CO. The oxygenate yield reached 749.8 mmol gPt-1 in a H2O aqueous solution over 0.1% Pt/TiO2 under 31 bar of mixed gas (20:5:6 CH4:CO:O2) at 150 °C for 3 h, while the CH3OH selectivity was 62.3%. On the basis of the control experiments and spectroscopic results, we identified the surface Pt-Ti alloy as the active sites. Moreover, CO promoted the dissociation of O2 on the surface of Pt-Ti alloyed layers and the subsequent activation of CH4 to form oxygenated products.

Constructing Highly Efficient ZnO Nanocatalysts with Exposed Extraordinary (110) Facet for CO2 Electroreduction

Electrochemical reduction of CO2 to highly valuable products is a promising way to reduce CO2 emissions. The shape and facets of metal nanocatalysts are the key parameters in determining the catalytic performance. However, the exposed crystal facets of ZnO with different morphologies and which facets achieve a high performance for CO2 reduction are still controversial. Here, we systematically investigate the effect of the facet-dependent reactivity of reduction of CO2 to CO on ZnO (nanowire, nanosheet, and flower-like). The ZnO nanosheet with exposed (110) facet exhibited prominent catalytic performance with a Faradaic efficiency of CO up to 84% and a current density of -10 mA cm-2 at -1.2 V versus RHE, far outperforming the ZnO nanowire (101) and ZnO nanoflower (103). Based on detailed characterizations and kinetic analysis, the ZnO nanosheet (110) with porous architecture increased the exposure of active sites. Further studies revealed that the high CO selectivity originated from the enhancement of CO2 adsorption and activation on the ZnO (110) facet, which promoted the conversion of CO2 toward CO. This study provides a new way to tailor the activity and selectivity of metal catalysts by engineering exposed specific facets.

One-Step Pyrolysis Construction of Bimetallic Atom-Cluster Sites for Boosting Bifunctional Catalytic Activity in Zn-Air Batteries

Due to their unique advantages, single atoms and clusters of transition metals are expected to achieve a breakthrough in catalytic activity, but large-scale production of active materials remains a challenge. In this work, a simple solvent-free one-step annealing method is developed and applied to construct diatomic and cluster active sites in activated carbon by utilizing the strong anchoring ability of phenanthroline to metal ions, which can be scaled for mass productions. Benefiting from the synergy between the different metals, the obtained sub-nano-bimetallic atom-cluster catalysts (FeNiAC -NC) exhibit high oxygen reduction reactions (ORR) activity (E1/2 = 0.936 V vs. RHE) and a small ORR/oxygen evolution reaction (OER) potential gap of only 0.594 V. An in-house pouch Zn-air battery is assembled using an FeNiAC -NC catalyst, which demonstrates a stability of 1000 h, outperforming previous reports. The existence of clusters and their effects on catalytic activity is analyzed by density functional theory calculations to reveal the chemistry of nano-bimetallic atom-cluster catalysts.

Catalyzing Sustainable Water Splitting with Single Atom Catalysts: Recent Advances

Electrochemical water splitting for sustainable hydrogen and oxygen production have shown enormous potentials. However, this method needs low-cost and highly active catalysts. Traditional nano catalysts, while effective, have limits since their active sites are mostly restricted to the surface and edges, leaving interior surfaces unexposed in redox reactions. Single atom catalysts (SACs), which take advantage of high atom utilization and quantum size effects, have recently become appealing electrocatalysts. Strong interaction between active sites and support in SACs have considerably improved the catalytic efficiency and long-term stability, outperforming their nano-counterparts. This review's first section examines the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). In the next section, SACs are categorized as noble metal, non-noble metal, and bimetallic synergistic SACs. In addition, this review emphasizes developing methodologies for effective SAC design, such as mass loading optimization, electrical structure modulation, and the critical role of support materials. Finally, Carbon-based materials and metal oxides are being explored as possible supports for SACs. Importantly, for the first time, this review opens a discussion on waste-derived supports for single atom catalysts used in electrochemical reactions, providing a cost-effective dimension to this vibrant research field. The well-known design techniques discussed here may help in development of electrocatalysts for effective water splitting.

A zero-valent palladium cluster-organic framework

Acquiring spatial control of nanoscopic metal clusters is central to their function as efficient multi-electron catalysts. However, dispersing metal clusters on surfaces or in porous hosts is accompanied by an intrinsic heterogeneity that hampers detailed understanding of the chemical structure and its relation to reactivities. Tethering pre-assembled molecular metal clusters into polymeric, crystalline 2D or 3D networks constitutes an unproven approach to realizing ordered arrays of chemically well-defined metal clusters. Herein, we report the facile synthesis of a {Pd3} cluster-based organometallic framework from a molecular triangulo-Pd3(CNXyl)6 (Xyl = xylyl; Pd3) cluster under chemically mild conditions. The formally zero-valent Pd3 cluster readily engages in a complete ligand exchange when exposed to a similar, ditopic isocyanide ligand, resulting in polymerization into a 2D coordination network (Pd3-MOF). The structure of Pd3-MOF could be unambiguously determined by continuous rotation 3D electron diffraction (3D-ED) experiments to a resolution of ~1.0 Å (>99% completeness), showcasing the applicability of 3D-ED to nanocrystalline, organometallic polymers. Pd3-MOF displays Pd03 cluster nodes, which possess significant thermal and aerobic stability, and activity towards hydrogenation catalysis. Importantly, the realization of Pd3-MOF paves the way for the exploitation of metal clusters as building blocks for rigidly interlocked metal nanoparticles at the molecular limit.

Fine-Tuning of Pt Dispersion on Al2O3 and Understanding the Nature of Active Pt Sites for Efficient CO and NH3 Oxidation Reactions

Fine-tuning the dispersion of active metal species on widely used supports is a research hotspot in the catalysis community, which is vital for achieving a balance between the atomic utilization efficiency and the intrinsic activity of active sites. In this work, using bayerite Al(OH)3 as support directly or after precalcination at 200 or 550 °C, Pt/Al2O3 catalysts with distinct Pt dispersions from single atoms to clusters (ca. 2 nm) were prepared and evaluated for CO and NH3 removal. Richer surface hydroxyl groups on AlOx(OH)y support were proved to better facilitate the dispersion of Pt. However, Pt/Al2O3 with relatively lower Pt dispersion could exhibit better activity in CO/NH3 oxidation reactions. Further reaction mechanism study revealed that the Pt sites on Pt/Al2O3 with lower Pt dispersion could be activated to Pt0 species much easier under the CO oxidation condition, on which a higher CO adsorption capacity and more efficient O2 activation were achieved simultaneously. Compared to Pt single atoms, PtOx clusters could also better activate NH3 into -NH2 and -HNO species. The higher CO adsorption capacity and the more efficient NH3/O2 activation ability on Pt/Al2O3 with relatively lower Pt dispersion well explained its higher CO/NH3 oxidation activity. This study emphasizes the importance of avoiding a singular pursuit of single-atom catalyst synthesis and instead focusing on achieving the most effective Pt species on Al2O3 support for targeted reactions. This approach avoids unnecessary limitations and enables a more practical and efficient strategy for Pt catalyst fabrication in emission control applications.

Emerging Xene-Based Single-Atom Catalysts: Theory, Synthesis, and Catalytic Applications

In recent years, the emergence of novel 2D monoelemental materials (Xenes), e.g., graphdiyne, borophene, phosphorene, antimonene, bismuthene, and stanene, has exhibited unprecedented potentials for their versatile applications as well as addressing new discoveries in fundamental science. Owing to their unique physicochemical, optical, and electronic properties, emerging Xenes have been regarded as promising candidates in the community of single-atom catalysts (SACs) as single-atom active sites or support matrixes for significant improvement in intrinsic activity and selectivity. In order to comprehensively understand the relationships between the structure and property of Xene-based SACs, this review represents a comprehensive summary from theoretical predictions to experimental investigations. Firstly, theoretical calculations regarding both the anchoring of Xene-based single-atom active sites on versatile support matrixes and doping/substituting heteroatoms at Xene-based support matrixes are briefly summarized. Secondly, controlled synthesis and precise characterization are presented for Xene-based SACs. Finally, current challenges and future opportunities for the development of Xene-based SACs are highlighted.

Universal and Energy-Efficient Approach to Synthesize Pt-Rare Earth Metal Alloys for Proton Exchange Membrane Fuel Cell

Traditional synthesis methods of platinum-rare earth metal (Pt-RE) alloys usually involve harsh conditions and high energy consumption because of the low standard reduction potentials and high oxophilicity of RE metals. In this work, a one-step strategy is developed by rapid Joule thermal-shock (RJTS) to synthesize Pt-RE alloys within tens of seconds. The method can not only realize the regulation of alloy size, but also a universal method for the preparation of a family of Pt-RE alloys (RE = Ce, La, Gd, Sm, Tb, Y). In addition, the energy consumption of the Pt-RE alloy preparation is only 0.052 kW h, which is 2-3 orders of magnitude lower than other reported methods. This method allows individual Pt-RE alloy to be embedded in the carbon substrate, endowing the alloy catalyst excellent durability for oxygen reduction reaction (ORR). The performance of alloy catalyst shows negligible decay after 20k accelerated durability testing (ADT) cycles. This strategy offers a new route to synthesize noble/non-noble metal alloys with diversified applications besides ORR.

Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Modulating Coordination of Iron Atom Clusters on N,P,S Triply-Doped Hollow Carbon Support towards Enhanced Electrocatalytic Oxygen Reduction

Constructing atom-clusters (ACs) with in situ modulation of coordination environment and simultaneously hollowing carbon support are critical yet challenging for improving electrocatalytic efficiency of atomically dispersed catalysts (ADCs). Herein, a general diffusion-controlled strategy based on spatial confining and Kirkendall effect is proposed to construct metallic ACs in N,P,S triply-doped hollow carbon matrix (MACs /NPS-HC, M=Mn, Fe, Co, Ni, Cu). Thereinto, FeACs /NPS-HC with the best catalytic activity for oxygen reduction reaction (ORR) is thoroughly investigated. Unlike the benchmark sample of symmetrical N-surrounded iron single-atoms in N-doped carbon (FeSAs /N-C), FeACs /NPS-HC comprises bi-/tri-atomic Fe centers with engineered S/N coordination. Theoretical calculation reveals that proper Fe gathering and coordination modulation could mildly delocalize the electron distribution and optimize the free energy pathways of ORR. In addition, the triple doping and hollow structure of carbon matrix could further regulate the local environment and allow sufficient exposure of active sites, resulting in more enhanced ORR kinetics on FeACs /NPS-HC. The zinc-air battery assembled with FeACs /NPS-HC as cathodic catalyst exhibits all-round superiority to Pt/C and most Fe-based ADCs. This work provides an exemplary method for establishing atomic-cluster catalysts with engineered S-dominated coordination and hollowed carbon matrix, which paves a new avenue for the fabrication and optimization of advanced ADCs.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

Metal Clusters Effectively Adjust the Local Environment of Polymeric Carbon Nitride for Bifunctional Overall Water Splitting

Compared with single-atom catalysts, clusters not only possess more metal-loadings and stability but also provide flexible active sites to break the linear scaling relationship of multistep reactions. However, exploring precise structure-activity relationships and the synergistic effect between clusters and nanosheets is still in its infancy. Here, based on first-principles and nonequilibrium Green's function simulation, the C2N-supported Fe and Co tetrahedral clusters exhibit remarkable bifunctional catalytic performance with a very low overpotential of hydrogen (0.12 and 0.07 V) /oxygen (0.20 and 0.55 V) evolution reactions (HER/OER), respectively. The C2N-regulated Fe and Co clusters have suitable d-band centers around the Fermi surface for HER. In turn, the Fe and Co clusters activate the subadjacent dual-carbon sites for OER. Simultaneously, the cluster enhances the electronic conductivity of C2N, and the initial current only needs ultralow bias voltage around 0.1-0.4 V. The desired metal cluster regulation strategy offers cost-effective potential for advancing clean energy technology.

Developing a class of dual atom materials for multifunctional catalytic reactions

Dual atom catalysts, bridging single atom and metal/alloy nanoparticle catalysts, offer more opportunities to enhance the kinetics and multifunctional performance of oxygen reduction/evolution and hydrogen evolution reactions. However, the rational design of efficient multifunctional dual atom catalysts remains a blind area and is challenging. In this study, we achieved controllable regulation from Co nanoparticles to CoN4 single atoms to Co2N5 dual atoms using an atomization and sintering strategy via an N-stripping and thermal-migrating process. More importantly, this strategy could be extended to the fabrication of 22 distinct dual atom catalysts. In particular, the Co2N5 dual atom with tailored spin states could achieve ideally balanced adsorption/desorption of intermediates, thus realizing superior multifunctional activity. In addition, it endows Zn-air batteries with long-term stability for 800 h, allows water splitting to continuously operate for 1000 h, and can enable solar-powered water splitting systems with uninterrupted large-scale hydrogen production throughout day and night. This universal and scalable strategy provides opportunities for the controlled design of efficient multifunctional dual atom catalysts in energy conversion technologies.

Recent Advances on Single-Atom Catalysts for Photocatalytic CO2 Reduction

The present energy crisis and environmental challenges may be efficiently resolved by converting carbon dioxide (CO2 ) into various useful carbon products. The development of more effective catalysts has been the main focus of current research on photocatalytic CO2 reduction. Due to their high atomic efficiency and superior catalytic activity, single-atom catalysts (SACs) have attracted considerable interest in catalytic CO2 conversion. This review discusses the current research developments, obstacles, and potential of SACs for photocatalytic CO2 reduction. And further, discusses the principle of photocatalytic carbon dioxide reduction. This work has compared and analyzed the effects of support materials and active site types in SACs on photocatalytic CO2 reduction performance. This work believes that by sharing these developments, some inspiration for the rational design and development of stable and effective photocatalytic CO2 reduction catalysts based on SACs can be provided.

Inter-Metal Interaction of Dual-Atom Catalysts in Heterogeneous Catalysis

Dual-atom catalysts (DACs) have been a new frontier in heterogeneous catalysis due to their unique intrinsic properties. The synergy between dual atoms provides flexible active sites, promising to enhance performance and even catalyze more complex reactions. However, precisely regulating active site structure and uncovering dual-atom metal interaction remain grand challenges. In this review, we clarify the significance of the inter-metal interaction of DACs based on the understanding of active center structures. Three diatomic configurations are elaborated, including isolated dual single-atom, N/O-bridged dual-atom, and direct dual-metal bonding interaction. Subsequently, the up-to-date progress in heterogeneous oxidation reactions, hydrogenation/dehydrogenation reactions, electrocatalytic reactions, and photocatalytic reactions are summarized. The structure-activity relationship between DACs and catalytic performance is then discussed at an atomic level. Finally, the challenges and future directions to engineer the structure of DACs are discussed. This review will offer new prospects for the rational design of efficient DACs toward heterogeneous catalysis.

Dual-Active-Sites Single-Atom Catalysts for Advanced Applications

Dual-Active-Sites Single-Atom catalysts (DASs SACs) are not only the improvement of SACs but also the expansion of dual-atom catalysts. The DASs SACs contains dual active sites, one of which is a single atomic active site, and the other active site can be a single atom or other type of active site, endowing DASs SACs with excellent catalytic performance and a wide range of applications. The DASs SACs are categorized into seven types, including the neighboring mono metallic DASs SACs, bonded DASs SACs, non-bonded DASs SACs, bridged DASs SACs, asymmetric DASs SACs, metal and nonmetal combined DASs SACs and space separated DASs SACs. Based on the above classification, the general methods for the preparation of DASs SACs are comprehensively described, especially their structural characteristics are discussed in detail. Meanwhile, the in-depth assessments of DASs SACs for variety applications including electrocatalysis, thermocatalysis and photocatalysis are provided, as well as their unique catalytic mechanism are addressed. Moreover, the prospects and challenges for DASs SACs and related applications are highlighted. The authors believe the great expectations for DASs SACs, and this review will provide novel conceptual and methodological perspectives and exciting opportunities for further development and application of DASs SACs.

Acidic CO2 Electrolysis Addressing the "Alkalinity Issue" and Achieving High CO2 Utilization

Electrochemical CO2 reduction reaction (CO2 RR) provides a promising approach for sustainable chemical fuel production of carbon neutrality. Neutral and alkaline electrolytes are predominantly employed in the current electrolysis system, but with striking drawbacks of (bi)carbonate (CO3 2- /HCO3 - ) formation and crossover due to the rapid and thermodynamically favourable reaction between hydroxide (OH- ) with CO2 , resulting in low carbon utilization efficiency and short-lived catalysis. Very recently, CO2 RR in acidic media can effectively address the (bi)carbonate issue; however, the competing hydrogen evolution reaction (HER) is more kinetically favourable in acidic electrolytes, which dramatically reduces CO2 conversion efficiency. Thus, it is a big challenge to effectively suppress HER and accelerate acidic CO2 RR. In this review, we begin by summarizing the recent progress of acidic CO2 electrolysis, discussing the key factors limiting the application of acidic electrolytes. We then systematically discuss addressing strategies for acidic CO2 electrolysis, including electrolyte microenvironment modulation, alkali cations adjusting, surface/interface functionalization, nanoconfinement structural design, and novel electrolyzer exploitation. Finally, the new challenges and perspectives of acidic CO2 electrolysis are suggested. We believe this timely review can arouse researchers' attention to CO2 crossover, inspire new insights to solve the "alkalinity problem" and enable CO2 RR as a more sustainable technology.

Performance Regulation of Single-Atom Catalyst by Modulating the Microenvironment of Metal Sites

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

Understanding the selection of catalytic pathway on graphene-supported nitrogen coordinated Ru-atom by ab initio molecular dynamics simulation

CONTEXT

In the paper, the ORR/OER on graphene-supported nitrogen coordinated Ru-atom (Ru-N-C) is simulated. We discuss nitrogen coordination influences electronic properties, adsorption energies, and catalytic activity in a single-atom Ru active site. The over potentials on Ru-N-C are 1.12 eV/1.00 eV for ORR/OER. We calculate Gibbs-free energy (ΔG) for every reaction step in ORR/OER process. In order to gain a deeper understanding of the catalytic process on the surface of single atom catalysts, the ab initio molecular dynamics (AIMD) simulations show that Ru-N-C has a structural stability at 300 K and that ORR/OER on Ru-N-C can occur along a typical four-electron process of reaction. AIMD simulations of catalytic processes provide detailed information about atom interactions.

METHODS

In this paper, we use density functional theory (DFT) with PBE functional to study the electronic properties and adsorption properties of graphene-supported nitrogen coordinated Ru-atom (Ru-N-C) Gibbs-free energy and Gibbs-free energy for very reaction step. The structural optimization and all the calculations are carried out by Dmol3 package, adopting the PNT basis set and DFT semicore pseudopotential. Ab initio molecular dynamics simulations (AIMD) were run for 10 ps. The canonical (NVT) ensemble, massive GGM thermostat, and a temperature of 300 K are taken into account. The functional of B3LYP and the DNP basis set are chosen for AIMD.

Single-Atom Nano-Islands (SANIs): A Robust Atomic-Nano System for Versatile Heterogeneous Catalysis Applications

Academician Tao Zhang from China and co-workers designed the first Pt₁ /FeO_x single-atom catalysts (SACs) in 2011, and they proposed the concept of "single-atom catalysis" in the field of heterogeneous catalysis. Generally, it is easy for active metal single-atom sites on a carrier to migrate and aggregate, which results in poor performance; or the chemical bond between the metal atom and carrier is too strong (immovable), which results in passivation of the active site. Recently, "nano-island" type SACs were designed, in which the active metal atoms are isolated on the "islands", and can move within the respective "island", but the migration across the "island" is blocked, to achieve a dynamic confinement design of single atoms (that is, a "moving but not aggregating" design philosophy). Herein, a new concept of "single-atom nano-islands (SANIs)" is proposed to describe these congeneric "atomic-nano" systems in heterogeneous catalysis fields. Particularly, the SANIs are divided into three categories: "one-island-one-atom", "one-island-multi-atoms", and "island-sea synergism" architectures. The scientific significance and application principles of SANIs in versatile heterogeneous catalysis fields (i.e., thermocatalysis, electrocatalysis, and photocatalysis) are summarized. The challenges and proposals of SANIs are also provided.

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.

A Fluorinated Covalent Organic Framework with Accelerated Oxygen Transfer Nanochannels for High-Performance Zinc-Air Batteries

The establishment of abundant three-phase interfaces with accelerated mass transfer in air cathodes is highly desirable for the development of high-rate and long-cycling rechargeable zinc-air batteries (ZABs). Covalent organic frameworks (COFs) exhibit tailored nanopore structures, facilitating the rational tuning of their specific properties. Here, by finely tuning the fluorinated nanopores of a COF, a novel air cathode for rechargeable ZABs is unprecedentedly designed and synthesized. COF nanosheets are decorated with fluorinated alkyl chains, which shows high affinity to oxygen (O₂ ), in its nanopores (fluorinated COF). The fluorinated COF nanosheets are stacked into well-defined O₂ -transport channels, which are then assembled into aerophilic "nano-islands" on the hydrophilic FeNi layered-double-hydroxide (FeNi LDH) electrocatalyst surface. Therefore, the mass-transport "highway" for O₂ and water is segregated on the nanoscale, which significantly enlarges the area of three-phase boundaries and greatly promotes the mass-transfer therein. ZABs based on the COF-modified air cathode deliver a small charge/discharge voltage gap (0.64 V at 5 mA cm^-2 ), a peak power density (118 mW cm^-2 ), and a stable cyclability. This work provides a feasible approach for the design of the air cathodes for high-performance ZABs, and will expand the new application of COFs.

Altering Ligand Microenvironment of Atomically Dispersed CrN4 by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium

Because of the instability and Fenton reactivity of non-precious metal nitrogen-carbon based catalyst when processing the oxygen reduction reaction (ORR), seeking for electrocatalysts with highly efficient performance becomes very highly desired to speed up the commercialization of fuel cell. Herein, chromium (Cr)-N₄  electrocatalyst containing extraterrestrial S formed axial S₁ -Cr₁ N₄  bonds (S₁ Cr₁ N₄ C) is achieved via an assembly polymerization and confined pyrolysis strategy. Benefiting from the adjusting  coordination configuration and electronic structure of the metal center through axial coordination, S₁ Cr₁ N₄ C exhibits enhanced the intrinsic activity (half-wave potential (E_1/2 ) is 0.90 V versus reversable hydrogen electrode, RHE) compared with that of CrN₄ C and Pt/C catalysts. More notably, the catalyst is almost inert in catalyzing the Fenton reaction, and thus shows the high stability. Density functional theory (DFT) results further reveal that the existence of axial S atoms in S₁ Cr₁ N₄ C moiety has the better ORR activity than Cr₁ N₄ C moieties. The axial S ligand in S₁ Cr₁ N₄ C moiety can break the electron localization around the planar Cr₁ N₄  active center, which facilitated the rate-limiting reductive release of OH* and accelerated overall ORR process. The present work opens up a new avenue to modulate the axial ligand type of the single-atoms (SAs) active center to enhance intrinsic SAs performances.

Ambient Electrosynthesis toward Single-Atom Sites for Electrocatalytic Green Hydrogen Cycling

With the ultimate atomic utilization, well-defined configuration of active sites and unique electronic properties, catalysts with single-atom sites (SASs) exhibit appealing performance for electrocatalytic green hydrogen generation from water splitting and further utilization via hydrogen-oxygen fuel cells, such that a vast majority of synthetic strategies toward SAS-based catalysts (SASCs) are exploited. In particular, room-temperature electrosynthesis under atmospheric pressure offers a novel, safe, and effective route to access SASs. Herein, the recent progress in ambient electrosynthesis toward SASs for electrocatalytic sustainable hydrogen generation and utilization, and future opportunities are discussed. A systematic summary is started on three kinds of ambient electrochemically synthetic routes for SASs, including electrochemical etching (ECE), direct electrodeposition (DED), and electrochemical leaching-redeposition (ELR), associated with advanced characterization techniques. Next, their electrocatalytic applications for hydrogen energy conversion including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, and oxygen reduction reaction are reviewed. Finally, a brief conclusion and remarks on future challenges regarding further development of ambient electrosynthesis of high-performance and cost-effective SASCs for many other electrocatalytic applications are presented.

Atomically Dispersed Ni-Cu Catalysts for pH-Universal CO2 Electroreduction

CO₂ electroreduction is of great significance to reduce CO₂ emissions and complete the carbon cycle. However, the unavoidable carbonate formation and low CO₂ utilization efficiency in neutral or alkaline electrolytes hinder its application at commercial scale. The development of CO₂ reduction under acidic conditions provides a promising strategy, but the inhibition of the hydrogen evolution reaction is difficult. Herein, the first work to design a Ni-Cu dual atom catalyst supported on hollow nitrogen-doped carbon is reported for pH-universal CO₂ electroreduction to CO. The catalyst shows a high CO Faradaic efficiency of ≈99% in acidic, neutral, and alkaline electrolytes, and the partial current densities of CO reach 190 ± 11, 225 ± 10, and 489 ± 14 mA cm^-2 , respectively. In particular, the CO₂ utilization efficiency under acidic conditions reaches 64.3%, which is twice as high as that of alkaline conditions. Detailed study indicates the existence of electronic interaction between Ni and Cu atoms. The Cu atoms push the Ni d-band center further toward the Fermi level, thereby accelerating the formation of *COOH. In addition, operando characterizations and density functional theory calculation are used to elucidate the possible reaction mechanism of CO₂ to CO under acidic and alkaline electrolytes.

Ionic Covalent Organic Frameworks-Derived Cobalt Single Atoms and Nanoparticles for Efficient Oxygen Electrocatalysis

Metal single atoms show outstanding electrocatalytic activity owing to the abundant atomic reactive sites and superior stability. However, the preparation of single atoms suffers from inexorable metal aggregation which is harmful to electrocatalytic activity. Here, ionic covalent organic frameworks (iCOFs) are employed as the sacrificial precursor to mitigate the metal aggregation and subsequent formation of bulky particles. Molecular dynamics simulation shows that iCOFs can trap and confine more Co ions as compared to neutral COFs, resulting in the formation of a catalyst composed of Co single atoms and uniformly distributed Co nanoparticles (Co_SA &Co_NP-10 ). However, the neutral COFs derive a catalyst composed of Co atomic clusters and large Co nanoparticles (Co_AC &Co_NP-25 ). The Co_SA &Co_NP-10 catalyst exhibits higher oxygen bifunctional electrocatalytic activities than Co_AC &Co_NP-25 , coinciding with the density functional theory results. Taking the Co_SA &Co_NP-10 as the air cathode in Zn-air batteries (ZABs), the aqueous ZAB presents a high power density of 181 mW cm^-2 , a specific capacity of 811 mAh g^-1 as well as a long cycle life of 407 h at a current density of 10 mA cm^-2 , while the quasi-solid state ZAB displays a power density of 179 mW cm^-2 and the cycle life of 30 h.

Heteroatom Coordination Regulates Iron Single-Atom-Catalyst with Superior Oxygen Reduction Reaction Performance for Aqueous Zn-Air Battery

Platinum group metal (PGM)-free M-N-C catalysts have exhibited dramatic electrocatalytic performance and are considered the most promising candidate of the Pt catalysts in oxygen reduction reaction (ORR). However, the electrocatalytic performance of the M-N-C catalysts is still limited by their inferior intrinsic activity and finite active site density. Regulating the coordination environment and increasing the pore structure of the catalyst is an effective strategy to enhance the electrocatalytic performance of the M-N-C catalysts. In this work, the coordination environment and pore structure exquisitely regulated Fe-N-C catalyst exhibit excellent ORR activity and durability. With the enhanced intrinsic activity and increased active site density, the optimized Fe-N/S-C catalyst shows impressive ORR activity (E_1/2  = 0.904 V vs reversible hydrogen electrode (RHE)) and superior long-term durability in an alkaline medium. As the advanced physical characterization and theoretical chemistry methods illustrate, the S-modified Fe-N_x (Fe-N₃ /S-C) moiety is confirmed as the improved active center for ORR, and the increased active site density further improved ORR efficiency. Based on the Fe-N/S-C cathode, a Zn-air battery is fabricated and shows superior power density (315.4 mW cm^-2 ) and long-term discharge stability at 20 mA cm^-2 . This work would open a new perspective to design atomically dispersed iron-metal site catalysts for advanced electro-catalysis.

Graphdiyne-Based Single-Atom Catalysts with Different Coordination Environments

As a special carbon material, graphdiyne (GDY) features the superiorities of incomplete charge transfer effect on the atomic level, tunable electronic structure and anchoring metal atoms directly with organometallic coordination bonds M (metal)-C (alkynyl carbon in GDY), providing it an ideal platform to construct single-atom catalysts (ACs). The coordination environment of single atoms anchored on GDY plays a key role in their catalytic performance. The mini-review highlights state-of-the-art progress in the rational design of GDY-based ACs and their applications, and mainly reveals the relationship between the coordination engineering of the GDY-based ACs and corresponding catalytic performance. Finally, some prospects concerning the future development of GDY-based ACs in energy conversion are also discussed.

Asymmetric Coordination Environment Engineering of Atomic Catalysts for CO2 Reduction

Single-atom catalysts (SACs) have emerged as well-known catalysts in renewable energy storage and conversion systems. Several supports have been developed for stabilizing single-atom catalytic sites, e.g., organic-, metal-, and carbonaceous matrices. Noticeably, the metal species and their local atomic coordination environments have a strong influence on the electrocatalytic capabilities of metal atom active centers. In particular, asymmetric atom electrocatalysts exhibit unique properties and an unexpected carbon dioxide reduction reaction (CO₂RR) performance different from those of traditional metal-N₄ sites. This review summarizes the recent development of asymmetric atom sites for the CO₂RR with emphasis on the coordination structure regulation strategies and their effects on CO₂RR performance. Ultimately, several scientific possibilities are proffered with the aim of further expanding and deepening the advancement of asymmetric atom electrocatalysts for the CO₂RR.

Functionalized MOF-Based Photocatalysts for CO2 Reduction

Metal-organic frameworks (MOFs) materials have become a research forefront in the field of photocatalytic CO₂ reduction attributed to their ultra-high specific surface area, adjustable structure, and abundant catalytic active sites. Particularly, MOFs can be facilely tuned to match CO₂ photoreduction by utilizing post-modification of metal nodes, functionalization of organic linkers, and combination with other active materials. Herein, the recent advances in the construction strategy of MOF-based photocatalysts materials for CO₂ reduction are highlighted. Some systematic modification strategies on MOF-based photocatalysts are also discussed, such as modification of metal sites and organic ligands, construction of heterojunction, introduction of single/dual-atom, and strain engineering. Finally, the future development directions of MOF-based photocatalysts in the field of CO₂ reduction are presented.

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal-organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.