Catalysis with Two-Dimensional Materials Confining Single Atoms: Concept, Design, and Applications

Phase transition and electrochemical properties of S-functionalized MXene anodes for Li-ion batteries: a first-principles investigation

The advancement of anode materials for achieving high energy storage is a crucial topic for high-performance Li-ion batteries (LIBs). Here, first-principles calculations were used to conduct a thorough and systematic investigation into lithium storage properties of MXenes with new S functional groups as LIB anode materials. Density of states, diffusion energy barriers, open circuit voltages and storage capacities were calculated to comprehensively evaluate the lithium storage properties of S-functionalized MXenes. Based on the computational results, Ti2CS2 and V2CS2 were selected as excellent candidates from ten M2CS2 MXenes. The diffusion energy barriers of M2CS2 within the range of 0.26-0.32 eV are lower than those of M2CO2 and M2CF2, indicating that M2CS2 anodes exhibit faster charge/discharge rates. By examining the stable crystal structures and comparing atomic positions before and after Li adsorptions, structural phase transitions during Li-ion adsorptions could happen for nearly all M2CS2 MXenes. The phase transitions predicted were directly observed using ab initio molecular dynamic simulations. The cycle stability, storage capacity and other lithium storage properties were enhanced by the reversible structural phase transition.

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.

Large-Scale Synthesis of High-Loading Single Metallic Atom Catalysts by a Metal Coordination Route

Single atom catalyst (SAC) is one of the most efficient and versatile catalysts with well-defined active sites. However, its facile and large-scale preparation, the prerequisite of industrial applications, has been very challenging. This dilemma originates from the Gibbs-Thomson effect, which renders it rather difficult to achieve high single atom loading (< 3 mol%). Further, most synthesizing procedures are quite complex, resulting in significant mass loss and thus low yields. Herein, a novel metal coordination route is developed to address these issues simultaneously, which is realized owing to the rapid complexation between ligands (e.g., biuret) and metal ions in aqueous solutions and subsequent in situ polymerization of the formed complexes to yield SACs. The whole preparation process involves only one heating step operated in air without any special protecting atmospheres, showing general applicability for diverse transition metals. Take Cu SAC for an example, a record yield of up to 3.565 kg in one pot and an ultrahigh metal loading 16.03 mol% on carbon nitride (Cu/CN) are approached. The as-prepared SACs are demonstrated to possess high activity, outstanding selectivity, and robust cyclicity for CO2 photoreduction to HCOOH. This research explores a robust route toward cost-effective, massive production of SACs for potential industrial applications.

Synergistic Al-Al Dual-Atomic Site for Efficient Artificial Nitrogen Fixation

Synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the Haber-Bosch process. However, it is commonly obstructed by the high activation energy. Here, we report the design and synthesis of an Al-Al bonded dual atomic catalyst stabilized within an amorphous nitrogen-doped porous carbon matrix (Al2NC) with high NRR performance. The dual atomic Al2-sites act synergistically to catalyze the complex multiple steps of NRR through adsorption and activation, enhancing the proton-coupled electron transfer. This Al2NC catalyst exhibits a high Faradaic efficiency of 16.56±0.3 % with a yield rate of 29.22±1.2 μg h-1 mgcat -1. The dual atomic Al2NC catalyst shows long-term repeatable, and stable NRR performance. This work presents an insight into the identification of synergistic dual atomic catalytic site and mechanistic pathway for the electrochemical conversion of N2 to NH3.

Ni Single Atom Decorated Porous Hollow Carbon Nanosphere-Based Electrodes for High Performance Symmetric Solid-State Supercapacitors

Ni single atom containing hollow carbon nanospheres with nitrogen doping has been synthesized by carbonization of Ni(NO3)2/phloroglucinol-formaldehyde polymer/silica composite. The samples have been characterized by powder X-ray diffraction, nitrogen adsorption/desorption, electron microscopic, Raman and X-ray photoelectron spectroscopic studies. The microstructure and surface area vary with the amount of Ni(NO3)2 employed in the syntheses and the carbonization environment. An optimized amount of nickel and argon as the carbonization gas afford Ni-1.0@N@HCN-Ar which possesses overall superior features. The uniformly dispersed Ni single atoms within the hollow porous carbon framework fully utilize all the electroactive sites thereby improving the supercapacitive performance. The specific capacitance of Ni-1.0@N@HCN-Ar reaches 777 F g-1 at 1 A g-1 with a Coulombic efficiency of 98.4 % and excellent recyclability. The energy and power density of Ni-1.0@N@HCN-Ar are found to be high; at 1 A g-1 its energy density is 155.4 Wh kg-1 with a power density of 600.3 W kg-1. At a high current density of 10 A g-1 the material shows a high energy density of 118.4 Wh kg-1 with excellent power density of 6003.4 W kg-1. A symmetric solid-state supercapacitor assembled with this material, Ni-1.0@N@HCN-Ar//Ni-1.0@N@HCN-Ar using H2SO4/PVA gel electrolyte shows a superior energy density value of 30 Wh kg-1 at a power density of 1200 W kg-1.

Multifunctional Antimonene-Silver Nanocomposites for Ultra-Multi-Mode and Multi-Analyte Sensing, Parallel and Batch Logic Computing, Long-Text Information Protection

To simulate life's emergent functions, mining the multiple sensing capabilities of nanosystems, and digitizing networks of transduction signals and molecular interactions, is an ongoing endeavor. Here, multifunctional antimonene-silver nanocomposites (AM-Ag NCs) are synthesized facilely and fused for molecular sensing and digitization applications (including ultra-multi-mode and multi-analyte sensing, parallel and batch logic computing, long-text information protection). By mixing surfactant, AM, Ag+ and Sodium borohydride (NaBH4) at room temperature for 5 min, the resulting NCs are comprised of Ag nanoparticles scattered within AM nanosheets and protected by the surfactant. Interestingly, AM-Ag NCs exhibit ultra-multi-mode sensing ability for multiplex metal ions (Hg2+, Fe3+, or Al3+), which significantly improved selectivity (≈2 times) and sensitivity (≈400 times) when analyzing the combined channels. Moreover, multiple sensing capabilities of AM-Ag NCs enable diverse batch and parallel molecular logic computations (including advanced cascaded logic circuits). Ultra-multi-mode selective patterns of AM-Ag NCs to 18 kinds of metal ions can be converted into a series of binary strings by setting the thresholds, and realized high-density, long-text information protection for the first time. This study provides new ideas and paradigms for the preparation and multi-purpose application of 2D nanocomposites, but also offers new directions for the fusion of molecular sensing and informatization.

Graphitic carbon nitride supported Ni-Co dual-atom catalysts beyond Ni1(Co1) single-atom catalysts for hydrogen production: a density functional theory study

Using density functional theory calculations we investigate the formation, structure and electronic properties of gh-C3N4-supported Ni-Co (Ni-Co/gh-C3N4) dual-atom catalysts and Ni1(Co1) single-metal catalysts, as a paradigmatic example of single-atom versus few-atom catalysts. An inverted mold assumption is proposed to identify the factors determining the number, shape and packing manner of metal atoms inside the pores of gh-C3N4. The area matching between virtual fragments and metal fillers and lattice inheritance from N coordination and metal aggregates allow for a stable Ni-Co/gh-C3N4, which would possess more active sites and a more complex structure-activity relation than single-atom doping. The hydrogen production behavior and catalytic activity of this catalyst are comprehensively discussed. Ni-Co/gh-C3N4 exhibits higher hydrogen evolution activity than Ni1(Co1)/gh-C3N4 at an appropriate H coverage, which is comparable to Pt under analogous conditions. This strategy, derived from the inverted mold assumption, is deemed to be a simple and easy-to-operate method for designing and building metal aggregates confined inside the pores of two-dimensional materials and in the cavities of nanoparticles for few-atom catalysts.

Oxygen Doping Cooperated with Co-N-Fe Dual-Catalytic Sites: Synergistic Mechanism for Catalytic Water Purification within Nanoconfined Membrane

Atom-site catalysts, especially for graphitic carbon nitride-based catalysts, represents one of the most promising candidates in catalysis membrane for water decontamination. However, unravelling the intricate relationships between synthesis-structure-properties remains a great challenge. This study addresses the impacts of coordination environment and structure units of metal central sites based on Mantel test, correlation analysis, and evolution of metal central sites. An optimized unconventional oxygen doping cooperated with Co-N-Fe dual-sites (OCN Co/Fe) exhibits synergistic mechanism for efficient peroxymonosulfate activation, which benefits from a significant increase in charge density at the active sites and the regulation in the natural population of orbitals, leading to selective generation of SO4 •-. Building upon these findings, the OCN-Co/Fe/PVDF composite membrane demonstrates a 33 min-1 ciprofloxacin (CIP) rejection efficiency and maintains over 96% CIP removal efficiency (over 24 h) with an average permeance of 130.95 L m-2 h-1. This work offers a fundamental guide for elucidating the definitive origin of catalytic performance in advance oxidation process to facilitate the rational design of separation catalysis membrane with improved performance and enhanced stability.

Single-Atom based Metal-Organic Framework Photocatalysts for Solar-Fuel Generation

The growing demand for fossil fuels and subsequent CO2 emissions prompted a search for alternate sources of energy and a reduction in CO2. Photocatalysis driven by solar light has been found as a potential research area to tackle both these problems. In this direction, SAC@MOF (Single-atom loaded MOFs) photocatalysis is an emerging field and a promising technology. The unique properties of single-atom catalysts (SACs), such as high catalytic activity and selectivity, are leveraged in these systems. Photocatalysis, focusing on the utilization of Metal-Organic Frameworks (MOFs) as platforms for creating single-atom catalysts (SACs) characterized by metal single-atoms (SAs) as their active sites, are noted for their unparalleled atomic efficiency, precisely defined active sites, and superior photocatalytic performance. The synergy between MOFs and SAs in photocatalytic systems is meticulously examined, highlighting how they collectively enhance photocatalytic efficiency. This review examines SAC@MOF development and applications in environmental and energy sectors, focusing on synthesis and stabilization methods for SACs on MOFs and also characterization techniques vital for understanding these catalysts. The potential of SAC@MOF in CO2 Photoreduction and Photocatalytic H2 evolution is highlighted, emphasizing its role in green energy technologies and advances in materials science and Photocatalysis.

Effective Photocatalytic Ethanol Reforming into High-Value-Added Multicarbon Compound Coupled with H2 Production Over Pt-S3 Sites at PtSA-ZnIn2S4 Interface

Selective photocatalytic production of high-value acetaldehyde concurrently with H2 from bioethanol is an appealing approach to meet the urgent environment and energy issues. However, the difficult ethanol dehydrogenation and insufficient active sites for proton reduction within the catalysts, and the long spatial distance between these two sites always restrict their catalytic activity. Here, guided by the strong metal-substrate interaction effect, an atomic-level catalyst design strategy to construct Pt-S3 single atom on ZnIn2S4 nanosheets (PtSA-ZIS) is demonstrated. As active center with optimized H adsorption energy to facilitate H2 evolution reaction, the unique Pt single atom also donates electrons to its neighboring S atoms with electron-enriched sites formed to activate the O─H bond in *CH3CHOH and promote the desorption of *CH3CHO. Thus, the synergy between Pt single atom and ZIS together will reduce the energy barrier for the ethanol oxidization to acetaldehyde, and also narrow the spatial distance for proton mass transfer. These features enable PtSA-ZIS photocatalyst to produce acetaldehyde with a selectivity of ≈100%, which will spontaneously transform into 1,1-diethoxyethane via acetalization to avoid volatilization. Meanwhile, a remarkable H2 evolution rate (184.4 µmol h-1) is achieved with a high apparent quantum efficiency of 10.50% at 400 nm.

Self-Organized Gradually Single-Atom-Layer of Metal Osmium for an Unprecedented Hydrogen Production from Seawater

Atomic thick two-dimensional (2D) materials with exciting physical, chemical, and electronic properties are gaining increasing attention in next-generation science and technology, showing great promise in catalysis and energy science. However, the precise design and synthesis of efficient catalytic systems based on such materials still face many difficulties, especially in how to control the preparation of structurally determined, highly active, atomic-scale distribution of material systems. Here, we report that a highly active zerovalent osmium single-atom-layer with a thickness of single atom size has been successfully and controllably self-organized on the surface of 2D graphdiyne (GDY) material. Detailed characterizations showed that the incomplete charge transfer effect between the Os atoms and GDY not only stabilized the catalytic system but also improved the intrinsic activity, making the Gibbs free energy reach the best and resulting in remarkable performance with a small overpotential of 49 mV at 500 mA cm-2, large specific j0 of 18.6 mA cm-2, and turnover frequency of 3.89 H2 s-1 at 50 mV. In addition, the formation of sp-C-Os bonds guarantees the high long-term stability of 800 h at a large current density of 500 mA cm-2 in alkaline simulated seawater.

Heterogeneous Rhodium Single-Atom-Site Catalyst Enables Chemoselective Carbene N-H Bond Insertion

Transition-metal-catalyzed carbene insertion reactions of a nitrogen-hydrogen bond have emerged as robust and versatile methods for the construction of C-N bonds. While significant progress of homogeneous catalytic metal carbene N-H insertions has been achieved, the control of chemoselectivity in the field remains challenging due to the high electrophilicity of the metal carbene intermediates. Herein, we present an efficient strategy for the synthesis of a rhodium single-atom-site catalyst (Rh-SA) that incorporates a Rh atom surrounded by three nitrogen atoms and one phosphorus atom doped in a carbon support. This Rh-SA catalyst, with a catalyst loading of only 0.15 mol %, exhibited exceptional catalytic performance for heterogeneous carbene insertion with various anilines and heteroaryl amines in combination with diazo esters. Importantly, the heterogeneous catalyst selectively transformed aniline derivatives bearing multiple nucleophilic moieties into single N-H insertion isomers, while the popular homogeneous Rh2(OAc)4 catalyst produced a mixture of overfunctionalized side products. Additionally, similar selectivities for N-H bond insertion with a set of stereoelectronically diverse diazo esters were obtained, highlighting the general applicability of this heterogeneous catalysis approach. On the basis of density functional theory calculations, the observed selectivity of the Rh-SA catalyst was attributed to the insertion barriers and the accelerated proton transfer assisted by the phosphorus atom in the support. Overall, this investigation of heterogeneous metal-catalyzed carbene insertion underscores the potential of single-atom-site catalysis as a powerful and complementary tool in organic synthesis.

Boosting CO2 Hydrogenation to Methanol over Monolayer MoS2 Nanotubes by Creating More Strained Basal Planes

The controlled creation, selective exposure, and activation of more basal planes while simultaneously minimizing the generation and exposure of edge sites are crucial for accelerating methanol synthesis from CO2 hydrogenation over MoS2 catalysts but remain a bottleneck. Here, we report a facile method to fabricate heteronanotube catalysts with single-layer MoS2 coaxially encapsulating the carbon nanotubes (CNTs@MoS2) through host-guest chemistry. Inheriting the long tubular structure of CNTs, the grown MoS2 nanotubes exhibit significantly more basal planes than bulk MoS2 crystals. More importantly, the tubular curvature not only promotes strain and sulfur vacancy (Sv) generation but also preferentially exposes more in-plane Sv while limiting edge Sv exposure, which is conducive to methanol synthesis. Both the strain and layer number of MoS2 can be easily and finely adjusted by altering CNT diameter and quantity of precursors. Remarkably, CNTs@MoS2 with monolayer MoS2 and maximum strain displayed methanol selectivity of 78.1% and methanol space time yield of 1.6 g gMoS2-1 h-1 at 260 °C and GHSV of 24000 mL gcat.-1 h-1, representing the best results to date among Mo-based catalysts. This study provides prospects for novel catalyst design by synthesizing coaxial tubular heterostructure to create additional catalytic sites and ultimately enhance conversion and selectivity.

Surface Engineered Single-atom Systems for Energy Conversion

Single-atom catalysts (SACs) are demonstrated to show exceptional reactivity and selectivity in catalytic reactions by effectively utilizing metal species, making them a favorable choice among the different active materials for energy conversion. However, SACs are still in the early stages of energy conversion, and problems like agglomeration and low energy conversion efficiency are hampering their practical applications. Substantial research focus on support modifications, which are vital for SAC reactivity and stability due to the intimate relationship between metal atoms and support. In this review, a category of supports and a variety of surface engineering strategies employed in SA systems are summarized, including surface site engineering (heteroatom doping, vacancy introducing, surface groups grafting, and coordination tunning) and surface structure engineering (size/morphology control, cocatalyst deposition, facet engineering, and crystallinity control). Also, the merits of support surface engineering in single-atom systems are systematically introduced. Highlights are the comprehensive summary and discussions on the utilization of surface-engineered SACs in diversified energy conversion applications including photocatalysis, electrocatalysis, thermocatalysis, and energy conversion devices. At the end of this review, the potential and obstacles of using surface-engineered SACs in the field of energy conversion are discussed. This review aims to guide the rational design and manipulation of SACs for target-specific applications by capitalizing on the characteristic benefits of support surface engineering.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Modulating Electronic Structure of Iridium Single-Atom Anchored on 3D Fe-Doped β-Ni(OH)2 Catalyst with Nanopyramid Array Structure for Enhanced Oxygen Evolution Reaction

Developing high-performance electrocatalysts for oxygen evolution reaction (OER) is crucial in the pursuit of clean and sustainable hydrogen energy, yet still challenging. Herein, a spontaneous redox strategy is reported to achieve iridium single-atoms anchored on hierarchical nanosheet-based porous Fe doped β-Ni(OH)2 pyramid array electrodes (SAs Ir/Fe-β-Ni(OH)2 ), which exhibits high OER performance with a low overpotential of 175 mV at 10 mA cm-2 and a remarkable OER current density in alkaline electrolyte, surpassing Fe-β-Ni(OH)2 /NF and IrO2 by 31 and 38 times at 1.43 V versus RHE, respectively. OER catalytic mechanism demonstrates that the conversion of * OH→* O and the active lattice O content can be significantly improved due to the modulation effect of the Ir single atoms on the local electronic structure and the redox behavior of FeNi (oxy) hydroxide true active species. This work provides a promising insight into understanding the OER enhancement mechanism for Ir single-atoms modified FeNi-hydroxide systems.

Ru Regulated Electronic Structure of Pdx Cuy Nanosheets for Efficient Hydrogen Evolution Reaction in Wide pH Range

The development of highly effective catalysts for hydrogen evolution reaction (HER) in a wide pH range is crucial for the sustainable utilization of green energy utilization, while the slow kinetic reaction rate severely hinders the progress of HER. Herein, the reaction kinetic issue is solved by adjusting the electronic structure of the Ru/Pdx Cuy catalysts. The champion catalyst displays a remarkable performance for HER with the ultralow overpotential (27, 28, and 97 mV) in 1.0 m KOH, 0.5 m H2 SO4 , and 1.0 m PBS at 10 mA cm-2 and high the mass activity (3036 A g-1 ), respectively, superior to those of commercial Pt/C benchmarks and most of reported electrocatalysts, mainly due to its low reaction activation energy. Density functional theory (DFT) calculations indicate that Ru doping contributes an electron-deficient 3d band, which promotes water adsorption. Additionally, this also leads to an upward shift of the d-band center of Pd and a downward shift of the d-band center of Cu, further optimizing the adsorption/dissociation of H2 O and H* . Results from this work may provide an insight into the design and synthesis of high-performance pH-universal HER electrocatalysts.

Emerging Two-Dimensional Carbonaceous Materials for Electrocatalytic Energy Conversions: Rational Design of Active Structures through High-Temperature Chemistry

Electrochemical energy conversion and storage technologies involving controlled catalysis provide a sustainable way to handle the intermittency of renewable energy sources, as well as to produce green chemicals/fuels in an ecofriendly manner. Core to such technology is the development of efficient electrocatalysts with high activity, selectivity, long-term stability, and low costs. Here, two-dimensional (2D) carbonaceous materials have emerged as promising contenders for advancing the chemistry in electrocatalysis. We review the emerging 2D carbonaceous materials for electrocatalysis, focusing primarily on the fine engineering of active structures through thermal condensation, where the design, fabrication, and mechanism investigations over different types of active moieties are summarized. Interestingly, all the recipes creating two-dimensionality on the carbon products also give specific electrocatalytic functionality, where the special mechanisms favoring 2D growth and their consequences on materials functionality are analyzed. Particularly, the structure-activity relationship between specific heteroatoms/defects and catalytic performance within 2D metal-free electrocatalysts is highlighted. Further, major challenges and opportunities for the practical implementation of 2D carbonaceous materials in electrocatalysis are summarized with the purpose to give future material design guidelines for attaining desirable catalytic structures.

Surface Electronic Properties-Driven Electrocatalytic Nitrogen Reduction on Metal-Conjugated Porphyrin 2D-MOFs

Two-dimensional (2D) metal organic framework (MOF) or metalloporphyrin nanosheets with a stable metal-N4 complex unit present the metal as a single-atom catalyst dispersed in the 2D porphyrin framework. First-principles calculations on the 3d-transition metals in M-TCPP are investigated in this study for their surface-dependent electronic properties including work function and d-band center. Crystal orbital Hamiltonian population (-pCOHP) analysis highlights a higher contribution of the bonding state in the M-N bond and antibonding state in the N-N bond to be essential for N-N bond activation. A linear relationship between ΔGmax and surface electronic properties, N-N bond strength, and Bader charge has been found to influence the rate-determining potential for nitrogen reduction reaction (NRR) in M-TCPP MOFs. 2D Ti-TCPP MOF, with a kinetic energy barrier of 1.43 eV in the final protonation step of enzymatic NRR, shows exclusive NRR selectivity over competing hydrogen reduction (HER) and nitrogenous compounds (NO and NO2). Thus, Ti-TCPP MOF with an NRR limiting potential of -0.35 V in water solvent is proposed as an attractive candidate for electrocatalytic NRR.

Design of MoS2 edge-anchored single-atom catalysts for propane dehydrogenation driven by DFT and microkinetic modeling

The design of efficient catalysts for direct propane dehydrogenation (PDH) to inhibit coke formation and deactivation of traditional Pt-based catalysts are challenging tasks. Herein, by exploiting the unique geometric feature and tunability of single atom catalysts (SACs), a wide range of 3d-5d transition metals anchored on the MoS2 edge in the single atom form (TM1-S4/edge) are comprehensively investigated for the PDH application by first-principles calculations, ab initio molecular dynamics (AIMD) simulations and microkinetic modeling. Five criteria are assessed in terms of the feasibility of preparation, practical stability, feasibility of recovery after air oxidation, activity and selectivity. We identified Ru1-S4/edge SAC as the most promising candidate with activity six times higher than that of the conventional Pt(111) catalyst. Interestingly, AIMD simulations show that the motif region of the highly reactive TM1-S4/edge SACs (such as Ru, Os, Rh, and Ir) exhibits a dynamic change, with a TM-coordinated S atom tending to flutter at reaction temperatures and return to its initial position when the species is adsorbed on TMs, thereby affecting the PDH activities. In addition to identifying the potential PDH catalyst from a practical application point of view, we believe that this study also provides a comprehensive picture for the theoretical screening of low-coordination single-atom catalysts.

Renaissance of Strong Metal-Support Interactions

Strong metal-support interactions (SMSIs) have emerged as a significant and cutting-edge area of research in heterogeneous catalysis. They play crucial roles in modifying the chemisorption properties, interfacial structure, and electronic characteristics of supported metals, thereby exerting a profound influence on the catalytic properties. This Perspective aims to provide a comprehensive summary of the latest advancements and insights into SMSIs, with a focus on state-of-the-art in situ/operando characterization techniques. This overview also identifies innovative designs and applications of new types of SMSI systems in catalytic chemistry and highlights their pivotal role in enhancing catalytic performance, selectivity, and stability in specific cases. Particularly notable is the discovery of SMSI between active metals and metal carbides, which opens up a new era in the field of SMSI. Additionally, the strong interactions between atomically dispersed metals and supports are discussed, with an emphasis on the electronic effects of the support. The chemical nature of SMSI and its underlying catalytic mechanisms are also elaborated upon. It is evident that SMSI modification has become a powerful tool for enhancing catalytic performance in various catalytic applications.

Layered Double Hydroxide Nanosheets: Synthesis Strategies and Applications in the Field of Energy Conversion

In recent years, layered double hydroxides (LDH) nanosheets have garnered substantial attention as intriguing inorganic anionic layered clay materials. These nanosheets have captured the attention of researchers due to their unique physicochemical properties. This review aims to showcase the latest advancements in laboratory research concerning LDH nanosheets, with a specific emphasis on their methods of preparation. This review provides detailed insights into the factors influencing the anionic conductivity of LDH, along with delineating the applications of LDH nanosheets in the realm of energy conversion. Notably, the review highlights the crucial role of LDH nanosheets in the oxygen evolution reaction (OER), a vital process in water splitting and diverse electrochemical applications. The review emphasizes the significant potential of LDH nanosheets in enhancing supercapacitor technology, owing to their high surface area and exceptional charge storage capacity. Additionally, it elucidates the prospective application of LDH nanosheets as anion exchange membranes in anion exchange membrane fuel cells, potentially revolutionizing fuel cell performance through improved efficiency and stability facilitated by enhanced ion transport properties.

Research on the environmental stability performance of chromite ore processing residue solidified products

Chromite ore processing residue (COPR) is a hazardous waste because of leachable chromium, especially Cr(vi). Therefore, ascorbic acid (AA) and blast furnace slag (BFS) have been used to detoxify and solidify COPR. On this basis, environmental stability experiments with high temperature and freeze-thaw cycles were carried out to explore the stability performance of a solidified body with 40% COPR. The environmental stability performance was analyzed through changes in edge length, mass loss, compressive strength development, and leaching concentration of Cr(vi). The result indicated that the high-temperature environment had much more effect on the solidified body than the freeze-thaw cycle environment in these four aspects: after being maintained at 900 °C for 2 h, the compressive strength of the solidified bodies reached its minimum value (35.76 MPa). However, in the freeze-thaw cycle experiments, the compressive strength of the solidified bodies consistently remained above 80 MPa, and the leaching of hexavalent chromium was below the limit (5 mg L-1). In addition, X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR) analysis verified that COPR was effectively solidified through physical and chemical means. Moreover, high temperature changes the molecular structure of the solidified body, thus reducing the compressive strength and curing ability of the solidified body, while the freeze-thaw cycle experiment has little effect on it.

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.

Contemporary advances in photocatalytic CO2 reduction using single-atom catalysts supported on carbon-based materials

The persistent issue of CO2 emissions and their subsequent impact on the Earth's atmosphere can be effectively addressed through the utilization of efficient photocatalysts. Employing a sustainable carbon cycle via photocatalysis presents a promising technology for simultaneously managing the greenhouse effect and the energy dilemma. However, the efficiency of energy conversion encounters limitations due to inadequate carrier utilization and a deficiency of reactive sites. Single-atom catalysts (SACs) have demonstrated exceptional performance in efficiently addressing the aforementioned challenges. This review article commences with an overview of SAC types, structures, fundamentals, synthesis strategies, and characterizations, providing a logical foundation for the design and properties of SACs based on the correlation between their structure and efficiency. Additionally, we delve into the general mechanism and the role of SACs in photocatalytic CO2 reduction. Furthermore, we furnish a comprehensive survey of the latest advancements in SACs concerning their capacity to enhance efficiency, long-term stability, and selectivity in CO2 reduction. Carbon-structured support materials such as covalent organic frameworks (COFs), graphitic carbon nitride (g-C3N4), metal-organic frameworks (MOFs), covalent triazine frameworks (CTFs), and graphene-based photocatalysts have garnered significant attention due to their substantial surface area, superior conductivity, and chemical stability. These carbon-based materials are frequently chosen as support matrices for anchoring single metal atoms, thereby enhancing catalytic activity and selectivity. The motivation behind this review article lies in evaluating recent developments in photocatalytic CO2 reduction employing SACs supported on carbon substrates. In conclusion, we highlight critical issues associated with SACs, potential prospects in photocatalytic CO2 reduction, and existing challenges. This review article is dedicated to providing a comprehensive and organized compilation of recent research findings on carbon support materials for SACs in photocatalytic CO2 reduction, with a specific focus on materials that are environmentally friendly, readily accessible, cost-effective, and exceptionally efficient. This work offers a critical assessment and serves as a systematic reference for the development of SACs supported on MOFs, COFs, g-C3N4, graphene, and CTFs support materials to enhance photocatalytic CO2 conversion.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS)

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

Single-atom nanozymes with peroxidase-like activity: A review

Single-atom nanozymes (SANs) are nanomaterials-based nanozymes with atomically dispersed enzyme-like active sites. SANs offer improved as well as tunable catalytic activity. The creation of extremely effective SANs and their potential uses have piqued researchers' curiosity due to their advantages of cheap cost, variable catalytic activity, high stability, and large-scale production. Furthermore, SANs with uniformly distributed active centers and definite coordination structures offer a distinctive opportunity to investigate the structure-activity correlation and control the geometric and electrical features of metal centers. SANs have been extensively explored in photo-, thermal-, and electro-catalysis. However, SANs suffer from the following disadvantages, such as efficiency, non-mimicking of the 3-D complexity of natural enzymes, limited and narrow range of artificial SANs, and biosafety aspects. Among a quite limited range of artificial SANs, the peroxidase action of SANs has attracted significant research attention in the last five years with the aim of producing reactive oxygen species for use in cancer therapy, and water treatment among many other applications. In this review, we explore the recent progress of different SANs as peroxidase mimics, the role of the metal center in enzymatic activity, possible prospects, and underlying limitations in real-time applications.

Piezo-Flexocatalysis of Single-Atom Pt-Loaded Graphitic Carbon Nitride

This study develops a single-atom Pt-loaded graphitic carbon nitride (SA-Pt/CN) and evaluates its piezo-flexocatalytic properties by conducting a hydrogen evolution reaction (HER) and Rhodamine B (RB) dye degradation test under ultrasonic vibration in the dark. SA-Pt/CN has a hydrogen gas yield of 1283.8 µmol g-1  h-1 , which is 23.3 times higher than that of pristine g-C3 N4 . Moreover, SA-Pt/CN enhances the dye degradation reaction rate by ≈2.3 times compared with the pristine sample. SA-Pt/CN exhibits lattice distortion and strain gradient enlargement caused by the single atom Pt at the N sites of g-C3 N4 , which disrupts the symmetric structure and contributes to the enhancement of piezoelectric and flexoelectric polarization. As far as it is known, this is the first study to investigate the piezo-flexocatalytic reaction of SA-Pt/CN without light irradiation and provides new insights into single-atom piezocatalysts.

Low-Temperature Propane Activation and Mineralization over a Co3O4 Sub-nanometer Porous Sheet: Atomic-Level Insights

Light alkanes make up a class of widespread volatile organic compounds (VOCs), bringing great environmental hazards and health concerns. However, the low-temperature catalytic destruction of light alkanes is still a great challenge to settle due to their high reaction inertness and weak polarity. Herein, a Co3O4 sub-nanometer porous sheet (Co3O4-SPS) was fabricated and comprehensively compared with its bulk counterparts in the catalytic oxidation of C3H8. Results demonstrated that abundant low-coordinated Co atoms on the Co3O4-SPS surface boost the activation of adsorbed oxygen and enhance the catalytic activity. Moreover, Co3O4-SPS has better surface metal properties, which is beneficial to electron transfer between the catalyst surface and the reactant molecules, promoting the interaction between C3H8 molecules and dissociated O atoms and facilitating the activation of C-H bonds. Due to these, Co3O4-SPS harvests a prominent performance for C3H8 destruction, 100% of which decomposed at 165 °C (apparent activation energy of 49.4 kJ mol-1), much better than the bulk Co3O4 (450 °C and 126.9 kJ mol-1) and typical noble metal catalysts. Moreover, Co3O4-SPS also has excellent thermal stability and water resistance. This study deepens the atomic-level insights into the catalytic capacity of Co3O4-SPS in light alkane purification and provides references for designing efficacious catalysts for thermocatalytic oxidation reactions.

A Comprehensive Review on Electrocatalytic Applications of 2D Metallenes

This review introduces metallenes, a cutting-edge form of atomically thin two-dimensional (2D) metals, gaining attention in energy and catalysis. Their unique physicochemical and electronic properties make them promising for applications like catalysis. Metallenes stand out due to their abundance of under-coordinated metal atoms, enhancing the catalytic potential by improving atomic utilization and intrinsic activity. This review explores the utility of 2D metals as electrocatalysts in sustainable energy conversion, focusing on the Oxygen Evolution Reaction, Oxygen Reduction Reaction, Fuel Oxidation Reaction, and Carbon Dioxide Reduction Reaction. Aimed at researchers in nanomaterials and energy, the review is a comprehensive resource for unlocking the potential of 2D metals in creating a sustainable energy landscape.

A single transition metal atom anchored on Nb2C as an electrocatalyst for the nitrogen reduction reaction

Nitrogen (N2) reduction to produce ammonia (NH3) under milder conditions is attractive as NH3 has been widely used in various fields. The electrocatalytic nitrogen reduction reaction (NRR) is considered to be a more moderate and green method for ammonia synthesis. Herein, using density functional theory (DFT) computations, we investigated the potential application of single-atom catalysts (SACs) toward the NRR, in which transition metal (TM, TM = Ti, V, Mn, Fe, Co, Y, Zr, Mo) atoms are supported on Nb2C (TM-Nb2C). Through our screening, Fe-Nb2C is highlighted from 8 candidate systems as the superior SAC for the NRR with a low limiting potential of -0.47 V. Meanwhile, a volcano plot between UL (NRR) and the ICOHP values of the N-H bond in *NH2 is established to determine the optimal ICOHP values that can be used as a simple descriptor of the NRR performance of Fe-Nb2C.

Highly sensitive detection of circulating tumour cells based on an ASV/CV dual-signal electrochemical strategy

Circulating tumour cells (CTCs), as a tumour marker, may provide more information in early diagnosis and accurate therapy of cancer patients. Electrochemical detection of CTCs has exhibited exceptional advantages. However, single-signal electrochemical detection usually has a high probability of false positives coming from interferents, operating personnel, and nonstandard analytical processes. Herein, a dual-signal strategy using anodic stripping voltammetry (ASV) and cyclic voltammetry (CV) for highly sensitive detection of CTCs was developed. When MCF-7 cells were present, aptamer DNA (DNA1)-magnetic beads (MBs) were captured by CTCs and detached from the biosensing electrodes. Following magnetic separation, polystyrene bead (PS)-CdS QDs labelled on MCF-7 cells were dissolved by HNO3 and the intensity of the oxidation peak current of Cd2+ ions was proportional to the amount of MCF-7 cells in ASV (y = 6.8929 lg Ccells + 1.0357 (Ccells, cells per mL; R2, 0.9947; LOD, 3 cells per mL)). Meanwhile, the anodic peak currents of the remaining electrode in CV were also proportional to the amount of MCF-7 cells (y = 3.7891 lg Ccells + 52.3658 (Ccells, cells per mL; R2, 0.9846; LOD, 3 cells per mL)). An ASV/CV dual-signal biosensor for electrochemical detection of CTCs was achieved, which overcame the limitations of any single-signal mode and improved the detection reliability and precision.

Two Dimensional Ir-Based Catalysts for Acidic OER

Electrochemical water splitting in acidic media is one of the most promising hydrogen production technologies, yet its practical applications in proton exchange membrane (PEM) water electrolyzers are limited by the anodic oxygen evolution reaction (OER). Iridium (Ir)-based materials are considered as the state-of-the-art catalysts for acidic OER due to their good stability under harsh acidic conditions. However, their activities still have much room for improvement. Two-dimensional (2D) materials are full of the advantages of high-surface area, unique electrical properties, facile surface modification, and good stability, making the development of 2D Ir-based catalysts more attractive for achieving high catalytic performance. In this review, first, the unique advantages of 2D catalysts for electrocatalysis are reviewed. Thereafter, the classification, synthesis methods, and recent OER achievements of 2D Ir-based materials, including pure metals, alloys, oxides, and perovskites are introduced. Finally, the prospects and challenges of developing 2D Ir-based catalysts for future acidic OER are discussed.

Progress on Single-Atom Photocatalysts for H2 Generation: Material Design, Catalytic Mechanism, and Perspectives

Solar energy utilization is of great significance to current challenges of the energy crisis and environmental pollution, which benefit the development of the global community to achieve carbon neutrality goals. Hydrogen energy is also treated as a good candidate for future energy supply since its combustion not only supplies high-density energy but also shows no pollution gas. In particular, photocatalytic water splitting has attracted increasing research as a promising method for H2 production. Recently, single-atom (SA) photocatalysts have been proposed as a potential solution to improve catalytic efficiency and lower the costs of photocatalytic water splitting for H2 generation. Owing to the maximized atom utilization rate, abundant surface active sites, and tunable coordination environment, SA photocatalysts have achieved significant progress. This review reviews developments of advanced SA photocatalysts for H2 generation regarding the different support materials. The recent progress of titanium dioxide, metal-organic frameworks, two-dimensional carbon materials, and red phosphorus supported SA photocatalysts are carefully discussed. In particular, the material designs, reaction mechanisms, modulation strategies, and perspectives are highlighted for realizing improved solar-to-energy efficiency and H2 generation rate. This work will supply significant references for future design and synthesis of advanced SA photocatalysts.

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.

Insight on Atomically Dispersed Cu Catalysts for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (ECO2R) with renewable electricity is an advanced carbon conversion technology. At present, copper is the only metal to selectively convert CO2 into multicarbon (C2+) products. Among them, atomically dispersed (AD) Cu catalysts have received great attention due to the relatively single chemical environment, which are able to minimize the negative impact of morphology, valence state, and crystallographic properties, etc. on product selectivity. Furthermore, the completely exposed atomic Cu sites not only provide space and bonding electrons for the adsorption of reactants in favor of better catalytic activity but also provide an ideal platform for studying its reaction mechanism. This review summarizes the recent progress of AD Cu catalysts as a chemically tunable platform for ECO2R, including the atomic Cu sites dynamic evolution, the catalytic performance, and mechanism. Furthermore, the prospects and challenges of AD Cu catalysts for ECO2R are carefully discussed. We sincerely hope that this review can contribute to the rational design of AD Cu catalysts with enhanced performance for ECO2R.

Combining First-Principles Modeling and Symbolic Regression for Designing Efficient Single-Atom Catalysts in the Oxygen Evolution Reaction on Mo2CO2 MXenes

In this study, we address the significant challenge of overcoming limitations in the catalytic efficiency for the oxygen evolution reaction (OER). The current linear scaling relationships hinder the optimization of the electrocatalytic performance. To tackle this issue, we investigate the potential of designing single-atom catalysts (SACs) on Mo2CO2 MXenes for electrochemical OER using first-principles modeling simulations. By employing the Electrochemical Step Symmetry Index (ESSI) method, we assess OER intermediates to fine-tune the activity and identify the optimal SAC for Mo2CO2 MXenes. Our findings reveal that both Ag and Cu exhibit effectiveness as single atoms for enhancing OER activity on Mo2CO2 MXenes. However, among the 21 chosen transition metals (TMs) in this study, Cu stands out as the best catalyst for tweaking the overpotential (ηOER). This is due to Cu's lowest overpotential compared to other TMs, which makes it more favorable for the OER performance. On the other hand, Ag is closely aligned with ESSI = ηOER, making the tuning of its overpotential more challenging. Furthermore, we employ symbolic regression analysis to identify the significant factors that exhibit a correlation with the OER overpotential. By utilizing this approach, we derive mathematical formulas for the overpotential and identify key descriptors that affect the catalytic efficiency in the electrochemical OER on Mo2CO2 MXenes. This comprehensive investigation not only sheds light on the potential of MXenes in advanced electrocatalytic processes but also highlights the prospect of improved activity and selectivity in OER applications.

Theoretical study of Mo2N supported transition metal single-atom catalyst for OER/ORR bifunctional electrocatalysis

The rational design and development of an efficient bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the key to developing new renewable energy storage and conversion technologies. Transition metal nitrides (TMNs) have shown excellent energy storage and electrochemistry potential due to their unique electronic structure and physicochemical properties. In this paper, based on the first-principles method of density functional theory (DFT), a series of efficient and stable bifunctional single-atom catalysts (SACs) were designed on Mo2N by introducing transition metal atoms as active sites, and the effects of different TM atoms on the catalytic performance of 2D-Mo2N (Two dimensional Mo2N) were evaluated. The calculation results show that TM@Mo2N exhibits excellent stability and good conductivity, which is conducive to electron transfer during the electrocatalytic reaction. Among these SACs, the Au@Mo2N single-atom catalyst has a very low OER overpotential (0.36 V), exhibiting high OER activity. Meanwhile, Au@Mo2N also exhibits excellent ORR performance with a low overpotential of 0.4 V, indicating that Au@Mo2N is the best OER/ORR bifunctional catalyst. This work provides a feasible solution for developing transition metal bifunctional electrocatalysts. Au@Mo2N is expected to replace traditional commercial Pt catalyst materials and become a catalyst with excellent performance in fuel cell modules.

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Identifying the active sites in unequal iron-nitrogen single-atom catalysts

Single-atom catalysts (SACs) have become one of the most attractive frontier research fields in catalysis and energy conversion. However, due to the atomic heterogeneity of SACs and limitations of ensemble-averaged measurements, the essential active sites responsible for governing specific catalytic properties and mechanisms remain largely concealed. In this study, we develop a quantitative method of single-atom catalysis-fluorescence correlation spectroscopy (SAC-FCS), leveraging the atomic structure-dependent catalysis kinetics and single-turnover resolution of single-molecule fluorescence microscopy. This method enables us to investigate the oxidase-like single-molecule catalysis on unidentical iron-nitrogen (Fe-N) coordinated SACs, quantifying the active sites and their kinetic parameters. The findings reveal the significant differences of single sites from the average behaviors and corroborate the oxidase-like catalytic mechanism of the Fe-N active sites. We anticipate that the method will give essential insights into the rational design and application of SACs.

Diatomite-Templated Synthesis of Single-Atom Cobalt-Doped MoS2 /Carbon Composites to Boost Sodium Storage

2D transition metal dichalcogenides (TMDCs) and single-atom catalysts (SACs) are promising electrodes for energy conversion/storage because of the layered structure and maximum atom utilization efficiency. However, the integration of such two type materials and the relevant sodium storage applications remain daunting challenges. Here, an ingenious diatomite-templated synthetic strategy is designed to fabricate single-atom cobalt-doped MoS2 /carbon (SA Co-MoS2 /C) composites toward the high-performance sodium storage. Benefiting from the unique hierarchical structure, high electron/sodium-ion conductivity, and abundant active sites, the obtained SA Co-MoS2 /C reveals remarkable specific capacity (≈604.0 mAh g-1 at 0.1 A g-1 ), high rate performance, and outstanding long cyclic stability. Particularly, the sodium-ion full cell composed of SA Co-MoS2 /C anode and Na3 V2 (PO4 )3 cathode demonstrates unexpected stability with the cycle number exceeded 1200. The internal sodium storage mechanism is clarified with the aid of density functional theory calculations and in situ experimental characterizations. This work not only represents a substantial leap in terms of synthesizing SACs on 2D TMDCs but also provides a crucial step toward the practical sodium-ion battery applications.

Incorporating Catalytic Units into Nanomaterials: Rational Design of Multipurpose Catalysts for CO2 Valorization

ConspectusCO2 conversion to valuable chemicals is effective at reducing CO2 emissions. We previously proposed valorization strategies and developed efficient catalysts to address thermodynamic stability and kinetic inertness issues related to CO2 conversion. Earlier, we developed molecular capture reagents and catalysts to integrate CO2 capture and conversion, i.e., in situ transformation. Based on the mechanistic understanding of CO2 capture, activation, and transformation at a molecular level, we set out to develop heterogeneous catalysts by incorporating catalytic units into nanomaterials via the immobilization of active molecular catalysts onto nanomaterials and designing nanomaterials with intrinsic catalytic sites.In thermocatalytic CO2 conversion, carbonaceous and metal-organic framework (MOF)-based catalysts were developed for nonreductive and reductive CO2 conversion. Novel Cu- and Zn-based MOFs and carbon-supported Cu catalysts were prepared and successfully applied to the cycloaddition, carboxylation, and carboxylative cyclization reactions with CO2, generating cyclic carbonates, carboxyl acids, and oxazolidinones as respective target products. Reductive conversion of CO2, especially reductive functionalization with CO2, is a promising transformation strategy to produce valuable chemicals, alleviating chemical production that relies on petrochemistry. We explored the hierarchical reductive functionalization of CO2 using organocatalysts and proposed strategies to regulate the CO2 reduction level, triggering heterogeneous catalyst investigation. Introducing multiple active sites into nanomaterials opens possibilities to develop novel CO2 transformation strategies. CO2 capture and in situ conversion were realized with an N-doped carbon-supported Zn complex and MOF materials as CO2 adsorbents and catalysts. These nanomaterial-based catalysts feature high stability and excellent efficiency and act as shape-selective catalysts in some cases due to their unique pore structure.Nanomaterial-based catalysts are also appealing candidates for photocatalytic CO2 reduction (PCO2RR) and electrocatalytic CO2 reduction (ECO2RR), so we developed a series of hybrid photo-/electrocatalysts by incorporating active metal complexes into different matrixes such as porous organic polymers (POPs), metal-organic layers (MOLs), micelles, and conducting polymers. By introducing Re-bipyridine and Fe-porphyrin complexes into POPs and regulating the structure of the polymer chain, catalyst stability and efficiency increased in PCO2RR. PCO2RR in aqueous solution was realized by designing the Re-bipyridine-containing amphiphilic polymer to form micelles in aqueous solution and act as nanoreactors. We prepared MOLs with two different metallic centers, i.e., the Ni-bipyridine site and Ni-O node, to improve the efficiency for PCO2RR due to the synergistic effect of these metal centers. Sulfylphenoxy-decorated cobalt phthalocyanine (CoPc) cross-linked polypyrrole was prepared and used as a cathode, achieving the electrocatalytic transformation of diluted CO2 benefiting from the CO2 adsorption capability of polypyrrole. We fabricated immobilized 4-(t-butyl)-phenoxy cobalt phthalocyanine and Bi-MOF as cathodes to promote the paired electrolysis of CO2 and 5-hydroxymethylfurfural (HMF) and obtained CO2 reductive products and 2,5-furandicarboxylic acid (FDCA) efficiently.

Relationship between Structure and Performance of Atomic-Scale Electrocatalysts for Water Splitting

Atomic-scale electrocatalysts greatly improve the performance and efficiency of water splitting but require special adjustments of the supporting structures for anchoring and dispersing metal single atoms. Here, the structural evolution of atomic-scale electrocatalysts for water splitting is reviewed based on different synthetic methods and structural properties that create different environments for electrocatalytic activity. The rate-determining step or intermediate state for hydrogen or oxygen evolution reactions is energetically stabilized by the coordination environment to the single-atom active site from the supporting material. In large-scale practical use, maximizing the loading amount of metal single atoms increases the efficiency of the electrocatalyst and reduces the economic cost. Dual-atom electrocatalysts with two different single-atom active sites react with an increased number of water molecules and reduce the adsorption energy of water derived from the difference in electronegativity between the two metal atoms. In particular, single-atom dimers induce asymmetric active sites that promote the degradation of H2 O to H2 or O2 evolution. Consequently, the structural properties of atomic-scale electrocatalysts clarify the atomic interrelation between the catalytic active sites and the supporting material to achieve maximum efficiency.

Interlayer-Confined NiFe Dual Atoms within MoS2 Electrocatalyst for Ultra-Efficient Acidic Overall Water Splitting

Confining dual atoms (DAs) within the van der Waals gap of 2D layered materials is expected to expedite the kinetic and energetic strength in catalytic process, yet is a huge challenge in atomic-scale precise assembling DAs within two adjacent layers in the 2D limit. Here, an ingenious approach is proposed to assemble DAs of Ni and Fe into the interlayer of MoS2 . While inheriting the exceptional merits of diatomic species, this interlayer-confined structure arms itself with confinement effect, displaying the more favorable adsorption strength on the confined metal active center and higher catalytic activity towards acidic water splitting, as verified by intensive research efforts of theoretical calculations and experimental measurements. Moreover, the interlayer-confined structure also renders metal DAs a protective shelter to survive in harsh acidic environment. The findings embodied the confinement effects at the atom level, and interlayer-confined assembling of multiple species highlights a general pathway to advance interlayer-confined DAs catalysts within various 2D materials.

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.

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.

CeO2/Cu2O/Cu Tandem Interfaces for Efficient Water-Gas Shift Reaction Catalysis

Metal-oxide interfaces on Cu-based catalysts play very important roles in the low-temperature water-gas shift reaction (LT-WGSR). However, developing catalysts with abundant, active, and robust Cu-metal oxide interfaces under LT-WGSR conditions remains challenging. Herein, we report the successful development of an inverse copper-ceria catalyst (Cu@CeO₂), which exhibited very high efficiency for the LT-WGSR. At a reaction temperature of 250 °C, the LT-WGSR activity of the Cu@CeO₂ catalyst was about three times higher than that of a pristine Cu catalyst without CeO₂. Comprehensive quasi-in situ structural characterizations indicated that the Cu@CeO₂ catalyst was rich in CeO₂/Cu₂O/Cu tandem interfaces. Reaction kinetics studies and density functional theory (DFT) calculations revealed that the Cu⁺/Cu⁰ interfaces were the active sites for the LT-WGSR, while adjacent CeO₂ nanoparticles play a key role in activating H₂O and stabilizing the Cu⁺/Cu⁰ interfaces. Our study highlights the role of the CeO₂/Cu₂O/Cu tandem interface in regulating catalyst activity and stability, thus contributing to the development of improved Cu-based catalysts for the LT-WGSR.

Thermodynamically Driven Tilt Grain Boundaries of Monolayer Crystals Using Catalytic Liquid Alloys

We report a method to precisely control the atomic defects at grain boundaries (GBs) of monolayer MoS2 by vapor-liquid-solid (VLS) growth using sodium molybdate liquid alloys, which serve as growth catalysts to guide the formations of the thermodynamically most stable GB structure. The Mo-rich chemical environment of the alloys results in Mo-polar 5|7 defects with a yield exceeding 95%. The photoluminescence (PL) intensity of VLS-grown polycrystalline MoS2 films markedly exceeds that of the films, exhibiting abundant S 5|7 defects, which are kinetically driven by vapor-solid-solid growths. Density functional theory calculations indicate that the enhanced PL intensity is due to the suppression of nonradiative recombination of charged excitons with donor-type defects of adsorbed Na elements on S 5|7 defects. Catalytic liquid alloys can aid in determining a type of atomic defect even in various polycrystalline 2D films, which accordingly provides a technical clue to engineer their properties.