Scalable two-step annealing method for preparing ultra-high-density single-atom catalyst libraries

Efficient catalytic oxidation of formaldehyde by defective g-C3N4-anchored single-atom Pt: A DFT study

Indoor volatile formaldehyde is a serious health hazard. The development of low-temperature and efficient nonhomogeneous oxidation catalysts is crucial for protecting human health and the environment but is also quite challenging. Single-atom catalysts (SACs) with active centers and coordination environments that are precisely tunable at the atomic level exhibit excellent catalytic activity in many catalytic fields. Among two-dimensional materials, the nonmagnetic monolayer material g-C3N4 may be a good platform for loading single atoms. In this study, the effect of nitrogen defect formation on the charge distribution of g-C3N4 is discussed in detail using density functional theory (DFT) calculations. The effect of nitrogen defects on the activated molecular oxygen of Pt/C3N4 was systematically revealed by DFT calculations in combination with molecular orbital theory. Two typical reaction mechanisms for the catalytic oxidation of formaldehyde were proposed based on the Eley-Rideal (E-R) mechanism. Pt/C3N4-V3N was more advantageous for path 1, as determined by the activation energy barrier of the rate-determining step and product desorption. Finally, the active centers and chemical structures of Pt/C3N4 and Pt/C3N4-V3N were verified to have good stability at 375 K by determination of the migration energy barriers and ab initio molecular dynamics simulations. Therefore, the formation of N defects can effectively anchor single-atom Pt and provide additional active sites, which in turn activate molecular oxygen to efficiently catalyze the oxidation of formaldehyde. This study provides a better understanding of the mechanism of formaldehyde oxidation by single-atom Pt catalysts and a new idea for the development of Pt as well as other metal-based single-atom oxidation catalysts.

General negative pressure annealing approach for creating ultra-high-loading single atom catalyst libraries

Catalyst systems populated by high-density single atoms are crucial for improving catalytic activity and selectivity, which can potentially maximize the industrial prospects of heterogeneous single-atom catalysts (SACs). However, achieving high-loading SACs with metal contents above 10 wt% remains challenging. Here we describe a general negative pressure annealing strategy to fabricate ultrahigh-loading SACs with metal contents up to 27.3-44.8 wt% for 13 different metals on a typical carbon nitride matrix. Furthermore, our approach enables the synthesis of high-entropy single-atom catalysts (HESACs) that exhibit the coexistence of multiple metal single atoms with high metal contents. In-situ aberration-corrected HAADF-STEM (AC-STEM) combined with ex-situ X-ray absorption fine structure (XAFS) demonstrate that the negative pressure annealing treatment accelerates the removal of anionic ligand in metal precursors and boosts the bonding of metal species with N defective sites, enabling the formation of dense N-coordinated metal sites. Increasing metal loading on a platinum (Pt) SAC to 41.8 wt% significantly enhances the activity of propane oxidation towards liquid products, including acetone, methanol, and acetic acid et al. This work presents a straightforward and universal approach for achieving many low-cost and high-density SACs for efficient catalytic transformations.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

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.

Universal Synthesis of Single-Atom Catalysts by Direct Thermal Decomposition of Molten Salts for Boosting Acidic Water Splitting

Single-atom catalysts (SACs) are considered prominent materials in the field of catalysis due to their high metal atom utilization and selectivity. However, the wide-ranging applications of SACs remain a significant challenge due to their complex preparation processes. Here, a universal strategy is reported to prepare a series of noble metal single atoms on different non-noble metal oxides through a facile one-step thermal decomposition of molten salts. By using a mixture of non-noble metal nitrate and a small-amount noble metal chloride as the precursor, noble metal single atoms can be easily introduced into the non-noble metal oxide lattice owing to the cation exchange in the in situ formed molten salt, followed by the thermal decomposition of nitrate anions during the heating process. Analyses using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure spectroscopy confirm the formation of the finely dispersed single atoms. Specially, the as-synthesized Ir single atoms (10.97 wt%) and Pt single atoms (4.60 wt%) on the Co3O4 support demonstrate outstanding electrocatalytic activities for oxygen evolution reaction and hydrogen evolution reaction, respectively.

Directional surface reconstruction of C and S Co-Doped Co2VO4/CoP for the cooperative enhancement of hydrogen production via seawater electrolysis

The endeavor to architect bifunctional electrocatalysts that exhibit both exceptional activity and durability heralds an era of boundless potential for the comprehensive electrolysis of seawater, an aspiration that, nevertheless, poses a substantial challenge. Within this work, we describe the precise engineering of a three-dimensional interconnected nanoparticle system named SCdoped Co2VO4/CoP (SCCo2VO4), achieved through a meticulously arranged hydrothermal treatment sequence followed by gas-phase carbonization and phosphorization. The resulting SCCo2VO4 electrode exhibits outstanding bifunctional electrocatalytic stability, attributed to the strategic anionic doping and abundant heterogeneous interfaces. Doping not only adjusts the electronic structure, enhancing electron transfer efficiency but also optimizes the surface-active sites. This electrode prodigiously necessitated an extraordinarily minimal overpotential of merely 92 and 350 mV to attain current densities of 10 and 50 mA cm-2 for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1 M KOH solution. Noteworthily, when integrated into an electrolyzer for the exhaustive splitting of seawater, the SCP-Co2VO4 manifested an exceptionally low cell voltage of 2.08 V@50 mA cm-2 and showcased a durability that eclipses that of most hitherto documented nickel-based bifunctional materials. Further elucidation through Density Functional Theory (DFT) analyses underscored that anion doping and the inherent heterostructure adeptly optimize the Gibbs free energy of intermediates comprising hydrogen, chlorine, and oxygen (manifested as OH, O, OOH) within the HER and OER paradigms, thus propelling the electrochemical kinetics of seawater splitting to unprecedented velocities. These revelations unfurl a pioneering design philosophy for the creation of cost-effective yet superior catalysts aimed at the holistic division of water molecules, charting a course towards the realization of efficient and sustainable hydrogen production methodologies.

A High-Entropy Single-Atom Catalyst Toward Oxygen Reduction Reaction in Acidic and Alkaline Conditions

The design of high-entropy single-atom catalysts (HESAC) with 5.2 times higher entropy compared to single-atom catalysts (SAC) is proposed, by using four different metals (FeCoNiRu-HESAC) for oxygen reduction reaction (ORR). Fe active sites with intermetallic distances of 6.1 Å exhibit a low ORR overpotential of 0.44 V, which originates from weakening the adsorption of OH intermediates. Based on density functional theory (DFT) findings, the FeCoNiRu-HESAC with a nitrogen-doped sample were synthesized. The atomic structures are confirmed with X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and scanning transmission electron microscopy (STEM). The predicted high catalytic activity is experimentally verified, finding that FeCoNiRu-HESAC has overpotentials of 0.41 and 0.37 V with Tafel slopes of 101 and 210 mVdec-1 at the current density of 1 mA cm-2 and the kinetic current densities of 8.2 and 5.3 mA cm-2, respectively, in acidic and alkaline electrolytes. These results are comparable with Pt/C. The FeCoNiRu-HESAC is used for Zinc-air battery applications with an open circuit potential of 1.39 V and power density of 0.16 W cm-2. Therefore, a strategy guided by DFT is provided for the rational design of HESAC which can be replaced with high-cost Pt catalysts toward ORR and beyond.

Efficient electro-Fenton degradation of organic pollutants via the synergistic effect of 1O2 and •OH generated on single FeN4 sites

The electro-Fenton with in situ generated 1O2 and •OH is a promising method for the degradation of micropollutants. However, its application is hindered by the lack of catalysts that can efficiently generate 1O2 and •OH from electrochemical oxygen reduction. Herein, N-doped stacked carbon nanosheets supported Fe single atoms (Fe-NSC) with FeN4 sites were designed for simultaneous generation of 1O2 and •OH to enhance electro-Fenton degradation. Due to the synergistic effect of 1O2 and •OH, a variety of contaminants (phenol, 2,4-dichlorophenol, sulfamethoxazole, atrazine and bisphenol A) were efficiently degraded with high kinetic constants of 0.037-0.071 min-1 by the electro-Fenton with Fe-NSC as cathode (-0.6 V vs Ag/AgCl, pH 6). Moreover, the superior performance for electro-Fenton degradation was well maintained in a wide pH range from 3 to 10 even with interference of various inorganic salt ions. It was found that FeN4 sites with pyridinic N coordination were responsible for its good performance for electro-Fenton degradation. Its 1O2 yield was higher than •OH yield, and the contribution of 1O2 was more significant than •OH for pollutant degradation.

Insights into the Optimization of Catalytic Active Sites in Lithium-Sulfur Batteries

ConspectusLithium-sulfur batteries (LSBs), recognized for their high energy density and cost-effectiveness, offer significant potential for advancement in energy storage. However, their widespread deployment remains hindered by challenges such as sluggish reaction kinetics and the shuttle effect of lithium polysulfides (LiPSs). By the introduction of catalytic materials, the effective adsorption of LiPSs, smooth surface migration behavior, and significantly reduced conversion energy barriers are expected to be achieved, thereby sharpening electrochemical reaction kinetics and fundamentally addressing the aforementioned challenges. However, driven by practical application targets, the demand for higher loadings and reduced electrolyte parameters inevitably exacerbates the burden on catalytic materials during their service. Additionally, given that catalytic materials contribute negligible electrochemical capacity, their incorporation inevitably increases the mass of nonactive components for reducing the energy density of LSBs. A meticulous insight into the lithium-sulfur catalytic reaction reveals that the conversion of LiPSs is dominated by active sites on the surfaces of catalytic materials. These microregions provide the necessary electron and ion transport for the conversion reaction of LiPSs, with their efficacy and quantity directly impacting the conversion efficiency. In light of these considerations, the strategic optimization of active sites emerges as a paramount pathway toward promoting the performance of LSBs while concurrently mitigating unnecessary mass. Here, we outline three strategies developed by our group to optimize active sites of catalytic materials: (1) Augmenting active sites by customizing structural modulation and precise dimensional control to maximize exposure. Emphasis has been placed on the approaches for material synthesis and the essence of reactions for achieving this strategy. (2) Regulating the microenvironment of active sites by integrating the coordination refinement, long-range atomic interactions, metal-support interactions, and other electronic regulation strategies, thereby providing an elevation in the intrinsic catalytic performance. (3) Implementing a self-cleaning mechanism for active sites to counteract deactivation by designing a tandem adsorption-migration-transformation pathway of sulfur contained within the molecular domain. Throughout this process, the intrinsic mechanisms driving performance enhancement through active site optimization strategies have been prominently emphasized, which encompass aspects such as electronic structure, atomic composition, and molecular configuration and significantly expand the comprehension of Li-S catalytic chemistry. Subsequently, considerations demanding heightened attention in future processes of active site optimization for catalytic materials have been delineated, including the in situ evolution patterns and resistance to the poisoning of active sites. It is noteworthy that given the similarity between Li-S catalysis chemistry and traditional electrocatalytic processes, this Account elucidates the concept of active site optimization by drawing insights from representative works and our own works in the field of electrocatalysis, which is relatively rare in previous reviews of LSBs. The proposed insights contribute to uncovering the intrinsic mechanisms of Li-S catalysis chemistry and introducing innovative ideas into active site optimization, ultimately advancing energy density and stability in LSBs.

Toward Functionality and Deactivation of Metal-Single-Atom in Heterogeneous Photocatalysts

Single-atom heterogeneous catalysts (SAHCs) provide an enticing platform for understanding catalyst structure-property-performance relationships. The 100% atom utilization and adjustable local coordination configurations make it easy to probe reaction mechanisms at the atomic level. However, the progressive deactivation of metal-single-atom (MSA) with high surface energy leads to frequent limitations on their commercial viability. This review focuses on the atomistic-sensitive reactivity and atomistic-progressive deactivation of MSA to provide a unifying framework for specific functionality and potential deactivation drivers of MSA, thereby bridging function, purpose-modification structure-performance insights with the atomistic-progressive deactivation for sustainable structure-property-performance accessibility. The dominant functionalization of atomically precise MSA acting on properties and reactivity encompassing precise photocatalytic reactions is first systematically explored. Afterward, a detailed analysis of various deactivation modes of MSA and strategies to enhance their durability is presented, providing valuable insights into the design of SAHCs with deactivation-resistant stability. Finally, the remaining challenges and future perspectives of SAHCs toward industrialization, anticipating shedding some light on the next stage of atom-economic chemical/energy transformations are presented.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

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.

Unveiling Low Temperature Assembly of Dense Fe-N4 Active Sites via Hydrogenation in Advanced Oxygen Reduction Catalysts

The single-atom Fe-N-C is a prominent material with exceptional reactivity in areas of sustainable energy and catalysis research. It is challenging to obtain the dense Fe-N4 site without the Fe nanoparticles (NPs) sintering during the Fe-N-C synthesis via high-temperature pyrolysis. Thus, a novel approach is devised for the Fe-N-C synthesis at low temperatures. Taking FeCl2 as Fe source, a hydrogen environment can facilitate oxygen removal and dichlorination processes in the synthesis, efficiently favouring Fe-N4 site formation without Fe NPs clustering at as low as 360 °C. We shed light on the reaction mechanism about hydrogen promoting Fe-N4 formation in the synthesis. By adjusting the temperature and duration, the Fe-N4 structural evolution and site density can be precisely tuned to directly influence the catalytic behaviour of the Fe-N-C material. The FeNC-H2-360 catalyst demonstrates a remarkable Fe dispersion (8.3 wt %) and superior acid ORR activity with a half-wave potential of 0.85 V and a peak power density of 1.21 W cm-2 in fuel cell. This method also generally facilitates the synthesis of various high-performance M-N-C materials (M=Fe, Co, Mn, Ni, Zn, Ru) with elevated single-atom loadings.

Accumulation of Long-Lived Photogenerated Holes at Indium Single-Atom Catalysts via Two Coordinate Nitrogen Vacancy Defect Engineering for Enhanced Photocatalytic Oxidation

Visible-light-driven photocatalytic oxidation by photogenerated holes has immense potential for environmental remediation applications. While the electron-mediated photoreduction reactions are often at the spotlight, active holes possess a remarkable oxidation capacity that can degrade recalcitrant organic pollutants, resulting in nontoxic byproducts. However, the random charge transfer and rapid recombination of electron-hole pairs hinder the accumulation of long-lived holes at the reaction center. Herein, a novel method employing defect-engineered indium (In) single-atom photocatalysts with nitrogen vacancy (Nv) defects, dispersed in carbon nitride foam (In-Nv-CNF), is reported to overcome these challenges and make further advances in photocatalysis. This Nv defect-engineered strategy produces a remarkable extension in the lifetime and an increase in the concentration of photogenerated holes in In-Nv-CNF. Consequently, the optimized In-Nv-CNF demonstrates a remarkable 50-fold increase in photo-oxidative degradation rate compared to pristine CN, effectively breaking down two widely used antibiotics (tetracycline and ciprofloxacin) under visible light. The contaminated water treated by In-Nv-CNF is completely nontoxic based on the growth of Escherichia coli. Structural-performance correlations between defect engineering and long-lived hole accumulation in In-Nv-CNF are established and validated through experimental and theoretical agreement. This work has the potential to elevate the efficiency of overall photocatalytic reactions from a hole-centric standpoint.

Dual single atomic Ni sites constructing Janus hollow graphene for boosting electrochemical sensing of glucose

Single atom catalysts (SACs) have attracted attention due to their excellent catalysis activity under specific reactions and conditions. However, the low density of SACs greatly limits catalytic performance. The three-dimensional graphene hollow nanospheres (GHSs) with very thin shell structure can be used as excellent carrier materials. Not only can its outer surface be used to anchor metal single atoms, but its inner surface can also provide rich sites. Here, a novel step-by-step assembly strategy is reported to anchor nickel single atoms (Ni SAs) on the inner and outer surfaces of GHSs (Ni SAs/GHSs/Ni SAs), which significantly increases the loading capacity of Ni SAs (4.8 wt%). Compared to conventional materials that only anchor Ni SAs to the outer surface of the carrier (Ni SAs/GHSs), Ni SAs/GHSs/Ni SAs exhibits significantly higher electrocatalytic activity toward glucose oxidation in alkaline media. The sensitivity of Ni SAs/GHSs/Ni SAs/GCE is nearly five times higher than that of Ni SAs/GHSs/GCE. Moreover, the sensor based on Ni SAs/GHSs/Ni SAs can detect glucose in a wide concentration range of 0.8 µM-1.1244 mM with a low detection limit of 0.19 µM (S/N = 3). This study not only provides an effective sensing material for glucose detection, but also opens a new avenue to construct high-density metal SACs.

Single-Pd-Site Catalyst Induced by Different Dimensional Nitrogen of N-Doping Carbon for Efficient Hydroaminocarbonylation of Alkynes

The unsaturated amides are traditionally synthesized by acylation of carboxylic acids or hydration of nitrile compounds but are rarely investigated by hydroaminocarbonylation of alkynes using heterogeneous single-metal-site catalysts (HSMSCs). Herein, single-Pd-site catalysts supported on N-doping carbon (NC) with different nitrogen dimensions inherited from corresponding metal-organic-framework precursors are successfully synthesized. 2D NC-supported single-Pd-site (Pd1/NC-2D) exhibited the best performance with near 100% selectivity and 76% yield of acrylamide for acetylene hydroaminocarbonylation with better stability, superior to those of Pd1/NC-3D, single-metal-site/nanoparticle coexisting catalyst, and nanoparticle catalyst. The coordination environment and molecular evolution of the single-Pd-site during the process of acetylene hydroaminocarbonylation on Pd1/NC-2D are detailly illuminated by various characterizations and density functional theoretical calculations (DFT). DFT also showed the energy barrier of rate-determining step on Pd1/NC-2D is lower than that of Pd1/NC-3D. Furthermore, Pd1/NC-2D catalyst illustrated the general applicability of the hydroaminocarbonylation for various alkynes.

3D Noble-Metal Nanostructures Approaching Atomic Efficiency and Atomic Density Limits

Noble metals have been widely used in catalysis, however, the scarcity and high cost of noble metal motivate researchers to balance the atomic efficiency and atomic density, which is formidably challenging. This article proposes a robust strategy for fabricating 3D amorphous noble metal-based oxides with simultaneous enhancement on atomic efficiency and density with the assistance of atomic channels, where the atomic utilization increases from 18.2% to 59.4%. The unique properties of amorphous bimetallic oxides and formation of atomic channels have been evidenced by detailed experimental characterizations and theoretical simulations. Moreover, the universality of the current strategy is validated by other binary oxides. When Cu2IrOx with atomic channels (Cu2IrOx-AE) is used as catalyst for oxygen evolution reaction (OER), the mass activity and turnover frequency value of Cu2IrOx-AE are 1-2 orders of magnitude higher than CuO/IrO2 and Cu2IrOx without atomic channels, largely outperforming the reported OER catalysts. Theoretical calculations reveal that the formation of atomic channels leads to various Ir sites, on which the proton of adsorbed *OH can transfer to adjacent O atoms of [IrO6]. This work may attract immediate interest of researchers in material science, chemistry, catalysis, and beyond.

Underpotential Deposition of 3D Transition Metals: Versatile Electrosynthesis of Single-Atom Catalysts on Oxidized Carbon Supports

Use of single-atom catalysts (SACs) has become a popular strategy for tuning activity and selectivity toward specific pathways. However, conventional SAC synthesis methods require high temperatures and pressures, complicated procedures, and expensive equipment. Recently, underpotential deposition (UPD) has been investigated as a promising alternative, yielding high-loading SAC electrodes under ambient conditions and within minutes. Yet only few studies have employed UPD to synthesize SACs, and all have been limited to UPD of Cu. In this work, a flexible UPD approach for synthesis of mono- and bi-metallic Cu, Fe, Co, and Ni SACs directly on oxidized, commercially available carbon electrodes is reported. The UPD mechanism is investigated using in situ X-ray absorption spectroscopy and, finally, the catalytic performance of a UPD-synthesized Co SAC is assessed for electrochemical nitrate reduction to ammonia. The findings expand upon the usefulness and versatility of UPD for SAC synthesis, with hopes of enabling future research toward realization of fast, reliable, and fully electrified SAC synthesis processes.

Strategies Toward High Selectivity, Activity, and Stability of Single-Atom Catalysts

Single-atom catalysts (SACs) hold immense promise in facilitating the rational use of metal resources and achieving atomic economy due to their exceptional atom-utilization efficiency and distinct characteristics. Despite the growing interest in SACs, only limited reviews have holistically summarized their advancements centering on performance metrics. In this review, first, a thorough overview on the research progress in SACs is presented from a performance perspective and the strategies, advancements, and intriguing approaches employed to enhance the critical attributes in SACs are discussed. Subsequently, a comprehensive summary and critical analysis of the electrochemical applications of SACs are provided, with a particular focus on their efficacy in the oxygen reduction reaction , oxygen evolution reaction, hydrogen evolution reaction , CO2 reduction reaction, and N2 reduction reaction . Finally, the outline future research directions on SACs by concentrating on performance-driven investigation, where potential areas for improvement are identified and promising avenues for further study are highlighted, addressing challenges to unlock the full potential of SACs as high-performance catalysts.

Highly Curved Defect Sites: How Curvature Effect Influences Metal-Free Defective Carbon Electrocatalysts

Topological defects are widely recognized as effective active sites toward a variety of electrochemical reactions. However, the role of defect curvature is still not fully understood. Herein, carbon nanomaterials with rich topological defect sites of tunable curvature is reported. The curved defective surface is realized by controlling the high-temperature pyrolytic shrinkage process of precursors. Theoretical calculations demonstrate bending the defect sites can change the local electronic structure, promote the charge transfer to key intermediates, and lower the energy barrier for oxygen reduction reaction (ORR). Experimental results convince structural superiority of highly-curved defective sites, with a high kinetic current density of 22.5 mA cm-2 at 0.8 V versus RHE for high-curvature defective carbon (HCDC), ≈18 times that of low-curvature defective carbon (LCDC). Further raising the defect densities in HCDC leads to the dual-regulated products (HCHDC), which exhibit exceptionally outstanding ORR activity in both alkaline and acidic media (half-wave potentials: 0.88 and 0.74 V), outperforming most of the reported metal-free carbon catalysts. This work uncovers the curvature-activity relationship in carbon defect for ORR and provides new guidance to design advanced catalysts via curvature-engineering.

A Bioinspired Single-Atom Fe Nanozyme with Excellent Laccase-Like Activity for Efficient Aflatoxin B1 Removal

The applications of natural laccases are greatly restricted because of their drawbacks like poor biostability, high costs, and low recovery efficiency. M/NC single atom nanozymes (M/NC SAzymes) are presenting as great substitutes due to their superior enzyme-like activity, excellent selectivity and high stability. In this work, inspired by the catalytic active center of natural enzyme, a biomimetic Fe/NC SAzyme (Fe-SAzyme) with O2-Fe-N4 coordination is successfully developed, exhibiting excellent laccase-like activity. Compared with their natural counterpart, Fe-SAzyme has shown superior catalytic efficiency and excellent stability under a wide range of pH (3.0-9.0), temperature (4-80 °C) and NaCl strength (0-300 mm). Interestingly, density functional theory (DFT) calculations reveal that the high catalytic performance is attributed to the activation of O2 by O2-Fe-N4 sites, which weakened the O─O bonds in the oxygen-to-water oxidation pathway. Furthermore, Fe-SAzyme is successfully applied for efficient aflatoxin B1 removal based on its robust laccase-like catalytic activity. This work provides a strategy for the rational design of laccase-like SAzymes, and the proposed catalytic mechanism will help to understand the coordination environment effect of SAzymes on laccase-like catalytic processes.

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

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

Advances in Synthesis and Applications of Single-Atom Catalysts for Metal Oxide-Based Gas Sensors

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented.

A first-principles study on Ni-decorated MoS2 for efficient formaldehyde degradation over a wide temperature range

The development of a high-efficiency, low-cost, and environmentally friendly catalyst for formaldehyde degradation is crucial for addressing the issue of indoor formaldehyde pollution. Given that modern individuals spend over 90% of their time indoors, effectively tackling indoor formaldehyde pollution holds significant importance. Therefore, this paper proposes an efficient catalyst for formaldehyde degradation: surface modification of MoS2 by single-atom Ni, which can convert formaldehyde into harmless H2O and CO2. The DFT method is employed to systematically investigate the oxidative degradation pathways of formaldehyde on the surface of Ni-doped MoS2. The research focuses on two common oxidative degradation pathways in both the L-H mechanism and E-R mechanism. Our findings demonstrate that these four reaction paths occur spontaneously within the temperature range of 300-800 K with a reaction equilibrium constant greater than 105. Moreover, even under extreme temperature conditions (100 K), the reaction rate remains favorable. Furthermore, our findings indicate that the minimum activation energy is merely 0.91 eV and H2O and CO2 only need to overcome an energy barrier of 0.71 eV for desorption from the catalyst surface. This substantiates its potential application both in indoor environments and under extreme temperature conditions. This theoretical research provides innovative ideas and strategies for effectively oxidizing formaldehyde.

Exfoliation of Metal-Organic Frameworks to Give 2D MOF Nanosheets for the Electrocatalytic Oxygen Evolution Reaction

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond-breaking strategy is demonstrated for synthesizing ultrathin metal-organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs' structure due to the instant temperature drop and drastic expansion of liquid N2, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.

Mated-Atom Nanozymes with Efficient Assisted NAD+ Replenishment for Skin Regeneration

Excessive accumulation of reduced nicotinamide adenine dinucleotide (NADH) within biological organisms is closely associated with many diseases. It remains a challenge to efficiently convert superfluous and detrimental NADH to NAD+. NADH oxidase (NOX) is a crucial oxidoreductase that catalyzes the oxidation of NADH to NAD+. Herein, M1M2 (Mi=V/Mn/Fe/Co/Cu/Mo/Rh/Ru/Pd, i = 1 or 2) mated-atom nanozymes (MANs) are designed by mimicking natural enzymes with polymetallic active centers. Excitingly, RhCo MAN possesses excellent and sustainable NOX-like activity, with Km-NADH (16.11 μM) being lower than that of NOX-mimics reported so far. Thus, RhCo MAN can significantly promote the regeneration of NAD+ and regulate macrophage polarization toward the M2 phenotype through down-regulation of TLR4 expression, which may help to recover skin regeneration. However, RhRu MAN with peroxidase-like activity and RhMn MAN with superoxide dismutase-like activity exhibit little modulating effects on eczema. This work provides a new strategy to inhibit skin inflammation and promote skin regeneration.

Continuous and low-carbon production of biomass flash graphene

Flash Joule heating (FJH) is an emerging and profitable technology for converting inexhaustible biomass into flash graphene (FG). However, it is challenging to produce biomass FG continuously due to the lack of an integrated device. Furthermore, the high-carbon footprint induced by both excessive energy allocation for massive pyrolytic volatiles release and carbon black utilization in alternating current-FJH (AC-FJH) reaction exacerbates this challenge. Here, we create an integrated automatic system with energy requirement-oriented allocation to achieve continuous biomass FG production with a much lower carbon footprint. The programmable logic controller flexibly coordinated the FJH modular components to realize the turnover of biomass FG production. Furthermore, we propose pyrolysis-FJH nexus to achieve biomass FG production. Initially, we utilize pyrolysis to release biomass pyrolytic volatiles, and subsequently carry out the FJH reaction to focus on optimizing the FG structure. Importantly, biochar with appropriate resistance is self-sufficient to initiate the FJH reaction. Accordingly, the medium-temperature biochar-based FG production without carbon black utilization exhibited low carbon emission (1.9 g CO2-eq g-1 graphene), equivalent to a reduction of up to ~86.1% compared to biomass-based FG production. Undoubtedly, this integrated automatic system assisted by pyrolysis-FJH nexus can facilitate biomass FG into a broad spectrum of applications.

Lutetium-Induced Ultrafine PtRu Nanoclusters with a High Electrochemical Surface Area for Direct Methanol Fuel Cells at Alleviated Temperatures

PtRu alloys have been recognized as the state-of-the-art catalysts for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). However, their applications in DMFCs are still less efficient in terms of both catalytic activity and durability. Rare earth (RE) metals have been recognized as attractive elements to tune the catalytic activity, while it is still a world-class challenge to synthesize well-dispersed Pt-RE alloys. Herein, we developed a novel hydrogen-assisted magnesiothermic reduction strategy to prepare a highly dispersed carbon-supported lutetium-doped PtRu catalyst with ultrafine nanoclusters and atomically dispersed Ru sites. The PtRuLu catalyst shows an outstanding high electrochemical surface area (ECSA) of 239.0 m2 gPt-1 and delivers an optimized MOR mass activity and specific activity of 632.5 mA mgPt-1 and 26 A cmPt-2 at 0.4 V vs saturated calomel electrode (SCE), which are 3.6 and 3.5 times of commercial PtRu-JM and an order higher than PtLu, respectively. These novel catalysts have been demonstrated in a high-temperature direct methanol fuel cell running in a temperature range of 180-240 °C, achieving a maximum power density of 314.3 mW cm-2. The AC-STEM imaging, in situ ATR-IR spectroscopy, and DFT calculations disclose that the high performance is resulted from the highly dispersed PtRuLu nanoclusters and the synergistic effect of the atomically dispersed Ru sites with PtRuLu nanoclusters, which significantly reduces the CO* intermediates coverage due to the promoted water activation to form the OH* to facilitate the CO* removal.

Understanding the Dynamic Aggregation in Single-Atom Catalysis

The dynamic response of single-atom catalysts to a reactive environment is an increasingly significant topic for understanding the reaction mechanism at the molecular level. In particular, single atoms may experience dynamic aggregation into clusters or nanoparticles driven by thermodynamic or kinetic factors. Herein, the inherent mechanistic nuances that determine the dynamic profile during the reaction will be uncovered, including the intrinsic stability and site-migration barrier of single atoms, external stimuli (temperature, voltage, and adsorbates), and the influence of catalyst support. Such dynamic aggregation can be beneficial or deleterious on the catalytic performance depending on the optimal initial state. Those examples will be highlighted where in situ formed clusters, rather than single atoms, serve as catalytically active sites for improved catalytic performance. This is followed by the introduction of operando techniques to understand the structural evolution. Finally, the emerging strategies via confinement and defect-engineering to regulate dynamic aggregation will be briefly discussed.

Using Cu-Based Metal-Organic Framework as a Comprehensive and Powerful Antioxidant Nanozyme for Efficient Osteoarthritis Treatment

Developing nanozymes with effective reactive oxygen species (ROS) scavenging ability is a promising approach for osteoarthritis (OA) treatment. Nonetheless, numerous nanozymes lie in their relatively low antioxidant activity. In certain circumstances, some of these nanozymes may even instigate ROS production to cause side effects. To address these challenges, a copper-based metal-organic framework (Cu MOF) nanozyme is designed and applied for OA treatment. Cu MOF exhibits comprehensive and powerful activities (i.e., SOD-like, CAT-like, and •OH scavenging activities) while negligible pro-oxidant activities (POD- and OXD-like activities). Collectively, Cu MOF nanozyme is more effective at scavenging various types of ROS than other Cu-based antioxidants, such as commercial CuO and Cu single-atom nanozyme. Density functional theory calculations also confirm the origin of its outstanding enzyme-like activities. In vitro and in vivo results demonstrate that Cu MOF nanozyme exhibits an excellent ability to decrease intracellular ROS levels and relieve hypoxic microenvironment of synovial macrophages. As a result, Cu MOF nanozyme can modulate the polarization of macrophages from pro-inflammatory M1 to anti-inflammatory M2 subtype, and inhibit the degradation of cartilage matrix for efficient OA treatment. The excellent biocompatibility and protective properties of Cu MOF nanozyme make it a valuable asset in treating ROS-related ailments beyond OA.

Partial Thermal Condensation Mediated Synthesis of High-Density Nickel Single Atom Sites on Carbon Nitride for Selective Photooxidation of Methane into Methanol

Direct selective transformation of greenhouse methane (CH4) to liquid oxygenates (methanol) can substitute energy-intensive two-step (reforming/Fischer-Tropsch) synthesis while creating environmental benefits. The development of inexpensive, selective, and robust catalysts that enable room temperature conversion will decide the future of this technology. Single-atom catalysts (SACs) with isolated active centers embedded in support have displayed significant promises in catalysis to drive challenging reactions. Herein, high-density Ni single atoms are developed and stabilized on carbon nitride (NiCN) via thermal condensation of preorganized Ni-coordinated melem units. The physicochemical characterization of NiCN with various analytical techniques including HAADF-STEM and X-ray absorption fine structure (XAFS) validate the successful formation of Ni single atoms coordinated to the heptazine-constituted CN network. The presence of uniform catalytic sites improved visible absorption and carrier separation in densely populated NiCN SAC resulting in 100% selective photoconversion of (CH4) to methanol using H2O2 as an oxidant. The superior catalytic activity can be attributed to the generation of high oxidation (NiIII═O) sites and selective C─H bond cleavage to generate •CH3 radicals on Ni centers, which can combine with •OH radicals to generate CH3OH.

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

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

Tip-like Fe-N4 Sites Induced Surface Microenvironments Regulation Boosts the Oxygen Reduction Reaction

Single atom catalysts with defined local structures and favorable surface microenvironments are significant for overcoming slow kinetics and accelerating O2 electroreduction. Here, enriched tip-like FeN4 sites (T-Fe SAC) on spherical carbon surfaces were developed to investigate the change in surface microenvironments and catalysis behavior. Finite element method (FEM) simulations, together with experiments, indicate the strong local electric field of the tip-like FeN4 and the more denser interfacial water layer, thereby enhancing the kinetics of the proton-coupled electron transfer process. In situ spectroelectrochemical studies and the density functional theory (DFT) calculation results indicate the pathway transition on the tip-like FeN4 sites, promoting the dissociation of O-O bond via side-on adsorption model. The adsorbed OH* can be facilely released on the curved surface and accelerate the oxygen reduction reaction (ORR) kinetics. The obtained T-Fe SAC nanoreactor exhibits excellent ORR activities (E1/2 =0.91 V vs. RHE) and remarkable stability, exceeding those of flat FeN4 and Pt/C. This work clarified the in-depth insights into the origin of catalytic activity of tip-like FeN4 sites and held great promise in industrial catalysis, electrochemical energy storage, and many other fields.

Current Advances on the Single-Atom Nanozyme and Its Bioapplications

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

Structure-Function Relationship of Highly Reactive CuOx Clusters on Co3 O4 for Selective Formaldehyde Sensing at Low Temperatures

Designing reactive surface clusters at the nanoscale on metal-oxide supports enables selective molecular interactions in low-temperature catalysis and chemical sensing. Yet, finding effective material combinations and identifying the reactive site remains challenging and an obstacle for rational catalyst/sensor design. Here, the low-temperature oxidation of formaldehyde with CuOx clusters on Co3 O4 nanoparticles is demonstrated yielding an excellent sensor for this critical air pollutant. When fabricated by flame-aerosol technology, such CuOx clusters are finely dispersed, while some Cu ions are incorporated into the Co3 O4 lattice enhancing thermal stability. Importantly, infrared spectroscopy of adsorbed CO, near edge X-ray absorption fine structure spectroscopy and temperature-programmed reduction in H2 identified Cu+ and Cu2+ species in these clusters as active sites. Remarkably, the Cu+ surface concentration correlated with the apparent activation energy of formaldehyde oxidation (Spearman's coefficient ρ = 0.89) and sensor response (0.96), rendering it a performance descriptor. At optimal composition, such sensors detected even the lowest formaldehyde levels of 3 parts-per-billion (ppb) at 75°C, superior to state-of-the-art sensors. Also, selectivity to other aldehydes, ketones, alcohols, and inorganic compounds, robustness to humidity and stable performance over 4 weeks are achieved, rendering such sensors promising as gas detectors in health monitoring, air and food quality control.

Salt Effect Engineering Single Fe-N2P2-Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn-Air Batteries

Developing efficient metal-nitrogen-carbon (M-N-C) single-atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn-air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect-induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra-axial Cl on interlinked porous carbon nanosheets, achieving a superior single-atom Fe catalyst (denoted as Fe-NP-Cl-C) for ORR and Zn-air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe-N2P2-Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe-NP-Cl-C catalyst exhibits distinguished ORR performance with a half-wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn-air battery based on Fe-NP-Cl-C delivers an extremely high peak power density of 260 mW cm-2 and a large specific capacity of 812 mA h g-1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single-atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.

In situ modulating coordination fields of single-atom cobalt catalyst for enhanced oxygen reduction reaction

Single-atom catalysts, especially those with metal-N4 moieties, hold great promise for facilitating the oxygen reduction reaction. However, the symmetrical distribution of electrons within the metal-N4 moiety results in unsatisfactory adsorption strength of intermediates, thereby limiting their performance improvements. Herein, we present atomically coordination-regulated Co single-atom catalysts that comprise a symmetry-broken Cl-Co-N4 moiety, which serves to break the symmetrical electron distribution. In situ characterizations reveal the dynamic evolution of the symmetry-broken Cl-Co-N4 moiety into a coordination-reduced Cl-Co-N2 structure, effectively optimizing the 3d electron filling of Co sites toward a reduced d-band electron occupancy (d5.8 → d5.28) under reaction conditions for a fast four-electron oxygen reduction reaction process. As a result, the coordination-regulated Co single-atom catalysts deliver a large half-potential of 0.93 V and a mass activity of 5480 A gmetal-1. Importantly, a Zn-air battery using the coordination-regulated Co single-atom catalysts as the cathode also exhibits a large power density and excellent stability.

Single Zn Atoms with Acetate-Anion-Enabled Asymmetric Coordination for Efficient H2 O2 Photosynthesis

Exploring unique single-atom sites capable of efficiently reducing O2 to H2 O2 while being inert to H2 O2 decomposition under light conditions is significant for H2 O2 photosynthesis, but it remains challenging. Herein, we report the facile design and fabrication of polymeric carbon nitride (CN) decorated with single-Zn sites that have tailorable local coordination environments, which is enabled by utilizing different Zn salt anions. Specifically, the O atom from acetate (OAc) anion participates in the coordination of single-Zn sites on CN, forming asymmetric Zn-N3 O moiety on CN (denoted as CN/Zn-OAc), in contrast to the obtained Zn-N4 sites when sulfate (SO4 ) is adopted (CN/Zn-SO4 ). Both experimental and theoretical investigations demonstrate that the Zn-N3 O moiety exhibits higher intrinsic activity for O2 reduction to H2 O2 than the Zn-N4 moiety. This is attributed to the asymmetric N/O coordination, which promotes the adsorption of O2 and the formation of the key intermediate *OOH on Zn sites due to their modulated electronic structure. Moreover, it is inactive for H2 O2 decomposition under both dark and light conditions. As a result, the optimized CN/Zn-OAc catalyst exhibits significantly improved photocatalytic H2 O2 production activity under visible light irradiation.

Quantitative Description of Metal Center Organization and Interactions in Single-Atom Catalysts

Ultra-high-density single-atom catalysts (UHD-SACs) present unique opportunities for harnessing cooperative effects between neighboring metal centers. However, the lack of tools to establish correlations between the density, types, and arrangements of isolated metal atoms and the support surface properties hinders efforts to engineer advanced material architectures. Here, this work precisely describes the metal center organization in various mono- and multimetallic UHD-SACs based on nitrogen-doped carbon (NC) supports by coupling transmission electron microscopy with tailored machine-learning methods (released as a user-friendly web app) and density functional theory simulations. This approach quantifies the non-negligible presence of multimers with increasing atom density, characterizes the size and shape of these low-nuclearity clusters, and identifies surface atom density criteria to ensure isolation. Further, it provides previously inaccessible experimental insights into coordination site arrangements in the NC host, uncovering a repulsive interaction that influences the disordered distribution of metal centers in UHD-SACs. This observation holds in multimetallic systems, where chemically-specific analysis quantifies the degree of intermixing. These fundamental insights into the materials chemistry of single-atom catalysts are crucial for designing catalytic systems with superior reactivity.

Ni Single-Atoms Based Memristors with Ultrafast Speed and Ultralong Data Retention

Memristor with low-power, high density, and scalability fulfills the requirements of the applications of the new computing system beyond Moore's law. However, there are still nonideal device characteristics observed in the memristor to be solved. The important observation is that retention and speed are correlated parameters of memristor with trade off against each other. The delicately modulating distribution and trapping level of defects in electron migration-based memristor is expected to provide a compromise method to address the contradictory issue of improving both switching speed and retention capability. Here, high-performance memristor based on the structure of ITO/Ni single-atoms (NiSAs/N-C)/Polyvinyl pyrrolidone (PVP)/Au is reported. By utilizing well-distributed trapping sites , small tunneling barriers/distance and high charging energy, the memristor with an ultrafast switching speed of 100 ns, ultralong retention capability of 106  s, a low set voltage (Vset ) of ≈0.7 V, a substantial ON/OFF ration of 103 , and low spatial variation in cycle-to-cycle (500 cycles) and device-to-device characteristics (128 devices) is demonstrated. On the premise of preserving the strengths of a fast switching speed, this memristor exhibits ultralong retention capability comparable to the commercialized flash memory. Finally, a memristor ratioed logic-based combinational memristor array to realize the one-bit full adder is further implemented.

Prediction of transition metal carbonitride monolayers MN4C6 (M = Cr, Mn, Fe, and Co) made up of a benzene ring and a planar MN4 moiety

Based on first-principles calculations, we predict a class of graphene-like magnetic materials, transition metal carbonitrides MN4C6 (M = Cr, Mn, Fe, and Co), which are made up of a benzene ring and an MN4 moiety, two common planar units in the compounds. The structural stability is demonstrated by the phonon and molecular dynamics calculations, and the formation mechanism of the planar geometry of MN4C6 is ascribed to the synergistic effect of sp2 hybridization, M-N coordination bond, and π-d conjugation. The MN4C6 materials consist of only one layer of atoms and the transition metal atom is located in the planar crystal field, which is markedly different from most two-dimensional materials. The calculations indicate that MnN4C6, FeN4C6, and CoN4C6 are ferromagnetic while CrN4C6 has an antiferromagnetic ground state. The Curie temperatures are estimated by solving the anisotropic Heisenberg model with the Monte Carlo method.

Hollow carbon-based materials for electrocatalytic and thermocatalytic CO2 conversion

Electrocatalytic and thermocatalytic CO2 conversions provide promising routes to realize global carbon neutrality, and the development of corresponding advanced catalysts is important but challenging. Hollow-structured carbon (HSC) materials with striking features, including unique cavity structure, good permeability, large surface area, and readily functionalizable surface, are flexible platforms for designing high-performance catalysts. In this review, the topics range from the accurate design of HSC materials to specific electrocatalytic and thermocatalytic CO2 conversion applications, aiming to address the drawbacks of conventional catalysts, such as sluggish reaction kinetics, inadequate selectivity, and poor stability. Firstly, the synthetic methods of HSC, including the hard template route, soft template approach, and self-template strategy are summarized, with an evaluation of their characteristics and applicability. Subsequently, the functionalization strategies (nonmetal doping, metal single-atom anchoring, and metal nanoparticle modification) for HSC are comprehensively discussed. Lastly, the recent achievements of intriguing HSC-based materials in electrocatalytic and thermocatalytic CO2 conversion applications are presented, with a particular focus on revealing the relationship between catalyst structure and activity. We anticipate that the review can provide some ideas for designing highly active and durable catalytic systems for CO2 valorization and beyond.

Well-defined N3 C1 -anchored Single-Metal-Sites for Oxygen Reduction Reaction

N-, C-, O-, S-coordinated single-metal-sites (SMSs) have garnered significant attention due to the potential for significantly enhanced catalytic capabilities resulting from charge redistribution. However, significant challenges persist in the precise design of well-defined such SMSs, and the fundamental comprehension has long been impeded in case-by-case reports using carbon materials as investigation targets. In this work, the well-defined molecular catalysts with N3 C1 -anchored SMSs, i.e., N-confused metalloporphyrins (NCPor-Ms), are calculated for their catalytic oxygen reduction activity. Then, NCPor-Ms with corresponding N4 -anchored SMSs (metalloporphyrins, Por-Ms), are synthesized for catalytic activity evaluation. Among all, NCPor-Co reaches the top in established volcano plots. NCPor-Co also shows the highest half-wave potential of 0.83 V vs. RHE, which is much better than that of Por-Co (0.77 V vs. RHE). Electron-rich, low band gap and regulated d-band center contribute to the high activity of NCPor-Co. This study delves into the examination of well-defined asymmetric SMS molecular catalysts, encompassing both theoretical and experimental facets. It serves as a pioneering step towards enhancing the fundamental comprehension and facilitating the development of high-performance asymmetric SMS catalysts.

Sequential co-reduction of nitrate and carbon dioxide enables selective urea electrosynthesis

Despite the recent achievements in urea electrosynthesis from co-reduction of nitrogen wastes (such as NO3-) and CO2, the product selectivity remains fairly mediocre due to the competing nature of the two parallel reduction reactions. Here we report a catalyst design that affords high selectivity to urea by sequentially reducing NO3- and CO2 at a dynamic catalytic centre, which not only alleviates the competition issue but also facilitates C-N coupling. We exemplify this strategy on a nitrogen-doped carbon catalyst, where a spontaneous switch between NO3- and CO2 reduction paths is enabled by reversible hydrogenation on the nitrogen functional groups. A high urea yield rate of 596.1 µg mg-1 h-1 with a promising Faradaic efficiency of 62% is obtained. These findings, rationalized by in situ spectroscopic techniques and theoretical calculations, are rooted in the proton-involved dynamic catalyst evolution that mitigates overwhelming reduction of reactants and thereby minimizes the formation of side products.

Modulating the Electronic Metal-Support Interactions to Anti-Leaching Pt Single Atoms for Efficient Hydrosilylation

Modulating the electronic metal-support interaction (EMSI) of the single-atomic sites against leaching via microenvironment regulation is critical to achieving high activity and stability but remains challenging. Herein, this work selectively confines Pt single atoms on CoFe layered double hydroxide (LDH) by three oxygen atoms around cation vacancy (Pt1 /LDHV ) or one oxygen atom at the regular surface (Pt1 /LDH) via cation vacancy engineering. By characterizing the structural evolution of the obtained catalysts before and after vacancy construction and single-atom anchoring, this work demonstrates how the microenvironments modulate the EMSI and the catalytic performance. Theoretical simulations further reveal a significantly enhanced EMSI effect by the three-coordinated Pt1 atoms on cation vacancies in Pt1 /LDHV , which endows a more prominent anti-leaching feature than the one-coordinated ones on the regular surface. As a result, the Pt1 /LDHV catalyst shows exceptional performance in anti-Markovnikov alkene hydrosilylation, with a turnover frequency of 1.3 × 105 h-1 . More importantly, the enhanced EMSI of Pt1 /LDHV effectively prevented the leaching of Pt atom from the catalyst surface and can be recycled at least ten times with only a 3.4% loss of catalytic efficiency with minimal Pt leaching, and reach a high turnover number of 1.0 × 106 .

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.

Precise Design and Modification Engineering of Single-Atom Catalytic Materials for Oxygen Reduction

Due to their unique electronic and structural properties, single-atom catalytic materials (SACMs) hold great promise for the oxygen reduction reaction (ORR). Coordinating environmental and engineering strategies is the key to improving the ORR performance of SACMs. This review summarizes the latest research progress and breakthroughs of SACMs in the field of ORR catalysis. First, the research progress on the catalytic mechanism of SACMs acting on ORR is reviewed, including the latest research results on the origin of SACMs activity and the analysis of pre-adsorption mechanism. The study of the pre-adsorption mechanism is an important breakthrough direction to explore the origin of the high activity of SACMs and the practical and theoretical understanding of the catalytic process. Precise coordination environment modification, including in-plane, axial, and adjacent site modifications, can enhance the intrinsic catalytic activity of SACMs and promote the ORR process. Additionally, several engineering strategies are discussed, including multiple SACMs, high loading, and atomic site confinement. Multiple SACMs synergistically enhance catalytic activity and selectivity, while high loading can provide more active sites for catalytic reactions. Overall, this review provides important insights into the design of advanced catalysts for ORR.

Precise arrangement of metal atoms at the interface by a thermal printing strategy

The kinetics and pathway of most catalyzed reactions depend on the existence of interface, which makes the precise construction of highly active single-atom sites at the reaction interface a desirable goal. Herein, we propose a thermal printing strategy that not only arranges metal atoms at the silica and carbon layer interface but also stabilizes them by strong coordination. Just like the typesetting of Chinese characters on paper, this method relies on the controlled migration of movable nanoparticles between two contact substrates and the simultaneous emission of atoms from the nanoparticle surface at high temperatures. Observed by in situ transmission electron microscopy, a single Fe3O4 nanoparticle migrates from the core of a SiO2 sphere to the surface like a droplet at high temperatures, moves along the interface of SiO2 and the coated carbon layer, and releases metal atoms until it disappears completely. These detached atoms are then in situ trapped by nitrogen and sulfur defects in the carbon layer to generate Fe single-atom sites, exhibiting excellent activity for oxygen reduction reaction. Also, sites' densities can be regulated by controlling the size of Fe3O4 nanoparticle between the two surfaces. More importantly, this strategy is applicable to synthesize Mn, Co, Pt, Pd, Au single-atom sites, which provide a general route to arrange single-atom sites at the interface of different supports for various applications.

Surface-Modified Pd/CeO2 Single-Atom Catalyst Shows Increased Activity for Suzuki Cross-Coupling

Single-atom catalysts (SACs) comprise catalytically active atoms dispersed on supports; they combine the high activity and site uniformity of homogeneous catalysts with the ease of separability of heterogeneous catalysts. However, SACs lack fine control over the active site, provided by ligands in homogeneous catalysts. In this work, we demonstrate that modification of the support with an organic monolayer is a viable approach to improving the catalytic performance. The addition of catechol-type monolayers to a Pd/CeO2 SAC increases its catalytic activity for Suzuki cross-coupling, a central reaction in the synthesis of fine chemicals and pharmaceuticals. Kinetic trials reveal that the coating reduces the activation energy from 49 ± 9 to 22 ± 5 kJ/mol and produces a 4-fold rate enhancement at 25 °C, an effect we attribute to π-π interactions between the reactant and the catechol coating. Further development of this approach could vastly increase the utility of SACs in organic synthesis.

Modulating Electron Density of Ni-N-C Sites by N-doped Ni for Industrial-level CO2 Electroreduction in Acidic Media

Electro-chemically reducing CO2 in a highly acidic medium is promising for addressing the issue of carbonate accumulation. However, the hydrogen evolution reaction (HER) typically dominates the acidic CO2 reduction. Herein, we construct an efficient electro-catalyst for CO formation based on a core-shell structure, where nitrogen-doped Ni nanoparticles coexist with nitrogen-coordinated Ni single atoms. The optimal catalyst demonstrates a significantly improved CO faradaic efficiency (FE) of 96.7 % in the acidic electrolyte (pH=1) at an industrial-scale current density of 500 mA cm-2 . Notably, the optimal catalyst maintains a high FE of CO exceeding 90 % (current density=500 mA cm-2 ) in the electrolyte with a wide pH range from 0.67 to 14. In-situ spectroscopic characterization and density functional theory calculations show that the local electron density of Ni-N-C sites is enhanced by N-doped Ni particles, which facilitates the formation of *COOH intermediate and the adsorption of *CO. This study demonstrates the potential of a hybrid metal/Ni-N-C interface in boosting acidic CO2 electro-reduction.