Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Atomic-level tailoring of single-atom tungsten catalysts for optimized electrochemical nitrate-to-ammonia conversion

Nitrate contamination of water resources poses significant health and environmental risks, necessitating efficient denitrification methods that produce ammonia as a desirable product. The electrocatalytic nitrate reduction reaction (NO3RR) powered by renewable energy offers a promising solution, however, developing highly active and selective catalysts remains challenging. Single-atom catalysts (SACs) have shown impressive performance, but the crucial role of their coordination environment, especially the next-nearest neighbor dopant atoms, in modulating catalytic activity for NO3RR is underexplored. This study aims to optimize the NO3RR performance of tungsten (W) single atoms anchored on graphene by precisely engineering their coordination environment through first and next-nearest neighbor dopants. The stability, reaction paths, activity, and selectivity of 43 different nitrogen and boron doping configurations were systematically studied using density functional theory. The results reveal W@C3, with W coordinated to three carbon atoms, exhibits outstanding NO3RR activity with a low limiting potential of -0.36 V. Intriguingly, introducing next-nearest neighbor B and N dopants further enhances the performance, with W@C3-BN achieving a lower limiting potential of -0.26 V. This exceptional activity originates from optimal nitrate adsorption strengths facilitated by orbital hybridization and charge modulation effects induced by the dopants. Furthermore, high energy barriers for NO2 and NO formation on W@C3 and W@C3-BN ensure their selectivity towards NO3RR products. These findings provide crucial atomic-level insights into rational design strategies for high-performance single-atom NO3RR catalysts via coordination environment engineering.

Graphene-supported MN4 single-atom catalysts for multifunctional electrocatalysis enabled by axial Fe tetramer coordination

Multifunctional electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are crucial for development of the key electrochemical energy storage and conversion devices, for which single-atom catalyst (SAC) has present great promises. Very recently, some experimental works showed that structurally well-defined ultra-small transition-metal clusters (such as Fe and Co tetramers, denoted as Fe4 and Co4, respectively), can efficiently modulate the catalytic behavior of SACs by axial coordination. Herein, taking the graphene-supported MN4 SACs as representatives, we theoretically explored the feasibility of realizing multifunctional SACs for ORR, OER and HER by this novel axial coordination engineering. Through extensive first-principles calculations, from 23 candidates, IrN4 decorated with Fe4 (IrN4/Fe4) is identified as the promising trifunctional catalyst with the theoretical overpotential of 0.43, 0.51 and 0.30 V for OER, ORR and HER, respectively. RhN4/Fe4 and CoN4/Fe4 are recognized as potential OER and ORR bifunctional catalysts. In addition, NiN4/Fe4 exhibits the best ORR activity with an overpotential of 0.30 V, far superior to the pristine NiN4 SAC (0.88 V). Electronic structure analyses reveal that the significantly enhanced ORR/OER activity can be ascribed to the orbital and charge redistribution of Ni/Ir active center, resulting from its electronic interaction with Fe4 cluster. This work could stimulate and guide the rational design of graphene-based multifunctional SACs realized by axial coordination of small TM clusters, and provide insights into the modulation mechanism.

A Chemoenzymatic Cascade for the Formal Enantioselective Hydroxylation and Amination of Benzylic C-H Bonds

We report the synthesis and characterization of an artificial peroxygenase (CoN4SA-POase) with CoN4 active sites by supporting single-atom cobalt on polymeric carbon nitrogen, which exhibits high activity, selectivity, stability, and reusability in the oxidation of aromatic alkanes to ketones. Density functional theory calculations reveal a different catalytic mechanism for the artificial peroxygenase from that of natural peroxygenases. In addition, continuous-flow systems are employed to combine CoN4SA-POase with enantiocomplementary ketoreductases as well as an amine dehydrogenase, enabling the enantioselective synthesis of chiral alcohols and amines from hydrocarbons with significantly improved productivity. This work, emulating nature and beyond nature, provides a promising design concept for heme enzyme-based transformations.

The Significance of the 'Insignificant': Non-covalent Interactions in CO2 Reduction Reactions with 3C-TM (TM=Sc-Zn) Single-Atom Catalysts

With energy shortages and excessive CO2 emissions driving climate change, converting CO2 into high-value-added products offers a promising solution for carbon recycling. We investigate CO2 reduction reactions (CO2RR) catalyzed by 10 single-atom catalysts (SACs), incorporating weak non-covalent interactions, specifically lone pair-π and H-π interactions. The SACs, consisting of transition metals coordinated by three carbon atoms in a defective graphene substrate (3C-TM, TM=Sc-Zn), leverage these interactions to influence the energy fluctuations of intermediates and the limiting potentials of CO2RR, without altering the overall reaction pathway. Our findings show that SACs based on early transition metals (Sc, Ti, V, Cr) can serve as catalysts for C1 products, including HCOOH, HCHO, CH3OH, and CH4, while those based on Fe and Co are suitable for CO formation. Driving force analysis helps bridge theoretical results with experimental observations and propose a modified approach for assessing hydrogen evolution reactions (HER) competition. SACs based on Ni and Cu exhibit moderate HER tolerance, while early transition metals excel in selective CO2 reduction. We also identify a linear scaling relationship between the free energies of *COOH and *CO. This study offers valuable insights for future experimental studies and large-scale computational screenings.

Safety Landscape of Therapeutic Nanozymes and Future Research Directions

Oxidative stress and inflammation are at the root of a multitude of diseases. Treatment of these conditions is often necessary but current standard therapies to fight excessive reactive oxygen species (ROS) and inflammation are often ineffective or complicated by substantial safety concerns. Nanozymes are emerging nanomaterials with intrinsic enzyme-like properties that hold great promise for effective cancer treatment, bacterial elimination, and anti-inflammatory/anti-oxidant therapy. While there is rapid progress in tailoring their catalytic activities as evidenced by the recent integration of single-atom catalysts (SACs) to create next-generation nanozymes with superior activity, selectivity, and stability, a better understanding and tuning of their safety profile is imperative for successful clinical translation. This review outlines the current applied safety assessment approaches and provides a comprehensive summary of the safety knowledge of therapeutic nanozymes. Overall, nanozymes so far show good in vitro and in vivo biocompatibility despite considerable differences in their composition and enzymatic activities. However, current safety investigations mostly cover a limited set of basic toxicological endpoints, which do not allow for a thorough and deep assessment. Ultimately, remaining research gaps that should be carefully addressed in future studies are highlighted, to optimize the safety profile of therapeutic nanozymes early in their pre-clinical development.

Well-defined asymmetric nitrogen/carbon-coordinated single metal sites for carbon dioxide conversion

Asymmetric nitrogen/carbon-coordinated single metal sites (M-NxC4-x) outperform symmetric M-N4 sites in carbon dioxide (CO2) electroreduction. However, the challenge of crafting well-defined M-NxC4-x sites complicates the understanding of their structure-catalytic performance relationship. In this study, we employ metallized N-confused tetraphenylporphyrin (M-NCTPP) to investigate CO2 conversion on M-N3C1 sites using both density functional theory and experimental methods. The optimal cobalt (Co)-N3C1 site (Co-NCTPP) achieves a current density of 500 mA cm-2 and a carbon monoxide Faraday efficiency exceeding 90 % at -1.25 V vs. the reversible hydrogen electrode, surpassing the performance of Co-N4 (Co-TPP). This research introduces a novel approach for designing and synthesizing high-activity heteroatom-anchored single metal sites, advancing fundamental understanding in the field.

Emerging ZnO Semiconductors for Photocatalytic CO2 Reduction to Methanol

Carbon recycling is poised to emerge as a prominent trend for mitigating severe climate change and meeting the rising demand for energy. Converting carbon dioxide (CO2) into green energy and valuable feedstocks through photocatalytic CO2 reduction (PCCR) offers a promising solution to global warming and energy needs. Among all semiconductors, zinc oxide (ZnO) has garnered considerable interest due to its ecofriendly nature, biocompatibility, abundance, exceptional semiconducting and optical properties, cost-effectiveness, easy synthesis, and durability. This review thoroughly discusses recent advances in mechanistic insights, fundamental principles, experimental parameters, and modulation of ZnO catalysts for direct PCCR to C1 products (methanol). Various ZnO modification techniques are explored, including atomic size regulation, synthesis strategies, morphology manipulation, doping with cocatalysts, defect engineering, incorporation of plasmonic metals, and single atom modulation to boost its photocatalytic performance. Additionally, the review highlights the importance of photoreactor design, reactor types, geometries, operating modes, and phases. Future research endeavors should prioritize the development of cost-effective catalyst immobilization methods for solid-liquid separation and catalyst recycling, while emphasizing the use of abundant and non-toxic materials to ensure environmental sustainability and economic viability. Finally, the review outlines key challenges and proposes novel directions for further enhancing ZnO-based photocatalytic CO2 conversion processes.

Nanomaterials for Flexible Neuromorphics

The quest to imbue machines with intelligence akin to that of humans, through the development of adaptable neuromorphic devices and the creation of artificial neural systems, has long stood as a pivotal goal in both scientific inquiry and industrial advancement. Recent advancements in flexible neuromorphic electronics primarily rely on nanomaterials and polymers owing to their inherent uniformity, superior mechanical and electrical capabilities, and versatile functionalities. However, this field is still in its nascent stage, necessitating continuous efforts in materials innovation and device/system design. Therefore, it is imperative to conduct an extensive and comprehensive analysis to summarize current progress. This review highlights the advancements and applications of flexible neuromorphics, involving inorganic nanomaterials (zero-/one-/two-dimensional, and heterostructure), carbon-based nanomaterials such as carbon nanotubes (CNTs) and graphene, and polymers. Additionally, a comprehensive comparison and summary of the structural compositions, design strategies, key performance, and significant applications of these devices are provided. Furthermore, the challenges and future directions pertaining to materials/devices/systems associated with flexible neuromorphics are also addressed. The aim of this review is to shed light on the rapidly growing field of flexible neuromorphics, attract experts from diverse disciplines (e.g., electronics, materials science, neurobiology), and foster further innovation for its accelerated development.

Single Atomic Cu-C3 Sites Catalyzed Interfacial Chemistry in Bi@C for Ultra-Stable and Ultrafast Sodium-Ion Batteries

Regulating interfacial chemistry at electrode-electrolyte interface by designing catalytic electrode material is crucial and challenging for optimizing battery performance. Herein, a novel single atom Cu regulated Bi@C with Cu-C3 site (Bi@SA Cu-C) have been designed via the simple pyrolysis of metal-organic framework. Experimental investigations and theoretical calculations indicate the Cu-C3 sites accelerate the dissociation of P-F and C-O bonds in NaPF6-ether-based electrolyte and catalyze the formation of inorganic-rich and powerful solid electrolyte interphase. In addition, the Cu-C3 sites with delocalized electron around Cu trigger an uneven charge distribution and induce an in-plane local electric field, which facilitates the adsorption of Na+ and reduces the Na+ migration energy barrier. Consequently, the obtained Bi@SA Cu-C achieves a state-of-the-art reversible capacity, ultrahigh rate capability, and long-term cycling durability. The as-constructed full cell delivers a high capacity of 351 mAh g-1 corresponding to an energy density of 265 Wh kg-1. This work provides a new strategy to realize high-efficient sodium ion storage for alloy-based anode through constructing single-atom modulator integrated catalysis and promotion effect into one entity.

Edge-doped substituents as an emerging atomic-level strategy for enhancing M-N4-C single-atom catalysts in electrocatalysis of the ORR, OER, and HER

M-N4-C single-atom catalysts (MN4) have gained attention for their efficient use at the atomic level and adjustable properties in electrocatalytic reactions like the ORR, OER, and HER. Yet, understanding MN4's activity origin and enhancing its performance remains challenging. Edge-doped substituents profoundly affect MN4's activity, explored in this study by investigating their interaction with MN4 metal centers in ORR/OER/HER catalysis (Sub@MN4, Sub = B, N, O, S, CH3, NO2, NH2, OCH3, SO4; M = Fe, Co, Ni, Cu). The results show overpotential variations (0 V to 1.82 V) based on Sub and metal centers. S and SO4 groups optimize FeN4 for peak ORR activity (overpotential at 0.48 V) and reduce OER overpotentials for NiN4 (0.48 V and 0.44 V). N significantly reduces FeN4's HER overpotential (0.09 V). Correlation analysis highlights the metal center's key role, with ΔG*H and ΔG*OOH showing mutual predictability (R2 = 0.92). Eg proves a reliable predictor for Sub@CoN4 (ΔG*OOH/ΔG*H, R2 = 0.96 and 0.72). Machine learning with the KNN model aids catalyst performance prediction (R2 = 0.955 and 0.943 for ΔG*OOH/ΔG*H), emphasizing M-O/M-H and the d band center as crucial factors. This study elucidates edge-doped substituents' pivotal role in MN4 activity modulation, offering insights for electrocatalyst design and optimization.

Symmetry-Broken Steered Delocalization State in a Single-Atom Photocatalyst

Single-atom catalysts (SACs) that feature uniform metal active sites with symmetry configurations hold great promise in photocatalysis, while their catalytic efficiency is often restricted by the insufficient inherent activity. Drawing inspiration from hard-soft acid-base theory, here we propose that the delocalized electronic state of single-atom centers can be selectively modulated by adjusting their coordination symmetry, thereby optimizing the adsorption and activation of the reactant molecules. By taking ceria-based Ru-SAC (Ru-CeO2) as an example, we show that after introducing symmetry breaking, the Ru-CeO2 with an asymmetric Ru-O4 configuration (named P-Ru-CeO2) exhibits highly delocalized electrons with a soft acidic nature, leading to a much higher photocatalytic performance than for pristine Ru-CeO2 and CeO2 counterparts. The corresponding inherent mechanism was systematically investigated by spectroscopy and theoretical studies. This work provides an effective strategy for the design and controllable modulation of atomically dispersed catalysts with symmetry-broken configurations, thereby advancing applications in photocatalysis.

Investigation of a Single Atom Iron Catalyst for the Electrocatalytic Reduction of Nitric Oxide to Hydroxylamine: A DFT Study

Hydroxylamine, as an important reducing agent, disinfectant, foaming agent, and biocide, plays a role in both human life and industrial production. However, its synthesis is confronted with challenges, such as high pollution and large consumption. Here, we propose a coordination tailoring strategy to design 47 graphene-supported single iron atom catalysts (SACs), namely, Fe@CxZy (Z = B, N, O, P, and S), for the reduction of nitric oxide to hydroxylamine. Using density functional theory calculations, we demonstrated the great impact of the coordination environment on the stability, catalytic selectivity, and activity of the Fe site. We identified that the experimentally available Fe@N4 possesses an ultralow theoretical limiting potential of -0.32 V compared to that of other catalysts. A comprehensive investigation of the electronic properties elucidates the underlying active origin and reaction mechanism of the nitric oxide reduction reaction to hydroxylamine on Fe@N4. These results not only explain the catalytic origin of synthesized SACs for the NH2OH production but also offer theoretical guidance for further optimizing high-performance catalysts.

Biomimetic Electronic Communication of Iodine Doped Single-Atom Fe Site for Highly Active and Stable Dopamine Oxidation

Rational design of highly active and stable catalysts for dopamine oxidation is still a great challenge. Herein, inspired by the catalytic pocket of natural enzymes, an iodine (I)-doped single Fe-site catalyst (I/FeSANC) is synthesized to mimic the catalytic center of heme enzymes in both geometrical and electronic structures, aiming to enhance dopamine (DA) oxidation. Experimental studies and theoretical calculations show that electronic communication between I and FeN5 effectively modulates the electronic structure of the active site, greatly optimizing the overlap of Fe 3d and O 2p orbitals, thereby enhancing OH adsorption. In addition, the electronic communication induced by iodine doping attenuates the attack of proton hydrogen on the active center, thereby enhancing the stability of I/FeSANC. This work provides new insights into the design of highly active and stable single-atom catalysts and enhances the understanding of catalytic mechanisms for DA oxidation at the atomic scale.

Single-Atom Doped Fullerene (MN4-C54) as Bifunctional Catalysts for the Oxygen Reduction and Oxygen Evolution Reactions

Development of high-performance oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts is crucial to realizing the electrolytic water cycle. C60 is an ideal substrate material for single atom catalysts (SACs) due to its unique electron-withdrawing properties and spherical structure. In this work, we screened for a novel single-atom catalyst based on C60, which anchored transition metal atoms in the C60 molecule by coordination with N atoms. Through first-principles calculations, we evaluated the stability and activity of MN4-C54 (M = Fe, Co, Ni, Cu, Rh, Ru, Pd, Ag, Pt, Ir, Au). The results indicate that CuN4-C54, which is based only on earth-abundant elements, exhibited low overpotentials of 0.46 and 0.47 V for the OER and ORR, respectively, and was considered a promising bifunctional catalyst, showing better performance than the noble-metal ones. In addition, according to the linear relationship of intermediates, we established volcano plots to describe the activity trends of the OER and ORR on MN4-C54. Finally, d-band center and crystal orbital Hamiltonian populations methods were used to explain the catalytic origin. Suitable d-band centers lead to moderate adsorption strength, further leading to good catalytic performances.

Designing Robust Single Atom Catalysts by Three-in-One Strategy: Sub-1-nm Space Confining, Bimetallic Bonding and Reaction-Induced Forming Active Sites

It is imperative to design robust single atom catalysts (SACs) that maintain the stability of the active component under diverse reaction conditions and prevent aggregation or deactivation. Confining the single atom active site within sub-nanometer (sub-1-nm) spaces has proven effective in enhancing the stability and activity of the catalyst, owing to the strong constraints and regulations imposed on atomic behavior at this scale. Bimetallic bond atomic sites, comprising two distinct metal compositions, often exhibit unique electronic structures and catalytic properties. Designing SACs under reaction-induced conditions, such as varying temperatures, pressures, and atmospheres, can facilitate a deeper understanding of the formation and migration behavior of active sites in real reactions, as well as the optimization mechanisms for performance enhancement. The objective of this review is to promote a robust SAC design strategy that encapsulates bimetallic bonding active sites within sub-1-nm spaces and investigates catalyst preparation and performance under reaction-induced conditions. This design strategy is anticipated to bolster the catalytic activity and stability of the catalyst while also offering fresh perspectives and optimization avenues for the catalytic processes involved in practical chemical reactions.

Deciphering the Nitrogen Activation Mechanisms on Group VIII Single Atoms at MoS2

The activation of nitrogen (N2) is vital for sustainable ammonia production and nitrogen fixation technologies. This study employs density functional theory (DFT) to investigate the nitrogen activation and reduction capabilities of Group VIII single-atom catalysts anchored on MoS2. Among these, osmium anchored on MoS2 (Os@MoS2) emerged as the most promising catalyst, exhibiting the highest N2 activation and the lowest nitrogen reduction reaction (NRR) overpotential (0.624 V). A pronounced "electron drift" effect was observed for Os@MoS2, leading to significant charge redistribution that weakens the N ≡ N triple bond, facilitating its activation. The N-N dissociation energy barrier at the *N-NH2 intermediate was calculated to be only 0.82 eV, confirming Os@MoS2's superior catalytic efficiency. Detailed analyses, including electrostatic potential maps, electron localization functions, spin density, and charge transfer, revealed the pivotal role of orbital interactions in driving N2 activation. Interestingly, the trends in adsorbed N2 bond energies and NRR overpotentials showed a consistent diagonal pattern across the Group VIII catalysts, emphasizing the importance of electronic and geometric factors. This work offers valuable insights into nitrogen activation mechanisms and provides a framework for designing efficient catalysts, highlighting Os@MoS2's potential in sustainable ammonia synthesis.

Electrocatalytic Dinitrogen Reduction to Ammonia Using Easily Reducible N-Fused Cobalt Porphyrins

Single-site molecular electrocatalysts, especially those that perform catalytic conversion of N2 to NH3 under mild conditions, are highly desirable to derive fundamental structure-activity relations and as potential alternatives to the current energy-consuming Haber-Bosch ammonia production process. Combining theoretical calculations with experimental evidence, it has been shown that easily reducible cobalt porphyrins catalyze the six-electron, six-proton reduction of dinitrogen to NH3 at neutral pH and under ambient conditions. Two easily reducible N-fused cobalt porphyrins - CoNHF and CoNHF(Br)2 - reveal NRR activity with Faradic efficiencies between 6-7.5 % with ammonia yield rates of 300-340 μmol g-1 h-1. Contrary to this, much harder-to-reduce N-fused porphyrins - CoNHF(Ph)2 and CoNHF(PE)2 - reveal no NRR activity. The present study highlights the significance of tuning the redox and structural properties of single-site NRR electrocatalysts for improved NRR activity under mild conditions.

Graphene-Metal Nanocrystal Hybrid Materials for Bioapplications

The development of functional nanomaterials is crucial for advancing personalized and precision medicine. Graphene-metal nanocrystal hybrid materials not only possess the intrinsic advantages of graphene-based materials but also exhibit additional optical, magnetic, and catalytic properties of various metal nanocrystals, showing great synergies in bioapplications, including biosensing, bioimaging, and disease treatments. In this Perspective, we discuss the advantages and design principles of graphene-metal nanocrystal hybrid materials and provide an overview of their applications in biological fields. Finally, we highlight the challenges and future directions for their practical implementation.

Pore modulation of single atomic Fe sites for ultrafast Fenton-like chemistry with amplified electron migration oxidation

The limited interaction between pollutants, oxidants, and the surface catalytic sites of single atom catalysts (SACs) restricts the water decontamination effectiveness. Confining catalytic sites within porous structures enables the localized enrichment of reactants for optimized reaction kinetics, while the specific regulatory mechanisms remain unclear. Herein, SACs with porous modification significantly improves the utilization of peroxymonosulfate (PMS) and pollutant degradation activity. Confining catalytic sites in porous structure effectively reduces the mass transfer distance between radicals (SO4•- and •OH) and pollutants, thereby improving reaction performance. Pore modulation changes the surface electronic structure, leading to a significant improvement in the electron migration process. The system shows significant potential in effectively oxidizing various common emerging pollutants, and exhibits robust resistance to interference from environmental matrices. Moreover, a quantitative evaluation using life cycle assessment (LCA) indicates that the pFe-SAC/PMS system showcases superior environmental importance and practicality.

Exploring Nitrogen Reduction Reaction Mechanisms with Graphyne-Confined Single-Atom Catalysts: A Computational Study Incorporating Electrode Potential and pH

This study reconciles discrepancies between practical electrochemical conditions and theoretical density functional theory (DFT) frameworks, evaluating three graphyne-confined single-atom catalysts (Mo-TEB, Mo@GY, and Mo@GDY). Using both constant charge models in vacuum and constant potential models with continuum implicit solvation, we closely mimic real-world electrochemical environments. Our findings highlight the crucial role of explicitly incorporating electrode potential and pH in the constant potential model, providing enhanced insights into the nitrogen reduction reaction (NRR) mechanisms. Notably, the superior NRR performance of Mo-TEB is attributed to the d-band center's proximity to the Fermi level and enhanced magnetic moments at the atomic center. This research advances our understanding of graphyne-confined single-atom catalysts as effective NRR platforms and underscores the significance of the constant potential model for accurate DFT studies of electrochemical reactions.

Factorial Optimization of CoCuFe-LDH/Graphene Ternary Composites as Electrocatalysts for Water Splitting

The layered double hydroxides (LDHs) have demonstrated significant potential as non-noble-metal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Their unique compositional and structural properties contribute to their efficiency and stability as catalysts. In this study, CoCuFe-LDH composites were grown on graphene (G) via a cost-effective and straightforward one-step hydrothermal process. A 2-level full-factorial model was employed to determine the impact of Co (1.5, 3, and 4.5 mmol) and graphene (10, 30, and 50 mg) concentrations on the onset potential of OER and HER, which were the chosen response variables. OER and HER activity variabilities were assessed in triplicate using Co[3]Cu[3]Fe[3]-LDH/G[30] (central point), which were determined at 0.01% and 0.02%, respectively. Statistical analyses demonstrated that Co[4.5]Cu[3]Fe[3]-LDH/G[10] and Co[1.5]Cu[3]Fe[3]-LDH/G[10] showed the lowest onset potential at 1.52 V and -0.32 V (V vs RHE) for the OER and HER, respectively, suggesting that a high cobalt concentration enhances OER performance, while optimal HER catalysis was achieved with lower cobalt concentrations. Moreover, the trimetallic composites exhibited good stability with negligible loss of catalytic activity over 24 h.

Probing Basal and Prismatic Planes of Graphitic Materials for Metal Single Atom and Subnanometer Cluster Stabilization

Supported metal single atom catalysis is a dynamic research area in catalysis science combining the advantages of homogeneous and heterogeneous catalysis. Understanding the interactions between metal single atoms and the support constitutes a challenge facing the development of such catalysts, since these interactions are essential in optimizing the catalytic performance. For conventional carbon supports, two types of surfaces can contribute to single atom stabilization: the basal planes and the prismatic surface; both of which can be decorated by defects and surface oxygen groups. To date, most studies on carbon-supported single atom catalysts focused on nitrogen-doped carbons, which, unlike classic carbon materials, have a fairly well-defined chemical environment. Herein we report the synthesis, characterization and modeling of rhodium single atom catalysts supported on carbon materials presenting distinct concentrations of surface oxygen groups and basal/prismatic surface area. The influence of these parameters on the speciation of the Rh species, their coordination and ultimately on their catalytic performance in hydrogenation and hydroformylation reactions is analyzed. The results obtained show that catalysis itself is an interesting tool for the fine characterization of these materials, for which the detection of small quantities of metal clusters remains a challenge, even when combining several cutting-edge analytical methods.

Ru3@Mo2CO2 MXene single-cluster catalyst for highly efficient N2-to-NH3 conversion

Single-cluster catalysts (SCCs) representing structurally well-defined metal clusters anchored on support tend to exhibit tunable catalytic performance for complex redox reactions in heterogeneous catalysis. Here we report a theoretical study on an SCC of Ru3@Mo2CO2 MXene for N2-to-NH3 thermal conversion. Our results show that Ru3@Mo2CO2 can effectively activate N2 and promotes its conversion to NH3 through an association mechanism, in which the rate-determining step of NH2* + H* → NH3* has a low energy barrier of 1.29 eV. Notably, with the assistance of Mo2CO2 support, the positively charged Ru3 cluster active site can effectively adsorb and activate N2, leading to 0.74 |e| charge transfer from Ru3@Mo2CO2 to the adsorbed N2. The supported Ru3 also acts as an electron reservoir to regulate the charge transfer for various intermediate steps of ammonia synthesis. Microkinetic analysis shows that the turnover frequency of the N2-to-NH3 conversion on Ru3@Mo2CO2 is as high as 1.45 × 10-2 s-1 site-1 at a selected thermodynamic condition of 48 bar and 700 K, the performance of which even surpasses that of the Ru B5 site and Fe3/θ-Al2O3(010) reported before. Our work provides a theoretical understanding of the high stability and catalytic mechanism of Ru3@Mo2CO2 and guidance for further designing and fabricating MXene-based metal SCCs for ammonia synthesis under mild conditions.

Rational design of graphdiyne-based single-atom catalysts for electrochemical CO2 reduction reaction

Graphdiyne (GDY) has achieved great success in the application of two-dimensional carbon materials in recent years due to its excellent electrochemical catalytic capacity. Considering the unique electronic structure of GDY, transition metal (TM1) (TM = Fe, Ru, Os, Co, Rh, Ir) single-atom catalysts (SACs) with isolated loading on GDY were designed for electrochemical CO2 reduction reaction (CO2RR) with density functional theoretical (DFT) calculations. The charge density difference and projected densities of states have been systematically calculated. The mechanism of electrochemical catalysis and the reaction pathway of CO2RR over Os1/GDY catalysts have also been investigated and high catalytic activity was found for the generation of methane. The calculated results provide a theoretical basis for the design of efficient GDY-based SACs for electrochemical CO2RR.

Stable, while Still Active? A DFT Study of Cu, Ag, and Au Single Atoms at the C3N4/TiO2 Interface

Hybrid DFT calculations are employed to compare the adsorption and stabilization of Cu, Ag, and Au atoms on graphitic C3N4 and on the heterojunction formed by g- C3N4 and TiO2. While Cu and Ag can be strongly chemisorbed in form of cations on g- C3N4, Au is only weakly physisorbed. On g- C3N4/TiO2, all coinage metal adatoms can be strongly chemisorbed, but, while Cu and Ag forms cations, Au form an Au- species. Ab Initio Molecular Dynamics simulations confirm that the metal adatoms on g-C3N4 are highly mobile at room temperature, while they remain confined in the interfacial spacing between C3N4 and TiO2 on the heterojunction, being both stably bound and reachable for the reactants in a catalytic cycle. Doping g- C3N4/TiO2 with metal single atoms permits thus to generate catalytic systems with tunable charge and chemical properties and improved stability with respect to bare C3N4. Moreover, the changes in the electronic structure of g- C3N4/TiO2 induced by the presence of the metal single atoms are beneficial also for photocatalytic applications.

Effect of Different N/C Coordination Electronic Structures on the Activity of Bifunctional Rare-Earth Ytterbium Electrocatalysts for Oxygen Electrodes

The research and development of bifunctional electrocatalysts for the oxygen electrode is of great significance to solve the problem of electrochemical energy. Herein, the effect of different structure-activity relationships on the performance of YbNxCy-gra catalysts was explored. The bifunctional activity of graphene with a vacancy defect supported by single-atom rare-earth ytterbium was studied by density functional theory (DFT) calculations. We systematically analyzed the stability, electronic properties, and catalytic performance of potential bifunctional catalysts. The results showed that all catalysts were thermodynamically and kinetically stable. Under acidic conditions, YbN2C2-oppo-gra and YbN2C2-pen-gra showed good ORR activity, and their overpotentials were 0.53 and 0.65 V, respectively. In an alkaline environment, most of the Yb(OH)NxCy-gra catalysts showed excellent ORR and OER bifunctional catalytic activity. Their overpotentials were all below 0.6 V. In particular, the ηORR and ηOER of the Yb(OH)N4C0-gra electrocatalyst were as low as 0.33 and 0.42 V. This verified the practicability and feasibility of hydroxyl-modified catalysts to enhance activity. This research provides theoretical insights into the further design and development of high-efficiency rare-earth-supported bifunctional catalysts.

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.

Unraveling the Photocatalytic Mechanism of N2 Fixation on Single Ruthenium Sites

Photocatalytic N2 fixation offers promise for ammonia synthesis, yet traditional photocatalysts encounter challenges such as low efficiency and short carrier lifetimes. Atomically precise ligand-metal nanoclusters emerge as a solution to address these issues, but the photophysical mechanism remains elusive. Inspired by the synthesis of Au4Ru2 NCs, we investigate the mechanism behind N2 activation on Au4Ru2, focusing on photoactivity and carrier dynamics. Our results reveal that vibration of the Ru-N bond in the low-frequency domain suppresses the deactivation process leading to a long lifetime of the excited N2. By the strategy of isoelectronic substitution, we identify the single Ru sites as the active sites for N2 activation. Furthermore, these ligand-protected M4Ru2 (M = Au, Ag, Cu) NCs show robust thermal stability in explicit solvation and decent photochemical activity for N2 activation and NH3 production. These findings have significant implications for the optimization of catalysts for sustainable ammonia synthesis.

Boosting the Electrocatalytic Oxygen Reduction Activity of MnN4-Doped Graphene by Axial Halogen Ligand Modification

Exploring highly active electrocatalysts as platinum (Pt) substitutes for the oxygen reduction reaction (ORR) remains a significant challenge. In this work, single Mn embedded nitrogen-doped graphene (MnN4) with and without halogen ligands (F, Cl, Br, and I) modifying were systematically investigated by density functional theory (DFT) calculations. The calculated results indicated that these ligands can transform the dyz and dxz orbitals of Mn atom in MnN4 near the Fermi-level into dz2 orbital, and shift the d-band center away from the Fermi-level to reduce the adsorption capacity for reaction intermediates, thus enhancing the ORR catalytic activity of MnN4. Notably, Br and I modified MnN4 respectively with the lowest overpotentials of 0.41 and 0.39 V, possess superior ORR catalytic activity. This work is helpful for comprehensively understanding the ligand modification mechanism of single-atom catalysts and develops highly active ORR electrocatalysts.

Investigating the interplay between charge transfer and CO2 insertion in the adsorption of a NiFe catalyst for CO2 electroreduction on a graphite support through DFT computational approaches

This article describes a density functional theory (DFT) study to explore a bio-inspired NiFe complex known for its experimental activity in electro-reducing CO2 to CH4 when adsorbed on graphite. The coordination properties of the complex are investigated in isolated form and when physisorbed on a graphene surface. A comparative analysis of DFT approaches for surface modeling is conducted, utilizing either a finite graphene flake or a periodic carbon surface. Results reveal that the finite model effectively preserves all crucial properties. By examining predicted structures arising from CO2 insertion within the mono-reduced NiFe species, whether isolated or adsorbed on the graphene flake, a potential species for subsequent electro-reduction steps is proposed. Notably, the DFT study highlights two positive effects of complex adsorption: facile electron transfers between graphene and the complex, finely regulated by the complex state, and a lowering of the thermodynamic demand for CO2 insertion.

Stabilizing Few-Atom Platinum Clusters by Zinc Single-Atom-Glue for Efficient Anti-Markovnikov Alkene Hydrosilylation

Few-atom metal clusters (FAMCs) exhibit superior performance in catalyzing complex molecular transformations due to their special spatial environments and electronic states, compared to single-atom catalysts (SACs). However, achieving the efficient and accurate synthesis of FAMCs while avoiding the formation of other species, such as nanoparticles and SACs, still remains challenges. Herein, we report a two-step strategy for synthesis of few-atom platinum (Pt) clusters by predeposition of zinc single-atom-glue (Zn1) on MgO nanosheets (Ptn-Zn1/MgO), where FAMCs can be obtained over a wide range of Pt contents (0.09 to 1.45 wt %). Zn atoms can act as Lewis acidic sites to allow electron transfer between Zn and Pt through bridging O atoms, which play a crucial role in the formation and stabilization of few-atom Pt clusters. Ptn-Zn1/MgO exhibited a high selectivity of 93 % for anti-Markovnikov alkene hydrosilylation. Moreover, an excellent activity with a turnover frequency of up to 1.6×104 h-1 can be achieved, exceeding most of the reported Pt SACs. Further theoretical studies revealed that the Pt atoms in Ptn-Zn1/MgO possess moderate steric hindrance, which enables high selectivity and activity for hydrosilylation. This work presents some guidelines for utilizing atomic-scale species to increase the synthesis efficiency and precision of FAMCs.

First-Principles Insights into the Selectivity of CO2 Electroreduction over Heterogeneous Single-Atom Catalysts

Heterogeneous metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have garnered considerable attention in the two-electron CO2 reduction reaction (2e-CO2RR). Interestingly, almost M-N-C SACs mainly produce CO, while Sb is one of the few SACs reported so far that can produce HCOOH. Nevertheless, the underlying factors for different selectivities on Sb-N-C SAC remain controversial, and the lack of in-depth understanding of limiting factors hampers further regulations. Here, by using constant-potential first-principles calculations, we revealed that the high HCOOH selectivity of Sb-N-C SAC is mainly attributed to their weak charge accumulation ability. Remarkably, considering the highly tunable geometric structure of M-N-C SACs, we provide that Sb-N-C SAC with the SbN3S1 center is a promising candidate for CO production. Our work provides the mechanism insight into 2e-CO2RR selectivity and further paves the way toward electrocatalyst regulation and design.

Hetero-Diatomic CoN4-NiN4 Site Pairs with Long-Range Coupling as Efficient Bifunctional Catalyst for Rechargeable Zn-Air Batteries

In this study, Co/Ni-NC catalyst with hetero-diatomic Co/Ni active sites dispersed on nitrogen-doped carbon matrix is synthesized via the controlled pyrolysis of ZIF-8 containing Co2+ and Ni2+ compounds. Experimental characterizations and theoretical calculations reveal that Co and Ni are atomically and uniformly dispersed in pairs of CoN4-NiN4 with an intersite distance ≈0.41 nm, and there is long-range d-d coupling between Co and Ni with more electron delocalization for higher bifunctional activity. Besides, the in situ grown carbon nanotubes at the edges of the catalyst particles allow high electronic conductivity for electrocatalysis process. Electrochemical evaluations demonstrate the superior ORR and OER bifunctionality of Co/Ni-NC catalyst with a narrow potential gap of only 0.691 V and long-term durability, significantly prevailing over the single-atom Co-NC and Ni-NC catalysts and the benchmark Pt/C and RuO2 catalysts. Co/Ni-NC catalyzed Zn-air batteries achieve a high specific capacity of 771 mAh g-1 and a long continuous operation period up to 340 h with a small voltage gap of ≈0.65 V, also much superior to Pt/C-RuO2.

Advances in Naked Metal Clusters for Catalysis

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

The oxygen evolution reaction on cobalt atom embedded nitrogen doped graphene electrocatalysts: a density functional theory study

The oxygen evolution reaction (OER) is essential for the development of renewable energy conversion and storage technologies. Eight N-doped graphenes containing variable numbers of embedded cobalt atoms (Coxy-NG, x = 1-4, y = 1-3, where x represents the number of embedded Co atoms and y represents different configurations) were designed and their OER electrocatalytic activities were systematically studied through density functional theory calculations. The significant roles of the number of Co atoms and their configuration in their OER performance were discussed in detail. Co31-NG occupies the peak of the activity volcano plot with a low overpotential of 0.31 V, which is smaller than Co11-NG with only one Co atom and even superior to the widely used IrO2 (0.56 V). The electronic structure and electron density analysis reveal that the outstanding electrocatalytic performance is due to the orbital hybridization between Co and N atoms and the increased positive charge on in-plane Co due to the out-of-plane Co atoms/clusters. This work clarifies the important role of transition atoms and provides excellent examples for reducing the overpotential through embedding several transition metal atoms onto single-atom electrocatalysts.

Discriminating the Active Ru Species Towards the Selective Generation of Singlet Oxygen from Peroxymonosulfate: Nanoparticles Surpass Single-Atom Catalysts

Singlet oxygen (1O2) is an exceptional reactive oxygen species in advanced oxidation processes for environmental remediation. Despite single-atom catalysts (SACs) representing the promising candidate for the selective generation of 1O2 from peroxymonosulfate (PMS), the necessity to meticulously regulate the coordination environment of metal centers poses a significant challenge in the precisely-controlled synthetic method. Another dilemma to SACs is their high surface free energy, which results in an inherent tendency for the surface migration and aggregation of metal atoms. We here for the first time reported that Ru nanoparticles (NPs) synthesized by the facile pyrolysis method behave as robust Fenton-like catalysts, outperforming Ru SACs, towards efficient activation of PMS to produce 1O2 with nearly 100 % selectivity, remarkably improving the degradation efficiency for target pollutants. Density functional theory calculations have unveiled that the boosted PMS activation can be attributed to two aspects: (i) enhanced adsorption of PMS molecules onto Ru NPs, and (ii) decreased energy barriers by offering adjacent sites for promoted dimerization of *O intermediates into adsorbed 1O2. This study deepens the current understanding of PMS chemistry, and sheds light on the design and optimization of Fenton-like catalysts.

Engineering Spin Polarization of the Surface-Adsorbed Fe Atom by Intercalating a Transition Metal Atom into the MoS2 Bilayer for Enhanced Nitrogen Reduction

The precise control of spin states in transition metal (TM)-based single-atom catalysts (SACs) is crucial for advancing the functionality of electrocatalysts, yet it presents significant scientific challenges. Using density functional theory (DFT) calculations, we propose a novel mechanism to precisely modulate the spin state of the surface-adsorbed Fe atom on the MoS2 bilayer. This is achieved by strategically intercalating a TM atom into the interlayer space of the MoS2 bilayer. Our results show that these strategically intercalated TM atoms can induce a substantial interfacial charge polarization, thereby effectively controlling the charge transfer and spin polarization on the surface Fe site. In particular, by varying the identity of the intercalated TM atoms and their vacancy filling site, a continuous modulation of the spin states of the surface Fe site from low to medium to high can be achieved, which can be accurately described using descriptors composed of readily accessible intrinsic properties of materials. Using the electrochemical dinitrogen reduction reaction (eNRR) as a prototypical reaction, we discovered a universal volcano-like relation between the tuned spin and the catalytic activity of Fe-based SACs. This finding contrasts with the linear scaling relationships commonly seen in traditional studies and offers a robust new approach to modulating the activity of SACs through interfacial engineering.

Electronegativity-dependent Pt anchoring and molecule adsorption for graphene-based supported Pt single atom

CONTEXT

To unravel the effects of the C vacancy, doping N type and number, the adsorption of HCHO and O2 was investigated on the graphene (Gr)-based supported Pt single atom by density functional theory calculations. The electronegativity of the vacancy and N-doped Gr was a crucial factor both for the anchoring for a Pt and the further adsorption of HCHO and O2 on the supported Pt. The electronegativity can be tuned by the C vacancy number (1V and 2V), the doping N type (graphitic-N, pyridinic-N and pyrrolic-N) and the doping pyridinic-N number (1N ~ 4N). The high electronegativity of the vacancy and N-doped Gr favored the anchoring for a Pt compared to the Gr, while too high electronegativity was detrimental for further adsorption of adsorbates on the supported Pt. The Bader charge analysis proved that the electronegativity followed the trend as pyrrolic-N > pyridinic-N > graphitic-N, and 4N-Gr > 2V-Gr > 3N-Gr > 2N-Gr > 1N-Gr > 1V-Gr > Gr. As a result, the pyridinic-N, the 1V-Gr, 1N-Gr and 2N-Gr with the suitable electronegativity achieved both stronger anchoring for a Pt and more favorable adsorption of HCHO and O2 on the supported Pt than the pristine Gr support.

METHODS

Periodic DFT calculation was performed using the VASP code. The PAW method and the GGA-PBE functionals were used. Part of work was also carried out by the DSPAW procedure of Device Studio.

CO2 electroreduction on single atom catalysts: the role of the DFT functional

One key process involving single atom catalysts (SACs) is the electroreduction of CO2 to fuels. The chemistry of SACs differs largely from that of extended catalytic surfaces, presenting an opportunity to improve the ability to activate very stable molecules, such as CO2. In this work, we performed a density functional theory (DFT) study of CO2 activation on a series of SACs, focusing on the role played by the adopted functional in activity predictions. The role of the exchange-correlation functional has been widely investigated in heterogenous catalysts, but it is less explored in SACs. We tested the widely used PBE and the PBE+U corrected functionals against the more robust hybrid PBE0 functional. The results show that PBE is reliable if one is interested in qualitative predictions, but it leads to some inaccuracies in other cases. A possible way to attenuate this effect is by adopting the PBE+U framework, as it gives results that are very similar to PBE0 at an acceptable computational cost. The results of this study further underline the importance of the computational framework adopted in predicting the activity of SACs. The work suggests that one needs to go beyond PBE for quantitative estimates, an important consideration when performing screening and high-throughput calculations.

Multifunctional black phosphorus pressure sensors with bending angle monitoring and direction recognition characteristics

Flexible pressure sensors, an important class of intelligent sensing devices, are widely explored in body-motion and medical health monitoring, artificial intelligence and human-machine interaction. As a unique layered nanomaterial, black phosphorus (BP) has excellent electrical, mechanical, and flexible characteristics, which make it a promising candidate for fabricating high-performance pressure sensors. Herein, hierarchically structured BP-based pressure sensors were constructed. The sensors exhibit high sensitivity, stability and a wide sensing range and respond to various human motions including finger pressure, swallowing, and wrist bending. The sensors can identify different handwriting processes with featured signals. In particular, benefiting from the unique structure of loose-dense layers, the sensors show a distinctive response to bending angles and directions, revealing a characteristic of direction recognition. This feature facilitates the sensors to monitor human motions. The sensors have been successfully powered by a home-made Cu2ZnSn(S,Se)4 thin-film solar cell, which demonstrates the sustainability, flexibility and low power consumption of integrated devices. This work offers a strategy to construct hierarchically structured pressure/strain sensors with direction recognition and provides further insights into manufacturing portable sensing devices for realistic and innovative applications.

A review on photocatalytic attribution and process of pyrolytic biochar in environment

Biochar has attracted significant attention due to its excellent environmental benefits and extensive applications. Recently, a consensus has been accepted that biochar can act as a photocatalyst and trigger effective photocatalytic reactions in the environment, which is important to energy conversion and the cycle of elements. However, its photocatalytic processes and the corresponding environmental impacts need to receive more and due attention. In this review, we provide a comprehensive summary of the underlying correlations among the pyrolytic evolution of biomass, the structure characteristic of biochar, and the resultant photocatalytic performance. Moreover, the photocatalytic processes and the influence of environmental factors were elaborately investigated on biochar. Finally, future tendencies and challenges in the photocatalysis of biochar have been prospected in the environmental field. This review has offered innovative insights into the photocatalytic essential of biochar and highly enhanced the understanding of its environmental impact.

Benchmarking a Molecular Flake Model on the Road to Programmable Graphene-Based Single-Atom Catalysts

Single-atom catalysts (SACs) of embedding an active metal in nitrogen-doped graphene are emergent catalytic materials in various applications. The rational design of efficient SACs necessitates an electronic and mechanistic understanding of those materials with reliable quantum mechanical simulations. Conventional computational methods of modeling SACs involve using an infinite slab model with periodic boundary condition, limiting to the selection of generalized gradient approximations as the exchange correlation (XC) functional within density functional theory (DFT). However, these DFT approximations suffer from electron self-interaction error and delocalization error, leading to errors in predicted charge-transfer energetics. An alternative strategy is using a molecular flake model, which carved out the important catalytic center by cleaving C-C bonds and employing a hydrogen capping scheme to saturate the innocent dangling bonds at the molecular boundary. By doing so, we can afford more accurate hybrid XC functionals, or even high-level correlated wavefunction theory, to study those materials. In this work, we compared the structural, electronic, and catalytic properties of SACs simulated using molecular flake models and periodic slab models with first-row transition metals as the active sites. Molecular flake models successfully reproduced structural properties, including both global distortion and local metal-coordination environment, as well as electronic properties, including spin magnetic moments and metal partial charges, for all transition metals studied. In addition, we calculated CO binding strength as a descriptor for electrochemical CO2 reduction reactivity and noted qualitatively similar trends between two models. Using the computationally efficient molecular flake models, we investigated the effect of tuning Hartree-Fock exchange in a global hybrid functional on the CO binding strength and observed system-dependent sensitivities. Overall, our calculations provide valuable insights into the development of accurate and efficient computational tools to simulate SACs.

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.

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.

Single-Site Ni-Grafted TiO2 with Diverse Coordination Environments for Visible-Light Hydrogen Production

Solar hydrogen production at a high efficiency holds the significant importance in the age of energy crisis, while the micro-environment manipulation of active sites on photocatalysts plays a profound role in enhancing the catalytic performance. In this work, a series of well-defined single-site Ni-grafted TiO2 photocatalysts with unique and specific coordination environments, 2,2'-bipyridine-Ni-O-TiO2 (T-Ni Bpy) and 2-Phenylpyridine-Ni-O-TiO2 (T-Ni Phpy), were constructed with the methods of surface organometallic chemistry combined with surface ligand exchange for visible-light-induced photocatalytic hydrogen evolution reaction (HER). A prominent rate of 33.82 μmol ⋅ g-1  ⋅ h-1 and a turnover frequency of 0.451 h-1 for Ni are achieved over the optimal catalyst T-Ni Bpy for HER, 260-fold higher than those of Ni-O-TiO2 . Fewer electrons trapped oxygen vacancies and a larger portion of long-lived photogenerated electrons (>3 ns, ~52.9 %), which were demonstrated by the electron paramagnetic resonance and femtosecond transient IR absorption, correspond to the photocatalytic HER activity over the T-Ni Bpy. The number of long-lived free electrons injected from the Ni photoabsorber to the conduction band of TiO2 is one of the determining factors for achieving the excellent HER activity.

Transition metals anchored on nitrogen-doped graphdiyne for an efficient oxygen reduction reaction: a DFT study

The search for highly active and low-cost single-atom catalysts for the oxygen reduction reaction (ORR) is essential for the widespread application of proton exchange membrane fuel cells. Transition metals anchored on nitrogen-doped graphdiyne (GDY) have attracted considerable interest as potentially excellent catalysts for the ORR. However, the relationship between the active site and nitrogen-doped GDY remains unclear. In this work, we conducted a systematic investigation of sp-hybridized N atoms anchoring single transition metal atoms of 3d and 4d on GDY (TMC2N2) as electrocatalysts for the ORR. Firstly, 18 kinds of TMC2N2 were determined to have good thermodynamic stability. Due to the extremely strong adsorption of *OH, TMC2N2 exhibits inferior ORR performance compared to traditional Pt(111). Considering that *OH adsorption hinders the catalytic activity of TMC2N2, we modified the OH ligand of TMC2N2 to develop the high-valent metal complex (TMC2N2-OH) aiming to enhance the electrocatalytic activity. The adsorption of intermediates on most TMC2N2-OH is weakened after the modification of the OH ligand, especially for the adsorption of *OH. Thus, by comparing the ORR overpotential of catalysts before and after ligand modification, we find that the catalytic activity of different TMC2N2-OHs improves to various degrees. MnC2N2-OH, TMC2N2-OH, and TcC2N2-OH exhibit relatively high ORR catalytic activity, with overpotentials of 0.93 V, 1.19 V, and 0.92 V, respectively. Furthermore, we investigated the cause of improved catalytic activity of TMC2N2-OH and found that the modified coordination environment of the catalyst led to adjusted adsorption of ORR intermediates. In summary, our work sheds light on the relationship between nitrogen-doped GDY and transition metal sites, thus contributing to the development of more efficient catalysts.

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.

Boosting the oxygen reduction reaction activity of dual-atom catalysts on N-doped graphene by regulating the N coordination environment

Development of low-cost and high-efficiency oxygen reduction reaction (ORR) catalysts is of significance for fuel cells and metal-air batteries. Here, by regulating the N environment, a series of dual-atom embedded N5-coordinated graphene catalysts, namely M1M2N5 (M1, M2 = Fe, Co, and Ni), were constructed and systematically investigated by DFT calculations. The results reveal that all M1M2N5 configurations are structurally and thermodynamically stable. The strong adsorption of *OH hinders the proceeding of ORR on the surface of M1M2N5, but M1M2N5(OH2) complexes are formed to improve their catalytic activity. In particular, FeNiN5(OH2) and CoNiN5(OH2) with the overpotentials of 0.33 and 0.41 V, respectively, possess superior ORR catalytic activity. This superiority should be attributed to the reduced occupation of d-orbitals of Fe and Co atoms in the Fermi level and the apparent shift of dyz and dz2 orbitals of Ni atoms towards the Fermi level after adsorbing *OH, thus regulating the active sites and exhibiting appropriate adsorption strength for reaction intermediates. This work provides significant insight into the ORR mechanism and theoretical guidance for the discovery and design of low-cost and high-efficiency graphene-based dual-atom ORR catalysts.

Structural Self-Regulation-Promoted NO Electroreduction on Single Atoms

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

Stability Issues in Electrochemical CO2 Reduction: Recent Advances in Fundamental Understanding and Design Strategies

Electrochemical CO2 reduction reaction (CO2 RR) offers a promising approach to close the anthropogenic carbon cycle and store intermittent renewable energy in fuels or chemicals. On the path to commercializing this technology, achieving the long-term operation stability is a central requirement but still confronts challenges. This motivates to organize the present review to systematically discuss the stability issue of CO2 RR. This review starts from the fundamental understanding on the destabilization mechanisms of CO2 RR, with focus on the degradation of electrocatalyst and change of reaction microenvironment during continuous electrolysis. Subsequently, recent efforts on catalyst design to stabilize the active sites are summarized, where increasing atomic binding strength to resist surface reconstruction is highlighted. Next, the optimization of electrolysis system to enhance the operation stability by maintaining reaction microenvironment especially mitigating flooding and carbonate problems is demonstrated. The manipulation on operation conditions also enables to prolong CO2 RR lifespan through recovering catalytically active sites and mass transport process. This review finally ends up by indicating the challenges and future opportunities.