Single Metal Atoms Research Articles

Non-noble metal single-atoms for oxygen electrocatalysis in rechargeable zinc-air batteries: recent developments and future perspectives.

Ever-growing demands for zinc-air batteries (ZABs) call for the development of advanced electrocatalysts. Single-atom catalysts (SACs), particularly those for isolating non-noble metals (NBMs), are attracting great interest due to their merits of low cost, high atom utilization efficiency, structural tunability, and extraordinary activity. Rational design of advanced NBM SACs relies heavily on an in-depth understanding of reaction mechanisms. To gain a better understanding of the reaction mechanisms of oxygen electrocatalysis in ZABs and guide the design and optimization of more efficient NBM SACs, we herein organize a comprehensive review by summarizing the fundamental concepts in the field of ZABs and the recent advances in the reported NBM SACs. Moreover, the selection of NBM elements and supports of SACs and some effective strategies for enhancing the electrochemical performance of ZABs are illustrated in detail. Finally, the challenges and future direction in this field of ZABs are also discussed.

Quantum chemical studies of transition metal single-atom catalysts: exploration of catalytic descriptors

This work provides important insight into the structure–activity relationships of transition metal single-atom catalysts. Various traditional, spectral and electronic descriptors are suggested.

The recent progress of cathode materials for heterogeneous electro-Fenton reactions

As one of environmentally friendly and efficient oxidation techniques, the electro-Fenton (EF) processes allow to remove and mineralize organic pollutants. However, the transitional EF processes require an acidic environment, adding ferrous ions, treatment of iron sludge, and costly solution pH adjustment. Heterogeneous EF (hetero-EF) processes enable EF to function over a broad working pH region and recycle across multiple reaction rounds which reduces overall cost and displays great potential for practical engineering applications. Here, this work reviewed the reported cathode materials for the hetero-EF processes. We firstly described the mechanisms of homogeneous and heterogeneous EF reactions. Then, we summarized different types of cathode materials for hetero-EF reactions, including metal-free carbon materials, transition metal single-atom catalysts (SACs)/carbon materials, alloys catalysts/carbon materials, metal compounds/carbon materials and other materials. Finally, we overviewed the challenges and proposal strategies of hetero-EF technique in order to expand its wide-scale applicability and practical significance for advanced wastewater treatment.

Ruthenium single-atom doping-driven modulation of Co3O4 spinel tetrahedral site 3d-orbital occupancy in lithium-oxygen batteries.

Metal single-atom catalysts have attracted widespread attention in the field of lithium-oxygen batteries due to their unique active sites, high catalytic selectivity, and near total atomic utilization efficiency. Isolated metal atoms not only serve as the active sites themselves, but also function as modulators, reversely regulating the surface electronic structure of the support to enhance its inherent electrocatalytic activities. Despite the potential of isolated metal atom-driven active sites, understanding the structure-activity relationship remains a challenge. In this study, we present a ruthenium single-atom doping-driven cost-effective and durable tricobalt tetroxide electrocatalyst with excellent oxygen electrode electrocatalytic activity. The lithium-oxygen battery with this catalyst as the oxygen electrode demonstrates high performance, achieving a capacity of up to 25 000 mA h g-1 and maintaining good stability over 400 cycles at a current density of 100 mA g-1. This improvement is attributed to the exquisite control of the morphology and structure of the discharge product, lithium peroxide. The aresults of physical characterization and theoretical calculations reveal that isolated ruthenium atoms bond with the tetrahedral cobalt site, resulting in spin polarization enhancement and rearrangement of d orbital energy levels in cobalt. This rearrangement reduces the dz2 orbital occupancy and promotes their transfer to the octahedral cobalt site, thereby enhancing its adsorption capacity for the oxygen-containing intermediates, and ultimately increasing the electrocatalytic activity of the oxygen evolution reaction. This work presents an innovative strategy to regulate the catalytic activity of metal oxides by introducing another metal single atom.

Computational screening of defective BC3-supported single-atom catalysts for electrochemical CO2 reduction.

The electrochemical CO2 reduction reaction (eCO2RR) driven by renewable electricity offers a green and sustainable technology for synthesizing chemicals and managing global carbon balance. However, developing electrocatalysts with high activity and selectivity for producing C1 products (CO, HCOOH, CH3OH, and CH4) remains a daunting task. In this study, we conducted comprehensive first-principles calculations to investigate the eCO2RR mechanism using B-defective BC3-supported transition metal single-atom catalysts (TM@BC3 SACs). Initially, we evaluated the thermodynamic and electrochemical stability of the designed 26 TM@BC3 SACs by calculating the binding energy and dissolution potential of the anchored TM atoms. Subsequently, the selectivity of the eCO2RR and hydrogen evolution reaction (HER) on stable SACs was determined by comparing the free energy change (ΔG) for the first protonation of CO2 with the ΔG of *H formation. The stability and selectivity screening processes enabled us to narrow down the pool of SACs to the 14 promising ones. Finally, volcano plots for the eCO2RR towards different C1 products were established by using the adsorption energy descriptors of key intermediates, and three SACs were predicted to exhibit high activity and selectivity. The limiting potentials (UL) for HCOOH production on Pd@BC3 and Ag@BC3 are -0.11 V and -0.14 V. CH4 is a preferred product on Re@BC3 with UL of -0.22 V. Elaborate electronic structure calculations elucidate that the activity and selectivity originate from the sufficient activation of the C-O bond and the strong orbital hybridization between crucial intermediates and metal atoms. The proposed catalyst screening criteria, constructed volcano plots and predicted SACs may provide a theoretical foundation for the development of computationally guided catalyst designs for electrochemical CO2 conversion to C1 products.

Mechanistic insights into high-throughput screening of tandem catalysts for CO2 reduction to multi-carbon products.

In carbon dioxide electrochemical reduction (CO2ER), since isolated catalysts encounter challenges in meeting the demands of intricate processes for producing multi-carbon (C2+) products, tandem catalysis is emerging as a promising approach. Nevertheless, there remains an insufficient theoretical understanding of designing tandem catalysts. Herein, we utilized density functional theory (DFT) to screen 80 tandem catalysts for efficient CO2ER to C2 products systematically, which combines the advantages of nitrogen-doped carbon-supported transition metal single-atom catalysts (M-N-C) and copper clusters. Three crucial criteria were designed to select structures for generation and transfer of *CO and facilitate C-C coupling. The optimal Cu/RuN4-pl catalyst exhibited an excellent ethanol production capacity. Additionally, the relationship between CO adsorption strength and transfer energy barrier was established, and the influence of the electronic structure on its adsorption strength was studied. This provided a novel and well-considered solution and theoretical guidance for the design of rational composition and structurally superior tandem catalysts.

Emerging electrospinning platform toward nanoparticle to single atom transformation for steering selectivity in ammonia synthesis.

The rising top-down synthetic methodologies for transition metal single-atom catalysts (SACs) require controlled movement of metal atoms through the substrates; however, their direct transportation towards the ideal carrier remains a huge challenge. Herein, we showed a "top down" strategy for Co nanoparticles (NPs) to Co SA transformation by employing electrospun carbon nanofibers (CNFs) as atom carriers. Under high-temperature conditions, the Co atoms migrate from the surfaces of Co NPs and are then anchored by the surrounding carbon to form a Co-C3O1 coordination structure. The synthesized Co SAs/CNF electrocatalyst exhibits excellent electrocatalytic nitrate reduction reaction (NO3RR) activity with an NH3 yield of 0.79 mmol h-1 cm-2 and Faraday efficiency (FE) of 91.3% at -0.7 V vs. RHE in 0.1 M KNO3 and 0.1 M K2SO4 electrolytes. The in situ electrochemical characterization suggests that the NOH pathway is preferred by Co SAs/CNFs, and *NO hydrogenation and deoxygenation easily occur on Co SAs due to the small adsorption energy between Co SAs and *NO, as calculated by theoretical calculations. It is revealed that a small energy barrier (0.45 eV) for the rate determining step (RDS) ranges from *NO to *NOH and a strong capability for inhibiting hydrogen evolution (HER) significantly promotes the NH3 selectivity and activity of Co SAs/CNFs.

First-principles study of an efficient non-noble metal single-atom catalyst Fe1/TiVCO2 for CO oxidation

MXenes, a class of two-dimensional layered materials of transition metal carbides, nitrides, and carbonitrides, have piqued the interest of researchers due to their unique physical and chemical properties.

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.

Electronic configuration regulation of single-atomic Mn sites mediated by Mo/Mn clusters for an efficient hydrogen evolution reaction.

Tuning the electron distribution of metal single-atom active sites via bimetallic clusters is an effective way to enhance their hydrogen evolution reaction (HER) activity, but remains a great challenge. A biochar-based electrocatalyst (BCMoMn800-2) with both MnN4 active sites and Mo2C/Mn7C3 clusters was synthesized using in situ enriched Mo/Mn biomass as a precursor to trigger the HER. Various characterization and density functional theory (DFT) calculation results indicated that the presence of Mo2C/Mn7C3 clusters in BCMoMn800-2 effectively induced the redistribution of charges at MnN4 sites, reducing the energy of H* activation during the HER. In 0.5 M H2SO4, the overpotential was 27.4 mV at a current density of 10 mA cm-2 and the Tafel slope was 31 mV dec-1, and its electrocatalytic performance was close to that of Pt/C. The electrocatalyst also exhibited excellent electrocatalytic stability and durability. This work might provide a new strategy for solid waste recycling and constructing efficient HER electrocatalysts.

Synergism between metal single-atom sites and S-vacant two-dimensional nanosheets for efficient hydrogen evolution uncovered by density functional theory and machine learning

H* is generated on Pd single atoms and then migrates to neighboring S sites on which H2 is produced, representing catalytic synergism via hydrogen spillover, which enables Pd1/Sv-MnPS3 to be active and durable for the HER.

Screening of red phosphorus supported transition metal single-atom catalysts for efficient photocatalytic water splitting H2 generation

Comprehensive theoretical calculations are performed to screen red phosphorus-supported transition metal single-atom catalysts to realize efficient photocatalytic H2 generation, offering important references for future design of photocatalysts.

Catalytic kinetic growth of a half-metallic hexagonal boron nitride-graphene lateral heterostructure using transition metal single-atom catalysts on Rh(111)

Transition metal single-atom catalysts (TM = Mn, Zr, Nb, Mo, Hf, Ta, and W) promote growth of a h-BN-G heterostructure on C-dimer-unpreferred Rh(111).

Insights into the electrochemical catalytic mechanism of atomically dispersed metal on h-BCN monolayer.

Exploiting efficient and inexpensive electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great significance for the rapid development of renewable energy technologies. The embedding of single-atom metals into two-dimensional (2D) carbon-based materials as electrocatalysts for fuel cells and metal-air batteries have become a major research focus. Herein, the catalytic properties of the ORR and OER of partial metal atoms embedded in a two-dimensional h-BCN monolayer were systematically investigated on thermodynamic and kinetic scales using density functional theory calculations. Electronic structure studies confirm that the d orbitals of metal atoms hybridize with the 2p orbitals of the O atom, modulating the adsorption free energies of oxygen-containing intermediates (OOH, O and OH). Comparison and analysis of catalytic activity results reveal that Co single-atom catalysts (Co-SACs@h-BCN) possess the lowest ORR and OER overpotentials and therefore are expected to be a kind of excellent bifunctional electrocatalyst. This study not only demonstrates that boron- and nitrogen-doped carbon-based materials embedded with partial metal atoms are superior 2D electrocatalysts, but also provides guidance for the investigation of efficient non-precious metal-based bifunctional electrocatalysts.

Mechanistic study of the competition between carbon dioxide reduction and hydrogen evolution reaction and selectivity tuning via loading single-atom catalysts on graphitic carbon nitride.

In the context of catalytic CO2 reduction (CO2RR), the interference of the inherent hydrogen evolution reaction (HER) and the possible selectivity towards CO have posed a significant challenge to the generation of formic acid. To address this hurdle, in this work, we have investigated the impact of different single-atom metal catalysts on tuning selectivity by employing density functional theory (DFT) calculations to scrutinize the reaction pathways. Single-atom catalysts supported on carbon-based systems have proven to be pivotal in altering both the activity and selectivity of the CO2RR. In this study, a series of single-atom-metal-loaded g-C3N4 monolayers (MCN, M = Ni, Cu, Zn, Ga, Cd, In, Sn, Pb, Ag, Au, Bi, Pd and Pt) were systematically examined. Through detailed DFT calculations, we explored their influence on reaction selectivity between the *COOH and *OCHO intermediates. Notably, NiCN favors the reaction via the *OCHO route, with a significantly lower rate-determining potential of 0.36 eV, which is approximately 73.5% lower than that of the CN system (1.36 eV). Most importantly, the Ni single-atom catalyst with lower coordination significantly enhances CO2 adsorption, promoting CO2RR over HER. Overall, this study, guided by DFT calculations, provides a theoretical prediction of how the selection of single-atom metal catalysts can effectively modulate the reaction pathway, thereby offering a potential solution for achieving high product selectivity in CO2RR.

Atomically dispersed silver atoms incorporated in spinel cobalt oxide (Co3O4) for boosting oxygen evolution reaction

Incorporating noble metal single atoms into lattice of spinel cobalt oxide (Co3O4) is an attractive way to fabricate oxygen evolution reaction (OER) electrocatalysts because of the high activity and economic benefit. The commonly used high valence noble metal dopants such as ruthenium, iridium and rhodium tend to supersede Co3+ at octahedral site of Co3O4 and result in great activity, the origins of admirable activity were also wildly investigated. However, bare explorations on doping noble metal single atom into tetrahedral site of Co3O4 to construct OER catalyst have been reported, corresponding catalytic activity and mechanism remain mystery. Here, a promising structure that tetrahedrally substituent Ag single atom embedded in Co3O4 nanoparticles on the surface of carbon nanotube (Ag-Co3O4/CNT) was presented, and its performance in OER was probed. The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) demonstrate the successful embeddedness of atomical Ag atom in Co3O4 lattice, the resultant electronic interaction is conducive to promote charge transfer for OER. Theoretical calculations further disclose that atomical Ag dopant prefers to replace tetrahedral Co2+ rather than octahedral Co3+. The substitution Ag acts as the active site through Ag-Co bridge and facilitates the desorption process, which improves the turnover frequency (TOF) and boosts the intrinsic activity of Ag-Co3O4/CNT. Benefiting from the essentials above, Ag-Co3O4/CNT displays remarkable activity (236 mV@10 mA cm−2) and robust stability for alkaline OER. This finding offers a potential direction for the design of noble metal single atom involved Co3O4 based OER electrocatalysts.

Single-atom Pd anchored in the porphyrin-center of ultrathin 2D-MOFs as the active center to enhance photocatalytic hydrogen-evolution and NO-removal

Single-atom catalysts were widely used to treat atmospheric pollution and alleviate energy crises through photocatalysis. However, how to prevent the aggregation of single atoms during the preparation and catalytic processes remained a great challenge. Herein, a novel ultrathin two-dimensional porphyrin-based single-atom photocatalyst Ti-MOF (abbreviated as TMPd) obtained through a simple hydrothermal synthesis strategy was used for photocatalytic hydrogen evolution and NO removal, in which the single-atom Pd tightly anchored in the center of porphyrin to ensure single-atom Pd stable existence. Compared with most reported MOFs-based photocatalysts, the TMPd showed an excellent hydrogen evolution rate (1.32 mmol g−1 h−1) and the NO removal efficiency (62%) under visible light irradiation. Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and synchrotron-radiation-based X-ray absorption fine-structure spectroscopy (XAFS) proved that pd in TMPd existed in an isolated state, and the atomic force microscope (AFM) proved the ultrathin morphology of TMPd. DFT calculations had demonstrated that single-atom Pd could serve as the active center and more effectively achieve electron transfer, indicating that single-atom Pd played a vital role in photocatalytic hydrogen evolution. In addition, a possible photocatalytic pathway of NO removal was proposed based on ESR and in-situ infrared spectra, in which the catalysts anchored with single-atom Pd could produce more active substances and more effectively oxidize NO to NO2− or NO3−. The results suggested that coordinating single-atom metal species as the active site in the center of porphyrin could be a feasible strategy to obtain various ultrathin porphyrin-based single-atom photocatalysts to acquire excellent photocatalytic performance further.

Transition Metal Functionalized C30N12S6 as High-Performance Trifunctional Catalysts with Integrated Descriptors toward Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions: A Case of High-Throughput First-Principles Screening within the Framework of TM-N2@C30N10S6.

In energy conversion and storage technologies, the design of highly efficient trifunctional electrocatalysts integrating with the high hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) activities is highly desirable. Herein, utilizing first-principles computations, a novel periodically ordered macropore C30N12S6 monolayer was proposed, and the stability analysis attests to its good stability. Single transition metal (TM) atom anchored onto this newly proposed C30N12S6 monolayer to form single-atom catalysts, as achieved by TM-N2@C30N10S6, among which the Co-N2@C30N10S6 is the most promising multifunctional catalyst toward HER/OER/ORR with low overpotential of 0.01/0.59/0.3 V; meanwhile, the Rh-N2@C30N10S6 can be used as a bifunctional OER/ORR catalyst with low overpotential of 0.37/0.44 V, overmatching the landmark Pt (111) and IrO2/RuO2 catalysts. Particularly, the TM-d orbital in TM@CNS is remarkably hybridized with the O-p orbital of oxygenated intermediates, so that the lone electrons initially located at the antibonding orbital pair up and fill the downward bonding orbital, allowing OH* to be suitably adsorbed on TM@CNS, enhancing the catalytic performance. The relevant attributes, such as good stabilities and metallic features, ensured their applications in ambient conditions. Moreover, multilevel descriptors were constructed to clarify the origin of activity on TM@CNS, such as ΔGOH* (Gibbs free energy of OH*), εd (d-band center), COHP (crystal orbital Hamilton population), Nd/Nd+s (number of d/d + s electrons) and φ (descriptor), among which the filling of outer d-electrons of TM atom significantly affects the value of ΔGOH* that can determine the overpotential and, thus, become a key descriptor.

From Porphyrin-Like Rings to High-Density Single-Atom Catalytic Sites: Unveiling the Superiority of p-C2N for Bifunctional Oxygen Electrocatalysis.

With effective utilization of the catalytic site, single-atom catalysts (SACs) supported by nitrogen atoms surrounding built-in pores of two-dimensional (2D) materials, such as porphyrin/phthalocyanine-based covalent organic frameworks, have been highly promising electrocatalysts in the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) processes for the air electrode of the metal-air battery. However, the number of stable single-atom anchoring sites, i.e., accessible single-atom metal sites, has been concerning as a result of the appearance of heterogeneous or large and even supersized pores in substrate materials. 2D porous graphitic carbon nitride (PGCN) with a stronger stability and smaller component is regarded as a more potential alternative owing to similar controllability and designability. In this work, inspired by the robust coordinated TM-N4 environment of porphyrin/phthalocyanine molecules, novel p-C2N with a high density of porphyrin-like organic units is rationally designed. In well-designed p-C2N, a higher homogeneity and uniformity of coordination sites can enhance the electrocatalytic activity in the whole catalytic material and better prevent SACs from sintering and agglomerating into thermodynamically stable nanoclusters. Utilizing density functional theory (DFT), the stability of the p-C2N monolayer, TM@p-C2N, and OER/ORR catalytic activities of TM@p-C2N (TM including Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) are systematically evaluated. Among them, Ir@p-C2N (0.31 V of the OER and 0.36 V of the ORR), Co@p-C2N (0.47 and 0.22 V), and Rh@p-C2N (0.55 and 0.27 V) are screened as promising SACs for the bifunctional ORR and OER. The proposal of p-C2N guides a new direction for the development of TM-N-C-based SAC bifunctional electrocatalysts.

Engineering Atomically Dispersed Cu-N1S2 Sites via Chemical Vapor Deposition to Boost Enzyme-Like Activity for Efficient Tumor Therapy.

Single-atom nanozymes (SAzymes), with well-defined and uniform atomic structures, are an emerging type of natural enzyme mimics. Currently, it is important but challenging to rationally design high-performance SAzymes and deeply reveal the interaction mechanism between SAzymes and substrate molecules. Herein, this work reports the controllable fabrication of a unique Cu-N1S2-centred SAzyme (Cu-N/S-C) via a chemical vapor deposition-based sulfur-engineering strategy. Benefiting from the optimized geometric and electronic structures of single-atom sites, Cu-N/S-C SAzyme shows boosted enzyme-like activity, especially in catalase-like activity, with a 13.8-fold increase in the affinity to hydrogen peroxide (H2O2) substrate and a 65.2-fold increase in the catalytic efficiency when compared to Cu-N-C SAzyme with Cu-N3 sites. Further theoretical studies reveal that the increased electron density around single-atom Cu is achieved through electron redistribution, and the efficient charge transfer between Cu-N/S-C and H2O2 is demonstrated to be more beneficial for the adsorption and activation of H2O2. The as-designed Cu-N/S-C SAzyme possesses an excellent antitumor effect through the synergy of catalytic therapy and oxygen-dependent phototherapy. This study provides a strategy for the rational design of SAzymes, and the proposed electron redistribution and charge transfer mechanism will help to understand the coordination environment effect of single-atom metal sites on H2O2-mediated enzyme-like catalytic processes.