Activity Of Single-atom Catalysts Research Articles

Semimetallic state transition in Two-Dimensional carbon nitride covalent networks and enhanced electrocatalytic activity for nitrate to ammonia conversion

Single-atom catalysts (SACs), due to their maximum atomic utilization rate, show tremendous potential for application in the electrocatalytic synthesis of ammonia from nitrate. Yet, the development of superior supports that preserve the high selectivity, activity, and stability of SACs remains an imperative challenge. In this work, based on first-principles calculations and tight-binding (TB) model analysis, a new two-dimensional (2D) carbon nitride monolayer, C7N6, is proposed. The C7N6 structure exhibits a strong covalent network, with dynamical, thermal, and mechanical stability. Surprisingly, the structural transition from C9N4 to C7N6 corresponds to a semimetallic state transition. Further symmetry analysis unveils that the Dirac states in C7N6 are protected by space–time inversion symmetry, and the physical origin of the Dirac cone was confirmed using the TB model. Additionally, a non-zero Z2 invariant and significant topological edge states demonstrate its topologically nontrivial nature. Considering the excellent structural and topological properties of C7N6, a three-step screening strategy is designed to identify eligible SACs for electrochemical nitrate reduction reaction (NO3RR), and Ti@C7N6 is identified as possessing the best activity, with the last proton-electron coupling step *NH2→*NH3 being the potential-determining step (PDS), for which the limiting potential is 0.48 V. Moreover, a free energy diagram shows that the *NOH reaction pathway is energetically preferred on Ti@C7N6, and ab initio molecular dynamics (AIMD) calculations at 500 K confirm its good thermal stability. Our study not only provides excellent CN-based support material but also offers theoretical guidance for constructing highly active and selective SACs for nitrate reduction.

Comprehensive review on single-atom catalysts in electrochemical hydrogen-evolution reaction: computational modelling and experimental investigation

ABSTRACT The growing demand for renewable energy sources has driven research efforts towards highly efficient electrochemical processes, especially the Hydrogen Evolution Reaction (HER) which is an essential component in renewable hydrogen production. Single Atom Catalysts (SACs) have been recognised as promising catalysts for exceptional HER activities owing to their unique electronic and geometrical features. This comprehensive review highlights the effective correlation between computational modelling and experimental research in fabricating new SACs and better understanding their catalytic mechanisms in the HER. Furthermore, the review briefly describes the choice of metals and tailoring the electronic structure, as well as exploring the influence of the reaction parameters on the catalytic activity of SACs for HER. Concluding remarks alongside future perspectives on the improvement of SACs for HER are emphasised. It is hoped that this review will pave the way for rational design of highly effective, low-cost and stable SACs for sustainable hydrogen production.

Insight into the d–p Orbital Hybridization of Ru and B Dispersed g‐C3N4 Catalysts for Nitrogen Reduction Reaction

AbstractElectrochemical nitrogen reduction reaction (NRR) is considered as an important strategy for ammonia using green energy. Single atom catalysts (SACs) with a metal atom as active site have been shown its advantages to high ammonia yield and low energy consumption. In this work, we proposed a new concept of SACs with the capacity of d‐p orbital hybridization. These computationally designed SACs contained a metal and non‐metal pair anchored on g‐C3N4 slab. Among these SACs studied in the present work, RuB@g‐C3N4 catalyst performed the best activity for NRR with low onset potential of −0.33 V by a two‐step scanning computations using density functional theory (DFT). Moreover, suppressing effectively to the competitive hydrogen evolution reaction (HER) also make it more selective. It was further revealed that the hybridization of d‐p orbitals between metal and nonmetal could regulate the electronics of active sites and facilitate the adsorption and charge transfer of adsorbed N2 molecules resulting in enhanced catalytic performance. This work demonstrated an alternative way to further enhance the catalytic activity of SACs by introducing a non‐metal atom.

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.

Predicting the activity of single atom catalysts

Predicting the activity of single atom catalysts

Dipole Effect on Oxygen Evolution Reaction of 2D Janus Single-Atom Catalysts: A Case of Rh Anchored on the P6m2-NP Configurations.

Catalytic performance of single-atom catalysts (SACs) relies fundamentally on the electronic nature and local coordination environment of the active site. Here, based on a machine-learning (ML)-aided density functional theory (DFT) method, we reveal that the intrinsic dipole in Janus materials has a significant impact on the catalytic activity of SACs, using 2D γ-phosphorus carbide (γ-PC) as a model system. Specifically, a local dipole around the active site is a key degree to tune the catalytic activity and can be used as an important descriptor with a high feature importance of 17.1% in predicting the difference of adsorption free energy (ΔGO* - ΔGOH*) to assess the activity of the oxygen evolution reaction. As a result, the catalytic performance of SACs can be tuned by an intrinsic dipole, in stark contrast to those external stimuli strategies previously used. These results suggest that dipole engineering and the revolutionary DFT-ML hybrid scheme are novel approaches for designing high-performance catalysts.

Electrochemical Probing the Site Reactivity in Iron Single-Atom Catalysts for Electrocatalytic Nitrate Reduction to Ammonia.

Single-atom catalysts (SACs), specifically iron single atoms dispersed on nitrogen-doped carbon (Fe-NC), have shown promising potential in the electrocatalytic reduction of nitrate to ammonia (NitRR), but there is a lack of understanding of their intrinsic activity. The conventional measurements often overlook the intrinsic performance of SACs, leading to significant underestimation. This study presents an in situ electrochemical probing protocol, using two poisoning molecules (SCN- and NO2-), to characterize the reactivity of Fe sites in Fe-NC SACs for NitRR. The technique aids in quantifying the yield rate of ammonia on Fe sites and the active site number. The findings reveal the intrinsic turnover frequency (TOF) based on the number and ammonia yield rate of Fe sites, challenging the current understanding of SACs' inherent performances. This unique approach holds considerable potential for determining the intrinsic activity of other SACs in complex reactions, opening new avenues for the exploration of electrocatalytic processes.

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.

Regulating electronic structure of Fe single-atom site by S/N dual-coordination for efficient Fenton-like catalysis

The activity of single-atom catalysts in peroxymonosulfate activation process is bound up with the local electronic state of metal center. However, the large electronegativity of N atoms in Metal-N4 restricts the electron transfer between center metal atom and peroxymonosulfate. Herein, we constructed Fe-SN-C catalyst by incorporating S atom in the first coordination sphere of Fe single-atom site (Fe-S1N3) for Fenton-like catalysis. The Fe-SN-C with a low valent Fe is found to exhibit excellent catalytic activity for bisphenol A degradation, and the corresponding rate constant reaches 0.405 min−1, 11.9-fold higher than the original Fe-N-C. Besides, the Fe-SN-C/PMS system exhibits ideal catalytic stability under the effect of wide pH range and background substrates by the fast generation of high-valent Fe species. Experimental results and theoretical calculations reveal that the dual coordination of S and N atoms notably increases the local electron density of Fe atoms and electron filling in eg orbital, causing a d band center shifting close to the fermi level and thereby optimizes the activation energy for peroxymonosulfate decomposition via Fe 3d-O 2p orbital interaction. This work provides further development of promising SACs for the efficient activation of peroxymonosulfate based on direct regulation of the coordination environment of active center metal atoms.

Fe-Nx sites coupled with core-shell FeS@C nanoparticles to boost the oxygen catalysis for rechargeable Zn-air batteries

The development of efficient single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) remains a formidable challenge, primarily due to the symmetric charge distribution of metal single-atom sites (M-N4). To address such issue, herein, Fe-Nx sites coupled synergistic catalysts fabrication strategy is presented to break the uniform electronic distribution, thus enhancing the intrinsic catalytic activity. Precisely, atomically dispersed Fe-Nx sites supported on N/S-doped mesoporous carbon (NSC) coupled with FeS@C core-shell nanoparticles (FAS-NSC@950) is synthesized by a facile hydrothermal reaction and subsequent pyrolysis. Due to the presence of an in situ-grown conductive graphitic layer (shell), the FeS nanoparticles (core) effectively adjust the electronic structure of single-atom Fe sites and facilitate the ORR kinetics via short/long-range coupling interactions. Consequently, FAS-NSC@950 displays a more positive half-wave potential (E1/2) of 0.871 V with a significantly boosted ORR kinetics (Tafel slope = 52.2 mV dec−1), outpacing the commercial Pt/C (E1/2 = 0.84 V and Tafel slope = 54.6 mV dec−1). As a bifunctional electrocatalyst, it displays a smaller bifunctional activity parameter (ΔE) of 0.673 V, surpassing the Pt/C-RuO2 combination (ΔE = 0.724 V). Besides, the FAS-NSC@950-based zinc-air battery (ZAB) displays superior power density, specific capacity, and long-term cycling performance to the Pt/C-Ir/C-based ZAB. This work significantly contributes to the field by offering a promising strategy to enhance the catalytic activity of SACs for ORR, with potential implications for energy conversion and storage technologies.

Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation.

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

Rational Design of HighPerformance M-N-C Single Atom Catalysts

High-performance electrocatalysts are required because the sluggish kinetics of Oxygen Reduction Reaction (ORR) at the cathode in either Fuel Cell (FC) or Metal-Air Battery (MAB). The current most effective ORR catalysts are noble Pt-group metals, whose high price and low abundance severely hamper the widespread application of FC and MAB. Dispersively non-noble transition metal coordinated with nitrogen atoms doped in carbon nanomaterials (M-N-C) Single Atom Catalysts (SACs) have been considered as the most promising catalysts for the ORR. Here, the effect of typical morphology on the activity and stability of SACs was discussed firstly. Moreover, the effect of metal type, coordination nitrogen number, heteroatom decoration on the central metal atom were discussed. Based on the rate determining step (RDS), we propose the ideal characters that high-performance SACs should possess.

Theoretical and experimental investigation of the coordination effect on photocatalytic CO2 reduction efficiency of cobalt single atom catalyst

Visible-light-driven carbon dioxide (CO2) reduction, which converts CO2 into value-added fuels, is an intriguing strategy to simultaneously mitigate the energy crisis and achieve a carbon–neutral economy. Single-atom catalysts (SACs) with homogeneously dispersed definite active sites and adjustable coordination environments show great potential in catalyzing photocatalytic CO2 reduction reaction (CO2RR). According to previously reported studies, the catalytic performance of SACs applied in various fields has a great relationship with their local coordination environment of metal centers. However, in-depth investigation of the coordination effect, which has been widely explored to regulate the electronic structure of SACs, on the activity, selectivity, and stability of CO2 photoreduction has never been reported. Herein, a series of Co single atom photocatalysts with diverse N coordination number (CoSA-Nx/C, x = 2, 3, and 4) were synthesized and their photocatalytic CO2RR performance was examined. Theoretical and experimental investigation demonstrated that the photocatalytic CO2RR activity of Co SACs can be effectively adjusted by fine-regulating the N coordination. The optimal CoSA-N2/C photocatalyst with two-coordinate N atoms with increased unoccupied Co 3d electronic orbitals possesses enhanced CO2 adsorption and activation as well as reduced energy barrier for *COOH intermediate generation, thus showing superior photoconversion of CO2-to-CO performance with 10110 μmol g−1h−1 CO evolution yield, 82.6 % CO selectivity, and long-term stability. This work promotes the future rationally design and synthesis of photocatalysts applied in targeted reactions at atomic scale.

Dual-atoms iron sites boost the kinetics of reversible conversion of polysulfide for high-performance lithium-sulfur batteries

Atomically dispersed metal catalysts have offered significant potential for accelerating sluggish kinetics of lithium polysulfides conversion and inhibiting the shuttle effect, so as to achieve the long-life cycle and high-rate performance of lithium sulfur batteries. However, the end-on adsorption structure between single metal site and polysulfide limits the adsorption capacity and catalytic activity of single atom catalysts. Here, we construct dual-atoms iron sites on nitrogen doped graphene to serve as highly efficient catalyst for lithium sulfur batteries. As expected, the dual-atoms sites can firmly bind polysulfides by forming double Fe-S bonds between polysulfides and the two adjacent iron atoms. Such double-bond adsorption structure is also favorable for electron transfer and polysulfides activation, resulting in reducing the energy barrier and accelerating the reaction kinetics. As a result, the as-obtained dual-atoms iron catalyst can effectively alleviate the shuttle effect and improve the utilization of active sulfur, thus the batteries present high initial capacity of 1615 mAh g−1 at 0.05 C and long-cycle life with a decay rate per cycle as low as 0.015% at 2C over 1000 cycles.

The enhanced peroxymonosulfate activation ability and mechanism on low-coordinated Fe-N3 single sites for organic pollutant degradation in wastewater

For boosting the intrinsic peroxymonosulfate (PMS) catalytic activity of single atom catalysts (SACs), we fabricate zeolitic imidazolate framework (ZIF)-derived Fe SAC with different nitrogen coordination numbers (denoted as Fe-Nx/C) to boost organic pollutant oxidation. X-ray absorption fine structure (XAFS) spectroscopy was undertaken to determine the coordination configuration of Fe–N path. As expected, the resultant Fe-N3/C exhibited more excellent PMS catalysis activity with a p-nitrophenol (4-NP) degradation kinetic rate constant (kobs) of 0.061 min−1 that is 2.4-fold enhancement relative to the Fe-N4/C. Electron paramagnetic resonance (EPR) measurements and quenching tests confirmed the 1O play the main role associated with high-valent Fe–oxo and a small amount of OH and SO4− species. Density functional calculations revealed that low coordinated Fe–N3 sites prefer to form a bridge coordination bond structure of O-Fe-O with the adsorption sites of O2 and O1 in PMS molecule, exhibiting stronger interaction with the oxidant and better electron donating capability to enhance PMS activation comparing to common Fe-N4 structure based on charge deformation density, electron localization function (ELF) and projected density of state (PDOS) analysis. These findings indicate that microstructure modulation is a useful avenue to prepare a high-performance Fenton-like SACs.

Rational Design of Graphene-Supported Single Atom Catalysts for High Performance Lithium-Oxygen Batteries

Aprotic lithium-oxygen batteries (LOBs) have been considered as one of the next-generation battery technologies, due to its ultrahigh theoretical energy density (~3600 Wh kg -1), 5-10 times higher than the state-of-the-art lithium-ion batteries (LIBs), supporting the development of electric vehicles (EVs). However, the battery system suffers from poor cyclability, low round-trip efficiency and inferior rate capability, mainly originated from the inertness of oxygen gas and the poor electrical conductivity of lithium peroxide (Li2O2), leading to the sluggish kinetics of oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER). In response to the challenges, developing a highly efficient catalyst at the cathode is vital to boost the reaction rate and reduce the reaction overpotentials, eventually address the problem. Heterogeneous single-atom catalysts (SACs) on solid support are emerging as a new frontier in this research field. Here, we use the density of state (DFT) calculation to determine the catalytic activities of a wide range of transition metal single atoms distributing on the nitrogen doped graphene support and found that Zn-SAC exhibits the highest ORR/OER activities. We discovered that Zn-N4 moieties, functioning as catalytic centers, bind with LiO2 not very strongly, reduce the reaction overpotentials, facilitates the reaction rate and enhance the stability of the catalyst. The catalytic activity of SACs is highly correlated to the Gibbs free energy of the adsorbed LiO2. A descriptor, F of the catalysts determining the catalytic activity was developed from the metal properties of electronegativity (EN), enthalpy of vaporization (EV) and number of electrons in d orbital, by using supervised machine learning (ML) method, providing guidance for designing useful SACs in ORR/OER process. This work systematically provides guidance to design highly efficient SAC for LOBs and provides fundamental insights in choosing the proper metal for the ORR/ OER application.

Tuning the Coordination Environment of Carbon-Based Single-Atom Catalysts via Doping with Multiple Heteroatoms and Their Applications in Electrocatalysis.

Carbon-based single-atom catalysts (SACs) are considered to be a perfect platform for studying the structure-activity relationship of different reactions due to the adjustability of their coordination environment. Multi-heteroatom doping has been demonstrated as an effective strategy for tuning the coordination environment of carbon-based SACs and enhancing catalytic performance in electrochemical reactions. Herein, recently developed strategies for multi-heteroatom doping, focusing on the regulation of single-atom active sites by heteroatoms in different coordination shells, are summarized. In addition, the correlation between the coordination environment and the catalytic activity of carbon-based SACs are investigated through representative experiments and theoretical calculations for various electrochemical reactions. Finally, concerning certain shortcomings of the current strategies of doping multi-heteroatoms, some suggestions are put forward to promote the development of carbon-based SACs in the field of electrocatalysis.

Reshaping the coordination and electronic structure of single atom sites on the right branch of ORR volcano plot

The coordination environment of atomic metal sites in single atom catalysts is of vital significance in tailoring the d-orbital states and thus the catalytic behavior towards oxygen reduction reaction (ORR), thereby offering great promise to boost activity via regulating the local chelation structure. Herein we designed a carbon coordination environment for the atomic M sites reside on the right leg of the volcano plot, to effectively counter balance the low adsorption energy of the metal center to oxygen species. Combining time-of-flight secondary ion mass spectrometry and density functional theory calculations, we unveiled the M-C coordination structure and the thus induced changes in electronic structure and ORR catalytic behavior. The reshape in coordination structure, i.e., the replacement of typical nitrogen coordination by low electronegativity carbon coordination, gives rise to exceptional intrinsic activity towards ORR. This work brings a new perspective to boost the activity of single atom catalysts on the weak bonding leg, with exceptional ORR activity being further expected through advancing the chelation structure.

The novel π–d conjugated TM2B3N3S6 (TM = Mo, Ti and W) monolayers as highly active single-atom catalysts for electrocatalytic synthesis of ammonia

Recently, single-atom catalysts (SACs) are receiving significant attention in electrocatalysis fields due to their excellent specific activities and extremely high atomic utilization ratio. Effective loading of metal atoms and high stability of SACs increase the number of exposed active sites, thus significantly improving their catalytic efficiency. Herein, we proposed a series (29 in total) of two-dimensional (2D) conjugated structures of TM2B3N3S6 (TM means those 3d to 5d transition metals) and studied the performance as single-atom catalysts for nitrogen reduction reaction (NRR) using density functional theory (DFT). Results show that TM2B3N3S6 (TM = Mo, Ti and W) monolayers have superior performance for ammonia synthesis with low limiting potentials of −0.38, −0.53 and −0.68 V, respectively. Among them, the Mo2B3N3S6 monolayer shows the best catalytic performance of NRR. Meanwhile, the π conjugated B3N3S6 rings undergo coordinated electron transfer with the d orbitals of TM to exhibit good chargeability, and these TM2B3N3S6 monolayers activate isolated N2 according to the “acceptance–donation” mechanism. We have also verified the good stability (i.e., Ef < 0, and Udiss > 0) and high selectivity (Ud = –0.03, 0.01 and 0.10 V, respectively) of the above four types of monolayers for NRR over hydrogen evolution reaction (HER). The NRR activities have been clarified by multiple-level descriptors (ΔG*N2H, ICOHP, and Ɛd) in the terms of basic characteristics, electronic property, and energy. Moreover, the aqueous solution can promote the NRR process, leading to the reduction of ΔGPDS from 0.38 eV to 0.27 eV for the Mo2B3N3S6 monolayer. However, the TM2B3N3S6 (TM = Mo, Ti and W) also showed excellent stability in aqueous phase. This study proves that the π-d conjugated monolayers of TM2B3N3S6 (TM = Mo, Ti and W) as electrocatalysts show great potentials for the nitrogen reduction.

Ternary single atom catalysts for effective oxygen reduction and evolution reactions

Synergistic effects have been demonstrated for binary single-atom catalysts (SACs), while it is still challenging to investigate the activity and performance of ternary SACs due to the lack of robust method to synthesize these materials. Herein, we provide a general approach for the preparation of ternary SACs, and the features of single atoms highly dispersed on the support have been identified and verified. As an illustration, trace platinum decorated iron-cobalt ternary single atom catalysts (PtFeCo/NC) possess an obviously enhanced bifunctional activity of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as well as high stability. The density functional theory calculations reveal that the synergistic effects induced by the introduction of Pt enable modulating the adsorption strength of reaction intermediates both on the Fe-N4 and Co-N4 sites, thus enhancing the ORR and OER activity, respectively. In addition, the PtFeCo/NC-based Zn-air battery delivers a higher power density than that of commercial Pt/C-RuO2, and can be steadily operated over 188 h.