Coordination tailoring towards efficient single-atom catalysts for N2 fixation: A case study of iron-nitrogen-carbon (Fe@N-C) systems

Single-atom transition metals supported on B-doped g-C3N4 monolayers for electrochemical nitrogen reduction

Electrochemical reduction of naturally abundant nitrogen (N2) under ambient conditions is a promising method for ammonia (NH3) synthesis, while the development of a highly active, stable and low-cost catalyst remains a challenge. Herein, the N2 reduction reaction of TM@g-BC3N4 in electrochemical nitrogen reduction has been systematically investigated using density functional theory (DFT) calculations and compared with that of TM@g-C3N4. It was found that TM atoms are more stably anchored to g-BC3N4 than to g-C3N4. The adsorption free energy of N2 molecules on Fe@g-BC3N4 has the greatest change compared with that on Fe@g-C3N4, decreasing by 1.08 eV. The spin charge density around the Fe atom in Fe@g-BC3N4 increases significantly compared with that in Fe@g-C3N4, and the total magnetic moment of the system increases by 3.26μB. The limiting potential (-0.57 V) of Fe@g-BC3N4 in nitrogen reduction is decreased by 0.06 V compared with that of Fe@g-C3N4 (-0.63 V), and the desorption free energy of ammonia molecules decreases from 1.72 eV to 0.46 eV. The Fe atom has higher catalytic activity, the ammonia molecule is easier for desorption, and nitrogen reduction performance is better. This provides an important reference for the application of g-C3N4-based single atom catalysts in the field of nitrogen reduction.

Design of 3d transition metal-embedded asymmetric HMo2CF for electrocatalytic conversion of N2 to NH3

The electrochemical reduction of N2 to NH3 (NRR) is challenging due to the lack of efficient catalysts under mild conditions. We constructed a series of 3d-transition-metal (3d-TM)-embedded asymmetric 2D MXene HMo2CF with one H or F vacancy (Hv or Fv) based on first-principles calculation. Due to the strong steric effect of the surface-covered H or F terminals, N2 is favored to be adsorbed on Hv or Fv through the end-on mode rather than the side-on mode. Compared to NRR on the exposed Mo at F-vacancy (denoted as MoFv) of HMo2CFv, TM-substituted (TM = V, Cr, Mn, and Fe) MoFv improved NRR activities by reducing the barriers to 0.70, 0.59, 0.58, and 0.72 eV, respectively, from the original 0.81 eV. Particularly, Mn- or Cr-embedded HMo2CFv exhibited the best catalytic performances among 3d-TMs (Ti-Ni) undergoing alternating or distal mechanism, where the potential determining step (PDS) occurs at the first hydrogenation of N2 to NNH with a barrier of 0.58 or 0.59 eV. For TM-substituted (TM = Ti to Ni) Mo adjacent to F-vacancy, the catalytic barriers varied slightly in the range of 0.72 to 0.82 eV. The adsorption energy comparison indicated that TM-embedded HMo2CF exhibited higher selectivity toward N2 end-on adsorption than H2 to initialize the following NRR process. Greater electron transfer between TM-N2 was ascribed to the moderate N2 adsorption. Our work is expected to provide an insightful method for designing efficient NRR catalysts.

Single-Atom Anchored g-C3N4 Monolayer as Efficient Catalysts for Nitrogen Reduction Reaction

Electrochemical N₂ reduction reaction (NRR) is a promising approach for NH₃ production under mild conditions. Herein, the catalytic performance of 3d transition metal (TM) atoms anchored on s-triazine-based g-C₃N₄ (TM@g-C₃N₄) in NRR is systematically investigated by density functional theory (DFT) calculations. Among these TM@g-C₃N₄ systems, the V@g-C₃N₄, Cr@g-C₃N₄, Mn@g-C₃N₄, Fe@g-C₃N₄, and Co@g-C₃N₄ monolayers have lower ΔG(*NNH) values, especially the V@g-C₃N₄ monolayer has the lowest limiting potential of -0.60 V and the corresponding limiting-potential steps are *N2+H++e-=*NNH for both alternating and distal mechanisms. For V@g-C₃N₄, the transferred charge and spin moment contributed by the anchored V atom activate N₂ molecule. The metal conductivity of V@g-C₃N₄ provides an effective guarantee for charge transfer between adsorbates and V atom during N₂ reduction reaction. After N₂ adsorption, the p-d orbital hybridization of *N₂ and V atoms can provide or receive electrons for the intermediate products, which makes the reduction process follow acceptance-donation mechanism. The results provide an important reference to design high efficiency single atom catalysts (SACs) for N₂ reduction.

Rational Design of Atomic Site Catalysts for Electrocatalytic Nitrogen Reduction Reaction: One Step Closer to Optimum Activity and Selectivity

The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber–Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH3 yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area: the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.

Graphical Abstract

Rational design of atomic site catalysts (ASCs) for nitrogen reduction reaction (NRR) has both scientific and industrial significance. In this review, the recent experimental and theoretical breakthroughs in the design principles of transition metal ASCs for NRR are comprehensively discussed, and the topic is also extended to double-atom catalysts and boron-based metal-free ASCs.

Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry

In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and cost-effectiveness make SACs ideal for photocatalytic CO₂ reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, CO₂ adsorption ability, active sites, and defects. Consequently, it is necessary to investigate these factors in depth to elucidate the working principle(s) of SACs for catalytic applications. Herein, the recent progress in the development of SACs for photocatalytic CO₂ reduction with H₂ O is reviewed. First, a brief overview of CO₂ photoreduction and SACs for CO₂ conversion is provided. Several synthesis strategies and useful techniques for characterizing SACs employed in heterogeneous catalysis are then described. Next, the challenges of SACs for photocatalytic CO₂ reduction and related optimization strategies, in terms of activity, product selectivity, and stability, are explored. The progress in the development of noble metal- and transition metal-based SACs and dual-SACs for photocatalytic CO₂ reduction is discussed. Finally, the prospects of SACs for CO₂ reduction are considered.

Distribution Pattern of Metal Atoms in Bimetal-Doped Pyridinic-N4 Pores Determines Their Potential for Electrocatalytic N2 Reduction

Doping two single transition-metal (TM) atoms on a substrate host opens numerous possibilities for catalyst design. However, what if the substrate contains more than one vacancy site? Then, the combination of two TMs along with their distribution patterns becomes a design parameter potentially complementary to the substrate itself and the bimetal composition. In this study, we investigate ammonia synthesis under mild electrocatalytic conditions on a transition-metal-doped porous C₂₄N₂₄ catalyst using density functional theory (DFT). The TMs studied include Ti, Mn, and Cu in a 2:4 dopant ratio (Ti₂Mn₄@C₂₄N₂₄ and Ti₂Cu₄@C₂₄N₂₄). Our computations show that a single Ti atom in both catalysts exhibits the highest selectivity for N₂ fixation at ambient conditions. This work is a good theoretical model to establish the structure-activity relationship, and the knowledge earned from the metal-N₄ moieties may help studies of related nanomaterials, especially those with curved structures.

Descriptors and graphical construction for in silico design of efficient and selective single atom catalysts for the eNRR

Outline a screening protocol that uses density functional theory calculations to simultaneously optimize with respect to multiple criteria, thereby successfully identifying catalysts that are highly selective and also result in low overpotentials for ammonia production through eNRR.

Transition Metal-Modified Co4 Clusters Supported on Graphdiyne as an Effective Nitrogen Reduction Reaction Electrocatalyst

Conversion of N₂ into NH₃ through the electrochemical nitrogen reduction reaction (NRR) under ambient conditions represents a novel green ammonia synthesis method. The main obstacle for NRR is lack of efficient, stable, and cost-effective catalysts. In this work, by using density functional theory calculations, 16 transition metal-modified Co₄ clusters supported on graphdiyne (GDY) as potential NRR catalysts were systematically screened. Through the examinations of stability, N₂ activation, selectivity, and activity, Ti-, V-, Cr-, Mn-, and Zr-Co₃@GDY were identified as the promising candidates toward NRR. Further explorations on the NRR mechanisms and the Pourbaix diagrams suggest that Ti-Co₃@GDY was the most promising candidate catalyst, as it has the lowest limiting potential and high stability under the working conditions. The high activities originate from the synergy effect, where the Co₃ cluster acts as the electron donor and the heteroatom serves as the single active site throughout the NRR process. Our results offer a new perspective for advancing sustainable NH₃ production.

Rational Design of Graphene-Supported Single-Atom Catalysts for Electroreduction of Nitrogen

Critically, the central metal atoms along with their coordination environment play a significant role in the catalytic performance of single-atom catalysts (SACs). Herein, 12 single Fe, Mo, and Ru atoms supported on defective graphene are theoretically deigned for investigation of their structural and electronic properties and catalytic nitrogen reduction reaction (NRR) performance using first-principles calculations. Our results reveal that graphene with vacancies can be an ideal anchoring site for stabilizing isolated metal atoms owing to the strong metal-support interaction, forming stable TMC_x or TMN_x active centers (x = 3 or 4). Six SACs are screened as promising NRR catalyst candidates with excellent activity and selectivity during NRR, and RuN₃ is identified as the optimal one with an overpotential of ≥0.10 V via the distal mechanism.

MX Anti-MXenes from Non-van der Waals Bulks for Electrochemical Applications: The Merit of Metallicity and Active Basal Plane

Two-dimensional transition-metal compounds (2DTMCs) are promising materials for electrochemical applications, but 2DTMCs with metallicity and active basal planes are rare. In this work, we proposed a simple and effective strategy to extract 2DTMCs from non-van der Waals bulk materials and established a material library of 79 2DTMCs, which we named as anti-MXenes since they are composed of one M atomic layer sandwiched by two X atomic layers. By means of density functional theory computations, 24 anti-MXenes were confirmed to be thermodynamically, dynamically, mechanically, and thermally stable. The metallicity and active basal plane endow these anti-MXenes with potential as excellent electrode materials, for example, as electrocatalysts for hydrogen evolution reactions (HER). Among the noble-metal free anti-MXenes with favorable H-binding, CuS can boost HER at the whole range of H coverages, while CoSi, FeB, CoB, and CoP show promise for HER at some specific H coverages. The active sites are the tetra-coordinating nonmetal atoms at the basal planes, thus rendering a very high density of active sites for these materials. CoB is also a promising anode material for lithium-ion batteries, showing low Li diffusion energy barriers, a very high capacity, and a suitable open circuit voltage. This work promotes the "computational exfoliation" of 2D materials from non-van der Waals bulks and exemplifies the applications of anti-MXenes in various electrochemical processes.

Screening a Suitable Mo Form Supported on Graphdiyne for Effectively Electrocatalytic N2 Reduction Reaction: From Atomic Catalyst to Cluster Catalyst

Single-atom catalysts (SACs) stand out from the atomically dispersed catalysts due to their high specific activity and 100% atomic utilization ratio. However, besides inheriting most of the advantages of SACs, multiple-atom centered site catalysts not only boost higher metal atom loading but also provide more flexible active sites. In this work, by using spin-polarized density functional theory calculations, we systematically investigated the electrochemical nitrogen reduction reaction (eNRR) performance catalyzed by Mo_x (x = 1-4) supported on graphdiyne (GDY). Our results showed that N₂ was favorably adsorbed on the substrates via a well-known "acceptance-donation" mechanism, which can be deeply understood by good multiple linear regressions between the adsorption Gibbs free energy of N₂ and lengths or integrated crystal orbital Hamilton populations of Mo-N and N-N bonds. According to the designed screening criteria, Mo₃@GDY was found to be most active toward NRR with high selectivity and stability. The predicted onset potential was only -0.32 V. The activity originates from a moderate N adsorption energy, which can balance the thermodynamics of the two potential potential-determining steps, i.e., N₂ + H⁺ + e^- = *N₂H and *NH₂ + H⁺ + e^- = NH₃. Moreover, the GDY serves as an electron reservoir during the whole NRR process, where it can provide electrons or accept electrons arbitrarily depending on the need of each elementary step, suggesting that the GDY sheet is a very suitable platform for electrocatalysis applications. The superior electrocatalytic performance of the triple-atom catalyst compared to that of the SACs, double-atom catalyst, and the quadruple-atom cluster catalyst toward NRR offers a huge opportunity for the exploration of a new generation of electrochemical catalysts, where the metal clusters should be highlighted.

Synergistic Effect of Surface-Terminated Oxygen Vacancy and Single-Atom Catalysts on Defective MXenes for Efficient Nitrogen Fixation

The production of ammonia (NH₃) from molecular dinitrogen (N₂) under ambient conditions is of great significance but remains as a great challenge. Using first-principles calculations, we have investigated the potential of using a transition metal (TM) atom embedded on defective MXene nanosheets (Ti_3-xC₂O_y and Ti_2-xCO_y with a Ti vacancy) as a single-atom electrocatalyst (SAC) for the nitrogen reduction reaction (NRR). The Ti_3-xC₂O_y nanosheet with Mo and W embedded, and the Ti_2-xC₂O_y nanosheet with Cr, Mo, and W embedded, can significantly promote the NRR while suppressing the competitive hydrogen evolution reaction, with the low limiting potential of -0.11 V for W/Ti_2-xC₂O_y. The outstanding performance is attributed to the synergistic effect of the exposed Ti atom and the TM atom around an extra oxygen vacancy. The polarization charges of the active center are reasonably tuned by the embedded TM atoms, which can optimize the binding strength of key intermediate *N₂H. The good feasibility of preparing such TM SACs on defective MXenes and the high NRR selectivity with regard to the competitive HER suggest new opportunities for driving NH₃ production by MXene-based SAC electrocatalysts under ambient conditions.