Design of a High-Performance Electrocatalyst for N2 Conversion to NH3 by Trapping Single Metal Atoms on Stepped CeO2

Design of high performance nitrogen reduction electrocatalysts by doping defective polyoxometalate with a single atom promoter

Single-atom catalysts (SACs) are emerging as promising candidates for electrochemical nitrogen reduction reaction (NRR). Previous studies have shown that the single-atom centers of SACs can not only serve as active sites, but also act as promoters to affect the catalytic properties. However, the use of single metal atoms as promoters in electrocatalysis has rarely been studied. In this work, the defective Keggin-type phosphomolybdic acid (PMA) is used as a substrate to support the single metal atoms. We aim to tune the electronic structures of the exposed molybdenum active sites on defective PMA by using these supported single atoms as promoters for efficient NRR. Firstly, the stability and N2 adsorption capacity were studied to screen for an effective catalyst capable of activating N2. Most of the SACs were found to have good stability and N2 adsorption capacity. Then, we compared the selectivity and NRR activity of the catalysts and found that catalysts with metal atom promoters have improved NRR selectivity and activity. Finally, electronic structure analysis was carried out to understand the promoting effect of the promoter on N2 activation and the activity of the NRR process. This work provides a new strategy for designing efficient catalysts for electrocatalytic reactions by introducing promoters.

First-Principles Study of Bimetallic Pairs Embedded on Graphene Co-Doped with N and O for N2 Electroreduction

The electrocatalytic nitrogen reduction reaction (NRR) is considered a viable alternative to the Haber-Bosch process for ammonia synthesis, and the design of highly active and selective catalysts is crucial for the industrialization of the NRR. Dual-atom catalysts (DACs) with dual active sites offer flexible active sites and synergistic effects between atoms, providing more possibilities for the tuning of catalytic performance. In this study, we designed 48 graphene-based DACs with N4O2 coordination (MM'@N4O2-G) using density functional theory. Through a series of screening strategies, we explored the reaction mechanisms of the NRR for eight catalysts in depth and revealed the "acceptance-donation" mechanism between the active sites and the N2 molecules through electronic structure analysis. The study found that the limiting potential of the catalysts exhibited a volcano-shaped relationship with the d-band center of the active sites, indicating that the synergistic effect between the bimetallic components can regulate the d-band center position of the active metal M, thereby controlling the reaction activity. Furthermore, we investigated the selectivity of the eight DACs and identified five potential NRR catalysts. Among them, MoCo@N4O2-G showed the best NRR performance, with a limiting potential of -0.20 V. This study provides theoretical insights for the design and development of efficient NRR electrocatalysts.

Electrochemical nitrogen fixation on single metal atom catalysts

The electrochemical reduction of nitrogen (eNRR) offers a promising alternative to the Haber-Bosch (H-B) process for producing ammonia under moderate conditions. However, the inertness of dinitrogen and the competing hydrogen evolution reaction pose significant challenges for eNRR. Thus, developing more efficient electrocatalysts requires a deeper understanding of the underlying mechanistic reactions and electrocatalytic activity. Single atom catalysts, which offer tunable catalytic properties and increased selectivity, have emerged as a promising avenue for eNRR. Carbon and metal-based substrates have proven effective for dispersing highly active single atoms that can enhance eNRR activity. In this review, we explore the use of atomically dispersed single atoms on different substrates for eNRR from both conceptual and experimental perspectives. The review is divided into four sections: the first section describes eNRR mechanistic pathways, the second section focuses on single metal atom catalysts (SMACs) with metal atoms dispersed on carbon substrates for eNRR, the third section covers SMACs with metal atoms dispersed on non-carbon substrates for eNRR, and the final section summarizes the remaining challenges and future scope of eNRR for green ammonia production.

Role of Peripheral Coordination Boron in Electrocatalytic Nitrogen Reduction over N-Doped Graphene-Supported Single-Atom Catalysts

In this work, we investigate the effect of peripheral B doping on the electrocatalytic nitrogen reduction reaction (NRR) performance of N-doped graphene-supported single-metal atoms using density functional theory (DFT) calculations. Our results showed that the peripheral coordination of B atoms could improve the stability of the single-atom catalysts (SACs) and weaken the binding of nitrogen to the central atom. Interestingly, it was found that there was a linear correlation between the change in the magnetic moment (μ) of single-metal atoms and the change in the limiting potential (U_L) of the optimum NRR pathway before and after B doping. It was also found that the introduction of the B atom suppressed the hydrogen evolution reaction, thereby enhancing the NRR selectivity of the SACs. This work provides useful insights into the design of efficient SACs for electrocatalytic NRR.

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.

Rare-Earth Single-Atom Catalysts: A New Frontier in Photo/Electrocatalysis

Single-atom catalysts (SACs) provide well-defined active sites with 100% atom utilization, and can be prepared using a wide range of support materials. Therefore, they are attracting global attention, especially in the fields of energy conversion and storage. To date, research has focused on transition-metal and precious-metal-based SACs. More recently, rare-earth (RE)-based SACs have emerged as a new frontier in photo/electrocatalysis owing to their unique electronic structure arising from the spin-orbit coupling of the 4f and valence orbitals, unsaturated coordination environment, and unique behavior as charge-transport bridges. However, a systematic review on the role of the RE active sites, catalytic mechanisms, and synthetic methods for RE SACs is lacking. Therefore, in this review, the latest developments in RE SACs having applications in photo/electrocatalysis are summarized and discussed. First, the theoretical advantages of RE SACs for photo/electrocatalysis are briefly introduced, focusing on the roles of the 4f orbitals and coupled energy levels. In addition, the most recent research progress on RE SACs is summarized for several important photo/electrocatalytic reactions and the corresponding catalytic mechanisms are discussed. Further, the synthetic strategies for the production of RE SACs are reported. Finally, challenges for the development of RE SACs are highlighted, along with future research directions and perspectives.

Electrocatalytic Reduction of N2 to NH3 Over Defective 1T'-WX2 (X=S, Se, Te) Monolayers

Defects in transition metal dichalcogenides (TMDs) can serve as active sites in catalytic reactions. In this work, by means of first-principles calculations, the catalytic activities of WX₂ (X=S, Se, Te) monolayers in the 1T' phase with both vacancy defects (missing chalcogen atoms, ^X V_d ) and antisite defects (replacing chalcogen atoms with W atoms, ^X A_d ) were evaluated for the nitrogen reduction reaction (NRR). Results showed that all these defective catalysts had great potential toward electrocatalytic ammonia synthesis by exhibiting low limiting potentials (U_L ). Over 1T'-WTe₂ @^Te V_d , 1T'-WS₂ @^S A_d , 1T'-WSe₂ @^Se A_d , and 1T'-WTe₂ @^Te A_d , the corresponding U_L values were -0.49, -0.21, -0.19, and -0.15 V, much smaller than that of the benchmark catalyst, the Ru (0001) surface (U_L =-0.98 V). Furthermore, the hydrogen evolution reaction (HER) was inhibited. 1T'-WX₂ monolayers with the antisite defects showed better NRR activity than those with the vacancy defects because of the smaller steric hindrance at the former. Results suggest that the steric effect at the active surface sites should be utilized to develop better catalysts.

Single-atom catalyst of TM@D-silicene-an effective way to reduce N2 into ammonia

Nowadays, the electrocatalytic nitrogen reduction reaction (NRR) still faces great challenges. It's significant to design the electrocatalysts with excellent activity and high selectivity. Herein, the 28 single atom catalysts of transition metal atoms anchored on defective silicene (TM@D-silicene) are designed for electrocatalytic ammonia synthesis under ambient conditions. Two independent screening schemes are proposed to screen the potential SAC candidate. The criteria of E_bin < 0 eV, ΔG_*N2< -0.1 eV, ΔG_*H > ΔG_*N2, and the lowest ΔG_*NNH in scheme I, as well as ΔG_*N2< -0.1 eV, smaller G_*NNH and larger G_*NH2 in scheme II are utilized in the screening procedure. Finally, Cr@D-silicene is picked out since it performs well in the aspects of N₂ adsorption, selectivity and catalytic activity of NRR. Moreover, the electronic properties are systematically investigated to clarify why the Cr@D-silicene is qualified for NRR from the perspective of the strong interaction between N₂ and Cr, the continuous activation of the N₂ molecule, charge transfer and distribution. This work provides a new idea for electrocatalytic ammonia synthesis by using single-atom catalysts.

Screening of Transition-Metal Single-Atom Catalysts Anchored on Covalent-Organic Frameworks for Efficient Nitrogen Fixation

Two-dimensional (2D) covalent-organic frameworks (COFs) offer abundant hollow sites for stably anchoring transition-metal (TM) atoms to promote single-atom catalysis (SACs), which is expected to overcome the poor stability of SACs on conventional substrate materials. Using first-principles calculations within density-functional theory, a number of TM atoms embedded on a 2D COF Pc-TFPN (TMPc-TFPN) as SACs for ammonia synthesis under ambient conditions are investigated. Through a "five-step" screening strategy, WPc-TFPN is highlighted from 26 TMPc-TFPNs as the best SACs for nitrogen reduction reaction (NRR) with a low limiting potential of -0.19 V. Meanwhile, multiple-level descriptors are developed to uncover the origins of NRR activity, among which a simple descriptor φ that involves the electronegativity and number of d electrons of TM atoms shows volcano plot trends of limiting potential of NRR. This work provides a rational strategy for fast screening SACs for the electrochemical N₂ fixation using 2D COFs containing TM-N₄ units as host materials, which could also be applied to other electrochemical reactions.

Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study

In this work, the H2 dissociation and acetylene hydrogenation on Cu doped CeO2(111) were studied using density functional theory calculations. The results indicated that Cu doping promotes the formation of oxygen vacancy (Ov) which creates Cu/O and Ce/O frustrated Lewis pairs (FLPs). With the help of Cu/O FLP, H2 dissociation can firstly proceed via a heterolytic mechanism to produce Cu-H and O-H by overcoming a barrier of 0.40 eV. The H on Cu can facilely migrate to a nearby oxygen to form another O-H species with a barrier of 0.43 eV. The rate-determining barrier is lower than that for homolytic dissociation of H2 which produces two O-H species. C2H2 hydrogenation can proceed with a rate-determining barrier of 1.00 eV at the presence of Cu-H and O-H species., While C2H2 can be catalyzed by two O-H groups with a rate-determining barrier of 1.06 eV, which is significantly lower than that (2.86 eV) of C2H2 hydrogenated by O-H groups on the bare CeO2(111), showing the high activity of Cu doped CeO2(111) for acetylene hydrogenation. In addition, the rate-determining barrier of C2H4 further hydrogenated by two O-H groups is 1.53 eV, much higher than its desorption energy (0.72 eV), suggesting the high selectivity of Cu doped CeO2(111) for C2H2 partial hydrogenation. This provides new insights to develop effective hydrogenation catalysts based on metal oxide.

Tantalum based single, double, and triple atom catalysts supported on g-C2N monolayer for effective nitrogen reduction reaction: a comparative DFT investigation

Density functional theory simulations demonstrate that single and triple Ta-atom catalysts anchored to C2N monolayer act as superior catalysts for the nitrogen reduction reaction via alternating and distal pathways.

Rare earth element based single-atom catalysts: synthesis, characterization and applications in photo/electro-catalytic reactions

Rare earth elements play an important role in various fields, which has attracted increasing interest from the scientific community. Meanwhile, single-atom catalysts show huge advantages in many aspects compared with traditional nanomaterials due to their 100% atomic utilization efficiency. Thus, the combination of the two concepts has yielded an efficient way to realize the high-value utilization of rare earth elements. In this mini-review, rare earth-based single-atom catalysts including their synthesis methods, characterization means and corresponding applications are constructively summarized and discussed. In particular, the important roles of rare earth elements as active centers in photo/electrocatalytic reactions are focused on. Finally, future prospects are also provided.

Recent Advances in Electrocatalysts for Alkaline Hydrogen Oxidation Reaction

With the rapid development of anion-exchange membrane technology and adequate supply of high-performance non-noble metal oxygen reduction reaction (ORR) catalysts in alkaline media, the commercialization of anion exchange membrane fuel cells (AEMFCs) become possible. However, the kinetics of the anodic hydrogen oxidation reaction (HOR) in AEMFCs is significantly decreased compared to the HOR in proton exchange membrane fuel cells (PEMFCs). Therefore, it is urgent to develop HOR catalysts with low price, high activity, and robust stability. However, comprehensive timely reviews on this specific subject do not exist enough yet and it is necessary to update reported major achievements and to point out future investigation directions. In this review, the current reaction mechanisms on HOR are summarized and deeply understood. The debates between the mechanisms are greatly harmonized. Recent advances in developing highly active and stable electrocatalysts for the HOR are reviewed. Moreover, the side reaction control is for the first time systematically introduced. Finally, the challenges and future opportunities in the field of HOR catalysis are outlined.

Electrocatalytic Reduction of N2 Using Metal-Doped Borophene

Ammonia synthesis is an essential process in chemistry and industry. However, it is limited by the lack of efficient catalysts and high energy costs. Developing highly efficient systems for ammonia synthesis is an important and long-standing challenge. In this paper, a large class of metal atoms (including 3d/4d transition metals and main group metals) anchored onto borophene have been studied as single atom catalysts for ammonia synthesis. After comprehensive computational screening and systematic evaluation, four candidates stand out. We predict that Mo, Mn, Tc, and Cr@BM-β₁₂ will have superior performance for catalytic reduction of N₂ to NH₃ with low limiting potentials of -0.26, -0.32, -0.38, and -0.48 V, respectively. Furthermore, we studied the activity of the competitive HER on M@BM-β₁₂. The results implied that the two materials Mo@BM-β₁₂ and Mn@BM-β₁₂ showed HER suppression. These properties exceed most currently reported nitrogen reduction reaction electrocatalysts. Our results suggest the possibility of efficient electrochemical reduction of N₂ to NH₃ in a lower energy process.

Single Nb or W Atom-Embedded BP Monolayers as Highly Selective and Stable Electrocatalysts for Nitrogen Fixation with Low-Onset Potentials

Conversion of dinitrogen (N₂) molecules into ammonia through electrochemical methods is a promising alternative to the traditional Haber-Bosch process. However, searching for an eligible electrocatalyst with high stability, low-onset potential, and superior selectivity is still one of the most challenging and attractive topics for the electrochemical N₂ reduction reaction (NRR). Here, by means of first-principles calculations and the conductor-like screening model, four comprehensive criteria were proposed to screen out eligible NRR electrocatalysts from 29 atomic transition metals embedded on the defective boron phosphide (BP) monolayer with B-monovacancy (M/BP single-atom catalysts, SAC, M = Sc-Zn, Y-Cd, and Hf-Hg). Consequently, the Nb/BP and W/BP SACs are identified as the promising candidates, on which the N₂ molecule can only be activated through the enzymatic pathway with the onset potentials of -0.25 and -0.19 V, and selectivities of 90.5 and 100%, respectively. It is worth noting that the W/BP SAC has the lowest overpotential among the 29 systems investigated. The electronic properties were also calculated in detail to analyze the activity origin. Importantly, the Nb/BP and W/BP SACs possess high thermal stabilities due to that their structures can be retained very well up to 1000 and 700 K, respectively. This work not only provides an efficient and reliable method to screen eligible NRR electrocatalysts but also paves a new way for advancing sustainable ammonia synthesis.

Highly stable Mo-doped Fe2P and Fe3P monolayers as low-onset-potential electrocatalysts for nitrogen fixation

Mo atom doping can modify the electronic properties of Fe₂P and Fe₃P monolayers, and significantly enhance their NRR activities with onset potentials as low as −0.30 V and −0.17 V, respectively.

Single metal atom anchored on a CN monolayer as an excellent electrocatalyst for the nitrogen reduction reaction

Based on first-principles calculations, we have studied the behavior of single-atom catalysts formed by a series of single metal atoms (from Ti to Cu) and a CN monolayer in nitrogen reduction reactions (NRRs). It was demonstrated that TM atoms could be anchored on CN and Ti@CN has good electrical conductivity, high stability and good catalytic performance. The onset potential of Ti@CN is as low as -0.38 V through the enzymatic mechanism, which well suppresses the competitive hydrogen evolution reaction. In addition, the determinate step of Ti@CN for the N2 reduction reaction is lower than that of the Ru(0001) stepped surface (-0.98 V). We further examine the effect of coordination on activity and propose a single Ti atom anchored on CN as a promising catalyst with high catalytic capability for N2 reduction to NH3. Our work offers a new opportunity and useful guidance for the NRR in an ambient environment.

Highly selective electroreduction of nitrate to ammonia on a Ru-doped tetragonal Co2P monolayer with low-limiting overpotential

A Ru-doped Co2P monolayer is constructed to modulate the d-band center close to the value at the volcano top and its limiting potential is −0.38 V, exhibiting excellent activity as well as selectivity.

Feasibility of N2 Reduction on the V Anchored 1T-MoS2 Monolayer: A Density Functional Theory Study

Developing efficient electrocatalysts for nitrogen reduction reaction (NRR) at ambient conditions is crucial for NH₃ synthesis. In this manuscript, the NRR performance of the transition metal anchored MoS₂ monolayer with 1T atomic structure (1T-MoS₂ ) is systematically evaluated by density functional theory computations. Our results reveal that the V decorated 1T-MoS₂ exhibits the outstanding catalytic activity toward NRR via distal mechanism where the corresponding onset potential is 0.66 V, being superior to the commercial Ru material. Furthermore, the powerful binding energy between the V atom and the 1T-MoS₂ provides the good resistance against clustering of the V dopant, indicating its stability. Overall, this work provides a potential alternative for the application of NH₃ synthesis.

Computational Design of Transition Metal Single-Atom Electrocatalysts on PtS2 for Efficient Nitrogen Reduction

Electrocatalytic nitrogen reduction is promising to serve as a sustainable and environmentally friendly strategy to achieve ammonia production. Single-atom catalysts (SACs) hold great promise to convert N₂ into NH₃ because of the unique molecular catalysis property and ultrahigh atomic utilization ratio. Here, we demonstrate a universal computational design principle to assess the N₂ reduction reaction (NRR) performance of SACs anchored on a monolayer PtS₂ substrate (SACs-PtS₂). Our density functional theory simulations unveil that the barriers of the NRR limiting potential step on different SAC centers are observed to be linearly correlated to the integral of unoccupied d states (UDSs) of SACs. As a result, the Ru SAC-PtS₂ catalyst with the largest number of UDSs exhibits a much lower barrier of the limiting step than those of other SACs-PtS₂ catalysts and the Ru(0001) benchmark. Our work bridges the apparent NRR activity and intrinsic electronic structure of SAC centers and offers effective guidance to screen and design efficient SACs for the electrochemical NRR process.

Ru–polyoxometalate as a single-atom electrocatalyst for N2 reduction to NH3 with high selectivity at applied voltage: a perspective from DFT studies

Polyoxometalate supported Ru₁ achieves high NRR selectivity at applied voltage while maintaining high activity.