Highly efficient N2 fixation catalysts: transition-metal carbides M2C (MXenes)

Two-Dimensional MXene-Based Electrocatalysts: Challenges and Opportunities

MXene, regarded as cutting-edge two-dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene-based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next-generation platform in fundamental research and real-world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene-based electrocatalysts.

Mesoporous Electrocatalysts with p-n Heterojunctions for Efficient Electroreduction of CO2 and N2 to Urea

The electrocatalytic synthesis of high-value-added urea by activating N2 and CO2 is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N2 and CO2 on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO2/Co3O4 heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g-1 h-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO2 targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N2 targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty eg orbitals of Co3+ in CeO2/Co3O4, which presents a low-spin state (LS: t2g6eg0). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through in situ Raman spectroscopy that the CeO2/Co3O4 catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.

Revealing the Adsorption Behavior of Nitrogen Reduction Reaction on Strained Ti2 CO2 by a Spin-Polarized d-band Center Model

Electrocatalytic reduction of dinitrogen to ammonia has attracted significant research interest. Herein, it reports the boosting performance of electrocatalytic nitrogen reduction on Ti2 CO2 MXene with an oxygen vacancy through biaxial tensile strain engineering. Specifically, tensile strain modified electronic structures and formation energy of oxygen vacancy are evaluated. The exposed Ti atoms with additional electron states near the Fermi level serve as active site for intermediate adsorption, leading to superior catalytic performance (Ulimit = -0.44 V) under 2.5% biaxial tensile strain through a distal mechanism. However, the two sides of the "Sabatier optimum" in volcano plot are not limited by two different electronic steps, but are induced by the diverse adsorption behaviors of intermediates. Crucially, the "Sabatier optimum" results from the different response speeds of the adsorption energy for *N2 and *NNH to strains. Moreover, the authors observe conventional d-band adsorption for *N2 and *NNH, non-linear adsorption for *NNH2 , and abnormal d-band adsorption for *N, *NH, *NH2 , and *NH3 , which can be explained by the competition between attractive orbital hybridization and repulsive orbital orthogonalization with the spin-polarized d-band model, which further clarifies the contributions of 3σ → dz2 and dxz /dyz → 2π* to the overall population of bonding and anti-bonding states.

A single transition metal atom anchored on Nb2C as an electrocatalyst for the nitrogen reduction reaction

Nitrogen (N2) reduction to produce ammonia (NH3) under milder conditions is attractive as NH3 has been widely used in various fields. The electrocatalytic nitrogen reduction reaction (NRR) is considered to be a more moderate and green method for ammonia synthesis. Herein, using density functional theory (DFT) computations, we investigated the potential application of single-atom catalysts (SACs) toward the NRR, in which transition metal (TM, TM = Ti, V, Mn, Fe, Co, Y, Zr, Mo) atoms are supported on Nb2C (TM-Nb2C). Through our screening, Fe-Nb2C is highlighted from 8 candidate systems as the superior SAC for the NRR with a low limiting potential of -0.47 V. Meanwhile, a volcano plot between UL (NRR) and the ICOHP values of the N-H bond in *NH2 is established to determine the optimal ICOHP values that can be used as a simple descriptor of the NRR performance of Fe-Nb2C.

Surface Modulation of Co3O4 Yolk-Shell Spheres with Tungsten Doping for Superior Acetone Sensitivity

This study introduces a promising technique to enhance the sensitivity of p-type semiconductors in gas-sensing applications. By utilizing a glycerate-templated synthesis approach, a unique hierarchical W-doped Co3O4 yolk-shell sphere (YSS)-based sensor was developed, exhibiting exceptional sensitivity toward acetone gas. The synthesized YSSs feature a yolk-shell structure with a diameter of approximately 500 nm and a large surface area of 117.46 m2/g, which allows for efficient gas interaction and high sensitivity toward acetone gas. Furthermore, the incorporation of tungsten (W), a non-noble metal, as a dopant significantly enhances the surface activity of Co3O4, leading to a remarkably high response of 16.5 toward 5 ppm acetone, which is substantially higher than that of the pure Co3O4 YSS (2.9). The W-doped Co3O4 YSS also exhibits excellent selectivity to other interfering gases and the ability to detect ultralow concentrations of acetone as low as 10 ppb. The proposed non-noble metal doping strategy presents a practical solution for enhancing the sensitivity and selectivity of p-type semiconductor-based gas sensors. This approach holds great potential for practical gas-sensing applications due to their affordability and abundance, making them a cost-effective and versatile alternative to noble metal-catalyzed sensors.

Work function regulation of surface-engineered Ti2CT2 MXenes for efficient electrochemical nitrogen reduction reaction

Electrochemical conversion of nitrogen to ammonia is a promising method in modern agriculture and industry due to its suitability and feasibility under mild conditions. Therefore, seeking electrocatalysts and understanding the catalytic mechanisms are of great importance. In this work, by combining the concept of the synergetic effect of the terminal vacancy and transition metal active center, we studied the whole catalytic mechanism of defective Ti₂CT₂ MXenes with functional groups (T = O, F, H, OH) by employing first-principles calculations. It is demonstrated that the electron transfer behavior of 2D transition metal carbides can be tuned by modifying the surface functional groups. Herein, the rarely investigated work function regulation is proved to effectively alter the electron transfer ability, thus the binding strength of key intermediates on the surface can be optimized. Besides, Ti₂CO₂ with an oxygen vacancy is identified as a promising candidate through a distal mechanism, where the calculated electronic properties reveal that the introduction of in-gap states is responsible for activating N₂ with physical adsorption. In addition, obvious orbital splitting of the σ and π* orbitals of N₂ is observed due to the hybridization of frontier orbitals. The symmetry matching rule of the frontier orbitals of π* 2p and the σ 2p orbitals of N with Ti d orbitals further illustrates the "acceptance-donation" interaction. These theoretical insights highlight the underlying mechanism of the synergetic effect of surficial vacancy and exposed transition metal atoms, and provide an alternative view of designing efficient NRR electrocatalysts.

Ti2N nitride MXene evokes the Mars-van Krevelen mechanism to achieve high selectivity for nitrogen reduction reaction

We address the low selectivity problem faced by the electrochemical nitrogen (N2) reduction reaction (NRR) to ammonia (NH3) by exploiting the Mars-van Krevelen (MvK) mechanism on two-dimensional (2D) Ti2N nitride MXene. NRR technology is a viable alternative to reducing the energy and greenhouse gas emission footprint from NH3 production. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. The MvK mechanism reduces this competition by eliminating the adsorption and dissociation processes at the sites for NH3 synthesis. We show that the new class of 2D materials, nitride MXenes, evoke the MvK mechanism to achieve the highest Faradaic efficiency (FE) towards NH3 reported for any pristine transition metal-based catalyst-19.85% with a yield of 11.33 μg/cm2/hr at an applied potential of - 250 mV versus RHE. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

Theoretical Evaluation of Effect of Bimetallic Au-based Alloy Catalysts on Initial N2 Electroreduction Pathways

Theoretical study of effect of bimetallic Au-based alloys catalysts on initial N2 electroreduction pathways at the present simulated electrode/aqueous interfaces based on DFT calculations is carried out in this work....

Theoretical Screening of Transition Metal Doped Defective MoS2 as Efficient Electrocatalyst for CO Conversion to CH4

CO is a key intermediate during electrochemical CO₂ conversion. The deep reduction of CO to value-added chemical products is a crucial strategy for effective carbon utilization. Single transition metal atoms supported by two-dimensional material present a novel paragon for various catalytic reactions. Herein, we employ first principle theory to study a series of single 3d-transition metal atoms supported by monolayered MoS₂ with S vacancy as efficient electrocatalyst for CO electroreduction to CH₄ . The screening result indicates that Cr doped defective MoS₂ (labeled as Cr/S_v -MoS₂ ) is beneficial to electroreduction of CO to CH₄ , with even less negative limiting potential (-0.32 V) than Cu that has been widely studied as the most promising electrocatalyst in experiment. The outstanding activity is derived from the regulation of the d-band-center of doped Cr and Mo atoms exposed on the surface. This discovery provides a theoretical basis for the preparation of future electrocatalysts for CORR.

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N₂RR), CO₂ reduction reaction (CO₂RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

Controlled 2H/1T phase transition in MoS2 monolayers by a strong interface with M2C MXenes: a computational study

Due to the high conductivity and abundant active sites, the metallic 1T phase of a two-dimensional molybdenum sulfide monolayer (1T-MoS₂) has witnessed a broad range of potential applications in catalysis, and spintronic and phase-switching devices, which, however, are greatly hampered by its poor stability. Thus, the development of particular strategies to realize the phase transition from the stable 2H phase to the metastable 1T phase for MoS₂ nanosheets is highly desirable. Herein, by means of density functional theory (DFT) computations, we systematically explored the potential of the interfacial interaction of 2H- and 1T-MoS₂ monolayers with a series of M₂C MXenes (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) for achieving the 2H/1T phase transformation. Our results revealed that the 2H → 1T transition for MoS₂ monolayers can occur thermodynamically by anchoring on Ti₂C, Zr₂C, or Hf₂C substrates with the extremely strong metal-S interaction, which can be well rationalized by the analysis of the charge transfer, work function, and density of states. Specially, these obtained stable 1T-MoS₂/M₂C hybrid materials exhibit excellent metallic features, outstanding magnetism, and enhanced mechanical properties. Our findings provide a new avenue to tune the phase transformation for MoS₂ monolayers by strong interfacial interactions, which helps to further widen the potential applications of MoS₂ monolayers.

Electronic and Optical Properties of Atomic-Scale Heterostructure Based on MXene and MN (M = Al, Ga): A DFT Investigation

After the discovery of graphene, a lot of research has been conducted on two-dimensional (2D) materials. In order to increase the performance of 2D materials and expand their applications, two different layered materials are usually combined by van der Waals (vdW) interactions to form a heterostructure. In this work, based on first-principles calculation, some charming properties of the heterostructure constructed by Hf2CO2, AlN and GaN are addressed. The results show that Hf2CO2/AlN and Hf2CO2/GaN vdW heterostructures can keep their original band structure shape and have strong thermal stability at 300 K. In addition, the Hf2CO2/MN heterostructure has I-type band alignment structure, which can be used as a promising light-emitting device material. The charge transfer between the Hf2CO2 and AlN (or GaN) monolayers is 0.1513 (or 0.0414) |e|. The potential of Hf2CO2/AlN and Hf2CO2/GaN vdW heterostructures decreases by 6.445 eV and 3.752 eV, respectively, across the interface. Furthermore, both Hf2CO2/AlN and Hf2CO2/GaN heterostructures have remarkable optical absorption capacity, which further shows the application prospect of the Hf2CO2/MN heterostructure. The study of this work provides theoretical guidance for the design of heterostructures for use as photocatalytic and photovoltaic devices.

High-throughput screening of single metal atom anchored on N-doped boron phosphide for N2 reduction

Developing eco-friendly and highly-efficient catalysts for the electrochemical nitrogen reduction reaction (NRR) under ambient conditions to replace the energy-intensive and environment-polluting Haber-Bosch process is of great significance, while remaining a long-standing challenge in the field of energy conversion today. Herein, through the first principles high-throughput screening, we systematically investigated the catalytic activity of a series of single metal atom immobilized on N-doped boron phosphide (N₃-BP) for N₂ reduction, denoted as MN₃-BP. In particular, a "four-step" screening strategy, involving the structural stability, N₂ chemisorption, low energy cost, as well as good selectivity, was adopted for the stringent screening of the promising MN₃-BP candidates for NRR. Our results unveil that among these candidates, MoN₃-BP eventually stands out, benefiting from its high selectivity and activity, as well as accompanying a considerably favorable limiting potential of -0.25 V for NRR. More impressively, the NRR activity origin of various candidates was revealed by the descriptor φ and ICOHP. Overall, our work not only accelerates the discovery of SACs for converting N₂ into sustainable NH₃ but also provides an exciting impetus for the rational design of NRR catalysts with high stability, high activity, and high selectivity.

Termination effects of single-atom decorated v-Mo2CTx MXene for the electrochemical nitrogen reduction reaction

The lack of the green, economical and high-efficient catalysts restrict the development of electrochemical nitrogen reduction reaction (NRR). By means of density functional theory (DFT) calculations, we have systematically investigated the NRR catalytic performance of single atoms decorated v-Mo₂CT₂ (T = O, F, OH, Cl, and Li) MXene (TM@v-Mo₂CT₂). Our calculation results reveal the introduction of single atom can significantly improve the NRR activity and selectivity on v-Mo₂CO₂, and Ir@v-Mo₂CO₂ system possesses the lowest limiting potential of only -0.33 V among all studied systems. The termination effects of TM@v-Mo₂CT₂ are further discussed and a descriptor of the adsorption energy of *NNH species (ΔE(*NNH)) is proposed to establish the relationship with NRR limiting potential (U_L(NRR)), in which a moderate (ΔE(*NNH)) is required for high NRR activity. Moreover, a good linear relationship between the ΔE(*NNH) and the excess electrons on Ir atom shows that different ΔE(*NNH) originates from the difference of valence state of Ir atom, which is due to the change of coordination environment. Importantly, the synergistic effects of Ir atom and the surface O-terminations during the first hydrogenation step lead to a promoted NRR performance. Our study might provide new possibilities for rational design of cost-effective MXene-based NRR electrocatalysts.

Boron-Doped MXenes as Electrocatalysts for Nitrogen Reduction Reaction: A Theoretical Study

Electrocatalytic nitrogen reduction reaction (NRR) is a promising and sustainable approach for ammonia production. Since boron as an active center possesses electronic structure similar to that of transition metals with d-orbitals (J. Am. Chem. Soc., 2019, 141 (7), 2884), it is supposed to be able to effectively activate the triple bond in N2. MXenes can be applied as substrates due to the large specific surface area, high conductivity, and tunable surface composition. In this work, the catalytic performance of a series of MXenes-supported single boron atom systems (labeled as B@MXenes) has been systematically studied by using density functional theory (DFT). B@Nb4C3O2, B@Ti4N3O2, and B@Ti3N2O2 were screened out owing to outstanding catalytic activity with limiting potentials of −0.26, −0.15, and −0.10 V, respectively. Further analysis shows that the unique property of boron that can intensely accept lone pair and back-donate the anti-bond of nitrogen contributes to the activation of inert triple bond. This work provides a new idea for the rational design of NRR catalyst and is of great significance for the future development of nitrogen reduction catalysts.

Electrochemical Fixation of Nitrogen by Promoting N2 Adsorption and N-N Triple Bond Cleavage on the CoS2/MoS2 Nanocomposite

An electrochemical N₂ reduction reaction (NRR), as an environmentally benign method to produce NH₃, is a suitable alternative to substitute the energy-intensive Haber-Bosch technology. Unfortunately, to date, it is obstructed by the lack of efficient electrocatalysts. Here, a CoS₂/MoS₂ nanocomposite with CoS₂ nanoparticles decorated on MoS₂ nanosheets is fabricated and adapted as a catalyst for the NRR. As unveiled by experimental and theoretical results, the strong interaction between CoS₂ and MoS₂ modulates interfacial charge distribution with electrons transferring from CoS₂ to MoS₂. Consequently, a local electrophilic region is formed near the CoS₂ side, which enables effective N₂ absorption. On the other hand, the nucleophilic area formed near the MoS₂ side is in favor of breaking stable N≡N, the potential-determining step (*N₂ → *N₂H) which brings about a much decreased energy barrier than that on pure MoS₂. As a result, this catalyst exhibits an excellent NRR performance, NH₃ yield and Faradaic efficiency of 54.7 μg·h^-1·mg^-1 and 20.8%, respectively, far better than most MoS₂-based catalysts.

Hydroxyl-Boosted Nitrogen Reduction Reaction: The Essential Role of Surface Hydrogen in Functionalized MXenes

MXenes, an emerging family of two-dimensional (2D) metal carbides and nitrides, have been demonstrated to be effective nitrogen reduction reaction (NRR) catalysts. So far, most of the theoretical studies toward NRR are based on bare MXenes; however, the structural stabilities are questionable. In this work, we studied the NRR process on several synthesized MXenes (Ti₂C, V₂C, Cr₂C, Zr₂C, Nb₂C, Mo₂C, Hf₂C, and Ta₂C) with hydroxyl (OH) termination since the structures are preferred under NRR operating conditions as per Pourbaix stability diagrams. It is found that OH plays an essential role in tuning the NRR chemistry, as a new surface-hydroxylation mechanism. Different from the widely accepted NRR mechanism where only protons are involved in the reaction, hydrogen (H) atoms from surface hydroxyl could be captured by the intermediate and participate into the NRR, while the remaining H vacancy can subsequently be self-repaired by the protons under the applied potential. The cooperative effect of surface hydroxylation can effectively boost the NRR, while Mo₂C(OH)₂ stands out with the most favorable limiting potential of -0.62 V and highest selectivity. Moreover, new scaling relationships based on the H vacancy energy are established, elucidating the possibility for structure-activity tuning. This study not only elaborates the essential role of surface OH functionalization in evaluating NRR performance but also affords new insights into advance sustainable NH₃ production.

Towards H2O catalyzed N2-fixation over TiO2 doped Run clusters (n = 5, 6): a mechanistic and kinetic approach

H2O driven N2 fixation is known as the best alternative pathway to synthesise NH3 under ambient conditions. The thermodynamic non-spontaneous reaction can be accomplished by a photocatalytic water splitting reaction over a TiO2 supported surface with oxygen vacancies. Previous experiments have also shown N2 activation over a neutral Ru cluster whose catalytic activity was remarkably enhanced by TiO2 doping. In this article, we have investigated the detailed mechanism and kinetics of the H2O catalyzed nitrogen reduction reaction (NRR) over bare and TiO2 doped Ru5 clusters in conjunction with DFT and TST calculations. The lack of photochemical activity of the small model cluster provoked us to explore an alternative route of NH3 formation via H2O catalysis. For this, we have considered H2 as co-reactant. The partial reduction of N2 into NH3 or N2H4 could be achieved by a H2O oxidation reaction, however, catalytic regeneration requires additional H2 which effectively makes the overall reaction catalyzed by H2O. Above all, the present investigation suggests that NH3 is most favorably produced through the distal mechanism. Analysis of the rate constants demonstrates that the doping with TiO2 accelerates the kinetics of NRR by a few orders of magnitude. Furthermore, an increase of the size of the metal cluster would not significantly enhance the overall performance of NRR.

Enhancing N2 Fixation Activity by Converting Ti3 C2 MXenes Nanosheets to Nanoribbons

Metal carbides M₂ C (MXenes) with two-dimensional (2D) structure have been indicated as promising materials for N₂ fixation, with the activity being related to edge planes. Here, it is instead demonstrated that the transformation from a 2D- (nanosheets) to a 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N₂ fixation activity due to the formation of exposed Ti-OH sites. A linear relationship is observed between ammonia formation rate and amount of oxygen on the surface of Ti₃ C₂ MXene.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

A DFT screening of single transition atoms supported on MoS2 as highly efficient electrocatalysts for the nitrogen reduction reaction

The development of low-cost and highly efficient materials for the electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is an attractive and challenging topic in chemistry. In this study, the electrocatalytic performance of a series of transition metal (TM) atoms supported on MoS2 nanosheets (TM@MoS2) was systematically investigated using density functional theory (DFT) calculations. It was found that Re supported on MoS2 (Re@MoS2) has the best NRR catalytic activity with a limiting potential of -0.43 V, along with high selectivity over the competing hydrogen evolution reaction (HER). Moreover, the ab initio molecular dynamics (AIMD) simulations at 500 K and density of states (DOS) calculations indicated the high thermodynamic stability and excellent electrical conductivity of Re@MoS2. A linear trend between several parameters of single atom catalysts (SACs) and the adsorption Gibbs free energy change of the NH species (ΔG*NH) was observed, indicating the later as a simple descriptor for the facilitated screening of novel SACs. These results pave the way for exploring novel, highly efficient electrocatalysts for the electrochemical NRR under ambient conditions.

Surface-defect-engineered photocatalyst for nitrogen fixation into value-added chemical feedstocks

Surface-defect-engineered photocatalyst for nitrogen fixation.