A theoretical study of electrocatalytic ammonia synthesis on single metal atom/MXene

Adsorption and Potential CO Gas-Sensing Performance of the Fe-Doped Ti2CO2-MXenes: A First-Principles Study

The adsorption behaviors and electronic properties of five gas molecules (CO, H2O, NH3, NO, and C2H6O) on the intrinsic Ti2CO2 and Fe-doped Ti2CO2 were calculated and studied based on first principles. The adsorption height, bond length change, adsorption energy, charge transfer, band structure, differential charge, work function, and recovery time of the two gas adsorption systems were discussed, and their sensing performance was evaluated. The results show that the CO gas molecules have the best adsorption energy and charge transfer on Ti2CO2 modified by the Fe atom (Ti2CO2-Fe). The electrical conductivity obviously increases with the decrease of the band gap, which changes from semiconductor to conductor behavior. The reduction of the work function in the Ti2CO2-Fe system weakens the binding of the electron, which improves the electron flow between the substrate and the gas molecules. In addition, the Ti2CO2-Fe system with H2O molecule participation remained the best adsorption effect on CO gas, and the fast recovery time was 625 s at 398 K. Therefore, Ti2CO2-Fe is a prospective material for the advancement of CO gas-sensitive sensors.

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.

Designing M13 Bacteriophage and Fe-Nanonest Self-Assembly System for Universal and Facile Preparation of Metal Single Atoms as Stable Mimicking Enzymes

Metal single-atom catalysts (MSACs) possess multiple advantages in chemical synthesis; their efficient fabrication routes, however, remain a challenge to date. Here, an interdisciplinary design using M13 bacteriophage virus as a biotemplate to carry Fe nanoclusters, which we figuratively call "Fe-nanonests", is proposed to enable facile and versatile synthesis of MSACs. The feasibility and generality of this self-assembly method was demonstrated by the observation of six different metal single atoms (MSAs) including Ag, Pt, Pd, Zn, Cu, and Ni. With Pd as a representative, key factors dominating the fabrication were determined. The Pd single atoms exhibited excellent horseradish peroxidase (HRP)-like activity, which was further improved by 50% via genetic editing of the M13 pVIII protein terminals. Excellent stability was also observed in the quantification of acid phosphatase, a cancer predictor. X-ray absorption near-edge structure spectroscopy has been applied to the analysis of Pd single atoms as well, and the Pd-N4 coordination explained the mechanism of high HRP-like catalytic activity. The MSAs synthesized by the M13 phage and Fe-nanonest self-assembly method show promising prospects in non-cold-chain medical detection applications.

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.

A Theoretical Study of Fe Adsorbed on Pure and Nonmetal (N, F, P, S, Cl)-Doped Ti3C2O2 for Electrocatalytic Nitrogen Reduction

The possibility of using transition metal (TM)/MXene as a catalyst for the nitrogen reduction reaction (NRR) was studied by density functional theory, in which TM is an Fe atom, and MXene is pure Ti₃C₂O₂ or Ti₃C₂O_2−x doped with N/F/P/S/Cl. The adsorption energy and Gibbs free energy were calculated to describe the limiting potentials of N₂ activation and reduction, respectively. N₂ activation was spontaneous, and the reduction potential-limiting step may be the hydrogenation of N₂ to *NNH and the desorption of *NH₃ to NH₃. The charge transfer of the adsorbed Fe atoms to N₂ molecules weakened the interaction of N≡N, which indicates that Fe/MXene is a potential catalytic material for the NRR. In particular, doping with nonmetals F and S reduced the limiting potential of the two potential-limiting steps in the reduction reaction, compared with the undoped pure structure. Thus, Fe/MXenes doped with these nonmetals are the best candidates among these structures.

Recent Status and Challenges in Multifunctional Electrocatalysis Based on 2D MXenes

Due to their chemical and electrical characteristics, such as metallic conductivity, redox-activity in transition metals, high hydrophilicity, and adjustable surface properties, MXenes are emerging as important contributors to oxygen reduction...

Symbolic Transformer Accelerating Machine Learning Screening of Hydrogen and Deuterium Evolution Reaction Catalysts in MA2Z4 Materials

Two-dimensional (2D) materials have been developed into various catalysts with high performance, but employing them for developing highly stable and active nonprecious hydrogen evolution reaction (HER) catalysts still encounters many challenges. To this end, the machine learning (ML) screening of HER catalysts is accelerated by using genetic programming (GP) of symbolic transformers for various typical 2D MA₂Z₄ materials. The values of the Gibbs free energy of hydrogen adsorption (ΔG_H*) are accurately and rapidly predicted via extreme gradient boosting regression by using only simple GP-processed elemental features, with a low predictive root-mean-square error of 0.14 eV. With the analysis of ML and density functional theory (DFT) methods, it is found that various electronic structural properties of metal atoms and the p-band center of surface atoms play a crucial role in regulating the HER performance. Based on these findings, NbSi₂N₄ and VSi₂N₄ are discovered to be active catalysts with thermodynamical and dynamical stability as ΔG_H* approaches to zero (-0.041 and 0.024 eV). In addition, DFT calculations reveal that these catalysts also exhibit good deuterium evolution reaction (DER) performance. Overall, a multistep workflow is developed through ML models combined with DFT calculations for efficiently screening the potential HER and DER catalysts from 2D materials with the same crystal prototype, which is believed to have significant contribution to catalyst design and fabrication.

MXenes as Superexcellent Support for Confining Single Atom: Properties, Synthesis, and Electrocatalytic Applications

Single atom catalysts (SACs) have shown their noticeable potential and gradually become a new favorite in catalytic field due to the particular selectivity, high catalytic performance, and strong durability. The most important factor in the synthesis of SACs is the selection of appropriate support and formation of metal-support interaction. Among a large number of nanomaterials, MXenes can be utilized as benign supports for fixing SACs because of their expansive specific surface area, regulable bandgap, superior electronic conductivity, and strong mechanical stability. The structure and property of MXenes can be manipulated by changing transition metal elements and surface termination. Here, the uniqueness and superiority of MXenes as superexcellent supports for confining SACs are analyzed from structure and property. The synthetic strategy of MXene-supported SACs is also summarized, especially emphasizing the immobilization of isolated atom against aggregation by utilizing the formidable metal-support covalent coordination interaction. In addition, the applications of MXene-supported SACs in electrocatalytic field are highlighted, including hydrogen evolution reaction, oxygen evolution reaction, overall water splitting, oxygen reduction reaction, and nitrogen reduction reaction. Finally, the challenges and prospects are pointed out for the further understanding and practical application of MXene-supported SACs in electrocatalysis.

Ammonia Synthesis Using Single-Atom Catalysts Based on Two-Dimensional Organometallic Metal Phthalocyanine Monolayers under Ambient Conditions

We have identified three novel metal phthalocyanine (MPc, M = Mo, Re, and Tc) single-atom catalyst candidates with excellent predicted performance for the production of ammonia from electrocatalytic nitrogen reduction reaction (NRR) through a combination of high-throughput screening and first-principles calculations on a series of 3d, 4d, and 5d transition metals anchored onto extended Pc monolayer catalysts. Analysis of the energy band structures and projected density of states of N₂-MPc revealed significant orbital hybridization and charge transfer between the adsorbed N₂ and catalyst MPc, which accounts for the high catalytic activity. Among 30 MPc catalysts, MoPc and TcPc monolayers were found to be the most promising new NRR catalysts, as they exhibit excellent stability, low onset potential, and high selectivity. A comprehensive reaction pathway search found that the maximum free energy changes for the MoPc and TcPc monolayers are 0.33 and 0.54 eV, respectively. As a distinctive nature of this work, the hybrid reaction pathway was considered extensively and searched systematically. The onset potential of the hybrid pathway is found to be smaller than or comparable to that of the commonly known pure pathway. Thus, the hybrid path is highly competitive with low onset potential and high activity. The hybrid pathway is expected to have an important impact on future research on the mechanism of NRR, and it will open up a new way to explore the mechanism of the NRR reaction. We hope that our work will provide impetus to the creation of new catalysts for reduction of N₂ to NH₃. This work provides new insights into the rational design of NRR catalysts and explores novel reaction pathways under ambient or mild conditions.

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 systematic investigation of the catalytic performances of monolayer carbon nitride nanosheets C1-xNx

Graphitic carbon nitrides (CNs) are potential candidate materials for the electro-catalytic industry due to their unique physical and chemical properties. However, to date, a full understanding of the electro-catalytic properties of CNs is still lacking. Herein, by using density functional theory calculations, we systematically investigate the catalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), N2 reduction reaction (NRR), and CO2 reduction reaction (CO2RR) of monolayer graphitic carbon nitrides (C1-xNx), C3N (x = 1/4), C2N (x = 1/3), and g-C3N4 (x = 4/7). We also evaluated the NRR activity of B doped C1-xNx, and the CO2RR activity of Cu and Pd modified C1-xNx. The cohesive energy and ab initio molecular dynamics (AIMD) results show that C3N, C2N, and g-C3N4 are stable at room temperature. The C3N-C1 site is predicted to deliver the best HER catalytic performance with a reaction Gibbs free energy (ΔGH*) of -0.03 eV (close to the ideal value (0 eV)). Among the studied C1-xNx materials, the C3N-C2 site is predicted to possess a favorable ηOER of 0.82 V for OER. Pure C3N, C2N, and g-C3N4 are not suitable for NRR and CO2RR. Due to the strong hybridization between the N 2p orbital and the B 2p orbital, the NRR performances of B doped BN-C2N, BN-C3N, and BN-g-C3N4 are greatly enhanced, with corresponding overpotential ηNRR of 0.57 V, 0.70 V, and 0.72 V, respectively. The transition metals Cu and Pd can enhance the CO2RR activity of C3N, C2N, and g-C3N4. The limiting potentials UL of pure C3N, C2N, and g-C3N4 are 0.96 V, 0.86 V, and 2.37 V, respectively, while these values are 0.63 V, 0.68 V, and 0.77 V with Cu or Pd modification. This work provides deep understanding of the catalytic properties of monolayer C1-xNx and guidance for synthesizing higher activity catalysts in the future.

Single molybdenum atom anchored on 2D Ti2NO2 MXene as a promising electrocatalyst for N2 fixation

The electrocatalytic synthesis of ammonia (NH3) at ambient temperature is an attractive and challenging subject in the chemical industry. The synthesis of NH3 under ambient conditions requires efficient and stable electrocatalysts with ultralow overpotential to ensure low energy consumption and high NH3 yield. Herein, electrocatalysts consisting of a single transition metal (TM) atom (TM = Mo, Mn, Fe, Co, Ni, or Cu) anchored on 2D M2NO2 MXene (M = Ti, V, and Cr), designated as TM/M2NO2, are designed for N2 reduction reaction (NRR) by density functional theory calculations. The results show that the bonding strength between Mo and Ti2NO2 is strong. The overpotential (ηNRR) of Mo/Ti2NO2 surface-catalyzed NRR is estimated to be as low as 0.16 V via an enzymatic mechanism, which is lower than those reported to date. For Mo/V2NO2 and Mo/Cr2NO2 catalysts, the NRR occurs through the consecutive mechanism and enzymatic mechanism, with corresponding ηNRR values of 0.38 V and 0.22 V, respectively. In addition, the reaction Gibbs free energy of NH3 desorption from the Mo/Ti2NO2 surface is only 0.12 eV. Electronic structure analysis indicates that Mo/Ti2NO2 shows metallic characteristics, which ensures the efficient transfer of electrons between Mo and Ti2NO2. Ab initio molecular dynamics simulations indicate that the Mo atom can be stably immobilized on the Ti2NO2 substrate to prevent its aggregation into Mo clusters. Further analysis illustrates that hydrogen adsorption is not favored on the Mo/Ti2NO2 surface. Mixing the N2 source with extra gases, such as NO2, NO, SO2, SO, and O2, should be avoided for NRR on Mo/Ti2NO2 surface. These predictions offer a new opportunity for the electrocatalytic synthesis of NH3 by N2 reduction in the future.

Single and double boron atoms doped nanoporous C2N-h2D electrocatalysts for highly efficient N2 reduction reaction: a density functional theory study

The electrocatalytical process is the most efficient way to produce ammonia (NH₃) under ambient conditions, but developing a highly efficient and low-cost metal-free electrocatalysts remains a major scientific challenge. Hence, single atom and double boron (B) atoms doped 2D graphene-like carbon nitride (C₂N-h2D) electrocatalysts have been designed (B@C₂N and B₂@C₂N), and the efficiency of N₂ reduction reaction (NRR) is examined by density functional theory calculation. The results show that the single and double B atoms can both be strongly embedded in natural nanoporous C₂N with superior catalytic activity for N₂ activation. The reaction mechanisms of NRR on the B@C₂N and B₂@C₂N are both following an enzymatic pathway, and B₂@C₂N is a more efficient electrocatalyst with extremely low overpotential of 0.19 eV comparing to B@C₂N (0.29 eV). In the low energy region, the hydrogenation of N₂ is thermodynamically more favorable than the hydrogen production, thereby improving the selectivity for NRR. Based on these results, a new double-atom strategy may help guiding the experimental synthesis of highly efficient NRR electrocatalysts.