Two-dimensional π-conjugated metal-organic nanosheets as single-atom catalysts for the hydrogen evolution reaction

A feasible strategy for designing cytochrome P450-mimic sandwich-like single-atom nanozymes toward electrochemical CO2 conversion

Carbon dioxide electroreduction (CO2ER) presents a promising strategy for environmentally friendly CO2 utilization due to its low energy consumption. Single-atom nanozymes (SANs), amalgamating the benefits of single-atom catalysts and nanozymes, have become a hot topic in catalysis. Inspired by the intricate structure of cytochrome P450, we designed 81 sandwich-like SANs using Group-VIII transition metals (TMN4-S-TM'N4) and evaluated their performance in CO2ER using density functional theory (DFT). Our investigation revealed that most SANs display superior catalytic activity and improved specific product selectivity in comparison to the Cu (211) surface. Notably, IrN4-S-TMN4 (TM = Co, Rh, Pd) exhibited selective CO2 reduction to CO with remarkable limiting potentials (UL) of -0.11, -0.07, and -0.09 V, respectively, demonstrating potential as artificial CO dehydrogenases. Furthermore, RuN4-S-RuN4 exhibited formate dehydrogenase-like activity, resulting in selective production of HCOOH at a UL of -0.10 V. Machine learning analysis elucidated that the exceptional activity and selectivity of these SANs stemmed from precise modulation of electron density on sulfur atoms, achieved by varying transition metals in the subsurface. Our research not only identifies exceptional SANs for CO2ER but also provides insights into innovative methods for regulating non-bonding interactions and achieving sustainable CO2 conversion.

Ligand-Mediated Hydrogenic Defects in Two-Dimensional Electrically Conductive Metal-Organic Frameworks

Compared to dense analogues, high-surface-area metals offer several key advantages in electrocatalysis and energy storage. Of the porous manifolds, metal-organic frameworks (MOFs) boast the highest known surface area of any material class, and a subset of known frameworks also conduct electricity. The premier conductive scaffolds, Ni₃(HITP)₂ and Ni₃(HIB)₂, are both predicted to be metallic, but experiments have yet to measure bulk metallicity. In this paper, we explore the thermodynamics of hydrogen vacancies and interstitials and demonstrate that interstitial hydrogen is a plausible and prevalent defect in the conductive MOF family. The existence of this defect is predicted to render both Ni₃(HITP)₂ and Ni₃(HIB)₂ as bulk semiconductors, not metals, and emphasizes that hydrogenic defects play a critical role in determining the bulk properties of conductive MOFs.

Screening out the Transition Metal Single Atom Supported on Onion-like Carbon (OLC) for the Hydrogen Evolution Reaction

A recent experiment has confirmed that onion-like nanospheres of carbon (OLC) covered with single Pt atoms show comparable hydrogen evolution reaction (HER) catalytic activity to the commercial Pt/C. In this work, we have performed screening calculations on the single transition metal (TM) atom supported on OLC (a total of 26 candidates) using the density functional theory (DFT) to find excellent HER catalysts. Our calculated results indicate that the Nb₁/CLO, Mo₁/CLO, Ru₁/CLO, Rh₁/CLO, Pd₁/CLO, and Ir₁/OLC show high-efficient catalysts performance for the HER, as experimental Pt₁/OLC does. We also try to seek an appropriate descriptor relevant to the Gibbs free energies, and the average local ionization energy (ALIE), which is first used to predict HER activity, shows a perfect linear correlation with Gibbs free energy. It is interesting to note that the ALIE descriptor is more successful than the commonly used d-band center.

Strain-promoted conductive metal-benzenhexathiolate frameworks for overall water splitting

Designing efficient catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a desirable strategy for overall water splitting and the generation of clean and renewable energies. Herein, the electrocatalytic HER and OER activity of the conductive metal-benzenhexathiolate (M-BHT) frameworks has been evaluated utilizing first-principles calculations. The in-plane π-d conjugation of M-BHT guarantees fast electron transfer during electrocatalytic reactions. Notably, Rh-BHT holds the promise of bifunctional HER/OER activity with the overpotentials of 0.07/0.36 V. Furthermore, the application of strain engineering tailors the adsorption of intermediates and promotes the overall water splitting performance. Rh-BHT with the +1% tensile strain shows the HER/OER overpotential of 0.02/0.37 V. This work not only demonstrates the prospects of conductive metal-organic frameworks in electrocatalysis but also offers new insights into designing efficient catalysts by strain engineering.

Novel Two-Dimensional Metal-Based π-d Conjugated Nanosheets as Photocatalyst for Nitrogen Reduction Reaction: The First-Principle Investigation

Photocatalytic nitrogen reduction reaction (NRR) is becoming a promising route for producing green and sustainable ammonia under ambient conditions. However, the development of highly efficient photocatalysts for NRR still remains a grand challenge as a result of the sluggish activation of inert N₂, the competitive hydrogen evolution reaction (HER), and inadequate photogenerated external potential, which usually cause extremely poor NRR performance and low light utilization efficiency. Herein, on the basis of density functional theory (DFT) computations, we rationally designed a series of two-dimensional (2D) π-d conjugated metal-B₃N₃ (M₃B₆N₆S₁₂) semiconductors. Using the electron "σ acceptance-π* backdonation" process, Mo₃B₆N₆S₁₂ and Nb₃B₆N₆S₁₂ can efficiently activate and reduce N₂ to ammonia with a rather low overpotential of 0.07 and 0.21 V, respectively. Importantly, a high photogenerated external potential enables spontaneous NRR on Mo₃B₆N₆S₁₂ and Nb₃B₆N₆S₁₂ under visible/infrared light irradiation, contributing to the extraordinary photoactivity of Mo₃B₆N₆S₁₂ and Nb₃B₆N₆S₁₂ as promising solar light-driven N₂ fixation catalysts. Meanwhile, the competing HER is effectively restrained. For the first time, we rationally propose a series of M₃B₆N₆S₁₂ semiconductors as promising metal-based photocatalysts for N₂ reduction with extraordinary photoactivity and a high photogenerated external potential. This work paves a new path for the rational designing of 2D metal-based NRR photocatalysts with high activity, good selectivity, and high stability.

Two-Dimensional Conductive Metal-Organic Frameworks as Highly Efficient Electrocatalysts for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LiSBs) which are expected to fulfill the increasing demands of high-density energy storage have been under intensive investigation. However, the development of LiSBs is facing many obstacles, such as the poor electronic conductivity of sulfur, shuttling effects of lithium polysulfides (LiPSs), sluggish Li₂S decomposition, and low discharging/charging efficiency. Suitable electrocatalysts that can solve the above problems are promising in the development of LiSBs. Herein, 13 two-dimensional (2D) metal-organic frameworks (MOFs) of nitrogen-, sulfur-, and oxygen-coordinated transition-metal (TM) atoms (Co, Ni, Cu, and Zn) are selected and constructed to reveal the structure-activity relationship of 2D MOFs in terms of the electrocatalytic performance. Among all the 2D MOFs investigated, Cu₃(HITP)₂, Zn₃(HITP)₂, and Cu₃(C₁₈H₉O₃N₃)₂ offer moderate binding strength to LiPSs, which effectively suppresses Li₂S_n dissolution and shuttling. Cu₃(HITP)₂ exhibits good electrical conductivity, a low Gibbs free energy barrier, effective electrocatalytic ability for Li₂S decomposition, and a high sulfur loading amount. A descriptor φ is proposed to correlate the binding energies of the 2D MOFs to the coordination environment and the electronegativity of the TM atoms in the LiPSs via an implicit volcano plot. These findings are helpful for understanding the electrocatalytic effect of 2D MOFs in LiSBs and represent a promising approach for the development of future LiSBs.

Single-Atom Catalysts: A Review of Synthesis Strategies and Their Potential for Biofuel Production

Biofuels have been derived from various feedstocks by using thermochemical or biochemical procedures. In order to synthesise liquid and gas biofuel efficiently, single-atom catalysts (SACs) and single-atom alloys (SAAs) have been used in the reaction to promote it. SACs are made up of single metal atoms that are anchored or confined to a suitable support to keep them stable, while SAAs are materials generated by bi- and multi-metallic complexes, where one of these metals is atomically distributed in such a material. The structure of SACs and SAAs influences their catalytic performance. The challenge to practically using SACs in biofuel production is to design SACs and SAAs that are stable and able to operate efficiently during reaction. Hence, the present study reviews the system and configuration of SACs and SAAs, stabilisation strategies such as mutual metal support interaction and geometric coordination, and the synthesis strategies. This paper aims to provide useful and informative knowledge about the current synthesis strategies of SACs and SAAs for future development in the field of biofuel production.

Design strategies of two-dimensional metal-organic frameworks toward efficient electrocatalysts for N2 reduction: cooperativity of transition metals and organic linkers

Two-dimensional (2D) metal-organic frameworks (MOFs) serve as emerging electrocatalysts due to their high conductivity, chemical tunability, and accessibility of active sites. We herein proposed a series of 2D MOFs with different metal atoms and organic linkers with the formula M₃C₁₂X₁₂ (M = Cr, Mo, and W; X = NH, O, S, and Se) to design efficient nitrogen reduction reaction (NRR) electrocatalysts. Our theoretical calculations showed that metal atoms in M₃C₁₂X₁₂ can efficiently capture and activate N₂ molecules. Among these candidates, W₃C₁₂X₁₂ (X = O, S, and Se) show the best NRR performance due to their high activity and selectivity as well as low limiting potential (-0.59 V, -0.14 V, and -0.01 V, respectively). Moreover, we proposed a d-band center descriptor strategy to screen out the high activity and selectivity of M₃C₁₂X₁₂ for the NRR. Therefore, our work not only demonstrates a class of promising electrocatalysts for the NRR but also provides a strategy for further predicting the catalytic activity of 2D MOFs.

Single-Atom (Iron-Based) Catalysts: Synthesis and Applications

Supported single-metal atom catalysts (SACs) are constituted of isolated active metal centers, which are heterogenized on inert supports such as graphene, porous carbon, and metal oxides. Their thermal stability, electronic properties, and catalytic activities can be controlled via interactions between the single-metal atom center and neighboring heteroatoms such as nitrogen, oxygen, and sulfur. Due to the atomic dispersion of the active catalytic centers, the amount of metal required for catalysis can be decreased, thus offering new possibilities to control the selectivity of a given transformation as well as to improve catalyst turnover frequencies and turnover numbers. This review aims to comprehensively summarize the synthesis of Fe-SACs with a focus on anchoring single atoms (SA) on carbon/graphene supports. The characterization of these advanced materials using various spectroscopic techniques and their applications in diverse research areas are described. When applicable, mechanistic investigations conducted to understand the specific behavior of Fe-SACs-based catalysts are highlighted, including the use of theoretical models.

Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal-organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Highly efficient catalytic properties of Sc and Fe single atoms stabilized on a honeycomb borophene/Al(111) heterostructure via a dual charge transfer effect

Theoretical design and experimental fabrication of highly efficient single-atom catalysts (SACs) containing isolated metal atoms monodispersed on appropriate substrates have surged to the forefront of heterogeneous catalysis in recent years. Nevertheless, the instability of SACs, i.e., preferential clustering in chemical reaction processes, dramatically hinders their practical applications. In this paper, using first-principles calculations, we predict that a honeycomb borophene/Al(111) heterostructure can be an ideal candidate to stabilize and enhance the catalysis of many transition metal (TM) SACs via a dual charge transfer mechanism. The Al(111) substrate donates electrons to the pre-covered two-dimensional honeycomb borophene (h-B) to stabilize the latter, and the deposited TM atoms further provide electrons to the h-B, enhancing the covalent binding between the h-B and the Al(111) substrate. Intriguingly, during CO oxidation, the h-B/Al(111) heterostructure can in turn serve as an efficient electron reservoir to accept electrons from or donate electrons to the deposited TM-SACs and the reactants. Such a flexible dual charge transfer mechanism not only facilitates stabilizing the TM-SACs rather than clustering, but also effectively reduces the reaction barriers. Particularly, in contrast to expensive noble metal atoms such as Pd and Pt, low-cost Sc- and Fe-SACs are found to be the most promising SAC candidates that can be stabilized on h-B/Al(111) for O₂ activation and CO oxidation, with fairly low reaction barriers (around 0.50-0.65 eV). The present findings may provide important theoretical guidance for the experimental fabrication of highly stable, efficient, and economic SACs stabilized on various heterostructure substrates.

Single-Atom and Dual-Atom Electrocatalysts Derived from Metal Organic Frameworks: Current Progress and Perspectives

Single-atom catalysts (SACs) have attracted increasing research interests owing to their unique electronic structures, quantum size effects and maximum utilization rate of atoms. Metal organic frameworks (MOFs) are good candidates to prepare SACs owing to the atomically dispersed metal nodes in MOFs and abundant N and C species to stabilize the single atoms. In addition, the distance of adjacent metal atoms can be turned by adjusting the size of ligands and adding volatile metal centers to promote the formation of isolated metal atoms. Moreover, the diverse metal centers in MOFs can promote the preparation of dual-atom catalysts (DACs) to improve the metal loading and optimize the electronic structures of the catalysts. The applications of MOFs derived SACs and DACs for electrocatalysis, including oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, carbon dioxide reduction reaction and nitrogen reduction reaction are systematically summarized in this Review. The corresponding synthesis strategies, atomic structures and electrocatalytic performances of the catalysts are discussed to provide a deep understanding of MOFs-based atomic electrocatalysts. The catalytic mechanisms of the catalysts are presented, and the crucial challenges and perspectives are proposed to promote further design and applications of atomic electrocatalysts.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Single-Layer Cu2WS4 with Promising Electrocatalytic Activity toward Hydrogen Evolution Reaction

Two-dimensional hydrogen evolution reaction (HER) electrocatalysts are outstanding alternatives to high-cost Pt due to their good material properties. However, the few existing two-dimensional HER electrocatalysts have shortcomings that restrict their performance. Here, we report a first-principles study of single-layer A₂BS₄ (A = Ag, Cu; B= Mo, W) as HER electrocatalysts and identify single-layer Cu₂WS₄ as a promising candidate. Single-layer A₂BS₄ is found to be chemically, dynamically, and thermally stable. They require only a small energetic cost to be created from their layered bulks, suggesting the possibility of their exfoliation in experiments. Most importantly, without significant density of vacancies and in the absence of large applied strain, the basal plane of single-layer Cu₂WS₄ shows excellent electrocatalytic activity toward HER. Such an activity is attributed to the introduced in-gap states and d band center shifting upon adsorbing hydrogen. These characteristics suggest that single-layer Cu₂WS₄ is an extraordinary two-dimensional HER electrocatalyst.

2D Metal-Organic Frameworks as Multifunctional Materials in Heterogeneous Catalysis and Electro/Photocatalysis

Metal-organic frameworks (MOFs) are composed of particles with 3D geometry and are currently among the most widely studied heterogeneous catalysts. To further increase their activity, one of the recent trends is to develop related 2D materials with a high aspect ratio derived from a large lateral size and a small thickness. Here, the use of these 2D MOFs as catalysts, electrocatalysts, and photocatalysts is summarized, illustrating the advantages of these 2D materials compared to analogous 3D MOFs. The state of the art is summarized in tables and, when possible, pertinent turnover number (TON) and frequency (TOF) values. This enhanced activity of 2D MOFs derives from the accessibility of the active sites, the presence of a higher density of defects, and exchangeable coordination positions around the MOFs, as well as from their ability to form thin films on electrodes or surfaces. The importance of providing convincing evidence of the stability of 2D MOFs under reaction conditions and general characterization data of the used 2D material after catalysis is highlighted. In the last part, views regarding challenges in the field and new developments that can be expected are presented.

Unexpected monoatomic catalytic-host synergetic OER/ORR by graphitic carbon nitride: density functional theory

Although single metal atoms (SMAs) have been extensively investigated as unique active sites in single-atom catalysts, the possible active sites of the host catalysts have been unfortunately neglected in previous studies. In single-atom catalysts, the SMAs can promote the chemical and catalytic activities of host atoms, which may act as secondary active sites, resulting in a significant synergistic effect on the catalytic performance. Using density functional theory calculations, we studied the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) on two different types of active sites: single metal (M1) atoms and the neighboring host atoms of several M1/g-C3N4 samples. The contributions of M1 and host atoms towards the reduction of the OER/ORR overpotentials of Fe1, Co1, Ni1, Cu1 and Zn1/g-C3N4, bifunctional electrocatalysts with the OER/ORR overpotentials of 0.50-0.70 V were investigated. Finally, new M1/g-C3N4 catalysts with high OER/ORR performances could be estimated based on the d-band centre of the M1 atoms in the future.