Transition Metal Single-atom Catalysts Research Articles

Bifunctional electrocatalytic activity of Fe-embedded biphenylene for oxygen reduction and evolution reactions.

Transition metal single-atom catalysts have attracted great attention because of their great potential applications in the chemical industry. Except for graphene, there are few single-layer materials that can act as substrates to support the dispersive metal atoms. Recently, a biphenylene layer, a new two-dimensional allotrope of graphene, was synthesized in experiments, providing a new substrate layer to fabricate single-atom catalysts (SACs). In this work, we predict three transition metal SACs MN4-biphenylene (M = Fe, Co, and Ni) based on first-principles calculations. The results indicate that FeN4-biphenylene is a promising bifunctional catalyst with low overpotentials for both the oxygen reduction reaction and oxygen evolution reaction, ηORR = 0.11 V and ηOER = 0.27 V. The high catalytic activities are explained by the position of the d-band center of the Fe atom in the biphenylene network and the strength of interaction between FeN4-biphenylene and the reaction intermediates.

Theoretical investigations of electrochemical nitrogen reduction on single transition metal atom catalysts supported by 1T-MoSe2

The search for stable and efficient single-atom catalysts (SACs) for the electrocatalytic nitrogen reduction reaction (eNRR) has garnered significant theoretical interest. In this study, we evaluate the eNRR performance of eighteen two-dimensional 1T-MoSe2-supported transition metal single-atom catalysts (TM@1T-MoSe2, TM=V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Tc, Ru, Rh, Pd, Ta, W, Re, Os, Ir, and Pt) using density functional theory calculations. We assess the stability of each TM@1T-MoSe2, as well as the limiting potential of eNRR and the ammonia selectivity on each stable TM@1T-MoSe2. Our results show that W@1T-MoSe2 is the most promising single-atom catalyst for eNRR, with a limiting potential of -0.23 V via the distal pathway starting from three co-adsorbed nitrogen molecules. Furthermore, the multi-adsorption of N2 on W@1T-MoSe2 effectively suppress the hydrogen evolution reaction, thus enhancing the selectivity of the eNRR. This research provides a promising avenue for the development of a new class of 1T-MoSe2-based single-atom catalysts for electrocatalytic ammonia synthesis.

Exploration of single Fe atom supported on anatase TiO2(001) for methane oxidation: A DFT study

Partial oxidation of methane is a promising alternative strategy for methanol production under mild reaction conditions; however, significant challenges hinder the development of appropriate catalysts. In this study, based on first-principles calculations, we demonstrate that a single Fe atom supported on anatase TiO 2 (001) provides double active sites (Fe and Ti 5C ) to activate gas-phase O 2 and form O-assisted intermediates. The triple state Fe-O/TiO 2 (001) system exhibited better activity for methane activation (Δ G max = 1.02 eV). Our findings offer new insights into the design of non-noble-3 d transition metal single-atom catalysts on TiO 2 (001) for partial methane oxidation via an inexpensive O 2 oxidant under mild reaction conditions.

Original exploration of transition metal single-atom catalysts for NOxreduction

Various atomically dispersed transition metal-coordinated graphite carbon materials are, for the first time, used for NOxreduction. Proper K-doping promotes NOxconversion and N2selectivity.

Boosting photo-assisted efficient electrochemical CO2 reduction reaction on transition metal single-atom catalysts supported on the C6N6 nanosheet.

CO2 reduction to value-added chemicals turns out to be a promising and efficient approach to resolve the increasing energy crisis and global warming. However, the catalytic efficiency of CO2 reduction reaction (CO2RR) to form C1 products (CO, HCOOH, CH3OH, CH4) needs to be quite efficient. Herein with the help of density functional theory, CO2RR towards C1 products was investigated on a transition metal (TM = Fe, Co, Ni) embedded C6N6 framework. The stable geometry of the catalysts, CO2 adsorption configurations, and CO2RR mechanisms were systematically studied for all the systems considered. The possible different adsorption configurations and adsorption energy calculations indicated that CO2 could be chemically adsorbed on the Co@C6N6 system. On the other hand, physical adsorption of CO2 is more preferable on Fe@C6N6 and Ni@C6N6 systems. As a competitive reaction, hydrogen evolution reaction (HER) was investigated and the systems were found to show more selectivity for CO2RR than for HER. OCHO formation turned out to be more favorable than COOH formation as initial protonation intermediates for CO2RR on the TM@C6N6 systems. The present work demonstrates that the Co@C6N6 catalyst can favor the electrocatalytic CO2RR among all systems. In addition, the photocatalytic activity of the systems was also investigated. The systems are found to be active for photoreduction of CO2 to CH3OH and CH4 in the presence of reducing agents such as H2 and H2O as they possess appropriate absorption spectrum in the visible region as well as suitable band edge positions. These findings open a way for designing single atom catalysts for important catalytic reactions.

First principles screening of transition metal single-atom catalysts for nitrogen reduction reaction

The discovery of metals as catalytic centers for nitrogen reduction reactions (NRR) has stimulated great enthusiasm for single-atom catalysts. However, the poor activity and low selectivity of available single-atom catalysts (SACs) are far away from the industrial requirement. Through the first principles screening, the doping engineering can effectively regulate the NRR performance of β-Sb monolayer. Especially, the origin of activated N2 is discovered from the perspective of the electronic structure of the active center. Among the 24 transition metal dopants, Re@Sb showed the best NRR catalytic performance with a low limiting potential. The Re@Sb also could significantly inhibit hydrogen evolution reaction (HER) and achieve a high theoretical Faradaic efficiency of 100%. Our findings not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.

Effect of Nitrogen Ligand Type on 3d Orbital Level Rearrangement of Cobalt Single Atom Catalysts

Transition metal single atom catalysts have recently emerged in acid oxygen evolution reactions (OER) due to their maximum atomic efficiency and high durability. Herein, we report a 3d orbital level rearrangement in square planar symmetry according to the ligand type of a cobalt single atom catalyst. We fabricate cobalt single atom catalyst supported on pyrrole type nitrogen doped crumpled graphene (Pyrrolic CoN4-CG) for acidic OER and it shows orbital rearrangement phenomenon because of their longer Co-N bond distance than pyridine type CoN4 sites. When pyrrole type nitrogen is introduced as a ligand of a cobalt single atom catalyst, the degree of oxidation during OER reaction is much greater than when pyridine type nitrogen ligand is introduced, which is confirmed by Operando X-ray absorption spectroscopy measurements. In addition, the reduction of OH- adsorption energy according to the orbital level rearrangement of Pyrrolic CoN4 and the change in the rate determination step are revealed by density functional theory calculations.

Theoretical Study on Nitrobenzene Hydrogenation by N-Doped Carbon-Supported Late Transition Metal Single-Atom Catalysts

A density functional theory (DFT) study was performed to investigate the activities of 12 late transition metal single-atom-supported nitrogen-doped carbon-based catalysts (M1-N4/C SACs) nitrobenzene hydrogenation. Three different dihydrogen (H2) dissociation mechanisms are proposed: homolytic cleavage at the metal (M) site, heterolytic cleavage at M–N sites, and heterolytic cleavage at M–N–C sites. The coadsorption of nitrobenzene (PhNO2) and H2 was proposed, and the effect of charge change of hydrogen atoms caused by the coverage of PhNO2 on the H2 dissociation activity was revealed. Six M1–N4/C SACs (M = Ru, Os, Fe, Ag, Ir, Rh) with a potentially high activity of H2 dissociation were screened. Then, the two crucial steps, H2 dissociation and the second N–O bond breaking (PhNOH* + H* → PhN* + H2O), were compared. The obtained results indicate that different M1–N4/C SACs might have different rate-determining steps for nitrobenzene hydrogenation and that Ru1–N4/C SAC could be one of the most promising catalysts with good catalytic activities for nitrobenzene hydrogenation.

Recognition of water-dissociation effect toward lattice oxygen activation on single-atom Co catalyst in toluene oxidation

The presence of water can promote the catalytic activity of some heterogeneous catalysts by oxygen activation, but the mechanism remains unclear especially for transition metal single-atom catalysts (SACs), which is further complicated due to the difficult identification of active sites. In this study, we prepared single-atom Co supported on MnO2 and investigated the promotion mechanism of oxidation reaction with the existence of water. According to the results of various characteristic experiments and density functional theory (DFT) calculations, it was found that the single-atom Co forms the Co−O−Mn entity as the actual active site on Co1/MnO2 and the dissociation of water on the active site facilitates the activation of lattice oxygen of support, which are responsible for the activity promotion. The unique water effect of Co1/MnO2 was further confirmed by comparing the performance of CoOx-nanoparticles catalyst (CoOx/MnO2). This work provides new insight into the lattice oxygen activation mechanism of transition metal SACs.

Theoretical Evaluation of Electrochemical Nitrate Reduction Reaction on Graphdiyne-Supported Transition Metal Single-Atom Catalysts.

The electrochemical reaction can be applied as a powerful method to eliminate the pollution of nitrate (NO3–) and as a feasible synthesis to enable the conversion of nitrate into ammonia (NH3) at room temperature. Herein, density functional theory calculations are applied to comprehensively analyze the electrochemical nitrate reduction reaction (NO3RR) on graphdiyne-supported transition metal single-atom catalysts (TM@GDY SACs) for the first time. It can be found that the vanadium-anchored graphdiyne (V@GDY) displays the lowest limiting potential of −0.63 V versus a reversible hydrogen electrode among the investigated systems in this work. Notably, the competing hydrogen evolution reaction is relatively restrained due to the comparatively weak adsorption of the H proton on the TM@GDY SACs. Moreover, higher energy intake is needed to overcome the energy barrier during the formation of byproducts (NO2, NO, N2O, and N2) on V@GDY without applying extra electrode potential, showing the selectivity of NH3 in the NO3RR process. The ab initio molecular dynamics simulation denotes that the V@GDY possesses excellent structure stability at the temperature of 600 K without much distortion, compared with the initial shape, indicating the promise for synthesis. This study not only offers a feasible NO3RR electrocatalyst but also paves the way for the development of the NO3RR process.

Computational study of transition metal single-atom catalysts supported on nitrogenated carbon nanotubes for electrocatalytic nitrogen reduction

Computational study of transition metal single-atom catalysts supported on nitrogenated carbon nanotubes for electrocatalytic nitrogen reduction

Screening of transition metal single-atom catalysts doped on γ-graphyne-like BN sheet for efficient nitrogen reduction reaction

Ammonia is an important industrial raw material. To promote the production of ammonia, it is urgent to develop efficient catalysts for nitrogen reduction reactions (NRR). Here, we have reported a novel electrocatalyst: γ -graphyne-like BN sheet-supported single metal atom (M/ γ BN, M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ru) for NRR and studied the effect of Hubbard U correction in single-atom catalysts. The results of adsorption show that Ti/ γ BN, V/ γ BN, and Ru/ γ BN have the best adsorption energy for nitrogen. A detailed analysis of the NRR mechanism indicates that V/ γ BN has the lowest energy barrier in the rate-determining step when it follows a distal mechanism. Further analysis shows that the superior catalytic performance in V/ γ BN sheet is mainly attributed to the electron donation and back-donation mechanism. More interestingly, V/ γ BN greatly inhibited HER selectivity. By analyzing the doping structure and adsorption system, it can be found that when considering Hubbard U correction, there will be an obvious correlation between energy and distance. This study not only provides a basis for understanding the mechanism of nitrogen reduction reaction catalyzed by single-atom catalysts but also provides a new design idea for the rational design of high-efficiency NRR catalysts. • A novel BN based single-atom catalyst, Metal/γ-graphyne-like BN as an NRR electrocatalyst. • V/γBN follows the distal mechanism with low overpotential (0.190 V) and barrier (0.412 eV). • Excellent NRR catalytic activity comes from electron donation and back - donation mechanism.

Orbital Dependence in Single-Atom Electrocatalytic Reactions.

Transition metal single-atom catalysts supported on N-doped graphene (TM-N-C) could serve as an ideal model for studying orbital dependence in electrocatalytic reactions because the atom on the catalytic active site has discrete single-atom-like orbitals. In this work, the catalytic efficiency of Fe-N-C for the oxygen evolution reaction (OER) under a small structural perturbation has been comprehensively investigated with density functional theory calculations. The results suggest that the subtle local environment of a single atom can significantly modulate the catalytic reactivity. Further analysis demonstrates that the energy level of the TM dz2 orbital center, rather than the d-band center, is responsible for the OER catalytic efficiency as the dz2 orbital participates mainly in the reactions. Essentially, the d-band theory can be extended to the sub-d orbital level, and a small perturbation of the crystal field, induced by lattice strain or z-direction displacement of the TM atom, can prominently change the sub-d orbital associated with the reaction and in turn affect the catalytic activity.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution.

Transition metal single‐atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn‐containing zeolitic imidazolate framework. Aberration‐corrected environmental TEM coupled with in‐situ EELS is used for direct visualization of the evolution process at 200–1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co‐single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation‐driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high‐temperature processes.

A clicking confinement strategy to fabricate transition metal single-atom sites for bifunctional oxygen electrocatalysis.

Rechargeable zinc-air batteries call for high-performance bifunctional oxygen electrocatalysts. Transition metal single-atom catalysts constitute a promising candidate considering their maximum atom efficiency and high intrinsic activity. However, the fabrication of atomically dispersed transition metal sites is highly challenging, creating a need for for new design strategies and synthesis methods. Here, a clicking confinement strategy is proposed to efficiently predisperse transitional metal atoms in a precursor directed by click chemistry and ensure successful construction of abundant single-atom sites. Concretely, cobalt-coordinated porphyrin units are covalently clicked on the substrate for the confinement of the cobalt atoms and affording a Co-N-C electrocatalyst. The Co-N-C electrocatalyst exhibits impressive bifunctional oxygen electrocatalytic performances with an activity indicator ΔE of 0.79 V. This work extends the approach to prepare transition metal single-atom sites for efficient bifunctional oxygen electrocatalysis and inspires the methodology on precise synthesis of catalytic materials.

A first-principles study on the electrochemical reaction activity of 3d transition metal single-atom catalysts in nitrogen-doped graphene: Trends and hints

Electrochemical reactions are essential in the processes of energy storage and conversion, and performance is tightly dependent on the electrocatalysts. Herein, we systematically investigate the activity of 3d transition metal embedded nitrogen-doped graphene (MNx-G) for single-atom catalysts (SACs) in the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The calculated volcano curves reveal the optimal SAC configuration for each reaction to be CoN3-G for the ORR, CoN4-G for the OER, and Ni/CuN3-G for the HER. Analysis based on the machine learning method suggests that high catalytic performance is dominated by the number of valence electrons occupying the d orbitals, the covalent radius, the electronegativity, the ratio of nearest-neighbor N and C atoms for the metal atoms, and the bond length between metal atoms and adsorbates. This work may shed some light on further studies of the ORR, OER, and HER with non-precious metal SACs.

Transition metal single-atom embedded on N-doped carbon as a catalyst for peroxymonosulfate activation: A DFT study

Single-atom catalysts perform excellently in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs), for which the generation of reactive oxygen species (ROS) is essential to the degradation of emerging organic pollutants in water. However, the detailed PMS activation mechanisms remain elusive. Density functional theory (DFT) calculation as a powerful approach can overcome the limitations of the experimental studies, providing a molecular-level perspective of catalytic process. This study conducted DFT calculations to clarify the electronic structures and PMS adsorption and activation mechanisms of a series of transition metal single-atom catalysts. According to the DFT study, significant electronic interaction and negative formation energy make nitrogen-doped carbon (N@C) supports suitable for stabilizing metal atoms (Me) to form MeN@C catalysts. As the active site, single metal atom adsorbs the oxygen atoms of PMS by electrostatic and magnetic interactions, and transfer electrons from MeN@C to activate PMS. Different adsorption configurations and the subsequent PMS activation lead to the generation of various ROS, including the SO4•- radical, •OH radical, singlet oxygen (1O2), high-valent metal-oxo species, and surface-activated PMS*. Electron transfer mediated by surface-activated PMS* may dominate in all MeN@C/PMS systems. The generation of free radicals can be difficult for some systems. High-valent metal-oxo species are readily formed by FeN@C/PMS and MnN@C/PMS, whereas 1O2 tends to be produced by CoN@C/PMS, NiN@C/PMS, and CuN@C/PMS. The findings will provide a theoretical basis for the design and synthesis of effective MeN@C catalysts for the AOPs to remove emerging organic pollutants from water and wastewater.

Unprecedentedly high activity and selectivity for hydrogenation of nitroarenes with single atomic Co1-N3P1 sites

Transition metal single atom catalysts (SACs) with M1-Nx coordination configuration have shown outstanding activity and selectivity for hydrogenation of nitroarenes. Modulating the atomic coordination structure has emerged as a promising strategy to further improve the catalytic performance. Herein, we report an atomic Co1/NPC catalyst with unsymmetrical single Co1-N3P1 sites that displays unprecedentedly high activity and chemoselectivity for hydrogenation of functionalized nitroarenes. Compared to the most popular Co1-N4 coordination, the electron density of Co atom in Co1-N3P1 is increased, which is more favorable for H2 dissociation as verified by kinetic isotope effect and density functional theory calculation results. In nitrobenzene hydrogenation reaction, the as-synthesized Co1-N3P1 SAC exhibits a turnover frequency of 6560 h−1, which is 60-fold higher than that of Co1-N4 SAC and one order of magnitude higher than the state-of-the-art M1-Nx-C SACs in literatures. Furthermore, Co1-N3P1 SAC shows superior selectivity (>99%) toward many substituted nitroarenes with co-existence of other sensitive reducible groups. This work is an excellent example of relationship between catalytic performance and the coordination environment of SACs, and offers a potential practical catalyst for aromatic amine synthesis by hydrogenation of nitroarenes.

Metal-organic framework derived single-atom catalysts for electrochemical CO2 reduction.

With maximum atomic utilization, transition metal single atom catalysts (SACs) show great potential in electrochemical reduction of CO2 to CO. Herein, by a facile pyrolysis of zeolitic imidazolate frameworks (ZIFs) assembled with tiny amounts of metal ions, a series of metal-nitrogen-carbon (M-N-C) based SACs (M = Fe, Ni, Mn, Co and Cu), with metal single atoms decorated on a nitrogen-doped carbon support, have been precisely constructed. X-ray photoelectron spectroscopy (XPS) for M-N-C showed that the N 1s spectrum was deconvoluted into five peaks for pyridinic (∼398.3 eV), M-N coordination (∼399.6 eV), pyrrolic (∼400.4 eV), quaternary (∼401.2 eV) and oxidized (∼402.9 eV) N species, demonstrating the existence of M-N bonding. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) indicates homogeneous distribution of metal species throughout the N-doped carbon matrix. Among the catalysts examined, the Fe-N-C catalyst exhibits the best catalytic performance in electrocatalytic CO2 reduction reaction (CO2RR) with nearly 100% faradaic efficiency for CO (FECO) at -0.9 V vs. the reversible hydrogen electrode (RHE). Ni-N-C is the second most active catalyst towards CO2RR performance, then followed by Mn-N-C, Co-N-C and Cu-N-C. Considering the optimum activity of Fe-N-C catalyst for the CO2RR, we then further investigate the effect of pyrolysis temperature on CO2RR of the Fe-N-C catalyst. We find the Fe-N-C catalyst pyrolyzed at 1000 °C exhibits the best catalytic activity in CO2RR with excellent CO selectivity.

Single Ni active sites with a nitrogen and phosphorus dual coordination for an efficient CO2 reduction.

Transition metal single-atom catalysts (SACs) have emerged as a research hotspot in CO2RRs. However, tuning the electronic configuration of a metal single-atom by employing new heteroatoms still remains a challenge. Herein, a carbon matrix loaded with a N and P co-coordinated Ni single-atom (denoted as Ni-NPC) was prepared for an efficient CO2RR. XANES and EXAFS were conducted to explore the coordination environment and charge distribution of the Ni-NPC catalyst. DFT calculations indicated that the Ni atom gained electrons from the P atom, and the Ni-NPC sample had a decreased energy barrier of +0.97 eV after doping with P atoms, which was favorable to overcome the limiting-step bottleneck for promoting CO2RR. Due to the rich Ni atomic active sites and superior P-doping effect, Ni-NPC exhibited a maximum FECO of 92% with a high current density of 22.6 mA cm-2 at -0.8V vs. RHE, which was far superior to those of NC, NPC and Ni-NC catalysts. Moreover, both the FECO and current density of the Ni-NPC catalyst remained stable for more than 16 h at -0.8 V vs. RHE, indicating a high stability for long-term CO2RR experiments.