Metal Single-atom Catalysts Research Articles

Manipulating Selectivity of Hydroxyl Radical Generation by Single-Atom Catalysts in Catalytic Ozonation: Surface or Solution.

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive ·OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O3 onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

Computational screening of single-atom catalysts for direct electrochemical NH3 synthesis from NO on defective boron phosphide monolayer

Selective electrocatalytic reduction of NO to ammonia provides a promising way to remove pollutant NO and realize green ammonia synthesis under ambient conditions. The key to promote the practical application of NO reduction reaction (NORR) is to develop economical and efficient electrocatalysts to replace Pt-based catalysts. In this work, we designed a series of transition metal (TM) single-atom catalysts (SACs) supported on BP monolayer with a B vacancy (TM@BP, TM = Ti ∼ Zn, Zr ∼ Ag, and Hf ∼ Au), and systematically investigated their performance of electrocatalytic NORR by means of density functional theory (DFT). Taking the changes of free energy of the first (NO → NOH/NO → NHO) and last (OH → H2O/NH2 → NH3) hydrogenation steps of NORR as the screening criteria, seven TM@BP (TM = Ti, V, Cu, Rh, Ag, Ir, and Au) were selected from eighteen candidates. All possible reaction pathways of NORR to produce NH3 and byproducts (N2O, N2 and H2) were considered. Our calculated results show that Ti@BP, V@BP, Cu@BP, and Au@BP not only exhibit similar catalytic activity to Pt-based catalysts, but also have excellent NORR selectivity toward NH3, so they are predicted to be the most promising catalysts. This work provides useful guidance for the design and exploration of efficient NORR electrocatalysts.

P-Orbital Bismuth Single-Atom Catalyst for Highly Effective Oxygen Electroreduction in Quasi-Solid Zinc-Air Batteries.

P-block metals have gradually been utilized to synthesize non-noble-metal catalysts for oxygen reduction reaction (ORR) due to the easily tunable localized p-orbitals and resulted versatile electronic structures. The high-density single-atom bismuth sites (Bi-NC) anchored onto nitrogen-doped three-dimensional porous carbon are proved to possess significant electrocatalytic ORR performance. Theoretical calculations unveil positively charged bismuth centers prominently improved the adsorption capacity of N-doped carbon to O2 . The p orbitals of Bi sites within Bi-NC easily generate hybrid states with p orbitals of O2 , thus promoting charge transfer and ultimately reducing the energy barrier of ORR. Benefiting from p-orbital electrons regulation of bismuth atoms, Bi-NC exhibit ORR half-wave potential of 0.86V (vs RHE). Additionally, both liquid and quasi-solid zinc-air batteries with Bi-NC as air-cathodes achieve higher power density and specific capacity than 20wt% Pt/C, and comparable stability and round-trip efficiency with 20wt% Pt/C. The discovery sheds light on the theoretical and practical guidance for p-block metallic single-atom catalysts.

Theoretical studies of non-noble metal single-atom catalyst Ni1/MoS2: Electronic structure and electrocatalytic CO2 reduction

Single-atom catalysts (SACs) have aroused significant interest in heterogeneous catalysis in recent years because of their high catalytic selectivity and tunable activity in various chemical reactions. Herein, non-noble metal SACs with 3d-series metal single atoms (M1) (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) supported on MoS2 are computationally screened by using first-principles quantum-chemical theory. The Ni1/MoS2 catalyst is found to be the most stable among those 3d-series SACs due to the optimal binding energy. In order to provide a fundamental understanding of the intrinsic stability and bonding interaction between the metal single atoms and MoS2 support, the electronic structure, including the spin density populations, charge density difference (CDD), electron localization function (ELF), band structure, density of states (DOS), and crystal orbital Hamiltonian populations (COHP) are systematically examined. The solid-state quantum theory of atoms in molecules (QTAIM) is also applied to further characterize the Ni—S and Mo—S covalent and ionic bonding nature between the metal single atoms and support. It is found that in addition to Ni—S bonding, there exists significant Ni—Mo bonding that is critical for the electronic structure, stability, and catalytic properties of Ni1/MoS2 catalyst. As a typical application of this Ni1/MoS2 catalyst, the electrocatalytic mechanism and reaction pathway of CO2 reduction reaction (CO2RR) on Ni1/MoS2 catalyst have been investigated. The MoS2-supported Ni single atoms are found to exhibit high catalytic activity for CO2RR to methanol. The calculational results provide theoretical insights towards the design of highly efficient SACs on MoS2-based functional materials.

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H2 is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.

Optimizing the semiconductor-metal-single-atom interaction for photocatalytic reactivity.

Metal single-atom (MSA) catalysts with 100% metal atom utilization and unique electronic properties are attractive cocatalysts for efficient photocatalysis when coupled with semiconductors. Owing to the absence of a metal-metal bond, MSA sites are exclusively coordinated with the semiconductor photocatalyst, featuring a chemical-bond-driven tunable interaction between the semiconductor and the metal single atom. This semiconductor-MSA interaction is a platform that can facilitate the separation/transfer of photogenerated charge carriers and promote the subsequent catalytic reactions. In this Review, we first introduce the fundamental physicochemistry related to the semiconductor-MSA interaction. We highlight the ligand effect on the electronic structures, catalytic properties and functional mechanisms of the MSA cocatalyst through the semiconductor-MSA interaction. Then, we categorize the state-of-the-art experimental and theoretical strategies for the construction of the efficient semiconductor-MSA interaction at the atomic scale for a wide range of photocatalytic reactions. The examples described include photocatalytic water splitting, CO2 reduction and organic synthesis. We end by outlining strategies on how to further advance the semiconductor-MSA interaction for complex photocatalytic reactions involving multiple elementary steps. We provide atomic and electronic-scale insights into the working mechanisms of the semiconductor-MSA interaction and guidance for the design of high-performance semiconductor-MSA interface photocatalytic systems.

A Perspective on Metal Single Atom- Based Electrocatalytsts towards Hydrogen Peroxide Production

The green production of hydrogen peroxide has attracted considerable attention owing to the urge for an alternative production method to the current production. Electrocatalytic reduction via the 2e- pathway has emerged as the brightest approach. However, the finding of catalysts with high efficiency and selectivity to provide a proper interaction between active sites and *OOH is remaining as the primary obstacle. Employing metal single-atom catalysts (SACs) offers a potential manner to address those challenges. The presented review aims to provide cutting-edge development of a single metal catalyst for electrocatalytic H2O2 production. Also, perspective is laid out to propose novel approaches for material development in the future

Structural Effects of Metal Single-Atom Catalysts for Enhanced Photocatalytic Degradation of Gemfibrozil.

The development of efficient catalysts is a highly necessary but challenging task within the field of environmental water remediation. Single-atom catalysts are promising nanomaterials within this respect, but in-depth studies encompassing this class of catalysts remain elusive. In this work, we systematically study the degradation of gemfibrozil, a persistent pollutant, on a series of carbon nitride photocatalysts, investigating both the effect of (i) catalyst textural properties and (ii) metal single atoms on the contaminant degradation. Tests in the absence of the catalyst result in negligible degradation rates, confirming the stability of the contaminant when dispersed in water. Then, photocatalytic tests at optimal pH, solvent, and wavelength reveal a correlation between the support surface area and the degradation. This points to the role of carbon nitride surface nanostructure on gemfibrozil degradation. In particular, the use of silver on mesoporous carbon nitride single-atom catalyst (Ag@mpgC3N4) leads to an unprecedented degradation of gemfibrozil (>90% within 60 min). The possible degradation intermediates and products were identified by mass spectrometry and were inert by cytotoxicity evaluation. We anticipate that, with further refinement and customization, the carbon nitride catalysts reported herein may find broad applications for light-driven degradation of other contaminants of emerging concern.

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.

Nitrogen atom coordination tuned transition metal catalysts for NO oxidation and reduction

Developing an efficient catalyst for NO oxidation and reduction at ambient temperature is a significant challenge. Recent studies have suggested that the N-coordinated transition metal (TM) single atom catalysts (SACs) have high catalytic activity and stability. Herein, we report the activation potential of a series of 3d TM atoms supported on N coordination-tuned graphene (GR) for NO oxidation and reduction. The results show that the N coordination pattern can greatly alter the catalytic reactivity of TM on the catalysts, and the TM atom on the catalysts with three-coordinated pyridinic nitrogen TM-N3@GR exhibit the strongest chemical activity. Among the TM-N3@GR catalysts, Ti–N3@GR is the most promising candidate. The rate constants and equilibrium constants were calculated to evaluate the kinetic and thermodynamic feasibility of the catalytic reaction, respectively. Our results demonstrate that the reduction of NO to N2 on Ti–N3@GR can occur at ambient temperature with a large exotherm of 6.99 eV, and the oxidation of NO to NO2 on Ti–N3@GR can easily proceed when the temperature reaches 360 K with a large equilibrium constant. Our studies are of great significance for understanding the performance of N coordination-tuned catalysts and designing Ti-based catalysts for NO oxidation and reduction.

Nature and Dynamic Evolution of Rh Single Atoms Trapped by CeO2 in CO Hydrogenation

Metal single-atom catalysts (SACs) have attracted extensive interest because of their maximum atom efficiency and potentially good performances. However, the understanding of the catalytic behavior and the stability of SACs in high-temperature catalytic reactions remains a major challenge, especially under reducing conditions. In this work, we investigated the nature and stability of Rh/CeO2 SACs with different Rh loadings (0.2–4 wt % Rh) during syngas conversion (260 °C, 2 MPa). Using CO-DRIFTS, we found that atomically dispersed Rh was dispersed on Rh/CeO2 catalysts with an Rh loading up to 3 wt %. The Rh/CeO2 catalysts were stable in H2 up to 350 °C, while Rh nanoclusters were formed in CO at 200 °C. During syngas conversion at 260 °C, the Rh single atoms (Rhiso) on a 0.5 wt % Rh/CeO2 SAC slowly agglomerated to form Rh nanoclusters (RhNC) and a Rhiso/RhNC ratio of ∼1:2.5 was obtained after ∼10 h time-on-stream run. Furthermore, the Rhiso/RhNC ratio did not change with further reaction, indicating that the reaction reached steady state after a 10 h run. After the catalysts reached steady state (∼10 h), the Rhiso/RhNC ratio on the spent Rh/CeO2 catalysts decreased with the increase in Rh loadings, while the reactivity increased first and then decreased. We observed an optimal value of Rhiso/RhNC ≈ 1:3 on the 1 wt % Rh/CeO2 catalyst, showing the maximum reaction rates of CO (rCO) and ethanol selectivity (SEtOH) of 85.8 μmol/s/gRh and 26.2%, respectively. This indicated that the combination of Rh single atoms and Rh nanoclusters is beneficial to the ethanol selectivity and reaction rate, which was confirmed by DFT simulations. Moreover, the ratios of Rhiso/RhNC on the spent catalysts under the steady state have good relationships with the CO conversion rate and product selectivity in the reaction.

Low-Temperature Acetylene Semi-Hydrogenation over the Pd1-Cu1 Dual-Atom Catalyst.

The atomically dispersed metal catalyst or single-atom catalyst (SAC) with the utmost metal utilization efficiency shows excellent selectivity toward ethylene compared to the metal nanoparticles catalyst in the acetylene semi-hydrogenation reaction. However, these catalysts normally work at relatively high temperatures. Achieving low-temperature reactivity while preserving high selectivity remains a challenge. To improve the intrinsic reactivity of SACs, rationally tailoring the coordination environments of the first metal atom by coordinating it with a second neighboring metal atom affords an opportunity. Here, we report the fabrication of a dual-atom catalyst (DAC) that features a bonded Pd1-Cu1 atomic pair anchoring on nanodiamond graphene (ND@G). Compared to the single-atom Pd or Cu catalyst, it exhibits increased reactivity at a lower temperature, with 100% acetylene conversion and 92% ethylene selectivity at 110 °C. This work provides a strategy for designing DACs for low-temperature hydrogenation by manipulating the coordination environment of catalytic sites at the atomic level.

Electrocatalytic Reduction of Carbon Dioxide to High-Value Multicarbon Products with Metal–Organic Frameworks and Their Derived Materials

The electrocatalytic carbon dioxide reduction reaction (CO2RR) holds great potential in promoting carbon neutral through effectively converting CO2 molecules to useful chemicals and fuels. The high-efficiency electrochemical conversion of CO2 to single-carbon products has been well realized, while more efforts are needed for the generation of high-value multicarbon products. Metal–organic frameworks (MOFs), featuring porous structures, high chemical tunability, and ultralarge surface area, have attracted increasing attention in the electrochemical CO2RR. Herein, we review the recent progress of electrocatalytic CO2RR on MOF-based materials toward multicarbon products. First, the structure of MOFs is briefly introduced. Then, the electrocatalytic CO2RR performance and the corresponding catalytic mechanism of pristine MOFs (classified according to the kind of organic ligands/linkers) and MOF-derived materials (including metal nanomaterials, single-atom catalysts and nanocomposites) toward the multicarbon product generation are systematically discussed. Finally, critical challenges and potential opportunities are highlighted to inspire the rational design and targeted synthesis of advanced MOF-based materials for high-performance electrocatalytic CO2 reduction toward multicarbon products.

Method of manufacturing metal single-atom catalysts

Method of manufacturing metal single-atom catalysts

Single-atom Zn on bipyridine-functionalized porous organic polymers towards highly efficient N-formylation of amines with CO2 under mild conditions

The development of efficient, cost-effective and heterogeneous catalysts for the valorization of CO2 to high-value chemicals is of vital importance in reducing CO2 emissions. Herein, single-atom Zn on bipyridine-based porous organic polymer was successfully fabricated through a facile copolymerization of divinyl-bipyridine and divinyl-benzene monomers, followed by complexation with Zn(OAc)2. The prepared materials with different concentrations of bipyridine moieties possessed large surface areas, hierarchical porosity, and strong coordination interaction between Zn(OAc)2 and the polymer scaffold. Catalytic evaluation demonstrated that the optimal single-atom Zn catalyst displayed extremely high catalytic performance towards the N-formylation of amines with CO2 and PhSiH3. Notably, under mild reaction conditions (30 °C, 1 bar CO2), a Zn loading as low as 0.5 mol‰ was sufficient for 94% yield of N-methylformanilide, with a record-high turnover frequency (TOF) of 376 h−1 compared to previously reported heterogeneous catalysts. Moreover, the solid catalyst could be easily recovered and reused for five cycles. Additionally, various amines could be efficiently transformed into corresponding formamide with moderate to high yields. The present work highlights a facile and cost-effective method for constructing highly efficient and recyclable single-atom metal catalysts for N-formylation of amines with CO2.

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

Theoretical and Experimental Understanding of Metal Single-Atom Electrocatalysts for Accelerating the Electrochemical Reaction of Lithium-Sulfur Batteries.

Metal single-atom materials have attracted tremendous attention in the research field of lithium-sulfur (Li-S) batteries because they can effectively improve the reaction kinetics of sulfur cathodes. However, it is still difficult to determine the best metal single-atom catalyst for Li-S batteries, due to the lack of a unified measurement and evaluation method. Herein, a series of metal single-atom- and nitrogen-doped graphene materials (M-NG, M = Fe, Co, Ni, Ir, Ru) have been prepared as the catalysts for promoting the reaction kinetics of the sulfur reduction reaction process. Using rotating disk electrode measurements and density functional theory-based theoretical calculations, Ni-NG was screened out to be the best catalyst. It is found that Ni-NG materials can provide a kinetically favorable pathway for the reversible conversion of polysulfide conversion, thus increasing the utilization of sulfur. By coating the Ni-NG materials on the separator as a multifunctional interlayer, a commercially available sulfur cathode presents a stable specific capacity of 701.8 mAh g-1 at a current rate of 0.5C over 400 cycles. Even with a high sulfur loading of 3.8 mg cm-2, a high areal capacity of 4.58 mAh cm-2 can be achieved. This work will provide a fundamental understanding of efficient single-atom catalyst materials for Li-S batteries.