Stable Single-atom Catalysts Research Articles

Enhanced Electron-Hole Separation in Phosphorus-Coordinated Co Atom on g-C3N4 toward Photocatalytic Overall Water Splitting.

Revealing the decoration mode of g-C3N4 and understanding the physical mechanism of overall water splitting is important for the further improvement of the photocatalytic activity of g-C3N4-based materials. With core level shift and molecular dynamics simulations based on first-principles calculations, Co1(PHx)3 anchored on the triazine of g-C3N4 is determined as a stable single-atom catalyst with high efficiency for photocatalytic overall water splitting. The separated spin-polarized charge density distribution of valence-band maximum and conduction-band minimum states is beneficial for the long lifetime of photoexcited electrons and holes. An anchored Co single atom site is the active site for oxygen evolution reaction, and nitrogen atoms act as active sites for hydrogen evolution reaction. This new decoration mode of g-C3N4 opens a possible way to functionalize g-C3N4 on both triazine and void sites to realize the separation of OER and hydrogenation reaction by water splitting.

Single-atom Co-N-C catalysts for high-efficiency reverse water-gas shift reaction

Designing efficient non-precious metal catalysts for CO2 hydrogenation is a significant challenge. Cobalt-based catalysts often failed to catalyze the reverse water-gas shift (RWGS) reaction with high CO selectivity and stability. Herein, nitrogen-doped carbon (N-C) immobilized single-atom Co-N4 catalyst with 5% Co loading were prepared by anchoring strategy through regulating the cobalt species coordination structure. The stable single-atom catalyst achieves almost 100% CO selectivity and a high CO2 conversion of 52.4% at 500 °C during CO2 hydrogenation, while the 20% Co-N-C nanoparticle catalyst favored CH4 formation. The experiments and density functional theory (DFT) calculations revealed that atomically dispersed Co-N4 site followed the hydrogen-assisted pathway in which the intermediate COOH* was desorbed and dissociated into CO, whereas the Co nanoparticle catalysts mainly followed the direct dissociation. This study provided a new strategy for designing Co-based RWGS catalysts with excellent performance.

A Tale of Two Sites: Neighboring Atomically Dispersed Pt Sites Cooperatively Remove Trace H2 in CO-Rich Stream.

Single-atom catalysts (SACs) exhibit distinct catalytic behavior compared with nano-catalysts because of their unique atomic coordination environment without the direct bonding between identical metal centers. How these single atom sites interact with each other and influence the catalytic performance remains unveiled as designing densely populated but stable SACs is still an enormous challenge to date. Here, a fabrication strategy for embedding high areal density single-atom Pt sites via a defect engineering approach is demonstrated. Similar to the synergistic mechanism in binuclear homogeneous catalysts, from both experimental and theoretical results, it is proved that electrons would redistribute between the two oxo-bridged paired Pt sites after hydrogen adsorption on one site, which enables the other Pt site to have high CO oxidation activity at mild-temperature. The dynamic electronic interaction between neighboring Pt sites is found to be distance dependent. These new SACs with abundant Pt-O-Pt paired structures can improve the efficiency of CO chemical purification.

Investigation on the Reaction Mechanism of Methane Oxidation over MgAl2O4‐Supported Single‐Atom Catalyst Prepared at High Temperature

AbstractStability and reactivity of single‐atom catalysts (SACs) are the points of concern in catalysis, especially under the harsh conditions, such as at elevated temperatures in oxidizing conditions. Previous work showed that thermally stable Pt1 SAC supported on K‐modified MgAl2O4 prepared by a vapor‐phase self‐assembly mechanism (Pt1/K/MgAl2O4) showed better performance than a Pt/MgAl2O4 nanocatalyst in catalytic CH4 oxidation (Chem, 2022, 8, 731–748). However, the detailed reaction mechanism remains unclear, which preventing the further development of single‐atom catalysts for CH4 oxidation. Herein, the sintering resistance behaviour of the Pt1/K/MgAl2O4 catalyst, was investigated by means of density functional theory (DFT) calculations, and it was found that the catalytic CH4 oxidation takes place via the Mars‐van Krevelen (MvK) mechanism, similar to that on conventional oxides. This viewpoint is further verified by experiments. Additionally, other noble metals (Au, Ir and Ru) and alkali elements (Na and Cs) are also investigated and it is found that the alkali types also affect the catalytic performance. This work clarifies the reaction mechanism of CH4 oxidation on metal SAC supported on MgAl2O4 synthesized at high temperature, providing a chance to manipulate the performances of metal SAC in CH4 oxidation.

Strong Electronic Metal-Support Interaction between Iridium Single Atoms and a WO3 Support Promotes Highly Efficient and Robust CO2 Cycloaddition.

The carbon dioxide (CO2 ) cycloaddition of epoxides to cyclic carbonates is of great industrial importance owing to the high economical values of its products. Single-atom catalysts (SACs) have great potential in CO2 cycloaddition by virtue of their high atom utilization efficiency and desired activity, but they generally suffer from poor reaction stability and catalytic activity arising from the weak interaction between the active centers and the supports. In this work, Ir single atoms stably anchored on the WO3 support (Ir1 -WO3 ) are developed with a strong electronic metal-support interaction (EMSI). Superior CO2 cycloaddition is realized in the Ir1 -WO3 catalyst via the EMSI effect: 100% conversion efficiency for the CO2 cycloaddition of styrene oxide to styrene carbonate after 15h at 40 °C and excellent stability with no degradation even after ten reaction cycles for a total of more than 150h. Density functional theory calculations reveal that the EMSI effect results in significant charge redistribution between the Ir single atoms and the WO3 support, and consequently lowers the energy barrier associated with epoxide ring opening. This work furnishes new insights into the catalytic mechanism of CO2 cycloaddition and would guide the design of stable SACs for efficient CO2 cycloaddition reactions.

Thermally-stable single-atom catalysts and beyond: A perspective.

Single-atom catalysis is a research Frontier and has attracted extensive interests in catalysis. Significant progresses have been carried out in the synthesis and characterization of metal single-atom catalysts (SACs). However, the stability and catalytic reactivity of metal SAC at elevated temperatures are not well documented because single atoms sinter at elevated temperatures. Therefore, the development of stable and reactive SAC at high temperatures remains a formidable challenge. In this perspective, we summarize recent efforts on the preparation of the thermally-stable SACs synthesized at elevated temperature via the reverse-Ostwald ripening mechanism, including the approaches of atom trapping and vapor-phase self-assembly. The reducibility of lattice oxygen, the loading upper limit and the location of the metal single atom are discussed, combining experiments with simulations. In addition, we demonstrate that the coordination structure of the metal single atom can be tailored to address the relationship of structure and performances of the metal SAC in reactions. We expect that this perspective can provide some insights to guide the study for the rational design of thermally-stable and active single atom catalysts, which are especially suitable for high-temperature reactions.

General Strategy toward Hydrophilic Single Atom Catalysts for Efficient Selective Hydrogenation.

Well dispersible and stable single atom catalysts (SACs) with hydrophilic features are highly desirable for selective hydrogenation reactions in hydrophilic solvents towards important chemicals and pharmaceutical intermediates. A general strategy is reported for the fabrication of hydrophilic SACs by cation‐exchange approach. The cation‐exchange between metal ions (M = Ni, Fe, Co, Cu) and Na+ ions introduced in the skeleton of metal oxide (TiO2 or ZrO2) nanoshells plays the key role in forming M1/TiO2 and M1/ZrO2 SACs, which efficiently prevents the aggregation of the exchanged metal ions. The as‐obtained SACs are highly dispersible and stable in hydrophilic solvents including alcohol and water, which greatly facilitates the catalysis reaction in alcohol. The Ni1/TiO2 SACs have been successfully utilized as catalysts for the selective C=C hydrogenation of cinnamaldehyde to produce phenylpropanal with 98% conversion, over 90% selectivity, good recyclability, and a turnover frequency (TOF) of 102 h−1, overwhelming most reported catalysts including noble metal catalysts.

High-Throughput Screening of Stable Single-Atom Catalysts in CO2 Reduction Reactions

High-Throughput Screening of Stable Single-Atom Catalysts in CO₂ Reduction Reactions

Single-atom transition metals supported on defective graphene for electrochemical reduction of acetonitrile to ethylamine

The electrochemical acetonitrile (CH3CN) reduction reaction provides an alternative to produce ethylamine (CH3CH2NH2) under ambient conditions but remains challenging. Using density functional theory (DFT) calculations, we explored the catalytic performance of four types of graphene-based single-atom catalysts (SACs) total of 32 materials toward the CH3CN reduction reaction. We identified that two highly stable SACs, namely Cr@N4 and Mn@N4, can greatly accelerate both reaction thermodynamics and kinetics with small energy barriers. In particular, these two SACs can effectively suppress the competitive hydrogen evolution reaction (HER) by selective adsorption of key intermediate of *CH3CN rather than *H, which feature superior catalytic selectivity. More impressively, the activity trends of various candidates can be directly correlated to the adsorption energy of *CH3CHN (ΔE(*CH3CHN)), and the underlying activity origin is ascribed by the enhancement of the adsorption for the *CH3CHN. Furthermore, detailed electronic property analyses indicate that the bonding/anti-bonding interactions of different active metal centers with *CH3CHN are the key factor determining the adsorption strength. The more bonding contributions below the Fermi level (Ef) lead to the stronger *CH3CHN combination on active metal centers, while the anti-bonding state is opposite. As a result, Cr@N4 stands out among these candidates, which can tune the performance of acetonitrile reduction reaction by balancing the reactivity of CH3CN and the desorption of CH3CH2NH2. Our results unveil that the excellent catalytic activity of Cr@N4 for CH3CN reduction reaction can be further confirmed by calculating the bond lengths and charge variations. With the elongation of CN bond in the adsorbed CH3CHxNHy, CH3CN molecule can be effectively activated on Cr@N4. Overall, this work addresses a promising active catalyst and will be of a guidance of great theoretical significance to construct carbon-based materials supported transition metal single-atom electrocatalysts for the CH3CN reduction reaction.

Computational Study of Low-Energy Pt-Ion Implantation into Graphene for Single-Atom Catalysis

Single-atom catalysts (SACs) represent the ultimate goal of nanocatalysis fields. However, complex synthesis processes and pyrolysis inactivation problems are the two main challenges that plague the development of SACs. In this work, we propose that the ultralow-energy ion-implantation (ULEII) method could be utilized to simply and efficiently synthesize stable SACs. Our simulation results of Pt-ion implantation into graphene indicate that the total doping efficiency, including direct displacement doping and indirect trap doping, can be effectively optimized by delicately adjusting the energy of incident ions. Further systematic molecular dynamics simulations and first-principles calculations demonstrate that irradiation-induced vacancy defects can effectively capture and anchor adsorbed metal atoms on the graphene surface. The stability and migration characteristics of various defects are also clearly elucidated. Theoretically, by selecting an optimal ion energy, the ULEII method can achieve a doping efficiency as high as 73.4% .

Boosting fenton-like reaction by reconstructed single Fe atom catalyst for oxidizing organics: Synergistic effect of conjugated π-π sp2 structured carbon and isolated Fe-N4 sites

The synthesis of a highly efficient and stable single atom catalyst (SAC) for peroxydisulfate (PDS) activation remains a challenge. Herein, a reconstructed iron single atom catalyst (Fe-SAC) with both abundant conjugated π-π sp2 structured carbon and reactive Fe-N4 sites was first reported to improve the decomposition of organics in PDS/SACs systems. Enhanced organics adsorption can be achieved by the conjugated π-π sp2 structured carbon via π-π interaction to decrease the migration distance of reactive species to organic molecules. In addition, PDS tended to be attached onto the Fe-N4 sites accompanied by the peroxymonosulfate (PMS) generation via S-O breakage of PDS, accelerating the generation of radicals. As a result, organics decomposition via PDS activation by reconstructed Fe-SAC was significantly higher (∼6 times) than the conventional Fe-SAC because of the synergistic effect of the resulting dual reaction sites. This work provides a new strategy to enhance the catalytic performances in PDS/SACs systems.

Photo-thermo semi-hydrogenation of acetylene on Pd1/TiO2 single-atom catalyst

Semi-hydrogenation of acetylene in excess ethylene is a key industrial process for ethylene purification. Supported Pd catalysts have attracted most attention due to their superior intrinsic activity but often suffer from low selectivity. Pd single-atom catalysts (SACs) are promising to significantly improve the selectivity, but the activity needs to be improved and the feasible preparation of Pd SACs remains a grand challenge. Here, we report a simple strategy to construct Pd1/TiO2 SACs by selectively encapsulating the co-existed small amount of Pd nanoclusters/nanoparticles based on their different strong metal-support interaction (SMSI) occurrence conditions. In addition, photo-thermo catalysis has been applied to this process where a much-improved catalytic activity was obtained. Detailed characterization combined with DFT calculation suggests that photo-induced electrons transferred from TiO2 to the adjacent Pd atoms facilitate the activation of acetylene. This work offers an opportunity to develop highly stable Pd SACs for efficient catalytic semi-hydrogenation process.

SiC Monolayers as Promising Substrates for the Development of Highly Stable Palladium Single Atom Catalysts: A Density Functional Theory Study

Single-atom catalysts have been touted as highly efficient catalysts, but the catalytic single-atom sites are unstable and tend to aggregate into nanoparticles during chemical reactions. In this study, we show that SiC monolayers are promising substrates for the development of highly stable single-atom catalysts (Pd1 /SiC) within the density functional theory. In presence of a Si-vacancy, the diffusion barrier energy of a Pd1 atom embedded in the SiC monolayer is substantially enhanced from 2.3 to 7.8 eV, which is much higher than the reported diffusion barrier energies of graphene, boron nitride and defective MgO of the same catalytic system. Ab initio molecular dynamic calculations at 500 K also confirm the enhanced stability of Pd1 /SiC monolayer (Si-vacancy) such that the Pd1 atom remains embedded in the vacancy. Additionally, the Pd1 /SiC monolayer (Si-vacancy) catalysts show a ∼34 % reduction of activation barrier energy for CO oxidation as compared to pristine catalysts. This work implies that nanostructured SiC materials are promising substrates for the synthesis of highly stable single-atom catalysts.

Single Atom Catalysts: An Overview of the Coordination and Interactions with Metallic Supports.

Catalyst utilization is a key economic factor in heterogeneous catalysis, particularly, when noble metals are used as the active phase. A huge saving on catalyst cost can be achieved with developing a single atomic layer of the active catalyst on a given cheap support. Besides the economic benefit, single atom catalysts (SACs) have also shown superior activity and selectivity relative to catalytic particles or nanoparticles; yet they are prone to aggregation and deactivation. The development of effective, stable, and commercially viable SACs is still a huge challenge. One of the remaining key obstacles is the ability to easily and effectively tune SACs-support interactions and coordination in a way that enables the production of robust, stable, and versatile SACs. Accordingly, the coordination and interactions between metallic supports and SACs and their impacts on SACs stability and activity are reviewed in this article.

Theoretical studies of MXene-supported single-atom catalysts: Os1/Ti2CS2 for low-temperature CO oxidation

We report herein a new sulfur-functionalized MXene Ti2C (Ti2CS2)-supported osmium-metal single-atom catalyst (SAC) Os1/Ti2CS2 with high low-temperature catalytic activity for CO oxidation. Using periodic density functional theory calculations, the most stable SAC, Os1/Ti2CS2, has been screened from a series of group 8–11 transition metal SACs M1/Ti2CS2 (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au). The calculations show that it is favorable for O2 and CO to be coadsorbed on the Os1 single atom (SA) of Os1/Ti2CS2 and the adsorption energy of the first O2 molecule is slightly higher than that of CO. Moreover, the termolecular co-adsorption of O2 + 2CO on Os1 SA is also possible, which is favorable for CO oxidation on Os1 SA through a novel three-molecule reaction mechanism. Accordingly, four different catalytic mechanisms, the Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), termolecular Langmuir-Hinshelwood-A (TLH-A) and termolecular Langmuir-Hinshelwood-B (TLH-B), are systematically studied for CO oxidation by O2 on Os1/Ti2CS2. The theoretical studies indicate that the TLH-B mechanism is the most feasible for CO oxidation with the reaction barrier energy of only 0.74 eV, which is far lower than for L-H, E-R and TLH-A with barrier energies of 1.06, 1.09 and 1.47 eV, respectively. The results provide fundamental understanding to the surface chemistry of MXene and designing new sulfur-functionalized two-dimensional MXene catalytic nanomaterials.

Vapor-phase self-assembly for generating thermally stable single-atom catalysts

<h2>Summary</h2> Preparation of thermally stable metal single-atom catalysts (SACs) is a challenge in heterogeneous catalysis, especially on conventional supports that provide a weak metal-support interaction. In this work, we report that a modified support MgAl₂O₄ can stabilize Pt single atoms by a mechanism of vapor-phase self-assembly in a high-temperature treatment (800°C, air). The experimental results on the formation mechanism and the structure are validated by DFT and <i>ab initio</i> molecular dynamics simulations. We infer that stable triangular K₃O₃ structures help stabilize Pt single atoms at high temperatures in oxidizing conditions, exhibiting excellent reactivity for methane oxidation. The obtained Pt/K/MgAl₂O₄ SAC presents excellent stability in methane oxidation after steam treatment at elevated temperatures, whereas the Pt/MgAl₂O₄ nanocatalyst suffers from rapid deactivation due to Pt nanoparticle growth. This work paves the way for preparing thermally stable and highly active SACs using conventional high-surface-area supports, despite the weak metal-support interaction.

Understanding Single-Atom Catalysis in View of Theory.

In the past decade, isolated single atoms have been successfully dispersed on various substrates, with their potential applications being intensively investigated in different reactions. While the essential target of research in single-atom catalysis is the precise synthesis of stable single-atom catalysts (SACs) with clear configurations and impressive catalytic performance, theoretical investigations have also played important roles in identifying active sites, revealing catalytic mechanisms, and establishing structure–activity relationships. Nevertheless, special attention should still be paid in theoretical works to the particularity of SACs. In this Perspective, we will summarize the theoretical progress made on the understanding of the rich phenomena in single-atom catalysis. We focus on the determination of local structures of SACs via comparison between experiments and simulations, the discovery of distinctive catalytic mechanisms induced by multiadsorption, synergetic effects, and dynamic evolutions, to name a few, the proposal of criteria for theoretically designing SACs, and the extension of original concepts of single-atom catalysis. We hope that this Perspective will inspire more in-depth thinking on future theoretical studies of SACs.

Rational construction of thermally stable single atom catalysts: From atomic structure to practical applications

As a new frontier in catalysis field, single-atom catalysts (SACs) hold unique electronic structure and high atom utilization, which have displayed unprecedented activity and selectivity toward a wide range of catalytic reactions. However, many reported SACs are susceptible to Ostwald ripening process in high temperature environment or long-term catalytic application, which will cause sintering and deactivation. This is due to the weak interaction between the metal atom and supports. The regeneration and recycling of deactivated catalysts will greatly increase the time and economic cost of industrial production. Therefore, it is necessary to develop SACs with excellent thermal stability to meet the industrial demands. Here, we discuss the fundamental comprehension of the stability of thermally stable SACs obtained from different synthesis methods. The influences of the speciation of metal centers and coordination environments on thermal stability are summarized. The importance of using novel in situ and operando characterizations to reveal dynamic structural evolution under synthesis and reaction conditions and to identify active sites of thermally stable SACs is highlighted. The mechanistic understanding of the unique role of thermally stable SACs in thermocatalytic application is also discussed. At last, a brief perspective on the remaining challenges and future directions of thermally stable SACs is presented.

Boosting Activity and Stability of Metal Single-Atom Catalysts via Regulation of Coordination Number and Local Composition.

Controlling the chemical environments of the active metal atom including both coordination number (CN) and local composition (LC) is vital to achieve active and stable single-atom catalysts (SACs), but remains challenging. Here we synthesized a series of supported Pt1 SACs by depositing Pt atoms onto the pretuned anchoring sites on nitrogen-doped carbon using atomic layer deposition. In hydrogenation of para-chloronitrobenzene, the Pt1 SAC with a higher CN about four but less pyridinic nitrogen (Npyri) content exhibits a remarkably high activity along with superior recyclability compared to those with lower CNs and more Npyri. Theoretical calculations reveal that the four-coordinated Pt1 atoms with about 1 eV lower formation energy are more resistant to agglomerations than the three-coordinated ones. Composition-wise decrease of the Pt-Npyri bond upshifts gradually the Pt-5d center, and minimal one Pt-Npyri bond features a high-lying Pt-5d state that largely facilitates H2 dissociation, boosting hydrogenation activity remarkably.

A Hydrothermally Stable Single-Atom Catalyst of Pt Supported on High-Entropy Oxide/Al2O3: Structural Optimization and Enhanced Catalytic Activity.

A catalyst with high-entropy oxide (HEO)-stabilized single-atom Pt can afford low-temperature activity for catalytic oxidation and remarkable durability even under harsh conditions. However, HEO is easy to harden during sintering, which results in a few defective sites for anchoring single-atom metals. Herein, we present a sol-gel-assisted mechanical milling strategy to achieve a single-atom catalyst of Pt-HEO/Al2O3. The strong interaction between HEO and Al2O3 effectively inhibits the growth of HEO microparticles, which leads to generation of more surface defects because of the nanoscale effect. Meanwhile, another strong interaction between Pt and HEO stabilizes single-atom Pt on HEO. Temperature-programmed techniques further verify that the reactivity of surface lattice oxygen species is enhanced because of the Pt-O-M bonds on the surface of HEO. Unlike conventional single-atom Pt catalysts, Pt-HEO/Al2O3 as a heterogeneous catalyst not only exhibits superior stability against hydrothermal aging but also presents long-term reaction stability for CO catalytic oxidation, which exceeds 540 h. The present work opens a new door for rational design of hydrothermally stable single-atom Pt catalysts, which are highly promising in practical applications.