Single-atom Catalysis Research Articles

Aiming for More Sustainable Cross-Coupling Chemistry by Employing Single-Atom Catalysis on Scale.

Scaling up syntheses from mg to kg quantities is a complex endeavor. Besides adapting laboratory protocols to industrial processes and equipment and thorough safety assessments, much attention is paid to the reduction of the process' environmental impact. For processes including transition metal catalyzed steps, e.g. cross-coupling chemistry, this impact strongly depends on the identity of the metal used. As such, a key approach is the replacement of single-use with reusable heterogeneous catalysts. Transition metal single-atom heterogeneous catalysts (SAC), a novel class of catalytic materials, might exhibit all the necessary properties to step up to this task. This article shall discuss current applications of SAC in cross-coupling chemistry from the point of a process chemist and shed light on the NCCR Catalysis contribution to the field. Investigations of the stability-activity-selectivity relationship of SACs in combination with early-stage life-cycle assessments (LCA) of potential processes lay the foundation for large-scale application tailored catalyst synthesis. Ultimately, prevailing challenges are highlighted, which need to be addressed in future research.

Single-Atom Nano-Islands (SANIs): A Robust Atomic-Nano System for Versatile Heterogeneous Catalysis Applications.

Academician Tao Zhang from China and co-workers designed the first Pt1 /FeOx single-atom catalysts (SACs) in 2011, and they proposed the concept of "single-atom catalysis" in the field of heterogeneous catalysis. Generally, it is easy foractive metal single-atom sites on a carrier to migrate and aggregate, which results in poor performance; or the chemical bond between the metal atom and carrier is too strong (immovable), which results in passivation of the active site. Recently, "nano-island" type SACsweredesigned, in which the active metal atoms are isolated on the "islands", and can move within the respective "island", but the migration across the "island" is blocked, to achieve a dynamic confinement design of single atoms (that is, a "moving but not aggregating" design philosophy). Herein, a new concept of "single-atom nano-islands (SANIs)" is proposed to describe these congeneric "atomic-nano"systems in heterogeneous catalysis fields. Particularly, the SANIs are divided into three categories: "one-island-one-atom", "one-island-multi-atoms", and "island-sea synergism" architectures. The scientific significance and application principles of SANIs in versatile heterogeneous catalysis fields (i.e., thermocatalysis, electrocatalysis, and photocatalysis) are summarized. The challenges and proposals of SANIs are also provided.

Asymmetric Coordination of Iridium Single-atom IrN3 O Boosting Formic Acid Oxidation Catalysis.

Rational design of the proximal coordination of an active site to achieve its optimum catalytic activity is the ultimate goal in single-atom catalysis, but still challenging. Here, we report theoretical prediction and experimental realization of an asymmetrically coordinated iridium single-atom catalyst (IrN3 O) for the formic acid oxidation reaction (FAOR). Theoretical calculations reveal that the substitution of one or two nitrogen with more electronegative oxygen in the symmetric IrN4 motif splits and downshifts the Ir 5d orbitals with respect to the Fermi level, moderating the binding strength of key intermediates on IrN4-x Ox (x=1, 2) sites, especially that the IrN3 O motif shows ideal activity for FAOR with a near-zero overpotential. The as-designed asymmetric Ir motifs were realized by pyrolyzing Ir precursor with oxygen-rich glucose and nitrogen-rich melamine, exhibiting a mass activity of 25 and 87 times greater than those of state-of-the-art Pd/C and Pt/C, respectively.

Theoretical exploration of Rh1/CeO2 catalysts with high performance using CO oxidation as a probe reaction

Rh1/CeO2 attracts much attention in single-atom catalysis due to its unique catalytic performance and maximum atomic efficiency. Since different possible structures of Rh1/CeO2 coexist in real catalysts, it is hard to discriminate their catalytic roles and reaction mechanism due to the difficulty of preparing pure modeling catalyst. To solve this difficult problem, the adsorption and reaction behaviors of CO and O2 on four modeling catalysts of Rh1/CeO2 were systematically investigated by using density functional theory calculation in this work. The catalytic role and CO oxidation mechanism on Rh1/CeO2 catalysts were elucidated. On Rh1/CeO2(111), CO reacts with adsorbed O2 via Eley-Rideal mechanism; and the rate-determined step of CO oxidation cycle is CO reaction with the rest O atom of O2 with a barrier of 0.99 eV. On Rh1/CeO2-δ(111), CO reacts with activated O2 via Langmuir-Hinshelwood mechanism and continue the oxidation cycle via Rh1/CeO2(111). On RhxCe1-xO2, CO directly reacts with surface oxygen without barrier to form RhxCe1-xO2-δ; while the barrier of CO oxidation cycle on RhxCe1-xO2-δ is 1.18 eV. Upon the discrimination of the catalytic roles of Rh1/CeO2 catalysts with different structures, several important insights into design and development of Rh1/CeO2 catalysts with high oxidation performance were proposed.

Critical Review on the Mechanisms of Fe2+ Regeneration in the Electro-Fenton Process: Fundamentals and Boosting Strategies

This review presents an exhaustive overview on the mechanisms of Fe3+ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe3+ conversion to Fe2+ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe2+ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe3+/Fe2+ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.

Vacancy Defects in 2D Transition Metal Dichalcogenide Electrocatalysts: From Aggregated to Atomic Configuration.

Vacancy defect engineering has been well leveraged to flexibly shape comprehensive physicochemical properties of diverse catalysts. In particular, growing research effort has been devoted to engineering chalcogen anionic vacancies (S/Se/Te) of 2D transition metal dichalcogenides (2D TMDs) toward the ultimate performance limit of electrocatalytic hydrogen evolution reaction (HER). In spite of remarkable progress achieved in the past decade, systematic and in-depth insights into the state-of-the-art vacancy engineering for 2D-TMDs-based electrocatalysis are still lacking. Herein, this review delivers a full picture of vacancy engineering evolving from aggregated to atomic configurations covering their development background, controllable manufacturing, thorough characterization, and representative HER application. Of particular interest, the deep-seated correlations between specific vacancy regulation routes and resulting catalytic performance improvement are logically clarified in terms of atomic rearrangement, charge redistribution, energy band variation, intermediate adsorption-desorption optimization, and charge/mass transfer facilitation. Beyond that, a broader vision is cast into the cutting-edge research fields of vacancy-engineering-based single-atom catalysis and dynamic structure-performance correlations across catalyst service lifetime. Together with critical discussion on residual challenges and future prospects, this review sheds new light on the rational design of advanced defect catalysts and navigates their broader application in high-efficiency energy conversion and storage fields.

Simultaneous Detection of Ovarian Cancer-Concerned HE4 and CA125 Markers Based on Cu Single-Atom-Triggered CdS QDs and Eu MOF@Isoluminol ECL.

Simultaneous detection of different disease markers is significant for clinical diagnosis. In this work, a dual-signal electrochemiluminescence (ECL) immunosensor was constructed for the simultaneous detection of carbohydrate antigen 125 (CA125) and human epithelial protein 4 (HE4) markers of ovarian cancer. The results showed that the Eu metal-organic framework-loaded isoluminol-Au nanoparticles (Eu MOF@Isolu-Au NPs) could generate a strong anodic ECL signal through synergistic interaction; as cathodic luminophore, the composite of carboxyl-functionalized CdS quantum dots and N-doped porous carbon-anchored Cu single-atom catalyst could catalyze H2O2 co-reactant to produce a large amount of •OH and O2•-, making the anodic and cathodic ECL signals significantly increase and become stable. Based on the enhancement strategy, a sandwich immunosensor was constructed for the simultaneous detection of ovarian cancer-associated CA125 and HE4 markers by combining antigen-antibody specific recognition and magnetic separation technique. The resulting ECL immunosensor displayed high sensitivity, a wide linear response range of 0.005∼500 ng mL-1, and low detection limits of 0.37 and 1.58 pg mL-1 for CA125 and HE4, respectively. Furthermore, it had excellent selectivity, stability, and practicability in the detection of real serum samples. This work establishes a framework for in-depth design and application of single-atom catalysis in ECL sensing.

The single-atom catalytic activity of the hydrogen evolution reaction of the experimentally synthesized boridene 2D material: a density functional theory study

Previous theoretical studies have suggested that two-dimensional (2D) MBene materials might display adequate monatomic catalytic activity for the hydrogen evolution reaction (HER). Recently, a study reported the experimental synthesis of a 2D MBene (Mo4/3B2), re-defined as boridene, albeit no effort has been devoted to explore the single-atom catalytic activity for HER of this experimentally synthesized 2D material. Therefore, we herein investigate the single-atom HER performance of the boridene. Interestingly, with Mo defects mixed with single Au and Zn atoms shows excellent hydrogen evolution performance, and the change in the Gibbs free energy ([Formula: see text]) value is close to 0eV, which can even match the performance of Pt-based materials. Through analysis of the charge density difference and density of states, the mechanism affecting the HER performance is explained at the electronic level. This work provides a new direction for the use of the Mo4/3B2 monolayer two-dimensional materials in the field of single-atom catalysis for HER. This study used the DFT calculations in Vienna ab initio simulation package. The GGA-Perdew-Burke-Ernzerhof functional with DFT-D2 correction is used to describe the exchange-correlation interactions. The projection augmented wave is used with the plane wave cutoff of 500eV. The convergences of energy and force are 10-5eV and 0.01eV/Å, respectively. A vacuum layer with a height of 20Å is set in the Z direction. For geometry optimization, self-consistent, and DOS calculations, the k-point grids sampled in Brillouin zones are 3 × 3 × 1, 9 × 9 × 1, and 9 × 9 × 1, respectively. The AIMD simulation is performed in the canonical ensemble (NVT), and the temperature was maintained at 300K by Nosé-Hoover thermostats with a time step of 2.0fs.

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Macrocycle-Based Covalent Organic Frameworks.

Macrocycles with well-defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle-based COFs (M-COFs) preserve the macrocycles' unique activities, enabling applications in various fields such as single-atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single-layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M-COFs, and presents an in-depth outlook on the challenges and opportunities in this emerging field.

Stable single iron atom within two-dimensional porphyrin sheet catalysis for CO oxidation: A computational investigation

CO produced by incomplete combustion of fuel poses a great threat to the environment and human health. Single atom catalysis can increase the active site of the catalyst, but it is unstable and easy to agglomerate at the higher temperatures. A stable single iron atom two-dimensional porphyrin sheet (Fe–TDPS) was designed, which was as CO oxidation catalyst to reduce costs and increase catalytic activity, and the reaction mechanism was investigated by density functional theory. Fe–TDPS has good stability at 1000 K with the ab initio molecular dynamics simulation. Absorption energies suggested that both CO and O2 preferred to anchor at the Fe–N4 site. The mechanisms of CO oxidation, which have reacted on the Fe–TDPS, were explored by Langmuir–Hinshelwood, Eley–Rideal, and ter-molecular Eley–Rideal mechanisms. Ter-molecular Eley–Rideal mechanism was feasible because of the small energy barrier. Results indicated that Fe–TDPS can be used as a catalyst for CO oxidation in the mild condition.

Interlayer Charge Transfer Regulates Single-Atom Catalytic Activity on Electride/Graphene 2D Heterojunctions.

Single-atom catalysts with structure and activity tunability have attracted significant attention for energy and environmental applications. Herein we present a first-principles study of single-atom catalysis on two-dimensional graphene and electride heterostructures. The anion electron gas in the electride layer enables a colossal electron transfer to the graphene layer, with the degree of transfer being controllable by the selection of electride. The charge transfer tunes the d-orbital electron occupancy of a single metal atom, enhancing the catalytic activity of hydrogen evolution reactions and oxygen reduction reactions. The strong correlation between the adsorption energy Eads and the charge variation Δq suggests that interfacial charge transfer is a critical catalytic descriptor for the heterostructure-based catalysts. The polynomial regression model proves the importance of charge transfer and accurately predicts the adsorption energy of ions and molecules. This study provides a strategy to obtain high-efficiency single-atom catalysts using two-dimensional heterostructures.

Ru Catalysts Supported on Bamboo-like N-Doped Carbon Nanotubes: Activity and Stability in Oxidizing and Reducing Environment.

The catalysts with platinum-group metals on nanostructured carbons have been a very active field of research, but the studies were mainly limited to Pt and Pd. Here, Ru catalysts based on nitrogen-doped carbon nanotubes (N-CNTs) have been prepared and thoroughly characterized; Ru loading was kept constant (3 wt.%), while the degree of N-doping was varied (from 0 to 4.8 at.%) to evaluate its influence on the state of supported metal. Using the N-CNTs afforded ultrafine Ru particles (<2 nm) and allowed a portion of Ru to be stabilized in an atomic state. The presence of Ru single atoms in Ru/N-CNTs expectedly increased catalytic activity and selectivity in the formic acid decomposition (FAD) but had no effect in catalytic wet air oxidation (CWAO) of phenol, thus arguing against a key role of single-atom catalysis in the latter case. A remarkable difference between these two reactions was also found in regard to catalyst stability. In the course of FAD, no changes in the support or supported species or reaction rate were observed even at a high temperature (150 °C). In CWAO, although 100% conversions were still achievable in repeated runs, the oxidizing environment caused partial destruction of N-CNTs and progressive deactivation of the Ru surface by carbonaceous deposits. These findings add important new knowledge about the properties and applicability of Ru@C nanosystems.

D-p Hybridization-Induced "Trapping-Coupling-Conversion" Enables High-Efficiency Nb Single-Atom Catalysis for Li-S Batteries.

Single-atom catalysts have been paid more attention to improving sluggish reaction kinetics and anchoring polysulfide for lithium-sulfur (Li-S) batteries. It has been demonstrated that d-block single-atom elements in the fourth period can chemically interact with the local environment, leading to effective adsorption and catalytic activity toward lithium polysulfides. Enlightened by theoretical screening, for the first time, we design novel single-atom Nb catalysts toward improved sulfur immobilization and catalyzation. Calculations reveal that Nb-N4 active moiety possesses abundant unfilled antibonding orbitals, which promotes d-p hybridization and enhances anchoring capability toward lithium polysulfides via a "trapping-coupling-conversion" mechanism. The Nb-SAs@NC cell exhibits a high capacity retention of over 85% after 1000 cycles, a superior rate performance of 740 mA h g-1 at 7 C, and a competitive areal capacity of 5.2 mAh cm-2 (5.6 mg cm-2). Our work provides a new perspective to extend cathodes enabling high-energy-density Li-S batteries.

Sodium-Directed Photon-Induced Assembly Strategy for Preparing Multisite Catalysts with High Atomic Utilization Efficiency.

Integrating different reaction sites offers new prospects to address the difficulties in single-atom catalysis, but the precise regulation of active sites at the atomic level remains challenging. Here, we demonstrate a sodium-directed photon-induced assembly (SPA) strategy for boosting the atomic utilization efficiency of single-atom catalysts (SACs) by constructing multifarious Au sites on TiO2 substrate. Na+ was employed as the crucial cement to direct Au single atoms onto TiO2, while the light-induced electron transfer from excited TiO2 to Au(Na+) ensembles contributed to the self-assembly formation of Au nanoclusters. The synergism between plasmonic near-field and Schottky junction enabled the cascade electron transfer for charge separation, which was further enhanced by oxygen vacancies in TiO2. Our dual-site photocatalysts exhibited a nearly 2 orders of magnitude improvement in the hydrogen evolution activity under simulated solar light, with a striking turnover frequency (TOF) value of 1533 h-1 that exceeded other Au/TiO2-based photocatalysts reported. Our SPA strategy can be easily extended to prepare a wide range of metal-coupled nanostructures with enhanced performance for diverse catalytic reactions. Thus, this study provides a well-defined platform to extend the boundaries of SACs for multisite catalysis through harnessing metal-support interactions.

Carbon nitride based materials: more than just a support for single-atom catalysis.

Recently, the missing link between homogeneous and heterogeneous catalysis has been found and it was named single-atom catalysis (SAC). However, the SAC field still faces important challenges, one of which is controlling the bonding/coordination between the single atoms and the support in order to compensate for the increase in surface energy when the particle size is reduced due to atomic dispersion. Excellent candidates to meet this requirement are carbon nitride (CN)-based materials. Metal atoms can be firmly trapped in nitrogen-rich coordination sites in CN materials, which makes them a unique class of hosts for preparing single-atom catalysts (SACs). As one of the most promising two-dimensional supports to stabilize isolated metal atoms, CN materials have been increasingly employed for preparing SACs. Herein, we will cover the most recent advances in single-atoms supported by CN materials. In this review, the most important characterization techniques and the challenges faced in this topic will be discussed, and the commonly employed synthetic methods will be delineated for different CN materials. Finally, the catalytic performance of SACs based on carbon nitrides will be reviewed with a special focus on their photocatalytic applications. In particular, we will prove CN as a non-innocent support. The relationship between single-atoms and carbon nitride supports is two-way, where the single-atoms can change the electronic properties of the CN support, while the electronic features of the CN matrix can tune the catalytic activity of the single sites in photocatalytic reactions. Finally, we highlight the frontiers in the field, including analytical method development, truly controlled synthetic methods, allowing the fine control of loading and multi-element synthesis, and how understanding the two-way exchange behind single-atoms and CN supports can push this topic to the next level.

Single-atom catalysis enabled by high-energy metastable structures.

Owing to limited degrees of freedom, the active sites of stable single-atom catalyst (SAC) often have one structure that is energetically much lower than other local-minimum structures. Thus, the SAC adopts one lowest-energy structure (LES) with an overwhelmingly larger proportion than any other high-energy metastable structure (HEMS), and the LES is commonly assumed to be solely responsible for the catalytic performance of an SAC. Herein, we demonstrate with SACs anchored on CeO2 that the HEMS of an SAC, even though its proportion remains several orders of magnitude lower than the LES throughout the catalytic reaction, can dictate catalysis with extraordinary activity arising from its unique coordination environment and oxidation state. Thus, we unravel the key role of HEMS-enabled catalysis in single-atom catalysis, which shakes the common assumption in the studies of SACs and urges new developments in both experiment and theory to identify and exploit catalysis via HEMSs.

Coexistence of Au single atoms and Au nanoparticles on NiAl-LDH for selective electrooxidation of benzyl alcohol to benzaldehyde.

Introducing different active sites into heterogeneous catalysts provides new prospects to address the challenges in single-atom catalysis. Herein, the Au single atoms together and the Au nanoparticles were loaded onto NiAl-LDH by a facile impregnation-reduction method for the first time, resulting in the formation of Au1+n-NiAl-LDH, in which abundant Au single atoms are located around the Au nanoparticles with ∼5 nm size. When applied in the electrocatalytic benzyl alcohol oxidation reaction (BAOR), the as-prepared Au1+n-NiAl-LDH exhibits a remarkable selectivity of 91% and 177.63 μmol for benzaldehyde in 5 hours, while in contrast examples using solely Au single atom loaded NiAl-LDH (Au1-NiAl-LDH) and solely Au nanoparticle loaded NiAl-LDH (Aun-NiAl-LDH) can only realize 87.36 μmol production (75% selectivity) and 48.90 μmol production (28% selectivity) of benzaldehyde, respectively. Such a dramatic difference can be attributed to the synergistic effects of Au single atoms and Au nanoparticles. DFT calculation results reveal that for Au1+n-NiAl-LDH, Au single atoms promote the dehydrogenation capacity of LDH laminates, while Au nanoparticles offer adsorption sites for the electrophilic attachment of benzyl alcohol.

G-C3N4Photocatalysts: Utilizing Electron–Hole Pairs for Boosted Redox Capability in Water Splitting

Since the first discovery of solar-driven water splitting catalyzed by TiO2semiconductors, extensive research works have been devoted over the decades. Currently, the design of a photocatalyst with dual redox potential is of prominent interest to fully utilize both photogenerated electrons and holes in the redox reactions. Among all, the coproduction of H2and O2from water using metal-free carbon nitride (g-C3N4) has been viewed as a rising star in this field. However, the hole-mediated oxidation reaction is commonly recognized as the rate-determining step, which drastically leads to poor overall water splitting efficiency. On top of that, rapid recombination and undesirable back reaction appeared as one of the challenging parts in overall water splitting. In this review, the up-to-date advances in modified g-C3N4-based photocatalysts toward efficient overall water splitting are summarized, which are mainly classified into structural and defect engineering, single-atom catalysis, cocatalyst loading, and heterojunction construction. This review also addresses the underlying idea and concept to tackle the aforementioned problem with the use of emerging modification strategies, hence serving as the guiding star for future research. Despite the outstanding breakthrough thus far, critical recommendations related to g-C3N4photocatalytic systems are prospected to pave the way toward the implementation in the practical energy production process.

Progress on the in situ imaging of growth dynamics of two-dimensional materials.

One key issue to promote the industrialization of two-dimensional (2D) materials is to grow high-quality and large-scale 2D materials. Investigations of the growth mechanism and growth dynamics are of fundamental importance for the growth of 2D material, in which in situ imaging is highly needed. By applying different in situ imaging techniques, details for growth process, including nucleation and morphology evolution, can be obtained. This review summarizes the recent progress on the in situ imaging of 2D material growth, in which the growth rate, kink dynamics, domain coalescence, growth across the substrate steps, single-atom catalysis, and intermediates have been revealed.