A heterogeneous single-atom palladium catalyst surpassing homogeneous systems for Suzuki coupling

Expedient synthesis of E-hydrazone esters and 1H-indazole scaffolds through heterogeneous single-atom platinum catalysis

Unprotected E-hydrazone esters are prized building blocks for the preparation of 1H-indazoles and countless other N-containing biologically active molecules. Despite previous advances, efficient and stereoselective synthesis of these compounds remains nontrivial. Here, we show that Pt single atoms anchored on defect-rich CeO₂ nanorods (Pt₁/CeO₂), in conjunction with the alcoholysis of ammonia borane, promotes exceptionally E-selective hydrogenation of α-diazoesters to afford a wide assortment of N-H hydrazone esters with an overall turnover frequency of up to 566 hours^-1 upon reaction completion. The α-diazoester substrates could be generated in situ from readily available carboxylic esters in one-pot hydrogenation reaction. Utility is demonstrated through concise, scalable synthesis of 1H-indazole-derived pharmaceuticals and their ¹⁵N-labeled analogs. The present protocol highlights a key mechanistic nuance wherein simultaneous coordination of a Pt site with the diazo N═N and ester carbonyl motifs plays a central role in controlling stereoselectivity, which is supported by density functional theory calculations.

Confinement Catalysis with 2D Materials for Energy Conversion

The unique electronic and structural properties of 2D materials have triggered wide research interest in catalysis. The lattice of 2D materials and the interface between 2D covers and other substrates provide intriguing confinement environments for active sites, which has stimulated a rising area of "confinement catalysis with 2D materials." Fundamental understanding of confinement catalysis with 2D materials will favor the rational design of high-performance 2D nanocatalysts. Confinement catalysis with 2D materials has found extensive applications in energy-related reaction processes, especially in the conversion of small energy-related molecules such as O₂ , CH₄ , CO, CO₂ , H₂ O, and CH₃ OH. Two representative strategies, i.e., 2D lattice-confined single atoms and 2D cover-confined metals, have been applied to construct 2D confinement catalytic systems with superior catalytic activity and stability. Herein, the recent advances in the design, applications, and structure-performance analysis of two 2D confinement catalytic systems are summarized. The different routes for tuning the electronic states of 2D confinement catalysts are highlighted and perspectives on confinement catalysis with 2D materials toward energy conversion and utilization in the future are provided.

Merging Single-Atom-Dispersed Iron and Graphitic Carbon Nitride to a Joint Electronic System for High-Efficiency Photocatalytic Hydrogen Evolution

Scalable and sustainable solar hydrogen production via photocatalytic water splitting requires extremely active and stable light-harvesting semiconductors to fulfill the stringent requirements of suitable energy band position and rapid interfacial charge transfer process. Motivated by this point, increasing attention has been given to the development of photocatalysts comprising intimately interfaced photoabsorbers and cocatalysts. Herein, a simple one-step approach is reported to fabricate a high-efficiency photocatalytic system, in which single-site dispersed iron atoms are rationally integrated on the intrinsic structure of the porous crimped graphitic carbon nitride (g-C₃ N₄ ) polymer. A detailed analysis of the formation process shows that a stable complex is generated by spontaneously coordinating dicyandiamidine nitrate with iron ions in isopropanol, thus leading to a relatively complicated polycondensation reaction upon thermal treatment. The correlation of experimental and computational results confirms that optimized electronic structures of Fe@g-C₃ N₄ with an appropriate d-band position and negatively shifting Fermi level can be achieved, which effectively gains the reducibility of electrons and creates more active sites for the photocatalytic reactions. As a result, the Fe@g-C₃ N₄ exhibits a highlighted intramolecular synergistic effect, performing greatly enhanced solar-photon-driven activities, including excellent photocatalytic hydrogen evolution rate (3390 µmol h^-1 g^-1 , λ > 420 nm) and a reliable apparent quantum efficiency value of 6.89% at 420 nm.

Single Atomic Cu-N2 Catalytic Sites for Highly Active and Selective Hydroxylation of Benzene to Phenol

Searching for an efficient single-atom catalyst for benzene hydroxylation to phenol is of critical importance, but it still remains a challenge. Herein, a single-atom catalyst with unique Cu-N₂ moieties (Cu₁-N₂/HCNS) was prepared and confirmed by HAADF-STEM and EXAFS. Turnover number (TON) over Cu₁-N₂/HCNS (6,935) is 3.4 times of Cu₁-N₃/HCNS (2,034) under the same reaction conditions, and both exhibit much higher phenol selectivity (close to 99%) and stability compared with Cu nanoparticles and nanoclusters. Experiments and DFT calculations reveal that atomically dispersed Cu species are active sites for benzene hydroxylation to phenol, and the Cu-N₂ is more active than Cu-N₃ owing to its much lower energy barrier concerning the activation of H₂O₂ led by its unique coordination state of local atomic structure. We envision that this work opens a new window for modulating coordination environments of single metallic atoms in catalysis design.

Versatile Applications of Metal Single-Atom @ 2D Material Nanoplatforms

Recently, emerging 2D material-supported metal single-atom catalysts (SACs) are receiving enormous attention in heterogeneous catalysis. Due to their well-defined, precisely located metal centers, unique metal-support interaction and identical coordination environment, these catalysts serve as excellent models for understanding the fundamental issues in catalysis as well as exhibiting intriguing practical applications. Understanding the correlations between metal-support combinations and the catalytic performance at the atomic level can be achieved on the SACs@2D materials nanoplatforms. Herein, recent advances of metal SACs on various types of 2D materials are reviewed, especially their exciting applications in the fields of chemicals, energy, and the environment. Based on the summary and perspectives, this work should contribute to the rational design of perfect metal SACs with versatile properties.

Tuning the Electrocatalytic Activity of Co3O4 through Discrete Elemental Doping

To gain constructive insight into the possible effect of doping on the electrocatalytic activity of materials, a catalytic framework with a discrete distribution of dopants is an appropriate model system. Such a system assures well-defined active centers, maximum atom utilization efficiency, and hence enhanced selectivity, catalytic activity, and stability. Herein, a comprehensive investigation of the electrocatalytic activity of iron-doped cobalt oxide (Fe-Co₃O₄) nanosheets is presented. In order to understand the contribution of dopants, a series of materials with controlled doping levels are investigated. By controlled iron inclusion into the structure of Co₃O₄, an apparent improvement in the oxygen evolution reaction activity which is reflected in the decrease of 160 mV in the overpotential to reach the current density of 10 mA/cm² is manifested. Additionally, it is shown that there exists an optimum doping content above which the catalytic activity fades. Further investigation of the system with density functional calculations reveals that, along with the optimization of adsorption energy toward the reaction intermediates, substantial downshift of the Fermi level and delocalization of electron density occurs on introducing iron ions into the structure.

Low-Temperature Fast Production of Carbon and Acetic Acid Dual-Promoted Pd/C Catalysts

The Pd/C catalysts are widely used in synthesis of fine chemicals in industry, but their production suffers from a complicated two-step process involving impregnation and reduction, and requires large amounts of solvents and reductant, which would lead to a series of issues such as time consumption, resource waste and environmental pollution. Herein, ultra-small Pd nanoparticles uniformly anchored on carbon nanotubes (Pd/CNTs) were synthesized by using a one-pot and low-temperature reduction strategy. The present process/technology is very sensitive to and controlled by the supports and solvents, and the carbon support and acetic acid synergistically play crucial and decisive roles in the fast production of Pd/C catalysts. Also, the used solvents can be recycled and reutilized, which meets the requirements of sustainable chemistry and green economy. When the as-obtained Pd/CNTs catalyst was used to catalyze the oxidation of benzyl alcohol to benzaldehyde, it achieved a conversion efficiency as high as 99.3 % and a high selectivity up to >99.9 %. The simple, scalable and environmentally friendly strategy can be extended to anchor Pd nanoparticles on various carbon substrates, which sheds a new light on the synthesis of Pd/C catalysts.

Increasing Solar Absorption of Atomically Thin 2D Carbon Nitride Sheets for Enhanced Visible-Light Photocatalysis

Atomically thin 2D carbon nitride sheets (CNS) are promising materials for photocatalytic applications due to their large surface area and very short charge-carrier diffusion distance from the bulk to the surface. However, compared to their bulk counterpart, CNS' applications always suffer from an enlarged bandgap and thus narrowed solar absorption range. Here, an approach to significantly increase solar absorption of the atomically thin CNS via fluorination followed by thermal defluorination is reported. This approach can greatly increase the visible-light absorption of CNS by extending the absorption edge up to 578 nm. The modulated CNS loaded with Pt cocatalyst as a photocatalyst shows a superior photocatalytic hydrogen production activity under visible-light irradiation to Pt-CNS. Combining experimental characterization with theoretical calculations shows that this approach can introduce cyano groups into the framework of CNS as well as the accompanied nitrogen vacancies at the edges, which leads to both narrowing the bandgap and changing the charge distribution. This study will provide an effective strategy to increase solar absorption of carbon-nitride-based photocatalysts for solar energy conversion applications.

Anchoring Cu1 species over nanodiamond-graphene for semi-hydrogenation of acetylene

The design of cheap, non-toxic, and earth-abundant transition metal catalysts for selective hydrogenation of alkynes remains a challenge in both industry and academia. Here, we report a new atomically dispersed copper (Cu) catalyst supported on a defective nanodiamond-graphene (ND@G), which exhibits excellent catalytic performance for the selective conversion of acetylene to ethylene, i.e., with high conversion (95%), high selectivity (98%), and good stability (for more than 60 h). The unique structural feature of the Cu atoms anchored over graphene through Cu-C bonds ensures the effective activation of acetylene and easy desorption of ethylene, which is the key for the outstanding activity and selectivity of the catalyst.

It is highly desired to explore cheap, non-toxic transition metals catalysts for semihydrogenation of acetylene. Here, isolated Cu atoms anchored onto a defective nanodiamond-graphene support exhibit robust catalytic performance in acetylene semihydrogenation in comparison with supported Cu clusters.

Local Structure and Coordination Define Adsorption in a Model Ir1 /Fe3 O4 Single-Atom Catalyst

Single-atom catalysts (SACs) bridge homo- and heterogeneous catalysis because the active site is a metal atom coordinated to surface ligands. The local binding environment of the atom should thus strongly influence how reactants adsorb. Now, atomically resolved scanning-probe microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and DFT are used to study how CO binds at different Ir1 sites on a precisely defined Fe3 O4 (001) support. The two- and five-fold-coordinated Ir adatoms bind CO more strongly than metallic Ir, and adopt structures consistent with square-planar IrI and octahedral IrIII complexes, respectively. Ir incorporates into the subsurface already at 450 K, becoming inactive for adsorption. Above 900 K, the Ir adatoms agglomerate to form nanoparticles encapsulated by iron oxide. These results demonstrate the link between SAC systems and coordination complexes, and that incorporation into the support is an important deactivation mechanism.

Directly transforming copper (I) oxide bulk into isolated single-atom copper sites catalyst through gas-transport approach

Single-atom metal catalysts have sparked tremendous attention, but direct transformation of cheap and easily obtainable bulk metal oxide into single atoms is still a great challenge. Here we report a facile and versatile gas-transport strategy to synthesize isolated single-atom copper sites (Cu ISAS/NC) catalyst at gram levels. Commercial copper (I) oxide powder is sublimated as mobile vapor at nearly melting temperature (1500 K) and subsequently can be trapped and reduced by the defect-rich nitrogen-doped carbon (NC), forming the isolated copper sites catalyst. Strikingly, this thermally stable Cu ISAS/NC, which is obtained above 1270 K, delivers excellent oxygen reduction performance possessing a recorded half-wave potential of 0.92 V vs RHE among other Cu-based electrocatalysts. By varying metal oxide precursors, we demonstrate the universal synthesis of different metal single atoms anchored on NC materials (M ISAS/NC, where M refers to Mo and Sn). This strategy is readily scalable and the as-prepared sintering-resistant M ISAS/NC catalysts hold great potential in high-temperature applications.

Single-atom catalysts attract lots of attention, but direct transformation of bulk metal oxide into single atoms remains challenging. Here the authors report a gas-transport route to transform monolithic copper (I) oxide into copper single-atoms catalyst with a high activity for oxygen reduction.

Freestanding Hierarchical Carbon Nitride/Carbon-Paper Electrode as a Photoelectrocatalyst for Water Splitting and Dye Degradation

Freestanding electrodes composed of 2D materials are highly attractive for many applications such as batteries, membranes, actuators, optical devices, and other energy-related devices owing to their low price, unique structure, high specific surface area, and excellent mechanical and electrical properties. Here, we report the facile large-scale fabrication of freestanding hierarchical carbon nitride/carbon electrodes (CN/C) by the in situ crystallization of CN precursors on conductive carbon paper, followed by thermal annealing. The resulting CN exhibits a vertically aligned morphology with a homogeneous layer distribution, improved crystallinity, and excellent contact with the carbon paper. The freestanding electrodes exhibit high electrical conductivity and good photoelectrochemical activity as anodes in water splitting photoelectrochemical cells. Furthermore, we show here as a proof-of-concept that the freestanding CN/C electrodes can be used as photoelectrocatalysts for the oxidative degradation of organic compounds in water, with enhanced activity compared to photocatalytic and electrocatalytic degradation, while the extracted electrons can be used for the simultaneous production of hydrogen at the cathode.

Construction of Single-Iron-Atom Nanocatalysts for Highly Efficient Catalytic Antibiotics

Bacterial infection caused by pathogenic bacteria has long been an intractable issue that threatens human health. Herein, the fact that nanocatalysts with single iron atoms anchored in nitrogen-doped amorphous carbon (SAF NCs) can effectively induce peroxidase-like activities in the presence of H₂ O₂ , generating abundant hydroxyl radicals for highly effective bacterial elimination (e.g., Escherichia coli and Staphylococcus aureus), is reported. In combination with the intrinsic photothermal performance of the nanocatalysts, noticeable bacterial-killing effects are extensively investigated. Especially, the antibacterial mechanism of critical cell membrane destruction induced by SAF NCs is unveiled. Based on the bactericidal properties of SAF NCs, in vivo bacterial infections propagated at wounds by E. coli and S. aureus pathogens can be effectively eradicated, resulting in better wound healing. Collectively, the present study highlights the highly efficient in vitro antibacterial and in vivo anti-infection performances by the single-iron-atom-containing nanocatalysts.

Atom-by-Atom Resolution of Structure-Function Relations over Low-Nuclearity Metal Catalysts

Controlling the structure sensitivity of catalyzed reactions over metals is central to developing atom-efficient chemical processes. Approaching the minimum ensemble size, the properties enter a non-scalable regime in which each atom counts. Almost all trends in this ultra-small frontier derive from surface science approaches using model systems, because of both synthetic and analytical challenges. Exploiting the unique coordination chemistry of carbon nitride, we discriminate through experiments and simulations the interplay between the geometry, electronic structure, and reactivity of palladium atoms, dimers, and trimers. Catalytic tests evidence application-dependent requirements of the active ensemble. In the semi-hydrogenation of alkynes, the nuclearity primarily impacts activity, whereas the selectivity and stability are affected in Suzuki coupling. This powerful approach will provide practical insights into the design of heterogeneous catalysts comprising well-defined numbers of atoms.

Atomically Dispersed Pt1-Polyoxometalate Catalysts: How Does Metal-Support Interaction Affect Stability and Hydrogenation Activity?

Unlike nanostructured metal catalysts, supported single-atom catalysts (SACs) contain only atomically dispersed metal atoms, hinting at much more pronounced metal-support effects. Herein, we take a series of polyoxometalate-supported Pt catalysts as examples to quantitatively investigate the stability of Pt atoms on oxide supports and how the Pt-support interaction influences the catalytic performance. For this entire series, we show that the Pt atoms prefer to stay at a 4-fold hollow site of one polyoxometalate molecule and that the least adsorption energy to obtain sintering-resistant Pt SACs is 5.50 eV, which exactly matches the cohesive energy of bulk Pt metal. Further, we compared their catalytic performance in several hydrogenation reactions and simulated the reaction pathways of propene hydrogenation by density functional theory (DFT) calculations. Both experimental and theoretical approaches suggest that despite the Pt₁-support interactions being different, the reaction pathways of various Pt₁-polyoxometalate catalysts are very similar and their effective reaction barriers are close to each other and as low as 24 kJ/mol, indicating the possibility of obtaining SACs with improved stability without compromising activity. DFT calculations show that all reaction elementary steps take place only on the Pt atom without involving neighboring O atoms and that hydrogenation proceeds from the molecularly adsorbed H₂ species. Pt SACs give a weaker H₂ adsorption energy than Pt clusters or surfaces, resulting in small adsorption equilibrium constants and small apparent activation barriers, which agree between experiment and theory.

Metal Nanoclusters: New Paradigm in Catalysis for Water Splitting, Solar and Chemical Energy Conversion

A sustainable future demands innovative breakthroughs in science and technology today, especially in the energy sector. Earth-abundant resources can be explored and used to develop renewable and sustainable resources of energy to meet the ever-increasing global energy demand. Efficient solar-powered conversion systems exploiting inexpensive and robust catalytic materials for the photo- and photo-electro-catalytic water splitting, photovoltaic cells, fuel cells, and usage of waste products (such as CO₂ ) as chemical fuels are appealing solutions. Many electrocatalysts and nanomaterials have been extensively studied in this regard. Low overpotentials, catalytic stability, and accessibility remain major challenges. Metal nanoclusters (NCs, ≤3 nm) with dimensions between molecule and nanoparticles (NPs) are innovative materials in catalysis. They behave like a "superatom" with exciting size- and facet-dependent properties and dynamic intrinsic characteristics. Being an emerging field in recent scientific endeavors, metal NCs are believed to replace the natural photosystem II for the generation of green electrons in a viable way to facilitate the challenging catalytic processes in energy-conversion schemes. This Review aims to discuss metal NCs in terms of their unique physicochemical properties, possible synthetic approaches by wet chemistry, and various applications (mostly recent advances in the electrochemical and photo-electrochemical water splitting cycle and the oxygen reduction reaction in fuel cells). Moreover, the significant role that MNCs play in dye-sensitized solar cells and nanoarrays as a light-harvesting antenna, the electrochemical reduction of CO₂ into fuels, and concluding remarks about the present and future perspectives of MNCs in the frontiers of surface science are also critically reviewed.

In situ spectroscopy-guided engineering of rhodium single-atom catalysts for CO oxidation

Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O₂ is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T₂₀ values up to 130 °C below the previously reported Rh₁/NPTA.

Single-atom catalysts have been studied for CO oxidation, but experimental in situ investigations are limited. Here, the authors use a suite of in situ/operando spectroscopy to identify key intermediates and define design principles to enhance the CO oxidation activity of atomically dispersed Rh on heteropoly acids.

Is the Suzuki-Miyaura Cross-Coupling Reaction in the Presence of Pd Nanoparticles Heterogeneously or Homogeneously Catalyzed? An Interfacial Surface-Enhanced Raman Spectroscopy Study

Pd-catalyzed Suzuki-Miyaura C-C cross-coupling is very central in chemistry. The question of whether the catalysis by using Pd nanoparticles (NPs) is heterogeneous (on the Pd surface) or homogeneous (by soluble Pd released from the NP surface) remains under fundamental physicochemical debate. This work reports on the in situ characterization of the Suzuki-Miyaura cross-coupling reactions by using surface-enhanced Raman spectroscopy. We find clear evidence of heterogeneous catalysis on the Pd surface. In contrast, the soluble Pd species leaching into the solution cannot catalyze the reaction, indicating a direct contact of the aryl halides with the metal surface is a prerequisite. Accordingly, the surface ligands and charge of the Pd NPs, which determine the molecule-metal contact, are very important in the couplings. By a simple exchange of the surface ligand or a surface modification of the support material, the catalytic activity of Pd NPs is improved due to the enhanced electric attraction between the metal and the reactant molecules.

Nanocatalytic Tumor Therapy by Single-Atom Catalysts

Initiating localized catalytic chemical reactions in tumor microenvironment (TME) can achieve appealing tumor-therapeutic efficacy concurrently with high specificity and desirable biosafety, which is mainly dependent on the high performance of biomedical nanocatalysts. This report demonstrates that PEGylated single-atom Fe-containing nanocatalysts (PSAF NCs) could effectively trigger the in situ tumor-specific Fenton reaction to generate abundant toxic hydroxyl radicals (•OH) selectively under the acidic TME. Based on density functional theory, it has been theoretically uncovered that the nanocatalysts could specifically catalyze the heterogeneous Fenton reaction via a proton-mediated H₂O₂-homolytic pathway. These generated radicals could not only lead to the apoptotic cell death of malignant tumors, but also induce the accumulation of lipid peroxides, causing tumor cell ferroptosis, which synergistically lead to an impressive tumor suppression outcome. In the meantime, the favorable biodegradability and biocompatibility of PSAF NCs also guarantee their desirable biosafety both in vivo and in vitro.

Catalytic farming: reaction rotation extends catalyst performance

The use of heterogeneous catalysis has key advantages compared to its homogeneous counterpart, such as easy catalyst separation and reusability. However, one of the main challenges is to ensure good performance after the first catalytic cycles. Active catalytic species can be inactivated during the catalytic process leading to reduced catalytic efficiency, and with that loss of the advantages of heterogeneous catalysis. Here we present an innovative approach in order to extend the catalyst lifetime based on the crop rotation system used in agriculture. The catalyst of choice to illustrate this strategy, Pd@TiO₂, is used in alternating different catalytic reactions, which reactivate the catalyst surface, thus extending the reusability of the material, and preserving its selectivity and efficiency. As a proof of concept, different organic reactions were selected and catalyzed by the same catalytic material during target molecule rotation.

Static Regulation and Dynamic Evolution of Single-Atom Catalysts in Thermal Catalytic Reactions

Single-atom catalysts provide an ideal platform to bridge the gap between homogenous and heterogeneous catalysts. Here, the recent progress in this field is reported from the perspectives of static regulation and dynamic evolution. The syntheses and characterizations of single-atom catalysts are briefly discussed as a prerequisite for catalytic investigation. From the perspective of static regulation, the metal-support interaction is illustrated in how the supports alter the electronic properties of single atoms and how the single atoms activate the inert atoms in supports. The synergy between single atoms is highlighted. Besides these static views, the surface reconstruction, such as displacement and aggregation of single atoms in catalytic conditions, is summarized. Finally, the current technical challenges and mechanistic debates in single-atom heterogeneous catalysts are discussed.

Non defect-stabilized thermally stable single-atom catalyst

Surface-supported isolated atoms in single-atom catalysts (SACs) are usually stabilized by diverse defects. The fabrication of high-metal-loading and thermally stable SACs remains a formidable challenge due to the difficulty of creating high densities of underpinning stable defects. Here we report that isolated Pt atoms can be stabilized through a strong covalent metal-support interaction (CMSI) that is not associated with support defects, yielding a high-loading and thermally stable SAC by trapping either the already deposited Pt atoms or the PtO₂ units vaporized from nanoparticles during high-temperature calcination. Experimental and computational modeling studies reveal that iron oxide reducibility is crucial to anchor isolated Pt atoms. The resulting high concentrations of single atoms enable specific activities far exceeding those of conventional nanoparticle catalysts. This non defect-stabilization strategy can be extended to non-reducible supports by simply doping with iron oxide, thus paving a new way for constructing high-loading SACs for diverse industrially important catalytic reactions.