Pd Single-Atom Catalysts on Nitrogen-Doped Graphene for the Highly Selective Photothermal Hydrogenation of Acetylene to Ethylene

Atomic-level tailoring of single-atom tungsten catalysts for optimized electrochemical nitrate-to-ammonia conversion

Nitrate contamination of water resources poses significant health and environmental risks, necessitating efficient denitrification methods that produce ammonia as a desirable product. The electrocatalytic nitrate reduction reaction (NO3RR) powered by renewable energy offers a promising solution, however, developing highly active and selective catalysts remains challenging. Single-atom catalysts (SACs) have shown impressive performance, but the crucial role of their coordination environment, especially the next-nearest neighbor dopant atoms, in modulating catalytic activity for NO3RR is underexplored. This study aims to optimize the NO3RR performance of tungsten (W) single atoms anchored on graphene by precisely engineering their coordination environment through first and next-nearest neighbor dopants. The stability, reaction paths, activity, and selectivity of 43 different nitrogen and boron doping configurations were systematically studied using density functional theory. The results reveal W@C3, with W coordinated to three carbon atoms, exhibits outstanding NO3RR activity with a low limiting potential of -0.36 V. Intriguingly, introducing next-nearest neighbor B and N dopants further enhances the performance, with W@C3-BN achieving a lower limiting potential of -0.26 V. This exceptional activity originates from optimal nitrate adsorption strengths facilitated by orbital hybridization and charge modulation effects induced by the dopants. Furthermore, high energy barriers for NO2 and NO formation on W@C3 and W@C3-BN ensure their selectivity towards NO3RR products. These findings provide crucial atomic-level insights into rational design strategies for high-performance single-atom NO3RR catalysts via coordination environment engineering.

Ambient-condition acetylene hydrogenation to ethylene over WS2-confined atomic Pd sites

Ambient-condition acetylene hydrogenation to ethylene (AC-AHE) is a promising process for ethylene production with minimal additional energy input, yet remains a great challenge due to the difficulty in the coactivation of acetylene and H2 at room temperature. Herein, we report a highly efficient AC-AHE process over robust sulfur-confined atomic Pd species on tungsten sulfide surface. The catalyst exhibits over 99% acetylene conversion with a high ethylene selectivity of 70% at 25 oC, and a record space-time yield of ethylene of 1123 molC2H4 molPd-1 h-1 under ambient conditions, which is nearly four times that of the typical Pd1Ag3/Al2O3 catalyst, and exhibiting superior stability of over 500 h. We demonstrate that the confinement of Pd-S coordination induces positively-charged atomic Pdδ+, which not only facilitates C2H2 hydrogenation but also promotes C2H4 desorption, thereby enabling a high conversion of C2H2 to C2H4 at room temperature while suppressing over-hydrogenation to C2H6.

Reinforcing the Efficiency of Plastic Upgrading through Full-Spectrum Photothermal Effect Integration of Heat Isolator

Photoreforming of polyethylene terephthalate (PET) to H2 is practically attractive strategy for upgrading waste plastics. The major challenge is to utilize the infrared energy in the solar spectrum to improve the efficiency for photoreforming of PET to H2. Herein, through the ingenious integration of tungsten phosphide nanoparticles and tungsten single atoms (WP/W SAs) with carbon nitride (g-C3N4), the constructed hybrid inherits both the desirable properties and structural merits of the respective building blocks. Specifically, the photothermal effect of WP/W SAs couples with the "heat isolator" role of g-C3N4 due to its low thermal conductivity, thereby forming localized high-temperature regions, reducing the activation energy and improving the kinetics in the photoreforming of PET to H2. Additionally, the green pretreatment of PET using alkali-free hydrothermal strategy is reported, achieving direct separation of the ethylene glycol and terephthalic acid. This work not only provides an alkali-free hydrothermal pretreatment for PET, but also integrates the photothermal effect with the thermal insulation and opens a new avenue for harnessing solar energy into to convert plastics into H2.

Photocatalytic Semi-Hydrogenation of Acetylene to Polymer-Grade Ethylene with Molecular and Metal-Organic Framework Cobaloximes

The semi-hydrogenation of acetylene in ethylene-rich gas streams is a high-priority industrial chemical reaction for producing polymer-grade ethylene. Traditional thermocatalytic routes for acetylene reduction to ethylene, despite progress, still require high temperatures and high H2 consumption, possess relatively low selectivity, and use a noble metal catalyst. Light-powered strategies are starting to emerge, given that they have the potential to use directly the abundant and sustainable solar irradiation, but are ineffective. Here an efficient, >99.9% selective, visible-light powered, catalytic conversion of acetylene to ethylene is reported. The catalyst is a homogeneous molecular cobaloxime that operates in tandem with a photosensitizer at room temperature and bypasses the use of non-environmentally friendly and flammable H2 gas feed. The reaction proceeds through a cobalt-hydride intermediate with ≈100% conversion of acetylene under competitive (ethylene co-feed) conditions after only 50 min, and with no evolution of H2 or over-hydrogenation to ethane. The cobaloxime is further incorporated as a linker in a metal-organic framework; the result is a heterogeneous catalyst for the conversion of acetylene under competitive (ethylene co-feed) conditions that can be recycled up to six times and remains catalytically active for 48 h, before significant loss of performance is observed.

Large-Scale Synthesis of High-Loading Single Metallic Atom Catalysts by a Metal Coordination Route

Single atom catalyst (SAC) is one of the most efficient and versatile catalysts with well-defined active sites. However, its facile and large-scale preparation, the prerequisite of industrial applications, has been very challenging. This dilemma originates from the Gibbs-Thomson effect, which renders it rather difficult to achieve high single atom loading (< 3 mol%). Further, most synthesizing procedures are quite complex, resulting in significant mass loss and thus low yields. Herein, a novel metal coordination route is developed to address these issues simultaneously, which is realized owing to the rapid complexation between ligands (e.g., biuret) and metal ions in aqueous solutions and subsequent in situ polymerization of the formed complexes to yield SACs. The whole preparation process involves only one heating step operated in air without any special protecting atmospheres, showing general applicability for diverse transition metals. Take Cu SAC for an example, a record yield of up to 3.565 kg in one pot and an ultrahigh metal loading 16.03 mol% on carbon nitride (Cu/CN) are approached. The as-prepared SACs are demonstrated to possess high activity, outstanding selectivity, and robust cyclicity for CO2 photoreduction to HCOOH. This research explores a robust route toward cost-effective, massive production of SACs for potential industrial applications.

Highly Stable Cesium Molybdenum Chloride Perovskite Nanocrystals for Photothermal Semihydrogenation Applications

Metal halide perovskite materials with excellent carrier transport properties have been regarded as a new class of catalysts with great application potential. However, their development is hampered by their instability in polar solvents and high temperatures. Herein, we report a solution-processed Cs2MoCl6 perovskite nanocrystals (NCs) capped with the Mo6+, showing high thermostability in polar solvents. Furthermore, the Pd single atoms (PdSA) can be anchored on the surface of Cs2MoCl6 NCs through the unique coordination structure of Pd-Cl sites, which exhibit excellent semihydrogenation of different alkyne derivatives with high selectivity at full conversion at room temperature. Moreover, the activity could be improved greatly under Xe lamp irradiation. Detailed experimental characterization and DFT calculations indicate the improved activity under light illumination is due to the synergistic effect of photo-to-heat conversion and photoinduced electron transfer from Cs2MoCl6 to PdSA, which facilitates the activation of the C≡C group. This work not only provides a new catalyst for high selective semihydrogenation of alkyne derivatives but also opens a new avenue for metal halides as photothermal catalysts.

Advances of Synergistic Electrocatalysis Between Single Atoms and Nanoparticles/Clusters

Combining single atoms with clusters or nanoparticles is an emerging tactic to design efficient electrocatalysts. Both synergy effect and high atomic utilization of active sites in the composite catalysts result in enhanced electrocatalytic performance, simultaneously provide a radical analysis of the interrelationship between structure and activity. In this review, the recent advances of single-atomic site catalysts coupled with clusters or nanoparticles are emphasized. Firstly, the synthetic strategies, characterization, dynamics and types of single atoms coupled with clusters/nanoparticles are introduced, and then the key factors controlling the structure of the composite catalysts are discussed. Next, several clean energy catalytic reactions performed over the synergistic composite catalysts are illustrated. Eventually, the encountering challenges and recommendations for the future advancement of synergistic structure in energy-transformation electrocatalysis are outlined.

Cu-Pd bimetal-decorated siloxene nanosheets for semi-hydrogenation of acetylene

Metallic Pd has been proved highly promising when paired with Cu for industrially important acetylene semi-hydrogenation. Herein, we demonstrate that high-surface-area siloxene can feasibly enable alloying between Pd and Cu via room-temperature reduction with Si-H bonds. Unprecedentedly small Cu nanoparticles with isolated Pd were in situ loaded on siloxene, addressing the core problem of low selectivity of Pd and low activity of Cu. This devised structure outclassed the traditional impregnated SiO2 in every aspect of the catalytic performance for the semi-hydrogenation of acetylene under industry conditions, with a 91% acetylene conversion and an impressive 93% selectivity to ethylene at 200 °C, and showed long-term stability with negligible activity decay at this harsh temperature. This work provides new insights for the design of economic bimetallic loaded catalysts for balancing the activity-selectivity dilemma, demonstrating the viability of siloxene as both a synthetic reagent and a carrier material for efficient catalysis.

Bi-modified Cu-Based Catalysts for Acetylene Hydrogenation: Leveraging Dispersion and Hydrogen Spillover

Removing trace acetylene from the ethylene stream through selective hydrogenation is a crucial process in the production of polymer-grade ethylene. However, achieving high selectivity while maintaining high activity remains a significant challenge, especially for nonprecious metal catalysts. Herein, the trade-off between activity and selectivity is solved by synergizing enhanced dispersion and hydrogen spillover. Specifically, a bubbling method is proposed for preparing SiO2-supported copper and/or bismuth carbonate with high dispersion, which is then employed to synthesize highly dispersed Bi-modified CuxC-Cu catalyst. The catalyst displays outstanding catalytic performance for acetylene selective hydrogenation, achieving acetylene conversion of 100% and ethylene selectivity of 91.1% at 100 °C. The high activity originates from the enhanced dispersion, and the exceptional selectivity is due to the enhanced spillover capacity of active hydrogen from CuxC to Cu, which is promoted by the Bi addition. The results offer an avenue to design efficient catalysts for selective hydrogenation from nonprecious metals.

A feasible interlayer strategy for simultaneous light and heat management in photothermal catalysis

Photothermal conversion represents one crucial approach for solar energy harvesting and its exploitation as a sustainable alternative to fossil fuels; however, an efficient, cost-effective, and generalized approach to enhance the photothermal conversion processes is still missing. Herein, we develop a feasible and efficient photothermal conversion strategy that achieves simultaneous light and heat management using supported metal clusters and WSe2 interlayer toward enhanced CO2 hydrogenation photothermal catalysis. The interlayer can simultaneously reduce heat loss in the catalytic layer and improve light absorption, leading to an 8-fold higher CO2 conversion rate than the controls. The optical and thermal performance of WSe2 interlayered catalysts on different substrates was quantified using Raman spectroscopy. This work demonstrates a feasible and generalized approach for effective light and heat management in solar harvesting. It also provides important design guidelines for efficient photothermal converters that facilitate the remediation of the energy and environmental crises faced by humans.

Circumventing the activity-selectivity trade-off via the confinement effect from induced potential barriers on the Pd nanoparticle surface

The request for both high catalytic selectivity and high catalytic activity is rather challenging, particularly for catalysis systems with the primary and side reactions having comparable energy barriers. Here in this study, we simultaneously optimized the selectivity and activity for acetylene semi-hydrogenation by rationally and continuously varying the doping ratio of Zn atoms on the surface of Pd particles in Pd/ZnO catalysts. In the reaction temperature range of 40-200 °C, the conversion of acetylene was close to ∼100%, and the selectivity for ethylene exceeded 90% (the highest ethylene selectivity, ∼98%). Experimental characterization and density functional theory calculations revealed that the Zn promoter could alter the catalyst's potential energy surface, resulting in a "confinement" effect, which effectively improves the selectivity yet without significantly impairing the catalytic activity. The mismatched impacts on activity and selectivity resulting from continuous and controllable alteration in the catalyst structure provide a promising parameter space within which the two aspects could both be optimized.

Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Decoding Active Sites for Highly Efficient Semihydrogenation of Acetylene in Palladium-Copper Nanoalloys

Accurately decoding the three-dimensional atomic structure of surface active sites is essential yet challenging for a rational catalyst design. Here, we used comprehensive techniques combining the pair distribution function and reverse Monte Carlo simulation to reveal the surficial distribution of Pd active sites and adjacent coordination environment in palladium-copper nanoalloys. After the fine-tuning of the atomic arrangement, excellent catalytic performance with 98% ethylene selectivity at complete acetylene conversion was obtained in the Pd34Cu66 nanocatalysts, outperforming most of the reported advanced catalysts. The quantitative deciphering shows a large number of active sites with a Pd-Pd coordination number of 3 distributed on the surface of Pd34Cu66 nanoalloys, which play a decisive role in highly efficient semihydrogenation. This finding not only opens the way for guiding the precise design of bimetal nanocatalysts from atomic-level insight but also provides a method to resolve the spatial structure of active sites.

Lattice Strain and Mott-Schottky Effect of the Charge-Asymmetry Pd1Fe Single-Atom Alloy Catalyst for Semi-Hydrogenation of Alkynes with High Efficiency

The ideal interface design between the metal and substrate is crucial in determining the overall performance of the alkyne semihydrogenation reaction. Single-atom alloys (SAAs) with isolated dispersed active centers are ideal media for the study of reaction effects. Herein, a charge-asymmetry "armor" SAA (named Pd1Fe SAA@PC), which consists of a Pd1Fe alloy core and a semiconducting P-doped C (PC) shell, is rationally designed as an ideal catalyst for the selective hydrogenation of alkynes with high efficiency. Multiple spectroscopic analyses and density functional theory calculations have demonstrated that Pd1Fe SAA@PC is dual-regulated by lattice tensile and Schottky effects, which govern the selectivity and activity of hydrogenation, respectively. (1) The PC shell layer applied an external traction force causing a 1.2% tensile strain inside the Pd1Fe alloy to increase the reaction selectivity. (2) P doping into the C-shell layer realized a transition from a p-type semiconductor to an n-type semiconductor, thereby forming a unique Schottky junction for advancing alkyne semihydrogenation activity. The dual regulation of lattice strain and the Schottky effect ensures the excellent performance of Pd1Fe SAA@PC in the semihydrogenation reaction of phenylethylene, achieving a conversion rate of 99.9% and a selectivity of 98.9% at 4 min. These well-defined interface modulation strategies offer a practical approach for the rational design and performance optimization of semihydrogenation catalysts.

Efficient Alkyne Semihydrogenation Catalysis Enabled by Synergistic Chemical and Thermal Modifications of a PdIn MOF

Recently, there has been a growing interest in using MOF templating to synthesize heterogeneous catalysts based on metal nanoparticles on carbonaceous supports. Unlike the common approach of direct pyrolysis of PdIn-MOFs at high temperatures, this work proposes a reductive chemical treatment under mild conditions before pyrolysis (resulting in PdIn-QT). The resulting material (PdIn-QT) underwent comprehensive characterization via state-of-the-art aberration-corrected electron microscopy, N2 physisorption, X-ray absorption spectroscopy, Raman, X-ray photoelectron spectroscopy, and X-ray diffraction. These analyses have proven the existence of PdIn bimetallic nanoparticles supported on N-doped carbon. In situ DRIFT spectroscopy reveals the advantageous role of indium (In) in regulating Pd activity in alkyne semihydrogenation. Notably, incorporating a soft nucleation step before pyrolysis enhances surface area, porosity, and nitrogen content compared to direct MOF pyrolysis. The optimized material exhibits outstanding catalytic performance with 96% phenylacetylene conversion and 96% selectivity to phenylethylene in the fifth cycle under mild conditions (5 mmol phenylacetylene, 7 mg cat, 5 mL EtOH, R.T., 1 H2 bar).

Potassium regulating electronic state of zirconia supported palladium catalyst and hydrogen spillover for improved acetylene hydrogenation

High-selectivity acetylene hydrogenation to produce ethylene is an important issue of removing acetylene impurity in ethylene for industrial polyethylene production. Developing high-efficiency catalyst with excellent ethylene selectivity and catalytic durability is desirable but still challenging. In this work, potassium doped palladium catalysts supported on zirconia with different K contents (Pd/ZrO2-xK) have been developed to catalyze acetylene hydrogenation, the Pd/ZrO2-16K exhibits impressive catalytic performance with acetylene conversion of 100 %, ethylene selectivity of 81 % and high catalytic durability. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) and density functional theory (DFT) calculations reveal that K doping effectively weakens the adsorption of ethylene by regulating the electronic state of catalyst to improve ethylene selectivity and substantially lowers the barriers of hydrogen activation and transfer reactions to favor hydrogen spillover, thus conferring a remarkably improved durability on the Pd/ZrO2-16K catalysts.

Pd-Mn/NC Dual Single-Atomic Sites with Hollow Mesopores for the Highly Efficient Semihydrogenation of Phenylacetylene

The direct pyrolysis of metal-zeolite imidazolate frameworks (M-ZIFs) has been widely recognized as the predominant approach for synthesizing atomically dispersed metal-nitrogen-carbon single-atom catalysts (M/NC-SACs), which have exhibited exceptional activity and selectivity in the semihydrogenation of acetylene. However, due to weak adsorption of reactants on the single site and restricted molecular diffusion, the semihydrogenation of large organic molecules (e.g., phenylacetylene) was greatly limited for M/NC-SACs. In this work, a dual single-atom catalyst (h-Pd-Mn/NC) with hollow mesopores was designed and prepared using a general host-guest strategy. Taking the semihydrogenation of phenylacetylene as an example, this catalyst exhibited ultrahigh activity and selectivity, which achieved a turnover frequency of 218 molC═CmolPd-1 min-1, 16-fold higher than that of the commercial Lindlar catalyst. The catalyst maintained high activity and selectivity even after 5 cycles of usage. The superior activity of h-Pd-Mn/NC was attributed to the 4.0 nm mesopore interface of the catalyst, which enhanced the diffusion of macromolecular reactants and products. Particularly, the introduction of atomically dispersed Mn with weak electronegativity in h-Pd-Mn/NC could drive the electron transfer from Mn to adjacent Pd sites and regulate the electronic structure of Pd sites. Meanwhile, the strong electronic coupling in Pd-Mn pairs enhanced the d-electron domination near the Fermi level and promoted the adsorption of phenylacetylene and H2 on Pd active sites, thereby reducing the energy barrier for the semihydrogenation of phenylacetylene.

Pt-doped Ru nanoparticles loaded on 'black gold' plasmonic nanoreactors as air stable reduction catalysts

This study introduces a plasmonic reduction catalyst, stable only in the presence of air, achieved by integrating Pt-doped Ru nanoparticles on black gold. This innovative black gold/RuPt catalyst showcases good efficiency in acetylene semi-hydrogenation, attaining over 90% selectivity with an ethene production rate of 320 mmol g-1 h-1. Its stability, evident in 100 h of operation with continuous air flow, is attributed to the synergy of co-existing metal oxide and metal phases. The catalyst's stability is further enhanced by plasmon-mediated concurrent reduction and oxidation of the active sites. Finite-difference time-domain simulations reveal a five-fold electric field intensification near the RuPt nanoparticles, crucial for activating acetylene and hydrogen. Kinetic isotope effect analysis indicates the contribution from the plasmonic non-thermal effects along with the photothermal. Spectroscopic and in-situ Fourier transform infrared studies, combined with quantum chemical calculations, elucidate the molecular reaction mechanism, emphasizing the cooperative interaction between Ru and Pt in optimizing ethene production and selectivity.

Heteroatom-Coordinated Palladium Molecular Catalysts for Sustainable Electrochemical Production of Hydrogen Peroxide

Currently, hydrogen peroxide (H2O2) manufacturing involves an energy-intensive anthraquinone technique that demands expensive solvent extraction and a multistep process with substantial energy consumption. In this work, we synthesized Pd-N4-CO, Pd-S4-NCO, and Pd-N2O2-C single-atom catalysts via an in situ synthesis approach involving heteroatom-rich ligands and activated carbon under mild reaction conditions. It reveals that palladium atoms interact strongly with heteroatom-rich ligands, which provide well-defined and uniform active sites for oxygen (O2) electrochemically reduced to hydrogen peroxide. Interestingly, the Pd-N4-CO electrocatalyst shows excellent performance for the electrocatalytic reduction of O2 to H2O2 via a two-electron transfer process in a base electrolyte, exhibiting a negligible amount of onset overpotential and >95% selectivity within a wide range of applied potentials. The electrocatalysts based on the activity and selectivity toward 2e- ORR follow the order Pd-N4-CO > Pd-N2O2-C > Pd-S4-NCO in agreement with the pull-push mechanism, which is the Pd center strongly coordinated with high electronegativity donor atoms (N and O atoms) and weakly coordinated with the intermediate *OOH to excellent selectivity and sustainable production of H2O2. According to density functional theory, Pd-N4 is the active site for selectivity toward H2O2 generation. This work provides an emerging technique for designing high-performance H2O2 electrosynthesis catalysts and the rational integration of several active sites for green and sustainable chemical synthesis via electrochemical processes.

Designing M13 Bacteriophage and Fe-Nanonest Self-Assembly System for Universal and Facile Preparation of Metal Single Atoms as Stable Mimicking Enzymes

Metal single-atom catalysts (MSACs) possess multiple advantages in chemical synthesis; their efficient fabrication routes, however, remain a challenge to date. Here, an interdisciplinary design using M13 bacteriophage virus as a biotemplate to carry Fe nanoclusters, which we figuratively call "Fe-nanonests", is proposed to enable facile and versatile synthesis of MSACs. The feasibility and generality of this self-assembly method was demonstrated by the observation of six different metal single atoms (MSAs) including Ag, Pt, Pd, Zn, Cu, and Ni. With Pd as a representative, key factors dominating the fabrication were determined. The Pd single atoms exhibited excellent horseradish peroxidase (HRP)-like activity, which was further improved by 50% via genetic editing of the M13 pVIII protein terminals. Excellent stability was also observed in the quantification of acid phosphatase, a cancer predictor. X-ray absorption near-edge structure spectroscopy has been applied to the analysis of Pd single atoms as well, and the Pd-N4 coordination explained the mechanism of high HRP-like catalytic activity. The MSAs synthesized by the M13 phage and Fe-nanonest self-assembly method show promising prospects in non-cold-chain medical detection applications.

Atomic Three-Dimensional Investigations of Pd Nanocatalysts for Acetylene Semi-hydrogenation

Deciphering the three-dimensional (3D) insight into nanocatalyst surfaces at the atomic level is crucial to understanding catalytic reaction mechanisms and developing high-performance catalysts. Nevertheless, better understanding the inherent insufficiency of a long-range ordered lattice in nanocatalysts is a big challenge. In this work, we report the local structure of Pd nanocatalysts, which is beneficial for demonstrating the shape-structure-adsorption relationship in acetylene hydrogenation. The 5.27 nm spherical Pd catalyst (Pdsph) shows an ethylene selectivity of 88% at complete acetylene conversion, which is much higher than those of the Pd octahedron and Pd cube and superior to other reported monometallic Pd nanocatalysts so far. By virtue of the local structure revelation combined with the atomic pair distribution function (PDF) and reverse Monte Carlo (RMC) simulation, the atomic surface distribution of the unique compressed strain of Pd-Pd pairs in Pdsph was revealed. Density functional theory calculations verified the obvious weakening of the ethylene adsorption energy on account of the surface strain of Pdsph. It is the main factor to avoid the over-hydrogenation of acetylene. The present work, entailing shape-induced surface strain manipulation and atomic 3D insight, opens a new path to understand and optimize chemical activity and selectivity in the heterogeneous catalysis process.

Acetylene Semi-Hydrogenation at Room Temperature over Pd-Zn Nanocatalyst

A reaction of fundamental and commercial importance is acetylene semi-hydrogenation. Acetylene impurity in the ethylene feedstock used in the polyethylene industry poisons the Ziegler-Natta catalyst which adversely affects the polymer quality. Pd based catalysts are most often employed for converting acetylene into the main reactant, ethylene, however, it often involves a tradeoff between the conversion and the selectivity and generally requires high temperatures. In this work, bimetallic Pd-Zn nanoparticles capped by hexadecylamine (HDA) have been synthesized by co-digestive ripening of Pd and Zn nanoparticles and studied for semi-hydrogenation of acetylene. The catalyst showed a high selectivity of ~85 % towards ethylene with a high ethylene productivity to the tune of ~4341 μmol g-1  min-1 , at room temperature and atmospheric pressure. It also exhibited excellent stability with ethylene selectivity remaining greater than 85 % even after 70 h on stream. To the best of the authors' knowledge, this is the first report of room temperature acetylene semi-hydrogenation, with the catalyst effecting high amount of acetylene conversion to ethylene retaining excellent selectivity and stability among all the reported catalysts thus far. DFT calculations show that the disordered Pd-Zn nanocatalyst prepared by a low temperature route exhibits a change in the d-band center of Pd and Zn which in turn enhances the selectivity towards ethylene. TPD, XPS and a range of catalysis experiments provided in-depth insights into the reaction mechanism, indicating the key role of particle size, surface area, Pd-Zn interactions, and the capping agent.

Enhancing Photocatalytic-Transfer Semi-Hydrogenation of Alkynes Over Pd/C3 N4 Through Dual Regulation of Nitrogen Defects and the Mott-Schottky Effect

The selective hydrogenation of alkynes is an important reaction; however, the catalytic activity and selectivity in this reaction are generally conflicting. In this study, ultrafine Pd nanoparticles (NPs) loaded on a graphite-like C3 N4 structure with nitrogen defects (Pd/DCN) are synthesized. The resulting Pd/DCN exhibits excellent photocatalytic performance in the transfer hydrogenation of alkynes with ammonia borane. The reaction rate and selectivity of Pd/DCN are superior to those of Pd/BCN (bulk C3 N4 without nitrogen defects) under visible-light irradiation. The characterization results and density functional theory calculations show that the Mott-Schottky effect in Pd/DCN can change the electronic density of the Pd NPs, and thus enhances the hydrogenation selectivity toward phenylacetylene. After 1 h, the hydrogenation selectivity of Pd/DCN reaches 95%, surpassing that of Pd/BCN (83%). Meanwhile, nitrogen defects in the supports improve the visible-light response and accelerate the transfer and separation of photogenerated charges to enhance the catalytic activity of Pd/DCN. Therefore, Pd/DCN exhibits higher efficiency under visible light, with a turnover frequency (TOF) of 2002 min-1 . This TOF is five times that of Pd/DCN under dark conditions and 1.5 times that of Pd/BCN. This study provides new insights into the rational design of high-performance photocatalytic transfer hydrogenation catalysts.

M supported on Al-defective Al2-δO3 (M = Fe, Co, Ni, Cu, Ag, Au) as catalysts for acetylene semi-hydrogenation: a theoretical perspective

Semi-hydrogenation of acetylene is of great importance for both industry and academia. High prices and limited supplements of noble metals leave room for developing base metal catalysts. Experiments revealed the atomically dispersed Cu supported by Al2O3 with excellent long-term stability and high ethylene selectivity, but the physical nature has rarely been investigated theoretically. DFT calculations and microkinetic modeling revealed that the surface OH species could stabilize Cu1/Al2-δO3 and enhance its catalytic performance. The selectivity of ethylene formation decreases with increasing copper clusters (e.g., Cu1/Al2-δO3> Cu4/Al2-δO3> Cu8/Al2-δO3), meaning that the atomically dispersed copper may be a potential candidate for acetylene semi-hydrogenation. The structures of a series of single site catalysts M1/Al2-δO3 (M = Fe, Co, Ni, Ag, Au) are similar to that of Cu1/Al2-δO3, but their performances in catalyzing acetylene semi-hydrogenation are different. M1/Al2-δO3 (M = Ag, Au) shows higher selectivity than Cu1/Al2-δO3, while M1/Al2-δO3 (M = Fe, Co, Ni) demonstrates a higher turnover frequency (TOF) of ethylene than Cu1/Al2-δO3. Moreover, our results indicate that the Ni1-Cu1/Al2-δO3 alloy shows both high activity and ethylene selectivity. The present results show a compensation between the reactivity and the selectivity, suggesting that alloys of VIIIB metals with IB metals like Ni1-Cu1/Al2-δO3 may be efficient candidate catalysts in acetylene selective hydrogenation.

Anchoring ultrasmall Pd nanoparticles by bipyridine functional covalent organic frameworks for semihydrogenation of acetylene

Acetylene hydrogenation is a well-accepted solution to reduce by-products in the ethylene production process, while one of the key technical difficulties lies in developing a catalyst that can provide highly dispersed active sites. In this work, a highly crystalline layered covalent organic framework (COF) material (TbBpy) with excellent thermal stability was synthesized and firstly applied as support for ultrasmall Pd nanoparticles to catalyze acetylene hydrogenation. 100% of C2H2 conversion and 88.2% of C2H4 selectivity can be obtained at 120 °C with the space velocity of 70 000 h-1. The reaction mechanism was elucidated by applying a series of characterization techniques and theoretical calculation. The results indicate that the coordination between Pd and N atom in the bipyridine functional groups of COFs successfully increased the dispersibility and stability of Pd particles, and the introduction of COFs not only improved the adsorption of acetylene and H2 onto catalyst surface, but enhanced the electron transfer process, which can be responsible for the high selectivity and activity of catalyst. This work, for the first time, reported the excellent performance of Pd@TbBpy as a catalyst for acetylene hydrogenation and will facilitate the development and application of COFs materials in the area of petrochemicals.

Continuous Production of Functionalized Graphene Inks by Soft Solution Processing

The continuous production of high-quality, few-layer graphene nanosheets (GNSs) functionalized with nitrogen-containing groups was achieved via a two-stage reaction method. The initial stage produces few-layer GNSs by utilizing our recently developed glycine-bisulfate ionic complex-assisted electrochemical exfoliation of graphite. The second stage, developed here, uses a radical initiator and nitrogen precursor (azobisisobutyronitrile) under microwave conditions in an aqueous solution for the efficient nitrogen functionalization of the initially formed GNSs. These nitrile radical reactions have great advantages in green chemistry and soft processing. Raman spectra confirm the insertion of nitrogen functional groups into nitrogen-functionalized graphene (N-FG), whose disorder is higher than that of GNSs. X-ray photoelectron spectra confirm the insertion of edge/surface nitrogen functional groups. The insertion of nitrogen functional groups is further confirmed by the enhanced dispersibility of N-FG in dimethyl formamide, ethylene glycol, acetonitrile, and water. Indeed, after the synthesis of N-FG in solution, it is possible to disperse N-FG in these liquid dispersants just by a simple washing-centrifugation separation-dispersion sequence. Therefore, without any drying, milling, and redispersion into liquid again, we can produce N-FG ink with only solution processing. Thus, the present work demonstrates the 'continuous solution processing' of N-FG inks without complicated post-processing conditions. Furthermore, the formation mechanism of N-FG is presented.

Metal-Organic Frameworks Encaged Ru Single Atoms for Rapid Acetylene Harvest and Activation in Hydrochlorination

Ruthenium (Ru)-based catalysts have been candidates in hydrochlorination for vinyl chloride monomer (VCM) production, yet they are limited by efficient acetylene (C₂H₂) utilization. The strong adsorption performance of HCl can deactivate Ru active sites which resulted in weak C₂H₂ adsorption and slow activation kinetics. Herein, we designed a channel that employed metal-organic framework (MOF)-encaged Ru single atoms to achieve rapid adsorption and activation of C₂H₂. Low-Ru (∼0.5 wt %) single-atom catalysts (named Ru-NC@MIL) were assembled by hydrogen-bonding nanotraps (the H-C≡C-H^δ+···O^δ- interactions between C₂H₂ and carboxylate groups/furan rings). Results confirmed that C₂H₂ could easily enter the encapsulation channels in an optimal mode perpendicular to the channel with a potential energy of 42.3 kJ/mol. The harvested C₂H₂ molecules can be quickly passed to Ru-N₄ active sites for activation by stretching the length of carbon-carbon triple bonds (C≡C) to 1.212 Å. Such a strategy guaranteed >99% C₂H₂ conversion efficiency and >99% VCM selectivity. Moreover, a stable long-term (>150 h) catalysis with high efficiency (∼0.85 kg_vcm/h/kg_cat.) and a low deactivation constant (0.001 h^-1) was also achieved. This work provides an innovative strategy for precise C₂H₂ adsorption and activation and guidance for designing multi-functional Ru-based catalysts.

Single-Atom Catalysts for Hydrogen Activation

Selective hydrogenation is one of the most important reactions in fine chemical industry, and the activation of H₂ is the key step for hydrogenation. Catalysts play a critical role in selective hydrogenation, and some single-atom catalysts (SACs) are highly capable of activating H₂ in selective hydrogenation by virtue of the maximized atom utilization and the highly uniform active sites. Therefore, more research efforts are needed for the rational design of SACs with superior H₂ -activating capabilities. Herein, the research progress on H₂ activation in typical hydrogenation systems (such as alkyne hydrogenation, hydroformylation, hydrodechlorination, hydrodeoxygenation, nitroaromatics hydrogenation, and polycyclic aromatics hydrogenation) is reviewed, the mechanisms of SACs for H₂ activation are summarized, and the structural regulation strategies for SACs are proposed to promote H₂ activation and provide schemes for the design of high-selectivity hydrogenation catalysts from the atomic scale. At the end of this review, an outlook on the opportunities and challenges for SACs to be developed for selective hydrogenation is presented.

Selective hydrogenation of acetylene to ethylene by alkali-metal palladium complex hydrides

A series of alkali-metal palladium complex hydrides have been shown to be catalytically active and selective for partial hydrogenation of acetylene to ethylene. The complex hydrides differ in composition, structure, and catalytic function as compared to the conventional Pd metal or alloy catalysts.

Tripodal Pd metallenes mediated by Nb2C MXenes for boosting alkynes semihydrogenation

2D metallene nanomaterials have spurred considerable attention in heterogeneous catalysis by virtue of sufficient unsaturated metal atoms, high specific surface area and surface strain. Nevertheless, the strong metallic bonding in nanoparticles aggravates the difficulty in the controllable regulation of the geometry of metallenes. Here we propose an efficient galvanic replacement strategy to construct Pd metallenes loaded on Nb₂C MXenes at room temperature, which is triggered by strong metal-support interaction based on MD simulations. The Pd metallenes feature a chair structure of six-membered ring with the coordination number of Pd as low as 3. Coverage-dependent kinetic analysis based on first-principles calculations reveals that the tripodal Pd metallenes promote the diffusion of alkene and inhibit its overhydrogenation. As a consequence, Pd/Nb₂C delivers an outstanding turnover frequency of 10372 h^-1 and a high selectivity of 96% at 25 ^oC in the semihydrogenation of alkynes without compromising the stability. This strategy is general and scalable considering the plentiful members of the MXene family, which can set a foundation for the design of novel supported-metallene catalysts for demanding transformations.

Space-confined growth of nanoscale metal-organic frameworks/Pd in hollow mesoporous silica for highly efficient catalytic reduction of 4-nitrophenol

The development of confined growth of metal-organic frameworks (MOFs) in a nano-space remains a challenge mainly due to the spatial size randomness and inhomogeneity of host materials and the limitation of MOF species. In this study, we developed a general "stepwise vacuum evaporation" strategy, which allows the nano-confined growth of MOFs in hollow mesoporous silica nanospheres (HMSN) by the vacuum forces and the capillary effect. A series of nanoscale MOFs including ZIF-8, ZIF-90, HKUST-1, MIL-53(Cr) and UiO-66-NH2 were confinely synthesized inside the cavities of HMSN, resulting in hierarchically porous composites with core-shell structures. Further functionalization was studied by anchoring Pd to obtain UiO-66-NH2/Pd@HMSN catalyst, which exhibited excellent activity in the catalytic reduction of 4-nitrophenol to 4-aminophenol under ambient condition.

Emerging Applications of Synchrotron Radiation X-Ray Techniques in Single Atomic Catalysts

Single atom catalysts (SACs) can achieve a maximum atom utilization efficiency of 100%, which provides significantly increased active sites compared with traditional catalysts during catalytic reactions. Synchrotron radiation technology is an important characterization method for identifying single-atom catalysts. Several types of internal information, such as the coordination number, bond length and electronic structure of metals, can all be analyzed. This review will focus on the introduction of synchrotron radiation techniques and their applications in SACs. First, the fundamentals of synchrotron radiation and the corresponding techniques applied in characterization of SACs will be briefly introduced, such as X-ray absorption near edge spectroscopy and extended X-ray absorption fine structure spectroscopy and in situ techniques. The detailed information obtained from synchrotron radiation X-ray characterization is described through four routes: 1) the local environment of a specific atom; 2) the oxidation state of SACs; 3) electronic structures at different orbitals; and 4) the in situ structure modification during catalytic reaction. In addition, a systematic summary of synchrotron radiation X-ray characterization on different types of SACs (noble metals and transition metals) will be discussed.

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.

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.

Cu Catalysts Doped with a Heteroatom into the Subsurface: Unraveling the Role of Subsurface Chemistry in Tuning the Catalytic Performance of C2H2 Selective Hydrogenation

Heteroatoms doped into the subsurface of transition metals play a vital role in heterogeneous catalysis via either expressing surface structures or even directly participating in the reaction. Herein, DFT calculations and microkinetic modeling are implemented to examine C₂H₂ selective hydrogenation over heteroatom (H, B, C, N, or P)-doped Cu(111) and Cu(211) subsurfaces, which are compared with pure Cu(111) and Cu(211) to unravel the role of subsurface chemistry in tuning the surface structure and further regulating catalytic performance. Our results indicate that the catalytic performance toward C₂H₂ selective hydrogenation is closely related to the type of doped subsurface heteroatom and the Cu surface coordination environment, which can be attributed to the simultaneous change of Cu surface geometric and electronic structures. Catalytic performance improvement over the heteroatom-doped Cu(111) is generally better than that over the doped Cu(211); especially, B- or N-doped Cu(111) has excellent C₂H₄ activity and selectivity and greatly inhibits green oil. For the heteroatom-doped Cu(211), better performance is only obtained on P-Cu(211), which is still lower than the B- and N-doped Cu(111). The subsurface heteroatom doping should focus on high-coordination Cu(111) instead of low-coordination Cu(211). AIMD simulations verified the thermal stability of B-Cu(111) and N-Cu(111); both were screened out to be the most suitable catalysts toward C₂H₂ hydrogenation. This work clearly unravels the role of subsurface chemistry in heterogeneous catalysis and contributes to the rational design of high-performance metal catalysts by tuning surface structures with the heteroatom into the subsurface.

An efficient NiCu@C/Al2O3 catalyst for selective hydrogenation of acetylene

The development of non-noble metal catalysts for selective hydrogenation still remains a challenge. Herein, NiCu@carbon core-shell nanoparticles supported on Al₂O₃ (NiCu@C/Al₂O₃) were prepared, which showed enhanced catalytic performance of acetylene-selective hydrogenation in comparison with NiCu/Al₂O₃ without carbon encapsulation. In detail, NiCu@C/Al₂O₃ displayed high ethylene selectivity (>86%) even at an acetylene conversion of 100% and excellent stability (>90 h). Thus, NiCu@C/Al₂O₃ exhibited great potential as an alternative to Pd-based catalysts for acetylene-selective hydrogenation.

Semi-Hydrogenation of Acetylene to Ethylene Catalyzed by Bimetallic CuNi/ZSM-12 Catalysts

The purpose of this work is to develop a low-cost and high-performance catalyst for the selective catalytic hydrogenation of acetylene to ethylene. Non-precious metals Cu and Ni were selected as active ingredients for this study. Using ZSM-12 as a carrier, Cu-Ni bimetallic catalysts of CuNix/ZSM-12 (x = 5, 7, 9, 11) with different Ni/Cu ratios were prepared by incipient wetness impregnation method. The total Cu and Ni loading were 2 wt%. Under the optimal reaction conditions, the acetylene conversion was 100%, and the ethylene selectivity was 82.48%. The CuNi7/ZSM-12 prepared in this work exhibits good performance in the semi-hydrogenation of acetylene to ethylene with low cost and has potential for industrial application.

Vacancy-Rich MXene-Immobilized Ni Single Atoms as a High-Performance Electrocatalyst for the Hydrazine Oxidation Reaction

Single-atom catalysts (SACs), on account of their outstanding catalytic potential, are currently emerging as high-performance materials in the field of heterogeneous catalysis. Constructing a strong interaction between the single atom and its supporting matrix plays a pivotal role. Herein, Ti₃ C₂ T_x -MXene-supported Ni SACs are reported by using a self-reduction strategy via the assistance of rich Ti vacancies on the Ti₃ C₂ T_x MXene surface, which act as the trap and anchor sites for individual Ni atoms. The constructed Ni SACs supported by the Ti₃ C₂ T_x MXene (Ni SACs/Ti₃ C₂ T_x ) show an ultralow onset potential of -0.03 V (vs reversible hydrogen electrode (RHE)) and an exceptional operational stability toward the hydrazine oxidation reaction (HzOR). Density functional theory calculations suggest a strong coupling of the Ni single atoms and their surrounding C atoms, which optimizes the electronic density of states, increasing the adsorption energy and decreasing the reaction activation energy, thus boosting the electrochemical activity. The results presented here will encourage a wider pursuit of 2D-materials-supported SACs designed by a vacancy-trapping strategy.

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Copper phyllosilicate-derived ultrafine copper nanoparticles with plenty of Cu0and Cu+ for the enhanced catalytic performance of ethylene carbonate hydrogenation to methanol

The hydrogenation of CO₂-derived carbonates to methanol is an alternative route for the indirect utilization of abundant C1 sources. Various Cu/SiO₂catalysts with different copper loading content prepared by using an ammonia evaporation hydrothermal method are implemented to evaluate the catalytic performance of ethylene carbonate (EC) hydrogenation to methanol and ethylene glycol (EG). The Cu loading content was identified to significantly affect the Cu nanoparticles (NPs) size and metal-support interaction. Highly dispersed Cu NPs restricted and embedded in copper phyllosilicate presented a smaller average particle size than the impregnated Cu/SiO₂-IM catalyst. ThexCu/SiO₂catalyst with ultrafine Cu NPs showed abundant Cu-O-Si interfaces, acidic sites, and coherent Cu⁰and Cu⁺species. The 5Cu/SiO₂catalyst achieved methanol yield of 76% and EG yield of 98% at EC conversion of 99%, and no obvious deactivation was observed after long-term operation. The superior catalytic performance of the 5Cu/SiO₂catalyst is attributed to the synergetic effect between the appropriate Cu⁰surface area which provides sufficient active hydrogen, and the atomic ratio of Cu⁺for the polarization and activation of carbon-oxygen bonds.

Rare-Earth Single-Atom Catalysts: A New Frontier in Photo/Electrocatalysis

Single-atom catalysts (SACs) provide well-defined active sites with 100% atom utilization, and can be prepared using a wide range of support materials. Therefore, they are attracting global attention, especially in the fields of energy conversion and storage. To date, research has focused on transition-metal and precious-metal-based SACs. More recently, rare-earth (RE)-based SACs have emerged as a new frontier in photo/electrocatalysis owing to their unique electronic structure arising from the spin-orbit coupling of the 4f and valence orbitals, unsaturated coordination environment, and unique behavior as charge-transport bridges. However, a systematic review on the role of the RE active sites, catalytic mechanisms, and synthetic methods for RE SACs is lacking. Therefore, in this review, the latest developments in RE SACs having applications in photo/electrocatalysis are summarized and discussed. First, the theoretical advantages of RE SACs for photo/electrocatalysis are briefly introduced, focusing on the roles of the 4f orbitals and coupled energy levels. In addition, the most recent research progress on RE SACs is summarized for several important photo/electrocatalytic reactions and the corresponding catalytic mechanisms are discussed. Further, the synthetic strategies for the production of RE SACs are reported. Finally, challenges for the development of RE SACs are highlighted, along with future research directions and perspectives.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Palladium catalyzed radical relay for the oxidative cross-coupling of quinolines

Traditional approaches for transition-metal catalyzed oxidative cross-coupling reactions rely on sp²-hybridized starting materials, such as aryl halides, and more specifically, homogeneous catalysts. We report a heterogeneous Pd-catalyzed radical relay method for the conversion of a heteroarene C(sp³)–H bond into ethers. Pd nanoparticles are supported on an ordered mesoporous composite which, when compared with microporous activated carbons, greatly increases the Pd d charge because of their strong interaction with N-doped anatase nanocrystals. Mechanistic studies provide evidence that electron-deficient Pd with Pd–O/N coordinations efficiently catalyzes the radical relay reaction to release diffusible methoxyl radicals, and highlight the difference between this surface reaction and C–H oxidation mediated by homogeneous catalysts that operate with cyclopalladated intermediates. The reactions proceed efficiently with a turn-over frequency of 84 h⁻¹ and high selectivity toward ethers of >99%. Negligible Pd leaching and activity loss are observed after 7 catalytic runs.

Traditional approaches for transition-metal catalyzed oxidative cross-coupling reactions rely on sp²-hybridized starting materials. Here the authors report a heterogeneous Pd-catalyzed radical relay method for the conversion of a heteroarene C(sp³)–H bond into ethers.