Cation vacancy stabilization of single-atomic-site Pt1/Ni(OH)x catalyst for diboration of alkynes and alkenes

Promotion of Single-Electron Transfer by Low-Coordinated Co Single Atoms to Facilitate Advanced Oxidation Processes in Wastewater Treatment

Heterogeneous catalysts are fascinating for advanced oxidation processes (AOPs) in wastewater treatment to reduce cost, metal contamination, and pH operation limitations. However, they usually encounter low catalytic efficiency because of the difficult single-electron-transfer (SET) pathway during AOPs. Herein, an efficient heterogeneous catalyst for AOPs is realized through the rational regulation of N coordination around Co single-atom (SA) centers in favor of SET. As guided by calculations, low N coordination enables a high density of electronic states at the Fermi energy level of SA Co to facilitate SET activation of peroxomonosulfate (PMS). A special oxide-compounding method is further applied to decrease the N coordination of SA Co on the carbon carriers from common Co1-N3/4 to the desired Co1-N2. Co1-N2 shows a delightful activity for AOP degradation of various organic pollutants with kinetic rate and turnover frequency values up to 0.862 min-1 and 389 h-1, respectively, greatly outperforming those of Co1-N3/4. It is also superior in a wide pH operation range and has strong resistance to environmental disturbances. Detailed mechanistic investigations confirm the generation of singlet oxygen (1O2) instead of common radical O species from the SET between PMS and Co1-N2, corroborating the calculated results and accounting for the enhanced AOP activity.

Advancing Heterogeneous Organic Synthesis With Coordination Chemistry-Empowered Single-Atom Catalysts

For traditional metal complexes, intricate chemistry is required to acquire appropriate ligands for controlling the electron and steric hindrance of metal active centers. Comparatively, the preparation of single-atom catalysts is much easier with more straightforward and effective accesses for the arrangement and control of metal active centers. The presence of coordination atoms or neighboring functional atoms on the supports' surface ensures the stability of metal single-atoms and their interactions with individual metal atoms substantially regulate the performance of metal active centers. Therefore, the collaborative interaction between metal and the surrounding coordination environment enhances the initiation of reaction substrates and the formation and transformation of crucial intermediate compounds, which imparts single-atom catalysts with significant catalytic efficacy, rendering them a valuable framework for investigating the correlation between structure and activity, as well as the reaction mechanism of catalysts in organic reactions. Herein, comprehensive overviews of the coordination interaction for both homogeneous metal complexes and single-atom catalysts in organic reactions are provided. Additionally, reflective conjectures about the advancement of single-atom catalysts in organic synthesis are also proposed to present as a reference for later development.

Design and regulation of defective electrocatalysts

Electrocatalysts are the key components of electrochemical energy storage and conversion devices. High performance electrocatalysts can effectively reduce the energy barrier of the chemical reactions, thereby improving the conversion efficiency of energy devices. The electrocatalytic reaction mainly experiences adsorption and desorption of molecules (reactants, intermediates and products) on a catalyst surface, accompanied by charge transfer processes. Therefore, surface control of electrocatalysts plays a pivotal role in catalyst design and optimization. In recent years, many studies have revealed that the rational design and regulation of a defect structure can result in rearrangement of the atomic structure on the catalyst surface, thereby efficaciously promoting the electrocatalytic performance. However, the relationship between defects and catalytic properties still remains to be understood. In this review, the types of defects, synthesis methods and characterization techniques are comprehensively summarized, and then the intrinsic relationship between defects and electrocatalytic performance is discussed. Moreover, the application and development of defects are reviewed in detail. Finally, the challenges existing in defective electrocatalysts are summarized and prospected, and the future research direction is also suggested. We hope that this review will provide some principal guidance and reference for researchers engaged in defect and catalysis research, better help researchers understand the research status and development trends in the field of defects and catalysis, and expand the application of high-performance defective electrocatalysts to the field of electrocatalytic engineering.

Rational Design of Ultrahigh-Loading Ir Single Atoms on Reconstructed Mn─NiOOH for Enhanced Catalytic Performance in Urea-Water Electrolysis

Investigating advanced electrocatalysts is crucial for improving the efficacy of water splitting to generate environmentally friendly fuel. The discovery of highly effective electrocatalysts, capable of driving oxygen evolution reaction (OER) and urea oxidation reaction (UOR) in urea-alkaline environments, is pivotal for advancing large-scale hydrogen production. This study aims to introduce a new method that involves creating nanosheets of high-loading iridium single atoms embedded in a manganese-containing nickel oxyhydroxide matrix (Ir@Mn─NiOOH). These nanostructures are derived from self-supported hydrate pre-catalyst nanosheets grown on nickel foam and then activated through electrochemical etching pretreatment. The Ir@Mn─NiOOH nanoarchitecture displays outstanding electrocatalytic activity, having a low overpotential of just 258 mV and a potential of 1.319 V (at 10 mA cm-2) for OER and UOR, respectively. Such extraordinary catalytic characteristics of Ir@Mn─NiOOH is mainly owing to the strong synthetic electronic interaction between Ir single atoms and Mn─NiOOH, which can change its electronic characteristics and boost electrochemical catalytic sites. This research presents a new way to produce exceptionally efficient catalysts by adding a synergistic effect to complex multi-electron processes.

Ultrahigh Single Au Atoms Loaded Porous Aromatic Frameworks for Enhanced Photocatalytic Hydrogen Evolution

Supported single-atom catalysts (SACs) are promising in heterogeneous catalysis because of their atom economy, unusual transformations, and mechanistic clarity. The metal SAs loading, however, limits the catalytic efficiency. Herein, an in situ pre-metallated monomer-based preparation strategy is shown to achieve ultrahigh Au SAs loading in catalyst formations. The polymerization of single-atom loaded monomers yield a new porous aromatic framework (PAF-164) with Au SAs loading up to a record high 45.3 wt.%. SACs of Au-PAFs exhibit excellent photocatalytic activity in hydrogen (H2) evolution, and the H2 evolution rate of Au100%-SAs-PAF-164 can reach 4.82 mmol g-1 h-1 with great recyclability.

Recent Progresses of Polarons: Fundamentals and Roles in Photocatalysis and Photoelectrocatalysis

Photocatalysis and photoelectrocatalysis are promising ways in the utilization of solar energy. To address the low efficiency of photocatalysts and photoelectrodes, in-depth understanding of their catalytic mechanism is in urgent need. Recently, polaron is considered as an influential factor in catalysis, which brings researchers a new approach to modify photocatalysts and photoelectrodes. In this review, brief introduction of polaron is given first, followed by which models and recent experimentally observations of polarons are reviewed. Studies about roles of polarons in photocatalysis and photoelectrocatalysis are listed in order to provide some inspiration in exploring the mechanism and improving the efficiency of photocatalysis and photoelectrocatalysis.

Enhanced Photocatalytic Hydrogen Evolution Activity Driven by the Synergy Between Surface Vacancies and Cocatalysts: Surface Reaction Matters

The incorporation of defects and cocatalysts is known to be effective in improving photocatalytic activity, yet their coupled contribution to the photocatalytic hydrogen evolution process has not been well-explored. In this study, We demonstrate that the incorporation of S vacancies and NiSe can contribute to the improvement of charge separation efficiency via the formation of a strong electric field within the bulk ZnIn2S4 (ZIS) and on its surface. More importantly, We also demonstrate that the synergy of S vacancies and NiSe benefits the overall hydrogen evolution activity by facilitating the H2O adsorption and dissociation process. This is particularly important for hydrogen evolution taking place under alkaline conditions where the proton concentration is low, allowing ZISv-NiSe (containing abundant S vacancies) to outperform ZIS-NiSe under alkaline conditions. In contrast, under acid conditions, since there are already sufficient amounts of protons available for reaction, the hydrogen evolution activity became governed by the hydrogen adsorption/desorption process rather than the H2O dissociation process. This leads to ZIS-NiSe exhibiting higher activity than ZISv-NiSe due to its more favorable hydrogen adsorption energy. The findings thus provide insights into how defect and cocatalyst modification strategies can be tailor-made to improve hydrogen evolution activity under different pH conditions.

Electronic Structure Regulated Carbon-Based Single-Atom Catalysts for Highly Efficient and Stable Electrocatalysis

Single-atom-catalysts (SACs) with atomically dispersed sites on carbon substrates have attained great advancements in electrocatalysis regarding maximum atomic utilization, unique chemical properties, and high catalytic performance. Precisely regulating the electronic structure of single-atom sites offers a rational strategy to optimize reaction processes associated with the activation of reactive intermediates with enhanced electrocatalytic activities of SACs. Although several approaches are proposed in terms of charge transfer, band structure, orbital occupancy, and the spin state, the principles for how electronic structure controls the intrinsic electrocatalytic activity of SACs have not been sufficiently investigated. Herein, strategies for regulating the electronic structure of carbon-based SACs are first summarized, including nonmetal heteroatom doping, coordination number regulating, defect engineering, strain designing, and dual-metal-sites scheming. Second, the impacts of electronic structure on the activation behaviors of reactive intermediates and the electrocatalytic activities of water splitting, oxygen reduction reaction, and CO2/N2 electroreduction reactions are thoroughly discussed. The electronic structure-performance relationships are meticulously understood by combining key characterization techniques with density functional theory (DFT) calculations. Finally, a conclusion of this paper and insights into the challenges and future prospects in this field are proposed. This review highlights the understanding of electronic structure-correlated electrocatalytic activity for SACs and guides their progress in electrochemical applications.

Anchoring Pt Single-Atom Sites on Vacancies of MgO(Al) Nanosheets as Bifunctional Catalysts to Accelerate Hydrogenation-Cyclization Cascade Reactions

The direct hydrogenation of 2-nitroacylbenzene to 2,1-benzisoxazole presents a significant challenge in the pharmaceutical and fine chemicals industries. In this study, a defect engineering strategy is employed to create bifunctional single-atom catalysts (SACs) by anchoring Pt single atoms onto metal vacancies within MgO(Al) nanosheets. The resultant Pt1/MgO(Al) SAC displays an exceptional catalytic activity and selectivity in the hydrogenation-cyclization of 2-nitroacylbenzene, achieving a 97.5 % yield at complete conversion and a record-breaking turnover frequency of 458.8 h-1 under the mild conditions. The synergistic catalysis between the fully exposed single-atom Pt sites within a unique Pt-O-Mg/Al moiety and the abundant basic sites of the MgO(Al) support is responsible for this outstanding catalytic performance. The current work, therefore, paves the way for developing bifunctional or multifunctional SACs that can enhance efficient organocatalytic conversions.

Realizing the Tailored Catalytic Performances on Atomic Pt-Promoted Transition Metal Moieties Implanted Layered Double Hydroxides for Water Electrolysis

High-performance production of green hydrogen gas is necessary to develop renewable energy generation technology and to safeguard the living environment. This study reports a controllable engineering approach to tailor the structure of nickel-layered double hydroxides via doped and absorbed platinum single atoms (PtSA) promoted by low electronegative transition metal (Mn, Fe) moieties (PtSA-Mn,Fe-Ni LDHs). We explore that the electron donation from neighboring transition metal moieties results in the well-adjusted d-band center with the low valence states of PtSA(doped) and PtSA(ads.), thus optimizing adsorption energy to effectively accelerate the H2 release. Meanwhile, a tailored local chemical environment on transition metal centers with unique charge redistribution and high valence states functions as the main center for H2O catalytic dissociation into oxygen. Therefore, the PtSA-Mn,Fe-Ni LDH material possesses a small overpotential of 42 and 288 mV to reach 10 mA·cm-2 for hydrogen and oxygen evolution, respectively, superior to most reported LDH-based catalysts. Additionally, the mass activity of PtSA-Mn,Fe-Ni LDHs proves to be 15.45 times higher than that of commercial Pt-C. The anion exchange membrane electrolyzer stack of PtSA-Mn,Fe-Ni LDHs(+,-) delivers a cell voltage of 1.79 V at 0.5 A·cm-2 and excellent durability over 600 h. This study presents a promising electrocatalyst for a practical water splitting process.

Electron Deficient Ir-O Bonds Promote Heterogeneous Ir-Catalyzed Anti-Markovnikov Hydroboration of Alkenes under Mild Neat Conditions

Tuning electronic characteristics of metal-ligand bonds based on reaction pathways to achieve efficient catalytic processes has been widely studied and proven to be feasible in homogeneous catalysis, but it is scarcely investigated in heterogeneous catalysis. Herein, we demonstrate the regulation of the electronic configuration of Ir-O bonds in an Ir single-atom catalyst according to the borane activation mechanism. Ir-O bonds in Ir1/Ni(OH)x are found to be more electron-poor than those in Ir1/NiOx. Despite the mild solvent-free conditions and ambient temperature, Ir1/Ni(OH)x exhibits outstanding performance for the hydroboration of alkenes, furnishing the desired alkylboronic esters with a turnover frequency value of ≤3060 h-1 and 99% anti-Markovnikov selectivity, which is significantly better than that of Ir1/NiOx (42 h-1). It is further proven that the more electron-poor Ir-O bonds as active centers are more oxidative and so benefit the activation of the H-B bond in the reductive pinacolborane.

Stabilizing Single-Atom Pt on Fe2O3 Nanosheets by Constructing Oxygen Vacancies for Ultrafast H2 Sensing

Single-atom catalysts (SACs) hold great promise in highly sensitive and selective gas sensors due to their ultrahigh atomic efficiency and excellent catalytic activity. However, due to the extremely high surface energy of SACs, it is still a huge challenge to synthesize a stable single-atom metal on sensitive materials. Here, we report an atomic layer deposition (ALD) strategy for the elaborate synthesis of single-atom Pt on oxygen vacancy-rich Fe2O3 nanosheets (Pt-Fe2O3-Vo), which displayed ultrafast and sensitive detection to H2, achieving the stability of Pt single atoms. Gas-sensing investigation showed that the Pt-Fe2O3-Vo materials enabled a significantly enhanced response of 26.5-50 ppm of H2, which was 17-fold higher than that of pure Fe2O3, as well as ultrafast response time (2 s), extremely low detection limit (86 ppb), and improved stability. The experimental and density functional theory (DFT) studies revealed that the abundant oxygen vacancy sites of Fe2O3 contributed to stabilizing the Pt atoms via electron transfer. In addition, the stabilized Pt atoms also greatly promote the electron transfer of H2 molecules to Fe2O3, thereby achieving an excellent H2 sensing performance. This work provides a potential strategy for the development of highly selective and stable chemical sensors.

Recent advances in catalytic oxidation of VOCs by two-dimensional ultra-thin nanomaterials

Catalytic oxidation, an end-of-pipe treatment technology for effectively purifying volatile organic compounds (VOCs), has received widespread attention. The crux of catalytic oxidation lies in the development of efficient catalysts, with their optimization necessitating a comprehensive analysis of the catalytic reaction mechanism. Two-dimensional (2D) ultra-thin nanomaterials offer significant advantages in exploring the catalytic oxidation mechanism of VOCs due to their unique structure and properties. This review classifies strategies for regulating catalytic properties and typical applications of 2D materials in VOCs catalytic oxidation, in addition to their characteristics and typical characterization techniques. Furthermore, the possible reaction mechanism of 2D Co-based and Mn-based oxides in the catalytic oxidation of VOCs is analyzed, with a special focus on the synergistic effect between oxygen and metal vacancies. The objective of this review is to provide valuable references for scholars in the field.

Precise Atomic Structure Regulation of Single-Atom Platinum Catalysts toward Highly Efficient Hydrogen Evolution Reaction

Noble metal single-atom-catalysts (SACs) have demonstrated significant potential to improve atom utilization efficiency and catalytic activity for hydrogen evolution reaction (HER). However, challenges still remain in rationally modulating active sites and catalytic activities of SACs, which often results in sluggish kinetics and poor stability, especially in neutral/alkaline media. Herein, precise construction of Pt single atoms anchored on edge of 2D layered Ni(OH)2 (Pt-Ni(OH)2-E) is achieved utilizing in situ electrodeposition. Compared to the single-atom Pt catalysts anchored on the basal plane of Ni(OH)2 (Pt-Ni(OH)2-BP), the Pt-Ni(OH)2-E possesses superior electron affinity and high intrinsic catalytic activity, which favors the strong adsorption and rapid dissociation toward water molecules. As a result, the Pt-Ni(OH)2-E catalyst requires low overpotentials of 21 and 34 mV at 10 mA cm-2 in alkaline and neutral conditions, respectively. Specifically, it shows the high mass activity of 23.6 A mg-1 for Pt at the overpotential of 100 mV, outperforming the reported catalysts and commercial Pt/C. This work provides new insights into the rational design of active sites for preparing high-performance SACs.

Corrosion Engineering of Part-Per-Million Single Atom Pt1/Ni(OH)2 Electrocatalyst for PET Upcycling at Ampere-Level Current Density

The plastic waste issue has posed a series of formidable challenges for the ecological environment and human health. While conventional recycling strategies often lead to plastic down-cycling, the electrochemical strategy of recovering valuable monomers enables an ideal, circular plastic economy. Here a corrosion synthesized single atom Pt1/Ni(OH)2 electrocatalyst with part-per-million noble Pt loading for highly efficient and selective upcycling of polyethylene terephthalate (PET) into valuable chemicals (potassium diformate and terephthalic acid) and green hydrogen is reported. Electro-oxidation of PET hydrolysate, ethylene glycol (EG), to formate is processed with high Faraday efficiency (FE) and selectivity (>90%) at the current density close to 1000 mA cm-2 (1.444 V vs RHE). The in situ spectroscopy and density functional theory calculations provide insights into the mechanism and the understanding of the high efficiency. Remarkably, the electro-oxidation of EG at the ampere-level current density is also successfully illustrated by using a membrane-electrode assembly with high FEs to formate integrated with hydrogen production for 500 h of continuous operation. This process allows valuable chemical production at high space-time yield and is highly profitable (588-700 $ ton-1 PET), showing an industrial perspective on single-atom catalysis of electrochemical plastic upcycling.

Identification of Synergies in Fe, Co-Coordinated Polyphthalocyanines Scaffolds for Electrochemical CO2 Reduction Reaction

The synergistic interaction between the isolated metal sites promoted the electrocatalytic activity of the catalysts. However, the structural heterogeneity of the isolated sites makes it challenging to evaluate this effect accurately. In this work, metal-coordinated polyphthalocyanine molecules (Fe-PPc, Co-PPc, FeCo-PPc) with long-range ordered and precise coordination structures are used as a platform to study the synergies of different isolated metal sites in the electrochemical CO2 reduction reaction. The combination means of experimental and theoretical calculation clearly reveal that the coexistence of Fe and Co sites in PPc significantly enhances the conjugation effect of the macrocycle. This enhancement subsequently causes the metal sites to lose more electrons, thereby improving their adsorption of CO2 and facilitating the formation of intermediate *COOH on them. As a result, FeCo-PPc achieves a CO partial current density of about 57.4 mA/cm2 with a high turnover frequency of over 49000 site-1 h-1 at -0.9 V (vs RHE).

Catalyzing Sustainable Water Splitting with Single Atom Catalysts: Recent Advances

Electrochemical water splitting for sustainable hydrogen and oxygen production have shown enormous potentials. However, this method needs low-cost and highly active catalysts. Traditional nano catalysts, while effective, have limits since their active sites are mostly restricted to the surface and edges, leaving interior surfaces unexposed in redox reactions. Single atom catalysts (SACs), which take advantage of high atom utilization and quantum size effects, have recently become appealing electrocatalysts. Strong interaction between active sites and support in SACs have considerably improved the catalytic efficiency and long-term stability, outperforming their nano-counterparts. This review's first section examines the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER). In the next section, SACs are categorized as noble metal, non-noble metal, and bimetallic synergistic SACs. In addition, this review emphasizes developing methodologies for effective SAC design, such as mass loading optimization, electrical structure modulation, and the critical role of support materials. Finally, Carbon-based materials and metal oxides are being explored as possible supports for SACs. Importantly, for the first time, this review opens a discussion on waste-derived supports for single atom catalysts used in electrochemical reactions, providing a cost-effective dimension to this vibrant research field. The well-known design techniques discussed here may help in development of electrocatalysts for effective water splitting.

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

Synergistic effect of interface and defect engineering of MoC/MoO2 nano dot encapsulated N-doped carbon nanoflowers for highly durable dye-sensitized solar cells

The design and fabrication of advanced counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) are limited by the scarcity of active sites and poor durability. Herein, we report the controlled preparation of a heterostructured nanoreactor CE based on defect-rich N-doped carbon nanoflowers (NCF) encapsulating MoC/MoO2 nano dots (NDs) in a well-defined heterophase (MoC/MoO2-NCF). The MoC/MoO2 NDs were uniformly dispersed on the NCF, and the NCF limited the size of the MoC/MoO2 NDs and prevented their agglomeration, thus maximizing the electrochemically active surface area of MoC/MoO2. Moreover, the synergistic effect between the MoC/MoO2 interface and the N-defects is conducive to the full exposure of the active sites. Furthermore, theoretical calculations revealed that the MoC/MoO2 heterojunction played a unique role in modulating the electronic structure and regulating the adsorption energy of tri-iodide in the iodide reduction reaction. The MoC/MoO2-NCF CEs in DSSCs demonstrated a power conversion efficiency (PCE) of 9.92% and high durability, exceeding the PCE (8.36%) and durability of Pt CEs. Overall, this study offers insights into the controlled synthesis of high-performance Mo-based composite CE materials for DSSCs.

Locally Ordered Single-Atom Catalysts for Electrocatalysis

Single-atom catalysts manifest nearly 100 % atom utilization efficiency, well-defined active sites, and high selectivity. However, their practical applications are hindered by a low atom loading density, uncontrollable location, and ambiguous interaction with the support, thereby posing challenges to maximizing their electrocatalytic performance. To address these limitations, the ability to arrange randomly dispersed single atoms into locally ordered single-atom catalysts (LO-SACs) substantially influences the electronic effect between reactive sites and the support, the synergistic interaction among neighboring single atoms, the bonding energy of intermediates with reactive sites and the complexity of the mechanism. As such, it dramatically promotes reaction kinetics, reduces the energy barrier of the reaction, improves the performance of the catalyst and simplifies the reaction mechanism. In this review, firstly, we introduce a variety of compelling characteristics of LO-SACs as electrocatalysts. Subsequently, the synthetic strategies, characterization methods and applications of LO-SACs in electrocatalysis are discussed. Finally, the future opportunities and challenges are elaborated to encourage further exploration in this rapidly evolving field.

Progress on Single-Atom Photocatalysts for H2 Generation: Material Design, Catalytic Mechanism, and Perspectives

Solar energy utilization is of great significance to current challenges of the energy crisis and environmental pollution, which benefit the development of the global community to achieve carbon neutrality goals. Hydrogen energy is also treated as a good candidate for future energy supply since its combustion not only supplies high-density energy but also shows no pollution gas. In particular, photocatalytic water splitting has attracted increasing research as a promising method for H2 production. Recently, single-atom (SA) photocatalysts have been proposed as a potential solution to improve catalytic efficiency and lower the costs of photocatalytic water splitting for H2 generation. Owing to the maximized atom utilization rate, abundant surface active sites, and tunable coordination environment, SA photocatalysts have achieved significant progress. This review reviews developments of advanced SA photocatalysts for H2 generation regarding the different support materials. The recent progress of titanium dioxide, metal-organic frameworks, two-dimensional carbon materials, and red phosphorus supported SA photocatalysts are carefully discussed. In particular, the material designs, reaction mechanisms, modulation strategies, and perspectives are highlighted for realizing improved solar-to-energy efficiency and H2 generation rate. This work will supply significant references for future design and synthesis of advanced SA photocatalysts.

Single-Atom and Dual-Atom Electrocatalysts: Synthesis and Applications

Distinguishing themselves from nanostructured catalysts, single-atom catalysts (SACs) typically consist of positively charged single metal and coordination atoms without any metal-metal bonds. Dual-atom catalysts (DACs) have emerged as extended family members of SACs in recent years. Both SACs and DACs possess characteristics that combine both homogeneous and heterogeneous catalysis, offering advantages such as uniform active sites and adjustable interactions with ligands, while also inheriting the high stability and recyclability associated with heterogeneous catalyst systems. They offer numerous advantages and are extensively utilized in the field of electrocatalysis, so they have emerged as one of the most prominent material research platforms in the direction of electrocatalysis. This review provides a comprehensive review of SACs and DACs in the field of electrocatalysis: encompassing economic production, elucidating electrocatalytic reaction pathways and associated mechanisms, uncovering structure-performance relationships, and addressing major challenges and opportunities within this domain. Our objective is to present novel ideas for developing advanced synthesis strategies, precisely controlling the microstructure of catalytic active sites, establishing accurate structure-activity relationships, unraveling potential mechanisms underlying electrocatalytic reactions, identifying more efficient reaction paths, and enhancing overall performance in electrocatalytic reactions.

Palladium Single-Atom (In)Stability Under Aqueous Reductive Conditions

Here, we investigate the stability and performance of single-atom Pd on TiO2 for the selective dechlorination of 4-chlorophenol. A challenge inherent to single atoms is their high surface free energy, which results in a tendency for the surface migration and aggregation of metal atoms. This work evaluates various factors affecting the stability of Pd single-atoms, including atomic dispersion, coordination environment, and substrate properties, under reductive aqueous conditions. The transition from single atoms to clusters vastly enhanced dechlorination kinetics without diminishing carbon-chlorine bond selectivity. X-ray absorption spectroscopy analysis using both in situ and ex situ conditions followed the dynamic transformation of single atoms into amorphous clusters, which consist of a unique unsaturated coordination environment and few nanometer diameter. The intricate relationship between stability and performance underscores the vital role of detailed characterization to properly determine the true active species for dehalogenation reactions.

Single-Atom Catalysis in Organic Synthesis

Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Stabilizing Single-Atomic Pt by Forming PtFe Bonds for Efficient Diboration of Alkynes

Precisely tailoring the oxidation state of single-atomic metal in heterogeneous catalysis is an efficient way to stabilize the single-atomic site and promote their activity, but realizing this approach remains a grand challenge to date. Herein, a class of stable single-atomic catalysts with well-tuned oxidation state of Pt by forming PtFe atomic bonds is reported, which are supported by defective Fe₂ O₃  nanosheets on reduced graphene oxide (PFARFNs). These as-synthesized materials can greatly enhance the catalytic activity, stability, and selectivity for the diboration of alkynes. The PFARFNs exhibit high conversion of 99% at 100 °C with an outstanding turnover frequency (TOF) of 545 h^-1 , and a relatively high conversion of 58% at room temperature (25 °C) with a TOF of 310 h^-1 , which has been hardly achieved previously. Through both experimental and theoretical investigation, it is demonstrated that the fast electron transfer from Fe to Pt in Fe-Pt-O atomic sites in PFARFNs can not only stabilize the single-atomic Pt, but also significantly improve their catalytic activity.

Synchronous coupling of defects and a heteroatom-doped carbon constraint layer on cobalt sulfides toward boosted oxide electrolysis activities for highly energy-efficient micro-zinc-air batteries

The sluggish kinetics of oxygen electrocatalysis reactions on cathodes significantly suppresses the energy efficiency of zinc-air batteries (ZABs). Herein, by coupling in situ generated CoS nanoparticles rich in cobalt vacancies (V_Co) with a dual-heteroatom-doped layered carbon framework, a hybrid Co-based catalyst (Co_1-xS@N/S-C) is designed and synthesized from Co-MOF precursor. Experimental analyses, together with density functional theory (DFT)-based calculations, demonstrate that the facilitated ion diffusion enabled by the introduced V_Co, together with the enhanced electron transport benefiting from the well-designed dual-heteroatom-doped laminated carbon framework, synergistically boost the bifunctional electrocatalytic activity of Co_1-xS@N/S-C (ΔE = 0.76 V), which is much superior to that of CoS@N/S-C without V_Co (ΔE = 0.89 V), CoS without V_Co (ΔE = 1.23 V), and the dual-heteroatom-doped laminated carbon framework. As expected, the further assembled ZAB employing Co_1-xS@N/S-C as the cathode electrocatalyst exhibits enhanced energy efficiency in terms of better cycling stability (510 cycles/170 hours) and a higher specific capacity (807 mA h g^-1). Finally, a flexible/stretched solid state micro-ZAB (F/SmZAB) with Co_1-xS@N/S-C as the cathode electrocatalyst and a wave-shaped GaIn-Ni-based liquid metal as the electronic circuit is further designed, which can display excellent electrical properties and long elongation. This work provides a new defect and structure coupling strategy for boosting the oxide electrolysis activities of Co-based catalysts. Furthermore, F/SmZAB represents a promising solution for a compatible micropower source in wearable microelectronics.

The synthesis of single-atom catalysts for heterogeneous catalysis

Heterogeneous catalysis is an important class of reactions in industrial production, especially in green chemical synthesis, and environmental and organic catalysis. Single-atom catalysts (SACs) have emerged as promising candidates for heterogeneous catalysis, due to their outstanding catalytic activity, high selectivity, and maximum atomic utilization efficiency. The high specific surface energy of SACs, however, results in the migration and aggregation of isolated atoms under typical reaction conditions. The controllable preparation of highly efficient and stable SACs has been a serious challenge for applications. Herein, we summarize the recent progress in the precise synthesis of SACs and their different heterogeneous catalyses, especially involving the oxidation and reduction reactions of small organic molecules. At the end of this review, we also introduce the challenges confronted by single-atom materials in heterogeneous catalysis. This review aims to promote the generation of novel high-efficiency SACs by providing an in-depth and comprehensive understanding of the current development in this research field.

Challenges and Opportunities in Engineering the Electronic Structure of Single-Atom Catalysts

Controlling the electronic structure of transition-metal single-atom heterogeneous catalysts (SACs) is crucial to unlocking their full potential. The ability to do this with increasing precision offers a rational strategy to optimize processes associated with the adsorption and activation of reactive intermediates, charge transfer dynamics, and light absorption. While several methods have been proposed to alter the electronic characteristics of SACs, such as the oxidation state, band structure, orbital occupancy, and associated spin, the lack of a systematic approach to their application makes it difficult to control their effects. In this Perspective, we examine how the electronic configuration of SACs can be engineered for thermochemical, electrochemical, and photochemical applications, exploring the relationship with their activity, selectivity, and stability. We discuss synthetic and analytical challenges in controlling and discriminating the electronic structure of SACs and possible directions toward closing the gap between computational and experimental efforts. By bringing this topic to the center, we hope to stimulate research to understand, control, and exploit electronic effects in SACs and ultimately spur technological developments.

Solid-State Reaction Heterogeneity During Calcination of Lithium-Ion Battery Cathode

During solid-state calcination, with increasing temperature, materials undergo complex phase transitions with heterogeneous solid-state reactions and mass transport. Precise control of the calcination chemistry is therefore crucial for synthesizing state-of-the-art Ni-rich layered oxides (LiNi_1-x-y Co_x Mn_y O₂ , NRNCM) as cathode materials for lithium-ion batteries. Although the battery performance depends on the chemical heterogeneity during NRNCM calcination, it has not yet been elucidated. Herein, through synchrotron-based X-ray, mass spectrometry microscopy, and structural analyses, it is revealed that the temperature-dependent reaction kinetics, the diffusivity of solid-state lithium sources, and the ambient oxygen control the local chemical compositions of the reaction intermediates within a calcined particle. Additionally, it is found that the variations in the reducing power of the transition metals (i.e., Ni, Co, and Mn) determine the local structures at the nanoscale. The investigation of the reaction mechanism via imaging analysis provides valuable information for tuning the calcination chemistry and developing high-energy/power density lithium-ion batteries.

Process coupling of CO2 reduction and 5-HMF oxidation mediated by defect-enriched layered double hydroxides

Aiming at the comprehensive utilization of waste carbon resources and renewable carbon resources, we put forward the photocatalytic coupling process of CO₂ reduction and 5-hydroxymethylfurfural (5-HMF) oxidation mediated by the anionic compound of layered double hydroxides (LDHs). Specifically, a ZnNiFe-LDH was synthesized by co-precipitation method, during which CO₂ was stored between LDH layers in the form of carbonate. Then, a certain amount of metal vacancies were introduced into LDH nanosheets by selectively etching Zn²⁺ ions. ICP-AES, EPR and XPS showed that the concentration of Zn vacancies gradually increased with the etching time prolonging, which thus optimized the electronic structure of LDH layers. Under the catalysis of the electron-rich metal cations and hydroxyl groups on the layers, the interlayer carbonate was in situ reduced into CO coupled accompanied with the 5-HMF oxidation to 2.5-furandiformaldehyde (DFF). Compared with the unetched ZnNiFe-LDHs, the CO and DFF yields over the LDHs etched for 3 h were increased by 2.84 and 2.82 times under UV-vis irradiation with a density of 500 mW cm^-2. Finally, combined with isotope-labeled ¹³CO₂ experiments and in situ FTIR characterization, we revealed the possible coupling mechanism and defect-induced performance enhancement mechanism.

Coupling Transition Metal Compound with Single-Atom Site for Water Splitting Electrocatalysis

Single-atom site catalysts (SACs) provide an ideal platform to identify the active centers, explore the catalytic mechanism, and establish the structure-property relationships, and thus have attracted increasing interests for electrocatalytic energy conversion. Substantial endeavors have been devoted to the construction of carbon-supported SACs, and their progress have been comprehensively reviewed. Compared with carbon-supported SACs, transition metal compounds (TMCs)-supported SACs are still in their infancy in the field of electrocatalysis. However, they have also aroused ever-increasing attention for driving electrocatalytic water splitting, and emerged as an indispensable class of SACs in recent years, predominately owing to their inherently structural features, such as rich anchoring sites, surface defects, and lattice vacancy. Herein, in this review, we have systematically summarized the recent advances of a variety of TMC supported SACs toward electrocatalytic water splitting. The advanced characterization techniques and theoretical analyses for identifying and monitoring the atomic structure of SACs are firstly manifested. Subsequently, the anchoring and stabilization mechanisms for TMC supported SACs are also highlighted. Thereafter, the advances of TMC supported SACs for driving water electrolysis are systematically unraveled.

Cation-Tuning Engineering on Metal Oxides for Oxygen Electrocatalysis

Cation-tuning engineering has become a new frontier in altering the electronic structure of electrocatalysts, which has been employed to enhance their electrochemical performance. Significant efforts have been made to promote the electrochemical performance of transition metal-based materials during oxygen electrocatalysis and related energy devices such as Zn-air batteries. Herein, the advantages of cation-tuning engineering, including cation vacancies/defects and cation doping, in the modification of the electronic structure of transition metal oxide catalysts are discussed. Additionally, practical applications of the cation-tuning engineering strategy are reviewed in detail with a special emphasis on oxygen reduction reaction and oxygen evolution reaction. Lastly, challenges and future opportunities in this field are also proposed.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Coupling Transition Metal Catalysts with Ir for Enhanced Electrochemical Water Splitting Activity

Developing advanced electrocatalysts is of great significance for boosting electrochemical water splitting to produce hydrogen. The electrocatalytic activity of a catalyst is associated with the surface/interface, geometric structure, and electronic properties. Coupling Ir with transition metal compounds is an effective strategy to improve their electrocatalytic performance. In this review, we summarize the recent progress of Ir coupled transition metal compound catalysts for the application in driving electrochemical water splitting. The significant role of Ir played in the promotion of electrocatalytic performance is firstly illustrated. Then, the applications of Ir-based catalysts in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are comprehensively discussed, with an emphasis on correlating the structure-function relationships. Lastly, the challenges and future directions for the fabrication of more advanced Ir coupled electrocatalysts are also presented.

Tailoring the Dispersion of Metals on ZnO with Preadsorbed Water

The dispersity of metal particles over oxide surfaces is generally critical for the applications of the metal/oxide hybridized systems. In this work, we have experimentally investigated the hydration effect of preadsorbed water species over the Cu and Pd particles deposited on the ZnO(10-10) surface. Using scanning tunneling microscopy (STM), we clearly saw that both Cu and Pd grow as three-dimensional particles on the clean ZnO(10-10) surface but disperse into single atoms and few-atom clusters on the water-covered surfaces. Moreover, X-ray photoelectron spectroscopy (XPS) measurements revealed that Cu is readily oxidized by interacting with the molecular water while Pd tends to bind the surface hydroxyls and keep neutral status. Our work has demonstrated the effective role of the surface water in tuning the morphologies as well as electronic states of the supported metals, which may bring new insights to a number of important surface processes with water in presence.

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.

Recent progresses on single-atom catalysts for the removal of air pollutants

The booming industrialization has aggravated emission of air pollutants, inflicting serious harm on environment and human health. Supported noble-metals are one of the most popular catalysts for the oxidation removal of air pollutants. Unfortunately, the high price and large consumption restrict their development and practical application. Single-atom catalysts (SACs) emerge and offer an optimizing approach to address this issue. Due to maximal atom utilization, tunable coordination and electron environment and strong metal-support interaction, SACs have shown remarkable catalytic performance on many reactions. Over the last decade, great potential of SACs has been witnessed in the elimination of air pollutants. In this review, we first briefly summarize the synthesis methods and modulation strategies together with the characterization techniques of SACs. Next, we highlight the application of SACs in the abatement of air pollutants including CO, volatile organic compounds (VOCs) and NOx, unveiling the related catalytic mechanism of SACs. Finally, we propose the remaining challenges and future perspectives of SACs in fundamental research and practical application in the field of air pollutant removal.

Single-atom site catalysts for environmental remediation: Recent advances

Single-atom site catalysts (SACs) can maximize the utilization of active metal species and provide an attractive way to regulate the activity and selectivity of catalytic reactions. The adjustable coordination configuration and atomic structure of SACs enable them to be an ideal candidate for revealing reaction mechanisms in various catalytic processes. The minimum use of metals and relatively tight anchoring of the metal atoms significantly reduce leaching and environmental risks. Additionally, the unique physicochemical properties of single atom sites endow SACs with superior activity in various catalytic processes for environmental remediation (ER). Generally, SACs are burgeoning and promising materials in the application of ER. However, a systematic and critical review on the mechanism and broad application of SACs-based ER is lacking. Herein, we review emerging studies applying SACs for different ERs, such as eliminating organic pollutants in water, removing volatile organic compounds, purifying automobile exhaust, and others (hydrodefluorination and disinfection). We have summarized the synthesis, characterization, reaction mechanism and structural-function relationship of SACs in ER. In addition, the perspectives and challenges of SACs for ER are also analyzed. We expect that this review can provide constructive inspiration for discoveries and applications of SACs in environmental catalysis in the future.

Oxidase-like ZnCoFe Three-Atom Nanozyme as a Colorimetric Platform for Ascorbic Acid Sensing

Great enthusiasm in single-atom catalysts for various catalytic reactions continues to heat up. However, the poor activity of the existing single/dual-metal-atom catalysts does not meet the actual requirement. In this scenario, the precise design of triple-metal-atom catalysts is vital but still challenging. Here, a triple-atom site catalyst of FeCoZn catalyst coordinated with S and N, which is doped in the carbon matrix (named FeCoZn-TAC/SNC), is designed. The FeCoZn catalyst can mimic the activity of oxidase by activating O₂ into ^•O₂^- radicals by virtue of its atomically dispersed metal active sites. Employing this characteristic, triple-atom catalysts can become a great driving force for the development of novel biosensors featuring adequate sensitivity. First, the property of FeCoZn catalyst as an oxidase-like nanozyme was explored. The obtained FeCoZn-TAC/SNC shows remarkably enhanced catalytic performance than that of FeCoZn-TAC/NC and single/dual-atom site catalysts (FeZn, CoZn, FeCo-DAC/NC and Fe, Zn, Co-SAC/NC) because of trimetallic sites, demonstrating the synergistic effect. Further, the utility of the oxidase-like FeCoZn-TAC/SNC in biosensor field is evaluated by the colorimetric sensing of ascorbic acid. The nanozyme sensor shows a wide concentration range from 0.01 to 90 μM and an excellent detection limit of 6.24 nM. The applicability of the nanozyme sensor in biologically relevant detection was further proved in serum. The implementation of TAC in colorimetric detection holds vast promise for further development of biomedical research and clinical diagnosis.

Photoinduced loading of electron-rich Cu single atoms by moderate coordination for hydrogen evolution

Single-atom catalysts offer maximal atom utilization efficiencies and high-electronegativity heteroatoms play a crucial role in coordinating reactive single metal atoms to prevent agglomeration. However, these strong coordination bonds withdraw electron density for coordinated metal atoms and consequently affect their catalytic activity. Herein we reveal the high loading (11.3 wt%) and stabilization of moderately coordinated Cu-P₃ structure on black phosphorus support by a photochemical strategy with auxiliary hydrogen. Single-atom Cu sites with an exceptional electron-rich feature show the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\triangle {G}_{{{{{{\rm{H}}}}}}*}$$\end{document}△GH* close to zero to favor catalysis. Neighboring Cu atoms work in synergy to lower the energy of key water adsorption and dissociation intermediates. The reported catalyst shows a low overpotential of only 41 mV at 10 mA cm⁻² and Tafel slope of 53.4 mV dec⁻¹ for the alkaline hydrogen evolution reaction, surpassing both isolated Cu single atoms and Cu nanoclusters. The promising materials design strategy sheds light on the design and fabrication of high-loading single metal atoms and the role of neighboring single atoms for enhanced reaction kinetics.

While atomically dispersed metals can maximize reaction catalytic sites, it is challenging to achieve high atomic densities without agglomeration. Here, authors prepared Cu single-atoms on black phosphorous using a photochemical strategy and auxiliary H₂ as proton reduction electrocatalysts.

Single-Atom Nanozymes: Fabrication, Characterization, Surface Modification and Applications of ROS Scavenging and Antibacterial

Nanozymes are nanomaterials with intrinsic natural enzyme-like catalytic properties. They have received extensive attention and have the potential to be an alternative to natural enzymes. Increasing the atom utilization rate of active centers in nanozymes has gradually become a concern of scientists. As the limit of designing nanozymes at the atomic level, single-atom nanozymes (SAzymes) have become the research frontier of the biomedical field recently because of their high atom utilization, well-defined active centers, and good natural enzyme mimicry. In this review, we first introduce the preparation of SAzymes through pyrolysis and defect engineering with regulated activity, then the characterization and surface modification methods of SAzymes are introduced. The possible influences of surface modification on the activity of SAzymes are discussed. Furthermore, we summarize the applications of SAzymes in the biomedical fields, especially in those of reactive oxygen species (ROS) scavenging and antibacterial. Finally, the challenges and opportunities of SAzymes are summarized and prospected.

Emerging single atom catalysts in gas sensors

Single atom catalysts (SACs) offer unprecedented opportunities for high-efficiency reactions taking place in many important fields of catalytic processes, electrochemistry, and photoreactions. Due to their maximized atomic utilization and unique electronic and chemical properties, SACs can provide high activity and excellent selectivity for gas adsorption and electron transport, leveraging SACs that enhance the detection sensitivity and selectivity to target gases. In the past few years, SACs including both noble (Pt, Pd, Au, etc.) and non-noble (Mn, Ni, Zn etc.) metals have been demonstrated to be very useful in optimizing sensing performances. However, a comprehensive review on this topic is still missing. Herein, we summarize the synthesis technologies of SACs that are applicable to gas sensors. The electronic and chemical interactions between SACs and host sensing materials, which are crucial to sensor functions, are discussed. Then, we highlight the application progress of various SACs in gas sensors. Prospects in the creation of new sensing materials with emerging SACs and versatile supports are also present. Finally, the challenges and prospects of SACs in the future development of sensors are analyzed.

Role of Polarons in Single-Atom Catalysts: Case Study of Me1 [Au1, Pt1, and Rh1] on TiO2(110)

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh1, Pt1, and Au1 metals on the reduced TiO2(110) surface, a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt1 and Au1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11244-022-01651-0.

Controlled Modification of Axial Coordination for Transition-Metal Single-Atom Electrocatalyst

Single-atom catalysts (SACs) have emerged as a new frontier in areas such as electrocatalysis, photocatalysis, and enzymatic catalysis. Aided by recent advances in the synthetic methodologies of nanomaterials, atomic characterization technologies, and theoretical calculation modeling, various SACs have been prepared for a variety of catalytic reactions. To meet the requirements of SACs with distinctive performance and appreciable selectivity, much research has been carried out to adjust the coordination configuration and electronic properties of SACs. This concept summarizes the latest advances in the experimental and computational efforts aimed at tuning the axial coordination of SACs. Series of atoms, functional groups or even macrocycles are oriented into the atomic metal center, and how this affects the electrocatalytic performance is also reviewed. Finally, this concept presents perspectives for the further precise design, preparation and in-situ detection of axially coordinated SACs.

Light-Induced Structural Dynamic Evolution of Pt Single Atoms for Highly Efficient Photocatalytic CO2 Reduction

Revealing the structural evolution of the real active site during photocatalysis is very important for understanding the catalytic mechanism, but it remains a great challenge. By employing single atoms (SAs) as the mechanism research platform, we investigated the variation of the SA structure under light and the corresponding reaction pathway controlment mechanism. In particular, taking the defect anchoring strategy, Pt SAs are anchored on the metal ion vacancy-rich ZnNiTi layered double hydroxide-etched (ZnNiTi-LDHs-E) support. It is proved by CO-Fourier transform infrared and X-ray absorption fine structure characterization methods that the Pt SAs could gain photoelectrons to form cationic Pt(IV), electron-rich Pt(II), and near-neutral Ptδ+ species at different light intensities. By in situ inducing the above different Pt SAs in photocatalytic CO2 reduction, a dramatic product distribution is observed: (1) under weak light, Pt(IV) SAs cannot activate CO, so CO cannot be further transformed into hydrocarbons; (2) under the moderate light, electron-rich Pt(II) SAs could cooperate with adjacent LDH surface sites (Ni2+/Ti4+) to open up the C-C coupling route for C2H6 generation; and (3) Pt SAs in the state of near-neutral Ptδ+ could directly hydrogenate CO into CH4. This work reveals the structural evolution of Pt SAs in photocatalysis and the corresponding effect on catalytic performance, which provides a new idea for the construction of highly efficient photocatalysts.

Activated MoS2 by Constructing Single Atomic Cation Vacancies for Accelerated Hydrogen Evolution Reaction

Regulating the electronic structure of MoS2 by constructing cationic vacancies is an effective method to activate and improve its intrinsic properties. Herein, we synthesize the MoS2-based composite with abundant single atomic Mo cation vacancies through uniformly loading nickel-cobalt-Prussian blue analogues (NiCoPBA) (NiCoPBA-MoS2-VMo) by immersing a single Ni atom-decorated MoS2 (Ni-MoS2) into K3[Co(CN)6] solution. Subsequently, NiCoP-MoS2-VMo with improved conductivity is obtained by phosphating the composite as a high-efficiency hydrogen evolution reaction (HER) catalyst. Experiments and theoretical calculations indicate that the electrons of NiCoP are spontaneously transferred to the substrate MoS2-VMo nanosheets in NiCoP-MoS2-VMo, and the moderately oxidized NiCoP is beneficial to the adsorption of OH*. Meanwhile, the mono-atomic Mo cation vacancies of the catalyst modulate the electronic structure of S, optimizing the adsorption of hydrogen in the reaction process. Therefore, NiCoP-MoS2-VMo has enhanced chemical adsorption for H* (on MoS2-VMo) and OH*(on NiCoP), expediting the water-splitting step and HER catalysis, and benefiting from the regulation of the electronic structure induced by the construction of metallic Mo vacancies in MoS2, the as-prepared catalyst displays an overpotential of only 67 mV at 10 mA cm-2 with long-term stability (no current decay over 20 h). This work affords not only a kind of efficient HER catalysts but also a new valuable route for developing inexpensive and high-performance catalysts with single atomic cation vacancies.