Atomically Dispersed Ru on Ultrathin Pd Nanoribbons

Manipulating d-Band Center of Nickel by Single-Iodine-Atom Strategy for Boosted Alkaline Hydrogen Evolution Reaction

Ni-based electrocatalysts have been predicted as highly potential candidates for hydrogen evolution reaction (HER); however, their applicability is hindered by an unfavorable d-band energy level (Ed). Moreover, precise d-band structural engineering of Ni-based materials is deterred by appropriative synthesis methods and experimental characterization. Herein, we meticulously synthesize a special single-iodine-atom structure (I-Ni@C) and characterize the Ed manipulation via resonant inelastic X-ray scattering (RIXS) spectroscopy to fill this gap. The complex catalytic mechanism has been elucidated via synchrotron radiation-based multitechniques (SRMS) including X-ray absorption fine structure (XAFS), in situ synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy, and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). In particular, RIXS is innovatively applied to reveal the precise regulation of Ni Ed of I-Ni@C. Consequently, the role of such single-iodine-atom strategy is confirmed to not only facilitate the moderate Ed of the Ni site for balancing the adsorption/desorption capacities of key intermediates but also act as a bridge to enhance the electronic interaction between Ni and the carbon shell for forming a localized polarized electric field conducive to H2O dissociation. As a result, I-Ni@C exhibits an enhanced alkaline hydrogen evolution performance with an overpotential of 78 mV at 10 mA/cm2 and superior stability, surpassing the majority of the reported Ni-based catalysts. Overall, this study has managed to successfully tailor the d-band center of materials from the SRMS perspective, which has crucial implications for nanotechnology, chemistry, catalysis, and other fields.

RuZn NPs with electroactivity and oxidase-like property for dual-mode anti-cancer drug monitoring

6-mercaptopurine (6-MP) as the effective anti-cancer drug was used for the treatment of Crohn's disease and acute lymphoblastic leukaemia, but the response to maintenance therapy was variable with individual differences. In order to control the dosage and decrease the side effects of 6-MP, a sensitive and stable assay was urgently needed for 6-MP monitoring. Herein, RuZn NPs with electrochemical oxidation property and oxidase-like activity was proposed for dual-mode 6-MP monitoring. Burr-like RuZn NPs were prepared and explored to not only exhibit an electrochemical oxidation signal at 0.78 V, but also displayed excellent oxidase-like performances. RuZn NPs were utilized for the dual-mode monitoring of 6-MP, attributing to the formation of Ru-SH covalent bonding. The colorimetric method showed good linearity from 10 μM to 5 mM with the limit of detection (LOD) of 300 nM, while the electrochemical method provided a higher sensitivity with the LOD of 37 nM in range from 100 nM to 200 μM. This work provided a new way for the fabrication of dual-functional nanotags with electroactivity and oxidase-like property, and opened a dual-mode approach for the 6-MP detection applications with complementary and satisfactory results.

An all-metallic nanovesicle for hydrogen oxidation

Vesicle, a microscopic unit that encloses a volume with an ultrathin wall, is ubiquitous in biomaterials. However, it remains a huge challenge to create its inorganic metal-based artificial counterparts. Here, inspired by the formation of biological vesicles, we proposed a novel biomimetic strategy of curling the ultrathin nanosheets into nanovesicles, which was driven by the interfacial strain. Trapped by the interfacial strain between the initially formed substrate Rh layer and subsequently formed RhRu overlayer, the nanosheet begins to deform in order to release a certain amount of strain. Density functional theory (DFT) calculations reveal that the Ru atoms make the curling of nanosheets more favorable in thermodynamics applications. Owing to the unique vesicular structure, the RhRu nanovesicles/C displays excellent hydrogen oxidation reaction (HOR) activity and stability, which has been proven by both experiments and DFT calculations. Specifically, the HOR mass activity of RhRu nanovesicles/C are 7.52 A mg(Rh+Ru)-1 at an overpotential of 50 mV at the rotating disk electrode (RDE) level; this is 24.19 times that of commercial Pt/C (0.31 mA mgPt-1). Moreover, the hydroxide exchange membrane fuel cell (HEMFC) with RhRu nanovesicles/C displays a peak power density of 1.62 W cm-2 in the H2-O2 condition, much better than that of commercial Pt/C (1.18 W cm-2). This work creates a new biomimetic strategy to synthesize inorganic nanomaterials, paving a pathway for designing catalytic reactors.

3D Noble-Metal Nanostructures Approaching Atomic Efficiency and Atomic Density Limits

Noble metals have been widely used in catalysis, however, the scarcity and high cost of noble metal motivate researchers to balance the atomic efficiency and atomic density, which is formidably challenging. This article proposes a robust strategy for fabricating 3D amorphous noble metal-based oxides with simultaneous enhancement on atomic efficiency and density with the assistance of atomic channels, where the atomic utilization increases from 18.2% to 59.4%. The unique properties of amorphous bimetallic oxides and formation of atomic channels have been evidenced by detailed experimental characterizations and theoretical simulations. Moreover, the universality of the current strategy is validated by other binary oxides. When Cu2IrOx with atomic channels (Cu2IrOx-AE) is used as catalyst for oxygen evolution reaction (OER), the mass activity and turnover frequency value of Cu2IrOx-AE are 1-2 orders of magnitude higher than CuO/IrO2 and Cu2IrOx without atomic channels, largely outperforming the reported OER catalysts. Theoretical calculations reveal that the formation of atomic channels leads to various Ir sites, on which the proton of adsorbed *OH can transfer to adjacent O atoms of [IrO6]. This work may attract immediate interest of researchers in material science, chemistry, catalysis, and beyond.

Air-Stable, Large-Area 2D Metals and Semiconductors

Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.

Bridging Together Theoretical and Experimental Perspectives in Single-Atom Alloys for Electrochemical Ammonia Production

Ammonia is an essential commodity in the food and chemical industry. Despite the energy-intensive nature, the Haber-Bosch process is the only player in ammonia production at large scales. Developing other strategies is highly desirable, as sustainable and decentralized ammonia production is crucial. Electrochemical ammonia production by directly reducing nitrogen and nitrogen-based moieties powered by renewable energy sources holds great potential. However, low ammonia production and selectivity rates hamper its utilization as a large-scale ammonia production process. Creating effective and selective catalysts for the electrochemical generation of ammonia is critical for long-term nitrogen fixation. Single-atom alloys (SAAs) have become a new class of materials with distinctive features that may be able to solve some of the problems with conventional heterogeneous catalysts. The design and optimization of SAAs for electrochemical ammonia generation have recently been significantly advanced. This comprehensive review discusses these advancements from theoretical and experimental research perspectives, offering a fundamental understanding of the development of SAAs for ammonia production.

Slow-release synthesis of Cu single-atom catalysts with the optimized geometric structure and density of state distribution for Fenton-like catalysis

Single-atom Fenton-like catalysis has attracted significant attention, yet the quest for controllable synthesis of single-atom catalysts (SACs) with modulation of electron configuration is driven by the current disadvantages of poor activity, low selectivity, narrow pH range, and ambiguous structure-performance relationship. Herein, we devised an innovative strategy, the slow-release synthesis, to fabricate superior Cu SACs by facilitating the dynamic equilibrium between metal precursor supply and anchoring site formation. In this strategy, the dynamics of anchoring site formation, metal precursor release, and their binding reaction kinetics were regulated. Bolstered by harmoniously aligned dynamics, the selective and specific monatomic binding reactions were ensured to refine controllable SACs synthesis with well-defined structure-reactivity relationship. A copious quantity of monatomic dispersed metal became deposited on the C3N4/montmorillonite (MMT) interface and surface with accessible exposure due to the convenient mass transfer within ordered MMT. The slow-release effect facilitated the generation of targeted high-quality sites by equilibrating the supply and demand of the metal precursor and anchoring site and improved the utilization ratio of metal precursors. An excellent Fenton-like reactivity for contaminant degradation was achieved by the Cu1/C3N4/MMT with diminished toxic Cu liberation. Also, the selective ·OH-mediated reaction mechanism was elucidated. Our findings provide a strategy for regulating the intractable anchoring events and optimizing the microenvironment of the monatomic metal center to synthesize superior SACs.

Nanoarchitectonics of Metallene Materials for Electrocatalysis

Controlling the synthesis of metal nanostructures is one approach for catalyst engineering and performance optimization in electrocatalysis. As an emerging class of unconventional electrocatalysts, two-dimensional (2D) metallene electrocatalysts with ultrathin sheet-like morphology have gained ever-growing attention and exhibited superior performance in electrocatalysis owing to their distinctive properties originating from structural anisotropy, rich surface chemistry, and efficient mass diffusion capability. Many significant advances in synthetic methods and electrocatalytic applications for 2D metallenes have been obtained in recent years. Therefore, an in-depth review summarizing the progress in developing 2D metallenes for electrochemical applications is highly needed. Unlike most reported reviews on the 2D metallenes, this review starts by introducing the preparation of 2D metallenes based on the classification of the metals (e.g., noble metals, and non-noble metals) instead of synthetic methods. Some typical strategies for preparing each kind of metal are enumerated in detail. Then, the utilization of 2D metallenes in electrocatalytic applications, especially in the electrocatalytic conversion reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, fuel oxidation reaction, CO₂ reduction reaction, and N₂ reduction reaction, are comprehensively discussed. Finally, current challenges and opportunities for future research on metallenes in electrochemical energy conversion are proposed.

Intermetallic Single-Atom Alloy In-Pd Bimetallene for Neutral Electrosynthesis of Ammonia from Nitrate

Harvesting recyclable ammonia (NH₃) from the electrocatalytic reduction of nitrate (NO₃RR) offers a sustainable strategy to close the ecological nitrogen cycle from nitration contamination in an energy-efficient and environmentally friendly manner. The emerging intermetallic single-atom alloys (ISAAs) are recognized to achieve the highest site density of single atoms by isolating contiguous metal atoms into single sites stabilized by another metal within the intermetallic structure, which holds promise to couple the catalytic benefits from intermetallic nanocrystals and single-atom catalysts for promoting NO₃RR. Herein, ISAA In-Pd bimetallene, in which the Pd single atoms are isolated by surrounding In atoms, is reported to boost neutral NO₃RR with a NH₃ Faradaic efficiency (FE) of 87.2%, a yield rate of 28.06 mg h^-1 mg_Pd^-1, and an exceptional electrocatalytic stability with increased activity/selectivity over 100 h and 20 cycles. The ISAA structure induces substantially diminished overlap of Pd d-orbitals and narrowed p-d hybridization of In-p and Pd-d states around the Fermi level, resulting in a stronger NO₃^- adsorption and a depressed energy barrier of the potential-determining step for NO₃RR. Further integrating the NO₃RR catalyst into a Zn-NO₃^- flow battery as the cathode delivers a power density of 12.64 mW cm^-2 and a FE of 93.4% for NH₃ production.

Activable Ru-PdRu nanosheets with heterogeneous interface for High-efficiency alcohol oxidation reaction

Fabricating 2D nanomaterials with heterogeneous structure is a feasible way to improve catalytic performance owing to its large surface area and tunable electron structure. However, such a category has not been widely reported in the field of alcohol oxidation reaction (AOR). In this work, we reported a new type of heterostructure nanosheet with Ru nanoparticles decorated around the edge of PdRu nanosheets (Ru-PdRu HNSs). Particularly, strong electronic interaction and sufficient active sites attributed to the construction of heterogeneous interface, is the key to the superior electrocatalytic behavior of Ru-PdRu HNSs towards methanol oxidation reaction (MOR), ethylene glycol oxidation reaction (EGOR), and glycerol oxidation reaction (GOR). Remarkably, owing to the enhanced electron transfer brought by the introduction of the Ru-PdRu heterogeneous interface, these novel nanosheets are highly durable. Apart from being able to maintain the highest current density after 4000 s chronoamperometry test, Ru-PdRu HNSs can be reactivated with negligible activity loss in MOR and GOR test after four consecutive i-t experiments. Impressively, in the EGOR test, after reactivation, the current density is step-wisely increased, making it one of the best AOR electrocatalysts.

Hydrogen Bond Network Induced by Surface Ligands Shifts the Semi-hydrogenation Selectivity over Palladium Catalysts

Tuning the metal-ligand interfaces of heterogeneous catalysts has emerged as an effective strategy to optimize their catalytic performance. However, improving the selectivity via organic modification remains a challenge so far. In this work, we demonstrate a simple ligand modification by preparing cysteamine-coated ultrathin palladium nanosheets. The as-prepared catalyst exhibits excellent selectivity with durability during catalytic hydrogenation of terminal alkynes, superior to most previously reported ligand-protected palladium catalysts. Further study reveals that a zwitterionic transformation occurs on the palladium interface under the H₂ conditions, generating a rigid hydrogen bond network. Such an unexpected effect beyond the traditional steric effect derived from van der Waals interactions makes the catalytic surface favor the hydrogenation of alkynes over alkenes without significantly sacrificing the catalytic activity. These results not only provide a unique steric effect concept for surface coordination chemistry but also provide a practical application to improve the selectivity and activity comprehensively.

Embedding Ru Clusters and Single Atoms into Perovskite Oxide Boosts Nitrogen Fixation and Affords Ultrahigh Ammonia Yield Rate

Ammonia is a key chemical feedstock worldwide. Compared with the well-known Haber-Bosch method, electrocatalytic nitrogen reduction reaction (ENRR) can eventually consume less energy and have less CO₂ emission. In this study, a plasma-enhanced chemical vapor deposition method is used to anchor transition metal element onto 2D conductive material. Among all attempts, Ru single-atom and Ru-cluster-embedded perovskite oxide are discovered with promising electrocatalysis performance for ENRR (NH₃ yield rate of up to 137.5 ± 5.8 µg h^-1  mg_cat ^-1 and Faradaic efficiency of unexpected 56.9 ± 4.1%), reaching the top record of Ru-based catalysts reported so far. In situ experiments and density functional theory calculations confirm that the existence of Ru clusters can regulate the electronic structure of Ru single atoms and decrease the energy barrier of the first hydrogenation step (*NN to *NNH). Anchoring Ru onto various 2D perovskite oxides (LaMO-Ru, MCr, Mn, Co, or Ni) also show boosted ENRR performance. Not only this study provides an unique strategy toward transition-metal-anchored new 2D conductive materials, but also paves the way for fundamental understanding the correlation between cluster-involved single-atom sites and catalytic performance.

Defect-Rich PdIr Bimetallene Nanoribbons with Interatomic Charge Localization for Isopropanol-Assisted Seawater Splitting

Metallene with outstanding physicochemical properties is an efficient two-dimensional electrocatalysts for sustainable hydrogen (H₂ ) production applications. However, the controllable fabrication of extended atomically thin metallene nanoribbons remains a formidable challenge. Herein, this work proposes a controllable preparation strategy for atomically thin defect-rich PdIr bimetallene nanoribbons (PdIr BNRs) with a thickness of only 1.5 nm for the efficient and stable isopropanol-assisted seawater electrolytic H₂ production. When using PdIr BNRs as catalyst to build an isopropanol-assisted seawater electrolysis system, a voltage of only 0.38 V is required at @10 mA cm^-2 to achieve energy-saving H₂ production, while producing high value-added acetone at the anode. The aberration-corrected high-resolution transmission electron microscopy (HRTEM) clearly reveals that the PdIr BNRs possess abundant structural defects, which can additionally serve as highly catalytically active sites. Density functional theory (DFT) calculations combined with X-ray absorption spectroscopy studies reveal that the introduction of Ir atoms can induce the formation of a localized charge region and shift the d-band center of Pd down, thereby reducing the adsorption energy on the catalyst in favor of the rapid desorption of H₂ . This work opens the way for the controllable design and construction of defect-rich atomically thin metallene nanoribbons for efficient electrocatalytic applications.

Spatially Confined PdHx Metallenes by Tensile Strained Atomic Ru Layers for Efficient Hydrogen Evolution

Hydride metallenes show great potential for hydrogen-related catalytic applications due to favorable electronic structures modulated by interstitial hydrogen atoms and large active surface areas of metallenes. Metallene nanostructures generally have compressive strain relative to bulk, which can affect both the stability and the catalytic behavior of hydride metallenes but in general cannot be controlled. Here, we demonstrate highly stable PdH_x metallenes with a tensile strained Ru surface layer and reveal the spatial confinement effect of the Ru skin by multiple spectroscopic characterizations and molecular dynamics simulations. These PdH_x@Ru metallenes with a 4.5% expanded Ru outer layer exhibit outstanding alkaline hydrogen evolution reaction activity with a low overpotential of 30 mV at 10 mA cm^-2 and robust stability with negligible activity decay after 10,000 cycles, which are superior to commercial Pt/C and most reported Ru-based electrocatalysts. Control experiments and first-principles calculations reveal that the tensile strained Ru outer layer lowers the energy barrier of H₂O dissociation and provides a moderate hydrogen adsorption energy.

Tailoring of Active Sites from Single to Dual Atom Sites for Highly Efficient Electrocatalysis

Single atom catalysts (SACs) have been attracting extensive attention in electrocatalysis because of their unusual structure and extreme atom utilization, but the low metal loading and unified single site induced scaling relations may limit their activity and practical application. Tailoring of active sites at the atomic level is a sensible approach to break the existing limits in SACs. In this review, SACs were first discussed regarding carbon or non-carbon supports. Then, five tailoring strategies were elaborated toward improving the electrocatalytic activity of SACs, namely strain engineering, spin-state tuning engineering, axial functionalization engineering, ligand engineering, and porosity engineering, so as to optimize the electronic state of active sites, tune d orbitals of transition metals, adjust adsorption strength of intermediates, enhance electron transfer, and elevate mass transport efficiency. Afterward, from the angle of inducing electron redistribution and optimizing the adsorption nature of active centers, the synergistic effect from adjacent atoms and recent advances in tailoring strategies on active sites with binuclear configuration which include simple, homonuclear, and heteronuclear dual atom catalysts (DACs) were summarized. Finally, a summary and some perspectives for achieving efficient and sustainable electrocatalysis were presented based on tailoring strategies, design of active sites, and in situ characterization.

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.

In Situ Surface Restraint-Induced Synthesis of Transition-Metal Nitride Ultrathin Nanocrystals as Ultrasensitive SERS Substrate with Ultrahigh Durability

It is a major challenge to synthesize crystalline transition-metal nitride (TMN) ultrathin nanocrystals due to their harsh reaction conditions. Herein, we report that highly crystalline tungsten nitride (W₂N, WN, W₃N₄, W₂N₃) nanocrystals with small size and excellent dispersibility are prepared by a mild and general in situ surface restraint-induced growth method. These ultrafine tungsten nitride nanocrystals are immobilized in ultrathin carbon layers, forming an interesting hybrid nanobelt structure. The hybrid WN/C nanobelts exhibit a strong localized surface plasmon resonance (LSPR) effect and surface-enhanced Raman scattering (SERS) effect, including a lowest detection limit of 1 × 10^-12 M and a Raman enhancement factor of 6.5 × 10⁸ comparable to noble metals, which may be one of the best records for non-noble metal SERS substrates. Moreover, they even can maintain the SERS performance in a variety of harsh environments, showing outstanding corrosion resistance, radiation resistance, and oxidation resistance, which is not available on traditional noble metal and semiconductor SERS substrates. A synergistic Raman enhancement mechanism of LSPR and interface charge transfer is found in the carbon-coated tungsten nitride substrate. A microfluidic SERS channel integrating the enrichment and detection of trace substances is constructed with the WN/C nanobelt, which realizes high-throughput dynamic SERS analysis.

Biaxially Compressive Strain in Ni/Ru Core/Shell Nanoplates Boosts Li-CO2 Batteries

Regulating surface strain of nanomaterials is an effective strategy to manipulate the activity of catalysts, yet not well recognized in rechargeable Li-CO₂ batteries. Herein, biaxially compressive strained nickel/ruthenium core/shell hexagonal nanoplates (Ni/Ru HNPs) with lattice compression of ≈5.1% and ≈3.2% in the Ru {10-10} and (0002) facets are developed as advanced catalysts for Li-CO₂ batteries. It is demonstrated that tuning the electronic structure of Ru shell through biaxially compressive strain engineering can boost the kinetically sluggish CO₂ reduction and evolution reactions, thus achieving a high-performance Li-CO₂ battery with low charge platform/overpotential (3.75 V/0.88 V) and ultralong cycling life (120 cycles at 200 mA g^-1 with a fixed capacity of 1000 mAh g^-1 ). Density functional theory calculations reveal that the biaxially compressive strain can downshift the d-band center of surface Ru atoms and thus weaken the binding of CO₂ molecules, which is energetically beneficial for the nucleation and decomposition of Li₂ CO₃ crystals during the discharge and charge processes. This study confirms that strain engineering, though constructing a well-defined core/shell structure, is a promising strategy to improve the inherent catalytic activity of Ru-based materials in Li-CO₂ batteries.

Synergistic effect of oxidation etching and phase transformation triggered by controllable ion-bath microenvironments toward constructing ultra-thin porous nanosheets for accelerated industrial water splitting at high current density

Precisely tailoring the structure of inorganic materials at the micron and nanometer scales, especially in collaboration with component customization to design efficient, stable and low-cost transition-metal-based catalysts for industrial electrocatalytic water splitting (EWS) is a key renewable energy technology, but still facing a daunting challenge. Here, the controllable escape of Ni atom is adopted to disturb the hydrothermal ion-bath environment, thereby resulting in the coexistence of high valence Ni and Fe ions. Combined with a one-step hydrothermal coordination strategy, the timeline-adjusted ion-bath microenvironment can effectively trigger the phase transformation of carbonate hydroxide hydrate nanosheets (NFCH) to nickel ferrite intercalated NFCH ultra-thin porous nanosheets (NF-CH-O). Thanks to the high-energy phase boundary synergistic effect and the rapid mass transfer advantages of ultra-thin porous nanostructures, the as-prepared NF-CH-O nanosheets exhibit remarkable oxygen and hydrogen evolution reaction (OER/HER) catalytic activity and stability, with low overpotentials of 207/191 mV at 50 mA cm^-2, respectively, as well as the activity retention for 100 h. The alkaline water electrolyzer set up with NF-CH-O as both anodic and cathodic electrodes only requires a cell potential of 1.688 V to reach 50 mA cm^-2 in a continuous operation of 100 h. More impressively, NF-CH-O only requires overpotentials of 266, 292 mV and 1.877 V to drive high current densities up to 500 mA cm^-2 for OER, HER and EWS, respectively, and exhibits excellent stability with a reduction in the activity of less than 10% over cycles of more than 65 h. This work highlights the room-temperature controllable ion-bath oxidative etching strategy to design efficient bifunctional catalysts with ultra-thin porous structure and high-current-density activity. Meanwhile, combined with the advantages of direct growth on the substrate for mass production, such meticulous consideration of nanostructured design will be more competitive in the H₂-production industry.

Reversing the Catalytic Selectivity of Single-Atom Ru via Support Amorphization

Supported single-atom catalysts (SACs), with the extremely homogenized active sites could achieve high hydrogenation selectivity toward one of the functional groups coexisting in the reactant molecule. However, as to the target group, the control of selective recognition and activation by SACs still remains a challenge. Herein, the phase engineering of the support is adopted to control the chemo-recognition behavior of SACs in selective hydrogenation. Single-atom Ru on amorphous porous ultrathin TiO₂ nanosheets (Ru₁/a-TiO₂) is constructed, in which Ru is more positively charged than that in the crystalline counterpart (Ru₁/c-TiO₂). Moreover, in the nitro/vinyl selective hydrogenation process, Ru₁/a-TiO₂ shows superior nitro selectivity, opposite to the vinyl selectivity of Ru₁/c-TiO₂. Density functional theory calculations for single-atom Ru of different charge states show that the reactant adsorption configuration could be inverted in the amorphous TiO₂, accounting for the chemo-recognition behavior controlled by the phase of support.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Assessing the Catalytic Behavior of Platinum Group Metal-Based Ultrathin Nanowires Using X-ray Absorption Spectroscopy

Ultrathin metal-based nanowires have excelled as electrocatalysts in small-molecule reactions, such as the oxygen reduction reaction (ORR), the methanol oxidation reaction (MOR), and the ethanol oxidation reaction (EOR), and have consistently outperformed analogous Pt/C standards. As such, a detailed understanding of the structural and electronic properties of ultrathin nanowires is essential in terms of understanding structure-property correlations, which are crucial in the rational design of ever more sophisticated electrocatalysts. X-ray absorption spectroscopy (XAS) represents an important and promising characterization technique with which to acquire unique insights into the electronic structure and the local atomic structure of nanomaterials. Herein, we discuss tangible examples of how both ex situ and in situ XAS experiments have been recently applied to probing the complex behavior of ultrathin nanowires used in electrocatalysis. Moreover, based on this precedence, we provide ideas about the future potential and direction of these ongoing efforts.

Single-Atom Ru Implanted on Co3 O4 Nanosheets as Efficient Dual-Catalyst for Li-CO2 Batteries

Li-CO2 battery has attracted extensive attention and research due to its super high theoretical energy density and its ability to fix greenhouse gas CO2 . However, the slow reaction kinetics during discharge/charge seriously limits its development. Hence, a simple cation exchange strategy is developed to introduce Ru atoms onto a Co3 O4 nanosheet array grown on carbon cloth (SA Ru-Co3 O4 /CC) to prepare a single atom site catalyst (SASC) and successfully used in Li-CO2 battery. Li-CO2 batteries based on SA Ru-Co3 O4 /CC cathode exhibit enhanced electrochemical performances including low overpotential, ultra high capacity, and long cycle life. Density functional theory calculations reveal that single atom Ru as the driving force center can significantly enhance the intrinsic affinity for key intermediates, thus enhancing the reaction kinetics of CO2 reduction reaction in Li-CO2 batteries, and ultimately optimizing the growth pathway of discharge products. In addition, the Bader charge analysis indicates that Ru atoms as electron-deficient centers can enhance the catalytic activity of SA Ru-Co3 O4 /CC cathode for the CO2 evolution reaction. It is believed that this work has important implications for the development of new SASCs and the design of efficient catalyst for Li-CO2 batteries.

RuCu Cage/Alloy Nanoparticles with Controllable Electroactivity for Specific Electroanalysis Applications

Electrochemical nanotags with controllable and multiresponse electroactivity have a great capacity for overcoming the drawbacks of limited target monitoring and inaccurate detection results for electrochemical sensors. In this contribution, double electro-oxidative Ru and Cu metals were integrated into RuCu nanostructures for the generation of dual electro-oxidative signals. A facial approach was proposed for the controllable fabrication of RuCu cage nanoparticles (NPs) and RuCu alloy NPs by simply adjusting the pH value of the reaction system. RuCu cage NPs and RuCu alloy NPs demonstrated inherent different electro-oxidative responses owing to the remarkable distinction of structures with different metal valences. RuCu cage NPs showed a single electro-oxidization peak at 0.84 V, assigned to the exposure of more Ru⁰ electroactive sites on the hollow cage structures. RuCu alloy NPs illustrated dual electro-oxidization peak at 0.84 and -0.16 V, attributing to the presence of Ru⁰ and Cu⁺ electroactive sites on the alloy structures, respectively. RuCu cage NPs and RuCu alloy NPs served as specific electroactive tags, achieving the selective monitoring of Na₂S and ratiometric electrochemical detection of xanthine in monosodium glutamate, respectively. The limits of detection were as low as 27 pM for Na₂S and 70 nM for xanthine. The rational design of multimetal nanostructures holds enormous potential for the generation of multiresponse electroactivity with the impetus for exploring the capacity of specific electrochemical sensing.

General Design Concept for Single-Atom Catalysts toward Heterogeneous Catalysis

As a new and popular material, single-atom catalysts (SACs) exhibit excellent activity, selectivity, and stability for numerous important reactions, and show great potential in heterogeneous catalysis due to their high atom utilization efficiency and the controllable characteristics of the active sites. The composition and coordination would determine the geometric and electronic structures of SACs, and thus greatly influence the catalytic performance. Based on atom economy, rational design and controllable synthesis of SACs have become central tasks in the fields of low-cost and green catalysis. Herein, an introduction to the recent progress in the precise synthesis of SACs including the regulation of the coordination structure and the choice of different systems is presented. Thereafter, the potentials of SACs in different applications are comprehensively summarized and discussed. Furthermore, a detailed discussion of the recent developments regarding the large-scale preparation of SACs is provided, including the major issues and prospects for industrialization. Finally, the main challenges and opportunities of rapid large-scale industrialization of SACs are briefly discussed.

Highly efficient ammonia synthesis at low temperature over a Ru-Co catalyst with dual atomically dispersed active centers

The desire for a carbon-free society and the continuously increasing demand for clean energy make it valuable to exploit green ammonia (NH3) synthesis that proceeds via the electrolysis driven Haber-Bosch (eHB) process. The key for successful operation is to develop advanced catalysts that can operate under mild conditions with efficacy. The main bottleneck of NH3 synthesis under mild conditions is the known scaling relation in which the feasibility of N2 dissociative adsorption of a catalyst is inversely related to that of the desorption of surface N-containing intermediate species, which leads to the dilemma that NH3 synthesis could not be catalyzed effectively under mild conditions. The present work offers a new strategy via introducing atomically dispersed Ru onto a single Co atom coordinated with pyrrolic N, which forms RuCo dual single-atom active sites. In this system the d-band centers of Ru and Co were both regulated to decouple the scaling relation. Detailed experimental and theoretical investigations demonstrate that the d-bands of Ru and Co both become narrow, and there is a significant overlapping of t2g and eg orbitals as well as the formation of a nearly uniform Co 3d ligand field, making the electronic structure of the Co atom resemble that of a "free-atom". The "free-Co-atom" acts as a bridge to facilitate electron transfer from pyrrolic N to surface Ru single atoms, which enables the Ru atom to donate electrons to the antibonding π* orbitals of N2, thus resulting in promoted N2 adsorption and activation. Meanwhile, H2 adsorbs dissociatively on the Co center to form a hydride, which can transfer to the Ru site to cause the hydrogenation of the activated N2 to generate N2H x (x = 1-4) intermediates. The narrow d-band centers of this RuCo catalyst facilitate desorption of surface *NH3 intermediates even at 50 °C. The cooperativity of the RuCo system decouples the sites for the activation of N2 from those for the desorption of *NH3 and *N2H x intermediates, giving rise to a favorable pathway for efficient NH3 synthesis under mild conditions.

Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti3C2Clx) for Efficient CO2 Electroreduction to Methanol

Single atom catalysts possess attractive electrocatalytic activities for various chemical reactions owing to their favorable geometric and electronic structures compared to the bulk counterparts. Herein, we demonstrate an efficient approach to producing single atom copper immobilized MXene for electrocatalytic CO₂ reduction to methanol via selective etching of hybrid A layers (Al and Cu) in quaternary MAX phases (Ti₃(Al_1-xCu_x)C₂) due to the different saturated vapor pressures of Al- and Cu-containing products. After selective etching of Al in the hybrid A layers, Cu atoms are well-preserved and simultaneously immobilized onto the resultant MXene with dominant surface functional group (Cl_x) on the outmost Ti layers (denoted as Ti₃C₂Cl_x) via Cu-O bonds. Consequently, the as-prepared single atom Cu catalyst exhibits a high Faradaic efficiency value of 59.1% to produce CH₃OH and shows good electrocatalytic stability. On the basis of synchrotron-based X-ray absorption spectroscopy analysis and density functional theory calculations, the single atom Cu with unsaturated electronic structure (Cu^δ+, 0 < δ < 2) delivers a low energy barrier for the rate-determining step (conversion of HCOOH* to absorbed CHO* intermediate), which is responsible for the efficient electrocatalytic CO₂ reduction to CH₃OH.

Self-Activated Catalytic Sites on Nanoporous Dilute Alloy for High-Efficiency Electrochemical Hydrogen Evolution

Design and synthesis of effective electrocatalysts for hydrogen evolution reaction (HER) in wide pH environments are critical to reduce energy losses in water electrolyzers. Here, by using a self-activation strategy, we construct an atomic nickel (Ni) decorated nanoporous iridium (Ir) catalyst, which can create the reaction-favorable chemical environment and maximize the electrochemical active surface area (ECSA), enabling efficient HER over a wide pH range. By using operando X-ray absorption spectroscopy and theoretical calculations, the atomic Ni sites are identified as the synergistic sites, which not only accelerate the water dissociation under operation conditions but also activate the surface Ir sites thus leading to the efficient H₂ generation. This work highlights the significance of atomic-level decorating strategy which can optimize the activity of surface Ir atoms with negligible sacrifice of the ECSA.

Pulsed Laser Confinement of Single Atomic Catalysts on Carbon Nanotube Matrix for Enhanced Oxygen Evolution Reaction

The design of atomically dispersed single atom catalysts (SACs) must consider high metal-atom loading amount, effective confinement, and strong interactions with matrix, which can maximize their catalytic performance. Here reported is a promising method to synthesize SACs on highly conductive multiwall carbon nanotube (MWCNT) supports using pulsed laser confinement (PLC) process in liquid. Atomic cobalt (Co) and phosphorus (P) with a high loading density are homogeneously incorporated on the outer wall of the MWCNT (Co-P SAC MWCNT). Density functional theory (DFT) calculations in combination with systematic control experiments found that the incorporated Co and P adatoms act as an adsorption energy optimizer and a charge transfer promoter, respectively. Hence, favorable kinetics and thermodynamics in Co-P SAC MWCNT can be simultaneously achieved for water oxidation resulting in a superior catalytic performance than the benchmark RuO₂ catalyst. Crucially, total processing time for assembling Co-P SAC MWCNT via PLC process is less than 60 min, shedding light on the promising practical applications of our SAC design strategy.

Copper-based single-atom alloys for heterogeneous catalysis

Heterogeneous catalysts, as crucial industrial commodities, play an important role in industrial production, especially in energy catalysis. Traditional noble metal catalysts cannot meet the increasing demand. Therefore, the exploration of cost-effective catalysts with high activity and selectivity is important to promote chemical production. Single-atom alloy (SAA) catalysts reduce the use of precious metals compared with traditional catalysts. The unique structure of SAAs, extremely high atom utilization and high catalytic selectivity give them a prominent position in heterogeneous catalysis. SAAs are widely used in selective hydrogenation/dehydrogenation, carbon dioxide reduction reaction (CO2RR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitric oxide reduction reaction (NORR). Here, the applications and research progress of copper-based single-atom alloys in the various catalytic reactions mentioned above are mainly introduced, and the factors (such as synthesis method, composition content, etc.) affecting the catalytic performance are analyzed using a combination of various characterization and testing methods.

Atomically dispersed Ru in Pt3Sn intermetallic alloy as an efficient methanol oxidation electrocatalyst

We successfully fabricate a novel concave nanostructure that is composed of atomically dispersed Ru atoms in Pt₃Sn nanoconcaves (Ru-Pt₃Sn NCs), which shows enhanced performance in methanol electroxidation compared to commercial Pt/C. This could be ascribed to the stable intermetallic structure and active surface structure, as well as the synergy among Pt, Sn and Ru.

Preparation of highly dispersed Pt–Sn/Al2O3 catalysts via supercritical fluid deposition and their catalytic performance

Highly dispersed Pt–Sn/Al₂O₃, which shows excellent catalytic performance was produced by SFD and their particle size effect was studied.

Photoexcited Electron Dynamics of Nitrogen Fixation Catalyzed by Ruthenium Single-Atom Catalysts

It is still a grand challenge to exploit efficient catalysts to achieve sustainable photocatalytic N₂ reduction under ambient conditions. Here, we developed a ruthenium-based single-atom catalyst anchored on defect-rich TiO₂ nanotubes (denoted Ru-SAs/Def-TNs) as a model system for N₂ fixation. The constructed Ru-SAs/Def-TNs exhibited a catalytic efficiency of 125.2 μmol g^-1 h^-1, roughly 6 and 13 times higher than those of the supported Ru nanoparticles and Def-TNs, respectively. Through ultrafast transient absorption and photoluminescence spectroscopy, we revealed the relationship between catalytic activity and photoexcited electron dynamics in such a model SA catalytic system. The unique ligand-to-metal charge-transfer state formed in Ru-SAs/Def-TNs was found to be responsible for its high catalytic activity because it can greatly promote the transfer of photoelectrons from Def-TNs to the Ru-SAs center and the subsequent capture by Ru-SAs. This work sheds light on the origin of the high performance of SA catalysts from the perspective of photoexcited electron dynamics and hence enriches the mechanistic understanding of SA catalysis.

Precise fabrication of single-atom alloy co-catalyst with optimal charge state for enhanced photocatalysis

While the surface charge state of co-catalysts plays a critical role for boosting photocatalysis, studies on surface charge regulation via their precise structure control remain extremely rare. Herein, metal-organic framework (MOF) stabilized bimetallic Pd@Pt nanoparticles, which feature adjustable Pt coordination environment and a controlled structure from core-shell to single-atom alloy (SAA), have been fabricated. Significantly, apart from the formation of a Mott-Schottky junction in a conventional way, we elucidate that Pt surface charge regulation can be alternatively achieved by changing its coordination environment and the structure of the Pd@Pt co-catalyst, where the charge between Pd and Pt is redistributed. As a result, the optimized Pd₁₀@Pt₁/MOF composite, which involves an unprecedented SAA co-catalyst, exhibits exceptionally high photocatalytic hydrogen production activity, far surpassing its corresponding counterparts.

In Situ Investigations of Al/Perovskite Interfacial Structures

Interfacial properties of perovskite layers and metal electrodes play a crucial role in device performance and long-term stability of perovskite solar cells. In this work, we performed a comprehensive study of the interfacial structures and ion migration at the interface of a CH₃NH₃PbI₃ perovskite layer and an Al electrode using in situ synchrotron radiation photoemission spectroscopy measurements. It was found that the Al electrode can react with the perovskite layers, leading to the formation of aluminum iodide species and the bonding between Al and N, as well as the reduction of Pb²⁺ ions to metallic Pb species at the interface. Moreover, during the Al deposition, iodide ions can migrate from the CH₃NH₃PbI₃ subsurface to the Al electrode, while the reduced Pb remains at the subsurface. The depth profile photoemission measurements, made by varying the photon energies of incident synchrotron radiation X-rays, demonstrate that the reaction occurs at the Al/CH₃NH₃PbI₃ interface at least with a thickness of ∼3.5 nm below the perovskite surface. This study provides an atomic-level fundamental understanding of the Al/CH₃NH₃PbI₃ interfacial structures and insight into the degradation mechanisms of perovskite solar cells when using Al metal as the electrode.

Chemical Synthesis of Single Atomic Site Catalysts

Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Zero-Dimensional/Two-Dimensional AuxPd100-x Nanocomposites with Enhanced Nanozyme Catalysis for Sensitive Glucose Detection

Here, we report facile fabrication of two-dimensional (2D) Pd nanosheet (NS)-supported zero-dimensional (0D) Au nanoparticles via galvanic replacement. In the synthesis, the surface-clean Pd NSs premade not only acted as a sacrifice template for replacing Pd atoms by Au³⁺ ions, but served as a support substrate to support Au nanoparticles. The morphology, structure, and composition of products relied on the Au/Pd feed atomic ratio. Interestingly, the as-obtained 0D/2D Au_xPd_100-x (x = 4.5, 9.8, and 21) nanocomposites showed remarkably enhanced peroxidase-mimic catalysis in the model oxidation reaction, which followed the typical Michaelis-Menten theory. Compared to Pd NSs, the enhanced catalysis of Au_xPd_100-x was closely related to both the increased specific surface area and the modified electronic structure of Pd NSs, which resulted in a change in the catalytic pathway, that is, from hydroxyl radical generation to rapid electron transfer. The work provides a simple yet efficient avenue to build highly efficient heterogeneous catalysts based on metallic NSs, as exemplified by the superior nanozyme activity of 0D/2D bimetallic nanostructures for glucose detection.

Engineering the Atomic Layer of RuO2 on PdO Nanosheets Boosts Oxygen Evolution Catalysis

We report an atomic-scale controllable synthesis of the face-centered cubic Ru overlayers on Pd nanosheets (Pd@Ru NSs) by a solution-based epitaxial growth method. The thickness of Ru overlayers can be accurately tuned at an atomic level, which has been confirmed by atomic force microscopy and high-angle annular dark-field scanning transmission electron microscopy. After annealing in air, the Pd@Ru NSs were transformed to PdO@RuO₂ NSs with rutile RuO₂ epitaxially grown on the PdO. The oxygen evolution reaction (OER) activity and stability strongly depend on the atomic layers of RuO₂ and ∼4 atomic layers of RuO₂ (PdO@RuO₂-4layers) exhibit superior stability and optimal activity for OER with only 257 mV of the overpotential to reach 10 mA cm^-2. Density functional theory calculations well reproduce the thickness dependence of OER activity and reveal that O* binds more weakly on the PdO@RuO₂-4layers that boosts the rate-determining step for formation of HOO*, assuring the best OER performance.

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

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

Quasi Pd1Ni single-atom surface alloy catalyst enables hydrogenation of nitriles to secondary amines

Hydrogenation of nitriles represents as an atom-economic route to synthesize amines, crucial building blocks in fine chemicals. However, high redox potentials of nitriles render this approach to produce a mixture of amines, imines and low-value hydrogenolysis byproducts in general. Here we show that quasi atomic-dispersion of Pd within the outermost layer of Ni nanoparticles to form a Pd1Ni single-atom surface alloy structure maximizes the Pd utilization and breaks the strong metal-selectivity relations in benzonitrile hydrogenation, by prompting the yield of dibenzylamine drastically from ∼5 to 97% under mild conditions (80 °C; 0.6 MPa), and boosting an activity to about eight and four times higher than Pd and Pt standard catalysts, respectively. More importantly, the undesired carcinogenic toluene by-product is completely prohibited, rendering its practical applications, especially in pharmaceutical industry. Such strategy can be extended to a broad scope of nitriles with high yields of secondary amines under mild conditions.

Atomically Dispersed Pt on Screw-like Pd/Au Core-shell Nanowires for Enhanced Electrocatalysis

Engineering noble metal nanostructures at the atomic level can significantly optimize their electrocatalytic performance and remarkably reduce their usage. We report the synthesis of atomically dispersed Pt on screw-like Pd/Au nanowires by using ultrafine Pd nanowires as seeds. Au can selectively grow on the surface of Pd nanowires by an island growth pattern to fabricate surface defect sites to load atomically dispersed Pt, which can be confirmed by X-ray absorption fine structure measurements and aberration corrected HRTEM images. The nanowires with 2.74 at % Pt exhibit superior HER properties in acidic solution with an overpotential of 20.6 mV at 10 mA cm^-2 and enhanced alkaline ORR performance with a mass activity over 15 times greater than the commercial platinum/carbon (Pt/C) catalysts.

Simple physical preparation of single copper atoms on amorphous carbon via Coulomb explosion

Metal single atom (MSA) materials exhibit excellent properties and are receiving widespread interest for their effectiveness in promoting a variety of catalytic reactions. The current strategies for preparing MSA catalysts involve complicated operation flows and suffer from low loading of the single atoms prepared, owing to the surface defect density of the substrate. In this paper, we report a simple physical method for preparing high-density copper single atom catalysts on amorphous carbon by Coulomb explosion. The results of the in situ observation showed that copper atoms on particle surfaces were emitted under electron beam irradiation and were captured by defects in a surrounding amorphous carbon film as isolated single atoms. By controlling the time and intensity of the Coulomb explosion, the ratio of copper single atoms to clusters aggregated from single atoms on the amorphous carbon can be controlled. Our work will provide new ideas for a universal simple physical preparation of MSA catalysts.