Direct methane activation by atomically thin platinum nanolayers on two-dimensional metal carbides

Finding Natural, Dense, and Stable Frustrated Lewis Pairs on Wurtzite Crystal Surfaces for Small-Molecule Activation

The surface frustrated Lewis pairs (SFLPs) open up new opportunities for substituting noble metals in the activation and conversion of stable molecules. However, the applications of SFLPs on a larger scale are impeded by the complex construction process, low surface density, and sensitivity to the reaction environment. Herein, wurtzite-structured crystals such as GaN, ZnO, and AlP are found for developing natural, dense, and stable SFLPs. It is revealed that the SFLPs can naturally exist on the (100) and (110) surfaces of wurtzite-structured crystals. All the surface cations and anions serve as the Lewis acid and Lewis base in SFLPs, respectively, contributing to the surface density of SFLPs as high as 7.26×1014 cm-2. Ab initio molecular dynamics simulations indicate that the SFLPs can keep stable under high temperatures and the reaction atmospheres of CO and H2O. Moreover, outstanding performance for activating the given small molecules is achieved on these natural SFLPs, which originates from the optimal orbital overlap between SFLPs and small molecules. Overall, these findings not only provide a simple method to obtain dense and stable SFLPs but also unfold the nature of SFLPs toward the facile activation of small molecules.

Gas-phase synthesis of Ti2CCl2 enables an efficient catalyst for lithium-sulfur batteries

MXenes have aroused intensive enthusiasm because of their exotic properties and promising applications. However, to date, they are usually synthesized by etching technologies. Developing synthetic technologies provides more opportunities for innovation and may extend unexplored applications. Here, we report a bottom-up gas-phase synthesis of Cl-terminated MXene (Ti2CCl2). The gas-phase synthesis endows Ti2CCl2 with unique surface chemistry, high phase purity, and excellent metallic conductivity, which can be used to accelerate polysulfide conversion kinetics and dramatically prolong the cyclability of Li-S batteries. In-depth mechanistic analysis deciphers the origin of the formation of Ti2CCl2 and offers a paradigm for tuning MXene chemical vapor deposition. In brief, the gas-phase synthesis transforms the synthesis of MXenes and unlocks the hardly achieved potentials of MXenes.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

Sulfur-Stabilizing Ultrafine High-Entropy Alloy Nanoparticles on MXene for Highly Efficient Ethanol Electrooxidation

High-entropy alloys (HEAs) are significantly promising candidates for heterogeneous catalysis, yet the controllable synthesis of ultrafine HEA nanoparticles (NPs) remains a formidable challenge due to severe thermal sintering during the high-temperature fabrication process. Herein, we report a sulfur-stabilizing strategy to construct ultrafine HEA NPs with an average diameter of 4.02 nm supported on sulfur-modified Ti3C2Tx (S-Ti3C2Tx) MXene, on which the strong interfacial metal-sulfur interactions between HEA NPs and the S-Ti3C2Tx supports significantly increase the interfacial adhesion strength, thus greatly suppressing nanoparticle sintering by retarding both particle migration and metal atom diffusion. The representative quinary PtPdCuNiCo HEA-S-Ti3C2Tx exhibits excellent catalytic performance toward alkaline ethanol oxidation reaction (EOR) with an ultrahigh mass activity of 7.03 A mgPt+Pd-1, which is 4.34 and 5.17 times higher than those of the commercial Pt/C and Pd/C catalysts, respectively. In situ attenuated total reflection-infrared spectroscopy studies reveal that the high intrinsic catalytic activity for the EOR can be ascribed to the synergy of different catalytically active sites of HEA NPs and the well-designed interfacial metal-sulfur interactions.

Photocatalytic Oxidative Coupling of Methane over Au1 Ag Single-Atom Alloy Modified ZnO with Oxygen and Water Vapor: Synergy of Gold and Silver

C-H dissociation and C-C coupling are two key steps in converting CH4 into multi-carbon compounds. Here we report a synergy of Au and Ag to greatly promote C2 H6 formation over Au1 Ag single-atom alloy nanoparticles (Au1 Ag NPs)-modified ZnO catalyst via photocatalytic oxidative coupling of methane (POCM) with O2 and H2 O. Atomically dispersed Au in Au1 Ag NPs effectively promotes the dissociation of O2 and H2 O into *OOH, promoting C-H activation of CH4 on the photogenerated O- to form *CH3 . Electron-deficient Au single atoms, as hopping ladders, also facilitate the migration of electron donor *CH3 from ZnO to Au1 Ag NPs. Finally, *CH3 coupling can readily occur on Ag atoms of Au1 Ag NPs. An excellent C2 H6 yield of 14.0 mmol g-1  h-1 with a selectivity of 79 % and an apparent quantum yield of 14.6 % at 350 nm is obtained via POCM with O2 and H2 O, which is at least two times that of the photocatalytic system. The bimetallic synergistic strategy offers guidance for future catalyst design for POCM with O2 and H2 O.

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 μmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 μmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

Combining First-Principles Modeling and Symbolic Regression for Designing Efficient Single-Atom Catalysts in the Oxygen Evolution Reaction on Mo2CO2 MXenes

In this study, we address the significant challenge of overcoming limitations in the catalytic efficiency for the oxygen evolution reaction (OER). The current linear scaling relationships hinder the optimization of the electrocatalytic performance. To tackle this issue, we investigate the potential of designing single-atom catalysts (SACs) on Mo2CO2 MXenes for electrochemical OER using first-principles modeling simulations. By employing the Electrochemical Step Symmetry Index (ESSI) method, we assess OER intermediates to fine-tune the activity and identify the optimal SAC for Mo2CO2 MXenes. Our findings reveal that both Ag and Cu exhibit effectiveness as single atoms for enhancing OER activity on Mo2CO2 MXenes. However, among the 21 chosen transition metals (TMs) in this study, Cu stands out as the best catalyst for tweaking the overpotential (ηOER). This is due to Cu's lowest overpotential compared to other TMs, which makes it more favorable for the OER performance. On the other hand, Ag is closely aligned with ESSI = ηOER, making the tuning of its overpotential more challenging. Furthermore, we employ symbolic regression analysis to identify the significant factors that exhibit a correlation with the OER overpotential. By utilizing this approach, we derive mathematical formulas for the overpotential and identify key descriptors that affect the catalytic efficiency in the electrochemical OER on Mo2CO2 MXenes. This comprehensive investigation not only sheds light on the potential of MXenes in advanced electrocatalytic processes but also highlights the prospect of improved activity and selectivity in OER applications.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Photocatalytic Oxidative Coupling of Methane to Ethane Using Water and Oxygen on Ag3PO4-ZnO

Photocatalytic oxidative coupling is an effective way of converting CH4 to high-value-added multi-carbon chemicals under mild conditions, where the breaking of the C-H bond is the main rate-limiting step. In this paper, the Ag3PO4-ZnO heterostructure photocatalyst was synthesized for photocatalytic oxidative coupling of methane (OCM) to C2H6. In addition, an excellent C2H6 yield (16.62 mmol g-1 h-1) and a remarkable apparent quantum yield (15.8% at 350 nm) at 49:1 CH4/Air and 20% RH are obtained, which is more than three times that of the state-of-the-art photocatalytic systems. Ag3PO4 improves the adsorption and dissociation ability of O2 and H2O, benefiting the formation of surface hydroxyl species. As a result, the C-H bond activation energy of CH4 on ZnO was obviously reduced. Meanwhile, the improved separation of photogenerated carriers on the Ag3PO4-ZnO heterostructure also accelerates the OCM process. Moreover, Ag nanoparticles (NPs) derived from Ag3PO4 reduction by photoelectrons promote the coupling of *CH3, which can inhibit the overoxidation of CH4 and increase C2H6 selectivity. This research provides a guide for the design of catalyst and reaction systems in the photocatalytic OCM process.

2D MXenes polar catalysts for multi-renewable energy harvesting applications

The synchronous harvesting and conversion of multiple renewable energy sources for chemical fuel production and environmental remediation in a single system is a holy grail in sustainable energy technologies. However, it is challenging to develop advanced energy harvesters that satisfy different working mechanisms. Here, we theoretically and experimentally disclose the use of MXene materials as versatile catalysts for multi-energy utilization. Ti₃C₂T_X MXene shows remarkable catalytic performance for organic pollutant decomposition and H₂ production. It outperforms most reported catalysts under the stimulation of light, thermal, and mechanical energy. Moreover, the synergistic effects of piezo-thermal and piezo-photothermal catalysis further improve the performance when using Ti₃C₂T_X. A mechanistic study reveals that hydroxyl and superoxide radicals are produced on the Ti₃C₂T_X under diverse energy stimulation. Furthermore, similar multi-functionality is realized in Ti₂CT_X, V₂CT_X, and Nb₂CT_X MXene materials. This work is anticipated to open a new avenue for multisource renewable energy harvesting using MXene materials.

Palladium metallene confined on MXene with increased hydroxyl binding strength for highly efficient ethanol electrooxidation

Rational design and synthesis of high-performance electrocatalysts for ethanol oxidation reaction (EOR) is crucial to large-scale commercialization of direct ethanol fuel cells, but it is still an incredible challenge. Herein, a unique Pd metallene/Ti3C2Tx MXene (Pdene/Ti3C2Tx)-supported electrocatalyst is constructed via an in-situ growth approach for high-efficiency EOR. The resulting Pdene/Ti3C2Tx catalyst achieves an ultrahigh mass activity of 7.47 A mgPd-1 under alkaline condition, as well as high tolerance to CO poisoning. In situ attenuated total reflection-infrared spectroscopy studies combined with density functional theory calculations reveal that the excellent EOR activity of Pdene/Ti3C2Tx catalyst is attributed to the unique and stable interfaces which reduce the reaction energy barrier of *CH3CO intermediate oxidation and facilitate oxidative removal of CO poisonous species by increasing the Pd-OH binding strength.

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B₂O₃ and BN) emerge as promising catalysts for selective oxidation of light alkanes by O₂ to form value-added products, resulting from their unique advantage in suppressing CO₂ formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al₂O₃-supported B₂O₃ catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B₂O₃/Al₂O₃ catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH₃^•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH₃^• and CH₃OO^• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Conjugated dual size effect of core-shell particles synergizes bimetallic catalysis

Core-shell bimetallic nanocatalysts have attracted long-standing attention in heterogeneous catalysis. Tailoring both the core size and shell thickness to the dedicated geometrical and electronic properties for high catalytic reactivity is important but challenging. Here, taking Au@Pd core-shell catalysts as an example, we disclose by theory that a large size of Au core with a two monolayer of Pd shell is vital to eliminate undesired lattice contractions and ligand destabilizations for optimum benzyl alcohol adsorption. A set of Au@Pd/SiO₂ catalysts with various core sizes and shell thicknesses are precisely fabricated. In the benzyl alcohol oxidation reaction, we find that the activity increases monotonically with the core size but varies nonmontonically with the shell thickness, where a record-high activity is achieved on a Au@Pd catalyst with a large core size of 6.8 nm and a shell thickness of ~2-3 monolayers. These findings highlight the conjugated dual particle size effect in bimetallic catalysis.

Nonoxidative Coupling of Methane to Produce C2 Hydrocarbons on FLPs of an Albite Surface

The characteristics of active sites on the surface of albite were theoretically analyzed by density functional theory, and the activation of the C-H bond of methane using an albite catalyst and the reaction mechanism of preparing C₂ hydrocarbons by nonoxidative coupling were studied. There are two frustrated Lewis pairs (FLPs) on the (001) and (010) surfaces of albite; they can dissociate H₂ under mild conditions and show high activity for the activation of methane C-H bonds. CH₄ molecules can undergo direct dissociative adsorption on the (010) surface, whereas a 50.07 kJ/mol activation barrier is needed on the (001) surface. The prepared albite catalyst has a double combination function of the (001) and (010) surfaces; these surfaces produce a spillover phenomenon in the process of CH₄ activation reactions, where CH₃ overflows from the (001) surface with CH₃ adsorbed on the (010) surface to achieve nonoxidative high efficiently C-C coupling with an activation energy of 18.51 kJ/mol. At the same time, this spillover phenomenon inhibits deep dehydrogenation, which is conducive to the selectivity of the C₂ hydrocarbons. The experimental results confirm that the selectivity of the C₂ hydrocarbons is maintained above 99% in the temperature range of 873 K to 1173 K.

Substrate mediated properties of gold monolayers on SiC

In light of their unique physicochemical properties two-dimensional metals are of interest in the development of next-generation sustainable sensing and catalytic applications. Here we showcase results of the investigation of the substrate effect on the formation and the catalytic activity of representative 2D gold layers supported by non-graphenized and graphenized SiC substrates. By performing comprehensive density functional theory (DFT) calculations, we revealed the epitaxial alignment of gold monolayer with the underlying SiC substrate, regardless of the presence of zero-layer graphene or epitaxial graphene. This is explained by a strong binding energy (∼4.7 eV) of 2D Au/SiC and a pronounced charge transfer at the interface, which create preconditions for the penetration of the related electric attraction through graphene layers. We then link the changes in catalytic activity of substrate-supported 2D Au layer in hydrogen evolution reaction to the formation of a charge accumulation region above graphenized layers. Gold intercalation beneath zero-layer graphene followed by its transformation to quasi-free-standing epitaxial graphene is found to be an effective approach to tune the interfacial charge transfer and catalytic activity of 2D Au. The sensing potential of substrate-supported 2D Au was also tested through exploring the adsorption behaviour of NH₃, NO₂ and NO gas molecules. The present results can be helpful for the experimental design of substrate-supported 2D Au layers with targeted catalytic activity and sensing performance.

Progress and Perspectives of Single-Atom Catalysts for Gas Sensing

Single-atom catalysts (SACs) attract extensive attention in the field of heterogeneous catalysis in recent years due to the maximum atom utilization and unique physical and chemical properties. The gas sensing is actually a heterogeneous catalysis process but the SACs are new to this area. Although SACs show huge potential in gas sensing, the SACs gas sensing area currently is still at the infancy stage. This work critically reviews the recent advances and current status of single-atom gas sensing materials. General synthesis routes, characterization methods, and sensing performance indexes are introduced. At the end, the challenges and future prospects on SACs gas sensing are presented from the authors' perspectives. This work is anticipated to provide insights and guideline for the chemical sensing community.

Mo2TiC2 MXene-Supported Ru Clusters for Efficient Photothermal Reverse Water-Gas Shift

Driving metal-cluster-catalyzed high-temperature chemical reactions by sunlight holds promise for the development of negative-carbon-footprint industrial catalysis, which has yet often been hindered by the poor ability of metal clusters to harvest and utilize the full spectrum of solar energy. Here, we report the preparation of Mo2TiC2 MXene-supported Ru clusters (Ru/Mo2TiC2) with pronounced broadband sunlight absorption ability and high sintering resistance. Under illumination of focused sunlight, Ru/Mo2TiC2 can catalyze the reverse water-gas shift (RWGS) reaction to produce carbon monoxide from the greenhouse gas carbon dioxide and renewable hydrogen with enhanced activity, selectivity, and stability compared to their nanoparticle counterparts. Notably, the CO production rate of MXene-supported Ru clusters reached 4.0 mol·gRu-1·h-1, which is among the best reported so far for photothermal RWGS catalysts. Detailed studies suggest that the production of methane is kinetically inhibited by the rapid desorption of CO from the surface of the Ru clusters.

Probing La and Ce Excited-State Reactivity with Resonant Two-Photon Ionization Spectroscopy

Dehydrogenation and C-C bond cleavage of 1-butyne by the excited states of La and Ce atoms are investigated in laser-ablation metal molecular beams. The excited states of the metal atoms are prepared by resonant excitation, detected by resonant two-photon ionization spectroscopy, and the reaction products are monitored by photoionization time-of-flight mass spectrometry. The reactivities of La* [5d²(³F)6p (⁴G_5/2°)] and Ce* [4f5d(³F°)6s6p(³P°) (⁵H₅)] excited states are observed to be higher than those of the initial states of the corresponding metal atoms. The higher reactivities of the excited states are attributed to their higher energies and favorable electron configurations to form two covalent bonds of the metal-insertion intermediates. Although both excited La and Ce atoms show increased reactivities, the enhancement for Ce is much more pronounced than that of La, which cannot be explained by electron configurations alone. The larger reactivity enhancement from the initial states to the excited state of the Ce atom than that of La is due to the longer lifetime of the Ce excited state.

Pt-Co Electrocatalysts: Syntheses, Morphologies, and Applications

Pt-Co electrocatalysts have attracted significant attention because of their excellent performance in many electrochemical reactions. This review focuses on Pt-Co electrocatalysts designed and prepared for electrocatalytic applications. First, the various synthetic methods and synthesis mechanisms are systematically summarized; typical examples and core synthesis parameters are discussed for regulating the morphology and structure. Then, starting with the design and structure-activity relationship of catalysts, the research progress of the morphologies and structures of Pt-Co electrocatalysts obtained based on various strategies, the structure-activity relationship between them, and their properties are summarized. In addition, the important electrocatalytic applications and mechanisms of Pt-Co catalysts, including electrocatalytic oxidation/reduction and bifunctional catalytic reactions, are described and summarized, and their high catalytic activities are discussed on the basis of their mechanism and active sites. Moreover, the advanced electrochemical in situ characterization techniques are summarized, and the challenges and direction concerning the development of high-performance Pt-Co catalysts in electrocatalysis are discussed.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Deciphering the Space Charge Effect of the p-n Junction between Copper Sulfides and Molybdenum Selenides for Efficient Water Electrolysis in a Wide pH Range

Space charge transfer is crucial for an efficient electrocatalytic process, especially for narrow-band-gap metal sulfides/selenides. Herein, we designed and synthesized a core-shell structure which is an ultrathin MoSe₂ nanosheet coated CuS hollow nanoboxes (CuS@MoSe₂) to form an open p-n junction structure. The space charge effect in the p-n junction region will greatly improve electron mass transfer and conduction, and also have abundant active interfaces. It was used as a bifunctional electrocatalyst for water oxidation at a wide pH range. It exhibits a low overpotential of 49 mV for the HER and 236 mV for the OER at a current density of 10 mA·cm^-2 in acidic pH, 72 mV for the HER and 219 mV at 10 mA·cm^-2 for the OER in alkaline pH, and 62 mV for the HER and 230 mV at 10 mA·cm^-2 for the OER under neutral conditions. The experimental results and density functional theory calculations testify that the p-n junction in CuS@MoSe₂ designed and synthesized has a strong space charge region with a synergistic effect. The built-in field can boost the electron transport during the electrocatalytic process and can stabilize the charged active center of the p-n junction. This will be beneficial to improve the electrocatalytic performance. This work provides the understanding of semiconductor heterojunction applications and regulating the electronic structure of active sites.

A general method for rapid synthesis of refractory carbides by low-pressure carbothermal shock reduction

Refractory carbides are attractive candidates for support materials in heterogeneous catalysis because of their high thermal, chemical, and mechanical stability. However, the industrial applications of refractory carbides, especially silicon carbide (SiC), are greatly hampered by their low surface area and harsh synthetic conditions, typically have a very limited surface area (<200 m² g^-1), and are prepared in a high-temperature environment (>1,400 °C) that lasts for several or even tens of hours. Based on Le Chatelier's principle, we theoretically proposed and experimentally verified that a low-pressure carbothermal reduction (CR) strategy was capable of synthesizing high-surface area SiC (569.9 m² g^-1) at a lower temperature and a faster rate (∼1,300 °C, 50 Pa, 30 s). Such high-surface area SiC possesses excellent thermal stability and antioxidant capacity since it maintained stability under a water-saturated airflow at 650 °C for 100 h. Furthermore, we demonstrated the feasibility of our strategy for scale-up production of high-surface area SiC (460.6 m² g^-1), with a yield larger than 12 g in one experiment, by virtue of an industrial viable vacuum sintering furnace. Importantly, our strategy is  also applicable to the rapid synthesis of refractory metal carbides (NbC, Mo₂C, TaC, WC) and even their emerging high-entropy carbides (VNbMoTaWC₅, TiVNbTaWC₅). Therefore, our low-pressure CR method provides an alternative strategy, not merely limited to temperature and time items, to regulate the synthesis and facilitate the upcoming industrial applications of carbide-based advanced functional materials.

Highly Durable Heterogeneous Atomic Catalysts

ConspectusSingle-atom catalysts (SACs), in which surface metal atoms are isolated on the surface of a support, have received a tremendous amount of attention recently because this structure would utilize precious metals fully, without occluding atoms inside nanoparticles, and enable unique surface reactions which typical nanoparticle catalysts cannot induce. Various synthesis methods and characterization techniques have been reported that yield enhanced activity and selectivity. The single-atom structures were realized on various supports such as metal oxide/carbide/nitride, porous materials derived from zeolite or metal-organic frameworks, and carbon-based materials. Additionally, when the metal atoms are isolated on other metal nanoparticles, this material is denoted as a single-atom alloy (SAA). The single-atom structure, however, cannot catalyze the surface reaction that necessitates ensemble sites, where several metal atoms are located nearby. Very recently, ensemble catalysts, in which all of the metal atoms are exposed at the surface with neighboring metal atoms, have been reported, overcoming the limitation of single-atom catalysts. We call all of these materials (SACs, SAAs, and ensemble catalyst) heterogeneous atomic catalysts, indicating that the surface metal atomic structure is intentionally controlled. To use these atomic catalysts for practical applications, high durability should be guaranteed, which has received relatively less attention.In this Account, we discuss recent examples of heterogeneous atomic catalysts with high durability. Structural stability, indicating whether the surface atomic structure is thermodynamically stable, should be carefully considered. Typically, metal atoms are immobilized on a highly defective support, stabilizing both the metal atom and the support. The surface metal atoms might become destabilized upon the adsorption of chemical intermediates. This transient behavior should be carefully monitored; density functional theory (DFT) calculations are particularly useful in estimating this stability. Aside from structural stability, the catalyst performance can be degraded significantly by poisoning with impurities. If the single-atom sites are susceptible to impurities with stronger adsorption, the surface reaction would not occur efficiently, leading to a decrease in activity without structure degradation. A long-term durability test should be performed for target reactions. Heterogeneous atomic catalysts have been used for various electrochemical, photocatalytic, and thermal reactions. Although electricity, light, and heat are just different forms of energy, the specific conditions which the catalyst should satisfy are different. Whereas precious metal atoms are mostly used as surface-active sites, the properties of the support are different depending on the type of reaction. For example, the support should have high conductivity for electrochemical reactions, it should be able to absorb light for photocatalytic reactions, and it should be durable at high temperature in the presence of steam for thermal reactions. Highly durable heterogeneous atomic catalysts are certainly possible with a great potential for practical applications. These new catalysts can accelerate the current paradigm shift toward more sustainable chemical production.

Roles of Metal Ions in MXene Synthesis, Processing and Applications: A Perspective

With a decade of effort, significant progress has been achieved in the synthesis, processing, and applications of MXenes. Metal ions play many crucial roles, such as in MXene delamination, structure regulation, surface modification, MXene composite construction, and even some unique applications. The different roles of metal ions are attributed to their many interactions with MXenes and the unique nature of MXenes, including their layered structure, surface chemistry, and the existence of multi-valent transition metals. Interactions with metal ions are crucial for the energy storage of MXene electrodes, especially in metal ion batteries and supercapacitors with neutral electrolytes. This review aims to provide a good understanding of the interactions between metal ions and MXenes, including the classification and fundamental chemistry of their interactions, in order to achieve their more effective utilization and rational design. It also provides new perspectives on MXene evolution and exfoliation, which may suggest optimized synthesis strategies. In this respect, the different effects of metal ions on MXene synthesis and processing are clarified, and the corresponding mechanisms are elaborated. Research progress on the roles metal ions have in MXene applications is also introduced.