Effect of Bystander Hydrogen Atoms on Hydrogen Desorption on Single-Atom Alloy Surfaces: Insights from Simulated Temperature-Programmed Desorption Spectra

Single-atom alloy (SAA) catalysts exhibit unique and excellent catalytic properties in heterogeneous hydrogenation/dehydrogenation reactions. A thorough understanding of the microscopic surface processes is essential to improve the catalytic performance. Here, from a new perspective of the temperature-programmed desorption (TPD) spectra of hydrogen (H) on two common SAA surfaces, Pt@Cu(111) and Pd@Cu(111), we reveal and confirm the key influence of H atoms attached to Pt/Pd dopants, i.e., the H atom bystander, on the desorption process of surface H atoms. It is found that only after considering the effect of the H atom bystander can the simulated TPD spectra well reproduce the experimentally observed higher desorption temperature on Pt@Cu(111) than on Pd@Cu(111) and the leftward shift of the TPD peak with increasing H atom coverage; otherwise, the features are inconsistent with experiments. Our work provides direct evidence for the effect of bystander H atoms from a simulation perspective.

Phase-Transition-Induced Surface Reconstruction of Rh1 Site in Intermetallic Alloy for Propane Dehydrogenation

The fine-tuning of the geometric and electronic structures of active sites plays a crucial role in catalysis. However, the intricate entanglement between the two aspects results in a lack of interpretable design for active sites, posing a challenge in developing high-performance catalysts. Here, we find that surface reconstruction induced by phase transition in intermetallic alloys enables synergistic geometric and electronic structure modulation, creating a desired active site microenvironment for propane dehydrogenation. The resulting electron-rich four-coordinate Rh1 site in the RhGe0.5Ga0.5 intermetallic alloy can accelerate the desorption of propylene and suppress the side reaction and thus exhibits a propylene selectivity of ∼98% with a low deactivation constant of 0.002 h-1 under propane dehydrogenation at 550 °C. Furthermore, we design a computational workflow to validate the rationality of the microenvironment modulation induced by the phase transition in an intermetallic alloy.

Identification of Intersite Distance Effects in Au-Ag Single-Atom Alloy Catalysts Using Single Nanoparticle Collision Electrochemistry

Regulating the atomic density of single-atom alloys (SAAs) promotes the potential to significantly enhance the electrocatalytic activity. However, conventional methods for study on the electrocatalytic performance of SAAs versus the intersite distance demand exhaustive experiments and characterization. Herein, we present a combinatorial synthesis and analysis method to investigate the intersite distance effect of SAA electrocatalysts. We employ single-nanoparticle collision electrochemistry to realize in situ electrodeposition of a precisely tunable Au atomic density onto individual parent Ag nanoparticles, followed by instantaneous electrocatalytic measurement of the newborn Au-Ag SAAs. In this work, the utility of our method is confirmed by the identification of intersite distance effects of Au-Ag SAAs toward the oxygen reduction reaction. When the site distance between two neighboring Au atoms is 1.9 nm, Au-Ag SAAs exhibit optimal activity. This work provides a simple and efficient method for screening other SAA electrocatalysts with ideal intersite distance at the single-nanoparticle level.

Biomimetic Frustrated Lewis Pair Catalysts for Hydrogenation of CO to Methanol at Low Temperatures

The industrial production of methanol through CO hydrogenation using the Cu/ZnO/Al2O3 catalyst requires harsh conditions, and the development of new catalysts with low operating temperatures is highly desirable. In this study, organic biomimetic FLP catalysts with good tolerance to CO poison are theoretically designed. The base-free catalytic reaction contains the 1,1-addition of CO into a formic acid intermediate and the hydrogenation of the formic acid intermediate into methanol. Low-energy spans (25.6, 22.1, and 20.6 kcal/mol) are achieved, indicating that CO can be hydrogenated into methanol at low temperatures. The new extended aromatization-dearomatization effect involving multiple rings is proposed to effectively facilitate the rate-determining CO 1,1-addition step, and a new CO activation model is proposed for organic catalysts.

Single-Atom Alloys Materials for CO2 and CH4 Catalytic Conversion

The catalytic conversion of greenhouse gases CH4 and CO2 constitutes an effective approach for alleviating the greenhouse effect and generating valuable chemical products. However, the intricate molecular characteristics characterized by high symmetry and bond energies, coupled with the complexity of associated reactions, pose challenges for conventional catalysts to attain high activity, product selectivity, and enduring stability. Single-atom alloys (SAAs) materials, distinguished by their tunable composition and unique electronic structures, confer versatile physicochemical properties and modulable functionalities. In recent years, SAAs materials demonstrate pronounced advantages and expansive prospects in catalytic conversion of CH4 and CO2. This review begins by introducing the challenges entailed in catalytic conversion of CH4 and CO2 and the advantages offered by SAAs. Subsequently, the intricacies of synthesis strategies employed for SAAs are presented and characterization techniques and methodologies are introduced. The subsequent section furnishes a meticulous and inclusive overview of research endeavors concerning SAAs in CO2 catalytic conversion, CH4 conversion, and synergy CH4 and CO2 conversion. The particular emphasis is directed toward scrutinizing the intricate mechanisms underlying the influence of SAAs on reaction activity and product selectivity. Finally, insights are presented on the development and future challenges of SAAs in CH4 and CO2 conversion reactions.

Overcoming limitations in propane dehydrogenation by codesigning catalyst-membrane systems

Propylene production through propane dehydrogenation (PDH) is endothermic, and high temperatures required to achieve acceptable propane conversions lead to low selectivity and severe carbon-induced deactivation of conventional catalysts. We developed a catalyst-membrane system that removes the hydrogen by-product and can thus achieve propane conversions that exceed equilibrium limits. In this codesigned system, a silica/alumina (SiO2/Al2O3) hollow-fiber hydrogen membrane was packed with a selective platinum-tin (Pt1Sn1/SiO2) PDH catalyst on the tube side with hydrogen diffusing from the tube to the shell side. We demonstrate that the catalyst-membrane system can achieve propane conversions >140% of the nominal equilibrium conversion with a propylene selectivity >98% without deactivation of the system components. We also show that by introducing oxygen on the shell side of the catalyst-membrane system, we can couple the endothermic PDH reaction on the tube side with exothermic hydrogen oxidation on the shell side. This coupling results in higher rates of hydrogen transport, leading to further enhancements in the propane conversion as well as desired thermoneutral system operation.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Design of MoS2 edge-anchored single-atom catalysts for propane dehydrogenation driven by DFT and microkinetic modeling

The design of efficient catalysts for direct propane dehydrogenation (PDH) to inhibit coke formation and deactivation of traditional Pt-based catalysts are challenging tasks. Herein, by exploiting the unique geometric feature and tunability of single atom catalysts (SACs), a wide range of 3d-5d transition metals anchored on the MoS2 edge in the single atom form (TM1-S4/edge) are comprehensively investigated for the PDH application by first-principles calculations, ab initio molecular dynamics (AIMD) simulations and microkinetic modeling. Five criteria are assessed in terms of the feasibility of preparation, practical stability, feasibility of recovery after air oxidation, activity and selectivity. We identified Ru1-S4/edge SAC as the most promising candidate with activity six times higher than that of the conventional Pt(111) catalyst. Interestingly, AIMD simulations show that the motif region of the highly reactive TM1-S4/edge SACs (such as Ru, Os, Rh, and Ir) exhibits a dynamic change, with a TM-coordinated S atom tending to flutter at reaction temperatures and return to its initial position when the species is adsorbed on TMs, thereby affecting the PDH activities. In addition to identifying the potential PDH catalyst from a practical application point of view, we believe that this study also provides a comprehensive picture for the theoretical screening of low-coordination single-atom catalysts.

Electron Localization in Rationally Designed Pt1Pd Single-Atom Alloy Catalyst Enables High-Performance Li-O2 Batteries

Li-O2 batteries (LOBs) are considered as one of the most promising energy storage devices due to their ultrahigh theoretical energy density, yet they face the critical issues of sluggish cathode redox kinetics during the discharge and charge processes. Here we report a direct synthetic strategy to fabricate a single-atom alloy catalyst in which single-atom Pt is precisely dispersed in ultrathin Pd hexagonal nanoplates (Pt1Pd). The LOB with the Pt1Pd cathode demonstrates an ultralow overpotential of 0.69 V at 0.5 A g-1 and negligible activity loss over 600 h. Density functional theory calculations show that Pt1Pd can promote the activation of the O2/Li2O2 redox couple due to the electron localization caused by the single Pt atom, thereby lowering the energy barriers for the oxygen reduction and oxygen evolution reactions. Our strategy for designing single-atom alloy cathodic catalysts can address the sluggish oxygen redox kinetics in LOBs and other energy storage/conversion devices.

Recent advances in single-atom alloys: preparation methods and applications in heterogeneous catalysis

Single-atom alloys (SAAs) are a different type of alloy where a guest metal, usually a noble metal (e.g., Pt, Pd, and Ru), is atomically dispersed on a relatively more inert (e.g., Ag and Cu) host metal. As a type of atomic-scale catalyst, single-atom alloy catalysts have broad application prospects in the field of heterogeneous catalysis for hydrogenation, dehydrogenation, oxidation, and other reactions. Numerous experimental and characterization results and theoretical calculations have confirmed that the resultant electronic structure caused by charge transfer between the host metal and guest metal and the special geometric structure of the guest metal are responsible for the high selectivity and catalytic activity of SAA catalysts. In this review, the common methods for the preparation of single-atom alloys in recent years are introduced, including initial wet impregnation, physical vapor deposition, and laser ablation in liquid technique. Afterwards, the applications of single-atom alloy catalysts in selective hydrogenation, dehydrogenation, oxidation reactions, and hydrogenolysis reactions are emphatically reviewed. Finally, several challenges for the future development of SAA catalysts are proposed.

Dual-Atom Catalyst with N-Colligated Zn1Co1 Species as Dominant Active Sites for Propane Dehydrogenation

Dual-atom catalysts (DACs) with paired active sites can provide unique intrinsic properties for heterogeneous catalysis, but the synergy of the active centers remains to be elucidated. Here, we develop a high-performance DAC with Zn1Co1 species anchored on nitrogen-doped carbon (Zn1Co1/NC) as the dominant active site for the propane dehydrogenation (PDH) reaction. It exhibits several times higher turnover frequency (TOF) of C3H8 conversion and enhanced C3H6 selectivity compared to Zn1/NC or Co1/NC with only a single-atom site. Various experimental and theoretical studies suggest that the enhanced PDH performance stems from the promoted activation of the C-H bond of C3H8 triggered by the electronic interaction between Zn1 and Co1 colligated by N species. Moreover, the dynamic sinking of the Zn1 site and rising of the Co1 site, together with the steric effect of the dissociated H species at the bridged N during the PDH reaction, provides a feasible channel for C3H6 desorption through the more exposed Co1 site, thereby boosting the selectivity. This work provides a promising strategy for designing robust hetero DACs to simultaneously increase activity and selectivity in the PDH reaction.

Atomic Distance Engineering in Metal Catalysts to Regulate Catalytic Performance

It is very important to understand the structure-performance relationship of metal catalysts by adjusting the microstructure of catalysts at the atomic scale. The atomic distance has an essential influence on the composition of the environment of active metal atom, which is a key factor for the design of targeted catalysts with desired function. In this review, we discuss and summarize strategies for changing the atomic distance from three aspects and relate their effects on the reactivity of catalysts. First, the effects of regulating bond length between metal and coordination atom at one single-atom site on the catalytic performance are introduced. The bond lengths are affected by the strain effect of the support and high-shell doping and can evolve during the reaction. Next, the influence of the distance between single-atom sites on the catalytic performance is discussed. Due to the space matching of adsorption and electron transport, the catalytic performance can be adjusted with the shortening of site distance. In addition, the effect of the arrangement spacing of the surface metal active atoms on the catalytic performance of metal nanocatalysts is studied. Finally, a comprehensive summary and outlook of the relationship between atomic distance and catalytic performance is given.

Elucidating the Reactivity of Oxygenates on Single-Atom Alloy Catalysts

Doping isolated transition metal atoms into the surface of coinage-metal hosts to form single-atom alloys (SAAs) can significantly improve the catalytic activity and selectivity of their monometallic counterparts. These atomically dispersed dopant metals on the SAA surface act as highly active sites for various bond coupling and activation reactions. In this study, we investigate the catalytic properties of SAAs with different bimetallic combinations [Ni-, Pd-, Pt-, and Rh-doped Cu(111), Ag(111), and Au(111)] for chemistries involving oxygenates relevant to biomass reforming. Density functional theory is employed to calculate and compare the formation energies of species such as methoxy (CH3O), methanol (CH3OH), and hydroxymethyl (CH2OH), thereby understanding the stability of these adsorbates on SAAs. Activation energies and reaction energies of C-O coupling, C-H activation, and O-H activation on these oxygenates are then computed. Analysis of the data in terms of thermochemical linear scaling and Bro̷nsted-Evans-Polanyi relationship shows that some SAAs have the potential to combine weak binding with low activation energies, thereby exhibiting enhanced catalytic behavior over their monometallic counterparts for key elementary steps of oxygenate conversion. This work contributes to the discovery and development of SAA catalysts toward greener technologies, having potential applications in the transition from fossil to renewable fuels and chemicals.

Structural Self-Regulation-Promoted NO Electroreduction on Single Atoms

Simultaneously elevating loading and activity of single atoms (SAs) is desirable for SA-containing catalysts, including single-atom catalysts (SACs). However, the fast self-nucleation of SAs limits the loading, and the activity is confined by the adsorption-energy scaling relationships on monotonous SAs. Here, we theoretically design a novel type of SA-containing catalyst generated by two-step structural self-regulation. In the thermodynamic self-regulation step, divacancies in graphene spontaneously pull up SAs from transition metal supports (dv-g/TM; TM = fcc Co, hcp Co, Ni, Cu), leading to the expectably high loading of SAs. The subsequent kinetic self-regulation step involving an adsorbate-assisted and reversible vacancy migration dynamically alters coordination environments of SAs, helping circumvent the scaling relationships, and consequently, the as-designed dv-g/Ni can catalyze NO-to-NH3 conversion at a low limiting potential of -0.25 V vs RHE.

Modeling Single-Atom Catalysis

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited.

Ferric Single-Site Catalyst Confined in a Zeolite Framework for Propane Dehydrogenation

Non-oxidative dehydrogenation of propane is a highly efficient approach for industrial preparation of propene that is commonly catalyzed by noble Pt or toxic Cr catalysts and suffers from coking. In this work, ferric catalyst confined in a zeolite framework was synthesized by a hydrothermal procedure. The isolated Fe in the framework formed distorted tetrahedra, which were beneficial for the selective dehydrogenation of propane and reached over 95 % propene selectivity and over 99 % total olefins selectivity. This catalyst had a silanol-free structure and was oxygen tolerant, hydrothermally stable, and coke free, with a deactivation constant of 0.01 h-1 . This study provided guidance for the synthesis of structural heteroatomic zeolite and efficient propane non-oxidative dehydrogenation over early transition metals.

Mechanism research reveals the role of Fen (n = 2-5) supported C2N as single-cluster catalysts (SCCs) for the non-oxidative propane dehydrogenation in the optimization of catalytic performance

Single cluster catalysts show excellent potential for propane dehydrogenation, compensating for the limited catalytic performance of single-atom catalysts in reactions involving multiple reaction steps and intermediates. Herein, density functional theory is used to investigate the catalytic activity and mechanism for non-oxidized propane dehydrogenation on Fen-C2N (n = 2-5). Firstly, the stability of Fen-C2N (n = 2-5) is evaluated by comparing the mean values of binding energy and cohesive energy. The results show that Fen-C2N (n = 2-4) can exist stably, which is also verified by the molecular dynamics calculation at 873 K. Band structure analysis shows that the screened catalysts have metal properties, which are conducive to charge transfer. Fukui function analysis is used to predict the optimal adsorption site. The electronic properties of propane and propylene adsorbed on catalysts are further studied by the partial density of states and deformation charge density. The activation barrier (Ea) and reaction energy (ΔE) of the main reaction steps are evaluated. The results show that Fe2-C2N (Ea = 0.97 eV, ΔE= 0.22 eV) has the best catalytic activity. The Ea for further propylene dehydrogenation is also used to evaluate the yield of propylene. Compared with Fe-C2N, Fe2-C2N can regulate the adsorption strength of propane and propylene, showing better catalytic ability and higher selectivity for propylene. The above research provides ideas for the design of new catalysts with high selectivity and activity for non-oxidative propane dehydrogenation.

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.

Thermodynamically and Kinetically Stabilized Pt Clusters Against Sintering on CeO2 Nanofibers Through Enclosing CeO2 Nanocubes

Sintering is a major concern for the deactivation of supported metals catalysts, which is driven by the force of decreasing the total surface energy of the entire catalytic system. In this work, a double-confinement strategy is demonstrated to stabilize 2.6 nm-Pt clusters against sintering on electrospun CeO2 nanofibers decorated by CeO2 nanocubes (m-CeO2 ). Thermodynamically, with the aid of CeO2 -nanocubes, the intrinsically irregular surface of polycrystalline CeO2 nanofibers becomes smooth, offering adjacent Pt clusters with decreased chemical potential differences on a relatively uniform surface. Kinetically, the Pt clusters are physically restricted on each facet of CeO2 nanocubes in a nanosized region. In situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation reveals that the Pt clusters can be stabilized up to 800 °C even in a high density, which is far beyond their Tammann temperature, without observable size growth or migration. Such a sinter-resistant catalytic system is endowed with boosted catalytic activity toward both the hydrogenation of p-nitrophenol after being aged at 500 °C and the sinter-promoting exothermic oxidation reactions (e.g., soot oxidation) at high temperatures over 700 °C. This work offers new opportunities for exploring sinter-resistant nanocatalysts, starting from the rational design of whole catalytic system in terms of thermodynamic and kinetic aspects.

Intrinsic and external active sites of single-atom catalysts

Active components with suitable supports are the common paradigm for industrial catalysis, and the catalytic activity usually increases with minimizing the active component size, generating a new frontier in catalysis, single-atom catalysts (SACs). However, further improvement of SACs activity is limited by the relatively low loading of single atoms (SAs, which are heteroatoms for most SACs, i.e., external active sites) because of the highly favorable aggregation of single heteroatoms during preparation. Research interest should be shifted to investigate SACs with intrinsic SAs, which could circumvent the aggregation of external SAs and consequently increase the SAs loading while maintaining them individual to further improve the activity. In this review, SACs with external or intrinsic SAs are discussed and, at last, the perspectives and challenges for obtaining high-loading SACs with intrinsic SAs are outlined.

Designing main-group catalysts for low-temperature methane combustion by ozone

The catalytic combustion of methane at a low temperature is becoming increasingly key to controlling unburned CH₄ emissions from natural gas vehicles and power plants, although the low activity of benchmark platinum-group-metal catalysts hinders its broad application. Based on automated reaction route mapping, we explore main-group elements catalysts containing Si and Al for low-temperature CH₄ combustion with ozone. Computational screening of the active site predicts that strong Brønsted acid sites are promising for methane combustion. We experimentally demonstrate that catalysts containing strong Bronsted acid sites exhibit improved CH₄ conversion at 250 °C, correlating with the theoretical predictions. The main-group catalyst (proton-type beta zeolite) delivered a reaction rate that is 442 times higher than that of a benchmark catalyst (5 wt% Pd-loaded Al₂O₃) at 190 °C and exhibits higher tolerance to steam and SO₂. Our strategy demonstrates the rational design of earth-abundant catalysts based on automated reaction route mapping.

Reactivity of Single-Atom Alloy Nanoparticles: Modeling the Dehydrogenation of Propane

Physical catalysts often have multiple sites where reactions can take place. One prominent example is single-atom alloys, where the reactive dopant atoms can preferentially locate in the bulk or at different sites on the surface of the nanoparticle. However, ab initio modeling of catalysts usually only considers one site of the catalyst, neglecting the effects of multiple sites. Here, nanoparticles of copper doped with single-atom rhodium or palladium are modeled for the dehydrogenation of propane. Single-atom alloy nanoparticles are simulated at 400-600 K, using machine learning potentials trained on density functional theory calculations, and then the occupation of different single-atom active sites is identified using a similarity kernel. Further, the turnover frequency for all possible sites is calculated for propane dehydrogenation to propene through microkinetic modeling using density functional theory calculations. The total turnover frequencies of the whole nanoparticle are then described from both the population and the individual turnover frequency of each site. Under operating conditions, rhodium as a dopant is found to almost exclusively occupy (111) surface sites while palladium as a dopant occupies a greater variety of facets. Undercoordinated dopant surface sites are found to tend to be more reactive for propane dehydrogenation compared to the (111) surface. It is found that considering the dynamics of the single-atom alloy nanoparticle has a profound effect on the calculated catalytic activity of single-atom alloys by several orders of magnitude.

Fe/Cu diatomic catalysts for electrochemical nitrate reduction to ammonia

Electrochemical conversion of nitrate to ammonia offers an efficient approach to reducing nitrate pollutants and a potential technology for low-temperature and low-pressure ammonia synthesis. However, the process is limited by multiple competing reactions and NO₃^- adsorption on cathode surfaces. Here, we report a Fe/Cu diatomic catalyst on holey nitrogen-doped graphene which exhibits high catalytic activities and selectivity for ammonia production. The catalyst enables a maximum ammonia Faradaic efficiency of 92.51% (-0.3 V(RHE)) and a high NH₃ yield rate of 1.08 mmol h^-1 mg^-1 (at - 0.5 V(RHE)). Computational and theoretical analysis reveals that a relatively strong interaction between NO₃^- and Fe/Cu promotes the adsorption and discharge of NO₃^- anions. Nitrogen-oxygen bonds are also shown to be weakened due to the existence of hetero-atomic dual sites which lowers the overall reaction barriers. The dual-site and hetero-atom strategy in this work provides a flexible design for further catalyst development and expands the electrocatalytic techniques for nitrate reduction and ammonia synthesis.

Tracking C-H bond activation for propane dehydrogenation over transition metal catalysts: work function shines

The activation of the C-H bond in heterogeneous catalysis plays a privileged role in converting light alkanes into commodity chemicals with a higher value. In contrast to traditional trial-and-error approaches, developing predictive descriptors via theoretical calculations can accelerate the process of catalyst design. Using density functional theory (DFT) calculations, this work describes tracking C-H bond activation of propane over transition metal catalysts, which is highly dependent on the electronic environment of catalytic sites. Furthermore, we reveal that the occupancy of the antibonding state for metal-adsorbate interaction is the key factor in determining the ability to activate the C-H bond. Among 10 frequently used electronic features, the work function (W) exhibits a strong negative correlation with C-H activation energies. We demonstrate that e^-W can effectively quantify the ability of C-H bond activation, surpassing the predictive capacity of the d-band center. The C-H activation temperatures of the synthesized catalysts also confirm the effectiveness of this descriptor. Apart from propane, e^-W applies to other reactants like methane.

Atomic Aerogel Materials (or Single-Atom Aerogels): An Interesting New Paradigm in Materials Science and Catalysis Science

The concept of "single-atom catalysis" is first proposed by Tao Zhang, Jun Li, and Jingyue Liu in 2011. Single-atom catalysts (SACs) have a very high catalytic activity and greatly improved atom utilization ratio. At present, SACs have become frontier materials in the field of catalysis. Aerogels are highly porous materials with extremely low density and extremely high porosity. These pores play a key role in determining their surface reactivity and mechanical stability. The alliance of SACs and aerogels can fully reflect their structural advantages and lead to new enhancement effects. Herein, a general concept of "atomic aerogel materials" (AAMs) (or single-atom aerogels (SAAs)) is proposed to describe this interesting new paradigm in both material and catalysis fields. Based on the basic units of "gel," the AAMs can be divided into two categories: carrier-level AAMs (with micro-, nano-, or sub-nanometer pore structures) and atomic-level AAMs (with atomic-defective or oxygen-bridged sub-nanopore structures). The basic unit of the former (i.e., single-atom-functionalized aerogels) is the carrier materials in nanostructures, and the latter (i.e., single-atom-built aerogels) is the single metal atoms in atomic structures. The atomic-defective or oxygen-bridged AAMs will be important development directions in versatile heterogeneous catalytic or noncatalytic fields. The design proposals, latent challenges, and coping strategies of this new "atomic nanosystem" in applications are pointed out as well.

Designing single-site alloy catalysts using a degree-of-isolation descriptor

Geometrically isolated metal atoms in alloy catalysts can target efficient and selective catalysis. However, the geometric and electronic disturbance between the active atom and its neighbouring atoms, that is, diverse microenvironments, makes the active site ambiguous. Herein, we demonstrate a methodology to describe the microenvironment and determine the effectiveness of active sites in single-site alloys. A simple descriptor, degree-of-isolation, is proposed, considering both electronic regulation and geometric modulation within a PtM ensemble (M = transition metal). The catalytic performance of PtM single-site alloy is examined thoroughly using this descriptor for an industrially important reaction, propane dehydrogenation. The volcano-shaped isolation-selectivity plot reveals a Sabatier-type principle for designing selective single-site alloys. Specifically, for a single-site alloy with a high degree-of-isolation, alternation of the active centre has a great impact on tuning selectivity, validated by the outstanding consistency between experimental propylene selectivity and the computational descriptor.

Machine Learning Descriptors for Data-Driven Catalysis Study

Traditional trial-and-error experiments and theoretical simulations have difficulty optimizing catalytic processes and developing new, better-performing catalysts. Machine learning (ML) provides a promising approach for accelerating catalysis research due to its powerful learning and predictive abilities. The selection of appropriate input features (descriptors) plays a decisive role in improving the predictive accuracy of ML models and uncovering the key factors that influence catalytic activity and selectivity. This review introduces tactics for the utilization and extraction of catalytic descriptors in ML-assisted experimental and theoretical research. In addition to the effectiveness and advantages of various descriptors, their limitations are also discussed. Highlighted are both 1) newly developed spectral descriptors for catalytic performance prediction and 2) a novel research paradigm combining computational and experimental ML models through suitable intermediate descriptors. Current challenges and future perspectives on the application of descriptors and ML techniques to catalysis are also presented.

Structure-dependence and metal-dependence on atomically dispersed Ir catalysts for efficient n-butane dehydrogenation

Single-site pincer-ligated iridium complexes exhibit the ability for C-H activation in homogeneous catalysis. However, instability and difficulty in catalyst recycling are inherent disadvantages of the homogeneous catalyst, limiting its development. Here, we report an atomically dispersed Ir catalyst as the bridge between homogeneous and heterogeneous catalysis, which displays an outstanding catalytic performance for n-butane dehydrogenation, with a remarkable n-butane reaction rate (8.8 mol·g_Ir^-1·h^-1) and high butene selectivity (95.6%) at low temperature (450 °C). Significantly, we correlate the BDH activity with the Ir species from nanoscale to sub-nanoscale, to reveal the nature of structure-dependence of catalyst. Moreover, we compare Ir single atoms with Pt single atoms and Pd single atoms for in-depth understanding the nature of metal-dependence at the atomic level. From experimental and theoretical calculations results, the isolated Ir site is suitable for both reactant adsorption/activation and product desorption. Its remarkable dehydrogenation capacity and moderate adsorption behavior are the key to the outstanding catalytic activity and selectivity.

Promotion Effect of Well-Defined Deposited Water Layer on Carbon Monoxide Oxidation Catalyzed by Single-Atom Alloys

Single-atom alloys (SAAs) exhibit excellent catalytic performance and unique electronic structures, emerging as promising catalysts for potential industrial reactions. While most of them have been widely employed under reducing conditions, few are applied in oxidation reactions. Herein, using density functional theory calculations and microkinetic simulations, we demonstrate that a well-defined one water layer can improve CO oxidation on model SAAs, with reaction rates increased by orders of magnitude. It is found that the formation of hydrogen bonds and the transfer of charges effectively enhance the adsorption and activation of oxygen molecules at the H₂O/SAA interfaces, which not only increases the surface coverage of O₂ species but also reduces the energy barrier of CO oxidation. The proposed strategy in this work would extend the application range of SAA catalysts to oxidation reactions.

Single-atom alloy Ir/Ni catalyst boosts CO2 methanation via mechanochemistry

A new catalytic approach is pioneered to achieve CO₂ methanation via a single atom alloy Ir/Ni catalyst using a ball-milling method. This Ir/Ni catalyst exhibits a TOF_CH4 of 10244 h^-1 and a 220 h lifetime at 350 °C without deactivation, illustrating excellent catalytic efficiency in the presence of mechanical energy.

Recent advances in the theoretical studies on the electrocatalytic CO2 reduction based on single and double atoms

Excess of carbon dioxide (CO2) in the atmosphere poses a significant threat to the global climate. Therefore, the electrocatalytic carbon dioxide reduction reaction (CO2RR) is important to reduce the burden on the environment and provide possibilities for developing new energy sources. However, highly active and selective catalysts are needed to effectively catalyze product synthesis with high adhesion value. Single-atom catalysts (SACs) and double-atom catalysts (DACs) have attracted much attention in the field of electrocatalysis due to their high activity, strong selectivity, and high atomic utilization. This review summarized the research progress of electrocatalytic CO2RR related to different types of SACs and DACs. The emphasis was laid on the catalytic reaction mechanism of SACs and DACs using the theoretical calculation method. Furthermore, the influences of solvation and electrode potential were studied to simulate the real electrochemical environment to bridge the gap between experiments and computations. Finally, the current challenges and future development prospects were summarized and prospected for CO2RR to lay the foundation for the theoretical research of SACs and DACs in other aspects.

An automated reaction route mapping for the reaction of NO and active species on Ag4 clusters in zeolites

A computational investigation of the catalytic reaction on multinuclear sites is very challenging. Here, using an automated reaction route mapping method, the single-component artificial force induced reaction (SC-AFIR) algorithm, the catalytic reaction of NO and OH/OOH species over the Ag₄²⁺ cluster in a zeolite is investigated. The results of the reaction route mapping for H₂ + O₂ reveal that OH and OOH species are formed over the Ag₄²⁺ cluster via an activation barrier lower than that of OH formation from H₂O dissociation. Then, reaction route mapping is performed to examine the reactivity of the OH and OOH species with NO molecules over the Ag₄²⁺ cluster, resulting in the facile reaction path of HONO formation. With the aid of the automated reaction route mapping, the promotion effect of H₂ addition on the SCR reaction was computationally proposed (boosting the formation of OH and OOH species). In addition, the present study emphasizes that automated reaction route mapping is a powerful tool to elucidate the complicated reaction pathway on multi-nuclear clusters.

Isolated Rh atoms in dehydrogenation catalysis

Isolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga₉Rh₂ phase and the hitherto not-studied Ga₃Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems.

Quantum Alchemy Based Bonding Trends and Their Link to Hammett's Equation and Pauling's Electronegativity Model

We present an intuitive and general analytical approximation estimating the energy of covalent single and double bonds between participating atoms in terms of their respective nuclear charges with just three parameters, [E_AB ≈ a - bZ_AZ_B + c(Z_A^7/3 + Z_B^7/3) ]. The functional form of our expression models an alchemical atomic energy decomposition between participating atoms A and B. After calibration, reasonably accurate bond dissociation energy estimates are obtained for hydrogen-saturated diatomics composed of p-block elements coming from the same row 2 ≤ n ≤ 4 in the periodic table. Corresponding changes in bond dissociation energies due to substitution of atom B by C can be obtained via simple formulas. While being of different functional form and origin, our model is as simple and accurate as Pauling's well-known electronegativity model. Analysis indicates that the model's response in covalent bonding to variation in nuclear charge is near-linear, which is consistent with Hammett's equation.

A Priori Design of Dual-Atom Alloy Sites and Experimental Demonstration of Ethanol Dehydrogenation and Dehydration on PtCrAg

Single-atom catalysts have received significant attention for their ability to enable highly selective reactions. However, many reactions require more than one adjacent site to align reactants or break specific bonds. For example, breaking a C-O or O-H bond may be facilitated by a dual site containing an oxophilic element and a carbophilic or "hydrogenphilic" element that binds each molecular fragment. However, design of stable and well-defined dual-atom sites with desirable reactivity is difficult due to the complexity of multicomponent catalytic surfaces. Here, we describe a new type of dual-atom system, trimetallic dual-atom alloys, which were designed via computation of the alloying energetics. Through a broad computational screening we discovered that Pt-Cr dimers embedded in Ag(111) can be formed by virtue of the negative mixing enthalpy of Pt and Cr in Ag and the favorable interaction between Pt and Cr. These dual-atom alloy sites were then realized experimentally through surface science experiments that enabled the active sites to be imaged and their reactivity related to their atomic-scale structure. Specifically, Pt-Cr sites in Ag(111) can convert ethanol, whereas PtAg and CrAg are unreactive toward ethanol. Calculations show that the oxophilic Cr atom and the hydrogenphilic Pt atom act synergistically to break the O-H bond. Furthermore, ensembles with more than one Cr atom, present at higher dopant loadings, produce ethylene. Our calculations have identified many other thermodynamically favorable dual-atom alloy sites, and hence this work highlights a new class of materials that should offer new and useful chemical reactivity beyond the single-atom paradigm.

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.

Approaching Molecular Definition on Oxide-Supported Single-Atom Catalysts

ConspectusSingle-atom catalysts (SACs) offer unique advantages such as high (noble) metal utilization through maximum possible dispersion, large metal-support contact areas, and oxidation states usually unattainable in classic nanoparticle catalysis. In addition, SACs can serve as models for determining active sites, a simultaneously desired as well as elusive target in the field of heterogeneous catalysis. Due to the complexity of heterogeneous catalysts bearing a variety of different sites on metal particles and the respective support as well as at their interface, studies of intrinsic activities and selectivities remain largely inconclusive. While SACs could close this gap, many supported SACs remain intrinsically ill-defined due to complexities arising from the variety of different adsorption sites for atomically dispersed metals, hampering the establishment of meaningful structure-activity correlations. In addition to overcoming this limitation, well-defined SACs could even be utilized to shed light on fundamental phenomena in catalysis that remain ambiguous when studies are obscured by the complexity of heterogeneous catalysts.In this Account, we describe approaches to break down the complexity of supported single-atom catalysts through the careful choice of oxide supports with specific binding motives as well as the adsorption of well-defined ligands such as ionic liquids on single metal sites. An example of molecularly defined oxide supports is polyoxometalates (POMs), which are metal oxo clusters with precisely known composition and structure. POMs exhibit a limited number of sites to anchor atomically dispersed metals such as Pt, Pd, and Rh. Polyoxometalate-supported single-atom catalysts (POM-SACs) thus represent ideal systems for the in situ spectroscopic study of single atom sites during reactions as, in principle, all sites are identical and thus equally active in catalytic reactions. We have utilized this benefit in studies of the mechanism of CO and alcohol oxidation reactions as well as the hydro(deoxy)genation of various biomass-derived compounds. More so, the redox properties of polyoxometalates can be finely tuned by changing the composition of the support while keeping the geometry of the single-atom active site largely constant. We further developed soluble analogues of heterogeneous POM-SACs, opening the door to advanced liquid-phase nuclear magnetic resonance (NMR) and UV-vis techniques but, in particular, to electrospray ionization mass spectrometry (ESI-MS) which proves powerful in determining catalytic intermediates as well as their gas-phase reactivity. Employing this technique, we were able to resolve some of the long-standing questions about hydrogen spillover, demonstrating the broad utility of studies on defined model catalysts.

High-Throughput Computational Screening of Bioinspired Dual-Atom Alloys for CO2 Activation

CO₂ activation is an integral component of thermocatalytic and electrocatalytic CO₂ conversion to liquid fuels and value-added chemicals. However, the thermodynamic stability of CO₂ and the high kinetic barriers to activating CO₂ are significant bottlenecks. In this work, we propose that dual atom alloys (DAAs), homo- and heterodimer islands in a Cu matrix, can offer stronger covalent CO₂ binding than pristine Cu. The active site is designed to mimic the Ni-Fe anaerobic carbon monoxide dehydrogenase CO₂ activation environment in a heterogeneous catalyst. We find that combinations of early transition metals (TMs) and late TMs embedded in Cu are thermodynamically stable and can offer stronger covalent CO₂ binding than Cu. Additionally, we identify DAAs that have CO binding energies similar to Cu, both to avoid surface poisoning and to ensure attainable CO diffusion to Cu sites so that the C-C bond formation ability of Cu can be retained in conjunction with facile CO₂ activation at the DAA sites. Machine learning feature selection reveals that the more electropositive dopants are primarily responsible for attaining the strong CO₂ binding. We propose seven Cu-based DAAs and two single atom alloys (SAAs) with early TM late TM combinations, (Sc, Ag), (Y, Ag), (Y, Fe), (Y, Ru), (Y, Cd), (Y, Au), (V, Ag), (Sc), and (Y), for facile CO₂ activation.

Interlayer Charge Transfer Regulates Single-Atom Catalytic Activity on Electride/Graphene 2D Heterojunctions

Single-atom catalysts with structure and activity tunability have attracted significant attention for energy and environmental applications. Herein we present a first-principles study of single-atom catalysis on two-dimensional graphene and electride heterostructures. The anion electron gas in the electride layer enables a colossal electron transfer to the graphene layer, with the degree of transfer being controllable by the selection of electride. The charge transfer tunes the d-orbital electron occupancy of a single metal atom, enhancing the catalytic activity of hydrogen evolution reactions and oxygen reduction reactions. The strong correlation between the adsorption energy E_ads and the charge variation Δq suggests that interfacial charge transfer is a critical catalytic descriptor for the heterostructure-based catalysts. The polynomial regression model proves the importance of charge transfer and accurately predicts the adsorption energy of ions and molecules. This study provides a strategy to obtain high-efficiency single-atom catalysts using two-dimensional heterostructures.

Nested Metal Catalysts: Metal Atoms and Clusters Stabilized by Confinement with Accessibility on Supports

Supported catalysts that are important in technology prominently include atomically dispersed metals and metal clusters. When the metals are noble, they are typically unstable-susceptible to sintering-especially under reducing conditions. Embedding the metals in supports such as organic polymers, metal oxides, and zeolites confers stability on the metals but at the cost of catalytic activity associated with the lack of accessibility of metal bonding sites to reactants. An approach to stabilizing noble metal catalysts while maintaining their accessibility involves anchoring them in molecular-scale nests that are in or on supports. The nests include zeolite pore mouths, zeolite surface cups (half-cages), raft-like islands of oxophilic metals bonded to metal oxide supports, clusters of non-noble metals (e.g., hosting noble metals as single-atom alloys), and nanoscale metal oxide islands that selectively bond to the catalytic metals, isolating them from the support. These examples illustrate a trend toward precision in the synthesis of solid catalysts, and the latter two classes of nested catalysts offer realistic prospects for economical large-scale application.

Role of the Support Effects in Single-Atom Catalysts

In recent years, single-atom catalysts (SACs) have received a significant amount of attention due to their high atomic utilization, low cost, high reaction activity, and selectivity for multiple catalytic reactions. Unfortunately, the high surface free energy of single atoms leads them easily migrated and aggregated. Therefore, support materials play an important role in the preparation and catalytic performance of SACs. Aiming at understanding the relationship between support materials and the catalytic performance of SACs, the support effects in SACs are introduced and reviewed herein. Moreover, special emphasis is placed on exploring the influence of the type and structure of supports on SAC catalytic performance through advanced characterization and theoretical research. Future research directions for support materials are also proposed, providing some insight into the design of SACs with high efficiency and high loading.

Synthesis of rhodium intermetallic catalysts by enlarging the inter-particle distance on high-surface-area carbon black supports

Here, we report a "critical distance" method for the synthesis of 9 kinds of sub-5 nm rhodium (Rh)-based intermetallic catalysts. Enlarging the distance between intermetallic particles on high-surface-area carbon black supports could significantly suppress the metal sintering in high-temperature annealing. The prepared Rh₂Sn intermetallic catalysts exhibited enhanced activity in catalyzing the hydrogenation of nitrobenzene.

Demystifying the Chemical Ordering of Multimetallic Nanoparticles

ConspectusMultimetallic nanoparticles (NPs) have highly tunable properties due to the synergy between the different metals and the wide variety of NP structural parameters such as size, shape, composition, and chemical ordering. The major problem with studying multimetallic NPs is that as the number of different metals increases, the number of possible chemical orderings (placements of different metals) for a NP of fixed size explodes. Thus, it becomes infeasible to explore NP energetic differences with highly accurate computational methods, such as density functional theory (DFT), which has a high computational cost and is typically applied to up to a couple of hundred metal atoms. Here, we demonstrate a methodology advancing NP simulations by effectively exploring the vast materials space of multimetallic NPs and accurately identifying the ones with the most thermodynamically preferred chemical orderings. With accuracies reaching that of DFT, our methodology is applicable to practically any NP size, shape, and metal composition. We achieve this by significantly advancing the bond-centric (BC) model, a physics-based model that has been previously shown to rapidly predict bimetallic NP cohesive energies (CEs). Specifically, the BC model is trained in a way to understand how the bimetallic bond strength changes under different coordination environments present on a NP and how the metal composition of every site affects the detailed coordination environment using fractional coordination numbers. This newly modified BC model leads to an improvement from 0.331 (original model) to 0.089 eV/atom in CE predictions when compared to DFT values on a robust data set of 90 different NPs consisting of PtPd, AuPt, and AuPd NPs with varying compositions and chemical orderings. By incorporating the modified BC model into an in-house-developed genetic algorithm (GA) we can effectively and accurately predict the most stable chemical orderings of large, realistic bimetallic NPs consisting of thousands of metal atoms. This is demonstrated on AuPd bimetallic NPs, a challenging system due to the similarity in the cohesion of the two metals. By training our BC model using a unique DFT calculation on a bimetallic NP (one calculation for two metals combining together), we expand to explore the chemical ordering of multimetallic NPs. We first demonstrate the application of our methodology on a AuPdPt NP and validate our stability predictions with literature data. Then, we effectively explore the vast materials space of multimetallic NPs consisting of combinations of Au, Pt, and Pd as a function of metal composition. Our thermodynamic stability trends are presented in a ternary diagram revealing detailed, and yet, unexpected chemical ordering trends. Our computational framework can aid both experimental and computational researchers toward effectively screening multimetallic NP stability. Moreover, we provide an outlook of how this framework can be applied to catalyst discovery, high-entropy alloys, and single-atom alloys.

Auto-accelerated dehydrogenation of alkane assisted by in-situ formed olefins over boron nitride under aerobic conditions

Oxidative dehydrogenation (ODH) of alkane over boron nitride (BN) catalyst exhibits high olefin selectivity as well as a small ecological carbon footprint. Here we report an unusual phenomenon that the in-situ formed olefins under reactions are in turn actively accelerating parent alkane conversion over BN by interacting with hydroperoxyl and alkoxyl radicals and generating reactive species which promote oxidation of alkane and olefin formation, through feeding a mixture of alkane and olefin and DFT calculations. The isotope tracer studies reveal the cleavage of C-C bond in propylene when co-existing with propane, directly evidencing the deep-oxidation of olefins occur in the ODH reaction over BN. Furthermore, enhancing the activation of ethane by the in-situ formed olefins from propane is successfully realized at lower temperature by co-feeding alkane mixture strategy. This work unveils the realistic ODH reaction pathway over BN and provides an insight into efficiently producing olefins.