Electrocatalytic CO2 Reduction over Atomically Precise Metal Nanoclusters Protected by Organic Ligands

Recent developments in atomically precise metal nanocluster-based photocatalysts for hydrogen production

Photocatalytic hydrogen production offers a sustainable approach for utilising light energy, providing a promising solution to global energy challenges. The efficiency of this process relies on developing photocatalysts with broad light responsiveness and effective charge carrier separation capabilities. Atomically precise metal nanoclusters (NCs) have emerged as a highly favourable class of materials for this role due to their unique atomic arrangements, ultrasmall size, quantum confinement effects, and plenty of surface-active sites. These exceptional properties endow NCs with semiconductor-like behaviour, allowing for the generation of electrons and holes under light excitation, thus driving the hydrogen production reaction. Moreover, their robust light-absorption properties across the UV to near-IR spectrum, coupled with tuneable optical properties controlled by their composition and structure, promise NCs as next-generation photocatalysts. This review explores recent developments in the application of NCs for photocatalytic hydrogen production, emphasising strategies to enhance charge carrier separation and transfer efficiency, as well as photostability. The discussion also highlights the challenges and future opportunities in using NCs for efficient hydrogen production.

N-Heterocyclic Carbene Polymer-Stabilized Au Nanowires for Selective and Stable Reduction of CO2

The structural stability of nanocatalysts during electrochemical CO2 reduction (CO2RR) is crucial for practical applications. However, highly active nanocatalysts often reconstruct under reductive conditions, requiring stabilization strategies to maintain activity. Here, we demonstrate that the N-heterocyclic carbene (NHC) polymer stabilizes Au nanowire (NW) catalysts for selective CO2 reduction to CO over a broad potential range, enabling coupling with Cu NWs for one-step tandem conversion of CO2 to C2H4. By combining the hydrophobicity of the polystyrene chain and the strong binding of NHC to Au, the polymer stabilizes Au NWs and promotes CO2RR to CO with excellent selectivity (>90%) across -0.4 V to -1.0 V (vs RHE), a significantly broader range than unmodified Au NWs (-0.5 V to -0.7 V). Stable CO2RR at negative potentials yields a high jCO of 142 A/g Au at -1.0 V. In situ ATR-IR analysis indicates that the NHC polymer regulates the water microenvironment and suppresses hydrogen evolution at high overpotential. Moreover, NHC-Au NWs maintain excellent stability during 10 h of CO2RR testing, preserving the NW morphology and catalytic performance, while unmodified NWs degrade into nanoparticles with reduced activity and selectivity. NHC-Au NWs can be coupled with Cu NWs in a flow cell to catalyze CO2RR to C2H4 with 58% efficiency and a partial current density of 70 mA/cm2 (overall C2 product efficiency of 65%). This study presents an adaptable strategy to enhance the catalyst microenvironment, ensure stability, and enable efficient tandem CO2 conversion into value-added hydrocarbons.

Understanding the Position Effects of Monoatom Doping in Silver Nanoclusters on Oxygen Reduction by Single Entity Electrochemistry

Alloying nanoclusters (NCs) with monoatom doping represents an effective strategy to enhance catalytic performances due to the synergistic interactions between the dopant and host atoms. However, in-depth understanding the position effects of monoatom doping within alloying NCs, particularly at the atomic level, remains elusive. Here, we employed single entity collision electrochemistry method to investigate the electrocatalytic behaviors of individual monoatom-doped bimetallic M1Ag24 (M = Ag, Au, Pt, and Cu) NCs toward oxygen reduction reaction (ORR). By relying on high-resolution and high-throughput electrochemical measurements, we successfully discriminated the effects of monoatom variation in M1Ag24 NCs on ORR activity at the single atom resolution and identified different M1Ag24 NCs across characteristic populations. Our experimental findings and theoretical calculations reveal the electrocatalytic reaction dynamics associated with intracluster migration of Au monoatom during the dynamic alloying process of Au1Ag24 NCs. This work demonstrates a novel approach for in situ identifying the position effects of foreign doping atoms on the electrocatalytic activity of alloy NCs at the single atom level.

Tuning Atomically Precise Gold Nanoclusters for Selective Electroreduction of CO2

The electroreduction of greenhouse gas CO2 into high-value-added chemicals using renewable electricity is a promising way to mitigate climate change and realize carbon cycling. Atomically precise thiolate-protected gold nanoclusters have shown great potential for selective electrochemical conversion of CO2 toward CO due to their quantum confinement effect and unique electronic structures. Additionally, the atomic precision of gold nanocluster is advantageous for investigating the CO2 reduction mechanism, which is typically challenging to understand due to the complexity of the catalytic interface, and unknown structure of the active site in more conventional catalysts. By summarizing CO2 reduction catalyzed by gold nanoclusters, we aim to identify key factors that contribute to the activity, selectivity, and stability of nanocluster catalysts, as well as elucidate the CO₂ reduction pathway, thereby contributing to the design of more active and selective nanocluster catalysts for CO2 reduction.

Remove the innermost atom of a magnetic multi-shell gold nanoparticle for near-unity conversion of CO2 to CO

Few reports on paramagnetic metal nanoparticles with atomic precision and their difficult tailoring retard the insightful investigation of metal nanoparticle paramagnetism. Herein, we introduced a thiol-iodine mixture ligand-protecting strategy to successfully synthesize multi-shell paramagnetic [Au127I4(TBBT)48 (I: iodine, TBBT: 4-tert-butylphenylthiolate)]. The innermost Au atom was successfully removed via thiol induction without altering the structure framework to produce diamagnetic Au126I4(TBBT)48 with local ligand arrangement changed (butterfly effect), which could be further transformed into paramagnetic [Au126I4(TBBT)48]+ via hydrogen peroxide oxidation. The spin populations of both paramagnetic nanoparticles are more densely distributed on surface iodine than sulfur. Diamagnetic Au126I4(TBBT)48 exhibited a Faradaic efficiency of ~100% at -0.57 volt during the electrocatalytic reduction of carbon dioxide to carbon monoxide, while paramagnetic Au127I4(TBBT)48 and [Au126I4(TBBT)48]+ exhibited the maximum Faradaic efficiency of 87% at -0.67 volt and 90% at -0.57 volt, respectively, indicating the spin-catalytic activity correlation.

Ligand-Dependent Intracluster Interactions in Electrochemical CO2 Reduction Using Cu14 Nanoclusters

The electrochemical CO2 reduction reaction (CO2RR) has been extensively studied because it can be leveraged to directly convert CO2 into valuable hydrocarbons. Among the various catalysts, copper nanoclusters (Cu NCs) exhibit high selectivity and efficiency for producing CO2RR products owing to their unique geometric/electronic structures. However, the influence of protective ligands on the CO2RR performance of Cu NCs remains unclear. In this study, it is shown that different thiolate ligands, despite having nearly identical geometries, can substantially affect the electrochemical stability of Cu14 NCs in the CO2RR. Notably, Cu14 NCs protected by 2-phenylethanethiolate exhibit greater stability and achieve a relatively higher selectivity (≈40%) for formic acid production compared with the cyclohexanethiolate-protected counterpart. These insights are crucial for designing Cu NCs that are both stable and highly selective, enhancing their efficacy for electrochemical CO2 reduction.

Highly Selective CO2 Conversion to CH4 by a N-Doped HTiNbO5/NH2-UiO-66 Photocatalyst without a Sacrificial Electron Donor

Photocatalytic reduction of CO2 to value-added chemicals is a promising technology for reducing atmospheric CO2, but selectively producing a specific product still remains a great challenge. In this study, a Z-scheme heterojunction, N-doped HTiNbO5/NH2-UiO-66(Zr) (referred to as NH-NU), is developed to integrate the advantages of semiconductor photocatalysts and porous CO2 adsorbents for CO2-to-CH4 conversion. The NH-NU Z-scheme heterojunctions are fabricated via a simple one-pot solvothermal method, enabling the formation of a tight and uniform interface between the two phases, thereby facilitating the separation and transfer of the photoinduced charge carriers, as confirmed by TEM, EPR, electrochemical studies, and work functions. As a result, the as-prepared photocatalyst demonstrates a significant increase in selectivity for CH4 production through CO2 photoreduction, achieving a 10-fold enhancement compared to that of the pristine MOF, NH2-UiO-66. Moreover, there is no obvious decrease in the photocatalytic activity for CH4 production across four consecutive cycles. In situ FT-IR spectroscopy and DFT calculations reveal that charge-enriched N-doped NH-NU-3 composites stabilize various C1 intermediates in multistep elementary reactions, leading to superior selectivity in the CO2-to-CH4 conversion process. This work establishes that efficient and selective heterogeneous catalytic processes can be achieved through the stabilization of reaction intermediates by designing suitable Z-scheme heterojunctions.

Asymmetric Charge Distribution in Atomically Precise Metal Nanoclusters for Boosted CO2 Reduction Catalysis

Recently, atomically precise metal nanoclusters (NCs) have been widely applied in CO2 reduction reaction (CO2RR), achieving exciting activity and selectivity and revealing structure-performance correlation. However, at present, the efficiency of CO2RR is still unsatisfactory and cannot meet the requirements of practical applications. One of the main reasons is the difficulty in CO2 activation due to the chemical inertness of CO2. Constructing symmetry-breaking active sites is regarded as an effective strategy to promote CO2 activation by modulating electronic and geometric structure of CO2 molecule. In addition, in the subsequent CO2RR process, asymmetric charge distributed sites can break the charge balance in adjacent adsorbed C1 intermediates and suppress electrostatic repulsion between dipoles, benefiting for C-C coupling to generate C2+ products. Although compared to single atoms, metal nanoparticles, and inorganic materials the research on the construction of asymmetric catalytic sites in metal NCs is in a newly-developing stage, the precision, adjustability and diversity of metal NCs structure provide many possibilities to build asymmetric sites. This review summarizes several strategies of construction asymmetric charge distribution in metal NCs for boosting CO2RR, concludes the mechanism investigation paradigm of NCs-based catalysts, and proposes the challenges and opportunities of NCs catalysis.

Generalizable Organic-to-Aqueous Phase Transfer of a Au18 Nanocluster with Luminescence Enhancement and Robust Photocatalysis in Water

For the majority of gold nanoclusters (NCs), their water insolubility, low photoluminescence (PL) intensity, and less understood photostability are three critical factors that limit their application in the biomedical and photocatalysis fields. In this study, we report a polymer wrapping method for phase transfer of organic soluble NCs into aqueous phase without degrading the electronic and optical properties, and such materials are further demonstrated for robust photocatalysis in water. We first synthesized a Au18(DMBT)14 NC (DMBT = 2,4-dimethylbenzenethiolate) and found that the aromatic ligands confer a greatly enhanced antioxidation capability of the NC compared to the Au18(CHT)14 counterpart (CHT = cyclohexanethiolate), with the critical role of aromatic ligand interactions identified by X-ray crystallography. The organic soluble Au18(DMBT)14 was successfully transferred into the aqueous phase by an amphiphilic polymer (Pluronic F127, abbrev. F127) wrapping method, producing Au18-D@F127 nanoparticles [each containing a few NCs; Au18-D is an abbreviation for Au18(DMBT)14] with a 10-fold enhancement in PL intensity, and similar results were also obtained for Au18(CHT)14. This method is broadly applicable to various NCs, rendering their water solubility and significantly enhancing the PL intensity of otherwise weakly emissive gold NCs. The exceptional antioxidation stability of Au18(DMBT)14 enables the application of Au18-D@F127 NPs for the photocatalytic activation of persulfate ions and subsequent photodegradation of water pollutants efficiently.

Rational design and synthesis of atomically precise nanocluster-based nanocomposites: a step towards environmental catalysis

Atomically precise metal nanoclusters (NCs) and metal-organic frameworks (MOFs) possess distinct properties that can present challenges in certain applications. However, integrating these materials to create new composite functional materials has gained significant interest due to their unique characteristics through a range of applications, particularly in catalysis. Considering MOFs as hosts and NCs as guests, several synergistic effects have been observed in composites, particularly in environmental catalytic reactions. However, the precise role of encapsulated NCs within the MOF pore structure is still in its infancy. Besides, stabilizing NCs, whether through intact ligands or without ligands via the MOF host, presents challenges that are currently being investigated. This feature article reviews recent advancements in the synthesis of NC@MOF composites, focusing on cutting-edge strategies for selecting MOFs and the roles of NC ligands, as well as characterization and catalytic applications.

Ag nanoparticles induced abundant Cuδ+ sites in Cu2Se nanoflower rods to promote efficient carbon dioxide electroreduction to ethanol

Electrocatalytic carbon dioxide reduction reaction (eCO2RR) is one of the attractive approaches to CO2 utilization. Nevertheless, it remains a challenge to prepare highly efficient and selective electrocatalysts to realize deep conversion of multi-carbon products. The absence of active sites due to the reconfiguration of copper-based catalysts leads to migration deactivation during catalysis, rendering low catalytic efficiency. In this work, Cu2Se nanowires (NWs) were obtained from the Cu foam. After further electroreduction for 90 s, Ag nanoparticles (NPs) were modified on the flower rod-shaped Cu2Se NWs (Ag2.69%/Cu2Se NWs). This interfacial modification strategy, balanced the Cuδ+ active valence state on the surface of the Ag2.69%/Cu2Se NWs, led to a high Faradaic efficiency (FE) of ethanol (∼70 %) at -0.52 V (vs. reversible hydrogen electrode (RHE)). Besides, the Ag2.69%/Cu2Se NWs achieved a high partial current (∼13.9 mA) for ethanol, 18.5 μmol/h ethanol yield and an energy efficiency of 47.1 % in the H-type electrolytic cell. The reaction mechanism was investigated at the molecular level through density functional theory (DFT) calculation. The interface-modified Ag NPs provided stable *COOH intermediates during the reaction process, effectively achieving surface *CO enrichment. Besides, the presence of Ag NPs resulted in the generation of abundant Cuδ+ active species. The *CO migrated to the Cuδ+ active sites to accelerate the asymmetric coupling of *CO and *CHO, which significantly improved the FE of CO2 electroreduction to ethanol. Our work offers a promising approach for designing Cu2Se-based nanowires electrocatalysts for CO2 reduction with excellent activity and selectivity.

The impact of ligand chain length on the HER performance of atomically precise Pt6(SR)12 nanoclusters

Atomically precise metal cluster-based electrocatalysts have been paid significant attention for an efficient hydrogen evolution reaction (HER). Herein, we have synthesized atomically precise Pt6(SR)12 nanoclusters using 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA), 8-mercaptooctanoic acid (MOA), and 11-mercaptoundecanoic acid (MUA) thiol ligands in aqueous media at room temperature to understand the impact of ligand chain length on the HER performance. The composition of Pt6(SR)12 metal clusters was confirmed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Electrochemical studies confirmed that the HER performance of Pt6(SR)12 NCs is notably affected by the ligand chain length, and Pt6(MPA)12 exhibits an overpotential of 19 mV at a current density of 10 mA cm-2, which is several-fold higher than the Pt NCs developed in the recent past. The interfacial charge transfer kinetics and the HER performance decrease with the increase in the chain length of the thiol ligands. Density functional theory calculations showed that the Gibbs free energy for hydrogen adsorption decreases with decreasing chain length of the thiol ligand. The ligands used to synthesize Precise metal clusters for electrocatalysis play an essential role in their efficiency.

Fundamentals and Challenges of Ligand Modification in Heterogeneous Electrocatalysis

The development of efficient catalytic materials in the energy field could promote the structural transformation from traditional fossil fuels to sustainable energy. In heterogeneous catalytic reactions, ligand modification is an effective way to regulate both electronic and steric structures of catalytic sites, thus paving a prospective avenue to design the interfacial structures of heterogeneous catalysts for energy conversion. Although great achievements have been obtained for the study and applications of heterogeneous ligand-modified catalysts, the systematical refinements of ligand modification strategies are still lacking. Here, we reviewed the ligand modification strategy from both the mechanistic and applicable scenarios by focusing on heterogeneous electrocatalysis. We elucidated the ligand-modified catalysts in detail from the perspectives of basic concepts, preparation, regulation of physicochemical properties of catalytic sites, and applications in different electrocatalysis. Notably, we bridged the electrocatalytic performance with the electronic/steric effects induced by ligand modification to gain intrinsic structure-performance relations. We also discussed the challenges and future perspectives of ligand modification strategies in heterogeneous catalysis.

Subtle Tuning of Catalytic Well Effect in Phthalocyanine Covalent Organic Frameworks for Selective CO2 Electroreduction into C2H4

In the electrocatalytic CO2 reduction reaction (CO2RR), the strategic design of a catalytic well capable of regulating the overall confinement effects of catalytic sites holds significant promise for enhancing multiple-electron transfer and C─C coupling efficiency, particularly for the generation of C2+ products. Here, a series of Cu-salphen-based covalent organic frameworks (COFs) featuring hydroxyl-induced catalytic well are synthesized, which demonstrate successful application in electrocatalytic CO2RR to yield multiple-electron transferred products. The meticulously engineered catalytic well, facilitated by multi-hydroxyl groups, manifests robust confinement effects, facilitating selective adsorption, enrichment, and activation of CO2, intermediate stabilization, and reduction of energy barriers for electrocatalytic CO2RR. Specifically, product selectivity can be finely tuned from CH4 to C2H4 by modulating the levels of catalytic well, with CuPc-DFP-4OH-Cu exhibiting the most pronounced catalytic well effect, yielding a high 56.86% faradaic efficiency (FE) for C2H4 at -0.7 V, while CuPc-DFP-Cu, with the weakest catalytic well effect, achieves a 75.24% FE for CH4 at -1.0 V. Notably, the attained FE for C2H4 (56.86%) surpasses that of all reported COFs to date. Complemented by theoretical calculations and in situ tests, this study delves deeply into the pivotal roles of hydroxyl-induced catalytic well with confinement effects.

Ultrasonic treatment-assisted reductive deposition of Cu and Pd nanoparticles on ultrathin 2D Bi2S3 nanosheets for selective electrochemical reduction of CO2 into C2 compounds

In this work, we have ultrasonically deposited Cu and Pd nanoparticles on Bi2S3 nanoparticles, prepared using an ultrasonication assisted hydrothermal method. We implemented intense ultrasonic waves bearing frequency of 20 kHz and power of 750 W at the acoustic wavelength of 100 mm to reduce Cu and Pd nanoparticles on the Bi2S3 surface. The XRD confirmed the formation of highly crystalline Bi2S3 nanoparticles with a pure orthorhombic phase and the deposition of copper (Cuo) and palladium (Pdo) nanoparticles was indicated by the strengthening and broadening of the peaks. XPS also confirmed the formation of Cuo and Pdo nanoparticles on Bi2S3. The Transmission Electron Microscopy (TEM) also exhibited the deposition of Cu and Pd nanoparticles on the Bi2S3 nanosheets which was further confirmed using high resolution TEM analysis. The electrochemical CO2 reduction by Cu-Pd/Bi2S3 electrocatalyst using Cu foam as the conducting support led to the formation of acetaldehyde and ethylene as the major products. The rate of formation of ethylene was found to be 488.5 μ mol g-1h-1 at an applied potential of -0.6 V (vs. RHE), with the best Faradaic efficiency of 57.09 % at -0.4 V (vs. RHE). Among the liquid phase products, acetaldehyde was the major product showing the maximum Faradaic efficiency of 6.473 % at -0.2 V (vs. RHE), with a total formation rate of 64.27 μ mol g-1h-1. The results revealed that the Cu-Pd/Bi2S3 electrocatalyst was more selective to C2 products while the pure Bi2S3 nanoparticles majorly produced C1 compounds.

Structural Evolution of Stapes Controls the Electrochemical CO2 Reduction on Bimetallic Cu-doped Gold Nanoclusters

Ligand protected gold nanoclusters have been proposed for electrochemical CO2 reduction (eCO2R) as an alternative to polycrystalline catalysts, showing higher selectivity control due to the tailored composition and precise microenvironment. Here, two gold cluster families are studied with different staple motifs (Au25(SR)18 and Au144(SR)60, where SR = thiolate) doped with Ag or Cu to understand the interplay between the composition and the performance of these catalysts. Detailed cluster characterization and Density Functional Theory simulations demonstrate that the dynamic aspects involving ligand removal are crucial to unraveling the role of the dopant, the cluster curvature, and the staple structure. The best activity performance toward CO is obtained for Cu-doped Au144(SR)60 at U = -0.8 VRHE as ligands are only partially depleted and the staple can bend to stabilize *CO intermediate, while Cu-containing Au25(SR)18 can produce formate but shows worse CO selectivity. This study points toward the importance of ligand stability during eCO2R on bimetallic gold nanoclusters, paving the way for improving the design and operation of this family of catalysts.

Experimental and Theoretical Insights into Single Atoms, Dual Atoms, and Sub-Nanocluster Catalysts for Electrochemical CO2 Reduction (CO2RR) to High-Value Products

Electrocatalytic carbon dioxide (CO2) conversion into valuable chemicals paves the way for the realization of carbon recycling. Downsizing catalysts to single-atom catalysts (SACs), dual-atom catalysts (DACs), and sub-nanocluster catalysts (SNCCs) has generated highly active and selective CO2 transformation into highly reduced products. This is due to the introduction of numerous active sites, highly unsaturated coordination environments, efficient atom utilization, and confinement effect compared to their nanoparticle counterparts. Herein, recent Cu-based SACs are first reviewed and the newly emerged DACs and SNCCs expanding the catalysis of SACs to electrocatalytic CO2 reduction (CO2RR) to high-value products are discussed. Tandem Cu-based SAC-nanocatalysts (NCs) (SAC-NCs) are also discussed for the CO2RR to high-value products. Then, the non-Cu-based SACs, DACs, SAC-NCs, and SNCCs and theoretical calculations of various transition-metal catalysts for CO2RR to high-value products are summarized. Compared to previous achievements of less-reduced products, this review focuses on the double objective of achieving full CO2 reduction and increasing the selectivity and formation rate toward C-C coupled products with additional emphasis on the stability of the catalysts. Finally, through combined theoretical and experimental research, future outlooks are offered to further develop the CO2RR into high-value products over isolated atoms and sub-nanometal clusters.

N-Heterocyclic carbene-functionalized metal nanoparticles and nanoclusters for nanocatalysis

N-Heterocyclic carbenes (NHCs) have recently emerged as a popular ligand for the functionalization of metal nanoparticles and atomically precise metal clusters. The strong electron-donating properties of NHCs and robust NHC-metal covalent bonding endow metal nanostructures with improved stability and enhanced catalytic performances. In this review, we focus on NHC-coordinated metal nanoparticles and nanoclusters for the electrochemical CO2 reduction reaction (eCO2RR), selective hydrogenation of unsaturated compounds, and asymmetrical catalytic reactions. We discuss the underlying factors that may be at play in determining the improved activity of NHC-functionalized metals and address a few promising perspectives of NHC functionalization for new and better catalytic metal nanostructures.

Chiral Amphiphilic Au23 Cluster and Its Specific Recognition to Remote Di-amines

The atomic precision of metal nanoclusters and variability of surface ligands pave the way for their rational design and functionalization, whereas the property strengthening in multiple ways has been long challenging. Herein, improved amphiphilicity, chirality, thermostability, and strong CPL (circularly polarized luminescence) properties have been accomplished by facile ligand exchange of [Au23(CHT)16]- with HCapt (HCHT and HCapt denote cyclohexanethiol and captopril). In addition, the obtained chiral [Au23(SR)16]- (short for [Au23(CHT)16-x(Capt)x]-) clusters show specific binding affinity to remote-diamines (such as arginine and single/double strand DNA), originating from the hydrogen bonding and Van der Waals interaction among the surface Capt ligands and the di-amine groups.

State of Play of Critical Mineral-Based Catalysts for Electrochemical E-Refinery to Synthetic Fuels

The pursuit of decarbonization involves leveraging waste CO2 for the production of valuable fuels and chemicals (e.g., ethanol, ethylene, and urea) through the electrochemical CO2 reduction reactions (CO2RR). The efficacy of this process heavily depends on electrocatalyst performance, which is generally reliant on high loading of critical minerals. However, the supply of these minerals is susceptible to shortage and disruption, prompting concerns regarding their usage, particularly in electrocatalysis, requiring swift innovations to mitigate the supply risks. The reliance on critical minerals in catalyst fabrication can be reduced by implementing design strategies that improve the available active sites, thereby increasing the mass activity. This review seeks to discuss and analyze potential strategies, challenges, and opportunities for improving catalyst activity in CO2RR with a special attention to addressing the risks associated with critical mineral scarcity. By shedding light onto these aspects of critical mineral-based catalyst systems, this review aims to inspire the development of high-performance catalysts and facilitates the practical application of CO2RR technology, whilst mitigating adverse economic, environmental, and community impacts.

Strong effect-correlated electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) holds great potential to alleviate the greenhouse effect and our dependence on fossil fuels by integrating renewable energy for the electrosynthesis of high-value fuels from CO2. However, the high thermodynamic energy barrier, sluggish reaction kinetics, inadequate CO2 conversion rate, poor selectivity for the target product, and rapid electrocatalyst degradation severely limit its further industrial-scale application. Although numerous strategies have been proposed to enhance ECR performances from various perspectives, scattered studies fail to comprehensively elucidate the underlying effect-performance relationships toward ECR. Thus, this review presents a comparative summary and a deep discussion with respect to the effects strongly-correlated with ECR, including intrinsic effects of materials caused by various sizes, shapes, compositions, defects, interfaces, and ligands; structure-induced effects derived from diverse confinements, strains, and fields; electrolyte effects introduced by different solutes, solvents, cations, and anions; and environment effects induced by distinct ionomers, pressures, temperatures, gas impurities, and flow rates, with an emphasis on elaborating how these effects shape ECR electrocatalytic activities and selectivity and the underlying mechanisms. In addition, the challenges and prospects behind different effects resulting from various factors are suggested to inspire more attention towards high-throughput theoretical calculations and in situ/operando techniques to unlock the essence of enhanced ECR performance and realize its ultimate application.

Structure-Activity Relationships of the Structural Analogs Au8Cu1 and Au8Ag1 in the Electrocatalytic CO2 Reduction Reaction

Owing to the significant attention directed toward alloy metal nanoclusters, it is crucial to explore the relationship between their structures and their performance during the electrocatalytic CO2 reduction reaction (eCO2RR) and discover potential synergistic effects for the design of novel functional nanoclusters. However, a lack of suitable analogs makes this investigation challenging. In this study, we synthesized a well-defined pair of structural analogs, [Au8Cu1(SAdm)4(Dppm)3Cl]2+ and [Au8Ag1(SAdm)4(Dppm)3Cl]2+ (Au8Cu1 and Au8Ag1, respectively), and characterized them. Single-crystal X-ray diffraction analysis revealed that Au8M1 (M=Cu/Ag) consists of a tetrahedral Au3M1 core capped by three (Dppm)Au staples, one Au2(SR)3 staple, one lone SR ligand, and a terminal Cl ligand. Ag and Cu were doped at the same site in the Au8M1 nanoclusters, which has rarely been reported. Au8Cu1 exhibited a significantly higher CO Faradaic efficiency (FECO; ~82.2 %) during eCO2RR than that of Au8Ag1 (FECO; ~33.1 %). Density functional theory calculations demonstrated that *COOH is the key intermediate in the reduction of CO2 to CO. The formation of *COOH on Au8Cu1 is more thermodynamically stable than on Au8Ag1, and Au8Cu1 shows a smaller *CO formation energy than that on Au8Ag1, which promotes the reduction of CO2. We believe that the structural analogs Au8Cu1 and Au8Ag1 offer a suitable template for the in-depth investigation of structure-property correlations at the atomic level.

In Situ Pyrolysis of ZIF-67 to Construct Co2N0.67@ZIF-67 for Photocatalytic CO2 Cycloaddition Reaction

The development of heterogeneous catalysts with abundant active sites is pivotal for enhancing the efficiency of photothermal CO2 conversion. Herein, we report the construction of Co2N0.67@ZIF-67 through the in situ pyrolysis of ZIF-67 under low-temperature pyrolysis conditions. During the pyrolysis process, the crystal structure of ZIF-67 is predominantly preserved concurrently with the formation of Co2N0.67 nanoparticles (NPs) within the ZIF-67 pores. The optimal catalyst Co2N0.67@ZIF-67(450,2) not only possesses high photothermal efficiency but also can efficiently activate CO2. Benefiting from these characteristics, Co2N0.67@ZIF-67(450,2) exhibited significant catalytic activity in the photocatalytic cycloaddition of CO2 and epichlorohydrin. The yield of (chloromethyl)ethylene carbonate reached 95%, which is more than 4 times higher than that of ZIF-67 under visible light irradiation (300 W·m2 Xe lamp, 3 h). This study could offer an alternative approach to enhance the photocatalytic activity of MOFs through low-temperature pyrolysis.

Recent progress in atomically precise metal nanoclusters for photocatalytic application

Photocatalysis is a widely recognized green and sustainable technology that can harness inexhaustible solar energy to carry out chemical reactions, offering the opportunity to mitigate environmental issues and the energy crisis. Photocatalysts with wide spectral response and rapid charge transfer capability are crucial for highly efficient photocatalytic activity. Atomically precise metal nanoclusters (NCs), an emerging atomic-level material, have attracted great interests owing to their ultrasmall size, unique atomic stacking, abundant surface active sites, and quantum confinement effect. In particular, the molecule-like discrete electronic energy level endows them with small-band-gap semiconductor behavior, which allows for photoexcitation in order to generate electrons and holes to participate in the photoredox reaction. In addition, metal NCs exhibit strong light-harvesting ability in the wide spectral UV-near IR region, and the diversity of optical absorption properties can be precisely regulated by the composition and structure. These merits make metal NCs ideal candidates for photocatalysis. In this review, the recent advances in atomically-precise metal NCs for photocatalytic application are summarized, including photocatalytic water splitting, CO2 reduction, organic transformation, photoelectrocatalytic reactions, N2 fixation and H2O2 production. In addition, the strategy for promoting photostability, charge transfer and separation efficiency of metal NCs is highlighted. Finally, a perspective on the challenges and opportunities for NCs-based photocatalysts is provided.

Dithiophosphonate-Protected Eight-Electron Superatomic Ag21 Nanocluster: Synthesis, Isomerism, Luminescence, and Catalytic Activity

The structure-property relationship considering isomerism-tuned photoluminescence and efficient catalytic activity of silver nanoclusters (NCs) is exclusive. Asymmetrical dithiophosphonate NH4[S2P(OR)(p-C6H4OCH3)] ligated first atomically precise silver NCs [Ag21{S2P(OR)(p-C6H4OCH3)}12]PF6 {where, R = nPr (1), Et (2)} were established by single-crystal X-ray diffraction and characterized by electrospray ionization mass spectrometry, NMR (31P, 1H, 2H), X-ray photoelectron spectroscopy, UV-visible, energy-dispersive X-ray spectroscopy, Fourier transforms infrared, thermogravimetric analysis, etc. NCs 1 and 2 consist of eight silver atoms in a cubic framework and enclose an Ag@Ag12-centered icosahedron to constitute an Ag21 core of Th symmetry, which is concentrically inscribed within the S24 snub-cube, P12 cuboctahedron, and the O12 truncated tetrahedron formed by 12 dithiophosphonate ligands. These NCs facilitate to be an eight-electron superatom (1S21P6), in which eight capping Ag atoms exhibit structural isomerism with documented isoelectronic [Ag21{S2P(OiPr)2}12]PF6, 3. In contrast to 3, the stapling of dithiophosphonates in 1 and 2 triggered bluish emission within the 400 to 500 nm region at room temperature. The density functional theory study rationalized isomerization and optical properties of 1, 2, and 3. Both (1, and 2) clusters catalyzed a decarboxylative acylarylation reaction for rapid oxindole synthesis in 99% yield under ambient conditions and proposed a multistep reaction pathway. Ultimately, this study links nanostructures to their physical and catalytic properties.

Constructing an Asymmetric Covalent Triazine Framework to Boost the Efficiency and Selectivity of Visible-Light-Driven CO2 Photoreduction

The photocatalytic reduction of CO2 represents an environmentally friendly and sustainable approach for generating valuable chemicals. In this study, a thiophene-modified highly conjugated asymmetric covalent triazine framework (As-CTF-S) is developed for this purpose. Significantly, single-component intramolecular energy transfer can enhance the photogenerated charge separation, leading to the efficient conversion of CO2 to CO during photocatalysis. As a result, without the need for additional photosensitizers or organic sacrificial agents, As-CTF-S demonstrates the highest photocatalytic ability of 353.2 µmol g-1 and achieves a selectivity of ≈99.95% within a 4 h period under visible light irradiation. This study provides molecular insights into the rational control of charge transfer pathways for high-efficiency CO2 photoreduction using single-component organic semiconductor catalysts.

Strategies for Enhancing the Photocatalytic and Electrocatalytic Efficiency of Covalent Triazine Frameworks for CO2 Reduction

Converting carbon dioxide (CO2) into fuel and high-value-added chemicals is considered a green and effective way to solve global energy and environmental problems. Covalent triazine frameworks (CTFs) are extensively utilized as an emerging catalyst for photo/electrocatalytic CO2 reduction reaction (CO2RR) recently recognized for their distinctive qualities, including excellent thermal and chemical stability, π-conjugated structure, rich nitrogen content, and a strong affinity for CO2, etc. Nevertheless, single-component CTFs have the problems of accelerated recombination of photoexcited electron-hole pairs and restricted conductivity, which limit their application for photo/electrocatalytic CO2RR. Therefore, emphasis will then summarize the strategies for enhancing the photocatalytic and electrocatalytic efficiency of CTFs for CO2RR in this paper, including atom doping, constructing a heterojunction structure, etc. This review first illustrates the synthesis strategies of CTFs and the advantages of CTFs in the field of photo/electrocatalytic CO2RR. Subsequently, the mechanism of CTF-based materials in photo/electrocatalytic CO2RR is described. Lastly, the challenges and future prospects of CTFs in photo/electrocatalytic CO2RR are addressed, which offers a fresh perspective for the future development of CTFs in photo/electrocatalytic CO2RR.

Multiflower-like ReS2/NiAl-LDH Heterojunction for Visible-Light-Driven Photocatalytic CO2 Reduction

The development of high-efficiency heterojunction photocatalysts has been recognized as an effective approach to facilitate photocatalytic CO2 reduction. In this research, we successfully synthesized a novel multiflower-like ReS2/NiAl-LDH heterojunction through a hydrothermal method. Remarkably, when exposed to visible-light irradiation, 2-ReS2/NiAl-LDH demonstrated an exceptional CO production rate of 272.26 μmol·g-1·h-1, which was 4.0 and 10.8 times higher than that of pristine NiAl-LDH and ReS2. The intertwined structure of ReS2 and NiAl-LDH promoted the efficient transfer and separation of photogenerated carriers, thereby significantly enhancing the photocatalytic CO2 reduction capabilities of the ReS2/NiAl-LDH. Furthermore, the carrier transfer pathway for the 2-ReS2/NiAl-LDH heterojunction was elucidated, suggesting a type II scheme mechanism, as evidenced by photochemical deposition experiments. The findings of this study offer valuable insights and pave the way for future research in the design and construction of LDH-based and ReS2-based heterojunctions for efficient photocatalytic CO2 reduction.

Atomically precise Au and Ag nanoclusters doped with a single atom as model alloy catalysts

Gold and silver nanoclusters (NCs) composed of <200 atoms are novel catalysts because their catalytic properties differ significantly from those of the corresponding bulk surface and can be dramatically tuned by the size (number of atoms). Doping with other metals is a promising approach for improving the catalytic performance of Au and Ag NCs. However, elucidation of the origin of the doping effects and optimization of the catalytic performance are hampered by the technical challenge of controlling the number and location of the dopants. In this regard, atomically precise Au or Ag (Au/Ag) NCs protected by ligands or polymers have recently emerged as an ideal platform because they allow regioselective substitution of single Au/Ag constituent atoms while retaining the size and morphology of the NC. Heterogeneous Au/Ag NC catalysts doped with a single atom can also be prepared by controlled calcination of ligand-protected NCs on solid supports. Comparison of thermal catalysis, electrocatalysis, and photocatalysis between the single-atom-doped and undoped Au/Ag NCs has revealed that the single-atom doping effect can be attributed to an electronic or geometric origin, depending on the dopant element and position. This minireview summarizes the recent progress of the synthesis and catalytic application of single-atom-doped, atomically precise Au/Ag NC catalysts and provides future prospects for the rational development of active and selective metal NC catalysts.

Symmetry Breaking Enhancing the Activity of Electrocatalytic CO2 Reduction on an Icosahedron-Kernel Cluster by Cu Atoms Regulation

Recently, CO2 hydrogenation had a new breakthrough resulting from the design of catalysts to effectively activate linear CO2 with symmetry-breaking sites. However, understanding the relationship between symmetry-breaking sites and catalytic activity at the atomic level is still a great challenge. In this study, a set of gold-copper alloy Au13 Cux (x=0-4) nanoclusters were used as research objects to show the symmetry-controlled breaking structure on the surface of nanoclusters with the help of manipulability of the Cu atoms. Among them, Au13 Cu3 nanocluster displays the highest degree of symmetry-breaking on its crystal structure compared with the other nanoclusters in the family. Where the three copper atoms occupying the surface of the icosahedral kernel unevenly with one copper atom is coordinately unsaturated (CuS2 motif relative to CuS3 motif). As expected, Au13 Cu3 has an excellent hydrogenation activity of CO2 , in which the current density is as high as 70 mA cm-2 (-0.97 V) and the maximum FECO reaches 99 % at -0.58 V. Through the combination of crystal structures and theoretical calculations, the excellent catalytic activity of Au13 Cu3 is revealed to be indeed closely related to its asymmetric structure.

Multilevel-Regulated Metal-Organic Framework Platform Integrating Pore Space Partition and Open-Metal Sites for Enhanced CO2 Photoreduction to CO with Nearly 100% Selectivity

Rational design and regulation of atomically precise photocatalysts are essential for constructing efficient photocatalytic systems tunable at both the atomic and molecular levels. Herein, we propose a platform-based strategy capable of integrating both pore space partition (PSP) and open-metal sites (OMSs) as foundational features for constructing high-performance photocatalysts. We demonstrate the first structural prototype obtained from this strategy: pore-partitioned NiTCPE-pstp (TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene, pstp = partitioned stp topology). Nonpartitioned NiTCPE-stp is constructed from six-connected [Ni3(μ3-OH)(COO)6] trimer and TCPE linker to form 1D hexagonal channels with six coplanar OMSs directed at channel centers. After introducing triangular pore-partitioning ligands, half of the OMSs were retained, while the other half were used for PSP, leading to unprecedented microenvironment regulation of the pore structure. The resulting material integrates multiple advanced properties, including robustness, wider absorption range, enhanced electronic conductivity, and high CO2 adsorption, all of which are highly desirable for photocatalytic applications. Remarkably, NiTCPE-pstp exhibits excellent CO2 photoreduction activity with a high CO generation rate of 3353.6 μmol g-1 h-1 and nearly 100% selectivity. Theoretical and experimental studies show that the introduction of partitioning ligands not only optimizes the electronic structure to promote the separation and transfer of photogenerated carriers but also reduces the energy barrier for the formation of *COOH intermediates while promoting CO2 activation and CO desorption. This work is believed to be the first example to integrate PSP strategies and OMSs within metal-organic framework (MOF) photocatalysts, which provides new insight as well as new structural prototype for the design and performance optimization of MOF-based photocatalysts.

Copper Doping Boosts Electrocatalytic CO2 Reduction of Atomically Precise Gold Nanoclusters

Unraveling the atomistic synergistic effects of nanoalloys on the electrocatalytic CO2 reduction reaction (eCO2RR), especially in the presence of copper, is of paramount importance. However, this endeavor encounters significant challenges due to the lack of the crystallographically determined atomic-level structure of appropriate monometallic and bimetallic analogues. Herein, we report a one-pot synthesis and structure characterization of a AuCu nanoalloy cluster catalyst, [Au15Cu4(DPPM)6Cl4(C≡CR)1]2+ (denoted as Au15Cu4). Single-crystal X-ray diffraction analysis reveals that Au15Cu4 comprises two interpenetrating incomplete, centered icosahedra (Au9Cu2 and Au8Cu3) and is protected by six DPPM, four halide, and one alkynyl ligand. The Au15Cu4 cluster and its closest monometal structural analogue, [Au18(DPPM)6Br4]2+ (denoted as Au18), as model systems, enable the elucidation of the atomistic synergistic effects of Au and Cu on eCO2RR. The results reveal that Au15Cu4 is an excellent eCO2RR catalyst in a gas diffusion electrode-based membrane electrode assembly (MEA) cell, exhibiting a high CO Faradaic efficiency (FECO) of >90%, and this efficiency is substantially higher than that of the undoped Au18 (FECO: 60% at -3.75 V). Au15Cu4 exhibits an industrial-level CO partial current density of up to -413 mA/cm2 at -3.75 V with the gas CO2-fed MEA, which is 2-fold higher than that of Au18. The density functional theory (DFT) calculations demonstrate that the synergistic effects are induced by Cu doping, where the exposed pair of AuCu dual sites was suggested for launching the eCO2RR process. Besides, DFT simulations reveal that these special dual sites synergistically coordinate a moderate shift in the d-state, thus enhancing its overall catalytic performance.

Nanocluster Surface Microenvironment Modulates Electrocatalytic CO2 Reduction

The catalytic activity and product selectivity of the electrochemical CO2 reduction reaction (eCO2 RR) depend strongly on the local microenvironment of mass diffusion at the nanostructured catalyst and electrolyte interface. Achieving a molecular-level understanding of the electrocatalytic reaction requires the development of tunable metal-ligand interfacial structures with atomic precision, which is highly challenging. Here, the synthesis and molecular structure of a 25-atom silver nanocluster interfaced with an organic shell comprising 18 thiolate ligands are presented. The locally induced hydrophobicity by bulky alkyl functionality near the surface of the Ag25 cluster dramatically enhances the eCO2 RR activity (CO Faradaic efficiency, FECO : 90.3%) with higher CO partial current density (jCO ) in an H-cell compared to Ag25 cluster (FECO : 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO2 . Remarkably, the hydrophobic Ag25 cluster exhibits jCO as high as -240 mA cm-2 with FECO >90% at -3.4 V cell potential in a gas-fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO2 RR over 120 h. Operando surface-enhanced infrared absorption spectroscopy and theoretical simulations reveal how the ligands alter the neighboring water structure and *CO intermediates, impacting the intrinsic eCO2 RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.

Activity of Different AunSn+1 Staples in the Ligand Exchange of Au23(SR)16- with a Single Foreign Thiolate Ligand

Ligand exchange has been widely used to synthesize novel thiolated gold nanoclusters and to regulate their specific properties. Herein, density functional theory (DFT) calculations were conducted to investigate the kinetic profiles of the ligand exchange of the [Au23(SCy)16]- nanocluster with an aromatic thiolate (2-napthalenethiol). The three types of staple motifs (i.e., trimetallic Au3S4, monometallic AuS2, and the bridging thiolates) of the Au23 cluster precursor could be categorized into eight groups of S sites with different chemical environments. The ligand exchange of all of them occurs favorably via the SN1-like pathway, with one site starting with the Au-S dissociation and seven other sites starting with the H-transfer steps. By contrast, the SN2-like pathway (i.e., the synergistic SCy-to-SAr exchange prior to the H-transfer step) is unlikely in the target systems. Meanwhile, the Au-S bond on the capping Au atom of the bicapped icosahedral Au15 core is the most active one, while the S sites on Au3S4 (except for the one remote from the metallic core) are all competitive exchanging sites. The ligand exchange activity of the bridging thiolate and the remote S site on Au3S4 is significantly less reactive. The calculation results correlate with the multiple ligand exchange within only a few minutes and the preferential etching of the AuS2 staple with the foreign ligands reported in earlier experiments. The relative activity of different staples might be helpful in elucidating the inherent principles in the ligand exchange-induced size-evolution of metal nanoclusters.

Hydride-doped Ag17Cu10 nanoclusters as high-performance electrocatalysts for CO2 reduction

The atomically precise metal electrocatalysts for driving CO2 reduction reactions are eagerly pursued as they are model systems to identify the active sites, understand the reaction mechanism, and further guide the exploration of efficient and practical metal nanocatalysts. Reported herein is a nanocluster-based electrocatalyst for CO2 reduction, which features a clear geometric and electronic structure, and more importantly excellent performance. The nanocatalysts with the molecular formula of [Ag17Cu10(dppm)4(PhC≡C)20H4]3+ have been obtained in a facile way. The unique metal framework of the cluster, with silver, copper, and hydride included, and dedicated surface structure, with strong (dppm) and labile (alkynyl) ligands coordinated, endow the cluster with excellent performance in electrochemical CO2 reduction reaction to CO. With the atomically precise electrocatalysts in hand, not only high reactivity and selectivity (Faradaic efficiency for CO up to 91.6%) but also long-term stability (24 h), are achieved.

Benchmark Models for Elucidating Ligand Effects: Thiols Ligated Isostructural Cu6 Nanoclusters

Atomically precise copper nanoclusters (Cu NCs) have attracted tremendous attention for their huge potential in many applications. However, the uncertainty of the growth mechanism and complexity of the crystallization process hinder the in-depth understanding of their properties. In particular, the ligand effect has been rarely explored at the atomic/molecular level due to the lack of feasible models. Herein, three isostructural Cu6 NCs ligated with diverse mono-thiol ligands (2-mercaptobenzimidazole, 2-mercaptobenzothiazole, and 2-mercaptobenzoxazole, respectively) are successfully synthesized, which provide an ideal platform to unambiguously address the intrinsic role of ligands. The overall atom-by-atom structural evolution process of Cu6 NCs is mapped out with delicate mass spectrometry (MS) for the first time. It is intriguingly found that the ligands, albeit only atomic difference (NH, O, and S), can profoundly affect the building-up processes, chemical properties, atomic structures, as well as catalytic activities of Cu NCs. Furthermore, ion-molecule reactions combined with density functional theory (DFT) calculations demonstrate that the defective sites formed on ligand can significantly contribute to the activation of molecular oxygen. This study provides fundamental insights into the ligand effect, which is vital for the delicate design of high-efficient Cu NCs-based catalysts.

Lattice Compression Revealed at the ≈1 nm Scale

Lattice tuning at the ≈1 nm scale is fascinating and challenging; for instance, lattice compression at such a minuscule scale has not been observed. The lattice compression might also bring about some unusual properties, which waits to be verified. Through ligand induction, we herein achieve the lattice compression in a ≈1 nm gold nanocluster for the first time, as detected by the single-crystal X-ray crystallography. In a freshly synthesized Au52 (CHT)28 (CHT=S-c-C6 H11 ) nanocluster, the lattice distance of the (110) facet is found to be compressed from 4.51 to 3.58 Å at the near end. However, the lattice distances of the (111) and (100) facets show no change in different positions. The lattice-compressed nanocluster exhibits superior electrocatalytic activity for the CO2 reduction reaction (CO2 RR) compared to that exhibited by the same-sized Au52 (TBBT)32 (TBBT=4-tert-butyl-benzenethiolate) nanocluster and larger Au nanocrystals without lattice variation, indicating that lattice tuning is an efficient method for tailoring the properties of metal nanoclusters. Further theoretical calculations explain the high CO2 RR performance of the lattice-compressed Au52 (CHT)28 and provide a correlation between its structure and catalytic activity.

Contemporary Strategies for Immobilizing Metallophthalocyanines for Electrochemical Transformations of Carbon Dioxide

Metallophthalocyanine (PcM) coordination complexes are well-known mediators of the electrochemical reduction of carbon dioxide (CO2). They have many properties that show promise for practical applications in the energy sector. Such properties include synthetic flexibility, a high stability, and good efficiencies for the reduction of CO2 to useful feedstocks, such as carbon monoxide (CO). One of the ongoing challenges that needs to be met is the incorporation of PcM into the heterogeneous materials that are used in a great many CO2-reduction devices. Much progress has been made in the last decade and there are now several promising approaches to incorporate PcM into a range of materials, from simple carbon-adsorbed preparations to extended polymer networks. These approaches all have important advantages and drawbacks. In addition, investigations have led to new proposals regarding CO2 reduction catalytic cycles and other operational features that are crucial to function. Here, we describe developments in the immobilization of PcM CO2 reduction catalysts in the last decade (2013 to 2023) and propose promising avenues and strategies for future research.

Size Effects of Atomically Precise Gold Nanoclusters in Catalysis

The emergence of ligand-protected, atomically precise gold nanoclusters (NCs) in recent years has attracted broad interest in catalysis due to their well-defined atomic structures and intriguing properties. Especially, the precise formulas of NCs provide an opportunity to study the size effects at the atomic level without complications by the polydispersity in conventional nanoparticles that obscures the relationship between the size/structure and properties. Herein, we summarize the catalytic size effects of atomically precise, thioate-protected gold NCs in the range of tens to hundreds of metal atoms. The catalytic reactions include electrochemical catalysis, photocatalysis, and thermocatalysis. With the precise sizes and structures, the fundamentals underlying the size effects are analyzed, such as the surface area, electronic properties, and active sites. In the catalytic reactions, one or more factors may exert catalytic effects simultaneously, hence leading to different catalytic-activity trends with the size change of NCs. The summary of literature work disentangles the underlying fundamental mechanisms and provides insights into the size effects. Future studies will lead to further understanding of the size effects and shed light on the catalytic active sites and ultimately promote catalyst design at the atomic level.

Poly-Hydride [AuI 7 (PPh3 )7 H5 ](SbF6 )2 cluster complex: Structure, Transformation, and Electrocatalytic CO2 Reduction Properties

Hydride Au^I bonds are labile due to the mismatch in electric potential of an oxidizing metal and reducing ligand, and therefore the structure and structure-activity relationships of nanoclusters that contain them are seldom studied. Herein, we report the synthesis and characterization of [Au₇ (PPh₃ )₇ H₅ ](SbF₆ )₂ (abbrev. Au₇ H₅ ²⁺ ), an Au cluster complex containing five hydride ligands, which decomposed to give [Au₈ (PPh₃ )₇ ]²⁺ (abbrev. Au₈ ²⁺ ) upon exposure to light (300 to 450 nm). The valence state of Au^I and H^- was verified by density functional theory (DFT) calculations, NMR, UV/Vis and XPS. The two nanoclusters behaved differently in the electrocatalytic CO₂ reduction reaction (CO₂ RR): Au₇ H₅ ²⁺ exhibited 98.2 % selectivity for H₂ , whereas Au₈ ²⁺ was selective for CO (73.5 %). Further DFT calculations showed that the H^- ligand inhibited the CO₂ RR process compared with the electron-donor H.