Effects of ligand tuning and core doping of atomically precise copper nanoclusters on CO(2) electroreduction selectivity

Recent progress in the electrocatalytic applications of thiolate-protected metal nanoclusters

Ultrasmall metal nanoclusters (NCs) with atomic precision possess a size range between individual atoms and plasmonic nanomaterials. These atomically precise materials represent an emerging class of nanocatalysts, offering unique opportunities to explore electrocatalytic properties and establish precise structure-property correlations at the atomic scale. Among the large number of metal NCs that are stabilized by various ligands, thiolate-protected metal NCs are a particularly prominent class for electrocatalytic investigations. Recent experimental and theoretical studies have demonstrated the significant potential of these materials in enhancing various electrocatalytic reactions, including hydrogen evolution, oxygen reduction and CO2 reduction reactions. However, comprehensive and in-depth discussions regarding their catalytic properties, particularly from a theoretical standpoint, are limited and require further explorations. In this review, we focus on the recent progress in thiolate-protected metal NCs in the field of electrocatalysis. The influences of structure, ligand, doping and interface control on their electrocatalytic activity/selectivity and the reaction mechanisms are discussed. Importantly, the perspectives we propose regarding future research endeavors are expected to offer valuable references for subsequent investigations in this area.

Atomic-Level Elucidation of Lattice-Hydrogens in Copper Catalysts for Selective CO2 Electrochemical Conversion toward C2 Products

Copper is the most efficient and practical electrocatalyst for the electrochemical reduction of carbon dioxide (ECR) to give multicarbon (C2+) products, but the mechanism by which such products are formed - though known to involve lattice-hydrogens - remains elusive, and the selectivity of the reaction is poor. Herein, we report the synthesis of [AuCu24(dppp)6H22]+, a copper hydride nanocluster bearing exposed Cu3H3 units in specific surface cavities, and our use of it to study the mechansim and selectivity of the reduction of CO2 to C2+ products. Results of in situ infrared spectroscopy and theoretical calculations showed that these Cu3H3 units can effectively lower the energy barrier to the formation of the *COCOH intermediate, which allowed the competition between the C1 and C2 pathways to be elucidated. Isotope labeling experiments and catalyst recrystallization studies corroborated the theoretical simulations, identifying the lattice-hydrogen (H-) in the Cu3H3 active unit as being indispensable for the formation of C2H4. The molecular design guidelines which this work has facilitated constitute a new approach towards the of copper-based catalysts that convert CO2 to C2+ products based on lattice-hydrogen engineering.

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.

Machine Learning Potential for Copper Hydride Clusters: A Neutron Diffraction-Independent Approach for Locating Hydrogen Positions

Determining hydrogen positions in metal hydride clusters remains a formidable challenge, which relies heavily on unaffordable neutron diffraction. While machine learning has shown promise, only one deep learning-based method has been proposed so far, which relies heavily on neutron diffraction data for training, limiting its general applicability. In this work, we present an innovative strategy─SSW-NN (stochastic surface walking with neural network)─a robust, non-neutron diffraction-dependent technique that accurately predicts hydrogen positions. Validated against neutron diffraction data for copper hydride clusters, SSW-NN proved effective for clusters where only X-ray diffraction data or DFT predictions are available. It offers superior accuracy, efficiency, and versatility across different metal hydrides, including silver and alloy hydride systems, currently without any neutron diffraction references. This approach not only establishes a new research paradigm for metal hydride clusters but also provides a universal solution for hydrogen localization in other research fields constrained by neutron sources.

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.

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Structural Changes in Metal Chalcogenide Nanoclusters Associated with Single Heteroatom Incorporation

Atomically precise nanoclusters (NCs) are promising building blocks for designing materials and interfaces with unique properties. By incorporating heteroatoms into the core, the electronic and magnetic properties of NCs can be precisely tuned. To accurately predict these properties, density functional theory (DFT) is often employed, making the rigorous benchmarking of DFT results particularly important. In this study, we present a benchmarking approach based on metal chalcogenide NCs as a model system. We synthesized a series of bimetallic, iron-cobalt chalcogenide NCs [Co6-xFexS8(PEt3)6]+ (x = 0-6) (PEt = triethyl phosphine) and investigated the effect of heteroatoms in the octahedral metal chalcogenide core on their size and electronic properties. Using ion mobility-mass spectrometry (IM-MS), we observed a gradual increase in the collision cross section (CCS) with an increase in the number of Fe atoms in the core. DFT calculations combined with trajectory method CCS simulations successfully reproduced this trend, revealing that the increase in cluster size is primarily due to changes in metal-ligand bond lengths, while the electronic properties of the core remain largely unchanged. Moreover, this method allowed us to exclude certain multiplicity states of the NCs, as their CCS values were significantly different from those predicted for the lowest-energy structures. This study demonstrates that gas-phase IM-MS is a powerful technique for detecting subtle size differences in atomically precise NCs, which are often challenging to observe using conventional NC characterization methods. Accurate CCS measurements are established as a benchmark for comparison with theoretical calculations. The excellent correspondence between experimental data and theoretical predictions establishes a robust foundation for investigating structural changes of transition metal NCs of interest to a broad range of applications.

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.

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.

Recent Progress in Atomically Precise Cu-M Alloy Nanoclusters

Metal nanoclusters (NCs) with dimensions of approximately 3 nm serve as a crucial link between metal-organic complexes and metal nanoparticles, garnering significant interest due to their distinctive molecule-like characteristics. These include well-defined molecular structures, clear HOMO-LUMO transitions, quantized charge, and robust luminescence emission. Atomically precise alloy NCs, in contrast to homometallic NCs, exhibit a wealth of structures and intriguing properties, with their novel attributes often intricately tied to the positions of alloyed elements within the structure, facilitating the exploration of structure-property relationships. A notable subgroup within this category comprises Cu-M (where M represents metals such as Au, Ag, Rh, Ir, Pd, Pt, Zn, Al etc.) alloy NCs. In this review, we initially outline recent advancements in the development of efficient synthetic techniques for Cu-M alloy NCs, emphasizing the underlying physical and chemical properties that enable precise control over their sizes and surface characteristics. Subsequently, we delve into recent progress in structural elucidation techniques for Cu-M alloy NCs. This structural insight is instrumental in comprehensively understanding the structure-property correlations at the molecular level. Finally, we showcase various examples of Cu-M alloy NCs to illustrate their photoluminescent and catalytic properties, shedding light on their diverse functionalities and potential applications.

Promoting CO2 Electroreduction to Hydrocarbon Products via Sulfur-Enhanced Proton Feeding in Atomically Precise Thiolate-Protected Cu Clusters

Thiolate-protected Cu clusters with well-defined structures and stable low-coordinated Cu+ species exhibit remarkable potential for the CO2RR and are ideal model catalysts for establishing structure-electrocatalytic property relationships at the atomic level. However, extant Cu clusters employed in the CO2RR predominantly yield 2e- products. Herein, two model Cu4(MMI)4 and Cu8(MMI)4(tBuS)4 clusters (MMI=2-mercapto-1-methylimidazole) are prepared to investigate the synergistic effect of Cu+ and adjacent S sites on the CO2RR. Cu4(MMI)4 can reduce CO2 to deep-reduced products with a 91.0 % Faradaic efficiency (including 53.7 % for CH4) while maintaining remarkable stability. Conversely, Cu8(MMI)4(tBuS)4 shows a remarkable preference for C2+ products, achieving a maximum FE of 58.5 % with a C2+ current density of 152.1 mA⋅cm-2. In situ XAS and ex situ XPS spectra reveal the preservation of Cu+ species in Cu clusters during CO2RR, extensively enhancing the adsorption capacity of *CO intermediate. Moreover, kinetic analysis and theoretical calculations confirm that S sites facilitate H2O dissociation into *H species, which directly participate in the protonation process on adjacent Cu sites for the protonation of *CO to *CHO. This study highlights the important role of Cu-S dual sites in Cu clusters and provides mechanistic insights into the CO2RR pathway at the atomic level.

Single-atom tailored atomically-precise nanoclusters for enhanced electrochemical reduction of CO2-to-CO activity

The development of facile tailoring approach to adjust the intrinsic activity and stability of atomically-precise metal nanoclusters catalysts is of great interest but remians challenging. Herein, the well-defined Au8 nanoclusters modified by single-atom sites are rationally synthesized via a co-eletropolymerization strategy, in which uniformly dispersed metal nanocluster and single-atom co-entrenched on the poly-carbazole matrix. Systematic characterization and theoretical modeling reveal that functionalizing single-atoms enable altering the electronic structures of Au8 clusters, which amplifies their electrocatalytic reduction of CO2 to CO activity by ~18.07 fold compared to isolated Au8 metal clusters. The rearrangements of the electronic structure not only strengthen the adsorption of the key intermediates *COOH, but also establish a favorable reaction pathway for the CO2 reduction reaction. Moreover, this strategy fixing nanoclusters and single-atoms on cross-linked polymer networks efficiently deduce the performance deactivation caused by agglomeration during the catalytic process. This work contribute to explore the intrinsic activity and stability improvement of metal clusters.

Macrocyclic Sulfur Ligand Stabilized Trans-Palladium Dichloride Complex: Syntheses, Structure, Chlorine Rotation, and Application in α-Olefination of Nitriles by Primary Alcohols

Herein, we have reported the synthesis of a macrocyclic organosulfur ligand (L1) having a seventeen-membered macrocyclic ring. Subsequently, the corresponding trans-palladium complex (C1) of bulky macrocyclic organosulfur ligand (L1) was synthesized by reacting it with PdCl2 (CH3 CN)2 salt. The newly synthesized ligand and complex were characterized using various analytical and spectroscopic techniques. The complex showed a square planar geometry with trans orientation of two ligands around the palladium center. The complex possesses intramolecular SCH…Cl interactions of 2.648 Å between the macrocyclic ligand and palladium dichloride. The potential energy surface (PES) for the rotational process of C1 suggested a barrier of ~23.81 kcal/mol for chlorine rotation. Furthermore, the bulky macrocyclic organosulfur ligand stabilized palladium complex (C1) was used as a catalyst (2.5 mol %) for α-olefination of nitriles by primary alcohols. The α,β-unsaturated nitrile compounds were found to be the major product of the reaction (57-78 % yield) with broad substrate scope and large functional group tolerance. Notably, the saturated nitrile product was not observed during the reaction. The mechanistic studies suggested the formation of H2 and H2 O as only by-products of the reaction, thereby making the protocol greener and sustainable.

Unveiling the Remarkable Stability and Catalytic Activity of a 6-Electron Superatomic Ag30 Nanocluster for CO2 Electroreduction

Nanocluster catalysts face a significant challenge in striking the right balance between stability and catalytic activity. Here, we present a thiacalix[4]arene-protected 6-electron [Ag30(TC4A)4(iPrS)8] nanocluster that demonstrates both high stability and catalytic activity. The Ag30 nanocluster features a metallic core, Ag104+, consisting of two Ag3 triangles and one Ag4 square, shielded by four {Ag5@(TC4A)4} staple motifs. Based on DFT calculations, the Ag104+ metallic kernel can be viewed as a trimer comprising 2-electron superatomic units, exhibiting a valence electron structure similar to that of the Be3 molecule. Notably, this is the first crystallographic evidence of the trimerization of 2-electron superatomic units. Ag30 can reduce CO2 into CO with a Faraday efficiency of 93.4% at -0.9 V versus RHE along with excellent long-term stability. Its catalytic activity is far superior to that of the chain-like AgI polymer ∞1{[H2Ag5(TC4A)(iPrS)3]} (∞1Agn), with the composition similar to Ag30. DFT calculations elucidated the catalytic mechanism to clarify the contrasting catalytic performances of the Ag30 and ∞1Agn polymers and disclosed that the intrinsically higher activity of Ag30 may be due to the greater stability of the dual adsorption mode of the *COOH intermediate on the metallic core.

The smallest superatom Au4(PPh3)4I2 with two free electrons: synthesis, structure analysis, and electrocatalytic conversion of CO2 to CO

Atomically precise metal nanoclusters (NCs) have emerged as a new class of ultrasmall nanoparticles with both free valence electrons and precise structures (from the metal core to the organic ligand shell) and provide great opportunities to understand the relationship between their structures and properties, such as electrocatalytic CO2 reduction reaction (eCO2RR) performance, at the atomic level. Herein, we report the synthesis and the overall structure of the phosphine and iodine co-protected Au4(PPh3)4I2 (Au4) NC, which is the smallest multinuclear Au superatom with two free e- reported so far. Single-crystal X-ray diffraction reveals a tetrahedral Au4 core stabilized by four phosphines and two iodides. Interestingly, the Au4 NC exhibits much higher catalytic selectivity for CO (FECO: > 60%) at more positive potentials (from -0.6 to -0.7 V vs. RHE) than Au11(PPh3)7I3 (FECO: < 60%), a larger 8 e- superatom, and Au(i)PPh3Cl complex; whereas the hydrogen evolution reaction (HER) dominates the electrocatalysis when the potential becomes more negative (FEH2 of Au4 = 85.8% at -1.2 V vs. RHE). Structural and electronic analyses reveal that the Au4 tetrahedron becomes unstable at more negative reduction potentials, resulting in decomposition and aggregation, and consequently the decay in catalytic performance of Au based catalysts towards the eCO2RR.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Assembly of copper-clusters into a framework: enhancing the structural stability and photocatalytic HER performance

We synthesized a novel Cu₈ cluster and employed it as a building block to further obtain a copper cluster-based MOF material (CCMOF). Compared with the Cu₈ cluster precursor, the periodically constructed CCMOF exhibited better structural stability, and showed enhanced photocatalytic H₂ evolution performance due to efficient mass/charge transportation benefitting from the ordered porous framework structure. This two-step grafting construction strategy is of great significance to the exploration of copper clusters in catalytic applications.