The field of catalysis using atomically precise metal nanoclusters (NCs) is rapidly expanding due to their unique structural properties. However, the inherent instability and often low catalytic activity of these NCs hinder their practical applications. Researchers have explored various strategies to improve NC catalytic performance, including heterometallic doping, ligand engineering, and size regulation. Despite these efforts, the controlled synthesis of NCs with precise structures remains challenging, largely because the clusters are susceptible to external influences during the assembly process. This inherent difficulty highlights the need for innovative approaches to enhance both the activity and stability of NC-based catalysts. Single-atom (SA) catalysts have emerged as highly efficient catalysts for various reactions. Their distinct electronic states, different from those of NCs, suggest that integrating SAs and metal NCs within a single system could offer synergistic advantages, potentially leading to substantial improvements in catalytic performance. Introducing SAs can modulate the electron distribution and the free energy of intermediate adsorption, optimizing the catalytic activity. However, common methods for preparing nanocluster-single atom (NC-SA) catalysts, such as pyrolysis, often result in catalysts with ill-defined structures, complicating the understanding of structure-activity relationships. Therefore, a controlled method to synthesize a NC-SA catalyst with high activity and stability remains a significant challenge. This study aims to address this gap by developing a facile approach to functionalize atomically precise metal NCs with extra SAs, aiming to achieve remarkable improvements in catalytic activity and stability.

Extensive research has been conducted to enhance the catalytic properties of metal nanoclusters. Studies have focused on techniques such as heterometallic doping, which involves incorporating different metal atoms into the nanocluster structure to modify its electronic and geometric properties [6-10]. Ligand engineering, the manipulation of the molecules surrounding the nanocluster core, has also been shown to influence catalytic activity [11-15]. Size regulation, controlling the number of atoms in the nanocluster, affects the cluster's electronic structure and thus its reactivity [16-18]. Despite these advances, synthesizing metal nanoclusters with precisely controlled structures remains a significant hurdle. The use of single-atom catalysts (SACs) has also garnered significant attention, owing to their high atom utilization efficiency and unique catalytic properties [22-26]. The integration of SACs with metal nanoclusters has been explored as a promising avenue to improve catalytic performance [27-37]. This approach leverages the distinct electronic properties of SACs and NCs to create a synergistic effect. However, many existing NC-SA catalyst synthesis methods, including pyrolysis, yield catalysts with poorly defined structures, hindering the elucidation of structure-activity relationships. This study addresses this limitation by introducing a novel approach to create a well-defined NC-SA catalyst with exceptional activity and stability.

The researchers synthesized uniformly dispersed metal nanoclusters modified with single-atom sites within a poly-carbazole matrix. This was achieved through a co-electropolymerization strategy. The process began with the synthesis of the alkynyl carbazole ligand (HCzPA) and the subsequent synthesis of Au8 nanoclusters ([Au8(dppp)4(CzPA)2], where dppp is 1,3-bis(diphenylphosphino)propane and CzPA is 9-(4-ethynylphenyl)carbazole). The carbazole-substituted phenanthroline monomer, chelated with various single-atom metals (DCP@M, M = Fe, Co, Ni, Cu, Zn), was then electrochemically co-polymerized with the Au8 clusters. Cyclic voltammetry (CV) was employed to drive the polymerization, with the potential range set between 0 V and 2.0 V (vs. Ag/AgCl). A range of control samples were prepared, including co-polymerization of Au8 and DCP (Poly-(Au8-DCP)), self-polymerization of monomers Au8 (Poly-Au8) and DCP@M (Poly-DCP@M). The structure and composition of the synthesized materials were extensively characterized using various techniques. High-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray spectroscopy (EDS) mapping were used to determine the morphology and elemental distribution. X-ray absorption near-edge structure (XANES) and Fourier-transformed extended X-ray absorption fine structure (EXAFS) analyses investigated the local coordination environment and chemical states of the Fe and Au species. X-ray photoelectron spectroscopy (XPS) examined the electronic states of the Au and Fe species, and CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) explored the electron cloud density of the accessible Au sites. Density functional theory (DFT) calculations were performed to model the electronic structures and reaction pathways. Electrocatalytic activity was evaluated using a three-electrode H-type cell with 0.5 M KHCO3 electrolyte under a CO2 atmosphere. Gas chromatography (GC) and 1H nuclear magnetic resonance (NMR) spectroscopy were used to analyze the products. Electrochemical double-layer capacitance (Cdl) was measured to estimate the electrochemical active surface areas (ECSAs). In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy provided insights into intermediates during the CO2RR process. Finally, CO2 isotopic experiments were conducted to confirm the carbon source of the reduction products.

The co-electropolymerization strategy yielded a highly homogeneous NC-SA hybrid catalyst, Poly-(Au8-DCP@Fe). This catalyst exhibited a significantly enhanced electrocatalytic performance toward CO2 reduction reaction (CO2RR), achieving a maximum CO Faradaic efficiency (FE) of 90.89% at -0.57 V (vs. RHE). This represents a substantial ~18-fold increase compared to the pristine Au8 crystal. Detailed characterization revealed that the introduction of Fe single atoms (SAs) effectively modulated the electronic structure of the Au8 nanoclusters. XPS analysis showed a higher oxidation state of Au atoms in Poly-(Au8-DCP@Fe) compared to Poly-(Au8-DCP), indicating electron transfer from the Au8 clusters to the Fe SAs. This electron redistribution was further supported by CO-DRIFTS, which showed a red-shift of adsorbed CO on Au8 in Poly-(Au8-DCP@Fe), indicative of decreased electron density. DFT calculations corroborated the experimental findings, demonstrating that the Fe SAs optimized the adsorption energy of the key intermediate *COOH in the CO2RR, thereby lowering the reaction barrier. The calculated free energy diagram indicated that Poly-(Au8-DCP@Fe) had a more optimal ΔG for the rate-limiting step of CO2 reduction to *COOH (0.81 eV) than other samples, resulting in enhanced activity and selectivity. In situ ATR-FTIR experiments further supported these findings by showing the stronger *COOH and *CO signals for Poly-(Au8-DCP@Fe) compared to other control samples. The long-term stability test showed that the CO FE remained above 80% for over 6 hours, demonstrating the effectiveness of the polymer matrix in preventing agglomeration. Control experiments with other metals (Co, Ni, Cu, Zn) and with a different comonomer (BCP) showed that the enhanced activity was specifically due to the combination of Au8 clusters and Fe SAs and not solely a result of the polymer matrix or the single atoms alone. The higher ECSA of Poly-(Au8-DCP@Fe) also contributed to its improved catalytic performance. Isotopic labeling experiments using 13CO2 confirmed that the CO product originated from the CO2 reduction.

This study demonstrates a novel approach for significantly enhancing the electrocatalytic CO2RR activity of atomically precise Au nanoclusters by introducing Fe single-atom sites through a co-electropolymerization strategy. The substantial improvement in CO FE (~18-fold increase) is attributed to the precise modulation of the electronic structure of the Au8 nanoclusters by the Fe SAs. This electronic modulation optimizes the adsorption of key intermediates, facilitating the CO2RR and suppressing the competing hydrogen evolution reaction. The results provide compelling evidence of the synergistic effect between single atoms and nanoclusters in catalysis. The facile electropolymerization method offers a highly controllable approach to creating highly homogeneous NC-SA hybrid catalysts with enhanced activity and stability. This work contributes significantly to the field by providing a new strategy for designing and fabricating highly efficient catalysts for CO2 reduction. The findings suggest that the precise tailoring of electronic structure through single-atom modification is a powerful tool for improving the intrinsic catalytic properties of metal nanoclusters.

This research successfully synthesized a highly active and stable NC-SA hybrid catalyst, Poly-(Au8-DCP@Fe), for CO2RR via a facile co-electropolymerization approach. The catalyst exhibited a remarkable 18-fold increase in CO2-to-CO conversion activity compared to its unmodified counterpart, attributed to the synergistic interplay between Au8 nanoclusters and Fe single atoms. This work introduces a new strategy for enhancing the inherent catalytic activity of metal nanoclusters and opens avenues for future research on designing efficient catalysts for other important chemical transformations. Further investigation could explore other metal combinations and polymer matrices to optimize catalytic performance and expand the applicability of this approach to a broader range of reactions.

The study primarily focuses on the CO2RR activity of the Poly-(Au8-DCP@Fe) catalyst. While the long-term stability test demonstrates high stability for over 6 hours, further investigations could assess its stability over even longer periods under various operating conditions. Although the DFT calculations provided valuable insights into the reaction mechanism, the computational model may not fully capture all aspects of the complex catalytic process. The study also primarily focused on Fe as the single-atom dopant; exploring other single-atom dopants could reveal further insights into structure-activity relationships and optimize catalyst performance.