Active sites of ligand-protected Au25 nanoparticle catalysts for CO2 electroreduction to CO

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.

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.

-SR removal or -R removal? A mechanistic revisit on the puzzle of ligand etching of Au25(SR)18 nanoclusters during electrocatalysis

Accurate identification of active sites is highly desirable for elucidation of the reaction mechanism and development of efficient catalysts. Despite the promising catalytic performance of thiolated metal nanoclusters (NCs), their actual catalytic sites remain elusive. Traditional first-principles calculations and experimental observations suggested dealkylated S and dethiolated metal, respectively, to be the active centers. However, the real kinetic origin of thiolate etching during the electrocatalysis of NCs is still puzzling. Herein, we conducted advanced first-principles calculations and electrochemical/spectroscopic experiments to unravel the electrochemical etching kinetics of thiolate ligands in prototype Au25(SCH3)18 NC. The electrochemical processes are revealed to be spontaneously facilitated by dethiolation (i.e., desorption of -SCH3), forming the free HSCH3 molecule after explicitly including the solvent effect and electrode potential. Thus, exposed under-coordinated Au atoms, rather than the S atoms, serve as the real catalytic sites. The thermodynamically preferred Au-S bond cleavage arises from the selective attack of H from proton/H2O on the S atom under suitable electrochemical bias due to the spatial accessibility and the presence of S lone pair electrons. Decrease of reduction potential promotes the proton attack on S and significantly accelerates the kinetics of Au-S bond breakage irrespective of the pH of the medium. Our theoretical results are further verified by the experimental electrochemical and spectroscopic data. At more negative electrode potentials, the number of -SR ligands decreased with concomitant increase of the vibrational intensity of S-H bonds. These findings together clarify the atomic-level activation mechanism on the surface of Au25(SR)18 NCs.

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 (FE_CO: > 60%) at more positive potentials (from -0.6 to -0.7 V vs. RHE) than Au₁₁(PPh₃)₇I₃ (FE_CO: < 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 (FE_H2 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.

Programmable kernel structures of atomically precise metal nanoclusters for tailoring catalytic properties

The unclear structures and polydispersity of metal nanoparticles (NPs) seriously hamper the identification of the active sites and the construction of structure-reactivity relationships. Fortunately, ligand-protected metal nanoclusters (NCs) with atomically precise structures and monodispersity have become an ideal candidate for understanding the well-defined correlations between structure and catalytic property at an atomic level. The programmable kernel structures of atomically precise metal NCs provide a fantastic chance to modulate their size, shape, atomic arrangement, and electron state by the precise modulating of the number, type, and location of metal atoms. Thus, the special focus of this review highlights the most recent process in tailoring the catalytic activity and selectivity over metal NCs by precisely controlling their kernel structures. This review is expected to shed light on the in-depth understanding of metal NCs' kernel structures and reactivity relationships.

Transplanting Gold Active Sites into Non-Precious-Metal Nanoclusters for Efficient CO2-to-CO Electroreduction

Electrocatalytic CO₂ reduction reaction (CO₂RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni₄ nanocluster (NC). While the Ni₄ NC exclusively produces H₂, the Au-transplanted NC selectively produces CO over H₂. The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO₂-to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 mol_CO/mol_NC/s and 25,228 A/g_Au, respectively, at an overpotential of 0.32 V) and high durability toward the CO₂RR over 25 h.

Effects of ligand tuning and core doping of atomically precise copper nanoclusters on CO2 electroreduction selectivity

Atomically precise nanoclusters (NCs) provide opportunities for correlating the structure and electrocatalytic properties at atomic level. Herein, we report the single-atom doping effect and ligand effect on CO₂ electroreduction (eCO₂RR) by comparing monogold-doped Au₁Cu₂₄ and homocopper Cu₂₅ NCs protected by triphenylphosphine or/and tris(4-fluorophenyl)phosphine. Catalytic results revealed that the electronic distribution of Cu₂₅ NCs is enormously contracted by doping Au atoms, entitling it to exhibit the unique inhibition of hydrogen evolution reaction. And the inductive effect of ligand strongly favors the formation of formate in eCO₂RR. Overall, this work will provide guidance for the rational design of the copper-based catalysts in the eCO₂RR.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au₁₃ unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Single-Atom-Kernelled Nanocluster Catalyst

To propose the concept of single-atom-kernelled nanocluster, we synthesized a Pd-based trimetal nanocluster with a single-Ag atom-kernel for the first time by introducing some steric hindrance factors and employing a joint alloying strategy that combines the coreduction with an antigalvanic reduction (AGR). Although the AGR-derived Pd-based trimetal nanoclusters with single-silver atom kernels have low contents of gold, they show higher activity and selectivity than those of the bimetal precursor nanocluster in the electrocatalytical reduction of CO₂ to CO. Furthermore, it is revealed that the kernel single atoms from both Au₄Pd₆(TBBT)₁₂ and Au₃AgPd₆(TBBT)₁₂ are not the active sites for catalysis, but greatly influence the catalytical performance by effecting the electronic configuration. Thus, it is demonstrated that the single-atom-kernelled nanocluster can not only improve the precious metal utilization (even to 100%) but also better the properties and provide insight into the structure-property correlation for metal nanoclusters.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au₂₈ (C₂ B₁₀ H₁₁ S)₁₂ (tht)₄ Cl₄ (Au₂₈ -S), was isolated by a facile self-reducing synthesis. Au₂₈ -S adopts an identical Au₂₈ metal framework to that of the reported alkynyl-protected Au₂₈ -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au₂₈ -S shows better catalytic activity for the electrochemical reduction of CO₂ to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

The Factors Dictating Properties of Atomically Precise Metal Nanocluster Electrocatalysts

Metal nanoparticles occupy an important position in electrocatalysis. Unfortunately, by using conventional synthetic methodology, it is a great challenge to realize the monodisperse composition/structure of metal nanoparticles at the atomic level, and to establish correlations between the catalytic properties and the structure of individual catalyst particles. For the study of well-defined nanocatalysts, great advances have been made for the successful synthesis of nanoparticles with atomic precision, notably ligand-passivated metal nanoclusters. Such well-defined metal nanoclusters have become a type of model catalyst and have shown great potential in catalysis research. In this review, the authors summarize the advances in the utilization of atomically precise metal nanoclusters for electrocatalysis. In particular, the factors (e.g., size, metal doping/alloying, ligand engineering, support materials as well as charge state of clusters) affecting selectivity and activity of catalysts are highlighted. The authors aim to provide insightful guidelines for the rational design of electrocatalysts with high performance and perspectives on potential challenges and opportunities in this emerging field.

Alkynyl and halogen co-protected (AuAg)44 nanoclusters: a comparative study on their optical absorbance, structure, and hydrogen evolution performance

We report the synthesis, structure, and electrochemical hydrogen evolution reaction (HER) performance of two alkynyl and halogen coprotected AuAg alloy nanoclusters, namely Au₂₄Ag₂₀(^tBuPh-CC)₂₄Cl₂ (NC 1 for short) and Au₂₂Ag₂₂(^tBuCC)₁₆Br_3.28Cl_2.72 (NC 2 for short). Single crystal X-ray structural analysis revealed that the two nanoclusters possess a rather similar core@shell@shell keplerate metal core configuration to M₁₂@M₂₀@M₁₂ with the main difference in the outermost shell (Au₁₂vs. Au₁₀Ag₂). Interestingly, such a subtle difference in the two-metal-atoms results in different optical absorbance features and drastically different HER performances. Both NCs have excellent long-term stability for the HER, but NC 1 possesses superior activity to NC 2, and density functional theory calculations disclosed that the binding energy of hydrogen to form the key *H intermediate for NC 1 is much lower and hence it adopts a more energetically feasible HER pathway.

High-Performance Ligand-Protected Metal Nanocluster Catalysts for CO2 Conversion through the Exposure of Undercoordinated Sites

Previous experimental breakthroughs reveal the potential to create novel heterogeneous catalysts for the electroreduction of CO2 to a high-value product CO using ligand-protected Au-based nanoclusters. Since the chemical composition and geometric structures have been precisely defined, it is possible to adopt robust design guidelines for the development of practical catalysts and to fundamentally elucidate the underlying reaction mechanism. In this short review, the computational progress made to understand the experimentally observed reduction process on the following subset of materials—Au25(SR)18−, Au24Pd(SR)18, Au23(SR)16− and Au21Cd2(SR)16−—is described. A significant finding from our first-principles mechanistic studies is that CO2 conversion on the fully ligand protected nanoclusters is thermodynamically unfavorable due to the very weak binding of intermediates on the surface region. However, the reaction becomes feasible when either Au or S sites are exposed through the removal of a ligand. The results particularly point to the role of undercoordinated S sites in the creation of highly functional heterogeneous catalysts that are both active and selective for the CO2 conversion process. The incorporation of dopants could significantly influence the catalytic reactivity of the nanoclusters. As demonstrated in the case of the monopalladium substitution in Au25(SR)18−, the presence of the foreign atom leads to an enhancement of CO production selectivity due to the greater stabilization of the intermediates. With the Cd substitution doping of Au23(SR)16−, the improvement in performance is also attributed to the enhanced binding strength of the intermediates on the geometrically modified surface of the nanocluster.

A Heteroleptic Gold Hydride Nanocluster for Efficient and Selective Electrocatalytic Reduction of CO2 to CO

It has been a long-standing challenge to create and identify the active sites of heterogeneous catalysts, because it is difficult to precisely control the interfacial chemistry at the molecular level. Here we report the synthesis and catalysis of a heteroleptic gold trihydride nanocluster, [Au₂₂H₃(dppe)₃(PPh₃)₈]³⁺ [dppe = 1,2-bis(diphenylphosphino)ethane, PPh₃ = triphenylphosphine]. The Au₂₂H₃ core consists of two Au₁₁ units bonded via six uncoordinated Au sites. The three H atoms bridge the six uncoordinated Au atoms and are found to play a key role in catalyzing electrochemical reduction of CO₂ to CO with a 92.7% Faradaic efficiency (FE) at -0.6 V (vs RHE) and high reaction activity (134 A/g_Au mass activity). The CO current density and FE_CO remained nearly constant for the CO₂ reduction reaction for more than 10 h, indicating remarkable stability of the Au₂₂H₃ catalyst. The Au₂₂H₃ catalytic performance is among the best Au-based catalysts reported thus far for electrochemical reduction of CO₂. Density functional theory (DFT) calculations suggest that the hydride coordinated Au sites are the active centers, which facilitate the formation of the key *COOH intermediate.

Electrochemical CO2 reduction catalyzed by atomically precise alkynyl-protected Au7Ag8, Ag9Cu6, and Au2Ag8Cu5 nanoclusters: probing the effect of multi-metal core on selectivity

We report the first all-alkynyl-protected Au2Ag8Cu5 cluster, which adopts a M@M8@M6 core configuration similar with Au7Ag8/Ag9Cu6 clusters. The three clusters exhibited strong metal core effect toward CO2RR, which was understood by DFT calculations.

Homoleptic Alkynyl-Protected Ag15 Nanocluster with Atomic Precision: Structural Analysis and Electrocatalytic Performance toward CO2 Reduction

We report the fabrication of homoleptic alkynyl-protected Ag₁₅ (C≡C-^t Bu)₁₂ ⁺ (abbreviated as Ag₁₅ ) nanocluster and its electrocatalytic properties toward CO₂ reduction reaction. Crystal structure analysis reveals that Ag₁₅ possesses a body-centered-cubic (BCC) structure with an Ag@Ag₈ @Ag₆ metal core configuration. Interestingly, we found that Ag₁₅ can adsorb CO₂ in the air and spontaneously self-assembled into one-dimensional linear material during the crystal growth process. Furthermore, Ag₁₅ can convert CO₂ into CO with a faradaic efficiency of ca. 95.0 % at -0.6 V and a maximal turnover frequency of 6.37 s^-1 at -1.1 V along with excellent long-term stability. Finally, density functional theory (DFT) calculations disclosed that Ag₁₅ (C≡C-^t Bu)₁₁ ⁺ with one alkynyl ligand stripping off from the intact cluster can expose the uncoordinated Ag atom as the catalytically active site for CO formation.

Photodriven Catalytic Hydrogenation of CO2 to CH4 with Nearly 100% Selectivity over Ag25 Clusters

The conversion of chemically inert carbon dioxide and its photoreduction to value-added products have attracted enormous attention as an intriguing prospect for utilizing the principal greenhouse gas CO₂. Herein, we explore the use of Ag₂₅ clusters with well-defined atomic structures for high-selectivity photocatalytic hydrogenation of CO₂ to methane. Ag₂₅ clusters, with molecular-like properties and surface plasmon resonance, exhibit competitive catalytic activity for light-driven CO₂ reduction that yield an almost 100% product selectivity of methane at a relatively mild temperature (100 °C). DFT calculations reveal that the absorption of CO₂ on Ag₂₅ clusters is energetically favorable. The methanation of the Ag₂₅ cluster catalyst has been investigated by operando infrared spectroscopy, verifying that methane was produced through a -H-assisted multielectron reaction pathway via the transformation of formyl and formaldehyde species to form surface CH_x. This work presents a highly efficient strategy for high-performance CO₂ methanation via well-defined metal cluster catalysts.

Distinct chemical fixation of CO2 enabled by exotic gold nanoclusters

Atomically precise metal nanoclusters, especially the metal nanoclusters with an exotic core structure, have given rise to a great deal of interest in catalysis, attributing to their well-defined structures at the atomic level and consequently unique electronic properties. Herein, the catalytic performances of three gold nanoclusters, such as Au₃₈S₂(S-Adm)₂₀ with a body-centered cubic (bcc) kernel structure, Au₃₀(S-Adm)₁₈ with a hexagonal close-packed (hcp) core structure, and Au₂₁(S-Adm)₁₅ with a face-centered cubic (fcc) kernel structure, were attempted for the CO₂ cycloaddition with epoxides toward cyclic carbonates. Due to the excess positive charge with a strong Lewis acidity and large chemical adsorption capacity, the bcc-Au₃₈S₂(S-Adm)₂₀ nanocluster outperformed the hcp-Au₃₀(S-Adm)₁₈ and fcc-Au₂₁(S-Adm)₁₅ nanoclusters. Additionally, the synergistic effect between the gold nanocluster and co-catalyst played a crucial role in CO₂ cycloaddition.

Hydrogen Evolution Electrocatalyst Design: Turning Inert Gold into Active Catalyst by Atomically Precise Nanochemistry

Electrocatalytic hydrogen evolution reaction (HER) holds promise in the renewable clean energy scheme. Crystalline Au and Ag are, however, poor in catalyzing HER, and the ligands on colloidal nanoparticles are generally another disadvantage. Herein, we report a thiolate (SR)-protected Au₃₆Ag₂(SR)₁₈ nanocluster with low coverage of ligands and a core composed of three icosahedral (I_h) units for catalyzing HER efficiently. This trimeric structure, together with the monomeric I_h Au₂₅(SR)₁₈^- and dimeric I_h Au₃₈(SR)₂₄, constitutes a unique series, providing an opportunity for revealing the correlation between the catalytic properties and the catalyst's structure. The Au₃₆Ag₂(SR)₁₈ surprisingly exhibits high catalytic activity at lower overpotentials for HER due to its low ligand-to-metal ratio, low-coordinated Au atoms and unfilled superatomic orbitals. The current density of Au₃₆Ag₂(SR)₁₈ at -0.3 V vs RHE is 3.8 and 5.1 times that of Au₂₅(SR)₁₈^- and Au₃₈(SR)₂₄, respectively. Density functional theory (DFT) calculations reveal lower hydrogen binding energy and higher electron affinity of Au₃₆Ag₂(SR)₁₈ for an energetically feasible HER pathway. Our findings provide a new strategy for constructing highly active catalysts from inert metals by pursuing atomically precise nanoclusters and controlling their geometrical and electronic structures.

Controlled syngas production by electrocatalytic CO2 reduction on formulated Au25(SR)18 and PtAu24(SR)18 nanoclusters

Syngas, a gaseous mixture of CO and H₂, is a critical industrial feedstock for producing bulk chemicals and synthetic fuels, and its production via direct CO₂ electroreduction in aqueous media constitutes an important step toward carbon-negative technologies. Herein, we report controlled syngas production with various H₂/CO ratios via the electrochemical CO₂ reduction reaction (CO₂RR) on specifically formulated Au₂₅ and PtAu₂₄ nanoclusters (NCs) with core-atom-controlled selectivities. While CO was predominantly produced from the CO₂RR on the Au NCs, H₂ production was favored on the PtAu₂₄ NCs. Density functional theory calculations of the free energy profiles for the CO₂RR and hydrogen evolution reaction (HER) indicated that the reaction energy for the conversion of CO₂ to CO was much lower than that for the HER on the Au₂₅ NC. In contrast, the energy profiles calculated for the HER indicated that the PtAu₂₄ NCs have nearly thermoneutral binding properties; thus, H₂ production is favored over CO formation. Based on the distinctly different catalytic selectivities of Au₂₅ and PtAu₂₄ NCs, controlled syngas production with H₂/CO ratios of 1 to 4 was demonstrated at a constant applied potential by simply mixing the Au₂₅ and PtAu₂₄ NCs based on their intrinsic catalytic activities for the production of CO and H₂.

Atomically Precise Gold Nanoclusters as Model Catalysts for Identifying Active Sites for Electroreduction of CO2

Accurate identification of active sites is critical for elucidating catalytic reaction mechanisms and developing highly efficient and selective electrocatalysts. Herein, we report the atomic-level identification of active sites using atomically well-defined gold nanoclusters (Au NCs) Au₂₅ , Au₃₈ , and Au₁₄₄ as model catalysts in the electrochemical CO₂ reduction reaction (CO₂ RR). The studied Au NCs exhibited remarkably high CO₂ RR activity, which increased with increasing NC size. Electrochemical and X-ray photoelectron spectroscopy analyses revealed that the Au NCs were activated by removing one thiolate group from each staple motif at the beginning of CO₂ RR. In addition, density functional theory calculations revealed higher charge densities and upshifts of d-states for dethiolated Au sites. The structure-activity properties of the studied Au NCs confirmed that dethiolated Au sites were the active sites and that CO₂ RR activity was determined by the number of active sites on the cluster surface.

The ligand effect on the interface structures and electrocatalytic applications of atomically precise metal nanoclusters

Metal nanoclusters, also known as ultra-small metal nanoparticles, occupy the gap between discrete atoms and plasmonic nanomaterials, and are an emerging class of atomically precise nanomaterials. Metal nanoclusters protected by different types of ligands, such as thiolates, alkynyls, hydrides, and N-heterocyclic carbenes, have been synthesized in recent years. Moreover, recent experiment and theoretical studies also indicated that the metal nanoclusters show great promise in many electrocatalytic reactions, such as hydrogen evolution, oxygen reduction, and CO₂reduction. The atomically precise nature of their structures enables the elucidation of structure-property relationships and the reaction mechanisms, which is essential if nanoclusters with enhanced performances are to be rationally designed. Particularly, the ligands play an important role in affecting the interface bonding, stability and electrocatalytic activity/selectivity. In this review, we mainly focus on the ligand effect on the interface structure of metal nanoclusters and then discuss the recent advances in electrocatalytic applications. Furthermore, we point out our perspectives on future efforts in this field.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Dynamics of weak interactions in the ligand layer of meta-mercaptobenzoic acid protected gold nanoclusters Au68(m-MBA)32 and Au144(m-MBA)40

Atomically precise metal nanoclusters, stabilized and functionalized by organic ligands, are emerging nanomaterials with potential applications in plasmonics, nano-electronics, bio-imaging, nanocatalysis, and as therapeutic agents or drug carriers in nanomedicine. The ligand layer has an important role in modifying the physico-chemical properties of the clusters and in defining the interactions between the clusters and the environment. While this role is well recognized from a great deal of experimental studies, there is very little theoretical information on dynamical processes within the layer itself. Here, we have performed extensive molecular dynamics simulations, with forces calculated from the density functional theory, to investigate thermal stability and dynamics of the ligand layer of the meta-mercaptobenzoic acid (m-MBA) protected Au68 and Au144 nanoclusters, which are the first two gold nanoclusters structurally solved to atomic precision by electron microscopy [Azubel et al., Science, 2014, 345, 909 and ACS Nano, 2017, 11, 11866]. We visualize and analyze dynamics of three distinct non-covalent interactions, viz., ligand-ligand hydrogen bonding, metal-ligand O[double bond, length as m-dash]C-OHAu interaction, and metal-ligand Ph(π)Au interaction. We discuss their relevance for defining, at the same time, the dynamic stability and reactivity of the cluster. These interactions promote the possibility of ligand addition reactions for bio-functionalization or allow the protected cluster to act as a catalyst where active sites are dynamically accessible inside the ligand layer.

Gold Nanoclusters as Electrocatalysts for Energy Conversion

Gold nanoclusters (Aun NCs) exhibit a size-specific electronic structure unlike bulk gold and can therefore be used as catalysts in various reactions. Ligand-protected Aun NCs can be synthesized with atomic precision, and the geometric structures of many Aun NCs have been determined by single-crystal X-ray diffraction analysis. In addition, Aun NCs can be doped with various types of elements. Clarification of the effects of changes to the chemical composition, geometric structure, and associated electronic state on catalytic activity would enable a deep understanding of the active sites and mechanisms in catalytic reactions as well as key factors for high activation. Furthermore, it may be possible to synthesize Aun NCs with properties that surpass those of conventional catalysts using the obtained design guidelines. With these expectations, catalyst research using Aun NCs as a model catalyst has been actively conducted in recent years. This review focuses on the application of Aun NCs as an electrocatalyst and outlines recent research progress.

An overview on the current understanding of the photophysical properties of metal nanoclusters and their potential applications

Photophysics of atomically precise metal nanoclusters (MNCs) is an emerging area of research due to their potential applications in optoelectronics, photovoltaics, sensing, bio-imaging and catalysis. An overview of the recent advances in the photophysical properties of MNCs is presented in this review. To begin with, we illustrate general synthesis methodologies of MNCs using direct reduction, chemical etching, ligand exchange, metal exchange and intercluster reaction. Due to strong quantum confinement, the NCs possess unique electronic properties such as discrete optical absorption, intense photoluminescence (PL), molecular-like electron dynamics and non-linear optical behavior. Discussions have also been carried out to unveil the influence of the core size, nature of ligands, heteroatom doping, and surrounding environments on the optical absorption and photophysical properties of metal clusters. Recent findings reveal that the excited-state dynamics, nonlinear optical properties and aggregation induced emission of metal clusters offer exciting opportunities for potential applications. We discuss briefly about their versatile applications in optoelectronics, sensing, catalysis and bio-imaging. Finally, the future perspective of this research field is given.

Enhancing CO2 Electroreduction with Au/Pyridine/Carbon Nanotubes Hybrid Structures

Selective electrochemical reduction of CO₂ by using renewable electricity has received considerable attention because of the potential to convert a harmful greenhouse gas into useful chemicals. A high-performance electrocatalyst for CO₂ reduction is constructed based on metal nanoparticles/organic molecule hybrid materials. On the nanoscale, Au nanoparticles are uniformly anchored on carbon nanotubes to afford substantially increased current density, improved selectivity for CO, and enhanced stability. On the molecular level, the catalytic performance is further enhanced by introducing axial pyridine groups to the surface of the carbon nanotubes. The resulting hybrid catalyst exhibits around 93 % faradaic efficiency for CO production over a wide potential range (-0.58 to -0.98 V), a high mass activity of 251 A g_Au ^-1 at -0.98 V in aqueous solution at near-neutral pH, and strong stability with continuous electrolysis for 10 h at -0.58 V. DFT calculations indicate that the synergistic effects of Au and axial pyridine could dramatically stabilize the key intermediate (*COOH) formed in the rate-limiting step of CO₂ reduction, which effectively lowers the overpotential.

Metal Nanoclusters: New Paradigm in Catalysis for Water Splitting, Solar and Chemical Energy Conversion

A sustainable future demands innovative breakthroughs in science and technology today, especially in the energy sector. Earth-abundant resources can be explored and used to develop renewable and sustainable resources of energy to meet the ever-increasing global energy demand. Efficient solar-powered conversion systems exploiting inexpensive and robust catalytic materials for the photo- and photo-electro-catalytic water splitting, photovoltaic cells, fuel cells, and usage of waste products (such as CO₂ ) as chemical fuels are appealing solutions. Many electrocatalysts and nanomaterials have been extensively studied in this regard. Low overpotentials, catalytic stability, and accessibility remain major challenges. Metal nanoclusters (NCs, ≤3 nm) with dimensions between molecule and nanoparticles (NPs) are innovative materials in catalysis. They behave like a "superatom" with exciting size- and facet-dependent properties and dynamic intrinsic characteristics. Being an emerging field in recent scientific endeavors, metal NCs are believed to replace the natural photosystem II for the generation of green electrons in a viable way to facilitate the challenging catalytic processes in energy-conversion schemes. This Review aims to discuss metal NCs in terms of their unique physicochemical properties, possible synthetic approaches by wet chemistry, and various applications (mostly recent advances in the electrochemical and photo-electrochemical water splitting cycle and the oxygen reduction reaction in fuel cells). Moreover, the significant role that MNCs play in dye-sensitized solar cells and nanoarrays as a light-harvesting antenna, the electrochemical reduction of CO₂ into fuels, and concluding remarks about the present and future perspectives of MNCs in the frontiers of surface science are also critically reviewed.

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

The frontiers between homogeneous and heterogeneous catalysis are progressively disappearing.

CO2 electrochemical reduction at thiolate-modified bulk Au electrodes

Simple modification of polycrystalline bulk Au by an appropriate thiol can selectively enhance electrochemical CO₂RR at the expense of HER.

First-Principles Modeling in Heterogeneous Electrocatalysis

The last decade has witnessed tremendous progress in the development of computer simulation based on quantum mechanical description of the interactions between electrons and between electrons and atomic nuclei with electrode potentials taken into account–promoting the possibility to model electrocatalytic reactions. The cornerstone of this development was laid by the widely used computational hydrogen electrode method which involves a posteriori correction of standard constant charge first principles studies in solvent environment. The description of this technique and its contribution to our effort to understand electrocatalytic reactions on the active sites of metal-based nanoparticles are reviewed. The pathways and energetics of the relevant elementary reactions are presented. We also discussed a recent attempt in the literature to account for the inflow and outflow of electrons from the electrode as electrochemical reactions proceed, which has been greatly assisted by the development of density functional theory within the grand canonical framework. Going beyond the computational hydrogen electrode method by explicit incorporation of electrode potential within the calculations permits access to more detailed insights without requiring extra computational burden.