Molecular mechanism for the activation of Au25(SCH2CH2Ph)18 nanoclusters by imidazolium-based ionic liquids for catalysis

Transition metal oxide clusters: advanced electrocatalysts for a sustainable energy future

The comprehensive utilization of sustainable green energy is essential to face the global energy and environmental crisis. The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and electrocatalytic urea synthesis (EUS) are the pivotal electrocatalytic processes, necessitating the development of low-cost electrocatalysts with high efficiency. Small-sized transition metal oxide (TMO) clusters have attracted a lot of attention because of their exceptional qualities, such as exhibiting a dense array of low-coordinated metal active sites (e.g. abundant metal cation defects and oxygen vacancy), amorphous structures with high surface energy, high atom utilization efficiency, and cost-effectiveness. Furthermore, the synergistic actions between metal clusters and TM-Nx single atom active sites remarkably boost up the electrocatalytic performances, corroborated by density functional theory (DFT). More efforts in this comprehensive feature article are expected to achieve insights into the fundamental understanding of electrocatalytic reaction mechanisms in our lab and serve as a guide for creating cutting-edge electrocatalysts of transition metal oxide clusters.

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Structure control and evolution of atomically precise gold clusters as heterogeneous precatalysts

Metal clusters have distinct features from single atom and nanoparticle (>1 nm) catalysts, making them effective catalysts for various heterogeneous reactions. Nevertheless, the ambiguity and complexity of the catalyst structure preclude in-depth mechanistic studies. The evolution of metal species during synthesis and reaction processes represents another challenge. One effective solution is to precisely control the structure of the metal cluster, thus offering a well-defined pre-catalyst. The well-defined chemical formula and configurations make atomically precise metal nanoclusters optimal choices. To fabricate an atomically precise metal nanocluster-based heterogeneous catalyst with enhanced performance, careful structural design of both the nanocluster and support material, an effective assembling technique, and a pre-treatment method for these hybrids need to be developed. In this review, we summarize recent advances in in the development of heterogeneous catalysts using atomically precise gold and alloy gold nanoclusters as precursors. We will begin with a brief introduction to the structural properties of atomically precise nanoclusters and structure determination of cluster/support hybrids. We will then introduce heterogeneous catalysts prepared from medium size (tens to hundreds of metal atoms) and low nuclearity nanoclusters. We will illustrate how ligand modification, support-cluster interaction, hybrid fabrication, and heteroatom (Pt, Pd Ag, Cu, Cd, Fe) introduction affect the structural properties and pretreatment/reaction-induced structural evolution of gold nanocluster pre-catalysts. Lastly, we will highlight the synthetic method of NCs@MOF hybrids and their effectiveness in circumventing the adverse cluster structural evolution. These findings are expected to shed light on the structure-activity relationship studies and future catalyst design strategies using atomically precise metal nanocluster pre-catalysts.

Ionic Liquids-Driven Cluster-to-Cluster Conversion of Polyhydrido Copper(I) Clusters Cu7H5 to Cu8H6 and Cu12H9

Although ionic liquids (ILs) are of prime interest for the synthesis of various nanomaterials, they are scarcely utilized for the polyhydrido copper(I) [Cu(I)H] clusters. Herein, two air-stable Cu(I)H clusters, [Cu8H6(dppy)6](NTf2)2 (Cu8H6) and {Cu12H9(dppy)6[N(CN)2]3} (Cu12H9), are synthesized in high yields for the first time from the ILs-driven conversion of an unprecedented cluster [Cu7H5(dppy)6](ClO4)2 (Cu7H5) by a facile three-layers diffusion crystal (TLDC) method, strategically introducing IL-NTf2 and IL-N(CN)2 as two types of unusual interfacial crystallized templates, respectively. Their structures are fully characterized by various spectroscopic methods and X-ray crystallography, which shows that the anion of IL plays an important role as an anion template and an anion ligand in controlling the structural conversion of Cu(I)H clusters. Their photophysical properties are also investigated, and it is found that all reported clusters exhibit red luminescence with λem ranging from 600 to 690 nm.

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

A brief account is given of highlights of our computational efforts - often in collaboration with experimental groups - to understand spectroscopic and chemical properties of ionic liquids (ILs). Molecular dynamics, including their inhomogeneous character, responsible for key spectral features observed in dielectric absorption, infra-red (IR) and fluorescence correlation spectroscopy (FCS) measurements are elucidated. Mechanisms of chemical processes involving imidazolium-based ILs are illustrated for CO2 capture and related reactions, transesterification of cellulose, and Au nanocluster-catalyzed Suzuki cross-coupling reaction with attention paid to differing roles of IL ions. A comparison with experiments is also made.

Developing CHCA/PPD as a novel matrix for enhanced matrix-assisted laser desorption/ionization-mass spectrometry imaging for analysis of antibiotics in grass carp tissues

Antibiotics have important medical value, but they need to be monitored when used as veterinary drugs. We report α-cyano-4-hydroxycinnamic acid/p-phenylenediamine (CHCA/PPD) hybrid as a novel matrix for enhanced matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) in situ the spatial distribution of antibiotic drugs in grass carp tissues.

METHOD

We have used MALDI-TOF-MSI in positive ion mode for the analysis of quinolones and sulfonamides in grass carps. A novel CHCA/PPD matrix was prepared and applied using a simple method to improve the analysis.

RESULTS

Compared with the traditional matrix, CHCA/PPD significantly improved the detection intensity of quinolones and sulfonamides with better sensitivity (17.20%-94.30%) and reproducibility. For demonstration, this novel matrix was successfully applied to visualize enrofloxacin (ENR) in grass carp tissues, with the entire abundance differences clearly observed based on MALDI-MSI. The concentration levels in different tissues were determined, with a calibration curve of 10-2000 μg/ml (R²  > 0.993).

CONCLUSION

This study was the first to introduce CHCA/PPD as a novel matrix, and the classical acid-base mixing was used to improve the ionization effect of the traditional matrix CHCA in MALDI. Based on CHCA/PPD, MALDI-MSI detected ENR in different grass carp tissues for the first time and realized the spatial distribution and concentration detection.

Mass Spectrometry of Au10(4-tert-butylbenzenethiolate)10 Nanoclusters Using Superconducting Tunnel Junction Cryodetection Reveals Distinct Metastable Fragmentation

Cryodetection mass spectrometry (MS) was used to study the Au₁₀(TBBT)₁₀ (TBBT = 4-tert-butylbenzenethiolate) catenane nanocluster. The matrix-assisted laser desorption ionization (MALDI) process generates distinct fragments that can be arranged into two distinct regimes: (i) in-source fragmentation, which occurs rapidly in a relatively short (<170 ns) time frame, and (ii) metastable fragmentation, which occurs postacceleration during a time-of-flight (TOF) mass analysis over a longer time frame (>170 ns-250 μs). Using MALDI-TOF MS with superconducting tunnel junction (STJ) cryodetection, distinct metastable nanocluster fragments were resolved at lower energies deposited into the detector. The results also demonstrated that STJ cryodetection MS can be used to acquire multiple (>10), simultaneous tandem mass spectra in a single experiment. Simulated fragmentation of the Au₁₀ nanocluster using ab initio molecular dynamics (AIMD) revealed the different fragmentation processes and confirmed the MS results. Using both the empirical MS data and AIMD calculations, fragmentation pathways are proposed for Au₁₀(TBBT)₁₀, which terminate with two small, stable ringed species.

Computational Modeling of the Interaction of Silver Clusters with Carbohydrates

Silver nanoparticles are recognized for their numerous physical, biological, and pharmaceutical applications. In the present study, the interaction of silver clusters with monosaccharide molecules is examined to identify which molecule works better as a reducing agent in the application of a green synthesis approach. Geometry optimization of clusters containing one, three, and five silver atoms is performed along with the optimization of α-d-glucose, α-d-ribose, d-erythrose, and glyceraldehyde using density functional theory. Optimized geometries allow identifying the interaction formed in the silver cluster and monosaccharide complexes. An electron localization function analysis is performed to further analyze the interaction found and explain the reduction process in the formation of silver nanoparticles. The overall results indicate that glyceraldehyde presents the best characteristics to serve as the most efficient reducing agent.

Experimental and DFT calculation study of interaction between silver nanoparticle and 1-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid

The mechanism of stabilization of silver nanoparticles (Ag NPs) by 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) is elucidated from experimental spectroscopic investigations and density functional theory (DFT) calculations. FTIR spectrum of the synthesized IL stabilized silver nanoparticles reveals small red shift in B–F stretching frequency while C–H stretching remains unshifted. There is no shift in NMR peaks of IL before and after the synthesis of IL stabilized Ag NPs. This suggests that Ag NPs are surrounded by anions of ILs. The optimized structure obtained from density functional theory (DFT) calculations also shows the anionic part of the IL surrounding the Ag nanocluster. This is supported by the IR frequency data calculated using DFT. The calculated binding energy and interaction energy obtained between cluster and IL is considerably attractive. Density of State (DOS) calculation shows that the HOMO-LUMO gap of the Ag cluster-IL composite is significantly lesser than that of the IL alone.

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Heterogeneous gold catalysts for selective hydrogenation: from nanoparticles to atomically precise nanoclusters

Gold nanocatalysts with different sizes (nanoparticles and nanoclusters) show different catalytic performances for various selective hydrogenation reactions. The recent breakthrough in a controllable synthesis of atomically precise gold nanoclusters provides unprecedented opportunities for understanding the catalytic behavior at the atomic/molecular levels. Herein, we review the progress in catalytic hydrogenation over gold nanoparticles and atomically precise gold nanoclusters in the last five years. We also compare the results obtained from different reactions so that a better understanding of their catalytic behavior can be obtained. Finally, we provide some future perspectives on gold nanocatalysis.

Heterogeneous Cross-Coupling over Gold Nanoclusters

Au clusters with the precise numbers of gold atoms, a novel nanogold material, have recently attracted increasing interest in the nanoscience because of very unique and unexpected properties. The unique interaction and electron transfer between gold clusters and reactants make the clusters promising catalysts during organic transformations. The AunLm nanoclusters (where L represents organic ligands and n and m mean the number of gold atoms and ligands, respectively) have been well investigated and developed for selective oxidation, hydrogenation, photo-catalysis, and so on. These gold clusters possess unique frameworks, providing insights into the catalytic processes and an excellent arena to correlate the atomic frameworks with their intrinsic catalytic properties and to further investigate the tentative reaction mechanisms. This review comprehensively summarizes the very latest advances in the catalytic applications of the Au nanoclusters for the C-C cross-coupling reactions, e.g., Ullmann, Sonogashira, Suzuki cross-couplings, and A3-coupling reactions. It is found that the proposed catalytically active sites are associated with the exposure of gold atoms on the surface of the metal core when partial capping organic ligands are selectively detached under the reaction conditions. Finally, the tentative catalytic mechanisms over the ligand-capped Au nanoclusters and the relationship of structure and catalytic performances at the atomic level using computational methods are explored in detail.

Reversible Control of Chemoselectivity in Au38(SR)24 Nanocluster-Catalyzed Transfer Hydrogenation of Nitrobenzaldehyde Derivatives

Chemoselective hydrogenation of nitrobenzaldehyde derivatives is one of the important catalytic processes being studied in hydrogenation catalysis. In this work, we report for the first time the catalytic reaction over atomically precise gold nanocluster catalysts (Au₂₅, Au₃₈, Au₅₂, and Au₁₄₄) using potassium formate as the hydrogen source. A complete selectivity for hydrogenation of the aldehyde group, instead of the nitro group, is obtained. A distinct dependence on the size of nanocluster catalysts is also observed, in which the Au₃₈(SCH₂CH₂Ph)₂₄ gives rise to the highest catalytic activity. The catalyst also shows good versatility and recyclability. Interestingly, the ligand-off nanocluster changes its catalytic selectivity to the nitro hydrogenation, which is in contrast with the ligand-on catalyst. In addition, the selectivity can be restored by treating the ligand-off nanocluster catalyst with thiol. This reversible control of chemoselectivity is remarkable and may stimulate future work on the exploitation of such nanoclusters for hydrogenation catalysis with control over selectivity.

Hydride-induced ligand dynamic and structural transformation of gold nanoclusters during a catalytic reaction

Quasi-homogeneous ligand-protected gold nanoclusters (Au NCs) with atomic precision and well-defined structure offer great opportunity for exploring the catalytic nature of nanogold catalysts at a molecular level. Herein, using real-time electrospray ionization mass spectrometry (ESI-MS), we have successfully identified the desorption and re-adsorption of p-mercaptobenzoic acid (p-MBA) ligands from Au25(p-MBA)18 NC catalysts during the hydrogenation of 4-nitrophenol in solution. This ligand dynamic (desorption and re-adsorption) would initiate structural transformation of Au25(p-MBA)18 NC catalysts during the reaction, forming a mixture of smaller Au NCs (Au23(p-MBA)16 as the major species) at the beginning of catalytic reaction, which could further be transformed into larger Au NCs (Au26(p-MBA)19 as the major species). The adsorption of hydrides (from NaBH4) is identified as the determining factor that could induce the ligand dynamic and structural transformation of NC catalysts. This study provides fundamental insights into the catalytic nature of Au NCs, including catalytic mechanism, active species and stability of Au NC catalysts during a catalytic reaction.

Engineering Functional Metal Materials at the Atomic Level

With continuous research efforts devoted into synthesis and characterization chemistry of functional nanomaterials in the past decades, the development of metal materials is stepping into a new era, where atom-by-atom customization of property-dictating structural attributes is expected. Herein, the state-of-the-art modulation of functional metal nanomaterials at the atomic level, by size- and structure-controlled synthesis of thiolate-protected metal (e.g., Au and Ag) nanoclusters (NCs), is exemplified. Metal NCs are ultrasmall (<3 nm) particles with hierarchical primary, secondary, and tertiary structures, reminiscent of natural proteins or enzymes. Given the proven dependence of their physicochemical properties on their size and structure, documented synthetic methodologies delivering NCs with atomic-level monodispersity and tailorable size and structural attributes at individual hierarchical levels are categorized and discussed. Such assured atomic-level modulation could confer metal NCs with novel application opportunities in diverse fields, which are also exemplified by their size- and structure-dictated catalytic and biomedical performance. The precise synthesis and application chemistry developed based on the hierarchical structure scheme of metal NCs could increase the acceptance of metal NCs as a new family of functional materials.

Gold nanocatalysts supported on carbon for electrocatalytic oxidation of organic molecules including guanines in DNA

Gold (Au) is chemically stable and resistant to oxidation. Although bulk Au is catalytically inert, nanostructured Au exhibits unique size-dependent catalytic activity. When Au nanocatalysts are supported on conductive carbon (denoted as Au@C), Au@C becomes promising for a wide range of electrochemical reactions such as electrooxidation of alcohols and electroreduction of carbon dioxide. In this mini-review, we summarize Au@C nanocatalysts with specific attention on the most recent achievements including the findings in our own laboratories, and show that Au nanoclusters (AuNCs, <2 nm) on nitrided carbon are excellent electrocatalysts for the oxidation of organic molecules including guanines in DNA. The state-of-the-art synthesis and characterization of these nanomaterials are also documented. Synergistic interactions among Au-containing multicomponents on carbon supports and their applications in electrocatalysis are discussed as well. Finally, challenges and future outlook for these emerging and promising nanomaterials are envisaged.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Dual effects of water vapor on ceria-supported gold clusters

Atomically precise nanocatalysts are currently being intensely pursued in catalysis research. Such nanocatalysts can serve as model catalysts for gaining fundamental insights into catalytic processes. In this work we report a discovery that water vapor provokes the mild removal of surface long-chain ligands on 25-atom Au25(SC12H25)18 nanoclusters in a controlled manner. Using the resultant Au25(SC12H25)18-x/CeO2 catalyst and CO oxidation as a probe reaction, we found that the catalytic activity of cluster/CeO2 is enhanced from nearly zero conversion of CO (in the absence of water) to 96.2% (in the presence of 2.3 vol% H2O) at the same temperature (100 °C). The cluster catalysts exhibit high stability during the CO oxidation process under moisture conditions (up to 20 vol% water vapor). Water vapor plays a dual role in gold cluster-catalyzed CO oxidation. FT-IR and XPS analyses in combination with density functional theory (DFT) simulations suggest that the "-SC12H25" ligands are easier to be removed under a water vapor atmosphere, thus generating highly active sites. Moreover, the O22- peroxide species constitutes the active oxygen species in CO oxidation, evidenced by Raman spectroscopy analysis and isotope experiments on the CeO2 and cluster/CeO2. The results also indicate the perimeter sites of the interface of Au25(SC12H25)18-x/CeO2 to be active sites for catalytic CO oxidation. The controlled exposure of active sites under mild conditions is of critical importance for the utilization of clusters in catalysis.

Theoretical insights into the interaction between RunPt13-n (n=4, 7 and 9) clusters and [BMIM]+ based ionic liquids: Effect of anion

Density functional theory has been performed to systematically study the interactions between Ru_nPt_13-n (n=4, 7 and 9) clusters and [BMIM]⁺ based ionic liquids. Ionic liquids [BMIM][Br], [BMIM][BF₄], [BMIM][PF₆], [BMIM][CF₃SO₃], and [BMIM][NTf₂] have different effects on the stability of Ru₇Pt₆. Ionic liquids with median size anions of PF₆^- and CF₃SO₃^- can better improve the stability of Ru₇Pt₆ than those with the small anions of Br^- and BF₄^- and large anion of NTf₂^-. Based on negative relaxation energies, the stabilities of Ru₄Pt₉, Ru₇Pt₆, and Ru₉Pt₄ are all enhanced after interacting with [BMIM][CF₃SO₃]. The stability enhanced degree is in agreement with the interaction strength. For Ru₇Pt₆-n{[BMIM][CF₃SO₃]} (n=1, 2, 3, 4), the interaction between ionic liquid and cluster plays the primary role in stabilizing the cluster in Ru₇Pt₆-[BMIM][CF₃SO₃]. With the increase of the number of [BMIM][CF₃SO₃], the role of the interaction in stabilizing the cluster is getting weaker, while the role of steric protection is getting more important.

Tailoring the Electronic and Catalytic Properties of Au25 Nanoclusters via Ligand Engineering

To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au25(SNap)18](-) [TOA](+), where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au25(SCH2CH2Ph)18](-), albeit their skeletons (i.e., Au25S18 framework) are similar. A red shift by ∼10 nm in the HOMO-LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au13 kernel. The unsupported [Au25(SNap)18](-) nanoclusters show high thermal and antioxidation stabilities (e.g., at 80 °C in the present of O2, excess H2O2, or TBHP) due to the effects of aromatic ligands on stabilization of the nanocluster's frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au25(SR)18/CeO2 (R = Nap, Ph, CH2CH2Ph, and n-C6H13) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4'-nitro-1,1'-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure-activity relationships of the Au25 nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications.