Finding the Most Catalytically Active Platinum Clusters With Low Atomicity

Atomically Precise Nanometer-Sized Pt Catalysts with an Additional Photothermy Functionality

Atomically precise ~1-nm Pt nanoparticles (nanoclusters, NCs) with ambient stability are important in fundamental research and exhibit diverse practical applications (catalysis, biomedicine, etc.). However, synthesizing such materials is challenging. Herein, by employing the mixture ligand protecting strategy, we successfully synthesized the largest organic-ligand-protected (~1-nm) Pt23 NCs precisely characterized with mass spectrometry and single-crystal X-ray diffraction analyses. Interestingly, natural population analysis and Bader charge calculation indicate an alternate, varying charge -layer distribution in the sandwich-like Pt23 NC kernel. Pt23 NCs can catalyze the oxygen reduction reaction under acidic conditions without requiring calcination and other treatments, and the resulting specific and mass activities without further treatment are sevenfold and eightfold higher than those observed for commercial Pt/C catalysts, respectively. Density functional theory and d-band center calculations interpret the high activity. Furthermore, Pt23 NCs exhibit a photothermal conversion efficiency of 68.4 % under 532-nm laser irradiation and can be used at least for six cycles, thus demonstrating great potential for practical applications.

Characterization of changes in the electronic structure of platinum sub-nanoclusters supported on graphene induced by oxygen adsorption

As the sizes of noble metal catalysts, such as platinum, have been successfully minimized, fundamental insights into the electronic properties of metal sub-nanoclusters are increasingly sought for optimizing their catalytic performance. However, it is difficult to rationalize the catalytic activities of metal sub-nanoclusters owing to their more complex electronic structure compared with those of small molecules and bulky solids. In this study, the adsorption of molecular oxygen on a Pt13 sub-nanocluster supported on a graphene layer was analyzed using density functional theory. Unlike bulk Pt, the Pt13 sub-nanocluster has multiple adsorption sites, and the adsorption energy depends strongly on the type of adsorption site. The O2 adsorption energy does not correlate with the transferred charge between O2 and the Pt13 moiety; therefore, to elucidate the differences in the adsorption sites, we propose an original approach for analyzing the electronic structure change in metal sub-nanoclusters caused by molecular adsorption. Our analysis of the integrated local density of state (LDOS) revealed that O2 adsorption on the Pt13 sub-nanocluster has a distinct feature, different from that on a smaller Pt2 cluster or rather a larger Pt slab. The change in the electronic structure of the Pt13 moiety was primarily observed near the Fermi level, different from that of the Pt slab whose DOS was distributed over a wide energy range. Furthermore, the change in the integrated LDOS correlated well with the O2 adsorption energy on the Pt13 sub-nanocluster.

Switch Volmer-Heyrovsky to Volmer-Tafel Pathway for Efficient Acidic Electrocatalytic Hydrogen Evolution by Correlating Pt Single Atoms with Clusters

As cost-effective catalysts, platinum (Pt) single-atom catalysts (SACs) have attracted substantial attention. However, most studies indicate that Pt SACs in acidic hydrogen evolution reaction (HER) follow the slow Volmer-Heyrovsky (VH) mechanism instead of the fast kinetic Volmer-Tafel (VT) pathway. Here, this work propose that the VH mechanism in Pt SACs can be switched to the faster VT pathway for efficient HER by correlating Pt single atoms (SAs) with Pt clusters (Cs). Our calculations reveal that the correlation between Pt SAs and Cs significantly impacts the electronic structure of exposed Pt atoms, lowering the adsorption barrier for atomic hydrogen and enabling a faster VT mechanism. To validate these findings, this work purposely synthesize three catalysts: l-Pt@MoS2, m-Pt@MoS2 and h-Pt@MoS2 with low, moderate, and high Pt-loading, having different distributions of Pt SAs and Cs. The m-Pt@MoS2 catalyst with properly correlating Pt SAs and Cs exhibits outstanding performance with an overpotential of 47 mV and Tafel slope of 32 mV dec-1. Further analysis of the Tafel values confirms that the m-Pt@MoS2 sample indeed follows the VT reaction mechanism, aligning with the theoretical findings. This study offers a deep understanding of the synergistic mechanism, paving a way for designing novel-advanced catalysts.

Overcoming the Electrode Challenges of High-Temperature Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are becoming a major part of a greener and more sustainable future. However, the costs of high-purity hydrogen and noble metal catalysts alongside the complexity of the PEMFC system severely hamper their commercialization. Operating PEMFCs at high temperatures (HT-PEMFCs, above 120 °C) brings several advantages, such as increased tolerance to contaminants, more affordable catalysts, and operations without liquid water, hence considerably simplifying the system. While recent progresses in proton exchange membranes for HT-PEMFCs have made this technology more viable, the HT-PEMFC viscous acid electrolyte lowers the active site utilization by unevenly diffusing into the catalyst layer while it acutely poisons the catalytic sites. In recent years, the synthesis of platinum group metal (PGM) and PGM-free catalysts with higher acid tolerance and phosphate-promoted oxygen reduction reaction, in conjunction with the design of catalyst layers with improved acid distribution and more triple-phase boundaries, has provided great opportunities for more efficient HT-PEMFCs. The progress in these two interconnected fields is reviewed here, with recommendations for the most promising routes worthy of further investigation. Using these approaches, the performance and durability of HT-PEMFCs will be significantly improved.

On the structure and electronic properties of Ptn clusters: new most stable structures for n = 16-17

The lowest energy structures and electronic properties of Ptn clusters up to n = 17 are investigated by using a genetic algorithm in combination with density functional theory calculations. There are several putative global minimum structures for platinum clusters which have been reported by using different approaches, but a comprehensive study for n = 15-17 has not been carried out so far. Herein, we perform a consensus using GGA (PBE), meta-GGA (TPSS) and hybrid (B3PW91, PBE0, PBEh-3c, M06-L) functionals in conjunction with the Def2-TZVP basis set. New most stable structures are found for Pt16 and Pt17, which are slightly lower in energy than the previously reported global minima. Molecular dynamics simulations show that the clusters are rigid at room temperature. We analyze the structural, electronic, energy and vibrational data of the investigated clusters in detail.

Ultra-small Mo-Pt subnanoparticles enable CO2 hydrogenation at room temperature and atmospheric pressure

We present a partially-oxidised bimetallic Mo-Pt subnanoparticle (Mo4Pt8Ox) enabling thermally-driven CO2 hydrogenation to CO at room temperature and atmospheric pressure. A mechanistic study explained the full catalytic cycle of the reaction from CO2 activation to catalyst reactivation. DFT calculations revealed that alloying with Mo lowers the activation barrier by weakening the CO adsorption. This finding could be a first step for low-energy CO2 conversion.

Size-Dependent Geometrical Structures of Platinum Oxide Cluster Cations Studied by Ion Mobility-Mass Spectrometry

Structures of platinum oxide cluster cations (Pt_nO_m⁺) were studied by ion mobility-mass spectrometry in combination with theoretical calculations. Structures of oxygen-equivalent Pt_nO_n⁺ (n = 3-7) clusters were discussed from the comparison between their collision cross sections (CCSs) obtained by mobility measurement and simulated CCSs of their structural candidates from structural optimization calculations. Assigned structures of Pt_nO_n⁺ were found to be composed of Pt frameworks and bridging O atoms, which follows the previous theoretical prediction on the neutral clusters. The structures change from planar (n = 3 and 4) to three-dimensional (n = 5-7) with increasing cluster size by deforming platinum frameworks. Comparison with other group-10 metal oxide cluster cations (M_nO_n⁺; M = Ni and Pd) showed that the Pt_nO_n⁺ structures have a similar tendency to Pd_nO_n⁺ rather than Ni_nO_n⁺.

Pt17 nanocluster electrocatalysts: preparation and origin of high oxygen reduction reaction activity

We recently found that [Pt₁₇(CO)₁₂(PPh₃)₈]^z (Pt = platinum; CO = carbon monoxide; PPh₃ = triphenylphosphine; z = 1+ or 2+) is a Pt nanocluster (Pt NC) that can be synthesized with atomic precision in air. The present study demonstrates that it is possible to prepare a Pt₁₇-supported carbon black (CB) catalyst (Pt₁₇/CB) with 2.1 times higher oxygen reduction reaction (ORR) activity than commercial Pt nanoparticles/CB by the adsorption of [Pt₁₇(CO)₁₂(PPh₃)₈]^z onto CB and subsequent calcination of the catalyst. Density functional theory calculation strongly suggests that the high ORR activity of Pt₁₇/CB originates from the surface Pt atoms that have an electronic structure appropriate for the progress of ORR. These results are expected to provide design guidelines for the fabrication of highly active ORR catalysts using Pt NCs with a diameter of about 1 nm and thereby enabling the use of reduced amounts of Pt in polymer electrolyte fuel cells.

Structure effects of Pt15 clusters for the oxygen reduction reaction: first-principles calculations

In the present work, the lowest energy structures and electronic properties of Pt₁₅ clusters are investigated using molecular dynamics simulations. The results showed that the most stable configuration is a capped pyramidal structure, which is 0.8 kal mol^-1 lower in energy than a layered structure previously reported [V. Kumar and Y. Kawazoe, Evolution of Atomic and Electronic Structure of Pt Clusters: Planar, Layered, Pyramidal, Cage, Cubic, and Octahedral Growth, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 205418.]. The result is further confirmed by using both the PW91/cc-pVDZ-PP and PBE/PW approaches including the other representative isomers for Pt₁₅. Due to the interesting structure arrangements found, we have investigated the catalytic activities for the oxygen reduction reaction. We found that the most stable Pt₁₅ clusters are plausible catalyts for the ORR according to their interaction with oxygen species, which is consistent with experiments of Pt clusters with atomicity below 20. The results of the structure, electronic, adsorption and vibrational properties of the clusters are provided.

Cytotoxic sub-nanometer aqueous platinum clusters as potential antitumoral agents

Ligand-free sub-nanometer metal clusters (MCs) of Pt, Ir, Rh, Au and Cu, are prepared here in neat water and used as extremely active (nM) antitumoral agents for HeLa and A2870 cells. The preparation just consists of adding the biocompatible polymer ethylene-vinyl alcohol (EVOH) to an aqueous solution of the corresponding metal salt, to give liters of a MC solution after filtration of the polymer. Since the MC solution is composed of just neat metal atoms and water, the intrinsic antitumoral activity of the different sub-nanometer metal clusters can now fairly be evaluated. Pt clusters show an IC₅₀ of 0.48 μM for HeLa and A2870 cancer cells, 23 times higher than that of cisplatin and 1000 times higher than that of Pt NPs, and this extremely high cytotoxicity also occurs for cisplatin-resistant (A2870 cis) cells, with a resistance factor of 1.4 (IC₅₀ = 0.68 μM). Rh and Ir clusters showed an IC₅₀ ∼ 1 μM. Combined experimental and computational studies support an enhanced internalization and cytotoxic activation.

Alloying at a Subnanoscale Maximizes the Synergistic Effect on the Electrocatalytic Hydrogen Evolution

Bonding dissimilar elements to provide synergistic effects is an effective way to improve the performance of metal catalysts. However, as the properties become more dissimilar, achieving synergistic effects effectively becomes more difficult due to phase separation. Here we describe a comprehensive study on how subnanoscale alloying is always effective for inter-elemental synergy. Thirty-six combinations of both bimetallic subnanoparticles (SNPs) and nanoparticles (NPs) were studied systematically using atomic-resolution imaging and catalyst benchmarking based on the hydrogen evolution reaction (HER). Results revealed that SNPs always produce greater synergistic effects than NPs, the greatest synergistic effect was found for the combination of Pt and Zr. The atomic-scale miscibility and the associated modulation of electronic states at the subnanoscale were much different from those at the nanoscale, which was observed by annular-dark-field scanning transmission electron microscopy (ADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively.

Gram-Scale Synthesis of Carbon-Supported Sub-5 nm PtNi Nanocrystals for Efficient Oxygen Reduction

The preparation of a high performance and durability with low-platinum (Pt) loading oxygen reduction catalysts remains a challenge for the practical application of fuel cells. Alloying Pt with a transition metal can greatly improve the activity and durability for oxygen reduction reaction (ORR). In this work, we present a one-pot wet-chemical strategy to controllably synthesize carbon supported sub-5 nm PtNi nanocrystals with a ~3% Pt loading. The as-prepared PtNi/C-200 catalyst with a Pt/Ni atomic ratio of 2:3 shows a high oxygen reduction activity of 0.66 A mgpt−1 and outstanding durability over 10,000 potential cycles in 0.1 M KOH in a half-cell condition. The PtNi/C-200 catalyst exhibits the highest ORR activity, with an onset potential (Eonset) of 0.98 V and a half-wave potential (E1/2) of 0.84 V. The mass activity and specific activity are 3.89 times and 9.16 times those of 5% commercial Pt/C. More importantly, this strategy can be applied to the gram-scale synthesis of high-efficiency electrocatalysts. As a result, this effective synthesis strategy has a significant meaning in practical applications of full cells.

Structures of Small Platinum Cluster Anions Ptn-: Experiment and Theory

The structures of platinum cluster anions Pt6--Pt13- have been investigated by trapped ion electron diffraction. Structures were assigned by comparing experimental and simulated scattering functions using candidate structures obtained by density functional theory computations, including spin-orbit coupling. We find a structural evolution from planar structures (Pt6-, Pt7-) and amorphous-like structures (Pt7--Pt9-) to structures based on distorted tetrahedra (Pt9--Pt11-). Finally, Pt12- and Pt13- are based on hcp fragments. While the structural parameters are well described by density functional theory computations for all clusters studied, the predicted lowest energy structure is found in the experiment only for Pt6-. For larger clusters, higher energy isomers are necessary to obtain a fit to the scattering data.

Controlled Synthesis of Au25 Superatom Using a Dendrimer Template

Superatoms are promising materials for their potential in elemental substitution and as new building blocks. Thus far, various synthesis methods of thiol-protected Au clusters including an Au₂₅ superatom have been investigated. However, previously reported methods were mainly depending on the thermodynamic stability of the aimed clusters. In this report, a synthesis method for thiol-protected Au clusters using a dendrimers template is proposed. In this method, the number of Au atoms was controlled by the stepwise complexation feature of a phenylazomethine dendrimer. Therefore, synthesis speed was increased compared with the case without the dendrimer template. Hybridization for the Au₂₅ superatoms was also achieved using the complexation control of metals.

Poly-phenylene jacketed tailor-made dendritic phenylazomethine ligand for nanoparticle synthesis

Synthesizing metal clusters with a specific number of atoms on a preparative scale for studying advanced properties is still a challenge. The dendrimer templated method is powerful for synthesizing size or atomicity controlled nanoparticles. However, not all atomicity is accessible with conventional dendrimers. A new tailor-made phenylazomethine dendrimer (DPA) with a limited number of coordination sites (n = 16) and a non-coordinating large poly-phenylene shell was designed to tackle this problem. The asymmetric dendron and adamantane core four substituted dendrimer (PPDPA16) were successfully synthesized. The coordination behavior confirmed the accumulation of 16 metal Lewis acids (RhCl₃, RuCl₃, and SnBr₂) to PPDPA16. After the reduction of the complex, low valent metal nanoparticles with controlled size were obtained. The tailor-made dendrimer is a promising approach to synthesize a variety of metal clusters with desired atomicity.

Relativistic Effects in Platinum Nanocluster Catalysis: A Statistical Ensemble-Based Analysis

Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt₇ subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.

Atomically precise structures of Pt2(S-Adam)4(PPh3)2 complexes and catalytic application in propane dehydrogenation

As a bridge between single metal atoms and metal nanoclusters, atomically precise metal complexes are of great significance for controlled synthesis and catalytic applications at the atomic level. Herein, novel Pt₂(S-Adam)₄(PPh₃)₂ complexes were prepared via the conventional synthetic methods of metal nanoclusters. The atomically precise crystal structures of the binuclear Pt complexes with three kinds of packing modes in a unit cell were determined by X-ray crystallography. The two Pt atoms are bridged by two S atoms of thiolates, constructing a rhombus on a plane. Moreover, the ultraviolet visible absorption spectra of Pt₂(S-Adam)₄(PPh₃)₂ complexes show an apparent absorption peak centered at 454 nm. Furthermore, the Pt complexes were used as precursors to prepare catalysts for non-oxidative propane dehydrogenation. The as-prepared Pt-based catalysts with a particle size of approximately 1 nm demonstrated a propane conversion of about 18% and significantly enhanced selectivity for propylene, up to 93%. Our work will be beneficial to the basic understanding of platinum complexes, as well as the improvement of the catalytic dehydrogenation of propane.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Enhanced oxygen reduction activity of size-selected platinum subnanocluster catalysts: Ptn (n = 3–9)

Ptn subnanoclusters (n = 3–9) on a carbon substrate exhibit 1.6–2.2 times higher activity than the standard Pt/C catalysts. EXAFS experiments and DFT calculations show plausible structures and energetics for reaction intermediates in the processes.

Unique Functions and Applications of Rigid Dendrimers Featuring Radial Aromatic Chains

Dendrimers, which are highly branched polymers and regarded as huge single molecules, are interesting substances from the aspect of not only polymer chemistry but also molecular chemistry. Various applications in materials science and life science have been investigated by taking advantage of the radially layered structures and intramolecular nanospaces of dendrimers. Most dendrimers have flexible structures that originate from their organic chains which contain many sp³-type atoms, while relatively rigid dendrimers composed only of sp²-type atoms have rarely been reported. It has been recently clarified that such rigid dendrimers exhibit a specific aromatic property not found in other materials. Dendritic phenylazomethines (DPAs), as one of the rigid dendrimers, have only sp²-type C and N atoms and possess a radially branched π-conjugation system in their own macromolecular chains. Such geometric and electronic structures heighten the electron density at the core of the dendrimer and induce an intramolecular potential gradient, which affords unique reactivities that lead to extraordinary functions. This unique property of the rigid dendrimers can be regarded as a new atypical electronic state based on radial aromatic chains not found in conventional aromatic compounds containing spherical aromaticity, Möbius aromaticity, metal aromaticity, and conductive polymers. Therefore, this as-yet-unknown characteristic is expected to contribute to the further development of fundamental and materials chemistry.In this Account, we highlight the rigid DPA dendrimers and their peculiar atomically precise and selective assembly behaviors that originate from the radial aromatic chains. One of the most noteworthy attainments based on the radial aromatic chains is the precise synthesis of a multimetallic multinuclear complex of a dendrimer containing a total of 13 elements. Next, we describe the electrochemical and catalytic functionalization of such multinuclear dendrimer complexes and the construction of supramolecular nanoarchitectures by the polymerization of DPAs. These complexes exhibit encapsulation-release switching of guests and additive-free catalytic ability similar to proteins and enzymes. Such selective and accurate control of the intramolecular assembly of guests and the intermolecular arrangement of hosts realized by the radial aromatic chains of dendrimers will enable supramolecular chemistry and biochemistry to be linked from a new aspect. In addition, the multimetallic multinuclear complexes of dendrimers afford a novel approach to precisely synthesize sub-nanoparticles with ultrasmall particle sizes (1 nm) that have been technically difficult to obtain by conventional nanotechnology. We discuss the method for the synthesis of these sub-nanoparticles with well-controlled atomicity and composition using DPA complexes as a template and recent advances to reveal their specific physical and chemical properties. These results suggest that the unique electronic states induced in such radial aromatics could play an important role in the development of next-generation chemistry.

Screening of transition metal doped copper clusters for CO2 activation

Activation of CO₂ is the first step towards its reduction to more useful chemicals. Here we systematically investigate the CO₂ activation mechanism on Cu₃X (X is a first-row transition metal atom) using density functional theory computations. The CO₂ adsorption energies and the activation mechanisms depend strongly on the selected dopant. The dopant electronegativity, the HOMO-LUMO gap and the overlap of the frontier molecular orbitals control the CO₂ dissociation efficiency. Our calculations reveal that early transition metal-doped (Sc, Ti, V) clusters exhibit a high CO₂ adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. Thus, early transition metal-doped copper clusters, particularly Cu₃Sc, may be efficient catalysts for the carbon capture and utilization process.

Simple and high-yield preparation of carbon-black-supported ∼1 nm platinum nanoclusters and their oxygen reduction reactivity

The improvement of oxygen reduction reaction (ORR) catalysts is essential before polymer electrolyte fuel cells can be used widely. To this end, we established a simple method for the size-selective synthesis of a series of ligand-protected platinum nanoclusters with ∼1 nm particle size (Pt_n NCs; n = ∼35, ∼51, and ∼66) and narrow size distribution (±∼4 Pt atoms) under atmospheric conditions. Using this method, each ligand-protected ∼1 nm Pt NC was obtained in a relatively high yield (nearly 80% for Pt_∼66). We succeeded in adsorbing each ligand-protected ∼1 nm Pt NC on carbon black (CB) and then removing most of the ligands from the surface of the Pt NCs via calcination while maintaining the original size. The obtained Pt_∼35/CB, Pt_∼51/CB, and Pt_∼66/CB exhibited ORR mass activities that were 1.6, 2.1, and 1.6 times higher, respectively, than that of commercial CB supported-Pt nanoparticles, and also display high durability.

Development of Highly Sensitive Raman Spectroscopy for Subnano and Single-Atom Detection

Direct detection and characterisation of small materials are fundamental challenges in analytical chemistry. A particle composed of dozens of metallic atoms, a so-called subnano-particle (SNP), and a single-atom catalyst (SAC) are ultimate analysis targets in terms of size, and the topic is now attracting increasing attention as innovative frontier materials in catalysis science. However, characterisation techniques for the SNP and SAC adsorbed on substrates requires sophisticated and large-scale analytical facilities. Here we demonstrate the development of an ultrasensitive, laboratory-scale, vibrational spectroscopic technique to characterise SNPs and SACs. The fine design of nano-spatial local enhancement fields generated by the introduction of anisotropic stellate-shaped signal amplifiers expands the accessibility of small targets on substrates into evanescent electromagnetic fields, achieving not only the detection of isolated small targets but also revealing the effects of intermolecular/interatomic interactions within the subnano configuration under actual experimental conditions. Such a development of “in situ subnano spectroscopy” will facilitate a comprehensive understanding of subnano and SAC science.

Role of atomicity in the oxygen reduction reaction activity of platinum sub nanometer clusters: A global optimization study

Metal nanoclusters are an important class of materials for catalytic applications. Sub nanometer clusters are relatively less explored for their catalytic activity on account of undercoordinated surface structure. Taking this into account, we studied platinum-based sub nanometer clusters for their catalytic activity for oxygen reduction reaction (ORR). A comprehensive analysis with global optimization is carried out for structural prediction of the platinum clusters. The energetic and electronic properties of interactions of clusters with reaction intermediates are investigated. The role of structural sensitivity in the dynamics of clusters is unraveled, and unique intermediate specific interactions are identified. ORR energetics is examined, and exceptional activity for sub nanometer clusters are observed. An inverse size versus activity relationship is identified, challenging the conventional trends followed by larger nanoclusters. The principal role of atomicity in governing the catalytic activity of nanoclusters is illustrated. The structural norms governing the sub nanometer cluster activity are shown to be markedly different from larger nanoclusters.

Metal carbonyl clusters of groups 8-10: synthesis and catalysis

In this review article, we discuss advances in the chemistry of metal carbonyl clusters (MCCs) spanning the last three decades, with an emphasis on the more recent reports and those involving groups 8-10 elements. Synthetic methods have advanced and been refined, leading to higher-nuclearity clusters and a wider array of structures and nuclearities. Our understanding of the electronic structure in MCCs has advanced to a point where molecular chemistry tools and other advanced tools can probe their properties at a level of detail that surpasses that possible with other nanomaterials and solid-state materials. MCCs therefore advance our understanding of structure-property-reactivity correlations in other higher-nuclearity materials. With respect to catalysis, this article focuses only on homogeneous applications, but it includes both thermally and electrochemically driven catalysis. Applications in thermally driven catalysis have found success where the reaction conditions stabilise the compounds toward loss of CO. In more recent years, MCCs, which exhibit delocalised bonding and possess many electron-withdrawing CO ligands, have emerged as very stable and effective for reductive electrocatalysis reactions since reduction often strengthens M-C(O) bonds and since room-temperature reaction conditions are sufficient for driving the electrocatalysis.

Low-Temperature H2 Reduction of Copper Oxide Subnanoparticles

Subnanoparticles (SNPs) with sizes of approximately 1 nm are attractive for enhancing the catalytic performance of transition metals and their oxides. Such SNPs are of particular interest as redox-active catalysts in selective oxidation reactions. However, the electronic states and oxophilicity of copper oxide SNPs are still a subject of debate in terms of their redox properties during oxidation reactions for hydrocarbons. In this work, in situ X-ray absorption fine structure (XAFS) measurements of Cu₂₈ O_x SNPs, which were prepared by using a dendritic phenylazomethine template, during temperature-programmed reduction (TPR) with H₂ achieved lowering of the temperature (T₅₀ =138 °C) reported thus far for the Cu^II →Cu^I reduction reaction because of Cu-O bond elongation in the ultrasmall copper oxide particles.

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.

Selective Hydroperoxygenation of Olefins Realized by a Coinage Multimetallic 1-Nanometer Catalyst

The science of particles on a sub-nanometer (ca. 1 nm) scale has attracted worldwide attention. However, it has remained unexplored because of the technical difficulty in the precise synthesis of sub-nanoparticles (SNPs). We recently developed the "atom-hybridization method (AHM)" for the precise synthesis of SNPs by using a suitably designed macromolecule as a template. We have now investigated the chemical reactivity of alloy SNPs obtained by the AHM. Focusing on the coinage metal elements, we systematically evaluated the oxidation reaction of an olefin catalyzed by these SNPs. The SNPs showed high catalytic performance even under milder conditions than those used with conventional catalysts. Additionally, the hybridization of multiple elements enhanced the turnover frequency and the selectivity for the formation of the hydroperoxide derivative. We discuss the unique quantum-sized catalysts providing generally unstable hydroperoxides from the viewpoint of the miniaturization and hybridization of materials.

Visible-Light Acceleration of H2 Evolution from Aqueous Solutions of Inorganic Hydrides Catalyzed by Gold-Transition-Metal Nanoalloys

Production of hydrogen (H₂) upon hydrolysis of inorganic hydrides potentially is a key step in green energy production. We find that visible-light irradiation of aqueous solutions of ammonia-borane (AB) or NaBH₄ containing "click"-dendrimer-stabilized alloyed nanocatalysts composed of nanogold and another late transition-metal nanoparticle (LTMNP) highly enhances catalytic activity for H₂ generation while also inducing alloy to Au core@M shell nanocatalyst restructuration. In terms of visible-light-induced acceleration of H₂ production from both AB and NaBH₄, the Au₁Ru₁ alloy catalysts show the most significant light-boosting effect. Au-Rh and Au-PtNPs are also remarkable with total H₂ release time from AB and NaBH₄ down to 1.3 min at 25 °C (AuRh), 3 times less than in the dark, and Co is the best earth-abundant metal alloyed with nanogold. This boosting effect is explained by the transfer of plasmon-induced hot electron from the Au atoms to the LTMNP atoms facilitating water O-H oxidative addition on the LTMNP surface, as shown by the large primary kinetic isotope effect k_H/k_D upon using D₂O obtained for both AB and NaBH₄. The second simultaneous and progressive effect of visible-light irradiation during these reactions, alloy to Au core@M shell restructuration, enhances the catalytic activity in the recycling, because, in the resulting Au core@M shell, the surface metal (such as Ru) is much more active than the original Au-containing alloy surface in dark reactions. There is no light effect on the rate of hydrogen production for the recycled nanocatalyst because of the absence of Au on the NP surface, but it is still very efficient in hydrogen release during four cycles because of the initial light-induced restructuration, although it is slightly less efficient than the original nanoalloy in the presence of light. The dendritic triazole coordination on each LTMNP surface appears to play a key role in these remarkable light-induced processes.

Synthetic strategies of supported atomic clusters for heterogeneous catalysis

Supported atomic clusters with uniform metal sites and definite low-nuclearity are intermediate states between single-atom catalysts (SACs) and nanoparticles in size. Benefiting from the presence of metal–metal bonds, supported atomic clusters can trigger synergistic effects among every metal atom, which contributes to achieving unique catalytic properties different from SACs and nanoparticles. However, the scalable and precise synthesis and atomic-level insights into the structure–properties relationship of supported atomic clusters is a great challenge. This perspective presents the latest progress of the synthesis of supported atomic clusters, highlights how the structure affects catalytic properties, and discusses the limitations as well as prospects.

Supported atomic clusters with precise nuclearity are intermediate states between single-atom catalysts and nanoparticles in size. Here the authors summarize and discuss synthetic strategies of supported atomic clusters with unique catalytic properties for heterogeneous reactions.

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Tuning the ORR activity of Pt-based Ti2CO2 MXenes by varying the atomic cluster size and doping with metals

The rational design of ideal catalysts for the oxygen reduction reaction (ORR) is of great significance for solving the electrocatalytic potential problems in proton exchange membrane fuel cells (PEMFCs). Pt_n (n = 1-4) and Pt₃Au alloy subnanoclusters supported on a defective Ti₂CO₂ monolayer with oxygen vacancies (denoted as v-Ti₂CO₂) are simulated by using density functional theory to investigate their ORR performance. The geometries, energetics, and electronic properties of the different systems are analyzed. It is found that the supported Pt₃Au alloy subnanocluster possesses the best ORR activity. The underlying mechanisms of the improved ORR activity originates from the moderate hybridization between the O 2p and the 5d orbitals of Au and Pt according to the density of states analysis. Our study suggests a facile route for designing low-cost MXene-based electrocatalysts by alloying transition metals with Pt catalysts, which may stimulate realization of suitable alternative catalysts for ORR catalysis.

Enhanced Catalytic Performance of Subnano Copper Oxide Particles

Subnanoparticles (SNPs) with ultrasmall particle sizes (<1 nm) have potential to provide catalytic activity that is superior to that of nanoparticles. Size-controlled Cu_nO_x (n = 12, 28, and 60) materials supported on zirconia, prepared using a dendritic macromolecular reactor, exhibited increased ionicity of the Cu-O bonds with a decrease in size of the particles, which was suggested on the basis of the peak intensity in the Cu 2p_3/2 region. The polarization of the Cu-O bonds in the ultrasmall copper oxides provides size-dependent catalytic activity in aerobic oxidation of the CH₃ group bonded with aromatic rings. The smallest Cu₁₂O_x materials achieved an excellent large turnover number (TON = 40 206) without any significant deactivation.

Photo/electrocatalysis and photosensitization using metal nanoclusters for green energy and medical applications

Owing to the rapidly increasing demand for sustainable technologies in fields such as energy, environmental science, and medicine, nanomaterial-based photo/electrocatalysis has received increasing attention. Recently, synthetic innovations have allowed the fabrication of atomically precise metal nanoclusters (NCs). These NCs show potential for green energy and medical applications. The present article primarily focuses on evaluation of the recent developments in the photo/electrocatalytic and photosensitizing characteristics of metal and alloy NCs. The review comprises two sections: (i) photo/electrocatalysis for green energy and (ii) photosensitization for biomedical therapy applications. Finally, the challenges associated with the use of metal NCs are presented on the basis of current developments.

Tin oxide subnanoparticles: a precisely-controlled synthesis, subnano-detection for their detailed characterisation and applications

Subnanometric metal particles exhibit anomalous chemical activity, suggesting innovative applications as next-generation materials. However, a precise synthesis and detailed characterisation of these subnano-materials remain a major challenge. Here we summarise recent works on the synthesis of size-controlled tin (Sn) oxide subnanoparticles (SNPs) using the dendrimer template method, and on their detailed characterisation. Size-controlled Sn oxide SNPs (Sn12, Sn28 and Sn60) have been synthesised and they showed a size-dependent compositional change containing not only stable Sn(iv) states but also metastable Sn(ii) states so as to form subnano-scaled particle shapes. Detailed vibrational characterisation of SNPs was achieved by employing subnano-sensitive Raman spectroscopy for spectroscopic characterisation. Combined with density functional theory studies, the inherent subnano-structures of Sn oxide SNPs have been elucidated for the first time. Furthermore, the size-dependent activity of Sn oxide SNPs upon CO oxidation was rationally explained from the simulated structure of Sn oxide SNPs. A detailed understanding of the chemical and physical nature of subnano-materials facilitates the rational design of SNPs for practical applications such as catalysis, biosensors, and electronics.

Ultrahigh sensitive Raman spectroscopy for subnanoscience: Direct observation of tin oxide clusters

Subnanometric metal clusters exhibit anomalous catalytic activity, suggesting innovative applications as next-generation materials, although identifying and characterizing these subnanomaterials in atomic detail remains a substantial challenge because of the severely weak signal intensity for the conventional analytical methods. Here, we report a subnanosensitive vibrational technique established based on the surface-enhanced Raman spectroscopy, demonstrating the first-ever detailed vibrational characterization of subnanomaterials. Furthermore, combining with density functional theory calculations, we reveal that inherent surface structures of the tin oxide subnanoclusters determine the size-specific spectral and catalytic characteristics of these clusters. The high-sensitivity characterization methodology elaborated here can provide a comprehensive understanding of the chemical and structural natures of subnanomaterials, which facilitate the rational design of subnanomaterials on the atomic scale for practical applications, such as in catalysts, biosensors, and electronics.

Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

Single-size platinum Pt6 subnanoclusters exhibit superior mass-specific and surface-specific activities for the oxygen reduction reaction. The enhanced activity is attributed to polarized electron distributions based on rigorous structure characterization by X-ray absorption fine structure spectroscopy and density functional theory.

Size, Composition, and Support-Doping Effects on Oxygen Reduction Activity of Platinum-Alloy and on Non-platinum Metal-Decorated-Graphene Nanocatalysts

Recent investigations reported in the open literature concerning the functionalization of graphene as a support material for transition metal nanoparticle catalysts have examined isolated systems for their potential Oxygen Reduction Reaction (ORR) activity. In this work we present results which characterize the ability to use functionalized graphene (via dopants B, N) to upshift and downshift the adsorption energy of mono-atomic oxygen, O* (the ORR activity descriptor on ORR Volcano Plots), for various compositions of 4-atom, 7-atom, and 19-atom sub-nanometer binary alloy/intermetallic transition metal nanoparticle catalysts on graphene (TMNP-MDG). Our results show several important and interesting features: (1) that the combination of geometric and electronic effects makes development of simple linear mixing rules for size/composition difficult; (2) that the transition from 4- to 7- to 19-atom TMNP on MDG has pronounced effects on ORR activity for all compositions; (3) that the use of B and N as dopants to modulate the graphene-TMNP electronic structure interaction can cause shifts in the oxygen adsorption energy of 0.5 eV or more; (4) that it might be possible to make specific doped-graphene-Ni_xCu_y TMNP systems which fall close to the Volcano Peak for ORR. Our results point to systems which should be investigated experimentally and may improve the viability of future fuel cell or other ORR applications, and provide new paths for future investigations of more detail for TMNP-MDG screening.

Ultrafast, Controllable Synthesis of Sub-Nano Metallic Clusters through Defect Engineering

Supported metallic nanoclusters (NCs, < 2 nm) are of great interests in various catalytic reactions with enhanced activities and selectivities, yet it is still challenging to efficiently and controllably synthesize ultrasmall NCs with a high-dispersal density. Here we report the in situ synthesis of surfactant-free, ultrasmall, and uniform NCs via a rapid thermal shock on defective substrates. This is achieved by using high-temperature synthesis with extremely fast kinetics while limiting the synthesis time down to milliseconds (e.g., ∼1800 K for 55 ms) to avoid aggregation. Through defect engineering and optimized loading, the particle size can be robustly tuned from >50 nm nanoparticles to <1 nm uniform NCs with a high-dispersal density. We demonstrate that the ultrasmall NCs exhibit drastically improved activities for catalytic CO oxidation as compared to their nanoparticulated counterparts. In addition, the reported method shows generality in synthesizing most metallic NCs (e.g., Pt, Ru, Ir, Ni) in an extremely facile and efficient manner. The ultrafast and controllable synthesis of uniform, high-density, and size-controllable NCs paves the way for the utilization and nanomanufacturing of NCs for a range of catalytic reactions.