Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

Quantitative analysis of air-oxidation reactions of thiolate-protected gold nanoclusters

The interaction of dioxygen (O2) with inorganic nanomaterials is one of the most essential steps to understanding the reaction mechanism of O2-related reactions. However, quantitative analyses for O2-binding processes and subsequent oxidation reactions on the surface are still elusive, whereas the reaction of O2 with molecules such as transition metal complexes has been widely explored. Herein, we have quantitatively evaluated reaction processes of air-oxidation reactions of atomically precise thiolate-protected Au25 nanoclusters ([Au25(SR)18]-) as a model of O2 activation by inorganic nanomaterials. Kinetic analyses on the air-oxidation reaction of [Au25(SR)18]- revealed a controlling factor for O2-activation processes, which could be finely tunable by the protecting thiolate ligands.

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.

Advances in Cu nanocluster catalyst design: recent progress and promising applications

The quest for cleaner pathways to the production of fuels and chemicals from non-fossil feedstock, efficient transformation of raw materials to value-added chemicals under mild conditions, and control over the activity and selectivity of chemical processes are driving the state-of-the-art approaches to the construction and precise chemical modification of sustainable nanocatalysts. As a burgeoning category of atomically precise noble metal nanoclusters, copper nanoclusters (Cu NCs) benefitting from their exclusive structural architecture, ingenious designability of active sites and high surface-to-volume ratio qualify as potential rationally-designed catalysts. In this Minireview, we present a detailed coverage of the optimal design strategies and controlled synthesis of Cu NC catalysts with a focus on tuning of active sites at the atomic level, the implications of cluster size, shape and structure, the ligands and heteroatom doping on catalytic activity, and reaction scope ranging from chemical catalysis to emerging photocatalysis and electrocatalysis.

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.

Bridging the gas and condensed phases for metal-atom encapsulating silicon- and germanium-cage superatoms: electrical properties of assembled superatoms

With the development of nanocluster (NC) synthesis methods in the gas phase, atomically precise NCs composed of a finite number of metal and semiconductor atoms have emerged. NCs are expected to be the smallest units for nanomaterials with various functions, such as catalysts, optoelectronic materials, and electromagnetic devices. The exploration of a stable NC called a magic number NC has revealed a couple of important factors, such as a highly symmetric geometric structure and an electronic shell closure, and a magic number behavior is often enhanced by mixing additional elements. A synergetic effect between geometric and electronic structures leads to the formation of chemically robust NC units called superatoms (SAs), which act as individual units assembled as thin films. The agglomeration of non-ligated bare SAs is desirable in fabricating the assembled SAs associated with intrinsic SA nature. The recent development of an intensive pulsed magnetron sputtering method opens up the scalable synthesis of SAs in the gas phase, enabling the fabrication of SA assembly coupled with the non-destructive deposition of a soft-landing technique. This perspective describes our recent progress in the investigation of the formation of binary cage SA (BCSA) assembled thin films composed of metal-atom encapsulating silicon-cage SAs (M@Si₁₆) and germanium-cage SAs (M@Ge₁₆), with a focus on their electrical properties associated with a conduction mechanism toward the development of new functional nanoscale materials.

Exploring the Potential Energy Surface of Pt6 Sub-Nano Clusters Deposited over Graphene

Catalytic systems based on sub-nanoclusters deposited over different supports are promising for very relevant chemical transformations such as many electrocatalytic processes as the ORR. These systems have been demonstrated to be very fluxional, as they are able to change shape and interconvert between each other either alone or in the presence of adsorbates. In addition, an accurate representation of their catalytic activity requires the consideration of ensemble effects and not a single structure alone. In this sense, a reliable theoretical methodology should assure an accurate and extensive exploration of the potential energy surface to include all the relevant structures and with correct relative energies. In this context, we applied DFT in conjunction with global optimization techniques to obtain and analyze the characteristics of the many local minima of Pt₆ sub-nanoclusters over a carbon-based support (graphene)-a system with electrocatalytic relevance. We also analyzed the magnetism and the charge transfer between the clusters and the support and paid special attention to the dependence of dispersion effects on the ensemble characteristics. We found that the ensembles computed with and without dispersion corrections are qualitatively similar, especially for the lowest-in-energy clusters, which we attribute to a (mainly) covalent binding to the surface. However, there are some significant variations in the relative stability of some clusters, which would significantly affect their population in the ensemble composition.

Atomically Precise Platinum Carbonyl Nanoclusters: Synthesis, Total Structure, and Electrochemical Investigation of [Pt27(CO)31]4- Displaying a Defective Structure

The molecular Pt nanocluster [Pt27(CO)31]4- (14-) was obtained by thermal decomposition of [Pt15(CO)30]2- in tetrahydrofuran under a H2 atmosphere. The reaction of 14- with increasing amounts of HBF4·Et2O afforded the previously reported [Pt26(CO)32]2- (32-) and [Pt26(CO)32]- (3-). The new nanocluster 14- was characterized by IR and UV-visible spectroscopy, single-crystal X-ray diffraction, direct-current superconducting quantum interference device magnetometry, cyclic voltammetry, IR spectroelectrochemistry (IR SEC), and electrochemical impedance spectroscopy. The cluster displays a cubic-close-packed Pt27 framework generated by the overlapping of four ABCA layers, composed of 3, 7, 11, and 6 atoms, respectively, that encapsulates a fully interstitial Pt4 tetrahedron. One Pt atom is missing within layer 3, and this defect (vacancy) generates local deformations within layers 2 and 3. These local deformations tend to repair the defect (missing atom) and increase the number of Pt-Pt bonding contacts, minimizing the total energy. The cluster 14- is perfectly diamagnetic and displays a rich electrochemical behavior. Indeed, six different oxidation states have been characterized by IR SEC, unraveling the series of 1n- (n = 3-8) isostructural nanoclusters. Computational studies have been carried out to further support the interpretation of the experimental data.

Research Progress of Highly Efficient Noble Metal Catalysts for the Oxidation of 5-Hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) is considered to be one of the most pivotal multifunctional biomass platform chemicals. This Review discusses recent advances in catalytic oxidation of HMF towards high-value products. The reaction mechanism of different noble metals and the path of HMF oxidation to high-value products have been deeply investigated in the noble metal catalytic system. The reaction mechanisms of different noble metals and HMF conversion paths were compared in detail. Moreover, the factors affecting the performance of different noble metal catalysts were summarized. Finally, effective strategies were put forward to improve the catalytic performance of noble metal catalysts. The purpose is to provide a valuable reference for the academic research on the preparation of oxidation products from biomass-based HMF and the industrial application of noble metal catalysts.

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.

Design and Performance of a Soft-Landing Instrument for Fragment Ion Deposition

We report the development of a new high-flux electrospray ionization-based instrument for soft landing of mass-selected fragment ions onto surfaces. Collision-induced dissociation is performed in a collision cell positioned after the dual electrodynamic ion funnel assembly. The high duty cycle of the instrument enables high-coverage deposition of mass-selected fragment ions onto surfaces at a defined kinetic energy. This capability facilitates the investigation of the reactivity of gaseous fragment ions in the condensed phase. We demonstrate that the observed reactions of deposited fragment ions are dependent on the structure of the ion and the composition of either ionic or neutral species codeposited onto a surface. The newly developed instrument provides access to high-purity ion fragments as building blocks for the preparation of unique ionic layers.

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.

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.

Platinum nanocluster catalysts supported on Marimo carbon via scalable dry deposition synthesis

The development of efficient fuel cells greatly promotes reducing the consumption of fossil energy, and it is crucial to enhance the platinum (Pt) catalytic activity by optimizing both the nanoparticle size and support effect.