Isomerizations of a Pt4 cluster revealed by spatiotemporal microscopic analysis

Vibrational wave-packet dynamics of the silver pentamer probed by femtosecond NeNePo spectroscopy

Vibrational wave-packet dynamics on the ground electronic state of the neutral silver pentamer (Ag5) are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the 2A1 state of Ag5via photodetachment of mass-selected, cryogenically cooled Ag5- anions using a fs pump pulse. The temporal evolution of the vibrational wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag5+. Frequency analysis of the fs NeNePo transients for pump-probe delay times from 0.2 to 8 ps reveals three primary beating frequencies at 157 cm-1, 101 cm-1 and 56 cm-1 as well as four weaker features. A comparison of these experimentally obtained beating frequencies to harmonic normal mode frequencies calculated from electronic structure calculations confirms that Ag5 in the gas phase adopts a planar trapezoidal geometry, similar to that previously observed in solid argon. The dependence of the ionization yield on the laser polarization indicates a s-d wave electron photodetachment from a 'p-type' occupied molecular orbital of Ag5. Franck-Condon analysis shows that both processes, photodetachment and subsequent photoionization determine the beating frequencies probed in the time-dependent cation yield. The present study extends the applicability of fs NeNePo spectroscopy to characterize the vibrational spectra in the far-IR frequency range in the absence of perturbations from a medium or a messenger atom to mass-selected neutral metal clusters with more than three atoms in the ground electronic states.

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.

Free-Energy Landscape and Isomerization Rates of Au4 Clusters at Finite Temperatures

In metallic nanoparticles, the geometry of atomic positions controls the particle's electronic band structure, polarizability, and catalytic properties. Analyzing the structural properties is a complex problem; the structure of an assembled cluster changes from moment to moment due to thermal fluctuations. Conventional structural analyses based on spectroscopy or diffraction cannot determine the instantaneous structure exactly and can merely provide an averaged structure. Molecular simulations offer an opportunity to examine the assembly and evolution of metallic clusters, as the preferred assemblies and conformations can easily be visualized and explored. Here, we utilize the adaptive biasing force algorithm applied to first-principles molecular dynamics to demonstrate the exploration of a relatively simple system, which permits a comprehensive study of the small metal cluster Au₄ in both neutral and charged configurations. Our simulation work offers a quantitative understanding of these clusters' dynamic structure, which is significant for single-site catalytic reactions on metal clusters and provides a starting point for a detailed quantitative understanding of more complex pure metal and alloy clusters' dynamic properties.

Dynamic hetero-metallic bondings visualized by sequential atom imaging

Traditionally, chemistry has been developed to obtain thermodynamically stable and isolable compounds such as molecules and solids by chemical reactions. However, recent developments in computational chemistry have placed increased importance on studying the dynamic assembly and disassembly of atoms and molecules formed in situ. This study directly visualizes the formation and dissociation dynamics of labile dimers and trimers at atomic resolution with elemental identification. The video recordings of many homo- and hetero-metallic dimers are carried out by combining scanning transmission electron microscopy (STEM) with elemental identification based on the Z-contrast principle. Even short-lived molecules with low probability of existence such as AuAg, AgCu, and AuAgCu are directly visualized as a result of identifying moving atoms at low electron doses.

The dynamic assembly and disassembly of atoms and molecules is challenging to characterize in real time, with atomic resolution and elemental identification. Here, the authors report direct observation of more than twenty homo and hetero-metallic compounds, including labile Ag-Cu dimers and Au-Ag-Cu trimers.

Structural dynamics of Ru clusters during nitrogen dissociation in ammonia synthesis

The dynamic evolution of catalyst structures greatly influences the reactivity, especially sub-nanometer clusters, exhibiting complex configurational fluctuation. In the present work, we study the structural dynamics of a Ru₁₉ cluster during the dissociation of N₂ and calculate the reaction free energies using ab initio molecular dynamics (AIMD). Our AIMD calculation predicts a peak-shaped reaction entropy curve due to the adsorption-induced phase transition of the Ru₁₉ cluster. The low melting points of sub-nanometer clusters make it possible to activate N₂ at low temperatures. This work demonstrates that the dynamic changes of cluster structures have a non-negligible effect on reaction free energy and offer an opportunity for achieving ammonia synthesis under mild conditions.

Unraveling the Nature of Extraframework Catalytic Ensembles in Zeolites: Flexibility and Dynamics of the Copper-Oxo Trimers in Mordenite

Extraframework cations define the chemical versatility of zeolite catalysts. Addressing their structural complexity and dynamic behavior represents one of the main fundamental challenges in the field. Herein, we present a computational approach for the identification and analysis of the accessible pool of intrazeolite extraframework complexes with a Cu/MOR catalyst as an industrially important model system. We employ ab initio molecular dynamics for capturing the ensemble of reactive isomers with the [Cu₃O₃]²⁺ stoichiometry confined in the mordenite channels. The high structural diversity of the generated isomers was ensured by concentrating the kinetic energy along the low-curvature directions of the potential energy surface (PES). Geometrically distinct [Cu₃O₃]²⁺ complexes were identified via a series of clustering procedures ensuring that one structure of each local minima is retained. The proposed procedure has resulted in a set of previously unknown peroxo-complexes, which are >50 kJ/mol more stable than the recently hypothesized chair-shaped structure. Our analysis demonstrates that the most stable peroxo-containing clusters can be formed under operando conditions from molecular oxygen and the Cu₃O unit, similar to that in methane monooxygenase (MMO) enzymes.

Strong response of Pt clusters to the environment and conditions, formation of metastable states, and simple methods to trace the reversible changes

Subnanometric metal particles, the so-called "clusters", are known to be responsive to their surroundings, but the detection of occurring changes, understanding the causes, and predicting the consequences are still extremely difficult for such small particles. Our study was aimed at estimating the potential of adsorption-based methods for these purposes. Using carbon monoxide as a probing molecule, which readily adsorbs on both bare and H-covered Pt surface, we have probed the adsorption properties of highly dispersed Pt/γ-Al₂O₃ samples after treatments under different atmospheres and temperatures (H₂ or inert gas, 25-500 °C). The combined results of CO-chemisorption measurements, CO TPD, CO TPO, H₂-by-CO displacement, and H₂ TPD suggest that the system shuttles between two states: one with oxygen vacancies in the support and the other one with redox-active oxygen near the Pt clusters. These extreme states can be reversibly created and deleted, giving rise to innumerable intermediate structures that differ in the amount, binding strength, and/or reactivity of adsorbed species. Two adsorbates could act cooperatively, resulting in hydrogen spillover onto the support and making the adsorbate-metal-support interactions even more complex. Implications for better understanding the dynamic behavior of oxide-supported clusters and nanoparticles are discussed.

Metal atom-guided conformational analysis of single polynuclear coordination molecules

Microscopic observation of single molecules is a rapidly expanding field in chemistry and differs from conventional characterization techniques that require a large number of molecules. One of such form of single-molecule microscopy is high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which is especially suitable for coordination compounds because of its atomic number-dependent contrast. However, to date, single-molecule observations using HAADF-STEM has limited to simple planar molecules. In the present study, we demonstrate a direct structural investigation of nonplanar dendronized polynuclear Ir complexes with subnanometer resolution using Ir as an atomic label. Decreasing the electron dose to the dendrimer complexes is critical for the single-molecule observation. A comparison with simulated STEM images of conformational isomers is performed to determine the most plausible conformation. Our results enlarge the potential of electron microscopic observation to realize structural analysis of coordination macromolecules, which has been impossible with conventional methods.

Highly Dispersed Molybdenum Oxycarbide Clusters Supported on Multilayer Graphene for the Selective Reduction of Carbon Dioxide

Molybdenum oxycarbide clusters are novel nanomaterials that exhibit attractive catalytic activity; however, the methods for their production are currently very restrictive. This work represents a new strategy for the creation of near-subnanometer size molybdenum oxycarbide clusters on multilayer graphene. To adsorb Mo-based polyoxometalates of the type [PMo₁₂ O₄₀ ]^3- as a precursor for Mo oxycarbide clusters, the novel tripodal-phenyl cation N,N,N-tri(4-phenylbutyl)-N-methylammonium ([TPBMA]⁺ ) is synthesized. [TPBMA]⁺ exhibits superior adsorption on multilayer graphene compared to commercially available cations such as tetrabutylammonium ([nBu₄ N]⁺ ) and tetraphenylphosphonium ([PPh₄ ]⁺ ). Using [TPBMA]⁺ as an anchor, highly dispersed precursor clusters (diameter: 1.0 ± 0.2 nm) supported on multilayer graphene are obtained, as confirmed by high-resolution scanning transmission electron microscopy. Remarkably, this new material achieves the catalytic reduction of CO₂ to selectively produce CO (≈99.9%) via the reverse water-gas-shift reaction, by applying carbothermal hydrogen reduction to generate Mo oxycarbide clusters in situ.

Quantum Materials Exploration by Sequential Screening Technique of Heteroatomicity

Subnanoparticles (SNPs) exhibit unique properties and functions due to their extremely small particle sizes which extend into the quantum scale. Although the synthesis of SNPs requiring precise control of atomicity and composition has not been accomplished, we recently developed an atom-hybridization method (AHM) that realizes such atomic-level control using a macromolecular template. As a next step in the quest for innovative quantum materials, the practical creation of functional subnanomaterials will become a central subject. In this study, we established a new screening technique for functional SNPs by focusing on the simple indium-tin binary system with sequential compositions using the latest AHM. As a result, it was revealed that a thermodynamically unstable indium species was produced only at a certain composition leading to a durable luminescent function. Such a phenomenon in subnanosized substances will play an important role in the development of the as-yet-unknown field of quantum materials.

Subnano-transformation of molybdenum carbide to oxycarbide

Ultrasmall particles exhibit structures and/or properties that are different from those of the corresponding bulk materials; in this context especially ultrasmall precious-metal particles have been extensively investigated. In this study, we targeted the transition base-metal Mo and succeeded in systematically producing Mo oxycarbide/carbide particles with diameters of 1.7 ± 0.7, 1.4 ± 0.5, 1.3 ± 0.4, 1.2 ± 0.3, 1.0 ± 0.3, and 0.8 ± 0.2 nm on a carbon support using the carbothermal hydrogen reduction method at 773 K and a diphenylazomethine-type dendrimer as a template. The formation and properties of the particles were confirmed using X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, and X-ray absorption fine structure (XAFS) studies. We found that Mo particles with a diameter of 1.3 nm or greater formed carbides such as β'-Mo₂C, whereas smaller particles formed oxycarbides, indicating a size-dependent transformation in the phase or composition of the particles. Thus, this work demonstrated a new concept, subnano-transformation, which would be a new class of phase transformation based on the concept of the size dependence in such an ultrasmall scale. In addition, the movement of Mo atoms within a cluster and on the fringes of a nanoparticle was also demonstrated during continuous time-course high-resolution HAADF-STEM observation.

Superatomic Gallium Clusters in Dendrimers: Unique Rigidity and Reactivity Depending on their Atomicity

Superatoms have been investigated due to their possible substitution for other elements. The solution-phase synthesis of superatoms has attracted attention to realize the availability of superatoms. However, the previous approach is basically limited to the formation of a single cluster. Here, superatoms are investigated and the number of valence electrons in these superatoms is changed by designing the number of gallium atoms present. Based on the dendrimer template method, clusters consisting of 3, 12, 13, and other numbers of atoms have been synthesized. The halogen-like superatomic nature of Ga₁₃ is structurally and electrochemically observed as completely different to the other clusters. The gallium clusters of 13 and 3 atoms, which can fill the 2P and 1P superatomic orbitals, respectively, exhibit different reactivities. The 3-atom gallium cluster is suggested as being reduced to Ga₃ H₂ ^- due to the lower shift of energy levels in the unoccupied orbitals. The results for these gallium clusters provide candidates for superatoms.

Ensembles of Metastable States Govern Heterogeneous Catalysis on Dynamic Interfaces

Heterogeneous catalysis is at the heart of the chemical industry. Being able to tune and design efficient catalysts for processes of interest is of the utmost importance, and for this, a molecular-level understanding of heterogeneous catalysts is the first step and indeed a prime focus of modern catalysis research. For a long time, the single most thermodynamically stable structure of the catalytic interface attained under the reaction conditions had been envisioned as the reactive phase. However, some catalytic interfaces continue to undergo structural dynamics in the steady state, triggered by high temperatures and pressures and binding and changing reagents. Among particularly dynamic interfaces are such widely used catalysts as crystalline and amorphous surfaced supporting (sub)nanometallic clusters. Recently, it became clear that this dynamic fluxionality causes the supported clusters to populate many distinct structural and stoichiometric states under catalytic conditions. Hence, the catalytic interface should be viewed as an evolving statistical ensemble of many structures (rather than one structure). Every member in the ensemble contributes to the properties of the catalyst differently, in proportion to its probability of being populated. This new notion flips the established paradigm and calls for a new theory, new modeling approaches, operando measurements, and updated design strategies. The statistical ensemble nature of surface-supported subnanocluster catalysts can be exemplified by oxide-supported and adsorbate-covered Pt, Pd, Cu, and CuPd clusters, which are catalytic toward oxidative and nonoxidative dehydrogenation. They have access to a variety of 3D and quasi-2D shapes. The compositions of their thermal ensembles are dependent on the cluster size, leading to size-specific catalytic activities and the famous "every atom counts" phenomenon. The support and adsorbates affect catalyst structures, and the state of the reacting species causes the ensemble to change in every reaction intermediate. The most stable member of the ensemble dominates the thermodynamic properties of the corresponding intermediate, whereas the kinetics can be determined by more active but less populated metastable catalyst states, and that suggests that many earlier studies might have overlooked the actual active sites. Both effects depend on the relative time scales of catalyst restructuring and reaction dynamics. The catalyst may routinely operate off-equilibrium. Ensemble phenomena lead to surprising exceptions from established rules of catalysis, such as scaling relations and Arrhenius behavior. Catalyst deactivation is also an ensemble property, and its extent of mitigation can be predicted through the new paradigm. These findings were enabled by advances in theory, such as global optimization and subsequent utilization of multiple local minima and pathways sampling as well as operando catalyst characterization. The fact that the per-site and per-species resolution is needed for the description and prediction of catalyst properties gives theory the central role in catalysis research, as most experiments provide ensemble-average information and cannot detect the crucial minority species that may be responsible for the catalytic activity.

Distinctive stability of a free-standing monolayer clay mineral nanosheet via transmission electron microscopy

The distinctive stability of the monolayer clay mineral demonstrated by electron diffraction.