Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping

Tailoring Superatomic Stability with Transition Metals in Silicon Cages: Shrinking to M@Si15 (M = Re, Os, Ir)

The design of materials with intriguing electronic properties is crucial for advancing nanoscale technologies, where precise control over atomic structure and electronic behavior is essential. Metal-encapsulating silicon cage superatoms (SAs) provide a new paradigm for molecular-scale material design, allowing fine-tuning of both structure and electronic characteristics. The formation of superatoms mimicking halogens, noble gases, and alkali metals has been well-studied, particularly with M@Si16, where early transition metals from groups 3 to 5 stabilize within a Si16 cage, achieving a 68-electron configuration. For late transition metals with excess electrons, a Si15 cage offers enhanced stability by fulfilling the 68-electron rule with one fewer Si atom. This research synthesizes Si15 cage-SAs with rhenium (Re) from group 7 and iridium (Ir) from group 9 on p-type and n-type organic substrates. The stability of Re@Si15 and Ir@Si15 is evaluated via oxidative reactivity with X-ray photoelectron spectroscopy and theoretical calculations, including osmium (Os) from group 8. Re@Si15-, Os@Si150, and Ir@Si15+ exhibit superatomic behaviors similar to halogens, noble gases, and alkali metals due to the 68-electron shell closure. Among them, Re@Si15- on p-type organic substrates shows superior electronic and geometric properties. These findings advance our understanding of M@Sin systems for transition metals, addressing longstanding questions about their properties at n = 15 and 16.

Photoemission spectroscopy and microscopy for Ta@Si16 superatoms and their assembled layers

Superatoms (SAs) with specific compositions have the potential to significantly advance the field of nanomaterials science, leading to next-generation nanoscale functionalities. In this study, we fabricated assembled layers with tantalum metal-atom encapsulating silicon cage (Ta@Si16) SAs on an organic C60 substrate through deposition, and we characterized their electronic and optical properties by photoelectron spectroscopy and microscopy. The alkaline nature of Ta@Si16 SAs reveals their electronic behaviors, such as charge transfer and electromagnetic near-field sensing, through two-photon photoemission (2PPE) spectroscopy and microscopy with a femtosecond laser. The evolution of the work function for Ta@Si16 SAs on C60, observed by 2PPE spectroscopy, demonstrates charge transfer complexation between the topmost C60 layer and the first Ta@Si16 layer, consistent with the electron-donating alkaline characteristics of Ta@Si16 SAs. Specifically, a small amount of Ta@Si16 SA deposition leads to a dramatic increase in 2PPE intensity, attributable to electromagnetic near-field enhancements, suggesting applications as sensitizers for nonlinear imaging in photoemission microscopy. For the assembled Ta@Si16 SA layers, a plasmonic response of hνp = 17.9 eV is spectroscopically identified, including their valence and conduction band structures, and the plasmonic energetics are discussed in the context of metal doping in bulk silicon.

Boron-doped scandium clusters B@Sc n-1 -/0/+ with n = 2-13: uncovering the smallest endohedrally doped cages

A comprehensive study using density functional theory with the PBE functional and the Def2-TZVP basis set investigates the pure Sc n +/0/- and doped Sc n-1B+/0/- clusters with n = 1-13 in three charged states. B@Sc6 +/0/- clusters emerge as the smallest doped cages identified so far, distinguished by their near-perfect octahedral geometry, with a B atom centrally enclosed in the Sc6 +/0/- cages. Structural analysis reveals size-dependent trends, with a critical size at n = 6, marking a transition from exohedral to endohedral configuration, and a shift in the substitution-addition pattern of the B atom within the pure Sc host. Incorporation of a B atom induces electron redistribution, stabilizes high spin states and reduces energetic degeneracy. B-doping enhances the stability of the initial Sc n +/0/- clusters, showing a consistent preference for cationic isomers. A molecular orbital (MO) analysis provides a detailed explanation for the observed energy degeneracy among various stable spin states by delving into their electronic configurations.

Geometric and Electronic Properties of P Atom-Doped Al Nanoclusters: Alkaline-like Superatom of P@Al12

The geometric and electronic characteristics of phosphorus-atom doped aluminum nanoclusters, AlnPm (n = 7-17, m = 1 and 2), were investigated through a combination of experiments and theoretical calculations. The size dependences of the ionization energy (Ei) for AlnPm NCs exhibit a local minimum of 5.37 eV at Al12P1, attributed to an endohedral P@Al12 superatom (SA). This SA originates from an excess electron toward the 2P shell closing (40e), coexisting with an exohedral isomer featuring a vertex P atom. The stability of the endohedral P@Al12 is further enhanced in its cationic state compared to the exohedral isomer, when complexed with a fluorine (F) atom, forming an SA salt denoted as P@Al12+F- with an elevated Ei ranging from 6.42 to 7.90 eV. In contrast, for the anionic Al12P1-, the exohedral form is found to be more stable than the endohedral one using anion photoelectron spectroscopy and calculations. The geometric and electronic robustness of neutral P@Al12 SAs against electron donation and acceptance is discussed in comparison to rare-gas-like Si@Al12 SAs.

Cage doping of Ti, Zr, and Hf-based 13-atom nanoclusters: two sides of the same coin

Transition metal nanoclusters can exhibit unique and tunable properties which result not only from their chemical composition but also from their atomic packing and quantized electronic structures. Here, we introduce a promising family of bimetallic TM@Ti12, TM@Zr12, and TM@Hf12 nanoclusters with icosahedral geometry, where TM represents an atom from groups 3 to 12. Density functional theory calculations show that their stability can be explained with familiar concepts of metal cluster electronic and atomic shell structures. The magnetic properties of these quasispherical clusters are entirely consistent with superatom electronic shells and Hund's rules, and can be tuned by the choice of the TM dopant. The computed cluster atomization energies were analyzed in terms of the elements' cohesive energy, Ecoh, and contributions from geometric distortion, Edis, surface energy, Es, and ionic bonding, Ei. Some clusters have anomalous stability relative to Ecoh + Edis + Es + Ei. We attribute this to superatomic character associated with a favorable atomic and electronic shell structure. This raises the possibility of designing stable superatoms and materials with tailored electronic and magnetic properties.

Alkaline Earth Metal Superatom of W@Si16: Characterization of Group 6 Metal Encapsulating Si16 Cage on Organic Substrates

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the "periodic law" of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

Reactivity of cationic silver clusters with O2: a probe of interplay between clusters' geometric and electronic structures

We explored the size-dependent reactivity of Agn+ (n = 2-22) with O2 under mild conditions and found that only a few sizes of Agn+, with even values of n = 4, 6, 12, 16, 18, and 22, are reactive. Possible structures of Agn+ (n = 2-22) were determined using a genetic algorithm with incomplete local optimizations at the DFT level, and the calculated bonding strengths of O2 on these structures are consistent with experimental observations. Analyses revealed a close relationship between the reactivity of Agn+ with O2 and its HOMO-LUMO gap: cationic silver clusters with a small HOMO-LUMO gap are reactive, which can be rationalized by the covalent character of chemical bonds between Agn+ and O2 involving their frontier orbitals. The peculiar size-dependent HOMO-LUMO gaps and reactivity with O2 correlate with the subtle interplay between the electronic configurations and geometric structures of these silver cluster cations.

Photoelectron velocity map imaging spectroscopy of group 14 elements and iron tetracarbonyl anionic clusters MFe(CO)4- (M = Si, Ge, Sn)

The heteronuclear group 14 M-iron tetracarbonyl clusters MFe(CO)4- (M = Si, Ge, Sn) anions have been generated in the gas phase by laser ablation of M-Fe alloys and detected by mass and photoelectron spectroscopy. With the support of quantum chemical calculations, the geometric and electronic structures of MFe(CO)4- (M = Si, Ge, Sn) are elucidated, which shows that all the MFe(CO)4- clusters have the M-Fe bonded, iron-centered, and carbonyl-terminal M-Fe(CO)4 structure with the C2v symmetry and a 2B2 ground state. The M-Fe bond can be considered a double bond, which includes one σ electron sharing bond and one π dative bond. The C-O bonds in those anionic clusters are calculated to be elongated to different extents, and in particular, the C-O bonds in SiFe(CO)4- are elongated more. The Si-Fe alloy thus turns out to be a better collocation to activate the C-O bonds in the gas phase among group 14. The present findings have important implications for the rational development of high-performance catalysts with isolated metal atoms/clusters dispersed on supports.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

New Insights into Adsorption Properties of the Tubular Au26 from AIMD Simulations and Electronic Interactions

Recently, we revealed the electronic nature of the tubular Au₂₆ based on spherical aromaticity. The peculiar structure of the Au₂₆ could be an ideal catalyst model for studying the adsorptions of the Au nanotubes. However, through Google Scholar, we found that no one has reported connections between the structure and reactivity properties of Au₂₆. Here, three kinds of molecules are selected to study the fundamental adsorption behaviors that occur on the surface of Au₂₆. When one CO molecule is adsorbed on the Au₂₆, the σ-hole adsorption structure is quickly identified as belonging to a ground state energy, and it still maintains integrity at a temperature of 500 K, where σ donations and π-back donations take place; however, two CO molecules make the structure of Au₂₆ appear with distortions or collapse. When one H₂ is adsorbed on the Au₂₆, the H-H bond length is slightly elongated due to charge transfers to the anti-bonding σ* orbital of H₂. The Au₂₆-H₂ can maintain integrity within 100 fs at 300 K and the H₂ molecule starts moving away from the Au₂₆ after 200 fs. Moreover, the Au₂₆ can act as a Lewis base to stabilize the electron-deficient BH₃ molecule, and frontier molecular orbitals overlap between the Au₂₆ and BH₃.

Anion photoelectron spectroscopy and quantum chemical calculations of bimetallic niobium-aluminum clusters NbAln-/0 (n = 3-12): identification of a half-encapsulated symmetric structure for NbAl12. Phys Chem Chem Phys 2023;25:6498-6509. [PMID: 36786014 DOI: 10.1039/d2cp04978c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Bimetallic niobium-doped aluminum clusters, NbAl_n^-/0 (n = 3-12), are investigated through a synergetic combination of size-selected anion photoelectron spectroscopy and theoretical calculations. It is found that the dominant structures of NbAl_n^- anions with n = 3-8 can be described by gradually adding Al atoms to the NbAl₃^- core. Starting from n = 9, the lowest-energy geometric structures of NbAl_9-12^- transform into bilayer structures. In particular, NbAl₁₂^- has a C_3v symmetric structure, which can be viewed as a NbAl₆ regular hexagon over a bowl-shaped Al₆ structure. More detailed analyses indicate that NbAl₉ and NbAl₁₂^- possess unusual stability, which may be attributed to their closed-shell electron configurations with superatomic features.

Catalytic NO Reduction by NO Pre-Adsorbed RhCeO2 NO- Clusters

A fundamental understanding on the dynamically structural evolution of catalysts induced by reactant gases under working conditions is challenging but pivotal in catalyst design. Herein, in combination with state-of-the-art mass spectrometry for cluster reactions, cryogenic photoelectron imaging spectroscopy, and quantum-chemical calculations, we identified that NO adsorption on rhodium-cerium bimetallic oxide cluster RhCeO₂ ^- can create a Ce³⁺ ion in product RhCeO₂ NO^- that serves as the starting point to trigger the catalysis of NO reduction by CO. Theoretical calculations substantiated that the reduction of another two NO molecules into N₂ O takes place exclusively on the Ce³⁺ ion while Rh behaves like a promoter to buffer electrons and cooperates with Ce³⁺ to drive NO reduction. Our finding demonstrates the importance of NO in regulating the catalytic behavior of Rh under reaction conditions and provides much-needed insights into the essence of NO reduction over Rh/CeO₂ , one of the most efficient components in three-way catalysts for NO_x removal.

Radiative cooling in silver and palladium doped gold clusters

The emission of photons from a thermally populated electronic excited state, via the process of recurrent fluorescence, has been recognized as a prominent cooling channel in hot molecules and small metal clusters. For the latter case, however, only monometallic species have been investigated to date. An active radiative cooling channel has a stabilizing effect and can favor the size and composition specific production of selected clusters. In this work, the influence of silver and palladium doping on the radiative cooling of gold cluster cations is studied. The quenching of metastable fragmentation due to radiation of laser-excited Au_n⁺, AgAu_n-1⁺ and PdAu_n-1⁺ (n = 11-15) clusters is investigated in a single-pass molecular beam setup. The observed high radiation rates, with values in the range from 10³ to 10⁵ s^-1, are consistent with recurrent fluorescence. The rates present a pronounced odd-even staggering with higher values for the clusters with closed-shell electronic configurations. While substitution of Au with Ag does not alter the odd-even pattern with cluster size, replacing Au with Pd shifts the pattern by one atom. The experimental observations are discussed in terms of the dissociation energy of the clusters, which sets their effective temperature during photon emission, and the low-lying electronic excited states involved in the photon emission process. Linear-response time-dependent density functional theory calculations on selected species are used to illustrate the significant effect of the electronic structure on the radiation rates. For n = 14, substitution of Au with Ag lowers the energy of the lowest-energy transition in the cluster, which in addition has a higher oscillator strength, favoring radiative cooling. The opposite effect is seen in Pd doped clusters. Based on this analysis, conclusions can be drawn about the significance of radiative cooling in laser-excited alloy clusters, with a concomitant fast stabilization at high internal energy conditions.

The nature of stability and adsorption interactions of binary Au-Li clusters with bridge adsorption structures

Earlier findings have confirmed that CO molecules have propensities to adsorb on low-coordinated gold atoms (top sites) of Au-based clusters, which can be treated by the Blyholder model wherein the σ donation and π-back donation take place. Here, the structural features and stability of (AuLi)_n (n = 1-9) clusters were first analyzed using the GA-DFT method. The new adsorption modes, vibration frequencies and electronic interactions for Au-Li clusters with CO were investigated in detail. More excitingly, we found that CO prefers to adsorb on the bridge sites of the Au-Li clusters rather than on the top sites, which are much lower in energies than the top adsorptions, and the C-O stretching frequencies are also red-shifted. AIMD simulations show that the adsorption structures still have good thermal stability at 500 K. The density of states reveals that the electronic structures of Au-Li clusters have excellent stability for the bridge adsorptions of CO molecules. The ETS-NOCV analysis and NPA charges show that the direction of charge flow is from Au-Li clusters → CO. Our study provides an idea to elucidate the new adsorption mechanism on Au-Li clusters and the connection between the geometries and reaction properties.

An Open-Shell Superatom Cluster Ta10- with Enhanced Stability by United d-d π Bonds and d-Orbital Superatomic States

We carried out a comprehensive study on the gas-phase reactions of Ta_n^- (n = 5-27) with nitrogen using a customized reflection time-of-flight mass spectrometer coupled with a velocity map imaging apparatus (Re-TOFMS-VMI). Among the studied tantalum clusters, Ta₁₀^- exhibits prominent mass abundance indicative of its unique inertness. DFT calculation results revealed a D_4d bipyramidal prolate structure of the most stable Ta₁₀^-, which was verified by photoelectron spectroscopy experiments. The calculations also unveiled that Ta₁₀^- has the largest HOMO-LUMO gap and second-order difference of binding energy among the studied clusters. This is associated with its well-organized superatomic orbitals, which consist of both 6s and 5d orbitals of tantalum atoms, allowing for splitting of superatomic 1D and 2P orbitals and an enlarged gap between the singly occupied molecular orbital (SOMO) and unoccupied β counterpart, which brings forth stabilization energy pertaining to Jahn-Teller distortion. Also, the SOMO exhibits a united d-d π orbital pattern that embraces the central Ta₈^- moiety.

Multiple CO2 reduction mediated by heteronuclear metal carbide cluster anions RhTaC2. Dalton Trans 2022;51:11491-11498. [PMID: 35833563 DOI: 10.1039/d2dt01612e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO₂ catalytic conversion and bimetallic carbides generated at the interface were proposed to contribute to the observed activity. Heteronuclear metal carbide clusters (HMCCs) that compositionally resemble the bimetallic carbides are suitable models to get a fundamental understanding of the reactivity of the related condensed-phase catalysts, while the reaction of HMCCs with CO₂ has not been touched in the gas phase. Herein, benefiting from the newly designed double ion trap reactors, the reaction of laser-ablation generated and mass-selected RhTaC₂^- clusters with CO₂ was studied. The experimental results identified that RhTaC₂^- can reduce four CO₂ molecules consecutively and generate the product RhTaC₂O₄^-. The pivotal roles of Rh-Ta synergy and the C₂ ligand in driving CO₂ reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC₂O₄^- was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO₂ atmosphere at elevated temperatures.

Adsorption properties of pyramidal superatomic molecules based on the structural framework of the Au20 cluster

The pyramidal Au₂₀ cluster is a highly inert and stable superatomic molecule, but it is not suitable as a potential catalyst for covalent bond activations, e.g., CO oxidation reaction. Herein, the adsorption and electronic properties of CO molecules on various pyramidal clusters based on the structural framework of Au₂₀ are investigated using density functional theory. According to the SVB model, we constructed isoelectronic superatomic molecules with different pyramid configurations by replacing the vertex atoms of the Au₂₀ using metal M atoms (M = Li, Be, Ni, Cu, and Zn group atoms). After the CO molecules are adsorbed on the vertex atoms of these metal clusters, we analyzed the CO adsorption energies, C-O bond stretching frequencies, and electronic properties of the adsorption structures. It was found that the adsorption of CO molecules results in minimal changes in the parent geometries of the pyramidal clusters, and most adsorption structures are consistent with the geometry of CO adsorption at the vertex site of the Au₂₀ cluster. There are significant red shifts when CO molecules are adsorbed on the Ni/Pd/Pt atoms of the clusters, and their CO adsorption energies were also greater. The molecular orbitals and density of states reveal that there are overlaps between the frontier orbitals of the clusters and CO, and the electronic structure of NiAu₁₉^- is not sensitive to CO. The ETS-NOCV analysis shows that the increase in the density of the bonding area caused by the orbital interactions between the fragments is higher than the decrease in the density of the bonding area caused by Pauli repulsion, presenting that the direction of charge flow in the deformation density is from CO → clusters. From energy decomposition analysis (EDA) and NPA charge, we find a predominant covalent nature of the contributions in CO⋯M interactions (σ-donation). Our study indicates that the SVB model provides a new direction to expand the superatomic catalysts from the superatom clusters, which also provides inference for the extension of the single atom catalysis.

Al13− and B@Al12− superatoms on a molecularly decorated substrate

Aluminum nanoclusters (Al_n NCs), particularly Al₁₃⁻ (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Al_n NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C₆₀) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C₆₆H₆₆), and the electronic states of Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al₁₃⁻ and boron-doped B@Al₁₂⁻ SAs through optimizing the cluster–surface interactions.

Anionic aluminium clusters are promising candidates for the fabrication of superatom-assembled nanomaterials. Here, the authors report enhanced stability for Al₁₃⁻ and boron-doped B@Al₁₂⁻ on a molecularly decorated p-type organic substrate.

Electronic structure and bonding in endohedral Zintl clusters

Endohedral Zintl clusters-multi-metallic anionic molecules in which a d-block or f-block metal atom is enclosed by p-block (semi)metal atoms-are very topical in contemporary inorganic chemistry. Not only do they provide insight into the embryonic states of intermetallic compounds and show promise in catalytic applications, they also shed light on the nature of chemical bonding between metal atoms. Over the past two decades, a plethora of endohedral Zintl clusters have been synthesized, revealing a fascinating diversity of molecular architectures. Many different perspectives on the bonding in them have emerged in the literature, sometimes complementary and sometimes conflicting, and there has been no concerted effort to classify the entire family based on a small number of unifying principles. A closer look, however, reveals distinct patterns in structure and bonding that reflect the extent to which valence electrons are shared between the endohedral atom and the cluster shell. We show that there is a much more uniform relationship between the total valence electron count and the structure and bonding patterns of these clusters than previously anticipated. All of the p-block (semi)metal shells can be placed on a ladder of total valence electron count that ranges between 4n+2 (closo deltahedra), 5n (closed, three-bonded polyhedra) and 6n (crown-like structures). Although some structural isomerism can occur for a given electron count, the presence of a central metal cation imposes a preference for rather regular and approximately spherical structures which maximise electrostatic interactions between the metal and the shell. In cases where the endohedral metal has relatively accessible valence electrons (from the d or f shells), it can also contribute its valence electrons to the total electron count of the cluster shell, raising the effective electron count and often altering the structural preferences. The electronic situation in any given cluster is considered from different perspectives, some more physical and some more chemical, in a way that highlights the important point that, in the end, they explain the same situation. This article provides a unifying perspective of bonding that captures the structural diversity across this diverse family of multimetallic clusters.

Exploring the Effects of a Doping Silver Atom on Anionic Gold Clusters' Reactivity with O2

Reactivities of AgAu_n-1^- (n = 3-10) with O₂ at a low temperature were studied using an instrument combining a magnetron sputter cluster source, a microflow reactor, and a time-of-flight mass spectrometer. Their reaction products as well as size-dependent kinetic rates were nearly identical to those of corresponding Au_n^- (n = 3-10). Previous experiments showed that the Ag atom in AgAu_n-1^- (n = 3-10) was fully or partially enclosed by the gold atoms. We studied the adsorption of O₂ on these reported structures using the B3LYP theory with relatively large basis sets. The theoretical results indicate that the adsorption sites as well as the adsorption energies of O₂ on AgAu_n-1^- (n = 3-10) are nearly identical to those on the corresponding Au_n^- (n = 3-10). The O₂ adsorption on a series of proposed isomers of AgAu_n-1^- (denoted as Au_n-1Ag^-), in which the silver atom was on the protruding site, was explored using the same theoretical methods. The O₂ tends to bond with the protruding Ag atoms, and the binding energies are apparently higher than those on the corresponding Au_n^- and AgAu_n-1^-. The adsorption and activation of O₂ on Au_n^-, AgAu_n-1^-, and Au_n-1Ag^- were correlated with their global electron detachment energies (VDEs) as well as the element types of the adsorption sites. Generally, low VDE values and silver sites facilitate the O₂ adsorption, and these two factors separately dominate in various cluster species. The revealed effects of a doping silver atom in small gold clusters are helpful to understand the role of the residual silver components in many nano gold catalysts.

The effect of size, charge state and composition on the binding of propene to yttrium-doped gold clusters

The catalytic activity of metal clusters can be easily tuned by their size, charge state, or the introduction of dopant atoms. Here, the dopant-, charge- and size-dependent propene adsorption on gold (Au n +) and yttrium doped gold (Au n-1Y+) clusters (n = 4-20) was investigated using combined gas-phase reaction studies and density functional theory computations. The increased charge transfer between the cluster and propene in the cationic clusters considerably enhances the propene binding on both pure and yttrium-doped species, compared to their neutral cluster counterparts, while yttrium-doping lowers the propene binding strength in a size-dependent way compared to the pure gold clusters. Chemical bonding and energy decomposition analysis indicate that there is no covalent bond between the cluster and propene. The preferred propene binding site on a cluster is indicated by the large lobes of its LUMO, together with the low coordination number of the adsorption site. In small yttrium-doped gold clusters propene can not only bind to the electron-deficient yttrium atom, but also to the partially positively-charged gold atoms. Therefore, by controlling the charge of the clusters, as well as by introducing yttrium dopants, the propene binding strength can be tuned, opening the route for new catalytic applications.

The doping engineering and crystal structure of rod-like Au8Ag17 nanoclusters

Alloy nanoclusters protected by ligands were widely studied due to the synergistic effect of metal atoms, and they exhibit enhanced properties in different fields, such as bio-imaging and catalysis. Herein, we obtained Au₈Ag₁₇(PPh₃)₁₀Cl₁₀ nanoclusters via one-step simple synthesis. The atomically precise crystal structure was determined by x-ray crystallography. It is found that the rod-like Au₈Ag₁₇ nanoclusters were composed of two Au₄Ag₉ icosahedrons via sharing the same Ag atom. Two Au atoms occupy the center of icosahedrons, and the other six Au atoms are all at the neck sites. Four kinds of Cl-Ag connecting modes were observed in Au₈Ag₁₇ nanoclusters. Moreover, the ultraviolet-visible absorption spectrum shows that the prominent absorption peaks of Au₈Ag₁₇ nanoclusters are at ∼395 and 483 nm. This work provides a feasible strategy to synthesize alloy nanoclusters with precise composition via doping engineering.

An IrVO4+ Cluster Catalytically Oxidizes Four CO Molecules: Importance of Ir-V Multiple Bonding

The generation and characterization of multiple metal-metal (M-M) bonds between early and late transition metals is vital to correlate the nature of multiple M-M bonds with the related reactivity in catalysis, while the examples with multiple M-M bonds have been rarely reported. Herein, we identified that the quadruple bonding interactions were formed in a gas-phase ion IrV⁺ with a dramatically short Ir-V bond. Oxidation of four CO molecules by IrVO₄⁺ is a highly exothermic process driven by the generation of stable products IrV⁺ and CO₂, and then IrV⁺ can be oxidized by N₂O to regenerate IrVO₄⁺. This finding overturns the general impression that vanadium oxide clusters are unwilling to oxidize multiple CO molecules because of the strong V-O bond and that at most two oxygen atoms can be supplied from a single V-containing cluster in CO oxidation. This study emphasizes the potential importance of heterobimetallic multiple M-M bonds in related heterogeneous catalysis.

Synthesis and Optical Properties of Unique Pt1Ag24 Nanoclusters with Mixed Exterior Motif Structures

The atomic arrangement of metal nanoclusters plays a significant role in the structure-property correlation. Herein, we present a novel Pt₁Ag₂₄(SR)₁₆(PPh₃)₃ nanocluster with a unique structure, different from two reported Pt₁Ag₂₄ nanoclusters. The nanocluster was prepared via one-pot synthesis and solvent extraction. It has a centered icosahedral Pt₁Ag₁₂ kernel and an open shell composed of three Ag₂(SR)₃(PPh₃) staple motifs and a unique trefoil-like Ag₆(SR)₇ motif. The three kinds of Pt₁Ag₂₄ nanoclusters have the same kernel but different shell configurations. The fine-tuning of structures is necessary and significant for the investigation of the relationship of structures and properties. The different UV-vis absorption spectra indicate that the optical properties of three Pt₁Ag₂₄ nanoclusters mainly depend on the exterior shell configuration and the metal-ligand interface. This work provides insights toward growth mechanism and the structure-property correlation of metal nanoclusters.

Argon Adsorption on Cationic Gold Clusters Aun+ (n ≤ 20)

The interaction of Au_n⁺ (n ≤ 20) clusters with Ar is investigated by combining mass spectrometric experiments and density functional theory calculations. We show that the inert Ar atom forms relatively strong bonds with Au_n⁺. The strength of the bond strongly varies with the cluster size and is governed by a fine interplay between geometry and electronic structure. The chemical bond between Au_n⁺ and Ar involves electron transfer from Ar to Au, and a stronger interaction is found when the Au adsorption site has a higher positive partial charge, which depends on the cluster geometry. Au₁₅⁺ is a peculiar cluster size, which stands out for its much stronger interaction with Ar than its neighbors, signaled by a higher abundance in mass spectra and a larger Ar adsorption energy. This is shown to be a consequence of a low-coordinated Au adsorption site in Au₁₅⁺, which possesses a large positive partial charge.

Tetrahedral Pt10- Cluster with Unique Beta Aromaticity and Superatomic Feature in Mimicking Methane

Utilizing a customized metal cluster source in tandem with a flow tube reactor and a reflectron time-of-flight mass spectrometer, we have obtained well-resolved pure metal clusters Pt_n^- and observed their gas-phase reactions with a few small gas molecules. Interestingly, the remarkable inertness of Pt₁₀^- was repeatedly observed in different reactions. Meanwhile, we have determined the structure of Pt₁₀^- within a regular tetrahedron. Considering that Pt possesses 5d⁹6s¹ electron configuration, the tetrahedral Pt₁₀^- exhibits unexpected stability at neither a magic number of valence electrons nor a shell closure of geometric structure. Comprehensive theoretical calculations unveil the stability of Pt₁₀^- is significantly associated with the all-metal aromaticity. In addition to the classical total aromaticity, which is mainly due to 6s electrons, there is unique beta-aromaticity ascribed to spin-polarized beta 5d electrons pertaining to singly occupied multicenter bonds. Further, we demonstrate the superatomic feature of such a transition metal cluster Pt₁₀^-, as Pt₆@Pt₄^-, in mimicking methane.

Superatomic and adsorption properties of Ni atom doped Au clusters

Due to their strong relativistic effects, Au clusters exhibit many unusual geometric structures. Among them, Au7-, Au8 and Au9+ have 18 valence electrons satisfying the magic numbers in the jellium model, respectively, but these three non-spherical clusters are not superatoms. In general, a single dopant atom can drastically change the structural and electronic properties of Au clusters. Here, we searched structures of NiAu7-, NiAu8 and NiAu9+ clusters using the genetic algorithm program (GA) combined with density functional theory (DFT). It was found that the alloy clusters are all 3D spherical structures. The molecular orbitals and density of states analysis indicate that they have completely filled superatomic shells (1S21P6), in which the electronic structure of the Ni atom is d10. Then, the electrostatic potential surfaces of the alloy clusters are analyzed, and the calculated results show that the NiAu8 superatom has remarkable σ-holes with positive potential regions. Moreover, these electron-deficient regions can be considered as interaction sites with some electron donors. After a Lewis base CO gas molecule is adsorbed on the Au-based superatom, we found that the C-O bond distance becomes slightly elongated and its stretching frequency presents a significant red-shift. This is due to the fact that 5d electrons of the Au atom of the NiAu8 transfer towards the anti-bond π orbitals of the CO molecule. Hence, this is an effective strategy for finding new types of superatoms and potential catalysts for covalent bond activation.

Vanadium Cluster Neutrals Reacting with Water: Superatomic Features and Hydrogen Evolution in a Fishing Mode

Hydrogen evolution reaction (HER) is known as the heart of various energy storage and conversation systems of renewable energy sources. Here we observe the cluster reactions of a light transition metal, vanadium, with water in a gas-phase flow tube reactor. While HER products of V₁ and V₂ were not observed, the effective HER of water on neutral V_n (n ≥ 3) clusters reveals reasonable and size-dependent reactivity of the vanadium clusters. Superatomic features and reaction dynamics of V₁₀, V₁₃, and V₁₆ are highlighted. Among the three typical superatoms, V₁₀ and V₁₆ exhibit an abnormal superatomic orbital energy level order, 1S|2S|1P|1D..., where the energy-reduced 2S orbital helps to accommodate the geometric structure and hence reinforce the cluster stability. In comparison, V₁₃ bears a less symmetrical structure and reacts readily with water, allowing for recombination of a hydroxyl atom with an adsorbed hydrogen atom, akin to a fishing-mode HER process. The joint experimental and theoretical study on neutral V_n clusters clarifies the availability of superatom chemistry for transition metals and appeals further development of cluster theory based on electronic cloud/orbital analysis instead of simply counting the valence electrons. Also, we provide insights into the HER mechanism of metal clusters and propose a strategy to design new materials for portable fuel cells of hydrogen energy.

Observation of Carbon-Carbon Coupling Reaction in Neutral Transition-Metal Carbonyls

Neutral titanium-metal carbonyl complexes with the chemical formula Ti(CO)_n (n = 4-7) are produced in the gas phase by the reactions of titanium atoms with carbon monoxide in a pulsed laser vaporization-supersonic expansion source. Their infrared absorption spectra in the carbonyl stretching frequency region are measured by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free electron laser. Infrared spectroscopy in conjunction with quantum chemical calculations confirm that all of these complexes have unexpected titanium ketenylidene OTiCCO(CO)_n-2 structures. Bonding analysis indicates that the OTiCCO core structure can be described by the bonding interactions between a TiO⁺ cation in the doublet ground state and a doublet ground state of CCO^-. The results reveal that the C-O bond breaking and C-C bond formation proceed efficiently in the reactions between laser-vaporized titanium atoms and carbon monoxide.

[AuAg26(SR)18S]- Nanocluster: Open Shell Structure and High Faradaic Efficiency in Electrochemical Reduction of CO2 to CO

For atomically precise metal nanoclusters, distinctive molecular architectures and promising applications are urgently required to be intensively explored. Herein, we have first reported the open shell structure of the [AuAg₂₆(S-Adm)₁₈S]^- nanocluster and its application in the electrochemical reduction of CO₂. The X-ray crystal structure of the AuAg₂₆ nanocluster is composed of a AuAg₁₂ icosahedron kernel and a Ag₁₄(SR)₁₈S open shell. The shell includes a Ag₆(SR)₃S, a Ag₅(SR)₆, and three Ag(SR)₃ motifs. It is the first time twisty propeller-like Ag₅(SR)₆ and trefoil-like Ag₆(SR)₃S motifs in metal nanoclusters have been observed. Due to the novel open shell configuration of Ag₁₄(SR)₁₈S, four triangular facets of the kernel are exposed. The AuAg₂₆ nanocluster shows excellent catalytic activity in the electrochemical reduction of CO₂ to CO. The Faradaic efficiency of CO is up to 98.4% under -0.97 V. The electrochemical in situ infrared study and DFT calculations demonstrate that the open shell structure of the AuAg₂₆ nanocluster is beneficial to the forming of intermediate COOH* in the electrochemical reduction of CO₂ to CO.

Stability of cationic silver doped gold clusters and the subshell-closed electronic configuration of AgAu14. J Chem Phys 2020;153:244304. [PMID: 33380086 DOI: 10.1063/5.0033487]

Silver doping is a valuable route to modulate the structural, electronic, and optical properties of gold clusters. We combine photofragmentation experiments with density functional theory calculations to investigate the relative stability of cationic Ag doped Au clusters, AgAu_N-1 ⁺ (N ≤ 40). The mass spectra of the clusters after photofragmentation reveal marked drops in the intensity of AgAu₈ ⁺, AgAu₁₄ ⁺, and AgAu₃₄ ⁺, indicating a higher relative stability of these sizes. This is confirmed by the calculated AgAu_N-1 ⁺ (N ≤ 17) dissociation energies peaking for AgAu₆ ⁺, AgAu₈ ⁺, and AgAu₁₄ ⁺. While the stability of AgAu₆ ⁺ and AgAu₈ ⁺ can be explained by the accepted electronic shell model for metal clusters, density of states analysis shows that the geometry plays an important role in the higher relative stability of AgAu₁₄ ⁺. For this size, there is a degeneracy lifting of the 1D shell, which opens a relatively large HOMO-LUMO gap with a subshell-closed 1S²1P⁴1P²1D⁶ electronic configuration.

Reactivity of Iron Hydride Anions Fe2Hn- (n = 0-3) with Carbon Dioxide

The hydrogenation of CO₂ into value-added complexes is of great importance for both environmental and economic issues. Metal hydrides are good models for the active sites to explore the nature of CO₂ hydrogenation; however, the fundamental insights into C-H bond formation are still far from clear because of the complexity of real-life catalysts. Herein, gas-phase reactions of the Fe₂H_n^- (n = 0-3) anions with CO₂ were investigated using mass spectrometry and quantum chemical calculations. The experimental results showed that the reduction of CO₂ into CO dominates all of these reactions, whereas Fe₂H^- and Fe₂H₂^- can induce the hydrogenation of CO₂ effectively to give rise to products Fe(HCO₂)^- and HFe(HCO₂)^-, respectively. The mechanistic aspects and the reactivity of Fe₂H_n^- with an increased number of H atoms in CO₂ hydrogenation were rationalized by theoretical calculations.

The structures of cationic gold clusters probed by far-infrared spectroscopy

Determining the precise structures of small gold clusters is an essential step towards understanding their chemical and physical properties. Due to the relativistic nature of gold, its clusters remain planar (2D) up to appreciable sizes. Ion mobility experiments have suggested that positively charged gold clusters adopt three-dimensional (3D) structures from n = 8 onward. Computations predict, depending on the level of theory, 2D or 3D structures as putative energy-minimum for n = 8. In this work, far-infrared multiple photon dissociation spectroscopy, using Ar as tagging element, is combined with density-functional theory calculations to determine the structures of Au_n⁺ (n≤ 9) clusters formed by laser ablation. While the Au frameworks in Au₆Ar_m⁺ and Au₇Ar_m⁺ complexes are confirmed to be planar and that in Au₉Ar_m⁺ three-dimensional, we demonstrate the coexistence of 3D and planar Au₈Ar_m⁺ (m = 1-3) isomers. Thus, it is revealed that at finite temperatures, the formal 2D to 3D transition takes place at n = 8 but is not sharp.

Iodization Threshold in Size-Dependent Reactions of Lead Clusters Pbn+ with Iodomethane

Utilizing a magnetron-sputtering (MagS) source in tandem with a multiple-ion laminar flow tube (MIFT) reactor and a customized triple quadrupole mass spectrometer (TQMS), we have prepared clean Pb_n⁺ (n = 1-13) clusters and measured their reactivity with iodomethane under high carrier gas pressures. Strong size dependences are found for the reactivity of these cationic Pb_n⁺ clusters with CH₃I. For the Pb_n⁺ with n ≤ 4, iodinated clusters Pb_nI⁺ were found to be the dominant products, in strong contrast to n > 4 where no such products were seen. Quantum chemical studies show that with an increasing number of Pb atoms, the Pb-Pb interatomic interactions become stronger compared with the Pb-I bonding in Pb_nI⁺ clusters. Furthermore, the reactions of Pb_1-4⁺ with CH₃I have fairly small transition state energy barriers, in contrast to those for Pb_n>4⁺ clusters which have magnitudes that will prevent reactions under the ambient conditions.

Methane activation by heteronuclear diatomic AuRh+ cation: comparison with homonuclear Au2+ and Rh2. Phys Chem Chem Phys 2020;22:6231-6238. [PMID: 32129335 DOI: 10.1039/c9cp05699h]

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH₄ with Au₂⁺, AuRh⁺ and Rh₂⁺ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au₂⁺ has the lowest reactivity, and it can activate methane but only produce H-Au₂-CH₃⁺ without H₂ release; Rh₂⁺ has the highest reactivity, and it can produce both carbene-type Rh₂-CH₂⁺ and carbyne-type H-Rh₂-CH⁺ with H₂ release; AuRh⁺ also has high reactivity to produce only AuRh-CH₂⁺ with H₂, avoiding the excessive dehydrogenation of CH₄. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

Dissociative chemisorption of O2 on Agn and Agn−1Ir (n = 3–26) clusters: a first-principle study

Lower dissociation barriers and higher reaction rates of O₂ on doped Ag_n−1Ir clusters, and a gradually weakened dopant effect.

Interaction of carbon monoxide with doped metal clusters

Highlight of experimental and computational studies about the interaction of CO with transition and coinage metal clusters, particularly discussing the influence of dopant atoms.

An Eight-Atom Iridium-Aluminum Oxide Cluster IrAlO6+ Catalytically Oxidizes Six CO Molecules

Fundamental understanding regarding oxygen storage capacity involving how and why an active site can buffer a large number of oxygen atoms in redox processes is vital to the design of advanced oxygen storage materials, while it is challenging because of the complexity of heterogeneous catalysis. Herein, we identified that an eight-atom iridium-aluminum oxide cluster IrAlO₆⁺ can transfer all the oxygen atoms to catalytically oxidize six CO molecules. This finding represents a breakthrough in cluster catalysis where at most three oxygen atoms from a heteronuclear metal oxide cluster can be catalytically involved in CO oxidation. We found that oxygen prefers to be stored on aluminum to form an O₃^-• radical in the energetically unfavorable IrAlO₆⁺ isomer and generate the low-coordinated iridium that is pivotal to capturing CO and triggering the catalysis. The powerful electron cycling capability of iridium and the cooperative iridium-aluminum interplay are emphasized to drive the oxygen atom-transfer behavior.

Relative Stability of Small Silver, Platinum, and Palladium Doped Gold Cluster Cations

The stability patterns of single silver, platinum, and palladium atom doped gold cluster cations, MAuN−1+ (M = Ag, Pt, Pd; N = 3–6), are investigated by a combination of photofragmentation experiments and density functional theory calculations. The mass spectra of the photofragmented clusters reveal an odd-even pattern in the abundances of AgAuN−1+, with local maxima for clusters containing an even number of valence electrons, similarly to pure AuN+. The odd-even pattern, however, disappears upon Pt and Pd doping. Computed dissociation energies agree well with the experimental findings for the different doped clusters. The effect of Ag, Pt, and Pd doping is discussed on the basis of an analysis of the density of states of the N = 3–5 clusters. Whereas Ag delocalizes its 5s valence electron in all sizes, this process is size-specific for Pt and Pd.