Reactivity of Metal Clusters - Chemical Reviews (ACS Publications)

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Reactivity of Metal Clusters Zhixun Luo,*,† A. W. Castleman, Jr.,*,‡ and Shiv N. Khanna*,§ †

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡ Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States ABSTRACT: We summarize here the research advances on the reactivity of metal clusters. After a simple introduction of apparatuses used for gas-phase cluster reactions, we focus on the reactivity of metal clusters with various polar and nonpolar molecules in the gas phase and illustrate how elementary reactions of metal clusters proceed one-step at a time under a combination of geometric and electronic reorganization. The topics discussed in this study include chemical adsorption, addition reaction, cleavage of chemical bonds, etching effect, spin effect, the harpoon mechanism, and the complementary active sites (CAS) mechanism, among others. Insights into the reactivity of metal clusters not only facilitate a better understanding of the fundamentals in condensed-phase chemistry but also provide a way to dissect the stability and reactivity of monolayer-protected clusters synthesized via wet chemistry.

CONTENTS 1. Introduction 2. Cluster Reaction Apparatuses 2.1. Multiple-Ions Laminar Flow Tube 2.2. Selected-Ion Flow Tube 2.3. Reaction/Collision Cells 2.4. Ion Traps and Tandem Quadrupole Reactors 3. Metal Cluster Reactivities with Nonpolar Molecules 3.1. Metal Clusters Reacting with Oxygen 3.1.1. Oxygen-Etching Effect 3.1.2. Oxygen Addition 3.1.3. Superoxo and Peroxo States 3.1.4. Cluster Odd−Even Alternation 3.2. Metal Clusters Reacting with Halogens 3.3. The Reactivity toward Hydrogen and Nitrogen 3.3.1. The Reactivity with Hydrogen 3.3.2. The Reactivity with Nitrogen 3.4. The Reactivity with CH4, C2H2, and C2H4 4. Reactivity of Metal Clusters with Polar Molecules 4.1. Reactions with Monoxides 4.1.1. Adsorption of Carbon Monoxide 4.1.2. Cleavage of N−O Bond 4.2. Reactions with Hydroxyl Compounds 4.2.1. Reactions with Water 4.2.2. Reactions with Alcohols 4.2.3. Hydrogen Evolution Origin 4.3. Reactions with NH3 and H2S 4.4. Reactions with Acetone and Formaldehyde 4.5. Reactions with Thiols: C−S Bond Activation © XXXX American Chemical Society

5. Theory and Mechanisms 5.1. Spin Effect 5.2. Harpoon Mechanism 5.3. Complementary Active Sites (CAS) Mechanism 5.4. Edge Effect Causes Active Sites 5.5. Ligands Induce Active Sites 6. Reactivity of Monolayer-Protected Metal Clusters 7. Metal Cluster Catalysis 8. Outlook Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION In retrospect, the mid-19th century is when cluster chemistry sparked a boom of research in areas including colloids, aerosols, and nucleation phenomena at small sizes. A resurgence of interest emerged in the 1980s, when supersonic expansions and molecule beam techniques became mature.1 From that time onward, various classes of clusters have been studied, providing a vast amount of information bridging the gap between atoms and macroscopic bulk materials. These studies have demonReceived: April 12, 2016

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DOI: 10.1021/acs.chemrev.6b00230 Chem. Rev. XXXX, XXX, XXX−XXX

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and reactants.68−91 Along with the development of various commercial mass spectrometers, the customized reaction cells, flowing after-glow tubes, ion traps, and gas-flow reactors have provided a wealth of information on the reactivity of metal clusters.92−96 In particular, advances in the technology of ion traps and flow-tube reactors have dominated the investigations of cluster chemistry during the past decades.2,10,97,98 Among others, there are also alternative methods of thermalizing clusters other than the flow tube or ion trap.99−101 We hereby summarize a few customized aparatuses that have been used to study cluster reactivity of metals in the gas phase.

strated that the behavior of clusters could not be simply deduced by linear extension of the properties from atoms to bulk matter.2,3 The distinct chemical properties of clusters emerge from a variety of factors, including their very high surface-to-volume ratio, quantum confinement at reduced sizes, and unique geometric and electronic structures. Insights into these factors in a well-defined manner are a prerequisite to the fundamental understanding of chemical properties at the atomic level. Recent advances in exploring catalysts for various chemical reactions and in developing functional materials via molecular aggregation have stimulated enormous research interest for chemists to seek precise atomic-level studies.4−9 Against this background, cluster science is expected to bring a third upsurge of research interest due to the fascinating properties of solidsupported clusters for catalysis and of monolayer-protected clusters (MPCs) for biotechnological applications.10,11 With the developments in instrumentation and technology, precise information regarding the breaking or formation of chemical bonds will be more clearly accessible.12,13 While the chemistry of solid-supported clusters and MPCs has become a topic of substantial current interest, information on structural and electronic properties is corroborated by free gas-phase clusters.14−25 The stability of MPCs is often understood within the conceptual framework of superatoms validated in the gas phase on the basis of electronic shells.3 In this respect, we summarize the research advances in the reactivity of metal clusters. After an introduction of gas-phase cluster reaction apparatuses, we discuss the reactivity of metal clusters with polar and nonpolar molecules, including related theoretical work and mechanisms. Extensive investigations of metal cluster reactivity, as partly illustrated in the present work or previous publications,26−28 are intended not only to enrich predictable and unanticipated chemical properties of clusters but also to contribute to an understanding of reaction processes in cluster-involved active sites of metals in general.29−49 Insights into the reactivity of metal clusters could facilitate a better understanding of the fundamentals in various interdisciplines and provide new views into condensed matter and MPCs synthesized via wet chemistry.

2.1. Multiple-Ions Laminar Flow Tube

Multiple-ions laminar flow tube (MIFT) is one of the typical experimental apparatuses for cluster reactions, as built by Castleman’s group,102 where metal clusters (generated by a LaVa source or MagS source)63 are transported through a reaction tube into which various reactant gases can be introduced. The reactants/products are sampled through a series of ion optics that culminate in a quadrupole mass spectrometer. Large amounts of carrier gas (e.g., He) are introduced into the flow tube, enabling sufficient collisional reactions and fast laminar flow, which requires a powerful differential pump to effectively remove the excessive buffer gas after the reactions. By providing a laminar flow and thermalized condition for the introduction of reactant gas, kinetics studies have enabled determination of the Arrhenius prefactor and activation energies. Such a typical MIFT system is helpful in the studies of multiple-collision reactivities, which are vital in probing magic species simply by checking whether or not they survive in the oxygen-etching reactions.103 2.2. Selected-Ion Flow Tube

The selected-ion flow tube (SIFT) is a technique termed by Adams and Smith,104 where mass-selected clusters derived from a certain ion source undergo collision reactions in vacuum. A combination of SIFT with mass spectrometry, abbreviated as SIFT-MS, has been widely used as a convenient technique for ion−molecule reaction investigations. For cluster reaction purposes, a typical SIFT system (such as that of the Schwarz group)105,106 allows the connection with multiple sources from which the subsequent cluster ions undertake mass selection by using a quadrupole mass filter and are then directed through a flow tube. The ion products extracted from the flow tube could be further guided toward a second quadrupole analyzer, passing a collision cell (e.g., a customized octupole), until the arrival to a final quadrupole and detector for mass analysis. Such a SIFT system has the advantage of allowing determination of the reactivity of size-selected clusters (even single-collision reactions) in the gas phase.98

2. CLUSTER REACTION APPARATUSES General apparatuses for gas-phase cluster reactions usually include components with three functions: cluster generation, cluster reaction, and ions detection. Insights into metal cluster reactivity at some sense profit from the development of mass spectrometry and various cluster sources, including Knudsen ovens,50,51 arc-discharge sputtering,52−55 magnetron sputtering (MagS),56,57 electrospray ionization (ESI),58 and laser vaporization (LaVa, also known as “Smalley source”).59−62 Among these approaches available for cluster generation, the LaVa source has been mostly used as a convenient and highly flexible method by which mixed clusters could be formed simply by using an alloy target of disk/rod components in different proportions. Recently, increasing research interest has been attracted by MagS sources,57,63 in view of their high efficiency and controllability. Cluster ions in gas phase largely undergo studies of metastability,5 photoexcited dissociation,64 collision-induced dissociation (CID),65 and cluster−molecule reactions.66,67 Among them, investigations into metal cluster reactivity have been generally accomplished by analyzing the ions produced through supersonic coexpansion and/or in fast flow of clusters

2.3. Reaction/Collision Cells

Functioning as a mini flow tube, tandem reaction cells (also known as collision cells) have become popular and convenient, such as those used by the Andersson group,107 Lievens group,108−110 and Fielicke group,111 where multiple-photondissociation (MPD) spectroscopy is combined to obtain vibrational spectral information on clusters in the gas phase. Customized reaction cells have also been applied by a few other research groups, such as Bowen’s,112 allowing studies of photoelectron spectroscopy of in situ synthetic clusters. 2.4. Ion Traps and Tandem Quadrupole Reactors

Two-dimensional and three-dimensional linear quadrupole ion traps, developed by Wolfgang Paul,113,114 generate a rf B


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Figure 1. (A) Typical mass spectrum of cationic aluminum cluster ions from a LaVa source. (B) The result of mass selecting Al16+ and introducing oxygen into the gas cell. The center of mass collision energy was 96.49 kJ/mol. (C) A histogram showing the product distributions from the reactions between Aln+ (n = 4−25) and oxygen. No oxygen-containing product ions were observed. The peaks in the histogram due to Aln‑4+ are shaded. Reproduced with permission from ref 130. Copyright 1986 American Institute of Physics.

Figure 2. Oxygen etching reactions of anionic Al clusters.

quadrupole field to store ions within defined boundaries, which are largely used in mass spectrometry. Ion traps and/or tandem quadrupole reactors are a kind of customized cluster system that uses dynamic electric fields to trap/guide charged clusters, allowing for gas-phase reactions at low pressures.115−117 Typical systems of the ion trap combined with tandem quadrupole reactors, such as those built by the Bernhardt group116 and also the He group,118 employ a RF-octupole ion trap inserted into a mass spectrometer arrangement, where the charged clusters are mass-selected in a quadrupole filter or via a mass gate.119 When running reactions, small clusters (e.g., generated by a LaVa source) are thermalized in a helium-filled ion guide before going through a quadrupole mass filter. Clusters attain thermal equilibration within a few milliseconds under the experimental conditions.116 The thermalized and mass-selected cluster ions are then transferred into a customized octupole ion trap, where well-defined fraction of reactants together with ∼1 Pa helium buffer gas could be introduced. Reactions in such ion traps can last for ∼0.1 s or even longer. After the reaction, all ions, including reactants, intermediates, and products, are released out of the ion trap and analyzed with a mass spectrometer.66,120−123

3.1. Metal Clusters Reacting with Oxygen

Cluster−oxygen reactions on metal and metal-alloy systems have been widely investigated.135−140 Through reactions with oxygen (a strong etchant), several interesting clusters of light metals (e.g., magnesium, aluminum, and their alloys) with resistance to oxygen etching have been identified as magic species, even though bulk metals of these clusters are highly reactive.2,141−143 These magic species (at closed electron shells) are also marked by large HOMO−LUMO gaps and high atomremoval energies (or, fragmentation energies), spin excitation energies, and/or ionization energies.63,144 Advances in this field have been intensified not only because of the importance of cluster reactions with oxygen providing insights into the reactivity and property of metals at reduced sizes, but also because of the knowledge gained through study of the gasphase reactivity of metal clusters to facilitate the understanding of interdiscipline frontiers, including condensed phase physics and surface chemistry. The following section will highlight the oxygen-etching effect and oxygen-addition reactions, cluster odd−even alternation phenomenon, as well as superoxo and peroxo states involved in metal cluster reactivity with oxygen. 3.1.1. Oxygen-Etching Effect. The observation of the oxygen-etching effect dates back to very early cluster investigations. Figure 1 shows one of the earliest cluster reaction studies between aluminum and oxygen by Jarrold and Bower in 1986.130 In this study, the cationic clusters Aln+ (n = 4−25) were generated by a LaVa source, ionized by a highenergy electron beam, and expanded into the vacuum system, where specific cluster sizes were mass-selected by a quadrupole mass filter, focused into a low-energy ion beam, and then passed through a cell where the reactant gas was introduced.130 Products and unreacted cluster ions were guided into a quadrupole mass analyzer and finally detected using a collision dynode and electron multiplier. A typical mass spectrum of nascent aluminum cluster ions showed that Al7+ appeared with relatively more abundance than its neighbors (Figure 1A), indicating its prominent stability. Later investigations have

3. METAL CLUSTER REACTIVITIES WITH NONPOLAR MOLECULES One of the aims in cluster chemistry is to find stable clusters that retain their integrity when interacting with other chemicals, thus giving rise to new building blocks for cluster-based materials. Extensive investigations simply applied nonpolar molecule gases (e.g., oxygen, nitrogen, hydrogen, halogens, and even noble gases) to probe cluster stability and reactivity. It is worth mentioning that, although numerous early investigations of cluster reactions were conducted on neutral metal clusters,124−129 the majority of cluster reactivities were revealed by examining cationic or anionic products via mass spectrometry.67,130−134 Simply, here we present first the reactivity of metal cluster ions toward nonpolar molecules. C


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Figure 3. (a) Nascent distribution of Aln− and AlnCu− clusters. (b) Nascent AlnCu−distribution. (c) Oxygen-etched AlnCu−distribution (1.25% partial pressures of oxygen). Peaks corresponding to pure aluminum clusters were superficially removed from panels b and c to discriminate AlnCu−.

Figure 4. (A) Nascent distribution of Aln− and AlnMgm− (a), nascent AlnMgm− distribution (b), and oxygen-etched AlnMgm− distribution (c), where the peaks corresponding to pure aluminum clusters were superficially removed to discriminate AlnMgm−. (B) A diagram showing the stability of Al− Mg alloy clusters correlative with the number of valence electrons.

a transition requires a spin excitation from singlet to triplet multiplicity in clusters with filled shells. This spin excitation is linked to the HOMO−LUMO gap, which creates an energy barrier when reacting with oxygen. In practice, clusters with HOMO−LUMO gaps exceeding 1.2 eV are found to be generally nonreactive to oxygen.149 Subsequent investigations of Al clusters reacting with ground-state molecular oxygen and singlet atomic oxygen have further explained the reason why the mass abundance of selective clusters could be enhanced,150 simply being caused by successive fragmentation of larger clusters along with the formation of a very stable molecule, Al2O, written as

further demonstrated the magic behvavior of the cationic cluster Al7+ with 20 valence electrons.145 The gas-phase reaction of Aln+ clusters with oxygen demonstrated that there were no oxygen-containing product ions being observed for all the clusters studied; instead, massselected Aln+ clusters were found to dissociate into smaller clusters. For example, the reaction of Al16+ with oxygen led to products Al12+ (90%) and Al11+ (10%), as shown in Figure 1B. Figure 1C displays a histogram of the products from reactions between aluminum cluster ions Aln+ (n = 4−25) and oxygen. Al+ was found to be a major product in such mass-selected reactions for the clusters up to n = 13, while at A113+ a transition occurs, and for n > 13, a loss of Al4 was found to be predominant products with rare exception. In brief, the mass loss (i.e., etching effect) for cationic aluminum clusters to react with oxygen follows the pathway “Aln+ + O2 → Alm+ + Aln−mO2”. For a similar system, Castleman et al. reported a study of the reactivity of anionic aluminum clusters with oxygen,146 as shown in Figure 2. Interestingly, it was found that the small aluminum cluster anions containing up to 12 Al atoms were rather reactive toward oxygen, but a few selected aluminum clusters, including Al13−, Al23−, and Al37−, were resistant to oxygen etching. Considering that aluminum generally has three valence electrons, the number of free electrons in an anionic cluster is 3n+1; hence, the observations of Al13−, Al23−, and Al37− could be accounted for by shell closings at 40, 70, and 112 electrons predicted by the jellium model.147,148 This finding unambiguously revealed that the electronic shell filling of Al clusters could account for the mass abundances seen in experiments; that is, electronic properties directly affect the overall stability and chemical reactivity of metal clusters.2 This relationship stems from the fact that the ground state of an oxygen molecule is spin-triplet. Any activation of the oxygen molecule requires the filling of the minority spin states, which results in a change in the spin multiplicity from triplet to singlet. Since the overall spin is conserved in free systems, such

Al n + 4 − + O2 → Al n− + 2Al 2O

(1)

This reactivity rationalizes that individual clusters could resist oxygen etching as dominant peaks and even exhibit increased abundance (such as Al13−) in mass spectrum due to fragments of larger unstable species. The inertness of magic Al clusters could be used as models to study the chemisorption of O2 on the Al(111) surface, which is recognized as a model reaction for surface oxidation and catalysis.151 Among others, Roach et al.152 reported a study of oxygen etching on alloy metal clusters AlnCu− and found that a few Al−Cu clusters with fully filled subshells and large HOMO− LUMO gaps were also resistant to oxygen etching. For example, Al22Cu− was observed as the largest peak in the products (Figure 3), which was interpreted as due to its relatively large vertical spin excitation (VSE) energy and HOMO−LUMO gap resulting from the geometry distortion that leads to a splitting of the shells in a spherical jellium, named as a crystal-field-like splitting of the electronic shells. Further insights into Al-based cluster reactivity have been attained when aluminum−magnesium alloys were examined. Since magnesium is divalent while aluminum is trivalent, the Al−Mg alloy clusters offer larger variations over the electron counts than pure aluminum clusters and hence provide more rigorous grounds to investigate the underlying principles of the D


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Figure 5. Mass spectra of Con− clusters in the absence (a) and presence of different flow rates of oxygen: (b) 0.5 STP cm3 min−1 and (c) 5.0 STP cm3 min−1. Reproduced from ref 153. Copyright 1997 American Chemical Society.

oxygen-etching effect. As expected, Al5Mg2− and Al11Mg3−, which correspond to magic numbers of 20 and 40 electrons, are found to exhibit reasonable stability (Figure 4),103 but Al7Mg3−, Al11Mg−, and Al11Mg2−, with electron counts of 28, 36, and 38, respectively, displayed unexpected stability. The stabilities of these nonmagic numbers of AlnMgm− clusters can be understood via a crystal-field-like splitting of degenerated shells due to the geometrical distortions of the clusters. These studies reinforced the importance of the near-free electron gas (NFEG) model (with closed shell n = 2, 8, 20, 40...) in rationalizing the electronic structure and stability of metal clusters. At the same time, also raised is a pending question regarding the development of a comprehensive model for metal clusters that could account for “magic numbers” according to the shell model (like noble gas) but also can successfully predict stability of all metal clusters where the shell model fails. The study of magic numbers in Al−Mg clusters and their inertness toward oxygen is also important in industrial anticorrosion and aerospace manufacturing.103 Similar to the drastic etching effect observed for aluminum and Al−Mg clusters, the chemical reactivity of cobalt cluster anions Con− (n = 2−8) toward O2 in a flow tube reactor was also found to have rapid rate coefficients, leading to fragmentation of parent clusters independent of the cluster sizes, as shown in Figure 5.153 Especially in the case of a large gas flow rate (Figure 5c), the parent clusters disappeared and the peaks shifted to a low mass region. Among the products of ConOx− species, an intense peak was found at CoO2−, which has been ascertained as a stable ionic molecule. Besides, other cobalt oxide anions containing one or two cobalt atoms were also observed. It was demonstrated that the primary reaction processes were those in which oxidation occurred by removing one (or two) cobalt atom from the cluster to form a small metal oxide anion, leaving the rest of the cobalt cluster as neutral products, written as Con− + O2 → Con − 1 + CoO2−

anionic CoO2−. The valence electrons in Al−Mg clusters form a nearly free electron gas, while the reactivity of Con clusters is driven by the 3d states, which are localized. Oxygen-etching reactions are an important probe to identify magic cluster species with shell closing, such as the aforementioned typical example of Al13−, for which the electronic orbitals mimic the atomic orbitals of Cl−. That is, 40 valence electrons form a completely filled shell and hence make the Al13− cluster behave as a superatomic noble gas. Several other superatomic cluster species with outstanding stability have also been ascertained through the examination of whether or not they survive in oxygen-etching reactions, including a few aluminum iodides, such as Al13I−, Al7I−,154 Al13I2n−, and Al14I2n+1−.155 3.1.2. Oxygen Addition. In addition to the oxygen-etching effect,156−158 the reactivities involving oxygen addition onto metal clusters have also been investigated in the past 20 years.98,159,160 The addition of oxygen is associated with an oxidation process, i.e., loss of electrons, charge distribution rearrangement, and orbital hybridization. Also, the oxygen addition/adsorption could follow the principle of increasing valency of metals but bear dramatic selectivity, depending on cluster sizes, bonding energies, and electronic behaviors. For example, selective anionic Aun− with an odd number of electrons (i.e., even number of Au atoms) showed significant O2 uptake, whereas some other clusters either showed very weak reactivity or did not exhibit any propensity for reactions with oxygen.161−163 In comparison, neutral and cationic gold clusters were found to be inert toward oxygen, with rare exceptions (e.g., Au10+).160 Considering an oxygen double bond, there is much larger electron density within an oxygen molecule than the molecular outer end, allowing for the relatively strong electron-withdrawing ability of the oxygen atom. Therefore, noble-metal clusters with additional unpaired electrons are favorable for such oxygen-addition reactivity. In addition to the investigations revealing the oxygenaddition reactivity of Au clusters, there were also a few reports showing the similarity of copper clusters in reacting with oxygen.164 The study by Lee and Ervin demonstrated that the addition of molecular oxygen through “Cun− + O2 → CunO2−” is a primary reaction channel for most copper cluster anions.159 Further reactions were also observed for small Cun− clusters (n = 2−5), allowing the formation of a subsequent product, CunO4−. Further, the effective bimolecular rate coefficients for

(2)

The reason why the reactivity of cobalt clusters with oxygen differs from that of aluminum and Al−Mg clusters is partly due to the intrinsic activity of different metals, and those with relatively low boiling point readily combust (e.g., aluminum and magnesium). Moreover, Al clusters react and readily form a very stable molecule, Al2O, while Co clusters react and produce E


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Figure 6. (A) Measured effective bimolecular rate coefficients for the reactions of copper cluster anions (circles), silver cluster anions (triangles), and gold cluster anions (squares) with oxygen as buffer gas at a pressure of ∼60 Pa. (B) A list showing the effective bimolecular rate coefficients for Cun−, Agn−, and Aun− reacting with oxygen at ∼60 Pa buffer gas pressure. KII is the reaction rate, and Kc is the collision rate. Reproduced from ref 159. Copyright 1994 American Chemical Society.

the reactions of Cun−, Agn−, and Aun− with oxygen have been measured at a certain buffer gas pressure, as shown in Figure 6.159 Other than oxgen addition, collision-induced dissociation (or fragmentation) of coinage metal clusters was also included in such gas-phase reactions.65 Oxygen addition was also found to dominate the reactivity of cationic Con+ clusters.165 The cationic Con+ (n = 2−9) clusters displayed a high reactivity toward O2 by taking successive oxidation pathways. It is notable that the oxygen addition on Con+ clusters does not follow a direct attachment; instead, the primary reaction mainly results in a replacement of a Co atom by an O2 molecule. Also, the formed oxide clusters allow for successive reactions toward oxygen, which resembles the etching effect observed for anionic Con−, as discussed above, whereas there is huge difference between cationic and anioinc cobalt clusters, probably because the latter readily form a stable molecule CoO2− in reacting with oxygen. For cationic counterparts, successive oxidation reactions were found to virtually terminate when the formed ConOm+ clusters displayed stoichiometric structures of (CoO)4(CoO2)n+ (n = 0−3) or Co2,3O4,5+. The loss of a Co atom in the reaction of Co clusters with O2 (usually termed as a switching reaction) was demonstrated to be consistent with Stevenson’s rule.165−167 According to Stevenson’s rule,166 the ionic products in decomposition prefer to bear lower values of ionization potential (IP), and the coordination of ligands can reduce the IP of metal clusters.168 Examination of the reaction rate constants for the mass-selected cobalt clusters illustrated a strong correlation between the cluster sizes and their reactivity, suggesting that the geometric structures and the release of stable molecules could be a major factor in determining the cobalt cluster reactivity.165 3.1.3. Superoxo and Peroxo States. Along with oxygen intake reactions for metal clusters, it is important to note that the bonding mechanism between metal clusters and oxygen could involve charge transfer with a concomitant activation of the O−O bond. The excitation of triplet dioxygen (3Σg−) to the

more reactive singlet state (a1Δg) can be achieved by the interaction with metal clusters leading to the formation of superoxo state (O2−•) and peroxo state (O22−) complexes due to chemisorption and charge transfer.169 For example, Klacar et al.170 examined the reactivity of oxygen with Agn clusters containing up to nine atoms and found that the molecular oxygen preferred a dissociation mode for its adsorption on larger-sized clusters (e.g., containing more than five Ag atoms), where the activation of O−O bond was initiated at a superoxo state. There could be coexistence of superoxo and peroxo states, but a transition from the former to the latter determines the O−O bond activation within a cluster−oxygen reaction process.172 For this, Wang, Zeng, and their co-workers171 reported an in-depth study of O2 chemisorption on even-sized Aun− clusters (Figure 7). They demonstrated spectroscopic and electronic evidence of the transition from superoxo to peroxo chemisorption for Au8−. It was noted that both superoxo and peroxo states coexisted in the cluster beam of O2Au8−, and the superoxo form involves a single O−Au bond (η1-O2) while the peroxo form involves two O−Au bonds (η2-O2). Also found was that the superoxo O2Au8− exhibits low binding energy within the O2-induced potential energy surface (PES) feature, while the peroxo O2Au8− displays a sharper and higher binding energy feature. It was also noted that, although both twodimensional (2D) and three-dimensional (3D) isomers of Au12− coexist in the cluster beam, O2 prefers the peroxo binding with the 3D isomer of Au12−.171 Superoxo and peroxo states have also been found as important chemical processes associated with other metal cluster reactions.163,173 3.1.4. Cluster Odd−Even Alternation. As has been shown above, with sufficient oxygen flowing, a dramatic loss of Al cluster signal is readily discerned for most species, especially even-numbered clusters (i.e., odd number of electrons), but odd-numbered clusters (i.e., even number of electrons) could survive in the rich-pressure condition, displaying slightly enlarged mass abundance. This odd/even F


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Figure 8. (A) Product ion mass spectra after reaction of Agn− with O2. Ion intensities are plotted as a function of the number of adsorbed oxygen atoms m. (B) Examples of measured oxidation kinetics for Ag2− and Ag3− at 300 K. Open symbols, experimental data; solid lines, kinetic fit. Reproduced from ref 181. Copyright 2004 American Chemical Society.

A collaborative effort between the Castleman and Khanna groups has given further insights into the odd−even selectivity for silver clusters reacting with oxygen, as shown in Figure 9.63 It was illustrated that the necessity or not for Ag clusters to become spin excited (and hence to accommodate the triplet spin of oxygen) plays a determining role in their reactivity. This is consistent with the experimental findings that odd-electron silver clusters reacted with oxygen while the even-electron systems were relatively inert (Figure 9A). Furthermore, an anionic 13-atom cluster was found to exhibit unexpected stability against reactivity with oxygen, which was rationalized by comparing the reactivity of Ag13− with proximate evenelectron clusters such as Ag15−. The inertness of Ag13− is associated with its large spin excitation energy and a crystalfield-like splitting of the orbitals caused by the unique triangular bilayer structure, as well as a relatively large gap despite not having a magic number of valence electrons, as shown in Figure 9B. These investigations revealed that the reactivity of metal clusters with oxygen is correlated with the excitation needed to activate an O−O bond. Silver and oxygen have negligible spin− orbit effects, and hence, their reactions follow the Wigner− Witmer rules of spin conservation. For the odd-electron systems, the spin of the extra electron could align opposite to the majority spin electrons of the 3O2 molecule, and the spin conservation does not require any spin excitation of the metal counterpart, whereas for even-electron cluster systems, the spin multiplicity of oxygen is decreased, and the reacting cluster has to follow the spin excitation of the remaining portion to conserve the total spin. The spin effect will be further demonstrated below (see section 5.1.).

Figure 7. Comparison of the experimental PES spectra of O2Au6− (a), O2Au8− (c and d), O2Au12− (f), O2Au14− (h), and O2Au18− (j) with the computed spectra and the corresponding structures b, e, g, i, and k. The experimental spectra of O2Au8− (c and d) were obtained under two different experimental conditions. The simulated spectra of the superoxo and peroxo isomers are represented in brown and blue colors, respectively. Reproduced from ref 171. Copyright 2012 American Chemical Society.

alternation was also seen in other experiments where most of the even-atom clusters reacted away in sharp contrast to the odd, indicative of a paired electron effect.174,175 Taking a glance over the studies of metal cluster in reacting with oxygen, there are actually abundant investigations showing the odd−even alternation.153,158,161,176−186 For example, among the Aun− cluster reactions with O2, molecular oxygen addition was found to be the main pathway for the even-sized clusters, while the odd-sized clusters were inert toward O2.159,161,163 Such even−odd alternation correlates with a similar pattern in the electron affinities of Aun clusters, validating that electron transfer between Aun− and O2 dominates their reactivity.161,171 Experimental evidence have also been obtained via photoelectron spectroscopy, revealing that even-sized gold clusters favor O2 chemisorption by noting the distinguishable O−O vibrational fingerprints.187,188 Bernhardt and co-workers181,189 reported the reactivity of anionic silver clusters Agn− with O2, as shown in Figure 8. It was demonstrated that, among the Agn− (n = 1−11) clusters they studied, the even-atom anions (i.e., odd number of valence electrons) were more reactive than the odd-atom cluster anions for the reaction of the first O2. Even-atom clusters were found to react readily with molecular oxygen by generating AgnO2− products, except Ag4−, while the odd clusters reacted to bind two O 2 molecules, except for the single atom. The corresponding kinetic data were examined (Figure 8B), and a sequential reaction channel was proposed: k1

3.2. Metal Clusters Reacting with Halogens

Halogenation reactions are important processes in chemical synthesis, where the halide products are useful intermediates in synthesizing numerous functionalized materials. There are numerous investigations demonstrating structures, stability, and superhalogen behavior in various metal halides.190−200 Studies of metal cluster reactivity with halogen shed light on the understanding of principles in related synthesis processes. For example, from Al cluster reactivity, several metal halide cluster products showed reasonable stability and the AlnI− generation was found to be energetically favorable. Investigations on the reaction mechanism and transition state structures for the dissociative chemisorption of I2 on aluminum clusters demonstrated that a halogen molecule approaches

k2

Ag n− + 2O2 → Ag nO2− + O2 → Ag nO4 −

(3) G


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Figure 9. (A) The mass spectrum of silver cluster anions produced via a MagS source (a) and the spectra after exposure to different quantities of oxygen (b−e). (B) Calculated molecular orbitals of Ag13−.

aluminum with an end-on orientation, due to the more effective orbital overlap compared with a side-on orientation.201 Also, it was found that halogen incorporation could leave the electronic properties unperturbed for some aluminum clusters.202 With an emphasis on the magic cluster Al13−, the lowest energy structure for Al13I− was ascertained to feature an icosahedral Al13 unit with the iodine atom located at an on-top site. Bergeron et al.155 found similar results for Al13Br− and Al13Cl−, the HOMO charge densities of which are dependent on the identity of halogen. Considering that the group IB coinage metals (i.e., Cu, Ag, Au) have electronic configurations characterized by a closed dshell and a single s-valence electron, it is believed that the coinage metal clusters may react with halogens in a similar fashion to alkali metals.203−213 Figure 10A displays a mass spectrum of Cun− clusters produced via a MagS source in the presence of reactant chlorine.214 It was noted that when chlorine was introduced into the fast-flow reactor, all of the original Cun− clusters disappeared due to the violent gas-phase reactivity between chlorine and copper cluster anions, except for the weak intensity of Cu7− (which bears a closed shell of eight electrons according to the jellium model).146,215,216 Among the observed CunCln+1− species, CuCl2− dominated the products, and the others (i.e., Cu2Cl3−, Cu3Cl4−, Cu4Cl5−, Cu5Cl6−, and Cu6Cl7−) in the mass spectrum exhibited an exponential decay with increasing values of n, no matter the quantity of chlorine introduced. The novel CunCln+1− species closely resembled the early findings of NanCln−1+ and CsnIn−1+ clusters.198,208,217 First-principle calculations figured out a series of cubiclike structures for these CunCln+1− clusters pertaining to a starting point in the formation of ionic crystals (Figure 10B).214 Within this reactivity, parallel and consecutive reaction channels have been deduced at an interesting circulatory sequence involved in Figure 10C. Initially, when a Cun− cluster reacts with a Cl2 molecule, it follows a few feasible channels that produce both neutral and anionic species, such as CuCl

Figure 10. A mass spectrum of Cu cluster anions at the reaction with chlorine, leading to the CunCln+1− products (A), where the insets show the structural sketch in forming the crystal. (B) the DFT-calculated structures of the CunCln+1− products. (C) The proposed reaction channels of Cun− clusters with chlorine.

and CuCl2−. Further, these first-step pathways give rise to consecutive reactions and hence larger n-values of the CunCln+1− species.214 These investigations reveal that dissociative chemisorption dominates the reactivity of halogen with copper clusters. H


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3.3. The Reactivity toward Hydrogen and Nitrogen

There is a similar case for silver clusters in reacting with chlorine (Figure 11),218 where AgnCln+1− species were observed

In this section, we present the reactivity of metal clusters with hydrogen and nitrogen.80,215,221−232 Hydrogen is known as a highly combustible diatomic gas and the lightest element on the periodic table. Hydrogen readily forms covalent compounds with many nonmetallic elements, and it is an important reducing agent of metallic ores. Chemisorption of H2 on metal clusters has been extensively studied in the past 30 years in view of the broad interest of hydrogen storage.111,125,233−251 Nitrogen is the lightest pnictogen and it is the most abundant uncombined element, having an electronegativity of 3.04. An N atom consists of five electrons in its outer shell allowing a triple bond in molecular nitrogen (N2). The strong NN triple bond results in the difficulty of converting N2 into other compounds. Nitrogen is usually unreactive at standard temperature and pressure; however, metal lithium (or magnesium) does burn in an N2 atmosphere, giving rise to lithium nitride (or magnesium nitride). 3.3.1. The Reactivity with Hydrogen. Metal−hydrogen cluster reactivity has been extensively studied with a focus on iron, 2 3 6 , 2 5 2 cobalt, 8 4 , 1 2 5 , 1 2 6 , 2 5 3 − 2 5 7 vanadium, and niobium.258−262 A typical example in Figure 12 illustrates the reactivity of cationic Fen+ and Vn+ clusters with hydrogen (D2 was used in order to avoid mass degeneracy).261,262 It was found that the presence of positive charge had a substantial influence on the reaction rate for the majority of iron and vanadium clusters,236,263,264 and the kinetics of D2 chemisorption on Fen+/Vn+ clusters exhibited a nonmonotonic dependence on n. It is interesting to mention that studies of hydrogen chemisorption onto cationic Fen+ (n = 4−22) found a generally enhanced (although size-selective) reactivity compared to that for neutral Fen. This behavior can be rationalized within a framework of the frontier orbital model of activated chemisorption of hydrogen by invoking an activation barrier and incorporating electrostatic interactions arising from the nonzero charge state of the Fen+ cluster.261 The rate of D2 chemisorption on neutral niobium clusters has also been found to exhibit a striking dependence on cluster sizes.125,126,252,265 For the Nb clusters, Zakin et al.266 compared the reactivity and chemisorption kinetics of cationic, anionic, and neutral species (Nbn−, Nbn, and Nbn+) via measurements of the relative rates of D2 activation by these niobium clusters, as shown in Figure 13. It has been concluded that some of the

Figure 11. Mass spectra of Agn− clusters in the absence (a) and presence (b) of reactant gas Cl2 (1.2 sccm flow rate).

in the small mass range (n ≤ 4), seen as AgCl2−, Ag2Cl3−, Ag3Cl4−, and Ag4Cl5−, verifying the same mechanism as in the formation of the above CunCln+1− series. Beside this, there were also reaction products of AgnCl2− and AgnCl− being observed, indicating that multiple reaction pathways coexisted in such flow tube conditions. Note that the AgnCl− and AgnCl2− species appearing in the larger mass range (8 ≤ n ≤ 14) displayed an odd−even alternation.63 In particular, the intensity ratio of Ag8Cl− to Ag8− is larger than that in its original mass distribution, also larger than that of the other counterparts with 8 ≤ n ≤ 14. This has been rationalized under an interpretation of the harpoon reaction considering that Ag8− bears nine valence electrons and its reactivity toward chlorine mimics that of “K + Br2”.219,220 The harpoon mechanism relating to metal clusters will be further discussed in section 5.2.

Figure 12. (a) Relative rate constants for reaction of cationic vanadium cluster Vn+ toward D2 as a function of cluster size, as determined by bare cluster depletion. The Y-axis represents the reactivity equal to ln Sx/[D2], where Sx is the fraction of bare clusters unreacted, i.e., the survival fraction, while [D2] is a factor directly proportional to the concentrations of D2 in the reactor. Typical uncertainties are estimated to be ±20%. Reproduced from ref 262. Copyright 1989 American Chemical Society. (b) Reactivity of cationic Fen+ toward D2 under identical conditions. Typical uncertainties are ±10%. Reproduced with permission from ref 261. Copyright 1988 American Institute of Physics. I


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Figure 13. Dependence of D2 chemisorption reactivity Rx on cluster size, for Nbx (a), Nbx− (b), and Nbx+ (c), respectively. Two data points are included for Nb9, Nb12, and Nbl2+ in order to reflect the rapid and slow components of cluster depletion. The three curves are plotted on the same vertical scale with typical uncertainties ±20%. Reproduced with permission from ref 266. Copyright 1988 American Institute of Physics.

niobium clusters react with D2 and exhibit different reaction rates at negative or positive charge state; in particular, the excess charge displays a profound influence for the clusters sized at 7 ≤ n ≤ 16, revealing the dependence on electronic structure of the reaction of niobium clusters with hydrogen. In addition, the maximum uptake of D2 by niobium clusters was found to be essentially independent of the charge state but varying with n. This is reasonable, as a high barrier may be present with size selectivity for the D−D bond activation of the clusters.126,235 Within numerous investigations, it is conclusive that hydrogen chemisorption is the dominant reaction pathway, and the hydrogen coadsorption could largely increase the photoionization threshold energies of the small metal clusters, allowing for dramatic cluster size dependence. The photoionization threshold energies were found to be even larger for multiple H2-chemisorptive clusters compared with the related bare metal clusters.221 Regarding the size dependence of hydrogen chemisorption, a typical study of the reactivity of neutral Al clusters with hydrogen demonstrated that clusters with closed electronic shells are less reactive to hydrogen than those with unfilled shells, indicating a pattern similar to that of the metal cluster reactivity with other diatomic gas molecules.221 Considering the H2 concentration as a constant throughout the reaction region, the reactivity can be quantified within a simple pseudo-first-order kinetic relationship −ln(fr ) = k[H 2]τ

Figure 14. A chemisorption study of D2 on neutral cobalt clusters. Controlled mass spectrum was collected with only pure helium injected as the reactant gas (a); the lower two mass spectra were taken with 3.6 sccm (b) and 5.0 sccm (c) flow of injected D2 reactant, respectively. The sharp peaks seen in the bottom-most trace for clusters with more than 10 atoms are all due to cobalt clusters with more than one molecule of D2 chemisorption. An enlarged spectrum shows the detail of the D2 chemisorption experiment on Co trimer (d) and the 9−16 atom clusters (e), where the dashed mass spectrum is the control experiment (only pure helium involved), while the solid trace is the observed mass spectrum with an average reactant flow of 7.4 and 5.0 sccm of D2, respectively. Reproduced with permission from ref 126. Copyright 1985 American Institute of Physics.

(4)

where f r is the fraction of bare cluster remaining, k refers to the rate constant for the addition of the first H2 to the cluster, [H2] is the concentration of the hydrogen, and τ is the reaction time approximated as “reaction channel length/flow velocity”. On the basis of eq 4, the constant k can be determined by measuring the mass spectra for a series of H2 flows and plotting out the logarithm values of the remaining bare cluster signal versus reagent flow, and then the rate constants can be calculated. In addition, dissociative chemisorption of D2 was also found to occur in the cases of cobalt and nickel clusters showing vivid sensitivity to the cluster sizes, as shown in Figure 14, where the detailed patterns of reactivity differed for each metal under the same conditions. Among the observed D2 chemisorption product species, Con(D2)m formation appears to shut off at five D2 molecules for Co11 and Co12; six for Co3, Co13, and Co14; and up to seven for Co15 and Co16. 3.3.2. The Reactivity with Nitrogen. Nitrogen gas, known as the most stable diatomic molecule, also allows for the addition on certain-sized metal clusters. For example, Lang and Bernhardt studied the reactions of small gold clusters Au3+

and Au5+ with N2 in a multicollision octupole ion trap.267 Lowtemperature reaction behaviors for mass-selected Au3+ and Au5+ clusters toward N2 were addressed, as is seen in Figure 15, where the products AunNm+ were marked with (n, m). It is

Figure 15. Ion mass distributions of Au3+ (a) and Au5+ (b) in the presence of pure N2 at TR = 200 K. The mass peaks are denoted by (x, y) corresponding to complexes of the stoichiometry AuxNy+. Reproduced with permission from ref 267. Copyright 1986 American Institute of Physics. J


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interesting that Au3N6+, Au5N8+, and Au5N6+ were observed as products, respectively, for mass-selected Au3+ and Au5+, indicating adsorption of multiple N2 molecules on the Aun+ clusters along with size-dependent selectivity. Among others, neutral and cationic cobalt clusters were found to be more favorable to undertake such chemisorptive reaction with N2 and typically form Con(N2)m species with n and m depending on the environmental temperature.260,268 The adsorption of molecular nitrogen on the cobalt cluster surfaces was demonstrated to help determine the geometrical structures of the related small cobalt clusters.268 What was interesting is that almost no reaction was observed for nitrogen toward anionic cobalt clusters,153 except for Co7− and Co8−, which adsorbed a single nitrogen molecule. Weak adsorption energies for N2 on Con− clusters provide smaller amounts of energies (e.g., 20−50 kJ/mol), which could be completely removed by buffer gas collisions before fragmentation occurs. A few other metals have also been studied showing similar reactivity toward nitrogen,125,233,255,258,269−272 such as tungsten,273 nickel,274 niobium,80,231 and molybdenum.275 Among these, niobium clusters readily react and attach N2 molecules. Even in the nascent niobium cluster distribution there could be contamination peaks of Nbn(N2)m in the small mass region.126 Also, well-defined product peaks of Mon(N2)1,2 were found to dominate the reaction products within “Mon + N2”, as shown in Figure 16. The temperature dependence of rate coefficients

where M refers to a generic metal, Mn(N2)x is the precursor, while MnN2x is the chemisorption product, and ka and kb are the rate constants for association/dissociation and chemisorption, respectively, depending on the metals and environmental temperature. It is still worth mentioning that nickel and copper clusters were found to be less reactive or almost nonreactive with nitrogen at room temperature.126 The tendency of N2 chemisorption on small metal clusters was due to limited but varied charge transfer between the orbitals of metal clusters and the nitrogen molecules.273 Different charge transfer rates allows the metal clusters to react with nitrogen slowly or rapidly. Having discussed the diversity of cluster reactivity of metals toward nitrogen and hydrogen, herewith we also summarize their similarities. First, metals take on similar patterns of reactivity with H2 and N2, i.e., chemisorption or dissociative chemisorption, although with different size dependence. Second, the critical factor enabling dissociative chemisorption of H2 or N2 on transition-metal clusters is applicable to both. It is worth mentioning that the dissociative chemisorption behavior of H2 or N2 on metal surfaces has long been a unique scientific topic in the chemistry of transition metals, where the surface d-orbitals play an important role. Through theoretical calculations, researchers have modeled the H2 chemisorption on surface sites of small metal clusters and found that the 3d orbital participation is crucial in lowering the activation barrier for the dissociative chemisorption of such diatomic gas molecules.276 It is still interesting to note that in the aforementioned studies of small metal clusters reacting with hydrogen and nitrogen, neither display a conspicuous odd− even effect, which largely differs from the metal cluster reactivity with oxygen. As mentioned earlier, oxygen is spintriplet in its ground state and the two half-filled molecular orbitals are antibonding in nature. Therefore, the activation of an oxygen molecule (3O2) requires filling its half-filled antibonding orbitals and reducing the multiplicity from triplet to singlet. For clusters with an odd number of electrons, the reaction can proceed without changes in the multiplicity of the cluster, while for clusters with an even number of electrons, the spin conservation requires a spin excitation of the cluster, leading to a barrier resulting in a dominant odd−even effect. Nitrogen and hydrogen tend to chemisorb on metal clusters, where the N−N and H−H bonds do not have to be broken, and even with dissociative chemisorption, there is no necessity to change the spin multiplicity of the metal clusters. 3.4. The Reactivity with CH4, C2H2, and C2H4

The homo- and heterolytic C−H bond cleavage (as well as functionalization) is well-known for its significance in chemistry and is regarded as a longstanding central challenge.277−290 Metal cluster reactivity shows its own novelty in this topic. For example, considerable investigations have been conducted upon the activation of methane by gas-phase palladium model systems.291−297 Experimental observations showed that neutral clusters Pdx (x ≤ 24) tend to adsorb methane with a few exceptions (e.g., Pd3 and Pd4), while in contrast, the reactivity of small palladium oxides toward methane is size-dependent for methane dehydrogenation.123,298 Recently, Bernhardt and coworkers123 showed an interesting study on the reactivity of methane with Pdx+ (x = 2−4) clusters. Mass spectrometric and reaction kinetic studies under different temperature conditions have elucidated the intrinsic propensity of palladium clusters in reacting with methane molecules, as depicted in Figure 17.123 The mass spectrum recorded in the case of Pd2+ displayed four

Figure 16. (a) Mass spectra of molybdenum clusters recorded in the presence (top) and absence (bottom) of 0.6% N2 in a reaction zone at 300 K. Product peaks due to Mo7(N2)1,2 and Mo13(N2)2 are prominent in the top trace. (b) Second-order rate coefficients for reaction of molybdenum clusters with N2 at 279, 300, and 372 K. Points connected by dashed lines are upper limiting values. Reproduced with permission from ref 275. Copyright 1995 American Institute of Physics.

coincides with the reaction mechanism where initially a weakly bonded molecular precursor state is formed.275 Simply, the metal cluster reactivity with nitrogen can be summarized as ka

kb

M n + x N2 ↔ M n(N2)x → M nN2x

(5) K


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reactive with C2H2, regardless of the cluster sizes. This interesting difference was explained by the activation energies for their chemisorption reactions. Cationic Con+ clusters showed similar size dependence as neutral Con clusters, except for Co4+ and Co5+, which display enhanced reactivity due to active sites induced by the influence of their positive charge. On the other hand, the adsorption rate of C2H2 to the cobalt cluster surfaces was found to be large enough to attain a stable chemisorption state instead of an activation of the C−H or C− C bond. Therefore, gas-phase reactivity does not necessarily depend on cluster sizes when the chemisorption state is not affected by the different frontier orbital energies. Figure 17. (Left) Ion mass distributions obtained after reaction of palladium clusters Pd2+ (upper) and Pd3+ (lower) with CD4 at room temperature (tR = 0.1 s). (Right) The proposed reaction mechanism for Pd2+ with CD4. Reproduced from ref 123. Copyright 2013 American Chemical Society.

4. REACTIVITY OF METAL CLUSTERS WITH POLAR MOLECULES As discussed in the above section, a shell-filling concept from traditional valence bond theory has been successfully applied to describe cluster stability and size-dependent reactivity, including molecule coadsorption and etching reactions. In this regard, chemical interactions are prone to energy minimization, which is attained when a cluster closes an incomplete electronic shell, either by ionization or forming a covalent/ionic bond. However, unexpected stability could be observed for some clusters without a closed electron shell according to the NFEG model, such as several interesting reports of superatoms that mimic an element or a group of the periodic table of elements.3,202,305 In addition to investigations of electronic structures, understanding how specific sizes and/or shapes affect the affinities and interactions of metal clusters toward a specific reagent will facilitate the efforts to design new materials/catalysts for specific applications. Also insights gained from metal cluster reactivity toward various polar molecules can be used to follow the course of reactions affected by numerous reactive centers/defect sites.

signal peaks corresponding to the bare unreacted Pd2+, the association complex Pd2CD4+ (weak), and the methaneactivated products Pd2C2D4+ (strong) and Pd2C3D8+ (weak). Similarly, Pd3+ reacted with CD4, resulting in the main product Pd3CD2+, together with a byproduct Pd3C2D6+, pointing to the additional adsorption of a second methane molecule. Tetramer Pd4+ was not observed to exhibit apparent reactivity under the room-temperature condition. With Pd2+ as a typical example, theoretical investigations addressed the activation of a first CH4 on Pd atoms and demonstrated the strong dependence of C−H bond cleavage to form hydrido−methyl complexes (H−Pdx− CH3). Such studies provide valuable information for homologous metal and metal oxides used as important catalysts in industrial processes,292−294,296,299−303 and are helpful in understanding the pivotal parameters and elementary reaction mechanisms involved in catalysis. Nonpolar gas molecules have also been found to be reactive with other metal clusters.259−262,304 For instance, utilizing a fast flow tube method, Kaya and co-workers260 investigated the reactivity of cobalt cluster cations Con+ (n = 2−22) with a few molecules, including CH4, C2H4, and C2H2 (also N2, H2). Their results indicated an interesting cluster size dependence for the reactants CH4 and C2H4 (also N2, H2), where Co4,5+ and Co10−15+ displayed much higher reactivity than their neighboring clusters; however, the Con+ clusters were found to be highly

4.1. Reactions with Monoxides

4.1.1. Adsorption of Carbon Monoxide. There are extensive investigations of metal cluster oxides reacting with N2O, NO, and CO in view of their importance in removal of pollutants, where the carbon/nitrogen monoxides are reduced, giving rise to less-poisonous oxygen-rich products.158,306−326 These interesting studies on cluster oxides, such as AgO,116 AuO,321,322 CeO2,309,327 FeO,325 and VO,306,328 reveal an

Figure 18. (A) Mass spectra of parent and product ions obtained in the reactions of Cu7Ti+ with NO under (a) single collision (∼2.7 × 10−2 Pa) and (b) multiple collision (∼0.27 Pa) conditions at the initial collision energy of 19.3 kJ/mol. (B) Relative intensities of the parent ion and the major product ions in the reactions of Cu7Ti+ with NO as a function of the NO pressure. Reproduced from ref 331. Copyright 2016 American Chemical Society. L


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Figure 19. (A) Distribution of Aln− (n = 7−73) clusters reacting with D2O. Nonpure aluminum clusters are shown in red. (B) Reaction of low-mass Aln− clusters (n = 7−20) with D2O. (C) Expansion of the shaded area in panel B. Red peaks are Al16− species; blue peaks are Al17−.

selectivity of Aln− reacting with oxygen (Al13−, Al23−, and Al37− have closed electronic shells and survive the oxygen etching), the magic cluster Al13−, having a strikingly high LUMO energy level, binds water quite weakly without surprise; in contrast, Al12− does bear low LUMO energy and particularly high binding energy with water. A reexamination of ionization energies provided a good match with superatom model shell closings.144,353 However, the closing of superatom shells and the location of LUMO were found to be insufficient to explain the observed selective reactivity of Aln− with water. Firstprinciples calculation revealed that, among the various Aln− species, clusters such as Al11− and Al13− bear insurmountable energy-transition states and hence have low reactivity toward water.354 While Al12− binds water tightly to form the product Al12H2O− in reasonable abundance, Al14− and Al16−, having open electronic shells, did not support the product observation with water adsorption. In order to interpret these findings, the complementary active sites (CAS) mechanism was proposed to rationalize the size-selective reactivity of Al cluster anions with water. Complementary active sites refer to adjacent locations on the cluster surface where, for example, an Al atom site acts as a Lewis acid and a second Al atom acts as a Lewis base. In principle, the initial interaction between a molecule (like water) and metal clusters (like Aln−) is the nucleophilic attack of a H2O molecule on the aluminum surface; therefore, the donation of lone-pair electrons from H2O to the LUMO (or LUMO+1 for odd-electron systems) of the Al cluster is required.102 After the initial adsorption, the presence of a neighboring Al site that could act as a Lewis base leads to a breaking of the OH bond, resulting in the adsorption of water. The presence of these neighboring pairs is governed by the geometry of the cluster that leads to a concentration of LUMO and HOMO on the adjacent sites, allowing the formation of such pairs. In clusters that have geometries that can support two such pairs in neighboring locations, the dissociated H atoms can combine to form H2 molecules, leading to the spontaneous production of hydrogen. This was shown to be the case, for example, for Al16−. This is also the main reason for the difference between Al12− (forming Al12H2O−) and Al16−18− (producing H2) in reacting with water.19 Note that the transition state for splitting H−OH in an Eley−Rideal-type

important oxygen atom transfer mechanism. More details about these investigations are available in a few previously published review papers.27,116,307,312,329,330 In comparison, less research interest was attracted to cluster reactivity of bare metals with such simple gas molecules. Nevertheless, metals do exhibit sizedependent reactivity toward these diatomic polar molecules, as reported in several previous investigations.107,162,211,311 By conducting CO adsorption studies on metal clusters in the gas phase, researchers aimed at obtaining information relating to the sizes, shapes, and electronic structures so as to extend the knowledge of cluster reactivity and bonding mechanisms to pollutant removal in the environment. 4.1.2. Cleavage of N−O Bond. Other than reasonable research interest in CO adsorption on metal clusters, the reactions of N2O and NO with neutral or ionic clusters have also been studied. For example, recently Hirabayashi and Ichihashi reported the reactivity of NO with two series of cluster cations, CunTi+ (n = 4−15) and CunV+ (n = 5−14, 16), by using a guided ion beam tandem mass spectrometer.331 It was found that most of these copper alloy clusters adsorb NO under single-collision conditions, followed by the release of Cu atoms; in comparison, multiple-collision reactions with NO were found to allow subsequent reactions by plucking out an N atom, as shown in Figure 18, where the reactivity of Cu7Ti+ is clearly illustrated. Among others, the cleavage of the N−O bond giving rise to N2 release was also observed in the reactivity of Ptn (n = 4−12) with N2O by Yamamoto et al.,332 who demonstrated that the associated oxygen could transfer to Ptn clusters, allowing the production of nitrogen via “Ptn + N2O → PtnO + N2”. 4.2. Reactions with Hydroxyl Compounds

4.2.1. Reactions with Water. Theoretical and experimental investigations into the reactivity of metals with water have attracted reasonable research interest, partly due to the importance of hydrogen evolution.333−352 Castleman, Khanna, and their colleagues102 reported an interesting study on aluminum cluster anions that were reacted with water in a gas flow tube. They found that Al16−, Al17−, and Al18− result in spontaneous production of H2 from water, while Al12 reacts to form the product Al12H2O−, as shown in Figure 19. Referring to the aforementioned NFEG theory in explaining the sizeM


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Figure 20. Reaction of Al cluster anions with methanol (A) and tert-butyl alcohol (B).

mechanism requires a dissociated H atom (rather than an Al atom) to act as the Lewis acid, whether or not producing H2. As a free H atom may freely move on the cluster surface, it could find the other paired active sites, resulting in the inferred release of H2. The CAS mechanism will be further discussed below. 4.2.2. Reactions with Alcohols. It is believed that the aforementioned reactivity of Al cluster is not restricted to water but rather occurs with any molecules that have an −OH functional group,64,355−368 such as the reactions of Al cluster anions with methanol (Figure 20A)369 and tert-butyl alcohol (Figure 20B).370 However, it was found that alcohols (especially methanol) exhibit an etching effect toward the Al clusters with few exceptions (such as Al13−). The etching by methanol closely resembles the reaction of Aln− toward oxygen, where selective species (e.g., Al13−) exhibit resistance to the etching effect and the cluster’s increasing abundances is due to its being a unique, stable product after the dissociation of larger Al clusters. In addition to the etching effect, alcohols allowed the binding to the Al clusters but were not observed to produce H2 in the fast-flow tube reactor at room temperature. Among the Aln− species that underwent the attachment of one or multiple alcohol molecules, Al15−, Al16−, Al17−, Al19−, and Al21− proved to be highly reactive species, undergoing chemisorption of different numbers of hydroxyl molecules. It was noted that Al15− attaches only one methanol molecule, but Al17− gives rise to attachment of three MeOH molecules, forming Al17(CH3OH)3−. This is because there are more active sites on Al17− than Al15−, and Al17− cluster offers less steric hindrance to attach multiple methanol molecules. This is also consistent with the mentioned CAS theory in interpreting the sizeselective reactivity of Al clusters with water. It is worth mentioning that although H2O has a slightly larger O−H bond dissociation energy compared to methanol (497.1 vs 437.6 kJ/ mol respectively),371 the −OH group in methanol seemingly does not undergo cleavage as easily as that in water. 4.2.3. Hydrogen Evolution Origin. A further insight into the reactivity with hydroxyl compounds is the origin of the hydrogen evolution reaction (HER), which has been an old project.372−374 Recently, Luo et al. reported a systematic study of Al clusters reacting with hydroxyl compounds by utilizing water, methanol/2-propanol, and their mixture as reactants (Figure 21).369 As demonstrated, typical HER of metal clusters involves a discharge reaction, i.e., a Volmer step (H+ + e− → Had) or the discharge reaction of H3O+ ions (H3O+ + e− → Had + H2O), where Had refers to an adsorbed H atom, which is mobile on the metal cluster surface; this is followed by a recombination reaction, either a Tafel step (Had + Had → H2) or a Heyrovsky step (H+ + Had + e−→ H2, or H2O + Had + e−→ H2 + OH−).369 Such insights gained from gas-phase cluster reactivity help to improve the understanding of the HER

Figure 21. Reaction of H2O and CH3OH (1:1 molar ratio) with Al clusters: (a) the original Aln− spectrum before the reaction and (b) the spectrum showing the products after simultaneous exposure to the reactants. (Inset) A sketch showing the competition and interaction between H2O and CH3OH when reacting with Aln−.

mechanism, which is known to be an important issue in chemistry in view of the potential value of H2 as fuel.369 4.3. Reactions with NH3 and H2S

Several early investigations have demonstrated the reactivity of metal clusters with ammonia,375−394 an example of which is shown in Figure 22, where the interesting clusters Al11NH3− and Al12NH3− dominated the products. Theoretical calculations revealed that the presence of ammonia leads to geometric distortion of Al12− together with dissociation of the NH3 molecule, the N and H atoms of which transfer onto the Al cluster, resulting in a reorganized minimum-energy structure. The dissociation of NH3 molecule resembles the dissociation of H2O as illustrated above, revealing a similar mechanism for such polar molecular systems other than just water. Identical experimental results have also been reported by Woodward,395 confirming the interesting reactivity of Al12− with NH3, as shown in Figure 23, where the reactivity of Al cluster anions with H2S gas was also addressed. Interestingly, Al12− appears very reactive both in the presence of NH3 and H2S. This was explained by its very prominent complementary active sites, low LUMO energy level, and high binding energy of the cluster. In contrast, Al13− and Al20− are less reactive than their neighboring clusters. It is thus concluded that polar S−H and N−H bonds undertake a similar mechanism as the O−H bond of water in reacting with aluminum clusters.395 Among others, the reactivity of gold cluster cations (Aun+) with H2S has also been studied by Sugawara et al.396 It was N


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Figure 22. Typical mass spectrum distributions of anionic Aln−clusters in the absence (a) and presence of NH3 (b). Minimum-energy structures of three distinct chemisorbed isomer classes (c). Reproduced with permission from ref 377. Copyright 2009 American Institute of Physics.

Figure 23. Mass spectra of Aln− clusters after reacting with (a) NH3 and H2S (b).

found that initial products were mainly AuSH+ for n = 2, while selective AunS+ and AunSH2+ were observed for the other Aun+ clusters. Also, gold cluster cations with an even number of atoms were found more reactive than adjacent odd-number clusters; no reactions for Au+ and Au3+ were observed in their study. The low reactivity of adatom Aun+ for n = 1, 3, 9, and 11 coincides with the low ionization potential of Aun and the weak binding energy of Aun+−Au. Further sulfuration reactions of AunS+ proceeded to give AunSm+ and finally stopped at those AunSm+xH2+ species when H2 release did not occur and the maximum number of sulfur atoms m + x increased with the cluster sizes. 4.4. Reactions with Acetone and Formaldehyde

Investigations of metal cluster reactivity have also been carried out for carbonyl-containing species of different bond strengths, e.g., formaldehyde (743.4 kJ/mol), acetone (771.4 kJ/mol), carbon dioxide (532.2 kJ/mol), and carbon monoxide (1076.4 kJ/mol).397 As results, CO bond cleavage was found for acetone and formaldehyde reacting with Al clusters (Figure 24),366 which is in sharp contrast to the case for carbon dioxide and carbon monoxide, which showed no reactivity even though carbon dioxide has a lower bond dissociation enthalpy than formaldehyde and acetone.366 Considering that the carbonyl bond is known as an important reaction center in organic

Figure 24. Aluminum cluster anion distributions after reaction with formaldehyde (A) and acetone (B). Aluminum clusters Aln− are labeled with blue numbers and formaldehyde additions [Aln(OCH2)−] are labeled with red numbers, while oxygen losses [Aln(CH2)−] are labeled with green numbers.

O


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Figure 25. Theoretically determined reaction coordinate diagrams of Aln− + OCH2 for (A) n = 9, (B) n = 12, and (C) n = 13. For each initial structure, the HOMO and LUMO (or LUMO+1) are shown in red and blue, respectively. The results reveal that Al9− and Al12− will react readily at the complementary active sites and subsequently lose an Al2O, while Al13− does not have active sites and has both a barrier to carbonyl cleavage and an endothermic Al2O release. (D, E) Theoretically determined reaction pathways for “Al11− + OCH2”. The LUMO and LUMO+1 are shown in blue and light blue, respectively, while the HOMO and HOMO−1 are shown in red and yellow, respectively.

Figure 26. Size distributions of Aln− clusters in the absence (a) and presence (b) of EtSH. The gas flow rate of EtSH was ∼7.2 sccm. Alternative assignments are made for some peaks. The inset images show the HOMO profiles of Al12S3− and Al17S2−.

indicating again that Al12− reacts more readily with formaldehyde. It is interesting to note that active sites on Al11− cleave formaldehyde, which differs from the previous finding that Al11− does not cleave water, because formaldehyde is more sensitive than water to the strength of the Al11− active sites. As shown in Figure 25D, the oxygen attacks the LUMO of Al11− along with a C−Al bond associated with the HOMO, while in the lower pathway (Figure 25E), oxygen binds to a second site with a dominant LUMO density and carbon binds to the HOMO−1. Both cases are energetically favorable and there is sufficient energy for Al2O loss. The production of Al2O was found to be much more energetically favorable, being >144.7 kJ/mol more exothermic than the loss of O, AlO, or Al3O.366

synthesis, this finding provides important information on C O bond chemistry. Comprehensive calculations have been performed on the carbonyl bond cleavage, with an emphasis on Al9−, Al11−, Al12−, and Al13−, as shown in Figure 25.366 It was found that Al9− readily reacts at the active sites and loses a stable Al2O molecule (Figure 25A), while Al13− does not have active sites and has a barrier to carbonyl cleavage (Figure 25C), and also Al2O release is endothermic. These are in agreement with experimental observations. The reaction pathway between Al12− and formaldehyde is similar to that of Al9− (Figure 25B), but the barrier for splitting the carbonyl is even lower than that for Al9− and the cleavage of the C−O bond gives rise to 3.03 eV energy, P


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Figure 27. Reactions of mass-selected Al13− clusters with 3O2 (A) and with a 1O2/3O2 mixture (B). The FT-ICR mass spectra show Al9− as the only major reaction product at m/z = 242.9 after up to 400 s of exposure to 3O2 and 1O2/3O2. (C) Calculated energy diagram for the interaction of 1O2 and 3O2 on the Al13− cluster surface. Adapted with permission from ref 142. Copyright 2008 American Association for the Advancement of Science.

the ethyl radical leaves the cluster. While the even-electron species showed a different number of attached H2S as Ag2n+1(H2S)1,2−, the odd-electron clusters (i.e., an even number of silver atoms) led to Ag2n(HS)− products such as Ag8(SH)−, Ag10(SH)−, and Ag12(SH)−, indicating that HAT between the leaving ethyl group and the silver sulfur cluster does not significantly occur on the cluster with unpaired electrons. These reactivity patterns are summarized by the following pathways:399

Theoretical analysis on the basis of the CAS mechanism demonstrated that the adjacent Lewis acid and Lewis base sites stabilize the resonance structure of Al11CH2O−, facilitating carbonyl cleavage by weakening the C−O bond. 4.5. Reactions with Thiols: C−S Bond Activation

Thiols are known as organosulfur compounds that contain an R−SH group and often exhibit reactivity comparable with that of alcohols (R−OH). Luo et al.398 reported an interesting study that explored the reactivity of thiols with Al clusters, providing insights into the C−S bond activation, as shown in Figure 26. The reaction that forms AlnSm− species is associated with a byproduct, ethylene (C2H4), that is a very stable molecule. Considering that an S atom has six valence electrons (3p orbitals), when it forms two single bonds with Al atoms, the electron cloud interaction between the S and C atoms is reasonably weakened, giving rise to C−S bond cleavage, according to such a reaction path as398

Ag n− + mC2H5SH → Ag n(HS)m− + mC2H5

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In a simple summary, the numerous investigations of metal cluster reactivity toward polar molecules have helped the understanding of the nature of chemical bonds that may be broken in this manner and also helped to identify what surfaces and clusters may possess the well-patterned Lewis acid/base sites optimal to promote this type of reactivity.400 This is particularly interesting in the development of alternative reactants and clusters (/metal surfaces) that would further the CAS mechanism for species with large bond energies, be they metal oxides, bimetallic interfaces, designed defect sites and step edges, or cluster-assembled materials.4,401−405

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Meanwhile, a hydrogen atom on the −CH3 group still competes with that in the −SH group and allows the adsorption on active sites of the Al cluster, leading to an alternative reaction pathway:398 Al n− + m EtSH → Al n(HSH)m− + mC2H4 → Al nSm− + mC2H4 + mH 2

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Ag n− + 2C2H5SH → Ag(H 2S)2− + 2C2H4 + Ag n − 1

Al n− + 2EtSH → Al n(EtS)2− + H 2 → Al nS2− + 2C2H4 + 2H 2

Ag n− + mC2H5SH → Ag n(H 2S)m− + mC2H4

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5. THEORY AND MECHANISMS

These reaction pathways validated the C−S bond activation in Al cluster reactions. Insights into the C−S bond breaking are important and ubiquitous in many chemical, environmental, and biological processes. Further, the gas-phase reactivity of ethanethiol with silver cluster anions was also examined;399 nevertheless, the primary cluster products were found to be AgnSH− and AgnSH2−, together with the interesting byproducts H3S− and (H3S)2−. The cleavage of the S−H bond of EtSH when reacting with Ag clusters was ascertained as one of the major reaction pathways, and the final products AgnSH2− require a hydrogen atom transfer (HAT) from the ethyl group to the cluster, along with ethylene release. Also found was that the even-electron Ag clusters (i.e., Ag2n+1−) showed greater abundance than the oddelectron clusters. In addition, product clusters Ag9(H2S)2− and Ag11(H2S)2− were also observed, indicating that Ag2n+1− clusters with an even number of electrons undergo HAT as

5.1. Spin Effect

As mentioned above (section 3.1.3), spin excitations hold a key in determining cluster reactivity of metals with oxygen. In view of the triplet spin of molecule oxygen (3O2), Kima and coworkers406 performed a comparison of the reactivity of Cu clusters with molecular oxygen and atomic oxygen and showed that atomic oxygen is more reactive for Cu cluster anions.164,406 Similarly, Schnöckel and co-workers142 found that the reaction rates of odd-numbered clusters obviously increased in the presence of singlet oxygen instead of triplet oxygen in its ground state. Figure 27 gives an example of Al13− reacting with molecule oxygen (3O2) and singlet oxygen (1O2), respectively, along with an illustration of the spin conservation in Al cluster reactivity. First, Al clusters form adducts [Aln·O2]− with the associated oxygen bound to its surface. Considering the spin conservation restrictions, 3[Al2n+1·O2]− is formed in a triplet Q


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Figure 28. Observed Aln− mass spectra (black) before and (red) after introduction of (A) 42.5 sccm of O2(X3Σg−) and (B) 42.5 sccm of a 12% O2(a1Δg)/O2(X3Σg−) mixture. (C) Observed and calculated branching ratios for (green) initial and final aluminum distributions with (black) O2(X3Σg−) and (red) O2(a1Δg)/O2(X3Σg−). Observed values are represented by circles. Calculated values are represented by colored lines.

state, while 2[Al2n·O2]− is in a doublet state, as the original oddand even-numbered Al cluster (i.e., Al2n+1− and Al2n−) are in singlet and doublet state, respectively. Next, the molecular oxygen dissociating on the surface of the structure-rearranged cluster may cause heating, resulting in the fragmentation of the cluster together with the production of the molecule Al2O. Because 3[Al2n+1·O2]− is in a triplet state while its energetically accessible fragments (Al2n‑1− and Al2O) are all in singlet states, a spin transition has to be completed within this inherently slow process. The direct formation of singlet-state 1[Al2n+1·O2]− from 1Al2n+1− and 3O2 is spin-forbidden; therefore, the reaction is unlikely to proceed because the spin−orbit coupling of such light metals as aluminum is small and hence prevents appreciable overlap between the potential energy surfaces of different spin states. However, in the case of Al2n− cluster systems, no such spin transition is needed, since the initially formed 2[Al2n·O2]− can directly react via 2Al2nO2− to form products 2Al2n−2− and Al2O. Figure 27C presents the calculated energy diagram for the interactions of 1O2 and 3O2 with an Al13− cluster to form Al9− and Al2O, respectively, revealing that it is necessary to undergo a spin excitation energy for the reaction with 3O2. Similarly, the spin accommodation of Al reacting with singlet oxygen and molecular oxygen has been further examined in a fast flow tube cluster system, as shown in Figure 28.150 Considering that the reactivity of Al clusters with ground-state molecular oxygen follows “Aln+4− + O2 →Aln− + 2Al2O”, as described in eq 1, Al9− will not be formed in such etching reactions since Al13− is unreactive with molecular O2 (X3Σg−). By introducing additional singlet-state O2 (a1Δg) along with ground-state triplet O2 (X3Σg−) into the multiple-ion flow tube, reasonable stability of Al9− was revealed. Reber et al.143 reported an interesting study on the cluster reactivity of AlnHm− with oxygen, as shown in Figure 29. It was noted that all the clusters containing even numbers of hydrogen atoms were etched away, while those containing odd numbers of hydrogen atoms survived. This is interesting, as the odd/ even electron counts of metal clusters can be simply altered by the addition of hydrogen atoms. First-principles calculations were performed to investigate the HOMO−LUMO gaps, VSE energies from a singlet state to triplet state (the lowest energy

Figure 29. (A) Mass spectrum of Al4Hn−. (B) Mass spectrum of Al4Hn− after exposure to oxygen. (C) Vertical spin excitation (V.S.E.) energy, HOMO−LUMO gap (H−L Gap), and adiabatic spin excitation (A.S.E.) energy of Al4Hn−.

required to excite the cluster without geometry rearrangement), and also adiabatic spin excitation (ASE) energies (the difference between the ground state of the triplet and the ground state of the singlet, allowing geometric rearrangement). It was found that there is a strong correlation between the reactivity and their spin excitation energies. These investigations demonstrated how the cluster reactivity of metals can be altered by changing their spin states.143 The spin effect as meticulously studied for aluminum clusters is also largely applicable to other metal clusters, such as the aforementioned reactions of Agn− with O2, where an evident even−odd oscillation has been revealed.63,159 Moreover, within a very recent study toward the kinetic measurement of reactions of Agn− (n = 6−69) with O2 run at 120 K,407 it was shown that the reactivity of large Agn− up to nanoscale is still dominated by the clusters’ global electronic properties along with spin effect. Nearly all even-sized Agn− adsorb oxygen molecules, but the odd-numbered Agn− with n = 6−69 are generally inert or have low reactivity. 5.2. Harpoon Mechanism

There is an interesting cluster reactivity linked to the harpoon mechanism that was discovered between halogens and alkali metals. The main feature is that such reactions have steric factors greater than 1; in other words, the reaction takes place faster than predicted by collision theory. According to gasphase collision theory, the reaction rate ν based on collisions for two species can be expressed as408 R


Chemical Reviews ν = Pσ

⎛ E ⎞ 8kBT NA exp⎜ − a ⎟[A][B] ⎝ RT ⎠ πμ

Review

proposed a CAS mechanism in revealing the size-selective reactivity of Al clusters, specifically “Al12− + 2H2O” and “Al17− + 2H2O”,102,354 as shown in Figure 31. In this mechanism, the Lewis acidity (rather than the charge acceptance) was demonstrated to be most important in the reaction with −OH chemicals, where the lone pair electrons of the oxygen often insert into the metal cluster that exhibits active sites (i.e., Lewis acid/base pairs) and hence is highly reactive with such protic species. The CAS mechanism has been found to be operative for many other polar molecules in reacting with such metal clusters, providing insights to account for size-selective reactivity of metal clusters. In particular, it visualizes how elementary reactions proceed one step at a time under a combination of geometric and electronic reorganization.

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where σ refers to the collisional cross section, kB is Boltzmann’s constant, μ is the reduced mass, [A] and [B] are the concentrations of the two reactants, and Ea is the activation energy, while R is the universal gas constant. In general, the steric factor P bears a range “0 ≤ P ≤ 1”, where the two limits indicate that either none or all of the relative orientations lead to a reaction. However, harpoon reactions could find a “P > 1”, such as the reaction of “K + Br2 → KBr + Br”, where the K atom plucks a Br atom out of the Br2 molecule. Such reactions begin with an electron leaping from the metal atom (i.e., a harpoon) to the halogen, resulting in a Coulomb attraction between the metal and halogen, and hence, the collisional cross section is largely extended, profiting their reactive encounter.220,409−413 Recently Luo et al.218 reported a study on the harpoon reactions of coinage metal clusters. Considering the electron configurations of Cu, [Ar]3d104s1, and Ag, [Kr]4d105s1, the harpoon mechanism for their reactive behavior was expressed as Ag8−(/Cu8−) + Cl 2 → Ag8Cl−(/Cu8Cl−) + Cl

5.4. Edge Effect Causes Active Sites

The presence of active sites is often associated with irregular charge distribution on the cluster surface. This is prominent in certain clusters with geometries that are akin to surface defects and edge effect. For this, a recent investigation by Reber et al.370 demonstrated that, at small sizes, Aln− clusters react with alcohols and find reactive pairs on specific active sites, while at larger sizes such reactive pairs could accumulate on the edges between facets. For example, Al20− (Figure 32) displays a defect-free double-cage structure, regarded as a double icosahedron with two Al atoms embedded. Theoretical calculations showed that the LUMO and LUMO+1 states of Al20− are delocalized along the equator of the prolate cluster, which is indicative of Lewis acid sites allowing oxygen to readily bind during cleavage. The transition state displays a larger energy than the binding energy of the nondissociated methanol. The high barrier suggests that HER is inhibited, despite the O− H cleavage process being exothermic. In comparison, the structure of Al23− bears a hexagonal packing of aluminum atoms with 3-fold longitudinal edges, where the Lewis base sites are located with some additional density on the more obtuse equatorial surface. Al25− is triangular and also has well-defined edges along different sections of the cluster. Similarly, the LUMO of Al27− are located primarily along the edges of the cluster, showing the location where the methanol molecule is binding.

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Calculations were performed on the steric factor based on classical collision theory and quantum chemistry, by checking out the interaction energies between a Cu8−/Ag8− cluster and an approaching Cl2 molecule.408 The results on the basis of two methods both displayed an extended collisional cross section (i.e., a steric factor P > 1) for the reactions of Ag8− and Cu8− with chlorine. Such investigations validated that the harpoon mechanism is operative in the reactivity of metal clusters, involving long-range transfer of valence electrons and Coulombic attraction and then reacting and ejecting a chlorine atom until the halide products formed, as shown in Figure 30.

5.5. Ligands Induce Active Sites

Considering that ligand-stabilized nanoclusters are often synthesized in a protic environment of solvents such as water and thiols, the ability for a cluster to react or not with such protic molecules is critical in identifying stable clusters. According to the CAS mechanism, species that have a nonuniform distribution of charge density exhibit distinct active sites in which a pair of adjacent metal atoms can be responsible for the dissociative chemisorption of the −OH compounds. Clusters with such complementary Lewis acid/ base pairs (which accept/donate electrons) are highly reactive with protic species. Thus, the chemical stability of a small metal cluster is maximized when (i) the cluster has a closed electronic shell and possesses a HOMO−LUMO gap larger than ∼115.8 kJ/mol (i.e., 1.2 eV)149 and (ii) the charge density is evenly distributed over the surface of the cluster, preventing the presence of active sites. These two criteria then result in other properties correlated with reactivity, such as ionization potentials, detachment energies, and reaction barriers. Through a joint experimental and theoretical study on the reactivity of methanol with AlnIm− clusters (Figure 33),414 recently it was reinforced that ligands do help enhance the stability of some

Figure 30. A drawing of the harpoon mechanism cluster reactivity of “Ag8−(/Cu8−) + Cl2 →Ag8Cl−(/Cu8Cl−) + Cl”.

5.3. Complementary Active Sites (CAS) Mechanism

The aforementioned reactivity of Aln− clusters with water, methanol, and formaldehyde has revealed that metal clusters may undergo a different but regular fundamental mechanism in reacting with polar −XH (X = O, S, N, etc.) molecules. This can be summarized by the following principles: (i) a free H atom on the cluster surface with a neighboring active site is likely to be attained for many metal clusters after reacting with a hydroxy molecule or similar polar molecules and (ii) those with paired active sites favor the release of H2. This was rationalized by Khanna, Castleman, and their co-workers, who S


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Figure 31. (A) Reaction coordinates for “Al12− + 2H2O”. (B) Reaction coordinate for “Al17− + 2H2O” and the formation of H2 from Al17−.

Figure 32. (Left) Reaction pathway for Al20− with methanol: (a) The Al20− cluster with LUMO and LUMO+1 plotted in red−black and blue−white, (b) methanol with intact O−H bound to the Al20− cluster, with the HOMO charge density plotted, (c) the transition state for O−H cleavage, and (d) the final state, where O−H is broken. (Right) LUMO charge density of Al23−, Al25−, and Al27−, and their energy structures when the O atom is bound to the selected Al atom, respectively.

iodine could induce an active site on the adatom, making this cluster reactive.356 Among others, it is interesting to note a report by Caraiman and Bohme, who found that benzenedoped metal clusters exhibit a faster oxygen-addition reaction comparing with the same sized bare metal clusters.416 The activation of molecular oxygen was improved by the transfer of an oxygen atom to the metal−benzene adduct, which was shown as an intrinsic property of the early metal cations maintained in the presence of benzene. Such investigations have not only helped to explain why the ligand-protected clusters prefer closed electronic shells but also pointed out that certain ligands used for stabilization/protection could perplex the charge density of the metallic core and even likely induce reactive Lewis acid and base sites.356 Figure 33. Reactivity of AlnIm− clusters with MeOH revealing the factors that determine if a ligand activates or passivates a superatom cluster.

6. REACTIVITY OF MONOLAYER-PROTECTED METAL CLUSTERS In recent years, there has been an increasing research interest in monolayer-protected clusters (MPCs) due to their unique stability and properties.417−422 The synthesis of MPCs via wet chemistry is generally achieved by the reduction of noble-metal salt solutions, typically employing a mild reducing agent in the presence of thiol compounds,423 regarded as “bottom-up approaches”. While several synthesis recipes exist with similarity to that of metal nanoparticles, the reduction process bears the

clusters;415 however, in some cases, the electronegative ligand may perturb the charge density of the metallic core, producing active sites and hence activating the cluster.414 It was demonstrated that two adjacent ligands on the geometric closed shell of Al13− could activate this magic cluster when the ligand induces adjacent active sites for Al13−. Also in Al14I3−, T


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technologies.438 The link between gas-phase metal clusters and MPCs is still open to further exploration.

higher requirement of an equilibrium between different oxidation states of the metal and the forms of the reducing agent. On the other hand, “top-down approaches” are also applicable to the synthesis of MPCs, achieved by etching reactions of larger metallic nanoparticles.424 The MPCs can then be separated by gel electrophoresis to gain uniform sizes and desired monodispersity. Among others, templates reactions (e.g., via polymers, dendrimers, oligonucleotides, polyelectrolytes, and proteins) were also used for direct synthesis of clusters instead of starting from “naked” metal ions in solution, where the interface between metals and the template functions as stabilizer, allowing for tunable cluster sizes. Several pioneering studies of MPCs have shed light on their reactivity,425,426 resulting in exciting questions on how to link the reactivities of gas-phase clusters and MPCs. To some extent, the reactivity of MPCs and the reaction rates depend on the chemical character of the stabilizer (e.g., chain length, density, and thickness) that shields the bare metal clusters, but if this shielding is insufficient, the stability/reactivity of the metallic core itself will be involved. A typical example is the postsynthesis peptide ligand place-exchange reactions of gold MPCs.421 Another example showed that chlorine radicals formed by UV irradiation of CCl4 solvents react with alkanethiolate-stabilized Au/Ag MPCs, resulting in metal salts and halogenated disulfides.427 Besides, several investigations on metal clusters and their complexes shed light on plasmon-enhanced reactivity428−430 and intracluster reactivity,431,432 as well as chemical interactions between clusters. For example, Wang et al.433 performed a study to elucidate the nature of chemical interactions within a variety of linear [AuX2]− complexes, for which they found ubiquitous linear or quasilinear structures attributed to wellbalanced overlap in both σ and π bonding orbitals, as well as minimal repulsion between the two cluster species. The unique stability of these complexes was related to the strong/weak covalency of the Au−X bond. By that analogy, the special stability of Au(CN)2− was demonstrated to be a result of both strong covalent and ionic interactions. Among others, Tofanelli et al.434 reported an interesting study on the relationships between structure, magnetism, and oxidation state for the three stable oxidation states (−1, 0, and +1) of Au25(SR)18. It is worth mentioning that X-ray absorption near-edge spectroscopy (XANES)435,436 facilitates detailed analysis of the ligand− ligand and ligand−cluster packing interactions on the basis of high-quality single-crystal X-ray structures.434 To bridge the reactivity of gas-phase metal clusters and MPCs synthesized via wet chemistry, it is appealing to provide further insights on how ligands balance the electronic and geometric structures and hence enhance/reduce the stability/ reactivity of metal clusters. To this end, Shafai et al.437 reported a theoretical study to examine the effect of ligands on the geometric and electronic structure of a typical metal cluster, Au13, which bears isomers for gas-phase bare clusters with planar, flake, cuboctahedral, and icosahedral geometries, respectively. Among them, the planar structure has the lowest total energy in the gas phase; however, for MPCs the icosahedron geometry was found to become the lowest-energy structure, allowing for Jahn−Teller distortion, ascribed to Au− ligand charge transfer interactions and a compressive strain. Along with the significance of finding stable cluster species for materials via gas-phase reactions, possibly the abnormal reactivity of MPCs can also be useful in appropriate

7. METAL CLUSTER CATALYSIS It is particularly important to determine the exact size of particles responsible for promoting or passivating a desired reaction. However, developing catalysts for the selective activation of certain chemical bonds (e.g., C−H) is a challenge in chemistry. In recent years, Fischer−Tropsch synthesis by cluster catalysis has been found to yield alkanes, alkenes, and various oxygenates, contributing to industrially significant heterogeneous processes.106,439−445 Other than extensive catalytic investigations of metal oxides and oxygen-centered radicals, 277,446−463 bare metal clusters have also been recognized as promising catalysts.10 In general, the metal cluster must be “activated” for catalysis by substitution of one or more ligands, but the size selectivity is heavily influenced by steric and electronic effects. Below we provide descriptions of the typical catalysis of platinum and gold clusters. Platinum-based heterogeneous catalysts have attracted reasonable research interest in many important commercial and industrial chemical processes,464−470 such as catalytic CO oxidation,471−485 where the conversion into CO2 is thermodynamically allowed, but an activation energy is needed for the dissociation of O2.486 Among the extensive investigations to optimize the efficiency and selectivity of industrial catalysts, there is an important aim endeavoring to gain insight into various aspects (such as size-dependence) relating to catalytic activity.231,265,274 For example, in order to increase Pt utilization for catalysis, reducing particle sizes is a general idea; however, likely poisoning or oxidation could limit the smallest size of Pt particles (ca. ∼2.6 nm) with a low utility (e.g., ∼20% of Pt atomic utilization).464 Furthermore, it was found that, when reducing the diameter of Pt nanoparticles down to ∼1 nm, there occurs a collapse in the crystalline structure, and the quantum size effect at such small size could cause a decline in the catalytic activity, such as for the hydrogen-oxidation reaction (HOR),487 while for the oxygen-reduction reaction (ORR), an interesting study of the core−shell Al13@Pt42 cluster revealed that the covalent Pt−Al bonding activates the Pt atoms at the edge sites, enabling improved utility of the novel 55-atom cluster (e.g., up to 70% of Pt atomic utilization).464 These investigations provide considerable evidence that an alloying technique improves the ORR activity and stability of catalysts.488−491 Beside platinum, the catalysis of gold clusters has been meticulously studied in retrospect to two significant observations in the 1980s: (i) the discovery of supported gold catalysts for CO oxidation492 and (ii) the catalysts for ethyne hydrochlorination.493 Neutral and ionic Au clusters, including Au-containing heteroatom systems, have been significantly identified as effective catalysts,305,323,433,437,438,459,494−509 revealing the bonding nature and redox activity of gold, as well as an interesting relativistic effect involved in gold clusters.510−512 By far, gold clusters have proven themselves to possess powerful catalytic capability in various chemical reactions, including CO oxidation,513 epoxidation,514,515 C−C bond formation,516 C−H activation,517 selective hydrogenation or reduction,518 water− gas shift,519 and the AuI/AuIII catalytic cycle.520 Among them, reasonable research interest was attracted to C−H activation, including detailed size-dependence and the effects of local charges that could effectively alter gold catalytic efficiency.277,439,459,508,509,517,521 These studies (including those U


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Figure 34. (A) Temperature-programmed reaction experiments for the CO-oxidation on selected Aun clusters on defect-rich MgO(100) films. The model catalysts are saturated at 90 K with 13CO and 18O2, and the isotopomer 13C18O16O is detected with a mass spectrometer, as a function of temperature. (B) The reactivity of Aun expressed as the number of formed CO2 per cluster. Reproduced with permission from ref 535. Copyright 2006 John Wiley and Sons. (C) The optimized atomic structures of model catalysts comprising (a and b) Au8, (c) Au4, and (d) Au3Sr clusters adsorbed at an F-center defect on MgO(100). Reproduced with permission from ref 536. Copyright 2003 John Wiley and Sons.

tite.548−552 Au25 and Pd1Au24 were even immobilized on multiwall carbon nanotubes (CNTs), as shown in Figure 35,

we have not referred here) revealed that gold clusters are very versatile catalysts,298,513,522−531 as having been highlighted in a few previously published review papers.509,532−534 Recently, catalysis of solid-supported metal clusters has attracted enormous research interest.537,538 There are two reasons for this interest. First, defects in support can strongly bind the clusters, thereby reducing sintering and leaching. Second, the support can enhance the catalytic activity, e.g., by exchanging charges with the deposited clusters. For example, Heiz and co-workers536,539−541 studied the catalytic activity of MgO-supported gold clusters for CO oxidation and provided an insight into size-dependent catalysis of Aun (e.g., n = 1−20) clusters. Figure 34A displays the temperature-dependent reaction curves and the number of CO2 molecules produced per cluster as a function of the number of atoms in the Aun cluster. It was noted that there is no CO2 being detected in case of n = 1 and 2, little CO2 for n = 3−6, no CO2 for n = 7, and around one CO2 per cluster for n = 8. For larger n (n > 8), the yield has irregular oscillations with an ascending trend corresponding to the value of n.539 Among n = 1−22, the most vigorous is Au18, which produces about two CO2 molecules per cluster (Figure 34B). Optimized cluster structures of model catalysts comprising a different number of gold atoms adsorbed at an F-center defect on MgO(100) are shown in Figure 34C. Significant progress has also been made on the high-pressure kinetics of Au6−Au10 clusters deposited on alumina, exhibiting high activity and selectivity for propylene epoxidation.525,542 It is worth mentioning that the catalytic activity generally goes down with cluster size, but it is not a directly proportional relationship; instead, the limit of size effect for catalysis was demonstrated to follow a volcano curve (that is, a volcano-shaped curve is plotted for the activity of catalysts as a function of a parameter relating to the ability of the catalyst surface),543 revealing different aspect of metal cluster catalysis compared with the size-selective reactivity of small metal clusters. Well-defined sizes of Au MPCs (e.g., Au10, Au11, Au18, Au25, and Au39) have also been applied for catalysis on other solid supports,544−548 such as mesoporous silica and hydroxyapa-

Figure 35. (Upper) Synthetic scheme of Au25/CNT and Pd1Au24/ CNT. (Lower) Catalytic performance of Au25/CNT and Pd1Au24/ CNT for conversion in benzyl alcohol oxidation. Reproduced from ref 544. Copyright 1994 American Chemical Society.

where the tunable catalytic performance of Au25/CNT for the aerobic oxidation of benzyl alcohol was attained.544 Insights gained from the gold cluster catalysis investigations, including gas-phase bare clusters, solid-supported clusters, and ligandstabilized nanoclusters (i.e., MPCs), shed light on metal cluster applications in industrial processes and biological techniques.553−556

8. OUTLOOK Cluster reactivity is associated with many chemical processes and material changes, such as combustion, catalysis, crystal nucleation and growth, solidification and phase transformation, V


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sol−gel, and sputtering film formation. The research field of metal cluster reactivity has been largely expanded, but there are still pending issues that require further exploration. Since free clusters offer an interaction-free environment, one of the objectives in cluster chemistry is to extend the investigations of size-selective metal clusters and to understand how fundamental insights can be acquired through a joint experimental and theoretical cluster methodology. It is important to apply the knowledge gained from cluster reactivity to condensed-phase chemistry (e.g., the nucleation origins) and the gas−particle conversion mechanism of aerosols.557 To further metal cluster reactivity and related investigations, follow-up development of apparatuses could be helpful to researchers in these fields, such as intense cluster sources capable of producing larger clusters, higher-resolution mass spectrometers with improved ion optics, better trapping/ transport efficiencies, sensitive detectors, cluster soft-landing deposition systems, and characterization instruments. Recently, by grazing-incidence X-ray absorption near edge structure (GIXANES) measurements, the electrochemical behavior of size-controlled Cu 20 and Cu 5 clusters via soft-landing deposition has been studied, showing promising application to realize low-overpotential electrocatalytic conversion of CO2.558 Another future aim of cluster reactivity investigations could still be further exploration of catalysis in newer systems, although gold catalysis has become a relatively mature field. Further research could be highlighted in relatively noncomplex methods of synthesis to more specific preparations and design at reduced sizes (even single-atom catalysis),559−561 aided by a better understanding of how selected metal clusters can be synthesized and aided by the insights gained from the fundamentals of cluster chemistry and theoretical studies. In this respect, single-site catalysts represent the ultimate limit of surface-dispersed catalysts and are often regarded as the Holy Grail in catalysis. Attention could be also paid to the key challenge concerning the nature of active sites of metal clusters,562 which perplexes the size selectivity, in contrast to the homogeneously catalyzed reactions by molecular complexes. In particular, it is worth noting that a few cluster ions have been found to function as isovalent species for noblemetal catalysis (e.g, ZrO+ vs Pd), thereby offering a pathway to develop new cluster species of cheaper elements that could mimic and replace expensive catalysts in chemical industry.10 This will be fully expanded with development of superatom chemistry.2,3 Probing metal cluster reactivity not only aims to reveal reaction mechanisms of the primal condensed phase but also provides a better understanding of the properties of cluster building blocks2,318,505 in forming new materials via cluster assembly. Recently, increasing research interest has been directed toward the synthesis of various MPCs; however, less attention has been paid toward reaction dynamics and influences of the conformations of the ligand shell, strong/ weak interactions, and how the reactivity changes for the clusters in the gas phase compared to those of the same sizes but protected with a ligand? With development of clusterrelated theory, especially that of superatoms, MPCs are becoming a promising research topic contributing to structural chemistry, biochemistry, material science, and a few interdisciplines. Looking ahead, the field of cluster chemistry may be further consummated along a few directions, leading the way of precise chemistry and atomic-level controlled nanomaterials.

AUTHOR INFORMATION Corresponding Authors

*Z.L. e-mail: [email protected]. *A.W.C. e-mail: [email protected] *S.N.K. e-mail: [email protected]. ORCID

Zhixun Luo: 0000-0002-9819-9155 Notes

The authors declare no competing financial interest. Biographies Zhixun Luo received his Ph.D. degree in 2009 on the basis of his work in Prof. Jiannian Yao’s group at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). In July 2009, he then joined Prof. Castleman’s group at The Pennsylvania State University, with a focus on investigations exploring stable cluster species as well as cluster reactivity. After 4 years of postdoctoral research at Penn State, he returned to ICCAS as a professor of physical chemistry under the support of the national Thousand Youth Talents Program and joined the State Key Laboratory for Structural Chemistry of Unstable and Stable Species. He is interested in exploring the physical/chemical properties of matter of finite dimension, elucidated through cluster research methods. The research interests in his group include structures and properties of atomic and molecular clusters/aggregates, as well as their assembly into materials. A. Welford Castleman, Jr. received his Ph.D. degree (1969) at the Polytechnic Institute of New York. In 1982, he became a full professor in the departments of chemistry and physics at the Pennsylvania State University, and in the late 1990s, he was appointed Eberly Distinguished Chair in Science. He was elected a member of the National Academy of Sciences in 1998, a fellow of the American Academy for Arts and Sciences, and a fellow of the New York Academy of Sciences. He received the Wilhelm Jost Memorial Lectureship 2000 Award from the German Chemical Society, the Thomas W. Phelan Fellows Award in 2007, and the Irving Langmuir Award in Chemical Physics from the American Chemical Society. He devotes his career to cluster science, to bridge the gas and condensed phase through investigation of the dynamics of formation and laser photophysics and spectroscopy, and to illustrate the gas-phase cluster reactivity. Shiv N. Khanna is a Commonwealth Professor of Physics at Virginia Commonwealth University (VCU), having been a visiting associate professor at Northeastern University (1983−1984) and a scientific collaborator at the Swiss Federal Institute of Technology in Switzerland (1980−1983). He served as chair of the physics department during 1995−1998. He is internationally recognized for his work on clusters (groups containing few atoms) and nanoscale materials. In a significant contribution, he and co-workers discovered that selected clusters can take on the chemical behavior of atoms in the periodic table and that materials with novel characteristics could be developed using such clusters, named “superatoms”, as building blocks. They proposed that the conventional periodic table of elements, which has remained at the heart of chemistry and material science for nearly a century, may finally need modification, with “superatoms” forming a third dimension. These developments have opened a pathway to novel nanomaterials and have been featured in more than 200 reports by various news agencies, including Chemical & Engineering News, Scientif ic American, Nature, Science, and New Scientist. His current work focuses on forming materials where clusters serve as the building blocks. He has coauthored more than 300 research publications in refereed journals, and has edited 6 monographs. These publications W


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(16) Castleman, A. W., Jr.; Davis, R. E.; Munkelwitz, H. R.; Tang, I. N.; Wood, W. P. Kinetics of Association Reactions Pertaining to H2SO4 Aerosol Formation. Int. J. Chem. Kinet. 1975, 1, 629−640. (17) Castleman, A. W., Jr.; Tang, I. N. Kinetics of Association Reaction of SO2 with Hydroxyl Radical. J. Photochem. 1976, 6, 349− 354. (18) Castleman, A. W., Jr.; Keesee, R. G. Metallic-Ions and Clusters Formation, Energetics, and Reaction. Z. Phys. D: At., Mol. Clusters 1986, 3, 167−176. (19) Hofmannsievert, R.; Castleman, A. W., Jr. Reaction of SO3 with Water Clusters and the Formation of H2SO4. J. Phys. Chem. 1984, 88, 3329−3333. (20) Morgan, S.; Castleman, A. W., Jr. Evidence of Delayed Internal Ion Molecule Reactions Following the Multiphoton Ionization of Clusters - Variation in Reaction Channels in Methanol with Degree of Solvation. J. Am. Chem. Soc. 1987, 109, 2867−2870. (21) Passarella, R.; Shul, R. J.; Keesee, R. G.; Castleman, A. W., Jr. Gas-Phase Reactions of Sulfides, Mercaptans, and Dimethyl Methylphosphonate with Ionic Species Derived from Argon and Water. Int. J. Mass Spectrom. Ion Processes 1987, 81, 227−233. (22) Upschulte, B. L.; Shul, R. J.; Passarella, R.; Keesee, R. G.; Castleman, A. W., Jr. Diagnostics of Flow Tube Techniques For Ion Molecule Reactions. Int. J. Mass Spectrom. Ion Processes 1987, 75, 27− 45. (23) Breen, J. J.; Kilgore, K.; Wei, S.; Tzeng, W. B.; Keesee, R. G.; Castleman, A. W., Jr. Reactions of Hydrogen Halides with Clusters of Ammonia Molecules. J. Phys. Chem. 1989, 93, 7703−7707. (24) Breen, J. J.; Tzeng, W. B.; Kilgore, K.; Keesee, R. G.; Castleman, A. W., Jr. Intracluster Reactions in Phenylacetylene Ammonia Clusters Initiated through Resonant Enhanced Ionization. J. Chem. Phys. 1989, 90, 19−24. (25) Dermota, T. E.; Zhong, Q.; Castleman, A. W., Jr. Ultrafast Dynamics in Cluster Systems. Chem. Rev. (Washington, DC, U. S.) 2004, 104, 1861−1886. (26) Riley, S. J.; Parks, E. K. In Physics and Chemistry of Small Clusters; Jena, P., Rao, B. K., Khanna, S. N., Eds.; Springer: New York, 1987; pp 727−739. (27) Lang, S. M.; Bernhardt, T. M. Gas Phase Metal Cluster Model Systems for Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2012, 14, 9255−9269. (28) Lang, S. M.; Popolan, D. M.; Bernhardt, T. M. In Chemisry and Physics of Solid Surfaces; Chemical Society, 2007; pp 53−90. (29) Knight, W. D.; Clemenger, K.; de Heer, W. A.; Saunders, W. A.; Chou, M. Y.; Cohen, M. L. Electronic Shell Structure and Abundances of Sodium Clusters. Phys. Rev. Lett. 1984, 52, 2141−2143. (30) Martin, T. P.; Bergmann, T.; Gö hlich, H.; Lange, T. Observation of Electronic Shells and Shells of Atoms in Large Na Clusters. Chem. Phys. Lett. 1990, 172, 209−213. (31) Smith, A. K.; Basset, J. M. Transition-Metal Cluster Complexes as Catalysts - Review. J. Mol. Catal. 1977, 2, 229−241. (32) Geusic, M. E.; Morse, M. D.; O'Brien, S. C.; Smalley, R. E. Surface-Reactions of Metal-Clusters 0.1. The Fast Flow Cluster Reactor. Rev. Sci. Instrum. 1985, 56, 2123−2130. (33) Jena, P.; Rao, B. K.; Khanna, S. N. Physics and Chemistry of Small Clusters; Plenum Press: New York, 1987; p 955. (34) Bernstein, E. R. Atomic and Molecular Clusters; Elsevier: Amsterdam, 1990; p 806. (35) Jena, P.; Khanna, S. N.; Rao, B. K. Physics and Chemistry of Finite Systems: From Clusters to Crystals; Springer: Berlin, 1992; p 1436. (36) Parent, D. C.; Anderson, S. L. Chemistry of Metal and Semiconductor Cluster Ions. Chem. Rev. 1992, 92, 1541. (37) De Heer, W. A. The Physics of Simple Metal Clusters: Experimental Aspects and Simple Models. Rev. Mod. Phys. 1993, 65, 611−676. (38) Ng, C.-Y.; Baer, T.; Powis, I. Cluster Ions; Wiley: Chichester, U.K., 1993; p 494. (39) Reynolds, P. J. On Clusters and Clustering: from Atoms to Fractals; North-Holland: The Netherlands, 1993; p 401.

have been cited more than 8000 times. He has delivered more than 130 lectures at national and international conferences and universities and in industry. He has been a recipient of the Outstanding Faculty Award from the State Council of Higher Education in Virginia. This is the highest honor bestowed by the Commonwealth of Virginia. He is a fellow of the American Physical Society and of the American Association for the Advancement of Science and has been the recipient of the University Distinguished Scholarship Award of VCU. He has also twice been the recipient of the Distinguished Scholar Award from the College of Humanities and Sciences at VCU.

ACKNOWLEDGMENTS We thank the national Thousand Youth Talents Program, and acknowledge the financial support from Young Professionals Program in ICCAS (Y3297B1261) and CAS projects (Grant No. Y31M0112C1, Y62A0412B1, Y5294512C1) and National Laboratory Frontier Crossing Project (O51Z011BZ3). S.N.K. gratefully acknowledges support from the Department of Energy under award number DE-SC0006420. REFERENCES (1) Castleman, A. W., Jr.; Bowen, K. H. Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter. J. Phys. Chem. 1996, 100, 12911−12944. (2) Castleman, A. W., Jr.; Khanna, S. N. Clusters, Superatoms, and Building Blocks of New Materials. J. Phys. Chem. C 2009, 113, 2664− 2675. (3) Luo, Z.; Castleman, A. W., Jr. Special and General Superatoms. Acc. Chem. Res. 2014, 47, 2931−2940. (4) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. When Gold Is Not Noble: Nanoscale Gold Catalysts. J. Phys. Chem. A 1999, 103, 9573−9578. (5) Satoh, N.; Nakashima, T.; Yamamoto, K. Metastability of Anatase: Size Dependent and Irreversible Anatase-rutile Phase Transition in Atomic-level Precise Titania. Sci. Rep. 2013, 3, 1959. (6) Vajda, S.; White, M. G. Catalysis Applications of Size-Selected Cluster Deposition. ACS Catal. 2015, 5, 7152−7176. (7) Copp, S. M.; Schultz, D. E.; Swasey, S.; Gwinn, E. G. Atomically Precise Arrays of Fluorescent Silver Clusters: A Modular Approach for Metal Cluster Photonics on DNA Nanostructures. ACS Nano 2015, 9, 2303−2310. (8) Yuan, X.; Zhang, B.; Luo, Z.; Yao, Q.; Leong, D. T.; Yan, N.; Xie, J. Balancing the Rate of Cluster Growth and Etching for Gram- Scale Synthesis of Thiolate- Protected Au25 Nanoclusters with Atomic Precision. Angew. Chem., Int. Ed. 2014, 53, 4623−4627. (9) Watanabe, Y. Atomically Precise Cluster Catalysis Towards Quantum Controlled Catalysts. Sci. Technol. Adv. Mater. 2014, 15, 063501. (10) Castleman, A. W., Jr.; Wei, S. Cluster Reactions. Annu. Rev. Phys. Chem. 1994, 45, 685−719. (11) Tyo, E. C.; Vajda, S. Catalysis by Clusters with Precise Numbers of Atoms. Nat. Nanotechnol. 2015, 10, 577−588. (12) Schuler, B.; Fatayer, S.; Mohn, F.; Moll, N.; Pavliček, N.; Meyer, G.; Peña, D.; Gross, L. Reversible Bergman Cyclization by Atomic Manipulation. Nat. Chem. 2016, 8, 220−224. (13) Zhang, C. X.; Chen, C. H.; Dong, H. X.; Shen, J. R.; Dau, H.; Zhao, J. Q. A Synthetic Mn4Ca-Cluster Mimicking the Oxygenevolving Center of Photosynthesis. Science 2015, 348, 690−693. (14) Tang, I. N.; Castleman, A. W., Jr. Mass-Spectrometric Study of Gas-Phase Clustering Reactions - Hydration of Monovalent Bismuth Ion. J. Chem. Phys. 1974, 60, 3981−3986. (15) Tang, I. N.; Lian, M. S.; Castleman, A. W., Jr. MassSpectrometric Study of Gas-Phase Clustering Reactions - Hydration of Monovalent Strontium Ion. J. Chem. Phys. 1976, 65, 4022−4027. X


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