Gas-phase Metal Clusters Research Articles

Toward Designing Reactive Metal Clusters for Dinitrogen Activation: A Guideline Based on N2 Initial Adsorption.

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N2 activation mechanism. However, the effective design and search of reactive clusters in N2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N2 initial adsorption energy (ΔEads) is an important parameter to control the N2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of ΔEads, we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV2S2-, TaV2C2-, and TaV2C3-, the high N2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of ΔEads revealed in this work not only reshapes our understanding of N2 activation reactions in the gas phase but also could have implication for other N2 activation processes in the condensed phase. The more general establishment of this new perspective on N2 activation reactivity warrants future experimental and computational studies.

Adsorption Forms of NO Molecules Adsorbed on Surfaces of Gas-Phase Metal Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

Adsorption Forms of NO Molecules Adsorbed on Surfaces of Gas-Phase Metal Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

The reactivity of O2 with copper cluster anions Cun− (n = 7−20): Leveling effect of spin accommodation

The activation of molecular oxygen is an important step in metal-catalyzed oxidation reactions and a hot subject for the research of gas-phase metal clusters. It is known that the Ag and Au clusters readily react with O2 when they have open shell electronic structures. Distinct from this, here we observed Cun− (n = 7−20) clusters of both open and closed shells possess high reactivity with O2 with few exceptions. In a combination with ab initio calculations, we demonstrate that the activation of O2 on the even- and odd-sized Cun− clusters follows the single and double electron transfer models, respectively. Such phenomenon of metal clusters with different basicity to activate oxygen is enabled by the leveling effect of spin accommodation. The activity of Cun− clusters is correlated to the HOMO level, and for the close-shell clusters is also governed by the vertical spin excitation energy (VSE). In encountering the attack of dioxygen, the activity of the copper cluster anions not only depends on their basicity to donate electrons, but also closely associated with the cluster sizes. Small copper clusters Cun− (n = 7−13) can dissociate O2 spontaneously, while large clusters require extra energies and display close relationship between the reaction rates and electronic vertical detachment energies (VDE). Our work illuminates a novel reaction mechanism between Cun− clusters and O2, which sheds light in manipulating the activity and stability of coinage clusters by controlling the spin and charge states.

Dinitrogen Activation and Functionalization by Heteronuclear Metal Cluster Anions FeV2C2- at Room Temperature.

It is of great importance to study the mechanisms to activate dinitrogen (N2), the very inert molecule, under mild conditions. Gas-phase metal clusters are being actively generated to react with N2 to identify new reaction types and mechanisms. Herein, an unprecedented, mechanistically unique metal atom (Fe or V) ejection in the thermal reaction of FeV2C2- with N2 has been identified using mass spectrometry, photoelectron imaging spectroscopy, and quantum chemistry calculations. Strong evidence suggests that the complete cleavage of the N≡N triple bond and subsequent functionalization of two N atoms via C-N coupling were achieved in this reaction. The complementary cooperation between V atoms with strong electron-donating ability and an Fe atom with large electron-withdrawing ability as well as the geometric flexibility of the Fe-V-V ring drives the whole reaction. The important role of C ligands in N≡N cleavage was also revealed. This study emphasizes the importance of heteronuclear metal systems for N2 fixation.

Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu18.

Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cun- (n = 7-37) and systematically studied their reactivity with O2, NO, and CO. We found remarkable stability of an open-shell cluster Cu18-, which is comparable with the closed-shell clusters Cu17- and Cu19- within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu18- is mainly contributed by the central copper atom, while the other 18 delocalized valence electrons occupy the lower-energy superatomic orbitals of the cluster. The finding of such an open-shell superatom Cu18-, with an electron configuration of 1S21P61D102S1||1F0, is interesting in the sense that an elementary cluster of coinage metal atoms could still behave as a superatom mimicking coinage metals like silver or gold atoms with an empty f orbital. The superatomic stability of this Cu18- cluster is reinforced by the unique electrostatic interaction between the Cu- core and Cu17 shell, which provides new insights into the chemistry of metal clusters.

A Facile N≡N Bond Cleavage by the Trinuclear Metal Center in Vanadium Carbide Cluster Anions V3C4.

Cleavage of the triple N≡N bond by metal clusters is of fundamental interest and practical importance in nitrogen fixation. Previous studies of N≡N bond cleavage by gas-phase metal clusters emphasized the importance of the dinuclear metal centers. Herein, the dissociative adsorption of N2 and subsequent C-N coupling on trinuclear carbide cluster anions V3C4- under thermal collision conditions have been characterized by employing mass spectrometry (collision induced dissociation), cryogenic photoelectron imaging spectroscopy, and quantum chemistry calculations. A theoretical analysis identified a crucial adsorption intermediate with N2 bonded with the V3 metal core in the end-on/side-on/side-on (ESS) mode, which most likely enables the facile cleavage of the N≡N bond. Such a vital N2 coordination in the ESS mode is a result of symmetry-matched interactions between the occupied orbitals of the metal core and both of the two empty π* orbitals of N2. Furthermore, carbon ligands also play a considerable role in enhancing the reactivity of the metal core toward N2. This study strongly suggests a new mechanism of N≡N bond cleavage by gas-phase metal clusters.

Chemistry and Structure of Silver Molecular Nanoparticles.

Silver and gold molecular nanoparticles (mNPs)are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal-organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag44 and Ag17 archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag44 archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production ofmetal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

Condensation and dissociation rates for gas phase metal clusters from molecular dynamics trajectory calculations.

In gas phase synthesis systems, clusters form and grow via condensation, in which a monomer binds to an existing cluster. While a hard-sphere equation is frequently used to predict the condensation rate coefficient, this equation neglects the influences of potential interactions and cluster internal energy on the condensation process. Here, we present a collision rate theory-molecular dynamics simulation approach to calculate condensation probabilities and condensation rate coefficients. We use this approach to examine atomic condensation onto 6-56-atom Au and Mg clusters. The probability of condensation depends upon the initial relative velocity (v) between atom and cluster and the initial impact parameter (b). In all cases, there is a well-defined region of b-v space where condensation is highly probable, and outside of which the condensation probability drops to zero. For Au clusters with more than 10 atoms, we find that at gas temperatures in the 300-1200 K range, the condensation rate coefficient exceeds the hard-sphere rate coefficient by a factor of 1.5-2.0. Conversely, for Au clusters with 10 or fewer atoms and for 14- and 28-atom Mg clusters, as cluster equilibration temperature increases, the condensation rate coefficient drops to values below the hard-sphere rate coefficient. Calculations also yield the self-dissociation rate coefficient, which is found to vary considerably with gas temperature. Finally, calculations results reveal that grazing (high b) atom-cluster collisions at elevated velocity (>1000 m s-1) can result in the colliding atom rebounding (bounce) from the cluster surface or binding while another atom dissociates (replacement). The presented method can be applied in developing rate equations to predict material formation and growth rates in vapor phase systems.

Sequential Dihydrogen Desorption from Hydride-Protected Atomically Precise Silver Clusters and the Formation of Naked Clusters in the Gas Phase.

We report the formation of naked cluster ions of silver of specific nuclearities, uncontaminated by other cluster ions, derived from monolayer-protected clusters. The hydride and phosphine co-protected cluster, [Ag18(TPP)10H16]2+ (TPP, triphenylphosphine), upon activation produces the naked cluster ion, Ag17+, exclusively. The number of metal atoms present in the naked cluster is almost the same as that in the parent material. Two more naked cluster ions, Ag21+ and Ag19+, were also formed starting from two other protected clusters, [Ag25(DPPE)8H22]3+ and [Ag22(DPPE)8H19]3+, respectively (DPPE, 1,2-bis(diphenylphosphino)ethane). By systematic fragmentation, naked clusters of varying nuclei are produced from Ag17+ to Ag1+ selectively, with systematic absence of Ag10+, Ag6+, and Ag4+. A seemingly odd number of cluster ions are preferred due to the stability of the closed electronic shells. Sequential desorption of dihydrogen occurs from the cluster ion, Ag17H14+, during the formation of Agn+. A comparison of the pathways in the formation of similar naked cluster ions starting from two differently ligated clusters has been presented. This approach developed bridges the usually distinct fields of gas-phase metal cluster chemistry and solution-phase metal cluster chemistry. We hope that our findings will enrich nanoscience and nanotechnology beyond the field of clusters.

Laser synthesis and spectroscopy of acetonitrile/silver nanoparticles

Silver nanoparticles with acetonitrile ligands are produced in a laser ablation flow reactor. Excimer laser ablation produces gas phase metal clusters which are thermalized with helium or argon collisions in the flowtube, and reactions with acetonitrile vapor coordinate this ligand to the particle surface. The gaseous mixture is captured in a cryogenic trap; warming produces a solution of excess ligand and coated particles. TEM images reveal particle sizes of 10–30nm diameter. UV–vis absorption and fluorescence spectra are compared to those of standard silver nanoparticles with surfactant coatings. Deep-UV ligand absorption is strongly enhanced by nanoparticle adsorption.

Cluster reaction of [Ag8]−/[Cu8]− with chlorine: Evidence for the harpoon mechanism?

To examine the question whether the harpoon mechanism can account for the reactive behavior of microscopic charged systems, we have investigated the reactivity of coinage metal clusters in gas phase. Our studies reveal that the reactivity between [Cu8]−/[Ag8]− and chlorine gas is consistent with the harpoon mechanism. An increased reactive cross section is noted through our theoretical estimation based on two methods, ascribed to a long-range transfer of valence electrons from the [Cu8]−/[Ag8]− cluster to chlorine. Insights into this reactivity will be of interest to other researchers working on obtaining a better understanding of the reaction mechanisms of such superatomic species.

Gas phase metal cluster model systems for heterogeneous catalysis

Since the advent of intense cluster sources, physical and chemical properties of isolated metal clusters are an active field of research. In particular, gas phase metal clusters represent ideal model systems to gain molecular level insight into the energetics and kinetics of metal-mediated catalytic reactions. Here we summarize experimental reactivity studies as well as investigations of thermal catalytic reaction cycles on small gas phase metal clusters, mostly in relation to the surprising catalytic activity of nanoscale gold particles. A particular emphasis is put on the importance of conceptual insights gained through the study of gas phase model systems. Based on these concepts future perspectives are formulated in terms of variation and optimization of catalytic materials e.g. by utilization of bimetals and metal oxides. Furthermore, the future potential of bio-inspired catalytic material systems are highlighted and technical developments are discussed.

Time-resolved analysis of strong-field induced plasmon oscillations in metal clusters by spectral interferometry with few-cycle laser fields

We propose a scheme for ultrafast real-time imaging of laser-induced collective electron oscillations (Mie plasmons) in gas phase metal clusters by interferometrically stable scanning of two intense few-cycle optical laser pulses. The feasibility of our nonlinear spectral interferometry method with experimentally accessible observables is tested in a theoretical case study on simple-metal clusters (Na(147)). The results show that the plasmon period and lifetime as well as the phase and relative amplitude of the collective electron motion can be extracted with sub-fs resolution. The access to nonlinear response effects, as the demonstrated increase of the plasmon lifetime with laser intensity due to ionization-induced contraction of the electron cloud, opens up vast opportunities for interrogating ultrafast many-particle dynamics in nanosystems under strong laser fields with unprecedented resolution.

Gas scattering effects and microstructural evaluation of electron beam evaporated titanium coatings in neon and argon at different gas pressures

Titanium coatings were deposited on stainless steel and glass substrates by electron beam evaporation in vacuum (4×10 −4 Pa) and at different neon and argon gas pressures (from 0.33 to 2.00 Pa). The effect of background gas pressure and type on film microstructure was evaluated by scanning electron microscopy (SEM). Since it was not feasible to maintain a constant evaporation rate for all runs, the ‘deposition efficacy’, a parameter defined as the ratio between coating thickness and mass of material evaporated, was statistically modelled using non-linear regression analysis to determine the effects of gas scattering. The results revealed that neon and argon promote similar scattering of the titanium vapour and this may point to a lower likelihood of gas-phase metal cluster formation in neon. The coating microstructure was found to change from a tapered grain structure, obtained under vacuum conditions, to a faceted one at low gas pressure (0.33 Pa), becoming more porous as gas pressure increased. For a given gas pressure, no significant differences could be observed in coating microstructure between the use of argon or neon. These findings may be significant when considering the use of neon as an alternative gas to argon, for improving reactive gas ionisation (through Penning mechanisms) in Plasma-Assisted Physical Vapour Deposition.

Solvation Energy and Gas-Phase Stability Influences on Alkali Metal Cluster Ion Formation in Electrospray Ionization Mass Spectrometry

The ability to observe abundant gas-phase metal cluster ions in electrospray ionization mass spectrometry (ESI-MS) is highly dependent on experimental conditions. Alkali halides (MX) and other alkali metal salts were used to investigate the formation of cluster ions in ESI-MS. All compounds were found to give cluster ions of the form (M(n)(+1)X(n))(+) and (M(n)X(n+1))(-), with only two alkali salts yielding doubly charged cluster ions. In homologous alkali halide series, the relative abundances of cluster ions increased with increasing size of either the cation (positive ion mode) or the anion (negative ion mode). Calculations using an electrostatic model show that the gas-phase stability of cluster ions is greater for smaller cations or anions when a fixed counterion is employed. This stability calculation goes in a direction just opposite to the trend in cluster ion abundances observed in ESI-MS. Studies of equimolar mixtures consisting of two alkali halides reveal two distinct trends. When the equimolar mixture was composed of differing ions that participate in the droplet charge excess with the same counterion, the less solvated ions were found to form more abundant cluster ions. When the ions participating in the charge excess were fixed, the preferred counterion in observed clusters was the one that is more solvated in solution and forms more stable clusters in the gas phase. These observations can be rationalized by an extended form of the charged residue model where the weakly solvated ions that are part of the charge excess are preferentially enriched in offspring droplets during uneven fission. By contrast, transfer of a particular counterion located in the bulk of the droplets to the offspring droplets is not disfavored when this counterion is strongly solvated.

Structural Characterization of Rhodium-Containing Hydrodesulfurization (HDS) Catalysts Derived from a Laser Vaporization Cluster Source

A series of highly dispersed Rh-based materials have been prepared by deposition of laser-generated gas-phase clusters onto an industrial-grade alumina. While the diameter of the gas-phase metal clusters could be kept at or below 1 nm, the average diameter of the resulting supported Rh particles was always at least 1.5 nm. The reaction of the clusters with either O2or C2H4failed to prevent Rh aggregation on the alumina surface. The addition of O2upstream of the Rh target was sufficient to form bulk Rh2O3, whereas downstream addition of O2or C2H4to the Rh clusters was insufficient to form Rh oxide particles. Most of the Rh-containing particles further aggregated during either sulfiding or HDS testing. Nevertheless, the resulting rhodium sulfides were comparable in activity to sulfided, commercial Co–Mo or Ni–Mo catalysts on a per gram of catalyst basis and superior on a per gram of metal basis. At low loadings (0.1 wt%), the cluster-derived catalysts were more hydrogen-efficient than the commercial catalysts.

Metal Cluster Oxidation: Sticking Probabilities and Ionization Potential Shiffs

ABSTRACTThis study deals with the initial oxidation of some 3d metal clusters in gas phase. In the experiments neutral metal clusters in a supersonic beam are made to collide with one or a few oxygen molecules in a reaction cell. The reaction products are detected with photoionization and mass spectrometry. By comparing the abundance of unreacted and reacted clusters, absolute sticking probabilities (S) can be determined. Transition metals with open d shells, e.g. Ni, have high, almost constant S for O2 on clusters containing more than 20 atoms. The size evolution of S for Cu, on the other hand, shows repeated minima and maxima. Cluster sizes which have high ionization potentials (IPs) and closed electronic shells, according to the spherical jellium model, are the ones with the lowest S. The IPs of CunO2 clusters have also been investigated. Compared with the IPs of the pure Cu clusters, the IPs of the oxidized clusters are generally higher and the large drops at the shell openings are less distinct.

Oxidation of small transition metal clusters

The initial oxidation of gas phase metal clusters is studied in a new experimental configuration. A beam of neutral metal clusters is produced in a laser vaporization source, and after skimming the beam passes a reaction cell where the clusters undergo one or a few collisions with oxygen molecules. The reaction products are detected with laser ionization and mass spectrometry. We have determined the sticking probability, S, of O 2 on Fe N, Co N and Cu N clusters, with N κ 10–60. Fe N and Co N show a similar size dependence with low sticking probability, S < 0.2, for the smallest sizes, then increasing almost monotonically to N κ 25, where S levels out at a constant value of 0.7 (Fe N) or 1.0 (Co N). Cu N shows a different size behavior with reactivity minima for clusters with closed electronic shells.

Chemical Reactivity. Chemical Reactions of Metal Cluster Cations in a Fourier‐Transform ion Cyclotron Resonance Mass Spectrometer

AbstractMetal cluster cations are generated in an external ion source by sputtering the respective metal foil with 20 keV Xe+ primary ions and are then injected by a series of electric lenses into the analyzer cell of a home‐built Fourier‐Transform Ion Cyclotron Resonance Mass Spectrometer (FT‐ICR MS). There, they are trapped in a magnetic field of 7 T for several tens of seconds through electrostatic deceleration and pulsed addition of a bath gas. Collisions serve to remove excess kinetic as well as inner energy and cool them to nearly room temperature. By the application of a radio frequency (RF) ejection pulse, one specific ion mass may be isolated. – Chemical reactions of the stored metal cluster ions may be studied by admitting a reactive gas with a continuous leak valve. We have investigated a variety of combinations metal/gas and found often surprising size‐specific effects. For some systems, the reactivity as a function of cluster size changes in an abrupt way whereas for others it varies smoothly. Transition metal clusters predominantly show dehydrogenation reactions, the degree of which depends on the nature of the metal as well as on cluster size. In addition, it is demonstrated that naked gas phase metal cluster ions may possess catalytic activity: Fe4+ ions bind up to four molecules of ethene under hydrogen elimination and liberate benzene through collision‐induced dissociation at low energy. The assertion of catalysis is proven by a complex sequence of five consecutive mass spectrometry steps (an (MS)5 experiment), in which the originally trapped Fe4+ clusters actually do initiate the synthesis of benzene twice.

Mathematical inorganic chemistry: from gas-phase metal clusters to superconducting solids

This Account surveys application of mathematical methods to studies of gas-phase bare metal clusters to infinite solid-state materials including bulk metals and superconducting solids. Further details on each of these applications are available in the references cited herein.