Charge State Of Clusters Research Articles

CO2 activation by copper oxide clusters: size, composition, and charge state dependence.

The interaction of CO2 with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O2/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuxOy+ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO2 in a flow tube reactor results in the preferred formation of near-stoichiometric CuxOy(CO2)+/- complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO2 to all cations while CO2 is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO2 activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu2O4(CO2)- and Cu3O4(CO2)- shows that CO2 activation exclusively results in the formation of a CO3 unit. Subsequent CO2 dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.

Reversible Photobleaching of Silver Clusters in Silica‐Based Glass under Ultraviolet Irradiation

AbstractInorganic glasses doped with luminescent silver clusters are promising materials for photonic applications as white light generation, optical data storage, and spectral conversion. This work reports the photostability study of luminescent silver clusters dispersed in silica‐based glass under continuous ultraviolet irradiation. The photobleaching process model is proposed and the quantum yield of photobleaching is derived from the experimental data. The proposed mechanism of photobleaching is photoionization of silver clusters. Degradation of cluster luminescence is reversible and restores after the heat treatment, indicating the possibility to release trapped electrons and return the initial charge state of clusters. The effect of heat treatment temperature on the luminescence restoration is studied, the amount of restored luminescent clusters depends linearly on the heat treatment temperature.

Mechanistic Insights into Oxidation-Induced Size Conversion of [Au6(dppp)4]2+ to [Au8(dppp)4Cl2]2.

Oxidation-induced conversion of gold nanoclusters is an important strategy for preparing novel atomically precise clusters and elucidating the kinetic correlations of different clusters. Herein, the oxidation-induced growth from [Au6(dppp)4]2+ to [Au8(dppp)4Cl2]2+ (reported by Konishi and co-workers) has been studied by density functional theory calculations. A successive oxidation → Cl- coordination → oxidation → Cl- coordination sequence occurs first to activate the Au6 structure, resulting in the high Au(core)-Au(corner) bond cleavage activity and the subsequent formation of [Au2(dppp)2Cl]+ and [Au4(dppp)2Cl]+ fragments. Then, the dimerization of two Au4 fragments and the rearrangement of the diphosphine coordination occur to generate the thermodynamically stable [Au8(dppp)4Cl2]2+ products. The proposed mechanism agrees with the experimental outcome for the fast reaction rate and the residual of the Au2 components. Specifically, a multivariate linear regression analysis indicates the strong correlation of the oxidation potential of Au6, Au8, Au23, and Au25 clusters with the HOMO energy, the number of Au atoms, and cluster charge state. The main conclusions [e.g., oxidation-induced Au(corner)-Au(core) bond activation, easy 1,2-P transfer steps, etc.] of this study might be widely applicable in improving our understanding of the mechanism of other cluster-conversion reactions.

Redundancy Strategy of Cascaded Energy Storage System Based on Model Prediction Control

The cascaded energy storage system is difficult to achieve global optimal control, because its controlled object includes not only external variables such as output power, voltage and current, but also various internal variables such as power module capacitor voltage and battery cluster charge state. Redundancy strategy is proposed in this paper based on model prediction control. Firstly, the predictive model of the system in discrete domain is constructed, and the fast model predictive control method of the cascaded energy storage system is studied to realize the global optimal control and improve the dynamic response performance of the cascaded energy storage system. Secondly, in order to realize the continuous and reliable operation of the cascaded energy storage system under the fault state, the redundancy control method of the cascaded energy storage system is studied. Finally, the validity and effectiveness have been testified by relevant simulations.

The Factors Dictating Properties of Atomically Precise Metal Nanocluster Electrocatalysts.

Metal nanoparticles occupy an important position in electrocatalysis. Unfortunately, by using conventional synthetic methodology, it is a great challenge to realize the monodisperse composition/structure of metal nanoparticles at the atomic level, and to establish correlations between the catalytic properties and the structure of individual catalyst particles. For the study of well-defined nanocatalysts, great advances have been made for the successful synthesis of nanoparticles with atomic precision, notably ligand-passivated metal nanoclusters. Such well-defined metal nanoclusters have become a type of model catalyst and have shown great potential in catalysis research. In this review, the authors summarize the advances in the utilization of atomically precise metal nanoclusters for electrocatalysis. In particular, the factors (e.g., size, metal doping/alloying, ligand engineering, support materials as well as charge state of clusters) affecting selectivity and activity of catalysts are highlighted. The authors aim to provide insightful guidelines for the rational design of electrocatalysts with high performance and perspectives on potential challenges and opportunities in this emerging field.

Kinetic Spectra of Polyatomic Clusters Formed via the Ion Sputtering of Metals

A theory is developed for the ion sputtering of metals resulting in the formation of excited neutral and charged clusters with their subsequent fragmentation to a stable state. The effect fragmentation has on the kinetic spectra and charge state of clusters is considered. The results are presented in the form of convenient formulas. Calculations for the kinetic spectra of polyatomic clusters of niobium and silver are given as examples.

Effect of Charge on the Catalytic Activity of CO Oxidation by zeolite Supported Single Site Palladium: A Density Functional Study

We have studied the oxidation of CO on palladium monomer in three oxidation states, viz. 0, +1 and +2, supported on FAU zeolite, using density functional theory. When metal complexes and clusters are attached to oxide or zeolite supports, they may well combine the industrial benefits of solid catalysts (heftiness for high temperature operation, absence of corrosiveness and comfort of separation from products) with the selectivity of soluble molecular catalysts. Further, single-atom catalysts (SACs) with discrete and isolated metal atoms anchored to supports can perform as active centres having significant and unique performances, such as drastic cost-reduction, notable catalytic activity and selectivity. CO oxidation has been extensively studied, as designing suitable catalysts is one of the foremost challenges in the field of catalytic research to remove poisonous carbon monoxide. CO oxidation over Pdn/FAU (n=0, +1 and +2) have been investigated which elucidate the effect of cluster charge state on the catalytic activity. The conventional bimolecular Langmuir-Hinshelwood mechanism with co-adsorbed CO and O2 has been considered. Our calculations indicate that Pd2+/FAU displays better catalytic activity among the considered systems. This study is anticipated to provide useful information for the development of highly active catalyst for CO oxidation.

Catalytic Activities of Au6, , and Clusters for CO oxidation: A density functional study

We have studied the CO oxidation over neutral, anionic, and cationic gold hexamer clusters using density functional theory which elucidates the effect of cluster charge state on the catalytic activity. Herein, we have considered the conventional bimolecular Langmuir–Hinshelwood mechanism with coadsorbed CO and O2 at the neighboring sites in all the clusters. Among the three clusters, entails lower barriers during the various steps of the oxidation mechanism. The stability of all the species including the transition states with respect to the interacting species in indicates no thermal activation. Our study suggests better catalytic activity of as compared to the neutral and cationic counterparts. © 2014 Wiley Periodicals, Inc.

Interaction of vanadium oxide cluster anions with water: an experimental and theoretical study on reactivity and mechanism

Vanadium oxide cluster anions (V(x)O(y)(-), x = 2-3; y = 3-7) are produced by laser ablation and reacted with water in a fast flow reactor. A time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Reaction channels of molecular hydrogen elimination (for V(2,3)O(3)(-)), water association (for V(2)O(5)(-) and V(3)O(6,7)(-)) and the coexistence of both channels (for V(2)O(4)(-) and V(3)O(4,5)(-)) are observed. V(2)O(6)(-) and V(3)O(8)(-) are nearly inert toward water. Density functional theory (DFT) calculations are performed to study the reaction mechanism of V(2)O(3)(-) in different spin states with water and the results support the experimental observation. The reaction mechanism of V(2)O(3)(+) with water is also studied, which is in agreement with the experimental report in previous literature [Eur. J. Inorg. Chem., 2008, 4961] that molecular hydrogen elimination is a minor reaction channel for V(2)O(3)(+) + H(2)O. The influence of cluster charge states and oxidation states of vanadium atoms on the cluster reactivity are presented based on the experimental and theoretical studies.

Density Functional Study of Hydrogen Binding on Gold and Silver−Gold Clusters

A theoretical study was carried out on the binding of hydrogen on small bimetallic Ag(m)Au(n) (m + n < or = 5) and pure Au(n) (n < or = 5) clusters with neutral, negative, and positive charge state. It is found that the composition and charge state of clusters have strong influence on the most favorable binding site. The adiabatic ionization potentials, electron affinities, and hydrogen binding energies of cluster hydrides increase with the Au content increasing for the given cluster size. The cationic silver-gold cluster hydrides prefer ejection of Au-containing products whereas the anionic silver-gold cluster hydrides prefer ejection of Ag-containing products. The magnitude of metal-H frequency in combination with the metal-H bond length indicates that, with the same type of the binding site, the Au-H interaction is stronger than the Ag-H interaction.

Structural, Magnetic Coupling and Oxidation State Trends in Models of the CaMn4 Cluster in Photosystem II

Density functional theory calculations are reported on a set of isomeric structures I, II and III that share the structural formula [CaMn4C9H10N2O16]q+.(H2O)3 (q= -1, 0, 1, 2, 3). Species I has a skeletal structure, which has been previously identified as a close match to the ligated CaMn4 cluster in Photosystem II, as characterized in the most recent 3.0 angstroms crystal structure. Structures II and III are rearrangements of I, which largely retain that model's bridging ligand framework, but feature metal atom positions broadly consistent with, respectively, the earlier 3.5 and 3.2 angstroms crystal structures for the Photosystem II water-oxidising complex (WOC). Our study explores the influence of the cluster charge state (and hence S state) on several important properties of the model structures; including the relative energies of the three models, their interconversion, trends in the individual Mn oxidation states, preferred hydration sites and favoured modes of magnetic coupling between the manganese atoms. We find that, for several of the explored cluster charge states, modest differences in the bridging-ligand geometry exert a powerful influence over the individual manganese oxidation states, but throughout these states the robustness of the tetrahedron formed by the Ca and three of the Mn atoms remains a significant feature and contrasts with the positional flexibility of the fourth Mn atom. Although structure I is lowest in energy for most S states, the energy differences between structures for a given S state are not large. Overall, structure II provides a better match for the EXAFS derived metal-metal distance parameters for the earlier S states (S0 to S2), but not for S3 in which a significant structural change is observed experimentally. In this S state structure III provides a closer fit. The implications of these results, for the possible action of the WOC, are discussed.

The Measurements of the Secondary Ion Emissions on the Charge State of Fast Ions and Clusters

A sample for studying secondary ion emissions was made by pressing powders of carbon nanotubes with purity of 98%. The emission yields of different species of atoms/molecules are measured by using a conventional time of flight (TOF) technique under bombardments of C + q , Si + q ( q =1–6), and Ni + q ( q =1–5), C 2 + q , Si 2 + q ( q =1,3,5), and Ni 2 + q ( q =3,5), C 3 + q , Si 3 + q , and Ni 3 + q ( q =2,4) with energy of 1.5 MeV per atom, respectively. With increasing charge states of the projectiles, the secondary ion emission yields per projectile increase and q 3 -dependence are observed. The size and the charge state of the clusters are larger, the secondary ion emissions are more enhanced. The electronic process plays an important role in secondary ion emissions under fast projectile bombardments.

The activity of the tetrahedral Au 20 cluster: charging and impurity effects

Several issues concerning the activity of the magical Au 20 cluster are addressed with the help of Density Functional Theory (DFT) simulations. First, the cluster is found to be extremely robust against distortions when supported at an oxide (MgO) surface, keeping its tetrahedral shape. Second, the activity toward CO oxidation is studied as a function of the cluster charge state. Whereas the neutral cluster is found to be only moderately active, the anion becomes extremely active, because of the presence of a weakly bound unpaired electron. The attempt to induce cluster charging upon attachment to a charged surface F +-center was unsuccessful, with a cluster only slightly more active than the unsupported, neutral one. Finally, some initial steps towards the rational design of active catalysts were taken by the study of the effect of having different impurities. Promising results are obtained by the addition of an extra Na atom, which preferentially adsorbs inside the tetrahedron. Two competing isomers with an endohedral Na atom within a Au 20 cage are found, and both of them are very active.

Measurement of carbon cluster charge state passing through thin foils using a luminance plate

A measurement system based on Coulomb explosion imaging for analyzing the structure of small carbon clusters has been developed. A cluster ion accelerated to the MeV energy region passed through thin foil, and the fragment atoms exploded with Coulomb repulsion were detected with a luminance screen detector. The trace image of the fragment atoms on the screen of the detector monitored by the CCD camera was downloaded as digital data to a hard disc. In preliminary experiments, both chain and ring structures were observed for C 3 and C 6, respectively. In addition, we measured the average charge states of constituent atoms of the carbon cluster after it passed through the thin foil; these charges are related to the cluster structure.

Formation of Nanometer-Scale Serine Clusters by Sonic Spray

Ion mobility and mass spectrometry techniques have been used to study clusters of serine formed by sonic spray ionization. Broad distributions of cluster sizes and charge states are observed, ranging from clusters containing only a few serine units to clusters that contain more than 600 serine units (i.e., protonated clusters of the form [mSer + nH]n+ with m = 8 to >600 and n = 1 to 10). Experimental collision cross sections, derived from the mobility data, are dependent upon cluster size but are not significantly influenced by the cluster charge state. A comparison of calculated cross sections for different trial geometries for several cluster sizes indicates that large clusters adopt tightly packed, roughly spherical geometries. The most abundant cluster size in the spectrum corresponds to the [8Ser + H]+ cluster; however, this ion comprises less than 1% of the total serine abundance. The measured cross section, Ω([8Ser + H]+) = 190 Å2, is in good agreement with the 191.2 ± 0.2 Å2 value reported previously from electrospray ionization. Isotopic labeling studies indicate that [8Ser + H]+ retains a strong chiroselective preference. Evidence for some chiroselectivity in larger clusters is presented.

On the theory of neutral and charged cluster ion sputtering of metal

A calculation method for large neutral and/or charged cluster (with number of atoms N > 5) elastic sputtering of a metal during ion bombardment is proposed. The result is presented as a simple assymptotic formula for the probability of cluster ejection and cluster charge state. A conclusion is made on the exponential nature of the dependence of the total cluster yield on the number of atoms the cluster consists of.

Surface-induced reactions of polyatomic ions and cluster ions

We have recently constructed the tandem mass spectrometer set-up BESTOF (consisting of a B-sector field combined with an E-sector field, a surface and a time of-flight mass spectrometer) which allows the investigation of ion/surface reactions with high primary mass and energy resolution, i.e. energy spreads of as low as 80 meV fwhm have been achieved. Using BESTOF we have extended previous investigations in several respects. Firstly, we have revisited previously investigated surface-induced reactions of various polyatomic ions including the acetone ion, the benzene ion and interacting with stainless steel surfaces. In addition, we have started to investigate the reactions of polyatomic hydrocarbon ions with carbon tiles from the Tore Supra tokamak in Cadarache. Thirdly, we have investigated surface-induced reactions (using stainless steel and gold surfaces) of fullerene ions as a function of cluster size n and cluster charge state z (up to z = 5) thereby extending previous measurements of Kappes and co-workers for singly charged fullerene ions to multiply charged fullerene ions. Finally, we have extended these fullerene-cluster ion studies to molecular clusters of benzene and acetone. Here we will first describe the characteristics and the performance of this newly constructed tandem mass spectrometer system BESTOF. We will then discuss in an illustrative manner results obtained with this machine selecting as examples the acetone monomer ion and the acetone dimer ions. Besides demonstrating the capabilities of the newly constructed ion/surface interaction apparatus BESTOF and besides illustrating these capabilities by presenting as an example surface-induced dissociations (SID) and surface-induced (pick-up) reactions (SIR) of the acetone monomer ion impacting on a hydrocarbon-covered stainless steel surface, we have here also extended these studies to surface-induced reactions of the acetone dimer ion. In contrast to several pioneering studies involving surface-induced dissociation of cluster ions, we have been here able for the first time to observe and analyse surface-induced (catalysed) reactions (SIR) of cluster ions. In particular, we have been able - as predicted by Cleveland and Landman using molecular dynamics simulations - to observe for the deuterated acetone dimer ion impact on the one hand `collisions which catalyse chemical reactions between cluster constituents' (intra-cluster ion-molecule reactions) leading to the production of the deuteronated acetone monomer ion and on the other hand `collisions between the cluster ion and the surface material' (hydrogen pick-up reactions) leading to the formation of the protonated acetone monomer ion . In competition with these two reactive SIR channels, also the straightforward fragmentation (SID) of the dimer ion into the two fragments could be observed as a minor reaction channel.

Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Gold clusters: reactions and deuterium uptake

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n<15) gold cations react readily with D2, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Au 10 + is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH3 and H.