Dynamical aspects and the role of IVR for the reactivity of noble metal clusters towards molecular oxygen

Here, we present the dynamical aspects and the role of internal energy redistribution (IVR) in the reactivity of noble metal clusters towards O₂. We show on the example of Ag₃O₂ ^- / Ag₃O₂ / Ag₃O₂ ⁺ that NeNePo spectroscopy carried out under zero electron kinetic energy (ZEKE) conditions can be a powerful tool to investigate the geometry relaxation and IVR induced by photodetachment in real time. Furthermore, we demonstrate that difference in the reactivity of Ag₆ ^- and Au₆ ^- towards O₂ can be attributed to different nature of the IVR process. Dissipative IVR in Ag₆ ^- favors fast complex stabilization, whereas resonant IVR found for Au₆ ^- might be an important factor determining the catalytic activity of Au₆ ^- cluster in the CO oxidation.     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Catalytically active gold clusters and nanoparticles for CO oxidation

Electronic states of gold nanoparticles in mordenite and their transformations under redox treatments have been studied by the methods of FTIR spectroscopy of adsorbed CO and diffuse reflectance UV-visible spectroscopy. Different states of ionic and metallic gold were detected in the zeolite channels and on the external surface of the zeolite - Au⁺ and Au³⁺ ions, charged clusters , and neutral nanoparticles Au_m. Catalytic tests of the samples revealed the existence of two types of active sites of gold in CO oxidation - gold clusters <2 nm (low-temperature activity) and gold nanoparticles (high temperature activity).     

Catalyst poisoning in the conversion of CO and N 2O to CO 2 and N 2 on Pt 4 - in the gas phase

Pt₄ ^- catalyses the conversion of CO and N₂O to CO₂ and N₂ in the gas phase, as observed by Fourier transform ion cyclotron (FT-ICR) mass spectrometry. The partial pressures of CO and N₂O determine the extent of poisoning and the turnover numbers that can be achieved. The catalytic conversion terminates as soon as two CO are adsorbed on the cluster. With N₂O, the reactivity of Pt₄O₂ ^- and Pt₄O₃ ^- is reduced to 41% and 34% compared to Pt₄O^-, respectively, and with Pt₄O₄ ^- this value is reduced to 1%. In contrast, Pt₄ ⁺ shows no apparent catalytic activity. Density functional theory calculations of Pt₄ ^+/- with CO and N₂O adsorbates reveal significantly different stabilities of the reaction intermediates for the different charge states.     

Decay pathways of small gold clusters

The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au ⁺ _n, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation. For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters, dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer evaporation. In this respect, Au ⁺ ₉ shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au ⁺ ₇ and Au ⁺ ₁₁. This nonamer anomaly is typical for copper-group cluster ions M ⁺ ₉ (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M ^- ₇. It is discussed in terms of the well-known electronic shell closing at n _e = 8 atomic valence electrons. Received 2 November 2000     

闭壳层金掺杂钛氧团簇阴离子解离氢分子研究

Dissociation of molecular hydrogen (H₂) is extensively studied to understand the mechanism of hydrogenation reactions. In this study, H₂ dissociation by Au₁-doped closed-shell titanium oxide cluster anions AuTi₃O₇^- and AuTi₃O₈^- has been identified by mass spectrometry and quantum chemistry calculations. The clusters were generated by laser ablation and massselected to react with H₂ in an ion trap reactor. In the reaction of AuTi₃O₈^- with H₂, the ion pair Au⁺-O₂^2- rather than Au⁺-O^2- is the active site to promote H₂ dissociation. This finding is in contrast with the previous result that the lattice oxygen is usually the reactive oxygen species in H₂ dissociation. The higher reactivity of the peroxide species is further supported by frontier molecular orbital analysis. This study provides new insights into gold catalysis involving H₂ activation and dissociation.     

Ab initio calculation of the geometric,electronic and magnetic properties of neutral and anionic Au n Pd (n = 1–9) clusters

The ab initio method based on density functional theory at the B3PW91 level has been applied to study the geometric, electronic, and magnetic properties of neutral and anionic Au _n Pd (n?=?1–9) clusters. The results show that the most stable geometric structures adopt a three-dimensional structure for neutral Au₇Pd and Au₈Pd clusters, but for anionic clusters, no three-dimensional lowest-energy structures were obtained. The relative stabilities of neutral and anionic Au _n Pd clusters were analysed by means of the dependent relationships between the binding energies per atom, the dissociation energies, the second-order difference of energies, the HOMO–LUMO energy gaps and the cluster size n, and a local odd–even alternation phenomenon was found. Natural population analysis indicates the sequential transfer from the Pd atom to the Au _n frame in Au_1,2,3,5Pd and Au_2,3Pd^? clusters, and from the Au _n frame to the Pd atom in other clusters. Much to our surprise, irrespective of whether it is the total magnetic moment or the local magnetic moment, the magnetic moment presents an odd–even alternation phenomenon as a function of the cluster size n. The magnetic effects are mainly localized on the various atoms (Au or Pd) for different cluster size n.     

Evolution of structures,stabilities, and electronic properties of anionic [AunRb]− (n = 1–10) clusters: comparison with pure gold clusters

The geometric structures, stabilities, and electronic properties of small size anionic [Au_nRb]^? and Au_n+1^? (n = 1–10) clusters have been systematically investigated by using density functional theory. The optimised geometries show that the structures of [Au_nRb]^? clusters favour the three-dimensional structure at n ≥ 8. The Rb atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. One Au atom capped on [Au_n-1Rb]^? structures is the dominant growth pattern for n = 2–8 and Rb atom capped on Au_n^? structures for n = 9–10. The averaged atomic bonding energies, fragmentation energies, second-order difference of energies, and highest occupied molecular orbital–lowest unoccupied molecular orbital gaps exhibit a pronounced even–odd alternations phenomenon. The charges in [Au_nRb]^? clusters transfer from the Rb atoms to Au_n host. In addition, it is found that the most favourable dissociation channel of the [Au_nRb]^? clusters is to eject a Rb atom and the highest energy dissociation path is Rb^? anion ejection.     

密度泛函理论研究Hg与Auqn(n=1—6, q=0,+1,-1) 团簇的相互作用

采用密度泛函理论中的广义梯度近似对Hg与小团簇Au ^q_n (n=1—6, q=0, +1, -1)的相互作用进行了系统研究. 结果表明,除Au₅^+,-团簇外,前线分子轨道理论可以成功预测大部分Au _n Hg ^q 复合物的最低能量结构. Au_n团簇对Hg的吸附受团簇尺寸大小和团簇所携带电荷的影 关键词： 密度泛函理论 汞 金团簇 吸附能     

Structural and electronic properties of silver-doped gold clusters Au n Ag v (2 ≤ n ≤ 10; v = 0, ±1): comparison with pure gold clusters

The structural and electronic properties of silver-doped gold clusters Au _n Ag ^v (2?≤?n?≤?10; v?=?0,?±1) have been systematically investigated using density functional theory. The results show that the ground state optimal structures of the cationic and neutral clusters are found to be planar up to n?=?3 and 9, respectively. However, for the anionic clusters, no three-dimensional lowest-energy structures are obtained according to DFT calculations. The calculated binding energy and dissociation energy as a function of cluster size exhibit odd–even alternations. The natural population analysis indicates that in Au _n Ag ^v clusters charges transfer from the Ag atom to the Au frames. The trends for the vertical detachment energies, adiabatic electron affinities, adiabatic ionization potentials, and chemical hardness of Au _n Ag ^v clusters, as the cluster size increases, are studied in detail and compared with the available experimental data.     

DFT study of CO adsorption on neutral and charged Pd<Subscript>n</Subscript>(n = 1–7) clusters

Density functional calculations have been performed to investigate CO adsorption on neutral, cationic and anionic Pd_n (n=1–7) clusters. From the results, it is observed that the binding of CO molecule to neutral and cationic palladium clusters takes place via 1-, 2- and 3-fold coordination. On the other hand, only terminal adsorption of CO molecule is possible in anionic clusters barring bridging adsorption in Pd₇ ^- cluster.     

AunY(n＝1—9)掺杂团簇的结构和电子性质研究

采用相对论有效原子实势(RECP)近似和密度泛函(B3LYP)方法，选择LANL2DZ基组，优化得到了Au_nY(n＝1—9)二元掺杂团簇稳定的基态结构和电子性质.研究结果表明，掺杂Y原子的Au_nY(n＝1—9)团簇随n的变化，其电离势、电子亲合能和费米能级与Au_n(n＝2—9)一样具有“奇-偶”振荡效应；团簇离子的稳定性具有“幻数”现象，Au₂Y⁺和Au₆Y⁺比其他团簇离子更稳定，与质谱实验结果一致；同一团簇中，团簇最稳定的异构体(基态)是趋于Y原子有最大的邻近的Au原子数. 关键词： Au-Y团簇 密度泛函 平衡几何结构 电子性质     

Metastability of gold-carbonyl cluster complexes, Au N(CO) M -

Smaller gold-cluster anions, typified by Au ₇ ^- , adsorb multiple CO molecules in a high-pressure, room-temperature flow-reactor, tending toward previously unknown saturation compositions, Au₇(CO) ₄ ^- . The weakness of the gold-carbonyl adsorption bond is evidenced indirectly by the high CO partial pressure required and more directly by the high probability of fragmentation in the field-free flight region of the reflectron-type time-of-flight mass spectrometer. The analysis of this metastability reveals that the actual distribution f_N,M of products Au₇(CO) _M ^- in the reactor may be highly non-statistical, e.g. with only even-M species present. Received 17 April 2001     

Geometric,stable and electronic properties of Aun-2Y2（n=3-8） clusters

Employing first-principles methods,based on the density function theory,and using the LANL2DZ basis sets,the ground-state geometric,the stable and the electronic properties of Aun-2Y2 clusters are investigated in this paper.Meanwhile,the differences in property among pure gold clusters,pure yttrium clusters,gold clusters doped with one yttrium atom,and gold clusters doped with two yttrium atoms are studied.We find that when gold clusters are doped by two yttrium atoms,the odd-even oscillatory behaviours of Aun-1Y and Aun disappear.The properties of Aun-2Y2 clusters are close to those of pure yttrium clusters.     

通过盐酸刻蚀和生长作用来选择性合成不同尺寸大小的金纳米团簇

Controllable syntheses of different-sized gold nanoclusters are of great significance for their fundamental science and practical applications.In this work,we achieve the controllable and selective syntheses of Au₇ and Au₁₃ clusters through adding HCl to the traditional Au₁₁ synthetic route at different reaction time.Time-dependent mass spectra and UVVis spectra were employed to monitor these two HCl-directed processes,and revealed the distinct roles of HCl as an etchant or a growth promotor,respectively.Furthermore,parallel experiments on independent synthetic routes involving only non-chlorine H⁺(acetic acid) or Cl^-(tetraethy lammonium chloride) instead of HCl were performed,which illustrated the main role of H⁺-etching and Cl^--assisted growth in HCl-directed cluster synthetic routes.We propose the HCl-etching is mainly achieved via the H⁺ action to break the Au (I)-PPh₃ part of clusters,while the HCl-promoted growth is realized via the attachment of Au-Cl species to the pre-formed clusters.     

Probing the structural,electronic and magnetic properties of small Au4M (M = Sc–Zn) clusters

Density-functional method PW91 has been selected to investigate the structural, electronic and magnetic properties of Au₄M (M =Sc–Zn) clusters. Geometry optimisations show that the M atoms in the ground-state Au₄M clusters favour the most highly coordinated position. The ground-state Au₄M clusters possess a solid structure for M = Sc and Ti and a planar structure for M = V–Zn. The characteristic frequency of the doped clusters is much greater than that of pure gold cluster. The relative stability and chemical activity are analysed by means of the averaged binding energy and highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap for the lowest energy Au₄M clusters. It is found that the dopant atoms can enhance the thermal stability of the host cluster except for Zn atom. The Au₄Ti, Au₄Mn and Au₄Zn clusters have relatively higher chemical stability. The vertical detachment energy, electron affinity and photoelectron spectrum are calculated and simulated theoretically for all the ground-state structures. The magnetism calculations reveal that the total magnetic moment of Au₄M cluster is mainly localised on the M atom and vary from 0 to 5 μ_B by substituting an Au atom in Au₅ cluster with different transition-metal atoms.     

Structural,electronic and magnetic properties of Au<Subscript>n</Subscript>Pt (n = 1−12) clusters in comparison with corresponding pure Au<Subscript>n+1</Subscript> (n = 1−12) clusters

An all-electron scalar relativistic calculation on Au _n Pt (n = 1−12) clusters has been performed by using density functional theory with the generalized gradient approximation at PW91 level. Our results reveal that all the lowest energy geometries of Au _n Pt  (n = 1−12) clusters may be generated by substituting Pt atom for one gold atom of the Au _n+1 cluster at the highest coordinated site. Compared with corresponding pure Au _n+1 cluster, the lowest energy geometries of Au _n Pt clusters are distorted slightly and still keep the planar structures due to the strong scalar relativistic effect in small gold cluster. The Au-Pt bonds are stronger and most Au-Au bonds far from Pt atom are weaker than the corresponding Au-Au bonds in pure Au _n+1 cluster. By substituting Pt atom for one gold atom of Au _n+1 cluster at the highest coordinated site, the relatively stable and inactive odd-numbered Au _n+1 cluster becomes the relatively unstable and reactive odd-numbered Au _n Pt cluster, and the relatively unstable and reactive even-numbered Au _n+1 cluster becomes the relatively stable and inactive even-numbered Au _n Pt  cluster chemically and electronically. All the Au _n Pt clusters prefer low spin multiplicity. The even-numbered Au _n Pt clusters are found to exhibit zero magnetic moment and the odd-numbered Au _n Pt clusters are found to possess magnetic moment with the value of 1 μ _B. The odd-even alterations of magnetic moments and electronic configurations for Au _n Pt clusters are very obvious and may be simply understood in terms of the electron pairing effect.     

Derivatives of the thiolate-protected gold cluster Au25(SR)18-1

Loss of small fragments (like AuL, Au₂L₃, Au₄L₄) have been found systematically in several MALDI and FAB experiments on thiolate-protected gold clusters of different sizes. When using the cluster Au₂₅L₁₈ ^-1 as parent cluster, the fragmented cluster Au₂₁L₁₄ ^-1 has been reported to be obtained in high proportion (L = SCH₂CH₂Ph). Here we analyse a few possible fragmentation patterns of the well-known parent cluster Au₂₅L₁₈ ^-1 (L = SCH₃). Using DFT calculations we study the different atomic configurations obtained after a AuL fragment is lost from Au₂₅L₁₈ ^-1. We found energetically favourable configurations that can be written as Au₁₃ [Au₂L₃]_6-z [AuL₂] _z ^-1, where the modification can be described as a replacement of the long protecting unit by a short one (Au₂L₃  →  AuL₂). A full replacement (z = 6) gives rise to a protected Au₁₉L₁₂ ^-1 cluster. This mechanism does not modify the super-atomic electronic structure of the gold core, i.e., all these fragments remain an 8 electron super-atom clusters exactly like the parent Au₂₅L₁₈ ^-1. We suggest that the Au₁₉L₁₂ ^-1 cluster could be realized by using a bulky thiolate, such as the tert-butyl thiolate SC(CH₃)₃ .     

Experimental g factor of hydrogenlike silicon-28

Using the first principles calculations, the mixed Au _n Zn _m ⁺ (n + m ?? 6) cluster cations and their monocarbonyls Au _n Zn _m CO⁺ have been investigated at the PW91 level. For the small Au _n Zn _m ⁺ , most ground-state isomers are planar structures. A significant odd-even oscillation of the highest occupied-lowest unoccupied molecular orbital energy gaps with the number of Au atoms is observed. Upon CO adsorption, the top site and C head-on adsorptions are most favorable in energy. Moreover, the optimized geometries indicate that the CO molecule prefers binding to Au atom of the Au _n Zn _m ⁺ clusters, which can be understood by the frontier molecular orbital theory in detail. From the theoretical calculations, the CO charge population, CO binding energy (BE) and the Gibbs free-energy change ??G generally decrease with the increase of the Zn content. It is found that the BE is highly related to the electron transfer between CO and the cluster cations. Furthermore, a linear correlation between ??G and the CO BE is found. The red shift in the CO stretching frequency is sensitive to the cluster size and composition. Our calculation suggests that CO reactive collision on Au₃Zn⁺ and Au₂Zn ₂ ⁺ may lead to the dissociation of the clusters with a Zn atom loss.     

Ternary alloying effect on the melting of metal clusters

Canonical ensemble Monte Carlo simulations are applied to investigate the melting of the icosahedral 55-atom Ag-Cu-Au clusters. The clusters are modeled by the second-moment approximation of the tight-binding (TB-SMA) many-body potentials. Results show that the introduction of the only Cu atom of the third alloying metal in the bimetallic Ag₄₃Au₁₂ cluster, forming the Ag₄₂Cu₁Au₁₂ cluster, can greatly increase the melting point of the cluster by about 100 K. It is also found that the substitution of the only Cu atom of the third alloying metal in the Ag₁Au₅₄ clusters, forming the Ag₁Cu₁Au₅₃ cluster, can result in an increase of 40 K in the melting point. It can be concluded that the melting points of the bimetallic clusters can be tuned by the third metal impurities doping. In addition, the surface segregation of Ag atoms in the Ag-Cu-Au trimetallic clusters occurs even after melting.