Structural characterization of catalytically active metal clusters in polymer membranes

Metal clusters were generated and stabilized in pore free, mechanically stable poly(amide imide) (PAI) polymer membranes in high dispersion and high concentration of typically 15wt-%. These membranes have been successfully tested for catalytic applications. Pure Pd-loaded and bimetallic Pd/Ag, Pd/Cu, Pd/Co, Pd/Pb PAI films were investigated by means of x-ray absorption spectroscopy, small and wide angle x-ray scattering and transmission electron microscopy to characterize the structure of the metal clusters in the protective polymer. The measurements consistently show a homogeneous distribution of metallic nanoclusters of 10–30 Å size within the membranes. Also, a smaller amount of larger aggregates up to 300 Å is observed in most of the films. The precise cluster size distribution critically depends on the solvents used as well as on other preparation parameters such as the stirring time of the metal precursor/polymer solution. In case of Pd/Ag and Pd/Pb bimetallic films no clear evidence for the formation of bimetallic clusters in the membrane, i.e. alloying of both metal components, is found. In Pd/Cu and Pd/Co membranes, chlorine from CuCl₂ and CoCl₂ precursors reacts with Pd which may influence the Pd catalytic behavior. Reduction and oxidation of the metal nanoclusters is quantitatively studied by means of x-ray absorption spectroscopy. Membrane properties are discussed with respect to catalytic applications.     

Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

A brief review of our laboratory's recent scanning tunneling microscopy (STM) studies on nanoclusters supported on TiO₂(110) is presented. Particular emphasis is placed on the system Au/TiO₂(110). The nucleation and growth of the clusters, which were vapor-deposited on TiO₂(110) under ultra high vacuum (UHV) conditions, were investigated using STM. It was found that Au, Pd, and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashion on TiO₂(110), but that at low coverages, quasi-two dimensional (quasi-2D) Au and Pd clusters were observed. These quasi-2D clusters are characterized by heights of 1–2 atomic layers. Annealing studies show that Au and Pd clusters form large microcrystals with well-defined hexagonal shapes. Al clusters, which have a strong interaction with the substrate, are oxidized upon deposition, “wetting” the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clusters have been studied using scanning tunneling spectroscopy (STS) to measure the cluster band gaps. The electronic structure was found to be cluster size-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties correlates with the transition from quasi-2D to 3D cluster growth.     

In-situ UV-visible study of Pd nanocluster formation in solution

The formation of Pd nanoclusters in solution is studied. This system has two types of light-absorbing species: Pd ions which absorb light via electronic transitions and Pd clusters and aggregates which absorb light via valence-conduction transitions and also scatter light due to their nanometric dimensions. Here we monitor these dynamic changes using UV-visible spectroscopy. The reduction and clustering concentration profiles are extracted from the raw data using a combination of net analyte signal (NAS) and principal component analysis (PCA) methods. PdCl2, Pd(OAc)2 and Pd(NO3)2 are used as Pd2+ precursors and various tetra-n-octylammonium carboxylates are applied as reducing and stabilising agents. This in situ approach enables the quantification of both the reduction of the Pd2+ ions and the growth of the Pd clusters. Kinetic models that account for ion reduction, cluster growth and aggregation are presented and the influence of the counteranions and the reducing agents on these processes is discussed.     

中介尺度Au纳米团簇熔化的分子动力学模拟

采用分子动力学模拟技术，研究了原子个数为16～8628的 Au纳米团簇的熔化过程.采用 Johnson的EAM (embedded atom method) 模型,模拟结果表明，金属纳米团簇存在一中介尺度区域.对Au纳米团簇而言，当原子个数N >456时，团簇的热力学性质与团簇尺寸呈线性关系，熔化首先从表面开始，逐步向中心区域推进，且满足Tmb－Tmc(N)＝aN(－1/3)的关系.另外，计算了中介区域的团簇的尺寸、熔化温度、表面能、熵、焓等热力学量以及均方根位移（RMSD）等动力学量，为研究纳米团簇提供定量数据.     

Structure and mobility of metal clusters in MOFs: Au, Pd, and AuPd clusters in MOF-74

Understanding the adsorption and mobility of metal-organic framework (MOF)-supported metal nanoclusters is critical to the development of these catalytic materials. We present the first theoretical investigation of Au-, Pd-, and AuPd-supported clusters in a MOF, namely MOF-74. We combine density functional theory (DFT) calculations with a genetic algorithm (GA) to reliably predict the structure of the adsorbed clusters. This approach allows comparison of hundreds of adsorbed configurations for each cluster. From the investigation of Au(8), Pd(8), and Au(4)Pd(4) we find that the organic part of the MOF is just as important for nanocluster adsorption as open Zn or Mg metal sites. Using the large number of clusters generated by the GA, we developed a systematic method for predicting the mobility of adsorbed clusters. Through the investigation of diffusion paths a relationship between the cluster's adsorption energy and diffusion barrier is established, confirming that Au clusters are highly mobile in the MOF-74 framework and Pd clusters are less mobile.     

Temperature-dependent structuring of Au-Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100)

Au-Pt bimetallic nanoclusters on a thin film of Al(2)O(3)/NiAl(100) undergo significant structural evolution on variation of the temperature. Au and Pt deposited sequentially from the vapor onto thin-film Al(2)O(3)/NiAl(100) at 300 K form preferentially bimetallic nanoclusters (diameter ≦ 6.0 nm and height ≦ 0.8 nm) with both Au and Pt coexisting at the cluster surface, despite the order of metal deposition. These bimetallic clusters are structurally ordered, have a fcc phase and grow with their facets either (111) or (001) parallel to the θ-Al(2)O(3)(100) surface. Upon annealing the clusters to 400-500 K, the Au atoms inside the clusters migrate toward the surface, resulting in formation of a structure with a Pt core and an Au shell. Annealing the sample to 500-650 K reorients the bimetallic clusters--all clusters have their (001) facets parallel to the oxide surface--and induces oxidation of Pt. Such annealed bimetallic clusters become encapsulated with the aluminium-oxide materials and a few Au remain on the surface.     

金钯二元小团簇的几何结构与电子性质

在UBP86/LANL2DZ和UB3LYP/def2-TZVP水平下详细研究了AumPdn(m+n≤6)团簇的几何结构和电子性质.阐明了团簇的结构特征、平均结合能、垂直电离势、垂直电子亲和能、电荷转移以及成键特征.除单取代混合团簇(AunPd和AuPdn,n=5或6)外,五和六原子混合团簇中钯原子趋于聚集到一起形成Pdcore,金原子分布在Pdcore周围形成PdcoreAushell结构.含一个和两个钯原子团簇的电子性质与纯金团簇类似,呈现一定奇偶振荡.混合团簇的电子性质,如最高占据分子轨道(HOMO),最低未占据分子轨道(LUMO),垂直电离势,垂直电子亲和能,Fermi能级和化学硬度等均与团簇空间结构和金、钯原子数之比直接相关.混合团簇中存在钯原子到金原子间的电荷转移,表明团簇中存在明显金钯间成键作用.分析团簇的电荷分布、前线轨道和化学硬度表明,金钯混合团簇对小分子如O2、H2和CO等的反应活性要强于纯金团簇.     

钯掺杂的25-原子和38-原子金纳米团簇

In this work,we describe two synthetic procedures for preparing palladium doped 25-atom nanoclusters (referred to as Pd1Au24(SR)18,where ― SR represents thiolate,R=C2H4Ph).Pure Pd1Au24(SC2H4Ph)18 nanoclusters are isolated by solvent extraction and size exclusion chromatography.Mass spectrometry and optical spectroscopy analyses demonstrate that the Pd1Au24(SC2H4Ph)18 nanocluster adopts the same core-shell structure as that of the homogold Au25(SC2H4Ph)18 nanocluster,that is,a Pd-or Au-centered icosahedron surrounded by six Au2(SR)3 "staple"-like motifs.Similar doping behavior has also been observed in 38-atom M38(SR)24 (M:metal) nanoclusters,indicating the unique behavior of Pd dopant being preferentially located in the icosahedral center.The catalytic activity of Pd1Au24(SC2H4Ph)18 has also been evaluated for the selective hydrogenation of α,β-unsaturated ketone (e.g.,benzalacetone) to α,β-unsaturated alcohol,and a 42% conversion of benzalacetone is attained.     

利用壳层厚度调节核壳Au@Pd纳米粒子的SERS活性

设计合成了一种尺寸可控, 且外壳上无“针孔”的核壳钯包金(Au@Pd)纳米粒子, 通过改变核的尺寸和外壳的厚度来调控其光学性质, 并用TEM、HRTEM、UV-Vis和SERS等手段对其进行了表征. 通过研究Au@Pd纳米粒子的SERS活性随Pd壳层厚度变化的规律, 发现薄壳Au@Pd纳米粒子远远优于Pd金属本身的SERS活性, 其原因主要是内层金核电磁场增强的长程效应.     

Monitoring Stability and Sizes of Au/Pd Core/Shell Nanoparticles by SEC

This paper describes how size exclusion chromatography (SEC) can be used to rapidly characterize Au/Pd core/shell nanoparticles (NPs). We monitored the sizes of Au/Pd core/shell NPs by effecting SEC separation using a mobile phase of 10 mM sodium dodecyl sulfate (SDS); the plot of retention time with respect to the standard size of the Au NPs was linear (R ² = 0.991) for diameters falling in the range from 12.1 to 59.9 nm; for five consecutive runs, the relative standard deviations of these retention times were less than 0.4%. Under the optimized separation conditions, we found that the addition of the surfactant SDS stabilized the Au/Pd core/shell NP samples. In addition, SEC analysis revealed that the sizes of the Au/Pd core/shell NPs could be controlled via modification of the rate of addition of the reducing agent and the use of adequate volumes of the seed and shell precursor metal ion solutions. When using these conditions to analyze the Au/Pd core/shell NPs produced through seed-assisted synthesis, a good correlation existed between the sizes determined through SEC and transmission electron microscopy. Our results suggest that SEC is a useful technique for monitoring the sizes of NPs and nanomaterials in general.     

Microwave-assisted in situ synthesis of fluorescent gold nanoclusters with BSA/montmorillonite and application on latent fingermark imaging

A method for in situ preparation of fluorescent gold nanoclusters(Au NCs) with bovine serum albumin/montmorillonite composite powder(Au NC-BSA/MMT) was developed, and the products were used to detect latent fingermarks. In this work, Au NCs were "grown" both inside and on the surface of BSA/MMT clay using one-step reduction of HAu Cl4 by BSA. The as-prepared Au NC-BSA/MMT nanocomposites emit intensive red fluorescence under the excitation of UV-visible light and show stable chemical features and low toxicity. The obtained fluorescent powders were characterized by UV-visible absorption spectroscopy,fluorescence spectroscopy, infrared spectroscopy, transmission electron microscopy/high-resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction to depict their sizes, structural information and optical features. Given their environmentally friendly preparation, simple operation, low cost, efficient UVvisible radiation-dependent photoluminescence and good affinity with finger residues, the in situ synthesized Au NC-BSA/MMT nanocomposite powders were used as an alternative fluorescent developing reagent for developing latent fingermarks deposited on various object surfaces(such as glass, aluminum foil, painted metal, plastic products and weighing papers) for individual identification. As results, the developed fingermarks with clear patterns and satisfactory level-2(minutiae points) and level-3(sweat pores) ridge details were obtained. Notably, treated prints could be excited by red light and emitted near infrared fluorescence, which was beneficial to avoid background interference and reduce the damage caused by UV light. With the advantages of the simple preparation process and good enhancement performance for latent fingermarks, the proposed method might be used in the preparation of various fluorescent probes for detecting trace evidence in forensic sciences.     

Melting and freezing characteristics and structural properties of supported and unsupported gold nanoclusters

Molecular dynamics simulations in conjunction with MEAM potential models have been used to study the melting and freezing behavior and structural properties of both supported and unsupported Au nanoclusters within a size range of 2 to 5 nm. In contrast to results from previous simulations regarding the melting of free Au nanoclusters, we observed a structural transformation from the initial FCC configuration to an icosahedral structure at elevated temperatures followed by a transition to a quasimolten state in the vicinity of the melting point. During the freezing of Au liquid clusters, the quasimolten state reappeared in the vicinity of the freezing point, playing the role of a transitional region between the liquid and solid phases. In essence, the melting and freezing processes involved the same structural changes which may suggest that the formation of icosahedral structures at high temperatures is intrinsic to the thermodynamics of the clusters, rather than reflecting a kinetic phenomenon. When Au nanoclusters were deposited on a silica surface, they transformed into icosahedral structures at high temperatures, slightly deformed due to stress arising from the Au-silica interface. Unlike free Au nanoclusters, an icosahedral solid-liquid coexistence state was found in the vicinity of the melting point, where the cluster consisted of coexisting solid and liquid fractions but retained an icosahedral shape at all times. These results demonstrated that the structural stability in the structures of small Au nanoclusters can be enhanced through interaction with the substrate. Supported Au nanoclusters demonstrated a structural transformation from decahedral to icosahedral motifs during Au island growth, in contrast to the predictions of the minimum-energy growth sequence: icosahedral structures appear first at very small cluster sizes, followed by decahedral structures, and finally FCC structures recovered at very large cluster sizes. The simulations also showed that island shapes are strongly influenced by the substrate, more specifically, the structural characteristic of a Au island is not only a function of size, but also depends on the contact area with the surface, which is controlled by the wetting of the cluster to the substrate.     

Formation of bimetallic Ag-Pd nanoclusters via the reaction between Ag nanoclusters and Pd2+ ions

We have investigated systematically the mechanistic aspects of the Ag-Pd bimetallic cluster formation within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles by using in-situ X-ray absorption spectroscopy (XAS). A two-step sequential reduction method is employed for the synthesis of Ag-Pd bimetallic clusters. The first step involves preparation of Ag nanoclusters, by mixing the Ag+ ions containing the AOT microemulsion system with a reducing agent hydrazine (N2H4) containing the AOT microemulsion system. In the second step, the addition of Pd2+ ions to Ag nanoclusters led to the formation of Ag-Pd bimetallic clusters via the reaction between Ag nanoclusters and Pd2+ ions in AOT reverse micelles. The reduction of silver ions and the formation of corresponding Ag nanoclusters are monitored as a function of the dosage of the reducing agent, hydrazine. In-situ XAS allowed probing of the reaction between Ag nanoclusters and Pd2+ ions during the formation of Ag-Pd bimetallic clusters. Analysis of Ag and Pd K-edge XAS spectra reveals that in the final stage Ag-Pd clusters, in which "Ag" atoms prefer to be surrounded by "Pd" and "Pd" atoms prefer to be surrounded by "Pd", were formed. On the basis of XAS results presented here, we are able to propose a structural model for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of Ag-Pd bimetallic clusters. We also discussed the atomic distribution of Ag and Pd atoms in Ag-Pd bimetallic clusters based on the calculated XAS structural parameters.     

DFT studies of the characteristics of Pd−Pt core-shell clusters

It is reported that Pd?Pt core-shell type nanoclusters in which the inner atoms of the Pd cluster are substituted by Pt significantly enhance the catalytic activity for cycloocatdiene hydrogenation. In order to discuss the electronic states of core-shell clusters, DFT calculations were carried out for Pd₁₃, Pt₁₃, Pt/Pd₁₂, Pd/Pt₁₂ Pd₃₈ and Pd₆/Pt₃₂ clusters. From these calculations, it was found that the charge transfer between the core atoms and the shell atoms played an important role for the modification of the electronic state of the surface atoms in them.     

A General and High‐Yield Galvanic Displacement Approach to Au?M (M=Au,Pd, and Pt) Core–Shell Nanostructures with Porous Shells and Enhanced Electrocatalytic Performances

In this work, we utilize the galvanic displacement synthesis and make it a general and efficient method for the preparation of Au? M (M=Au, Pd, and Pt) core–shell nanostructures with porous shells, which consist of multilayer nanoparticles. The method is generally applicable to the preparation of Au? Au, Au? Pd, and Au? Pt core–shell nanostructures with typical porous shells. Moreover, the Au? Au isomeric core–shell nanostructure is reported for the first time. The lower oxidation states of Au^I, Pd^II, and Pt^II are supposed to contribute to the formation of porous core–shell nanostructures instead of yolk‐shell nanostructures. The electrocatalytic ethanol oxidation and oxygen reduction reaction (ORR) performance of porous Au? Pd core–shell nanostructures are assessed as a typical example for the investigation of the advantages of the obtained core–shell nanostructures. As expected, the Au? Pd core–shell nanostructure indeed exhibits a significantly reduced overpotential (the peak potential is shifted in the positive direction by 44 mV and 32 mV), a much improved CO tolerance (I_f/I_b is 3.6 and 1.63 times higher), and an enhanced catalytic stability in comparison with Pd nanoparticles and Pt/C catalysts. Thus, porous Au? M (M=Au, Pd, and Pt) core–shell nanostructures may provide many opportunities in the fields of organic catalysis, direct alcohol fuel cells, surface‐enhanced Raman scattering, and so forth.     

Synthesis, structure and properties of metal nanoclusters

Metal nanoclusters have physical properties differing significantly from their bulk counterparts. Metallic properties such as delocalization of electrons in bulk metals which imbue them with high electrical and thermal conductivity, light reflectivity and mechanical ductility may be wholly or partially absent in metal nanoclusters, while new properties develop. We review modern synthetic methods used to form metal nanoclusters. The focus of this critical review is solution based chemical synthesis methods which produce fully dispersed clusters. Control of cluster size and surface chemistry using inverse micelles is emphasized. Two classes of metals are discussed, transition metals such as Au and Pt, and base metals such as Co, Fe and Ni. The optical and catalytic properties of the former are discussed and the magnetic properties of the latter are given as examples of unexpected new size-dependent properties of nanoclusters. We show how classical surface science methods of characterization augmented by chemical analysis methods such as liquid chromatography can be used to provide feedback for improvements in synthetic protocols. Characterization of metal clusters by their optical, catalytic, or magnetic behavior also provides insights leading to improvements in synthetic methods. The collective physical properties of closely interacting clusters are reviewed followed by speculation on future technical applications of clusters. (125 references).     

In Situ Spectroscopic Analysis of Nanocluster Formation

The importance of small metal clusters in catalysis and the problems in understanding the clustering process in solution are outlined. A new analysis method for UV/Vis spectra is presented and applied to monitor the kinetics of ion reduction and cluster formation in situ. This method, which is based on a combination of two chemometric techniques, takes into account the entire UV/Vis spectrum and offers better interpretation possibilities than the traditional “band‐assignment” approach. This is particularly true for nanoclusters because these have significant spectral contributions also outside the broad plasmon band that is usually associated with them. The reduction of palladium, gold, and silver ions and the formation of the corresponding clusters is monitored in the presence of two different reducing agents, sodium borohydride and tetraoctylammonium acetate. While Pd²⁺ is found to reduce and cluster directly, the spectral decomposition of the Au³⁺ reduction profiles shows two species corresponding to the Au⁺ intermediate and the Au⁰ clusters. The rates of reduction and clustering for Pd, Au, and Ag are compared and the possibilities of synthesising multimetallic clusters of these metals by coreduction are discussed.     

金纳米团簇在盐酸和十二硫醇刻蚀作用下的原位生长动力学研究

了解金属纳米团簇的形成机制对于进一步发展其化学制备方法是必要的。我们利用盐酸(HCl)和十二硫醇(RSH)共同刻蚀L₃ (L₃: 1, 3-双二苯基膦丙烷)包覆的多分散性的Au_n (15 ≤ n ≤ 60)团簇成功制备出单分散性的Au₁₃(L₃)₂(SR)₄Cl₄纳米团簇,并结合原位同步辐射X射线吸收谱、原位真空紫外-可见吸收光谱和质谱技术,研究了Au₁₃(L₃)₂(SR)₄Cl₄纳米团簇的动力学形成过程。结果表明,Au团簇从多分散到单分散的转变经历了3个明显不同的动力学步骤。首先,尺寸较大的多分散金属团簇Au_n主要在HCl刻蚀作用下,形成尺寸较小的亚稳的中间产物Au₈–Au₁₁团簇。然后,这些中间产物与反应溶液中已有的Au(Ⅰ)-Cl物种反应,并与SR发生部分配体交换,逐渐长大为由SR和L₃保护的Au₁₃团簇。最后,形成的Au₁₃团簇经过一个较缓慢的结构重组过程,最终形成稳定的Au₁₃(L₃)₂(SR)₄Cl₄的纳米团簇。     

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-??-cyclodextrin (H₂N-??-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H₂N-??-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).     

New types of multishell nanoclusters with a Frank-Kasper polyhedral core in intermetallics

A comprehensive study of the occurrence of two-shell clusters with the first shell as a Frank-Kasper polyhedron Z12, Z14, Z15, or Z16 (Frank-Kasper nanoclusters) is performed for 22,951 crystal structures of intermetallics containing only metal atoms. It is shown that besides the familiar Bergman and Mackay clusters, two more types of high-symmetrical icosahedron-based nanoclusters are rather frequent; they both have a 50-atom second shell. Moreover, two types of high-symmetrical Frank-Kasper nanoclusters with a Friauf-polyhedron (Z16) core are revealed; these nanoclusters have 44 and 58 atoms in the second shell. On the contrary, Z14 and Z15 Frank-Kasper polyhedra have been found to be rare and improper to form distinct nanoclusters in crystals. The second shells of Frank-Kasper nanoclusters have been revealed possessing their own stability: they can be realized in nanoclusters with different internal polyhedra and can shift around the core shell. The role of Frank-Kasper nanoclusters in assembling intermetallic crystal structures is illustrated by several examples.