Supported Metal Clusters Research Articles

Hexairidium carbonyl clusters in the micropores of faujasite zeolites: evidence from transmission electron microscopy

[Ir6(CO)16] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)2(acac)] followed by treatment in CO + H2. [Ir6(CO)15]2− in zeolite NaX was prepared similarly. Each sample was characterized by high‐resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost no scattering centers indicating iridium outside these pores. The supported [Ir6(CO)16] and [Ir6(CO)15]2−, which have previously been characterized by infrared and extended X‐ray absorption fine structure spectroscopies, are among the most uniform and structurally best defined supported metal clusters.

Modeling heterogeneous catalysts: metal clusters on planar oxide supports

Model catalysts consisting of Au and Ag clusters of varying size have been prepared on single crystal TiO2(110) and ultra-thin films of TiO2, SiO2 and Al2O3. The morphology, electronic structure, and catalytic properties of these Au and Ag clusters have been investigated using low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) and spectroscopy (STS) with emphasis on the unique properties of clusters <5.0 nm in size. Motivating this work is the recent literature report that gold supported on TiO2 is active for various reactions including low-temperature CO oxidation and the selective oxidation of propylene. These studies illustrate the novel and unique physical and chemical properties of nanosized supported metal clusters.

Kinetic modeling of the CO oxidation reaction on supported metal clusters

The CO oxidation on Pd is generally considered to be structure insensitive and is not expected to depend on the size of the Pd particles. However, several size effects have already been evidenced for this reaction. Near the temperature where the steady-state reaction rate is maximum, the reactivity per Pd surface atom (turnover number (TON)) increases for small particles. At low temperature, in transient molecular beam experiments, a second peak of CO2 appears after the CO beam is closed, while the oxygen is still supplied to the sample. This peak shifts, and its shape changes with temperature and oxygen pressure. This peak is explained by the presence of CO strongly bound to the particle edges. This strongly bound CO reactswell after the CO adsorbed on the facets has desorbed. A kinetic model is presented which explains the evolution of this peak with temperature and oxygen pressure.

Novel interactions of supported clusters: contact epitaxy

The study of clusters of ‘model’ metal systems such as Cu and Ag provide a valuable route to explore critical issues in materials epitaxy. Our investigations have led to observations of novel interactions between supported metal clusters in both homo- and heteroepitaxial configurations. In the experiments, clusters of both Cu and Ag were produced by inert gas condensation and deposited on the clean (001)Cu surface under ultrahigh vacuum. Following deposition, the Cu clusters were observed to be of initially random orientation on the substrate surface, undergoing reorientation upon annealing by a mechanism involving sintering and grain growth. In the case of Ag clusters, the formation of a heteroepitaxial layer between the particle and substrate was observed upon initial contact. The phenomenon, which we call ‘contact epitaxy’, may be understood from molecular dynamics simulations of a ‘soft impact’ between the nanoparticle and substrate which indicate that the ordered layers form within picoseconds of impact. The experiments were performed in an ultrahigh vacuum transmission electron microscope equipped with an in-situ nanoparticle sputtering system.

Bonding trends in free and supported metal clusters

Bonding trends in free and supported metal clusters

129Xe NMR Spectroscopy of Metal Carbonyl Clusters and Metal Clusters in Zeolite NaY

[Ir4(CO)12] and [Ir6(CO)16] were synthesized in the pores of zeolite NaY by reductive carbonylation of sorbed [Ir(CO)2(acac)], and [Rh6(CO)16] was similarly synthesized from [Rh(CO)2(acac)]. The supported metal carbonyl clusters were decarbonylated to give supported clusters modeled on the basis of extended X-ray absorption fine structure spectra as Ir4, Ir6, and Rh6, respectively. The supported metal carbonyl clusters and the supported metal clusters formed by their decarbonylation were investigated by 129Xe NMR spectroscopy at temperatures in the range of 100−305 K. As the temperature increased, the chemical shift decreased. The curves representing the chemical shift as a function of temperature for xenon sorbed on the zeolite that contained clusters modeled as Ir4, Ir6, and Rh6 were all essentially the same and hardly different from that observed for the bare zeolite NaY. This comparison leads to the conclusion that xenon is less strongly adsorbed on the decarbonylated metal clusters than on the zeolite framework. Larger chemical shifts were observed for the zeolites containing the metal carbonyl clusters, with the largest being observed for the zeolite containing [Ir4(CO)12]. These results are explained on the basis of the cluster sizes and NaY zeolite geometry. We suggest that the contact between xenon and [Ir4(CO)12] cluster is better than that between xenon and [Ir6(CO)16] or xenon and [Rh6(CO)16] clusters because these two larger clusters almost fill the zeolite supercages and exclude xenon, whereas [Ir4(CO)12] in the supercages is small enough to allow entry of the xenon.

Supported Iridium Cluster Catalysts for Propene Hydrogenation: Identification by X-Ray Absorption Spectra Measured During Catalysis

The identification of Ir4 and Ir6 as catalytically active species is reported. Supported metal carbonyl clusters, [Ir4(CO)12]/γ-Al2O3, [Ir6(CO)15]2−/γ-Al2O3, and [HIr4(CO)11]−/MgO, were treated in He to prepare supported metal clusters modeled as Ir4/γ-Al2O3 (1), Ir6/γ-Al2O3 (2), and Ir4/MgO (3). The samples were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy during propene hydrogenation catalysis; the partial pressures of propene and hydrogen were both 0.5 bar. From the EXAFS data the Ir-Ir shell coordination numbers of 1, 2, and 3 were determined as 3.2, 4.0, and 3.0, respectively.

Structure and melting of small Ni clusters on Ni surfaces

Using the Voter and Chen (VC) version of the embedded atom model (EAM), we performed computer simulations to obtain the structures and binding energies of small Ni clusters on Ni(001), Ni(110) and Ni(111) surfaces. The predicted Ni cluster structures on Ni(001) and Ni(111) generally agree with the results obtained by Liu and Adams using the Foiles, Baskes and Daw (FBD) version of the EAM (the only exception is one structure formed on the Ni(001) surface), but the corresponding binding energies differ significantly. The temperature dependence of the behaviour of the seven-atom Ni cluster on the Ni(111) surface shows that the predicted cluster melting temperature also depends significantly on which version of the EAM is used. Hence EAM predictions of the properties of supported metal clusters depend crucially on the parameterization of the model, i.e. on the kind of data used in optimizing the embedding function and pair interaction. Although ab initio results for Ni clusters on Ni surfaces are not available for comparison, it seems plausible that the EAM description of supported transition metal clusters, like the EAM description of free transition metal clusters, may in general be more accurate if the VC version of the model is used rather than the FBD version since the former uses diatomic data as well as bulk properties in optimizing the EAM functions, and its parameterization should therefore be more appropriate for the cluster level.

Nucleation and Growth of Supported Metal Clusters at Defect Sites on Mgo and NaCl (001) Surfaces: The Cases of Pd and Ag

ABSTRACTNucleation and growth of metal clusters at defect sites is discussed in terms of rate equation models, which are applied to the cases of Pd and Ag on MgO(001) and NaCl(001) surfaces. Pd/MgO has been studied experimentally by variable temperature atomic force microscopy (AFM). The island density of Pd on Ar-cleaved surfaces was determined in-situ by AFM for a wide range of deposition temperature and flux, and stays constant over a remarkably wide range of parameters; for a particular flux, this plateau extends from 200 K ≤ T ≤ 600 K, but at higher temperatures the density decreases. The range of energies for defect trapping, adsorption, surface diffusion and pair binding are deduced, and compared with earlier data for Ag on NaCl, and with recent calculations for these metals on both NaCl and MgO

Growth, Structure and Morphology of Supported Metal Clusters Studied by Surface Science Techniques

This paper reviews on the basic theoretical background of the nucleation, growth and morphology of supported clusters, from thermodynamics and kinetics approaches. The application of surface science techniques to study, in situ, the formation and the structure of supported metal clusters is discussed from several examples. The combination of surface science techniques with microscopy techniques (ex situ TEM or in situ STM and AFM) is very powerful to study supported metal clusters. The influence of the substrate on the structure of metal clusters is also discussed from recent molecular dynamics calculations and experimental observations.

Partially Decarbonylated Tetrairidium Clusters on γ-Al2O3: Structural Characterization and Catalysis of Toluene Hydrogenation

Supported metal clusters were prepared as [Ir4(CO)12] was adsorbed intact fromn-pentane solution onto γ-Al2O3powder that had been partially dehydroxylatedin vacuoat 400°C. The supported clusters were characterized by infrared spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The supported [Ir4(CO)12], which was stable after heating in He at temperatures up to 100°C, was decarbonylated to various degrees by treatment in He at temperatures higher than 100°C, with the decarbonylation being complete at 300°C. EXAFS data indicated an average Ir–Ir first-shell coordination number of about 3.0 at an average bond distance 2.67 Å at each stage of the decarbonylation, demonstrating that the decarbonylation proceeded without disruption of the tetrahedral cluster frame, ultimately giving Ir4/γ-Al2O3. Chemisorption of hydrogen on the supported Ir4clusters was characterized by an H/Ir atomic ratio of about 0.13, a value much less than that characteristic of larger iridium clusters, which indicates that the supported clusters have reactivities different from those of bulk metallic iridium or iridium particles large enough to have bulklike properties. The [Ir4(CO)12] clusters were partially reconstructed from Ir4/γ-Al2O3by treatment in CO at 150–200°C. The supported tetrairidium clusters at various stages of decarbonylation were found to be catalytically active for toluene hydrogenation at 60°C and atmospheric pressure. The catalytic activity of supported [Ir4(CO)12] was negligible, and the activity increased with increasing decarbonylation, until the degree of decarbonylation reached about 70%, whereupon the catalytic reaction rate became almost independent of the degree of decarbonylation. The data suggest that the last remaining CO ligands have almost no effect on the toluene hydrogenation reaction because the clusters have attained a sufficient degree of unsaturation to provide bonding sites for the reactant ligands.

STM study of Pd growth on TiO2(100)-(1 × 3)

STM has been used to study the initial growth of palladium on TiO2(100)-(1 × 3). The results demonstrate that STM is capable of imaging individual atoms which form part of a supported metal cluster. The data were obtained from small (35Ådiameter by 8Åhigh) clusters at a dispersed-equivalent coverage of about 0.01 ML. At higher coverages the clusters coalesce.

Adsorption of Sulfur on Ag/Al2O3and Zn/Al2O3Surfaces: Thermal Desorption, Photoemission, and Molecular Orbital Studies

The interaction of S2 with Ag/Al2O3 and Zn/Al2O3 surfaces has been investigated using thermal desorption mass spectroscopy, core- and valence-level photoemission, and ab initio self-consistent-field calculations. The sticking coefficient of S2 on clean alumina is small (<0.1) at temperatures between 300 and 700 K. In the S2/Al2O3 system, there is an energy mismatch between the orbitals of the molecule and the bands of the oxide, and the reactivity of S2 on pure alumina is very low. The adsorption of sulfur on Ag/Al2O3 and Zn/Al2O3 surfaces induces strong perturbations in the electronic properties of the admetals and oxide support. In the Ag/Al2O3 and Zn/Al2O3 systems, the supported metal clusters (or particles) provide a large number of electronic states that are very efficient at donating charge into the S2(2πg) orbitals, inducing in this way the breaking of the S−S bond and the formation of admetal sulfides (AgSx and ZnSy). The larger the electron transfer from the supported metal into the S2(2πg) orbit...

Surveying a potential energy surface by eigenvector-following Applications to global optimisation and the structural transformations of clusters

We have developed a method to search potential energy surfaces which avoids some of the difficulties associated with trapping in local minima. Steps are directly taken between minima using eigenvector-following. Exploration of this space by low temperature Metropolis Monte Carlo is a useful global optimisation tool. This method successfully finds the lowest energy icosahedral minima of Lennard- Jones clusters from random starting configurations, but cannot find the global minimum in a reasonable time for difficult cases such as the 38-atom Lennard-Jones cluster where the face-centred-cubic truncated octahedron is lowest in energy. However, by performing searches at higher temperatures, we have found a pathway between the truncated octahedron and the lowest energy icosahedral minima. Such a pathway may be illustrative of some of the structural transformations that are observed for supported metal clusters by electron microscopy.

MgO-Supported Tetrairidium Clusters: Evidence of the Metal−Support Interface Structure from X-ray Absorption Spectroscopy

Iridium clusters were prepared by chemisorption of [Ir4(CO)12] on MgO powder supports that had been calcined at 300, 500, and 700 °C to vary the degree of hydroxylation. The initially adsorbed species, identified as tetrairidium carbonyl clusters by infrared and Raman spectroscopies, were decarbonylated by treatment in He at 300 °C and then treated in H2 at 300 °C. The decarbonylated clusters at each stage were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge spectroscopy (XANES). After treatment in He, the clusters on each support nearly retained the tetrahedral metal frame of the [Ir4(CO)12] precursor. After treatment in H2, the clusters on MgO that had been calcined at 500 and at 700 °C still nearly retained this frame, whereas the clusters on MgO calcined at 300 °C underwent slight agglomeration. Ir−O interactions in the decarbonylated samples were indicated by the EXAFS data. In the family of samples treated in He but not H2, the Ir−O bonding distance (approximately 2.1 Å) decreased as the degree of dehydroxylation of the support increased. This trend, associated with an increasing loss of electron density from the iridium clusters, was also evidenced by an increase in the Ir white line area as the support dehydroxylation increased. Another Ir−O EXAFS contribution, near 2.6−2.7 Å, is attributed to a nonbonding interaction influenced by cluster and support geometry. The Ir−O bonding distances increased after treatment of the samples in H2, the increase being greatest for the MgO support that had been treated at 700 °C, from 2.07 to 2.15 Å. The Ir4/MgO clusters are among the most nearly uniform, stable, and well-characterized supported metal clusters.

Helium diffraction study of the nucleation and growth of supported metal clusters

A new method for the in situ study of the nucleation and growth of metal clusters on an ionic substrate from vapor beam deposition is presented. It is based on the diffraction of helium atoms from the ionic substrate. From the attenuation of the first order diffraction peak during the growth of metal clusters the nucleation and growth kinetics are obtained. The only additional information needed is the final size distribution of the clusters which can be obtained from transmission electron microscopy. This method has been applied to the nucleation and growth of Pd clusters on MgO (100). With the same method but using the diffraction (specular peak) from the metal clusters it is possible to follow the evolution of the cluster shape during an annealing experiment.

Deposition and Surface Dynamic of Metals Studied by the Embedded-Atom Molecular Dynamics Method

Abstract The embedded atom method (EAM) was used to simulate the dynamics of deposition of supported metal clusters on fcc transition metals and the dynamic behavior of surfaces. The formation of metal clusters was observed before deposition and the geometries, stabilities and dynamics of a series of metal clusters were discussed as obtained by EAM and compared with first-principle calculations. The calculations indicate that the structure of a cluster in the vapor phase is governed by its zero kelvin stability. We find that if the cluster forms in the vapor phase, it is difficult to get epitaxial growth since the metal cohesive energy is large and the chester can not be broken by thermal motions. The effect of impurities on the deposition is also discussed. The anharmonicity, roughening and reconstruction of surface were also investigated. Three-body interactions were found to he important to understand these phenomena

ChemInform Abstract: Synthesis and Characterization of Supported Metal Cluster Catalysts with Well‐Defined Structures

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Supported Metal Clusters: Synthesis, Structure, and Catalysis

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Conductance resonance of coupled supported metal clusters

The conductance resonance of a tunneling structure with a few metal clusters, deposited on an insulating film, is studied by the generalized Breit-Wigner formula in a tight-binding approximation. We find that in the conductance resonance the multiple peak structure comes from the interaction between supported metal clusters on an insulating film and that the different arrangements of metal clusters can cause a difference of the conductance resonance peaks. Therefore it is possible to predict the effect of the interaction between metal clusters on the conductance resonance and develop some new microelectronic devices by artificially arranging metal clusters on the surface of the insulating film.