The Change of the Electronic Structure of Metal Clusters Upon Hydrogen Chemisorption (original) (raw)
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Theoretical study of structure and stability of h+x (h2)n clusters
Chemical Physics Letters, 1981
Structural parameters, energies, and spectroscopic characteristics of two series of layerwise hydrogenated aluminum clusters Al 44 H n (n = 27-44) and Al 89 H т (т = 15, 24, 39, and 63) have been calculated by the density functional theory method. It has been shown that increasing number of H atoms in both series entails rapid enhancement of structural distortions up to cooperative rearrangements accompanied by a change in the shape and composition of the surface layer and internal core of the cluster. At the end of the first series Al 44 H n , several surface atoms migrate to the outer sphere of the cage to form valence-unsaturated "outer-surface" AlH n and Al 2 H n moieties, which can be active sites at the stages of deeper hydrogenation. Simultaneously, the inner core [Al] 5 disintegrates, and its atoms are introduced into the surface layer. A family of "inverted" Al 42 H 42 isomers with the hollow [Al 42 ] cage has been localized; the isomers contain the endohedral AlH 4 group and "inner" Al−H bonds with their hydrogen end directed to the center of the inner cavity. At the end of the second series, five alanate groups AlH 4 and two Al 3 H 2 fragments bonded to the surface through hydrogen bridges are formed in the outer sphere of the cluster. The results are of interest for DFT modeling of hydrogenation of nanosized aluminum clusters at the molecular level.
Molecular orbital description of catalysis by metal clusters
Journal of Catalysis, 1973
Extended Hiickel and complete neglect of differential overlap (CNDO) molecular orbital calculations were performed for clusters of metallic atoms to determine possible catalytic properties. Data for pure clusters of Ag, Pd, Cd, Cu, and Ni are reported, as well as for alloy systems of Pd-Ni and Cu-Ni. The results indicate the presence of d-band holes in Ni and Pd clusters with a greater amount in Ni. Clusters of Ag and Cd have as low energy form the linear geometry in preference to other three-dimensional geometries. Clusters have a low bond energy (BE) and a different electronic configuration than their bulk metals. The effect of substrates on metal clusters is examined and is shown to be capable of altering electron occupancy of the cluster.
An experimental set-up for generation and characterization of metal clusters
An experimental facility has been desig ned, fabricated and installed for the production and mass characterization of metal clusters. The metal clusters can be produced in a flow tube reactor by laser vapourisation of target metals and characteri zed by an indigenou sly designed and fabric ated time of flight mass spectrometer (TOFMS). Thi s setup can be used to carry out a variety of size-depe ndent physico-chemical properties of metal clusters in gas-phase, viz., stability pattern, ionization potential, fragmentation behaviour and chemical reac tivity, etc. The perfo rmance of the mass spectrometer has been tested by ionizin g different organic mo lecules like aniline, acetone, benzene, diethyl ether, etc., using third harmonic (355nm) of Nd:YAG laser. The resolution of thi s TOFMS is-500. Vapourisation of Fe target in the flow tube reactor res ults in th e monomer signal of Fe atoms. which red uces on addition of ethy lene indicating the reacti on of ethylene molecules wit h Fe atoms. Nano-sized Fe and AI metal particles have been deposited on silicon usin g thi s machine and characterised by scanning electron mi croscopy.
Potential use of some metal clusters as hydrogen storage materials—a conceptual DFT approach
Journal of Molecular Modeling, 2011
Structure, stability and reactivity of several metal clusters with or without hydrogen doping were studied using standard ab initio and density functional theory (DFT) calculations. Conceptual DFT-based reactivity descriptors and the associated electronic structure principles lend additional support towards understanding the stability of metal clusters upon hydrogen doping. Related aromaticity was analyzed through nucleus-independent chemical shift values. Site selectivity towards electrophilic and nucleophilic attacks were analyzed in terms of the corresponding local reactivity descriptors. Most of the metal clusters have the capacity to trap hydrogen molecules. Keywords Hydrogen storage. Metal cluster. Conceptual DFT. Aromaticity 1 Sensex is an abbreviation for the Bombay Exchange Sensitive Indexthe benchmark index of the Bombay Stock Exchange (BSE).
Chemical Reactivity of Free and Supported Metal Clusters
ChemInform, 2005
The study of the chemical properties of metal clusters is a growing field of research. It is motivated by possible connections with industrial catalytic processes. In addition, one aims to control and tune chemical and catalytic reactions by simply changing cluster size. In this context, the extreme localization of valence electrons of clusters is most important and leads to strong quantum size effects. Clusters are therefore also considered as building blocks for nanocatalysis. The use of model systems will allow obtaining a fundamental understanding of their physical and chemical properties. Such model systems can consist of free size-selected clusters or clusters stabilized on surfaces. Size selection is important, as the evolution of the cluster's properties with size is distinct. During the last 15 years studies on free clusters consisting of a couple of tens of atoms revealed strongly size-dependent reactivities (1-4). These studies showed the rate constants of chemical reactions to vary by orders of magnitudes, when cluster size is changed only by a single atom. Besides the work reviewed in this contribution there are only a few other studies showing that chemical reactivities of deposited clusters change as function of the exact cluster size. The group of Waste showed Ag4 to be responsible for the latent image in photography (5, 6). Jarrold describes a size-dependent oxide formation of small Sin clusters (7). In both examples, the model catalysts were produced by depositing clusters from the gas phase onto solid surfaces. A completely different 153
Hydrogen dissociation on small aluminum clusters
Chem Phys, 2010
Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Aln clusters considered, with n =2-6. The aluminum dimer, which shows a Π3u electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox et al., J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al6 cluster. The electronic structure of the Aln-H2 systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Aln clusters (with n even) exhibit the lowest barriers to H2 dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σu molecular orbital of H2.
The Journal of Physical Chemistry A, 2008
The adsorption of H 2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H 2 is found to be exclusively dissociative on Sc n O + , V n + , Fe n + , and Co n + , and both atomic and molecularly chemisorbed hydrogen is present in Ni n H m + complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2-or 3-fold coordinated to cationic metal clusters.
Photoelectron Spectroscopy of Clusters and Adsorbates on Clusters
Berichte der Bunsengesellschaft für physikalische Chemie, 1992
The electronic and geomctric structure of free metal cluster anions is studied by photodetachment. The cluster anions are produced in a PACIS (Pulsed Arc Cluster Ion Source). From mass separated bunches of anions electrons are detached by a UV-laser pulse. Photoelectron spectra of the detached electrons are measured with a "magnetic bottle" type time of flight spectrometer. First data on Ga; clusters are presented and compared to similar measurements on Aln-clusters. At the high resolution (::::; 30 meV) , accessible with this experimental set up comparison of the data with calculations delivers new insights into the geometric structure of the particles. A first application of this technique to molecules adsorbed on metal clusters is presented.
Homogeneous catalysis by metal clusters
Journal of Molecular Catalysis, 1984
The use of transition-metal cluster compounds in homogeneous catalytic reactions has received considerable attention in the last few years, mainly because discrete metal clusters may: (i) serve as reasonable models for metallic particles in processes such as chemisorption and heterogeneous catalysis and (ii) exhibit reactivity patterns different from those found in heterogeneous or mononuclear homogeneous catalysts [ 11. Catalysis by well-defined polynuclear species offers the possibility of multi-site interactions with the substrates, thus opening mechanistic pathways not available to mononuclear species [2] ; therefore, the question of whether the actual catalytically-active species is a cluster or a mononuclear intermediate derived therefrom, should be answered by observing a reaction not encountered for mononuclear complexes, or by observing correlations between catalytic activity and structural features of the cluster.
Fundamental aspects of catalysis on supported metal clusters
Journal of Materials Chemistry, 2004
In this review, we examine the role of oxide support defects, cluster size-dependence, cluster structural fluxionality, and impurity doping on the catalytic properties of size-selected metal clusters on surfaces. By combining experimental results from the oxidation of CO on sizeselected gold clusters with ab-initio calculations, a detailed picture emerges of the electronic and structural dynamics of this process. For Au 8 , Au 4 , and Au 3 Sr clusters on F-center defects on MgO(100), optimized atomic structures and local density of states calculations support the experimental results for the oxidation of CO. Fundamental aspects such as charge transfer from oxide defect sites and the adsorption and activation of reactant molecules are elucidated. Using a pulsed molecular beam set up, turnover frequencies for the oxidation of CO and the reduction of NO on Pd clusters were determined. This new experimental scheme allows for the determination of mechanistic details of much greater sophistication than with one-cycle experiments. Isolating known catalytic phenomena such as spillover, reverse spillover, and adlineation should be attainable at the atomic level using these pulsed molecular beam experiments on size-selected metal clusters on surfaces.