Adsorbates, metal clusters

Adatoms, density versus temperature, 222 Adsorbates, metal clusters, 28-29 A1 clusters, 25-27,29 Al,... 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

Figure 2. The BeisXe D3/1 cluster used to model chemisorption into the four-fold hollows. The Be 13X3 cluster used to model chemisorption into the three-fold hollows is generated by a 60° rotation of the six adsorbate atoms about the vertical axis. The D3d metal cluster is formed by rotating the bottom three metal atoms by 180° about the vertical axis.

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

When a supported metal on an oxide is prepared from an adsorbed precursor incorporating a noble metal bonded to an oxophilic metal, the result may be small noble metal clusters, each more-or-less nested in a cluster of atoms of the oxophilic metal, which is cationic and anchored to the support through metal-oxygen bonds [44,45]. The simplest such structure is modeled on the basis of EXAFS data as Re4Pt2, made from Re2Pt(CO)i2 (Fig. 6) [45]. 

The adsorbate-covered clusters yield a UPS difference spectrum with two peaks on either side of the metal d-states. The dominant changes in the intensity ratio of these peaks occur up to 50-atom Ag clusters which can be rationalized in terms of the cluster d band width and IP, which both depend on cluster size. 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

The recent interest in the exploration of electrocatalytic phenomena from first principles can be traced to the early cluster calculations of Anderson [1990] and Anderson and Debnath [1983]. These studies considered the interaction of adsorbates with model metal clusters and related the potential to the electronegativity determined as the average of the ionization potential and electron affinity—quantities that are easily obtained from molecular orbital calculations. In some iterations of this model, changes in reaction chemistry induced by the electrochemical environment... 

Meier DC, Goodman DW. 2004. The influence of metal cluster size on adsorption energies CO adsorbed on Au clusters supported on Ti02. J Am Chem Soc 126 1892-1899. 

Photoinduced deposition of various noble metals onto semiconductor particles has been extensively reported [310-315]. Several factors are controlling this reaction. To control the morphology of metal clusters with desired size and distribution pattern on a given surface area of titania, the most relevant factors are the surfactant, pH, local concentration of cations, and the source of cation [316], In the case of the Ag clusters, the reaction steps proposed include the creation of electron (e )-hole (p+) pairs, the reaction of holes with OH surface species, and the reaction of electrons with adsorbed Ag+ ions ... 

Figure 5-11 shows a simple model of the compact double layer on metal electrodes. The electrode interface adsorbs water molecules to form the first mono-molecular adsorption layer about 0.2 nm thick next, the second adsorption layer is formed consisting of water molecules and hydrated ions these two layers constitute a compact electric double layer about 0.3 to 0.5 nm thick. Since adsorbed water molecules in the compact layer are partially bound with the electrode interface, the permittivity of the compact layer becomes smaller than that of free water molecules in aqueous solution, being in the range from 5 to 6 compared with 80 of bulk water in the relative scale of dielectric constant. In general, water molecules are adsorbed as monomers on the surface of metals on which the affinity for adsorption of water is great (e.g. d-metals) whereas, water molecules are adsorbed as clusters in addition to monomers on the surface of metals on which the affinity for adsorption of water is relatively small (e.g. sp-metals). 

AFads = F(total system) - F(metal cluster) - F(adsorbate)... 

The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems [87] or in porous materials [88]. The ionic precursors are included prior to irradiation. The penetration in depth of ionizing radiation permits the ion reduction in situ, even for opaque materials. The surface of solid supports, adsorbing metal ions, is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. 

Figure 2.44. Charge density difference plotted in a plane containing the metal atoms and the carbon skeleton of the ethylene molecule. The difference is taken between interacting and non-interacting molecules and metal cluster for the adsorbed cases. For the gas phase molecule (top), the difference between the singlet and triplet state is shown. From Ref. [85].

There are several prerequisites which have to be fulfilled for the one electron ECP approach to be applicable. In the case of metal clusters the atomic configuration must be known, i.e. one must safely be able to assume a dns2 or a drL+1s1 configurations on the atoms in the cluster. The d orbitals should not form covalent bonds neither within the cluster nor between the cluster and the adsorbate. Ferromagnetic metals and copper are likely to have these properties. For other metals this is not so clear. Indications are that e.g. the ground state of the Pts cluster is low spin with developed covalent intra cluster d-d bonds[22]. 

In Fig. 5, the symbol I—I indicates a probable wavenumber range associated with the MSSR-allowed completely symmetrical modes, assuming that the surface complex allows the adsorbed hydrocarbon group to retain its full symmetry. The symbol (—) indicates other prominent, usually strong, absorptions of the ligand in the metal-cluster compound—absorptions that might also show up with... 

Muetterties et al. (130) published an extensive survey relating the properties of metal clusters and their ligands to analogous properties of metal surfaces and adsorbed species. Moskovits (131) has expressed caution, particularly in relating the reactivities of cluster ligands and the surface equivalents and more recently,... 

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