Clusters exposed atoms

A, while the distance from the siroheme iron to the nearest cluster iron atom is 4.4 A. One of the cluster sulfur atoms is in van der Waals contact with the siroheme ring. The X-ray structure and spectroscopic experiments (Madden et al., 1989) suggest that a cysteine thiol sulfur serves as the bridging ligand between the cluster and the siroheme. The sixth coordination site of the siroheme appears to be vacant and solvent exposed. This site presumably represents the location of substrate binding to sulfite reductase. 

For optimal use of Pt, a preferred size of Pt crystal can be predicted. For catalytic activity of exposed atoms (STY), this would take place around point C (Fig. 6.7 d). The opposite trends of dispersion and catalytic activity (related to metal character of atom in the dispersion) give the resulting curve C for overall catalysis observed as function of cluster size. Catalysts prepared by different methods will, generally, yield different crystallite size distribution and shape of metal crystals. This will result in dissimilarities in sorptive and catalytic behavior, even if allowance is made for the difference in active surface area285. ... 

Bagus et were the first to give a detailed ab initio description of the halogen-silver bond using Hartree-Fock wave functions. The silver surface was modeled as a four- or ten-atom cluster exposing the... 

Fig.1a,b Free clusters a 3x3x2 cluster used to represent the (100) surface plane of metal oxides with rock-salt structure b ZUijOb cluster exposing the Zn-terminated ZnO(OOOl) and the O-terminated ZnO(OOO-l) faces of wurtzite-hke ZnO. (Unless otherwise noted, in this and the following figures the metal atoms are represented by black spheres and the O atoms by gray spheres)... 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Figure 1.12 The three different types of cluster size dependence of catalytic conversion. Rates are considered normalized per exposed surface atom (schematic).

Exposed film is developed with an aqueous reducing agent such as hydroquinone, which can reduce Ag cations when catalyzed by Ag atoms. The reducing agent reacts selectively with those Ag cations located next to already-reduced Ag atoms, generating clusters of Ag ... 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface technique, but also the most difficult to quantify. When a surface is exposed to a beam of ions (Ar", 0.5-5 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection (desorption) of atoms, ions, and multi-atomic clusters. In SIMS, positive or negative secondary ions are detected directly with a quadrupole mass spectrometer. 

Other detection modes in bright CL or BL reactions are multichannel detectors, which provide simultaneous detection of the dispersed radiation and produce a permanent image of a wide area. Photographic films or plates are emulsions that contain silver halide crystals in which incident photons produce stable clusters of silver atoms within the crystals. Internal amplification is provided in the development process by an electron donor that reduces the remaining silver ions to silver atoms within the exposed crystals. A complexing agent is used to remove the... 

Lack of reactivity in copper sulfide cluster anions has been associated with structural features such as linear S-Cu-S bonding. Reactivity of metal sulfide cluster anions is associated with exposed under-coordinated metal atoms able to bond to coordinating molecules this is the essence of this chapter. 

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