Exposed metal sites

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Adsorbed CO and NO were used as probes to Investigate the effects of Co concentration and sulfide on the nature and numbers of exposed metal sites on reduced catalysts containing 1 to 6 wt% Co and 8 wt%. Mo on three alumina supports. Exposure of Mo Ions decreased with Increased Co concentration. Exposure of Co Ions typically reached a maximum at 2-4% Co. Sulfide decreased exposure of all metal Ion sites and Increased exposure of reduced metals. Effects of preadsorbed pyridine and 2,6-lutldlne, known poisons, on the exposure of metal sites, plus other evidence. 

Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

The single crystal catalysts, -1 cm in diameter and 1 mm thick, are typically aligned within 0.5 of the desired orientation. Thermocouples are generally spot-welded to the edge of the crystal for temperature measurement. Details of sample mounting, cleaning procedures, reactant purification, and product detection techniques are given in the related references. The catalytic rate normalized to the number of exposed metal sites is the specific activity, which can be expressed as a turnover frequency (TOF), or number of molecules of product produced per metal atom site per second. 

Interactions of hydrogen with exposed metal sites... 

The binding of H2 to exposed metal sites has also been probed for [Cu3(BTC)2]oo (HKUST-1 in Section 11.2.4). Rietveld analysis of the neutron powder diffraction data for D2-loaded samples of [Cu3(BTC)2] 00 reveals that there are six possible adsorption sites, the most favourable being the unsaturated axial site of the binuclear Cu fragment (Peterson et al., 2006). The CU-D2 distance of 2.39 A suggests significant interaction between H2 and Cu(II), but this is much weaker than that observed for molecular o-bonded t -H2 complexes. Interestingly, the heat of adsorption of H2 binding in HKUST-1... 

Interactions of hydrogen with metal-organic frameworks (MOFs) without exposed metal sites... 

V Eli- This is due to passivation of active sites required for H2 dissociation by a relatively small OH coverage. A complete oxide film at Pt will, however, react with H2 in an autocatalytic way, depending on opening up holes in the oxide which exposes metal sites where the dissociation of H2 can occur. The oxide film is then stripped in a mixed cathodic/anodic (oxidation of H) process. 

Chemisorption of simple probe molecules provides important information on the number of exposed metal sites, the nature of adsorbate-adsorbent interactions, and the electronic state of the surface. Frequently employed probe molecules include CO, H2, NO, and O2. Simple base molecules such as NH3 and pyridine and acid molecules such as CO2 are used to probe the acid-base properties of the surface. Studies of chemisorption have been concentrated on molybdenum nitrides. In general, chemisorption behavior of metal nitride surfaces is very similar to that of carbide surfaces. 

Colombo V, Galli S, Choi HJ, Han GD, Maspero A, Palmisano G, et al. High thermal and chemical stabiUty i n pyrazolate-bridged metal-organic frameworks with exposed metal sites. Chem Sci 2011 2 1311-9. 

Heterogeneous basic catalysis has been fundamental in the development of two-phase systems. The acidic and basic nature of any oxide material lies at the interface between the bulk material and its exposed surface where oxygen atoms may be protonated or deprotonated and exposed metal sites have the potential to function as Lewis acid (Figure 2). ... 

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