Low-coordinated sites

The presence of a site with a low metal-metal coordination is compatible with the non-crystalline nature of the cobalt deposits [64]. It is to be expected that these sites exhibit different chemical reactivity than the usual adsorption sites. This can be verified by subsequent deposition of a small amount (0.1 A) of Pd atoms, which are known to nucleate exclusively on the cobalt particles [64]. The corresponding IR spectrum is shown as the bottom trace in Fig. 6. It is seen that an additional peak appears at 2105 cm which is readily assigned to CO bound terminally to Pd. More importantly, the growth of this Pd feature is completely at the expense of the carbonyl species, indicating that Pd nucleates almost exclusively at these low coordinated sites and prevents the formation of the carbonyl species. 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

Density Functional Theory (DFT) has shown that low-coordinated sites on the gold nanoparticles can adsorb small inorganic molecules such as O2 and CO, and the presence of these sites is the key factor for the catal5dic properties of supported gold nanoclusters. Other contributions, induced by the presence of the support, can provide parallel channels for the reaction and modulate the final efficiency of Au-based catalysts. Also these calculations extended for the adsorption of O and CO on flat and... 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

The rules in overall decreasing order of importance essentially state that the ideal structures for carboranes will be based on most spherical deltahedra (rule 1) the BE hydrogens will tend to be placed in the lowest possible coordination environments (rule 2) when elements to the right of boron in the periodic table are incorporated into the deltahedron or deltahedral fragment, they will tend to preempt low-coordination sites (e.g., carbon) or, if electron-deficient, high coordination sites (rule 3) and, lastly, boron will eschew seven-coordinate BH or six-coordinate... 

There are a number of heteroatoms that can substitute for either boron or carbon in the carboranes. The groups that are as electron-deficient as BH groups are listed vertically to the left of the center line in Table V, whereas those that are as capable as carbon in donating electrons are listed to the right of the center line. The transition elements for the most part electronically substitute for boron and occupy high-coordination sites, but upon electron demand the transition element may also substitute for carbon and concomitantly occupy low-coordination sites. Several transition element moieties, by contrast, are one more electron deficient than boron and occupy, as would be anticipated, high-coordination positions and require additional electron donors (CH groups) to counter the electronic deficit (XIII-24). 

As one illustrative example, since Co is seen to inhabit BH or highest-coordination sites in general, it is noteworthy that a cobalt in its more reduced form (i.e., furnishing an additional electron into the cage) substitutes, in such cases, for a CH group and is, therefore, found, as would be expected, in a low-coordination site (1-23). 

Before leaving structures II-, III-, and IV-25, although III-25 is unstable with respect to VI-25 for an isomer of SB9Hi2, there is reason to suspect that the IV-25 isomer of SB9Hi2 may one day be discovered unless sulfur (as compared to carbon) has a greater predilection than carbon for low-coordination sites (which has not yet been ascertained). This suggests that the known %tdo-SB9H 11 (11-25) should be reconsidered. 

In this work we will present the results of the optical transitions associated to F centers formed at low-coordinated sites of the MgO (100) surface. As these excitations will appear at lower energy than those in the regular surface we will check whether they can be responsible of some of the unassigned features of the spectrum. 

A novel ten-vertex bimetallocarborane, (CsHo C C BeHs, is produced upon the polyhedral expansion of 1,7-C2B6H8 (16). The expected monometallic nine-vertex species, C5H5CoC2B6H8, is also produced in this reaction (see Fig. 4). An X-ray crystal structure of the bimetallic product (61) showed the 2 cobalt atoms to occupy adjacent positions on the two equatorial belts of the bicapped square antiprism. The carbon atoms occupy the low-coordinate caps. Thermal rearrangement (30) produced an isomeric complex in which the metals are separated from each other, but the carbon atoms remain in the low-coordinate sites. 

The characterization technique of CO Temperature-Programmed Desorption has been studied with Pt reforming catalysts. Critical factors in the experimental procedure and the catalyst pretreatment conditions were examined. The CO desorption spectrum consists mainly of two peaks which are probably combinations of other peaks and the result of various binding energy states of CO to Pt. These in turn could be due either to the interaction between Pt and the alumina support or the results of high and low coordination sites on the Pt crystallites. No significant relationship between the character of the CO desorption profile and the activity of commercial catalysts was observed. 

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