Coordination to surface

The ratioed, scale-expanded insert shows these more clearly. There is a negative band caused by the gaseous SO2 upon which a positive doublet is superimposed. Pumping at room temperature caused the doublet to disappear. The 1375 and 1358 cm l bands are attributed to SO2 weakly coordinated to surface oxide ions... 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

Carbon dioxide, C02, is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C02 adsorption onto oxide surfaces leads to a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of C02 and possible surface species are summarized in Table VI. The wave-number range below 1000 cm"1 was usually not accessible in studies on adsorbed C02 because of the strong absorption of the oxides at lower wave numbers. 

The catalysts derived from supported iron clusters exhibit in Fischer-Tropsch synthesis a high selectivity for propylene. Those catalysts are also selective for the stoechiometric homologation of ethylene to propylene and of propylene to n and iso butenes. The results are explained on the basis of a new mode of C-C bond formation which implies < - olefin coordination to surface methylene fragments or methylene insertion into a metal alkyl bond. 

Mechanistically it is logical to observe high selectivity for propylene if we assume that coupling of methylene to ethylene as well as ethylene coordination to surface carbene are fast reactions. Propylene for steric hindrance would react more slowly than ethylene with surface carbene reaction (3) whereas reaction (4) would be less favored for electronic reasons. It is difficult at this point to speculate why the selectivity for propylene is associated with small iron particles. One possibility is that the small iron particles displace the equilibrium olefin(acarbene mechanism besides these small Fe particles would have small hydrogenation properties which thus avoid methane formation from the carbene and saturated hydrocarbon formation from the olefin. 

It should be noted that with benzylamine, C-alkylated products are formed (Table 8) whereas with aniline (Table 5) only N-alkylation is reported. This is surprising, given that aniline is a weaker nucleophile than benzylamine and its aromatic ring is richer in electrons, and thus more suitable for alkylation. The latter situation might change, however, when the nitrogen is coordinated to surface A1 or interacting with surface OH. 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

Based on our surface spectroscopic results, a bidentate mode of coordination of Ca " and Tb " with the carboxylate group of Fc-peptide acid cystamine films is proposed (Scheme 6.6). There is precedence for bidentate metal coordination to surface-bound ligands. In a study of lanthanide selective sorbents, Fryxell and coworkers showed that self-assembly of glycinate monolayers on mesopo-rous materials bound Eu " in an 8-coordinated fashion. In this system, the close proximity of the ligands allowed four bidentate ligands to chelate the lanthanide cation [73]. 

Further studies showed that methanol adsorbs as surface methoxy species and methanol-like species strongly coordinated to surface metal cations (Lewis acid... 

---