Surface monodentate

Among the current spectroscopic methods extended x-ray absorption fine structure spectroscopy (EXAFS) has received the most attention, however other methods such as FTIR, may be equally as promising. EXAFS is considered to provide definitive information on inner- vs. outer-sphere bonding and is suitable for determining mode of attachment to the surface (monodentate, bidentate, binuclear) but does not resolve questions of surface speciation since it is not sensitive to H atoms. In addition, examination of the same system by different researchers has in some instances resulted in different conclusions. 

Metal ion binding to hydrous oxides can occur as monodentate or bidentate surface complexes (Eqs. 2.9a and 2.9b) where, respectively, one or two protons are released per mol of metal ion bound. Develop a simple graphical method to distinguish between monodentate and bidentate metal binding. 

In principle, it appears possible to distinguish between the formation of monodentate and bidentate surface complexes if the stoichiometry of the H+ release is know. A mean surface complex stoichiometry can be formulated... 

The photocatalytic oxidation of organic and inorganic compounds and the photo-catalytic production of H202 occurs also at the surface of iron(III)(hydr)oxides. It has been proposed (e.g., Hoffmann, 1990 Faust and Hoffmann, 1986) that the oxidation of S(IV) by 02 in atmospheric water is catalyzed by iron(III)(hydr)oxide particles. It is assumed that the reductant (HSO3) is specifically adsorbed at the surface of an iron(III)(hydr)oxide, forming either a monodentate or a bidentate surface complex ... 

ToF-SIMS, and AFM results, the formation of ordered monolayers of octade-cylphosphoric acid on a Ta205 surface involves both monodentate and bi-dentate phosphate species [135]. In the case of goethite, (y-AlOOH), it was found that methylphosphonic acid bound to the surface as a monodentate or a bidentate species depending on the pH and the concentration [163]. 

In Eq.(8), the monodentate nitrate species is written as an example of surface nitrate species which does not exclude possible bridging and bidentate species. 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

For some of the most intensively studied anions, increased information has not clarified the speciation or coordination of the adsorbed complex. For example, on the basis of IR data it has been claimed that sulphate adsorbs on goethite as both a bidentate and a monodentate surface complex (Parfitt and Smart, 1977 Persson and Lovgren, 1996). Some of the conflict may arise from differences in the conditions under which the experiments were carried out. Most IR spectroscopy measurements of adsorbed sulphate on Fe oxides were made on dried samples. Hug (1997) compared... 

ATR-FTIR data indicated that on goethite, glycophosphate adsorbs mainly as an inner-sphere, monodentate complex via the phosphonate group the carboxylate group does not appear to be involved in surface coordination in this case (Sheals et al 2002). 

The reaction of (alkyl)chlorosilanes with a silica surface has been discussed and reviewed in great detail in literature [10], Although 5 different reactions are possible with di-, tri- or tetrachlorosilanes, basically two important surface species are created. The first is a monodentate silyl group, created by the monomolecular reaction of 1 silanol with 1 chlorosilane, according to reaction (A) (cfr. Figure 2). The second surface specie is a bidentate silyl group, created either by a bimolecular reaction (B) or by a consecutive reaction (C). We have reported previously [11] that the surface of MCM-48, prepared by the gemini 16-12-16 surfactant, possesses 0.9 OH/nm2. 

Less is known about the combustion pathway. Using IR spectroscopy, formate, carboxylate, bidentate carbonate, monodentate carbonate, and acetate bands have been observed when butene is adsorbed on Surface degradation (oxidation) of butadiene has also been observed on a-Fe203 at room temperature (34). 

Figure 3 Adsorption energy of monodentate-B adsorbed formic acid on ZnO(lOlO) as a function of coverage. Calculated values (solid lines) are compared to a simple electrostatic model (dashed line) based on the atomic charges, lxn coverages refer to surface cells extended in the (000T) direction, and nxl to extensions in the (1120) direction, nxn cells have been extended in both the (0001) and the (1120) directions.

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