Surface chelation

SURFACE CHELATION AND ELECTRON INJECTION 9.16.4.1 Surface Chelation... 

The above examples of the dendrimeric Gdm complexes clearly illustrate how flexibility of the macromolecule is important in limiting proton relaxivity. This flexibility can originate from the intrinsic flexibility of the macromolecule itself, or/and from the non-rigid coupling of the chelate to the dendrimer surface. In both cases, the surface chelate benefits only partially from the slow motion of the dendrimer (or other macromolecule). In order... 

With bidentate ligands (mono nuclear or binuclear), surface chelates are formed. 

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

Metal centers bound to surface chelate, or surrounded by n protonated functional groups... 

Fig. 5.5c illustrates the effects of various ligands upon the dissolution rate, and that a surface chelate (ring structure of the ligand bound to the metal center at the surface) is more efficient in enhancing the dissolution rate. Furthermore, the acceleration increases in the series Salicylate > oxalate > malonate > phtalate > succinate which indicates that a 5-ring chelate is more efficient than a 6-ring or 7-ring chelate. 

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

Effect of surface chelation on the kinetics of electron transfer from the conduction band of Ti02 to methylviologen (MV2+). Oscillograms showing the time-dependent growth of the MV+ absorption at 630 nm after laser excitation (at 355 nm) of aqueous solutions (pH 4.85) containing colloidal Ti02 (1 g/e) and 10 3 M MV2+ ... 

Frei, H D. J. Fitzmaurice, and M. Gratzel (1990), "Surface Chelation of Semiconductors and Interfacial Electron Transfer , Langmuir 6,198-206. 

Adsorption and reaction of NO2. In contrast to the lack of reactivity of CO2 on the clean Au(lll) surface, NO2 is molecularly chemisorbed via its two oxygen atoms on clean Au(lll) at temperatures of 175 K and below to form a 0,0 -nitrito surface chelate with C2v symmetry (12). The NO2 sticking... 

I. Surface Chelation of Polypyridyl Complexes onto the TiO2 Oxide Surface... 

Frei, H. Fitzmaurice, D. J. Graetzel, M. Surface chelation of semiconductors and interfacia] electron transfer, Langmuir 1990, 6, 198. 

Infrared spectroscopic studies (185-188) indicate that nitric oxide is adsorbed onto metal surfaces, not only as the familiar nitrosyl ligand but also as [N202] (hyponitrite) units. On metal oxide surfaces, chelating nitrite has also been detected, and such species are believed to be intermediates in oxygen-exchange reactions between N 0 and bulk oxide in NiO or Fo203. The rate of this process is measurable under conditions (e.g., room temperature) in which gas-phase dissociation is minimal. Spectroscopic studies also suggest that nitric oxide reacts... 

G. Redmond D. Eitzmaurige M. Graetzel, Effect of surface chelation on the energy of an intraband surface state of a nanocrystalline Ti02 film. J. Phys. Chem. 1993, 97, 6951-6954. 

SA calcined at 750° was immersed in a mixed solution of 0.1 M zinc chloride and 2 M ammonium chloride and was left for 1 week. The surface protons were exchanged with zinc ion, and the exchanged amount of zinc ion and that of proton released were measured by titration. This 0.1 M ZnCla + 2 M NH4CI solution forms a surface chelate with the complex (chlorozinc) ion as shown in Fig. 16 20). 

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

Generally, the protonation of Al sites promotes the dissolution process with increasing H+ activity in acid solution (A1203, kaolinite, muscovite), whereas the rate of silica dissolution even decreases or remains constant (pH < 3). Obviously (lie more Al centers are exposed per unit surface area, the higher the proton-promoted dissolution rate and the more effective are surface chelates in catalyzing the weathering process. 

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