Ligand surface protonation

The surface concentration of the particular surface species (which is equal to the concentration of the precursor of the activated complex) (Eq. 3) can usually be determined from the knowledge of the number of surface sites and the extent of surface protonation or surface deprotonation or the surface concentration of ligands. Surface protonation or deprotonation can be measured from alkali-metric or acidimetric surface titrations, and ligands bound to the surface sites can lie determined analytically, from the change in the concentration of ligands in solution. 

Addition of a ligand, at constant pH, increases surface protonation while the addition of a metal ion (that is specifically adsorbed) lowers surface protonation. 

The dissolution rate of most oxides increases both with increasing surface protonation and with decreasing surface protonation, equivalent to the binding of OH ligands thus, in the alkaline range the dissolution rate increases with increasing pH (Chou and Wollast, 1984 Schott, 1990 Brady and Walther, 1990) (see Fig. 5.9c). 

Example 5.1 Change in Surface Protonation as a Consequence of Metal Ion or Ligand Adsorption... 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

The detachment of the Fe(II) into solution. This detachment is facilitated by the increase in surface protonation that is accompanied by the ligand binding. The fact that iron(II) is more readily detached than an iron(III) site from the mineral surface is due to the lower Madelung energy of the Fe(II)-oxygen bond than the Fe(III)-oxygen bond... 

Different modifications of hydrous oxides, even if present in solution with the same surface area concentrations, are characterized by significantly different reactivities (e.g., dissolution rate). This depends above all on the different coordination geometry of the surface groups. For a given pH (on surface protonation) the reactivity of a Fem-center is likely to increase with the number of terminal ligands (Wehrli et al., 1990), i.e., groups such as -Fe-OH are less acid and react faster than... 

Surface speciation can be expected to have a tremendous impact on rates of precursor complex formation. Rj, the rate of precursor complex formation, may depend upon the extent of surface protonation, since ligand exchange rates of >MeOH2, >MeOH, and >MeO may vary substantially ... 

These reactions take place via electrophilic attack of a surface proton on the ligand atom bonded to the metal center with liberation of an alkane, an alcohol (or a phenol), HX (X = halide) or NHR2 (11.1) ... 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Several studies have been conducted on the rates of dissolution of oxides. The work of Stumm and coworkers is noteworthy in this area (Stumm et al., 1983, 1985 Zutic and Stumm, 1984 Stumm, 1986). They have studied the effects of H+ and various complex-forming anions on oxide dissolution rates and found that dissolution rate (v) depends strongly on the relative concentrations of proton surface groups -OH2 and ligand surface complexes -L such that (Stumm et al., 1985)... 

Fig. 9. Schematic representation of the saturation transfer from receptor to a bound ligand. The protons of the receptor are saturated by a train of selective pulses. Upon binding (middle) the saturation is transferred to protons of the ligand. The different emphasis on the ligand protons represents the effectiveness of the transfer depending on proximity of ligand protons to protons on the receptor surface. This information is transferred back into solution upon dissociation of the complex.

We have been successful in decorating surfaces with neutral, metal complexes, (M" +)n(L )m having anionic ligands, L" that can be removed at low temperature by the reaction with surface protons to produce neutral species, HnL , by the following reaction ... 

As with the hquid phase reaction, one mechanism involves the reaction of the acac ligands with the acidic, surface protons by the following reaction ... 

The base can be a co-solvent, such as trialkylamine, or the Lewis base function can be designed into the metal complex using O-bearing ligands such as al-koxyamines, acetylacetonates, etc. The optimum amount of co-solvent is 1-2 equivalents of the metal complex present in the solvent. The nitrogen atoms in ethylenediamine are not sufficiently basic to activate the surface protons from silica, but ethylenediamine complexes of some metal cations, such as Cu(II), readily exchange for the protons in zeolites which are more acidic than silanol protons. ... 

Notice here that only two of the three (acac) ligands can interact with the surface protons, thus it appears that only 2/3 ° of the ligands can be lost by proton-assisted thermolysis. One of the samples, 1 wt% Ru(acac)3 appears to show a weight loss in the TG spectrum consistent with this prediction. It must be remarked that the data of this same sample agreed in the predicted and observed weight loss upon thermolysis (2.6%) whereas the other two samples showed different experimental and predicted weight losses. 

F ure 9.21. The net charge at the hydrous oxide surface is established by the proton balance (adsorption of or OH and their complexes) at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidi-metric titration curve and from a measurement of the extent of adsorption of specifically adsort)ed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). Addition of a ligand, at constant pH, increases surface protonation while the addition of a metal ion (i.e., specifically adsorbed) lowers surface protonation. (Adapted from Hohl et al., 1980.)... 

Figure 13.11. Ligand- and proton-promoted dissolution of AI2O3. (a) The ligand-catalyzed dissolution of a trivalent metal (hydr)oxide. (b) Measurement of Al(UI)(aq) as a function of time at constant pH at various oxalate concentrations. The dissolution Idnetics are given by a reaction of zero order. The dissolution rate, / l, is given by the slope of the (Al(III)(aq)] versus time curve, (c) Dissolution rate as a function of the surface ligand concentration for various ligands. The dissolution is proportional to the surface concentration of the ligand, <=MeL> or C(. (/ l = (d) Proton-promoted...

To exemplify inhibition effects, we choose a few case studies with Fe(III)(hydr)oxides because these oxides are readily dissolved with protons, ligands, and reductants and are of great importance in the iron cycles in natural waters. The reductive dissolution of Fe(III) minerals by a reductant such as H2S is much faster than ligand- or proton-promoted dissolution. The dissolution reaction, as shown by Dos Santos-Afonso and Stumm (1992), is initiated by the formation of =FeS and =FeSH surface complexes the subsequent electron transfer within the complex leads to the formation of Fe(II) centers in the... 

Figure 7. The effect of ligands and metal ions on surface protonation of a hydrous oxide is illustrated by two examples (1). Part a Binding of a ligand (pH 7) to hematite, which increases surface protonation. Part h Adsorption of Pb2+ to hematite (pH 4.4), which reduces surface protonation. Part c Surface protonation of hematite alone as a function of pH (for comparison). All data were calculated with the following surface complex formation equilibria (1 = 5 X 10"3 M >. Electrostatic correction was made by diffuse double layer model.

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