Surface proton adsorption

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

Thus, the shift in the titration curve, ACb, at constant pH, is directly related to the extent of Pb(II) binding to the oxide surface. The adsorption of a metal ion decreases the surface protonation. 

Hiemstra et al. (1989) have elaborated on a multisite proton adsorption model taking into account the various types of surface groups intrinsic log K values for the protonation of various types of surface groups can be estimated with this model. 

But factors other than the surface charge can become important such as the effects of specific adsorption of cations and anions on the degree of surface protonation (see Example 5.1). 

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 calculations are similar and the result is displayed in Fig. 5.14b for pH = 4.4. Obviously Pb-adsorption is accompanied by an increase in net charge and a marked decrease in surface protonation [sFeOHg]. Plausibly, this reduction in Cji, can decrease the dissolution rate. 

This may be due to the higher acidity of the surface sites (see Kint) values in Table I) and hence higher activation energy of proton adsorption for the silica-alumina and Y-zirconium phosphate. 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

In addition to proton adsorption, interactions between the ions of the inert electrolyte (counter ions, section 10.3) and the oxide surface lead to ion pair formation which influences the electrochemical properties of the oxides and the determination of pKa values. Ion pair formation involves outer sphere surface complexes (see Chap. 11), e.g. 

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. 

Onari, S. Arai,T. Kudo, K. (1977) Infrared lattice vibrations and dielectronic dispersion in a- Fe203. Phys. Rev. B16 1717 Onoda, G.Y. de Bruyn, P.L. (1966) Proton adsorption ot the ferric oxide/aqueous solution interface. I. A kinetic study of adsorption. Surface Sd. 4 48—63... 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent (z sc) and open-circuit potential (Voc) of the solar cell is essential. The conduction band of the TiO is known to have a Nernstian dependence on pH [13,18], The fully protonated sensitizer (22), upon adsorption, transfers most of its protons to the TiO surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic ruthenium complex and assists electron injection from the excited state of the sensitizer in the titania conduction band, favoring high photocurrents (18-19 inA/cm ). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction-band edge induced by the surface protonation. 

This subsequence is useful to consider if the time scale for proton adsorption-desorption reactions is comparable to or longer than that for outer-sphere surface complexation. It is a special case of the abstract scenario listed third in Table 4.3. Under the conditions given there, the protonation-proton dissociation reaction (A = SOH, B = H C = SOH2) is assumed to be much faster than outer-sphere surface complexation-dissociation, such that (kf ka, kb kd, k f kf, k b kb here)... 

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