Surface complexes energies

Figure Bl.25.2 shows the XPS spectra of two organoplatinum complexes which contain different amounts of chlorine. The spectrum shows the peaks of all elements expected from the compounds, the Pt 4f and 4d doublets (the 4f doublet is iimesolved due to the low energy resolution employed for broad energy range scans). Cl 2p and Cl 2s, N Is and C Is. Flowever, the C Is caimot be taken as characteristic for the complex only. All surfaces that have not been cleaned by sputtermg or oxidation in the XPS spectrometer contain carbon. The reason is that adsorbed hydrocarbons from the atmosphere give the optimum lowering of the surface free energy and hence, all surfaces are covered by hydrocarbon fragments [9].

The following assumptions are made (i) the activated complexes are in equilibrium with the reactants, (ii) the energy of a molecule is not altered when an activated complex is substituted for a nearest neighbour, and (iii) the products do not affect the course of reaction, except to define a boundary in surface processes. The various cases can be recognized from the magnitude of the pre-exponential term and calculated values [515] are summarized in Table 7. Low values of A indicate a tight surface complex whereas higher values are associated with a looser or mobile complex. 

It should be noted that dissociation of surface complexes of oxygen in polar solvents on semireduced ZnO films is presumably justified from the thermodynamic point of view as oxygen adsorption heat on ZnO and electron work function are [58] 1 and approximately 5 eV respectively while the energies of affinity of oxygen molecules to electron, to solvation of superoxide ion and surface unit charge zinc dope ions are 0.87, 3.5, and higher than 3 eV, respectively [43]. 

Greater adsorption of trace metals is found at higher pH and C02(g) concentrations. Sites available for Zn2+ sorption are less than 10% of the Ca2+ sites on the calcite surface, and Zn adsorption is independent of surface charge. This indicates a surface complex with a covalent character (Zachara et al., 1991). Furthermore, the surface complex remains hydrated and labile because Zn2+ is rapidly exchangeable with Ca2+, Zn2+ and ZnOH. At the dolomite-solution interface, the carbonate(C03)-metal (Ca/Mg) complex dominates surface speciation at pH > 8, but at pH 4-8, hydroxide (OH) -metal (Ca/Mg) dominates surface speciation (Pokrovsky et al., 1999). Calcite has an observed selectivity sequence Cd > Zn > Mn > Co > Ni > Ba = Sr, but their sorption reversibility is correlated with the hydration energies of the metal sorbates. Cadmium and Mn dehydrate soon after adsorption to calcite and form a precipitate, while Zn, Co and Ni form surface complexes, remaining hydrated until the ions are incorporated into the structure by recystallization (Zachara et al., 1991). 

Kulik, D. A., 2002, Gibbs energy minimization approach to model sorption equilibria at the mineral-water interface Thermodynamic relations for multi-site-surface complexation. American Journal of Science 302,227-279. ... 

Linear free energy relations between the tendency to form solute complexes and corresponding surface complexes. 

Stipp and Hochella (1991), on the basis X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED), have shown that CaC03 exposed to water, contains at the surface =C03H and =CaOH functional groups and van Capellen (1991) has proposed a surface complex formation model for carbonates. Similarly, Ronngren et al. (1991) have proposed =SH and =ZnOH functional groups for the surface of hydrous ZnS s). 

The conditions for the validity of a Langmuir type adsorption equilibrium are i) thermal equilibrium up to the formation of a monolayer, 0 = 1 ii) the energy of adsorption is independent of 0, (i.e., equal activity of all surface sites). There is no difference between a surface complex formation constant and a Langmuir adsorption... 

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

The "classical" theory of nucleation concentrates primarily on calculating the nucleation free energy barrier, AG. Chemical interactions are included under the form of thermodynamic quantities, such as the surface tension. A link with chemistry is made by relating the surface tension to the solubility which provides a kinetic explanation of the Ostwald Step Rule and the often observed disequilibrium conditions in natural systems. Can the chemical model be complemented and expanded by considering specific chemical interactions (surface complex formation) of the components of the cluster with the surface ... 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

As discussed in previous subchapters, the rate of the photochemical reductive dissolution of iron(III)(hydr)oxides depends on the concentration and type of surface complexes present and on the light intensity and its energy. Because the light intensity varies diurnally, also a diurnal variation in the iron(II) concentration can be expected in surface waters. This has been observed in acidic waters (McKnight and Bencala, 1988 Sulzberger et al., 1990). Fig. 10.17 shows such a diurnal variation in the concentration of dissolved Fe(II) in a slightly acidic alpine lake (Lake Cristallina) of Switzerland. 

TLM Activity Coefficients. In the version of the TLM as discussed by Davis et al. (11), mass action equations representing surface complexation reactions were written to include "chemical" and "coulombic" contributions to the overall free energy of reaction, e.g., the equilibrium constant for the deprotonation reaction represented by Equation 2 has been given as... 

In an interesting analysis of the effects of reduction of dimensionality on rates of adsorption/desorption reactions (26), the bimolecular rate of 10 M- s- has been reported as the lower limit of diffusion control. Based on this value, the rates given in Table III indicate the desorption step is chemical-reaction-controlled, likely controlled by the chemical activation energy of breaking the surface complex bond. On the other hand, the coupled adsorption step is probably diffusion controlled. 

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