Multinuclear surface complexes

As suggested in Figure 13.7, binuclear or multinuclear surface complexes tend to block surface sites. A much higher activation energy is involved in detaching simultaneously two metal centers from the surface hence dissolution is retarded by binuclear surface species. This retardation is especially pronounced when the effect of surface coordination with an inhibitor upon surface protonation is not unfavorable. For example, in the reaction of an anionic inhibitor, L, ... 

We need spectroscopic evidence to suggest more explicitly under what conditions bi- or multinuclear surface complexes are formed. A few references providing spectroscopic and other evidence for the formation of bi- or multinuclear surface complexes are given in Table II. 

Reviews (9, 63, 64) of the reactions between hydroxylated mineral surfaces and aqueous solutions brought out the richness of variety found in surface phenomena involving natural particles. Isolated surface complexes, the principal topic of this chapter, are expected when reaction times are short and the adsorbate content is low [Figure 6, inspired by Schindler and Stumm 63)]. Thus, surface complexes occupy a reasonably well-defined domain in the tableau of reaction time scale versus sorbate concentration. Localized clusters of adsorbate (47, 48, 65, 66) that contain two or more adsorbate ions bonded together can form if the amount sorbed is increased by accretion or bv the direct adsorption of polymeric species (multinuclear surface complexes). Surface clusters can erase the hyperfine structure in the ESR spectrum of an immobilized adsorbate (33, 67) or produce new second-neighbor peaks from ions like the absorber in its EXAFS spectrum (47, 66). 

Figure 2. Schematic representation of principal surface sorption processes, (a) Projection of the CoOOH structure in the ab plane. MSC, multinuclear surface complexation represented by an epitaxy of a-FeOOH (left) ISC, mononuclear monodentate (middle right), mononuclear bidentate (top right) and binuclear bidendate (lower right) inner-sphere complexation OSC, outer-sphere surface complexation (top) LD, lattice diffusion (center), (b) Example of epitaxy without sharing of oxygens (Van der Waals forces). The. ..AB-AB... close-packed anionic layer sequence of Co(OH)2(s) is coherently stacked on the. ..AB-BC-CA... layer sequence of CoOOH. Co(OH)2(s) has a 1H polytypic structure, and CoOOH a 3R. Small circles are...

---