Mononuclear surface

The active sites in our investigations are constituted by mononuclear surface transition-metal complexes (corresponding to low TMI loadings). Empirical models of such sites... 

The room temperature reaction of Aerosil 200 silica pretreated at 500°C with excess VO(0 Pr)3 results in complete reaction of the surface hydroxyls and quantitative formation of the mononuclear surface complex 1, eq 1 (4). 

Surface complex formation with ligands that form bidentate, mononuclear surface complexes, e.g., oxalate, salicylate, citrate, diphenols, etc. 

VII. The Relation of Mononuclear Surface Oxygen Species to Electron Spectroscopic and Catalysis Studies... 

Rapid formation of a (hypothetical) bidentate, mononuclear surface complex is the important first step followed by photoexcitation of this surface complex and subsequent electron transfer to Fe(III) with formation of an oxalate radical and Fe(II). The oxalate radical undergoes a rapid decarboxyla-... 

The various elementary steps involved in the surface photoredox reaction, leading to dissolution of hematite in the presence of oxalate, are outlined in Figure 12.10. The two-dimensional stmcture of the surface of an iron(III) hydroxide given in this figure is highly schematic. The charges indicated correspond to relative charges. An important step is the formation of a hypothetical bidentate, mononuclear surface complex. With pressure jump relaxation technique, it has... 

Case Examples. The effects of various oxoanions on EDTA-pro-moted dissolution of lepidocrocite (y-FeOOH) have been studied by Bondietti et al. (33). EDTA was chosen as a reference system because it is dissolution-active over a relatively wide pH range. Phosphate, arsenate, and selenite markedly inhibit the dissolution at near-neutral pH values. At pH <5 phosphate, arsenate, and selenite accelerate the dissolution. It is presumed that the bi-nuclear surface complexes formed at near-neutral pH values by these oxoanions (Table II) inhibit the dissolution. Figure 8a displays data on the effect of selenite on EDTA-promoted dissolution, and Figure 8b shows that calculations on surface speciation by Sposito et al. (35) support the preponderance of binuclear selenite surface complexes in the neutral-pH range. Mononuclear surface species prevail at lower pH values. 

Figure 13 Adsorption of Pu(V) by goethite as a function of pH in 0.1 M NaN03 solutions with EPu(V) = 1 X 10" M, and a sorbent/Hquid ratio of 0.55 g/L. The experimental data (open circles) are from Sanchez et al. (1985). The various curves are model fits to the experimental data assuming that adsorption is as one of three possible mononuclear surface species. The best fit of the data with DL, CC, and TL models is obtained assuming that Fe0H2-Pu020H" is the adsorbed species. From Turner (1995).

Collins et al. (1999a) found that Hg2+ sorbed to goethite as an iimer-sphere bidentate complex. Cheah et al. (1998) found that Cu " " sorbed to amorphous silica and Y-AI2O3 as monomeric and monodentate iimer-sphere surface complexes. However, bidentate complexes may also form on Y-AI2O3. Using polarized EXAFS, Dahn et al. (2003) determined that Ni " " sorbed to montmorillonite edge sites as an inner-sphere mononuclear surface complex. Inner-sphere surface complexes were observed with XAS for Cr " " adsorption on manganese (Manceau and Charlet, 1992) and iron oxides (Charlet and Manceau, 1992). 

Bostick et al. (2002) studied Cs+ adsorption onto vermiculite and montmorillonite with EXAFS and found that Cs+ formed both inner-and outer-sphere complexes on both aluminosihcates. The inner-sphere complexes bound to the siloxane groups in the clay structure. Combes et al. (1992) found that NpOj adsorbed onto goethite as a mononuclear surface complex. Waite et al. (1994) were successful in describing uranyl adsorption to ferrihydrite with the diffuse layer model using the inner-sphere, mononuclear, bidentate surface complex observed with EXAFS. 

Estimation of surface complexation constants for the selected mononuclear surface reactions using one set of low coverage sorption data (<0.25 pmol nr2) in conjunction with the objective curve fitting routine FITEQL (Westall, 1982). 

Mononuclear reactions. Adsorption producing mononuclear surface complexes, accompanied by deprotonation or hydrolysis of AlOH and/or Co(II), is represented by the generic reaction ... 

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