Electron transfer metal oxide surfaces

Of more apparent significance in the aquatic environment are redox processes induced or enhanced on absorbance of light by chromophores at metal oxide surfaces in which the metal of the oxide lattice constitutes the cationic partner. Light induced electron transfer within such a chromophore often results in disruption of the oxide lattice. The photoredox-induced dissolution of iron and manganese oxides by such a mechanism has been proposed as a possible means of supply of essential trace-metal nutrients to plants and aquatic organisms (29-31). ... 

Adsorption of Organic Reductants and Subsequent Electron Transfer on Metal Oxide Surfaces... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

When free radicals or other reactive organic species are generated in the electron transfer step, the pattern of surface coverage may have a dramatic impact on the yields of various possible organic oxidation products. Consider the oxidation of mercaptans (RSH) by a metal oxide surface. One-equivalent oxidation forms mercapto radicals (RS1) which quickly couple with neighboring radicals to form a disulfide product (36). 

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

Ultimately an understanding of electron transfer processes in dye-sensitized solar cells must be expressed in terms of a model which takes the specific nature of metal oxide surfaces into account [97]. Moreover, the nanostructured devices often involve oxide nanoparticles which approach the limit where quantum-size effects become important. It would be a great step forward if this could be incorporated into an electron-transfer model. 

It should also be pointed out that the rate of each of the reaction steps (precursor complex formation, electron transfer, and breakdown of successor complex) is affected by the chemical characteristics of the metal oxide surface sites and the nature of the reductant molecules. These aspects are discussed in detail in an excellent review by Stone (1986), and the reader is encouraged to refer to this article. 

Stone, A. T. (1986). Adsorption of organic reductants and subsequent electron transfer on metal oxide surfaces. ACS Symp. Ser, 323, 446-461. 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

Such reactions are possible at interaction of transition metal oxide surface in the redox reactant/surface site pairs possessing low electron transfer free energy and lead to formation of radical ions from the reactant molecules [64]... 

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