Surface sites, metal-oxides

Macroscopic Coefficients and Surface-Site Heterogeneity. Beniamin and heckles model (5) of heterogeneous metal oxide surface sites includes two observations of metal ion/surface site Interactions. 

At low pH, where resorption reactions are minimal, the photodissolution process may be represented as a two-step process involving adsorption of ligand L to metal oxide surface sites followed by detachment of reduced metal ions that is, for an iron oxyhydroxide ... 

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. 

Hydrated metal oxide surface sites (>MeOH) undergo analogous protona-tion/deprotonation reactions, creating a distribution of positive, negative, and neutral surface sites (16) ... 

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. 

Organic ligands without redox reactivity that coordinate metal oxide surface sites have been found to enhance rates of both reductive and non-reductive dissolution reactions ( 7). ... 

Figure 8.1. Reduction of tervalent metal oxide surface sites by phenol (HA) showing inner-sphere and outer-sphere mechanisms. [From Stone (1986), with permission.]...

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. 

Figure 16 Schematic representation of the reaction pattern for the oxidation of organic compounds with simultaneous oxygen evolution at metal-oxide anodes reactions (a), (b), (c), d) as in Figure 11 (e) combustion of the organic compound R via electrochemical oxidation mediated by physisorbed hydroxyl radicals (/) selective chemical oxidation of the organic compound at the higher metal oxide surface sites. (From Ref. 15. Copyright 1994, Pergamon Press Ltd. Reprinted with permission.)...

MgO and other alkaline earth oxides in pure and doped form play an important role in the eatalytic activation of methane [37]. It is likely that one or several of the metal oxide surface sites involved in the H2 splitting at room temperature are also relevant candidates for methane activation as introductory step for partial oxidation reactions such as oxidative eoupling of metheme (OCM). Particularly encouraging is the fact that H radical production... 

Owing to the amphoteric nature of metal oxide surface sites, acid-base titrations are commonly performed. Since siuface sites can accept up to two protons, titration plots of metal oxide powders are qualitatively similar to titration plots of diprotic weak acids (or weak bases) in solution. As with diprotic weak acids, the quantitative shape of metal oxide powder titration plots is determined by the two proton mass action constants, and K 2- e take the same proton mass action constant values used in Section II.C (p. al P- a2 8.0) a titration plot similar to the one shown... 

Since metal oxide surfaees are amphoteric, with different surface site types, different types of solutes will adsorb differently on each different t5q>e of surface site. In general, cation adsorption on all three types of surface site is possible. However, only the predominant adsorption reaction of a cation with an anionic surface site will be considered in the following derivation. Therefore, the principle reaction by which cation adsorption occurs will be written in terms of a cation solute Mreacting with an anionic metal oxide surface site SO, to give an adsorbed surface complex, SOM" . If the cation is monovalent, M, the surface complex is uncharged with the structure, SOM, which is formed as... 

One will note that a j 2 can be considered as a known variable since it can be readily obtained from an acid-base titration of the metal oxide surface sites as described in Section II.D. If two different types of surface site are present on a metal oxide surface, instead of a single break at each of flie two equivalence points as shown in Fig. 4, four equivalence points would be observed. 

Can this demand for a significant number of metal active sites be further quantified by a general expression in terms of cathode potential demand The answer is, in principle, yes, although the dependence of the relative populations of metal surface sites and oxidized surface sites on cathode potential could depend on (H20)/r-oh d somewhat different way, depending on the degree to which the... 

When a metal oxide surface is exposed to water, adsorption of water molecules takes place as shown in Equation 2.1. Cation sites can be considered as Lewis acids and interact with donor molecules like water through a combination of ion-dipole attraction and orbital overlap. Subsequent protonation and deprotonation of the surface hydroxyls produce charged oxide surfaces as shown in Equation 2.2 and Equation 2.3, respectively ... 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

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. 

In this case, precursor complex formation depends upon the lability of the incoming metal ion, rather than that of the oxide surface site, since the inner coordination ligands of the surface site are not exchanged (26). 

Investigations of the generation of super base sites on alkaline earth metal oxides by doping with alkali metals (246,247,253) led to the inference that when zero-valent alkali metals react with a metal oxide surface, the electron donated by the alkali metal to the oxide lattice resides in a defect site, such as an oxygen vacancy, generating a one-electron donor site (F center) (254,255) (Scheme 40). 

Large adsorbates, such as bi-isonicotinic acid, may bind to a surface at several sites which are sufficiently far apart not to interact strongly in a direct way. This kind of system is by necessity large and complex, and few detailed studies have been reported on such systems. Various structural aspects of bi-isonicotinic acid adsorption on rutile and anatase TiC>2 surfaces have been presented in several recent studies [68, 77, 78]. Bi-isonicotinic acid adsorption on TiC>2 surfaces is not only taken as a problem of direct interest to the photoelectrochemical applications, but also serves as a model system for surface science investigations of phenomena connected to the adsorption of large organic adsorbates on metal oxide surfaces. 

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