Metal oxide-water interfaces, reaction

Brown Jr., G.E., Parks, G.A., Bargar, J.R., and Towle, S.E., Use of X-ray absorption spectroscopy to study reaction mechanisms at metal oxide-water interfaces, in Mineral-Water Interfacial Reactions Kinetics and Mechanisms, in Sparks, D.L. and Grundl, T.J., Eds., American Chemical Society, Columbus, OH, 1998, p.14. 

Interfacial phenomena at metal oxide/water interfaces are fundamental to various phenomena in ceramic suspensions, such as dispersion, coagulation, coating, and viscous flow. The behavior of suspensions depends in large part on the electrical forces acting between particles, which in turn are affected directly by surface electrochemical reactions. Therefore, this chapter first reviews fundamental concepts and knowledge pertaining to electrochemical processes at metal oxide powder (ceramic powder)/aqueous solution interfaces. Colloidal stability and powder dispersion and packing are then discussed in terms of surface electrochemical properties and the particle-particle interaction in a ceramic suspension. Finally, several recent examples of colloid interfacial methods applied to the fabrication of advanced ceramic composites are introduced. 

Use of X-ray Absorption Spectroscopy To Study Reaction Mechanisms at Metal Oxide-Water Interfaces... 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

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. 

Geckeis, H. Rabung, T. 2002. Solid-water interface reactions of polyvalent metal ions at iron oxide-hydroxide surfaces. In Hubbard, A. (ed) Encyclopedia of Surface and Colloid Science. Dekker Inc., 4737-4748. 

Brown GE Jr (1990) Spectroscopic studies of chemisorption reaction mechanisms at oxide/water interfaces. In Mineral-Water Interface Geochemistiy. Hochella MF, White AF (eds), 23 309-63 Brown GE Jr, Parks GA, O Day PA (1995) Sorption at mineral-water interfaces macroscopic and microscopic perspectives. In Mineral Surfaces. Vaughan DJ, Patrick RAD (eds), 5 129-184 Birrgess J (1978) Metal Ions in Solution. Ellis Horwood, New York... 

The general mechanism will be roughly similar to that developed in section 15.2 for the oxidation of metals by oxygen with adsorption of water on the surface of oxide, two interface reactions that create and consume the defect and the diffusion of the defect. With these steps, it will be necessary to add desorption of the formed hydrogen at the external interface. 

This model may possibly be adapted to metal-water thermal explosions if one assumes that there are reactions between the molten metal and water (and substrate) that form a soluble salt bridge across the interface between the two liquids. This salt solution would then be the material which could superheat and, when finally nucleated, would initiate the thermal explosion. As noted, the model rests on the premise that there are chemical reactions which occur very quickly between metal and water to form soluble products. There is experimental evidence of some reactions taking place, but the exact nature of these is not known. Perhaps, in the case of aluminum, the hydroxide or hydrated oxides form. With substrates covered by rust or an inorganic salt [e.g., Ca(OH)2], these too could play an important role in forming a salt solution. 

Scherer, H.W. Zhang, Y. (1999) Studies on the mechanisms of fixation and release of ammonium in paddy soils after flooding. 1. Effect of iron oxides on ammonium fixation. J. Plant Nutr. Soil Sci. 162 593-597 Scherer, M.M. Balko, B.B. Tratnyek, P.G. (1998) The role of oxides in reduction reactions at the metal-water interface In Sparks, D.L. Gmndl.T.J. (eds.) Mineral-Water Interfacial Reactions, Kinetics and Mechanisms ACS Smposium Series 715, Am. Chem. Soc., 301-322... 

Macroscopic experiments allow determination of the capacitances, potentials, and binding constants by fitting titration data to a particular model of the surface complexation reaction [105,106,110-121] however, this approach does not allow direct microscopic determination of the inter-layer spacing or the dielectric constant in the inter-layer region. While discrimination between inner-sphere and outer-sphere sorption complexes may be presumed from macroscopic experiments [122,123], direct determination of the structure and nature of surface complexes and the structure of the diffuse layer is not possible by these methods alone [40,124]. Nor is it clear that ideas from the chemistry of isolated species in solution (e.g., outer-vs. inner-sphere complexes) are directly transferable to the surface layer or if additional short- to mid-range structural ordering is important. Instead, in situ (in the presence of bulk water) molecular-scale probes such as X-ray absorption fine structure spectroscopy (XAFS) and X-ray standing wave (XSW) methods are needed to provide this information (see Section 3.4). To date, however, there have been very few molecular-scale experimental studies of the EDL at the metal oxide-aqueous solution interface (see, e.g., [125,126]). 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

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