Solution-metal oxide interface kinetics

Adsorption-Desorption Kinetics at the Metal-Oxide-Solution Interface Studied by Relaxation Methods... 

Yasunaga, T., and Ikeda, T. (1986). Adsorption-desorption kinetics at the metal-oxide-solution interface studied by relaxation methods. ACS Symp. Ser. 323, 230-253. 

See especially Chaps. 2 and 3 in D. L. Sparks and D. L. Suarez, op. cit.10 A summary review of chemical relaxation methods is given by T. Yasunaga and T. Ikeda, Adsorption-desorption kinetics at the metal-oxide-solution interface studied by relaxation methods, Chap. 12 in J. A. Davis and K. F. Hays, op. cit.2... 

Yasunaga, T., and T. Ikeda. 1986. Adsorption-desorption kinetics of the metal-oxide-solution interface studied by relaxation methods, p. 230-253. In J.A. Davis and K.F. Hayes (ed.) Geochemical processes at mineral surfaces. Proc. Am. Chem. Soc. Symp. Ser. 323, Chicago, IL. 8-13 Sept. 1985. ACS, Washington, DC. 

Yasunaga, T. and T. Ikeda (1986), Adsorption-Desorption Kinetics at the Metal-oxide-Solution Interface studied by Relaxation Methods, in J. A. Davies and K. F. Hayes, Eds., Geochemical Processes at Mineral Surfaces, American Chemical Society, Washington, DC, pp. 230-253. 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

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. 

For the investigation of adsorption/desorption kinetics, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed SECM-induced desorption (SECMID) [5], but this represents the first use of SECM in an equilibrium perturbation mode of operation. The principles of SECMID are illustrated schematically in Figure 13.1, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface. For this type of investigation, the tip UME is placed close to the surface of the substrate, such that the tip/substrate separation, d, is of the order of, or less than, the electrode radius, a. The substrate is immersed in a solution of the adsorbate of interest and the adsorption/desorption process is initially at equilibrium. 

Under exposure to heavy-particle radiation, the protective oxide film forms on the metal as it does out-of-radiatioii, and the kinetics of the protective oxide formation are about the same in and out of radiation. Under irradiation, however, the film does not continue to increase in thickness. Radiation produces defects of unspecified nature in the protective oxide, and in the pre.sence of these defects, the oxide breaks up and/or reacts with the solution to form a nonprotective scale. The rate at which the protectiv c oxide breaks up is proportional to the concentration of defects in the oxide at the oxide-solution interface. Under these conditions, a steady state is established in which oxide is removed at a rate equal to the rate of formation and in which a steady-state thi( kness of film results. The corrosion rate is determined by the rate of transfer of reagents across this film. The defects are produced at a rate proportional to the intensity of radiation and are removed by thermal annealing at a rate proportional to the concentration of defects. In the derivation of the general equation, it was assumed that the rate of oxidation of the metal, R, at a given protective film thickness, X, is given by... 

Here SC stands for a semiconductor material, (SC + for the product of its oxidation, Ox for the oxidizer, and Red for the reduced form of Ox. The above reactions, called the conjugated reactions, proceed at a solid-solution interface simultaneously and with equal rate. In electrochemistry of metals they are considered as quite independent from each other. Once the kinetic parameters of these reactions are known, one can determine the rate (current) and potential of corrosion, using the condition, which follows from the above considerations ... 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

In general the anodic reaction consists of the transfer of ions - In most cases the metallic Ions - from the metal into the film and on through the film to the film-solution interface. The slow step postulated in the original theory was the transition from the metal into the oxide. Hie kinetic treatment accounted for the relation experimentally found between the current and voltage, e. g. ... 

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