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In 2005 Güntay et al. [4], investigated the radiation-induced decomposition of AgI colloids under a variety of conditions. Experiments included reactions in the

presence of an initial excess of Ag+(aq) and I−(aq), at pH 2 and 5, and with N2O or Argon sparging [4]. The results of this study showed that volatile iodine species could be formed via the decomposition of AgI by reaction with oxidizing free radicals such as •OH, •Cl and, •Cl2− but the production of these radicals is completely suppressed when a large excess of Ag+(aq) is present [4]. While the authors were particularly thorough in their investigation, the possibility of the formation of Ag2O and its subsequent reaction with I−(aq) was not evaluated. Due to the highly oxidizing conditions expected during irradiation it is reasonable to expect that Ag would be in contact with O2, thus, allowing for the formation of both AgI and Ag2O. It is well known that the degree of oxidation of Ag will have a significant influence on the interaction of Ag species with I−(aq)[5]. However, the interaction of Ag with I2 remains unaffected by Ag oxidation [6].

The interactions of Ag with I2 and Ag2O with I−(aq) have also been investigated by Krausmann et al. [6]. They developed a model to investigate the influence of a variety of factors on the silver oxide to silver iodide reaction kinetics including: pH, temperature,

initial iodine and iodide concentration, and agitation of the test solution. Their model showed that the accumulation of AgI monolayers on top of the Ag2O film was not rate limiting; therefore, diffusion or migration of either Ag(I) or I− through the AgI film was not rate-limiting. This is in agreement with our results discussed in Chapter 4, section 4.4. The substrate in our work does differ, as we electrochemically oxidized the Ag electrode to a pre-determined degree, while they allowed an Ag mesh to air-oxidize, and were thus unable to determine the exact amount or surface area of the reactant, Ag2O, for their model calculations. They reported a rate constant of (8.2 ± 3.6) mol⋅m−2⋅s−1 for the chemical reaction of Ag2O with I−(aq) [6], which is approximately two orders of

magnitude lower than the value obtained in this study. In our study, the rate constant was obtained using the geometric area of the electrode, and the true surface area may be many times larger than the geometric area due to roughening during Ag2O film growth [7,8]. The difference between the rate constant reported by Krausmann et al. and what we have reported is still larger than the difference that we expect when the geometric vs. true surface area is taken into account. It should be noted that Krausmann et al. investigated the homogeneous reaction of Ag2O powders/colloids dispersed in the aqueous phase with I−(aq) to form AgI, whereas the value we report is for the conversion of a Ag2O film. These authors also concluded that pH had little to no effect on the reaction kinetics, which is somewhat surprising since the dissolution of Ag2O increases by a factor of 10 with a pH change of ±1.0 away from pH 12, where Ag2O solubility is at a minimum [9].

Presently, we are not aware of any other reports using these particular techniques to examine the separate contributions of the interfacial chemical reaction kinetics and diffusion of a metal oxide-metal halide system. While there is a significant amount of

work available regarding the oxidation or reduction kinetics of small molecules or anions at metal surfaces [10,11], it appears that the techniques and the kinetic analysis used in this study are, in fact, a novel method for determining interfacial reaction kinetics.

5.5 Summary and Conclusions

The effect of mass transfer on the reaction kinetics was investigated by measuring the total reaction time as a function of electrode rotation rate. The total reaction time was found to be inversely proportional to the square root of the rotation rate, and

extrapolation of the rotation rate dependence to infinite rotation enabled us to determine a surface chemical reaction rate with reasonable accuracy. This surface reaction rate constant was approximately an order of magnitude larger than the net conversion rate constant obtained under stagnant conditions (Chapter 4), indicating that the conversion is largely mass-transfer limited under stagnant conditions.

The influence of phosphate in electrolytes on the conversion reaction was also investigated. The surface reaction rate constant in phosphate solutions was

approximately twice that in the absence of phosphate: 7.5 × 10−2 cm⋅s−1 vs.

4.2 × 10−2 cm⋅s−1. This difference is small, and is partially attributed to the difference in the initial surface area or surface structure ( )₀

2O Ag

A of the Ag2O films grown in the presence and absence of phosphate (Chapter 4, section 4.3.1). Although the XRD

analysis results suggest the incorporation of phosphorous into the Ag2O matrix during the initial anodic Ag2O film growth in phosphate solutions, this study suggests that its

presence does not seem to have any influence on the kinetics of the conversion.

1. A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons Inc., Hoboken, NJ, 2001, pp 337-339.

2. X. Zhang, S.D. Stewart, D.W. Shoesmith, and J.C. Wren, J. Electrochem. Soc., 154, F70 (2007).

3. S.D. Pretty, P.G. Keech, X.Zhang, J.J. Noël, and J.W. Wren, Electrochim. Acta, 56, 2754 (2011).

4. S. Güntay, R.C. Cripps, B. Jäckel, and H. Bruchertseifer, Nucl. Tech., 150, 303 (2005).

5. R.S. Neves, E. De Robertis, and A.J. Motheo, App. Surf. Sci., 253, 1379 (2006).

6. E. Krausmann, and Y. Drossinos, J. Nucl. Mat., 264, 113 (1999).

7. V.I. Birss, and G.A. Wright, Electrochim. Acta, 27, 1439 (1982).

8. B.M. Jovic, V.D. Jovic, and G.R. Stafford, Electrochim. Commun., 1, 247 (1999).

9. Lide, David R. (1998). Handbook of Chemistry and Physics, 87th ed., Boca Raton, FL: CRC Press. pp. 4–83.

10. G. Cohn, Chem. Rev., 42, 527 (1948).

11. M. Fleischmann, K. Korinek, D. Pletcher, J. Chem. Soc., Perkin Trans. 2, 1396 (1972).