Adsorption of thallium cations on RuO2–TiO2 electrodes

Abstract

The electrochemical ion-exchange properties of RuO 2 -TiO 2 film electrodes with different composition have been studied in acidic and alkaline media. Thalliumcation uptake has been observed only from the latter and its extent was found to be a function of electrode potential and composition. At potentials near 0.0 V (RHE), the amount of adsorbed Tl + exhibited a maximum, and decreased with increasing potential, reaching a broad minimum in the range 0.4-0.8 V. A further increase in the electrode potential, above about 1.0 V, led to an increase of adsorbed thallium species, essentially due to deposition of a few layers of Tl(III) hydroxide. In fact, the release of the latter species was found to be much slower than that of thallium ions adsorbed at 0.0 V. For the latter, in turn, the double injection/ejection mechanism, currently accepted to explain the charge-storage in oxide electrodes, seems to be confirmed. The high C values attained at 0.0 V indicate that the large ionic radius of Tl + does not prevent its diffusion through the thinner pore texture of the oxide coatings, possibly because of its poor hydration, related with lower charge density at the ion surfaces.

The bromine electrode Part II: reaction kinetics at polycrystalline Pt

“…This reduces the polarization range within which the bromide oxidation kinetics can be studied, in the positive direction. Another interesting consideration concerns the data given in Figure 1; as reported in the first part of the present work [14], significant variations in surface concentration of Br-related species were measured at two different pH (1 M HClO 4 and 1 M NaClO 4 adjusted to a pH of 2), probably due to a screening effect exerted by O-related species, more readily formed at the latter pH. Referring to the Br 2 /Br ) redox couple (centered at about 0.944 V SCE ), the higher peak currents, recorded at the less acidic pH, may be accounted for along the above lines.…”

“…A set of quasi-steady polarization curves, obtained at the Pt wire and recorded in solutions containing from 0.1 to 1 mol l )1 of Br ) /Br 3 ) , is shown in Figure 2. In consideration of the relatively narrow range of explored overpotentials (±150 mV around the OCP), the attainment of a limiting current seems to occur only for the more diluted solution (0.1 M), and anyway only for g higher than 0.1 V. On the other hand, data in Figure 2 do not show the expected exponential dependence of j on g: curves appear anomalously linear, possibly as a result of the presence of an adlayer of Br-species [14] at the electrode surface, which increases the ohmic resistance of charge transfer. The processing of the same data, in terms of Tafel plots, is shown in Figure 3; the E-log j plots appear to be continuously curved, in harmony with the linear representations of Figure 2; analogous results were described also in [6], and justified through an activation-control due to the chemical desorption step.…”

“…In spite of the good results obtained under the Langmuir adsorption isotherm approximation, we also showed [14] that a Frumkin isotherm may be utilized, which seems more appropriate given that a significant coverage was found for the adsorbed intermediates ($60%; [14] and references therein). Accordingly, a substitution of isotherms can be made, even if the mathematical treatment requires a further approximation, that is, to consider a coverage of around 66% (as a result, ln(h/(1)h)) » h); the current can thus be expressed with the following equation:…”

“…Again, for recombination control, a linear relation should be obtained by plotting (1-2a)Öj vs the ratio Fg/RT (the proper value of a was taken from [14]). The resulting analysis is shown in Figure 5 (only g values in the range 0.025 to 0.1 V were considered, i.e., the potential interval in which the high overpotential approximation of the Butler-Volmer equation should be respected).…”

“…This estimation is significantly lower than the value previously hypothesized (h » 0.66) to test the chemical recombination mechanism. While the latter was assumed for mathematical purposes (in order to simplify the equation and to carry out the test), the former suggests that lower h BrAE values may be attained at anodic overpotentials higher than under the CV conditions reported in [14]. Such a result is not surprising, considering that the coverage is related to the electrode potential (through the electrosorption isotherm), and it also depends on the concomitant presence of O-related species at the Pt electrode surface.…”