In Situ UV-Vis Reflectance Spectroscopy Study of Bromide Oxidation on a Platinum Rotating Disk Electrode in Aqueous Solutions

In situ reflection absorption UV-visible spectroscopy was used to detect solution phase products generated during the oxidation of bromide on the surface of a polycrystalline platinum rotating disk electrode, Pt(poly) RDE, as a function of the applied potential and rotation rate, ω, in 0.01M KBr in 0.1M HClO₄ aqueous solutions. 

The experimental set up employed for these measurements was similar to that described by Shi et al.¹ involving, in this case, a Cary 60 spectrophotometer and a modified set of reflection optics. The data collected were analyzed by theoretical simulations (COMSOL 4.3) that account for the formation of tribromide in solution using diffusion coefficients and homogeneous rates constants for the bromide|tribromide reaction reported earlier in the literature^{2, 3}, expect for the diffusion coefficient of bromide which was adjusted to achieve a better fit, i.e. D_Br^- =2.66 ×10^-5 cm²/s, as opposed to D_Br^- =2.08 ×10^-5 cm²/s as reported in the literature⁴.

Statistical analyses of the I_lim vs √ω (E = 1.4 V vs RHE), data yielded both for the experimental (see black symbols) and theoretical data (red symbols) best fit straight lines with virtually identical slopes and close to zero intercepts (see caption, Fig. 1).

Shown in black symbols in Fig. 2, are potential difference UV visible reflection absorption spectra recorded simultaneously at a wavelength, λ = 266 nm, which corresponds to the absorption maxima of the peak associated with tribromide in solution.² Also shown therein (see red symbols) are the results of the theoretical simulations. Although the slopes of the linear fits seem in very good agreement, the experimental results appear higher than the theoretical ones. The most likely origin of this effect relates to changes in the intensity of the reflected light induced both by the adsorption of bromide and the formation of a layer of oxide on the Pt surface at the high potentials at which the oxidation of bromide was monitored⁵.

REFERENCES

1. Shi, P.; Scherson, D. A., Anal. Chem. 2004, 76, 2398.
2. Liu, Q.; Margerum. D.W.  Environ. Sci. Technol. 2001, 35, 1127.
3. I. Ruff. J. Phys.Chem. 1972, 76, 2957.
4. J. Newman. Electrochemical Systems; third edition John Wiley & Sons. Inc., Hoboken, NJ, 2004.
5. Zhao, M.; Scherson, D. A; Anal. Chem. 1992, 64, 3064.