Normalized differential reflectance spectroscopy at polycrystalline platinum electrodes in aqueous acidic electrolytes: Quantitative aspects

The intensity of monochromatic light (λ = 633 nm) reflected off the surface of a polycrystalline Pt electrode, R, has been measured in situ as a function of potential, E, during continuous voltammetric cycles in the range -0.25 < E < 1.05 V vs. SCE, both in 0.5 M HClO ₄ and 0.5 M H ₂SO ₄ aqueous solutions. Plots of the normalized differential reflectance, ΔR/R = [R(E _sam) - R(E _ref)] / R(E _ref) vs. Q _n, where E _sam and E _ref refer to the sampling and reference potentials, respectively, and Q _n is the coulometrically determined relative oxide charge (in terms of e/Pt surface atom), were found to deviate from linearity in the PtOH formation region. Improved proportionality between ΔR/R and Q _n was achieved up to Q _n = 1, upon correcting ΔR/R for double layer effects using the method originally reported by Conway et al. [3]. In the case of 0.5 M H ₂SO ₄, however, remaining discrepancies could only be resolved by accounting for the charge due to bisulfate desorption induced by PtOH formation on the surface. On the basis of these results, it has been concluded that for both types of electrolytes, ΔR/R is proportional to the PtOH coverage, θ, and independent of the applied potential for Q _n ≤ 1, opening new prospects for monitoring θ quantitatively during the course of faradaic reactions.