Quantitative In Situ Differential Reflectance Spectroscopy Analysis of Polycrystalline Platinum Oxidation in an Aqueous Acidic Electrolyte

Abstract

A method is herein described that allows the coverage of the two forms of oxide believed to form on polycrystalline Pt, Pt(poly), surfaces in aqueous solutions to be determined based on the analysis of in situ differential reflectance spectroscopy, R/R, and voltammetric data recorded simultaneously. The model employed assumes R/R can be expressed as the sum of contributions due to each type of surface site, i, represented by the product of their potential dependent coverage, θ i (E), and a linear function of E obtained empirically. Values of θ i (E) for the unoxidized and oxidized sites are calculated by solving a system of three non-linear equations that relate θ i (E) to the charge associated with the formation of the oxide layer based on coulometric analyses and accepted values for the number of electrons transferred for each of the oxides. Data collected for Pt(poly) electrodes in aqueous 0.1 M HClO 4 was found to be consistent with the coexistence of the two forms of Pt oxide and unoxidized sites on the surface over a potential range extending from ca. Evidence has been obtained in our laboratories 1 that the differential reflectance of polycrystalline Pt, Pt(poly), electrodes in aqueous perchloric acid solutions up to potentials where only a single form of oxide is believed to form can be represented in terms of additive contributions due to the unoxidized or bare, and oxidized sites, expressed for each case in terms of the product of their corresponding potential dependent coverage, θ i (E), i = bare or oxidized, and a linear function of the applied potential, E. More recently, 2 this model was applied to the analysis of data collected on quasi perfect Pt(111) microfacets formed by melting and cooling the tip of Pt microwires, and found to account quantitatively for the optical response associated with the adsorption of hydrogen, hydroxyl, and bisulfate in acidic electrolytes.This brief communication seeks to extend this strategy to the oxidation of Pt(poly) surfaces in 0.1 M HClO 4 over a potential range wide enough to include the higher oxidation state Pt oxide.3 As will be shown, analyses of the data acquired made it possible to construct plots of the coverage of the unoxidized and the two forms of Pt oxide as a function of E. ExperimentalAll experiments were performed at room temperature (ca. 290 K) in Ar (PP300, 99.998%, Airgas)-purged aqueous 0.1 M HClO 4 (70%, Ultrapure, EMD) solutions in ultrapure water (18.3 M cm, EASYpure UV system, Barnstead). The instrumentation and techniques employed were the same as those specified in detail elsewhere, 4,5 including a low-power red laser (Coherent Inc. Red Eagle 635-10, λ = 635 nm, 10 mW) aimed at normal incidence toward the center of the disk of a rotating ring disk electrode, RRDE, assembly, (Area = 0.164 cm 2 , Pine Instruments, Model AFE7R8PTPT). Potential control of the RRDE was achieved with a bipotentiostat (AFRDE5, Pine Instruments) and data were collected with an acquisition card (NI, USB-6009) connected to a personal computer. After the electrode ...

Competition and Interhalogen Formation During Parallel Electrocatalytic Oxidation of Bromide and Chloride on Pt

“…From previous studies, it is known that the oxide layer on Pt initially forms up to a monolayer of OH , * and O*, coupled to a slow placeexchange between O and Pt as oxidation progresses. 60,61 For a fixed potential E, this oxide growth depends on time according to 41…”