Reaction of Br₂ with adsorbed CO on Pt, studied by the surface interrogation mode of scanning electrochemical microscopy

(Chemical Equation Presented) Scanning electrochemical microscopy surface interrogation (SI-SECM) in the cyclic voltammetry mode was successfully used to detect and quantify adsorbed CO on a Pt electrode by reaction with electrogenerated Br₂. The two-electrode setup used in this new technique allowed the production of Br₂ on an interrogator tip, which reported a transient positive feedback above a Pt substrate at open circuit as an indication of the reactivity of this halogen with CO_(ads). Br ^- and CO₂ are shown to be the main products of the reaction (in the absence of O₂), which may involve the formation of bromophosgene as a hydrolyzable intermediate. Under saturation conditions, CO_(ads) was reproducibly quantified at the polycrystalline Pt surface with θ_CO ≈ 0.5. The reaction is shown to be blocked by the action of pre-adsorbed cyanide, which demonstrates the surface character of the process. The formation of CO₂ as an end product was further tested in a bulk experiment: addition of Pt black to a mixture of Br₂ in 0.5 M H₂SO₄ through which CO was bubbled gave a precipitate of BaCO₃ in a saturated solution of Ba(OH)₂. The use of SI-SECM allowed access to a reaction that would otherwise be difficult to prove through conventional electrochemistry on a single electrode.