Adsorption kinetics of electrode processes and the mechanism of Kolbe electrosynthesis: Part I. Adsorption of carboxylic acids and the nature of the particles chemisorbed on platinum electrodes

Abstract

The adsorption of acetic acid, its halogenates and homologues on a platinum electrode has been studied in a wide range of potentials. The adsorption of carboxylic acids is observed throughout the potential range studied (0–2.7 V), and in aqueous solutions there are three characteristic regions: an irreversible destructive chemisorption, resulting from dehydrogenation or dehalogenation at the α-carbon atom, in the potential range 0–0.5 V with a maximum of adsorption at E r ≈ 0.2–0.3 V; a fully reversible adsorption of anions (or anion-like particles) from the potential of zero free charge to 1.5 V with a maximum at 0.8 V, associated with the anions being displaced by the adsorbing oxygen as the value of the potential becomes more positive; and an adsorption region, 1.6–2.7 V, with a maximum at Er ≈ 2.3 V, to a considerable extent corresponding to the reversible adsorption of carboxylic acid anions on the surface oxide layers formed. The drop at E r, ≈ 2.3 V is associated with the discharge of anions and the occurrence of Kolbe synthesis. In contrast to aqueous solutions, in absolute methanol solution monotonic growth of the surface coverage with the reversibly adsorbed carboxylic acid anions is observed up to the moment when the process of Kolbe electrosynthesis starts.