Effect of adsorbed layers on the anodic oxidation of simple organic compounds. Part 3.—Cu adsorption on Pt and its effect on the oxidation of HCOOH

Abstract

The adsorption and desorption of Cu on a smooth Pt electrode from 1 M H₂SO₄ solutions which contain a range of Cu²⁺ concentrations has been studied as a function of potential. The effect of adsorbed Cu on the oxidation rate of HCOOH has been examined at + 0.60 V (RHE). Cu²⁺ adsorption is controlled largely by an activation process. The slow step in the process is apparently the discharge of Cu²⁺ at the electrode to Cu⁺. Desorption of a preadsorbed Cu layer (+0.40 V) is extremely rapid at potentials+ 0.70 V, where no Cu²⁺ adsorption is normally found. Analysis of the oxidation state of the adsorbing layer by charge and coverage measurements yields a value of 1.92 electrons/site. This indicates that the major part of the layer is Cu_ads attached to a single Pt surface site. The rate of adsorption of Cu²⁺ is decreased slightly in the presence of 0.1 M HCOOH. At low coverages (θ^Cu_ads⩽0.19), a slight enhancement of the HCOOH oxidation rate is observed at +0.60 V. Above this coverage, the oxidation process is inhibited. Results are interpreted in terms of Cu²⁺ adsorption mechanisms on Pt and the effect of the adsorbed layer on the mechanisms of HCOOH electro-oxidation on Pt.