Kinetics and mechanism of electrochemical oxidation of carbon monoxide in molten carbonates. Confirmation of the existence of the CO 2–2 ion

Abstract

The kinetics and mechanism of the electrochemical oxidation of CO in molten alkali carbonate electrolyte have been determined by analysis of steady-state polarization and potential-scan data on 99.99 % pure gold electrodes at 800°C. Results show that of the two oxidation processes occurring, the first is due to physically dissolved CO, whereas the second process, whose half-wave potential is + 240 to + 300 mV (std. CO/CO₂) is due to CO combined chemically with the melt, the reacting species being the CO^2–₂ ion. For a 0.382 atm CO + 0.618 atm CO₂ mixture, the concentration of physically dissolved CO is estimated to be ∼2 × 10^–4 mol dm^–3. The concentration of CO^2–₂ ion is ∼8 times higher. Good agreement has been found between the results obtained independently from steady-state, potential-scan, electrolyte additive and gas-solubility studies. Exchange current densities are ∼2.2 × 10^–4 and 2.9 × 10^–4 A cm^–2 for 0.05 atm CO and 0.382 atm CO (at 0.618 atm CO₂), of which ∼40 and 30 % respectively result from oxidation of the CO^2–₂ ion.

The sequence of the individual reaction steps has been identified for each of these processes. The rate-determining reaction in the first process is: CO⁺+ CO^2–₃→ CO^–₂+ CO₂. The second process shows that, depending on the CO/CO₂ composition, the rate determining step is either: CO^2–₂→ CO^–₂+ e^–, or that identified for the first process.