Photocurrent kinetics at the electron emission from a metal into electrolyte solution: Part VII. Absolute rate constants of CO 2 electrochemical reduction on mercury

Abstract

The method of laser photoelectronic emission has been used to measure the oxidation and reduction rate constants of an intermediate of CO 2 electrochemical reduction up to a formate ion of mercury. The intermediate (CO 2 −) is produced during a homogeneous capture of emitted electrons by CO 2, is adsorbed on an electrode and takes part in electrode reactions. Its reduction in neutral solutions is faster than oxidation at <−1.48 V SCE. The reduction rate constant is (3–5)×10 6 s −1 at −1.6 V and is characterized by the transfer coefficient of 0.25–0.3 in the potential range from −1.5 to −1.8 V. The oxidation rate constant is (3–4)×10 6 s −1 at −1.35 V and its transfer coefficient is 0.55–0.65 in the range from −1.3 to −1.45 V. From the comparison between rate constants of the intermediate oxidation and of CO 2 reduction, free energy of the first stage of the total process has been found: CO 2+ e Me − → CO 2 ads −, equal to 0.26–0.28 eV at −1.47 V SCE. The electrode coverage by the intermediate in the working range of potentials (>−1.8 V) does not exceed 10 −5. The change in slope of CO 2 reduction polarization curves is accounted for the transition to a quasi-equilibrium of the first stage at >−1.48 V. A sharp acceleration of the intermediate reduction occurs at pH3.5. It is supposed that in neutral solutions the intermediate is CO 2 ads − which is protonated in acid solutions.