Electrochemical reduction of CO2 at copper electrodes and its relationship to the metal surface characteristics

The selectivity of H-2, CO, CH4 and C2H4 produced during the electrolysis Of CO2 in aqueous solution at copper electrodes has been investigated in relation to surface analysis of the electrodes after electrolysis by XPS and temperature-programmed photoelectron emission (TPPE) and surface potential (SP) measurements. The electrodes were prepared as follows: (A) only ultrasonic cleaning in acetone; (B) oxidation in air; (C) exposure to argon plasma. The electrolysis time was 3 h at a given potential. With electrode A, on increasing the potential in a negative direction the amount of CH4 increased, accompanied by a decrease in the amounts of H-2 and CO, and the SP value was positive and shifted in a more positive direction, the TPPE activity increased and in the O1s XPS spectra a peak assigned to adsorbed oxygen developed. The amount of C2H4 was greater in the potential range where both metallic Cu and Cu2O existed. With electrode B, the main product was H-2, the SP value was negative, the TPPE activity was much smaller and Cu2O existed more preferentially than metallic Cu. With electrode C, the surface underwent oxidation easily when immersed in solution. With increasing plasma treatment time the hydrocarbon products decreased, the SP value was lower and the TPPE activity was smaller. It is suggested that the adsorbed oxygen present at the electrode surfaces can lead to an increase in the number of electrons available at the surfaces and also contribute to the adsorption Of CO2, resulting in hydrocarbon production. Copyright (C) 2002 John Wiley Sons, Ltd.