It is still a challenge to find catalysts for the selective electrochemical reduction of CO2 to value added products. We present a new method for the in-situ modification of a copper catalyst by adding oxygen to the reactant gas. With an appropriate process procedure consisting of a sequence of electrolysis and purge steps, a Cu2O layer was formed during the purge steps. When it was reduced again during the next electrolysis step, Cu+ species remained stable on the surface and let to a higher faradaic efficiency for C2H4 formation and even a lower onset potential for CO2 reduction in general. This effect was observed for a wide range of O-2 content from 10 % to 60 % in the reactant gas. In comparison to electrolysis with pure CO2, formation of methane was largely suppressed. (C) 2016 Elsevier Ltd. All rights reserved.