We have studied the electrochemical reduction of CO2 on Cu overlayers on Pt(111) and Pt(211) surfaces. These systems were chosen to investigate the effect of strain on the catalytic activity of Cu surfaces and to obtain information about the role of steps in this process. The selectivity toward hydrocarbons on the copper overlayers is much lower than on polycrystalline copper due to a higher selectivity for hydrogen evolution. With the aim of understanding the lower activity toward CO2 electroreduction of the Cu overlayers, we also studied the surfaces under reaction condition using electrochemical scanning tunneling microscopy (EC-STM). In the presence of CO, the Cu overlayer changes from a flat to a granular structure exposing part of the Pt surface. The exposed Pt surface accounts for the high selectivity of these structures toward hydrogen evolution. These results illustrate the dynamic nature of the surface under reaction conditions and that in situ measurements are crucial to understand catalytic activity.