Developing electrocatalysts that are stable and efficient for CO 2 reduction is important for constructing a carbon-neutral energy cycle. New approaches are required to drive input electricity toward the desired CO 2 reduction reaction (CO 2 RR) rather than the competitive hydrogen evolution reaction (HER). In this study, we have used quantum mechanics to demonstrate that the space confinement formed in the gaps of adjacent gold or silver nanoparticles can be used to improve the Faradaic efficiency of CO 2 RR to CO. This behavior is due to the space confinement stabilizing *COOH, which is the key intermediate in the CO 2 RR. However, space confinement has almost no effect on *H, which is the key intermediate in the HER. Possible experimental approaches for the preparation of this type of gold or silver electrocatalyst have been proposed.