Evolution of Cu single atom catalysts to nanoclusters during CO 2 reduction to CO.

We synthesized Cu single atoms embedded in a N-doped porous carbon catalyst with a high Faradaic efficiency of 93.5% at -0.50 V ( vs . RHE) for CO 2 reduction to CO. The evolution of Cu single-atom sites to nanoclusters of about 1 nm was observed after CO 2 reduction at a potential lower than -0.30 V ( vs . RHE). The DFT calculation indicates that Cu nanoclusters improve the CO 2 activation and the adsorption of intermediate *COOH, thus exhibiting higher catalytic activity than CuN x sites. The structural instability observed in this study helps in understanding the actual active sites of Cu single atom catalysts for CO 2 reduction.