Subnanometer Copper Clusters of Five Atoms on Titania Boosting CO 2 Conversion to Methane

In this study, we report the development of an efficient electrochemical synthesis route for Cu clusters that significantly reduces the formation of deleterious Cu ions during preparation. This breakthrough enabled the controlled production of five‐atom copper clusters (Cu 5 ), which, when supported on TiO 2 , exhibit superior catalytic activity compared to previously reported Cu‐based catalysts. Notably, Cu 5 /TiO 2 achieves 100% selectivity toward CH 4 at temperatures as low as 300 °C, with a remarkably low activation energy of 19 kJ·mol −1 . Operando X‐ray absorption spectroscopy revealed that the reduction in the copper oxidation state during the reaction is due to reversible interactions with hydrogen molecules and CO 2 in a 4:1 ratio. These results are consistent with DFT calculations of the intrinsic activity of the Cu 5 cluster, which indicate that CO 2 methanation may proceed through both methoxide‐ and methylene‐mediated pathways. However, the methoxide route is predicted to be significantly more stable and energetically more favorable than the methylene route, suggesting a mechanistic preference for methoxide‐mediated CH 4 formation. These insights account for the outstanding catalytic performance of Cu 5 /TiO 2 and, combined with the high thermal stability previously reported for Cu 5 clusters, highlight their great potential as efficient catalysts for industrial CO 2 ‐to‐CH 4 conversion.