Single‐Atom Engineering of Atomically Precise Ag–Cu Alloy Nanoclusters Enables Site‐Gated Pathway Switching in CO ₂ Electroreduction

Atomically precise metal nanoclusters (NCs) provide a molecular platform for single‐atom‐level manipulation and unraveling structure‐property relationships. Herein, we realize site‐resolved single‐atom engineering on an Ag–Cu alloy NC platform, allowing for the precise decoupling of surface atomic identity as an independent structural variable. Starting from Ag 26 Cu 17 (SR) 30 (PPh 3 ) 7 (Ag 26 Cu 17 , SR = SPh‐ p ‐CF 3 ), three surface‐exposed Ag atoms are sequentially replaced by Cu through a controlled metal‐exchange reaction, generating an isostructural series of Ag 25 Cu 18 , Ag 24 Cu 19 , and Ag 23 Cu 20 NCs. Electrospray ionization mass spectrometry enables in situ tracking of the single‐atom substitution process, while single‐crystal X‐ray diffraction confirms complete retention of the cluster framework and ligand environment. Using CO 2 electroreduction as a structure‐sensitive probe, progressive Cu substitution at the three equivalent surface gate sites triggers a reaction pathway switch from CO‐selective reduction on Ag 26 Cu 17 to dominant deep‐reduction on Ag 23 Cu 20 , through modulating *CO stabilization and the *CO → *CHO energy barrier. Specifically, the faradaic efficiency (FE) for CO decreases from 90.42% to 13.46%, whereas the total FE for deep reduction products increases from 2.67% to 73.00%, with CH 4 as the major product (FE CH4 = 53.98%). Consequently, this work establishes Ag–Cu NCs as a robust platform for single‐atom engineering and atomic‐scale structure‐property analysis.