Atomically Precise Ag9Cu6 and Ag15 Nanoclusters for Nitrate Electroreduction to NH3: Probing the Cu Doping Effect

Ammonia (NH3) is an essential chemical feedstock and promising energy carrier. However, the traditional Haber-Bosch process remains highly energy intensive and carbon emitting, motivating the development of sustainable alternatives. Electrochemical nitrate reduction (eNO3RR) has emerged as an attractive route for green NH3 synthesis because nitrate is highly soluble and easier to activate than N2. Yet achieving high NH3 selectivity remains challenging due to the complex eight-electron, nine-proton transfer pathway. Although bimetallic catalysts can improve the catalytic performance, conventional heterogeneous systems often suffer from their non-uniform surface structures and ill-defined active sites, hindering the elucidation of structure-property relationships. Herein, we reveal the mechanism of heterometal doping on eNO3RR at the atomic level using atomically precise alkynyl-protected homometallic Ag15 and bimetallic Ag9Cu6 nanoclusters. Cu atoms selectively occupy the six outer octahedral vertex sites, forming a well-defined Ag1@Ag8@Cu6 framework and creating an accessible bimetallic surface microenvironment for electrocatalysis. Ag9Cu6 delivers markedly enhanced eNO3RR performance, achieving an NH3 Faradaic efficiency of 88.48% and a yield rate of 9.93 mg·h–1·cm–2 at –0.6 V vs. RHE, about 2.6 times higher than Ag15, together with excellent stability. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) clearly reveals hydrogenation of *NH2 is the rate-determining step as Ag15 suffers from accumulation of sluggish *NH2. In contrast, the Ag–Cu active sites in Ag9Cu6 promote deep hydrogenation and *NH3 desorption while suppressing the hydrogen evolution reaction. This work provides mechanistic insight into a design paradigm for atomically precise Ag–Cu bimetallic electrocatalysts toward selective NH3 synthesis.