Atomically precise (AgPd)27 nanoclusters for nitrate electroreduction to NH3: Modulating the metal core by a ligand induced strategy

Electrochemical nitrate reduction reaction (eNO₃^–RR) to synthesize NH₃ is a sustainable method to convert environmental contaminants into valuables. Pd based bimetallic nanocatalysts have demonstrated great promise as efficient catalysts, yet modulating the composition and configuration to improve the catalytic performance and achieve comprehensive mechanistic understanding remains challenging. Herein, by employing two ligands with different electron functional groups, we successfully prepared two atomically precise (AgPd)₂₇ bimetallic clusters of Ag₁₈Pd₉(C₈H₄F)₂₄ (Ag₁₈Pd₉) and Ag₂₂Pd₅(C₉H₁₀O₂)₂₆ (Ag₂₂Pd₅). The two clusters possess markedly different metal core composition and configuration, where Ag₁₈Pd₉ has a sandwich metal core structure with 9 Pd atoms located in the middle layer and Ag₂₂Pd₅ has a rod-shaped metal core structure composed of the M₁₃ configuration with 5 Pd atoms located at the center and vertices of the M₁₃ configuration. Unexpectedly, Ag₂₂Pd₅ exhibited remarkably superior eNO⁻₃RR performance than Ag₁₈Pd₉. Specifically, the highest Faradaic efficiency of NH₃ (FE_NH3) and its yield rate can reach 94.42 % and 1.41 mmol h⁻¹ mg⁻¹ at −0.6 V vs. RHE for Ag₂₂Pd₅, but the largest FE_NH3 and NH₃ yield rate is only 43.86 % and 0.41 mmol h⁻¹ mg⁻¹ at −0.5 V vs. RHE for Ag₁₈Pd₉. The in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) test provides the experimental evidence of the reaction intermediates hence revealing the reaction pathway, also shows that Ag₂₂Pd₅ has stronger capability for NO⁻₃ adsorption and NH₃ desorption than that of Ag₁₈Pd₉. Theoretical calculations indicate that the de-ligated clusters can expose the available AgPd bimetallic sites, synergistically serving as effective active sites and the different configurations result in significantly different catalytic activities, where the active sites in Ag₂₂Pd₅ are more favorable for NO⁻₃ adsorption and NH₃ desorption to accelerate the catalytic process.