Electrochemical NO₃^– Reduction Catalyzed by Atomically Precise Ag₃₀Pd₄ Bimetallic Nanocluster: Synergistic Catalysis or Tandem Catalysis?

Electrochemically converting NO3– compounds into ammonia represents a sustainable route to remove industrial pollutants in wastewater and produce valuable chemicals. Bimetallic nanomaterials usually exhibit better catalytic performance than the monometallic counterparts, yet unveiling the reaction mechanism is extremely challenging. Herein, we report an atomically precise [Ag30Pd4 (C6H9)26](BPh4)2 (Ag30Pd4) nanocluster as a model catalyst toward the electrochemical NO3– reduction reaction (eNO3–RR) to elucidate the different role of the Ag and Pd site and unveil the comprehensive catalytic mechanism. Ag30Pd4 is the homoleptic alkynyl-protected superatom with 2 free electrons, and it has a Ag30Pd4 metal core where 4 Pd atoms are located at the subcenter of the metal core. Furthermore, Ag30Pd4 exhibits excellent performance toward eNO3–RR and robust stability for prolonged operation, and it can achieve the highest Faradaic efficiency of NH3 over 90%. In situ Fourier-transform infrared study revealed that a Ag site plays a more critical role in converting NO3– into NO2–, while the Pd site makes a major contribution to catalyze NO2– into NH3. The bimetallic nanocluster adopts a tandem catalytic mechanism rather than a synergistic catalytic effect in eNO3–RR. Such finding was further confirmed by density functional theory calculations, as they disclosed that Ag is the most preferable binding site for NO3–, which then binds a water molecule to release NO2–. Subsequently, NO2– can transfer to the vicinal exposed Pd site to promote NH3 formation.将硝酸根（NO3–）化合物电化学转化为氨，是一条可持续的路径，既可去除废水中的工业污染物，又可制备高附加值化学品。双金属纳米材料通常较单金属对应物展现出更优异的催化性能，但阐明其反应机理仍极具挑战。在此，我们报道一种原子级精准的[Ag30Pd4(C6H9)26](BPh4)2（Ag30Pd4）纳米团簇作为模型催化剂，用于电化学硝酸根还原反应（eNO3–RR），以阐明银（Ag）与钯（Pd）位点的不同作用，并揭示完整的催化机理。Ag30Pd4是带有2个自由电子的全炔基保护超原子，其金属核为Ag30Pd4构型，其中4个Pd原子位于金属核的亚中心位置。此外，Ag30Pd4在eNO3–RR中展现出优异的催化性能与长时间运行的稳健稳定性，其氨产物的最高法拉第效率可达90%以上。原位傅里叶变换红外光谱研究表明，银位点在将NO3–转化为亚硝酸根（NO2–）的过程中发挥更为关键的作用，而钯位点则主要负责将NO2–催化还原为氨。该双金属纳米团簇在eNO3–RR中采用串联催化机制而非协同催化效应。这一发现进一步通过密度泛函理论计算得到验证，计算结果显示，银是NO3–最优选的结合位点，该位点可结合水分子并释放NO2–；随后，NO2–可转移至邻近的暴露钯位点，以促进氨的生成。