Mechanochemical-tuning size dependence of iridium single atom and nanocluster toward highly selective ammonium production

The electrochemical nitrate reduction reaction (NO₃RR) is an effective pathway to achieving sustainable NH₃ production. However, both the experimental investigations and the detailed mechanism are still lacking in terms of the size effect of nanocatalysts during NO₃RR, especially at the near atomic scale. Herein, we demonstrate a facile mechanochemical strategy to precisely immobilize Ir single-atom catalysts (SACs) or nanoclusters on spinel Co₃O₄. Compared with the conventional pyrolysis method, the established mechanochemical strategy can realize ultrafast (10 min) and mild synthesis at ambient temperature, avoiding hard-to-control agglomeration at high pyrolysis temperature. The results reveal that the atomic-scale Ir SAC-Co₃O₄ can significantly boost NO₃RR activity compared with nanocluster-scale Ir NC-Co₃O₄ by effectively enhancing the charge transfer and decreasing the energy barrier. Therefore, the present study not only provides new opportunities to fabricate nanocatalysts with controllable size but also gives new insights into the NO₃RR mechanism on the basis of the size effect.