Induction Periods in Nitroarene Reduction on Metal Nanoparticles (Nanozymes): A Kinetic Competition Map among the Catalytic, Oxygen Reduction, and Hydrogen Evolution Reactions

Induction periods are routinely observed in metal-nanoparticle (nanozyme)-catalyzed nitroarene reduction, yet it remains unclear whether they are universal and how their length depends on the metal, the hydride-generating reductant, and the substrate. Here, we show that the induction period is not an inherent feature of nitroarene reduction but a kinetically modulated state that arises from the competition among nitroarene reduction, the oxygen reduction reaction (ORR), and the hydrogen evolution reaction (HER). By combining real-time absorbance kinetics with localized surface plasmon resonance, open-circuit potential monitoring, and photographic assessment of H 2 bubble evolution, we relate the observed kinetic differences to the interfacial electron density, the hydridic character of surface-adsorbed H atoms (H*), and the availability of active surface sites. Across Ag, Au, Pt, and Pd nanoparticles (NPs), the highly hydridic NaBH 4 induces a prolonged ORR-dominated induction period for 4-nitrophenol reduction through a sequential pathway. In contrast, the milder NH 3 –BH 3 and the more reactive substrate 4-nitro-1-naphthol favor a concurrent pathway in which the ORR and nitroarene reduction proceed simultaneously. Under N 2 -saturated conditions, the reduction rates are governed by the balance between k ′ dissociation and k ′ HER: Pt NPs exhibit sluggish kinetics with NaBH 4 because of H*-induced site blocking and rapid HER, whereas Ag NPs are most active because they minimize the competing HER. With NH 3 –BH 3, in contrast, Pt NPs are most active because the moderated supply of e – and H* maintains sufficient surface availability. Together, these results provide a predictive competition map that can guide the rational design of nanocatalysts and nanozymes for use in complex aqueous and environmental media.