Continuous modulation of oxygen vacancies in MoO _{3− <i>x</i>} quantum dots enables to tunable nitrogen reduction reactivity

The electrochemical nitrogen reduction reaction (eNRR) is a sustainable pathway for ammonia production, yet its practical implementation is hindered by the inherent thermodynamic stability of nitrogen and the competitive hydrogen evolution reaction (HER). Therefore, designing efficient electrocatalysts with superior eNRR activity and selectivity remains challenging. In this work, ultrasmall molybdenum oxide quantum dots (MoO_3-x QDs) rich in oxygen vacancies (OVs) were synthesized through an ultrafast wet‒chemical approach. Owing to the exceptionally high surface-to-volume ratio, the sub-5-nm quantum-confined architecture of the MoO_3-x QDs has a high density of accessible active sites, facilitating charge transfer kinetics at the nanoscale. The number of OVs within MoO_3-x QDs can be precisely tailored by adjusting the amount of ligand introduced and effectively modifying the electronic structure of neighboring Mo sites in favor of the adsorption and hydrogenation of nitrogen, enhancing eNRR selectivity. Owing to the synergistic effects of quantum confinement and vacany engineering, the optimized MoO_3-x QD catalysts exhibit outstanding eNRR performance, achieving a good NH₃ yield rate of 38.55 μg h^-1 mg^-1 with a Faradaic efficiency of 8.2% at -0.15 V (vs. RHE), which surpasses that of most reported Mo-based eNRR catalysts under comparable conditions. Furthermore, in situ Fourier transform infrared (FTIR) characterization revealed that the eNRR reaction pathway of MoO_3-x QDs follows an associative distal mechanism. This work establishes a dual-modulation strategy that integrates quantum size effects with vacancy engineering, providing a promising avenue for designing transition metal oxide catalysts with enhanced activity and selectivity in multielectron transfer reactions.