Refining Defect States in W18O49 by Mo Doping: A Strategy for Tuning N2 Activation towards Solar-Driven Nitrogen Fixation

Photocatalysis may provide an intriguing approach to nitrogen fixation, which relies on the transfer of photoexcited electrons to the ultrastable N≡N bond. Upon N₂ chemisorption at active sites (e.g., surface defects), the N₂ molecules have yet to receive energetic electrons toward efficient activation and dissociation, often forming a bottleneck. Herein, we report that the bottleneck can be well tackled by refining the defect states in photocatalysts via doping. As a proof of concept, W₁₈O₄₉ ultrathin nanowires are employed as a model material for subtle Mo doping, in which the coordinatively unsaturated (CUS) metal atoms with oxygen defects serve as the sites for N₂ chemisorption and electron transfer. The doped low-valence Mo species play multiple roles in facilitating N₂ activation and dissociation by refining the defect states of W₁₈O₄₉: (1) polarizing the chemisorbed N₂ molecules and facilitating the electron transfer from CUS sites to N₂ adsorbates, which enables the N≡N bond to be more feasible for dissociation through proton coupling; (2) elevating defect-band center toward the Fermi level, which preserves the energy of photoexcited electrons for N₂ reduction. As a result, the 1 mol % Mo-doped W₁₈O₄₉ sample achieves an ammonia production rate of 195.5 μmol g_cat^-1 h^-1, 7-fold higher than that of pristine W₁₈O₄₉. In pure water, the catalyst demonstrates an apparent quantum efficiency of 0.33% at 400 nm and a solar-to-ammonia efficiency of 0.028% under simulated AM 1.5 G light irradiation. This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity.