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

TLDR

Photocatalysis offers a route to nitrogen fixation by transferring photoexcited electrons to the stable N≡N bond, yet N₂ chemisorption at surface defects often stalls electron transfer, creating a bottleneck. The study aims to overcome this bottleneck by refining defect states in photocatalysts through doping. Mo‑doped W₁₈O₄₉ ultrathin nanowires were synthesized, where low‑valence Mo at coordinatively unsaturated sites polarizes adsorbed N₂, promotes electron transfer, and raises the defect‑band center toward the Fermi level, enhancing N₂ activation and dissociation. The 1 mol % Mo‑doped sample achieved a 195.5 µmol g_cat⁻¹ h⁻¹ ammonia production rate—seven times higher than pristine W₁₈O₄₉—along with a 0.33 % quantum efficiency at 400 nm and 0.028 % solar‑to‑ammonia efficiency, confirming the effectiveness of subtle electronic structure modulation.

Abstract

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 gcat-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.