Wavelength-Dependent Nitrogen Fixation and Hydrogenation to Ammonia over Lithium Hydride Catalyst

Photocatalytic nitrogen (N₂) reduction offers a renewable route for mild-condition ammonia (NH₃) synthesis and has garnered increasing attention. Most efforts have focused on tailoring the band structures of photocatalysts, yet the photon-responsive behavior of key reactive intermediates (such as NH and NH₂) that could have a strong impact on the energetics of individual steps has remained essentially uninvestigated primarily due to their low abundance and high lability. Herein, we report a transition-metal-free lithium hydride (LiH) catalyst that forms detectable lithium hydride with hydrogen vacancies (LiH_1-x), lithium imide (Li₂NH), and lithium amide (LiNH₂) intermediate states during NH₃ formation and therefore exhibits dynamic photoresponsive behaviors. This catalyst provides an ideal platform to investigate the wavelength-tailored regulation of intermediates in this bellwether reaction of catalysis. Specifically, ultraviolet light activates LiH for N₂ reduction, forming LiH_1-x, Li₂NH, and LiNH₂ species. Those species/states can absorb ultraviolet and visible light, enabling efficient H₂ dissociation and sequential hydrogenation to produce NH₃. In this unique scenario, the reaction energetics of individual steps are wavelength dependent, thus decoupling the scaling relations that constrain thermal catalysis. Such a cumulative photon effect on the catalyst and reactive species leads to a prominent NH₃ concentration of ca. 0.25%, exceeding the thermodynamic limit of 0.13% at 644 K and atmospheric pressure.