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

Photocatalytic nitrogen (N 2 ) reduction offers a renewable route for mild-condition ammonia (NH 3 ) 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 NH 2 ) 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 (Li 2 NH), and lithium amide (LiNH 2 ) intermediate states during NH 3 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 N 2 reduction, forming LiH 1– x, Li 2 NH, and LiNH 2 species. Those species/states can absorb ultraviolet and visible light, enabling efficient H 2 dissociation and sequential hydrogenation to produce NH 3 . 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 NH 3 concentration of ca. 0.25%, exceeding the thermodynamic limit of 0.13% at 644 K and atmospheric pressure.