Photocatalytic nitrogen fixation under ambient conditions is currently widely explored in an attempt to develop a sustainable alternative for the Haber-Bosch process. In this work we combine defect-rich C3N4, one of the most investigated photocatalysts reported in literature for ammonia generation, with earth-abundant and bioinspired FeS2 to improve the activity for ammonia production. By this combination, an activity enhancement of approx. 400 % compared to unmodified C3N4 was achieved. The optimal FeS2 loading was established to be 1 wt.%, with ammonia yields of up to 800 µg L-1 after irradiation for 7 hours. By detailed material characterization of the electronic and material properties of the composites before and after the photocatalytic reaction, we reveal that NH3 generation occurs not photocatalytically from N2, but via a light-induced reduction of =N-CN groups adjacent to nitrogen vacancies in the structure of defect-rich C3N4. FeS2 acts similar to a co-catalyst, enhancing the ammonia yield by π-back-donation from Fe-centers to the imine nitrogen of the defect-rich C3N4, thereby activating the structure and boosting the ammonia generation from cyano groups.
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