Activity Enhancement of Defective Carbon Nitride for Photocatalytic Ammonia Generation by Modification with Pyrite

Abstract

Photocatalytic nitrogen fixation under ambient conditions is currently widely explored in an attempt to develop a sustainable alternative for the Haber-Bosch process. Still, a lack of fundamental understanding of reaction pathways and nitrogen activation mechanisms result in the slow development in this field. 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 photocatalytic ammonia production. By this approach, an activity enhancement of approx. 300 % compared to unmodified C3N4 was achieved. The optimal FeS2 loading was established to be 5 wt.%, with ammonia yields of up to 800 µg L-1 after irradiation for 7 hours. By detailed characterization of the electronic properties of the composites, we deduce that NH3 generation occurs via a novel mechanism involving mainly the reduction of the =N-CN group adjacent to nitrogen vacancies on defect-rich C3N4. FeS2 acts similar to a co-catalyst, improving the activity by π-back-donation from Fe-centers to the imine nitrogen of the defect-rich C3N4, reducing the activation barrier for terminal cyano group reduction upon illumination.