Light-Induced Ammonia Generation over Defective Carbon Nitride Modified with Pyrite

Abstract

The challenge to mitigate climate change and to limit global warming to well below 2 °C according to the Paris Agreement, requires a reduction of greenhouse gas emissions not only in transportation, but also in industry.[1] The Haber-Bosch process for the production of ammonia, NH3, is one of the most important industrial processes, but significantly contributes to the world’s annual CO2 emissions.[2] Photocatalytic reduction of dinitrogen (NRR) offers a sustainable alternative for the synthesis of ammonia. Some of the highest activities so far were achieved with carbon nitride (C3N4) photocatalysts, especially after the introduction of nitrogen vacancies.[3] Carbon nitrides are earth abundant, non-toxic, low-cost polymeric materials, with a suitable band gap for visible light absorption. To overcome the inherently fast charge carrier recombination that diminishes the efficiency, heterojunctions with a second photocatalyst are widely investigated.[4] Another common approach in the design of photocatalyst for the NRR is nature-inspired. Herein, mainly sulphur, iron and molybdenum containing materials are investigated, inspired by the composition of the active centre in nitrogenase enzymes.[5]In this contribution we present composite materials of FeS2 and vacancy-rich VN-C3N4, thereby combining both strategies of heterojunction formation and bio-inspiration, as well as that of defect engineering in C3N4, to increase the number of active sites.[6] The composites yield significantly higher amounts of ammonia after light irradiation for 7 h, compared to both FeS2 and VN-C3N4 alone. Hydrogen is produced as the only by-product. A combination of control experiments and material characterisation before and after light irradiation revealed that iron in FeS2 coordinates to VN-C3N4 at the defect site. Cyano-groups at these sites are activated via π-back-donation and subsequently converted into ammonia. Thus we could prove that – at least initially – ammonia is generated not from the N2 gas, but from the C3N4-framework itself.