Photoprompted Hot Electrons from Bulk Cross-Linked Graphene Materials and Their Efficient Catalysis for Atmospheric Ammonia Synthesis.

Abstract

Ammonia synthesis is the single most important chemical process in industry and has used the successful heterogeneous Haber-Bosch catalyst for over 100 years and requires processing under both high temperature (300-500 °C) and pressure (200-300 atm); thus, it has huge energy costs accounting for about 1-3% of human's energy consumption. Therefore, there has been a long and vigorous exploration to find a milder alternative process. Here, we demonstrate that by using an iron- and graphene-based catalyst, Fe@3DGraphene, hot (ejected) electrons from this composite catalyst induced by visible light in a wide range of wavelength up to red could efficiently facilitate the activation of N2 and generate ammonia with H2 directly at ambient pressure using light (including simulated sun light) illumination directly. No external voltage or electrochemical or any other agent is needed. The production rate increases with increasing light frequency under the same power and with increasing power under the same frequency. The mechanism is confirmed by the detection of the intermediate N2H4 and also with a measured apparent activation energy only ∼1/4 of the iron based Haber-Bosch catalyst. Combined with the morphology control using alumina as the structural promoter, the catalyst retains its activity in a 50 h test.