Strong Interfacial Electronic Interaction in the Transition-Metal/Mo₂C Catalyst for Enhanced Ammonia Synthesis at Ambient Pressure: Shift of the Rate Determining Step

The dissociation of nitrogen molecules over the catalyst surface is often treated as the reaction-limiting step for ammonia synthesis. Herein, we modified molybdenum carbide with transition metals (Fe and Co) to successfully shift the rate-determining step from dissociation of nitrogen molecules to the formation of −NHx species on the catalyst surface, which few catalysts have achieved. These catalysts illustrated stable performance for ammonia synthesis at ambient pressure. At 520 °C and 1 bar, the specific activity for Mo2C, Fe/Mo2C, and Co/Mo2C was calculated to be 8.58, 9.78, and 11.73 μmol h–1 m–2, respectively. Co/Mo2C showed the highest activity owing to its strong electron-donating ability to dissociate nitrogen molecules. X-ray photoelectron spectroscopy results suggested a strong interaction between molybdenum in the carbidic phase and the transition metals, signifying preferred migration of electrons from the transition metals to the Mo atoms. Such an interfacial phenomenon resembles to the electron metal–support interaction. Importantly, orders for hydrogen were positive, revealing that the catalyst surface was not poisoned by hydrogen. Further, characterization of spent catalysts disclosed that pyridinic and graphitic C–N bonds were present on the catalyst surface, which suggests that both Langmuir–Hinshelwood and Mars–van Krevelen mechanisms could co-exist and simultaneously contribute to generate ammonia.