(Ru-)Mo2C (MXene) catalysts for ammonia synthesis: From Haber-Bosch to chemical looping processes

Abstract

This study investigated (Ru-)Mo₂C (MXene) materials for ammonia thermo-catalytic synthesis under atmospheric or moderate pressure. Under a H₂-N₂ (3:1) flow, the Mo₂C MXene phase showed limited activity in ammonia synthesis at atmospheric pressure at 400 °C (0.01 mmol h⁻¹ g⁻¹), which increased significantly with temperature and pressure, reaching 2.07 mmol h⁻¹ g⁻¹ at 500°C and under 5 bar. Interestingly, Mo-based MXene was able to generate an appreciable quantity of ammonia without promoters such as ruthenium, the most active metal for the ammonia synthesis reaction. Unexpectedly, the addition of Ru to Mo₂C did not enhance its activity. The nitriding of the MXene under NH₃ or N₂ was then performed and characterized. A thermal treatment under NH₃ (600 °C, 5 bar) was efficient and, interestingly, nitriding also occurred in a lower extent under N₂ (600 °C, 5 bar) for the sample containing ruthenium. The N-containing MXene produced ammonia under pure H₂ flow from temperatures lower than 250 °C. Consecutive nitriding treatments and ammonia production under pure H₂ were successfully achieved and demonstrated for 5 cycles. This result is promising for chemical looping ammonia production process. This work highlights essential aspects that should be explored for future advances to consider using Mo₂CT_x MXene for the efficient thermal production of ammonia.