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

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 Mo2CTx MXene for the efficient thermal production of ammonia. •Mo2C MXene is used as catalyst for thermal ammonia synthesis.•Mo2C MXene is active for Haber-Bosch process without ruthenium.•The nitriding of (Ru-)Mo2C under NH3 and N2 is studied.•Chemical looping ammonia synthesis process can be envisioned with Mo2C.