(677d) Early Transition Metal Carbide and Nitride Supported Catalysts for Ammonia Synthesis

More than 500 million tons of NH₃ are produced annual via the Haber-Bosch process which converts N₂ and H₂ at high temperatures (400 – 500°C) and pressures (150 - 300 bars). Ammonia synthesis also accounts for 1-2% of global energy consumption.¹ The development of higher activity catalysts that can operate under less severe conditions would enhance the economics associated with and sustainability of NH₃ synthesis.

Research described in this paper investigates the performance of transition metal carbide and nitride supported metals for NH₃ synthesis. Previously, Mo₂C and Mo₂N have been reported to be more active than Ru-based catalysts, but slightly less active than the doubly-promoted Fe catalyst typically used in industrial processes.² To enhance the performance of the bulk carbides and nitrides, we introduced metals including Fe and Ru. While carbide and nitride supported metal catalysts have typically been produced using the passivated supports, the method that we used allows for direct interaction between the metal and support material.

The Mo₂C and Mo₂N supports were synthesized via a temperature program reaction method. To synthesize the Mo₂C, ammonium paramolybdate was reacted with flowing H₂ then a 15% CH₄/H₂ mixture. To synthesize the Mo₂N, ammonium paramolybdate was reacted with flowing NH₃. The resulting support materials were either passivated with 1% O₂/He, or a metal support (e.g. Fe and Ru) was introduced via dry impregnation, followed by reduction and passivation. The materials were evaluated for NH₃ synthesis at 14.7 psi and 400 - 500 °C using a stoichiometric mixture of N₂ and H₂, and characterized using techniques include X-ray diffraction and elemental analysis.

X-ray diffraction patterns for the Mo₂C contained peaks characteristic for α-MoC_1-x and β-Mo₂C, while patterns for the Mo₂N indicated the presence of γ-Mo₂N. Surface areas for the Mo₂C and Mo₂N supports were 113 and 85 m²/g, respectively. The addition of metal supports caused a significant reduction in the surface area. For example, the surface area for the Fe/Mo₂C catalyst was 30 m²/g. Nevertheless, all of the catalysts were highly active for NH₃ synthesis. Production rates for Mo₂C, Mo₂N, and Fe/Mo₂C were 120 μmol/hr g, 130 μmol/hr g, and 60 μmol/hr g, respectively. The specific activity for the Fe/Mo₂C catalyst was the highest (1.49 μmol/hr m²) among the catalysts evaluated and is higher than values previously reported in the literature.³ These and other results will be described in the presentation.

References:

[1] S. Back;Y. Jung, Phys. Chem. Chem. Phys., 2016, 18, 9161.

[2] S. T. Oyama, Catal. Today, 1992, 15, 179.

[3] R. Kojima, K. Aika, Appl. Catal. A: General 2001, 219, 141.