COx-free hydrogen production via ammonia decomposition over molybdenum nitride-based catalysts

Abstract

CO_x-free hydrogen generation via ammonia decomposition has received much attention as an important process for fuel cell applications. In the present study, non-precious Mo nitride-based catalysts with Co, Ni, and Fe additives were synthesized by temperature-programmed reaction of the corresponding oxide precursors with NH₃. N₂ adsorption, X-ray diffraction (XRD), NH₃-temperature programmed surface reaction (NH₃-TPSR), NH₃ pulse reaction, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were carried out to gain better insight into the chemical and textural properties of the catalysts. The XRD analysis confirmed the formation of the Mo₂N, Co₃Mo₃N, Ni₃Mo₃N, and Fe₃Mo₃N phases, which act as active species for ammonia decomposition reaction. The Co₃Mo₃N, Ni₃Mo₃N, and Fe₃Mo₃N catalysts were more active for ammonia decomposition than the Mo₂N catalysts, indicating that the Co, Ni, and Fe species promoted the reaction. The increase in particle size and the decrease in surface area by the Co, Ni, and Fe addition did not negatively affect the ammonia decomposition activity. Interestingly, the Co, Ni, and Fe addition facilitated the recombinative desorption of N atoms from the active components, resulting in the enhancement in the activity. In addition, kinetic analysis was also conducted in detail to investigate the effects of the NH₃ and H₂ partial pressures. The Co, Ni, and Fe addition alleviated the negative effect of the H₂ poisoning on the active sites.