In search of the bottlenecks of ammonia synthesis over Ru/Vulcan under ambient conditions

Abstract

Hydrogenation of the N–N bond under ambient conditions over 1 wt% Ru/Vulcan was monitored through operando Diffuse Reflectance Infrared Spectroscopy (DRIFTS) and DFT. IR signals centered at 3017 cm⁻¹ and 1302 cm⁻¹ were visible with attributes similar to the asymmetric stretching and bending vibrations of gas phase ammonia at 3381 cm⁻¹ and 1650 cm⁻¹. The intensities of the signals increased with consecutive H₂ : Ar and N₂ flow cycles at room temperature and atmospheric pressure due to accumulation of the formed NH_X on the catalyst surface. DFT estimations revealed that a compound with a molecular stoichiometry of N–NH₃ can give rise to an IR signal centered at 3051.9 cm⁻¹. The results of this study, combined with the known vapor liquid phase behavior of ammonia, suggest that under subcritical conditions, the bottlenecks of ammonia synthesis are both N–N bond dissociation and ammonia desorption from the pores of the catalyst.