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 H-2 : Ar and N-2 flow cycles at room temperature and atmospheric pressure due to accumulation of the formed NHX 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.