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

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 NH X on the catalyst surface. DFT estimations revealed that a compound with a molecular stoichiometry of N-NH 3 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. Interactions between atomic hydrogen and dinitrogen are facile under ambient conditions. N-NH 3 is the best candidate estimated by DFT corroborating the experimental IR shift. Desorption of NH 3 limits the rates under subcritical conditions.