(558bv) Understanding Fe-Based Catalysts for Electrochemical Ammonia Synthesis at Ambient Conditions | AIChE

(558bv) Understanding Fe-Based Catalysts for Electrochemical Ammonia Synthesis at Ambient Conditions


Hu, L. - Presenter, University of Central Florida
Feng, X., University of Central Florida
Electrochemical reduction of N2 to NH3 at ambient conditions may provide an alternative to the Haber−Bosch process for sustainable NH3 production when powered by solar- or wind-derived electricity, as the Haber-Bosch process relies on the consumption of fossil fuels and is energy-intensive (consuming 1−2% of the global energy supply). However, the development of this process has been hindered by the lack of efficient catalysts for the N2 reduction reaction (N2RR) at ambient conditions, due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). Fe-based catalysts have received a lot of attention as a promising candidate among the non-precious metal catalysts. Here, we present a study of the effect of the chemical states of Fe-based catalysts for N2 electroreduction to NH3. In contrast to pure metallic Fe catalyst, we developed an Fe/Fe oxide catalyst by in situ electroreduction of a pre-oxidized Fe foil in a neutral electrolyte. The resulting Fe/Fe oxide catalyst was composed of Fe and Fe3O4 nanocrystallites, as revealed by X-ray photoelectron spectroscopy and grazing incidence X-ray diffraction. The Fe/Fe3O4 catalyst shows greatly enhanced activity and selectivity for N2RR at ambient conditions than the original Fe foil, achieving a Faradaic efficiency of 8.29% for NH3 production at −0.3 V vs the reversible hydrogen electrode in a neutral electrolyte, which is around 120 times higher than that of the Fe foil. The high selectivity is enabled by an enhancement of the intrinsic N2RR activity as well as an effective suppression of the HER activity.[1] Comparative studies with Fe/Fe oxide samples derived at different pre-oxidation temperatures indicate that the N2RR activity is affected by Fe/Fe oxide ratio. Furthermore, the N2RR selectivity of the Fe/Fe3O4 catalyst is also superior to that of Fe, Fe3O4, and Fe2O3 nanoparticles. We are currently using in operando X-ray absorption spectroscopy to probe the chemical states of Fe-based nanocatalysts during N2RR, to establish a correlation between the chemical states and NH3 electrosynthesis activity on Fe-based catalysts.


[1] Hu, L.; Khaniya, A.; Wang, J.; Chen, G.; Kaden, W. E.; Feng, X. Ambient electrochemical ammonia synthesis with high selectivity on Fe/Fe oxide catalyst. ACS Catal. 2018, 8, 9312−9319.