(490g) Experimental and Theoretical Screening of Transition Metal Catalysts for Electrochemical Reduction of Nitrates to Ammonia

Ammonia is an important commodity chemical which is primarily used to manufacture fertilizers. Industrially, ammonia is manufactured by Haber-Bosch process. The process is highly energy intensive (500 - 600 °C, 150 - 200 atm), and leaves a massive carbon footprint (1.4 % of CO₂ emissions) as the H₂ is produced from Steam Reforming process. It is desired to synthesize NH₃ by an electrochemical route at ambient conditions.

Direct electrochemical reduction of dinitrogen would be ideal, but it suffers from mass transfer limitations (low solubility of N₂) and the competing H₂ evolution reaction. Both of them contribute to low Faradaic efficiency and low Yield of NH₃. The shortcomings of the electrochemical reduction of N₂ to NH₃ can be avoided in electrochemical reduction of nitrates to NH₃. Metal nitrates can be made by the oxidation of metallic oxides in the presence of NO₂. NO₂ can be obtained by the heterogeneous combustion of nitrogen in the excess of air at high temperatures, or by the electrochemical oxidation of nitrogen.

Transition metal catalysts are ideal for electrochemical reduction of nitrates to ammonia. In this study, we perform a theoretical and experimental screening of catalysts to identify the best electrocatalyst for the reduction of nitrates to ammonia. The catalysts considered for the study are Zn, Cu, Ni, Co, Fe, Mn, Cr, V, Ti, Ir, Pt, Au, Ag and Zr. The metrics used for the assessment are the Faradaic Efficiency and Yield of NH₃. NH₃ is quantified by UV-Visible spectroscopy using Indophenol method and Nessler’s reagent method. Rigorous control studies have been performed to avoid the error in quantification of NH₃ from some contamination sources. DFT studies are performed to predict the binding energies of nitrate ions on various transition metals and the reaction mechanism on various catalyst surfaces are studied.