(159v) Experimental and Theoretical Screening of Transition Metal Catalysts for Electrochemical Oxidation of Nitrogen to Nitrates | AIChE

(159v) Experimental and Theoretical Screening of Transition Metal Catalysts for Electrochemical Oxidation of Nitrogen to Nitrates

NH3 is an important commodity chemical, and it is manufactured by Haber-Bosch process which involves the reaction of N2 and H2 at high pressures (150 to 200 atm) and high temperatures (500 to 600 °C). More than 1 % of the world’s total energy is spent in the manufacture of NH3 and hence the process is highly energy intensive. The feedstock for the process, H2 is produced by Steam Reforming process and there is a massive Carbon footprint associated with it. It is desirable to synthesize NH3 in an electrochemical manner by using water, air and renewable electricity at ambient conditions. There are two methods for the electrochemical synthesis of ammonia, the direct and the indirect method. Direct electrochemical reduction of dinitrogen would be ideal, but the NH3 yield and the NH3 Faradaic efficiency is low in this case due to the competing hydrogen evolution reaction (HER) and high mass transfer resistances (low solubility of N2). Indirect method involves the conversion of N2 into nitrates and subsequently reducing the nitrates to NH3. Electrochemical reduction of NO3- to NH3 is less challenging when compared to direct N2 reduction to NH3 due to lower dissociation energy of N = O bond (204 kJ/mol) than N ≡ N (941 kJ/mol). Co was active for the electrochemical reduction of nitrates to ammonia, and we achieved a maximum NH3 Faradaic efficiency of 68.13 % at -0.6 V vs RHE (pH = 14) and a high NH3 current density of 245.34 mA/cm2 at -0.8 V vs RHE (pH = 14). Electrochemical oxidation of N2 to nitrates has not been explored in the literature and that is the focus of this study. Transition metal catalysts are ideal for electrochemical oxidation of Nitrogen to nitrates. In this study, we perform a theoretical and experimental screening to identify the best electrocatalyst for the oxidation of nitrogen to nitrates. 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 current density of Nitrates. Nitrates are quantified by Griess UV-Visible spectroscopy method. TiO2 was the most active catalyst among the considered metals. Rigorous control studies were performed to avoid the error in quantification of nitrates from contamination sources. We obtained a high nitrate Faradaic efficiency of 16% at 1 V vs RHE and TiO2 was stable in oxidation conditions under the study period of 24 h.