(664g) Selective Electrochemical Reduction of NO3- and CO2 to Urea on Silver Gas Diffusion Electrode
AIChE Annual Meeting
2023
2023 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Electrocatalysis & Photocatalysis I: Electrocatalytic CO2 and CO Reduction
Monday, November 6, 2023 - 5:30pm to 5:50pm
Electrochemical synthesis of urea from the concentrated stream of waste NO3- and CO2 from flue gas is attractive, as renewable energy can be used to make green urea. Electrochemical co-reduction of NO3- and CO2 can give a variety of products such as NH3, NO2-, H2, CO2RR products (CO, C2H4, HCOOH, etc.), formamide, methylamine, and urea. Electrochemical C-N coupling with selective synthesis of urea is challenging and a thorough catalyst screening to selectively produce urea is required. The above-mentioned areas are not explored well in the literature. In this work, we screened a wide range of metallic catalyst materials for urea synthesis. The catalysts that reduce CO2 to CO and HCOOH are active for urea synthesis. Ag is the best catalyst for selective synthesis of urea with near 100 % selectivity. The system can be operated dynamically over a potential range of -0.5 to -1.5 V vs. RHE without compromising the selectivity of urea. The concentration of NO3- and CO2 play a major role in determining the selectivity of urea. Ag GDE is used to overcome the mass transfer limitations and we report a high urea current density of ~-100 mA/cm2 with a urea Faradaic Efficiency of ~100 % at -1.2 V vs. RHE which is the highest so far reported in the literature. DFT calculations are performed to get insights on the electrochemical C-N bond formation. A combination of facile first and second C-N bond formation steps and an endergonic formamide formation step from *CONH2 results in improved kinetics and selectivity towards urea synthesis.