The Tandem Nitrate and CO₂ Reduction for Urea Electrosynthesis: Role of Surface N‐Intermediates in CO₂ Capture and Activation

Abstract

Electrochemical reduction of CO₂ and nitrate offers a promising avenue to produce valuable chemicals through the using of greenhouse gas and nitrogen-containing wastewater. However, the generally proposed reaction pathway of concurrent CO₂ and nitrate reduction for urea synthesis requires the catalysts to be both efficient in both CO₂ and nitrate reduction, thus narrowing the selection range of suitable catalysts. Herein, we demonstrate a distinct mechanism in urea synthesis, a tandem NO₃ ^- and CO₂ reduction, in which the surface amino species generated by nitrate reduction play the role to capture free CO₂ and subsequent initiate its activation. When using the TiO₂ electrocatalyst derived from MIL-125-NH₂, it intrinsically exhibits low activity in aqueous CO₂ reduction, however, in the presence of both nitrate and CO₂, this catalyst achieves an excellent urea yield rate of 43.37 mmol ⋅ g^-1 ⋅ h^-1 and a Faradaic efficiency of 48.88 % at -0.9 V vs. RHE in a flow cell. Even at a low CO₂ level of 15 %, the Faradaic efficiency of urea synthesis remains robust at 42.33 %. The tandem reduction procedure was further confirmed by in situ spectroscopies and theoretical calculations. This research provides new insights into the selection and design of electrocatalysts for urea synthesis.