The Tandem Nitrate and CO 2 Reduction for Urea Electrosynthesis: Role of Surface N-Intermediates in CO 2 Capture and Activation.

Electrochemical reduction of CO 2 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 CO 2 and nitrate reduction for urea synthesis requires the catalysts to be both efficient in both CO 2 and nitrate reduction, thus narrowing the selection range of suitable catalysts. Herein, we demonstrate a distinct mechanism in urea synthesis, a tandem NO 3 - and CO 2 reduction, in which the surface amino species generated by nitrate reduction play the role to capture free CO 2 and subsequent initiate its activation. When using the TiO 2 electrocatalyst derived from MIL-125-NH 2 , it intrinsically exhibits low activity in aqueous CO 2 reduction, however, in the presence of both nitrate and CO 2 , 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 CO 2 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.