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

Electrochemical reduction of CO 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 and nitrate reduction for urea synthesis requires the catalysts to be both efficient in both CO and nitrate reduction, thus narrowing the selection range of suitable catalysts. Herein, we demonstrate a distinct mechanism in urea synthesis, a tandem NO and CO reduction, in which the surface amino species generated by nitrate reduction play the role to capture free CO and subsequent initiate its activation. When using the TiO electrocatalyst derived from MIL-125-NH, it intrinsically exhibits low activity in aqueous CO reduction, however, in the presence of both nitrate and CO, this catalyst achieves an excellent urea yield rate of 43.37 mmol ⋅ g ⋅ h and a Faradaic efficiency of 48.88 % at -0.9 V vs. RHE in a flow cell. Even at a low CO 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.