A New Method for Urea Synthesis under Environmental Conditions Based on Nucleophilic Addition Reaction During Electrochemical CO₂ Reduction

Abstract

Urea (H2NCONH2) is a commercially indispensable chemical as a fertilizer and starting material for the manufacture of many other products. Currently, commercial urea can be prepared with an energy‐consuming process by combining liquid ammonia (NH3) and liquid carbon dioxide (CO2) under high‐temperature and high‐pressure conditions. An alternative approach for synthesis of urea may involve electrochemical synthesis reactions that occur under aqueous and ambient conditions. Here, we propose synthesis of urea via nucleophilic addition reaction during an electrochemical CO2 reduction process in NH4HCO3‐containing electrolytes. The urea formation at a rate of 0.11 mmol g−1 h−1 was achieved on a polycrystalline Au electrode (area: 1 cm2) at a potential of −1.4 V (vs. saturated calomel electrode). Furthermore, the detailed investigations including in‐situ spectroscopic analysis and isotopic labeling characterization reveal the capability of producing urea could be mainly attributed to a spontaneous reaction between the NH3 molecule and the CO2⋅− intermediate. This study promotes the exploration of electrochemical synthesis of high‐value‐added products.

Boosting the Urea Synthesis Rate on Ni Single-Atom Catalysts: The Impact of Acetonitrile Electrolyte in the Tandem CO2 Reduction/Nucleophilic Addition Reaction

“…The gas-phase product analyses (Figures S8 and S11) support all of the aforementioned conclusions regarding the C–O peak. In addition, the O–C–O vibrational band signal at 1360 cm –1 , which is attributed to the *COOHNH 2 intermediate adsorbed on the catalyst surface, was only observed in electrolyte B. ,, The *COOHNH 2 intermediate is formed during the nucleophilic addition reaction of the CO 2 •– intermediate with NH 3 , which eventually leads to the production of urea through a series of subsequent reactions. Acetonitrile improves the stability of the CO 2 •– intermediate during the reaction, which may account for its effect on the O–C–O vibrational band signal.…”

“…In addition, the O−C−O vibrational band signal at 1360 cm −1 , which is attributed to the *COOHNH 2 intermediate adsorbed on the catalyst surface, was only observed in electrolyte B. 16,52,53 The *COOHNH 2 intermediate is formed during the nucleophilic addition reaction of the CO 2…”

“…Jiao et al have successfully demonstrated that C–N bonds can be formed via coelectrolysis of CO and NH 3 on the surface of a Cu catalyst and that acetamide can be formed by nucleophilic addition of NH 3 to the surface-bound ketene intermediate . Recently, we proposed another route for the synthesis of urea by electrocatalytic CO 2 reduction in an NH 4 HCO 3 electrolyte and further verified that urea can be formed via a spontaneous nucleophilic addition reaction between NH 3 molecules and CO 2 •– intermediates . Compared to the conventional electrocatalytic coreduction reaction of CO 2 with nitrogen-containing molecules (such as N 2 , NO 2 – , or NO 3 – ) for the synthesis of urea, − the TCRR/NAR avoids the spontaneous reaction of nitrogen-containing intermediates with carbon-containing intermediates.…”

“…•− intermediates. 16 Compared to the conventional electrocatalytic coreduction reaction of CO 2 with nitrogen-containing molecules (such as N 2 , NO 2 − , or NO 3 − ) for the synthesis of urea, 17−19 the TCRR/NAR avoids the spontaneous reaction of nitrogen-containing intermediates with carbon-containing intermediates. In principle, the TCRR/NAR is sensitive to the intermediates and thus can effectively avoid side reactions.…”

Electrochemical urea production using carbon dioxide and nitrate: state of the art and perspectives

“…Ideally, an efficient electrocatalyst should promote C–N coupling while restricting or suppressing the side reactions, including hydrogen evolution, 72,85,86 carbon dioxide reduction 87 and nitrate reduction reactions. 30 However, it is not yet clear which family of electrocatalysts is the most selective for urea electrosynthesis from the perspective of the aforementioned considerations.…”