Oxygen Vacancy-Mediated Selective C–N Coupling toward Electrocatalytic Urea Synthesis

Abstract

The electrocatalytic C–N coupling for one-step urea synthesis under ambient conditions serves as the promising alternative to the traditional urea synthetic protocol. However, the hydrogenation of intermediate species hinders the efficient urea synthesis. Herein, the oxygen vacancy-enriched CeO2 was demonstrated as the efficient electrocatalyst with the stabilization of the crucial intermediate of *NO via inserting into vacant sites, which is conducive to the subsequent C–N coupling process rather than protonation, whereas the poor selectivity of C–N coupling with protonation was observed on the vacancy-deficient catalyst. The oxygen vacancy-mediated selective C–N coupling was distinguished and validated by the in situ sum frequency generation spectroscopy. The introduction of oxygen vacancies tailors the common catalyst carrier into an efficient electrocatalyst with a high urea yield rate of 943.6 mg h–1 g–1, superior than that of partial noble-metal-based electrocatalysts. This work provides novel insights into the catalyst design and developments of coupling systems.

Recent Progress in Electrocatalytic Urea Synthesis under Ambient Conditions

“…A similar phenomenon was observed on the InOOH electrocatalyst with oxygen vacancy (V O -InOOH), which showed 51% FE (urea) at −0.5 V from the coreduction of NO 3 – and CO 2 (Figure e,f) . Wang et al explored oxygen vacancy-enriched CeO 2 for urea synthesis from NO 3 – /CO 2 simultaneous reduction, on which the oxygen vacancy could stabilize the intermediate species . Amal and co-workers first reported Cu–N–C single atom catalysts for simultaneous electrochemical reduction of CO 2 and NO 3 – to urea, which achieved 28% FE (urea) with a current density of −27 mA cm –2 at −0.9 V .…”

“…The gaseous products, such as H 2 , CO, CH 4 , and C 2 H 4 , can be determined by gas chromatography (GC). For the soluble products above, the identification and quantification could be achieved by 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy, high-performance liquid chromatography–mass spectrometry (HPLC-MS) spectroscopy, the diacetyl monoxime method, or the urease decomposition method. − For the typical identification and quantification of urea by 1 H NMR spectroscopy, the electrolyte containing reaction products are extracted and used without any postprocessing. Subsequently, the electrolyte is mixed with the appropriate dimethyl sulfoxide- d 6 (DMSO- d 6 ) used as the deuterated reagent before being transferred into the NMR tube for test.…”

“…Thus, one urea molecule is decomposed by urease into CO 2 and two NH 3 molecules. Subsequently, the NH 3 concentration of the urea electrolyte can be detected by the indophenol blue method or ion chromatography. , Then the amount of produced urea can be calculated based on the quantification of NH 3 with and without urease. The accuracy of the urease method can be affected by the enzyme concentration, activity, hydrolysis time, temperature, solution environment, metal ions, and high concentration of ammonia .…”

“…45 Wang et al explored oxygen vacancy-enriched CeO 2 for urea synthesis from NO 3 − /CO 2 simultaneous reduction, on which the oxygen vacancy could stabilize the intermediate species. 35 Amal and coworkers first reported Cu−N−C single atom catalysts for simultaneous electrochemical reduction of CO 2 and NO 3 − to urea, which achieved 28% FE (urea) with a current density of −27 mA cm −2 at −0.9 V. 46 Other valuable products such as alkylamines were also expanded from electrochemical coreduc-tion of CO 2 and NO 3 − . For example, Wang et al first synthesized methylamine (CH 3 NH 2 ) using CoPc-NH 2 /CNT as the catalyst with 13% FE(CH 3 NH 2 ) at −0.92 V. 47 Meanwhile, the same team reported the first electrochemical conversion of CO 2 and NO 3…”

Identifying and tailoring C–N coupling site for efficient urea synthesis over diatomic Fe–Ni catalyst

“…The absorbance at 660 nm exhibits the linear relationship with the concentration of ammonia, thus the amount of ammonia can be obtained based on the calibration curve. The concentration of urea was quantified by the urease method 47 , 48 . Urease from Canavalia ensiformis (Jack bean) was purchased from Sigma, Urease activity: 20KU, Batch number: Lot#SLCJ5647.…”