Copper d-orbital regulation enables direct coupling of CO2 with NO2 for selective urea electrosynthesis

Researchers from Nanchang Hangkong University, Beihang University, Guangzhou University and South China Normal University have developed a catalyst that uses isolated copper atoms on titanium carbide to help turn carbon dioxide and nitrate into urea. By adjusting how electrons occupy the copper atoms' orbitals, the team promoted a reaction pathway in which an activated carbon dioxide intermediate directly combines with a nitrogen-containing species.

Urea is an important nitrogen fertilizer and a feedstock for chemical manufacturing. Its conventional production combines carbon dioxide with ammonia, whose manufacture is energy intensive, and requires elevated temperatures and pressures. Electrochemical synthesis from carbon dioxide and nitrate offers an alternative that could be powered by renewable electricity. A central challenge is making the carbon and nitrogen reactants combine efficiently instead of following separate reaction pathways.

Carbon dioxide and nitrate differ in how readily they become activated. This mismatch makes it difficult to form the carbon-nitrogen bonds needed for urea. Conventional pathways involving more extensively reduced carbon intermediates can also favor competing products. The team addressed this problem by controlling the early electron-transfer step that activates carbon dioxide, preserving an intermediate suited to direct carbon-nitrogen coupling.

The catalyst, called Cu_SACs/TiC, combines individual copper atoms with a titanium carbide support. Calculations indicate that strong electronic interactions with the support lower the energy of the copper d states, while spin-charge injection increases electron occupancy in a specific copper 3d orbital. These changes regulate electron transfer to adsorbed carbon dioxide, stabilizing a partially reduced intermediate while limiting further reduction toward carbon monoxide. Nearby titanium sites help convert nitrate into a reactive nitrogen-containing species, allowing the two intermediates to couple at the copper-titanium interface.

The researchers combined structural characterization, electrochemical measurements, spectroscopy under reaction conditions and density functional theory calculations to investigate this mechanism. Mass spectrometry and electron paramagnetic resonance measurements supported the formation of the activated carbon dioxide intermediate. In-situ Raman and FTIR spectroscopy tracked species involved in the reaction, while nitrogen isotope-labeling experiments supported nitrate as the nitrogen source of the urea product. Together, the results support a direct coupling pathway that differs from routes involving carbon monoxide or carboxyl intermediates.

In a laboratory electrochemical H-cell at room temperature, the optimized catalyst produced 558.8 μg h⁻¹ mg_cat⁻¹ of urea per hour per milligram of catalyst at an electrode potential of -0.3 V_RHE. It achieved a urea Faradaic efficiency of 50.1%, meaning that about half of the electrical charge passed contributed to urea formation. Its carbon selectivity reached 97.8%, indicating a strong preference for urea among the carbon-containing products. The catalyst outperformed copper nanoparticles on titanium carbide and the titanium carbide support alone.

The catalyst also maintained its urea production rate and FE over 30 successive reaction cycles. In a separate flow-cell experiment, it achieved a urea production rate of 551.5 μg h⁻¹ mg_cat⁻¹ and a FE of 47.7%. Production and efficiency remained stable during a 24-hour continuous test, providing evidence of durability under the reported laboratory conditions.