Dr Ranjith Thapa and his student Mr Narad Barman have co-authored a research paper on Electronic and Energy Descriptor for SACs as Tri-functional Catalysts Towards Urea Formation and Unveiling the C–N Coupling. Their work sheds new light on the catalytic pathways involved in urea synthesis using single-atom catalysts, offering deeper mechanistic understanding and improved catalyst design strategies. This research has significant implications for advancing sustainable production processes, especially in the fields of agriculture, pharmaceuticals, and the food industry.

Abstracts:

Single atom catalysts (SACs) have rapidly emerged as a cutting-edge trend in electro-catalysis for synthesizing nitrogen-based products such as ammonia, nitric acid and urea. In the present study, we considered NO2- as a specific N-based molecule, which participated in the simultaneous reduction with CO2 towards urea formation for 77 SACs. Our investigation demonstrates that, among possible nitrogen-containing intermediates generated during the simultaneous electrochemical reduction of CO₂ and NO2⁻, only the NH2 intermediate effectively couples with CO to form urea. In case of simultaneous reduction towards urea formation, the NH2 free energy serves as an effective energy descriptor for identifying suitable catalysts. Furthermore, the out of plane d-sub orbitals (dxz, dyz and dz2) of the transition metal (TM) were analysed to uncover the electronic origins of urea reactivity. Owing to the strong interaction between the d-sub orbitals of the TM and the sp³ hybrid orbitals of NH2, occupancy of the dyz orbitals play a significant role in determining catalytic activity. This is evidenced by a linear correlation (R2 = 0.73) between orbital occupancy of dyz and NH2 adsorption energy for all systems, identifying it as the electronic origin of urea reactivity. The interpretation about the descriptor is, NH2 sp³ (HOMO) donates σ-electron to TM d-orbital (LUMO), while TM d-orbital (HOMO) donates π*-electron back to NH2 sp3 (LUMO).

Explanation in layman language:

Urea is an essential nitrogen-rich fertilizer, but its industrial synthesis by the energy-intensive Bosch–Meiser process leads to significant CO₂ emissions. Electrochemical co-reduction of CO₂ and nitrogen-containing species (N₂, NO₂⁻, NO₃⁻) under ambient conditions offers a sustainable alternative, though competing reactions such as HER, CO₂RR, and nitrogen reduction pathways make selective urea production challenging. Transition metal single-atom catalysts (TM–SACs), particularly TM–N–C systems, show promise due to their high activity toward CO₂RR, NO₂RR, and related reactions while suppressing HER.
Because SAC reactivity is governed by the electronic structure of the metal center and its nitrogen-doped carbon support, identifying simple electronic descriptors is key for rational catalyst design. In TM–N–C systems, the TM d-orbitals split into sub-d orbitals, and this study demonstrates that hybridization of the dxz, dyz, and dz² orbitals with adsorbates controls catalytic performance. Among them, the dyz orbital best predicts activity, showing strong correlations for CO₂RR, NO₂RR, and their co-reduction across 77 SACs. For urea synthesis, two C–N coupling mechanisms are proposed, and the Gibbs free energy of adsorbed NH₂ emerges as an effective energy descriptor for evaluating urea formation activity. Molecular orbital analysis explains why dyz occupancy governs catalytic behavior.

Practical Implementations:

Our research outcome is urea a nitrogen-based fertilizer.

Applications of Urea
Agriculture (Major Use)

• Used as the most common nitrogen fertilizer (46% nitrogen)
• Promotes plant growth and increases crop yield.
• Applied to soil or as foliar spray.
• Chemical Industry

Used as a feedstock for producing:

• Melamine
• Urea-formaldehyde (UF) resins
• Urea–melamine–formaldehyde (UMF) resins
• Plastics, adhesives, and molded products

Pharmaceuticals

• Used in dermatological creams (10–40%) for moisturizing and treating dry skin, eczema, and psoriasis.
• Used in manufacturing some medicines.
• Laboratory and Biochemistry
• Used as a protein denaturant in electrophoresis.
• Helps break hydrogen bonds in proteins.

Food Industry

• Added in animal feed to supply non-protein nitrogen for cattle, enhancing digestion.
• Explosives
• Component of urea nitrate, a fertilizer-derived improvised explosive.
• Used in some commercial blasting formulations.

Link of the article: https://pubs.rsc.org/en/content/articlehtml/2025/sc/d5sc06657c
DOI: 10.1039/D5SC06657C (Edge Article) Chem. Sci., 2025