Electronic and energy descriptors for SACs as tri-functional catalysts towards urea formation and unveiling the C–N coupling mechanism

Abstract

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 the case of simultaneous reduction towards urea formation, the NH₂ free energy serves as an effective energy descriptor for identifying suitable catalysts, exhibiting a strong linear correlation with the limiting potential (R² = 0.93). Additionally, we observed a strong Brønsted–Evans–Polanyi (BEP) relationship (R² = 0.99) between the NH₂ adsorption energy and the activation energy of the coupling intermediate (CONH₂). Furthermore, the out of plane d-sub orbitals (d_xz, d_yz and d_z²) 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 d_yz orbitals plays a significant role in determining catalytic activity. This is evidenced by a linear correlation (R² = 0.73) between orbital occupancy of d_yz and NH₂ adsorption energy for all systems, identifying it as the electronic origin of urea reactivity. Our interpretation regarding the descriptor is that NH₂ sp³ (HOMO) donates a σ-electron to the TM d-orbital (LUMO), while the TM d-orbital (HOMO) donates a π*-electron back to NH₂ sp³ (LUMO).