As an important chemical, urea has a wide range of applications. However, the conventional Bosch-Meiser process suffers from high energy consumption and substantial carbon emissions. This study proposes a novel mechanochemical strategy for urea synthesis under ambient temperature and pressure using N₂, CO₂, and H₂O as feedstocks. Five transition metal oxide catalysts (TiO₂, ZnO, Cu₂O, Nb₂O₅, Fe₂O₃) were investigated for their mechanocatalytic effects in a ZrO₂ jar equipped with ZrO₂ balls, with TiO₂ exhibiting the highest performance. Urea production rate was up to 133.59 μg·L⁻¹·h⁻¹with the presence of TiO₂, which is 2.2 times larger than that the catalyst-free conditions. TiO₂ was characterized using TEM, EDS, XRD, XPS, EPR, and Raman spectroscopy, revealing an increase in oxygen vacancies after the mechanochemical reaction. FT-IR analysis was used to detect the adsorbed species on the TiO₂ surface, providing mechanistic insights. Density functional theory (DFT) calculations identified H₂O dissociation and N₂ activation as critical steps. The Vo in TiO₂ not only enhanced the adsorption of N₂ and H₂O but also facilitated H₂O dissociation to release free H atoms and weakened the N≡N bond. The C-N coupling between * N₂ and *CO was determined to be the rate-limiting step in urea formation. This study presents a green and energy-efficient mechanochemical approach for urea synthesis and elucidates the catalytic role of titanium dioxides in the process.

Key words: Urea synthesis, Mechanochemistry, TiO₂-mechanocatalyst, N₂ activation, Density functional theory