Mechanochemical Urea Synthesis Using Ammonia–Water and Carbon Dioxide Under Mild Conditions: An Experimental and Theoretical Study

Abstract

The production of urea predominantly relies on the energy-intensive Bosch–Meiser process, which operates at temperatures ranging from 150 to 200 °C and pressures of approximately 150 to 250 bar. More sustainable approaches to urea synthesis under milder conditions remain a significant challenge. Herein, we demonstrate that urea can be synthesized via a mechanochemical method using ammonia–water and CO₂ under an ambient environment. Without extra catalysts, the ZrO₂ texture of the jar and grinding balls has a crucial mechanocatalytic effect on direct urea synthesis. Experimental data coupled with theoretical calculation results indicate that the mechano-induced oxygen vacancies (O_V) within the (101) crystal plane of ZrO₂ play a pivotal role in urea formation. These vacancies notably reduce the energy barrier for the generation of *NH₂ and the subsequent decomposition of NH₂COOH, thereby facilitating a more energy-efficient urea synthesis process. This work presents a novel method for synthesizing urea under mild conditions, offering potential cost-effective alternatives to urea production.