Catalyst-Free Production of Urea from Nitrate and Carbon Dioxide in Water Microdroplets

We report a simple, one-step selective process for producing urea (NH₂CONH₂) from nitrate (NO₃^-) dissolved in aqueous microdroplets when sprayed with carbon dioxide (CO₂) as the nebulizing gas. This synthesis is accomplished without any catalyst or application of any external electric potential or radiation. The electric field at the gas-water interface is believed to drive the reaction process, resulting in urea being dissolved in the water microdroplets, which is the source of hydrogen for this conversion. The highest urea production rate of ∼118 μM h^-1 is achieved with optimized parameters. The selectivity of urea formation in this process is >99%. Mass spectrometric studies were conducted to identify the reaction intermediates involved in the conversion. Density functional theory (DFT) calculations support our experimental observations by providing insights into the reaction pathways for the transformation. This eco-friendly approach for urea synthesis consumes CO₂, thereby transforming a greenhouse gas into a value-added product.