Synthesis of urea from CO2 and N2 fixation under mild conditions using polarized hydroxyapatite as a catalyst

Abstract

Polarized hydroxyapatite (p-HAp) has been used as a catalyst for the synthesis of urea coupling N₂, CO₂ and water under mild reaction conditions when compared to classical nitrogen fixation reactions, such as the Haber–Bosch process. The reaction of 3 bar of N₂ and 3 bar of CO₂ under UV illumination at 120 °C (for 48 h) results in a urea yield of 1.5 ± 0.1 mmol per gram of catalyst (g_c) with a selectivity close to 80%, whereas the reaction is not successful without UV irradiation. However, the addition of small amounts of NO (314 ppm) produces 15.2 ± 0.6 and 4.6 ± 0.4 mmol g_c⁻¹ with and without UV illumination, respectively, with the selectivity in both cases being close to 100%. As nitrogen fixation without UV irradiation using p-HAp as a catalyst is a challenge, studies with NO have been conducted varying the reaction conditions (time, pressure and temperature). The results suggest a mechanism based on the production of NH₄⁺ through the oxidation of N₂.