The Synergistic Effect of Combining Electron Transfer and Photoactivation in Hydroxyapatite/ZrO₂ Nanocomposites...

Catalytically active hydroxyapatite (ca-HAp) decorated with zirconia nanoparticles (ZrO₂ NPs) is presented as a nanocomposite catalyst (ca-HAp/ZrO₂) capable of performing highly efficient nitrogen to ammonia (N₂-to-NH₃) fixation reactions under mild conditions. Accordingly, reactions were carried out in a batch reactor operating at 120 °C, 6 bar of N₂, and 20 mL of water, under UV irradiation (14 W) for 72 h. The yield of NH₃ obtained was 1.592 ± 0.146 mmol·g_c ^-1, which represents a N₂ fixation efficiency of 6.4%. Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies under in situ conditions (i.e., at elevated pressure and temperature and during UV irradiation) and density functional theory simulations (DFT) allowed us to elucidate the catalytic mechanism of the system. The ca-HAp/ZrO₂ nanocomposites exhibit a strong synergy arising from the initial photoactivation of N₂ by means of the π-backdonation mechanism in ZrO₂ (N₂ is anchored by four Zr⁴⁺ atoms) followed by the dinitrogen spillover toward the Ca-(I)²⁺ binding sites. Such sites, preferentially exposed in the (001) crystallographic planes of ca-HAp, show high activity due to the enhanced electron transfer properties of ca-HAp. These catalytic nanocomposites represent a viable alternative to the conventional catalysts used for N₂-to-NH₃ fixation reactions.