The Synergistic Effect of Combining Electron Transfer and Photoactivation in Hydroxyapatite/ZrO Nanocomposites Promotes Efficient Nitrogen-to-Ammonia Fixation Reaction.

Catalytically active hydroxyapatite (ca-HAp) decorated with zirconia nanoparticles (ZrO NPs) is presented as a nanocomposite catalyst (ca-HAp/ZrO) capable of performing highly efficient nitrogen to ammonia (N-to-NH) fixation reactions under mild conditions. Accordingly, reactions were carried out in a batch reactor operating at 120 °C, 6 bar of N, and 20 mL of water, under UV irradiation (14 W) for 72 h. The yield of NH obtained was 1.592 ± 0.146 mmol·g , which represents a N fixation efficiency of 6.4%. Near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies under 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/ZrO nanocomposites exhibit a strong synergy arising from the initial photoactivation of N by means of the π-backdonation mechanism in ZrO (N is anchored by four Zr atoms) followed by the dinitrogen spillover toward the Ca-(I) 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 N-to-NH fixation reactions.