Mixed Functional Group Ligands in UiO-66(Ce): A Strategy for Efficient Photocatalytic Nitrate to Ammonia Conversion

Abstract

Ammonia (NH3) is a key chemical feedstock used in agricultural fertilizers, energy applications, and many industrial processes. However, conventional production methods like the Haber–Bosch process are energy-intensive, costly, and environmentally harmful. Photocatalytic nitrate reduction offers a sustainable, low-energy alternative for NH3 synthesis. In this study, we developed Ce-UiO-66 metal–organic frameworks (MOFs) with mixed functional groups for efficient photocatalytic nitrate reduction. We synthesized and evaluated a series of (Ce)-X catalysts, where X = H, NH2, OH, H–NH2, NH2–OH, H–OH, and H–NH2–OH. Among them, Ce–NH2–OH (NO) showed the highest ammonia production rate at 212 μmol g–1 h–1. This enhanced performance results from synergistic effects: the amine group improves visible-light absorption by narrowing the bandgap, while the hydroxy group creates shallow electron traps that reduce charge recombination and improve charge transfer. The hydroxy group also increases the proportion of Ce3+ sites, which serve as active centers for the nitrate reduction reaction. This work presents a catalyst design strategy that combines structural and electronic engineering ammonia production, advancing sustainable clean energy technologies.