Multifunctional UiO-66 MOFs in Photocatalytic and Electrocatalytic Nitrogen Reduction: A New Horizon in Green Ammonia Synthesis

Abstract

The conventional production of ammonia (NH₃) rests on the energy-intensive Haber process, driving the scientific community to search for sustainable catalysts capable of reducing the inert N≡N bond under ambient conditions. Among the various porous materials examined for the nitrogen reduction reaction (NRR), UiO-66, a Zr-based MOF, stands out owing to its exceptional robustness, high BET surface area, tunable porosity and strong Lewis acidity at the Zr⁴⁺ nodes, making it a highly versatile platform for both photocatalytic and electrocatalytic NH₃ production. This review aims to deliver a comprehensive overview of UiO-66-based materials for photocatalytic (P-NRR) and electrocatalytic (E-NRR) nitrogen reduction, while providing a detailed account of the key parameters governing NRR, including its thermodynamics and kinetics, the challenges of activating the inert N≡N bond, and competing hydrogen evolution reaction (HER). Several synthesis methods for UiO-66 are critically discussed with respect to how each approach influences material properties, including crystallinity, surface area, stability, and porosity. Targeted modification strategies, such as metal doping, linker functionalization, defect engineering, and composite formation, are systematically explored, each aimed at introducing suitable active sites and establishing a favorable environment for N₂ adsorption and activation across both photocatalytic and electrocatalytic systems. P-NRR and E-NRR performances are critically analyzed, with due consideration given to NH₃ detection methods, including colorimetric assays, ion chromatography, and ¹H NMR spectroscopy, which are essential for reliable quantification. Finally, current challenges and future directions are outlined to guide the rational design of efficient, selective, and robust UiO-66-based catalysts for sustainable NH₃ synthesis.