Metal and Covalent Organic Frameworks for Photocatalytic Conversion of N2-to-NH3: Mechanisms, Materials, and Perspectives.

Ammonia is indispensable for food security and clean energy, yet its production via the Haber-Bosch process consumes vast amounts of fossil resources and contributes significantly to CO2 emissions. The photocatalytic nitrogen reduction reaction (NRR) driven by solar energy offers a sustainable alternative under ambient conditions; however, progress is limited by weak N2 adsorption, strong N≡N bond cleavage, competing hydrogen evolution, and low quantum efficiency. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have emerged as transformative photocatalyst platforms, combining high surface area, tunable porosity, πconjugated structures, and biomimetic active sites to enhance light harvesting, charge separation, and nitrogen activation. This review highlights recent advances in pristine MOF and COF frameworks, composites, and framework-derived catalysts, emphasizing strategies such as defect engineering, heteroatom doping, functionalization, and heterojunction construction toward photocatalytic NRR. Mechanistic insights from spectroscopy and density functional theory reveal associative, Mars-van Krevelen, and defect-assisted pathways, offering guidance for rational catalyst design. Beyond materials, techno-economic aspects, including scalability, durability, cost performance balance, and energy payback, are critically assessed relative to the Haber-Bosch process. This review highlights the importance of integrating molecular-level catalyst design with reactor-scale engineering to translate laboratory breakthroughs into scalable solar ammonia production.