Metal–Organic Frameworks for Dinitrogen Reduction to Ammonia: Active-Site Engineering, Mechanistic Pathways, and Reliability Challenges

Abstract Metal–organic frameworks (MOFs) provide a highly tunable platform for artificial dinitrogen reduction to ammonia because metal nodes, organic linkers, defects, pore environments, and interfaces can be engineered at molecular and atomic levels. This review critically examines MOF-based electrocatalytic and photocatalytic N2 reduction with emphasis on the identity of the working active site, mechanistic interpretation, and the reliability of NH3 assignment. Electrocatalytic systems are discussed as direct and intrinsic MOFs, post-synthetically or molecularly modified frameworks, MOF-based composites and microenvironment-engineered interfaces, and MOF-derived single-atom or M–N–C catalysts. Photocatalytic systems are organized as direct MOFs, defect-rich and electronically engineered frameworks, heterojunction/interfacial photocatalysts, and molecularly engineered POM/MOF or single-site systems. Across both routes, reported NH3 productivity is evaluated together with Faradaic efficiency, apparent quantum efficiency, competing hydrogen evolution, isotope labeling, analytical cross-validation, catalyst reconstruction, and structural stability. Particular attention is given to false-positive risks arising from nitrogen-containing contaminants and to the distinction between the synthesized material and the actual working catalyst. The review further discusses local microenvironment regulation, limitations of density functional theory, and the need to couple active-site design with gas–liquid–solid mass transport, conductive architectures, membranes, gas-diffusion electrodes, and flow reactors. Rather than ranking catalysts by apparent NH3 yield alone, this review identifies transferable design and validation principles for more reliable and engineering-relevant MOF-based nitrogen fixation.