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

Why It Matters

Solar‑driven nitrogen reduction could decarbonize ammonia synthesis, reducing reliance on fossil‑intensive Haber‑Bosch processes and cutting global CO₂ emissions.

Key Takeaways

• MOFs/COFs provide tunable active sites for efficient N₂ adsorption
• Defect engineering enhances charge separation and NRR activity
• Heterojunction construction suppresses competing hydrogen evolution
• Framework‑derived catalysts combine high surface area with stability
• Techno‑economic analysis shows solar ammonia could rival Haber‑Bosch at scale

Pulse Analysis

Ammonia remains a cornerstone of global agriculture and a potential clean‑energy carrier, yet the century‑old Haber‑Bosch process consumes 1‑2 % of worldwide energy and emits roughly 400 Mt of CO₂ annually. Photocatalytic nitrogen reduction (NRR) promises a low‑temperature, renewable pathway, but early attempts suffered from weak nitrogen adsorption and rapid hydrogen evolution. Recent advances in metal‑organic frameworks (MOFs) and covalent organic frameworks (COFs) have shifted the paradigm. Their crystalline, porous architectures enable precise placement of metal nodes and organic linkers, creating biomimetic sites that bind N₂ more strongly while facilitating light harvesting across the visible spectrum.

Researchers are now leveraging defect engineering, heteroatom doping and strategic functionalization to tailor electronic structures and promote charge carrier separation. Introducing vacancies or substituting atoms such as sulfur or phosphorus generates mid‑gap states that act as electron traps, prolonging lifetimes of photogenerated charges that drive N₂ activation. Moreover, constructing MOF‑MOF or MOF‑COF heterojunctions forms built‑in electric fields that preferentially direct electrons toward nitrogen reduction, markedly suppressing the competing hydrogen evolution reaction. Spectroscopic studies and density‑functional theory calculations have clarified three dominant pathways—associative, Mars‑van Krevelen and defect‑assisted—guiding rational catalyst design.

Beyond laboratory metrics, the review assesses techno‑economic viability, noting that solar‑driven ammonia could achieve parity with Haber‑Bosch if catalyst turnover frequencies exceed 10 h⁻¹ and system energy payback ratios approach unity. Scale‑up challenges include maintaining framework stability under prolonged illumination and integrating reactors with concentrated solar collectors. Nonetheless, the convergence of high‑throughput synthesis, computational screening and pilot‑scale photoreactors positions MOF/COF‑based NRR as a credible route toward a greener ammonia economy, potentially reshaping fertilizer markets and enabling carbon‑neutral energy storage solutions.