Solar-Driven Ammonia Production through Engineering of the Electronic Structure of a Zr-Based MOF

As a hydrogen carrier and a vital component in fertilizer production, ammonia (NH₃) is set to play a crucial role in the planet's future. While its industrial production feeds half of the global population, it uses fossil fuels and emits greenhouse gases. To tackle this issue, photocatalytic nitrogen fixation using visible light is emerging as an effective alternative method. This strategy avoids carbon dioxide (CO₂) emissions and harnesses the largest share of sunlight. In this work, we successfully incorporated a 5-nitro isophthalic acid linker into MOF-808 to introduce structural defects and open metal sites. This has allowed modulation of the electronic structure of the MOF and effectively reduced the band gap energy from 3.8 to 2.6 eV. Combination with g-C₃N₄ enhanced further NH₃ production, as these two materials possess similar band gap energies, and g-C₃N₄ has shown excellent performance for this reaction. The nitro groups serve as acceptors, and their integration into the MOF structure allowed effective interaction with the free electron pairs on N-(C)₃ in the g-C₃N₄ network nodes. Based on DFT calculations, it was concluded that the adsorption of N₂ molecules on open metal sites caused a decrease in their triple bond energy. The modified MOF-808 showed superior performance compared with the other MOFs studied in terms of N₂ photoreduction under visible light. This design concept offers valuable information about how to engineer band gap energy in MOF structures and their combination with appropriate semiconductors for solar-powered photocatalytic reactions, such as N₂ or CO₂ photoreduction.