Incorporating Zinc Metal Sites in Aluminum-Coordinated Porphyrin Metal-Organic Frameworks for Enhanced Photocatalytic Nitrogen Reduction to Ammonia

Rationally designing photocatalysts is crucial for the solar-driven nitrogen reduction reaction (NRR) due to the stable N≡N triple bond. Metal-organic frameworks (MOFs) are considered promising candidates but suffer from insufficient active sites and inferior charge transport. Herein, it is demonstrated that incorporating 3 d metal ions, such as zinc (Zn) or iron (Fe) ions, into Al-coordinated porphyrin MOFs (Al-PMOFs) enables the enhanced ammonia yield of 88.7 and 65.0 µg g cat −1 h −1 , 2.5- and 1.8-fold increase compared to the pristine Al-PMOF (35.4 µg g cat −1 h −1 ), respectively. The origin of ammonia (NH 3 ) is verified via isotopic labeling experiments. Incorporating Zn or Fe into Al-PMOF generates active sites in Al-PMOF, that is, Zn-N 4 or Fe-N 4 sites, which not only facilitates the adsorption and activation of N 2 molecules but suppresses the charge recombination. Photophysical and theoretical studies further reveal the upshift of the lowest unoccupied molecular orbital (LUMO) level to a more energetic position upon inserting 3d metal ions (with a more significant shift in Zn than Fe). The promoted nitrogen activation, suppressed charge recombination, and more negative LUMO levels in Al-PMOF( 3d metal) contribute to a higher photocatalytic activity than pristine Al-PMOF. This work provides a promising strategy for designing photocatalysts for efficient solar-to-chemical conversion.