Highly efficient visible-light photocatalytic nitrogen fixation via single-atom iron catalyst site and synergistic effect of Zr-cluster in zirconium-based porphyrinic metal-organic frameworks (PCN-222)

Zirconium-based porphyrinic metal-organic frameworks (MOFs), including PCN-222 (nonmetal), and PCN-222 (Fe) were employed as the photocatalyst for photocatalytic nitrogen (N2) fixation to NH3 under visible light irradiation. The as-synthesized MOFs of PCN-222 (nonmetal) and PCN-222 (Fe) exhibited a notable increase in photocatalytic N2 fixation compared to the pure (Metallo) porphyrin ligands of H2TCPP and FeTCPP, respectively. The comparative studies illustrated that using the Zr-cluster as a stable node and the active redox site is a successful strategy to increase the stability and photocatalytic activity of the MOF-based photocatalysts. On the other, the single-atom Fe site as an active center of absorption and activation of nitrogen molecules improved the N2 fixation activity. Remarkably, among them all, PCN-222 (Fe) demonstrated the highest photocatalytic N2 fixation activity with a rate of 1502.51 μmol g-1h−1 which could be related to the simultaneous presence of Zr-cluster and single-atom Fe site which promote the performance of the photocatalyst. So, the photocatalytic activity of PCN-222 (Fe) is 1.9 and 2.5 times higher than the activity of PCN-222 (nonmetal) and pure FeTCPP, respectively. High photocatalytic activity, sacrificial agent-free conditions, excellent efficiency in short response time, reusability, and mild reaction conditions are the attractive characteristics of the as-synthesized photocatalysts that make them suitable candidates for photocatalytic NH3 production.