Effect of Axial Ligands on Photocatalytic CO₂ Reduction of Zirconium-Based Porphyrins

Metalloporphyrins, with their unique large conjugated planar structure, have attracted research attention in catalysis, energy, and biomedicine. However, studies of examining the effects of axial ligands on metal centers are limited. Herein, we introduced oxalic acid and p-phthalic acid (PTA) as axial ligands into tetraphenylporphyrin zirconium (Zr-TPP). Through structural modulation of axial ligands, the zirconium-based porphyrins effectively enhance the separation and transport of photogenerated carriers and also induce an increase in charge density at the metal center atoms. These modified structures efficiently enhanced their photocatalytic performance, achieving over 90% selectivity for CO2-to-CO conversion. Specifically, the CO production with Zr-TPP is 272.43 μmol g–1 h–1, and it surpasses 336.5 μmol g–1 h–1 with Zr-TPP-PTA. The photocatalytic mechanism responsible for reducing CO2 to CO is explored through density functional theory calculations. This work explores axial ligand–metal interactions in porphyrin complexes, revealing that different axial ligands can affect the electronic structure of materials and improve their photocatalytic performance, providing ideas for the regulation of metalloporphyrin-based photocatalysts.