A molecularly engineered MOF photocatalyst for CO production from visible light-driven CO₂ reduction

The search for new robust and efficient heterogeneous photocatalysts for the reduction of CO₂ has emerged as a key focus in the realm of CO₂ reduction research. However, there is a significant challenge in fabricating a photocatalyst with remarkable photoreduction activity. In this context, accommodation of a photocatalytic redox-active molecular metal complex into a stable MOF framework by replacing the existing linker through post-synthetic exchange (PSE), also termed solvent-assisted ligand exchange (SALE), is a powerful tool for developing photocatalysts for CO₂ reduction. Herein, we demonstrate for the first time the successful incorporation of a Ru(II) bis-terpyridine complex, [Ru(cptpy)₂], into a stable Zr^IV-based metal-organic framework (MOF) consisting of a naphthalene diimide (NDI) linker via SALE. The obtained MOF, Zr-NDI@Ru-tpy or Zr-NDI@Ru-tpy-m was used for photocatalytic CO₂ reduction under visible light. The Zr-NDI@Ru-tpy shows an impressive CO production rate of 2449 μmol g^-1 h^-1 with a low hydrogen production rate of 101 μmol g^-1 h^-1, demonstrating a high selectivity of 97% for CO production. The turnover number (TON) for CO evolution by Zr-NDI@Ru-tpy is 123 in a photocatalytic run of 6 h. Furthermore, a plausible mechanism for CO₂ conversion into CO has been proposed using photophysical and electrochemical investigation, along with in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. This study shows that the insertion of a redox-active molecular catalyst into a MOF is a promising method to produce efficient and stable photocatalysts that are also recyclable.