Post-Synthetically Cluster-Metalated Hf-MOF for Aqueous Photocatalytic CO2 Reduction to Methanol

Recent advancements in single-site heterogeneous catalysis have attracted immense attention due to molecular-level control over structure-support interactions. Herein, a high-surface-area, thermally stable Hf-MOF-808, {[Hf6(μ3-O)5(μ3-OH)3(H2O)2(BTC)2(HCOO)5]}n (Hf-MOF), is postmodified with catalytically active NiII sites, and the photosensitizing [Ru(MBA)(bpy)2]Cl2 (MBA: 2-(5'-methyl-[2,2'-bipyridin]-5 yl)acetic acid, bpy: 2,2'-bipyridine) moiety via covalent grafting to develop the single-integrated photocatalytic system named Hf-Ni-MBA-Ru-MOF. Postsynthetic NiII metalation in the secondary building unit (SBU) and its coordination environment are characterized by several techniques, including X-ray absorption spectroscopy (XAS). The catalyst showed highly selective multielectron-proton CO2 reduction to methanol, yielding 404 μmol·g-1 after 10 h of visible-light irradiation. The high selectivity and production rate depicted by Hf-Ni-MBA-Ru-MOF in aqueous media under visible light are highly promising for sustainable CO2 conversion. The confinement of the [Ru(MBA)(bpy)2]2+ photosensitizer inside the MOF enhances the lifetime of the photoexcited electron, thereby increasing the efficacy of the catalytic CO2 reduction by transferring the excited-state electron to catalytically active NiII centers grafted to the Hf6-SBU . The excited-state electron transfer to the catalytic NiII site is explored by femtosecond transient absorption spectroscopy (FSTA) analysis supported by density functional theory (DFT) calculations. Furthermore, the intermediates in the multielectron reduction of CO2 to methanol are identified by the in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy study.