Poly-Single-Atom Photocatalysts on UiO-66 Scaffolds for Superior CO₂ Conversion under Visible Light

Conventional molecular photocatalysts, such as bipyridine-based complexes, offer high adaptability and selectivity for the CO₂ reduction reaction (CRR). However, due to their homogeneous phase operation, they face challenges such as rapid charge recombination, low performance, and recycling difficulties. Herein, we introduce an approach for knitting poly-single-atom photocatalysts (SAPCs) on metal-organic frameworks' (MOFs) external surface via on-surface photopolymerization of bipyridine (bpy)-based ligands onto a UiO-66 framework. By using [2,2'-bipyridine]-4,4'-diyl diacrylate monomer (A₂bpy), our approach efficiently produces poly-bipyridine ligands (poly-[bpy]) as a brush shell around the MOF nanocrystal's core. The poly-bipyridine brush is then postmetalated with Re(CO)₃Cl, to obtain a hybrid photocatalyst. The advanced structural analyses demonstrate high yields of both ligand photopolymerization and postfunctionalization (>45%) processes. Interestingly, the pores' accessibility of the UiO-66 core is well preserved, facilitating reactants' (e.g., H₂O/CO₂/HCO^3-) absorption and release. The hybrid heterogeneous photocatalysts displayed 8 times higher activity for the photocatalytic CRR under simulated sunlight, with respect to the homogeneous catalyst (Re(bpy)(CO)₃Cl), used as a benchmark and tested under the same experimental conditions. The better performance is attributed to several factors such as the UiO-66 pore affinity toward CO₂/carbonate adsorption, the active site's accessibility, and the enhanced charge separation within the hybrid system.