Microenvironment regulation of Ru(bda)L2 catalyst incorporated in metal-organic framework for effective photo-driven water oxidation

Abstract

A Ru(bda)L2 molecular catalyst (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, L represents ligands) was incorporated into a UiO-66 metal-organic framework (MOF) as isolating sites for chemical- and photo-driven water oxidation with the water nucleophilic attack mechanism. An impressive turn-over number of 566 was obtained for photo-driven water oxidation in the phosphate buffer saline, which was nearly 20 times that of the homogenous counterpart in the presence of ruthenium tris(bipyridine) and sodium persulfate. Infrared spectroscopy, X-ray diffraction and X-ray spectroscopy techniques were used to characterize the composition and structure of the catalysts. Kinetic isotope effect (KIE), proton inventory experiment and proton nuclear magnetic resonance were conducted to understand the photo-driven O2 evolution mechanism. It was observed that the protons participated in O2 evolution with a normal KIE value, and the uptake of phosphates by UiO-66 matrix improved the affinity towards H2O via hydrogen bonding. The high O2 output and the limited photosensitizer oxidative decomposition in PBS suggest that the phosphate acted as a proton mediator to assist proton and/or proton-couple electron transfer through a hydrogen-bonded network of water molecules. This study uses a heterogenous matrix to emulate an enzyme-like microenvironment capable of achieving efficient artificial photosynthesis.