The cocatalyst roles of three anionic Cd(II) porphyrinic metal-organic frameworks in the photocatalytic CO2 reduction to CO process carried out in Ru(bpy)3Cl2/CH3CN/H2O/Triethylamine or triethanolamine system

Three 3D, anionic Cd(II) porphyrinic metal-organic frameworks (PMOFs), 1(3.5:1)-2d-120 ([Cd 3·2 (H 2 TCPP) 2 ][(CH 3 ) 2 NH 2 ] 1.6 ), its Li + -exchanged product (e.g. Li + -1-120 , formula: [Cd 3·2 (H 2 TCPP) 2 ]Li 1.6 ) and 1(10:1)-4d-120 ([Cd 3·7 H 2 TCPP) 2 ][(CH 3 ) 2 NH 2 ] 0.6 ) were synthesized, all of which have the same PXRD pattern. Their VB and CB band-edge positions were determined in nonaqueous electrolyte as −0.59, 1.12; −0.60, 1.11; and −0.60, 1.16, respectively. Their light absorption properties in the range of 250–2500 ​nm were obtained by UV–Vis-IR reflectance spectroscopy. Transient photocurrent responses, electrochemical impedance spectra (EIS) and solid state photoluminescence (PL) spectra of the three PMOFs were obtained. When irradiated using light of 430 ​nm and −0.5-0.5 bias potentials (versus Ag/Ag + electrode in CH 3 CN), both positive and negative photocurrent were observed. Mott-Schottky plot measurements indicate that they are intrinsic semiconductors. Although these three PMOFs can photocatalytically reduce CO 2 to CO in the absence of Ru (bpy) 3 Cl 2 , their catalytic abilities are very weak. The roles of these PMOFs in CH 3 CN/trimethylamine (TEA) or triethanolamine (TEOA)/H 2 O (400 ​μL)/Ru (bpy) 3 Cl 2 . 6H 2 O was found to be mainly cocatalysts, and Ru (bpy) 3 Cl 2 was the actual photocatalyst in the presence of small amounts of water (0–400 ​μL). These PMOFs act as cocatalysts due to the increased reducing power upon light irradiation . Reducing the charge of the framework can reduce the efficiencies of the photocatalytic CO 2 reduction to CO. Li + -1-120 was found to be able to react with Ru (bpy) 3 Cl 2 when sufficient amounts of H 2 O are present in CH 3 CN and TEA system, preventing the photocatalytic property of Ru (bpy) 3 Cl 2 to be released. In addition, we found that TEOA is a worse reducing agent than TEA, leading to much lower photocatalytic efficiency of the system. Mechanisms were proposed for reactions occurred in CH 3 CN/TEA (or TEOA)/H 2 O (400 ​μL)/Ru (bpy) 3 Cl 2 . 6H 2 O/PMOF system, and strategies to improve the photocatalytic performance of CO 2 reduction to CO were proposed.