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

Abstract

Three 3D, anionic Cd(II) porphyrinic metal-organic frameworks (PMOFs), 1(3.5:1)-2d-120 ([Cd3·2(H2TCPP)2][(CH3)2NH2]1.6), its Li+-exchanged product (e.g. Li+-1-120, formula: [Cd3·2(H2TCPP)2]Li1.6) and 1(10:1)-4d-120 ([Cd3·7H2TCPP)2][(CH3)2NH2]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 CH3CN), both positive and negative photocurrent were observed. Mott-Schottky plot measurements indicate that they are intrinsic semiconductors. Although these three PMOFs can photocatalytically reduce CO2 to CO in the absence of Ru (bpy)3Cl2, their catalytic abilities are very weak. The roles of these PMOFs in CH3CN/trimethylamine (TEA) or triethanolamine (TEOA)/H2O (400 ​μL)/Ru (bpy)3Cl2.6H2O was found to be mainly cocatalysts, and Ru (bpy)3Cl2 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 CO2 reduction to CO. Li+-1-120 was found to be able to react with Ru (bpy)3Cl2 when sufficient amounts of H2O are present in CH3CN and TEA system, preventing the photocatalytic property of Ru (bpy)3Cl2 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 CH3CN/TEA (or TEOA)/H2O (400 ​μL)/Ru (bpy)3Cl2.6H2O/PMOF system, and strategies to improve the photocatalytic performance of CO2 reduction to CO were proposed.