Two new estertin modified tungstosilicates: synthesis, catalytic activity and photoelectrochemical property

Two new tungstosilicates (C(NH₂)₃)₅KNa₂H₃[Sn(CH₂CH₂COOCH₃)Co(H₂O)₂Si₂W₁₉O₆₉]·10H₂O (Si₂W₁₉-Co-SnRCOOCH₃) and (C(NH₂)₃)₅KNa₂H₃[Sn(CH₂CH₂COOCH₃)Mn(H₂O)₂Si₂W₁₉O₆₉]·13H₂O (Si₂W₁₉-Mn-SnRCOOCH₃) modified by organometal (OM) and transition metal (TM) ions were obtained, and they were self-assembled from mono-TM-containing (TM = Co, Mn) γ-[SiW₁₀O₃₅]^6- (γ-SiW₁₀)-based dimeric tungstosilicate and estertin Cl₃SnCH₂CH₂COOCH₃ in an acetate buffer solution, respectively. Their structures were determined with a series of physico-chemical and spectroscopic methods including X-ray crystallography, XRPD, etc. The estertin functional position in the tungstosilicate skeleton was analyzed by theoretical calculations using density functional theory (DFT). These two compounds have good catalytic activity for the oxidation of cyclohexanol to cyclohexanone. Meanwhile, they were composited with TiO₂ using a facial layer-by-layer (LBL) method. The assembled composite films {Si₂W₁₉-TM-SnRCOOCH₃/TiO₂}_n (TM = Co, Mn; n is the bilayer number, n = 1-6) exhibit efficient photoelectrocatalytic activity for the oxidation of methanol under irradiation of 100 mW cm^-2 using a Xe lamp as a light source. And the {Si₂W₁₉-Co-SnRCOOCH₃/TiO₂}₄ film produces the highest photocurrent among the composite films studied in this work. Combining the dark current measurement, surface photovoltage spectroscopy (SPV) and electrochemical impedance spectroscopy (EIS), the enhanced photoelectrocatalytic activity of {Si₂W₁₉-Co-SnRCOOCH₃/TiO₂}₄ composite film for the oxidation of methanol is attributed to the suppressed electron-hole recombination, increased carrier separation efficiency and interfacial charge transfer.