Preparation of a reduced layered tungstic acid H_xW ₂O₇ via acid treatment of Bi₂W ₂O₉ in the presence of Sn²⁺ ions

A reduced layered tungstic acid with a double-octahedral structure was prepared by acid treatment of Aurivillius-type Bi₂W₂O ₉ in the presence of Sn²⁺ ions. While the color of the product formed by acid treatment with no Sn²⁺ ions present, H2W2O7, was yellow, a blue powder was obtained after the acid treatment in the presence of Sn²⁺ ions. No notable change in the morphology was observed after acid treatment. The X-ray diffraction pattern of the product acid-treated in the presence of Sn²⁺ ions was very similar to that of H ₂W₂O₇. Essentially all the Bi³⁺ ions were lost upon acid treatment, indicating the occurrence of selective leaching of bismuth oxide sheets in Bi₂W₂O₉. A UV-visible absorption spectrum and XPS analysis demonstrated that the W ⁶⁺ ions were partially reduced to W⁵⁺ ions, and the number of protons in the product was correspondingly 2.4 per [W₂O ₇]. These results suggest the successful formation of a reduced layered tungstic acid, H_2.4W₂O₇.