Synergistic effect of Keggin-type [Xn+W11O39](12−n)− and TiO2 in macroporous hybrid materials [Xn+W11O39](12−n)−–TiO2 for the photocatalytic degradation of textile dyes

Abstract

Macroporous titania materials functionalized with monovacant Keggin-type polyoxometalates (POMs) [Xⁿ⁺W₁₁O₃₉]^(12−n)− (XW₁₁; Xⁿ⁺ = P⁵⁺, Si⁴⁺, Ge⁴⁺) were prepared by the sol–gel as well as the template technique. Lacunary XW₁₁ clusters were incorporated into wall structures of macroporous titania, resulting in the formation of hybrid titania materials. The structural integrity of the XW₁₁ clusters in the composites was characterized by UV diffuse reflectance spectra (UV/DRS), infrared spectra (IR), inductively coupled plasma atomic emission spectrometry (ICP-AES), ³¹P MAS NMR spectroscopy and thermogravimetric analysis (TGA). These investigations indicated that the primary lacunary Keggin structures remained intact in the hybrid composites. The porous structure of the composites was demonstrated via scanning electron microscopy (SEM) and N₂ adsorption–desorption isotherms, with the pore diameters in the range of 300 to 450 nm. The photocatalytic performances of the as-synthesized composites were evaluated by the degradation of aqueous textile dyes such as Rhodamine B, methyl orange and erythrosine B. S., and the intermediates and the final products of the degradation of Rhodamine B were detected by electrospray mass spectrometer (ES-MS) and ion chromatography (IC). The as-synthesized composites showed much higher photocatalytic activity than pure TiO₂ and pure POMs, which has been attributed to the synergistic effect resulting from the combination of POMs and TiO₂.