The composite films [Xn+W11O39](12−n)−/SiO2 (X = Si, Ge, P) (abbreviatedXW11/SiO2) were prepared by tetraethoxysilane (TEOS) hydrolysis sol-gel method via a spin-coating technique. Mono-vacant Keggin-type polyoxometalates (POMs) [Xn+W11O39](12-n)- were the inorganic precursors used in this method. Formation of the composite films is due to chemical grafting of organic silanol groups to the surface oxygen atoms at the vacant sites of [Xn+W11O39](12-n)-, resulting in the saturation of the surface of the lacunary POM. Therefore, a coordination structural model of the films was proposed. As for the films, retention of the primary Keggin structure was confirmed by UV-VIS, FT-IR, and MAS NMR spectra. The surface morphology of the films was characterized by scanning electron microscopy (SEM): the film surface is highly uniform, and the layer thickness is in the range of 250-350 nm. Aqueous formic acid (FA) (0-20 mmol/l) was degraded and mineralized into CO2 and H2O by irradiating the films in the near-UV area. The disappearance of FA follows Langmuir-Hinshelwood first-order kinetics.