N−Doped WO3 Photocatalyst by Thermal Decomposition of Ammonium Paratungstate (APT)

Titanium dioxide (TiO₂) might have wider applications in our daily life than you think. It has been applied in daily products, such as paints, toothpaste, food additive, dyes, sun creams, varnishes, textiles, paper and plastics. Another important application of TiO₂ is as a semiconductor. It has the function to purify air, water and soil polluted with various hazardous chemicals.

Nonetheless, TiO₂ photocatalysts has a shortage, where it requires a UV light irradiation wavelengths within 390 nm. In our previous reports, the visible light responsive TiO2 photocatalysts could be successfully prepared by using ion engineering techniques such as ion implantation or RF-magnetron sputtering deposition methods. Besides, the N-doping within the TiO2 semiconductor has recently been known to be effective for the preparation of the visible light responsive photocatalysts. In contrast, a tungsten trioxide (WO₃) semiconductor, has been considered as a photocatalyst functioning under visible light irradiation. However, since the photo-excited electrons in the conduction band of WO₃ semiconductor cannot reduce O₂, it has been believed that photocatalytic oxidation of organic compounds hardly occurs on such bare WO₃ surfaces. Nevertheless, WO3 loaded with Pt or CuO cocatalysts have recently been introduced to show the photocatalytic function under visible light irradiation.

In this study, the N-doped WO₃ semiconductor was prepared to expand its operation in visible light regions. The synthesis steps are as follows:

The N-doped WO₃ powders were prepared by thermal decomposition of ammonium paratungstate (APT), at 473–1073 K. Small quantity of Pt particle was then loaded on the N-doped WO₃ surface by a photodeposition method. The products were denoted as Pt(X)/N-WO3(Y) (X: loading amount of Pt (wt%), Y: calcination temperature (K)). A commercial N-free WO3 powder was used as a reference. The prepared N-WO3 samples were then characterized by XRD, diffuse reflectance UV–vis absorption (Shimadzu, UV-2200A) and XPS measurements at room temperature.

The photocatalytic reactivity of the Pt/N-WO3 samples was evaluated by decomposition of gaseous methanol under visible light or sunlight irradiations. The Pt/N-WO3 catalysts (50 mg) were placed onto a quartz cell (volume, ca. 33cm3).                                                     

The catalysts were degassed under high vacuum at 723 K for 2 h, treated in sufficient amounts of O2 at the same temperature for photocatalyst in the cell was cooled in a water bath. However, the water temperature increased up to 323–333 K during the photoreaction under sunlight irradiation. The amounts of CO2 produced in the photocatalytic reaction were then analyzed by a gas chromatography.

Conclusion

A novel N−doped WO₃ semiconductor could be prepared by a simple calcination of an ammonium paratungstate containing NH+ ions. The NWO3 prepared at 673–873 K efficiently absorbed visible light in longer wavelength regions as compared to a commercial N-free WO₃. The N-WO without Pt deposition did not show any photocatalytic reactivity. However, the Pt/N-WO3 samples showed much higher photocatalytic reactivity to decompose methanol of ca. 1000 ppm in gas phase under visible light having a wavelength longer than 450 nm or focused solar light irradiations. As well as, the Pt cocatalyst loaded on the N-WO₃ surfaces was found to work as an efficient oxidation catalyst at low temperatures of 323–333 K. These results indicate that the newly prepared N-WO₃ photocatalyst loaded with Pt cocatalyst can effectively utilize sunlight as light and/or heat sources.

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