Photocatalytic Degradation Of Acetone Over VO_x/LaF₃ Catalyst

The photocatalytic degradation of organic pollutants has attracted considerable attention in air and water purifications. Since the nontoxicity and chemical stability of TiO₂, T1O2 and other metal ion doped TiO₂ have been extensively studied as photodegradation catalysts. However, because of its wide band gap energy (E_g≈3.2 eV), the TiO₂ photocatalyst is known to be active only under ultraviolet lights (λ< 380 nm) and there is only about 2% of ultraviolet light in solar-light on the surface of earth. Hence, in order to make use of solar light as the light source in photodegradation reaction, a visible light active photocatalyst is desired.In the current investigation, VO_x/LaF₃ photocatalysts were designed for photodegradation of acetone under visible light. The effects of vanadium concentration and calcination temperature of VO_x/LaF₃ photocatalysts on the photodegradation of acetone in visible light were investigated by analyzing the product with gas chromatography. Our investigation proved that VO_x/LaF₃ photocatalysts are active and stable for photodegradation of acetone in visible lights. 16% VO_x/LaF₃ calcined at 673 K has the highest activity and the acetone conversion reached 58.4% under visible light. X-Ray diffraction (XRD), Laser Raman, UV-Vis spectra, N₂ absorption (BET specific surface area), and Fourier infrared (FT-IR) spectroscopic techniques have been employed to characterize the structure and composition of the VO_x/LaF₃ catalysts. The photocatalytic activities of VO_x/LaF₃ catalysts were compared and photocatalytic mechanism was proposed.The influence of specific surface area on catalytic activity is not themost important factor. The catalytic activity of the catalysts is found to besensitive to the phase structure of the catalysts. The XRD, Laser-Raman,and FT-IR characterizations of the VO_x/LaF₃ catalysts indicate that thereare V₂O₅ and LaVO₄ phases formed in VO_x/LaF₃ catalysts. The catalystscalcined at relatively low temratue (from 573 K to 673 K) contain V₂O₅and LaF₃ phase. At even high calcined temperature (723 K), V₂O₅domains are transferred to LaVO₄ phase. The UV-Vis spectra of thecatalysts show that the VO_x/LaF₃ catalysts calcined at relatively lowtemperature (lower than 723 K) have strong absorption in visible lightswith wavelength below 600 nm, which indicates that these catalysts areactive under visible light. This is consistent with the photocatalyticactivity investigations. The UV-vis spectrum shows that the absorptionedge of catalysts calcined at higher temperatue shifted to shorterwavelength, and these catalysts shows very low photodegradationactivities for acetone degradation reaction in visible light. V₂O₅ domains highly dispersed in LaF₃ matrix are active phase. The LaVO₄ phase is not active in the photodegradation reaction of acetone. Since that V₂O₅ has a band gap of about 2.0 eV, the visible light could excit electrons on V₂O₅ to generate e^--h⁺ pairs. LaF₃ (E_g= 10.3 eV) was employed as a matrix to hold V₂O₅ domains and to retard the recombination of e^--h⁺ pairs. Hence, the doping of V₂O₅ into LaF₃ matrix prolongs the life-time of e^--h⁺ pairs to let chemical reactions occur.