Vacuum ultraviolet photocatalytic degradation of volatile organic compounds : from material exploration to practical application

Abstract

Volatile organic compounds (VOCs) are classified as the major contributor to air pollution. Many strategies have been studied for VOCs removal, and photocatalytic oxidation (PCO) has been regarded as one of the most promising technologies, due to its environmentally friendliness, low cost, and energy consumption. Nevertheless, there are still several obstacles limiting its commercial application, including low efficiency, poor catalyst stability and difficulty in scale-up. To improve the efficiency of photocatalytic degradation of VOCs, a heterojunction catalyst of nanostructured TiO2/MnO2 was fabricated in this study, and toluene was chosen as a target VOC for the performance testing under the irradiation of vacuum ultraviolet (VUV) lamps. TiO2 nanosheets (NSs) were firstly fabricated by a hydrothermal method in the presence of fluorine (F), then MnO2 were coated on the TiO2 NSs via an in-situ synthesis method after removing the F-. TiO2/MnO2 exhibited excellent performance for both VOCs (96.0%) and residual ozone (O3) removal (99.9%), due to the formation of heterojunction structure and the synergetic effect of photocatalysis and ozonation of the TiO2/MnO2 photocatalyst. Next, F- in TiO2 NSs was found to benefit the PCO of toluene degradation. Therefore, the role of F- doping was systematically investigated. In this section, TiO2 NSs with different amounts of F- (denoted by F-TiO2) ranging from 0.62% to 8.12% (Atomic %) were synthesized. It was found that the optimum (7.04%) removal efficiency could reach 90.0% F-TiO2, which was much higher than that of pristine TiO2. The F- doping could effectively inhibit the recombination of photogenerated electron-hole pairs and enhance light absorption ability of the catalyst, especially in the UVC region. To further improve the removal efficiency of toluene and the residual O3, α-MnO2 nanowires were added again. Finally, a higher toluene degradation efficiency of 96.0 % was achieved with only 1 wt.% α-MnO2, and the O3 was completely eliminated. Afterwards, catalyst immobilization was further studied to facilitate the operation and minimize the catalyst dosage, including carbon cloth, nickel (Ni) foam, graphene oxide and glass beads. Among them, graphene oxide exhibited the best performance for both toluene degradation and O3 decomposition. However, the high cost and instability limited its practical application. Therefore, Ni foam was regarded as the best choice for commercialized air purification, due to its low cost and easy operation as well as a satisfactory toluene degradation and ozone removal efficiencies. Finally, to realize the commercial application of photocatalytic degradation of VOCs, scale-up synthesis of catalysts was investigated for commercial applications. A low-cost and efficient mesoporous Mn/CeO2 catalyst was fabricated via a scalable colloidal solution combustion method coupled with a dip-coating process. Under the VUV light irradiation, a high toluene removal efficiency of about 92.0% was achieved with a reaction rate of about 118 μmol/g/h. It was found that the quantities of Ce3+, Mn2+ and Mn3+ species of Mn/CeO2 play a key role in toluene oxidation and O3 decomposition. Later, 600 g catalysts were synthesized and exhibited a stable activity for toluene oxidation. Furthermore, the mechanism of toluene degradation over Mn/CeO2 was studied and discussed.