Preparation Of Keggin-type Transition Metal-substituted Heteropolyoxometallate/TiO₂ Film And Its Visible Photocatalytic Performance

Water environmental pollution caused by the toxic organic compounds has been a serious threat to human survival and health. Up to now, the advanced oxidation process is a better method to remove organic pollutants in water compared to the other conventional ones, because this process can oxidatively degrade and mineralize organic pollutants into non-toxic organic small molecules and CO2 and H2 O, does not cause the secondary pollution. Among the advanced oxidation processes, photocatalytic oxidation process is more superior and ideal because of being able to utilize the solar energy, but the key is to find the ideal photocatalyst. In the exploration and research of photocatalyst, Ti O2 has caused the most extensive attention and research interest due to low cost, high chemical stability and environment friendly, and is a photocatalyst with the most application prospect. However, the pure Ti O2, with a wide band gap(3.2 e V), can only absorb ultraviolet light, and the utilization efficiency towards solar energy is low because ultraviolet light in sunlight only accounts for less than 5%. Although the light absorption wavelength of Ti O2 can be extend to the visible light area and the light energy conversion efficiency has also been improved obviously(6~12%) through non-metallic and metal elements doping as well as organic dye or quantum dot surface modification, the efficiency is still very low compared to that of the green plants photosynthesis.Therefore, exploration and study of novel photocatalytic system is still very necessary and meaningful.As a novel photocatalytic system, a novel photocatalyst PW11M/Ti O2 nano film was prepared on the surface on a glass slide via sol-gel dipping pulling method using the Keggin type transition metal substituted heteropoly compound(PW11M) as a visible light active component and Ti O2 as a supporting architecture components in this paper. The light absorption properties, chemical structure, thermal stability, crystalline phase and surface morphology of the as-prepared catalysts were characterized using UV-vis DRS, IR, Raman, TG-DTG, XRD, SEM and TEM etc. At the same time, their visible photocatalytic activities were evaluated using the degradation of model pollutants such as rhodamine B(Rh B) and nitrobenzene(NB) as a probe. A novel photocatalysis mechanism was thus suggested and compared with that of the Ti O2 photocatalyst. The influences of the alcination temperature, the PW11 M dosage and the solution p H on the photocatalytic activity were also examined. Finally, the photocatalytic stability of the PW11M/Ti O2 nano film was assessed through the cyclic degradation experiment.The experimental results show that the PW11M/Ti O2 film has an obvious absorption in the visible range in addition to ultraviolet light. The absorption wavelength varies with the change of the substituted transition metal M. The surface structure and morphology of the film is closely related to the calcination temperature. When the calcination temperature is 100 ℃, the PW11M/Ti O2 film is amorphous. With increase of the calcination temperature, the PW11M/Ti O2 film converts gradually from the amorphous state to the crystalline state. Up to 500 ℃, the film is composed of “rice grain-shaped” crystals with a size of 100 ~ 600 nm. When used for degradation of the model organic pollutants, the PW11M/Ti O2 nano film showed an obvious visible photocatalytic activity. For the PW11Fe/Ti O2 system, for example, the degradation ratio of nitrobenzene was 92% at the reaction time of 60 min under visible light irradiation and the TOC removal reached 30% at 4 h; for the PW11Cu/Ti O2 system, the degradation ratio of Rh B was 98% at 60 min and the removal was 32% at 4 h; for the PW11Cr/Ti O2 system, the degradation ratio of Rh B was 88% and the COD removal was 72%; for the PW11Mn/Ti O2 system, the corresponding value was 99% and 33%, respectively. Electron paramagnetic resonance and hydroxyl radicals quenching experiments showed that hydroxyl radicals are the main reactive oxygen species that cause the degradation and mineralization of organic pollutants. When enhancing the calcination temperature, the photocatalytic activity of PW11M/Ti O2 was reduced, but increasing the amount of catalyst and the acidity of solution is in favour of improvement of the degradation rate of organic pollutants. After 10 times of recycles for degrading organics, the visible photocatalytic activity of PW11M/Ti O2 nano film was of little loss.