Preparation Of Visible Light Responsive Photocatalyst Bi₂₀ TiO₃₂ And Its Mechanism For Degradation Of Organic Pollutants

The important mission in using photocatalysis for the water treatment is to find photocatalysts with excellent performance. Preparation of visible light responsive catalyst is the key to make the best use of sunlight and put it into application, so the synthesis of high performance photocatalysts which is visible light responsive is the main subject.Bi and Ti are both cheap and non-poisonous metal which are called"green metal". Compound of Bi₂O₃ and TiO₂ can form many crystalline phase. One of the bismuth titanate is Bi₂₀TiO₃₂, which has not been studied systematically yet. In this subject, bismuth titanate Bi₂₀TiO₃₂ was prepared and characterized. Its performance and mechanism for photocatalytic degradation of organic pollutants was discussed.First of all, Bi₂₀TiO₃₂ was prepared by sol-gel method. Optimum experiments demonstrate that when the precursor ratio of Bi/Ti is M, the catalyst crystal can be made by being calcined at 300℃for 30 minutes. Under this condition, the main crystalface of Bi₂₀TiO₃₂ (201) will grow better, and the other peaks of it also develop well, while impurity peaks are very small. Higher calcination temperature or longer retention time will result in the loss of oxygen, which causes the transformation of crystalline phase. Under other precursor ratio of Bi/Ti, Bi₂₀TiO₃₂ can not develop well or more impurity phase will grow up. The prepared catalyst Bi₂₀TiO₃₂ is characterized to be nanocones, with the size around 90nm, and it is easy to coagulate. Its BET surface is 7.96m2/g, and zero point charge pH is tested to be 7.9. The curves of UV-diffuse reflection spectra and surface photovoltage spectra indicates Bi₂₀TiO₃₂ is highly visible light responsive that its sensitive wavelength extends to 530nm, namely the energy gap is 2.34 eV. It was calculated that the valence band is at 2.58V, so the holes'potential there is high enough to oxidize organics. The conduction band is at 0.24V, and the Fermi level was tested to be at 0.622V, which is close to the conduction band. It means that electrons have low work function and the catalyst has strong ability to supply electrons.Experiments on the comparison of the catalytic activity of the prepared catalysts were made. The results show that Bi₂₀TiO₃₂ has stronger adsorption ability, and its catalytic performance is higher than the mingled bismuth titanate. Under the irradiation of xenon lamp and the same weight of catalyst dose, the apparent rate constant of oxidation by P25 is higher than that of Bi₂₀TiO₃₂ for the degradation of phenol and methyl orange. Due to smaller specific surface area of Bi₂₀TiO₃₂, the specific activity of Bi₂₀TiO₃₂ is higher than that of P-25. While under the visible light irradiation (>400nm), the photodegradation speed of phenol and methyl orange by Bi₂₀TiO₃₂ is much faster than that achieved by P25. The apparent rate constant of degrading phenol by Bi₂₀TiO₃₂ is 0.0133min^-1, which is 33.25 times higher than that achieved by P25 (0.0004min^-1). The apparent rate constant of degrading methyl orange by Bi₂₀TiO₃₂ is 0.0119min^-1, which is 3.13 times higher than that achieved by P25 (0.0038min^-1). The experimental results demonstrate that Bi₂₀TiO₃₂ is qualified to be a favourable photocatalyst.The component and crystal structure of the photocatalyst exert a tremendous influence on the performance, while external factors also affect its activity, and optimization of the conditions will help to give a better place to play its role. It is found that being illuminated by the short wavelength light can get better degradation result. Enhancing the intensity of the light contribute to the degradation rate. pH value has remarkable impact on the degradation and on the catalyst in many aspects, the most important one of which is that the catalyst surface will be directly affected by the charge under different pH. That the catalyst is positively or negatively charged will affect its ability to attract organic pollutants, so for different types of organics, the optimum pH is different. Acidic condition is good for the degradation of methyl orange, while neutral is good for phenol oxidation. Aeration goes against the degradation of methyl orange or phenol, because the air bubbles cut off the contact between reactant and the catalyst. The optimum reaction temperature is 40℃, comparing with the result at 20℃and 60℃. Because lower temperature is disadvantageous for the molecular movement and for desorption of intermediate product from the surface. While higher temperature does not facilitate adsorption of the reactant. Adding less than 4% (V/V) of hydrogen peroxide restrained the degradation of methyl orange. When the hydrogen peroxide concentration is larger than 6% (V/V), the more the hydrogen peroxide, the faster the photocatalyic degradation speed is. The degradation of methyl orange is depressed by the ions in the tap water and river water. On the contrary, the degradation speed of methylene blue is promoted in the tap water and river water,which may be related to the oxidation mechanism. The important aspect in the study of the mechanism of photocatalytic process is the active species generated during the oxidation, because it is related with the reaction site on the catalyst, the attacked position of organics, the corresponding degradation pathways and the degradation products. GC-MS analysis shows the photodegradation product of 4-hydroxybenzylalcohol (HBA) by Bi₂₀TiO₃₂ is 4-hydroxybenzaldehyde (HBZ), which is the result of the cooperation of h+ and·OH.In order to investigate if there are different main active species for different pollutants in photodegradation by Bi₂₀TiO₃₂, three kinds of organics were chosen as the target pollutants, namely anion methyl orange, cation methylene blue and neutral phenol. The testing was made by adding scavenger of the active species. Experiments indicate that in deionized water, anion methyl orange has strong adsorption ability on the Bi₂₀TiO₃₂ catalyst surface, and holes are the main species, so the reaction site is on the surface of the catalyst. Superoxide anions also help to degrade methyl orange, but they are not the main species. It is basically the same with the case of phenol. Cation methylene blue is not easy to adsorbe onto the catalyst surface, and hydroxyl radicals are the main active species. The reaction site may be on the surface of the catalyst or in the solution near its surface.In tap water or river water, the degradation rate of methyl orange and methylene blue change differently, which has direct relation with the transformation of active species. In tap water or river water, owing to the competitive adsorption of anions, the utilization rate of holes is reduced promptly, so the degradation rate of methyl orange droped sharply. At this time, the main active species are not holes any more, but are secondary hydroxyl radicals. While due to the anions'medium function, methylene blue can adsorb onto the catalyst surface easily, and can be oxidized efficiently by holes. At this time, the main active species are holes instead of hydroxyl radicals. The quantization proportion of holes is higher, namely the concerntration of holes is larger, so the degradation rate speeds up.Another key problem for photocatalyst application is inactivation. It's found that the crystal phase doesn't change after reaction, which shows the structure of the catalyst is stable. Using ethanol and ultrasound can recover its activity, as illustrates that inactivation is just due to adsorption of intermediate products. Calcination can remove the intermediates and maintain the structure of the crystal, so it is also an effective method for the regeneration of catalyst.