Photoelectrocatalystic Degradation Of Organic Pesticide By TiO₂ Thin-Film Electrode

Due to high toxicity, high concentration and complicated composition of wastewater from manufacturing agrochemicals, its treatment and purification have become one of the biggest challenges.As a semiconductor oxides, TiO₂ is a photocatalyst that shows high photoefficiency and oxidation activity. TiO₂ has become a hot research area because it can decompose organic pollutants into CO₂ and H₂O in gases and liquids. The TiO₂ photo-oxidation process, however, has not been widely applied due to, for instance, low electron transfer rate to oxygen and high recombination rate of electron-hole pairs and, thus, low photo-oxidation rate of organic compounds on the catalyst surface.Photoelectrocatalysis is a method that applied the bias potential to take away the photoelectrons and decrease the combination of electronholes. So the method can increase the photoeeffciency evidently. Rare earth doped on the photocatalytic activities of TiO₂ was investigated. As the model pollutant of Beta-cypermethrin(BEC) and phomix to evaluate the photoelectrocatalysis activity of thin-film electrodes, the experimental results were as follows:1. Preparation of thin-film electrodesTiO₂ and Ce⁴⁺/TiO₂ thin-film electrodes were successfully prepared on foam nickel substrates by sol-gel technique. TiO₂ sols were prepared with Ti(OC₄H₉)₄ as precusor, alcohol as solvent, acetylacetone as complexation, polyethylene glycol(PEG400) as pore-forming agent, glacial acetic acid as inhibiting agent. The needed TiO₂ films on substrate of nickel foam were prepared via a sol-gel dip-coating method. TiO₂ thin-film electrode was obtained by annealing the deposited coatings for 2h at 500℃. It was found that TiO₂ films were stabile. Meanwhile, it was also showed that photoproduction current of Ce⁴⁺/TiO₂ films electrode increased obviously by single sweep voltammetry measurements.2. Photoelectrocatalytic activities of TiO₂ filmsA new photoelectrocatalytic reactor was designed,in which 125W mercury lamp was used as light source,and CHI600b used as power. A three-electrode system was used, in which photoelectrochemical thin-film electrode was used as working electrode(WE), Pt electrode as counting electrode(CE) and a saturated calomel electrode(SCE) as the reference electrode(RE). The photoelectrocatalysis activities of TiO₂ films were evaluated by the photoelectrodegradation of BEC and phomix, and the effects of various conditions on the photoelectrocatalytic characteristics were investigated. The experimental results were as follows:(1)BEC was choosed as the target. The experimental results showed that when heat-treated temperature was 500℃, the film layers was 5, concentration of BEC was 45mg·L^-1,initial pH was 6.08, concentration of NaCl concentration was 0.01mol·L^-1, potential was 0.6V(vs. SCE), degradation rate and COD removal were 80.3% and 71.6% in 120min, respectively. The results showed that the degradation reaction of the BEC almost followed Langmuir-Hinshehwood model, and the photoelectrocatalysis reaction is a first-order reaction at lower concentration ((?)90mg·L^-1). The reaction rate constant k₀ and the absorption constant K were determined to be 2.166 mg·(L·min)^-1 and 2.214×10^-2 L·mg^-1, respectively.(2)Phomix was choosed as the target. Photoelectrocatalytic activities of TiO₂ electrode was evalued by COD removal efficiency.It's found that when heat-treated temperature was 500℃, concentration of phomix was 70mg·L^-1,the film layers was 5, initial pH was 6.25, concentration of NaCl was 0.0lmol·L^-1, potential was +0.6V(vs. SCE), COD removal was 63.6% in 120min.3. Photoelectrocatalytic activities of Ce⁴⁺/TiO₂ filmsBEC was chosen as the research object.A three-electrode system was used,in which the photoelectroncatalytic thin-film electrode was used as working electrode, Pt electrode as counting electrode and a saturated calomel electrode as the reference electrode. Photoelectrocatalytic activities were evalued by 125W mercury lamp and 25W fluorescent lamp as light source, respectively.The results showed that when doping amount of Ce⁴⁺ was 0.5%(molar fraction), heat-treated temperature was 450℃, the film layers was 5, concentration of BEC and NaCl were 45mg·L^-1 and 0.01mol·L^-1 respectively,initial pH was 6.01, 125W mercury lamp as light source, potential was +0.1V(vs. SCE), degradation rate of BEC was 92.1% in 120 min.Compareing to 25W fluorescent lamp as light source with the same other conditions, the degradation rate of BEC by using TiO₂ films was 3% in 120min. When Ce⁴⁺/TiO₂ films was used, however,the rate was even reached 73% in 120min.