Synergetic Photoelectrocatalytic Degradation Of Organic Pollutants And Its Cathodic Processes

TiO₂ photocatalyst has been the focus of numerous investigations since 1972, particularly because of its application for the chemical conversion of light energy, the photocatalytic treatments of organic compounds, photochemical synthesis, the hydrophilicity of light inducement of interface and the synthesis of self-cleaning material. Due to the complete mineralization of a wide variety of organic compounds to CO₂, H₂O and inorganic constituents in aqueous/air media, the photocatalytic oxidation of TiO₂ is regarded as an interesting advanced oxidation technique. However, there are still some scientific and technical problems in the photocatalysis technology based on the TiO₂ semiconductor at present. One of the problems is that the holes and electrons excited by light are easily recombinated and lost their activities. Therefore, it is important to improve the efficiency of seperation of the charge carriers and the degradation of organic compound. Furthermore, Pt is always used as the cathode, and there are few reports about other materials used as the electrode. The mechanism of cathodic processes and the relationship with anodic reaction are even less researched deeply. Based on the above consideration, the anodic and cathodic reaction behavior and their dependance have been studied. This dissertation includes three parts.In the first part, a comparison of photoelectrocatalytic degradation of salicylic acid and aniline over anatase and rutile TiO₂/Ti photoanodes was investigated. The results showed that the photooxidation rates of the two compounds over anatase photoelectrode were larger than that at rutile photoelectrode, but this difference in rate was subtle for the photodegradation of salicylic acid compare to aniline. The rutile TiO₂ electrode had larger light to electricity conversion efficiency but lower faradaic efficiency compared to the anatase electrode.In the second part, the behavior of photoelectrocatalytic generation of H₂O₂ in cathode and degradation of aniline, using over TiO₂/Ti as photoanode and different electric materials as cathode, was investigated both in single and double reactors. The results showed, the yield of H₂O₂ can be improved by increasing applied potential,photocurrent and pH value, but controlled by photocurrent density, while it can be decreased in the presence of metal ions, like Cu2+ and Fe2+ etc. The cathodic processes and reaction types were mainly controlled by the anodic reactions. The cathodic reactions affect the anodic reactions by changing the quantity of photocurrent, but did not essentially change the reaction processes and mechanisms. Consequently, the anodic reaction was the rate-determined step in the whole photoelectrocatalytic reactions. Moreover, with graphite as cathode, it generated less H2O2, when using anatase TiC>2 as photoanode, compare to rutile as photoanode, but the capability of oxidating organic compound was better. Although when using the rutile TiC>2, it can produce more H2O2 compare to anatase, the capability of oxidation of H2O2 was much lower than the active species generated in anode. Thus the whole efficiency of degradation of organic compound was lower than that of anatase. When Pt was used as cathode, it generated less H2O2 than that of graphite as cathode, and the efficiency of degradation of aniline in the cathode was also lower.In the last part, we have studied the kinetics of photoelectrocatalytic degradation of aniline in anode and reduction of Ag cation in cathode. It was the anodic reactions that still determined photoelectrocatalytic reactions. Though there were a lot of factors infecting the reduction of Ag cation in cathode, this idea is viable and the maximal efficiency of recovery of Ag from solution was as estimated to be high as 98.43 %.