The Influencing Factors And Mechanism Research Of Photocatalysis And Photosensitization With Titania

The energy and environmental problems must be treated seriously by human being in 21th century. Because of the effective reactions using solar energy as driven force at room temperature, photocatalytical technology is considered to be the most popular way for the production of clean energy and detoxification of organic pollutants. Many semiconductor photocatalysts have been used widely in air and water decontamination, and TiO2 is the most promising one among them. In addition, the charge transfer in metal phthalocyanine sensitized TiO2 system will occur under visible irradiation, which is taken as the best method for TiO2 modification.This thesis focuses on the intrinsical mechanism research of photocatalysis and photosensitization, and we give our special idea on how to evaluate the relationship between the real and apparent photoactivity. Various active species generated during the photoreactions have been characterized at the same time. Based on the results we gained, a new photocatalyst of Fe modified TiO2 with high activity has been designed. Four sections are contained as follows.(1) The crystallinity degree is the only factor that will determine the intrinsic photocatalytic activity of TiO2. Ag+ was chosen as the electron acceptor for the photodegradation of 4-chlorophenol (4-CP) in the N2-purged aqueous suspension of TiO2 under UV light. With TiO2 samples prepared from different methods, it was found that the initial rates of 4-CP degradation, per the initial amount of Ag+ adsorbed, were almost the same at a given sintering temperature (Ts), and increased exponentially with Ts. It can be deduced that with the same amount of electron scavengers on the catalyst surface, different crystal phases of TiO2 actually have similar photocatalytic abilities at the same Ts. The observed variation in photoactivity among the samples is usually ascribed to difference in sorption capacity towards O2 in water. It recommended that to evaluate the photocatalytic activities of different TiO2 samples in an aerated aqueous suspension, the adsorption ability of O2 should be taken into account.(2) A new photocatalyst of iron modified TiO2 (Fe/TiO2) was prepared through adsorption-ashing two-step method, using Fe(Ⅲ) tetracarboxyphthalocyanine and anatase as the predecessor. Iron dots were highly dispersed and tightly attached onto the external surface of TiO2 in the form of Fe(Ⅲ)xOy network. Very trace of Fe loaded on TiO2 will accelerate the photodegradation of phenol and chlorophenol efficiently. Due to the aggregation of Fe clusters, the photoreaction was inhibited seriously as the Fe amount increased. Fe may act as a scavenger to trap the conduction band electrons, consequently to improve the charge separation efficiency. The stability of the catalyst is considerable expressed by the recycling experiment, and the lixiviation of iron hasn't been detected during the longtime photoreaction. For the (photo)-Fenton reaction initiated by Fe(Ⅲ) and H2O2, the activity of this composite photocatalyst was much more prominent. In addition, the photocurrent response enhanced obviously while a little Fe was joined on TiO2 surface, which proved again that the existence of Fe was in favor of the photoinduced charge transfer.(3) Aluminum tetracarboxyphthalocyanine (AlTCPc) adsorbed on TiO2 has been examined as an effective sensitizer for degradation of many aromatic pollutants in water under visible light (λ≥450 nm). The reaction rates is greatly influenced by experimental parameters, such as the physical properties of TiO2, the loading amount of dyes, the type and concentration of the substrates, the solution pH, and the center metal in phthalocyanine, etc.. The optimal loading of AlTCPc on TiO2 is all about 1.0 wt%, nearly independent of TiO2 chosen. The dye cation, superoxide, and hydroxyl radicals originated from the photosensitization were confirmed through ethanol quenching, changing electron captures and EPR spin trapping. In addition, the results demonstrates for the first time that the O2 capability of the solids is predominant to the sensitized reaction, and the activity of these composite sensitizers is not consistent with the photocatalytic properties of TiO2 itself.(4) The intensive research into AlTCPc-TiO2 was carried out imitating the first part. In the photodegradation of 4-CP with dye loaded TiO2 in aerated aqueous suspensions, anatase was much more active than rutile. and the photoactivity was consistent with total pore volume change as Ts increased, namely that the porous structure fits for the photosensitized reactions, while crystallinity seems to have no effect on photosensitization anymore. In N2-purged suspensions, the initial rates of 4-CP degradation, corrected by the initial amount of Ag+ adsorbed, increased first and then decreased with Ts. We deduce that the crystallinity degree is still the determined factor of the intrinsic photosensitized activity in AITCPc-TiO2 system, but the extraneous factors are much more complicated. The existence of dye on the TiO2 surface restrained the adsorption of electron captures and other active species, and the aggregation of dye is also bad for photosensitization, while these negative effects will be aggravated when TiO2 has small surface areas. It is noteworthy that in this composited system, the porous configuration and the O2 capability of TiO2 will dominate the apparent photoactivity.