Photocatalytic Activity And Mechanism Of Surface Modified TiO₂ With Borate And Heteropoly Acid

Using semiconductor photocatalytic technology to eliminate environmental organic pollutants is a kind of effective and clean way. Among plenty of photocatalysts, TiO2 has attracted much attention as it is effective, stable, economic and eco-friendly. However, TiO2 photocatalysis still suffers from some disadvantages, for example, the low quantum efficiency could not satisfy the practical application. Therefore, there have been taken many ways for improving the photocatalytic activity of TiO2. This thesis mainly focuses on the surface modification of TiO2 with borate and PW11Fe (iron(III) substituted polyoxotungstate). The effects of borate and PW11Fe on the photocatalytic activity of TiO2 for organic degradation in water, and the related mechanism have been discussed. The main contents and results are as follows:In the first part, we report that the addition of sodium borate to the aqueous suspension of anatase TiO2 at neutral pH can result in a significant enhancement in the rate of phenol degradation. Under the condition of 8.0 mM B4O72-, pH 7.0, the apparent rate constants of phenol degradation is 3.22 times as much as TiO2. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H2O2 decomposition, and chromate reduction in the presence of phenol. This borate-induced rate increase for phenol degradation was determined not only by the amount of borate adsorption but also by the structure of borate species (pH effect). A (photo)electrochemical measurement with the TiO2 film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O2 were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO2 was negatively shifted by 81 mV. Since the hole oxidation of water to O2 remained unchanged, it is proposed that a borate radical is produced, followed by regeneration through phenol oxidation. This borate-mediated hole transfer would promote the electron transfer to O2 and consequently improve the efficiency of the charge separation for phenol degradation at interfaces.In the second part, iron(III) substituted polyoxotungstate(PW11Fe) was synthesized by using the literature method. Phenol and 2,4-dichlorophenol were used as reacting substrates. In an aerated aqueous solution with initial pH from 2.0 to 7.0, PW11Fe/TiO2 was always much more active than TiO2, benificial to wastewater treatment. As the amount of PW11Fe increased, the rate of phenol degradation increased and then decreased after reaching a maximum at 2.0 mM PW11Fe. Moreover, during phenol degradation, several hydroxylated intermediates were produced, whose concentrations are greatly enhanced upon the addition of PW11Fe. Through phenol degradation under N2, it became clear that PW11Fe might be capable of promoting the electron transfer from the irradiated TiO2 to O2. This would result into improvement in the efficiency of the charge separation, and consequently in the rate of phenol degradation.