Photocatalytic Activity And Mechanism Of Inorganic Anion-Brookite And Cobalt Phosphate-Anatase For Organic Oxidation In Water

Semiconductor photocatalysis and technology for environmental remediation have been widely studied over more than 30 years. Among various semiconductors, titanium dioxide has attracted much attention as it is effective, stable, low-cost and non-toxic. However, the photogenerated charge carries of a semiconductor easily recombine to heat, and consequently the quantum yield is still not high enough to enable practical application. This thesis mainly focuses on the surface modification of brookite TiO₂ with four inorganic anions and anatase TiO₂ with cobalt phosphate. The modified TiO₂ shows a higher activity than TiO₂ for the photocatalytic degradation of organic substrate in aqueous solution. A possible mechanism the observed effect of these co-catalysts is proposed. The thesis mainly includes two parts as follows:In the first part, the effect of PO₄^3-, F^-, B₄O₇^2-, SO₄^2- on the brookite-photocatalyzed reaction has been examined for the first time. Through a hydrothermal method, brookite TiO₂ was prepared. By using phenol and 2,4-dichlorophenol as model substrates, the catalyst activity was evaluated. Upon the addition of 10 mM PO₄^3-、F^-、B₄O₇^2-、SO₄^2-, the rates of phenol degradation over TiO₂ were increased by 3.9,2.6,0.6 and 0.4 times, respectively. Such anion-induced rate enhancement of phenol degradation changed with the initial pH and anion concentration of the suspension, and had a positive correlation with the amount of anions adsorbed on brookite. Moreover, these anions also showed a positive effect on the formation of coumarin-OH adduct, a fluorescent probe of OH radicals. However, upon the addition of these anions, the brookite photocatalyzed formation and decomposition of H₂O₂ in aqeuous suspenions were all inhibited, ascribed to decrease in the amount of O₂ and H₂O₂ adsorption on brookite, respectively.In the second part, the effect of cobalt phosphate （CoPi） on the anatase-photocatalyzed reaction has been systematically studied. The catalysts were prepared by a photocatalytic method, including CoPi-deposited TiO₂ （CoPi/TiO₂）, Pt-deposited TiO₂ （Pt/TiO₂）, and CoPi-deposited Pt/TiO₂ （CoPi/Pt/TiO₂）. The catalyst activity was evaluated from the photocatalytic degradation of phenol, 4-chlorphenol（CP）, and 2,4-dichlorophenol（DCP） in 0.1 M K3PO4. In all reactions, CoPi/TiO₂ was less active than TiO₂, and Pt/TiO₂ was more active than TiO₂. Strikingly, CoPi showed a positive effect on the Pt/TiO₂-photocatalyzed degradation of CP and DCP, but not phenol. Similar activity trend among the catalysts was also observed from the photocatalytic reduction of O₂ to H₂O₂ in the presence of DCP. However, for DCP degradation in neutral aqueous solution without K₃PO₄, CoPi/Pt/TiO₂ became less active than either Pt/TiO₂ or TiO₂, ascribed to photo-dissolution of a detrimental Co²⁺ ion. It is proposed that the CoⅣ species formed from the hole oxidation of CoⅡ/Ⅲ in CoPi are surface-bound, and short-lived. They react only with the surface adsorbed organics on the oxide in the presence of both Pt （electron sink） and phosphate （CoPi repairer）. Moreover, there is a mutual promotion between the CoPi-mediated hole transfer and the Pt-mediated electron transfer, greatly improving the efficiency of the charge separation of TiO₂, and consequently increasing the rates of organic oxidation and O₂ reduction.