The development of a highly active TiO 2 photocatalyst for energy and environmental use is a great challenge. In this work, we report that the addition of sodium borate to an aqueous suspension of anatase TiO 2 at neutral pH can result in a significant enhancement in the rate of phenol degradation. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H 2 O 2 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 TiO 2 film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O 2 were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO 2 was negatively shifted by 81 mV. Since the hole oxidation of water to O 2 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 O 2 and consequently improve the efficiency of the charge separation for phenol degradation at interfaces.

Various methods that aim to improve the photocatalytic activity of TiO2 have been reported in the literature. Herein, we report that addition of CuWO4 into the aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO4 increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO4 was about 2.83 times that in the absence of CuWO4. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO4/TiO2 was much more stable than CuO/TiO2, due to the very high stability of CuWO4 against photocorrosion. The improved activity of TiO2 is not due to CuWO4 and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H2O2 in aqueous solution, CuWO4/TiO2 was also more active than TiO2. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO2 to CuWO4 that facilitates the charge separation of TiO2 and consequently accelerates reactions at interfaces.

UV irradiation of Au/TiO2 photocatalysts in the presence of borate and phosphate anions can produce H2O2 at a millimolar level in alkaline water solution. The positive effect of the anions is ascribed to the anion-mediated hole transfer from Au/TiO2 to an electron donor which thus accelerates the two-electron reduction of O2 to H2O2.

Carbonate anions are often present in aqueous solution, but their effect on the semiconductor-photocatalyzed reaction has been rarely studied. In this work, we report a positive effect of Na 2 CO 3 on the TiO 2 -photocatalyzed degradation of phenol, 2,4-dichlorophenol, and H 2 O 2 in an aerated aqueous suspension at initial pH 8.0. The rate of phenol degradation, upon the addition of 2.0 mM Na 2 CO 3 , 0.52 wt% Pt, and 2.0 mM Na 2 CO 3 plus 0.52 wt % Pt, was increased by 1.78, 3.38, and 6.63 times, respectively. Such positive effect of carbonate was also observed from a TiO 2 and Pt/TiO 2 film electrode for the photoelectrochemical oxidation of phenol, but not water. However, the rates of phenol degradation over TiO 2 and Pt/TiO 2 became decreased as carbonate concentration exceeded 5.0 and 2.0 mM, respectively. It is proposed that CO 3•− radicals are formed mainly from the hole oxidation of dicarbonate adsorbed on TiO 2 , followed by phenol degradation. At a high concentration, the CO 3•− radicals would recombine to a peroxocarbonate that easily decomposes into CO 2 and O 2 . The carbonate-mediated hole transfer from TiO 2 to phenol would incorporate with the Pt-mediated electron transfer from TiO 2 to O 2 , consequently resulting in a great improvement in the efficiency of charge separation for the reactions at interface.

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