Effects of phase composition and surface area on the photocatalytic paths on fluorinated titania

The effect of TiO2 surface fluorination in the photocatalytic degradation of the azo dye Acid Red 1 (AR1) was investigated employing a series of TiO2 photocatalysts including P25, a rutile and high surface area anatase samples. The effects of (i) pH lowering to 3.7 and (ii) TiO2 surface fluorination at this pH on the reaction rate under 254 nm or polychromatic, mainly visible light, irradiation and on AR1 adsorption on the photocatalyst surface were taken into account separately. In the case of P25 and rutile TiO2 photocatalysts the reaction rate did not vary upon pH lowering in the absence of fluoride, but almost doubled upon fluorination at pH 3.7, an effect to be attributed exclusively to the surface fluorine-induced modification of the reaction paths. By contrast, in the case of high surface area anatase samples a pH lowering from natural conditions down to 3.7 led to a remarkable increase of the reaction rate, which however was clearly inhibited upon surface fluorination at pH 3.7, with a trend perfectly matching that of AR1 adsorption on anatase. Thus, reaction paths favored by direct interaction of the substrate with the photocatalyst surface appear to prevail for high surface area anatase photocatalysts with respect to those which take advantage of a less reactive (and less adsorptive) fluorinated photocatalyst surface.