Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films

Abstract

The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO 2. Sol–gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 10 15 molecules of SA cm −2 ≡ 1 cm −1 integrated areas of the peaks in the FT-IR of SA over the range 2700–3000 cm −1, which is three times that reported previously by others. As the SA disappeared the concomitant amount of CO 2 generated was >90% that expected throughout the photomineralisation process for the sol–gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO 2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≅100% that expected based on the amount of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.