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

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 CO2. Sol–gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 1015 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 CO2 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 CO2 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.