Thin-film transmission IR spectroscopy as an in situ probe of the gas–solid interface in photocatalytic processes

A thin-film transmission infrared spectroscopy system was designed as an in situ probe for heterogeneous interfacial photocatalytic reactions. A photocatalyst, TiO2, supported on a silicon wafer substrate was located in an airtight cell containing BaF2 windows. This supported photocatalyst was simultaneously illuminated with both UV light and the infrared beam. The performance of the cell was tested using the photocatalytic decomposition of ethanol. The quality of the spectra for a porous TiO2 film of approximately 0.5 microns of thickness proved to be good for detecting bands of reactant and product species (ethanol, ethoxide, acetaldehyde, and acetate). This transmission cell was coupled to a gas phase infrared cell for the simultaneous analysis of reactants and products of this photodegradation of ethanol which appear in the gas phase. The affinity of the surface of TiO2 for water was investigated under air and argon environments in both the presence and the absence of UV illumination. In an air environment, UV illumination enhanced the adsorption of water on TiO2. A similar enhanced adsorption occurred in the argon environment without UV illumination. During UV illumination, bands of peroxide (O22−) species could be detected on the surface of TiO2 under the conditions of a dry air flow. Both peroxide (O22−) as well as superoxide (O2−) species could be detected after several hours in post-irradiated systems.