Surface structure and photocatalytic activity of nano-TiO₂ thin film for selective oxidation

Controlled titanium dioxide (TiO₂) thin films were deposited on stainless steel surfaces using the flame-aerosol synthetic technique, which is a one-step coating process that does not require further calcination. Solid state characterization of the coatings was conducted by different techniques, including X-ray diffraction (XRD) spectrum, scanning electron microscope, and atomic force microscope. The coated thin films were used in a gas phase photoreactor for the partial oxidation of hydrocarbons to alcohols and ketones as an alternative production method for the highly sought oxygenates. For this purpose, the oxidation reaction of cyclohexane to form cyclohexanol and cyclohexanone was chosen as a model reaction. The effects of the film thickness, anatase-to-rutile ratio, and particle morphology on the reactivity of the catalyst were studied. Experimental results revealed that there is an optimal film thickness (between 400 and 700 nm) for the photooxidation process that gave a maximum rate of reaction. The yield and selectivity of TiO₂ increased with the increase of the film thickness up to 350-400 nm. The activity decreased with a further increase in thickness. The influence of the crystallographic structure of TiO₂ on the partial oxidation of cyclohexane showed that the catalyst efficiency increased almost linearly with the increase of the anatase fraction between 20 and 95%. The coating technique that formed highly porous and soft aggregated TiO₂ thin films with rough morphology showed lower photoactivity than the technique that formed fine particle and transparent thin film.