Pigmentary TiO2 : a challenge for its use as photocatalyst in NOx air purification

The photocatalytic degradation of NOx in the gas phase was investigated comparing several commercial TiO2 sold as pigmentary-powders and characterized by crystallite sizes ranging from nano to micrometer dimensions. In particular the photocatalytic activity of the micro-sized sample was evaluated in comparison with the well-known activity of the nano-sized samples, being these last photocatalysts potentially dangerous due to the risk towards the human safety. The studied samples were precisely chosen among different commercially available products on the basis of the following features: pure anatase, uncoated surface, undoped material, not sold as photocatalytic materials. All samples reveal good photoactivity in the photodegradation of NOx in gas phase with an evident superiority of the nano-sized sample. However, the gap of activity between nano and micro-sized samples tends to be canceled when the starting NO2 concentration was reduced and fixed from 1000 to 200 ppb, a precise amount that is the first alert threshold for NO2 in air (World Health Organization). A proper kinetic model, based on the Langmuir–Hinshelwood mechanism and on the hypothesis of irreversible adsorption of the products on the catalysts surface, has been developed and discussed.