Pigmentary TiO2: a challenge in the world of photocatalysis

Titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index, in which it is surpassed only by a few other materials. More than 5 million tons of pigmentary TiO2 are consumed annually worldwide, and this number is expected to increase as consumption continues to rise. When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors and some gemstones like "mystic fire topaz". TiO2 is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets) as well as most toothpastes. Opacity is improved by optimal sizing of the titanium dioxide particles. The world of photocatalysis always focused the attention on ultra-small semiconductor particles that has become one of the fastest growing research areas in physical chemistry. Nanosized TiO2 is extremely efficient towards the degradation of pollutants both in air and in water and also possess enhanced photoredox chemistry [1]. However, in the last period, a growing uncertainty raised upon the use of nanosized powders due to possible risks towards the human safety as our tissues are not able to recognize crystallites of so small sizes which can penetrate into the human body either from skin or directly by breathing. To avoid this, a research on the possibility to use TiO2 characterized by larger-sized crystallites began a couple of years ago demonstrating the possibility to employ such a material for the photodegradation of pollutant molecules in the gas phase [2]. In the present work, the photoactivity towards the degradation of NOx in the gas phase was evaluated comparing several commercial pigmentary powdered TiO2 as both nanosized and microsized samples. In all cases samples were chosen with the following physico-chemical features: pure anatase, uncoated surface, undoped material. Samples were fully characterized investigating morphological, textural and surface properties and the results were correlated with those obtained by two commercial TiO2 samples specifically sold as photocatalytic powders (P25 by Evonik and PC105 by Cristal). All pigmentary samples are photoactive and we found a good correlation between photocatalytic performance and physico-chemical features. Obviously photocatalytic powders show the highest efficiencies towards the photodegradation of NOx especially at high pollutant amount.