Enhancement effect of TiO2 dispersion over alumina on the photocatalytic removal of NOx admixtures from O2-N2 flow

The efficiencies of bulk TiO2 Degussa P-25 and TiO2 dispersed over alumina support in the photocatalytic removal of NOx (NO + NO2) admixtures from flowing NO-O-2-N-2 mixture modeling polluted air are compared. The TiO2/Al2O3 samples with different Ti contents prepared by the sol-gel method are characterized by BET, XAFS and diffuse-reflectance UV-vis spectroscopy. At a TiO2 content above approximately 20 wt%, a thermally stable anatase phase is formed in TiO2/Al2O3 in which the Ti ions are six-coordinated. As the TiO2 content diminishes from similar to 20 to 1 wt%, the titanium coordination number gradually decreases, and the TiO2 phase is no longer detected. Most likely, isolated Ti ions in different coordinations are predominant in such samples. It is found that, upon UV irradiation, NO oxidation to NO2 in flowing NO-O-2-N-2 readily occurs over bulk TiO2 and TiO2/Al2O3 at TiO2 loadings higher than similar to 20 wt%. No photoreaction is observed on Al2O3 without TiO2 and on TiO2/Al2O3 at TiO2 loadings below similar to 10 wt%. Correlating these results with the XAFS and UV-vis data, it is concluded that the presence of a TiO2 phase on the alumina surface is a prerequisite for the photocatalytic activity in the NO oxidation. The outlet concentrations of NO and NO2 as a function of irradiation time are measured, and absorption capacities of the photocatalysts to NOx are quantitatively assessed. The performance of TiO2/Al2O3 is superior than that of bulk TiO2, because much larger amounts of NO2 formed by the photoreaction can be strongly trapped by the support. The highest adsorption capacity per unit catalyst weight is found for 50% TiO2/Al2O3. The NOx adsorption capacity of bulk TiO2 is found to increase upon humidification of the reaction gas mixture. (C) 2007 Elsevier B.V. All rights reserved.