| Popis: |
The efficiencies of bulk TiO 2 Degussa P-25 and TiO 2 dispersed over alumina support in the photocatalytic removal of NO x (NO + NO 2 ) admixtures from flowing NO–O 2 –N 2 mixture modeling polluted air are compared. The TiO 2 /Al 2 O 3 samples with different Ti contents prepared by the sol–gel method are characterized by BET, XAFS and diffuse-reflectance UV–vis spectroscopy. At a TiO 2 content above approximately 20 wt%, a thermally stable anatase phase is formed in TiO 2 /Al 2 O 3 in which the Ti ions are six-coordinated. As the TiO 2 content diminishes from ∼20 to 1 wt%, the titanium coordination number gradually decreases, and the TiO 2 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 NO 2 in flowing NO–O 2 –N 2 readily occurs over bulk TiO 2 and TiO 2 /Al 2 O 3 at TiO 2 loadings higher than ∼20 wt%. No photoreaction is observed on Al 2 O 3 without TiO 2 and on TiO 2 /Al 2 O 3 at TiO 2 loadings below ∼10 wt%. Correlating these results with the XAFS and UV–vis data, it is concluded that the presence of a TiO 2 phase on the alumina surface is a prerequisite for the photocatalytic activity in the NO oxidation. The outlet concentrations of NO and NO 2 as a function of irradiation time are measured, and absorption capacities of the photocatalysts to NO x are quantitatively assessed. The performance of TiO 2 /Al 2 O 3 is superior than that of bulk TiO 2 , because much larger amounts of NO 2 formed by the photoreaction can be strongly trapped by the support. The highest adsorption capacity per unit catalyst weight is found for 50% TiO 2 /Al 2 O 3 . The NO x adsorption capacity of bulk TiO 2 is found to increase upon humidification of the reaction gas mixture. |
