Photocatalytic removal of nitrogen oxides from air on TiO2 modified with bases and platinum

The efficiency of TiO2 (Degussa P-25) modified with an alkaline admixture (urea, BaO), sulfuric acid, or platinum in the photocatalytic oxidation of NO (50 ppm) with a flowing 7% O2 + N2 mixture under UV irradiation in a flow reactor at room temperature and atmospheric pressure is reported. Because of the progressive blocking of active sites of the photocatalyst by the reaction products (NO2, NO 3 − ), it is impossible to realize prolonged continuous removal of NO x (NO + NO2) from air without catalyst regeneration at elevated temperatures. The efficiency of the photocatalysts is characterized by specific photoadsorption capacity (SPC) calculated from the total amount of NO x adsorbed during 2-h-long irradiation. Modification of TiO2 with 5% BaO or 5% urea raises the SPC of the catalyst by a factor of 2–3. Presumably, this promoting effect is due to the basic properties of these dopants, which readily sorb NO2 and NO 3 − . A considerable favorable effect on SPC is also attained by adding 0.5% Pt to (5% BaO)/TiO2. The SPC of the (0.5% Pt)/TiO2 catalyst depends on the state of the platinum. The samples calcined in air at 500°C, which contain Pt+ and Pt2+, have an approximately 2 times higher SPC than unpromoted TiO2 and ensure a much larger NO2/NO ratio at the reactor outlet. Conversely, the samples reduced in an H2 atmosphere at 200°C, whose platinum is in the Pt0 state, show a lower SPC than the initial TiO2 and cause no significant change in the NO2/NO ratio.