Photocatalytic Conversion Nitrogen Oxides Over Titania

Using commercial available titania (TiO₂) spread on pieces of glass as catalyst, the effects of amount of catalyst, contact area, light intensity, nitrogen oxides concentration, flow rate, moisture, oxygen content and crystallite size and crystalline phase of titania on photocatalytic degradation of nitrogen oxides were examined, and a possible photocatalytic degradation mechanism was inferred by the fact that the product of the photocatalytic reaction was nitric acid proved by Fourier Transform Infrared Spectrometer. Kinetic equation of photocatalytic reaction of nitrogen oxides was established and verified. The reaction rate constant and adsorption equilibrium constant in the equation were determined.The commercial available titania doped with nonmetal element N, transition metal ions Fe³⁺ and notable metal atoms Pt were prepared. The property characterization and activity evaluation of doped TiO₂ were also performed. For the N-doped TiO₂, The crystal structure of TiO₂ was not changed after calcinations process. Analysis by X-ray photoelectron spectroscopy (XPS) indicated that N atoms were incorporated in the bulk phase of TiO₂ as N-Ti-O linkages. A significant shift of the absorption edge to a lower energy and a higher absorption in the visible light region were observed. These N-doped TiO₂ powders exhibited photocatalytic activity for the degradation of NOx under visible light irradiation. The sample mixed with 20 wt.% ammonium carbonate and calcined in 600℃for 1 hour showed best photocatalytic activity.In Fe-doped TiO₂, Fe³⁺ accommodated into the Ti lattices sites of TiO₂ in the doping process. As we know, Fe²⁺/Fe³⁺ energy level lies close to Ti³⁺/Ti⁴⁺level; thus, Fe³⁺ can provide a shallow trap for photo-generated hole and electron in TiO₂ and enhance the photogenerated electron-hole pair separation, consequently enhance the photocatalytic efficiency. The sample mixed with 0.2 at % Fe³⁺ and calcined at 600℃for 1 hour showed the best photocatalytic activity.Pt atoms were at the surface of Pt-doped TiO₂ in the form of Pt0 and PtIIO, the photocatalytic activity of Pt-doped TiO₂ for degradation of NOx was enhanced under both unicolor and simulated nature light because of the quick transfer of photogenerated electrons in TiO₂ semiconductors to the loaded Pt particles, resulting in a decrease in electron–hole recombination, as well as in efficient charge separation. The sample photodeposited 0.5 at % platinum showed the best photocatalytic activity.For Pt, N-codoped TiO₂, N-Ti-O linkages in the lattice of it formed in impregnating and calcinations process caused the absorbance of visible light and deposits of Pt at the surface of it provided more sites for photogenerated electrons to be efficiently transferred away from the TiO₂ to the loaded Pt particles, subsequently interacted with oxygen and enhanced the photogenerated electron-hole pair separation, thus improved the photocatalytic efficiency under visible irradiation notably. The photocatalytic conversion efficiency of NOx in an air stream at a flow rate of 0.5 L/min and contained 2.0 ppm NO₂ using the sample mixed with 20 wt.% ammonium carbonate and photodeposited 0.5 at % platinum under green light irradiation was 100%.For less than 1% product gone with the air stream, photocatalyst was deactivated after long-term operation because the NO3- ions adsorbed onto the surface of the catalyst and covered the active sites. Washing the deactivated photocatalyst with water would make the photocatalyst regenerate. Therefore, TiO₂ indicates potential for air purification and energy-saving in the future.