Adsorption of Nitrogen Oxides on TiO2 Surface as a Function of NO2 and N2O5 Fraction in the Gas Phase.

The present study was devoted to the investigation of adsorption of nitrogen oxides on TiO2, with the focus on the effect of NO x concentration, composition, and flow rate. The inlet NO with a concentration of 200-800 ppm in pure N2 was mixed with ozone, produced from pure oxygen, and directed to a reactor with catalytic TiO2 powder. The oxidation of NO by ozone allowed to prepare mixtures with variable concentrations of NO and NO2 or NO2 and N2O5 which were adsorbed on the catalyst surface during the oxidation phase and were desorbed when only NO was flowing through the reactor. Diffuse reflectance infrared Fourier transform spectroscopy studies showed NO3- as the main adsorbed nitrogen oxide specimen on the surface. The amount of adsorbed nitrogen oxide species increased with the increasing fraction of NO2 in the gas phase and was inversely proportional with the gas-phase NO concentration. An important finding was the abrupt increase in the nitrogen oxide adsorption capacity of TiO2 when the inlet concentration of ozone became sufficiently large to oxidize NO x to N2O5. On the basis of the results, a model of the surface processes is proposed, involving the production of NO3 and N2O5 on the surface of TiO2.