FTIR and TPD Study of the Room Temperature Interaction of a NO–Oxygen Mixture and of NO2 with Titanium Dioxide

In the present paper, kinetics and mechanism of NO and oxygen coadsorption on TiO2 at room temperature, which is the key step of the catalytic removal of NOx pollutants from air, were studied. NO adsorption on TiO2 in the absence of oxygen is weak and reversible, but it is found to strongly increase in the presence of oxygen. The ratio between the amount of adsorbed NO and O-2 in the course of adsorption is constant and close to three. A FTIR spectroscopic study reveals that the amount and composition of N-containing species on the TiO2 surface strongly depend on the contact time with the initial NO-O-2 mixture and on its composition. At relatively small exposures, IR bands assigned to NO- and nitrosyl complexes Tin+-NO (n = 3-4) are predominant in the spectra. With increasing contact time, NO- disappears, and IR bands of NO3- and possibly NO2- appear and grow. The thermal stability, of surface nitrates and nitrites correlates with their structure. IR spectra observed upon NO2 adsorption are similar to those after exposure to NO-O-2 mixtures. Exposure of the sample with preadsorbed (NO2)-N-14 to gaseous (NO)-N-15 results in a change in the IR spectra that suggests isotopic replacement of N-14 with N-15 in the adsorbed species. In the TPD profiles, after adsorption of NO-O-2 and NO desorption peaks of NO and NO2 dominate which presumably arise from the thermal decomposition of NO3- (NO2-) and nitrosyls Tin+-NO. A multistep scheme for the interaction of NO and O-2 with TiO2 is suggested which accounts for the results of both techniques applied.