Transmission FT-IR and Knudsen cell study of the heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles

The heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles was investigated. In particular, spectroscopic and kinetic measurements have been made to investigate surface reactions of NO2 on Al2O3, Fe2O3, and TiO2 at 298 K. Both gas-phase and surface-bound products are formed from the reaction of NO2 with these mineral oxide particles. At low coverages, FT-IR spectra of the mineral oxide surface exposed to gaseous NO2 show absorptions due to surface nitrite, specifically a chelating nitrito species. As the coverage increases, the surface becomes populated with surface nitrate bonded in several different bonding coordinations (monodentate, bidentate, and bridging). The predominant gas-phase product is NO, although there is a small amount (<1%) of detectable N2O. A Knudsen cell reactor coupled to a quadrupole mass spectrometer was used to measure the uptake coefficient, gamma, for NO2 on these oxide particles and to characterize gas-phase product formation. The Knudsen cell data showed NO to be the major gas-phase product with a delay in the onset of NO production. There was little production of N2O and no gas-phase HONO or HNO3 was detected. By monitoring the reaction until completion, the ratio of NO2 reacted to NO produced was determined to be similar to 2:1. These results complement the FT-IR data and suggest a two-step mechanism in which NO2(g) is initially adsorbed as a nitrite species which subsequently reacts with additional NO2 to form surface nitrate and gas-phase NO. Finally, the initial uptake coefficient was determined from the Knudsen cell data for NO2 on Al2O3, Fe2O3, and TiO2. Because NO2 can diffuse into the underlying layers of these oxide particles, the use of a geometric area does not give accurate values of the uptake coefficient. Gas diffusion must be taken into account to more accurately determine the uptake coefficient.