Reaction Products and Kinetics of the Reaction of NO2 with γ-Fe2O3

The reaction of NO2 with Fe2O3 has relevance for both atmospheric chemistry and catalysis. Most studies have focused on hematite, alpha-Fe2O3, as it is the thermodynamic stable state of iron oxide; however, other forms of Fe2O3 naturally occur and may have different chemistries. In this study, we have investigated the reaction products and kinetics for NO2 reacting with gamma-Fe2O3 powder using diffuse reflectance infrared Fourier transform spectroscopy and compared the results to those of previous studies of NO2 reacting with alpha-Fe2O3. Both alpha- and gamma-Fe2O3 produce surface-bound nitrate at the pressures examined in this study (24-212 mTorr); surface-bound nitrite products are observed at all pressures for gamma-Fe2O3 whereas nitrite was only observed on alpha-Fe2O3 at lower pressures. Surface-bound NO and Fe-NO products are observed on gamma-Fe2O3, which have not been observed with alpha-Fe2O3. The reaction kinetics show a first-order dependence on NO2 pressure and this is used to support the hypothesis of unimolecular reaction of adsorbed NO2 with the gamma-Fe2O3 surface as the slow step in the reaction mechanism. The difference in product formation between NO2 reacting with gamma-Fe2O3 and previous studies of alpha-Fe2O3 illustrate the fact that care must be taken in generalizing reactivity of different polymorphs.