Reaction products and kinetics of the reaction of NO_2 with γ-Fe_2O_3

摘要

The reaction of NO_2 with Fe_2O_3 has relevance for both atmospheric chemistry and catalysis. Most studies have focused on hematite, α-Fe_2O_3, as it is the thermodynamic stable state of iron oxide; however, other forms of Fe _2O_3 naturally occur and may have different chemistries. In this study, we have investigated the reaction products and kinetics for NO _2 reacting with γ-Fe_2O_3 powder using diffuse reflectance infrared Fourier transform spectroscopy and compared the results to those of previous studies of NO_2 reacting with α-Fe_2O_3. Both α- and γ-Fe _2O_3 produce surface-bound nitrate at the pressures examined in this study (24 - 212 mTorr); surface-bound nitrite products are observed at all pressures for γ-Fe_2O_3 whereas nitrite was only observed on α-Fe_2O_3 at lower pressures. Surface-bound NO~+ and Fe-NO products are observed on γ-Fe_2O_3, which have not been observed with α-Fe_2O_3. The reaction kinetics show a first-order dependence on NO_2 pressure and this is used to support the hypothesis of unimolecular reaction of adsorbed NO_2 with the γ-Fe _2O_3 surface as the slow step in the reaction mechanism. The difference in product formation between NO_2 reacting with γ-Fe_2O_3 and previous studies of α-Fe _2O_3 illustrate the fact that care must be taken in generalizing reactivity of different polymorphs.