Studies of N2O Adsorption and Decomposition on Fe–ZSM-5

Abstract

The interactions of N2O with H–ZSM-5 and Fe–ZSM-5 have been investigated using infrared spectroscopy and temperature-programmed reaction. Fe–ZSM-5 samples with Fe/Al ratios of 0.17 and 0.33 were prepared by solid-state exchange. It was determined that most of the iron in the samples of Fe–ZSM-5 is in the form of isolated cations, which have exchanged with Brønsted acid H+ in H–ZSM-5. The infrared spectrum of N2O adsorbed on H–ZSM-5 at 298 K exhibits bands at 2226 and 1308 cm−1 associated with vibrations of the N–N and N–O bonds, respectively. The positions of these bands relative to those seen in the gas phase suggest that N2O adsorbs through the nitrogen end of the molecule. The heat of N2O adsorption in H–ZSM-5 is estimated to be 5 kcal/mol. In the case of Fe–ZSM-5, additional infrared bands are observed at 2282 and 1344 cm−1 due to the interactions of N2O with the iron cations. Here, too, the directions of the shifts in the vibrational features relative to those for gas-phase N2O suggest that the molecule adsorbs through its nitrogen end. The heat of adsorption of N2O on the Fe sites is estimated to be 16 kcal/mol. The extent of N2O adsorption on Fe depends on the oxidation state of Fe. The degree of N2O adsorption is higher following pretreatment of the sample in He or CO at 773 K than following pretreatment in O2 or N2O at the same temperature. Temperature-programmed decomposition of N2O was performed on the Fe–ZSM-5 samples and revealed that N2O decomposes stoichiometrically to N2 and O2. A higher activity was observed when the catalysts were pretreated in He than when they were pretreated in N2O. For the He-pretreated samples, the activation energy for N2O decomposition was estimated to be 42 kcal/mol and the preexponential factor of the rate coefficient for this process was found to increase with Fe/Al ratio. This trend was attributed to the increasing autoreducibility of Fe3+ cations to Fe2+ cations with increasing Fe/Al ratio.