Reaction between N2O and CH4 over Fe ion-exchanged BEA zeolite catalyst: A possible role of nascent oxygen transients from N2O

Abstract

The reaction between N₂O and CH₄ over an Fe ion-exchanged BEA zeolite (Fe-BEA) catalyst was studied by using a pulse reaction technique, temperature-programmed desorption (TPD) and infrared (IR) spectroscopy. N₂O readily reacted with CH₄ in the presence of an N₂O + CH₄ mixture above 200 °C, while both the O₂ + CH₄ reaction and the catalytic decomposition of N₂O over the Fe-BEA catalyst required higher temperatures (above 400 °C). In the O₂-TPD studies, a desorption peak of O₂ was observed above 600 °C after O₂ treatment at 250 °C, while a new O₂ desorption peak appeared at the lower temperatures after N₂O treatment at 250 °C. However, the new O(a) species resulting from the N₂O treatment hardly reacted with CH₄ even at 350 °C, which was confirmed by the CH₄-pulsed experiments. On the other hand, a new IR band at 3683 cm⁻¹, which can be assigned to the OH group on Fe ion species, was observed after O₂ or N₂O treatment. The peak intensity at 3683 cm⁻¹ was not changed in the exposure of CH₄ only, but decreased in the exposure of N₂O + CH₄ mixture above 150 °C. At the same time, the CH_xO_y(a) species such as Fe–OCH₃ were formed, which were observed by IR measurements. The adsorbed surface species showed a high reactivity with N₂O even at low temperatures (∼200 °C). A possible mechanism is discussed in terms of active oxygen species such as nascent oxygen transients (O*(a)), which are formed in the exposure of N₂O + CH₄ mixture, and may play an important role in the activation/oxidation of CH₄ at initial steps to form CH_xO_y(a) species.