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 CHxOy(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 CHxOy(a) species.