Mechanistic origin of the different activity of Rh-ZSM-5 and Fe-ZSM-5 in N 2O decomposition

Abstract

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N 2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N 2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N 2O, N 2, and O 2. Classical reaction schemes failed to describe the experimental data. The preferred models of N 2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O 2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N 2O participate in the decomposition of gas-phase N 2O. Gas-phase O 2 is formed directly on N 2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O 2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N 2O adsorption and, most importantly, the faster desorption of O 2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N 2O activity.