Transient studies on the mechanism of N2O activation and reaction with CO and C3H8 over Fe-silicalite

The mechanism of the reaction of N2O and 13CO over Fe-silicalite was investigated with the use of the temporal analysis of products (TAP) reactor and compared with that of the reaction of N2O and C3H8 previously reported (Appl. Catal. A 267 (2004) 181). Upon direct N2O decomposition at 523-573 K, Fe-silicalite stored ca. 1018 atoms of oxygen per gram, with a ratio of 1 O atom per each 30-60 Fe atoms in the sample. Only a small fraction of the deposited oxygen was reactive for CO oxidation. Pump-probe experiments at different time delays (0-2 s) between the pulses of nitrous oxide and the reducing agent indicated the markedly different mechanisms of the N2O-13CO and N2O-C3H8 reactions in the temperature range of 623-673 K. Fe-silicalite is active for propane oxidation in the presence of short-lived oxygen species, that are produced when N2O and C3H8 are pulsed simultaneously. Time delays between the N2O and C3H8 pulses greater than 0.1 s are sufficient to transform these active oxygen species for hydrocarbon conversion into inactive ones. In contrast, the oxidation of CO by N2O does not depend on the lifetime of the oxygen species in the range of time delays investigated. The mechanisms of the N2O-mediated 13CO and C3H8 oxidations differ as a consequence of the different interactions of the two reducing agents with iron species in the zeolite. Pulse experiments support the occurrence of the scavenging mechanism with both propane and carbon monoxide. In this mechanism, short-lived oxygen deposited by N2O is efficiently eliminated by the reductant. Distinctive to propane, the strikingly high affinity of carbon monoxide for isolated Fe3+ ions in the zeolite gives rise to an additional pathway for N2O reduction in the presence of chemisorbed CO species. These particular Fe3+-CO species were identified by in situ UV/vis and EPR spectroscopies (J. Catal. 223 (2004) 13).