Identification by Isotopic Exchange of Oxygen Deposited on Fe/MFI by Decomposing N2O

Isotopic exchange was studied at 250°C between 18O2 and Fe/MFI that was prepared by sublimation and subsequently exposed to N216O. The pretreatment of the Fe/MFI, prior to its contact with N2O, is crucial for the kinetics of the subsequent isotopic exchange. Little exchange is observed with materials that were merely reduced at 500°C before dissociative adsorption of N2O. However, different adsorption sites are formed by reduction at T≥600°C or evacuation at 700°C after reduction at 500°C. After such treatment the dissociative adsorption of N2O results in a very active oxygen species, identified by rapid isotopic exchange with 18O2 at 250°C. The exchange kinetics of these 16O atoms with 18O2 is of the R1 type, as expected for ferryl oxygen ligated to Fe4+ ions in Fe/MFI. Only part of the oxygen from N2O displays this exchange activity; the number of the rapidly exchanging oxygen atoms is equal, within experimental error, to the number counted by integrating a sharp H2-TPR spike at 200°C. Adsorbed water blocks Fe sites for isotopic exchange.