Identification by isotopic exchange of oxygen deposited on Fe/MFI by decomposing N2O

Isotopic exchange was studied at 250degreesC between O-18(2) and Fe/MFI that was prepared by sublimation and subsequently exposed to (N2O)-O-16. 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 500degreesC before dissociative adsorption of N2O. However, different adsorption sites are formed by reduction at T greater than or equal to 600degreesC or evacuation at 700degreesC after reduction at 500degreesC. After such treatment the dissociative adsorption of N2O results in a very active oxygen species, identified by rapid isotopic exchange with O-18(2) at 250degreesC. The exchange kinetics of these O-16 atoms with O-18(2) is of the R-1 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 H-2-TPR spike at 200degreesC. Adsorbed water blocks Fe sites for isotopic exchange. (C) 2002 Elsevier Science (USA).