Active sites in HZSM-5 with low Fe content for the formation of surface oxygen by decomposing N 2O: is every deposited oxygen active?

Abstract

Surface-active centers were detected in HZSM-5 with a low content of iron (<1000 ppm) activated by steaming and high-temperature calcination in inert atmospheres (up to 1323 K). These centers lead to the formation of surface oxygen (O) ad species from N 2O and were characterized quantitatively by the transient response method. The total amount of active centers was proportional to the content of iron in the zeolites. Only a part of (O) ad deposited by decomposing N 2O was active in CO oxidation at 523 K and appeared as sharp O 2 peaks at 666 K during the TPD measurements. A binuclear Fe center is suggested featuring a “diamond core” structure, similar to that of the monooxygenase enzyme, as an active center. The active O atoms were assigned to the paired terminal oxygen atoms each bonded to one Fe site (ferryl groups) in the diferric [Fe 2O 2H] + cluster. Oxygen pretreatment at 823 K decreases somewhat the total amount of active sites but does not affect the ratio of active/inactive oxygen. Zeolite presaturated by water vapor at 473–523 K generates (O) ad species from N 2O completely inactive in the CO oxidation. Part of it appears as a broad peak at 940 K in the TPD profile. The total amount of the deposited oxygen corresponds to half of the stoichiometric amount of the surface Fe atoms and suggests that water blocks a half of the binuclear [Fe 2O 2H] + center, the remaining acting as a single Fe site.