Active sites of HZSM-5 with low Fe content for the formation of surface oxygen by decomposing N_2O:is every deposited oxygen active?

摘要

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 ot 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 shap 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.Teh active O atoms were assigned to the paired termainl 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.