Active Sites in HZSM-5 with Low Fe-content for the Formation of Surface Oxygen by Decomposing N2O: Is Every Deposited Oxygen Active?
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 N2O 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 N2O was active in CO oxidation at 523 K and appeared as sharp O2 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 [Fe2O2H]+ 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 N2O 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 [Fe2O2H]+ center, the remaining acting as a single Fe site.
Keywords: Fe-containing HZSM-5 ; a-Oxygen ; Ferryl groups in zeolite ; [Fe2O2] diamond core ; Monooxygenase ; N2O decomposition ; Oxo-bridged species ; Transient response method ; Temperature-programmed desorption of oxygen
Record created on 2006-04-18, modified on 2016-08-08