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Abstract: The hydrothermal stability of WIE-based Fe-ZSM-5 and Fe-BEA for N2O decomposition in the presence of NO, H2O and O2 was studied at 723K. Catalysts were characterized with UV–vis, IR and Al-XANES. The N2O conversion remains at a constant level with Fe–H-BEA even in the presence of large amounts of H2O. Deactivation was, however, observed with Fe–H-ZSM-5 already in the presence of 0.5vol.% water. The analysis of deactivated samples indicated that steam-dealumination and clustering of iron occurred during the reaction with Fe-ZSM-5. Replacement of protons for sodium in Fe-ZSM-5 slowed down the rate of catalyst deactivation significantly. Steam-dealumination is likely to precede the clustering of iron in Fe–H-ZSM-5. The excellent hydrothermal stability of WIE-based Fe–H-BEA relates to the high initial degree of dealumination in the as-received BEA. [Copyright &y& Elsevier]

Abstract: NO-assisted N2O decomposition over four different Fe-ZSM-5 samples prepared by wet ion exchange (WIE) or chemical vapor deposition (CVD) of FeCl3 was investigated by steady-state kinetics, in situ infrared (IR) spectroscopy, and transient response methods. Despite their lower iron loading, the samples prepared by WIE had the highest activity for N2O decomposition in the presence of NO. The in situ IR experiments showed that the most active sample was characterized by a high concentration of adsorbed NO, as well as adsorbed NO2 under reaction conditions. Step response experiments proved that NO2 is an intermediate of the catalytic cycle and functions as intermediate oxygen storage. IR and transient kinetic experiments showed that WIE catalysts behave qualitatively different in NO-assisted N2O decomposition than CVD catalysts. These differences are discussed in terms of the different structure of the iron species in the two types of samples. [Copyright &y& Elsevier]

Iron-containing zeolites were prepared by aqueous ion exchange, sublimation of FeCl3, or hydrothermal synthesis and were modified by steaming at 873 K. The samples contained different kinds of iron species ranging from isolated iron atoms to small clusters that contain a few atoms and somewhat larger oligomers. Yet, the dynamic behavior of all samples in N2O decomposition was qualitatively similar. Pulse experiments showed that a slow and fast pathway of O2 formation, which is the rate-limiting step of the reaction, exists. Both contributed to the overall activity. The fast pathway dominated on isolated iron sites, whereas the contribution of the slow pathway increased on catalysts with high iron loadings. Steaming led to an agglomeration of the iron clusters and increased the activity (with one exception). The steamed samples had a peculiar time-on-stream behavior. They exhibited a very high initial activity and then slowly decayed to steady state. This indicates that steaming did not merely agglomerate the iron clusters, but qualitatively changed the nature of the active sites. [Copyright &y& Elsevier]

The mechanism of N2O decomposition over an 18O-labeled Fe-ZSM-5 catalyst was studied. The isotope distribution of the O2 produced during N2O decomposition was statistical. This shows that O2 was formed from a pool of equilibrated surface oxygen atoms. The size of the pool was 1.0 O/Fe, i.e. most or all of the iron clusters took part in the reaction. The reactivity of the oxygen atoms in the pool was low. They did not exchange with 18O2 to a significant extent. [ABSTRACT FROM AUTHOR]

The reaction of Fe-zeolites prepared by sublimation of FeCl3 with N2O was studied by step- and pulse-response experiments. Steps from 0 to 5000 ppm N2O were performed at 673 K, after pretreatment of the catalyst in H2, He, or O2. When the catalysts were prereduced in H2, N2O was first consumed for the reoxidation of the catalyst, before steady-state N2O decomposition started. Also pretreatment in He reduced the catalyst. This autoreduction was not spontaneous, but caused by a small amount of hydrocarbons adsorbed on the catalyst. Prereduced catalysts had a higher steady-state activity, indicating that the incorporation of oxygen from N2O changed the structure of the active sites. Pulse responses proved that O2 desorption is the rate-limiting step of N2O decomposition. O2 desorption was relatively fast from a small fraction of sites, while a second oxygen pool desorbed only slowly from the catalyst. Only the former is responsible for catalytic activity. [Copyright &y& Elsevier]