Search

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]

Abstract: The mechanism of decomposition of N2O on Fe-ZSM-5 prepared by chemical vapor deposition was studied by step and pulse response experiments and by isotope labeling. The results show that only a very small fraction of the iron sites is taking part in the catalytic cycle. N2O deposits its oxygen atoms on these sites, the deposited oxygen atoms migrate over the iron cluster, exchange with other oxygen atoms and then desorb by recombination to O2. On strongly dehydroxylated iron sites also a direct recombination of two oxygen atoms, which were deposited from N2O, is possible. The concentration of these sites increases after treatment of the samples at high temperatures in inert gas. The relation between the sites involved in N2O decomposition and the α-sites, which were first described by the group of Panov, is discussed. We can conclude that the deposition of N2O on Fe-ZSM-5 CVD catalysts creates active oxygen species on the catalyst surface (in particular, after pretreatments at high temperature), but their properties are different from those of α-oxygen. [Copyright &y& Elsevier]

The effect of pretreatment conditions on the N2O-decomposition activity of Fe-ZSM-5 prepared by chemical vapor deposition (CVD) was studied by steady-state kinetics and step-response experiments. Pretreatment at 873 K in H2 or He significantly increased the N2O-decomposition activity of Fe-ZSM-5 CVD. In addition to the increased steady-state activity, the catalyst exhibited a very high initial activity in the step-response experiments, which slowly decayed to steady state. This “transient” activity was particularly high after pretreatment of the samples in He, whereas it vanished completely after pretreatment in O2 or exposure to water vapor. This led to the conclusion that the transient activity is related to strongly dehydroxylated Fe2+ sites created by autoreduction at high temperatures. On the basis of isotope-labeling data a mechanism for N2O decomposition is proposed. The first step is the reaction of Fe2+ with N2O to give a Fe3+O- species, which quickly transforms into a Fe2+–peroxo complex. Desorption of O2 occurs upon reaction of the complex with another N2O molecule (mainly during the transient) or via the recombination of two peroxo species after their migration over the iron oxide cluster (mainly during steady state). [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]