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The relation between the intrinsic mechanism of various N2O conversions over FeMFI catalysts and the nature of the active iron site(s) has been analyzed. To this end, direct N2O decomposition and N2O reduction with CO in the absence or presence of NO were investigated using a combination of transient pulse and steady-state techniques over steam-activated FeMFI zeolites with a similar iron content (0.6–0.7 wt% Fe) and different framework compositions (Si–Al, Si–Ga, Si–Ge, and Si). The forms of iron in the catalysts were characterized by UV/vis and HRTEM. The intrinsic reaction mechanism determines the optimal iron site distribution, which can be modulated by tuning the steaming temperature during activation. Oligonuclear iron oxo clusters in the zeolite channels are essential in direct N2O decomposition due to a faster desorption of O2 as compared to isolated ions. Such forms of active iron can be achieved at a lower steam-activation temperature over FeAlMFI and FeGaMFI (900 K) than over FeGeMFI and FeMFI (1150 K). Contrarily, zeolites with a more uniform distribution of isolated iron species lead to higher activities in N2O reduction with CO as compared to highly clustered catalysts. In this case, O-removal as CO2 is strongly accelerated vis-à-vis O2 desorption in direct N2O decomposition. The dual role of NO as a promotor in N2O decomposition and as an inhibitor in N2O reduction also supports the participation of different sites in both types of conversions. NO selectively inhibits N2O reduction over isolated iron ions, further evidencing the essential role of oligonuclear iron clusters in the NO-assisted N2O decomposition. [Copyright &y& Elsevier]

The reduction of N2O by CO was investigated over FeMFI zeolites prepared by different methods including sublimation, liquid ion exchange, and hydrothermal synthesis followed by steam activation. This leads to catalysts with different nature and distribution of iron species, as characterized by HRTEM, UV/vis, and EPR techniques. A common denominator in the FeZSM-5 samples is the heterogeneous iron constitution, with a significant degree of clustering in the form of iron oxide particles. Iron clustering was suppressed in steam-activated Fe-silicalite, presenting a remarkable uniform distribution of isolated iron species. In the presence of CO, the conversion of N2O over the catalysts is strongly accelerated with respect to direct N2O decomposition. The reaction rate increases linearly with the molar CO/N2O feed ratio and strongly depends on the preparation method applied. A correlation was found between the fraction of isolated Fe(III) species in the as-prepared catalysts and the activity for N2O reduction with CO. Steam-activated Fe-silicalite, containing mostly isolated iron ions in extraframework positions, shows the highest activity per mole of iron, while the highly clustered liquid-ion-exchanged catalyst presents the lowest activity. In situ UV/vis and EPR studies evidence the participation of mononuclear iron ions in the N2O–CO reaction, and also support the involvement of oligonuclear FexOy species. The reaction mechanism is iron site dependent. Over isolated sites, the reduction of N2O with CO occurs via coordinated CO species on Fe3+ ions. The reaction over oligonuclear sites proceeds via a redox Fe3+/Fe2+ process with intermediate formation of O− radicals. [Copyright &y& Elsevier]

Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraframework positions was essential for deriving structure-activity relationships in various N2O conversion reactions over iron zeolite catalysts. Characterization by UV–Vis/DRS revealed that any significant clustering of iron did not occur in this sample. Other steam-activated FeMFI zeolites, with different framework compositions or treated at higher temperatures, showed various degrees of clustering. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with C3H8 and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. [Copyright &y& Elsevier]

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N2O, N2, and O2. Classical reaction schemes failed to describe the experimental data. The preferred models of N2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N2O participate in the decomposition of gas-phase N2O. Gas-phase O2 is formed directly on N2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N2O adsorption and, most importantly, the faster desorption of O2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N2O activity. [Copyright &y& Elsevier]