On determining the nuclearity of iron sites in Fe-ZSM-5—a critical evaluation

Abstract

In order to interpret catalytic data on iron zeolites in terms of structure-activity relationships, reliable characterisation methods are needed. In particular, the nuclearity of the iron species is an important issue, since it is often invoked to explain catalytic activity. In the present contribution, we address the problem of the nuclearity of the iron species in Fe-ZSM-5 by a combination of techniques, that is, UV-Vis, EXAFS, Magnetic Circular Dichroism (MCD) and magnetisation measurements. Based on an in-depth analysis of these data, we show that some of the current interpretations of UV-Vis and EXAFS spectra need to be revised.

Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5

“…This process is followed by atomic oxygen migration which occurs through the highest free energy barrier. This result suggested that the latter process (reaction (5)) is the rate determining step of the overall kinetics. The catalyst is regenerated after a-oxygen recombination and molecular oxygen desorption.…”

“…Moreover, the reaction rate increases in time and this autocatalytic effect is assigned to the accumulation of NO x surface species. 5,11,25 Its formation comes from the interaction between N 2 O and the atomic a-oxygen labeled as (O) Fe (reaction (9)). …”

“…Based on kinetics, spectroscopic and modeling studies, mono-and binuclear iron-oxo species have been proposed as the potential active sites rather than clustered oxide species. 5,6 Modeling approaches play a crucial role due to the unusually low content of active iron species (around 3 Â 10 18 sites g À1 ) 7 preventing univocal spectroscopic experimental assignments. Indeed a detailed knowledge of a mechanism occurring between reactants and solids can be reached by combining energetic and kinetic of elementary steps with a macroscopic description of the reaction.…”

Spectroscopic Evidence of Extra-Framework Heterometallic Oxo-Clusters in Fe/Ga-ZSM-5 Catalysts

“…[14][15][16] Binuclear Fe species bound to framework Al as well as isolated Fe species associated with framework or extra-framework Al have been proposed as the active sites for benzene hydroxylation. 8,10,17 Raman spectroscopy is useful in the study of these materials because the vibrational modes of oxo/hydroxobridged metal sites are readily observed. 18 XAFS has also proven to be a powerful tool for establishing the connectivity of the Fe species present in Fe/ZSM-5.…”

“…8,9 Fe may be present inside the zeolite as mono-, bi-, or oligo-nuclear species, iron oxide clusters in varying degrees of agglomeration, mixed-oxide phases containing both Fe and Al, as well as bulk iron oxides on the external surfaces of the zeolite. [9][10][11][12][13] The preparation and pretreatment of the catalyst can affect the distribution and even the nature of the Fe sites. As a result of this heterogeneity, the species responsible for the unique catalytic activity of Fe/ZSM-5 remains elusive.…”