A.A. Battiston

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

XAFS Characterization of the Binuclear Iron Complex in Overexchanged Fe/ZSM5 -Structure and Reactivity

We have characterized, with emphasis on XAFS spectroscopy, one of the most promising DeNOx catalysts, i.e., Fe/ZSM5 prepared through the FeCl3sublimation technique. XAFS is a very useful tool for this purpose since it is element specific and can be used in situ, namely, in the presence of the reactants and at reaction temperature. In this communication it will be pointed out that the as-synthesized Fe/ZSM5 catalyst contains stable binuclear iron oxo/hydroxo-complexes. The reaction of these complexes with the probe molecule CO clearly shows that only one of the oxygen shells around iron changes while the iron contribution is not affected, the iron complex remaining binuclear.

Redox behaviour of over-exchanged Fe/ZSM5 zeolites studied with in-situ soft X-ray absorption spectroscopy

The oxidation and reduction behaviour of calcined over-exchanged Fe/ZSM5 has been studied using soft X-ray absorption by measuring the average iron valence under (2 mbar) helium, oxygen and deNOx (HC-SCR) conditions between room temperature and 350 °C. The results (probing depth of approximately 4 nm) show that Fe/ZSM5 is an extremely flexible redox system. The calcination procedure (severe calcination: heating rate 5 °C min−1, as normally used in the literature; mild calcination: heating rate 0.5 °C min−1) is proven to be important to optimise the reducibility of iron. Upon mild calcination Fe/ZSM5 has an average valence of 2.9 under oxygen (5% in helium), of 2.5 under pure helium at room temperature (RT), and of 2.1 under pure helium at 350 °C. Upon severe calcination Fe/ZSM5 shows higher average valences, in agreement with the assumption that part of the iron in this sample is positioned in small iron-oxide nanoparticles at the outer surface of the zeolite crystals. During heating in helium, the valence reaches a minimum value before slightly rising again (re-oxidation) when the temperature is kept constant. It is also found that the X-ray irradiation is able to affect the average valence by values up to 0.10. This study confirms that iron in ‘over-exchanged’ Fe/ZSM5 consists dominantly of highly reactive iron complexes, where the iron is (distorted) octahedral FeIII in the oxidised state. The implications for the reaction mechanism for the N2O decomposition and the nature of the α-oxygen sites are discussed, in relation to recent developments in the understanding of iron non-heme enzymes.

Bis(μ-OXO)dicopper as intermediate in the catalytic decomposition of No over Cu-ZSM-5

Abstract Novel spectroscopic methods were elaborated to tackle the intriguing question of the active site in Cu-ZSM-5 catalyzing the decomposition of NO. First, a DFT/ ab initio approach was developed, allowing to assign the experimental EPR spectra of Cu-ZSM-5 to representative Cu-zeolite structures. Second, an optical fiber UV-vis set-up was optimized, permitting to monitor the events taking place on the catalyst under reaction conditions. The computational study showed that both EPR signals result from bare Cu(II) ions, i.e. without coordinated extra-lattice oxygen ligands. Studying the NO decomposition activity in function of the Cu/Al ratio of the samples, indicated a sharp increase in TOF between Cu/Al = 0.2 and 0.3. Concomitantly, at the latter Cu/Al ratio, an EPR silent species is formed that is characterized by an intense band at 22700 cm 1 in UV-vis. EXAFS identified it as a dimeric Cu species with CuCu distance of 2.87 . Combining all spectroscopic data and comparing them with the well-characterized copper centers in enzymes and synthetic model complexes led to the identification of the bis(μ-oxo)dicopper core, i.e. [Cu 2 (μ-O) 2 ] 2+ . The operando UV-vis approach assigned the bis(μ-oxo)dicopper core as a key intermediate in the NO decomposition reaction, allowing the smooth formation and desorption of O 2 .

Structure/Reactivity correlation in Fe/ZSM5 dor deNOx Applications : In-Situ XAFS Characterization and Catalysis

Publisher Summary This chapter discusses in situ X-ray absorption fine structure (XAFS) characterization and catalysis in Fe/ZSM5. In situ XAFS spectroscopy performed at 350°C shows that in Fe/ZSM5 synthesized through the FeCl3 sublimation technique, Fe is predominantly present in the form of binuclear oxo or hydroxo-complexes. During reaction with i-C4H10, a clear reduction in the oxidation state of the iron is detected, accompanied by changes in only one of the three ferrous oxide (Fe–O) shells. This confirms the presence of reactive oxygen and demonstrates the possibility of XAFS of studying the local environment around iron in Fe/ZSM5 under experimentally realistic conditions.