N2O decomposition over Fe-FER: A Mössbauer study of the active sites (original) (raw)
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N2O Decomposition over Fe-Ferrierite: Primary and Secondary Reactions with Reducing Agents
Catalysis Letters, 2008
The decomposition of N 2 O either alone or with several reducing agents (CH 4 , CO, NO and D 2) was followed by temperature-programmed desorption of surface species formed during the reactions. TPD products formed in these ''primary'' reactions were compared with the TPD products after ''secondary'' reactions of the same set of reducing agents with the surface species formed during the primary reactions. Changes in TPD products after reactions measured under various conditions were analyzed in detail. These experiments provided a background for studies of the nature and reactivity of surface species formed during N 2 O reactions over Fe-ferrierite. The relevance of these experimental results to the mechanism of the N 2 O elimination is discussed.
Applied Surface Science, 2002
The effect of sulphur on the activity of fused iron catalyst in the reaction of the ammonia synthesis and decomposition has been studied. In both cases the non-linear dependence between the concentration of sulphur and the catalytic activity has been found. The deactivation has been very fast in the region of lower sulphur concentration and for higher concentration the quasi-stable level of the activity has been observed, depending on the temperature. The method of the determination of the number of active sites has been proposed, basing on the activity measurements on the poisoned and unpoisoned catalysts. The free enthalpy of the process of the active surface formation has been determined (21 kJ/mol for the ammonia synthesis and 63 kJ/mol for the decomposition). #
Journal of Catalysis, 2005
Fe-ZSM-5 catalysts (0.2-1.2 wt% Fe) were prepared by an exchanging of Na-ZSM-5 with Fe 2+ ions formed by the dissolution of iron in acidic medium, and characterized by UV-vis, EPR, and X-ray absorption spectroscopy and by TPR and TEM. Their catalytic properties were investigated for the selective catalytic reduction (SCR) of NO by isobutane (2000 ppm NO, 2000 ppm isobutane, 3% O 2 , 42,000 h −1 ) or by NH 3 (1000 ppm NO, 1000 ppm NH 3 , 1% O 2 , 750,000 h −1 ). The catalysts were highly active in both reactions, competing favorably with catalysts prepared by chemical vapor deposition of FeCl 3 into H-ZSM-5. The spectroscopic studies showed that at Fe contents less than or equal to 0.3 wt%, ca. 95% of the iron was present in mononuclear sites of different coordination. At higher Fe contents, small oligomeric clusters coexisted with mononuclear sites, and at 1.2% Fe, large, poorly ordered Fe oxide aggregates were also detected. By correlation of the activities with the concentration of Fe sites as determined from UV-vis spectra, it was established that mononuclear Fe ions are active sites for both SCR reactions, but oligomers contribute as well. At the same time, oligomers (and aggregate surfaces) are more active in unselective oxidation of the reductant, which limits the temperature window of selective NO reduction. This unselective attack by clustered species occurs at low temperatures with isobutane; hence the best performance was found for a catalyst of low Fe content (0.3 wt%), which is at variance with previous optimization strategies. With NH 3 , the unselective attack occurs at a much higher temperature; hence the best catalysts for NH 3 -SCR are those with the highest exposure of Fe sites. 2005 Elsevier Inc. All rights reserved. (W. Grünert). prepared by chemical vapor deposition (CVD) of anhydrous iron(III) chloride onto the H-form of the zeolite [10] have been shown to yield high activities most reproducibly. In the other reactions, a lower Fe content is more favorable, and the active species are sometimes developed only during a hightemperature treatment of the as-prepared catalysts.
Microporous and Mesoporous Materials, 2015
FeHBEA, FeHZSM-5 and FeHMOR with 1 wt% of Fe were prepared by combination of ion exchange and impregnation procedures. These zeolites have been characterized by X-ray diffraction, DR UV-vis, 57 Fe Mössbauer, XPS, NH 3 -TPD and FTIR. The similar XRD diffractograms, obtained for all the samples, have showed that introduction of iron ions into the zeolite supports did not induce significant changes in their structure. The FeHBEA zeolite possessed a pseudo-tetrahedral Fe(III) species as evidenced by combined used of DR UV-vis, 57 Fe Mössbauer and XPS studies. In contrast, in FeHZSM-5 and FeHMOR zeolites, the pseudo-tetrahedral and octahedral Fe(III) were identified. FTIR studies for the samples preadsorbed with NO proved the presence of Fe 3+ AOH groups that were reduced at room temperature by NO to Fe 2+-ANO and NO + . The catalytic activity of FeHBEA, FeHZSM-5 and FeHMOR in SCR of NO with ammonia and N 2 O decomposition strongly depended on the speciation of iron introduced into different zeolite structure and their acidity. The highest catalytic activity in SCR of NO with NH 3 was obtained for the FeHBEA containing iron as pseudo-tetrahedral Fe(III) and characterized by relatively high acidity. The smaller activity of FeHZSM-5 and FeHMOR in this process is probably related to the side reaction of direct ammonia oxidation by oxygen on a polynuclear octahedral Fe(III) present in both catalysts. In contrast, FeHZSM-5 and FeHMOR possess the higher catalytic activity than FeHBEA in N 2 O decomposition because of the presence not only well dispersed pseudo-tetrahedral Fe(III) but also much stronger acidic sites than that identified in FeHBEA.
Steam-activated FeMFI zeolites. Evolution of iron species and activity in direct N2O decomposition
Journal of Catalysis, 2003
In this paper the effect of the composition and steaming conditions of FeMFI catalysts on activity in direct N 2 O decomposition is investigated. MFI zeolites with different framework compositions (Fe-Al-Si, Fe-Ga-Si, and Fe-Si) and without iron (< 0.002 wt% Fe in the final catalyst) were activated at different temperatures (673-1273 K) and partial steam pressures (20-500 mbar). Extra-framework Fe is essential for high catalytic activity in direct N 2 O decomposition, while Lewis and Brønsted acidic sites play a minor role. Optimized activity was obtained at lower activation temperatures for the iron zeolites containing Al and Ga (873-923 K) than in the purely siliceous zeolite catalyst (1123 K), indicating relatively high stability of Fe in the silicalite framework. High partial steam pressures (> 100 mbar H 2 O in N 2) favor the extraction of framework iron, enabling the application of lower activation temperatures. The optimum temperature during steam activation is that at which extraction of framework iron is complete without extensive clustering of extra-framework iron species into oxide particles, as was demonstrated by transmission electron microscopy and electrochemical characterization of the samples. Additional experiments showed an increase in activity as a function of decreasing crystal size of the zeolite, indicating the presence of intracrystalline transport limitations. Furthermore, higher N 2 O decomposition activities can be obtained by the application of a novel alkaline post-treatment of the steamed catalyst to further improve the catalyst accessibility by creation of mesopores. Implications of the results for the nature of the active Fe-site in direct N 2 O decomposition are discussed. Our observations suggest that small intrazeolitic iron species in extra-framework positions are crucial in direct N 2 O decomposition.
Journal of the Chilean Chemical Society
The activity of Fe 2 O 3 /Al 2 O 3 catalysts prepared by impregnation of Al 2 O 3 with different amounts of Fe and calcination temperatures (650 and 900 ºC) in the direct N 2 O decomposition reaction was studied. High calcination temperature was introduced to study the effect of "aging", which are the conditions prevailing in the process-gas option for N 2 O abatement. The catalysts were characterized by BET, XRD, UV-DRS, and H 2-TPR. The incorporation of Fe promotes the alumina phase transition (g-Al 2 O 3 to a-Al 2 O 3) when the catalysts are calcined at 900 ºC, which is accompanied by a decrease in the specific area. The activity of the catalysts and the specific surface area depend on Fe loading and calcination temperature. It was found that highly dispersed Fe species are more active than bulk type Fe 2 O 3 particles. We conclude that Fe 2 O 3 /Al 2 O 3 catalysts prepared by impregnation method are active in the decomposition of N 2 O, to be used at low or high reaction temperatures (tail-gas or process-gas treatments, respectively), as part of nitric acid production plant.
Effect of noble metals in the decomposition of nitrous oxide over Fe-ferrierites
Catalysis Letters, 2007
The decomposition of nitrous oxide was studied over Fe-ferrierite, Me-ferrierites and Fe/Me-ferrierites (Me: Pt, Rh and Ru). Flow as well as batch experiments were carried out and showed a synergy between Fe and Me ions. Ions of noble metals in Fe-ferrierite increased the catalytic activity in the sequence Pt < Rh @ Ru. Addition of NO substantially decreased the decomposition of N 2 O over Rh/ferrierite and Ru/ferrierite, but not over bimetallic ferrierites. NO x species created during the decomposition of nitrous oxide alone as well as with addition of NO, and employment of nitrous oxide labeled with 18 O allowed us to assume a changing decomposition mechanism in the presence of Me ions in Fe-ferrierites.
Journal of Materials Chemistry a, 2014
In this work Fe-N-C catalysts were prepared by the oxalate-supported pyrolysis of FeTMPPCl or H 2 TMPP either in the presence or absence of sulfur. The well-known enhancing effect of sulfur-addition on the oxygen reduction activity was confirmed for these porphyrin precursors. The pyrolysis process was monitored in situ by high-temperature X-ray diffraction under synchrotron radiation (HT-XRD) and thermogravimetry coupled with mass-spectroscopy (TG-MS). It was found that the beneficial effect of sulfur could be attributed to the prevention of iron-carbide formation during the heat-treatment process. In the case of pyrolysis of the sulfur-free precursors an excessive iron-carbide formation leads to disintegration of FeN 4 -centers, hence limiting the number of ORR active sites on the final catalyst.
Journal of Catalysis, 2010
Fe in ferrierite Fe in ZSM-5 Fe in the beta zeolite DFT VASP a b s t r a c t The N 2 O decomposition over Fe-ferrierite, Fe-beta, and Fe-ZSM-5 has been recently studied [K. Jisa, J. Novakova, M. Schwarze, A. Vondrova, S. Sklenak, Z. Sobalik, J. Catal. 262 27] and a superior activity of Fe-ferrierite with respect to Fe-beta and Fe-ZSM-5 has been shown. In this study, we investigated (1) plausible active sites for the N 2 O decomposition over Fe-ferrierite and (2) the origin of the distinct reactivity of Fe-ferrierite, Fe-ZSM-5 and Fe-beta employing a combined theoretical (periodic DFT) and experimental (UV-vis-NIR spectroscopy, IR spectroscopy, 29 Si MAS NMR spectroscopy and catalytic batch experiments) approach. We evidenced that two Fe(II) cations accommodated in two adjacent six-membered rings in the eight-membered ring channel (b sites) of Fe-ferrierite (the calculated Fe-Fe distance is 7.4 Å) form the active site responsible for the superior activity of this catalyst in the N 2 O decomposition in the absence of NO. Similar structures can be formed in Fe-beta. However, the probability of their formation is very low. For Fe-ZSM-5, the geometrical arrangement of the cationic positions is far from that in Fe-ferrierite and it is not suitable for the N 2 O decomposition. Therefore, the predicted order of the activity of the Fe(II) exchanged zeolites agrees with our experimental findings and it is: Fe-ferrierite ) Febeta > Fe-ZSM-5. We further showed that the accommodation of divalent cations in rings forming cationic sites can lead to significant rearrangements of the local structures of the zeolite framework, and therefore, the precise structure of sites binding a divalent cation cannot be derived from results of Xray diffraction experiments, but can be inferred from theoretical calculations.