2.  State of the Art

2.1 Catalytic abatement of nitrogen oxides from exhaust gases

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Nitrogen oxides such as NO, NO2 (NOx) and N2O are noxious air pollutants in the atmosphere [1-30] produced by both natural and anthropogenic sources. However, the latter contribute approximately 75% of the total amount of NOx and N2O emitted into the atmosphere. The major anthropogenic sources can be classified as:

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  1. Mobile sources such as spark ignition automobiles, lean-burn diesel engines, locomotives etc.
  2. Stationary sources such as fossil and fuel-fired power plants, waste incinerators, industrial ovens, chemical processes such as nitric acid plants etc.

NOx released into the atmosphere contributes to ozone depletion and smog and reacts with O2 and moisture to form nitric acid that leads to acid rain [1,2,4-6].

N2O is a greenhouse gas [7,8], since it strongly absorbs infrared radiation in the atmosphere. Moreover, it has a much higher Global Warming Potential (GWP) than other greenhouse gases such as CO2 and CH4 [7,8].

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Legislations for the emission of nitrogen oxides, unburned hydrocarbons and particulates from mobile and stationary sources are becoming more stringent all over the world. Thus, the abatement of NOx and N2O from exhaust gases is essential to reach those legislation standards and also for a clean and healthy global environment. Catalytic methods for the reduction of nitrogen oxides offer a rational solution to the problem. The best available catalytic technologies for the reduction of NOx and N2O emission can be divided into three categories [1,8,9]: (1) selective catalytic reduction, (2) direct decomposition and (3) non-selective catalytic reduction.

For NOx, this is achieved by Selective Catalytic Reduction (SCR) using NH3 or hydrocarbons as reducing agents in the case of stationary or mobile emission sources respectively, while N2O can be removed from exhaust gases by both SCR and direct decomposition.A number of catalysts have been tested so far [5,10,11-25] which can be classified into three groups: supported metal oxides, noble metals and zeolites containing transition metal ions [21,22]. Among the various metal oxide catalysts [5,11-17], V2O5 supported on anatase TiO2 and promoted with either WO3 or MoO3 is commercially used for stationary emission sources since many years [3,26-30], though this system is not without problems [4]. Thus, despite a rather high resistance of this catalyst to SO2 poisoning, deactivation does take place at high concentrations of SO2 and ash in the flue gas. Moreover, this catalyst has high activity for oxidation of SO2 to SO3 which can react with slipped NH3 at high temperatures giving rise to ammonium salts that cause fouling and corrosion problems in the downstream equipment. To avoid these problems, the SCR unit should be placed downstream, in the tail-end, but there, the flue gas temperature drops below 423 K. This is too low for the SCR reaction and requires reheating to the typical SCR operating temperatures, which makes the overall operation very expensive. Furthermore, these catalysts are not suitable for lean-burn diesel and gasoline engines since they are not active at low temperatures and at high temperatures they promote unselective total oxidation of reducing agents. Hence, there is a strong need for the development of an efficient, economic and eco-friendly SCR catalyst that should be active over a wide temperature range, be resistant towards SO2 and water, have a long life with relatively low cost and should not be a problem for the disposal of the used catalyst.

Therefore, many efforts have been devoted to develop inexpensive low temperature SCR catalysts and a variety of catalysts have been proposed in the literature, among them mixed Fe-Mn oxides [23], supported MnO x -CeO2 [24], and recently supported Mn x O y -TiO2 (anatase) [4]. These catalysts have shown potential low temperature NH3-SCR activity and do not have the deficiencies that are associated with the commercial vanadia based and noble metal oxide catalysts [4,23,24]. Hence, they could be potential candidates for low temperature NH3-SCR catalysts for stationary sources in the future.

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Another group of catalysts are noble metal oxides such as Pt supported on Al2O3 which exhibits the best low temperature SCR activity in comparison to other metals. Though they are more resistant against H2O and SO2 poisoning [25], they have other drawbacks such as high prices, formation of substantial quantities of N2O and a narrow temperature window for NO reduction [25].

Zeolites containing transition metal ions are the third type of catalysts [2,10,31-36,67-73]. Many reasons were suggested in the literature for the superior performance of zeolites, in particular ZSM-5 zeolite as a support for SCR of NOx and N2O decomposition. These include the fact that the highly dispersed transition metal oxo-cations in the zeolite channels are accessible, possess a high degree of coordination of unsaturation and are in the appropriate oxidation state [74-78].

In the series of metal ion exchanged zeolites Cu- and Co-zeolites were extensively studied. In particular, the Cu-ZSM-5 system is very well studied for both NH3- and hydrocarbon-SCR of NO because of its promising catalytic activity [67-71]. Unfortunately, it is not resistant to deactivation by H2O and SO2 [73,79-81]. Co-zeolites are active only at higher temperatures, being not suitable for practical applications and are sensitive (mainly at lower temperatures) to H2O and SO2 [10]. Fe-containing zeolites have got a great deal of attention due to their high activity in a wide temperature window and their stability even in the presence of H2O and SO2 [2,31,37,73].

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A great variety of catalysts including metals, pure and mixed oxides and zeolitic systems have been reported for the decomposition or SCR of N2O [38-43]. Abatement of N2O emissions in chemical plants (eg. Nitric acid plants) is a challenging task since exhaust gases contain other components such as O2, NO, CO2, SO2 and H2O which can reduce the catalyst performance or even cause deactivation [40,41]. Fe-zeolites have attracted much attention for the abatement of N2O emissions, too, from chemical plants because of its insensitivity to O2, NO, CO2, SO2 and H2O [42,43,82]. Moreover, NO and SO2 enhance the catalysts performance in direct N2O decomposition, while O2 and CO2 do not [41]. Differently, H2O considerably inhibits the decomposition of N2O but only in the absence of other extra gas components (O2, NO, CO2 and/or SO2) [41]. Hence, a variety of Fe-containing zeolites including Fe-ZSM-5 [44,45,48,53], Ferrierite (FER) [44,46,47,53], Mordenite (MOR) [44,45,46,48,53], beta [44,46,48,50], Faujasite (FAU) [48,49], Y [40,44,45,53], Chabazite (CHA) [46], Cliniptilolite (HEU) [46], ZSM-11 [47], SBA-15 [51], SAPO-34 [54], HMS and MCM-41 [45,52] have been studied in both SCR of NOx or N2O and direct decomposition of N2O [44-54].

However, in summary, it turns out that Fe-MFI zeolites belong to the best catalysts known for the abatement of both NOx and N2O. Understanding the corresponding reaction mechanism on this catalytic system can provide vital information for the development of this system and consequently, the corresponding abatement process. Considerable research has been devoted to the reaction mechanism of removal of NOx and N2O decomposition. Therefore, catalytic abatement of nitrogen oxides from exhaust gases is discussed with respect to the process for the removal of NO and N2O over Fe-MFI zeolites with mechanistic insights.

2.1.1 Removal of NO

Selective catalytic reduction (SCR) of NO x

The major anthropogenic sources for NOx are power plants, stationary engines, lean-burn Otto engines, industrial boilers, process heaters, gas turbines, combustion of fuels, chemical industries etc. Emission gases from almost all of these sources contain an excess of O2 and depending on the source, moisture and SO2 can also be present. Under these conditions SCR of NOx with a reducing agent is the best available technology for NOx abatement. Typically, NH3 has been used as a reducing agent in the SCR of NOx due to its high selectivity towards reaction with NO in the presence of excess O2 and the promoting effect of O2 on the rate of this reaction. The stoichiometric NH3-SCR of NO reaction can be written as:

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4NO + 4NH3 + O2 → 4N2 + 6H2O (2.1)

This technology has been successfully used for stationary sources in industrially developed countries. The most commonly used catalytic system is V2O5/TiO2-WO3 or MoO3. However, this catalytic system is not adequate and is associated with some problems as mentioned above (page 5). Promisingly, these can be greatly suppressed using Fe-ZSM-5 as a SCR catalyst [37,46,83].

The reaction mechanism of the SCR of NO with NH3 has been extensively investigated over Fe or H-zeolites [37,46,84,85]. The schematic representation of this reaction mechanism is as follows.

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1. Quick adsorption of gaseous NH3 molecules on the Brønsted acid sites to form NH4 + ions with two or three hydrogen atoms bonded to the AlO4 tetrahedra of ZSM-5 zeolite

2NH3 (g) + 2H+ → 2NH4 + ( ad) (2.2)

2. Simultaneously, NO is oxidized to NO2 on Fe+3 sites in the presence of oxygen

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NO( g) + 1/2O2(ad) + Fe+3 sites → NO2(ad) (2.3)

3. One NO2 molecule diffuses to two adjacent NH4 + ions to form an active complex

NO2( ad) + 2NH4 + (ad) → NO2(NH4 +)2(ad) (2.4)

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4. The active complex subsequently reacts with one molecule of NO to form N2 and H2O and regenerates two Brønsted acid sites:

NO2(NH4 +)2(ad) + NO(g) → 2N2(g) + 3H2O(g) + 2H+ (2.5)

In this process the oxidation of NO to NO2 on Fe+3 sites was considered as the rate determining step [37].

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However, NH3-SCR technology is not successfully used for mobile applications such as lean burn Otto and gasoline engines due to the variety of the transient conditions, NH3 slip and the complications of maintaining an on-board NH3 source. Hence, an alternative hydrocarbon (HC)-SCR of NOx was proposed for this purpose [1,67].

Hydrocarbon-SCR of NO

For this process there is no need to carry around an additional reductant just to reduce the NOx from mobile engines since one could readily use the fuel (LPG or gasoline) to reduce NOx emissions also. Fe-ZSM-5 appeared to be more efficient for this reaction than any other catalysts tested so far [31,36]. Although there are complicated sequences of the reaction, it can be simply described by the following equation:

NO + hydrocarbon + O2 → N2 + CO2 + H2O (2.6)

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Considerable research has been done on the reaction mechanism of the SCR of NO with hydrocarbons over different zeolites and different reaction mechanisms involving different intermediates have been proposed [86-90]. Hence, the complicated multistep HC-SCR of NO reaction mechanism is simplified and generalized (for all hydrocarbons) as shown schematically below. Fe+3 ions play a major role in the HC-SCR of NO reaction. Though the structure is still at the debate stage, they are proposed as active iron centers for the oxidation of NO to NO2 and NO3 - (Eq. (2.7) and (2.8)) [86-89,91].

(2.7)

(2.8)

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These NO2/NO3 -species reacts with hydrocarbons to form N-containing deposits, the structure of which is ambiguous [86-89] and undergoes some subsequent rearrangements. These N-containing deposits are designated as active complex in the reaction.

NOx( ad) + CmHn(ad) → NOxCmHn(ad) (2.9)

NO2 reacts with N-containing deposits to produce N2, COx and H2O.

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NOxCmHn( ad) + NO2(g) → N2 + yCOp + zH2O (2.10)

Non-selective catalytic reduction of NO x

Non-selective catalytic reduction of NOx is an interesting technology from the practical point of view since, in this process the combustion gases such as CO and unburned hydrocarbons can be used as reducing agents for the abatement of NOx.Consequently, harmful emission gases (CO, NOx and hydrocarbons) will be cleaned off before being discharged into the atmosphere hence, this process is called as three way catalysis and the catalyst is called as three way catalyst (TWC) [1,9].

Using Pt, Pd and Rh based catalysts this technology has been successfully implemented in the combustion automobile applications for the abatement of NOx. However, under O2 rich conditions this process is not effective, since O2 competes with NOx to react with combustible gases. Consequently this technology is not suitable for lean-burn applications such as diesel engines. For the latter applications hydrocarbon-SCR of NOx with Fe-ZSM-5 could be the best process.

Direct catalytic decomposition of NO

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Direct catalytic decomposition of NO into its elements is an attractive and economical approach to reduce NO emissions since it does not demand the use of a reductant and avoids undesired emissions and slip of reductants. Unfortunately, none of the catalysts proposed in the literature shows a good activity and stability in direct NO decomposition under true reaction conditions.

2.1.2 Removal of N2O

Direct catalytic decomposition of N 2 O

The steamed Fe-ZSM-5 attracted much attention in recent years due to its significant activity, stability and durability in the direct N2O decomposition under realistic conditions, that means, in the presence of O2, NO, H2O and with high space velocities. Moreover, the potential of steamed Fe-ZSM-5 for direct N2O decomposition was confirmed in the simulated tail-gas at a pilot-scale [8]. Hence, reaction mechanism of N2O decomposition over different Fe-zeolites has been extensively studied [39,58,92-97] to comprehend the unique properties of Fe-zeolites. It is commonly accepted that the activation of N2O on active iron site, which is a Fe-oxo species (monomer or binuclear or oligonuclear), is the first step in the decomposition of N2O. This leads to the formation of highly active oxygen species, which is the so-called α-oxygen (Eq. (11)) [98-100].

(2.11)

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The α-oxygen can react with either N2O or with an other α-oxygen to form molecular O2 and subsequently regenerates the active site for the propagation of N2O decomposition as shown by (Eq. (2.12) and (2.13)).

(2.12)

(2.13)

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However, regeneration of active iron sites by forming molecular O2 is still not clear but it is widely accepted that O2 desorption is a rate-determining step in this reaction [39-41,58,95]. The unique performance of this catalytic system could be an efficient technology in the near future for the abatement of N2O in different sources.

Selective catalytic reduction (SCR) of N 2 O

SCR of N2O with different reducing agents such as CO, NH3, hydrocarbons etc. was investigated. As compared to the direct decomposition, the presence of a reductant in the feed not only enhances the N2O conversion but also shifts the conversion onset to lower temperatures. But studies dealing with the reaction mechanism are almost neglected. Reaction mechanism seems to be dependent on the reducing agent and the active iron sites [48,82,93-96,101,102]. In a study using labeled 15NH3 for SCR of 14N2O, differently labeled molecular nitrogens 14N2, 15N2 and 14N 15N were found in the products [64]. Hence, the authors suggested three reactions for the formation of different nitrogen isotopes, though they are not clear.

1. 14N2O decomposes on the active iron site

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(2.14)

2. Adsorbed oxygen reacts with 15NH3 to regenerate the active site and forms 15N2 and H2O.

(2.15)

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3. 14N2O and 15NH3 react together at the same active site, after 14N-14N splitting of 14N2O, forming 14N-15N.

(2.16)

Pophal et al. studied the propene-SCR of N2O on Fe-MFI zeolite but they did not study the reaction mechanism [101]. However, based on the FT-IR studies they proposed protonated propene might play an important role in the SCR of N2O.

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In propane SCR of N2O, propane reacts with the atomic oxygen, which is deposited by N2O and reduces the active iron site. Subsequently, this site is reoxidized by N2O [96].

Delahay et al. reported SCR of N2O with CO on Fe-beta zeolite [102]. They suggested that CO reacts with a binuclear Fe oxo cation to form CO2 and reduces the iron site. The reduced binuclear iron oxo site is reoxidized by N2O.

(2.17)

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(2.18)

2.2 Synthesis strategies and activity of Fe-zeolites

It is commonly accepted that the preparation method determines the activity in the SCR of NOx and N2O as well as N2O decomposition. Hence, different techniques were aimed to prepare Fe-zeolites with extraframework Fe species [2,31,32,37,45,53,103]. So far, a vast number of publications dealing with NH3 and hydrocarbon SCR of NOx, N2O and decomposition of N2O over Fe-MFI zeolites appeared. In contrast, limited research has been done on Fe-exchanged Mordenite, Ferrierite, beta, Y, LTL, Chabazite, Cliniptilolite, ZSM-11, ZSM-12, SBA-15, Al-HMS and MCM-41 catalysts. These studies revealed that Fe-MFI is more active and stable for these reactions than the other Fe-zeolites. However, the literature data are not directly comparable, since the applied experimental conditions were different in different research groups. Hence, only those reports which were performed under similar conditions are considered and studies under realistic conditions as in chemical plants are shortly mentioned. Furthermore, the Si/Al ratio, the ion exchange level of the Fe-zeolites, the pretreatment conditions, reducing agent, space velocity and the pore geometry of the zeolites can also influence the catalytic activity [34,44-46,53,55,58,103-105]. In this section, Fe-zeolites are discussed with respect to the preparation method and their activity in the removal of NOx and N2O.

2.2.1 Liquid ion exchange (LIE)

Traditional aqueous ion exchange aiming at the replacement of H+, Na+ or NH4 + cations of the zeolite matrix by Fe3+ or Fe2+ ions has been used to prepare extraframework iron containing Fe-zeolites. Sato and Iwamoto et al. investigated the influence of the zeolite framework on C2H4-SCR of NO reaction in the presence of O2 under dry conditions [53]. However, it should be mentioned that they are the first authors who proposed the hydrocarbon SCR of NOx and investigated the effect of zeolite framework on the reaction. For this study,Fe-exchanged MOR, FER, MFI, Y, LTL were prepared by conventional ion exchange from their Na form and tested under dry conditions. The Fe-MOR with an ion exchange level of 97% (Fe/Al ≈ 0.33) was found to be the most active catalyst among the Fe-zeolites with a maximum NO conversion of 20% at 523 K. On the basis of NO conversion at 473 K these catalysts were ranked as:

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Fe-MOR 97≈Fe-MOR 54 > Fe-FER 49 ≈ Fe-MFI 94 > Fe-Y 89 > Fe-LTL 53

The number in the sample labeling denotes the percentage of ion exchange level with respect to the exchange of protons with Fe3+ i.e., Fe/Al = 0.33 (ion exchange level ≈ 100%). The activity order of different Fe-zeolites may not be merely due to their structure as claimed by the authors, since the ion exchange levels and the Si/Al ratio of the Fe-zeolites are different [53].

R. Q. Long and R. T. Yang prepared a series of Fe-exchanged zeolites including Fe-ZSM-5, Fe-MOR, Fe-FER, Fe-beta, Fe-Y, Fe-CHA, Fe-HEU, Fe- Al-HMS and Fe-MCM-41 by conventional ion exchange and tested them in NH3-SCR of NO [2,45,46,52]. They always used the same feed composition and Gas Hourly Space Velocity (GHSV) for dry and wet conditions.

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The NOreduction rate was negligible in the absence of O2 and increases rapidly when the latter is introduced in the NH3-SCR feed. This indicates the requirement of O2 for the stoichiometric reaction (Eq. (2.1)). Under dry conditions, on 58% ion exchanged Fe-ZSM-5 (Fe% ≈ 1.59) nearly ≈ 100 % NO conversion was achieved in a broad temperature window from 673 to 823 K, higher than any other catalyst tested under these conditions (Table 2.1). When a small amount of Ce was introduced in Fe-ZSM-5 the activity further increased [2]. Therefore, the 58% Fe exchanged Fe-ZSM-5 (Fe% ≈ 1.59) and Ce-Fe-ZSM-5 (Fe% ≈ 1.14) catalysts were tested under realistic conditions with H2O and SO2 present in the feed [2,46]. The NO conversion only slightly decreased below 623 K but significantly increased and widened the temperature window at higher temperatures. Increase in activity was explained by increase in surface acidity, mainly Brønsted acidity of the catalysts. Especially, Ce-Fe-ZSM-5 was more stable during 60 h on stream under these conditions than the other two catalysts hence, the authors concluded that the Ce may be playing a stabilization role in Fe-ZSM-5 catalyst [2].

Under the same dry reaction conditions as forFe-ZSM-5 (Fe% ≈ 1.59) and Ce-Fe-ZSM-5 (Fe% ≈ 1.14), the ≈ 60% iron exchanged Fe-MOR (Fe% ≈ 2.41) and 81% iron exchanged Fe-HEU (Fe% ≈ 3.5) also exhibited high activity. Differently, the other Fe-zeolites such as, Fe-Y, Fe-FER, Fe-beta, Fe-CHA, and Fe-MCM-41 were found to be less active than Fe-ZSM-5, Fe-MOR and Fe-HEU catalysts [1,45,46]. Furthermore, under similar conditions, the latter three catalysts have shown much higher SCR activity than a commercial V2O5+WO3/TiO2 catalyst. The different behaviour of different Fe-zeolites in NH3-SCR reaction was explained by their unique structures. Mesoporous materials like Y, MCM-41 and HMS with large porediameter are favorable for the diffusion rates but are less active in the NH3-SCR reaction. Hence, the authors suggest that the SCR reaction could be free of diffusion limitation.

However, with conventional ion exchange, Fe/Al ratios only lower than 1 was achieved frequently. This is due to the fact that one Fe+3 ion must compensate three Brønsted acid sites to balance spatially separated negative charges of the zeolite matrix. Hence, only a small amount of Fe+3 ions could exchange with protons. The additional Fe+3 ions most likely form iron oxo- or hydroxo cations that can undergo complex chemical transformation during subsequent washing and calcination which lead to highly heterogeneous materials. Despite these problems considerable achievements with special techniques were reported [105,106].

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Pioneer work reported by Feng and Hall aimed at the replacement of Na+ by monovalent cations into a zeolite matrix rather than di and trivalent cations in order to achieve maximum iron exchange levels. In this regard they proposed a peculiar aqueous ion exchange method using FeC2O4 solution in an inert atmosphere in order to prevent oxidation of Fe+2 to Fe+3. The authors achieved the objective by hydrolysis of Fe+2 to Fe(OH)+ ions by FeC2O4 in H2O and showed, that the existence of such Fe(OH)+ ions is strongly pH sensitive. By this procedure an ion exchange level of 183% was achieved [73,81]. This catalyst showed superior performance in comparison to under-exchanged (≈ 22% ion exchanged) Fe-ZSM-5 and unusual stability even in the presence of 20% H2O and 150 ppm SO2 during 2500 h on stream in realistic conditions (Table 1). Unfortunately, these results could not be reproduced by the authors or by other groups [31,105,107]. The main reason for the non-reproducibility of the obtained results could be due to the difficulty of an accurate control of pH inside the zeolite pores.

Certainly, this work generated enormous research in the preparation of over-exchanged Fe-zeolites for SCR of NOx. Long and Yang reported an improved aqueous ion exchange method. In this method Fe+2 ions, which were generated in situ by reacting diluted HCl with iron powder, were exchanged with H+ of H-ZSM-5. The ion exchange was performed in an inert atmosphere in order to prevent oxidation of Fe+2 to Fe+3. An ion exchange level of 130% was achieved. At lower temperatures this catalyst (Fe-ZSM-5-130 with 3.58% Fe) showed superior performance in NH3-SCR in comparison to a 58% ion exchanged Fe-ZSM-5-58 (Fe% ≈ 1.59), which was prepared by conventional ion exchange methods [45]. However, at higher temperatures both catalysts show similar activity (Table 2.1).

Table 2.1. Summary of the performance of differently prepared Fe-ZSM-5 and different Fe-zeolites prepared by CVD are compared in NH3 and isobutaneSCR of NO

* The labeling of Fe-catalysts indicates the Fe Wt.% and the type of zeolite. The absence of Fe Wt.% in some samples indicates its unavailability.

2.2.2 Sublimation of FeCl3into the pores of the zeolite matrix (CVD)

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Inspired by Feng and Hall’s work, Chen and Sachtler prepared an over-exchanged Fe-ZSM-5 catalyst by chemical vapor deposition (CVD) of FeCl3 into the pores of H-ZSM-5 in an inert atmosphere [31]. Fourier Transformed Infra Red (FT-IR) spectroscopy was used to study the effect of sublimation of FeCl3 into the pores of the zeolite. They found that the Brønsted OH groups (3610 cm-1) and silanol groups (3750 cm-1) were completely consumed after sublimation. However, after subsequent washing and calcination steps, peaks at 3610 cm-1 weakly and 3750 cm-1 completely reappeared. Hence, the authors proposed that [Fe2Cl4]2+ ions replace two protons located within suitable distances according to the following reaction:

Fe2Cl6 + 2H+ → [Fe2Cl4]2+ + 2HCl (2.19)

With this technique the authors achieved Fe/Al = 1 (ion exchange level ≈ 300%) [31] and the catalyst showed a high activity and stability for isobutane-SCR of NO under wet conditions during 100 h on stream at 623 K (Table 2.1).

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These authors prepared a variety of Fe-zeolites (MFI, beta, MOR, FER, Y) by this method and studied the influence of their pore geometry and the reductant on the SCR of NO [44]. The five Fe-zeolites were tested under identical conditions in both isobutane and propane SCR of NO under dry conditions (Table 2.1). Therefore, the catalytic performance of these catalysts have been compared and tabulated in Table 2.1.

In both isobutane and propane SCR, Fe-MFI and Fe-beta exhibited higher activity than the other catalysts tested in this study (Table 2.1). The presence of 10% H2O in the SCR feed does not deteriorate the SCR activity of Fe-MFI but does deplete the activity of other Fe-zeolites studied for this work [44]. This indicates that MFI pore structure is more suitable for this reaction than any other zeolite pore structure, which can be clearly seen, too, from Table 2.1.

The CVD preparation technique attracted much attention in recent years by many research groups because of the high activity, stability and reproducibility, not only for the reduction of NO with isobutane [31,35,36,108] and NH3 [2,33,34] but also for the direct decomposition [42,43] or reduction of N2O with CO and propane [42,58,109].

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Grünert et al. investigated the effect of Si/Al ratio of the parent H-ZSM-5 support, washing intensity and calcination ramp of Fe-ZSM-5 catalyst on the activity of SCR of NO [36,55]. Fe-ZSM-5 catalysts were prepared by a method similar to the one of Chen et al. using different H-ZSM-5 supports with different ratios of Si/Al, 14 and 44, with different washing procedures and with different heating rates during calcination. The high Si/Al (44) containing catalyst showed lower activity than the low Si/Al (14) containing Fe-ZSM-5 catalyst in isobutane-SCR of NO. The low Si/Al (14) of the parent H-ZSM-5, high washing intensity (10 l H2O for 5 g of catalyst) and low heating rate during calcination process suppresses the formation of Fe x O y clusters.

Prins and Marturano et al. have shown that the distribution of iron species in Fe-ZSM-5 (prepared by CVD) is strongly dependent on the source of the parent ZSM-5 and hydrolysis processes of the zeolite after preparation [56]. Recently, Battiston et al. proposed that calcination is a crucial step in the final distribution of iron species in sublimed Fe-ZSM-5 sample [57]. Furthermore, van Santen and Zhu et al. reported that the pressure of FeCl3 vapor in the sublimation procedure plays a role in the formation of iron oxide clusters [92].

However, different characterization techniques, including FT-IR, H2-TPR, EXAFS, XANES, Mössbauer, EPR and UV/VIS spectroscopy revealed that samples prepared by CVD contain different iron species varying from isolated iron ions to oligomers and large iron oxide particles [36,55,108]. Heterogeneous distribution of iron species in Fe-ZSM-5 makes the catalyst complicated to comprehend the active iron centers in SCR of NOx. Consequently, based on their characterization studies on Fe-ZSM-5, which exhibited comparable SCR activities, different research groups proposed different active iron species for this reaction [36,55-57, 86,91,105, 110,111], which will be separately discussed in section 2.3.

2.2.3 Solid-state ion exchange (SSIE)

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This technique is based on the mechanical mixing of appropriate amounts of H-ZSM-5 support and Fe precursor (FeCl3. 6H2O or FeCl2. 4H2O) and subsequent heating (usually above the sublimation temperature of the Fe source) in an inert gas flow in an oven for a desired time followed by washing with H2O [36,103,106]. The method of Fe insertion into the zeolite pores is more or less similar to CVD but the required experimental setup is simple for SSIE.

Bell and Lobree et al. studied the effect of Fe/Al ratio on the nature and distribution of iron species in ZSM-5 zeolite matrix and reported that Fe/Al ratios below ≈ 0.6 favour the formation of isolated iron ions [103].

Long and Yang et al. studied the effect of preparation method on the NH3-SCR reaction over Fe-ZSM-5 and reported that the sample prepared by SSIE is more active than the CVD method [106].

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Grünert et al. also studied the effect of preparation method on the NH3-SCR reaction and the findings were different from that of Long and Yang et al. The authors found that the sample prepared by CVD method is more active than the SSIE method [36]. However, different characterization techniques revealed that the SSIE method leads to a highly heterogeneous distribution of iron species with extensive formation of clusters in Fe-ZSM-5. [36,55].

2.2.4 Other preparation techniques

For unambiguous assignment of active iron sites in SCR of NOx and N2O, it was essential to prepare a catalyst with a defined distribution of iron species. In this connection, recently Grünert et al. reported a novel mechano chemical route (MR) to prepare Fe-ZSM-5 catalyst [32]. This technique comprises of intense grinding of appropriate amounts of H-ZSM-5 support with FeCl3. 6H2O, followed by 2-3 short-time washing steps (0.5 l H2O per 2 g catalyst). Despite the much lower Fe content, this catalyst is more active in SCR of NO than the sample prepared by the CVD technique of Sachtler et al. indicating the importance of the mononuclear iron sites in the reaction (Table 2.1).

Another method for obtaining highly active Fe-MFI zeolites is hydrothermal synthesis followed by steam treatment. Here, Fe ions are added to the synthesis mixture and are incorporated in the zeolite lattice during crystallization. Subsequent steaming causes dislodgement of framework Fe to extraframework Fe species. J. Pérez-Ramírez et al. studied the steam activated Fe-silicalite and Fe-ZSM-5 with similar Fe content (≈ 0.68%) prepared by hydrothermal synthesis which exhibit different nature and distribution of iron species and thus resulted in different catalytic activity in SCR and decomposition of N2O [96]. Steam activated ex-Fe-silicalite with uniform distribution of isolated iron ions exhibited higher specific activity in the reduction of N2O with C3H8 and CO than the steam-activated ex-Fe-ZSM-5 catalyst with heterogeneous distribution of iron species. Nevertheless, ex-Fe-ZSM-5 showed remarkable performance in the decomposition of N2O in simulated tail gases (realistic conditions) as compared to ion exchanged and sublimed Fe-ZSM-5 [8,42,111].

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On comparing the different preparation techniques, it appears that traditional aqueous ion exchange method aiming at the replacement of H+ or Na+ cations of zeolite matrix by ferric ions leads to highly heterogeneous distribution of iron species in the zeolite with pooractivity in the HC-SCR of NOx or reduction of N2O and N2O decomposition [32,42,53,82,96]. That’s why, Feng and Hall introduced a novel idea to exchange H+, Na+ or NH4 + ions with monovalent metal cations to prepare over exchanged Fe-MFI zeolites. However, this technique is strongly pH sensitive which lead to the problem of reproducibility. Hence, this technique was almost disregarded. In contrast, sublimation method is much less sensitive to the origin of the zeolite i.e., the pH inside the zeolite pores. An advantage of this preparation method is the ease to achieve high ion exchange levels of Fe/Al up to ≈ 1 and more reproducibility. Though this technique leads to highly heterogeneous distribution of iron species in the zeolite, the obtained material is undoubtedly more active in various reactions. Improved liquid ion exchange method also leads to highly active materials for SCR of NOx with reproducible results. On the other hand, hydrothermal synthesis of Fe-MFI and steam activation allows the preparation of zeolites with framework and extraframework iron in a more reproducible manner.

Considering the pore size of different zeolite matrices, it appears that the activity of NH3- and HC-SCR of NO decreases with increasing pore size of the zeolite though the accessibility of iron sites to reactants increases [44-46,52]. In NH3-SCR of NO the Fe-ZSM-5 and Fe-MOR zeolites show higher activity in both dry and wet conditions than the other Fe-zeolites. While, in HC-SCR of NO, Fe-ZSM-5 and Fe-beta zeolites exhibit superior performance than the other studied Fe-zeolites and Fe-ZSM-5 was more stable in wet conditions. Interestingly, the best candidate in both SCR of NO reactions is Fe-ZSM-5. Furthermore, Fe-ZSM-5 shows superior performance in both direct catalytic decomposition and SCR of N2O reaction in dry and realistic conditions. Hence, the possible synergistic effect of the BrØnsted acid sites (mainly on the SCR of NOx) and intrinsic activity of the ZSM-5 pore structure for SCR of NOx and N2O and N2O decomposition make ZSM-5 a potential candidate for the support [112,113].

2.3 Structure-reactivity relationships in Fe-zeolites

As mentioned in the previous sections, the activity, stability and durability of Fe-MFI zeolites in SCR of NOx and N2O or N2O decomposition have motivated researchers to investigate the origin of the catalytic activity of this system. Most of the research on the nature of active iron sites was carried out for HC-SCR of NOx or N2O decomposition while only few studies dealt with the NH3-SCR of NOx. The majority of these studies was performed by applying physico-chemical techniques ex situ, in the absence of reactants, and the results were discussed in relation to separately measured catalytic data. This can lead to ambiguous conclusions on the nature of active iron sites. Very few in situ studies during interaction of feed components with iron species in the zeolite have been reported. In this section, studies dealing with the evaluation of the nature of active iron sites are discussed separately for SCR of NOx and decomposition or SCR of N2O.

2.3.1 Structure-reactivity relationships in the SCR of NO

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Among the various types of Fe species that can exist in Fe-containing zeolites, binuclear Fe-O-Fe species have been discussed with emphasis by several groups. The idea of binuclear iron oxide complex was first proposed by Garten et al. for Fe-Y zeolite [114] and Hall et al. confirmed these results over Fe-Y and Fe-MOR zeolites [40,115]. After that Panov et al. reported that N2O decomposition occurs on the bivalent iron dimers which produces α-oxygen with high oxidation potential [98-100]. Taking account of these reports and based on the TPR, EPR and IR results, Chen and Sachtler proposed an oxygen bridged binuclear iron complex such as [(HO)Fe-O-Fe(OH)]+2 as active iron species for SCR of NOx with hydrocarbons [31]. In fact, merely based on the CO-TPR data they proposed this binuclear iron complex [31] and later supported this argument by EPR spectroscopy. They assigned a signal at g' ≈ 2.03 to such a dimer species which was observed at room temperature. In relation to the temperature, the identification of an iron dimer by EPR line at g' ≈ 2.03 is highly questionable (see also section 4.1.2).

Marturano and Prins et al. studied the structure of iron species in Fe-ZSM-5 which was prepared by CVD technique using IR, Al MAS NMR and EXAFS spectroscopy [56]. Mainly based on the EXAFS data they suggested that the iron is mostly in the form of intrazeolite diferric (hydr)oxo-bridged binuclear clusters similar to those in MMO. Recently, Battiston et al. strongly supported the idea of binuclear iron oxide complex (Fe-O-Fe) as active iron species [35,116]. On the basis of in situ-EXAFS and XANES studies, these authors suggested that the binuclear iron species are the dominating species in sublimed overexchanged Fe-ZSM-5 and are proposed to be the active iron species in isobutane-SCR of NO reaction. However, when a variety of iron species coexists, only average coordination values can be derived from this technique. Discrimination of dimers is difficult and, as Grünert et al. have shown [64], the presence of clusters can be underestimated hence, the obtained results are ambiguous.

Recently, Grünert et al. studied sublimed overexchanged Fe-ZSM-5 catalysts by FT-IR, XRD, XPS, TPR, EXAFS, XANES and Mössbauer spectroscopy. Their findings were different from that of the above authors and suggested that the sublimed Fe-ZSM-5 contains a multitude of iron species including isolated Fe ions, oligomers (including dimers), iron oxide clusters and Fe2O3 particles [36,55]. Hence, they concluded that assignment of SCR of NO activity merely to binuclear iron species as ascribed by some authors might be doubtful. Considering the fact that different forms of iron species are coexisting in Fe-ZSM-5 zeolites, they suggested that isolated and oligomeric iron species including dimers are the active sites in both NH3- and isobutane-SCR of NO, whereby mononuclear iron species appeared to be more efficient than oligomers However, they also suggested that the cluster species are unselective at higher temperatures [32,36,55].

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Previously, Joyner et al. have observed different forms of iron in the ion exchanged Fe-ZSM-5 zeolites and based on their EXAFS results they proposed Fe4O4 nanoclusters as the active iron species in propane-SCR of NO [105].

Kucherov et al. assigned, based on in situ EPR studies, the catalytic activity of Fe-ZSM-5 and Fe-SAPO-34 catalysts in propene-SCR of NOx to distorted tetrahedral isolated Fe+3 ions [59]. Similarly, Sobalik and Wichterlova et al. ascribed the activity of Fe-FER zeolite in propane-SCR of NO merely to isolated iron sites [91]. Long and Yang concluded by EPR studies of differently prepared Fe-ZSM-5 zeolites, that isolated Fe+3 ions in tetrahedral coordination are the only active iron species in NH3-SCR of NO reaction [106].

By comparing all the results and discussion presented above, the high degree of debate and controversy on the nature of the active sites is readily evident. The only agreement seems to be that large Fe2O3 particles are not active sites for SCR of NO and must be avoided [36,86]. Moreover, there are only few studies dealing with the role of acidity in the SCR of NO, which may be an important property. However, Long and Yang reported that the higher the Brønsted acidity of the Fe-zeolite higher is the activity in the NH3-SCR of NO [46].

2.3.2 Structure-reactivity relationships in the decomposition and SCR of N2O

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Significant research has been done to evaluate the nature of active iron sites in N2O decomposition and in N2O-SCR. For N2O decomposition, catalytic activity is often ascribed to binuclear iron sites of different structures [58,56,93].

Panov et al. have suggested that a dinuclear iron complex with two α-sites is the active site for N2O decomposition that produces α-oxygens with high oxidation potential which can oxidize a wide range of organic molecules even at room temperature [98,99,117,118]. Sachtler et al. investigated the active iron sites in sublimed Fe-ZSM-5 zeolites in N2O decomposition [93].They formulated that the binuclear iron sites, similar to the active iron sites in SCR of NO, and larger clusters are more active for N2O decomposition than the mononuclear sites. Lioubov et al. reported a binuclear iron center similar to a diamond core structure of the MMO as the active site in N2O decomposition [94].

Centi and Vizzana have studied the activity, stability and nature of active sites of differently prepared Fe-ZSM-5 zeolites in SCR of N2O with propane under true industrial emission like conditions [82]. Samples prepared from liquid ion exchange method showed a large amount of iron oxide clusters and Fe2O3 particles while sublimed Fe-ZSM-5 showed mainly isolated Fe+3 ions and iron-oxo nanoclusters but not particles. The liquid ion-exchanged sample exhibited higher oxidation activity of propane and of SO2 to SO3 and the latter formed surface iron sulphate species that lead to the deactivation of the catalyst. In contrast, sublimed Fe-ZSM-5 showed a low oxidation activity and higher resistance to deactivation by SO2 during 600 h on stream. Hence, the authors concluded that the isolated Fe+3 ions and iron-oxo nanoclusters are the active iron sites for this reaction.

↓36

Delahay et al. studied a series of ion-exchanged Fe-beta zeolites for SCR of N2O with NH3 in the presence of O2 [95]. On the basis of their studies they proposed that the mononuclear iron oxo cations are the active sites for SCR of N2O with NH3.

Similarly, J. Pérez-Ramírez et al. studied the active site structure in N2O conversions over different Fe-MFI zeolites [96]. They have reported that steamed Fe-silicalite, which has exclusively isolated Fe+3 ions, was more active than a steamed Fe-ZSM-5 zeolite with pronounced cluster formation in SCR of N2O with different reducing agents. Interestingly, the steamed Fe-silicalite was less active in direct N2O decomposition than the steamed Fe-ZSM-5. Furthermore, they found different activation energies for the same reaction over these two catalysts. Hence, the authors concluded that in the reduction of N2O with a reductant the isolated Fe+3 ions are preferred over oligomers while the latter species are more active in direct N2O decomposition due to the easier oxygen recombination of two iron centres that are close together.

van Santen and Zhu et al. studied the effect of high temperature calcination and steaming on sublimed Fe-ZSM-5 catalyst for N2O decomposition [92].The authors observed disappearance of Brønsted acid sites in FT-IR spectra and high catalytic activity after high temperature calcination (973 K). Hence, it was concluded that upon high temperature calcination, iron oxide clusters react with Brønsted acid sites and form isolated cationic iron sites such as [FeO]+. Taking account of this observation they ascribed catalytic activity to cationic isolated iron sites and iron oxo nanoclusters.

↓37

In summary, despite considerable research on the nature of active iron sites in Fe-MFI zeolites for SCR of NOx/N2O and N2O decomposition, ambiguities and discrepancies still exist as described above. This could be due to the complexity of the Fe-MFI zeolites with respect to the iron constitution and the different sensitivity of the applied spectroscopic techniques to different iron species. It has been suggested on the basis of multitechnique studies that the Fe-ZSM-5 contains a multitude of iron species. In contrast, some authors concluded from their limited characterization studies the involvement of only one kind of iron species such as dimers in the reaction which seems to be highly unlikely. But it is rather likely that there is an involvement of different iron species in the reaction as suggested by other authors [36,82,96, 105]. It is also not yet clear whether the same active iron species that are involved in the NO-SCR are involved in the N2O reduction. Hence, this thesis was performed to clarify these contradictions and to give more insights into the structures of iron oxo sites formed by different preparation techniques and their role in the SCR of NOx/N2O and N2O decomposition by in situ spectroscopic studies.

2.4 Physico-chemical techniques for the characterization of Fe-containing zeolites

In the literature, Fe-MFI zeolites are comprehensively characterized for the structure of iron sites such as valence and coordination state of iron ions by using EPR [55,58,59,62-64,66, 93,104,106,119,120-130,131], UV/VIS-DRS [60-62,66,82,130,132-138], X-ray absorption spectroscopy (XANES and EXAFS) [35,36,56,57,62,64,66,86,93,105,108,111,116], Mössbauer spectroscopy [36,104,118,139] and XPS [36,83,104] and structural properties and surface acidity were investigated by Solid-state NMR [56,104,108,139,140], IR or FT-IR [36,104,141-143] and NH3-TPD measurements [103,141,144,145]. The Fe-MFI zeolites were also investigated under reaction conditions by in situ-EPR [59], in situ-X-ray absorption spectroscopy [35,57,105,116] and in situ-FT-IR spectroscopy [37,46,83-91] to identify the nature of active iron species and active reaction intermediates in SCR of NOx and N2O as well as in N2O decomposition.

In the following section, opportunities and limitations of the main characterization techniques used for Fe-containing zeolites are critically reviewed. Emphasis is dedicated to EPR, UV/VIS-DRS and FT-IR spectroscopy since these methods have been preferentially applied in this thesis. Special attention is paid to in situ spectroscopic studies, performed at elevated temperature and in the presence of reactants.

2.4.1 EPR spectroscopy of iron species

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EPR spectroscopy has been extensively used to identify the state of iron species in molecular sieves, since it is an efficient tool to identify isolated Fe3+ species of different coordination geometry [55,58,59,62-64,66,83,104,106,119-130] and Fe x O y clusters of different degrees of aggregation by analysis of the mutual magnetic interactions of the Fe sites [55]. Moreover, EPR spectroscopy has been also applied in situ under true SCR conditions. These studies were aimed at the identification of active iron species in SCR of NO by propane over Fe-MFI [59].

EPR is a unique technique to characterize geometrical and electronic peculiarities of different isolated Fe+3 ions in very low iron concentrations (which is often not possible by other techniques e.g. Mössbauer). Moreover, it provides information not only on the structure and valance state of isolated Fe+3 ions but also on electronic interactions between Fe+3 ions as well as with reactants. Based on the temperature dependence of the signal intensity one can derive information on the magnetic behavior such as para, ferri or antiferromagnetic interactions of the iron ions in zeolites.

The main advantage of EPR is that this technique can be used under true reaction conditions, i.e., at elevated temperatures and under reactant gas flow. Hence, the obtained results can be considered as authentic to draw conclusions on structure-activity relationships and, consequently, on the nature of active iron species in the SCR of NO or N2Oby different reducing agents and in direct N2O decomposition.

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In general, Fe+3 ions in zeolites are in high-spin electronic configuration since the possible ligands are oxygen, OH or H2O molecules which are weak field ligands. This results in a total spin of 5/2 and, thus, in a S ground state. The number and position of observable electronic transitions for Fe+3 ions depends strongly on the local crystal field symmetry, i.e zero field splitting (ZFS) parameters. The ZFS parameters (D and E) are expressed by second order terms in the spin-Hamiltonian:

H = gβHoS + D [Sz 2 – (1/3) S(S+1)] + E [Sx 2 – Sy 2] + higher order terms (2.20)

where g is the g-tensor, β is the Bohr magneton, Ho is the magnetic field vector, S is the electron spin operator, Sx, Sy and Sz are the spin matrices. The higher order terms come from the cubic field splitting constants and are usually much smaller than the ZFS parameters (D and E). The ZFS parameters are a measure of the deviation of the Fe+3 ion crystal field symmetry from cubic symmetry. For Fe+3 ions in cubic symmetry D = 0 and E = 0. In this case, the energy levels corresponding to the different values of the spin quantum number ms are degenerate in the absence of an external magnetic field. Under the influence of an external magnetic field, this degeneracy is removed and the energy levels split with equal distance. Then, the allowed five EPR transitions with Δms = ± 1 take place at the same resonant field value resulting in an isotropic EPR signal at g' ≈ 2 as shown in Fig. 1a. When distortion takes place from the ideal octahedral or tetrahedral symmetry to axial or rhombic symmetry respectively, the ZFS parameters for axial and rhombic distortion are D ≠ 0, E = 0 and D > E ≠ 0 respectively. In these cases, the orbital degeneracy is partly removed even in the absence of an external magnetic field giving rise to three so-called Kramer’s doublets (ms = ±5/2, ±3/2, ±1/2) as shown in Fig. 1b.When the zero field splitting between the Kramer’s doublets is large in comparison to the microwave energy (strong rhombic or axial distortion), the only allowed EPR transitions occur between ms = –1/2 and +1/2. In the case of maximum rhombic distortion for D >> hν, E/D ≈ 1/3, a single line at g' ≈ 4.3 is observed from the –1/2 → +1/2 transition together with a weak and broad feature at g' ≈ 9 which is due to a forbidden transition [120]. For D >> hν, E = 0 (strong axial distortion), a signal at g' ≈ 6 is observed [120].

↓40

Fig. 2.1. Schematic representation of energy level splitting for isolated Fe3+ ions in high symmetry (a) and axial distortion (b), adopted from [129].

For magnetically interacting Fe3+ ions, when the mutual distance between Fe3+ ions in a matrix is short enough, spin-spin dipolar and/or exchange interactions can average out the ZFS, resulting in a more or less isotropic signal in the range of g' ≈ 2. In general, the line width of those signals is much larger than that of g' ≈ 2 signals arising from isolated highly symmetric Fe3+ ions for which narrow lines are to be expected. Information, whether a signal at g' ≈ 2 arises from isolated or interacting Fe3+ species can be derived from the temperature dependence of the signal intensity.

For pure paramagnetic behavior (no magnetic interactions between Fe3+ ions) the EPR signal intensity follows Curie’s law and is inversely proportional to temperature. Deviations from this proportionality can provide information about the strength and type of magnetic interactions between Fe3+ ions in a sample. Thus, it has been shown that for antiferromagnetic compounds, long range antiferromagnetic ordering of the spins collapses above the Neel temperature and an EPR signal appears [131].

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It has been shown that in Fe-containing zeolites frequently three types of EPR signals are observed at g'-values of 6, 4.3 and 2. For illustration, the EPR spectrum of an Fe-ZSM-5 prepared by solid state ion exchange is shown in Fig. 2. The signal at g' ≈ 4.3 has been equivocally assigned to Fe+3 ions in tetrahedral coordination, either in framework [121-123] or extraframework positions [63,124,125], while the signal at g' ≈ 6 is frequently assigned to isolated Fe3+ species with higher coordination numbers [126,127]. However, it must be noted that just from the position of an EPR signal no conclusion on the number of coordinating ligands can be derived. The EPR signal at g' ≈ 2 has been typically assigned to iron oxide clusters. However, as mentioned above, isolated Fe3+ ions in positions of high symmetry (ZFS parameters D = E = 0) also contribute to an isotropic signal at g' ≈ 2 [123,128]. Moreover, it has been demonstrated, that small changes of D and E can induce dramatic changes in the number and position of observable Fe3+ signals [123,129]. Accordingly, R. Stösser et al. have shown that slight changes in the Fe site symmetry cause shift in the position of the EPR signal [146].

Fig. 2.2. X-band EPR spectrum at 293 K of an Fe-ZSM-5 prepared by solid-state ion exchange (Fe content: 5.2 wt.%).

2.4.2 UV/VIS-DRS spectroscopy of iron species

UV/VIS-DRS is an important technique which is partly complimentary to EPR. It is also able to distinguish between isolated Fe species of different structure and Fe x O y clusters of different nuclearity (which is sometimes not possible by other techniques e.g. EXAFS). It has been widely used to study Fe-zeolites [82,104,60,61,62,66,130,132-138]. In principle, two different types of electron transitions can be detected, mainly d-d and charge-transfer (CT) transitions. The d-d transitions of Fe+3 ions are symmetry and spin forbidden. Hence, often d-d transitions are weak and/or not observed. On the other hand, CT bands are allowed transitions and are usually intense. The wavelength of Fe+3 CT bands depends on the coordination number and on the degree of aggregation [58,82,66,147]. As an illustration example, the UV/VIS-DR spectrum of an Fe-ZSM-5 with 5.2% Fe is shown in Fig. 2.3.

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Fig. 2.3. Experimental UV/VIS diffuse reflectance spectrum and deconvoluted sub-bands of an Fe-ZSM-5 prepared by solid-state ion exchange (Fe content: 5.2 wt.%).

Usually, UV/VIS-DR spectra of iron species in solids contain several broad bands which overlap each other. This makes spectral analysis difficult, hence, for facilitating spectral assignment, spectra deconvolution has to be used [138]. In Fig. 3 the UV/VIS-DR spectrum of the sample exhibits strong absorption in the whole ultraviolet and visible region, which arises from Fe+3 ← O CT transitions. After deconvolution, six bands are resolved. This deconvolution was based on the following considerations. In general, Fe+3 ions give two CT bands associated to t1 → t2 and t1 → e transitions [132]. For isolated Fe+3 ions these two CT transitions fall in the high energy range of the spectrum (< 300 nm) [55,58,66,134,135]. Octahedral Fe+3 ions in small oligomeric iron-oxo clusters give bands between 300-400 nm and bands above 400 nm are characteristic for large iron oxide particles [82,135]. Assuming that the absorption coefficient is roughly equal in the maxima of the sub-bands, the percentage of each type of species can be derived from the area of the sub-bands. However, these values have to be regarded as an estimate due to the intrinsic uncertainty of the deconvolution procedure. Despite this fact the quantification provides a valuable indication of the relative amounts of various iron species structure among the zeolites investigated.

UV/VIS spectra of solids are recorded in diffuse reflectance mode. In general, in an UV/VIS-DRS spectrum the ratio of the light reflected from the sample and from an ideal non-absorbing reference standard is measured as a function of wavelength (λ). This can be expressed by Schuster-Kubelka-Munk equation:

↓43

F(R) = (1- R)2 / 2R= K/S (2.21)

where (R) is the ratio of the reflectivity of an infinitely thick layer of the sample and the one of a standard, K is the apparent absorption coefficient and S is the apparent scattering coefficient. Indeed, this equation is valid only under defined conditions. These are: sample should be an infinitely thick layer, diffuse monochromatic irradiation of the powder sample, isotropic light scattering, uniformly distributed TMI in low concentration (low absorption) and the absence of fluorescence.

The main advantage of the UV/VIS-DRS technique is that it provides information on d-d and CT transitions of iron species in the zeolite matrix, which strongly depend on the oxidation and coordination environment. Hence, this information can be used as a fingerprint for the identification of oxidation and coordination state of iron species in zeolites. Furthermore, for low absorbance [F (R) < 0.5] the Schuster-Kubelka-Munk function can be used for quantitative analysis [138].

↓44

To my knowledge, until now there are no reports on in situ applications of this technique during SCR of NOx and N2O over Fe-MFI zeolites. For the first time in situ UV/VIS studies for SCR of NOx and N2O were reported from this thesis [58].

2.4.3 FT-IR spectroscopy

FT-IR spectroscopy is an important and versatile technique to probe hydroxyl stretching and framework vibrations as well as surface acidity of micro and mesoporous materials. A great advantage of IR spectroscopy is that the construction of IR transparent windows withstands high temperatures and pressures. Hence, this technique is being widely used for in situ studies during catalysis (in the presence of reactants) to probe adsorbed species, active intermediates and the acid base-base properties of the catalyst surface.

Brønsted and Lewis acidity of Fe-zeolites are probed by FT-IR analysis of adsorbed probe molecules such as pyridine or NH3 [148]. Besides, formation of NOx species, intermediates or active adsorbed species/deposits during NH3- or isobutane-SCR of NO can be studied.

↓45

FT-IR spectroscopy belongs to the most frequently used technique for investigating the structural properties of MFI zeolites. Thus, it has been extensively used to probe both the hydroxyl stretching and framework vibrations. In general, bands around 3740 cm-1 are observed from terminal Si-OH groups, while a band around 3630 cm-1 is observed for Brønsted acidic bridging Si-O(H)-Al groups [104,141]. Usually, the number of Brønsted sites increases with the Al/Si ratio, since each Al atom replacing a Si framework atom creates a Brønsted site. Based on this information one can readily see the lattice arrangement of the zeolite. Grünert et al. found more silanol groups and less Brønsted OH groups in high Si/Al ratio (44) containing H-ZSM-5 zeolite than in H-ZSM-5 with low Si/Al ratio (14), indicating the presence of lattice defects in the former sample [36]. Infrared spectroscopy in the framework vibration range between 400-1200 cm-1 was also used to probe the framework substitution of iron ions. The band position of both symmetric and asymmetric Si-O-Fe stretching vibrations is shifted to lower wavenumbers as compared to Si-O-Si bands [141-143].

The role of acidity of Fe-MFI zeolites in the SCR of NOx is not completely understood and is still subject to debate [46,95,96,105]. FT-IR spectroscopy has been widely applied to investigate the surface acidity of Fe-MFI zeolites by adsorption of pyridine or NH3 [46,83,148]. This technique distinguishes between Lewis and Brønsted acidic sites and allows to discriminate between acidic sites of different strengths. For this purpose pyridine was considered to be a better probe molecule than NH3 [148]. In FT-IR studies of pyridine adsorption, Brønsted acid centres are detected by a band around 1540 cm-1, which originates from pyridinium ions (C5H5NH+) created via protonation of pyridine molecules by surface acidic hydroxyl groups[148]. On the other hand, Lewis acidic centres are characterised by bands around 1600 cm-1 and 1460-1445 cm-1, originating from pyridine coordinatively linked to a Lewis acidic sites. The band position around 1600 cm-1 reflects the strength of the Lewis acidity [148]. However, bands of hydrogen-bonded and physisorbed pyridine are expected in the similar range of Lewis acidic centres i.e., bands around 1440-1447 and 1580-1600 for hydrogen-bonded pyridine and 1439 and 1580 cm-1 for physisorbed pyridine [148,149,150]. Therefore, it is difficult to distinguish between differently adsorbed pyridine at normal conditions. However, exploiting the thermal stability of these pyridine species one can unambiguously determine the Lewis bonded pyridine by recording FT-IR spectra at sufficiently high temperatures [149]. Finally, the assignment of the band around 1620 cm-1 is ambiguously discussed in the literature. Buzzoni et al. ascribed it to Brønsted acidic sites in zeolites [150], while Busca et al. attributed it to Al Lewis acid sites of γ-Al2O3 and amorphous Al2O3/SiO2 [148].

In addition to the analysis of surface OH groups and acidity, IR spectroscopy has been extensively used in situ (in the presence of feed components) to probe the nature of adsorbed species and the reaction mechanism of the SCR of NOx and N2O [37,46,83-91]. Generally, adsorption of NO and co-adsorption of NO/O2 leads to the formation of adsorbed NOx (x ≥ 2). In the presence of NH3 or hydrocarbons, the formation of different adsorbed species/deposits can be detected. In general, adsorption of NO or NO/O2 on Fe-ZSM-5 leads to the formation of bands at around 1577, 1627, 1743, 1880, 2140 and 2198 cm-1. However, the assignment of some bands is ambiguously discussed. Table 2.2 gives an overview of the assignment of the different FT-IR bands that are reported in the literature. The 2198, 2140 and 1743 cm-1 bands are commonly assigned to [NO+][N2O4], NO+ (occupying cation exchange sites in the zeolite matrix) and N2O4 respectively [86-89,151]. But the ambiguity is concerning the 1577, 1627 and 1880 cm-1 bands (Table 2.2). For instance, Chen and Sachtler et al. observed 1625 and 1570 cm-1 bands [87], the 1625 cm-1 band assigned to nitro group coordinating to an iron ion and the 1570 cm-1 band is attributed to nitrate group. Similarly, Lobree and Bell et al. also observed bands at 1620 and 1577 cm- 1 and assigned these bands to NO2 and NO3 species respectively [88]. Different from the above author’s assignment, Hadjiivanov et al. assigned [89] the 1620 cm-1 band to bridging nitrates and the band at 1575 cm-1 to bidentate nitrate (Table 2.2). On the other hand the same 1880 cm-1 band was assigned to NO adsorbed on isolated Fe+3 ions [152], isolated Fe+2 ions [87,88] and iron oxide clusters [105] (Table 2.2). Hence, in this work an attempt was made to clarify these contradictions and to give more insights into the structures of adsorbed species formed by the adsorption of NO, NO/O2, NH3 and isobutane/NO/O2 by in situ FT-IR spectroscopic studies.

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Table 2.2. Summary of the assignment of the different FT-IR bands

Long and Yang studied the reaction mechanism of NH3-SCR of NO over Fe-ZSM-5 catalysts [37,46,83]. Upon subsequent adsorption of NH3, NO, NO/O2 and the complete NH3-SCR mixture they observed changes in the band positions and the appearance of new bands. On the basis of these findings they proposed a reaction mechanism, which is similar to that discussed in detail in the section 2.1.1. In this study the authors suggested that NO is first oxidized by Fe3+ ions to NO2 which subsequently reacts with NH4 + ions to form active NO2(NH4 +)2 complex. The active complex further reacts with NO to form N2 and H2O.

Sachtler et al. studied the reaction mechanism of HC-SCR of NOx over a Fe-ZSM-5 sample prepared by sublimation [86,87]. They proposed that Fe3+ sites are active centers to oxidize NO to NO2 and NO3 (NOx, x > 2). These NOx species react with hydrocarbons to form N-containing active intermediates, the structure of which is not yet conclusively known. However, this intermediate is reactive towards NO/O2 feed to form N2 (section 2.1.1).

2.4.4 Other spectroscopic techniques

Solid-state NMR spectroscopy

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Solid-state MAS 29Si-NMR and 27Al-NMR spectroscopy were implemented to probe the structural ordering of Fe+3 and Si+4 in Fe-silicalite and the coordination state of Si+4 and Al+3 in Fe-ZSM-5 zeolites after hydrothermal synthesis and steam activation [56,104,108,139,140]. Depending on Fe+3 and Si+4 interactions one can readily distinguish between long and short range local ordering, since the nucleus Si+4 feels the dipolar coupling with the paramagnetic Fe+3 sites which strongly depends on the spatial organization of the material. Accordingly, the presence of iron in the framework broadened the Si resonance and causes a shift of the signal linear with the iron concentration. This indicates a uniform distribution of iron species and a high degree of spatial ordering in Fe-ZSM-5 zeolite [104]. Depending on the tetrahedral or octahedral coordination of the 27Al-sites, peaks at different chemical shifts arise.Hence, 27Al-NMR spectroscopy was utilized to investigate the changes of aluminum coordination in the zeolites upon calcination and steam treatment. This information gives a hint on the position of Al in the zeolite such as in the framework or in the extraframework position.

Mössbauer spectroscopy

Mössbauer spectroscopy is used to study the nature and distribution of iron species in iron rich Fe-MFI zeolites, since it can detect the iron in almost all forms i.e., different oxidation states (Fe+3 and Fe+2), different coordination states (octahedral and tetrahedral) and different aggregation [36,104,118,139]. This ability distinguishes Mössbauer spectroscopy from other spectroscopic techniques. On the basis of isomer shift and quadrupole splitting parameters, the above mentioned different forms of iron species can be distinguished. On the other hand, the paramagnetic hyperfine structure in the presence of an external magnetic field provides information about the iron aggregation. However, small iron-oxo clusters such as oligomers require temperatures as low as < 1 K to get paramagnetic hyperfine structure resolved as compared to large aggregates. Large iron oxide clusters have different Debye temperatures as compared to isolated or oligomers [36]. Hence, this could lead to a biased spectral analysis. Accordingly, Grünert et al. have found that this technique overestimates large clusters and makes coexisting isolated Fe sites difficult to detect [36]. Furthermore, the low natural abundance of 57Fe isotope limits the technique to only iron rich samples and thus, samples with low Fe content are difficult to analyze.

X-ray Photoelectron Spectroscopy

Oxidation states of iron species in Fe-MFI zeolites are often investigated by X-ray Photoelectron Spectroscopy (XPS) [36,104,83]. This technique is surface sensitive and gives information about the external surface region of the material. Thus, it was found that Fe species which are present inside the zeolite pores could not be seen by this technique. In this technique, the binding energies of photoelectrons, liberated from the electronic core shells of Fe-atoms by X-rays are scanned. Depending on the valence state of surface Fe species, the Fe2p3/2 peaks appear at different binding energies. The binding energies of the Fe2p3/2 usually reported around ≈ 711 eV and ≈ 708 eV were attributed to Fe+3 and Fe+2 ions respectively [36,104,83].

X-ray absorption spectroscopy

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X-ray absorption spectroscopy has been extensively used to investigate the coordination, oxidation state as well as atomic bonding parameters such as bond lengths and coordination numbers of iron ions in Fe-MFI zeolites [35,36,56,57,62,64,65,86,93,105,108,111,116]. In particular, the near edge region of X-ray absorption (XANES) has been proven to be a valuable technique to gain qualitative information on coordination and oxidation state of iron ions. Thus, a well defined pre-edge peak, which arises from the 1s → 3d transition, at around 7110.5 eV was attributed to Fe+3 ions in tetrahedral or distorted octahedral coordination. The region behind the edge was used to analyse the degree of agglomeration of iron species.

This technique has been used in situ under conditions of isobutane SCR of NO at 623 K to monitor changes in both coordination and oxidation state of iron species and to derive information on active sites in Fe-ZSM-5 prepared by sublimation [35]. In this study, the sample was treated subsequently with He, O2, isobutane, NO, NO/O2 and isobutane/NO/O2. On the basis of the changes in the intensity of the pre-edge peak and the position of the Fe k edge peak, Koningsberger et al. studied the oxidation and coordination state of iron species in Fe-ZSM-5 zeolite [35]. Accordingly, XANES spectra of the catalyst after He, O2 and isobutane SCR of NO treatments were compared. The authors found that the spectrum after O2 treatment was similar to that of the spectrum after isobutane SCR of NO. Hence, they concluded that the mean oxidation state of iron under isobutane-SCR conditions is +3. These results were further supported by EXAFS and are discussed below.

EXAFS has been utilized to elucidate the coordination numbers, bond lengths, inter-atomic distances and degree of agglomeration of FeOx species. Thus, it has been used in situunder isobutane-SCR conditions [35]. As mentioned above, after subsequent feed treatments Koningsberger et al. studied the changes in the Fe-O and Fe-Fe coordination sphere [35]. Mainly they discussed changes in the Fe-O1 coordination sphere in the R range of 1.0 to 2.0 Å, which was attributed to Fe-O-Fe dimers. The changes in the intensity of Fe-O1 shell after He, O2, isobutane, NO and NO/O2 treatments were ascribed to the redox behavior of these dimers. Spectra of the catalyst after O2 treatment and isobutane-SCR reaction were compared and found to have similar spectra feature. Hence, they concluded that the mean oxidation state of iron is +3 in Fe-ZSM-5 catalyst under typical conditions of isobutane-SCR of NO.

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However, EXAFS can only give average coordination values of different iron species present in the sample and thus it is difficult to distinguish between different Fe sites (e.g. isolated ions, oligomers including dimers and large clusters) when they are coexisting in the same sample. Recently, Grünert et al. found that this technique is not sensitive enough to the poorly ordered intra-zeolitic iron oxide clusters [64]. Hence, using this technique, one cannot achieve unambiguous information about the degree of agglomeration of iron species, which is often encountered in Fe-molecular sieves. In summary, X-ray absorption spectroscopy is most sensitive for highly dispersed iron species but not for clusters hence, clusters could be underestimated.

Temperature-programmed desorption of NH 3

Temperature-programmed desorption of NH3 (NH3-TPD) was also applied to evaluate the intrinsic acidity of the catalysts [103,141,144,145]. The strength of the acidic sites in Fe-MFI zeolites can be qualitatively estimated by the desorption temperature of NH3: the stronger the acidic sites, the stronger NH3 is bound and, thus, the higher is the temperature needed for its desorption. However, in contrast to FT-IRof pyridine/NH3 adsorption, it is not possible to distinguish between Lewis and Brønsted acidity by NH3-TPD directly.

In summary, after critical review of the different characterization techniques it appears that, certainly, every technique has its own unique character to analyse Fe-zeolites but most of them are not able to distinguish between isolated Fe3+ species of different structure and iron oxide clusters of different nuclearity when they coexist in the same sample. For instance, Mössbauer spectroscopy is suitable for iron rich samples. However, it can also be used if samples are 57Fe enriched. In the presence of clusters, it is difficult to detect isolated Fe species and the large iron oxide clusters can be over estimated. X-ray photoelectron spectroscopy (XPS) is surface sensitive and it cannot detect Fe species that are present inside the pores. On the other hand, X-ray absorption spectroscopy (XAS) is sensitive only for highly dispersed Fe species but not for poorly ordered intra-zeolitic iron oxide clusters which are frequently present in the Fe-zeolites. By using NH3-TPD, intrinsic acidity of the zeolite can be measured but this technique cannot distinguish between Lewis and Brønsted acidity of the zeolite.

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However, from this critical review it also appears that EPR and UV/VIS-DRS are able to detect Fe3+ species in a variety of forms and at any concentration in the Fe-zeolite.First of all, EPR is a sensitive technique to identify isolated Fe3+ species of different coordination geometry by the position of their signals and Fe x O y clusters of different degree of aggregation by analysis of the mutual magnetic interactions of the Fe sites. The position of the charge-transfer (CT) bands of Fe3+ species in UV/VIS-DRS strongly depends on the coordination number and the degree of aggregation. Therefore, these two techniques are powerful tools to distinguish between isolated Fe3+ sites of different structures on the one hand and Fe x O y clusters of different nuclearity on the other hand.

FT-IR spectroscopy, on the other hand, is suitable to study the strength of the acidity and to distinguish between Lewis and Brønsted acidity of the zeolite by the position of the bands, which are characteristic for the interaction of the probe molecule (e.g., pyridine) with Lewis and Brønsted acid centres. Finally, considering all these special benefits of EPR, UV/VIS-DRS and FT-IR spectroscopy for characterizing complex materials like Fe-zeolites, these three techniques have been used for this thesis work to give more insights into the structure of Fe-oxo species formed by different preparation techniques and acidic properties of different Fe-zeolites with different framework composition as well as after different pre-treatments. Furthermore, these techniques have been used under in situ conditions (in the presence of reactants) to throw some light on the nature of active iron sites and adsorbed species.


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