Applied Catalysis B: Environmental 66 (2006) 208–216
`
`www.elsevier.com/locate/apcatb
`
`Investigation of the selective catalytic reduction of NO by
`NH3 on Fe-ZSM5 monolith catalysts
`Oliver Kro¨cher a,*, Mukundan Devadas a, Martin Elsener a, Alexander Wokaun a,
`Nicola So¨ger b, Marcus Pfeifer b, Yvonne Demel b, Lothar Mussmann b
`a Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
`b Umicore AG & Co. KG, Automotive Catalysts, Rodenbacher Chaussee 4, D-63403 Hanau, Germany
`
`Received 27 October 2005; received in revised form 20 March 2006; accepted 24 March 2006
`Available online 2 May 2006
`
`Abstract
`
`Fe-ZSM5 coated on cordierite monolith was investigated in the selective catalytic reduction (SCR) of NO with ammonia over a broad
`temperature range, applying simulated diesel exhaust gas conditions. The catalyst exhibited over 80% NOx reduction (DeNOx) from 400 to 650 8C
`at very good selectivity. The dosage of variable amounts of ammonia in the catalytic tests revealed that the SCR reaction is inhibited by ammonia.
`At very high temperatures DeNOx is reduced due to the selective catalytic oxidation (SCO) of ammonia to nitrogen and the oxidation to NO. Water-
`free experiments resulted in generally higher DeNOx values, which are explained by the inhibiting effect of water on the NO oxidation capability of
`Fe-ZSM5. The catalyst was stable upon thermal ageing and only 5–15% loss in DeNOx activity was observed after hydrothermal treatment. This
`loss in DeNOx is in parallel with a loss of ammonia storage capacity of the aged catalyst. Characterization by NH3 TPD and MAS 27Al NMR
`spectroscopy revealed dealumination of the zeolite by hydrothermal ageing, which reduces the Brønsted acidity of the catalyst.
`# 2006 Elsevier B.V. All rights reserved.
`
`Keywords: Fe-ZSM5; NOx conversion; Monolith; Stability; Urea SCR; Ammonia storage
`
`1. Introduction
`
`Iron zeolites of the type Fe-ZSM5 are successfully applied as
`catalysts for several chemical processes, e.g. the direct benzene
`to phenol hydroxylation [1,2], N2O decomposition [3,4] and NOx
`reduction in exhaust gases [5,6]. Among the latter diesel exhaust
`gases are especially challenging due to the changing tempera-
`tures, flows and concentrations. Currently, the selective catalytic
`reduction with urea (urea SCR) is judged to have the highest
`potential for coping the NOx emission problem of heavy-duty
`diesel engines [7–9]. Under the hydrothermal conditions in the
`exhaust pipe urea releases ammonia, which reacts with NO
`(>90% of NOx of an exhaust gas is formed by NO) according to
`the well-known standard SCR reaction [7]:
`4NH3 þ 4NO þ O2 ! 4N2 þ 6H2O
`
`(1)
`
`Fe-ZSM5 has been reported to be an active and selective
`catalyst for the SCR reaction in Refs. [5,6,10–13]. Despite of
`
`* Corresponding author. Tel.: +41 56 310 20 66; fax: +41 56 310 23 23.
`E-mail address: oliver.kroecher@psi.ch (O. Kro¨cher).
`
`0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
`doi:10.1016/j.apcatb.2006.03.012
`
`the valuable information provided in these laboratory
`investigations,
`some aspects have not been considered
`sufficiently for a reliable evaluation of Fe-ZSM5 as SCR
`catalyst:
`
`1. Water must be present in the basic feed in order to measure
`the actual performance of the catalyst and to investigate the
`‘‘real’’ functionality of the catalyst. However, omitting water
`may be useful
`in additional experiments to reveal
`the
`reaction mechanism.
`2. A good SCR catalyst combines a high NOx removal
`efficiency (DeNOx) with a high selectivity and low ammonia
`emissions after the catalyst. This means that suitable SCR
`catalysts must have acidic properties, which help to
`withdraw the ammonia in the catalyst and provide
`sufficiently high ammonia concentrations at
`the actives
`SCR sites at already low ammonia concentrations. Excessive
`ammonia dosage only slightly increases the ammonia
`concentration at the active sites. Therefore, the DeNOx
`remains nearly constant and all excessive ammonia is
`emitted. By plotting DeNOx versus ammonia slip these
`important catalyst properties are combined in one graph, e.g.
`
`Exhibit 2032.001
`
`

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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`209
`
`2. Experimental
`
`A commercially available Fe-ZSM5 powder was used as a
`model substance. The catalytic material was coated on a
`cordierite honeycomb of the size 4.66 in.  4.66 in.  3 in.
`with a cell density of 400 cpsi by Umicore automotive catalysts,
`Germany. For
`the catalytic investigation,
`the cordierite
`monolith was cut into pieces of the size 3.8 cm  1.7 cm 
`1.2 cm fitting to the sample holder of the tube reactor. Details
`about the plant set up are described in Ref. [14]. The gas hourly
`space velocity (GHSV = volumetric gas flow/coated monolith
`1, which represents the flow conditions
`volume) was 52,000 h
`in SCR converters on board of diesel vehicles. For tests of the
`standard SCR reaction, the composition of diesel exhaust gas
`was approximated by a model feed gas containing 10% O2, 5%
`H2O 1000 ppm of NO and balance N2. NH3 was added in the
`range 100–2000 ppm. The selective catalytic oxidation (SCO)
`reaction of ammonia to nitrogen was tested with a model feed
`consisting of 10% O2, 5% H2O and 1000 ppm NH3 but no NO.
`Water was omitted for supplementing investigations The
`concentrations of NO, NO2, N2O, NH3 and H2O in the gas
`phase were analysed by HR-FTIR spectroscopy (Nicolet
`Magna IR 560, OMNIC QuantPad software) equipped with a
`heated multiple pass gas cell.
`The thermal and hydrothermal stability of the Fe-ZSM5
`catalyst coated on cordierite was tested by ageing at 650 8C in
`the presence of 10% oxygen in nitrogen for 50 h (denoted as
`‘‘dry aged’’) and by ageing at 650 8C in 10% water and 10%
`oxygen in nitrogen for 50 h (denoted as ‘‘wet aged’’),
`respectively.
`The ammonia adsorption capacities of the fresh, the dry aged
`and the wet aged monolith catalysts were measured at six
`different temperatures between 200 and 450 8C by two different
`methods. (a) Thermal desorption: 1000 ppm ammonia was
`adsorbed on the fresh Fe-ZSM5 monolith catalyst at a fixed
`temperature in 10% O2, 5% H2O and balance N2. The GHSV
`1. In the desorption procedure, part
`was maintained at 52,000 h
`of the ammonia was removed by purging the sample at the
`adsorption temperature with pure N2, followed by increasing
`the temperature up to 450 8C in order to complete the ammonia
`desorption.
`(b) Combination of physical desorption and
`reaction of pre-adsorbed ammonia by NO according to the
`method of Kleemann et al. [16]: first, ammonia was adsorbed
`using a nitrogen gas flow containing 10% O2, 5% H2O and
`1000 ppm of NH3, at a fixed temperature until this concentra-
`tion was also reached at the catalyst outlet. Subsequently, the
`chemically accessible ammonia was removed by stopping the
`ammonia dosage and simultaneously dosing 10% O2, 5% H2O
`and 1000 ppm of NO at the same temperature until the NO
`consumption ceased. The physically desorbed ammonia was
`measured in parallel. Oxygen and water were always added in
`order to obtain ammonia storage capacities, being representa-
`tive for diesel exhaust gas conditions.
`Powder samples of the Fe-ZSM5 catalyst were used for
`characterization. Temperature programmed desorption of
`ammonia (NH3 TPD) was carried out in a TPD/TPR 2900
`analyser of Micromeritics measuring ammonia with a thermal
`
`Fig. 1. NH3 slip vs. DeNOx for the fresh Fe-ZSM5 monolith catalyst at (^)
`200 8C, (^) 250 8C, (~) 300 8C, (~) 350 8C, (&) 400 8C, (&) 450 8C, (*)
`500 8C, (*) 550 8C, () 600 8C, () 650 8C and (+) 700 8C.
`
`rectangular curve shape is a
`in Fig. 1. The almost
`consequence of the high SCR activities at low ammonia
`slip through the catalyst. Usually, ammonia emissions of
`about 10 ppm in average are regarded as harmless for
`automotive applications [14]. This necessitates a dosage
`control for adding the right amount of ammonia relating to
`the NOx concentration and the activity of the catalyst. For
`catalyst screening experiments dosing a constant amount of
`ammonia and measuring the activity of the catalyst as a
`function of temperature is a valuable tool. However, with
`regard to dosage control
`in automotive applications a
`completed description of
`the catalyst performance is
`necessary. In our opinion, the temperature dependency of
`the catalyst activity is best expressed with respect to the
`allowable ammonia slip of 10 ppm independent of the
`ammonia to NOx ratio dosed.
`3. Currently, a broad range of engine development strategies
`are followed, resulting in either low exhaust gas tempera-
`tures, e.g. for engines using excessive exhaust gas recycling
`(EGR), or very high exhaust gas temperatures, e.g. for
`engines without EGR but with hot diesel particulate filter
`(DPF) regeneration. Thus, to cover the maximum range of
`exhaust gas temperatures, the activity and selectivity of the
`catalysts should be checked from 150 to 700 8C.
`4. All above mentioned experimental studies report about Fe-
`ZSM5 powder catalysts, but monolith investigations are
`indispensable for gaining experimental data, which are
`representative of
`the situation in real world catalytic
`converters, i.e. low pressure drop, high geometric surface
`area and short diffusion distances [15].
`
`In this paper we report about Fe-ZSM5 coated on a cordierite
`monolith in the selective catalytic reduction and selective
`catalytic oxidation reaction. The experiments were performed
`mainly in the presence of water over a broad temperature range
`considering the dependency of DeNOx from the ammonia slip.
`Moreover, the stability as well as the ammonia storage capacity
`of coated Fe-ZSM5 were investigated.
`
`Exhibit 2032.002
`
`

`
`210
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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`conductivity detector. Fifty milligram of sample were degassed
`at 550 8C for 1 h in a He flow and cooled down to 100 8C. At
`this temperature adsorption of NH3 took place until saturation.
`Afterwards, the catalyst was flushed with He for 30 min. TPD
`measurements were performed from 100 to 650 8C with a
`heating rate of 20 8C/min, with He as carrier gas.
`27Al MAS NMR spectra were obtained on a Bruker
`Ultrashield 500 spectrometer at a magnetic field of 11.7 T
`equipped with a 4 mm MAS head probe. The aluminium
`resonance frequency at this field is 130 MHz. The sample
`rotation speed was 12.5 kHz. The 27Al chemical shifts were
`referenced to a saturated Al(NO3)3 solution. To obtain NMR
`spectra as quantitatively as possible in the presence of a
`heterogeneous distribution of quadrupolar coupling constants,
`the 27Al nuclei were excited with a single 208 pulse of 1 ms.
`Excitation pulses longer than 3 ms were seen to overemphasize
`the extra-framework aluminium signal intensity relative to the
`framework aluminium signal. The relaxation delay between the
`scans was set to 1 s. No saturation effects were observed in the
`spectrum for relaxation delays longer than 0.5 s.
`
`3. Results and discussion
`
`3.1. Catalytic performance of Fe-ZSM5 monolith catalysts
`
`3.1.1. Standard-SCR over Fe-ZSM5
`From Fig. 1, the high SCR activity of Fe-ZSM5 is clearly
`temperatures above 350 8C.
`discernible, especially at
`In
`accordance with the expected behaviour of a suitable SCR
`catalyst nearly all ammonia dosed goes into the SCR reaction,
`limited either by the activity of the catalyst at a given
`temperature or the stoichiometry of the SCR reaction. All
`additional ammonia causes an only marginal increase of the
`DeNOx. However, having a closer look at the ammonia slip at
`lower and intermediate temperatures up to 350 8C reveals an
`interesting behaviour. The steep part of the curves are slightly
`the NOx conversion first
`bent backward,
`indicating that
`increases as expected but then decreases again if ammonia is
`overdosed and ammonia slip is forced. Obviously, the SCR
`reaction is inhibited by ammonia. This effect
`is more
`pronounced in Fig. 2, where DeNOx values are plotted against
`the stoichiometric ratio a = NH3,in/NOx,in for different tem-
`peratures in the range 200–500 8C. These results suggest that
`the inhibition effect of ammonia could be due to competitive
`adsorption of ammonia and NO on the active sites. In fact, Eng
`and Bartholomew [17] and Stevenson et al. [18] observed a
`similar
`inhibition by ammonia on H-ZSM5, which was
`attributed to a competitive adsorption of ammonia and NO.
`Fig. 3 illustrates the differences between the DeNOx at
`10 ppm NH3 slip and the maximum DeNOx, plotted versus
`temperature in the range 200–700 8C. In accordance with Fig. 2,
`the difference between the maximum DeNOx and DeNOx at
`10 ppm ammonia slip decreases for lower temperatures due to
`the ammonia inhibition effect, which prevents that the catalyst
`activity is increased by adding ammonia in excess. In this
`temperature range DeNOx is rather limited by the reaction
`temperature. The maximum DeNOx is steadily increasing from
`
`Fig. 2. DeNOx vs. stoichiometric factor a for the fresh Fe-ZSM5 monolith
`catalyst at (^) 200 8C, (^) 250 8C, (~) 300 8C, (~) 350 8C, (&) 400 8C, (&)
`450 8C and (*) 500 8C.
`
`200 to 600 8C, reaching >90% for T > 450 8C. It decreases at
`temperatures beyond 600 8C. However, at 700 8C still around
`80% of conversion was achieved.
`
`3.1.2. Possible side-reactions during standard-SCR over
`Fe-ZSM5
`Generally, DeNOx is expected to be limited by the catalyst
`activity at lower temperatures, the stoichiometry of the SCR
`reaction at
`intermediate temperatures and by the catalyst
`selectivity at high temperatures. With increasing temperatures
`different side-reactions are conceivable producing N2O and
`NOx as well as increased amounts of nitrogen [19].
`For the production of N2O the direct oxidation of ammonia,
`2NH3 þ 2O2 ! N2O þ 3H2O
`and the reaction of ammonia with NO2 are discussed [19]:
`2NH3 þ 2NO2 ! N2O þ N2 þ 3H2O
`Other potential side-reactions of the SCR process are the
`selective catalytic oxidation of ammonia to nitrogen (SCO),
`4NH3 þ 3O2 ! 2N2 þ 6H2O;
`and the oxidation of ammonia to NO,
`4NH3 þ 5O2 ! 4NO þ 6H2O;
`which itself may be further oxidized to NO2.
`
`(2)
`
`(3)
`
`(4)
`
`(5)
`
`3.1.3. N2O formation
`Fig. 3 clearly shows that only negligible amounts of N2O
`(3 ppm) were formed during standard-SCR between 250 and
`400 8C and that no N2O was formed beyond 450 8C. This
`makes reaction (2) very unlikely, which is expected to produce
`increasingly amounts of N2O formation at higher temperatures.
`The reaction of ammonia with oxygen over Fe-ZSM5 was
`investigated without the influence of the SCR reaction by
`dosing ammonia without NO (Fig. 4a). No N2O was formed at
`any temperature, ruling out the relevance of reaction (2) for the
`formation of this side-product. Moreover, according to this
`
`Exhibit 2032.003
`
`

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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`211
`
`formation of N2O on Fe-ZSM5. However, having a closer look
`at the underlying chemistry, the contribution of reaction (2)
`cannot be strictly ruled out, as N2O might be formed as a short
`living intermediate at higher temperatures. In fact, Fe-ZSM5
`shows strongly increasing N2O decomposition activity at
`T > 400 8C
`2N2O ! 2N2 þ O2
`and N2O SCR activity at T > 350 8C [20]
`3N2O þ 2NH3 ! 4N2 þ 3H2O
`explaining the general absence of N2O at higher temperatures
`independent of the reaction conditions or the feed composition.
`
`(8)
`
`(7)
`
`Fig. 3. DeNOx vs. temperature for the fresh Fe-ZSM monolith (^) at 10 ppm
`NH3 slip with water in feed, (~) at maximum conversion with water in feed and
`(&) at 10 ppm NH3 slip without water. (~) N2O formation at 10 ppm NH3 slip
`with water in feed and (&) at 10 ppm NH3 slip without water.
`
`reaction equation the N2O formation should be independent of
`in the feed. However, experiments with
`the NO2 content
`increasing NO2 fractions in the feed, which will be subject of
`another publication, clearly showed the dependency of the N2O
`emissions on the NO2 concentration in the gas feed at 250–
`400 8C. This proves the relevance of reaction (3) for the
`
`3.1.4. SCO reaction and NO formation
`The investigation of the ammonia oxidation over Fe-ZSM5
`showed that ammonia conversion strongly increases with
`temperatures (Fig. 4a). Around 50% of the ammonia was
`oxidized at T = 600 8C and nearly 100% at 700 8C. The
`selectivity towards N2 was almost 100% till 600 8C, but beyond
`600 8C the selectivity towards nitrogen decreased accompanied
`with an increase in NO formation. The formation of NO beside
`the main product N2 in the ammonia oxidation experiment
`gives occasion to the assumption that during SCR over Fe-
`ZSM5 NO is not only consumed but also produced on the
`catalyst by reaction of ammonia with oxygen. This NO cannot
`be distinguished from the NO in the feed and, especially, if it is
`formed at the catalyst entrance it may also react downstream in
`the standard SCR reaction (1). Based on the observed products,
`it not possible to decide, if the oxidation of ammonia to nitrogen
`is a straightforward reaction or if it proceeds via NO as
`intermediate.
`
`3.1.5. NO2 formation
`Fe-ZSM5 has a distinct NO oxidation capability as shown
`in Fig. 5. The NO2 fraction increases with temperature as
`expected, but decreases again at higher temperatures due to
`the thermodynamic equilibrium between NO and NO2 lying
`on the side of NO. NO2 is essential for SCR activity of ZSM5-
`
`Fig. 4. SCO properties of the fresh Fe-ZSM5 monolith vs. temperature (a) in
`the presence of water and (b) without water. (^) NH3 oxidized, (~) selectivity
`towards NO, (&) selectivity towards N2 and (&) selectivity towards N2O.
`
`Fig. 5. NO oxidation to NO2 for the fresh Fe-ZSM monolith (&) with water in
`feed and (&) without water in feed.
`
`Exhibit 2032.004
`
`

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`212
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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`based catalysts [21], but, as the NO2 formed from NO is not
`observed under standard-SCR conditions (Fig. 3) it may be
`concluded that it is only a short living intermediate, which is
`immediately consumed. The decisive function of NO2 on the
`SCR reaction over Fe-ZSM5 was confirmed by adding NO2 to
`the feed, which drastically increased the DeNOx values up to
`low temperatures [22]. The
`NO2/NOx = 0.5, especially at
`SCR reaction involving NO2 is called fast-SCR [19] and is
`described by reaction equation(9). It may be regarded as the
`actual and general SCR stoichiometry over zeolite based
`catalysts, because either NO2 in the feed or the presence of an
`oxidation functionality in the zeolite for the oxidation of NO
`to NO2 is a pre-requisite for SCR activity:
`4NH3 þ 2NO þ 2NO2 ! 4N2 þ 6H2O
`
`(9)
`
`3.1.6. SCR stoichiometry
`The problem for the investigation of the SCR reaction is,
`that all reactions end up directly or via intermediates in
`nitrogen or NO, which cannot be distinguished from the feed
`components. Moreover, nitrogen cannot be detected by
`infrared spectroscopy. However, the contribution of all these
`side-reactions results in an increased consumption of ammonia
`relatively to the 1:1 stoichiometry of the SCR reaction. This
`excess consumption is derived from the curves in Fig. 6,
`showing the ratio of consumed ammonia to consumed NOx
`versus the ratio of dosed ammonia to dosed NOx at 200–
`700 8C, which is a direct scale for the compliance of the SCR
`stoichiometry. The values of 1–1.1 between 200 and 600 8C
`clearly demonstrate that Fe-ZSM5 follows the ideal SCR
`stoichiometry over a broad temperature range and that a
`maximum of 10% ammonia is consumed by ammonia
`oxidation. However, at 700 8C, values of about 1.5–1.7 were
`observed, indicating that the catalyst consumed an increasing
`part of ammonia mainly due to the oxidation to nitrogen at very
`high temperatures.
`
`Fig. 6. Observed ammonia to NOx stoichiometry vs. dosed ammonia to NOx
`ratio at (^) 200 8C, (~) 300 8C, (&) 400 8C, (*) 500 8C, (^) 600 8C and (~)
`700 8C.
`
`3.1.7. Water-free experiments
`In order to obtain a clearer picture of the functionality of the
`catalyst also water-free experiments have been performed.
`From Fig. 3 it is clearly discernible that Fe-ZSM5 shows a
`generally higher DeNOx activity if water is omitted. However,
`also much more N2O is formed reaching a maximum of 12 ppm
`at intermediate temperatures. As stated above this side-product
`is
`typically formed in larger amounts at
`intermediate
`temperatures if NO2 is present in the gas feed [22]. It is
`obvious from Fig. 5 that also much more NO2 is formed under
`water-free conditions than with water in the feed. Thus, the
`increase in N2O formation under water-free conditions is easily
`explained by the elevated concentration of NO2 on the catalyst.
`The higher oxidation capability of the catalyst under water
`free conditions, observable from the elevated NO2 concentra-
`tions in Fig. 5, is also apparent in the investigation of the
`ammonia oxidation without water as shown in Fig. 4b. By
`omitting the water, elevated NH3 conversions are observed over
`the whole temperature range. At
`lower temperatures the
`occurrence of NO and N2O is remarkable, which was not
`observed in the presence of water. Apparently, the ammonia
`oxidation to NO is promoted more than the following SCR
`reaction, resulting in the production of NO beside the main
`product N2. With increasing temperature NO decreases as the
`SCR reaction accelerates faster than the ammonia oxidation.
`Long and Yang [23] investigated the SCO reaction over
`powdered iron zeolites at temperatures up to 450 8C. They also
`found, that under water-free conditions the selectivity of the
`SCO reaction to nitrogen increases with temperature at the
`expense of a decrease in NO formation. This was explained by
`NO being the intermediate of the SCO reaction, which is
`reduced to nitrogen by NH3 in the SCR reaction.
`The formation of N2O seems to be coupled to the presence of
`gas phase NO. Due to the high oxidation capability of Fe-
`ZSM5, it is very likely, that part of the NO formed is further
`oxidized to NO2 due to the strong oxidizing properties of the
`catalyst under water-free conditions. The NO2, however, is not
`observable, as it is rapidly converted to nitrogen according to
`the ‘‘fast-SCR’’ reaction already at 200 8C [22]. Thus, the
`observed formation of N2O would be a side-product of the
`‘‘fast-SCR’’ reaction at low temperatures.
`
`3.2. Stability of Fe-ZSM5 monolith catalysts
`
`real-world conditions is an important
`Stability under
`parameter for the assessment of the suitability of a catalyst.
`Therefore, Long and Yang [24] investigated the durability of
`Fe-ZSM5 powder catalysts, aged in SO2 and H2O for 60 h at
`350 8C. They observed a 20% loss in NO conversion. Feng and
`Hall [25] studied the durability of the Fe-ZSM5 powder catalyst
`for a period of 2500 h at 500 8C. They observed a minimal loss
`in NO activity in a simulated exhaust gas stream. We checked
`both the thermal and hydrothermal stability of Fe-ZSM5, but
`coated on the cordierite honeycomb. Fig. 7a compares the NOx
`conversion at 10 ppm ammonia slip versus reaction tempera-
`ture of the aged catalysts with the results of the fresh catalyst
`taken from Fig. 3. The coated Fe-ZSM5 catalyst was stable
`
`Exhibit 2032.005
`
`

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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`213
`
`Fig. 8. Ammonia storage capacity of the fresh Fe-ZSM5 monolith catalyst
`determined by thermal desorption of ammonia.
`
`3.3. Total ammonia adsorption capacity of fresh and aged
`Fe-ZSM5 monolith catalysts
`
`In order to check this effect in more detail, the ammonia
`adsorption capacity of the fresh and aged monolith catalysts
`was measured and the corresponding powder samples were
`characterized by NH3 TPD and 27Al MAS NMR. First, the
`ammonia storage capacity was measured in simple thermal
`desorption experiments. As expected, the ammonia storage
`capacity decreased with increasing temperature (Fig. 8).
`Since the maximum temperature of 450 8C is expected to be
`too low for the quantitative desorption of strongly bound
`ammonia,
`the total ammonia adsorption capacity of
`the
`catalysts was also determined by a combination of physical
`desorption and ‘‘chemical’’ desorption, i.e. the reaction of pre-
`adsorbed ammonia with NO according to the method of
`Kleemann et al. [16]. The sum of the physically desorbed and
`reacted ammonia is depicted in Fig. 9, showing the same
`decreasing trend for the total amount of stored ammonia as
`Fig. 8. But, especially at T > 250 8C, a higher ammonia storage
`capacity was found for the combined method, indicating that in
`fact part of the ammonia remains on the catalyst surface in the
`exclusive thermal desorption experiments even at T = 450 8C.
`
`Fig. 9. Total ammonia storage capacity of the (&) fresh, (&) dry aged and ()
`wet aged Fe-ZSM5 monolith catalysts determined by the combination of
`thermal and chemical desorption of ammonia.
`
`Fig. 7. (a) DeNOx at 10 ppm NH3 slip and (b) maximum DeNOx for the (^)
`fresh, (&) dry aged and (*) wet aged Fe-ZSM5 monolith catalysts vs.
`temperature.
`
`under the dry ageing conditions. Only a small loss of NOx
`conversion at 10 ppm NH3 slip is discernible after dry aging
`over the entire temperature range. However, when the catalyst
`was aged in the presence of water a distinct loss in NOx
`conversion was observed, which increases with temperature.
`The picture partly changes for the maximum DeNOx of the
`catalysts (Fig. 7b), which was virtually the same above 500 8C.
`Only at lower temperatures the wet aged Fe-ZSM5 exhibited a
`5–15% loss in DeNOx. The aged catalysts exhibited the same
`inhibition of the SCR reaction by ammonia as observed for the
`fresh catalyst and also no N2O was formed at any temperature.
`The loss in NOx conversion for the wet aged catalyst must be
`due to the influence of water during ageing. The rather small
`loss of the maximum DeNOx compared to the DeNOx at
`10 ppm ammonia slip indicates that this loss is caused by a
`change in surface acidity of the catalyst and thereby influencing
`the ammonia surface coverage.
`Apart from the ageing experiments also the SO2 resistance
`of Fe-ZSM5 was checked. The coated Fe-ZSM5 catalyst was
`treated for 50 h at 500 8C with 100 ppm SO2 in 5% water,
`10% oxygen and nitrogen. A small increase in DeNOx was
`observed for the SO2 treated compared to the fresh catalyst
`(not shown here). This result is in line with Long and Yang
`[26] who also found an increase in DeNOx on sulphur
`treatment, which was explained by a small increase in surface
`acidity.
`
`Exhibit 2032.006
`
`

`
`214
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`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`The total amount of ammonia stored on the fresh catalyst is
`greater than on the aged catalysts (Fig. 9). The storage capacity
`in the catalysts decreases in the order fresh > dry aged > wet
`aged. As the ammonia storage capacity is a measure for the
`acidity of the samples, it is concluded that the acidity of Fe-
`ZSM5 is reduced by ageing, which supports the conclusions
`from the SCR results.
`According to the combined desorption method the total
`ammonia adsorption capacities depicted in Fig. 9 consist of two
`parts, which are plotted separately in Fig. 10, i.e. the ammonia
`
`reacted with NO in the SCR reaction and the chemically non-
`accessible ammonia desorbed physically. In the case of the
`fresh catalyst (Fig. 10a) there is a ‘‘step’’ in the amount of
`ammonia reacted above 350 8C and in the case of the aged
`catalysts (Fig. 10b and c) this ‘‘step’’ is observed above 250 8C.
`For fresh Fe-ZSM5, up to 350 8C a constant amount of
`ammonia is reacting via SCR (Fig. 10a), assigned to ammonia
`adsorbed on active SCR sites and mobile ammonia moving on
`the surface to the active sites. Beside this, a small amount of
`ammonia is desorbed unreacted before it gets in contact with
`NO. Above 350 8C the amount of reacting ammonia is
`considerably reduced. For the aged catalysts, up to 250 8C a
`constant amount of ammonia is reacting via SCR. If the acidity
`of an SCR catalyst is reduced, more ammonia is expected to be
`physically desorbed, which was found for the aged catalysts.
`The part of ammonia, which is physically desorbed, remains
`nearly constant over the whole temperature range for all three
`catalyst samples, except for T = 200 8C, where more ammonia
`is desorbed probably due to the very low SCR activity at this
`temperature. The observed ‘‘step’’ effect might be explained by
`two different acid sites on the surface, i.e. Brønsted and Lewis
`acid sites. Both types are covered at low temperature, but at
`high temperatures only the strong Brønsted acid sites are able to
`bind the ammonia.
`Temperature programmed desorption experiments with
`ammonia were performed with the fresh and aged catalysts
`as shown in Fig. 11. For the fresh Fe-ZSM5 sample, two well
`resolved peaks are centered at about 220 and 450 8C,
`respectively. The low temperature peak shifted from about
`220 8C to about 200 8C for the wet aged catalyst. Hidalgo et al.
`[27]
`found that
`the high temperature peak was always
`associated with ammonia adsorbed on acidic hydroxide groups
`(Brønsted acid sites). The interpretation of the low temperature
`peak is quite uncertain, i.e. it could be physisorbed ammonia
`[28], ammonia adsorbed on Si–OH [28] or ammonia from a
`non-zeolitic impurity [29]. The decrease in the high tempera-
`ture peak for the aged catalysts is attributed to the deal-
`umination effect, whereby framework alumina moves out of the
`
`Fig. 10. Ammonia storage capacities of the (a) fresh, (b) dry aged and (c) wet
`aged Fe-ZSM5 monolith catalysts determined by the combination of thermal
`and chemical desorption of ammonia. (&) Thermally desorbed ammonia and
`(&) ammonia reacted with NO.
`
`Fig. 11. NH3 TPD profiles of the (a) fresh, (b) dry aged and (c) wet aged Fe-
`ZSM5 powder catalysts.
`
`Exhibit 2032.007
`
`

`
`O. Kro¨cher et al. / Applied Catalysis B: Environmental 66 (2006) 208–216
`
`215
`
`the high temperature activity and
`temperature. However,
`thermal resistance of Fe-ZSM5 coated on cordierite monoliths
`suggest the use of this catalyst for the combination with a diesel
`particulate filter, which causes high exhaust gas temperatures
`during the regeneration process. Moreover, the usual platinum
`coating on the diesel particulate filter produces NO2, which
`supports the fast-SCR reaction. The observed DeNOx values
`are limited by the catalyst activity at low temperatures and by
`temperatures 700 8C,
`the selectivity at
`i.e. mainly the
`selective catalytic oxidation of ammonia to N2 (SCO reaction)
`and the oxidation to NO. In order to exploit the full DeNOx
`potential of the catalyst accurate dosing of the reducing agent is
`required with respect
`to the catalyst activity and the
`stoichiometry of the reaction, as overdosing of ammonia
`inhibits the SCR reaction at low temperatures. The distinct
`ammonia storage capacity of the catalyst was higher for the
`fresh catalyst than for aged catalysts, which could be attributed
`to the reduction of Brønsted acid sited needed for ammonia
`storage. This suggests the inclusion of the temporal change of
`the ammonia storage capacity in the dosing control model. No
`N2O was formed for both the fresh and aged catalysts in the
`SCR reaction. Supplementing water-free experiments revealed
`that water deactivates the SCR activity of the catalysts by
`hampering the oxidation capability of Fe-ZSM5.
`
`Acknowledgement
`
`D. Poduval (Schuit Institute of Catalysis, University of
`Eindhoven) is kindly acknowledged for carrying out the 27Al
`MAS NMR measurements.
`
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`Fig. 12. 27Al NMR spectra of the (a) fresh, (b) dry aged and (c) wet aged Fe-
`ZSM5 powder catalysts.
`
`zeolite lattice, due to the