Catalysis Today 100 (2005) 217–222
`
`www.elsevier.com/locate/cattod
`
`The effect of NO2 on the activity of fresh and aged zeolite
`catalysts in the NH3-SCR reaction
`
`Katariina Rahkamaa-Tolonena,*, Teuvo Maunulaa,
`Mari Lommab, Mika Huuhtanenb, Riitta L. Keiskib
`
`aECOCAT Oy, Catalyst Research, Typpitie 1, FIN-90650 Oulu, Finland
`bDepartment of Process and Environmental Engineering, University of Oulu, P.O. Box 4300, FIN-90014, Finland
`
`Available online 30 December 2004
`
`Abstract
`
`The activity of fresh and hydrothermally aged zeolite-based catalysts in the NH3-selective catalytic reduction (SCR) reaction with excess
`of oxygen were studied. In addition, the effect of NO2 in the gas feed as well as the acidity of the catalysts for the SCR activity was
`investigated. The studied catalysts were hydrogen, copper, iron and silver ion exchanged ZSM-5, mordenite, beta, ferrierite, and Y-zeolites.
`The investigation verifies that the zeolite-based catalysts are very promising for the ammonia SCR reaction. Especially, the activity at low and
`high temperatures was higher than the activity of commercial vanadia-based catalysts. From the studied catalysts, Fe-beta was the most
`potential one. The presence of NO2 in the inlet flow enhanced significantly the catalytic activity of fresh and hydrothermally aged zeolite
`catalysts. This suggests that the oxidation of NO to NO2 is probably the rate-determining step for the SCR reaction.
`# 2004 Elsevier B.V. All rights reserved.
`
`Keywords: NOx; NO2; NH3; Urea; SCR; Zeolites; Hydrothermal stability
`
`1. Introduction
`
`Nitrogen oxides remain a major source in air pollution.
`The emission limit values for heavy-duty vehicles are being
`made more stringent throughout the world. Engines that
`operate under lean burn (i.e., oxygen rich) conditions can
`provide significant fuel economy compared with stoichio-
`metric engines. In the presence of excess oxygen in the
`exhaust gas, however, NOx cannot be sufficiently removed
`by conventional three-way catalysts. Urea-selective cataly-
`tic reduction (SCR) is an attractive and proven after
`treatment method for
`future commercial heavy-duty
`vehicles. Unlike ammonia,
`the handling, storage, and
`transport of urea are efficient and safe. In addition, urea
`is non-toxic even at high concentrations in aqueous solution.
`Numerous development programs attempt to adapt the SCR
`technology for mobile diesel engines. In the urea-SCR
`system, urea will be hydrolyzed to ammonia and CO2 on a
`
`* Corresponding author. Tel.: +358 10 6535789; fax: +358 10 6535700.
`E-mail address: katariina.rahkamaa-tolonen@ecocat.com
`(K. Rahkamaa-Tolonen).
`
`0920-5861/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
`doi:10.1016/j.cattod.2004.09.056
`
`hydrolysis catalyst. Then ammonia reacts with NO to form
`nitrogen and water. The general reaction is as follows:
`4NO þ 4NH3 þ O2 ! 4N2 þ 6H2O:
`The effectiveness of the SCR technology for the control of
`NOx exhaust, using ammonia as a reductant, has been
`demonstrated in this study. Therefore, the crucial SCR
`catalysts can be studied without the effect of the urea
`hydrolysis variables.
`NO2 has been shown to enhance particularly the low
`temperature NOx reduction on conventional V2O5-WO3/
`the performance of
`TiO2-based catalysts [1]. However,
`V2O5-WO3/TiO2-based catalysts is not sufficient at low and
`high temperatures [2]. Many transition metal exchanged
`zeolites, such as Cu-ZSM-5, Co-ZSM-5, and Fe-ZSM-5, to
`mention a few, have been studied as a catalyst for the
`selective catalytic reduction of NOx [3,4]. It has been
`claimed that, e.g., Fe-MFI could exhibit higher activity in
`the SCR of NO by NH3 at stationary sources compared to the
`very classical V2O5-TiO2-based commercial catalysts [5].
`Zeolite-based catalysts have been studied extensively in
`NH3-SCR systems [6].
`
`Exhibit 2021.001
`
`

`
`218
`
`K. Rahkamaa-Tolonen et al. / Catalysis Today 100 (2005) 217–222
`
`Table 1
`Characteristics of zeolite-based SCR catalysts (M: metal)
`
`BET
`(m2/g)
`
`IER
`(%)
`
`M/Al
`(mol/mol)
`
`M (
`
`wt.%)
`
`Si/Al2
`ratio
`
`29
`22
`20
`20
`80
`29
`22
`20
`20
`80
`29
`22
`20
`20
`80
`22
`23
`22
`
`–
`–
`–
`–
`–
`–
`–
`–
`–
`–
`2
`1.6
`1.5
`1.1
`0.7
`3.0
`0.7
`0.9
`
`–
`–
`–
`–
`–
`–
`–
`–
`–
`–
`0.29
`0.18
`0.15
`0.11
`0.27
`0.34
`0.10
`0.12
`
`–
`–
`–
`–
`–
`–
`–
`–
`–
`–
`58
`36
`31
`23
`54
`34
`28
`36
`
`350
`510
`375
`275
`590
`343
`480
`407
`239
`649
`332
`433
`415
`270
`596
`452
`354
`544
`
`Catalyst
`
`ZSM-5
`Beta
`MOR
`FER
`Y
`H-ZSM-5
`H-beta
`H-MOR
`H-FER
`H-Y
`Cu-ZSM-5
`Cu-beta
`Cu-MOR
`Cu-FER
`Cu-Y
`Ag-beta
`Fe-ZSM-5
`Fe-beta
`
`of NO2 on the activity. A feed gas mixture contained
`600 ppm NO and 400 ppm NO2, and the other components
`were the same as in the experiment without NO2. In the case
`of fresh H-MOR and Ag-beta, the role of NO2 in the feed
`gas was studied in more detail. In this experiment, the feed
`gas mixture contained 0, 50, 100, 200, or 400 ppm NO2.
`The total amount of NO and NO2 was 1000 ppm (NOx).
`The catalysts were aged for 20 h at 600 8C in
`hydrothermal conditions (10% H2O in air) to evaluate the
`durability. Activity as well as adsorption and desorption
`properties of these catalysts were evaluated also after these
`hydrothermal pretreatments. The catalyst sample was
`flushed with nitrogen at 300 8C for 20 min before the
`NH3 adsorption step. After this the sample was cooled down
`to 200 8C and the sample was flushed with NH3 (500 ppm
`NH3 in N2) until the steady state was attained. The total gas
`1 at 25 8C. Thereafter, the sample was
`flow was 2 l min
`flushed with N2 for 5 min and the reactor temperature was
`1 under a 10%
`increased to 600 8C with a rate of 20 8C min
`O2/N2 mixture (TPO). The concentrations of desorbed and
`formed NH3, NO, NO2, and N2O were measured by a FT-IR
`gas analyzer. TPO with 10% oxygen was used instead of
`TPD (inert gas) to simulate the realistic conditions of NH3
`adsorption phenomena in lean SCR applications.
`
`3. Results
`
`3.1. Fresh samples
`
`The BET surface areas of the parent zeolites ZSM-5,
`MOR, beta, FER, and Y were 350, 375, 510, 275, and
`590 m2/g, respectively, and the pore volume was 0.23, 0.25,
`1, respectively.
`0.68, 0.18, and 0.45 cm3 g
`The hydrogen ion exchanged zeolites had a very low
`activity without NO2. The maximum NOx conversion of
`
`Zeolites are extensively used as shape-selective solid acid
`catalysts in many industrial processes [7]. Factors such as
`framework type and Si/Al2 ratio determine the catalytic
`properties of the material. The framework acidity can be
`modified by variation of the Si/Al2 ratio of the zeolite and the
`framework type or by substitution of Al by other trivalent
`elements. Modification of the framework acidity may lead to
`materials with improved catalytic properties. The acid
`properties of zeolites are considered as an important factor in
`controlling the catalytic activity in chemical reactions [8,9].
`It is assumed that SCR activity is enhanced by the surface
`acidity of catalysts.
`The aim of this study was to obtain knowledge about the
`activity of zeolite-based catalysts in the NH3-SCR reaction
`with excess of oxygen. The limited hydrothermal stability
`of zeolites may restrict their use, and therefore also the
`hydrothermal stability of catalysts was examined.
`In
`addition, the effect of NO2 in the gas feed as well as
`acidity of the catalysts for the SCR activity was investigated.
`The adsorption and desorption experiments of ammonia
`were used to study the acidity of fresh and aged zeolite
`catalysts and the influence of the acidity on the activity of
`zeolite catalysts.
`
`2. Experimental
`
`The zeolites used for the catalyst preparation were ZSM-
`5, mordenite (MOR), beta, ferrierite (FER), and Y-zeolites.
`The silica to alumina ratio of zeolites were 29, 20, 22, 20,
`and 80, respectively. The active cation (H, Cu, Fe, and Ag)
`was added into the zeolite structure by ion exchange. H-
`zeolite catalysts were prepared using NH4NO3, Cu-zeolite
`catalysts using (CH3COO)2Cu H2O, Fe-zeolite catalysts
`using FeCl3, and Ag-beta using AgNO3, respectively. The
`ion-exchanged metal ratio of aluminium was calculated as
`molar basis (n(M)/n(Al)), the value of which is insensitive to
`charge and as ion-exchange ratio (IER) in percent, (Table 1,
`metals analysed by XRF analysis (Philips MagiX)). The
`silica to alumina ratio of Fe-ZSM-5 was 23. Specific surface
`areas (BET) were measured by N2 adsorption using a
`volumetric Carlo Erba Sorptomatic 1990 analyzer.
`The powder sample (0.6 g) was inserted into a tubular
`quartz reactor heated by IR. The activity of a catalyst sample
`was examined with a simulated gas mixture (NH3 1000 ppm,
`NO 1000 ppm, H2O 8%, O2 10%, and N2 balance). The total
`1 at 25 8C and it corresponds to a
`gas flow was 1.2 l min
`1 with a typical monolith catalyst.
`space velocity of 25000 h
`The steady-state concentration were measured, by each
`50 8C, between 150 and 650 8C. Temperature was increased
`always 50 8C at a time and then it was kept constant for
`6 min at
`the reached temperature. The outlet gas was
`(GasmetTM FT-IR Gas
`analysed by a FT-IR analyzer
`Analyser).
`The activity experiments, where NO2 was added into the
`gas mixture, were also carried out in order to study the effect
`
`Exhibit 2021.002
`
`

`
`K. Rahkamaa-Tolonen et al. / Catalysis Today 100 (2005) 217–222
`
`219
`
`Fig. 2. NOx conversion over the fresh H-, Ag-, Cu-, and Fe-beta zeolites.
`Conditions: 1000 ppm NO, 1000 ppm NH3, 8% H2O, 10% O2, and balance
`with N2.
`
`Fe > Cu > Ag > H despite the fact that the amount of Ag
`(n(M)/n(Al)) was almost three times higher than the amount
`of Fe and two times higher than the amount of Cu. The
`following catalysts are the most
`suitable for high
`temperatures as fresh and their activity at 600 8C decreases
`following order: Cu-ZSM-5 (45%) > Fe-beta
`in the
`(34%)  Cu-FER
`(43%) > Ag-beta
`(41%) > Cu-beta
`(33%) > Fe-ZSM-5 (26%) > Cu-MOR (23%).
`the NOx
`When the reaction gas contained NO2,
`conversion was improved both at low (<300 8C) and at
`high (>450 8C) temperatures. This phenomenon is pre-
`sented for H-MOR and Cu-MOR catalysts in Fig. 3a and for
`H-FER and Cu-FER catalysts in Fig. 3b, respectively. It can
`be seen that H-MOR had a low activity at lower temperatures
`without NO2. The maximum NOx conversion was 34% at
`600 8C whereas the maximum NOx conversion was around
`60% at 350–500 8C in the presence of NO2. The activity of
`Cu-MOR was enhanced also at low and high temperatures in
`the presence of NO2. The same behaviour was observed in
`the case of H-FER and Cu-FER catalysts. The activity of H-
`FER zeolite increased dramatically in the presence of NO2.
`The presence of NO2 in the feed gas also diminished the
`differences between the catalysts. In addition, it decreases
`the observed ammonia slip over all the zeolite catalysts. All
`the catalysts had higher NOx conversions over a wider
`temperature range. The activity of most active fresh catalysts
`with NO2 decreases based on the maximum NOx conversion
`as follows: Cu-MOR (94%) > Fe-beta (92%) > Cu-FER
`(89%)  Cu-ZSM-5 (88%)  Cu-beta (87%)  Fe-ZSM-5
`(86%) > Ag-beta (82%) > Cu-Y (61%). Cu-Y-zeolite had
`again a substantially lower activity than the other catalysts.
`In addition, Cu-Y catalyst had a high ammonia slip
`compared to other catalysts and it also formed high amounts
`of N2O in the presence and absence of NO2 in the inlet flow.
`Also the other Cu-containing zeolites formed clearly higher
`amounts of N2O than the Fe-containing zeolites. This
`indicates that the selectivity to N2 is higher over the Fe-
`containing zeolites. The maximum selectivity to N2O was
`3% over the Fe-beta and Fe-ZSM-5 zeolites. Long and Yang
`[10] had similar findings on Fe-ZSM-5. According to them,
`
`Fig. 1. (a) NOx conversions of fresh Cu-ZSM-5, Cu-beta, Cu-MOR, Cu-
`FER, and Cu-Y-zeolites as a fuction of temperature. Conditions: 1000 ppm
`NO, 1000 ppm NH3, 8% H2O, 10% O2, and balance with N2. (b) NOx
`conversions of hydrothermally aged Cu-ZSM-5, Cu-beta, Cu-MOR, and
`Cu-FER zeolites as a fuction of temperature. Conditions: 1000 ppm NO,
`1000 ppm NH3, 8% H2O, 10% O2, and balance with N2; hydrothermal
`aging: 20 h at 600 8C in 10% H2O in air.
`
`29% was reached with H-beta at 350 8C. The activity of
`fresh catalysts without NO2 decreases based on the
`conversion
`as
`follows: Cu-MOR
`maximum NOx
`(93%) > Fe-beta
`(90%) > Cu-ZSM-5 (85%) > Cu-FER
`(75%) > Fe-ZSM-5
`(73%) > Cu-beta
`(69%) > Ag-beta
`(58%) > Cu-Y (45%). Based on the reaction initiation
`temperature (e.g., T50
`temperature, where 50% NOx
`conversion was reached) and wide temperature window,
`the activity of Cu-ZSM-5 among the Cu-containing catalysts
`was the highest without NO2 as can be seen in Fig. 1, where
`the catalytic activity of the fresh Cu-zeolites for NOx as a
`function of temperature is presented. The activity of Cu-Y-
`zeolite was substantially lower than the activity of other Cu-
`containing zeolites. The effect of ion-exchanged metal to the
`activity of zeolite material was investigated in the case of
`beta zeolite. As can be seen from Fig. 2, the ion-exchanged
`metal (Ag, Cu, and Fe) increased considerably the catalytic
`activity of beta zeolite. Fe-beta had the highest activity. It
`was active over a wide temperature range and it had the
`lowest ammonia slip (the concentration of NH3 after the
`catalyst) among the fresh catalysts. Cu-beta was more active
`than Ag-beta at temperatures lower than 400 8C whereas
`Ag-beta was more active at higher temperatures. Based on
`these results, it can be concluded that the catalytic activity
`of ion-exchanged cations on beta decreases as follows:
`
`Exhibit 2021.003
`
`

`
`220
`
`K. Rahkamaa-Tolonen et al. / Catalysis Today 100 (2005) 217–222
`
`decrease in the NOx conversion due to the production of NO
`as well as the formation NO2 and N2O indicates the
`oxidation on NH3. According to Alemany et al. [11], a
`decrease in the activity and selectivity of the V2O5-based
`catalyst, when the temperature of SCR reaction exceeds
`400 8C, is mainly due to production of NO and NO2 caused
`by ammonia oxidation.
`
`3.2. Aged samples
`
`The hydrothermal aging at 600 8C for 20 h decreased the
`activity of zeolite powder catalysts (Fig. 1b). The maximum
`NOx conversions were lower and they were reached at higher
`temperatures. Meanwhile, the activity of most of the catalysts
`was increased at high temperatures (T > 450 8C) compared to
`their activity as fresh. However, the activity of Cu-ZSM-5
`and Ag-beta decreased substantially at high temperatures.
`Based on the maximum conversions the activity of the aged
`catalysts without NO2 in the feed gas mixture decreases in
`the following order: Fe-beta (84%) > Cu-FER (72%)  Cu-
`MOR (72%) > Cu-ZSM-5 (66%)  Fe-ZSM-5 (65%) > Cu-
`beta (57%) > Ag-beta (38%). From the beta zeolite catalysts,
`the Fe-beta retained the catalytic activity considerably better
`than Cu- and Ag-beta after the hydrothermal aging. After
`aging, the following catalysts are the most suitable for high
`temperatures and their activity at 600 8C decreases in the
`following order: Cu-FER (69%) > Fe-beta (66%) > Fe-
`ZSM-5 (62%) > Cu-MOR (57%) > Cu-beta (54%) > Cu-
`ZSM-5 (33%) > Ag-beta (22%).
`The addition of NO2 to the inlet flow increased
`remarkably also the activity of hydrothermally aged
`catalysts and diminished the differences between the
`catalysts. All the copper containing zeolites had the reaction
`initiation temperature at 160–180 8C. The reaction initiates
`on Fe-ZSM-5 at around 150 8C and the NOx conversions of
`Fe-beta was already 70% at 150 8C. Fe-beta had also a very
`low ammonia slip. At the temperatures lower than 325 8C
`the maximum NH3 concentration after the Fe-beta catalyst
`was 26 ppm. At higher temperatures, the NH3 slip was near
`to zero. Ag-beta had clearly the highest reaction initiation
`temperature, which was 250 8C (50% converted). It had also
`the lowest maximum NOx conversion and highest NH3 slip.
`The effect of NO2 in the case of hydrothermally aged Fe-
`beta and Fe-ZSM-5 is demonstrated in Fig. 4. It can be seen
`the presence of NO2 enhanced especially the low
`that
`temperature activity. The activity of most active hydro-
`thermally aged catalysts with NO2 decreases based on the
`maximum NOx conversion as follows: Fe-beta (89%) > Cu-
`MOR (84%) > Fe-ZSM-5 (82%)  Cu-FER (81%) > Cu-
`beta (78%) > Cu- ZSM-5 (73%) > Ag-beta (70%).
`
`4. Discussion
`
`As was discussed above, the addition of NO2 to the inlet
`flow increased the catalytic activity of fresh and aged
`
`Fig. 3. (a) NOx conversions of fresh H-MOR and Cu-MOR in the presence
`and absence of NO2. Conditions: 600 ppm NO + 400 ppm NO2 or
`1000 ppm NO and 1000 ppm NH3, 8% H2O, 10% O2, and balance with
`N2. (b) NOx conversions of fresh H-FER and Cu-FER in the presence and
`absence of NO2. Conditions: 600 ppm NO + 400 ppm NO2 or 1000 ppm NO
`and 1000 ppm NH3, 8% H2O, 10% O2, and balance with N2.
`
`N2 was the only detectable N-containing product, and no
`N2O was observed for Fe-ZSM-5. The formation of
`undesired N2O (by N + NO) indicates that NO decomposi-
`tion has occurred or ammonia is partially oxidized by
`oxygen. However, the total oxidation activity is not too high
`because NH3 slip still exists except at high temperatures in
`the case of Cu-Y. Therefore, Cu-Y is not considered as a
`promising catalyst for NH3-SCR as well as not selected for
`further studies. The activity of hydrogen exchanged zeolites
`increased also significantly in the presence of NO2. H-FER
`had the highest NOx conversion, 81% at 300 8C (Fig. 3b).
`H-beta, which was the most active one without NO2 was now
`the second most active and had the maximum NOx
`conversion of 72% also at 300 8C.
`The effect of NO2 concentration in the inlet flow for the
`SCR activity was studied in detail with H-MOR and Ag-beta
`zeolites. It was observed that NOx conversion enhanced
`evenly over Ag-beta when the amount of NO2 in the feed gas
`mixture was increased from 0 to 400 ppm. Also the
`maximum NOx conversion was reached at lower tempera-
`tures. The presence of NO2 increased the reduction rate over
`the whole temperature range. In the case of H-MOR, the
`SCR activity did not increase so clearly at high temperatures
`(Fig. 3a). When the feed gas mixture contained 400 ppm
`NO2, ammonia started to oxidize at 500 8C. Therefore,
`the NOx conversion decreased at high temperatures. The
`
`Exhibit 2021.004
`
`

`
`K. Rahkamaa-Tolonen et al. / Catalysis Today 100 (2005) 217–222
`
`221
`
`Fig. 4. NOx conversions of hydrothermally aged Fe-beta and Fe-ZSM-5 in
`the presence and absence of NO2. Conditions: 600 ppm NO + 400 ppm NO2
`or 1000 ppm NO and 1000 ppm NH3, 8% H2O, 10% O2, and balance with
`N2.
`
`zeolite-based catalysts in NH3-SCR. The result indicated
`that most probably the reaction mechanism includes the
`oxidation of NO to NO2, which is a slow reaction step on
`zeolite-only catalysts. Therefore, the presence of NO2 in the
`feed gas mixture enhances the SCR reactions. This finding is
`in good agreement with the results of Coq et al. [12] who
`observed that NO2 reacts very fast with NO and NH3.
`According to Long and Yang, zeolites’ Bro¨nsted acid sites
`+ ions
`provide sites for ammonia adsorption, generating NH4
`+ ions with
`[4]. It was observed that the reactivity of NH4
`NO + O2 on Fe-ZSM-5 was much higher than that on H-
`ZSM-5, which is in accordance with the results of our study.
`The present study verifies that the SCR activity of hydrogen
`exchanged ZSM-5, MOR, beta, FER, and Y-zeolites was
`clearly lower than that on corresponding Cu, Fe, or Ag
`exchanged zeolites (Fig. 3). The addition of Cu, Fe or Ag
`to the zeolite increased the activities dramatically. The
`increase is related to the increase in NO oxidation to NO2.
`Therefore, the oxidation activity of Fe-beta is most probably
`higher than that on Cu-, Ag-, or H-beta (Fig. 2). NO2 is much
`+ ions, and therefore the
`more reactive than NO with NH4
`presence of it enhances the SCR activity. The oxidation of
`NO to NO2 is probably the rate-determining step for the SCR
`reaction.
`In the conditions where NH3 oxidation is low, NH3 and
`NOx react in the stoichiometry 1:1. Therefore, the lower the
`NOx conversion is, the higher is the NH3 slip. If NOx
`conversion and NH3 slip are both low and NOx conversion
`even negative, the NH3 oxidation activity is too high. The
`NOx reduction and NH3 oxidation rates are related to the
`cations and their amounts in zeolites. When the ion-
`exchange ratio is low, more acidic sites are free for NH3
`adsorption and cations are finely dispersed. If the amount of
`cations is high, there is a risk to have out of extra framework
`cations and metal oxide clusters, which can be too active for
`NH3 oxidation. The studied SCR catalysts were prepared by
`ion exchange, and therefore the IER (cation concentration)
`was balanced to the level where the cations are tightly on the
`zeolite structure.
`
`Fig. 5. Desorption of (a) NH3 and (b) NO over fresh zeolites during TPO
`(10% O2 in N2). Saturated with NH3 at 200 8C before TPO.
`
`The low SCR activity of H-Y and Cu-Y-zeolites
`correlates well with the results from ammonia adsorption
`and desorption in TPO experiments, which show that the
`parent Y-zeolite does not adsorb or desorb any ammonia
`(Fig. 5). Adsorbed NH3 is desorbed as NH3 at lower and
`mainly as NO of nitrogen oxides at higher temperatures in
`TPO experiments. The formation of N2O and NO2 in TPO
`was small with these catalysts. The selectivity for NH3
`desorption instead of nitrogen oxides was the highest with
`ZSM-5 and Beta of
`the non-ion-exchanged zeolites.
`However, the oxidation tendency was decreased in a few
`investigated cases when a cation was added into the zeolite
`(e.g., mordenite with Cu).
`The Y-zeolite has considerably higher Si/Al2 ratio than
`the other studied catalysts, and thus different acidity. The
`high Si/Al2 ratio of Y-zeolite does not promote the formation
`+ ions. It is known that a high Si/Al2 ratio
`of the NH4
`increases the bond strength and a low Si/Al2 ratio increases
`the adsorption capacity, respectively. Typically, the acidity
`of zeolites decreases after hydrothermal aging. The decrease
`in acidity is caused by the decreased surface area and
`dealumination. This was observed as lower amounts of
`adsorbed NH3 during the ammonia adsorption and
`desorption experiments after hydrothermal aging, because
`the adsorption capacity was decreased. Dealumination
`increases the Si/Al2 ratio, which in turn can increase the
`bond strength of adsorbents. This might explain the
`observed higher SCR activity at high temperatures after
`hydrothermal aging in the case of all other catalysts except
`
`Exhibit 2021.005
`
`

`
`222
`
`K. Rahkamaa-Tolonen et al. / Catalysis Today 100 (2005) 217–222
`
`Cu-ZSM-5 and Ag-beta. For example, with aged Cu-FER,
`the desorption of NH3, NO, and minor amount of N2O was
`observed at higher temperatures than before the aging.
`The strong acid sites correspond to the observed high
`temperature desorption. Another possible reason is the lower
`NH3 oxidation activity after aging. Thus, the formation of
`oxidation products, NO, N2O, and NO2, does not decrease
`the NOx conversion. After aging the highest NH3 oxidation
`activities, 16 and 9%, were observed in the case of Ag-beta
`and Cu-ZSM-5, respectively. The NH3 oxidation activity
`was calculated by the following Eq. (1):
`NH3 oxidation activityð%Þ
`¼ desorbedðNO þ NO2 þ N2OÞ
`adsorbedðNH3Þ
`These results are in accordance with the low SCR activity of
`zeolites after aging. The results allow us to conclude that in
`SCR reaction of NO with NH3, there is no doubt that the
`acidic function of the catalyst is one of the main factors
`which control the high activity. In particular, surface acidity
`plays an important role in the adsorption and activation of
`ammonia at high temperatures.
`
`100:
`
`(1)
`
`5. Conclusions
`
`The activity and hydrothermal stability of zeolite-based
`catalysts in the NH3-SCR reaction with excess of oxygen
`
`that based on the
`were studied. The results suggest
`hydrothermal stability, the best zeolite-based catalysts for
`NH3-SCR are Fe-beta, Cu-FER, Cu-MOR, and Fe-ZSM-5.
`These catalysts had the highest NOx conversions and widest
`temperature windows. Hydrothermal aging modifies the acid
`properties of zeolites. The higher the acidity retains, the
`better the hydrothermal stability for the SCR activity. The
`present study also indicates that the presence of NO2 in the
`feed gas mixture enhances the SCR reactions. The highest
`increase in the activity by NO2 was observed on H and metal
`promoted zeolites at low temperatures.
`
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`
`Exhibit 2021.006