The study of a synergetic effect over a H-ZSM-5/V2O5 hybrid catalyst on SCR reaction

Article abstract of DOI:10.1016/j.molcata.2005.03.028

The synergetic effect over a H-ZSM-5/V₂O₅ hybrid catalyst on selective catalytic reduction (SCR) reaction has been studied by steady state reaction (SSR), temperature programmed reaction (TPR) and NH ₃-temperature programmed desorption (TPD). The improved NO conversion is observed on the layered hybrid catalyst with V₂O ₅ as the first layer. TPR experiment suggested that V ₂O₅ plays a role as a supplier of oxygen source through N₂O formation, while H-ZSM-5 decomposes N₂O into N ₂ and α-oxygen. α-Oxygen may contribute to the increase in NO₂ species on H-ZSM-5 and improves the apparent SCR activity. NH₃-TPD by NO experiments shows the possibility of the contribution of spillover oxygen in the improvement of SCR activity.

Full text of DOI:10.1016/j.molcata.2005.03.028

Journal of Molecular Catalysis A: Chemical 236 (2005) 94–98
The study of a synergetic effect over a H-ZSM-5/V2O5
hybrid catalyst on SCR reaction
Seong Moon Jung∗,1, Olivier Demoulin, Paul Grangeꢀ
Unite´ de catalyse et chimie des mate´riaux divise´s, Universite´ catholique de Louvain, Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium
Received 2 December 2003; received in revised form 21 March 2005; accepted 22 March 2005
Available online 23 May 2005
Abstract
The synergetic effect over a H-ZSM-5/V2O5 hybrid catalyst on selective catalytic reduction (SCR) reaction has been studied by steady state
reaction (SSR), temperature programmed reaction (TPR) and NH3-temperature programmed desorption (TPD). The improved NO conversion
is observed on the layered hybrid catalyst with V2O5 as the first layer. TPR experiment suggested that V2O5 plays a role as a supplier of
oxygen source through N2O formation, while H-ZSM-5 decomposes N2O into N2 and ␣-oxygen. ␣-Oxygen may contribute to the increase in
NO2 species on H-ZSM-5 and improves the apparent SCR activity. NH3-TPD by NO experiments shows the possibility of the contribution of
spillover oxygen in the improvement of SCR activity.
© 2005 Elsevier B.V. All rights reserved.
Keywords: V2O5; H-ZSM-5; Hybrid catalyst; Synergetic effect; SCR reaction
1. Introduction
elimination with ammonia as a reducing agent has been ex-
tensively studied as a non-catalytic technology (SNCR) and
a catalytic technology (SCR) for stationary sources [1–6].
The SCR mechanisms can be classified into two cat-
egories: (i) Eley–Rideal, and (ii) Langmuir–Hinshelwood
[7–16]. The former mechanism has been emphasized at low
temperature over reducible materials including catalysts like
V2O5 based catalyst, which enables the adsorbed ammonia
to activate, whereas the Langmuir–Hinshelwood mechanism
has been used to explain the activity of highly acidic catalysts
with negligible reducibility such as zeolites and sulfate types,
at high temperature.
In the Eley–Rideal mechanism, the ammonia adsorbed
on the catalyst surface should be activated to react with NO
from the gas phase. Ramis et al. [7] have suggested that
the amide or amide radical forms through the dissociation
of ammonia adsorbed on Lewis acid sites, while Topsøe et
al. [8] have concluded that the NH3+ formed by the dissocia-
tion of ammonia adsorbed on Brønsted acid sites is the main
species. Although the adsorption site of ammonia for the for-
mation of active species is still under debate, the agreement
point between both authors is that one H atom of ammonia
The elimination of nitrogen oxides emitted from the com-
bustion process is particularly important in the reduction of
the environmental problems caused by the formation of acid
rain and ozone through secondary reactions in the atmosphere
[1]. Several processes have been proposed for the elimina-
tion of NOX through the widespread application of available
methods and/or via the development of new technologies [2].
Among the flue gas treatment methods, selective catalytic re-
duction (SCR) is well developed and used worldwide due to
its efficiency, selectivity and economy. The SCR process is
based on the reaction between NOX and a reducing agent such
as CO, H2 and ammonia, injected into the flue gas stream, to
produce innocuous water and nitrogen [1]. Especially, NOX
∗
Corresponding author at: Corporated Research & Development, LG
Chem, Ltd./Research Park, 104-1, Moonji-dong, Yuseong-gu, Daejeon 305-
380, Korea Tel.: +82 42 866 2892; fax: +82 42 863 7466.
E-mail address: jungsm@lgchem.com (S.M. Jung).
1
Tel.: +32 10 47 35 80; Fax: +32 10 47 36 49.
ꢀ
Deceased.
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.03.028

S.M. Jung et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 94–98
95
should be dissociated by the reducible site located near the
ammonia.
nitric oxide 0.1%; ammonia 0.105%; 2.5% oxygen in helium.
The reactor was vertical and made of quartz. The ramping rate
of temperature in TPR was 10 ◦C/min. The inlet and outlet
gas compositions were measured using a QMC 311 Balzers
quadrupole mass spectrometer coupled to the reactor.
NO oxidation was also carried out in a continuous flow
fixed bed reactor operating at atmospheric pressure. The to-
tal flow rate was 100 ml/min and the feed composition was:
nitric oxide 0.1%; oxygen 2.5%. The inlet and outlet NO2
compositions were measured using a detector with a chemi-
cal sensor (TX2000, Oldham).
An NH3-TPD (temperature programmed desorption)
spectrum was obtained by monitoring the desorbed species,
after ammonia adsorption (0.5 vol.% in helium) on the cat-
alyst at room temperature, under 50 ml/min of diluted nitric
oxide flow (0.5 vol.% in helium) while increasing the temper-
ature at a constant rate (10 ◦C/min). Outlet gas compositions
of NO and N2 were measured using a Balzers QMC 311
quadrupole mass spectrometer coupled to the reactor.
The mechanism involving the dissociation of ammonia
was already known in SNCR [4]. The NH2 radical formed
by the reaction of OH radical with ammonia at 800 ◦C is
known as the main species, which prefers the reaction with
NO to N2. Besides, in the SNCR mechanism, further disso-
ciation ammonia generates NH and this material prefers the
reaction with O2 to NO. The decrease in NO conversion is
usually identified over all SCR catalysts. As far as the SNCR
mechanism is concerned, it can be imagined that the excess
hydrogen dissociation of ammonia to NHX species due to the
highly reducible site would occur at high temperature and is
the main step to accelerate the ammonia oxidation.
In a previous paper [17], we reported that the hybrid cat-
alyst between TiO2 sulfate and V2O5 showed a high activity
due to the synergetic effect, which is attributed to the in-
creased amount of nitrate and NH2 species. In this hybrid
system, the effect of oxygen transfer was supposed to be the
main synergy in the mechanism.
In addition, the hybrid system consisting of H-ZSM-5 and
V2O5 also showed the modified SCR activity. Based on the
SCR mechanism of H-ZSM-5 [18–21], this promotional ef-
fect can be explained by the enhanced formation rate of NH2
and NO2 species. As a result, it is postulated that the oxy-
gen from V2O5 can transfer to the surface of strongly acidic
materials (TiO2 sulfate and H-ZSM-5) and that this oxygen
contributes to the increase in the rate of NH2 and nitrate for-
mation. The oxygen transfer in the hybrid system may be
explained by hypotheses such as (i) transfer of oxygen by
NO2 formation over V2O5, (ii) ␣-oxygen due to the decom-
position of N2O over H-ZSM-5 catalyst and (iii) spillover of
oxygen.
In this communication, we focus on the identification of
the oxygen source in the hybrid catalytic system with V2O5
and H-ZSM-5. Firstly, in order to verify the synergy, the SCR
reaction was performed using hybrid catalysts consisting in
V2O5 and H-ZSM-5 arranged in the reactor in different ways,
namely, disposed in separate beds with either V2O5 or H-
ZSM-5 in the top layer. Additionally, the various reaction
techniques were employed over pure and hybrid materials.
3. Results and discussion
Using the hybrid catalysts prepared by superposition of
the two solids, SCR reaction was carried out and the re-
sult are compared with those obtained with pure V2O5 and
H-ZSM-5, as shown in Fig. 1. Pure V2O5 shows a consid-
erable activity at temperatures lower than 350 ◦C. But the
NO conversion decreased due to the ammonia oxidation af-
ter 350 ◦C. It is reported that over pure V2O5, the decrease
in NO conversion at high temperature is attributed to the di-
rect oxidation of ammonia to NO since the reduced vanadium
surface is fully reoxidized. On pure H-ZSM-5, the SCR activ-
ity appeared at 400 ◦C, while ammonia oxidation was negli-
gible.
When H-ZSM-5 was placed in the first layer, the fea-
ture of NO consumption in the SCR reaction was almost
2. Experimental
H-ZSM-5 (PQ Co.) has a surface area of about 400 m2/g
and a SiO2/Al2O3 molar ratio of 30. used in this experiment is
a commercial oxide (Aldrich Co.). The specific surface area
of V2O5 (Aldrich Co.) is 2.5 m2/g. Hybrid H-ZSM-5/V2O5
catalysts are prepared through the superposition of H-ZSM-5
and V2O5 with an equal weight (0.075 g).
Temperature programmed reaction (TPR) and steady state
reaction (SSR) were carried out in a continuous flow fixed
bed reactor operating at atmospheric pressure. The catalyst
weight was 0.075 g in the case of a pure sample. The total flow
rate was 100 ml/min and the feed composition (volume) was:
Fig. 1. SCR activity of V2O5 (ꢁ), H-ZSM-5 (᭹), H-ZSM-5 (F)-V2O5 (S)
(♦), and V2O5 (F)- H-ZSM-5 (S) (ꢂ). Theoretical value (ꢃ).

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S.M. Jung et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 94–98
the same as the sum of activities observed on pure V2O5
and H-ZSM-5. However, when V2O5 was placed in the
first layer, the NO conversion at all temperature ranges was
higher than the sum of activities of pure V2O5 and pure
H-ZSM-5.
IntheSCRmechanismoverH-ZSM-5[18–21], itisknown
that the NO to NO2 oxidation is the rate-determining step.
Considering this suggestion, it could be imagined that the
role of V2O5 can be attributed to the increase in the oxi-
dation rate of NO2. It is also conceivable that two differ-
ent pathways exist by which V2O5 can increase the oxi-
dation rate of NO to NO2: (1) direct oxidation of NO by
V2O5 (V2O5 + NO → V2O4 + NO2) and (2) indirect oxida-
tion via oxygen transfer (V2O5 → V2O4 + O∗; O∗ migrates
to H-ZSM-5 so as to oxidise NO).
Fig. 3. TPR result obtained over pure V2O5 under a flow over NO
(1000 ppm), NH3 (1000 ppm) and O2 (2.5%) in He.
3.2. The indirect formation of NO2 via oxygen transfer
3.1. The direct formation of NO2 over V2O5
TPR experiment were performed in order to elucidate the
possibility that V2O5 acts more like an O reservoir, increas-
ing the oxidation rate of NO to NO2 considered as the rate
determining step of the SCR reaction over H-ZSM-5.
Misono proposed a bifunctional mechanism in SCR by
propylene over a physical mixture of Mn2O3 and Ce-ZSM-5
[22]. Inthissystem, theroleofeachcomponentwasexplained
as follows: Mn2O3 oxidizes NO to NO2 and Ce-ZSM-5 is re-
sponsibleforthereactionbetweenNO2 andpropyleneandthe
decomposition of intermediates to N2. Considering this syn-
ergetic scheme, the direct formation of NO2 over V2O5 can
be postulated over the hybrid system of V2O5 and H-ZSM-5
in the SCR reaction. To identify the contribution of V2O5 on
NO2 formation, the direct NO oxidation was carried out over
pure H-ZSM-5 and pure V2O5 in the presence of oxygen. The
NO2 concentrations observed are shown in Fig. 2. H-ZSM-5
catalysts showed a higher NO2 concentration at room temper-
ature, while no NO2 formation was detected on V2O5. When
temperature increased up to 200 ◦C, the NO2 formation over
H-ZSM-5 rapidly decreased. But when temperature further
increased, NO2 formation gradually increased. In the case of
V2O5, only 8ppm NO2 was detected at 450 ◦C. This result
suggests that the direct oxidation of NO to NO2 over V2O5
is not sufficient to explain the synergetic effect observed in a
layered hybrid system.
The distribution of the products over V2O5 in the
NO + NH3 + O2 reaction was investigated firstly. The change
of composition according to temperature increase is shown in
Fig. 3. The behavior of NO is similar to that observed in the
steady state SCR reaction, shown in Fig. 1. It was shown that
the increase in the NO concentration above 350 ◦C is accom-
panied by either the increase in the N2O concentration or by
the decrease in N2 concentration. The N2 concentration curve
in TPR above 350 ◦C is almost symmetrical with that below
350 ◦C, whereas the increasing slope of NO above 350 ◦C
was steeper than the decreasing slope of NO below 300 ◦C.
The different behavior in the NO concentration change be-
low and above 350 ◦C suggested the contribution of ammonia
oxidation reaction. In other words, 1 mole of NO and 1 mole
of NH3 react to be converted to 1 mole of N2 below 350 ◦C,
whereas above 350 ◦C, NH3 gradually reacts with O2 to form
NO preferentially. Thus, the total NO concentration above
350 ◦C is attributed to the sum of the NO produced due to
ammonia oxidation and the unreacted NO.
The N2O formation began between 300 and 350 ◦C and is
maximized at 450 ◦C. Above 450 ◦C, the N2O concentration
decreased, whereas NO formation still increased. In this ex-
periment, NO2 is not detected. Thus, it was shown that the
ammonia oxidation began around 300–350 ◦C and N2O is the
only by-product, not included in reactant.
To find the contribution of N2O on the promoted SCR ac-
tivity over the layered hybrid sample, a TPR experiment was
also performed over the layered hybrid catalyst with V2O5
and H-ZSM-5 as the first and the second layers, respectively.
The results are depicted in Fig. 4. Below 350 ◦C, the distri-
bution of product is similar to that observed for pure V2O5
as shown in Fig. 2. Above 350 ◦C, the high consumption rate
of NO is maintained up to 480 ◦C. Especially, the consump-
tion rate of NO between 350 and 480 ◦C led to the increase
Fig. 2. NO2 concentration (ppm) formed according to the temperature over
H-ZSM-5 (ꢁ), and V2O5 (᭹), under a flow of NO (1000 ppm) and O2 (2.5%)
in He.

S.M. Jung et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 94–98
97
Fig. 4. TPR result obtained over layered hybrid sample composed of V2O5
(F)- H-ZSM-5 (S) under a flow of NO (1000 ppm), NH3 (1000 ppm) and O2
(2.5%).
in the N2 concentration as well as the consumption of N2O.
From this result, it is suggested that N2O may participate in
the reaction of NO with ammonia. Kapteijn et al. [23]. exten-
sively studied the behaviour of the N2O over solid catalysts,
including the transition metal exchanged zeolites. They re-
ported that the nitrous oxide began to decompose into N2 and
activated oxygen at 350 ◦C, according to Eq. (1):
Fig. 5. N2O decomposition over H-ZSM-5.
step, the behavior of N2O can also be interpreted as shown
in Scheme 1: N2O adsorbed on coordinatively unsaturated
Al and Si, after dehydration, and then scission of N2O takes
place by the neighbor negative charge. The dehydrated site of
the surface is replenished by the surface oxygen (O∗), which
is produced by the decomposition of N2O. Up to 470 ◦C,
since the surface oxygen combined with the structure may be
stable, the conversion of N2O was decreased. This result sug-
gests the following: the N2O conversion below 470 ◦C may
not be detected in steady state reaction, since the replace-
ment of surface oxygen into the dehydrated site re-stabilizes
the surface structure and makes the surface inactive for N2O
decomposition. Above 470 ◦C, however, the surface oxygen
captured in the structure is emitted as O2, as shown in Fig. 5.
N2O ꢄ N2 + O∗
(1)
In the above equation, the activated oxygen (O∗) is sup-
posed to be a strong oxidant. Thus, the contribution of N2O
on SCR reaction over H-ZSM-5 can be depicted as follows:
N2 + O∗ + NO → NO2 + N2
NO2 + NH3 → N2 + H2O + OH
(2)
(3)
To verify the reaction of N2O over H-ZSM-5, the N2O
decomposition was performed. Fig. 5 shows the dependency
of N2O concentration on temperature as well as the product
distributions due to the decomposition of N2O. Up to 300 ◦C,
there was no change in the N2O concentration. The evolution
of the water adsorbed on the surface was only detected below
300 ◦C. The first consumption of N2O occurred between 350
and 470 ◦C, with the appearance of N2 and H2O, although
the N2O concentration gradually recovered from 450 ◦C to
reach 500 ppm. The second consumption of N2O appeared at
470 ◦C and continued up to 600 ◦C. In the second consump-
tion of N2O, O2 instead of H2O was detected with N2. This
result clearly indicates that pure H-ZSM-5 itself can show an
activity for the N2O decomposition.
In order to explain the pathways of N2O decomposition
based on the conventional mechanism, the in-depth zeolite
structure should be considered. It is known that the dehy-
dration of the H form zeolite at high temperature creates a
coordinatively unsaturated site and excess negative charge
in the structure, as shown in Scheme 1 [24]. As mentioned
above, the active site for the N2O decomposition should be
followed by dehydration. Actually, as shown in Fig. 5, H2O
appeared in the first consumption of N2O. Thus, in the first
Scheme 1. Pathways of N2O decomposition over pure H-ZSM-5.

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S.M. Jung et al. / Journal of Molecular Catalysis A: Chemical 236 (2005) 94–98
ied. Vanadium oxide disposed in the first layer upon the zeo-
lite in a reactor shows an improved activity, as compared with
the sum of the individual activities of the pure phases. TPR
and NH3-TPD show that the improved reactivity is closely
related to the enhanced formation of oxidation intermediates
by V2O5: (i) N2O, which is the source of ␣-oxygen, and (ii)
spillover oxygen. As a result, it can be suggested that the
oxidation rate of NO into NO2 increased by the transferred
oxygen from V2O5, contributing to the increase in the SCR
reaction rate over the hybrid catalysts.
Fig. 6. NH3-TPD under NO flow (5000 ppm/He) observed over: V2O5 (a),
H-ZSM-5 (b), and layered hybrid sample composed of V2O5 (F) H-ZSM-5
(S) (c).
Acknowledgements
This work was supported by ECSC project (7220-
ED/093). WealsoacknowledgethefinancialsupportofFNRS
(Fonds National de la Recherche Scientifique, Belgium).
This reaction path can be described as follows:
O∗-M + N2O → O2 + N2 + [ ]-M
2O∗-M → O2 + 2[ ]-M
(4)
(5)
(6)
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