Very active CeO2-zeolite catalysts for NOx reduction with NH3

Article abstract of DOI:10.1039/b205294f

Selective catalytic reduction of NO with NH₃ over high weight percentage CeO₂-zeolites showed excellent NO_x conversions at very high space velocities under simulated exhaust gas conditions in the presence of H₂O

Full text of DOI:10.1039/b205294f

Very active CeO2-zeolite catalysts for NOx reduction with NH3
K. Krishna,* G. B. F. Seijger, C. M. van den Bleek and H. P. A. Calis
Reactor and Catalysis Engineering, Faculty of Applied Sciences, Delft University of Technology,
Julianalaan 136, 2628, BL Delft, The Netherlands. E-mail: k.krishna@tnw.tudelft.nl; Fax: 0031 15 2788267;
Tel: 0031 15 2783859
Received (in Cambridge, UK) 31st May 2002, Accepted 1st August 2002
First published as an Advance Article on the web 13th August 2002
Selective catalytic reduction of NO with NH3 over high
weight percentage CeO2-zeolites showed excellent NOx
conversions at very high space velocities under simulated
exhaust gas conditions in the presence of H2O.
similar to the most active Fe-ZSM-5 catalysts reported in the
literature.2,3 The NH3 conversions followed the same trend as
that of NOx below 350 °C. Above 350 °C, 100% of NH3 is
converted over all the catalysts which rules out NH3 slip under
the SCR conditions. Among the catalysts tested the highest
conversions are observed over 75 wt% CeO2-H-BEA at 300,
350 and at 600 °C. The structure and acidity of the zeolites did
not have a significant influence on NOx conversion under the
reaction conditions used. In spite of the relatively lower amount
and the strength of acidity of H-BEA, observed from NH3-TPD,
75 wt% CeO2-H-BEA resulted in a better NOx reduction
performance. SEM micrographs revealed that the crystallite size
of H-BEA is smaller than that of other zeolites used. The large
external surface area (145 m2 g21) of zeolite beta, highest
among the zeolites studied, could lead to improved mixing of
the two solids and also to the accessibility of the reactants/
intermediates to the acidic sites. It is known that the SCR
reaction mainly takes place near the pore entrances of the
zeolites.8 Though Brønsted acidity is necessary for SCR to
achieve high conversions, smaller crystallites could expose
relatively more pore openings to the reactants, which results in
enhanced NOx conversions. It was not possible to compare the
catalysts in the temperature region 400–550 °C due to the high
NOx conversions ( > 85%). At 600 °C the NOx conversion
decreased over all the catalysts due to extensive oxidation of
NH3 to N2 and NOx, which can be seen from the direct oxidation
of NH3 (examples in Table 1). Enhancement in the NOx
conversion was reported when the feed contained a particular
NO/NO2 ratio (1+1).8 Under the reaction conditions, CeO2
increases the concentration of NO2 in the simulated exhaust gas
mixture, which further reacts with the NH3 activated over
zeolites leading to higher conversions. Irrespective of the
zeolites used all the catalysts showed similar NO conversions to
NO2 with a maximum at 450 °C (Fig. 1) which also explains
similar conversion trends of NOx in SCR over the catalysts.
Selective catalytic reduction (SCR) of NO with NH3 is an
indispensable technology in NOx (NO + NO2) emission control.
The temperatures associated with exhaust gases vary widely and
are often in the range 350–550 °C, where commercial V2O5/
TiO21 systems are not efficient. Recent studies have focussed on
the use of Fe-ZSM-52,3 catalysts because of their activity and
stability. Ce3+ ion exchanged zeolites were found to be active
for NOx reduction using NH3.4 The CeO2-zeolite systems
prepared by physical mixing and co-precipitation methods were
studied in the SCR with hydrocarbon reductants.5–7 The CeO2-
zeolite catalyst systems did not receive attention using NH3 as
a reductant. We found that, CeO2 in the presence of small
amount of H-zeolite showed excellent SCR activity with NH3.
In this paper, preliminary results on the SCR of NO with NH3
under simulated exhaust gas conditions over high weight
percentage CeO2-zeolite catalysts, prepared by simple physical
mixing are presented. NOx conversions of > 85% are observed
over a broad temperature range at very high space velocities. A
synergetic effect was found between an intimately mixed
oxidation component and the proton exchanged zeolite. The
presence of water vapour enhanced the NOx conversions above
450 °C.
H-BEA (Si/Al = 11, Uetikon), H-MOR (Si/Al = 10,
Zeolyst), Na-ZSM-5 (Si/Al = 11) and K-FER (Si/Al = 9.2)
(TOSOH, Japan), were used in the present study. Na+ and K+
forms of the zeolites were converted to the H-form. The
required amount of CeO2 (75 and 50 wt%, surface area = 128
m2 g21) and zeolite were thoroughly mixed in a mortar and
calcined at 600 °C for 5 h. The catalysts (0.3 g, sieve fraction
300–500 mm) were screened for SCR of NOx and the oxidation
of NH3 and NO in a tubular quartz reactor (5 mm internal
diameter) between 300 and 600 °C. The gas hourly space
velocity (GHSV) was 5 3 105 h21, based on the catalyst bulk
density of 1 g cm23. The SCR synthetic exhaust gas mixture
contained 1000 ppm NO, 1000 ppm NH3, 10 vol% O2, 9 vol%
H2O (when used) and balance N2. NH3 and water were removed
by placing phosphoric acid and water traps before some
analysers. The concentration of NOx, N2O and NH3 were
monitored with a chemiluminescence detector, gas chromatog-
raphy and by a microwave analyser (MIPAN) respectively. The
NOx conversion was calculated after 1 h of time on stream at
each reaction temperature. The materials were characterised by
chemical analysis, N2 adsorption, NH3-TPD and SEM.
Fig. 1 shows the conversion of NOx and NH3 at different
reaction temperatures over 75 wt% CeO2, physically mixed
with 25 wt% of the zeolites H-BEA, H-MOR, H-ZSM-5 and H-
FER. All the catalysts tested with simulated exhaust gases
having excess H2O and at high space velocities, in general
showed excellent NOx conversions with NH3 between 300 and
600 °C. In the reaction temperature range 350–550 °C, NOx
conversions exceeding 80% are observed over all the catalysts.
N2O formation was negligible (max. 9 ppm) over the catalysts
studied. The activity of the 75 wt% CeO2-H-zeolite catalysts is
Fig. 1 Conversion of NOx and NH3 in SCR (1000 ppm NO, 1000 ppm NH3,
10 vol% O2 and 9 vol% H2O balance N2) and oxidation of NO to NO2 (1000
ppm NO, 2 vol% O2 balance N2). GHSV = 5 3 105 h21
.
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Table 1 NH3 oxidation over CeO2-zeolite catalysts
Selectivity
(%)
Reaction
temperature/°C NH3 (%)
Conversion of to NOx
Catalysta
75 wt% CeO2-H-MOR
75 wt% CeO2-H-BEA
50 wt% CeO2-H-BEA
75 wt% CeO2-H-BEA
75 wt% CeO2-Na-ZSM-5
CeO2
600
600
600
600/wet
550
100
100
100
96
100
100
19.6
17.5
13.3
3.2
64.6
75.4
550
a Reaction conditions: 1000 ppm NH3, 10 vol% O2, balance N2. GHSV =
5 3 105 h21. Wet = in the presence of 9 vol% H2O in the above feed.
Fig. 3 NOx conversion over CeO2-ZSM-5 catalyts. Filled symbols in the
Fig. 2 presents the NOx conversion vs. temperature over
CeO2, 50 and 75 wt% CeO2-H-BEA, H-BEA and over a catalyst
having CeO2 (75 wt%) as the top bed and H-BEA (25 wt%) as
the bottom bed separated by quartz wool. No appreciable NOx
conversion is observed over CeO2 (13%) and H-BEA (20%). 20
ppm of N2O was observed over CeO2 at 500 and 550 °C. By
decreasing the CeO2 content from 75 to 50 wt%, a decrease in
NOx conversion at 300 and 350 °C is observed over CeO2-H-
BEA catalysts. The NOx conversions remained > 85% over
both the catalysts between 350 and 550 °C. A small improve-
ment in the NOx conversion over 50 wt% CeO2-H-BEA at 600
°C could be expected as seen from Fig. 2, due to the lower CeO2
content and decreased direct NH3 oxidation to NOx (Table 1).
Fig. 2 also gives the NOx conversion over 75 wt% CeO2-H-
BEA tested in the presence and absence of water. NOx
conversions remained similar and high up to 450 °C under both
conditions. In the absence of water in the feed, NOx conversion
decreased from 90 to 34% when the reaction temperature was
increased from 450 to 600 °C. When water is present in the
simulated exhaust gas mixture, a relatively small decrease in
NOx conversion, from 93% at 450 °C to 72% at 600 °C is
observed. Similar results were obtained over 75 wt% CeO2-H-
ZSM-5 (Fig. 3). Water did not affect the conversion of NO to
NO2 significantly (Fig. 2). Screening the 75 wt% CeO2-H-BEA
catalyst for NH3 oxidation at 600 °C showed 100 and 96% NH3
conversion in the absence and presence of water, respectively
(Table 1). The selectivity to NOx is high in the absence of water
(17.5%) compared with the NOx selectivity in the presence of
water (3.2%). Oxidation of NH3 to N2 generally proceeds in two
steps.9 First, a part of NH3 is oxidised to NOx and subsequently
NOx is reduced with unconverted NH3 in the presence of acidic
sites to N2. If extensive oxidation of NH3 to NOx takes place,
which is the case in the absence of H2O, less NH3 will be
available for further reduction, and thus decreases the overall
NOx conversion. The improved NOx conversions in the
presence of 9 vol% H2O, open symbols in the absence of water, GHSV =
5 3 105 h21 (*This catalyst was aged at 600 °C, **GHSV = 2 3 105 h21
500–800 mm particles, reactor internal diameter 7 mm).
,
presence of water are therefore due to strong suppression of the
direct oxidation of NH3 over CeO2. The catalyst having separate
beds of CeO2 and H-BEA, resulted in a lower NOx conversions
at all reaction temperatures. The NOx conversion of > 45% (14
ppm N2O at 600 °C) is observed in a narrow temperature range.
This clearly shows that the diffusion of reaction intermediates
between oxidation and acidic sites is most effective when both
the components are in intimate contact due to synergetic
effect.
A synergetic effect between the acidic sites of zeolite and the
oxidation component is further evident from Fig. 3. When the
Na-form of the zeolite is used, over the resulting 75 wt% CeO2-
Na-ZSM-5 catalyst, the NOx conversion ( < 65%) at all the
reaction temperatures is inferior to that of 75 wt% CeO2-H-
ZSM-5 under the same conditions. In the absence of Brønsted
acidic sites, due to extensive oxidation of unadsorbed NH3 to
NOx, which is seen from the selectivity of NOx in direct
oxidation of NH3 over CeO2 and 75 wt% CeO2-Na-ZSM-5
(Table 1), the NOx conversions were even negative above 500
°C. Due to preferential adsorption of NH3 over Brønsted acid
sites in 75 wt% CeO2-H-zeolite catalyst, its direct oxidation
over CeO2 to NOx decreases resulting in superior NOx
conversions. This is evident from the broad temperature range
where > 80% NOx conversions are maintained over 75 wt%
CeO2-H-ZSM-5 even in the absence of water in the feed. The
NOx conversions at 300 and 350 °C can be further improved by
decreasing the space velocity (Fig. 3). The catalysts tested at a
space velocity of 2 3 105 h21, in the presence of water showed
conversions of > 80% in the whole temperature range tested
(300–600 °C).
In conclusion, the high weight percentage CeO2-H-zeolite
composite mixtures are excellent selective catalytic reduction
catalysts with NH3 as a reductant. NOx conversions above 85%
are obtained over a broad temperature range (350–550 °C) and
at high space velocities (5 3 105 h21). The synergetic effect
between the intimately mixed CeO2 and the zeolite Brønsted
acidity is responsible for superior NOx conversions.
The financial support of Netherlands Technology Foundation
(STW) is greatly acknowledged.
Notes and references
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Catal. Lett., 1999, 63, 227 and references therein.
5 M. Misono, Cattech, 1998, 2, 183 and references therein.
6 T. Liese, E. Loffler and W. Grunert, J. Catal., 2001, 197, 123.
7 K. Krishna, G. B. F. Seijger, C. M van den Bleek, H. van Bekkum and H.
P. A. Calis, Chem. Commun., 2002, 948.
8 J. Eng and C. H. Bartholomew, J. Catal., 1997, 171, 27.
9 M. Amblard, R. Burch and B. W. L. Southward, Catal. Today, 2000, 59,
365.
Fig. 2 NOx conversion over CeO2-H-BEA catalysts in SCR and in NO
oxidation to NO2 (reaction conditions as in Fig. 1). Filled symbols in the
presence of 9 vol% H2O, open symbols in the absence of H2O. *CeO2
(0.225 g, top bed) + H-BEA (0.075 g, bottom bed). GHSV = 5 3 105
h21
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