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Applied Catalysis B: Environmental 59 (2005) 221–226
The selective catalytic oxidation of NH3 over Fe-ZSM-5
Aaron Akah a, Colin Cundy b, Arthur Garforth a,*
a School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Manchester M60 1QD, UKb School of Chemistry, Centre for Microporous Materials, The University of Manchester, Manchester M60 1QD, UK
Received 3 August 2004; received in revised form 20 October 2004; accepted 25 October 2004
Available online 8 March 2005
Abstract
The abatement of NH3 from waste streams has become an important environmental issue. Selective catalytic oxidation (SCO) of NH3 to N2
has emerged as a potential technology for taking care of NH3 slips and NH3 in waste streams. In this work, we describe the catalytic activity of
Fe-zeolite catalysts prepared by incipient wetness technique, ion exchange and hydrothermal synthesis in the SCO of NH3 to N2 using a fixed
bed flow reactor. Selective catalytic oxidation was carried out at 573–723 K and 105 Pa with gas hourly space velocities (GHSV) between
24 000 and 240 000 h�1. Results obtained showed that Fe-ZSM-5 catalysts prepared by incipient wetness technique were active for NH3
conversion (77–100%) and selectivity to N2 (65–100%). Fe-ASA and Fe-Beta showed good catalytic activity and selectivity, but their activity
and selectivity were less than that of Fe-ZSM-5. The effects of water vapour, Fe loading, and activation method on the performance of these
catalysts was also investigated.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Selective catalytic oxidation; Ammonia; ZSM-5; Impregnation
1. Introduction
Many chemical processes use reactants containing NH3
or produce NH3 as a by-product. They all act as potential
sources of NH3 slips [1]. The environmental criteria about
the control of waste gas through an industrial system have
become stricter, so there is a need to reduce the
concentration of residual gases such as NH3, NOx, and
SO2. Present techniques used to abate NH3 include
adsorption, biological purification and catalytic decomposi-
tion [2]. An ideal technology that can be applied is the
selective catalytic oxidation of NH3 to N2 and H2O. The
selective catalytic oxidation (SCO) is a relatively new
technology for NH3 remediation. In this process NH3 in
waste streams is selectively oxidised to N2 and H2O only.
SCO of NH3 is also possible in the liquid phase [3]. The
principal reaction that is expected to take place during the
SCO of NH3 is
4NH3 þ 3O2 ! 2N2 þ 6H2O
* Corresponding author. Tel.: +44 161 200 8850.
E-mail address: arthur.garforth@manchester.ac.uk (A. Garforth).
0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2004.10.020
however others may occur:
2NH3 þ 2O2 !N2O þ 3H2O
4NH3 þ 5O2 ! 4NO þ 6H2O
4NH3 þ 7O2 ! 4NO2 þ 6H2O
Transition metal oxides such as V2O5 could selectively
convert NH3 to N2 but were found to be less active vis-a-
vis the noble metals. An excellent early review by Il’chenko
[4] on NH3 oxidation looked at the activities of metals and
metal oxides for NH3 oxidation at low temperatures. For
metals at 300 8C, for example, the specific catalytic activity
of the overall process decreased in the sequence:
Pt > Pd > Cu > Ag > Au > Fe > W > Ti and low tem-
peratures favoured the formation of N2 and N2O. However,
selectivity to N2 decreased with increasing temperature. The
specific catalytic activities of the metal oxides decreased in
the sequence: Co3O4, MnO2 > CuO > NiO > Bi2O3 >Fe2O3 > V2O5 > TiO2 > ZnO > WO3. For nearly all oxi-
des, it was observed that selectivity to N2 decreased as the
catalyst activity increased. However, for CuO, the N2 selec-
tivity increased with catalyst activity.
A. Akah et al. / Applied Catalysis B: Environmental 59 (2005) 221–226222
Different types of materials have been known to be active
for the oxidation of NH3 to N2 such as Pt, Rh, Pd exchanged
ZSM-5 [5] and Fe exchanged ZSM-5 [6–9]. Early results
on NH3 oxidation showed that noble metals such as Pt and
Pd were active for oxidation of NH3 to NO and N2O
[5–7,10,11].
In this work, we studied the SCO performance (activity,
selectivity and durability) of Fe-ZSM-5 catalysts prepared
using impregnation, ion exchange and hydrothermal
synthesis. The effect of water on the performance of these
catalysts was also investigated. The catalytic materials were
characterised by Fe content by atomic absorption spectro-
scopy, N2 physisorption and NH3 temperature desorption
(NH3-TPD).
2. Experimental
2.1. Ion exchange and incipient wetness
Fe-ZSM-5 samples were prepared using ion exchange
and incipient wetness techniques. The starting materials
used for the impregnation and ion exchange of Fe-ZSM-5
were as follows.
With impregnation, Fe(NO3)3�9H2O (typically 0.2–4 g,
99.99%, Aldrich) was dissolved in a minimum volume
(typically 7 mL) of deionised water and added to 10 g of
NH4-ZSM-5 (Si/Al = 15, from PQ Zeolites BV) with
continued stirring to form a paste. After thorough mixing,
the paste was then allowed to age overnight at room
temperature. Following the aging process, the paste was then
dried at 363 K for 6 h. The obtained sample was then
pelleted, crushed and sieved to obtain a mesh size of 250–
425 mm. Activation was done in situ for 6 h at 773 K using
various gas streams, namely: Ar, 2% O2/Ar, 2% H2 in Ar or
500 ppm NH3 in Ar. The same technique was used to
impregnate zeolite Beta and ASA (amorphous silica
alumina).
For ion exchange, 2 g of NH4-ZSM-5 was added to
200 mL of 0.05–0.1 M FeCl2 solution with constant stirring
with a magnetic stirrer and hot plate. The mixture was
allowed to stand for about 24 h, after which it was filtered
and washed five times with deionised water. The catalyst
obtained was then dried at 393 K in air for 12 h. The pelleted
samples were crushed and sieved to obtain a mesh size of
250–425 mm activated as described above.
2.2. Fe-ZSM-5 synthesis
The isomorphously substituted Fe-ZSM-5 was synthe-
sised hydrothermally following the procedure described by
Bruckner [12], using tetra propyl ammonium bromide
(TPABr) as template. The materials used were sodium
metasilicate (Na2SiO3�5H2O, technical grade, BDH), iron
(III) sulphate (Fe2(SO4)3�5H2O, 97%, Aldrich) and sulphu-
ric acid (H2SO4, 98%, P and R laboratory supplies).
3. Characterisation of catalysts
3.1. Elemental analysis
The amount of Fe in the catalyst was determined by acid
extraction of 50 mg of Fe-ZSM-5 with 2 N HCl followed by
dilution to 1000 mL and subsequent analysis by atomic
absorption spectroscopy. The analytical error for this
elemental analysis was �0.3%.
3.2. NH3 temperature programmed desorption (TPD)
NH3-TPD experiments were carried out in a conventional
flow-through reactor with argon as carrier gas (50 mL/min)
[13]. Typically 100 mg of sample was pretreated by heating
in a flowing stream of argon from 298 to 773 K at
3 K min�1. This was done to remove water and impurities.
The temperature was held at 773 K for 6 h and then cooled to
398 K at a ramp rate of 5 K min�1 under flowing gas (Ar). At
398 K, the Ar flow was switched to a flow of 1000 ppm NH3/
Ar and the sample equilibrated with NH3 at 105 Pa for 4 h. A
mass spectrometer (Hiden Analytical) was used to show that
the NH3 concentration had stabilised (m/z = 15, 16, 17). The
reactor was then purged with Ar for a further 1 h to remove
weakly adsorbed NH3.
A linear temperature programme was used to follow
the desorption of NH3 with evolved gas detected by a
quadrupole mass spectrometer. For quantitative analysis,
experiments with different heating rates (2.5, 5 and
10 K min�1) were used.
3.3. Catalyst performance measurements
Catalysis was carried out in a fixed bed Pyrex micro reactor
inserted into an electric furnace and operated at atmospheric
pressure and a temperature range of 623–723 K. The reactant
gas was obtained by blending different gas flows with the use
of a Brooks Microprocessor Control and Readout Unit (model
0152/0154) and mass flow controllers (models 5850S, 5851S
and 5853S). The typical reactant gas composition was as
follows: 0–1000 ppm NH3, 0–1000 ppm He (internal stan-
dard), 2% O2, in a balance of Ar. The overall flow rate was
maintained at 50–500 cm3/min, i.e., gas hourly space
velocities (GHSV) ranging from 24 000–240 000 h�1 (normal
temperature and pressure). GHSV was calculated using the
bulk volume of the catalyst bed.
For the analysis of the inlet and outlet gas compositions,
the reactor outlet was connected in a parallel arrangement to a
Rae Systems photo ionisation detector (PID), a Hiden
Analytical quadrupole mass spectrometer and a gas chroma-
tograph (HP 6890) equipped with a Porapak Q and a 5 A
molecular sieve packed column (dimension, 1.5 m long and
0.2 cm internal diameter) connected to thermal conductivity
detectors. The quadrupole mass spectrometer was used to
monitor the effluent gas from the reactor, by scanning various
fragments of m/z = 2–150. In particular, NH3 (m/z = 17 min
A. Akah et al. / Applied Catalysis B: Environmental 59 (2005) 221–226 223
Fig. 1. NH3-TPD for catalyst samples.
the contribution of H2O), H2O (m/z = 18), N2 (m/z = 28), and
N2O (m/z = 44) were monitored. In a typical NH3 oxidation
experiment, a stream of NH3 (500 ppm) and O2 (2%) in Ar
(balance) was fed to the reactor. Experiments and N-balances
were performed at each investigated temperature and always
closed within 96 � 4%.
4. Results and discussion
4.1. NH3-TPD
The strength of the interaction of NH3 with acid sites is
reflected in the temperature profile of the desorption rate.
The area under the peaks is proportional to the total number
of acid sites in the catalyst under study. As shown in Fig. 1,
the catalysts studied here showed several different profiles.
Two major desorption events were noted for H-ZSM-5 with
a maximum at 500 and 700 K indicating the presence of
weak and strong acid sites, respectively. After impregnation
of Fe, the number of weaker acid sites was much reduced
whereas only a slight reduction in the number of stronger
sites had occurred compared to the parent (H-ZSM-5). The
Al-free synthesised Fe-ZSM-5, as expected showed very
Table 1
NH3 oxidation over catalysts activated in situ using 500 ppm NH3/Ar
Sample T (K) NH3 conversion
Empty tube 623 0
673 0
723 0
H-ZSM-5 623 16
673 22
723 32
2% IMP Fe-ZSM-5 623 78
673 88
723 100
2% IMP Fe-ASA 623 29
673 60
723 62
2% IMP Fe-Beta 623 28
673 70
723 93
Reaction conditions: 0.1 g catalyst, [NH3] = 500 ppm, [O2] = 2%, Ar = balance, to
ASA = amorphous silica alumina.
little weak acidity and no strong acidity. The 2% IMP Fe-
Beta catalyst has weak and strong acid sites and showed a
similar profile to 2% IMP Fe-ASA (known to have
predominantly Lewis acid sites [14]). However, the peak
maximum was broader and encompassed both low
temperature and high temperature peak events for Fe-
ZSM-5.
4.2. Selective catalytic oxidation (SCO) of NH3
4.2.1. Catalyst screening
Different catalysts were tested at 623–723 K for the SCO
of NH3 (Table 1). Selectivity to N2 increased with
temperature for all samples except H-ZSM-5. As expected
no conversion was observed with an empty Pyrex tube [15]
in contrast with Long and Yang [6]. From the results, Fe-
ZSM-5 was more active and selective than Fe-Beta and Fe-
ASA. The difference in activity is attributed to the ability of
Fe-ZSM-5 to form stabilised binuclear Fe-species, which are
the active sites for the SCO of NH3 to N2 [10].
4.2.2. The effect of Fe loading on SCO
Activity and selectivity increased with an increasing Fe
content (Fig. 2). Selectivity to N2 at low temperature
(%) N2 selectivity (%) N2 yield (%)
0 0
0 0
0 0
72 12
66 15
62 20
90 70
100 88
100 100
61 18
88 53
88 55
63 18
92 64
100 93
tal flow rate = 200 mL/min and GHSV = 96 000 h�1. IMP = impregnation,
A. Akah et al. / Applied Catalysis B: Environmental 59 (2005) 221–226224
Fig. 2. Effect of Fe content on SCO performance (conversion and N2 selectivity), flow rate = 200 mL/min, GHSV = 96 000 h�1.
Fig. 3. Effect of activation on NH3 conversion and N2 selectivity on 1% Fe-ZSM-5, flow rate = 300 mL/min, GHSV = 144 000 h�1.
progressively improved with Fe loading as well as
improving overall catalyst performance (Fig. 2). Based on
literature reports [17], the results are consistent with the
formation of more Fe binuclear species as temperature
increased. These species are thought to be responsible for
the formation of active sites. It is also possible that increased
temperature may result in an increase in the turnover
frequency.
4.2.3. The effect of activation technique on catalyst
performance
Catalysts were activated in situ under a range of gaseous
environments namely: inert (Ar), oxidising (2% O2/Ar) and
reducing (500 ppm NH3/Ar and 5% H2/Ar). Catalysts
activated in NH3 showed superior performance at low
reaction temperatures (Fig. 3) as a more reducing environ-
ment favoured the auto-reduction of Fe3+ to Fe2+ [16].
Fig. 4. Effect of water on catalyst activity, flow rate = 200 mL/min,
GHSV = 96 000 h�1, 2% Fe-ZSM-5, 5% H2O, T = 723 K.
4.2.4. Effect of H2O on SCO of NH3
In the presence of H2O the catalyst activity decreased and
stabilised at 70% after 4 h (Fig. 4). When H2O was removed,
the catalyst recovered its activity rapidly. More importantly,
the catalysts activity was slightly improved. This agrees with
work by Dubkov et al. [17] where H2O promotes the
reduction of Fe3+ to Fe2+ and leads to increased activity. The
presence of H2O in the feed had only a slightly negative
effect on N2 selectivity (�5%) and the residual NH3
conversion (70%) suggested significant numbers of acid
sites were still available.
4.2.5. The effect of Fe loading on catalyst life-time
The stability of Fe-ZSM-5 increased with Fe content
(Fig. 5). For 0.5% Fe-ZSM-5 the activity decreased rapidly
with time, whereas 1% Fe-ZSM-5 was stable initially. This
loss of activity was probably due to the formation of inactive
Fig. 5. Fe-ZSM-5 activity with time at 723 K, flow rate = 200 mL/min,
GHSV = 96 000 h�1.
A. Akah et al. / Applied Catalysis B: Environmental 59 (2005) 221–226 225
Fig. 6. Effect of feed flow rate on catalyst performance at 623 K.
Table 2
Effect of the different type of Fe incorporation
Sample NH3 conversion (%) N2 selectivity (%)
623 K 673 K 723 K 623 K 673 K 723 K
H-ZSM-5 16 22 32 72 66 62
SYN-Fe-ZSM-5 (Fe = 1.5%) 12 26 44 34 55 88
0.5% IE Fe-ZSM-5 25 55 70 76 93 98
0.5% IMP Fe-ZSM-5 10 30 60 72 100 100
Reaction conditions: 0.1 g catalyst, [NH3] = 500 ppm, [O2] = 2%, Ar = balance, total flow rate = 200 mL/min and GHSV = 96 000 h�1. SYN = synthesised.
iron oxide species. The activity was recovered completely
by reactivating the catalysts in situ, which indicated that
metal sintering did not occur. Higher loadings of Fe yielded
good activity (100%) and stability (>25 h) as well as 100%
selectivity to N2.
4.2.6. Comments on the SCO reaction mechanism
H-ZSM-5 catalysed the oxidation of NH3 to NO
predominantly. The addition of Fe believed to have led to
the formation of both Fe oxides and binuclear Fe species
[16], which allowed the production of N2 and NO. During
SCO, NH3 was first oxidised to NO, which was subsequently
reduced by more NH3 to N2. This is illustrated in Fig. 6, as
the W/F increased, the production of NO went through a
maximum and the N2 yield significantly increased.
Furthermore the results at 623 K suggest a faster first step
to produce NO and a slower rate determining second step to
produce N2.
4.2.7. Effect of different methods of incorporation of Fe
into the zeolite catalysts
The incorporation of Fe directly in the synthesis of Al-
free (SYN-Fe-ZSM-5) was also carried out. The results are
presented in Table 2 and show that the incorporation of Fe by
ion exchange, impregnation and synthesis all improved the
SCO of NH3 compared to the acidic form of ZSM-5 (H-
ZSM-5).
Interestingly, the Al-free SYN-Fe-ZSM-5 gave modest
improvements in SCO results and good selectivity to N2 at
723 K. At 0.5% Fe content, the ion exchanged Fe-ZSM-5
showed the most activity with both the ion exchanged and
impregnated catalysts being very selective to N2.
5. Conclusions
Activity and selectivity to N2 were optimised for a series
of Fe-impregnated ZSM-5. The results of the SCO of NH3
showed that Fe-ZSM-5 prepared by incipient wetness
technique was active for this process with NH3 conversion
and N2 selectivity reaching 100% at 723 K. Activity was
found to depend on the amount of Fe and nature of the Fe
present, the method of activation of the catalyst (redu-
cing > inert > oxidising) and contact time.
The most probable reason for loss of activity during the
first few hours on stream was the formation of inactive iron
oxide species as the catalyst approached equilibrium
between Fe2+ and Fe3+ sites. The loss of activity in the
presence of water was probably due to a site blocking
mechanism and the effect of water on the catalyst
performance was temporary. The subsequent increase in
conversion observed might be explained by improved
dispersion of Fe species caused by low temperature
‘‘steaming’’. Dubkov et al. [17] observed this effect and
found that more active sites were produced in the presence of
water at relatively lower temperature.
The catalysts reported here represent an alternative
convenient and cheaper method compared to ion exchange
[18] and chemical vapour deposition [19]. Whilst the
preparation of Fe-ZSM-5 by ion exchange is problematic
due to the precipitation of FeOOH, the chemical vapour
deposition route using of FeCl3 is expensive.
Acknowledgements
The authors would like to express sincere thanks to Mr.
Richard Plaisted for the Fe-ZSM-5 synthesis and The
Commonwealth Scholarship Commission in the UK for
funding the project.
References
[1] J. Phillips, Control Pollution Prevention Options for Ammonia
Emissions, U.S. Environmental Protection Agency, 1995.
[2] T. Curtin, LenihanF S., Chem. Commun. (2003) 1280.
[3] R. Ukropec, B.F.M. Kuster, J.C. Schouten, R.A. van Santen, Appl.
Catal. B 23 (1999) 45–57.
A. Akah et al. / Applied Catalysis B: Environmental 59 (2005) 221–226226
[4] N.I. Il’chenko, Russ. Chem. Rev. 45 (1976) 1119.
[5] Y. Li, J.N. Armor, Select. Appl. Catal. B 13 (1997) 131.
[6] R.Q. Long, R.T. Yang, Chem. Commun. (Cambridge) 17 (2000)
1651.
[7] R.Q. Long, R.T. Yang, Abstracts of Papers ACS 222 (2001) U326.
[8] R.Q. Long, R.T. Yang, Catal. Lett. 78 (2002) 353.
[9] R.Q. Long, R.T. Yang, J. Catal. 207 (2002) 158.
[10] R.Q. Long, R.T. Yang, J. Catal. 201 (2001) 145.
[11] T. Curtin, F. O’Regan, C. Deconinck, N. Knuttle, B.K. Hodnett, Catal.
Today 55 (2000) 189.
[12] A. Bruckner, Verified Syntheses of Zeolitic Materials, 2 ed. Elsevier,
Amsterdam, 2001, p. 205.
[13] A.C. Akah, Ph.D. Thesis, University of Manchester Institute of
Science and Technology, Manchester, 2004.
[14] S. Ali, A. Garforth, D.H. Harris, R.A. Shigeshi, Surf. Sci. Ser. 108
(2003) 295.
[15] W.D. Monnery, K.A. Hawboldt, A.E. Pollock, W.Y. Svrcek, Ind. Eng.
Chem. Res. 40 (2001) 144.
[16] J. Perez-Ramirez, G. Mul, F. Kapteijn, J.A. Moulijn, A.R. Overweg, A.
Domenech, A. Ribera, I.W.C.E. Arends, J. Catal. 207 (2002) 113.
[17] K.A. Dubkov, N.S. Ovanesyan, A.A. Shteinman, E.V. Starokon, G.I.
Panov, J. Catal. 207 (2002) 341.
[18] W.K. Hall, X. Feng, J. Dumesic, R. Watwe, Catal. Lett. 52 (1998) 13.
[19] H.-Y. Chen, W.M.H. Sachtler, Catal. Today 42 (1998) 73.