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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.

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