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Applied Catalysis B: Environmental 72 (2007) 82–91

A comparison of NOx adsorption on Na, H and BaZSM-5 films

Indra Perdana a,1, Derek Creaser a,*, Olov Ohrman b,2, Jonas Hedlund b

a Department of Chemical Reaction Engineering, Chalmers University of Technology, SE-412 96, Gothenburg, Swedenb Division of Chemical Technology, Lulea University of Technology, SE-971 87, Lulea, Sweden

Received 9 May 2006; received in revised form 22 September 2006; accepted 30 September 2006

Available online 21 November 2006

Abstract

NOx adsorption in ZSM-5 films containing Na+, H+ or Ba2+ as counter ions was studied. NaZSM-5 films showed a superior NOx adsorption

capacity over the entire temperature range (30–350 8C) in comparison with the other films. Besides the possibility to form strongly bound surface

nitrate species, the presence of the Na+ ions in ZSM-5 resulted in a large enhancement of various weakly adsorbed species. In the HZSM-5 film, the

NOx adsorption was mainly due to physisorption, surface nitric acid and nitrosium ion (NO+) formation. Besides weakly adsorbed species and

surface nitric acid, the NOx adsorption in BaZSM-5 films also resulted in formation of strongly bound surface nitrate species. The nitrate species in

the BaZSM-5 film were found to be resistant to NO exposure and were mainly formed through an NO2 disproportionation pathway.

# 2006 Elsevier B.V. All rights reserved.

Keywords: NOx adsorption; Monolith; ZSM-5 films; Counter ions

1. Introduction

Stringent regulations on emissions from combustion

processes have triggered substantial development in exhaust

gas after-treatment systems, in particular for operation under

lean combustion conditions at which NOx reduction is

problematic. Among several available methods, techniques

based on NOx adsorption materials [1] are among the most

promising ones for NOx abatement. Besides basic oxides,

zeolites have also been considered as NOx adsorbents [2–4] due

to their thermal stability and well-defined microporous

structures with high surface area.

NOx adsorption has been studied experimentally on a variety

of zeolite types with different counter ions [4–11]. The

formation of surface adsorbed species has been most frequently

investigated with in situ infrared (IR) spectroscopy. Depending

on the nature of the zeolite, various adsorbed species may exist.

In most cases, the presence of a metal counter ion in the zeolite

has been found to lead to the formation of surface nitrate

species after NOx exposure [2,5,6,11–13]. In addition, other

* Corresponding author. Fax: +46 31 7723035.

E-mail address: creaser@chalmers.se (D. Creaser).1 On leave from Department of Chemical Engineering, Gadjah Mada Uni-

versity, Indonesia.2 Present address: Haldor Topsøe A/S, R&D, Denmark.

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.09.016

surface species can also be present due to interactions with

surface hydroxyl groups [8–10,13].

To study NOx adsorption and desorption, zeolite powders

[2–4,10,14] or zeolite washcoats on cordierite monoliths

containing binder materials [7,15] have been used. With the

use of zeolite powders, it can however be difficult to obtain an

evenly distributed flow of the adsorbate containing gas through

the powder bed without an excessive pressure drop. As a result

the zeolite supported on a structured monolith support as a

washcoat can be experimentally more practical. Although the

reported results from zeolite washcoat studies are relevant for

catalyst development, the adsorption and transport properties

may be influenced by the type and quantity of binder material

used in the washcoat. A direct adsorption study using a film of

intergrown zeolite crystals free of binder material can provide a

more definitive understanding of the NOx adsorption and

transport properties in the zeolite. In addition, zeolite films are

also interesting materials due to their many unique potential

applications, such as for sensors and membranes [16–20].

Recently, techniques for coating monoliths and other structured

support materials with binder-free MFI type zeolite films

[17,21,22] have been reported and the films were found to be

catalytically active [22–25].

NOx adsorption–desorption in a binder-free NaZSM-5 film

grown on a cordierite monolith has been reported [13,23] and it

was shown that various adsorbed species were present over a

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–91 83

wide temperature range [13]. The results also indicated that

mass transport resistance in the films was an important factor

during transient NOx adsorption–desorption processes.

The affinity of ZSM-5 films in NOx adsorption is strongly

dependent on the structure and surface chemistry. Hence, the

presence of different counter ions in the ZSM-5 films is

expected to have a significant impact on the NOx adsorption–

desorption properties. The present work is therefore aimed at

studying the differences in observed surface species with H+

and Ba2+ counter ions compared to those previously reported

with Na+ [13] during NOx adsorption in ZSM-5 films supported

on cordierite monoliths. The NOx adsorption was studied by

temperature programmed desorption (TPD) and in situ DRIFT

spectroscopy.

2. Experimental

2.1. Sample preparation and characterization

In the present work, three types (NaZSM-5, HZSM-5 and

BaZSM-5) of film and powder samples were prepared. The film

samples were used for NOx adsorption experiments in a gas

flow reactor, whereas the powder samples were used for in situ

DRIFT spectroscopy measurements.

To prepare film samples, cordierite monoliths (approxi-

mately 100 mm in length) with a cross-section containing 188

channels (channel dimension 1 mm � 1 mm) were used as

supports. A detailed description of the sample preparation

procedure has been reported elsewhere [22]. The monoliths

were seeded with 60 nm silicalite-1 seeds [26,27] and

hydrothermally treated 12 times in a synthesis solution at

75 8C for 48 h in each step. The synthesis solution had a molar

composition of 3TPAOH:25SiO2:0.25Al2O3:1Na2O:1600

H2O:100EtOH. Following each hydrothermal treatment, the

samples were rinsed in an aqueous ammonia solution and were

treated with ultrasound in order to remove bulk crystals on top

of the film surface. After the final hydrothermal treatment, the

coated monoliths were polished on each side to a length of

75 mm. In addition, ZSM-5 powder samples were prepared in

an identical synthesis solution as used for film growth by

hydrothermal treatment for 72 h at 100 8C in the absence of

support and silicalite-1 seeds [22]. Finally, template molecules

in the monolith and powder samples were removed by

calcination at 550 8C. The samples were heated from room

temperature to 550 8C with a heating rate of 1.75 8C min�1 and

the temperature was kept constant at 550 8C for 6 h. Finally, the

samples were cooled down to room temperature at a cooling

rate of 1.75 8C min�1.

To prepare HZSM-5 samples, the NaZSM-5 samples were

ion-exchanged following a procedure reported elsewhere [24].

The samples were treated in a 10 wt.% NH4NO3 solution at

100 8C in a container equipped with a reflux condenser for 1 h

and were rinsed in deionized water for 1 h. The exchange was

carried out three times followed by a single calcination

treatment. The samples were heated from room temperature to

400 8C with a heating rate of 0.2 8C min�1 and the temperature

was kept constant at 400 8C for 24 h. Then, the temperature was

slowly decreased to room temperature with a cooling rate of

0.5 8C min�1.

To prepare BaZSM-5 samples, the NaZSM-5 film and

powder samples were ion-exchanged in a 2 wt.% Ba(NO3)2

solution in a container equipped with a reflux condenser at

80 8C for 2 h. To remove excess Ba(NO3)2 from the samples,

the film samples were treated in deionized water three times for

2 h each. The powder samples were rinsed with deionized water

in a stirred container three times for 2 h each. Following the

rinsing, the samples were calcined at 550 8C. The samples were

heated from room temperature to 550 8C with a heating rate of

1.8 8C min�1. After keeping the temperature constant at 550 8Cfor 1 h, the temperature was decreased to room temperature

with a cooling rate of 1.8 8C min�1. The ion-exchange and

calcination procedure was repeated three times.

Inductively coupled plasma atomic emission spectroscopy

(ICP-AES, Analytica AB, Sweden) was used to determine the

elemental composition of the powders. The exchange level in

the zeolite was expressed as a ratio of the amount of exchanged

atom to aluminium atom.

The zeolite loading (gzeolite/gsample) on the monolith samples

was determined by N2 gas adsorption measurements (BET

measurement). The surface area of the cordierite support was

negligible compared to the total surface area of the film sample.

With a knowledge of the specific surface area (415 m2/g [28])

for the ZSM-5 powder, the ZSM-5 loading in the film samples

could be calculated from the following equation [29]:

Zeolite loading ðgsample=gzeoliteÞ

¼measured BET area of ZSM-5 coated sample ½m2=gsample�

specific surface area of synthesized ZSM-5 powder ½m2=gzeolite�(1)

However, during film synthesis bulk crystals were found to

deposit on top of the film [22] in particular at the ends of the

monoliths. In order to remove as much sedimented crystals on

the film surface as possible the samples were treated with

ultrasound after each step [22]. To minimize the amount of

sediments at the monolith ends (top and bottom), the monoliths

were polished a few millimeters at both ends after the final

synthesis step [22]. Therefore, the measured weight difference

before and after synthesis cannot be used to estimate the zeolite

loading of the samples that were tested later on. Details about

the application of this technique for measurements of zeolite

loading on support materials are reported elsewhere [25,28–

30].

2.2. NOx adsorption–desorption measurements

NOx adsorption measurements with monolith samples were

performed in a gas flow reactor. As described in our previous

report [13], the reactor was a horizontal quartz glass tube with a

length of 880 mm and a diameter of 22 mm. The gas

temperature in the reactor was regulated with a temperature

controller connected to an electrical heater and thermocouples.

The monolith samples were placed inside the reactor and were

pre-treated with 8% O2 in argon at 500 8C for 15 min with

a total flow rate of 3000 ml min�1 (STP). Following the

Fig. 1. Top view of HZSM-5 film (a) and side view of HZSM-5 (b).

Table 1

Results from ICP-AES elemental and BET surface area measurements

Before cation

exchange

(mg/kg)a

After cation

exchange

(mg/kg)a

BET surface

area

(m2/gsample)b

Zeolite loading

(gzeolite/gsample)b

NaZSM-5

Si 414,000 – 58.1 0.14

Al 6,240 –

Na 6,000 –

HZSM-5

Si 428,000 447,000 58.6 0.14

Al 5,520 5,280

Na 5,210 <100

BaZSM-5

Si 414,000 427,000 74.7 0.18

Al 6,240 6,090

Na 6,000 813

Ba – 11,800

a Measurements on powder samples synthesized with similar synthesis

procedure as that for monolith film sample preparation.b Measurements on monolith samples with zeolite films.

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–9184

pre-treatment, the samples were cooled down to the specific

adsorption temperature which varied from 30 to 350 8C.

For NO2 adsorption experiments, a gas mixture containing

600 ppm NO2 in argon with a total flow rate of 2600 ml min�1

(STP) was exposed to the samples. The gas mixture was

prepared in a gas mixer (Environics 2000) consisting of several

mass flow controllers. Following the adsorption, the samples

were flushed with argon (inert gas) at the adsorption

temperature. To release adsorbed species, temperature pro-

grammed desorption (TPD) was carried out. The TPD consisted

of heating from the adsorption temperature to 550 8C with a

temperature ramp of 20 8C min�1 in a steady stream of pure

argon. In some experiments, the effect of an NO containing

atmosphere on the adsorbed species was further investigated by

exposing a sample that first had been equilibrated with NO2 to a

stream of 600 ppm NO in argon. During adsorption and

desorption, the outlet gas composition was analyzed online

using a chemiluminesence detector (Ecophysics CLD 700 EL

ht). The amount of adsorbed/desorbed NOx was quantified from

the area under the corresponding concentration curves of

adsorption, flushing and TPD.

The adsorbed species during NO2 adsorption was further

investigated for powders using in situ DRIFT spectroscopy (Bio

Rad FTS 6000 spectrometer). As reported previously [13], the

zeolite powders were placed in a sample holder assembly in a

Harrick Praying Mantis DRIFT cell. The gases were supplied

by individual mass flow controllers with a total flow rate of

200 ml min�1 (STP). The samples were pre-treated with 8% O2

in argon at 500 8C for 15 min. The adsorption experiments were

performed by introducing 600 ppm NO2 in argon followed by

argon flushing. Finally, to release the adsorbed species from the

samples, the temperature was increased to 500 8C with a ramp

of 20 8C min�1 and was then kept constant for about 20 min.

The spectra recorded were the average of 100 scans with a

resolution of 2 cm�1 with a background in an argon

atmosphere.

3. Results and discussion

3.1. Characterization by electron microscopy, N2

adsorption and ICP-AES

Details about synthesized ZSM-5 film properties have

been reported previously [13]. Following the repeated

hydrothermal treatments, the ZSM-5 films reached a

thickness of approximately 1700 nm thick. Scanning electron

microscopy (SEM) images taken after cation exchange and

high temperature treatment during experiments showed that

the exchange and temperature treatment did not deteriorate

the ZSM-5 films on the monolith surfaces. Instead, as visible

in Fig. 1, SEM images from the HZSM-5 sample shows a

homogeneous polycrystalline ZSM-5 film covering the entire

cordierite monolith surface. The images are very similar to

those recorded from NaZSM-5 films [13] and Ba exchanged

ZSM-5 films.

Table 1 contains the results of the ICP-AES measurements.

Based on these results for the powder samples, treated with

the identical method as that for the film samples, the ion-

exchange methods used in this present work could reach a

cation-exchange capacity of >98% and about 86% for H

and Ba exchanged ZSM-5, respectively. The degree of ion-

Fig. 3. The amount and distribution of adsorbed species at varying tempera-

tures after exposure to 600 ppm NO2 over NaZSM-5, BaZSM-5 and HZSM-5

monolith samples.

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–91 85

exchange was calculated on the assumption that one proton is

exchanged for one cationic sodium and one cationic Ba for two

sodium ions. The ICP-AES measurements on the synthesized

powder samples showed that the ZSM-5 had a Si/Al molar ratio

of about 64 and an Al/Na molar ratio of about 0.9, indicating

that they were pre-dominately NaZSM-5 [13]. Since the

powder and film samples were prepared in an identical

synthesis solution, it is here assumed that the Si/Al and Al/Na

molar ratios of the films are similar to those of the powder

samples. In another ongoing study, we are investigating the

exact elemental composition in thin ZSM-5 films, for example,

to elucidate if Al gradients are present and the results will be

reported elsewhere.

In addition, the results in Table 1 indicate that the ion-

exchange treatments did not significantly change the zeolite

loading of the original NaZSM-5 film. Also, the BET surface

area calculated from the N2 adsoption measurements was

similar for the monolith samples with HZSM-5 and NaZSM-5

films, but somewhat larger for the BaZSM-5 sample.

3.2. NOx adsorption capacity

To study NOx adsorption in ZSM-5 films, monolith samples

were exposed and equilibrated in a stream of 600 ppm NO2 in

argon. Following the equilibration with NO2, the samples were

flushed with argon (inert gas) and were heated (TPD treatment)

to desorb or decompose the adsorbed compounds. Fig. 2 shows

an example of the outlet NOx concentration profile during such

a sequence for the BaZSM-5 film sample. Besides the

breakthrough of NO2, NO was also formed during the NO2

adsorption over the BaZSM-5 film. Similar behaviours were

also observed for HZSM-5 and NaZSM-5 films. Fig. 3 is a

stacked bar diagram showing the quantities and distribution of

NOx compounds involved in the aforementioned treatments

following NO2 adsorption on NaZSM-5, HZSM-5 and BaZSM-

5 film samples. In the present report, results for the NaZSM-5

film are partly taken from our previous work [13].

Fig. 2. Measured NOx composition out of the reactor for BaZSM-5 film

exposed to 600 ppm NO2 for 60 min at 30 8C followed by a 30 min argon

flush, 30 min NO2 re-adsorption, 30 min argon flush at constant temperature and

finally a temperature ramp of 20 8C min�1.

An inert gas flushing, following NO2 equilibration always

results in desorption of adsorbed NOx mostly NO2. Fig. 3 shows

that the desorption is significant at low adsorption temperatures,

but the amount becomes negligible at adsorption temperatures

above 200 8C. The NOx desorption during inert gas flushing is

due to the release of physisorbed species. However, it should be

noted that due to mass transport limitation in the ZSM-5 films,

the inert gas flushing may not completely remove the

physisorbed species, particularly at low temperatures.

A temperature increase during TPD following the sequence

of NO2 adsorption and inert gas flushing results in another

desorption or decomposition of NOx adsorbed species. As seen

in Fig. 3, the quantity of NOx desorbed during TPD following

the adsorption at low temperatures is large and the amount

decreases as the adsorption temperature increases. However, at

high adsorption temperatures, the amount of the desorbed NOx

during TPD makes up a larger portion of the total amount of

NOx desorbed during TPD and flushing. This indicates that the

NOx desorption due to the temperature increase results from

the decomposition of chemisorbed species formed in the ZSM-

5 films during the NO2 adsorption. Depending on the

adsorption temperature, the chemisorbed species might

comprise various adsorbed species which differ in thermal

stability.

As clearly shown in Fig. 3, it was found that exchanging

cationic Na in ZSM-5 films with either H+ or Ba2+ cations

results in a significant reduction in the adsorption capacity.

The ion-exchange causes a decrease in the quantity of species

removed by argon flushing and it also significantly lowers the

amount of NOx later released during TPD. This relationship

indicates that the quantity of the physisorbed species in the

ZSM-5 films may be partly dependent on the quantity of

the chemisorbed species. The chemisorbed species formed

might play a role as active sites for further NO2 adsorption but

with weaker interactions. This kind of adsorption effect

results in an additional capacity for physisorption in the ZSM-

5 films.

As mentioned previously, NO is a byproduct during

exposure of the ZSM-5 films to NO2 at the adsorption

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–9186

temperatures studied. As seen in Fig. 3, the amount of NO

produced during NO2 adsorption in the NaZSM-5 film

decreases very little with increasing adsorption temperature.

However, the amount of byproduct NO with HZSM-5 and

BaZSM-5 films appears to decrease more with temperature,

becoming almost immeasurable above 200 8C. As reported

previously [13], the NO production is related to the formation

of surface nitric acid in the presence of residual water [2,6], by

the following reaction:

3NO2þH2O @ 2HNO3þNO: (2)

This reaction may also occur through the steps of NO2

disproportionation and in turn the interaction of the NO+

surface intermediate with residual water.

2NO2 @ NOþ þNO3�: (3)

H2O þ 2NOþ @ 2Hþ þNO2þNO: (4)

In the case of the NaZSM-5 film, the amount of byproduct

NO is still significant during NO2 adsorption even at 350 8C.

The NO production is thus related to the formation of thermally

stable nitrate species on metal sites through surface nitric acid

[2,3,6], by the following reaction:

nðHþNO3�Þ þ Mnþ�ðZeo�O�Þn @ MnþðNO3

�Þnþ nðZeo�O��HþÞ (5)

where M is a metal cation and n is its charge. However, it seems

that the development of NO in HZSM-5 and BaZSM-5 films is

not or at least less associated with the formation of the nitrate

species, but due to the formation of surface nitric acid (reaction

(2) above) which is less thermally stable compared to the nitrate

species.

As mentioned previously, the appearance of NO during NO2

adsorption in the ZSM-5 films is evidence for the presence of

residual water in the samples. The presence of counter ion

alkali or alkaline earth metals in the ZSM-5 can enhance the

adsorption affinity for water [31–33]. It is known to be difficult

to completely remove water from zeolites by heating under a

non-vacuum system [6]. During NO2 exposure, the residual

water can immediately react to form surface nitric acid [2,6]

which can in turn enhance the adsorption capacity by adsorbing

other NOx containing species via hydrogen bonds [13]. The

nitric acid formed can further interact either with NO2 or other

nitric acid molecules [34]. However, compared to the Ba2+

cation, Na+ is smaller in size and stronger in polarizing power.

In addition, as mentioned earlier, ICP-AES measurements of

the ZSM-5 powder samples show that the ZSM-5 have a Si/Al

molar ratio of about 64 which is considered to be high [35] and

the value is expected to be similar for the film samples. ZSM-5

with such as Si/Al molar ratio should contain only one to two

alumina sites per unit cell. For that reason, Ba2+ is more poorly

anchored in the ZSM-5 framework. Thus, at the same

temperature the amount of residual water in BaZSM-5 might

be less than that in NaZSM-5. As a result, less surface nitric

acid can be formed in the BaZSM-5 film leading to a low

adsorption capacity of the BaZSM-5 film for other weakly

adsorbed NOx species. Another possible reason for the low

adsorption capacity of the BaZSM-5 film at low temperatures is

that the stable nitrate species formed on the Ba sites may, due to

their large dimension, hinder other NOx molecules to be

adsorbed in the interior of the ZSM-5 films.

In comparison with the HZSM-5 film, the adsorption

capacity of the BaZSM-5 film at 350 8C is higher (not clearly

evident in Fig. 3). Due to the absence of alkali or alkaline earth

metal, it is obvious that there is no possibility to form nitrate

species in the HZSM-5 film. On the other hand, the presence of

Ba2+ ions in the ZSM-5 should promote the formation of stable

nitrate species from NO2 exposure. However, as shown by

Fig. 3, the quantity of these nitrate species should be much less

in comparison with those in the NaZSM-5 film. According to

ICP-AES element analysis of the powder ZSM-5 samples

prepared in a similar way to the film sample preparation, about

86% of the Na+ in the synthesized sample can be exchanged

with Ba2+ by the ion-exchange method used in the present

work. Since the ZSM-5 used in the present work has a low Al

content, it might lead to heterogeneous Brønsted sites [36]. The

catonic Ba would be likely to situate in a unit cell with two

aluminum sites or in between two adjacent unit cells having

only one aluminum site each. Depending on the position and

distance between the aluminum sites, it might cause Ba2+

cations to be unevenly distributed in the ZSM-5 framework.

3.3. Adsorbed species and their stability in ZSM-5 films

In the following section, the formation of adsorbed species

in the ZSM-5 films following NO2 adsorption is discussed in

more detail. The adsorbed species are classified with respect to

their thermal stability by NOx-TPD. Since the adsorbed species

can differ in thermal stability, a temperature increase during

TPD following a sequence of NO2 equilibration and inert gas

flushing over the ZSM-5 film samples can develop a specific

NOx concentration profile. Fig. 4 shows concentration profiles

of NOx desorption at the reactor outlet as a result of temperature

increase following NO2 equilibration and inert gas flushing at

various adsorption temperatures for the different ZSM-5 films.

NO concentrations were negligible throughout the desorption

period and thus are not depicted in the figure. In addition to

NOx-TPD observations, in situ DRIFT spectroscopy measure-

ment results from powder samples of the ZSM-5 are also

discussed.

Also, it is possible to estimate the heat of adsorption from the

TPD concentration profile [37]. Our calculation method is

similar to that used for powder samples [38]. The flow in the

channels was modeled to follow a one-dimensional single

channel model [39]. The desorption was from a monolayer of a

single adsorbed species with negligible re-adsorption and a

negligible activation energy for adsorption. The actual TPD

following NO2 adsorption resulted in a multi-peak desorption,

indicating the presence of various adsorbed species and thus it

was difficult to determine the initial condition of the model for

the desorption of each adsorbed species. Therefore, the heat of

adsorption was estimated only for the adsorbed species easily

removed early during TPD following NO2 equilibration and

Fig. 4. NOx concentration profiles during TPD after NO2 equilibration and

argon flushing on NaZSM-5 (a), HZSM-5 (b) and BaZSM-5 (c) at various

temperatures: 30 8C (i) to 350 8C (ii).

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–91 87

inert gas flushing at 30 8C. The calculation is based on the slope

of the concentration increase from the first desorption peak (see

Fig. 4). Since transport resistances both in the ZSM-5 film and

in the gas phase were neglected from the model, it is expected

that the resulting heat of adsorption would be overestimated.

3.3.1. NaZSM-5

As reported previously [13] and shown in Fig. 4(a), the

adsorbed species in the NaZSM-5 film can be classified into at

least three types of thermally stable species: low, intermediate

and high. The calculation for the estimated heat of adsorption

indicated that the adsorbed species that can be easily desorbed

early during TPD following NO2 equilibration at 30 8C have an

average heat of adsorption of about 49.7 kJ mol�1. This value is

at the upper limit of the heat of adsorption for physisorption

[40] probably due to the influence of transport resistance in the

NaZSM-5 film which is neglected in the model. The highly

thermally stable species are composed of two nitrate species

that also differ in thermal stability. The less stable nitrate

species was found to involve surface nitric acid in its formation

following reactions (2) and (5) above [13]. In situ DRIFT

spectroscopy measurements upon NO2 exposure to the

NaZSM-5 powder sample at 30 8C (see Fig. 5(a)) showed

the presence of nitrate species assigned to a feature at

1449 cm�1 [9,13] and surface nitric acid at 1551 and

1668 cm�1. These peaks from the nitric acid were found to

be unstable during NO exposure [13], since reaction (2) above

was reversed. Also, it was found that the presence of surface

nitric acid and hydroxyl groups could enhance the NOx

adsorption capacity for weakly adsorbed species. A detailed

discussion about the absorbed species and their interactions for

NaZSM-5 were reported previously [13].

3.3.2. HZSM-5

3.3.2.1. Thermal stability of adsorbed species. Fig. 4(b)

shows NOx desorption during TPD following NO2 equilibra-

tion at different temperatures for the HZSM-5 film. A

temperature increase following the equilibration at 30 8Cresults in a steep concentration increase that leads to the first

TPD peak. The concentration increase is similar to that which

arises from the NaZSM-5 film, suggesting it results from

desorption of similar adsorbed species. Since the release

occurs very quickly, it might be due to the release of

physisorbed NO2 still remaining in the ZSM-5 film after inert

gas flushing. The average heat of adsorption value of this

species estimated from the desorption profile, early during

TPD, was about 51.0 kJ mol�1 which is very close to that from

the NaZSM-5 film. Mass transport resistance in the ZSM-5

film could cause the release of the physisorbed species during

the inert gas flushing to be slow. However, the temperature

increase could activate the movement of the physisorbed

species in the ZSM-5 framework. The presence of physisorbed

NO2 on HZSM-5 in a 600 ppm NO2 atmosphere is also shown

by the in situ DRIFT spectroscopy measurements. As shown

in Fig. 5(b), a feature with a band at 1628 cm�1 is assigned to

the physisorbed NO2 and is similar to that observed on the

NaZSM-5 sample [13].

Besides the release of physisorbed species, the first

desorption peak from the HZSM-5 film might also be due to

the release of other weakly adsorbed species that differ slightly

in thermal stability. However, the peak from the HZSM-5 is

narrower and lower in intensity compared to that from the

NaZSM-5 film. This indicates that a lower quantity of the

weakly adsorbed species contributes to the appearance of the

first desorption peak of the HZSM-5 film compared to that from

the NaZSM-5 film. As seen in the Fig. 4(b), a further

temperature increase results in another NOx desorption peak

with a maximum at about 400 8C. Increasing the temperature to

above 400 8C results in a steep NOx concentration decrease that

is followed by an insignificant desorption above 500 8C.

However, it is interesting that TPD following NO2 adsorption at

350 8C results in a negligible NOx desorption (see Fig. 4(b)).

This observation indicates that the second peak from the

HZSM-5 film might belong to a specific surface species that is

relatively stable at high temperatures. Since cationic alkali or

Fig. 5. Infrared spectra following 600 ppm NO2 exposure at 30 8C for 5 min on NaZSM-5 (a), HZSM-5 (b) and BaZSM-5 (c). Dashed line is zero level.

Fig. 6. NOx-TPD profiles of HZSM-5 (a) and BaZSM-5 (b) after a sequence of

(i) 600 ppm NO2 equilibration and argon flushing at 200 8C, (ii) 600 ppm NO2

equilibration, argon flushing, 600 ppm NO equilibration and argon flushing at

200 8C and (iii) NO2 equilibration and argon flushing at 300 8C.

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–9188

alkaline earth metals are absent in the HZSM-5, this species

cannot be stable nitrates. It is more likely that the species is

surface nitric acid.

3.3.2.2. Stability of adsorbed species against NO atmos-

phere. The absorbed species formed in the HZSM-5 film were

further investigated by introducing 600 ppm NO to the NO2

equilibrated HZSM-5 film. Following the NO exposure, the

sample was then flushed with inert gas followed by TPD.

Fig. 6(a) shows the effect of NO exposure on the TPD profile for

the NO2 equilibrated HZSM-5. Following NO2 equilibration

and inert gas flushing at 200 8C, the NO exposure significantly

lowered the NOx desorption quantity and the TPD peak was at

slightly higher temperature compared to the desorption without

NO exposure. Thus, the peaks likely result from the desorption

of surface species having similar thermal stability. As

mentioned previously, the peaks are likely to result from the

decomposition of surface nitric acid in the HZSM-5. However,

as apparent from the figure, the NO feed seems unable to

completely remove the adsorbed species, by for example, the

reversal of reaction (2). TPD following NO2 adsorption at

300 8C caused a similar desorption peak to that after NO

exposure but with a lower intensity. Since part of the nitric acid

is stable against NO, there may be two possible ways to form

surface nitric acid in the HZSM-5. The first is due to a water–

NO2 interaction following reactions (2) or (3) and (4) above.

The second is a result of deprotonization following NO2

disproportionation.

Zeo�O��Hþ þNOþ þNO3�@ Zeo�O��NOþ þHþNO3

�:

(6)

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–91 89

The TPD observations indicate that the nitric acid species

formed through these two mechanisms are likely to have similar

thermal stability. A similar deprotonization reaction mechan-

ism for nitric acid formation was reported for H type mordenite

[10]. The presence of nitric acid and the nitrosium ion (NO+)

during NO2 adsorption on HZSM-5 is also confirmed by the in

situ DRIFT spectroscopy measurement with the HZSM-5

powder sample. As shown in Fig. 5(b), exposure of the

HZSM-5 powder to NO2 at 30 8C results in the development

of a feature with a band at 2129 cm�1 which represents the

stretching vibration of NO+ in HZSM-5 [12,41–43]. Mean-

while, a band at 1598 cm�1 is similar to that found for NaZSM-

5 (1551 cm�1) and can be attributed to the stretching vibration

of nitric acid molecules [13].

As noted above, the absence of cationic alkali or alkaline

earth metals causes HZSM-5 to be less hydrophilic. As a result,

the formation of surface nitric acid due to NO2–water

interaction is very limited, as indicated by the small amount

of NO formation during NO2 adsorption (Fig. 3). Although the

nitric acid can interact with Brønsted acid sites, there are few of

these interactions leading to a small quantity of weakly

adsorbed NOx containing species in the HZSM-5. As seen in

Fig. 5(b), the spectroscopy measurement after NO2 adsorption

on HZSM-5 shows a broad infrared spectrum in the range of

3500–3000 cm�1 that indicates interactions between nitric acid

molecules and OH groups with a small intensity. A temperature

increase to above 500 8C can, however, completely decom-

poses all nitric acid species in the HZSM-5 film (see Figs. 4(b)

and 6(a)).

3.3.3. BaZSM-5

3.3.3.1. Thermal stability of adsorbed species. Fig. 4(c)

shows NOx concentration profiles from TPDs with the NO2

equilibrated BaZSM-5 film. A temperature increase following a

sequence of NO2 equilibration and inert gas flushing at 30 8Cresults in NOx desorption that has a similar profile to that of the

HZSM-5 film. Two distinguishable concentration peaks are

visible. In comparison with the HZSM-5 film, the first

desorption peak of the BaZSM-5 film is broader due to a

high temperature shoulder. This indicates that the peak results

from the desorption of not only physisorbed species, but also

other weakly chemisorbed species. The average heat of

adsorption value estimated for the adsorbed species that could

be released early during TPD was about 53.9 kJ mol�1 which is

very close to those for the HZSM-5 and the NaZSM-5 films.

Similar to that seen with the HZSM-5 film, the second

desorption peak of the BaZSM-5 film has a maximum at about

the same temperature. Therefore, this peak can be attributed to

the decomposition of surface nitric acid in the BaZSM-5 film. A

further temperature increase to above 450 8C seems to cause

additional NOx desorption, indicated by the development of a

shoulder at high temperature. Moreover, TPD after NO2

equilibration at 300 8C results in additional NOx desorption

which is not observed from the HZSM-5 film. This indicates the

presence of another adsorbed species more stable than surface

nitric acid. As mentioned previously, stable nitrate species

might be formed on Ba sites. In situ DRIFT spectroscopy

measurements at 30 8C on the BaZSM-5 powder exposed to

600 ppm NO2 confirmed the presence of nitrate species. As

shown in Fig. 5(c), features with bands at 1450–1505 cm�1 may

characterize the vibrations of the nitrates.

In comparison with the surface nitrate species formed in the

NaZSM-5 film, the surface nitrate species in the BaZSM-5 film

apparently have lower thermal stability. As seen in Fig. 4(a and

c), high temperature NOx desorption (above 350 8C) for the

BaZSM-5 film has a peak at lower temperature. Also, the peak

for the BaZSM-5 film is lower in intensity indicating a small

amount of the nitrate species.

3.3.3.2. Stability of adsorbed species against NO

atmosphere. Fig. 6(b) shows the effect of NO exposure on

the stability of the adsorbed species in the BaZSM-5 film.

Preceding the NOx desorption by TPD, 600 ppm NO was fed to

the NO2 equilibrated BaZSM-5 film and then the film was

flushed with inert gas. As illustrated by the figure, the exposure

of the NO2 equilibrated BaZSM-5 film to NO at 200 8C results

in a large decrease in the NOx desorption quantity. However, the

NO feed does not completely eliminate the NOx desorption,

instead it causes another higher temperature peak to become

more discernible, which in the TPD after only NO2

equilibration was visible only as a slight high temperature

shoulder. This high temperature peak is also clear in the TPD

following NO2 equilibration at 300 8C. These observations

indicate that the peak at high temperature results from the

desorption or decomposition of adsorbed species more stable

than surface nitric acid. The large decrease in NOx desorption

due to the NO introduction is likely a result of the

decomposition of surface nitric acid and nitrate species

following the reversal of reactions (2) and (5). Meanwhile,

the peak at higher temperature should correspond to the

presence of nitrate species on Ba sites. These results indicate

that the formation of nitrate species in the BaZSM-5 does not

necessarily occur with the involvement of surface nitric acid

(reactions (2) and (5)) due to their resistance against NO

exposure.

In situ DRIFT spectroscopy measurements on the BaZSM-5

powder sample following NO2 adsorption at 30 8C indicated a

broad infrared spectrum developing in the wavenumber range

of 2200–3000 cm�1 (Fig. 5(c)). A similar infrared spectrum

was also observed with NaZSM-5 following NO2 adsorption

[13] and it was found that the intensity of the spectrum easily

collapsed after NO introduction. Therefore, the broad infrared

spectrum likely corresponds to the presence of surface nitric

acid in BaZSM-5 due to water–NO2 interaction. In NaZSM-5

the surface nitric acid formed might immediately interact with

the counter ion Na+ to form nitrate species and OH Brønsted

groups (reaction (5)) which become new sites for the adsorption

of additional NOx containing species [13,34]. In contrast, the

formation of the surface nitric acid in the BaZSM-5 is

apparently not followed by immediate reaction with cationic Ba

to form Brønsted sites. As a consequence, in situ spectroscopy

measurements on the BaZSM-5 powder exposed to NO2 show

an insignificant infrared spectrum development in the Brønsted

OH vibration frequency region (see Fig. 5(c)). In the interior of

I. Perdana et al. / Applied Catalysis B: Environmental 72 (2007) 82–9190

BaZSM-5 with low aluminum content, the formation of nitrates

through surface nitric acid might not be favorable. Although

surface nitric acid can be formed near barium sites, distances

between alumina sites might hinder the nitrate formation

according to reaction (5), compared to that with NaZSM-5. As

seen in Fig. 5(c), a feature with a frequency band at 1574 cm�1

is similar to that found in both NaZSM-5 (1551 cm�1) and

HZSM-5 (1598 cm�1) and is assigned to the stretching

vibration from nitric acid molecules.

The fact that nitrate species in BaZSM-5 films are stable in

an NO atmosphere suggests that they are formed primarily not

via surface nitric acid. An alternative pathway for their

formation is via the NO2 disproportionation reaction (3).

Subsequently, the NO+ binds to negatively charged sites in the

ZSM-5 framework, replacing the charge compensating Ba2+

which is further bound to NO3�.

2ðZeo�O�Þ�Ba2þ þ 2NOþ þ 2NO3�

@ 2ðZeo�O��NOþÞ þ Ba2þðNO3�Þ2: (7)

The in situ spectroscopy measurement on the powder sample

after NO2 adsorption shows the development of a feature at

2132 cm�1 (see Fig. 5(c)). This feature is often attributed to the

stretching vibration of NO+ in zeolites [12,41–43]. In addition,

another peak at 1739 cm�1 is clearly visible and it corresponds

to the formation N2O4 which is an intermediate product of the

NO2 disproportionation reaction.

However, as demonstrated by reaction (7), to form a

complete nitrate species, each Ba2+ counter ion needs two

charge compensating nitrates. In the interior of the ZSM-5, the

formation of the complete nitrate species might be hindered

either sterically or due to the fact that the Ba2+ cations are

positioned far from the framework negative charge sites due to

the high Si/Al ratio. Instead, each Ba2+ counter ion may capture

only one charge-compensating nitrate.

2ðZeo�O�Þ�Ba2þ þNOþ þNO3�

@ Zeo�O��NOþ þZeo�O��Ba2þ�NO3�: (8)

The reasons offered here to rationalize why nitrate formation is

less favorable in the BaZSM-5 film may also explain why the

few nitrates in BaZSM-5 have a lower thermal stability than

those in NaZSM-5 which is evident by comparing the high

temperature peaks in their respective TPD profiles (Fig. 4(a and

c)).

4. Conclusions

The present report dealt with a comparison of NOx

adsorption in ZSM-5 films with Na+, H+ and Ba2+ as counter

ions. Depending on the cations, adsorbed species having

different thermal stabilities could exist in the ZSM-5 films.

Besides the cations, residual water in the ZSM-5 also played

an important role in the adsorption. The presence of Na+

counter ions in the ZSM-5 was found to provide a high NOx

adsorption capacity over a wide temperature range. In HZSM-

5 film, in addition to physisorbed species, NO2 adsorption

could occur at relatively high temperatures and formed

species with moderate thermal stability, such as surface nitric

acid and nitrosium (NO+) ions. Surface nitric acid in the

HZSM-5 could be formed either by NO2–residual water

interactions or following an NO2 disproportionation pathway.

On the other hand, highly thermally stable nitrates,

moderately thermally stable nitric acid and weakly thermally

adsorbed species including physisorbed NO2 appeared to be

present in the BaZSM-5 film. The nitrates formed in the

BaZSM-5 were found to be mostly stable in an NO

atmosphere and thus did not apparently involve surface

nitric acid in their formation. Instead, NO2 disproportionation

led mainly to the development of the nitrate species.

However, the fewer nitrate species formed in the BaZSM-5

film and their lower thermal stability compared to those in the

NaZSM-5 film suggested that surface nitrate formation in the

BaZSM-5 was less favourable either due to sterical

hinderance or due to the poor anchoring of the Ba2+ cation

in the ZSM-5. The formation of fewer hydroxyl groups also

led to a lower adsorption capacity of the BaZSM-5 for weakly

adsorbed species.

Acknowledgements

The authors are grateful for the financial support of the

Swedish Research Council. I. Perdana and D. Creaser also

thank the SIDA-Swedish Research Links program for support.

Also the authors thank Alessandra Mosca at the Division of

Chemical Technology, Lulea University of Technology for

preparation of one of the monolith samples and some of the

characterization measurements.

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