The effect of metal oxide additives (WO3, MoO3, V2O5, Ga2O3) on the oxidation of NO and SO2 over Pt/Al2O3 and Pt/BaO/Al2O3 catalysts

Article abstract of DOI:10.1016/j.molcata.2003.08.025

Using NO_x storage catalyst can overcome the problem with exhausts from diesel and lean burn engines, which contains a large surplus of oxygen that prevents effective reduction of NO_x. The effect of metal oxide additives on enhancing NO oxidation and/or suppressing SO₂ oxidation in alumina supported Pt catalysts was studied. In the absence of SO₂, WO₃/Pt/Al₂O₃ and MoO₃/Pt/Al₂O₃ promoted the highest NO oxidation. In the presence of SO₂, the NO oxidation activity was decreased for all catalysts. The decrease in NO oxidation activity due to SO₂ exposure was highest for the Pt/Al₂O₃ and the Ga₂O₃/Pt/Al₂O₃ catalysts followed by WO₃/Pt/Al₂O₃ and V₂O₅/Pt/Al₂O₃ catalysts and finally MoO₃/Pt/Al₂O₃ catalyst. The MoO₃/Pt/Al₂O₃ catalyst was the most efficient catalyst in enhancing the NO oxidation while suppressing the SO₂ oxidation. For all catalysts, SO₂ oxidation was promoted in the presence of NO in the reaction gas mixture Pt/BaO/Al₂O₃ and Pt/BaO/MoO₃/Al₂O₃ catalysts showed equal NO_x storage capacity in the sulfur free cycles. When SO₂ was included in the reaction gas mixture, the NO_x storage capacity declined faster for the Pt/BaO/MoO₃/Al₂O₃ catalyst than for the Pt/BaO/Al₂O₃ catalyst, due to sulfur poisoning which resulted in a faster decrease in the regeneration capacity during rich periods.

Full text of DOI:10.1016/j.molcata.2003.08.025

Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
The effect of metal oxide additives (WO , MoO , V O , Ga O ) on the
3
3
2
5
2
3
oxidation of NO and SO over Pt/Al O and Pt/BaO/Al O catalysts
2
2
3
2
3
a,b,∗
a,c
a,b
Jazaer Dawody
, Magnus Skoglundh , Erik Fridell
a
Competence Centre for Catalysis, Chalmers University of Technology, SE-41296 Göteborg, Sweden
b
Department of Applied Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden
c
Surface Chemistry, Chalmers University of Technology, SE-41296 Göteborg, Sweden
Received 12 July 2003; accepted 28 August 2003
Abstract
For NO
x
storage, theoxidationofNOtoNO
2
isanimportantstepinthestoragemechanism. Inthefirstpartofthiswork, modelWO
and Pt/Al catalysts were prepared and used in flow reactor experiments. Heating ramp
oxidation and steady state oxidation experiments were performed in order to investigate the catalytic activity for NO oxidation both in the
presence and absence of SO in the reaction gas mixture.
In the absence of SO , the WO and MoO containing catalysts showed a significantly higher NO oxidation activity than the other catalysts.
When SO was included in the reaction gas mixture, the NO oxidation activity decreased for all catalysts, but the MoO containing catalyst
was less affected by the presence of SO and also showed the lowest SO oxidation activity.
In the second part of this work, model Pt/BaO/Al and Pt/BaO/MoO /Al catalysts were prepared and used in heating ramp oxidation
and transient flow reactor experiments in order to investigate the effects of NO oxidation activity enhancement and suppression of the activity
for SO oxidation on NO storage capacity. Both catalysts showed equal NO storage capacity in the sulphur free cycles. When SO was
included in the reaction gas mixture, the NO storage capacity declined faster for the Pt/BaO/MoO /Al catalyst than for the Pt/BaO/Al
catalyst, due to sulphur poisoning which resulted in a faster decrease in the regeneration capacity during rich periods.
2003 Elsevier B.V. All rights reserved.
3 2 3
/Pt/Al O ,
MoO
3
/Pt/Al
2
O
3
, V
2
O
5
/Pt/Al
2
O
3
, Ga
2
O
3
/Pt/Al
2
O
3
2 3
O
2
2
3
3
2
3
2
2
2
O
3
3
2 3
O
2
x
x
2
x
3
2
O
3
2 3
O
©
Keywords: NOx storage catalysts; Sulphur poisoning; NO oxidation; SO2 oxidation; Metal oxide additives
1
. Introduction
nitrogen oxides (NOx). To overcome this problem different
techniques have been developed. One technique is selec-
tive catalytic reduction using ammonia as a reducing agent
Vehicle combustion engines contribute significantly to
the anthropogenic emissions of CO2. The increasing aware-
ness of climate changes and the connection between accu-
mulated CO2 in the atmosphere and global warming [1,2]
emphasises the need to reduce the vehicle fuel consump-
tion. Diesel and lean burn engines have in this connection
become important alternatives to the conventional gasoline
engine, which operates close to stoichiometric air-to-fuel
ratios. The main advantage with lean operation is the rel-
atively low fuel consumption which can be 20–30% lower
compared to stoichiometric combustion [3]. The exhausts
from diesel and lean burn engines contain however, a large
surplus of oxygen which prevents effective reduction of
(NH3-SCR). Although the NOx reduction potential of this
technique is high, it has not yet been established in the mar-
ket. The major obstacles are connected with the delivery
systems of the reducing agents and the necessity of having
a separate tank with ammonia or urea in the vehicle.
Another concept is to use NOx storage catalysts, which
function with alternating lean and rich conditions. During
lean conditions NO is oxidised to NO2 and trapped in a spe-
cific NOx storage compound until the catalyst becomes satu-
rated. To regenerate the catalyst, the conditions are switched
during a short period (seconds) to rich conditions where the
stored nitrates are released and decomposed to NO2 and NO,
and reduced to N2 [4–6].
∗
One major problem with the NO storage catalyst is its
sensitivity to sulphur poisoning where sulphur compounds
Corresponding author. Tel.: +46-31-7723153; fax: +46-31-7723134.
x
E-mail address: jazaer.dawody@fy.chalmers.se (J. Dawody).
1
381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.08.025

2
16
J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
are converted in the catalyst and trapped as sulphates with
the NOx storage compound [7]. Sulphate formation origi-
nates from the undesired SO2 oxidation, which takes place
simultaneously with NO oxidation during lean conditions.
Since sulphates are thermally more stable than nitrates, the
amount as well as the size of stored sulphate particles in-
crease with increasing sulphur exposure time resulting in
deactivation of the NOx storage function [7].
with BaO as storage component and the NOx storage capac-
ity was investigated by performing lean/rich transient flow
reactor experiments.
2. Experimental
2.1. Catalyst preparation
Several studies have been performed with the aim of un-
derstanding, avoiding or at least suppressing the sulphur
poisoning in NOx storage catalysts. The outcome of these
studies shows that parameters such as regeneration gas com-
position, lean-to-rich time ratio, temperature, choice of the
storage material, as well as the noble metal(s) and the acid-
ity of the support material play an important role both for
the storage of NOx and the poisoning by sulphur [8–16].
During recent years different ways to minimise sulphur
poisoning of NOx storage catalysts have been suggested.
In some of these studies, sulphate decomposition and SOx
desorption were enhanced by catalyst washcoat modification
with components such as TiO2, Rh/ZrO2, Fe and Li in order
to prevent the growth of sulphate particles and/or stimulate
hydrogen generation for enhancing the reduction of barium
sulphates [17–20].
2
Seven cordierite monoliths (400 cells/in. ) with a length
of 15 mm containing 188 channels were washcoated with
alumina and five of the samples were impregnated with plat-
inum. Four of the platinum containing samples were also
provided with either, MoO , WO , V O or Ga O . Finally,
one of the remaining two alumina coated samples was pro-
vided with V O and the other with MoO .
The catalysts were prepared according to the method
described in detail in [34]. Briefly, the monoliths were im-
pregnated with alumina by immersing the monolith in an
alumina slurry, blowing away the excess of the slurry from
the channels, drying in air at 95 C for few seconds and
calcining in air at 500 C for 2 min. This procedure was
3
3
2
5
2
3
2
5
3
◦
◦
repeated until the desired amount of alumina was obtained.
Thereafter, the samples were calcined at 500 C for 2 h.
◦
It has also been shown that interaction between sulphur
and the noble metal particles during rich conditions is of
significant importance for the deactivation [13,16,21].
Formation of sulphates may also be suppressed by de-
signing the catalyst so that the NO2 formation is enhanced
in relation to SO2 oxidation, which is an important step for
NOx storage. It might be possible to suppress the oxida-
tion of SO2 to SO3 by changing the structure of the active
catalyst particles, since this reaction seems to be structure
sensitive [22]. Another way may be to add additives which
inhibit or at least suppress the oxidation of SO2.
The desired amount of Pt was added to the alumina coated
samples by filling the channels with an aqueous solution of
◦
platinum nitrate [Pt(NO ) ] and drying at 90 C for 12 h,
3
2
◦
and then calcining in air at 500 C for 2 h. To provide the
metal oxide additives, each alumina and Pt coated monolith
was impregnated with one of the following additive aque-
ous solutions: ammonium molybdate [(NH ) Mo O ], am-
4
2
2
7
monium tungsten oxide [(NH ) W O ·xH O], gallium
4
10 12 41
2
2
−
nitrate [Ga(NO ) ] or vanadium oxalate [VO(C O ) ]. Af-
3
3
2
4
ter the impregnation with the additive precursor solutions,
◦
the samples were dried and finally calcined in air at 500 C
Transition metal oxides such as V2O , MoO3 and WO3
for 2 h.
5
are widely used in NH3-SCR catalysts due to their redox
properties, which seem to be important in enhancing NO
conversion. In addition, they contribute to increasing the
surface acidity [23–27] which might suppress the SO2
oxidation.
x 2 3
For the NO storage experiments, one Pt/BaO/Al O and
one Pt/BaO/MoO /Al O catalyst were prepared. The size
3
2
3
of these catalysts as well as the washcoat impregnation
method was the same as for the catalysts described above.
The Pt/BaO/Al O catalyst was prepared by impregnation
2
3
Ga2O3 has been used in selective catalytic reduction
of NOx with hydrocarbons (HC-SCR) [28–33]. For a
Ga2O3-Al2O3 catalyst, two types of Lewis acid sites were
defined [33]. According to Pasel et al. [28], the presence
of SO2 in the gas feed did not strongly affect the catalytic
activity in NO conversion for a Ga2O3 loaded sulphated
zirconia catalyst.
The aim of this study is to investigate if a set of metal
oxides can act as promotors to enhance the NO oxidation
and/or suppress the SO2 oxidation. In the first part of the
study, alumina supported Pt catalysts with and without metal
oxide additives were prepared and the catalytic activity of
both separate and simultaneous NO and SO2 oxidation were
investigated. In the second part of the study, one unmodified
and one MoO3 modified NOx storage catalyst were prepared
with alumina, followed by impregnation with an aqueous
barium nitrate [Ba(NO ) ] solution and finally Pt precursor
3
2
solution. For the Pt/BaO/MoO /Al O catalyst, the impreg-
3
2
3
nation with molybdate solution was performed before the
impregnation with BaO in order to avoid BaO dissolving
since the molybdate solution is slightly acidic and to ensure
that equal amount BaO in both catalysts are accessible for
barium nitrate formation. Further, in order to avoid BaO dis-
solving when impregnating with the acidic platinum nitrate
solution, the catalysts were immersed in a solution of am-
monium carbamate (NH CO NH ) and the excess solution
4
2
2
was removed from the channels by air immediately before
impregnation with the Pt precursor solution.
The compositions of the washcoat materials for all cata-
lysts prepared are given in Table 1.

J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
217
Table 1
Catalyst washcoat compositions
Samples
Al2O3 (mg)
Pt (mg)
Metal additive (mmol)
BaO-content (mg)
Pt/Al2O3
623
585
545
622
569
607
574
469
490
13
12
13
13
12
−
−
−
MoO3/Pt/Al2O3
WO3/Pt/Al2O3
V2O5/Pt/Al2O3
Ga2O3/Pt/Al2O3
V2O5/Al2O3
MoO3/Al2O3
Pt/BaO/MoO3/Al2O3
Pt/BaO/Al2O3
0.35
0.39
0.18
0.18
0.18
0.35
0.47
–
−
−
−
−
−
−
−
12
13
125
125
2
.2. Catalyst characterisation
2.3. Activity studies
The platinum dispersion of the platinum containing cat-
alysts and the CO adsorption capacity of the MoO3/Al2O3
The activity of the catalysts was tested in another flow
reactor similar to the previously described one, but with
other instruments to analyse the outflow gas compositions.
A chemiluminescense detector (CLD 700) was used to mon-
itor NOx (NO and NO2), while SO2, CO2 and N2O were
measured by separate non-dispersive IR Maihak UNOR 610
instruments. The inlet gas composition was controlled by
an Environics 2000 gas mixer. The gas flow and space ve-
and V2O /Al2O3 samples were determined by temperature
5
programmed desorption (TPD) of CO. The TPD experiments
were conducted in a flow reactor with a quartz tube in which
the catalyst was placed. A thermocouple used to control
the temperature was placed a few mm in front of the cat-
alyst. Another thermocouple was placed inside the catalyst
to measure the catalyst temperature. A mass spectrometer
−1
locity in all experiments were 3000 ml/min and 38 000 h
respectively, with Ar as the carrier gas.
,
(Balzer QME 120) was connected to the reactor to analyse
the outflow gas composition. The gas flow into the reactor
was controlled with mass flow controllers.
Prior to each temperature ramp and steady state ex-
periment the catalysts were pre-treated at 500 C, first
◦
Prior to each measurement, the catalyst was first
pre-oxidised and then pre-reduced at 500 C with 2% O2 in
Ar and 4% H2 in Ar, respectively. CO was adsorbed on the
catalyst surface by exposing the catalyst to 2000 ppm CO
by reduction in 2% H2 for 20 min, flushing with Ar for
5 min and finally oxidation in 8% O2 for 20 min. After
each pre-treatment, the catalyst was cooled down to room
temperature in Ar. The gas compositions for all oxidation
experiments are given in Table 3.
◦
◦
in Ar at 4 C for 10 min. The sample was then heated with
◦
◦
a speed of 40 C/min up to 550 C in Ar and the desorbed
species were analysed by the MS. The Pt dispersion was
calculated by assuming an adsorption of 0.7 CO molecules
per Pt surface atom [35].
2.3.1. Heating ramp experiments
Each experiment started at room temperature by exposing
the pre-treated catalyst to the reaction gas mixture until sat-
uration (i.e. the outlet concentration of NOx and/or SO2 was
equal to the inlet concentration), thereafter, the temperature
The specific surface areas of the catalysts were determined
by nitrogen adsorption according to the BET method using
Digisorb 2600 (Micromertics) instrument.
The platinum dispersion and the specific surface areas of
the catalysts are given in Table 2.
◦
◦
was linearly increased by 5 C/min to 450 C in the same
reaction gas mixture. The experiments were performed both
with separate and simultaneous NO and SO2 oxidations.
Table 2
Catalyst characterisation data
Samples
BET surface area
Pt dispersion (%)
fresh catalyst
Desorbed CO (␮mol)
fresh catalyst
Pt dispersion (%)
used catalyst
Desorbed CO (␮mol)
used catalyst
2
(m /g catalyst)
Pt/Al2O3
33
32
31
31
34
n.d.
n.d.
9.6
7.7
6.2
9.4
4.5
4.6
18.3
12
3.4
12.6a
1.5
2.2
11.5
0.64
2.0
20
n.d.
n.d.
a
MoO3/Pt/Al2O3
WO3/Pt/Al2O3
Ga2O3/Pt/Al2O3
V2O5/Pt/Al2O3
V2O5/Al2O3
10.4
10.7
14.9a
−
4.7
18.4a
−
MoO3/Al2O3
−
6.1
−
a
The amount of CO adsorbed on Pt = the difference between the integrated amount of CO desorbed from MeO/Pt/Al2O3 and MeO/Al2O3 (MeO =
V2O5 or MoO3).

2
18
J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
Table 3
Summary of gas compositions
Experiment
NO (vol. ppm)
SO2 (vol. ppm)
O2 (vol.%)
C3H6 (vol.%)
NO heating ramp oxidation
SO2 heating ramp oxidation
450
−
−
8
8
8
8
8
8
8
−
−
30
30
−
−
NO + SO2 heating ramp oxidation
450
450
−
−
NO steady state oxidation
−
SO2 steady state oxidation
30
30
10
10
−
NO + SO2 steady state oxidation
NOx storage reduction lean condition
NOx storage reduction rich condition
450
400
400
−
650
650
2
.3.2. Steady state experiments
Steady state experiments were performed by saturating
the temperature is increased. This decrease is due to the
NO2 concentration becoming limited by the thermodynamic
equilibrium concentration. For the WO3 and MoO3 contain-
ing catalysts, the NO2 concentrations maxima are reached
the pre-treated catalyst with the reaction gas mixture at room
temperature in the same way as in the heating ramp exper-
iments. Thereafter, the activity for NO and SO2 oxidation
was measured at 125, 150, 175, 200, 225 C for 20 min at
each temperature.
◦
at ∼220 C with 92 and 81% NO conversion, respectively.
◦
The maximum in NO2 concentration for the other three cat-
◦
alysts is reached at 300–330 C with maximum conversions
of 78, 77 and 67% for the Ga2O3 containing, Pt/Al2O3 and
2
.3.3. Transient experiments
V2O containing catalysts, respectively.
5
Prior to the transient experiments, the NOx storage cat-
In order to investigate the influence of SO2 on the NO
oxidation activity, 30 ppm SO2 was included in the above
mentioned reaction gas mixture. The results from these ex-
periments are shown in Fig. 2 which displays the outlet NO2
concentration as a function of temperature. It is obvious that
the NO oxidation activity for all catalysts decreases in by
the presence of SO2. The formation of NO2 is shifted to-
wards higher temperatures. The NO2 formation rate is lower
and the maximum in NO2 formation is lower compared with
the corresponding experiments in absence of SO2. The re-
sults from these experiments show two interesting features:
(i) the Pt/Al2O3 catalyst seem to be more affected by the
presence of SO2 than for all catalysts with metal oxide ad-
ditives; and (ii) despite the presence of SO2 the MoO3 and
WO3 containing catalysts still show much higher NO oxi-
dation activity than the other catalysts.
◦
alysts were pre-treated at 550 C by reduction in 2% H2
◦
for 30 min and cooling down to 300 C in Ar. Thereafter,
the catalyst was stabilised in a lean gas mixture for 20 min
and regenerated in rich gas mixture for 4 min. The lean
gas mixture composition was 400 ppm NO, 8% O2 and
50 ppm C3H in Ar with a flow of 3000 ml/min. For the
6
rich gas composition, the oxygen supply was switched of,
and compensated by Ar to maintain constant flow. Tran-
6
sient experiments with lean/rich cycles were conducted at
◦
3
00 C with 16 and 4 min long lean and rich periods, re-
spectively. Four sulphur free lean/rich cycles were initially
performed, thereafter 10 ppm SO2 was included in both
the lean and the rich gas mixtures during the remaining
cycles.
3
3
3
. Results
4
3
2
1
00
00
00
00
0
.1. Heating ramp experiments
.1.1. NO oxidation
The catalytic activity for NO oxidation was investigated
Ga2O3
MoO3
in oxygen excess both in absence and presence of SO2.
Fig. 1 shows the concentration of NO2 as a function of
temperature in the absence of SO2. As is clear from the fig-
ure, all catalysts show very low NO oxidation activity for
unmodified
V2O5
WO3
◦
temperatures up to ∼130 C. After further temperature in-
1
00
200
300
°
400
◦
crease, NO2 starts to form at ∼140 and 165 C for the WO3
Temperature ( C)
and MoO3 containing catalysts, respectively. The activity
for NO oxidation is further lower for the Ga2O3 containing
Fig. 1. NO2 concentration as a function of temperature from separate NO
oxidation experiments. Catalysts: MoO3/Pt/Al2O3 (᭺); Ga2O3/Pt/Al2O3
catalyst followed by the Pt/Al2O3 and finally the V2O con-
taining catalyst. In all cases the NO2 concentration increases
until it reaches a maximum, and then starts to decrease as
5
(
+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3 (ꢀ). Temperature
◦
ramp = 5 C/min. Gases: 450 vol. ppm NO and 8% O2 in Ar. Total
flow = 3000 ml/min.

J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
219
400
300
200
100
0
Ga O3
2
Ga O3
2
3
2
1
0
0
0
MoO3
MoO3
unmodified
V2O5
unmodified
V O
2
5
WO3
WO3
1
00
200
300
400
500
100
200
300
400
Temperature (˚C)
Temperature (˚C)
Fig. 4. SO2 concentrations as a function of temperature from simultane-
ous NO and SO2 oxidation experiments. Catalysts: MoO3/Pt/Al2O3 (᭺);
Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3
Fig. 2. NO2 concentrations as a function of temperature from simultane-
ous NO and SO2 oxidation experiments. Catalysts: MoO3/Pt/Al2O3 (᭺);
Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3
◦
◦
(ꢀ). Temperature ramp = 5 C/min. Gases: 450 vol. ppm NO, 30 vol. ppm
(
ꢀ). Temperature ramp = 5 C/min. Gases: 450 vol. ppm NO, 30 ppm SO2
SO2 and 8% O2 in Ar. Total flow = 3000 ml/min.
and 8% O2 in Ar. Total flow = 3000 ml/min.
two catalysts in Fig. 3 is higher than the inlet SO2 concentra-
tion at temperatures <250 C. The peaks which appear at low
3
.1.2. SO2 oxidation
In the same way as with NO oxidation, SO2 heating ramp
◦
temperatures in the beginning of the heating ramps can be re-
lated to desorption of weakly adsorbed SO2. Concerning the
SO2 oxidation activity, it is obvious that the WO3 containing
catalyst shows high SO2 oxidation activity not only in the
separate SO2 oxidation experiments, but also in the simulta-
neous SO2 and NO oxidation experiments (see Figs. 3 and
oxidation experiments were performed both in the absence
and presence of NO in the reaction gas mixture. The results
from the separate SO2 oxidation experiments are shown in
Fig. 3, while Fig. 4 displays the results for the SO2 oxidation
in the presence of NO. It is worth to point out that a decrease
in the SO2 concentration indicates SO2 consumption, and
since the experiments were performed with oxygen in ex-
cess, we believe that any decrease in the outlet SO2 concen-
tration is related to SO2 adsorption and/or oxidation to SO3.
Prior to each experiment, the pre-treated catalysts were sat-
urated in the reaction gas mixture at room temperature until
the outlet NO concentration was equal to the inlet concentra-
tion and the SO2 concentration became very close to the inlet
concentration (30 ppm). Table 4 displays the time which was
required for detecting 20 ppm SO2 in the outlet flow for all
samples. As it is clear from the table, the Ga2O3 promoted
catalyst followed by the Pt/Al2O3 catalyst adsorb much more
SO2 during the saturation step than the other catalysts. This
may explain why the concentration of outlet SO2 for these
4). As can be seen in Table 4, the saturation time with SO2 for
the Ga2O3 containing catalyst and the unmodified catalyst
in the experiment with simultaneous NO and SO2 exposure
is shorter compared to the corresponding saturation time in
the separate SO2 oxidation experiment. This is seen from the
appearance of the temperature ramp curves in Fig. 4, where
the low temperature desorption peaks almost disappear and
the outlet SO2 concentrations decline below the inlet con-
centration at the beginning of the temperature ramp. The
lowest SO2 oxidation activity both in the presence and in the
absence of NO is shown by the MoO3 containing catalyst.
3
3
.2. Steady state experiments
.2.1. NO oxidation
Figs. 5 and 6 show the outlet concentration of NO2 as a
function of time at 125, 150, 175, 200 and 225 C with a step
of 20 min at each temperature in the absence and presence
Ga2O3
◦
MoO3
unmodified
V2O5
WO3
6
4
2
0
0
0
Table 4
The time required for detecting 20 ppm SO2 in the outlet flow during
the saturation step performed on the catalysts prior to the heating ramp
experiments both in the presence and absence of NO in the reaction gas
mixture
1
00
200
300
400
NO
SO2 saturation time with SO2 saturation time
Temperature (˚C)
NO in the feed (min)
without in the feed (min)
Pt/Al2O3
Ga2O3/Pt/Al2O3 52
WO3/Pt/Al2O3
V2O5/Pt/Al2O3
MoO3/Pt/Al2O3
48
64
93
31
18
7
Fig. 3. SO2 concentration as a function of temperature from separate SO2
oxidation experiments. Catalysts: MoO3/Pt/Al2O3 (᭺); Ga2O3/Pt/Al2O3
28
21
8
(
+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3 (ꢀ). Temperature
◦
ramp = 5 C/min. Gases: 30 vol. ppm SO2 and 8% O2 in Ar. Total
flow = 3000 ml/min.

2
20
J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
o
C
80
60
o
C
2
25
2
00
Ga O
MoO3
4
3
2
1
00
00
00
00
0
Ga O3
2
3
2
MoO3
o
C
unmodified
V O
unmodified
175
V O5
2
5
2
WO3
WO3
4
2
0
0
o
C
°
°
175 C
1
25
C
150 °C
°
°
C
1
50
200
C
225
o
C
1
25
0
20
40
60
Time (min)
80
100
0
20
40
60
Time (min)
80
100
120
Fig. 5. NO2 concentration as a function of time measured at the temper-
atures: 125, 150, 175, 200 and 225 C. Catalysts: MoO3/Pt/Al2O3 (᭺);
◦
Fig. 7. SO2 concentration as a function of time measured at the temper-
atures: 125, 150, 175, 200 and 225 C. Catalysts: MoO3/Pt/Al2O3 (᭺);
◦
Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3
◦
Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3 (ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3
(
ꢀ). The temperature between the steps was increased with 15 C/min.
◦
(
ꢀ). The temperature between the steps was increased with 15 C/min.
Gases: 450 vol. ppm NO, 8% O2 in Ar with a total flow of 3000 ml/min.
Gases: 30 vol. ppm SO2, 8% O2 in Ar with a total flow of 3000 ml/min.
of SO2, respectively. The highest NO oxidation activities
in NO steady state oxidation experiments are shown by the
WO3 containing catalyst followed by the MoO3/Pt/Al2O3
catalyst. The order in activity for the other three catalysts is:
3
.2.2. SO2 oxidation
SO2 steady state oxidation was performed in the absence
and presence of NO in the same way as in NO steady state
oxidation. The results from SO2 oxidation experiments in
the absence of NO are shown in Fig. 7. The SO2 peaks
which appear between the steady state steps are due to
SO2 desorption caused by the temperature increase. Even
in the steady state experiments, higher amounts of SO2
were adsorbed on the Ga2O3 containing and the Pt/Al2O3
catalysts, which give the high desorption SO2 peaks when
the temperature was increased between the steady state
steps. As it is clear from the figure, the measured SO2
concentrations at all temperatures for all catalysts are sim-
ilar, except for the WO3 containing catalyst which shows
Ga2O3/Pt/Al2O3 > Pt/Al2O3 > V2O /Pt/Al2O3. The ad-
5
dition of SO2 to the gas mixture deteriorates the NO oxida-
tion activity resulting in decreased outlet NO2 concentration
levels at all temperatures for all catalysts (see Fig. 6). The
order in NO oxidation activity is still in agreement with the
results from the simultaneous NO and SO2 heating ramp ex-
periments with the exception for the Pt/Al2O3 catalyst which
◦
shows higher NO2 formation at 200 and 225 C compared
to the concentration levels at the same temperatures in the
heating ramp oxidation experiment.
The results from NO steady state oxidation experiments
conducted in the presence of SO2 show that the MoO3 con-
taining catalyst and the WO3/Pt/Al2O3 catalyst still show
significantly higher NO oxidation activity than the other cat-
alysts as they also do in the corresponding heating ramp
oxidation experiments.
∼
30% higher oxidation of SO2 at these temperatures
compared to the other catalysts. When NO is included in
the reaction gas mixture (Fig. 8), the Pt/Al2O3 catalyst
shows the highest SO2 oxidation activity at all temperature
steps.
Ga2O3
°
2
25 °C
125
C
°
30
150
C
MoO3
°
unmodified
175
C
2
1
00
00
00 C
°
2
°
°
V O5
2
200
C
225
C
WO3
20
10
Ga O
2
3
MoO3
1
75 °C
unmodified
V O
1
25 °C
1
50 °C
2
5
WO3
0
20
40
60
Time (min)
80
100
0
20
40
60
80
100
Time (min)
Fig. 6. NO2 concentration as a function of time in the presence of
SO2 measured at the temperatures: 125, 150, 175, 200 and 225 C.
Fig. 8. SO2 concentration as a function of time in the presence of NO
measured at the temperatures: 125, 150, 175, 200 and 225 C. Cata-
◦
◦
Catalysts: MoO3/Pt/Al2O3 (᭺); Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3
lysts: MoO3/Pt/Al2O3 (᭺); Ga2O3/Pt/Al2O3 (+); V2O5/Pt/Al2O3 (ᮀ);
(
ᮀ); WO3/Pt/Al2O3 (᭡); Pt/Al2O3 (ꢀ). The temperature between the
WO3/Pt/Al2O3 (᭡); Pt/Al2O3 (ꢀ). The temperature between the steps
◦
◦
steps was increased with 15 C/min. Gases: 450 vol. ppm NO, 30 vol. ppm
SO2, 8% O2 in Ar with a total flow of 3000 ml/min.
was increased with 15 C/min. Gases: 450 vol. ppm NO, 30 vol. ppm SO2,
8% O2 in Ar with a total flow of 3000 ml/min.

J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
221
2
1
00
00
0
MoO3
MoO3
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
unmodified
unmodified
0
100
200
300
1
00
200
300
400
ο
Time (min)
Temperature ( C)
Fig. 11. Normalized NOx storage capacity as a function of SO2 exposure
time for Pt/BaO/MoO3/Al2O3 (᭺) and Pt/BaO/Al2O3 (ꢀ) catalysts.
Fig. 9. NO2 concentration as a function of temperature from heating ramp
NO oxidation performed on Pt/BaO/MoO3/Al2O3 (᭺) and Pt/BaO/Al2O3
◦
(
ꢀ) catalysts. Ramp rate = 5 C/min. Gases: 450 ppm NO, 8% O2 in Ar
subtracting the integrated amount of outlet NOx from the
integrated amount of inlet NOx for each lean period.
Some interesting features in the NOx storage experiments
are: (1) the steady state NOx levels for both catalysts imme-
diately become higher when SO2 is added to the gas mix-
ture; (2) both catalysts have relatively high storage capacity
in the sulphur free cycles; (3) the amount of unreduced NOx
during the rich periods is gradually increased with increased
sulphur exposure time, especially for the MoO3 containing
catalyst.
In order to investigate the impact of sulphur exposure on
the NOx storage capacity and at the same time compare the
catalysts with each other, a mean NOx storage value was
calculated from the four sulphur free cycles and the NOx
storage in the remaining cycles normalised to this value. The
resulting NOx storage as a function of time during SO2 ex-
posure is shown in Fig. 11. Obviously, the MoO3 containing
catalyst becomes deactivated by sulphur more quickly than
the Pt/BaO/Al2O3 catalyst which is manifested in the faster
decay of the NOx storage capacity.
with a total flow of 3000 ml/min.
3
.3. NOx storage experiments
3
.3.1. Heating ramp experiments
Heating ramp experiments with NO/O2 mixture were con-
ducted with model Pt/Ba/MoO3/Al2O3 and Pt/BaO/Al2O3
catalysts using the same procedure and gas composi-
tion as in Section 3.1.1 and Table 3, respectively. As
can be seen in Fig. 9, the NO oxidation activity for the
Pt/BaO/MoO3/Al2O3 catalyst is slightly higher than for the
Pt/BaO/Al2O3 catalyst, but significantly lower than for the
MoO3/Pt/Al2O3 catalyst (see Fig. 1).
3
.3.2. Transient NOx storage experiments
The results from the transient experiments using the two
barium containing catalysts are shown in Fig. 10. The con-
tinuous and broken lines denote NOx and NO2 outlet con-
centrations for the Pt/BaO/MoO3/Al2O3 and Pt/BaO/Al2O3
catalysts, respectively. In the initial lean periods, the storage
of NOx is seen from the relatively slow increase of the NOx
signal up to a steady state level. During the rich period the
outlet NOx signal becomes lower as the incoming and stored
NOx is reduced. The amount of stored NOx is calculated by
4. Discussion
4.1. NO oxidation
Our aim was to prepare catalysts with constant Pt-loading
3
2
1
00
00
00
0
NO x
(0.062 mmol), constant amount additives (0.345 mmol) and
constant amount of washcoat corresponding to 20 wt.% of
the catalyst mass.
From the catalyst characterisation data in Table 2, it is
clear that the specific surface area of the samples is almost
constant, which means that the addition of metal oxide ad-
ditives does not result in any significant decrease in surface
area.
NO 2
0
100
200
300
400
The Pt dispersion values which are displayed in Table 2
are obtained by taking into account that CO in addition to
Pt, also can be adsorbed to the metal oxide additives. For
this reason, temperature programmed CO desorption exper-
iments were performed on alumina supported metal oxide
samples without Pt. It was found that significant amounts
Time (min)
Fig. 10. Measured NOx and NO2 over a Pt/BaO/MoO3/Al2O3 (continuous
line) and a Pt/BaO/Al2O3 (broken line) catalyst during transient experi-
ments. The composition of lean gas mixture was 400 ppm NO, 650 ppm
C3H6, 8% O2 and for rich condition, O2 was switched off. 10 ppm SO2
was added to lean and rich gas mixtures from the fifth cycle. The total
gas flow was constant during the entire experiment time.
of CO adsorb on V O and MoO . Thus, the Pt dispersions
2
3
5

2
22
J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
for the MoO3/Pt/Al2O3 and V2O /Pt/Al2O3 catalysts were
ing ramp experiments. The NO2 outlet concentrations at all
temperature steps for all samples can be considered as steady
state levels, except for the MoO3 containing catalyst which
show a slightly decreasing NO2 concentration with time at
200 and 225 C. The step at 225 C was repeated and the
period was increased to 215 min. The NO2 concentration in
this experiment (not shown) continued to decrease with time
while, the NO concentration increased and the NOx con-
centration was stable. This means that the decreased outlet
concentration of NO2 cannot be related to NO2 adsorption.
One possible suggestion for this deactivation is Pt oxidation
formation [37].
5
calculated by subtracting the integrated amount of des-
orbed CO from the Pt-free samples from the corresponding
amount CO for the Pt-containing samples. The Pt disper-
sion achieved using this method does not take into account
for CO adsorbed on interfacial Pt–Me sites or spill over of
the CO from Pt- to Me-sites, which may occur during the
experiment. The Pt dispersions for the MoO3/Pt/Al2O3 and
◦
◦
V2O /Pt/Al2O3 samples are therefore somewhat uncertain.
5
As it is seen from the Table, the Pt dispersions for the
samples before performing the activity measurements are
roughly in the same range. Further, the Pt dispersions for
all samples except for the V2O and MoO3 containing cat-
The decrease in NO oxidation activity when both NO and
SO2 are present in the feed is also observed in the steady
state experiments (compare Figs. 3 and 4). The results from
these experiments are also in agreement with the heating
ramp experiments, except for the Pt/Al2O3 catalyst which
seems to become more active after performing the heating
ramp experiment in presence of both NO and SO2. This is
seen in the increased NO2 outlet concentrations at 200 and
5
alysts are significantly lowered after performing the activity
measurements. The decrease in Pt dispersion may indicate
increased Pt particle sizes, probably due to the exposure to
sulphur [36].
The results from the temperature ramp and steady state ox-
idation of NO show that WO3 and MoO3 seem to be the most
active additives in enhancing the activity for NO oxidation.
As it is seen from Fig. 1, the addition of either of these two
metal oxides results in higher NO oxidation activity where
NO2 forms with significantly higher rate and reaches higher
outlet concentration values in comparison to the other cata-
lysts. Obviously, WO3 has the highest promoting effect on
the NO oxidation activity in sulphur free atmosphere, since
the NO2 formation for the WO3 containing catalyst starts at
markedly lower temperature and reaches a higher conversion
◦
225 C in the steady state experiment with both NO and SO2
present in the feed which was performed directly after the
corresponding ramp experiment. These experiments where
repeated (not shown) and the results indicate a further in-
crease in NO oxidation activity for this catalyst. Possibly,
the simultaneous exposure of NO and SO2 at temperatures
◦
up to 450 C has caused sample sintering which means an
increase in the Pt particle sizes [36]. The activity enhance-
ment can be related to the increase in the stability against
PtOx formation when the particles become larger [37]. An-
other suggestion for the NO oxidation activity enhancement
is the formation of surface alumina sulphates due to sulphur
exposure. Skoglundh et al. [38] have observed an increase
in the oxidation activity for a Pt/Al2O3 catalyst after sulphur
exposure due to the formation of sulphated alumina.
(92%) compared to the other catalysts. Fig. 1 shows another
feature which is worth to mention, namely that the presence
of V2O slightly suppresses the NO oxidation activity. What
5
this depends on is not clear. It can be a consequence of the
high Pt dispersion for this catalyst. According to Olsson and
Fridell [37] alumina supported catalysts with smaller Pt par-
ticles show lower NO oxidation activity than catalysts with
larger Pt particles since the probability for platinum oxide
formation is higher on smaller Pt particles.
Concerning the effect of Ga2O3 on enhancing the NO
oxidation, the results show that Ga2O3 slightly promotes this
reaction in the absence of SO2.
The NO oxidation activity for all catalysts deteriorates
when SO2 is included in the reaction gas mixture (see
Fig. 2). For each catalyst, the decrease in NO2 formation
due to SO2 exposure can be obtained by subtracting the
NO2 outlet concentration curve obtained from the experi-
ments with simultaneous SO2 and NO exposure from the
corresponding NO oxidation experiment. Such a calculation
shows that the NO2 formation decreases in the following
order: Pt/Al2O3 > Ga2O3/Pt/Al2O3 > WO3/Pt/Al2O3 >
The decrease in the NO2 outlet concentration as a func-
tion of time shown by the MoO3/Pt/Al2O3 catalyst at si-
multaneous NO and SO2 steady state oxidation experiment
is shown by the WO3/Pt/Al2O3, Pt/Al2O3 catalysts as well.
Even for this experiment, the decrease in NO2 outlet con-
centration is temperature dependent where, the decrease in
◦
NO2 concentration is only seen at 200 and 225 C as in the
previous case. Further, the decrease in NO2 formation for
the MoO3/Pt/Al2O3 catalyst which was seen previously (in
SO2 free atmosphere) is further enhanced in the presence of
SO2. Since the presence of SO2 has altered the stable NO2
levels for the other catalysts, then the presence of SO2 may
have increased the Pt oxidation.
Enhancing the oxidation resistance for Pt catalysts has
previously been studied using XAFS and XANES [39,40].
The authors claim that the addition of electrophilic cations
to alumina supported Pt catalysts increases the resistance
against Pt oxidation due to the increase in surface acidity.
According to these studies, MoO3 may enhance the Pt oxi-
dation resistance. If the decrease in the NO2 formation with
time delay during the steady state oxidation is a consequence
V2O /Pt/Al2O3 > MoO3/Pt/Al2O3. This means that the
5
NO oxidation activity is most affected in the absence of
metal oxide additives. Of the tested metal oxides, MoO3
seems to be the most promising SO2 oxidation inhibitor.
The results from the NO steady state oxidation experi-
ments conducted in the absence of SO2 and shown in Fig. 3
are in agreement with the results from the separate NO heat-

J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
Table 5
223
of Pt oxidation, then our result is in disagreement with the
Percentage SO2 converted in the separate SO2 and simultaneous NO and
SO2 temperature ramp experiments
study mentioned above at least for these reaction conditions.
On the other hand, the suggested increase in surface acidity
in the XAFS and XANES studies were tested by performing
propane oxidation experiments [39]. The results from these
experiments show an increase in propane oxidation activity
for electrophilic cation modified Pt/Al2O3 catalysts. In our
experiments the presence of NO2 which is a strong oxida-
tion agent may block the protection effect caused by elec-
trophilic additives at least at some reactions conditions.
From the results and discussion mentioned above, it is
clear that MoO3 and WO3 have a significant enhancing
effect on the NO oxidation also in the presence of SO2.
These two metal oxides are widely used as promoters to
SO2 conversion (%)
with NO in the feed
SO2 conversion (%)
without NO in the feed
Pt/Al2O3
52
49
44
40
34
34
24
39
28
25
Ga2O3/Pt/Al2O3
WO3/Pt/Al2O3
V2O5/Pt/Al2O3
MoO /Pt/Al O
3
2
3
The result from this calculation is shown in Table 5,
which shows two interesting features: (i) the SO2 oxidation
is promoted in the presence of NO in all catalysts. A pos-
sible explanation for this feature is when NO is included
in the gas mixture, NO2 will be formed which is a stronger
oxidation agent than oxygen. This means that the environ-
ment will be more oxidative, which results in higher SO2
oxidation capacity. NO2 is used in the lead chamber pro-
cess for production of sulphuric acid synthesis just because
of the strong oxidation property of NO2 [43]; (ii) the WO3
containing catalyst shows high SO2 oxidation activity in
temperature ramp and steady state SO2 oxidation experi-
ments. In simultaneous NO and SO2 steady state oxidation,
the Pt/Al2O3 catalyst shows the highest SO2 oxidation ac-
tivity. As it was mentioned previously, this catalyst for some
reason became more active after the exposure to SO2 in
enhance the conversion of NO in supported V2O catalysts
5
for SCR applications [23,24,41,42]. Some researchers re-
late the enhancement in NO oxidation to the strong redox
properties of these metal oxides. This means that parallel to
the NO conversion which occur on the active sites, further
NO conversion occur on the promoter sites in form of redox
cycles which increase the amount of converted NO. The
redox activity of these two metal oxides was investigated
by performing NO oxidation temperature ramp experiments
using MoO3/Al2O3 and WO3/Al2O3 samples. The results
(
not shown) from those experiments show that neither of
the two samples were active for NO oxidation, i.e. the
enhancement in NO oxidation for the MoO3 and WO3 con-
taining catalysts cannot be related to direct NO oxidation
on the promotor sites. On the other hand, NO oxidation can
take place on the border site of Mo–Pt and W–Pt or via a
spill over process from Pt to Mo and W which means that
the presence of these metal oxides may play an important
roll in enhancing the NO oxidation activity.
◦
oxidative atmosphere at high temperatures (up to 450 C).
The SO2 oxidation behaviour in separate SO2 steady state
oxidation differs from the case when NO is included in the
gas mixture. In the first mentioned one, all catalysts except
the WO3/Pt/Al2O3 catalyst show steady SO2 levels at all
temperatures. The peak in between the steps is due to SO2
desorption. In the presence of NO, the SO2 oxidation is pro-
◦
moted and steady SO2 levels are not reached above 175 C.
4
.2. SO2 oxidation
The increase in the SO2 concentrations at each temperature
step can possibly be related to the Pt oxidation mentioned
previously.
As previously, mentioned in the Section 3, the SO2 ad-
sorption time at room temperature prior to the heating ramp
and steady state oxidation experiment varied between the
catalysts. This makes the interpretation of the results from
the SO2 heating ramp oxidation experiments complicated.
The Pt/Al2O3 and Ga2O3/Pt/Al2O3 catalysts adsorbed much
higher amounts of SO2 during the saturation step. In order to
investigate the effect of metal oxide addition on SO2 oxida-
tion, the SO2 conversion was calculated both in the absence
and presence of NO according to the following formula:
4.3. NOx storage
From the NO oxidation experiments which were per-
formed prior to the transient lean and rich experiments, two
features are observed. First, the presence of BaO leads to
lower NO oxidation activity both for Pt/BaO/Al2O3 and for
Pt/BaO/MoO3/Al2O3. This may be due to dissolution of
some amount of BaO during the impregnation with the acidic
platinum nitrate solution which may cover some Pt sites or
to the formation of more stable Pt-oxides when barium is
present [37]. Second, MoO3 still show some promoting ef-
fect on the NO oxidation, but the effect is much lower than
for the corresponding catalyst without BaO. The lower pro-
moting effect might be explained by less contact between Pt
and MoO3 in the modified preparation method, or that the
interaction between MoO3 and the basic BaO may reduce
the acidity of MoO3. The reason to include MoO3 in the
ꢀ
t
f
[
SO2]inlet(t − t0) −
[SO2]outlet dt
t
0
f
SO2conv =
[
SO2] (t − t0)
inlet
f
where t0 and t are initial and final time, respectively. We as-
f
sume that only a minor part of the adsorbed SO2 will remain
on the surface when the temperature ramp is terminated at
◦
4
50 C and due to the strong oxidative environment, the dif-
ference between the inlet and the outlet SO2 concentration
level is related to SO2 oxidation to SO3 and sulphate.

2
24
J. Dawody et al. / Journal of Molecular Catalysis A: Chemical 209 (2004) 215–225
NOx storage catalyst was the high NO oxidation activity and
the relatively low SO2 oxidation activity for MoO3/Pt/Al2O3
sample compared with the other metal oxide additives. It
was thought that an increased NO2 formation rate would fa-
cilitate and improve the NOx storage and suppressing SO2
oxidation will end up with lower amount of the thermally
stable sulphates which poison the NOx storage component.
Features which are seen from the transient experi-
ments performed with both the Pt/BaO/Al2O3 and the
Pt/BaO/MoO3/Al2O3 catalysts and worth to discuss (see
Fig. 10) are: (i) the regeneration during the rich conditions is
not complete even in the sulphur free cycles (NOx concen-
tration is not close to zero); (ii) the presence of SO2 seems to
affect the direct reduction where the steady state NOx level
increases directly after the SO2 exposure has been started;
SO2 oxidation in alumina supported Pt catalysts. The main
conclusions are summarised below.
Catalysts without NOx storage material:
•
•
•
In the absence of SO2, WO3/Pt/Al2O3 and MoO3/Pt/Al2O3
show the highest NO oxidation promoting.
In the presence of SO2, the NO oxidation activity is de-
creased for all catalysts.
The decrease in NO oxidation activity due to SO2 expo-
sure was highest for the Pt/Al2O3 and the Ga2O3/Pt/Al2O3
catalysts followed by WO3/Pt/Al2O3 and V2O /Pt/Al2O3
5
catalysts and finally MoO3/Pt/Al2O3 catalyst.
The MoO3/Pt/Al2O3 catalyst was the most efficient cata-
lyst in enhancing the NO oxidation while suppressing the
SO2 oxidation.
For all catalysts, SO2 oxidation was promoted in the pres-
ence of NO in the reaction gas mixture.
•
•
(iii) when SO2 exposure has begun, the NOx concentration
during the release period increases due to reduction dete-
rioration; (iv) the deterioration of the NO reduction due
to sulphur exposure is higher in the Pt/BaO/MoO3/Al2O3
catalyst, where the amount of detected NOx increase more
with time compared to the Pt/BaO/Al2O3 catalyst; (v) the
concentration of detected NO2 increases with time which
indicate a lower NOx storage capacity due to formation of
barium sulphates.
The amount of stored NOx in each storage cycle has been
calculated according to the calculation method described
in [7]. The results from these calculations show a slightly
higher storage capacity for Pt/BaO/Al2O3, compared with
the Pt/BaO/MoO3/Al2O3 sample, both in absence and pres-
ence of SO2, in spite of a higher NO2 formation for the later
catalyst.
NOx storage catalysts:
•
•
•
The Pt/BaO/MoO3Al2O3 catalyst showed a slightly
higher NO oxidation activity than the Pt/BaO/Al2O3
catalyst.
The Pt/BaO/Al2O3 and Pt/BaO/MoO3/Al2O3 catalysts
store almost the same amount of NOx in the SO2 free
cycles.
The Pt/BaO/MoO3/Al2O3 catalyst was sulphur poisoned
faster than the Pt/BaO/Al2O3 catalyst due to reduction in
the regeneration capacity in the rich periods where the out-
let NOx concentration during these periods was increased
with time.
From Fig. 11, it is obvious that the deterioration rate of
NOx storage capacity for both catalysts follows the same
tendency up to the seventh cycle. Thereafter, the NOx stor-
age capacity for the Pt/BaO/MoO3/Al2O3 catalyst deterio-
rates faster than for Pt/BaO/Al2O3 catalyst. From Fig. 10,
it is clear that the decreased reduction capacity during rich
condition for the MoO3 containing catalyst starts to be sig-
nificant from the seventh sulphur exposure cycle. Obviously,
the faster poisoning in the Pt/BaO/MoO3/Al2O3 catalyst is
a consequence of the deteriorated reduction capacity. What
that depends on is not clear, but somehow the presence of
MoO3 accelerates the deterioration of the Pt reduction.
The reason that the NOx storage catalyst with the added
promotor was not found to be more sulphur resistant is prob-
ably connected to the interaction between sulphur and Pt.
We have earlier found that a key component in sulphur de-
activation is connected to formation of sulphur species on
Pt during rich conditions [21]. This mechanism is obviously
not positively influenced by addition of molybdenum oxide.
Acknowledgements
This work has been performed within the Competence
Centre for Catalysis, which is financially supported by the
Swedish National Energy Agency and the member compa-
nies: AB Volvo, Johnson Matthey-CSD, Saab Automobile
Powertrain AB, Perstorp AB, AVL-MTC AB, Akzo Nobel
Catalysts and The Swedish Space Corporation.
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