Diesel Lean NOx-Trap Thermal Aging and Performance Evolution Characterization
Oil & Gas Science and Technology - Rev. IFP Energies nouvelles, Vol.
Diesel Lean NO -Trap Thermal Aging and x Performance Evolution Characterization
S. Benramdhane 1
C.-N. Millet 1
E. Jeudy 1
J. Lavy 1
V. Blasin-Aubé 0
M. Daturi 0
0 Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen , CNRS, 6 Bd Maréchal Juin, 14050 Caen - France
1 IFP Energies nouvelles, Rond-point de l'échangeur de Solaize , BP 3, 69360 Solaize - France
- Diesel Lean NOx-Trap Thermal Aging and Performance Evolution Characterization - The work described in this paper focuses on the impact of thermal aging on NOx trap structure and functions. They were evaluated on a Synthetic Gas Bench (SGB) and correlated with the analysis of the structural and chemical evolution of the catalyst. A FTIR Operando study allowed to further analyse the mechanisms occurring on the catalyst surface and highlight the most critical points. NOx trap samples were hydrothermally aged in a furnace up to 900°C under an oxidising flow. The main following impacts on the material were highlighted: reduction of the surface area, sintering of Pt yielding a decrease of the NO oxidation efficiency and hence of the NOx storage capacity, and a loss of CO and HC conversion, barium structural evolution into BaAl2O4 also being partly responsible for the loss of NOx storage capacity. Other possibilities for loss of NSC are: the transition of γ-Al2O3 to δ-Al2O3 and the evolution of Ba crystalline structure which needs further analysis. Both XRD and surface IR Operando studies show that hydrothermal aging has a rather homogeneous impact on all materials: alumina phase transition and BaAl2O4 production, which all lead to a partial decrease of all storage sites.
Environmental, ecological and health concerns result in
increasingly stringent regulations of pollutant emissions from
vehicle engines. Diesel and lean-burn gasoline engines are
attractive alternatives to conventional gasoline engines to
improve fuel economy and reduce CO2 emissions for light
duty vehicles. A major challenge is the abatement of NOx
(NO + NO2) emissions. Two main approaches are being
developed to answer to this challenge. The first technology
deals with the continuous Selective Catalytic Reduction
(SCR) of NOx. It takes advantage of the ability of some
catalysts to allow the selective reaction of a limited amount of
reductant (NH ) with NOx rather than O2. The second
nology is based on a cycle of NOx storage and reduction in a
Lean NOx Trap (LNT) [
], which operates on a principle of
alternating phases. The concept is based on the adsorption of
NOx during long periods of oxygen excess followed by
shorter periods of oxygen deficiency in the presence of
reducing agents during which the stored NOx are released
and reduced to N2, N2O or NH3. Reductants are mainly
hydrocarbons issued from a specific fuel injection strategy
into the cylinder or the exhaust pipe. Lean/rich transitions are
managed by the engine control unit. Main components of the
catalytic washcoat are usually alumina for the support, Pt, Pd
and Rh as noble metals to provide oxidative and reductive
functions, and a basic additive (often a barium salt) known
for its high affinity for NOx [
] to provide their storage.
NOx traps also show some undesired reactivity in regards
to sulphur compounds which are present in exhaust gases
from both Diesel and gasoline engines. It is commonly agreed
] that sulphur dioxide is first oxidized to sulphur trioxide
over platinum. Then, the SO3 reacts with barium and alumina
to form barium and aluminium sulphates which are more stable
than the corresponding nitrates. This causes gradual saturation
of the storage material with sulphur and leading to significant
NOx storage activity loss. Periodic desulfation (DeSOx)
hence requires higher temperatures which are detrimental to
the life of the catalyst. For this reason, sulphur deactivation
and the corresponding thermal aging are key obstacles to a
widespread implementation of the LNT [
Thermal deterioration is due both to the growth of precious
metal particles and to the formation of mixed oxides such as
aluminates, cerates, and zirconates by the reaction of NOx
storage material with the support or other compounds in the
]. Some authors studied thermally aged NOx
trap during lean/rich cycling at 600, 700 and 800°C in a flow
reactor. They demonstrated that the three following
deactivation mechanisms dominate above 800°C: loss of dispersion
of the precious metals, phase transitions of the adsorber
material and loss of total surface area [
The impact of thermal aging on the catalytic activity is
more significant at low temperatures where reaction kinetics
are the limiting step. It is hence more problematic on Diesel
vehicles where typical catalyst temperatures are in the range
150-300°C, compared to 300-600°C for gasoline.
The work described in this paper focuses on the impact of
thermal aging on the functionalities of a commercial Diesel
Lean NOx-Trap. They were evaluated on a Synthetic Gas
Bench (SGB) and correlated with the analysis of the structural
and chemical evolution of the catalyst. A FTIR Operando
study allowed further analyzing the mechanisms occurring on
the catalyst surface and highlighting the most critical points.
The commercial NOx trap that was investigated in this study
was supplied by Renault. It has a square monolithic
honeycomb structure of 400 cells per square inch (cpsi). The thin
ceramic walls are coated with a NOx storage / reduction
catalyst. The catalytic coating was characterized by X-Ray
Fluorescence analysis (XRF), X-Ray Diffraction (XRD) and
Scanning Electron Microscopy (SEM). TEM observations
were performed on a JEOL 2100F 200 kV microscope
equipped with a X-ray dispersive Spectrometer. X-ray
Diffraction (XRD) patterns were obtained with a PANalytical
X’Pert Pro MPD Diffractometer with Bragg-Brentano X-ray
tube anticathode Cu (wavenumber kA1 = 1.5406 Å). Samples
were loosely packed in a shallow cavity (0.2 cm deep and
1 cm in diameter). The washcoat was separated from the
cordierite support before being analyzed.
1.2 IR Operando Apparatus
The purpose of FTIR Operando study was to analyze NOx
and carbonates storage sites of the LNT samples under
representative running conditions so as to determine the impact of
thermal aging on the storage sites. The description of the
Operando setup, with gas line and analysis tools (IR, MS and
chemiluminescence) and the IR reactor cell, is described in
]. The material was pressed into self-supporting wafers of
10 mg.cm-2 and placed into the quartz reactor-cell equipped
with KBr windows. For the analysis of the surface, Operando
measurements were carried out with a Nicolet FT-IR Nexus
spectrometer equipped with a MCT detector. FT-IR spectra
were collected with a resolution of 4 cm-1. The analysis of the
outlet gases was performed by means of a Pfeiffer Omnistar
mass spectrometer. Likewise, FT-IR spectra of the gas phase
were collected using a gas microcell. The sample was activated
in the same way as SGB (described in Sect. 2.3).
The lean reacting gas composition is 900 ppmC HC (C3H6),
800 ppm CO, 270 ppm H2, 300 ppm NOx, 5% CO2, 15% O2
and 2% H O in Ar as a carrier gas. The total flow is adjusted
to 25 cm3.min-1. This composition is equivalent to the one
used in the Synthetic Gas Bench and is very similar to the real
The purpose of the Synthetic Gas Bench study was to
quantify the NOx storage and reduction capacities of the LNT
samples under representative running conditions so as to
determine the impact of thermal aging. The following types
of experiments were performed:
– sample pre-treatment,
– catalyst light-off during a temperature ramp,
– isothermal NOx storage experiments with different gas
– lean and rich pulses to mimic storage / reduction cycles
occurring in real driving conditions.
LNT samples cut from the monolith for our study were
25 mm in diameter and 50 mm in length for the Synthetic
Gas Bench analysis. Experiments were carried out in a flow
reactor under atmospheric pressure and realistic flow
conditions representative of Diesel exhaust gas by synthetic gas
mixture. The catalyst sample was placed in a quartz tube. A
thermocouple in front of the catalyst was used to control the
temperature and another one was inserted downstream from
the catalyst. The quartz tube was placed in an electrically
heated oven. Valves allowed to rapidly switch from one gas
composition to another, and to generate alternating rich and
lean phases. Gas compositions were chosen close to a lean
Diesel environment (equivalence ratios ER = 0.3) and to a
rich pulse environment (ER = 1.1) as detailed in Table 1.
Other compositions were also tested when considered useful
to better understand the catalyst behaviour. Propylene was
used to represent unburned hydrocarbons emitted by engine
Samples were pre-treated to stabilize their active surface
and thus their catalytic activity so as to ensure good test
reproducibility and a constant concentration of surface residual
impurities before starting any analysis.
The gas flow was switched between 290 s lean feed periods
(ER = 0.3) and 15 s rich pulses (ER = 1.1, see Tab. 1).
Temperature was increased at 5°C/min rate from 50°C up to
620°C and then stabilized at 620°C for 2 hours. The catalyst
was cooled back to room temperature in a synthetic air flow.
To ensure that samples were being evaluated under similar
initial states, all experiments were terminated by a step at
620°C in an air flow until stabilisation was reached.
1.3.2 Catalyst Light-off During a Temperature Ramp
For this experiment, the temperature was increased from 50
to 620°C at a 10°C/min rate. The CO and HC conversion
efficiencies of the LNT were determined as a function of the
temperature measured upstream of the catalyst. The
temperature at which 50% conversion was achieved was defined as
the “light-off” temperature for the particular pollutant.
1.3.3 Isothermal NOx Storage
The gas temperature was increased up to the desired value
under a N2 flow. The experiment was then switched to a lean
feed and NOx were monitored downstream from the catalyst.
The gas mixture was switched back to N2 after the NOx trap
became saturated and the temperature was increased so as
to thermally release the complete amount of stored NOx
(N2-TPD: temperature programmed desorption). The NOx
Storage Capacity (NSC), the NO to NO2 oxidation efficiency
and the DeNOx efficiency of the LNT were determined at
different temperatures and storage times.
1.3.4 Lean-Rich Cycling Conditions
The method to evaluate NOx storage, reduction and global
conversion efficiency of the NOx trap is by cycling between
lean and rich conditions to mimic engine operation. In this
test, the gas temperature was increased up to the desired
value (300°C) under an N2 flow. The gas flow was then
switched between 290 s lean feed periods (ER = 0.3) and 15 s
rich pulses (ER = 1.1). Once the lean/rich cycling exhibited a
repeatable behaviour (storage efficiency variation below
1%), the gas mixture was switched back to N2 and a
temperature programmed desorption was performed.
The NOx storage efficiency during the lean phase, the NOx
reduction efficiency during the rich phase and the overall
NOx conversion efficiency were thus determined.
The NOx storage efficiency represents the proportion of
NOx stored in the trap during the lean phase:
⎛ t1 ⎞
⎜ ∫ NOxout ⎟
ηstorage NOx (%) =⎜ ⎜⎜ 1 − tt01 ⎟⎟ × 100
⎜⎜ ∫ NOxsat ⎟⎟
⎝ t0 ⎠
where the lean phase starts at time t0 and ends at time t1,
NOxout are NOx mesured downstream from the NOx trap, and
NOxsat is the level of NOx that would be reached if the lean
phase is long enough to saturate the trap. NOxsat can also be
determined from the amount of N2O emitted because the
DeNOx is completely non-selective in our lean conditions.
The NOx reduction efficiency is determined during the
ηred NOx (%) = ⎜⎜ 1 − t2
⎜⎜ ∫ NOxin + NOxstored ⎟⎟
⎝ t1 ⎠
⎟⎟ × 100
where the rich phase starts at time t1 and ends at time t2,
NOxin are NOx mesured upstream of the NOx trap, and
NOxstored is the quantity of NOx that were stored during the
NOxstored = ∫ (NOxsat − NOxout )
The NOx conversion efficiency is determined during the
⎛ t2 ⎞ ⎞
⎜ ∫ NOxout ⎟
ηconv NOx (%) = ⎜⎜ 1 − tt02 ⎟⎟ × 100
⎜⎜ ∫ NOxin ⎟⎟
⎝ t0 ⎠
1.4 Hydrothermal Aging
NOx trap samples were hydrothermally aged in a furnace at
750, 800, 850 or 900°C for 5 h under a flow of 10% O2, 10%
H2O and N2. A new sample was used for each temperature.
2 RESULTS AND DISCUSSION
2.1 Catalyst Composition and Characterisation
Main components of the washcoat are Al2O3, ZrO2 and
CeO2. Trapping materials are mainly Ba and Sr. Active metals
are Pt and Rh. A combination of these two noble metals is
required to achieve good NOx storage and reduction
performances, together with good sulfur regeneration ability from
sulphur deposit [
]. SEM reveals that the washcoat hence
intimately bound to the cordierite support.
Figure 1 displays XRD spectra of the fresh and thermal
aged catalysts. The Pt peak at 2θ = 40° sharpens with aging
temperature, indicating that Pt particle sintering increases [
Ceria particles only sinter from the 800°C aging, according to
the peak at 2θ = 47°. TEM results on Pt and ceria coarsening
are in agreement with those obtained by XRD.
In the fresh catalyst, BaCO3 and BaC2O4 are the dominant
Ba-phases observed in the XRD patterns, but can not be
distinguished from each other. Hydrothermal aging at all
temperatures makes the Ba-phase react in solid phase with
BaAl2O4 CeO2 Al2O3
2-Theta - Scale
IR surface difference spectra of the fresh and hydrothermally aged (800°C) NOx-trap after exposure to a flow of NO + O2 at 200°C. The
negative peak in the aged sample is due to bulk carbonate removal upon NOx adsorption.
γ-Al2O3 leading to BaAl2O4 as can be seen at 2θ = 20°.
Another interesting feature of the XRD patterns is the change
in the shape of the alumina peak at 2θ = 68° in the samples
aged at 850 and 900°C: the split of the alumina peak at 2θ =
67.5° indicates the transition of a fraction of γ-Al2O3 (cubic
structure) to δ-Al2O3 (tetragonal structure), as was already
found by Toops et al. [
] above 860°C. This transition
induces a decrease of the surface area. This is in agreement
with surface area measurements (Tab. 2): hydrothermal aging
makes the surface area of the washcoat decreasing from 20%
at 750°C up to 42% at 900°C compared to the fresh catalyst.
2.2 Surface Characterisation by IR Operando
The NOx storage properties of the samples were evaluated by
IR Operando during isothermal NOx storage experiments
under a 300 ppm NO and 15% O2 flow to saturation. Figure 2
shows the IR spectrum of NOx species on the surface of
saturated fresh and hydrothermally aged (800°C) catalysts.
The various wavenumber of surface species are based on
literature and experiments carried out in in situ conditions
and are listed in the table below.
IR surface difference spectra of the fresh and hydrothermal (800°C) aged NOx-trap after exposure to the lean flow (ER = 0.3; Tab. 1) at 200°C.
In Figure 2, we can distinguish the formation of bidentate
nitrates coordinated with alumina cations (bands at 1
6061 260 cm-1) [
], nitrates coordinated at the interface
between alumina and barium oxide (bands at 1 585 and
1 297 cm-1) , bidentate nitrates coordinated with Ba2+
cations (1 555 and 1 303 cm-1) [
], monodentate nitrates
coordinated with Al2O3 (1 526 and 1 311 cm-1) [
monodentate nitrates coordinated with Ba2+ cations (1 452 and
1325 cm-1) [
] and polydentate nitrates coordinated with
Ba2+ cations (1 424 and 1 350 cm-1) . The feature centred
at 1 026 cm-1 contains the ν1 mode of the nitrate species. We
can also distinguish the formation of nitrites species: bridge
and/or bidentates alumina and/or barium nitrites (1 230 cm-1).
A band at 1 214 cm-1 was assigned to ionic nitrites [
The presence of these ionic nitrites may indicate that NO2
oxidation into NO3 on the storage sites becomes a limiting
step in NOx storage at 200°C (cf. Tab. 3).
The significantly reduced IR spectra show that hydrothermal
aging leads to a partial decrease of all storage sites. It also
appears to be some information as to which phases are
being impacted: the nitrate peak at 1 026 cm-1 is nearly
gone altogether and the dominant peak is now at 1 226 cm-1
(ionic nitrites), replacing almost totally the nitrates. That
means that the nitrite to nitrate oxidation function of the
material has been mainly impacted by the hydrothermal
treatment. Both XRD and surface IR Operando studies
show that hydrothermal aging has a rather homogeneous
impact on all materials: particle sintering, alumina phase
transition and BaAl2O4 production, which all lead to a partial
decrease of all storage sites.
An isothermal storage experiment was also performed
with a flow composition close to automotive exhaust
catalysis (ER = 0.3; Tab. 1). Figure 3 shows the spectra of the
nitrate and carbonate species on the surface of saturated
fresh and hydrothermally aged (800°C) catalysts. The
profiles shows the same storage sites as those observed with
the binary mixture (NO/O2), partially concealed by
competition between nitrates and carbonates as well as
hydrocarbon species: acrylates coordinated with Al2O3 (1 637 and
1 424 cm-1), acetates (1 574 and 1 452 cm-1) [
compounds are formed by the partial oxidation of propylene.
Barium carbonates (1 380 cm-1) [
] were also presents
(cf. Tab. 3).
2.3 Isothermal NOx Storage on the Synthetic
Figure 4 shows the NO oxidation efficiency of fresh and
aged catalysts as a function of the inlet gas temperature (200
As also shown by IR studies, hydrothermal aging induces
a loss of NO oxidation efficiency which is more significant at
200°C than at 300°C. The impact increases with the aging
temperature. It is due to Pt sintering that progressively
reduces the number of sites available to oxidize NO to NO .
NO2 formation is kinetically controlled at 200°C and is hence
strongly sensitive to Pt sintering, as was demonstrated by
Takahashi et al. [
] in engine close conditions: NO oxidation
efficiency is divided by a factor of 2 between the fresh and
the 700°C aged samples, and is close to zero on the catalyst
hydrothermally aged at 900°C.
In this figure, the impact of aging weakens at 300°C and
above when thermodynamical equilibrium becomes
dominant and favours NO formation: the NO oxidation efficiency
only decreases by 16% between the fresh and the 900°C aged
Figure 5 shows the NOx storage capacity of fresh and aged
catalysts as a function of the inlet gas temperature (200 and
Hydrothermal aging makes the NOx storage capacity
decrease and shows again a stronger impact at 200°C. This
was expected since it partly depends on the catalyst NO
oxidation efficiency, as the most effective pathway for NOx
storage from NO/O2 mixtures is the “nitrite” route, which
implies the stepwise oxidation of NO leading to the
formation of nitrite ad-species [
]. This explains why the catalyst
hydrothermally aged at 900°C sample had no more storage
capacity at 200°C (no more NO oxidation). At 300°C, NO2
was formed and NOx was stored.
The NOx storage capacity also depends on the storage
material performance [
]. A specific isothermal NOx
storage test was performed to better distinguish the impact of the
storage material and the oxidation Pt sites on aged samples.
300 ppm NO2 in nitrogen was used in order to bypass the NO
oxidation step. This experiment was performed on the fresh
NOx trap and on the 800 and 900°C hydrothermally aged
samples. Results are displayed in Figure 6 and show that the
NOx storage capacity decreases by up to 45% due to
hydrothermal aging. This storage site reduction is partly
attributed to BaAl2O4 formation, as was suggested by XRD
patterns at an aging temperature of 750°C and above . The
transition of γ-Al2O3 to δ-Al2O3 above 850°C also reduces
the alumina NOx storage capacity, as was observed in other
surface IR tests (results not displayed).
Pt sintering changes the Pt/Ba interface and may hence be
partly responsible for the NSC decrease, as was demonstrated
by Rodriguez [
2.4 Catalyst Light-off
Figure 7 shows the light-off temperatures (T50) of CO and
HC for fresh and aged catalysts.
The impact of hydrothermal aging on catalyst light-off is
not significant below 800°C, probably because the reductant
amount is small. Light-off temperatures of CO and C H
undergo 10 and 20°C increase after the 850 and 900°C aging
respectively. The loss of CO and HC conversion can be
attributed to thermal sintering of Pt particles, resulting in a
loss of adjacent active Pt sites, and to the loss of surface area
due to thermal sintering of the γ-Al2O3 washcoat.
2.5 Lean-Rich Cycling Conditions
The impact of thermal aging was also investigated in
conditions close to real use, i.e. by periodically switching the
gas composition between 290 s lean feed periods and 15 s
rich pulses to regenerate the NOx trap. Results are
summarized in Figure 8.
The NOx storage efficiency during NOx storage/reduction
cycles at 300°C decreased with the hydrothermal aging
temperature: from 90% for the fresh sample down to 42% for the
HA 900 sample. On the other hand, the NOx reduction
efficiency during rich pulses remained constant and close to
100%. The decrease of the global NOx conversion efficiency
was then mainly due to the NOx storage efficiency reduction.
Similar observations were made at 200°C. It also appeared
that the rich pulse duration was not sufficient anymore to
ensure complete NOx-trap regeneration: an amount of NOx
(8% of the NSC) remained stored in the aged trap in the same
condition. This might be due to a greater proportion of bulk
Ba storage sites leading to more stable nitrates, whose
diffusion is kinetically limited. Another possible reason is Pt
sintering and its effect on the Pt/Ba interface: as demonstrated
by Rodrigues [
], the proximity of platinum and barium is
important because Pt helps to remove and reduce NOx from
the trap when it is present in its surrounding.
This paper presents the effect of hydrothermal aging on NOx
trap structure and functions. The main following impacts on
the tested material were highlighted:
– NO oxidation decreases because of Pt sintering;
– NSC decreases because of the formation of BaAl2O4
which diminishes storage sites, and of Pt sintering which
changes Pt/Ba interface. Other parameters impacting the
NSC loss are: the transition of γ-Al2O3 to δ-Al2O3 and the
evolution of Ba crystalline structure which needs further
analysis. In Operando FTIR, nitrates adsorbed on Ba, Sr,
Al2O3, CeO2 and Ba/Al2O3 interface. Hydrothermal aging
leads to a partial decrease of all storage sites;
– Regenerations appeared to be less efficient, which was
attributed to a slower NOx release during rich pulses. The
change of Pt/Ba interface after Pt sintering could also
partly explain this failure.
These phenomena are of paramount importance to
understand the catalyst failure on duty, even if in real
S Benramdhane et al. / Diesel Lean NOx-Trap Thermal Aging and Performance Evolution Characterization
application conditions more parameters intervene. The effects
of temperature and sulphur + temperature will be investigated
on the Synthetic Gas Bench in aging conditions closer to
real driving conditions and will be described in a next paper.
A NOx trap aged on a vehicle (after 80 000 km duty) will
also be characterized for comparison.
The authors wish to thank the Analysis Department (XRD,
TEM, XF, etc.) at IFP Energies nouvelles for the help during
the study. Support of ANRT for S. Benramdhane PhD grant
is also gratefully acknowledged as well as Renault for
supplying the catalysts.
1 Takahashi N. , Shinjoh H. , Iijima T. , Suzuki T. , Yamazaki K. , Yokota K. , Suzuki H. , Miyoshi N. , Matsumoto S. , Tanizawa T. , Tanaka T. , Tateishi S. , Kasahara K. ( 1996 ) The new concept 3- way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst , Catal. Today 27 , 63 - 69 .
2 Gill L. , Blakeman P. , Twigg M. , Walker A. ( 2004 ) The use of NOx adsorber catalysts on diesel engines , Topics Catal . 28 , 1 - 4 , 157 - 164 .
3 Engström P. , Amberntsson A. , Skoglundh M. , Fridell E. , Smedler G. ( 1999 ) Sulphur dioxide interaction with NOx storage catalysts , Appl. Catal. B: Env. 22 , 4 , L241 - L248 .
4 Mahzoul H. , Limousy L. , Brilhac J.F. , Gilot P. ( 2000 ) Experimental study of SO2 adsorption on barium-based NOx adsorbers , J. Anal. Appl. Pyrolysis 56 , 2 , 179 - 193 .
5 Rohr F. , Peter S.D. , Lox E. , Kögel M. , Sassi A. , Juste L. , Rigaudeau C. , Belot G. , Gélin P. , Primet M. ( 2005 ) On the mechanism of sulphur poisoning and regeneration of a commercial gasoline NOx-storage catalyst , Appl. Catal. B: Env . 56 , 201 - 212 .
6 Elbouazzaoui S. , Courtois X. , Marecot P. , Duprez D. ( 2004 ) Characterisation by TPR, XRD and NOx storage capacity measurements of the ageing by thermal treatment and SO2 poisoning of a Pt/Ba/Al NOx-trap model catalyst , Topics Catal. 30/31 , 493 - 496 .
7 Fekete N. , Kemmler R. , Voigtländer D. , Krutzsch B. , Zimmer E. , Wenniger G. , Strehlau W. , Tillaart J.A. , Leyrer J. , Lox E.S. , Müller W. ( 1997 ) Evaluation of NOx storage catalysts for lean burn gasoline fueled passenger cars , SAE Technical paper 970746.
8 Jang B.H. , Yeon T.H., Han H.S. , Park Y.K. , Yie J.E. ( 2001 ) Deterioration mode of barium-containing NOx storage catalyst , Catal. Lett. 77 , 1 - 3 , 21 - 28 .
9 Toops T.J. , Bunting B.G. , Nguyen K. , Gopinath A. ( 2007 ) Effect of engine-based thermal aging on surface morphology and performance of Lean NOx Traps , Catal. Today 123 , 285 - 292 .
10 Hepburn J.S. , Thanasiu E. , Dobson D.A. , Watkins W.L. ( 1996 ) Experimental and modeling investigations of NOx trap performance , SAE Technical paper 962051.
11 Lesage T., Verrier C. , Bazin P. , Saussey J. , Daturi M. ( 2003 ) Studying the NOx-trap mechanism over a Pt-Rh/Ba/Al2O3 catalyst by operando FT-IR spectroscopy , Phys. Chem. Chem. Phys. 5 , 4435 - 4440 .
12 Amberntsson A. , Fridell E. , Skoglundh M. ( 2003 ) Influence of platinum and rhodium composition on the NOx storage and sulphur tolerance of a barium based NOx storage catalyst , Appl. Catal. B: Env. 46 , 3 , 429 - 439 .
13 Graham G.W. , Jen H.W. , Chun W. , Sun H.P. , Pan X.Q. , McCabe R.W. ( 2004 ) Coarsening of Pt particles in a model NOx trap , Catal. Lett. 93 , 129 - 134 .
14 Westerberg B. , Fridell E. ( 2001 ) J. Mol. Catal. A: Chemistry 165 , 249 .
15 Coronado J.M. , Anderson J.A. ( 1999 ) J. Mol. Catal. A: Chemistry 138 , 83 .
16 Lesage T., Verrier C. , Bazin P. , Saussey J. , Daturi M. ( 2003 ) Phys . Chem. Chem. Phys. 5 , 4435 .
17 Szailer T., Kwak J.H. , Kim D.H. , Szanyi J. , Wang C. , Peden C.H.F. ( 2006 ) Catal . Today 114 , 86 .
18 Abdulhamid H., Dawody J. , Fridell E. , Skoglundh M. ( 2006 ) J. Catal . 244 , 169 .
19 Fanson P.T. , Horton M.R. , Delgass W.N. , Lauterbach J . ( 2003 ) Appl . Catal. B: Env. 46 , 393 .
20 Milt V.G. , Querini C.A. , Miró E.E. , Ulla M.A. ( 2003 ) J. Catal . 220 , 424 .
21 Kim D.H. , Kwak J.H. , Szanyi J. , Burton S.D. , Peden C.H.F. ( 2007 ) Water-induced bulk Ba(NO3)2 formation from NO2 exposed thermally aged BaO/Al2O3 , Appl. Catal. B: Env. 72 , 233 - 239 .
22 Su Y., Amiridis M.D. ( 2004 ) In situ FTIR studies of the mechanism of NOx storage and reduction on Pt/Ba/Al2O3 catalysts, Catal. Today 96 , 31 - 41 .
23 Halkides T.I. , Kondrarides D.I. , Verykios X.E. ( 2002 ) Mechanistic study of the reduction of NO by C3H6 in the presence of oxygen over Rh/TiO2 catalysts , Catal. Today 73 , 213 - 221 .
24 Ouyang F. , Haneda M. , Sun W. , Kindaichi Y. , Hamada H. ( 2008 ) Roles of Surface Nitrogen Oxides in Propene Activation and NO Reduction on Ag/Al2O3, Kinet . Catal. 49 , 236 - 244 .
25 Courson C. , Khalfi A. , Mahzoul H. , Hodjati S. , Moral N. , Kiennemann A. , Gilot P. ( 2002 ) Experimental study of the SO2 removal over a NOx trap catalyst , Catal. Commun . 3 , 10 , 471 - 477 .
26 Lesage T., Saussey J. , Malo S. , Hervieu M. , Hedouin C. , Blanchard G. , Daturi M. ( 2007 ) Operando FTIR study of NOx storage over a Pt/ K/Mn/Al2O3-CeO2 catalyst, Appl. Catal. B: Env. 72 , 166 - 177 .
27 Takeuchi M. , Matsumoto S. ( 2004 ) NOx storage-reduction catalysts for gasoline engines , Topics Catal . 28 , 151 - 156 .
28 Rodriguez F. ( 2001 ) Thèse, Université de Pierre et Marie Curie (Paris VI).