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Applied Catalysis B: Environmental 77 (2007) 73–78

New approach to enhance the NOx storage performance at

high temperature using basic MgAl2O4 spinel support

Naoki Takahashi *, Shin’ichi Matsunaga, Toshiyuki Tanaka,Hideo Sobukawa, Hirofumi Shinjoh

TOYOTA Central Research & Development Labs., Inc., Nagakute, Aichi 480-1192, Japan

Received 12 July 2006; received in revised form 7 July 2007; accepted 9 July 2007

Available online 13 July 2007

Abstract

With the purpose of improving the NOx storage performance of NOx storage and reduction (NSR) catalysts above 673 K, a new approach using

a support with a higher basicity than g-Al2O3 was proposed and exemplified by using the MgAl2O4 spinel as the support for an NSR catalyst

containing the potassium NOx storage material. The NOx storage amounts on the fresh and thermally-aged K/Pt/MgAl2O4 catalysts at 873 K were

apparently higher than those on the K/Pt/Al2O3 catalysts. The higher potassium basicity resulting from the interaction between the basic MgAl2O4

support and the potassium NOx storage material enhanced the stability of the stored NOx over the fresh and thermally-aged K/Pt/MgAl2O4

catalysts, which leads to their competitive NOx storage amounts versus those of the K/Pt/Al2O3 catalysts. These results indicated a breakthrough

that improved the performance of the NSR catalysts at high temperature.

# 2007 Elsevier B.V. All rights reserved.

Keywords: NOx storage and reduction catalyst; MgAl2O4 spinel; Potassium; Thermal-aging; Basicity

1. Introduction

To meet the stringent emission regulations, a new after-

treatment system for lean-burn gasoline engines is required,

because the conventional three-way catalysts are not able to

detoxify NOx under excess oxygen conditions, although the lean-

burn engine contributes to the prevention of global warming. A

NOx storage and reduction (NSR) catalyst system has been

proposed and is currently being improved as one of the most

feasible and attractive solutions to this technical challenge [1,2].

A typical NSR catalyst consists of barium or potassium

compounds as a NOx storage material, Pt and g-Al2O3 [3,4]. In

an oxidative atmosphere, NO is first oxidized to NO2 over Pt,

then combined with the NOx storage material, and finally stored

as a nitrate ion. During the following reduction stage, the stored

nitrate ion is released as NO or NO2 from the NOx storage

material, and then reduced to nitrogen [1,3].

The conventional NSR catalysts for the lean-burn engine are

mainly operated at a temperature around 673 K [4], because

* Corresponding author at: 41-1 Yokomichi, Nagakute, Nagakute-cho, Aichi

480-1192, Japan. Tel.: +81 561 63 6293.

E-mail address: e0790@mosk.tytlabs.co.jp (N. Takahashi).

0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2007.07.007

these catalysts have a maximum NOx removal efficiency from

573 to 673 K. Our previous study revealed that the NOx storage

amount in the lean atmosphere was the same as the NOx

reduction amount from the subsequent rich spike over 673K

[5]; thus, the NSR capability above 673 K was restricted by the

NOx storage amount. In the near future, a motor vehicle with a

lean-burn gasoline engine will possibly cruise on the highway

at 100 km/h or faster, thus the NSR catalyst needs to work at

temperatures of 873 K or higher. Therefore, it is very important

to explore a new approach to enhance the high temperature

NSR performance of the catalysts in the lean phase.

Considering that the stability of the stored NOx could be

increased by adopting more basic elements [3,6], one promising

approach to obtain a higher NOx storage ability in the high-

temperature range has been proposed by enhancing the basicity

of the potassium NOx storage material in some way. It is well-

known that the electronic states of metals are affected and

changed by interactions with the support oxides or additives in

the supported metal catalysts [7–9]. These facts suggested that

the basicity of the potassium NOx storage material could be

enhanced if it is supported on a support with a higher basicity

than g-Al2O3. The mixed oxide composed of an alkaline earth

metal and aluminum is one of the support candidates, and the

N. Takahashi et al. / Applied Catalysis B: Environmental 77 (2007) 73–7874

Mg–Al mixed oxide is chosen as the specific example in this

study.

The NOx storage on Mg–Al mixed oxides with some loaded

precious metals but without NOx storage materials has been

reported [10–12]. These published studies aimed at upgrading

the NOx storage ability below 573 K, and the Mg–Al mixed

oxides were assumed to be the NOx storage sites. Compared to

these studies, this research mainly concerns the enhancement of

the so-called high-temperature NOx storage capability at

873 K. By employing a Mg–Al mixed oxide as the support to

enhance the basicity of the potassium NOx storage material, we

tried to determine the feasibility of this proposed approach in

order to improve the high-temperature NOx storage perfor-

mance, which has not yet been published.

2. Experimental

2.1. Mg–Al mixed oxide synthesis and characterization

The Mg–Al mixed oxide was synthesized by the sol–gel

method [13]. Specific amounts of magnesium acetate tetra-

hydrate ((CH3COO)2Mg�4H2O, Wako Pure Chemical Indus-

tries) and aluminium isopropoxide (Al[OCH(CH3)2]3, Wako

Pure Chemical Industries) were first dissolved in 2-propanol

((CH3)2CHOH, Wako Pure Chemical Industries). The obtained

solution was kept at 353 K for 2 h by refluxing the volatile

solvent. Ion-exchanged water was then added for hydrolysis,

and the reflux continued for another 2 h. The obtained gel was

dried using a rotary evaporator kept in a water bath at 323 K

until the liquid was fully vaporized. The dried compound was

kept at ambient temperature for 1 day, then heated to 1123 K at

the rate of 300 K/h, and finally calcined at 1123 K for 5 h in air.

The molar ratio of Mg to Al was 0.5.

Its specific surface area was obtained by the BET one-point

method using a Microdata Microsorp 4232II. Its crystalline

structure was analyzed by powder X-ray diffraction using Cu Ka

radiation with a Rigaku RINT-2100 diffractometer, and the

obtained pattern was analyzed by comparison with the stored

database. The temperature programmed desorption (TPD) of

CO2 for characterizing the basic property of the Mg–Al mixed

oxide and pure g-Al2O3 (150 m2/g) was measured using a Best

Sokki CATA5000-4, a conventional flow-type fixed bed reactor,

at atmospheric pressure. 1 g of the support (300–700 mm) was

packed in a quartz tube with a 10 mm inner diameter and then in-

situ pretreated at 873 K for 30 min under flowing N2. After the

pretreatment, the support was exposed to CO2 at 373 K until the

outlet concentration reached the same level as that of the inlet

gas, and then the inlet gas was switched to the N2 flow until no

CO2 was detected in the outlet gas. The support was finally

heated to 873 K at a rate of 20 K/min to obtain the TPD spectrum.

The outlet CO2 concentration was continuously monitored using

a non-dispersive infrared (NDIR) type analyzer.

2.2. Catalyst preparation and the characterizations

7 g of either the synthesized Mg–Al mixed oxide or g-Al2O3

support was wash-coated on a hexagonal cell monolithic

substrate (62 cells/cm2) having a diameter of 30 mm and length

of 50 mm [6]. The coated sample was immersed in a nitric acid

solution of dinitro-diamino platinum (Pt(NH3)2(NO2)2, Tanaka

Precious Metals) for 1 h, then the remaining solution was

removed by air flushing, dried at 393 K overnight, and finally

calcined at 523 K for 1 h in air. The coated substrate was

thereafter impregnated in an aqueous solution of potassium

acetate (CH3COOK, Wako Pure Chemical Industries) for

10 min, purged with air to remove the remaining solution, dried

at 393 K overnight, and finally calcined at 773 K for 3 h. The

platinum and potassium were separately loaded, and the

loading amounts of the Pt and potassium were determined by

the solution concentrations.

The Pt loadings were 0.07, 0.18 and 0.35 g, while the

loadings of the potassium as K2O were 1.0, 2.0 or 3.3 g,

respectively. The NO oxidation activity, NOx-TPD and NOx

storage amounts of these fresh or thermally-aged monolith

catalysts were measured as follows.

To estimate the Pt’s catalytic performance, the NO oxidation

activity at 873 K was analyzed. A lean gas, containing 7% O2,

400 ppm NO, 11% CO2, 5% H2O with N2 as the balance gas,

was used for the reaction. The gas flow rate was set at 30

000 cm3/min, corresponding to a gas hourly space velocity

(GHSV) of 51,500 h�1. When the NOx concentration in the

outlet gas reached a constant value, the NOx and NO

concentrations were measured by a chemiluminescent NOx

meter attached to a Horiba MEXA-7100 evaluation system. The

NOx concentration in this study is defined as the integration of

NO and NO2 in the gases; thus, the NO2 concentration is the

difference between the NOx and NO concentrations [14].

The stability of the stored NOx on the catalysts was

evaluated using the NOx-TPD method. The catalyst was first

exposed to the same lean gas as the NOx oxidation analysis

until the NOx concentration in the outlet gas reached a constant

value at 573 K; then the NOx saturated catalyst was heated

under the same lean gas flow from 573 to 973 K at a rate of

10 K/min for the TPD spectrum. The inlet carrier contains

400 ppm of NO; thus, a NOx concentration higher than

400 ppm is detected and quantified as the NOx desorbed from

the catalyst.

2.3. NOx storage measurement and thermal-aging

treatment

The NOx storage at 873 K was measured using a

conventional fixed-bed reactor [14] with simulated exhaust

gases at atmospheric pressure. The fresh catalyst was pretreated

for 15 min at 773 K with an NO-free stoichiometric model gas,

containing 0.28% CO, 0.16% H2, 0.09% C3H6, 0.62% O2,

14.25% CO2 and 5% H2O with N2 as the balance gas. Following

the in-situ pretreatment, the catalyst was heated to 873 K under

the same stoichiometric model gas and then exposed to the lean

gas until the NOx concentration in the outlet gas reached a

constant value. The composition and flow rate of the reaction

gas were the same as those for the NO oxidation analysis.

The thermal-aging treatment of the catalyst was conducted

using a cyclic feedstream, which simulated the actual engine

Fig. 1. XRD pattern of the synthesized Mg–Al mixed oxide.

Fig. 3. NOx storage performance of the fresh K/Pt/MgAl2O4 catalyst at 873 K

vs. K2O amounts. Pt loading amount 0.07 g (*), 0.18 g (�) and 0.35 g (^).

N. Takahashi et al. / Applied Catalysis B: Environmental 77 (2007) 73–78 75

exhaust gas under lean and rich conditions, except that no sulfur

compound was present. The catalyst was heated from ambient

temperature to 1023 K in 45 min and then kept at this

temperature for 5 h. The 4 min lean (6% O2, 530 ppm NO,

0.2% C3H6, 10% CO2 and 3% H2O with N2 as the balance gas)

and 1 min rich (6% CO, 530 ppm NO, 0.2% C3H6, 10% CO2

and 3% H2O with N2 as the balance gas) atmospheres were

alternatively switched during the entire thermal-aging proce-

dure. The NOx storage measurement on the thermally-aged

catalyst was conducted using the same procedure as that for the

fresh one, without any in-situ pretreatment.

3. Results and discussion

3.1. NOx storage site

The XRD pattern of the synthesized Mg–Al mixed oxide in

Fig. 1 indicated that there was no peak other than those

attributed to the MgAl2O4 spinel and that its lattice constant of

0.8083 nm totally matched the database value. Therefore, the

synthesized material was confirmed to be the MgAl2O4 spinel

without any defect and isolated MgO or Al2O3, and hereafter,

this synthesized support oxide was termed MgAl2O4.

Fig. 2 illustrates the outlet NOx concentrations using the K/

Pt/MgAl2O4 (heavy line) and K/Pt/Al2O3 (thin line) catalysts

and the inlet NOx concentration (dashed line) versus time. The

Fig. 2. NOx concentrations in the outlet and inlet gases of the catalysts. Outlet

NOx concentration of the K/Pt/MgAl2O4 catalyst ( ). Outlet NOx con-

centration of the K/Pt/Al2O3 catalyst (—). Inlet NOx concentration ( ).

two catalysts contain 0.07 g Pt and 1.0 g K2O. As shown in this

figure, upon switching to the lean atmosphere at time 0, the

outlet NOx concentrations with the two catalysts gradually

increase with time. The difference between the inlet and outlet

NOx concentrations corresponds to the NOx storage on the

catalyst. When the outlet NOx concentration reaches approxi-

mately the same level as the inlet NOx, the NOx storage sites

are assumed to be occupied with nitrate. It took about 600 s for

the K/Pt/MgAl2O4 catalyst, but only 300 s for the K/Pt/Al2O3

catalyst to become saturated with NOx. This result means that

the NOx storage amount on the K/Pt/MgAl2O4 catalyst was

much higher than that on the K/Pt/Al2O3 catalyst. The

integration amount of the stored NOx is noted as the ‘‘NOx

storage amount’’. Hereafter, the NOx storage performance is

indicated by the ratio of the NOx storage amount on each

catalyst to that on the fresh K/Pt/Al2O3 catalyst, which contains

0.07 g Pt and 1.0 g K2O as the reference.

The NOx storage amount versus the K2O amounts on the

fresh K/Pt/MgAl2O4 or K/Pt/Al2O3 catalysts with different Pt

loadings is shown in Figs. 3 and 4, respectively. The NOx

storage amount on both catalysts increased versus the K2O

amount, which did not vary much versus the Pt amount. At any

K2O amount, the NOx storage amount on the K/Pt/MgAl2O4

Fig. 4. NOx storage performance of the fresh K/Pt/Al2O3 catalyst at 873 K vs.

K2O amounts. Pt loading amount 0.07 g (*), 0.18 g (�) and 0.35 g (^).

Fig. 6. NOx-TPD spectra of the fresh K/Pt/MgAl2O4 and K/Pt/Al2O3 catalysts.

K/Pt/MgAl2O4 catalysts ( ), K/Pt/Al2O3 catalysts (—).

N. Takahashi et al. / Applied Catalysis B: Environmental 77 (2007) 73–7876

catalyst was apparently higher than that on the K/Pt/Al2O3

catalyst, and the difference decreased when the K2O amount

increased.

In our previous study, the NOx storage amount on the barium

compound was negligible at 873 K, even if the catalyst was

fresh [14]. Pauling’s electronegativities of barium and

magnesium are 0.9 and 1.2, respectively, thus the basicity of

magnesium is lower than that of barium. In addition, the

magnesium in this study completely formed an oxide with

alumina, while the barium in the previous study was supported

as a carbonate or nitrate on alumina. All catalysts showed

higher NOx storage amounts with increasing K2O amounts.

Moreover, when extrapolating the NOx storage amounts to a

zero K2O loading in Figs. 3 and 4, the NOx storage amounts

became negligible. These results indicated that the NOx storage

sites on these catalysts are the potassium compounds.

3.2. Influence of MgAl2O4 support on high-temperature

NOx storage

The CO2 desorption spectra for characterizing the basicity of

the MgAl2O4 (heavy line) and g-Al2O3 (thin line) are shown in

Fig. 5. The peak temperature of the CO2 desorption from

MgAl2O4 was 450 K, while that from g-Al2O3 was 440 K.

Moreover, the CO2 concentration from the MgAl2O4 was much

higher than that from g-Al2O3. The total amount of the CO2

desorption from the MgAl2O4 was 2.5 times that from the g-

Al2O3, although the BET specific surface area of the former

(110 m2/g) is about two-thirds that of the latter (150 m2/g). These

results indicate that the MgAl2O4 support has a stronger basicity

and greater number of basic sites than the g-Al2O3 support.

Fig. 6 shows the NOx-TPD spectra obtained for the fresh

catalysts. It is clear that the start and end desorption

temperatures of the K/Pt/MgAl2O4 catalyst were higher than

those for the K/Pt/Al2O3 catalyst, and the integration of the

NOx desorption amount on the former catalyst (2000 mmol)

was also higher than that on the latter (1060 mmol). These

results mean that the stability of the stored NOx on the

potassium NOx storage material loaded on the MgAl2O4

support was significantly enhanced. Therefore, the NOx storage

amount on the K/Pt/MgAl2O4 was consistently higher than that

on the K/Pt/Al2O3 catalyst for the same K2O amount, as shown

in Figs. 3 and 4. These results demonstrate the feasibility of our

Fig. 5. CO2-TPD spectra of the MgAl2O4 and g-Al2O3 supports. MgAl2O4

( ), g-Al2O3 (—).

approach to both improving the stability of the stored NOx and

enhancing the NOx storage capacity at high temperature by

adding the basic oxide to the support material.

3.3. Influence of the Pt loading on high-temperature NOx

storage

It has been reported that the role of Pt is to oxidize NO to

NO2 during the NOx storage process [3,4,14]. To clarify why Pt

loading does not affect the NOx storage amounts of the fresh K/

Pt/MgAl2O4 catalysts at 873 K, the NO oxidation activity of the

three catalysts, containing 2.0 g K2O with 0.07, 0.18 or 0.35 g

Pt, were analyzed. The outlet NO2 concentrations for them at

this temperature were 34, 33 and 33 ppm, respectively. This

result means that the NO oxidation capability of these catalysts

is not affected by their Pt loading and that all of them had the

same thermodynamic equilibrium value (34 ppm).

In our previous study [14], the NO2 concentration using the

Ba/Pt/Al2O3 catalyst reached thermodynamic equilibrium

above 673 K. In this study, due to the use of the potassium

NOx storage material and basic MgAl2O4 support, the basicity

of the catalysts was higher than that from our previous study. It

was reported that the basicity of the catalyst suppresses the

catalytic activity of Pt for NO oxidation at low temperature

[15]. The reaction temperature of 873 K is assumed to be high

enough for Pt to activate the NO oxidation. Therefore, the NOx

storage amount on the fresh catalysts did not totally rely on the

Pt loading, as shown in Fig. 4.

3.4. Influence of MgAl2O4 support on the NOx storage

versus the thermal-aging

The NOx storage amount on the thermally-aged K/Pt/

MgAl2O4 (black circles) and K/Pt/Al2O3 (white circles)

catalysts, containing 0.35 g Pt, versus the K2O amount is

shown in Fig. 7. The NOx storage amount on the K/Pt/MgAl2O4

catalyst at any K2O loading was higher than that on the K/Pt/

Al2O3 catalyst, which is the same as the fresh catalysts.

However, distinct from the fresh catalysts, the difference in the

NOx storage between these two thermally-aged catalysts

increased upon increasing the loading amount of K2O.

Fig. 7. NOx storage performances of the thermally-aged K/Pt/MgAl2O4 and K/

Pt/Al2O3 catalysts at 873 K vs. K2O amount. K/Pt/MgAl2O4 catalyst (*), K/Pt/

Al2O3 catalyst (*).Fig. 9. NOx-TPD spectra of the thermally-aged K/Pt/MgAl2O4 and K/Pt/Al2O3

catalysts. K/Pt/MgAl2O4 catalysts ( ), K/Pt/Al2O3 catalysts (—).

N. Takahashi et al. / Applied Catalysis B: Environmental 77 (2007) 73–78 77

To compare the thermal stability of the two catalysts, the NOx

storage residual ratios versus the K2O amount are illustrated in

Fig. 8. The NOx storage residual ratio is the percentage of the

NOx storage amount on the thermally-aged catalyst to that on the

fresh one. These values with the K/Pt/MgAl2O4 catalyst (black

circles) are consistently higher than those of the K/Pt/Al2O3

catalyst (white circles). In addition, the NOx storage residual

ratio on the K/Pt/MgAl2O4 catalyst increased versus the K2O

amount, while that for the K/Pt/Al2O3 remained nearly

unchanged. The value for the 3.3 g K2O containing K/Pt/

MgAl2O4 catalyst was almost 100%, indicating that this catalyst

did not deteriorate due to the thermal-aging treatment.

Fig. 9 shows the NOx-TPD spectra obtained for the

thermally-aged catalysts, containing 0.35 g Pt and 2.0 g

K2O. As in the case of the fresh catalysts, the start and end

desorption temperatures of the K/Pt/MgAl2O4 catalyst (heavy

line) were higher than those for the K/Pt/Al2O3 catalyst (thin

line), and the integration of the NOx desorption amount on the

former catalyst (630 mmol) was also higher than that on the

latter (45 mmol). The ratio of the NOx desorption amount over

the thermally-aged K/Pt/MgAl2O4 catalyst to that over the fresh

one was 53%, while it was only 7% for the K/Pt/Al2O3 catalyst.

These results indicate that the durability of the stored NOx on

the K/Pt/MgAl2O4 catalyst is overwhelmingly superior to those

on the K/Pt/Al2O3 catalyst.

Fig. 8. NOx storage residual ratio of the K/Pt/MgAl2O4 and K/Pt/Al2O3

catalysts at 873 K vs. K2O amount. K/Pt/MgAl2O4 catalyst (*), K/Pt/Al2O3

catalyst (*).

The outlet NO2 concentrations for both the thermally-aged

catalysts at 873 K were 34 ppm, indicating that the Pt’s

catalytic activities of these two catalysts show no difference and

do not deteriorate due to thermal aging treatment at this

temperature. Therefore, the above-mentioned characteristic of

the NOx storage might be caused by the difference in the state

of the potassium. It is reported that the NOx storage function

deteriorates when potassium reacts with the support during the

durability test [4]. This information implies that the solid-phase

reaction between the potassium NOx storage material with the

MgAl2O4 somehow differs from that with the g-Al2O3 support.

For example, these two supports have a spinel structure; however,

all the cation sites of MgAl2O4 are occupied by Mg2+ or Al3+,

while approximately 11% of those sites of the g-Al2O3 are

vacant. In other words, the MgAl2O4 support does not have any

lattice defects and should have a higher resistance to the solid-

phase reaction with potassium than the pure g-Al2O3. XRD

analyses were then conducted on the thermally-aged catalysts,

but no apparent XRD peak was assigned to any potassium

compounds. The potassium compounds perhaps exist in some

amorphous state, and thus could not be detected. A further

investigation is necessary to reveal the difference in the state of

the potassium and the influence from the MgAl2O4 support.

4. Conclusions

We report a new approach to improve the NOx storage ability

at high temperature by enhancing the basicity of the potassium

NOx storage material using a basic support. The feasibility of this

approach was exemplified by adopting MgAl2O4 as the support.

In addition, the K/Pt/MgAl2O4 catalyst containing 3.3 g of

K2O did not significantly deteriorate during the thermal-aging

treatment. These results suggested that this approach is a

promising breakthrough solution to improve the performance of

NSR catalysts at high temperature.

Acknowledgements

The authors gratefully acknowledge Ms. Chika Ando for the

catalyst preparation and Dr. Fei Dong for his significant help

with this manuscript’s preparation.

N. Takahashi et al. / Applied Catalysis B: Environmental 77 (2007) 73–7878

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