www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 70 (2007) 198–204

The low-temperature performance of NOx storage

and reduction catalyst

Naoki Takahashi *, Kiyoshi Yamazaki, Hideo Sobukawa, Hirofumi Shinjoh

TOYOTA Central Research and Development Laboratories Inc., 41-1 Yokimichi Nagakute Nagakute-cho, Aichi 480-1192, Japan

Available online 21 June 2006

Abstract

The catalytic performance and the behavior of NOx storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been

mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the

NOx storage amount in the lean atmosphere was the same as the NOx reduction amount from the subsequent rich spike (RS) above the temperature

of 400 8C, while the former was greater than the latter below the temperature of 400 8C. This indicated that when the temperature was below 400 8Ccompared with the NOx storage stage, the reduction of the stored NOx is somehow restricted. We found that the reduction efficiencies with the

reducing agents decrease in the order H2 > CO > C3H6 below 400 8C, thus not all of the NOx storage sites could be fully regenerated even using an

excessive reducing agent of CO or C3H6, which was supplied to the NSR catalyst, while all the NOx storage sites could be fully regenerated if an

adequate amount of H2 was supplied. We also verified that the H2 generation more favorably occurred through the water gas shift reaction than

through the steam reforming reaction. This difference in the H2 generation could reasonably explain why CO was more efficient for the reduction of

the stored NOx than C3H6, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though

promoting the H2 generation reaction.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Lean-burn engines; NOx storage and reduction catalyst; H2; Water gas shift reaction; Steam reforming reaction

1. Introduction

Reducing CO2 emissions from motor vehicles is a very

important effort for the global scale environmental protection;

meanwhile it is also regarded as an additional benefit from the

development of more fuel-efficient engines for the automobile

industry. The lean-burn gasoline engine system has attracted a

lot of attention for its remarkable potential to improve the fuel

economy compared with the conventional stoichiometric

operated engine systems. However, the wide use of the lean-

burn engine system is somewhat restricted by environmental

regulations. This is mainly because the conventional three-way

catalysts are not able to detoxify NOx in the lean-burn engine

exhaust, in which oxygen is excessive. The NOx storage and

reduction (NSR) catalyst system is proposed and already

regarded as one of the most feasible and attractive solutions to

this technical challenge [1,2].

* Corresponding author. Tel.: +81 561 63 6293; fax: +81 561 63 6150.

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

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

doi:10.1016/j.apcatb.2005.10.029

A typical NSR catalyst consists of precious metals (mainly

Pt), alkaline and alkaline earth metal oxides as NOx storage

compounds (usually BaO), and a metal oxide as support

(alumina). As shown in Fig. 1, the scheme of the NOx storage

and reduction reaction on the NSR catalyst [1] in the NOx

storage stage under an oxidative or lean-burn atmosphere, NOx

(NO) is first oxidized to NO2 over precious metals, then react or

combined with the NOx storage compounds, and finally stored

as a nitrate ion. In the following reduction stage under a

stoichiometric or reductive atmosphere (rich), the stored nitrate

ion is released as NOx (NO or NO2) from the NOx storage

compounds, and then reduced to nitrogen [3]. The engine

control system periodically provides a fuel-rich spike (RS)

containing reducing agents such as hydrogen (H2), carbon

monoxide (CO) and hydrocarbons (HC) to convert the stored

nitrate ions over the NSR catalyst [4].

The already reported NSR catalysts for the lean-burn

gasoline engine are mainly operated at a temperature around

400 8C [5]. Erkfeldt et al. pointed out that decreasing the

exhaust gas temperature of a lean-burn engines will be a very

meaningful way to reduce the thermal loss, thus improving the

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204 199

Fig. 1. Outline of the NOx storage and reduction reaction.

Table 1

Compositions of simulated exhaust gases for NOx storage and reduction

performance

NO

(ppm)

O2

(%)

H2

(%)

CO

(%)

C3H6

(ppm)

CO2

(%)

H2O

(%)

N2

Lean 1 400 7 0 0.01 200 11 5 Balance

RS1 400 0 1.6 6 1070 11 5 Balance

fuel economy [6]. Therefore, corresponding with this improve-

ment in the lean-burn engine for a higher fuel efficiency, the

enhancement of the low-temperature performance of the NSR

catalyst is very necessary and highly expected.

It is reported that the NOx storage and reduction on the NSR

catalyst apparently decrease if the reaction temperature goes

below 400 8C [5]. It is well known that the reducing agents,

such as CO and hydrocarbons, co-exist with oxygen, and the

NOx in the exhaust, thus a series of reactions occur on the NSR

catalyst. For further improving the low temperature activity and

optimizing the formulation of the NSR catalysts, it is very

necessary and meaningful to clarify which is the restricted

stage, the NOx storage or the following reduction of the stored

NOx in the entire NOx storage and reduction process? How

significant is the influence on the performance and behavior of

the NOx storage and reduction stages from the reducing agents

contained in the exhaust? To answer these questions, the NOx

storage and reduction on a model NSR catalyst with simulated

exhaust gases was designed, and experimentally investigated on

the basis of the temperature and the reducing agents in this

research.

2. Experimental

2.1. Catalyst preparation

The model NSR catalyst was prepared on a hexagonal cell

monolithic substrate (62 cells/cm2) having a diameter of

30 mm and length of 50 mm [7]. This catalyst consisted of

the oxide support (mainly alumina), ceria-zirconia-based

oxygen storage material, barium and potassium oxides as the

NOx storage compounds, and the supported metals of platinum

and rhodium. The total loading amount of platinum and

rhodium was 0.4 wt.% and that of barium and potassium was

5 wt.%. Nitric acid solution of Pt(NH3)2(NO2)2 and aqueous

solution of RhCl3, barium acetate and potassium acetate were

used for impregnation. Our previous studies showed that the

change in the composition of the NSR catalyst would not

qualitatively affect the basic features and behaviors of the NOx

storage and reduction performance. This research is mainly

studying the performance of the NSR catalysts instead of the

catalyst itself, thus only one NSR catalyst was used in the

experiments.

2.2. Measurement of NOx storage and reduction

The NOx storage and reduction experiments were conducted

using a conventional fixed-bed reactor [1,3,8] with simulated

exhaust gases under atmospheric pressure. The fresh catalyst

was first pretreated for 15 min at 500 8C with the 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. Prior to the NOx storage/release testing, to

clean away the residual NOx, the rich model gas (referred as

RS1 in Table 1) was flowed through the catalyst for 5 min

during cooling down or heating up to the test temperature.

Thereafter, the NSR catalyst was exposed to the lean

atmosphere until the NOx concentration in the outlet gas

reached constant, and subsequently switched to 3-s RS under

the lean atmosphere until the NOx concentration in the outlet

gas reached constant. This NOx storage/release testing

proceeded with the temperature stepping from 250 to 600 8C.

The gas flow rate for each measurement was set as

30 000 cm3/min, corresponding to the GHSV of 51 500 h�1.

The NOx concentration in this research is taken as the

integration of NO and NO2 in the gases, and is measured by the

chemiluminescent NOx meter attached to the Horiba MEXA-

7100 evaluation system. As listed in Table 1, the gas

compositions were determined according to the analysis results

of the lean-burn gasoline exhaust, and C3H6 was used as the

hydrocarbon species.

2.3. Influence on the behavior of NSR reaction by the

reducing agents

The gases as listed in Table 2 were used for investigating

the influence on the reduction of the stored NOx by the

reducing species in the RS. In these experiments, the in situ

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204200

Table 2

The gas compositions used to investigate the effect of reducing agent type on the stored NOx reduction

NO (ppm) O2 (%) H2 (%) CO (%) C3H6 (%) CO2 (%) H2O (%) N2

Lean 2 400 7 0 0 0 11 5 Balance

RS2 400 0 6 0 0 11 5 Balance

RS3 400 0 0 6 0 11 5 Balance

RS4 400 0 0 0 0.67 11 5 Balance

Blank-RS 400 0 0 0 0 11 5 Balance

Fig. 2. NOx concentration in the outlet and inlet gases using Lean 1 and RS1

gases at 250 8C: (—) outlet NOx concentration; (� � �) inlet NOx concentration.

pretreated NSR catalyst was alternately and periodically

switched to a 60-s lean atmosphere and a 3-s RS. The CO and

C3H6 concentrations of the reaction gases were measured

using the non-dispersive infrared CO analyzer and a flame

ionization HC detector equipped Horiba MEXA-7100,

respectively. The H2 concentration in the reaction gas was

measured by a sector-type mass spectrometer attached to a

Horiba MSHA-1000L.

2.4. H2 generation over the NSR catalyst

The gases in Table 3 were used for investigating the H2

generation over the NSR catalyst by the water gas shift (WGS)

reaction or steam reforming (SR) reaction under the transient

conditions. The lean and rich atmospheres were alternately and

periodically switched every 30 s with the temperature

decreasing from 400 to 200 8C at the rate of 10 8C/min.

3. Results and discussion

3.1. The performance of NOx storage and reduction with

temperature

As shown in Fig. 2, with the simulated gases in Table 1, the

evolution of the NOx concentration in the outlet gas with time at

250 8C shows the storage and reduction stages of the entire NOx

storage and reduction processes. When the lean atmosphere is

switched on, the outlet NOx concentration gradually increased

with time and then reached an approximately constant level

around 1400 s. The difference in the NOx concentration between

the inlet and outlet gases at this point could be attributed to the

selective NOx reduction by HC on Pt of the NSR catalyst [8]. The

shadow area ‘‘A’’relates to the NOx amount stored on the catalyst.

Upon the 3-s RS supplied to the NOx stored NSR catalyst, the

NOx concentration in the outlet gas momently jumps higher than

Table 3

The gas compositions used to investigate hydrogen generation performance on

the NSR catalyst

O2 (%) CO (%) C3H6 (%) H2O (%) N2

Water gas shift

Lean A 7 0 0 10 Balance

Rich A 0 6 0 10 Balance

Steam reforming

Lean B 7 0 0 10 Balance

Rich B 0 0 0.67 10 Balance

that in the inlet gas, quickly falls to nearly zero, and then

gradually increased again with time. Apparently, this is closely

related to the regeneration of the NOx storage sites or the

reduction of the stored NOx by the reducing agents in the RS. The

shadow area ‘‘B’’ relates to the amount of the regenerated NOx

storage sites on the catalyst. In this study, the NOx storage

amounts indicated by the shadow areas ‘‘A’’ and ‘‘B’’ are noted as

‘‘NOx storage amount’’ or ‘‘RS-NOx storage amount’’,

respectively. The NOx storage amount corresponds to the NOx

adsorption during the storage stage, while the RS-NOx storage

amount corresponds to the reduction of the stored or adsorbed

NOx during the reduction stage.

Fig. 3 plots the ratios of the NOx storage amount and RS-

NOx storage amount to the NOx storage amount at 400 8C

Fig. 3. NOx storage performance with Lean 1 and RS1 gases vs. reaction

temperature: (*) NOx storage amount; (*) RS-NOx storage amount.

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204 201

versus the reaction temperature. The ratio of the NOx storage

amount on the catalyst showed a maximum value both at 300 and

400 8C, while the ratio of the RS-NOx storage amount had a

maximum value at 400 8C. For the temperatures over 400 8C, the

NOx storage amount are nearly the same as the RS-NOx storage

amount. This suggests that the NOx storage sites on the catalyst

are nearly completely regenerated upon the 3-s RS over 400 8C.

For temperatures below 400 8C, the NOx storage amount on

the catalyst is greater than the RS-NOx storage amount. The

ratio of the RS-NOx storage amount to the NOx storage amount

at 400 8C decreases as the reaction temperature is decreased,

i.e., 71% at 300 8C and 47% at 250 8C. This result indicates that

the NOx storage sites were not completely regenerated during

the 3-s RS if the temperature is below 400 8C. The actual RS-

NOx storage amount under the actual exhaust atmosphere is

probably lower than that in this research, because the RS with

the actual lean-burn gasoline engines is one order of magnitude

shorter than that in this experiment, i.e., less than 1 s. Based on

these results, we can conclude that, compared with the storage

stage, the reduction of the stored NOx is somewhat restricted or

a kind of bottleneck for the entire NOx storage and reduction

process when the reaction temperature is below 400 8C.

Therefore, to improve the performance of the NSR catalysts,

the efforts should be focus on promoting the reactions involved

in the reduction stage instead of the NOx storage stage.

3.2. The influence of the reducing agents on the reduction

of the stored NOx

The performance of the NOx storage and reduction on the

NSR catalyst indicated that the reduction of the stored NOx is

the restricted stage or bottleneck for the entire process. It is very

important to clarify the individual reduction reaction involved

in the NOx reduction stage, thus figure out the solutions to

enhance the overall activity of the NSR catalyst. As shown in

Fig. 4, the conversions of H2, CO and C3H6, obtained from

Fig. 3 experiment, are approximately close to 100% at 400 8C,

while the conversions apparently decrease at 250 and 300 8C, as

H2 > CO > C3H6. These results indicated that the reduction

activity or efficiency with these three reducing agents is

Fig. 4. Conversion of the reducing agents in the rich spike vs. reaction

temperature: (*) H2; (*) CO; (�) C3H6.

different below 400 8C. As already mentioned, the NOx

reduction could occur under the lean atmosphere, thus, it is

difficult to clarify the effect of the reducing agents based only

on the analysis of the wave shape of the NOx in the outlet gas.

Therefore, to remove the influence of the reducing agents or any

possible selective NOx reduction, the lean gas without any

reducing agents is necessary for further investigation. As

summarized in Table 2, the simulated RS gases containing H2,

CO or C3H6 are labeled as RS2, RS3 and RS4, respectively. A

model gas without any reducing agents, consisting of NO, CO2

and H2O, with N2 as the balance gas was labeled the Blank-RS.

Along with being alternately switched to a 60-s lean

atmosphere and a 3-s RS at 250 8C, the NOx concentration

evolutions in the outlet gases with time are shown in Fig. 5. For

the convenience of the quantitative analysis, the concentration

of the chemical equivalent is used in this study. The

concentration of the chemical equivalent with the reducing

agents is based on the O2 concentration, assuming the complete

oxidation of the reducing agent, through the following

reduction reactions (1)–(3):

H2þð1=2ÞO2 ! H2O (1)

CO þ ð1=2ÞO2 ! CO2 (2)

C3H6þð9=2ÞO2 ! 3H2O þ 3CO2 (3)

For RS2, RS3 and RS4 in Table 2, the concentration of the

chemical equivalent with H2, CO and C3H6 was 3%. If all the

NOx contained in the lean period (Lean 2 in Table 2) is totally

stored on the NSR catalyst as nitrate ions, and then completely

reduced to N2 during the RS, the required concentration of the

chemical equivalent in the RS is estimated to be 1%. Therefore,

the reducing agents (3%) supplied in Fig. 5 are excessive

compared with the stored NOx amount.

The RS removes the stored NOx and refreshes the NOx

storage sites on the catalyst as described by reaction (4), and

these regenerated NOx storage sites then regain their NOx

storage ability:

MNO3þR ! NOxþMOyþROz

ðM : Ba or K; R : reducing agentsÞ (4)

The evolution of the NOx concentration under the lean

atmosphere with different reducing agents are shown in Fig. 5.

The NOx concentration in the outlet gas monotonically increased

with time and finally reached a constant level with the Blank-RS

(Fig. 5D). During the lean period, the increase in the NOx

concentration in the outlet gas with the reducing agents decreases

in the order no reducing agent Blank-RS > C3H6 > CO > H2.

This indicates that the reduction abilities of the reducing agents

decrease as H2 > CO > C3H6. With H2 used as the reducing

agent, the evolution of the NOx concentration versus time for

each NOx storage/reduction cycle was almost the same (Fig. 5A),

and indicates that the RS with H2 can regenerate nearly all the

NOx storage sites on the catalyst. As shown in Fig. 5, the NOx

conversion with different reducing agents during the RS reaches

a constant level when it gets close to the cycle end.

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204202

Fig. 5. NOx concentration in the outlet gas at 250 8C vs. time using Lean 2 and RS: (A) alternating with the RS2 (H2); (B) alternating with the RS3 (CO); (C)

alternating with the RS4 (C3H6); (D) alternating with the Blank-RS.

Besides the activity of the reducing agents, the amount of

the reducing agents contained in the RS is also very important

for the efficient reduction of the stored NOx and the fuel

efficiency. As shown in Fig. 6, the NOx conversion in the

seventh lean period is plotted versus the concentrations of the

chemical equivalent with the reducing agents. When the

concentration of the chemical equivalent is below 0.5%, the

reducing gas species only has a slight influence on the NOx

conversion. The NOx conversion apparently differs when the

concentration of the chemical equivalent with the reducing

agents ranges from 0.5 to 1.0%. In this concentration range,

Fig. 6. NOx conversion of the seventh lean period at 250 8C vs. the concentra-

tion of the chemical equivalent of the reducing agents in the RS: (*) H2; (*)

CO; (�) C3H6; (~) no reducing agents.

supply of the reducing agents becomes the determining factor

for the regeneration of the NOx storage sites. For the reducing

agent of H2, the NOx conversion rises along with the H2

concentration and finally reached over 95%, which is

apparently different from that with CO or C3H6 as the

reducing agent. For CO and C3H6 used as the reducing agent,

no effect from increasing the concentration of CO or C3H6 was

observed in this concentration range, and the release of the

stored NOx is the determining factor. In this case, the reduction

of the stored NOx with H2 or CO is 2.5 or 1.5 times faster than

that with C3H6, respectively, because the NOx conversion is

proportional to the reaction rate in reaction (4).

As indicated in Fig. 1, the release of the stored NOx

needs the reducing agents to locate on the interface between

the precious metals and the NOx storage compounds or on

the surface of the NOx storage compounds. It is well known

that the hydrogen atom spills over from the precious metals

to the oxide supports on catalysts [9–11]. Therefore, the

relative high mobility of the hydrogen atom on the catalyst

surface probably facilitates the regeneration of the NOx

storage sites with the H2 RS which results in its highest

performance.

In Figs. 2 and 5, the momentary sharp jumps of the NOx

concentration upon RS are observed. Even when the Blank-RS

is employed, the maximum NOx concentration in the outlet gas

is also higher than the NOx concentration in the inlet gas

(Fig. 5D). This result indicates that the momentary sharp jump

is caused by the desorption or release of the stored NOx from the

catalyst.

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204 203

Fig. 7. The emitted NOx concentration of the sixth RS at 250 8C vs. the

concentration of the chemical equivalent of the reducing agents in the RS: (*)

H2; (*) CO; (�) C3H6; (~) no reducing agents.

Fig. 8. H2 concentration in the outlet gas vs. time when the lean and rich

conditions were alternately switched every 30 s: ( ) water gas shift reaction;

(—) steam reforming reaction. The dashed line represents the reaction tem-

perature, which decreased at the rate of 10 8C/min.

The difference in the NOx concentration in the outlet gas

between the end point of the lean period and the peak point of

RS, or the sharp jump in the NOx concentration at the switching

point, is noted as the emitted NOx concentration hereafter. This

emitted NOx concentration reveals the actual performance of

the NSR catalyst. Fig. 7 shows the emitted NOx concentration

of the sixth RS versus the concentration of the chemical

equivalent of the reducing agents in the RS. The emitted NOx

concentration can be quite high if the desorbed NOx is not fully

or efficiently reduced to N2, as shown by reaction (5):

NOxþR ! ð1=2ÞN2þROx ðR : reducing agentsÞ (5)

The emitted NOx concentration rises with the increasing

concentration of the chemical equivalent with C3H6 up to

1.0%, while maintaining a somewhat stable value when the

concentration is above 1.0%. The emitted NOx concentration

gradually decreases with the increasing H2 concentration,

while it varies only slightly with CO as the reducing agent.

The effect of the reducing agents on the reduction of the

released NOx is different from each other, and the reactivity

decreases in the order H2 > CO > C3H6. Among the reducing

agents investigated in this study, H2 had the highest activity for

both the release of the stored NOx and the reduction of the

released NOx.

3.3. H2 generation over the NSR catalyst at low

temperatures

The above results showed that H2 is the most active and

efficient reducing agent among H2, CO and C3H6. In addition,

to the inherent properties of these two molecules, is there any

other factor that makes the difference between CO and C3H6?

This will be very important for the optimization of the practical

NSR catalyst. It is well known that H2 could be generated on

catalysts in a rich atmosphere with CO and C3H6. For example,

CO reacted with H2O on the Pt catalyst supported on cerium

oxides to produce H2, i.e., the so-called water gas shift (WGS)

reaction, as shown by reaction (6) [12–15]. Steam reforming

(SR) is another reaction to form H2 through the reaction

between HC and H2O on the Rh catalyst supported on the

thermal-resistant zirconium oxide in our NSR catalyst, as

shown by reaction (7) [5]:

CO þ H2O ! CO2þH2 (6)

C3H6þ 6H2O ! 3CO2þ 9H2 (7)

As described in Section 2.1, the NSR catalyst contains the

necessary compounds and works in the atmosphere for the

WGS and SR reactions. The formed H2 on the NSR catalyst will

facilitate the reduction of the stored NOx, which are one of the

critical factors related to the performance of the NSR catalyst.

Therefore, the H2 generation performance of the WGS and SR

reactions were also investigated.

The lean and rich atmospheres as listed in Table 3 were

alternately and periodically switched every 30 s, and its

temperature was decreased from 400 to 200 8C at the rate of

10 8C/min. Fig. 8 shows the H2 concentration in the outlet gas

versus time. The temperature range was from 280 to 230 8C.

The H2 concentration of the WGS reaction was higher than that

of the SR reaction. Moreover, the response of the increasing H2

concentration with the WGS reaction was faster than that with

the SR reaction, and the difference increased with the

decreasing temperature. Thus, the difference in the H2

generation performance between the WGS and the SR reactions

over the NSR catalyst seems to be one of the reasons why the

stored NOx reduction activity of CO was higher than that of

C3H6 as the RS reducing agent. Therefore, a higher reducing

performance of the stored NOx on the NSR catalyst could be

achieved if the H2 generation activity by the WGS and SR

reactions is enhanced.

4. Conclusions

The NOx storage and the subsequent reduction of the stored

NOx at low temperatures over the NSR catalyst for lean-burn

gasoline engines have been systematically investigated and

discussed in this study. We can conclude that the reduction of

the stored NOx was the restricted stage for the entire NOx

storage and reduction process below 400 8C. At low

temperatures, the activities of the reducing agents to reduce

N. Takahashi et al. / Applied Catalysis B: Environmental 70 (2007) 198–204204

the stored NOx are different and decreases in the order

H2 > CO > C3H6. A part of the NOx storage sites was not

regenerated when an excess of CO or C3H6 was supplied to the

NSR catalysts, while all the NOx storage sites could be fully

regenerated when adequate H2 was supplied. The experimental

results indicated that the release rate of the stored NOx was the

determining factor for the reduction of the stored NOx when CO

or C3H6 was used as the reducing agent. We also found that the

H2 generation by the water gas shift reaction on the NSR

catalysts was more apparent and efficient than that through the

steam reforming reaction. It is regarded as one of the reasons

why the activity of CO to reduce the stored NOx was higher than

that of C3H6. Therefore, the promotion of the H2 generation

activity by the water gas shift reaction is expected to be a

promising approach to improve the performance of the NSR

catalyst at low temperatures.

Acknowledgement

The authors would like to thank Dr. Fei Dong for the

manuscript preparation.

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