JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010, p. 542

Foundation item: Project supported by the National High-Tech Research and Development Program (2009AA064801) and the National Basic Research Program of

China (2010CB732304) supported by the Ministry of Science and Technology of China

Corresponding author: WENG Duan (E-mail: [e-mail]; Tel.: [phone])

DOI: 10.1016/S1002-0721(09)60150-2

NO

x

-assisted soot oxidation over K/CuCe catalyst

WENG Duan (㖕 ッ), LI Jia (ᴢ Շ), WU Xiaodong (ਈᰧϰ), SI Zhichun (ৌⶹ㷶)

(State Key Laboratory of New Ceramics & Fine Process, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China)

Received 9 March 2010; revised 22 April 2010

Abstract: CeO

2

and CuO

x

-CeO

2

supported potassium catalysts were synthesized by wetness impregnation method. The catalysts were char-

acterized by BET, NO-TPO, NO

x

-TPD and soot-TPO measurements. By the decoration of potassium and copper, the maximum soot combus-

tion temperature of the ceria-based catalyst decreased to 338 and 379 °C in the presence and absence of NO under a loose contact mode, re-
spectively. The pronouncedly enhanced NO oxidation ability by copper introduction and NO

x

storage capacity by potassium modification

were especially important in the NO

x

-assisted soot oxidation reaction with the K/CuCe catalyst.

Keywords: K/CuCe; soot combustion; NO

x

; rare earths

The extensive applications of diesel engines worldwide

have aroused many interests in effective after-treatment sys-
tems for the abatement of health and environmental risks.
The catalytic filter is likely the most promising one among
different alternatives proposed, which enables soot particu-
late (one of the main targets in diesel exhaust purifications)
combustion at lower temperatures (300–400 °C)

[1–5]

. How-

ever, the three-phase (soot, catalyst and contaminate gas) re-
action involves in this system has made the elimination of
soot a complex issue. Different ways to favor the contact
between the solid phases, as well as detailed investigations
on reaction mechanisms on how gaseous reactant get in-
volved, are fulfilled to further improve the effectiveness of
this kind of technique

[6–8]

.

CeO

2

and Ceria-based mixed oxides have recently gained

more attention on catalytic soot combustion for the attempt
to utilize active oxygen on the surface of CeO

2

[9–12]

. Their

activity can be promoted by doping transition metals such as
Co

[13,14]

, Mn

[15,16]

and Cu

[17,18]

. The enhanced catalytic activ-

ity is mainly attributed to the good redox properties and
strong interaction between transition metals and cerium

[19,20]

.

Potassium salts have also been widely used for the preferred
high mobility favoring the contact between soot and the
catalysts

[4,21,22]

. Even though the potassium loss at elevated

temperatures is a drawback not easy to avoid, introduction
of network stabilizers and necessary changes in potassium
status may compensate for this problem

[23]

. In our previous

work

[24]

, the K/Cu/Ce catalyst has been proven with high

resistance against sulfur dioxide. In the present work,
Cu-Ce mixed oxides were adopted as an improved support
in the respect to CeO

2

, and the NO adsorption, desorption

behaviors involved in soot combustion were emphasized.

1 Experimental

1.1 Catalysts preparation

The nitrates Cu(NO

3

)

2

·3H

2

O and Ce(NO

3

)

3

·6H

2

O were

mixed according to the molar ratio of Cu:Ce=1:9

[19]

and the

mixture was added dropwise to ammonia solution, which
contained hydrogen peroxide and ammonia in distilled water
according to the volume ratio of 1:4:4. The pH of the mixed
solution was kept at 10 to ensure the complete precipitation
of CuO

x

-CeO

2

mixed oxides. Then the precipitate was fil-

tered, dried, and calcined in static air at 500 °C for 3 h to ob-
tain the mixed oxides sample (CuCe). The ceria support (Ce)
was synthesized by a similar method.

The as-prepared CuCe and Ce support were impregnated

with corresponding amount of KNO

3

solution followed by

calcination at 500 °C for 3 h to obtain the potas-
sium-containing catalysts (K/CuCe and K/Ce). The amount
of potassium loading was 8 wt.% according to Peralta et
al.

[25]

.

1.2 Soot combustion measurements

The activities for soot oxidation were evaluated in a tem-

perature-programmed oxidation (TPO) reaction apparatus.
The catalyst was mixed with soot (Printex-U, Degussa) ac-
cording to a mass ratio of 10:1 using a spatula for 2 min to
produce a loose contact mode, which was more comparable
to practical application. 110 mg of the mixture was packed
between two quartz wool plugged in a tubular quartz reactor.
The experiment was carried out at a heating rate of 20 °C/min

from room temperature (RT) to 600°C in 10% O

2

/N

2

or 1000

ppm NO/10% O

2

/N

2

(500 ml/min). Concentrations of CO

2

and CO in the outlet gases were determined on-line by a

WENG Duan et al., NO

x

-assisted soot oxidation over K/CuCe catalyst

543

five-component analyzer FGA4015 equipped with infrared
sensor. Repeated experiments were performed to testify the
reproducibility of the working system, and the difference of
the maximal soot oxidation rate temperature (T

m

)

was within

10 °C.

1.3 Catalysts characterization

The specific surface areas of the samples were measured

using the N

2

adsorption at –196 ºC by the four-point Brun-

auer-Emmett-Teller (BET) method using an automatic sur-
face analyzer (F-Sorb 3400, Gold APP Instrument). The
samples were degassed in flowing N

2

at 200 ºC for 2 h.

NO temperature-programmed oxidation (NO-TPO) was

performed in a fixed-bed reactor. 250 mg of the catalyst was
applied. The experiment was carried out in 1000×10

–6

NO/10% O

2

/N

2

from RT to 600 ºC at a heating rate of 10

ºC/min. The effluent gaseous NO and NO

2

were monitored

using Nicolet Nexus 380 spectrometer.

NO

x

temperature-programmed desorption (NO

x

-TPD) was

executed on the same apparatus. Prior to the TPD experi-
ment, 250 mg of the sample was treated in 1000×10

–6

NO/10% O

2

/N

2

with a total flow rate of 250 ml/min at 250 and

400 °C for 30 min, respectively. Then the reactor was cooled
down to RT and then flushed by 250 ml/min N

2

for 30 min.

The desorption test was performed in 250 ml N

2

at a heating

rate of 10 ºC/min. NO

2

and NO production during the experi-

ment were monitored by Nicolet Nexus 380 spectrometer.

2 Results and discussion

2.1 NO-TPO

Fig. 1 shows the outlet NO, NO

2

and total NO

x

(NO+NO

2

)

concentrations during the NO-TPO test. From Fig. 1(a) and
(c), the potassium-free catalysts present two NO

x

peaks. The

one centered at 150 °C is ascribed to the kinetically favored
desorption of gaseous NO adsorbed on the surface of the
catalysts

[26]

. The other at higher temperature is corresponding

to the decomposition of surface nitrates/nitrites and NO oxi-
dation to NO

2

. The absence of the lower temperature peak on

the potassium-containing catalysts is due to the decreased
Lewis acidity. The decreased production of NO

2

and ele-

vated NO

2

desorption temperature over K/Ce display the in-

hibition effect by potassium on the ceria support. Contrarily,
the synergistic effect between copper oxide and ceria possi-
bly weakens the effect and thus brings about the effective
NO oxidation and NO

x

adsorption. According to Refs.

[27,28], NO

2

in the reaction gas may facilitate soot oxidation.

So it is expected that the K/CuCe catalyst exhibits a higher
soot oxidation activity than the pure ceria and K/CeO

2

cata-

lysts due to the higher NO oxidation ability enhanced by
copper incorporation.

2.2 NO

x

-TPD

250 and 400 °C are adopted as the typical adsorption tem-

peratures to investigate the NO

x

adsorption/desorption be-

haviors of the catalysts.

The NO

x

-TPD curves of the catalysts pretreated with NO/

O

2

at 250 °C are shown in Fig. 2. Two NO desorption peaks

are observed on the Ce catalyst: the desorption peak at
100–200 °C is related to the desorption of gaseous adsorbed
NO and the other peak at 350–480 °C is ascribed to the
thermodynamic-driven decomposition of nitrated-derived
NO

2

as well as NO

2

dissociation on reducible metal sites

[29]

.

Fig. 1 NO-TPO curves over Ce (a), K/Ce (b), CuCe (c) and K/CuCe catalysts (d)

544

JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010

Only a small amount of NO is desorbed from K/Ce. As is
known that the amount of NO

x

desorbed and the temperature

desorption that occurred are closely related to the number
and type of available sites for NO oxidation and NO

x

ad-

sorption, the absence of gaseous adsorbed NO and decreased
amount of nitrate and nitrite-derived NO indicate a coverage

effect of surface cerium by potassium salts. By the incorpo-
ration of copper, the CuCe mixed oxides show an enhanced
desorption of NO and especially NO

2

with the peak tem-

perature shifting towards 310–450 °C and 250–400 °C, re-
spectively. Similar with the K/Ce catalyst, only NO desorp-
tion is observed on K/CuCe due to the weakened oxidation

Fig. 2 NO

x

-TPD curves over Ce (a), K/Ce (b), CuCe (c) and K/CuCe (d) catalysts (preadsorption at 250 °C)

Fig. 3 NO

x

-TPD curves over Ce (a), K/Ce (b), CuCe (c) and K/CuCe (d) catalysts (preadsorption at 400 °C)

WENG Duan et al., NO

x

-assisted soot oxidation over K/CuCe catalyst

545

ability by introduction of potassium. The NO desorption
peak at 400– 570 °C is attributed to the decomposition of ni-
trates coordinated to Cu-Ce interfacial active sites.

The NO

x

-TPD curves of the catalysts pretreated with NO/

O

2

at 400 °C are shown in Fig. 3 and the calculated amounts

of NO

x

desorbed are present in Table 1. Compared with Fig.

2, the obvious difference of Fig. 3 is that the NO

x

production

from the potassium-containing catalyst is significantly
higher than those pretreated at 250 °C. As indicated in Fig.
1(a), the maximal NO

2

production appears at 400 °C on the

Ce catalyst and hereby much more NO

2

is desorbed as shown

in Fig. 3(a). Different from the small desorption amount after
adsorption at 250 °C, the NO

x

desorbed after preadsorption at

400 °C increases sharply to 304 μmol/g cat. on K/Ce. Simi-
larly, the elevated preadsorption temperature promotes the ad-
sorption of NO

x

on K/CuCe and the release of NO

2

at ca. 350

°C. The NO desorption profile of K/CuCe exhibits a bimodal
shaped peak above 300 °C. The amounts of NO and NO

2

de-

sorbed from K/CuCe are 357 and 45 μmol/g cat., respectively,
which are in accordance with the sum of CuCe and K/Ce
catalyst (341 and 51 μmol/g cat.). Corresponding to the adja-
cent desorption temperatures and the comparable desorption
amounts, the decomposition of nitrates associated to Cu-Ce
and K-Ce interfacial active sites accounts for the desorption
peaks centered at 430 and 550 °C, respectively.

2.3 Soot-TPO

The soot oxidation activities of the catalysts were evalu-

ated at different atmospheres. As listed in Table 2, K/CuCe
shows the highest catalytic activity with the T

m

at 338 °C.

The introduction of NO as the reactant gas results in a shift
of T

m

by 41 °C toward lower temperature on this catalyst.

The assisting effect of NO

x

on different catalysts follows the

order of CuCe>K/CuCe>Ce, which is in accordance with the
NO oxidation ability. On the other hand, the similar T

m

of

K/Ce in the presence and absence of NO demonstrates a lim-
ited utilization of NO

x

during soot oxidation as predicted by

the NO-TPO and NO

x

-TPD results, which is severely re-

stricted by the limited amount of NO

2

production. Thus, the

still high activity of K/Ce may rely largely on the volatile
nature of eutectic potassium component which can effec-
tively improve the poor contact between the catalyst and soot.

Table 1 Desorption of NO

x

from different catalysts based on the

NO

x

-TPD plots

Catalysts

N

NO

/(μmol/g cat.)

N

NO2

/(μmol/g cat.)

N

NOx

/(μmol/g cat.)

Ce

42

42

84

K/Ce 304

7

311

CuCe 37

44

81

K/CuCe 357

45

402

Table 2 T

m

, selectivity and BET surface area of the catalysts

NO/O

2

O

2

Catalysts

T

m

/°C

S

CO2

*

/%

T

m

/°C

S

CO2

*

/%

S

BET

/

(m

2

/g)

Ce 459

96

502

97.0

113

K/Ce 390

92 395 92.2 22

CuCe 404

99 511 99.4 47

K/CuCe 338 96

379 94.6 42

* Calculated by C

CO2

/(C

CO

+C

CO2

) in the outlet gas

This reason also accounts for the relatively low activity of
the potassium-free catalysts.

The selectivity to CO

2

production and the specific surface

area of different catalysts are also listed in Table 2. The
weakened oxidation ability of K/Ce is confirmed by the cor-
responding selectivity value, which is in turn strengthened
by the incorporation with copper. The feature of specific area
is less important due to the smaller pore diameters of the
catalyst (below 10nm) than the diameter of soot particle
(usually above 25nm)

[30]

.

The corresponding TPO curves of the catalysts are shown

in Fig. 4, which highlights the assisting effect of NO

x

on soot

oxidation. Generally, a sharp rise of NO appears simultane-
ously with the ignition of soot with catalysts. This NO pro-
duction resulting from the nitrate-derived NO

2

by soot is

considered as the initial activation for soot ignition. The
exothermic soot oxidation reaction, in turn, accelerates the
further decomposition of nitrates to produce more NO

x

due

to the heat transfer limitations. Ultimately the extensive oxi-
dation of soot by O

2

is initiated. The NO peak intensity fol-

lows the order of K/CuCe§K/Ce>CuCe>Ce. It can be seen
that the majority of NO rise on K/Ce arises from the ther-
mal-driven decomposition of nitrate, which contributes little
to the soot oxidation.

Fig. 4 Outlet CO

2

and corresponding NO concentration plots obtained during TPO runs over Ce and CuCe (a) and K/Ce and K/CuCe (b) catalysts

546

JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010

3 Conclusions

The K/CuCe catalyst synthesized by impregnation exhib-

ited a superior soot oxidation activity with a maximum soot
oxidation temperature at 338 °C in NO/O

2

reaction condition.

The incorporation of copper enhanced the utilization of NO

x

by increasing the NO oxidation ability of ceria catalyst. The
enhanced NO

x

storage capacity by adding potassium com-

ponent, as well as the improved contact between soot and
catalyst, ultimately brought about the pronouncedly de-
creased soot oxidation temperature. Both the NO

2

produced

by NO oxidation and that derived from the decomposition of
nitrates were critical to assist soot oxidation.

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