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: duanweng@tsinghua.edu.cn; Tel.: +86-10-62772726) 

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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