JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010, p. 542
NOx-assisted soot oxidation over K/CuCe catalyst
WENG Duan ( ), LI Jia ( ), WU Xiaodong ( ), SI Zhichun ( )
(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: CeO2 and CuOx-CeO2 supported potassium catalysts were synthesized by wetness impregnation method. The catalysts were char-
acterized by BET, NO-TPO, NOx-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 NOx storage capacity by potassium modification
were especially important in the NOx-assisted soot oxidation reaction with the K/CuCe catalyst.
Keywords: K/CuCe; soot combustion; NOx; rare earths
The extensive applications of diesel engines worldwide
have aroused many interests in effective after-treatment sys- 1 Experimental
tems for the abatement of health and environmental risks.
The catalytic filter is likely the most promising one among
1.1 Catalysts preparation
different alternatives proposed, which enables soot particu-
The nitrates Cu(NO3)2·3H2O and Ce(NO3)3·6H2O were
late (one of the main targets in diesel exhaust purifications)
mixed according to the molar ratio of Cu:Ce=1:9[19] and the
combustion at lower temperatures (300 400 °C)[1 5]. How-
mixture was added dropwise to ammonia solution, which
ever, the three-phase (soot, catalyst and contaminate gas) re-
contained hydrogen peroxide and ammonia in distilled water
action involves in this system has made the elimination of
according to the volume ratio of 1:4:4. The pH of the mixed
soot a complex issue. Different ways to favor the contact
solution was kept at 10 to ensure the complete precipitation
between the solid phases, as well as detailed investigations
of CuOx-CeO2 mixed oxides. Then the precipitate was fil-
on reaction mechanisms on how gaseous reactant get in-
tered, dried, and calcined in static air at 500 °C for 3 h to ob-
volved, are fulfilled to further improve the effectiveness of
tain the mixed oxides sample (CuCe). The ceria support (Ce)
this kind of technique[6 8].
was synthesized by a similar method.
CeO2 and Ceria-based mixed oxides have recently gained
The as-prepared CuCe and Ce support were impregnated
more attention on catalytic soot combustion for the attempt
with corresponding amount of KNO3 solution followed by
to utilize active oxygen on the surface of CeO2[9 12]. Their
calcination at 500 °C for 3 h to obtain the potas-
activity can be promoted by doping transition metals such as
sium-containing catalysts (K/CuCe and K/Ce). The amount
Co[13,14], Mn[15,16] and Cu[17,18]. The enhanced catalytic activ-
of potassium loading was 8 wt.% according to Peralta et
ity is mainly attributed to the good redox properties and
al.[25].
strong interaction between transition metals and cerium[19,20].
Potassium salts have also been widely used for the preferred
1.2 Soot combustion measurements
high mobility favoring the contact between soot and the
The activities for soot oxidation were evaluated in a tem-
catalysts[4,21,22]. Even though the potassium loss at elevated
perature-programmed oxidation (TPO) reaction apparatus.
temperatures is a drawback not easy to avoid, introduction
The catalyst was mixed with soot (Printex-U, Degussa) ac-
of network stabilizers and necessary changes in potassium
cording to a mass ratio of 10:1 using a spatula for 2 min to
status may compensate for this problem[23]. In our previous
produce a loose contact mode, which was more comparable
work[24], the K/Cu/Ce catalyst has been proven with high
to practical application. 110 mg of the mixture was packed
resistance against sulfur dioxide. In the present work,
between two quartz wool plugged in a tubular quartz reactor.
Cu-Ce mixed oxides were adopted as an improved support
The experiment was carried out at a heating rate of 20 °C/min
in the respect to CeO2, and the NO adsorption, desorption
from room temperature (RT) to 600°C in 10% O2/N2 or 1000
behaviors involved in soot combustion were emphasized.
ppm NO/10% O2/N2 (500 ml/min). Concentrations of CO2
and CO in the outlet gases were determined on-line by a
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
WENG Duan et al., NOx-assisted soot oxidation over K/CuCe catalyst 543
five-component analyzer FGA4015 equipped with infrared Fig. 1 shows the outlet NO, NO2 and total NOx (NO+NO2)
sensor. Repeated experiments were performed to testify the concentrations during the NO-TPO test. From Fig. 1(a) and
reproducibility of the working system, and the difference of (c), the potassium-free catalysts present two NOx peaks. The
the maximal soot oxidation rate temperature (Tm) was within one centered at 150 °C is ascribed to the kinetically favored
10 °C. desorption of gaseous NO adsorbed on the surface of the
catalysts[26]. The other at higher temperature is corresponding
1.3 Catalysts characterization
to the decomposition of surface nitrates/nitrites and NO oxi-
The specific surface areas of the samples were measured dation to NO2. The absence of the lower temperature peak on
using the N2 adsorption at  196 ºC by the four-point Brun- the potassium-containing catalysts is due to the decreased
auer-Emmett-Teller (BET) method using an automatic sur- Lewis acidity. The decreased production of NO2 and ele-
face analyzer (F-Sorb 3400, Gold APP Instrument). The vated NO2 desorption temperature over K/Ce display the in-
samples were degassed in flowing N2 at 200 ºC for 2 h. hibition effect by potassium on the ceria support. Contrarily,
NO temperature-programmed oxidation (NO-TPO) was the synergistic effect between copper oxide and ceria possi-
performed in a fixed-bed reactor. 250 mg of the catalyst was bly weakens the effect and thus brings about the effective
applied. The experiment was carried out in 1000×10 6 NO oxidation and NOx adsorption. According to Refs.
NO/10% O2/N2 from RT to 600 ºC at a heating rate of 10 [27,28], NO2 in the reaction gas may facilitate soot oxidation.
ºC/min. The effluent gaseous NO and NO2 were monitored So it is expected that the K/CuCe catalyst exhibits a higher
using Nicolet Nexus 380 spectrometer. soot oxidation activity than the pure ceria and K/CeO2 cata-
NOx temperature-programmed desorption (NOx-TPD) was lysts due to the higher NO oxidation ability enhanced by
executed on the same apparatus. Prior to the TPD experi- copper incorporation.
ment, 250 mg of the sample was treated in 1000×10 6
2.2 NOx-TPD
NO/10% O2/N2 with a total flow rate of 250 ml/min at 250 and
400 °C for 30 min, respectively. Then the reactor was cooled 250 and 400 °C are adopted as the typical adsorption tem-
down to RT and then flushed by 250 ml/min N2 for 30 min. peratures to investigate the NOx adsorption/desorption be-
The desorption test was performed in 250 ml N2 at a heating haviors of the catalysts.
rate of 10 ºC/min. NO2 and NO production during the experi- The NOx-TPD curves of the catalysts pretreated with NO/
ment were monitored by Nicolet Nexus 380 spectrometer. O2 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
2 Results and discussion
NO and the other peak at 350 480 °C is ascribed to the
thermodynamic-driven decomposition of nitrated-derived
2.1 NO-TPO
NO2 as well as NO2 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 effect of surface cerium by potassium salts. By the incorpo-
known that the amount of NOx desorbed and the temperature ration of copper, the CuCe mixed oxides show an enhanced
desorption that occurred are closely related to the number desorption of NO and especially NO2 with the peak tem-
and type of available sites for NO oxidation and NOx ad- perature shifting towards 310 450 °C and 250 400 °C, re-
sorption, the absence of gaseous adsorbed NO and decreased spectively. Similar with the K/Ce catalyst, only NO desorp-
amount of nitrate and nitrite-derived NO indicate a coverage tion is observed on K/CuCe due to the weakened oxidation
Fig. 2 NOx-TPD curves over Ce (a), K/Ce (b), CuCe (c) and K/CuCe (d) catalysts (preadsorption at 250 °C)
Fig. 3 NOx-TPD curves over Ce (a), K/Ce (b), CuCe (c) and K/CuCe (d) catalysts (preadsorption at 400 °C)
WENG Duan et al., NOx-assisted soot oxidation over K/CuCe catalyst 545
ability by introduction of potassium. The NO desorption
Table 1 Desorption of NOx from different catalysts based on the
peak at 400 570 °C is attributed to the decomposition of ni-
NOx-TPD plots
trates coordinated to Cu-Ce interfacial active sites.
Catalysts NNO/(źmol/g cat.) NNO2/(źmol/g cat.) NNOx/(źmol/g cat.)
The NOx-TPD curves of the catalysts pretreated with NO/
Ce 42 42 84
O2 at 400 °C are shown in Fig. 3 and the calculated amounts
K/Ce 304 7 311
of NOx desorbed are present in Table 1. Compared with Fig.
CuCe 37 44 81
2, the obvious difference of Fig. 3 is that the NOx production
K/CuCe 357 45 402
from the potassium-containing catalyst is significantly
higher than those pretreated at 250 °C. As indicated in Fig.
Table 2 Tm, selectivity and BET surface area of the catalysts
1(a), the maximal NO2 production appears at 400 °C on the
Catalysts NO/O2 O2 SBET/
Ce catalyst and hereby much more NO2 is desorbed as shown
Tm/°C SCO2*/% Tm/°C SCO2*/% (m2/g)
in Fig. 3(a). Different from the small desorption amount after
Ce 459 96 502 97.0 113
adsorption at 250 °C, the NOx desorbed after preadsorption at
400 °C increases sharply to 304 źmol/g cat. on K/Ce. Simi- K/Ce 390 92 395 92.2 22
larly, the elevated preadsorption temperature promotes the ad- CuCe 404 99 511 99.4 47
K/CuCe 338 96 379 94.6 42
sorption of NOx on K/CuCe and the release of NO2 at ca. 350
°C. The NO desorption profile of K/CuCe exhibits a bimodal * Calculated by CCO2/(CCO+CCO2) in the outlet gas
shaped peak above 300 °C. The amounts of NO and NO2 de-
sorbed from K/CuCe are 357 and 45 źmol/g cat., respectively, This reason also accounts for the relatively low activity of
which are in accordance with the sum of CuCe and K/Ce the potassium-free catalysts.
catalyst (341 and 51 źmol/g cat.). Corresponding to the adja- The selectivity to CO2 production and the specific surface
cent desorption temperatures and the comparable desorption area of different catalysts are also listed in Table 2. The
amounts, the decomposition of nitrates associated to Cu-Ce weakened oxidation ability of K/Ce is confirmed by the cor-
and K-Ce interfacial active sites accounts for the desorption responding selectivity value, which is in turn strengthened
peaks centered at 430 and 550 °C, respectively. by the incorporation with copper. The feature of specific area
is less important due to the smaller pore diameters of the
2.3 Soot-TPO
catalyst (below 10nm) than the diameter of soot particle
The soot oxidation activities of the catalysts were evalu- (usually above 25nm)[30].
ated at different atmospheres. As listed in Table 2, K/CuCe The corresponding TPO curves of the catalysts are shown
in Fig. 4, which highlights the assisting effect of NOx on soot
shows the highest catalytic activity with the Tm at 338 °C.
The introduction of NO as the reactant gas results in a shift oxidation. Generally, a sharp rise of NO appears simultane-
of Tm by 41 °C toward lower temperature on this catalyst. ously with the ignition of soot with catalysts. This NO pro-
duction resulting from the nitrate-derived NO2 by soot is
The assisting effect of NOx on different catalysts follows the
order of CuCe>K/CuCe>Ce, which is in accordance with the considered as the initial activation for soot ignition. The
NO oxidation ability. On the other hand, the similar Tm of exothermic soot oxidation reaction, in turn, accelerates the
K/Ce in the presence and absence of NO demonstrates a lim- further decomposition of nitrates to produce more NOx due
ited utilization of NOx during soot oxidation as predicted by to the heat transfer limitations. Ultimately the extensive oxi-
the NO-TPO and NOx-TPD results, which is severely re- dation of soot by O2 is initiated. The NO peak intensity fol-
lows the order of K/CuCe K/Ce>CuCe>Ce. It can be seen
stricted by the limited amount of NO2 production. Thus, the
still high activity of K/Ce may rely largely on the volatile that the majority of NO rise on K/Ce arises from the ther-
nature of eutectic potassium component which can effec- mal-driven decomposition of nitrate, which contributes little
tively improve the poor contact between the catalyst and soot. to the soot oxidation.
Fig. 4 Outlet CO2 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
ble nitrates stored on BaCoCe in soot catalytic oxidation. Ca-
3 Conclusions
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