Chemical Engineering Journal 226 (2013) 105–112

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Preparation of MnOx–CeO2–Al2O3 mixed oxides for NOx-assisted sootoxidation: Activity, structure and thermal stability

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.04.006

⇑ Corresponding author. Tel./fax: +86 10 62792375.E-mail address: wuxiaodong@tsinghua.edu.cn (X. Wu).

Fan Lin a, Xiaodong Wu a,⇑, Shuang Liu a, Duan Weng a, Yuying Huang b

a Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, Chinab Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China

h i g h l i g h t s

� Alumina is introduced to MnOx–CeO2

at different stages of a sol–gelmethod. Manganese species in themixed oxides are identified as Mn3O4

by EXAFS. MnOx–CeO2/Al2O3 showsgood soot oxidation activity after asevere thermal aging.� The soot–NO2 reaction dominates the

soot conversion at low temperatures.� The NO2 productivity is determined

by the active surface area andNO M NO2 equilibrium.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2012Received in revised form 9 March 2013Accepted 2 April 2013Available online 11 April 2013

Keywords:MnOx–CeO2–Al2O3

Diesel soot oxidationNO2 productionMetal oxide sinteringActive surface area

a b s t r a c t

Three kinds of MnOx–CeO2–Al2O3 mixed oxides were prepared by introducing Al2O3 to MnOx–CeO2 at dif-ferent stages of a sol–gel method. MnCeAl and MnCe/Al were received by adding aluminum nitrate andalumina powders to the Mn–Ce precursor solution, respectively, while MnCe + Al was obtained by mixingthe sol–gel-synthesized MnOx–CeO2 and Al2O3 powders mechanically. The catalysts were calcined in airat 800 �C for 20 h to obtain the thermally aged samples. The catalysts were characterized by soot and NOtemperature-programmed oxidation (soot-TPO and NO-TPO), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), extended X-ray absorption fine structure (EXAFS), energy dispersive spectroscopy(EDS) and H2 temperature-programmed reduction (TPR). The introduction of Al2O3 degrades the soot oxi-dation activity of the fresh catalysts compared with MnOx–CeO2 mixed oxides, but it helps to maintainrelatively high catalyst surface area and high MnOx dispersion after the rigorous aging treatment. Amongthe catalysts prepared, MnCe/Al exhibits the highest thermal stability with a small increase of Tm by 17 �Cfor soot oxidation after aging. The active surface area, which refers to the exposure area of MnOx on thecatalyst, is suggested to determine the NO oxidation and soot oxidation activities of the aged catalysts.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction eliminate PM is using a catalytic filter to trap and oxidize it. With

Diesel engines, which have good power performance and highfuel efficiency, are a promising power source for light vehicleswhich to date are still driven majorly by gasoline engines. How-ever, their emission problems should be solved before the wide uti-lization. One of major pollutants in diesel exhaust is particulatematter (PM) mainly composed of soot [1,2]. An efficient way to

the help of catalysts, the soot oxidation temperature can be signif-icantly decreased [3].

MnOx–CeO2 mixed oxides show high catalytic activities for die-sel soot oxidation in both excess O2 [4,5] and NO + O2 [6,7], due tothe synergistic effect between ceria and manganese oxide. How-ever, a serious drawback of this mixed oxide catalyst is the poorthermal stability resulting from the phase separation and sinteringof metal oxides at the calcination temperature above 500 �C [8].The BET surface area of MnOx–CeO2 drops to 8 m2/g and bulkMn3O4 particles begin to form at 650 �C, resulting in a severe deac-

106 F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112

tivation of the catalyst [9]. To date, however, few papers have fo-cused on improving the thermal stability of MnOx–CeO2 mixedoxides.

Alumina has been widely used as a carrier for catalysts becauseof its high surface area, excellent thermal stability and good adhe-sion to ceramic supports. It has been reported that the addition ofAl2O3 improves the oxygen storage capacity and thermal stabilityof CeO2–ZrO2 solid solutions [10–13]. Marikawa et al. [12] pro-posed that Al2O3 acts as a diffusion barrier to suppress the sinter-ing of CeO2–ZrO2 and hereby improve the durability at hightemperatures. Wang et al. [13] found that the sample by co-precip-itation of the Ce3+, Zr4+ and Al3+ precursors exhibits the superioroxygen storage capacity after aging. Although Al2O3 supportedtransition metal–cerium mixed oxides catalysts have been widelystudied for various applications [14–17], the stabilization effectof Al2O3 on MnOx–CeO2 catalysts is rarely addressed. In our previ-ous work [18], the addition of Al2O3 powders was found to enhancethe thermal stability of MnOx–CeO2 catalysts effectively by provid-ing a high surface area and preventing the sintering of manganeseoxide and ceria. However, the detailed mechanism of stabilizationeffect of alumina was not clarified. In this work, Al2O3 was intro-duced to MnOx–CeO2 mixed oxides at different stages of a sol–gel process to investigate the effects of mixing strategies on thethermal stability of the catalysts. The dependency of catalyticactivity on the surface area of active metal oxides was discussed.

2. Experimental

2.1. Catalyst preparation

MnOx–CeO2 mixed oxides (MnCe), with a Mn:Ce atom ratio of15:85, were synthesized by a citric acid-aided sol–gel method asdescribed in [7]. The metal ion precursors were Ce(NO3)3�6H2O(Yili, China) and C4H6MnO4�4H2O (Chemical Reagents, China). Theobtained solid gel was calcined at 550 �C for 3 h.

Al2O3 was introduced into MnOx–CeO2 in different ways toobtain three types of MnOx–CeO2–Al2O3 catalysts. All the sampleshad a Mn:Ce atom ratio of 15:85 and a weight ratio of(Mn2O3 + CeO2):Al2O3 = 2:1. The calcination condition was thesame as that of MnCe. MnCeAl was synthesized by adding Al(NO3)3

(Xiandaidongfang, China) into the Mn–Ce precursor solution.MnCe/Al was prepared by adding commercial c-Al2O3 (Weifu, Chi-na, SBET = 140 m2/g) to the Mn–Ce precursor solution. MnCe + Alwas prepared by mixing the as-received MnCe and Al2O3 powdersmechanically via ball milling for 30 min.

All the as-received samples were assigned to a label of ‘‘-f’’.After aging at 800 �C for 20 h in a dry air flow (200 ml/min) to ob-tain the aged catalysts labeled with a suffix of ‘‘-a’’.

2.2. Activity measurement

The NO temperature-programmed oxidation (TPO) tests werecarried out in a fixed-bed reactor with the effluent gases monitoredby an infrared spectrometer (Thermo Nicolet iS10). 100 mg of cat-alyst powders were diluted with 300 mg of silica pellets and thenwere sandwiched by quartz wools in a tubular quartz reactor.The gas mixture of 1000 ppm NO/10% O2/N2 (500 ml/min) wasfed with a gas hourly space velocity (GHSV) of 30,000 h�1. Thereactor temperature was ramped to 650 �C at a heating rate of10 �C/min. The temperature at which the maximal NO2 concentra-tion was attained in the outlet gas was denoted as Tm,NO2.

The activities of the catalysts for soot oxidation were evaluatedin the same reaction apparatus to that used in NO-TPO tests. Prin-tex-U (Degussa) was used as a model soot. Its particle size was25 nm and the specific surface area was 100 m2/g. 10 mg of soot

and 100 mg of catalyst powders were mixed by a spatula for2 min for ‘‘loose contact’’ conditions, and then the mixture was di-luted with 300 mg of silica pellets (0.3–0.7 mm in size) to reducethe specific pressure drop across the reactor and prevent reactionrunaway [19–21]. If the soot-catalyst mixture is undiluted, themass and heat transfer limitations will turn to be predominant,which are not typical for practical supported catalysts and couldlead to a result of misinterpretation [7,22–24]. The inlet gas mix-ture was 1000 ppm NO/10% O2/N2 (500 ml/min). Tm representedthe maximal soot oxidation rate temperature. The downstreamCO2/(CO2 + CO) ratio during soot oxidation was defined as theselectivity to CO2 (SCO2).

2.3. Catalyst characterization

The powder X-ray diffraction (XRD) patterns were collected bya Japan Science D/max-RB diffractometer using Cu Ka radiation(k = 0.15418 nm). The X-ray diffractograms were recorded at0.02� intervals in the range of 20� 6 2h 6 80� with a scanningvelocity of 4�/min.

The X-ray absorption data were recorded at room temperatureat beam line BL14W1 of the Shanghai Synchrotron Radiation Facil-ity (SSRF), China. Beamline was equipped with a double-crystal Si(111) monochromator. The commercial MnO, Mn3O4, MnO2 andCeO2 were used as standard reference samples. The extended X-ray absorption fine structure (EXAFS) spectra at the Mn K-edge(Ce LIII-edge for CeO2) were measured in the transmission mode.During the measurement, the synchrotron was operated at energyof 3.5 GeV. The scanning range was from 6390 to 7330 eV for MnK-edge and 5570 to 6120 eV for Ce LIII-edge, respectively. Data pro-cessing was performed using the program ATHENA.

The specific surface areas of the samples were measured usingthe N2 adsorption at �196 �C by the four-point Brunauer–Em-mett–Teller (BET) method using an automatic surface analyzer(F-Sorb 3400, Gold APP Instrument). The samples were degassedin vacuum at 200 �C for 2 h before the measurement.

The percentage of the metal elements on the catalyst surface re-gion was determined by energy dispersive spectroscopy (EDS)attachment of the JEOL JSM-6460LV SEM system. The scan areaswere about 500 lm � 500 lm, and the window integrals (IMn, ICe

and IAl) were the basis for the semi-quantitative mass determina-tion of Mn and Ce.

The H2 temperature-programmed reduction (TPR) tests werecarried out on Miromeritics AutoChem II 2920 with a thermal con-ductivity detector (TCD). About 50 mg of the sample was used ineach test. Before the test, the sample was pre-oxidized in 1% O2/He at 500 �C for 30 min. The reactor temperature was ramped from50 to 920 �C at a heating rate of 10 �C/min in 10% H2/Ar (50 ml/min).

3. Results

3.1. Catalytic activities for NO oxidation and soot oxidation

Diesel exhaust contains generally both soot and NOx as two ma-jor pollutants. NO2 is a stronger oxidant than O2 for soot oxidation,and acts as an intermediate to transport active oxygen to soot inthe presence of NOx and O2 [1,25]. The NO oxidation activities ofthe catalysts were measured, and the results are shown in Fig. 1and Table 1. The dash line in the figures represents the theoreticequilibrium NO2 concentration derived from 1000 ppm NO in ex-cess O2. The Tm,NO2 follows the order of MnCe-f < MnCe/Al-f < MnCe + Al-f < MnCeAl-f. After aging, the sequence of Tm,NO2

turns to be MnCe/Al-a < MnCe + Al-a < MnCeAl-a < MnCe-a. Theseresults indicate the superior thermal stability of the Al2O3-contain-

Fig. 1. Evolutions of NO2 during NO-TPO tests with the (a) fresh and (b) agedcatalysts.

F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112 107

ing catalysts (especially MnCe/A) for NO oxidation than MnOx–CeO2 mixed oxides.

Fig. 2 shows the evolutions of the downstream COx (CO2 + CO)and NO2 during the soot-TPO tests. Among the catalysts investi-gated, MnCe/Al-a shows an outstanding thermal stability with asmall Tm increase of 17 �C. Meanwhile, MnCe-a is severely de-graded with the Tm shifting toward to higher temperature by102 �C. The CO2 selectivity of MnCe-a and MnCeAl-a decreases be-low 90% as shown in Table 1, indicating that partial oxidation ofsoot becomes an obvious reaction over these catalysts.

Table 1Catalytic activities of the catalysts for NO oxidation and soot oxidation.a

Catalysts NO oxidation Soot oxidation inNO + O2

Soot oxidation in 10%O2

Tm,NO2

(�C)DTm,NO2

(�C)bTm

(�C)SCO2

(%)DTm

(�C)bTm

(�C)SCO2

(%)DTm

(�C)b

MnCeAl 380(400)

20 462(506)

96(84)

44 560(565)

96(88)

5

MnCe/Al 330(350)

20 431(448)

99(97)

17 560(557)

99(97)

�3

MnCe + Al 340(378)

38 442(480)

98(97)

36 568(568)

98(96)

0

MnCe 320(447)

127 418(520)

99(89)

102 482(555)

100(93)

73

Uncatalyzed – – 587 71 – 587 82 –

a The values in parenthesis represent the data of the corresponding aged samples.b 4Tm,NO2 = Tm,NO2,aged–Tm,NO2,fresh and 4Tm = Tm,aged–Tm,fresh.

The soot oxidation activity of both the fresh and aged catalystsfollows the same order of the NO oxidation activity, implying astrong dependency on the NO2 production ability. As shown inFig. 2b and d, the downstream NO2 concentrations during thesoot-TPO test are much lower than those produced in the NO-TPO tests, and rise back to the equilibrium level gradually afterthe soot oxidation reaches the maximum rate. It implies the con-sumption of NO2 by reaction with soot at low temperatures andthe competition from the soot–O2 reaction at high temperatures.In the latter part of soot oxidation process, oxygen becomes thedominant oxidant and the active oxygen-assisted soot combustionplays an important role. The mechanism of NO2-assisted soot oxi-dation will be discussed later.

The soot-TPO tests were also carried out in 10% O2 to evaluatethe activities of the catalysts in the absence of NO. The COx profilesare shown in Fig. 3, and the data are summarized in Table 1.Oxygen spillover effect is critical to this reaction. It has been pro-posed that the activated oxygen spills over from the catalyst tothe adjacent soot particles to react with carbon [26,27]. Thus, thesurface content and redox property of active metal oxides (CeO2

and especially MnOx) determines the reactivity. The introductionof inert Al2O3 decreases the surface contents of active metal oxidessignificantly and hereby severely degrades the soot oxidationactivities of the fresh Al2O3-containing catalysts in excess O2, caus-ing the Tm of all the Al2O3-containing catalysts close to that of theuncatalyzed soot combustion (Table 1). It also indicates that it isthe Al2O3 carrier rather than the SiO2 diluents affecting the contactbetween the active sites and soot. The activity of MnCe-a decreasesdramatically because of the severe loss of the catalyst surface areaand sintering of metal oxides. It seems that the main contributionof the Al2O3-containing catalysts and the aged MnOx–CeO2 is tooxidize CO–CO2, resulting in high CO2 selectivity compared to theuncatalyzed reaction.

3.2. Structural and textural properties

Fig. 4 shows the powder XRD patterns of the catalysts, and Ta-ble 2 lists the structural parameters. All the fresh samples exceptMnCeAl-f present typical diffraction peaks of fluorite-like struc-tural CeO2 phase. MnCeAl-f displays weak diffraction peaks, indi-cating a low crystallinity of ceria-based mixed oxides. It iscaused by the doping of Al3+ precursor which hindered the crystal-lization of CeO2 within the calcination. No peaks of manganese oxi-des are observed, suggesting a high dispersion of MnOx on the freshsamples. All the samples experienced remarkable sintering ofmetal oxides during aging. It is clearly shown in Table 2 that thealumina mixed mechanically does not inhibit the sintering of ceria,while the alumina powder support prevents the metal oxide sin-tering to some extent. The alumina serves as the thermal diffusionbarrier and effectively inhibits the growth of ceria crystallites,resulting in the smallest CeO2 crystallite size for MnCeAl-a. Thecharacteristic peak of bulk manganese oxides, ascribed to Mn3O4,is only observed in the diffraction pattern of MnCe-a, indicatingrelatively high dispersions of MnOx on those aged Al2O3-modifiedsamples. The specific surface area of MnCe-a decreases dramati-cally, while the addition of Al2O3 powders maintains MnCe/Al-aand MnCe + Al-a relatively high surfaces areas. On the other hand,the Al3+ doping method appears to contribute less to stabilizing thesurface area of MnCeAl-a, probably related to the unsatisfactorythermal stability of the home-made alumina.

As shown in Table 2, the lattice constants of ceria in the mixedoxides are smaller than that of CeO2 (ca. 0.5408 nm). Some re-searches ascribed the shrinkage of the unit cell to the replacementof Cex+ (ionic radii:Ce4+:0.097 nm; Ce3+:0.114 nm) by Mnx+ ions(Mn3+:0.065 nm; Mn2+:0.083 nm) to form the solid solutions [28–31]. Another possible reason is the oxidation of Ce3+–Ce4+ via inter-

Fig. 2. Evolutions of COx and NO2 during the soot-TPO tests with the (a and b) fresh and (c and d) aged catalysts in 1000 ppm NO/10% O2/N2.

108 F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112

acting with manganese oxide. Larachi et al. found using XPS thatthe addition of manganese to cerium generally increases the abun-dance of surface Ce4+ species because of the electron transfer fromCe to Mn [32]. Thus, the incorporation of manganese oxide into theceria lattice still remains controversial. Although many researchersbelieved that homogenous MnOx–CeO2 solid solutions are formed[6,28–31], there is also an argument by the oxygen isotherms thatthe mixed oxides of cerium and manganese do not form solid solu-tions [33]. In this study, the state of MnOx in the mixed oxides wasidentified by EXAFS. Fig. 5 shows the Fourier transform (FT) EXAFSspectra of the catalysts and references oxides. The FT spectra forMn in the fresh and aged mixed oxides samples are quite differentfrom that of CeO2, indicating that Mnx+ ions are not incorporatedinto the ceria lattice. Actually, all the catalysts present similar FTspectra which are close to that of Mn3O4. Combining with theXRD results which reveal the Mn3O4 phase on MnCe-a, it is plausi-ble to believe that manganese species exist mainly in the form ofMn3O4 in the mixed oxides, regardless as highly dispersed clustersor sintered crystallites.

The introduction of inert Al2O3 would inevitably decrease thesurface contents of active components (Mnx+ and less importantCex+) on the catalysts. Since the catalytic reactions mainly takeplace on the surface of the catalyst, only those surface active sitesare considered functional. The content and dispersion of activecomponents on the catalyst surface would be influenced by themixing way of alumina. Thus, the contents of different metals inthe outer layer of catalyst particles were detected by EDS, andthe results are listed in Table 2. The detection depth of EDS was

about 1 lm and the diameters of most aggregated catalyst parti-cles were larger than 5 lm in SEM images (not shown), so the datacould be used to estimate the element contents in the outer layerof catalyst particles though not on the exact external surface. Thenominal contents of Mn and Ce are 15% and 85% for MnCe, and6% and 34% for the Al2O3-modified samples, respectively. The de-tected Mn/Ce atomic ratios on MnOx–CeO2 are in accordance withthe nominal contents, indicating the homogeneous distributions ofMn and Ce in the mixed oxides. As expected, the mixing way ofAl2O3 does affect the element distributions. MnCe/Al exhibits thehighest atomic ratios of Mn and Ce, because they tend to depositon the surface of Al2O3 powders during the impregnation process.MnCe + Al has the lowest active metal contents in the outer layer ofcatalyst particles, which may be caused by the coverage of MnOx–CeO2 by the mechanically mixed alumina powders.

3.3. H2–TPR

The redox properties of the pre-oxidized catalysts were evalu-ated by H2–TPR and the results are shown in Fig. 6. As shown inFig. 6a, the low-temperature (<200 �C) peak appearing on most ofthe fresh samples is ascribed to the reduction of surface oxygenchemisorbed in the oxygen vacancies which are promoted by thesynergistic effect between ceria and manganese oxide. This reduc-tion peak is small on MnCeAl-f despite of its high surface area. It islikely caused by the Al3+ doping which leads to the low crystallinityof ceria and manganese oxide. The peaks at 200–450 �C are as-cribed to the successive reduction of MnO2/Mn2O3–Mn3O4,

Fig. 3. Evolutions of COx during the soot-TPO tests with the (a) fresh and (b) agedcatalysts in 10% O2/N2.

Fig. 4. XRD patterns of the (a) fresh and (b) aged samples: (1) MnCeAl, (2) MnCe/Al,(3) MnCe + Al and (4) MnCe.

F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112 109

Mn3O4–MnO and surface oxygen removal of ceria. The peaks at500–800 �C and temperatures above 800 �C are ascribed to thereduction of bulk oxygen of ceria promoted by MnOx and that ofpure ceria, respectively. These high-temperature peaks appear tobe less important for the catalytic oxidation of soot in O2. Thelow-temperature (<200 �C) disappears on most of the aged sam-ples due to the sintering of metal oxides in Fig. 6b. Nevertheless,MnCe/Al-a preserves some low-temperature reducibility, withbroad reduction peaks at 125–400 �C ascribed to the reduction ofmultiple oxidation states of manganese. The relatively high NOoxidation activity of MnCe/Al-a seems to be associated with theavailability of readily reducible oxygen at low temperatures.

4. Discussion

4.1. Relationship between NO oxidation ability and soot oxidationactivity

According to the literature [25], the catalytic oxidation of sootat the temperatures below 450 �C can be empirically ascribed tothe assistance of NO2. It is confirmed by the uncatalyzed soot oxi-dation in NO2 + O2 in Fig. 7. The total COx (CO + CO2) productionequals to the NO2 consumption approximately at the temperaturesbelow 385 �C. It is plausible to speculate that soot oxidation beginswith C + NO2 reaction which produces surface oxygenated C(O) viaEqs. (1) and (2) [34]. Afterwards, the C(O) can either decompose torelease CO (Eq. (3)) or be oxidized by O2 to produce CO2 (Eq. (4)). Inother words, NO2 only takes part in the first two steps which form

the surface oxygenated species C(O). Whether CO or CO2 being pro-duced depends on the reaction between C(O) and O2. Therefore,one NO2 molecule consumption leads to one CO or CO2 moleculeformation at the beginning of the soot oxidation, which is evi-denced by the evolution of NO2 and COx at the temperatures lowerthan 385 �C, with a ratio close to 1. At higher temperatures, O2

becomes the predominant oxidant with the complete consumptionof NO2 by reacting with soot and by the thermal decomposition. Itshould be noted that such a simple reaction mechanism requiresfurther modification and more evidences

Cþ NO2 ! CðONOÞ ð1Þ

CðONOÞ ! CðOÞ þ NO ð2Þ

CðOÞ þ O2 ! CO2 ð3Þ

CðOÞ ! CO ð4Þ

4.2. Role of active surface area in NO oxidation

In our previous researches [9,18], the catalyst surface area isfound to be critical to NO2 productivity of MnOx–CeO2 mixed oxi-des. The introduction of Al2O3 serves to increase the surface area ofthe catalyst. However, the surface Al2O3 does not contribute to thecatalytic oxidation of NO. MnOx species interacted with ceria areconsidered to be the main active sites on the mixed oxides cata-lysts [9,18]. Introducing Al2O3 at different catalyst preparation

Table 2Solid features of the fresh and aged samples.a

Samples db (nm) Dc (nm) SBET (m2/g) Atomic ratio in the outer layer of catalyst particlesd (%)

Mn Ce Al

MnCeAl – (0.5407) – (5.5) 67 (11) 7.0 (6.4) 37.1 (38.3) 55.9 (55.3)MnCe/Al 0.5400 (0.5408) 7.0 (15) 128 (42) 7.5 (7.4) 39.6 (38.3) 52.9 (54.3)MnCe + Al 0.5402 (0.5407) 5.2 (19) 73 (34) 5.4 (5.6) 32.1 (31.6) 62.5 (62.8)MnCe 0.5402 (0.5407) 5.2 (19) 80 (0.5) 15.6 (15.6) 84.4 (84.4) –

a The values in parenthesis represent the corresponding data of the aged samples.b Lattice constants of CeO2 calculated by Cohen’s method.c Mean crystallite sizes of CeO2 calculated according to Williamson–Hall equation.d Measured by EDS and expressed in the ratio of M (M = Mn, Ce and Al) to total cations.

Fig. 5. Fourier transformed k-weighted EXAFS oscillation of Mn K-edge (except CeLIII-edge for CeO2) for (a) the fresh, (b) aged and (c) reference samples.

Fig. 6. H2-TPR profiles of the (a) fresh and (b) aged catalysts.

110 F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112

stages results in different concentrations of surface MnOx on thecatalysts. Therefore, it is necessary to quantify the ‘‘active surface’’on different catalysts. In order to estimate the exposed area of sur-face manganese species, the weighted surface area occupation ofeach cation (Mnx+, Ce4+ and Al3+) together with its coordinated oxy-gen atoms could be estimated by the following equation:

Warea;M ¼MM þMO � y

x

qMxOy

ð5Þ

where Warea;M was the weighted surface area occupation of oxide ofmetal M; MM and MO were the atomic weight of metal M and oxygen,respectively; and qMxOy

was the density of the oxide MxOy. It wasassumed that the metals existed mainly in the form of Mn3O4, CeO2

and Al2O3, respectively, and the densities from PDF database

Fig. 7. Profiles of the uncatalyzed soot-TPO in 600 ppm NO2/10% O2/N2.

Fig. 8. Dependence of NO2 concentration in NO-TPO and COx concentration in soot-TPO at 350 �C on the active surface area of the catalysts.

F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112 111

(qMn3O4¼ 4:88 g=cm3, qCeO2

¼ 7:21 g=cm3;qAl2O3¼ 3:67 g=cm3Þwere

used to obtain the weighted surface area occupation ofWarea;Mn : Warea;Ce : Warea;Al ¼ 65 : 7:1 : 5:1: The surface area occupiedby MnOx was used to represent the so-called ‘‘active surface area’’on a catalyst, which was calculated via Eq. (6).

Sactive ¼ SBET �CMn �Warea;Mn

CMn �Warea;Mn þ CCe �Warea;Ce þ CAl �Warea;Alð6Þ

where Sactive was the active surface area; SBET was the BET surfacearea of catalyst; CMn, CCe and CAl were the surface atomic ratio(detected by EDS) of Mn, Ce and Al, respectively. Since Ce and Alare the predominant metal components as shown in Table 2,Sactive is mainly determined by the BET surface area of catalyst andthe surface atomic ratio of Mn.

As shown in Fig. 2, at a temperature such as 350 �C, most of thecatalysts exhibit a notable soot oxidation activity in NO + O2 buthave almost no activity in O2. It implies that the soot oxidationactivity of the catalysts depends strongly on their NO2 productionability at this temperature. Meanwhile, the NO2 production at350 �C over the fresh catalysts is determined by the theoretic equi-librium of NO M NO2 redox cycle in excess O2 according to the NO-TPO results in Fig. 1a. That is, the NO oxidation activity of the freshcatalysts except MnCeAl-f is under thermodynamic control. Theunexpected lower NO oxidation activity of MnCeAl-f is ascribedto its poor redox property due to the low crystallinity of metal oxi-des. In other words, the active sites on this sample are less active

than those on the other samples. Comparatively, the NO oxidationactivity of the aged catalysts at 350 �C is more likely obtained un-der kinetic controlled regime as shown in Fig. 1b. Thus, the NO2

concentration (CNO2) in the NO-TPO and the COx concentration(CCOx) in the soot-TPO at 350 �C were plotted versus the active sur-face area on different catalysts in Fig. 8. It can be seen that, for theaged catalysts, both the NO oxidation and soot oxidation activitiesincrease quasi linearly with the active surface area. These trendsdemonstrate the decisive role of the active surface area in theNO2 production and hereby the NO2-assisted soot oxidation. It isalso noted that the lines do not pass through the origin, whichshould be related to the thermal equilibrium of NO M NO2 redoxcycle and the existence of those readily oxidizable complexes onthe surface of soot.

5. Conclusions

Three kinds of Al2O3-modified MnOx–CeO2 mixed oxides wereprepared by different mixing ways: Al3+ doping at the precursorstage, MnOx–CeO2 deposition on Al2O3 powders and the mechani-cally mixture of the oxide powders. Manganese species in themixed oxides are identified to be Mn3O4 by EXAFS as the main ac-tive sites with interacting with ceria. Deposition of MnOx–CeO2 onAl2O3 powders is found to be an appropriate modification methodthat preserves the synergetic Mn–Ce effect and relatively high sur-face area of catalyst after aging at 800 �C for 20 h, with a smallincrease of Tm (17 �C) for soot oxidation in the presence of NOand O2. Such a modification is of great importance in real condi-tions. For the aged catalysts, the soot–NO2 reaction dominatesthe soot conversion at a low temperature such as 350 �C. In thiscase, the soot oxidation activity of catalysts depends linearly onthe NO2 productivity which is determined by the active surfacearea.

Acknowledgements

The authors would like to acknowledge Projects 51072096 sup-ported by National Natural Science Foundation of China and2010CB732304, 2013AA061902 by the Ministry of Science andTechnology of China.

References

[1] B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Science and technology of catalyticdiesel particulate filters, Catal. Rev. 43 (2001) 489–564.

[2] D. Fino, Diesel emission control: Catalytic filters for particulate removal, Sci.Technol. Adv. Mat. 8 (2007) 93–100.

[3] S. Liu, X.D. Wu, D. Weng, R. Ran, NOx-assisted soot oxidation on Pt–Mg/Al2O3

catalysts: magnesium precursor, Pt particle size, and Pt–Mg interaction, Ind.Eng. Chem. Res. 51 (2012) 2271–2279.

[4] Q. Liang, X.D. Wu, D. Weng, H.B. Xu, Oxygen activation on Cu/Mn–Ce mixedoxides and the role in diesel soot oxidation, Catal. Today 139 (2008) 113–118.

[5] W.J. Shan, N. Ma, J.L. Yang, X.W. Dong, C. Liu, L.L. Wei, Catalytic oxidation ofsoot particulates over MnOx–CeO2 oxides prepared by complexation–combustion method, J. Nat. Gas Chem. 19 (2010) 86–90.

[6] K. Tikhomirov, O. Kröcher, M. Elsener, A. Wokaun, MnOx–CeO2 mixed oxidesfor the low-temperature oxidation of diesel soot, Appl. Catal. B 64 (2006) 72–78.

[7] X.D. Wu, F. Lin, H.B. Xu, D. Weng, Effects of adsorbed and gaseous NOx specieson catalytic oxidation of diesel soot with MnOx–CeO2 mixed oxides, Appl.Catal. B 96 (2010) 101–109.

[8] X.F. Tang, Y.G. Li, X.M. Huang, Y.D. Xu, H.Q. Zhu, J.G. Wang, W.J. Shen, MnOx–CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: effect ofpreparation method and calcination temperature, Appl. Catal. B 62 (2006)265–273.

[9] X.D. Wu, S. Liu, D. Weng, F. Lin, Textural–structural properties and sootoxidation activity of MnOx–CeO2 mixed oxides, Catal. Commun. 12 (2011)345–348.

[10] J. Kašpar, P. Fornasiero, Nanostructured materials for advanced automotive de-pollution catalysts, J. Solid State Chem. 171 (2003) 19–29.

[11] R.D. Monte, P. Fornasiero, S. Desinan, J. Kašpar, J.M. Gatica, J.J. Calvino, E. Fonda,Thermal stabilization of CexZr1�xO2 oxygen storage promoters by addition of

112 F. Lin et al. / Chemical Engineering Journal 226 (2013) 105–112

Al2O3: Effect of thermal aging on textural, structural, and morphologicalproperties, Chem. Mater. 16 (2004) 4273–4285.

[12] A. Morikawa, T. Suzuki, T. Kanazawa, K. Kikuta, A. Suda, H. Shinjo, A newconcept in high performance ceria–zirconia oxygen storage capacity materialwith Al2O3 as a diffusion barrier, Appl. Catal. B 78 (2007) 210–221.

[13] J. Wang, J. Wen, M.Q. Shen, Effect of interaction between Ce0.7Zr0.3O2 and Al2O3

on structural characteristics, thermal stability, and oxygen storage capacity, J.Phys. Chem. C 112 (2008) 5113–5122.

[14] S.M. Saqer, D.I. Kondarides, X.E. Verykios, Catalytic oxidation of toluene overbinary mixtures of copper, manganese and cerium oxides supported on c-Al2O3, Appl. Catal. B 103 (2011) 275–286.

[15] I. Spassova, T. Tsontcheva, N. Velichkova, M. Khristova, D. Nihtianova, Catalyticreduction of NO with decomposed methanol on alumina-supported Mn–Cecatalysts, J. Colloid Interf. Sci. 374 (2012) 267–277.

[16] X.D. Wu, F. Lin, D. Weng, J. Li, Simultaneous removal of soot and NO overthermal stable Cu–Ce–Al mixed oxides, Catal. Commun. 9 (2008) 2428–2432.

[17] M. Wu, X. Wang, Q. Dai, Y. Gu, D. Li, Low temperature catalytic combustion ofchlorobenzene over Mn–Ce–O/Al2O3 mixed oxides catalyst, Catal. Today 158(2010) 336–342.

[18] X.D. Wu, S. Liu, D. Weng, F. Lin, R. Ran, MnOx–CeO2–Al2O3 mixed oxides forsoot oxidation: Activity and thermal stability, J. Hazard. Mater. 187 (2011)283–290.

[19] B.R. Stanmore, J.F. Brilhac, P Gilot, The oxidation of soot: a review ofexperiments, mechanisms and models, Carbon 39 (2001) 2247–2268.

[20] V. Sánchez Escribano, E. Fernández López, J.M. Gallardo-Amores, C. del HoyoMartínez, C. Pistarino, M. Panizza, C. Resini, G. Busca, A study of a ceria–zirconia-supported manganese oxide catalyst for combustion of diesel sootparticles, Combust. Flame 153 (2008) 97–104.

[21] P.A. Kumar, M.D. Tanwar, N. Russo, R. Pirone, D. Fino, Synthesis and catalyticproperties of CeO2 and Co/CeO2 nanofibres for diesel soot combustion, Catal.Today 184 (2012) 279–287.

[22] M.A. Peralta, M.S. Gross, B.S. Sánchez, C.A. Querini, Catalytic combustion ofdiesel soot: Experimental design for laboratory testing, Chem. Eng. J. 152(2009) 234–241.

[23] M.A. Peralta, M.S. Gross, M.A. Ulla, C.A. Querini, Catalyst formulation to avoidreaction runaway during diesel soot combustion, Appl. Catal. A 367 (2009) 59–69.

[24] N. Zouaoui, M. Issa, D. Kehrli, M. Jeguirim, CeO2 catalytic activity for sootoxidation under NO/O2 in loose and tight contact, Catal. Today 189 (2012) 65–69.

[25] B.R. Stanmore, V. Tschamber, J.-F. Brilhac, Oxidation of carbon by NOx, withparticular reference to NO2 and N2O, Fuel 87 (2008) 131–146.

[26] S.B. Simonsen, S. Dahl, E. Johnson, S. Helveg, Ceria-catalyzed soot oxidationstudied by environmental transmission electron microscopy, J. Catal. 255(2008) 1–5.

[27] A. Bueno-López, K. Krishna, M. Makkee, J.A. Moulijn, Enhanced soot oxidationby lattice oxygen via La3+-doped CeO2, J. Catal. 230 (2005) 237–248.

[28] L. Jia, M. Shen, J. Hao, T. Rao, J. Wang, Dynamic oxygen storage release overMn0.1Ce0.9Ox and Mn0.1Ce0.6Zr0.3Ox complex structural characterization, J.Alloy. Compd. 454 (2008) 321–326.

[29] H. Li, G. Qi, X. Zhang, X. Huang, W. Li, W. Shen, Low-temperature oxidation ofethanol over a Mn0.6Ce0.4O2 mixed oxide, Appl. Catal. B 103 (2011) 54–61.

[30] M. Wu, X. Wang, Q. Dai, D. Li, Catalytic combustion of chlorobenzene over Mn–Ce/Al2O3 catalyst promoted by Mg, Catal. Commun. 11 (2010) 1022–1025.

[31] D. Delimaris, T. Ioannides, VOC oxidation over MnOx–CeO2 catalysts preparedby a combustion method, Appl. Catal. B 84 (2008) 303–312.

[32] H.Y. Chen, A. Sayari, A. Adnot, F. Larachi, Composition-activity effects of Mn–Ce–O composites on phenol catalytic wet oxidation, Appl. Catal. B 32 (2001)195–204.

[33] G. Zhou, P.R. Shah, R.J. Gorte, A study of cerium–manganese mixed oxides foroxidation catalysis, Catal. Lett. 120 (2008) 191–197.

[34] H. Muckenhuber, H. Grothe, The heterogeneous reaction between soot andNO2 at elevated temperature, Carbon 44 (2006) 546–559.