La1−xCexMn1−yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot

Materials Chemistry and Physics xxx (2014) 1e9

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La1
xCexMn1
yCoyO3 perovskite oxides: Preparation, physicochemical properties and catalytic activity for the reduction of diesel soot Shaohua Wu, Chonglin Song*, Feng Bin, Gang Lv, Jinou Song, Cairong Gong State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China

h i g h l i g h t s  Cerium substitution at A-site in Mn100 enhances the catalytic activity.  Cerium substitution leads to the formation of the CeO2 phase.  Cobalt substitution at B-site in Ce20Mn generally decreases the catalytic activity.  Cerium substitution increases the a-O2 amount and low-temperature reducibility.  Cobalt substitution decreases the a-O2 amount and low-temperature reducibility.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 August 2013 Received in revised form 18 January 2014 Accepted 21 July 2014 Available online xxx

La1
xCexMn1
yCoyO3 catalysts were prepared by the “glucose method”. The structures and physicochemical properties for these catalysts were characterized using X-ray diffraction (XRD), nitrogen adsorption, scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), H2-temperature-programmed reduction (H2-TPR) and O2-tempreature-programmed desorption (O2-TPD). Results showed that cerium substitution at the A-site in LaMnO3 produced a CeO2 phase. The cobalt can be introduced into the B-site in La0.8Ce0.2MnO3 at any substitution ratio because of the similar ionic radii between cobalt and manganese. The catalytic activity for soot combustion in air was evaluated using a TG/DTA analyzer. Cerium substitution at A-site enhances the catalytic activity, while cobalt substitution at B-site inhibits the catalytic activity. The activation energy for soot combustion was calculated using the Horowitz method. The activation energy for non-catalytic soot combustion was 164.1 kJ mol
1. The addition of catalysts decreased the activation energy by about 26e63 kJ mol
1. Among the applied catalysts, Ce20Mn exhibited the lowest activation energy (101.1 kJ mol
1). © 2014 Elsevier B.V. All rights reserved.

Keywords: Oxides Powder diffraction Electron microscopy X-ray photo-emission spectroscopy Differential thermal analysis Oxidation

1. Introduction Catalytic particulate filter is the most efficient after-treatment device for the reduction of particulate matter (PM) from diesel engines. With the help of the catalyst, the temperature of PM oxidation (between 550 and 600  C) can be significantly decreased to the temperature of diesel exhaust gases, generally between 150 and 450  C [1]. A number of catalysts, including noble metals, single metal oxides, and mixed oxide systems or eutectic mixtures based on oxides, have been proposed for catalytic PM combustion [2]. Perovskite oxides are considered as

* Corresponding author. Tel.: þ86 22 27406842; fax: þ86 22 27403750. E-mail address: [email protected] (C. Song).

interesting candidates for this application, as they combine good stability and intrinsic catalytic activity and are much cheaper than noble metal [3]. A perovskite-type oxide has an ABO3 crystal structure, where large ionic radius cations are 12 coordinate to oxygen atoms and occupy A-sites, and smaller ionic radius cations are 6 coordinate to oxygen atoms and occupy B-sites [4]. In general, the A-site is occupied by a lanthanide ion, usually La, while the B-site is occupied by a transition metal ion [4]. For an un-substituted perovskite, catalytic activity is predominantly attributed to the metal ion at the B-site. The A-site metal has a strong effect on stability, providing a possibility for improving catalytic activity through synergetic interactions with metals at the B-sites [4]. Among various perovskites, lanthanum-based samples represented as LaBO3 are known to be excellent oxidation catalysts in a variety of reactions,

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Please cite this article in press as: S. Wu, et al., La1
xCexMn1
yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

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S. Wu et al. / Materials Chemistry and Physics xxx (2014) 1e9

including soot combustion [5]. For binary LaBO3 perovskites, LaMnO3 and LaCoO3 perovskites have been widely studied [6]. They have been shown to possess high catalytic activity in purifying diesel engine exhaust [6]. Partial substitution at the A- and/or B-site can also induce structural modifications related to the generation of crystal lattice defects and oxygen vacancies. This subsequently allows for the tailoring of thermal stability and catalytic activity [7]. Levasseur et al. [8] and Alifanti et al. [9] reported that upon partial substitution of lanthanum by cerium in perovskite catalysts, the reducibility and oxygen desorption could be significantly enhanced, leading to a higher catalytic activity. Alifanti et al. [10] has confirmed that the mutual substitution of cobalt by manganese at the B-site in LaMnO3 results in an increase of the specific surface area, and most of the mixed cobaltemanganese perovskites exhibit good activity for methane combustion. In this present work, the simultaneous substitution of cerium at the A-site and cobalt at the B-site in a LaMnO3 lattice will be examined to determine their effects on catalytic activity for soot combustion. A series of La1
xCexMn1
yCoyO3 catalysts were prepared and characterized by X-ray diffraction, nitrogen adsorption, scanning electron microscopy, Fourier transform infrared spectra, temperature-programmed reduction of hydrogen and temperature-programmed desorption of oxygen. The catalytic activity for soot combustion was evaluated by thermogravimetric analysis. To better understand the intrinsic mechanism for soot combustion over different catalysts, the Horowitz method was used to calculate the activation energy based on the TGA curves. This study will benefit designs for commercial perovskite-type catalysts used in the purification of diesel exhaust PM. 2. Experimental 2.1. Catalyst preparation LaMnO3 and La0.8Ce0.2Mn1
yCoyO3 (y ¼ 0, 0.1, 0.3, 0.5) catalysts were prepared by the glucose complexation. Briefly, lanthanum nitrates, manganese acetates, cerium nitrates and cobalt acetates were dissolved in deionized water to obtain an aqueous solution of La3þ, Ce3þ, Mn3þ and Co3þ in the desired stoichiometric ratio. A given amount of glucose was added into the above solution to form a homogeneous solution with a concentration of 0.1 mol L
1 for all the cations. The molar amount of glucose added is 1.2 times that of all cations. After stirring for 2 h at 70  C in a rotary evaporator, the solutions turned into gel. The gel obtained was dehydrated in an oven with an 80  Cs air stream at a flow rate of 80 ml min
1 overnight. The resulting spongy, friable, hygroscopic material was heated in a furnace under static air from room temperature to 400  C at a heating rate of 5  C min
1, maintained the material at 400  C for 3 h for total nitrate decomposition. Then, the temperature was increased to 800  C at a heating rate of 5  C min
1 and hold for 4 h. 2.2. Catalyst characterization The BrunauereEmmetteTeller (BET) specific surface area of the perovskite catalysts was derived from the corresponding nitrogen adsorption isotherm obtained at
196  C using a Quantachrome NOVA-2000 analyzer. Prior to analysis, samples were pretreated in vacuum at 300  C for 2 h to remove all moisture. Crystalline phases were determined by powder X-ray diffraction using a Rigaku D/ MAC/max 2500v/pc diffractometer with Cu Ka radiation (40 kV, 200 mA, l ¼ 1.5418 Å). Diffractograms were measured in the 2q range of 10e90 with a step scan of 0.02 . Phase recognition was realized according to JCPDS files. The morphology of the samples was obtained using a scanning electron microscope (SEM Philips XL-30). X-ray photo-emission spectroscopy (XPS) was performed

on a PHI-1600 ESCA spectrometer with Mg Ka (hn ¼ 1253.6 eV, 1 eV ¼ 1.603  10
19 J) X-ray source. The binding energies were calibrated using the C1s peak of contaminant carbon (BE ¼ 284.6 eV) as an internal standard. FT-IR absorbance spectra were obtained at room temperature in the wave number range from 1400 to 350 cm
1 using a VERTEX70 spectrometer. The measured wafer was prepared by pressing a mixture of the sample powder and KBr with a weight ratio of 1:100 into a disk (10 mm diameter). The resolution was set at 2 cm
1 and 64 scans were collected during the measurements. A ChemBet Pulsar TPR/TPD system was used to perform oxygen temperature-programmed desorption and temperature-programmed reduction of hydrogen. For O2-TPD experiments, 100 mg of the sample was placed in a quartz reactor and pretreated at 750  C for 30 min under a flow of 5% O2 in helium (50 ml min
1). The samples were cooled to room temperature under the same atmosphere. The gas was switched to pure helium (50 ml min
1) and maintained to purge all the gasphase oxygen from the system. The O2-TPD analysis was carried out by increasing the temperature from room temperature to 1000  C with a heating rate of 10  C min
1. The concentration of oxygen in the outlet gases was analyzed online by a mass spectrometry (Dycor LC-D200) while monitoring the m/e signal 32(O2). The amount of oxygen desorbed from the sample was determined by deconvolution and integration of the desorption peaks. For H2TPR experiments, 20 mg of the sample was put in a quartz reactor. Prior to each TPR analysis, the sample was pretreated at 550  C for 1 h in 5% O2/He mixture with a flow rate of 30 ml min
1 and then cooled to room temperature under the same atmosphere. The gas flow was switched to pure helium (30 ml min
1) and the reactor was flushed for 30 min. The H2-TPR measurement was carried out by increasing the temperature to 900  C with a heating rate of 10  C min
1 in 5% H2/He mixture at a constant flow rate of 30 ml min
1. The hydrogen consumption was monitored and quantified using a thermal conductivity detector (TCD). 2.3. Catalytic activity measurement The catalytic activity of the prepared catalysts for soot combustion was evaluated by TG/DTA technique. A type of Printex-U soot (Degussa AG) was used as a surrogate for diesel exhaust soot. Properties of this model soot are similar to those of real diesel soot particulate. Its primary particle size, specific surface area and ignition temperature were 25 nm, 100 m2 g
1 and above 300  C, respectively. Before measurement, the catalyst (9 mg) and model carbon (1 mg) were mixed in an agate mortar with a spatula to reproduce the tight contact mode, which is reproducible and provides a good basis for activity screening studies [11]. To ensure the same contact degree between air and carbon, inert SiO2 (9 mg) was added to the model carbon (1 mg) for the non-catalytic soot combustion experiments. The obtained reaction mixture was then loaded in an alumina crucible and heated from 50 C to 700  C (air flow: 200 ml min
1, heating rate: 10 C min
1). In this work, soot ignition temperature (denoted as Ti), maximum soot oxidation rate temperature (denoted as Tm), final soot combustion temperature (denoted as Tf) and temperature intervals DT ¼ Tf
Ti were used to evaluate the activity of the catalysts. Based on the TG/DTA curves, the activation energies for catalytic and non-catalytic soot combustion were calculated according to the Horowitz method [12]. 3. Results and discussion 3.1. Structure and morphology The X-ray diffraction patterns of the applied catalysts are shown in Fig. 1 and the results are listed in Table 1. The Mn100 catalyst

Please cite this article in press as: S. Wu, et al., La1
xCexMn1
yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

S. Wu et al. / Materials Chemistry and Physics xxx (2014) 1e9

Fig. 1. X-ray diffraction: (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo10, (d) Ce20MnCo30, (e) Ce20MnCo50.

presents a rhombohedral perovskite-type structure, characterized by the main diffraction peaks at 2q values of 32.6 , 46.7 and 58.1 (PDF 50-0298). This is the only phase identified for Mn100. For Ce20Mn, however, a new peak at 2q value of 27.7 is observed, corresponding to the presence of CeO2 (PDF 34-0394). This is because not all the cerium loaded is incorporated into the perovskite lattice, which leads to a decrease in the intensity of perovskite diffraction peaks. Quantitative analysis using a matrix-flushing method [13] reveals that the formed CeO2 accounts for 12.7 wt.% in the Ce20Mn catalyst. By contrast, cobalt-containing samples do not show evidence of cobalt-oxide segregation. Only diffraction peaks corresponding to CeO2 and perovskite are able to be observed, suggesting that cobalt is impregnated into the perovskite lattice, because of the similar radii of both Co3þ (0.074 nm) and Mn3þ (0.067 nm) cations [14,15]. The amount of CeO2 formed in cobalt-containing samples (10.1e10.7 wt.%) is a little lower than that in Ce20Mn. In this work, the multiphase sample, including perovskite and CeO2, is studied as a catalytic system in terms of the physicalechemical properties and catalytic activities. The crystallite size of these catalysts, calculated using the Scherrer method through the XRD profiles, is listed in Table 1. The crystallite size for Mn100 is 32.9 nm, while for Ce20Mn, it is only 25.4 nm. The decreased crystallite size is attributed to the dispersing effect of cerium oxides [16]. Cobalt introduction into the lattice of Ce20Mn also decreases the crystallite size to the range of 19.8e20.0 nm. Fig. 2 shows the SEM images of the applied perovskite catalysts. The average particle sizes of Mn100 and Ce20Mn are around 80 nm

3

and 70 nm, respectively, which are much larger than their crystallite sizes measured by XRD. This indicates that the particles of Mn100 and Ce20Mn are aggregated as polycrystals. For cobaltcontaining catalysts, the average particle size is about 20 nm, which is close to the crystallite size measured by XRD. SEM images also show that the crystals of cobalt-containing catalysts bunch together. In general, a decrease in the crystallite size corresponds to an increase in the BET specific surface area (SSA). As listed in Table 1, the Mn100 catalyst with the largest crystallite size exhibits the least SSA of 12.9 m2 g
1. The cobalt containing catalysts, which show the smallest crystallite size among the catalysts, exhibit the largest SSA (22.4e23.7 m2 g
1). All the catalysts applied have lower SSAs than that reported in the literature (16.5 m2 g
1 for LaMnO3 by Royer et al. [17], 32.6 m2 g
1 for La0.8Ce0.2MnO3 and 28.6 m2 g
1 for La0.8Ce0.2Co0.3MnO3 by Alifanti et al. [10]). Such relatively lower BET specific surface areas are attributed to the agglomerated and compact structures observed in the SEM images. The chemical states of elements for the catalysts were characterized by XPS. The results are shown in Fig. 3, and the binding energies of La 3d5/2, Ce 3d5/2, Mn 2p3/2, Co 2p3/2 and O 1s core levels obtained from spectral deconvolution are summarized in Table 2. All catalysts show binding energies of about 834.2 and 838.2 eV for the La 3d5/2 level, indicating that lanthanum ions are present in the 3þ oxidation state regardless of Ce and/or Co substitutions. The binding energies of Mn 2p3/2 (641.7e642.1 eV) and Co 2p3/2 (779.3e779.5 eV) suggest that both cations are to a large extent in the 3þ oxidation state [18]. Presence of the Mn4þ ions can not be well established due to the proximity of its peak to that of the Mn3þ ion [9]. Most of the cerium ions in the catalysts are present as Ce4þ, validated by the Ce 3d5/2 peaks locating at 882.5, 889.2 and 898.2 eV. The O 1s XPS signal show two peaks, centered on 529 and 531 eV, in all catalysts. The low binding energy peak is assigned to lattice oxygen, while the high binding energy one can be associated with adsorbed oxygen or surface hydroxyl groups [10]. Fig. 4 shows the IR spectra of the prepared perovskite catalysts. All of them present two strong and well-defined adsorption bands, typical of perovskite oxide ABO3. The first band at around 600 cm
1 is assigned to the stretching vibration of the MneO bond in the BO6 octahedron; the second one at about 400 cm
1 is ascribed to the deformation modes of the same polyhedral [19]. For cobaltcontaining samples, a shoulder band at around 667 cm
1 appears, which can be assigned to symmetric stretching vibration of the BO6 octahedron, whereas the main band at around 600 cm
1 is attributed to the antisymmetric stretching vibration of these octahedrons [20,21]. Ce20Mn exhibits weaker and wider vibration bands compared with Mn100, and a decrease in the intensity of the vibration bands (600 cm
1 and 400 cm
1) is observed with increasing cobalt substitution. These results suggest that cerium and/or cobalt substitution lead to a much more symmetric structure of the BO6 octahedral units [22].

3.2. O2-TPD results Table 1 CeO2 amount, crystallite size and BET specific surface area of La1
xCexMn1
yCoyO3 catalysts. Samples

Nominal composition

CeO2 (wt.%)

BET (m2 g
1)

Crystallite size (nm)

Mn100 Ce20Mn Ce20MnCo10 Ce20MnCo30 Ce20MnCo50

LaMnO3 La0.8Ce0.2MnO3 La0.8Ce0.2Mn0.9Co0.1O3 La0.8Ce0.2Mn0.7Co0.3O3 La0.8Ce0.2Mn0.5Co0.5O3

e 12.7 10.3 10.7 10.1

12.9 19.4 22.4 23.7 23.2

32.9 25.4 20.0 19.9 19.8

O2-TPD profiles of Mn100 and Ce20MnCoy (y ¼ 0, 10, 30 and 50) catalysts are shown in Fig. 5. Two desorption peaks, corresponding to two types of oxygen species, can be observed. The lowtemperature species, namely a-O2, is ascribed to adsorbed oxygen at oxygen vacancies and desorbs below 600  C [23]. The hightemperature species, namely b-O2, is assigned to lattice oxygen, the desorption of which occurs above 600  C, accompanied with partial reduction of B site cations to lower oxidation state in the ABO3 structure [24]. The high-temperature oxygen species is,

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yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

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Fig. 2. SEM images: (A) Mn100, (B) Ce20Mn, (C) Ce20MnCo10, (D) Ce20MnCo30, (E) Ce20MnCo50.

however, less important for soot combustion because the reaction proceeds at distinctly lower temperature. The amounts of oxygen desorbed were measured by deconvolution of the O2 desorption curves using Lorentzian peak shapes in a computer peak-fitting routine. As listed in Table 3, the amount of aO2 desorbed from Mn100 (172 mmol g
1) increases dramatically to

(316 mmol g
1) for Ce20Mn, while cobalt-containing samples possess the less amount of a-O2 than Ce20Mn. Thus, the amount of a-O2 desorbed from the applied catalysts follows the sequence of: Mn100 < Ce20MnCo10 < Ce20MnCo50 < Ce20MnCo30 < Ce20Mn. The increase in the amount of a-O2 is attributed to oxygen vacancies formed by partial substitution at the A- and/or B-sites [25]. These results validate the accepted view that the intensity of a-O2 depends on the nature of metal B of the ABO3 structure, as well as the degree of substitution at the A- and/or B-sites [26]. The amount of b-O2 (also listed in Table 3) is higher than a-O2. Ce20Mn, which desorbs the largest amount of a-O2, possesses the least amount of b-O2. An increase in the amount of b-O2 is found when cobalt is introduced into the lattice of Ce20Mn. The desorption of lattice oxygen b-O2 is linked to the morphology of the materials, and b-O2 desorbing sites can actually be ascribed to ‘surface anionic vacancies’ formed after desorption of lattice oxygen from the surface [27]. Substitution at the A- and/or B-sites in perovskite lattice can induce some changes to these ‘surface anionic vacancies’, resulting in variations in b-O2 desorption [27]. 3.3. H2-TPR analysis

Fig. 3. XPS survey scan spectra: (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo30.

The H2-TPR results of Mn100 and Ce20MnCoy (y ¼ 0, 10, 30 and 50) catalysts are shown in Fig. 6 and listed in Table 4. All the catalysts exhibit two major reduction peaks, corresponding to two successive reduction stages. The a peak at the lower temperature (<450 C) is assigned to the reduction of Mn4þ cations to Mn3þ cations and some Mn3þ cations on the surface to Mn2þ cations [17].

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yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

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Table 2 XPS binding energies (eV) of core electrons in La1
xCexMn1
yCoyO3 catalysts. Samples

La 3d5/2

Mn100 Ce20Mn Ce20MnCo10 Ce20MnCo30 Ce20MnCo50

834.2 834.0 834.2 834.1 833.8

838.2 838.1 837.8 837.8 838.3

Ce 3d5/2

Mn 2p3/2

882.8 882.5 882.5 882.3

641.3 641.2 641.3 641.1 641.3

889.1 889.4 889.2 889.4

The b peak at the higher temperature (>700  C) is related to the complete reduction of Mn3þ cations to Mn2þ cations [17]. Furthermore, a shoulder peak at around 720  C, namely the g peak, can be observed on cobalt-containing catalysts. Royer et al. [17] confirmed that the a peaks for the LaCoO3 and Mn100 are located at about 380  C, while the b peaks for the LaCoO3 and Mn100 are located at 600  C and 800  C, respectively. Therefore, for cobaltcontaining catalysts in our study, the g peak can be attributed to the complete reduction of Co2þ cations to metal cobalt. In this study, the low-temperature reducibility of the catalyst is more important in soot oxidation than high-temperature reducibility since the reaction actually occurs at low temperature (<500  C), as evidenced by TG analysis below. Among these catalysts, Mn100 possesses the lowest reducibility because it shows the highest reduction temperature (402  C) and the least amount of H2 consumption (243 mmol g
1) of a peak. Although the reduction temperature of a peak for Ce20Mn (344  C) is close to that for the cobalt containing catalysts (324e349  C), the amount of H2 consumption of a peak for Ce20Mn (292 mmol g
1) is much larger than that for cobalt containing catalysts (250e270 mmol g
1). Therefore, Ce20Mn possesses the higher low-temperature reducibility than the cobalt containing catalysts. To better understand the relative low-temperature reducibility of these catalysts, their initial H2 consumption rates (where 25% oxygen in the samples was consumed for a peaks) have been calculated according to Ji et al. [28] As listed in Table 4, the initial H2 consumption rate increases in the order of Mn100 < Ce20 MnCo10 < Ce20MnCo50 < Ce20MnCo30 < Ce20Mn, indicating that the low-temperature reducibility of these catalysts increases in the sequence of Mn100 < Ce20MnCo10 < Ce20MnCo50 < Ce20Mn Co30 < Ce20Mn.

898.3 898.1 898.3 898.2

Co 2p3/2

O 1s

779.4 779.3 779.5

531.4 531.4 531.6 531.5 531.5

529.1 528.8 528.9 528.9 529.2

3.4. Catalytic activity for soot combustion The DTG profiles of soot combustion over SiO2 and La1-xCexMn1catalysts are shown in Fig. 7, and the characteristic temperatures of Ti, Tm, Tf and DT (Tf
Ti) are listed in Table 5. Soot oxidation in the absence of a catalyst occurs at considerably higher temperatures with the TG curve giving a Tm at about 554  C, which is 124e139  C higher than in the presence of the employed catalysts. This result indicates that all the catalysts promote soot oxidation. The reaction temperature interval DT varies little over

yCoyO3

Fig. 5. O2-TPD profiles: (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo10, (d) Ce20MnCo30, (e) Ce20MnCo50.

Table 3 Amounts of oxygen desorbed during the O2-TPD experiments on La1
xCexMn1
yCoyO3 catalysts.

Fig. 4. FT-IR spectra: (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo10, (d) Ce20MnCo30, (e) Ce20MnCo50.

Sample

a-O2 (mmol g
1)

b-O2 (mmol g
1)

Mn100 Ce20Mn Ce20MnCo10 Ce20MnCo30 Ce20MnCo50

172 316 251 264 256

1030 504 901 766 1024

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S. Wu et al. / Materials Chemistry and Physics xxx (2014) 1e9 Table 5 Characteristic temperature and kinetic parameters of soot combustion over SiO2 and La1-xCexMn1-yCoyO3 catalysts.

Fig. 6. H2-TPR profiles: (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo10, (d) Ce20MnCo30, (e) Ce20MnCo50. Table 4 H2 consumption of a peak and initial H2 consumption rates on La1
xCexMn1
yCoyO3 catalysts during H2-TPR experiments. Sample

H2 consumption (mmol g
1)

Initial H2 consumption rate (mmol g
1 s
1)

Mn100 Ce20Mn Ce20MnCo10 Ce20MnCo30 Ce20MnCo50

243 292 255 267 263

0.25 0.49 0.30 0.35 0.32

these catalysts, suggesting a similar reaction rate. The Tm is 452  C for the Mn100 sample and upon the introduction of cerium, the Ce20Mn exhibits a higher catalytic activity since the DTG peak of soot combustion shifts towards lower temperature, giving a Tm temperature of 415  C. No gain in activity is found upon cobalt addition, as shown by all the cobalt-containing samples exhibiting the DTG peaks of soot combustion at higher temperatures than that of Ce20Mn. The activities of the catalysts therefore follow the sequence of Mn100 < Ce20MnCo10 < Ce20MnCo50 < Ce20 MnCo30 < Ce20Mn.

Fig. 7. DTG profiles of soot combustion: (a) SiO2, (b) Mn100, (c) Ce20Mn, (d) Ce20MnCo10, (e) Ce20MnCo30, (f) Ce20MnCo50.

Sample

Ti ( C)

Tm ( C)

Tf ( C)

DT ( C)

E (kJ mol
1)

SiO2 Mn100 Ce20Mn Ce20MnCo10 Ce20MnCo30 Ce20MnCo50

510 410 368 389 385 385

554 452 415 430 424 429

556 475 438 460 448 454

46 65 70 70 62 69

164.1 138.3 101.1 121.1 119.8 120.5

Soot oxidation supposedly takes place at the ‘triple contact point’, where a solid reactant, a catalyst and gaseous reactant meet together [16]. Theoretically, an increase in the specific surface area (SSA) of the catalyst favors the transfer of mass and heat from the catalyst to the soot, and thereby substantially enhances the catalytic activity for soot combustion [1]. In our study, however, the SSA of the catalyst does not correlate well with the activity of the catalyst. As shown in Fig. 8A, although the cobalt-containing catalysts (Ce20MnCo10, Ce20MnCo30, Ce20MnCo50) possess larger SSAs (22.4e23.7 m2 g
1) than Ce20Mn (19.4 m2 g
1), they exhibit lower activities as the Tm over them (424e430  C) is higher than that over Ce20Mn (415  C). This phenomenon indicates that the activity of the catalyst is not solely determined by the specific surface area (SSA) of the catalyst. The a-O2 and low-temperature reducibility are other two main factors affecting soot oxidation [29]. On the one hand, because the a-O2 is related to the oxygen vacancies in ABO3, the increase in the amount of a-O2 is beneficial for the activation of oxygen molecules and formation of active oxygen species, leading to a high catalytic activity [30]. On the other hand, since the soot oxidation is a kind of deep oxidation reaction and the catalysis nature of this reaction is redox process, the enhanced low-temperature reducibility can improve the soot oxidation [31]. To estimate these correlations, the Tm for these catalysts used in this study is plotted as functions of a-O2 and lowtemperature reducibility in Fig. 8B and C, respectively. Because the low-temperature reducibility of the catalyst can be determined by its initial H2 consumption rate during H2-TPR process as discussed above, the initial H2 consumption rate is used to represent the lowtemperature reducibility herein. Obviously, the larger the amount of a-O2 or the initial H2 consumption rate for the catalyst is, the lower the Tm is. For example, Ce20Mn with the largest amount of aO2 (316 mmol g
1) and initial H2 consumption rate (0.49 mmol g
1 s
1) shows the lowest Tm (415  C), while Mn100 with the least amount of a-O2 (172 mmol g
1) and initial H2 consumption rate (0.25 mmol g
1 s
1) exhibits the highest Tm (452  C). This observation validates that an increase in a-O2 or lowtemperature reducibility can enhance the activity of the catalyst on soot combustion. To date, there are several literatures addressing the effects of Aor B-site substitution on the activity of perovskite catalysts on soot oxidation. Table 6 lists the results of these literatures. It can be seen that despite the different feed gases in activity tests in their investigations, the Tm for A-site substituted catalysts is lower than that for un-substituted ones, indicating that partial substitution at A-site can lead to a higher activity. Similar result is obtained in our study since Ce20Mn possesses lower Tm (415  C) than Mn100 (452  C). With respect to the B-site substitution, however, there exists some controversy. In Table 6, Li et al. [32] discovered that the copper doping into La0.9K0.1CoO3 at B-site decreases the Tm from 438 to 360  C, corresponding to a higher activity. Similarly, He et al. [33] believed that B-site substitution could promote the catalytic activity since the LaMn0.8Co0.2O3 exhibits a lower Tm (417  C) than LaMnO3 (428  C). On the contrary, Bin et al. [4] concluded that the

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yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

S. Wu et al. / Materials Chemistry and Physics xxx (2014) 1e9

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Fig. 8. Correlations between SSA and Tm (A), between a-O2 and Tm (B) and between initial H2 consumption rate and Tm (C): (a) Mn100, (b) Ce20Mn, (c) Ce20MnCo10, (d) Ce20MnCo30, (e) Ce20MnCo50.

B-site substitution inhibits the catalytic activity, arising from the LaCo0.7Fe0.3O3 possessing a higher Tm (417  C) than LaCoO3 (406  C). Similar results are obtained in our study. As listed in Table 6, all cobalt containing catalysts exhibit higher Tm values (424e430  C) than Ce20Mn (415  C), suggesting a decrease in activity due to Bsite substitution. For the studies mentioned above, the difference in the catalytic activity after B-site substitution may be ascribed to the compositions of the catalysts and the preparation methods, which results in a change of the physicalechemical properties of the catalysts, such as the BET specific surface area, the amount of a-O2 and the low-temperature reducibility. 3.5. Kinetic analysis To better understand the intrinsic mechanism for soot combustion over catalysts, the activation energies (E) are calculated

Table 6 Literature results for the influences of A- or B-site substitution on the activity of perovskite catalysts on soot oxidation. Reference Li et al.[32]

Catalysts

LaCoO3 La0.9K0.1CoO3 La0.9K0.1Co0.9Cu0.1O3 He et al.[33] LaMnO3 La0.8Ce0.2MnO3 LaMn0.8Co0.2O3 Bin et al.[4] LaCoO3 La0.8K0.2CoO3 LaCo0.7Fe0.3O3 This work Mn100 Ce20Mn Ce20MnCoy (y ¼ 10, 30 and 50)

Preparation method

Tm ( C)

Citric acid 443 complexation 438 360 SoleGel 428 397 417 Citric acid 406 complexation 397 417 Glucose 452 complexation 415 424e430

Feed gas for activity test NO þ O2 þ N2

O2 þ N 2

using a Horowitz method [12]. This method of analysis is based on the dependence of the reaction rate on concentration and temperature:

dC ¼
kC n dt

(1)

k ¼ Ae
RT

(2)

E

Where k is the specific rate constant; n is the order of reaction; t is the time; C is the concentration of the reactant, corresponding to mr/mr þ mp (mr represents the weight of the reactant, mp represents the weight of product); A is the frequency factor; E is the activation energy; R is the gas constant and T is the temperature. Combining Equations (1) and (2) gives: E dmr ¼
Ae
RT mr dt

(3)

Using a series of substitution and approximations, Equation (3) can be transformed to:

A RTe2
E=RTe e ¼1 b E

(4)

or

mr Eq ¼ m0 RTe2

O2þNO þ He

ln ln

(5)

Air

Where b is the heating rate; Te, at which the ratio of mr to m0 (m0 is the initial weight of the reactants) equals 1/e, is the value of a characteristic temperature, and q is the value of T
Te. The activation energy E can be calculated using a plot of ln lnmr =m0 against q with

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xCexMn1
yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

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S. Wu et al. / Materials Chemistry and Physics xxx (2014) 1e9

References

Fig. 9. Horowitz plot for the determination of activation energy: (a) SiO2, (b) Mn100, (c) Ce20Mn, (d) Ce20MnCo10, (e) Ce20MnCo30, (f) Ce20MnCo50.

the slope as E=RTe2 . The activation energy for non-catalytic soot combustion was also measured for comparison. Curves of ln lnmr =m0 vs. q are plotted in Fig. 9. The experimental data fit statistically with Equation (5) with a correlation coefficient exceeding 0.98 for all samples tested. The kinetic parameters calculated through Fig. 9 are summarized in Table 5. The activation energy for non-catalytic soot combustion is 164.1 kJ mol
1. When Mn100 is added, the activation energy decreases to 138.3 kJ mol
1. Among the catalysts applied, Ce20Mn is found to exhibit the lowest activation energy for soot combustion (E ¼ 101.1 kJ mol
1). Cobaltcontaining samples exhibit slightly higher activation energy than Ce20Mn. Thus the activation energy for the samples decreases as follows: Mn100 > Ce20MnCo10 > Ce20MnCo50 > Ce20MnCo30 > Ce20Mn, which is consistent with the results of catalytic activity. 4. Conclusion La1
xCexMn1
yCoyO3 catalysts were synthesized, characterized, and evaluated for the removal of soot. All catalysts exhibit relatively lower specific surface areas compared with previous literature, because of their agglomerated structures. Cerium substitution leads to the formation of the CeO2 phase, while cobalt can be introduced to the lattice at any substitution ratio since the ionic radii of cobalt is similar to that of manganese. The BET specific surface area (SSA), the amount of a-O2 and the low-temperature reducibility of the catalyst affect the catalytic activity for soot oxidation. Cerium substitution at the A-site in Mn100 enhances the low-temperature reducibility of catalysts, increases the SSA, and increases the amount of a-O2. As a result, the Tm falls from 452  C for Mn100 to s for Ce20Mn, corresponding to the decreased activation energy from 138.3 kJ mol
1 to 101.1 kJ mol
1. Contrary to cerium substitution, the cobalt substitution at the B-site in Ce20Mn decreases the amount of a-O2 and lowers the low-temperature reducibility, causing a slight shift of the Tm towards higher temperature. Acknowledgments This study was supported by the National Natural Science Foundation of China (No. 51206119), the National Key Basic Research and Development Program (2013CB228506), and the Tianjin Research Program of Application Foundation and Advanced Technology (13JCZDJC35800).

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xCexMn1
yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029

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Please cite this article in press as: S. Wu, et al., La1
xCexMn1
yCoyO3 perovskite oxides: Preparation, physico-chemical properties and catalytic activity for the reduction of diesel soot, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.07.029