Lanthanum-Based Perovskite-Type Oxides La1−xCexBO3 (B = Mn and Co) as Catalysts: Synthesis and Characterization

Erdenee, Nyamdavaa; Enkhnaran, Uyanga; Galsan, Sevjidsuren; Pagvajav, Altantsog

doi:https://doi.org/10.1155/2017/9120586

Journal of Nanomaterials

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Abstract Introduction Experimental Results and Discussion Conclusion Conflicts of Interest Acknowledgments References Copyright Related Articles

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Nanomaterials for Efficient Energy Conversion and Storage

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Research Article | Open Access

Volume 2017 | Article ID 9120586 | https://doi.org/10.1155/2017/9120586

Lanthanum-Based Perovskite-Type Oxides La_1−xCe_xBO₃ (B = Mn and Co) as Catalysts: Synthesis and Characterization

Nyamdavaa Erdenee,¹Uyanga Enkhnaran,^1,2Sevjidsuren Galsan,¹and Altantsog Pagvajav¹

Academic Editor: Hui Zhang

Received25 Jan 2017

Accepted07 Mar 2017

Published20 Mar 2017

Abstract

La_1−xCe_xCoO₃ ( = 0, 0.2, 0.4) and La_1−xCe_xMnO₃ ( = 0, 0.2) perovskite-type oxides were prepared by sol-gel process. Characterization techniques EDS, FTIR, XRD, BET, and XPS experiments were performed to survey the composition, bulk structure, and the surface properties of perovskites. The reduction behavior, thermal stability, and catalytic activity were studied by H₂-TPR and catalytic performance. All synthesized samples showed well crystalline perovskite structure, 8–22 nm crystallite sizes, and SSA with 2–27 m²g⁻¹. The XRD results showed that the Ce substitution promoted the structural transformation for LaCoO₃ from rhombohedral into cubic and for LaMnO₃ no change in lattice geometry. Substitution with cerium ( = 0.2) showed smaller crystallite size, higher SSA, and the highest reducibility and catalytic activity for LaCoO₃.

1. Introduction

Lanthanum-based perovskite-type oxides with general formula ABO₃ (A = La³⁺, Pr³⁺, Nd³⁺, etc.; A = Co³⁺, Mn³⁺, Sr³⁺, etc.) may potentially replace noble metal catalysts due to their high catalytic activity, thermal stability, and low costs [1–3]. On the other hand, these materials present strong limitations for broader application in catalysis such as low surface area resulting from high calcination temperature and oxide instability at high operation temperature [4, 5]. Generally, an increase in the specific surface area of a perovskite-type oxide improves its catalytic activity by increasing the contact area between the catalyst and gas. Particles of perovskite-type oxides, which are the main factor of surface area, are still not small enough, largely because the conventional synthesis processes require calcination at high temperature. Thus, the obtained perovskite particles are heavily agglomerated and sintered, resulting in the low specific surface area [6, 7]. At present, a lot of efforts were carried out for the synthesis of perovskite with improved physical and chemical properties. LaBO₃ (B = Mn and Co) oxides have been evaluated with materials synthesized through methods such as pechini [8], sol-gel [9], citrate gel [10], and wet impregnation [5] methods. Among these, the sol-gel method is one of the most effective for the synthesis of nanostructured perovskite-type oxides [11].

It has been reported that the ABO₃ perovskites can be properly modified by the partial substitution of atoms at A or B which dramatically enhance the activity and significant structural changes, such as lattice distortions, stabilization of multiple oxidation states, or generation of cationic and anionic vacancies. Many studies have reported that partial substitution at the A-site by a cation of different valence (e.g., La³⁺ by Ce⁴⁺ or Sr²⁺) can form oxygen vacancies or change the oxidation state of the B-site cation, which enhances substantially the catalytic activity [12, 13]. Cerium is usually reported as a good promoter in perovskite lattice. An increase in the cerium substitution level up to 10% on the structure is expected to the enhancement in the activity that explained by oxygen excess in the lattice, cationic vacancies, structural defects, and the presence of multiple B oxidation states [14].

In this study, nanosized Ce-substituted perovskite-type oxides with up to 0.4 were synthesized by sol-gel method and described structural change including Ce distribution. The prepared powder samples were systematically characterized by X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), energy dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction of H₂ (H₂-TPR). Catalytic activity and selectivity were examined on dehydrogenation propane reaction. Dehydrogenation of propane is attractive in terms of the direct conversion of economic feedstock, which can contribute to the future chemical industry. Perovskite catalysts possess high activities to propylene on this reaction [15].

Our results demonstrated that cerium substituted and nonsubstituted lanthanum-based perovskite-type oxides (La_1−xCe_xMnO₃ and La_1−xCe_xCoO₃) show promising structural, surface, electronic states and catalytic properties for catalyst application.

2. Experimental

2.1. Synthesis of the Perovskite

All La_1−xCe_xCoO₃ and La_1−xCe_xMnO₃ samples were prepared by the sol-gel method, which allows the formation of amorphous citrates of metals with a wide flexibility of compositions [16]. In our preparation procedure, the corresponding nitrates lanthanum (III) nitrate hexahydrate (La(NO₃)₃6H₂O) (Roth, 99.995%), cerium (III) nitrate hexahydrate (Ce(NO₃)₃6H₂O) (Aldrich, 99.95%), and cobalt (II) nitrate monohydrate (Co(NO₃)₂H₂O) (Roth, 99.995%), and manganese (II) nitrate hexahydrate (Mn(NO₃)₂6H₂O) (Roth, 99.995%) in the appropriate quantities were dissolved in deionized water to give 0.1 M solutions. Citric acid monohydrate was added in 10 wt.% excess over the stoichiometric quantity to insure full complexation of the metal ions. Water was removed on a rotary evaporator at 80°C until the appearance of a gel. The obtained viscous material was dried overnight in a vacuum oven at 100°C. During this treatment, an intense production of nitrogen oxides occurred. The resulting strongly and highly hygroscopic and amorphous material was then crushed and calcined in air for 5 h at 750°C to obtain the desired phases.

2.2. Characterization

XRD data were obtained by using Maxima_X, XRD-7000 equipment with CuK_α radiation at room temperature. The structural parameters were determined by Rietveld analysis of the diffraction profiles. XAS measurements for Ce L₃, La L₃, and Co K-edges recorded at beam lines BL17C1 of National Synchrotron Center, Hsinchu, Taiwan. The data fitting was performed using the software package IFEFFIT. The specific surface areas were obtained by N₂ adsorption at 77 K, evaluated using the BET equation, on ASAP 2020. EDS elemental analysis was performed using an INCA system. The XPS data was obtained using a VG Scientific ESCALAB MKII spectrometer. The binding energy of the Au (4f_7/2) at eV was used to calibrate the binding energy scale of the spectrometer. XPS spectra smoothening and baseline subtraction were carried out using CasaXPS software. The experiments of the temperature-programmed reduction of H₂ (H₂-TPR) were carried out on Chem BET TPR/TPD Chemisorption Analyzer from Quantachrome.

2.3. Catalytic Activity Evaluation

Before the H₂-TPR experiment, the sample (ca. 0.10 g) was pretreated in an Ar stream at 150°C for 30 min and then cooled to room temperature. After that, a 5% H₂/Ar mixture stream was switched on and the temperature was increased at a rate of 10°C/min from room temperature to 900°C. The consumption of H₂ in the reactant stream was monitored with a thermal conductivity detector (TCD). The desorbed gases were monitored with a TCD and an online mass spectrometer (MS). The oxidative dehydrogenation of propane reaction was performed at atmospheric pressure in a fixed bed quartz reactor. For each testing, catalyst (ca. 0.20 g) was loaded in a quartz tubular reactor . The remaining space of the reactor was filled with quartz sand to minimize possible homogeneous reaction [17]. Typical feed gases used were V(C₃H₈) = 2 mL min⁻¹, V(O₂) = 2 mL/min, and V(N₂) = 16 mL min⁻¹ with a total flow rate of 20 mL min⁻¹. The reaction temperature was changed from 200 to 600°C at 25°C intervals. The highest temperature was consistent with the calcination temperature. The reaction products were analyzed on line by gas chromatography (GC) with a TCD detector using two packed columns, OV-1 column for CH₄, H₂, C₂H₄, C₃H₆, and C₃H₈ separation and TDX-01 column for O₂, CO, CO₂, and CH₄ separation.

3. Results and Discussion

3.1. Elemental Analysis

The elemental compositions of the La_1−xCe_xCoO₃ and La_1−xCe_xMnO₃ ( = 0, 0.2) perovskites determined by EDS analysis are summarized in Table 1. The EDS analysis shows the composition is almost the same (within experimental error) as the nominal composition of the samples.

3.2. FT-IR Spectra

As shown in Figure 1, synthesized La_1−xCe_xBO₃ (B = Mn and Co) perovskites had vibration band around 600 cm⁻¹, which could be attributed to the characteristic absorption band of the stretching vibration of Co-O and Mn-O band of BO₆ octahedron. These strong absorption bands are indicating the formation of the perovskite-type structures and found to be shifted towards higher frequency with increases in Ce substitution concentration. Moreover, the intensity of this vibration band increases for suggesting the change of oxidation state or oxygen vacancies. For the Ce-substituted LaCoO₃ perovskites, a peak observed at 663 cm⁻¹ which can be related to the existence of cobalt cations into Co²⁺ and Co³⁺ valence states may be attributed to vibration of Co-O bonds in Co₃O₄ structure.

(a)

(b)

The absorption peak at 2361 cm⁻¹ and 1644 cm⁻¹ is due to the deformation mode of absorbed molecular water of the carrier KBr(H₂O)n and CO₂, respectively [18, 19]. The broad band in the region of 3400 cm⁻¹ and 1640 cm⁻¹ is related to the H-O stretching and H-O-H bending vibration, which are associated with citrates and/or water molecules coordinated with the metal ions.

3.3. Structural Properties

Phase identification of the La_1−xCe_xCoO₃ and La_1−xCe_xMnO₃ ( = 0, 0.2 and 0.4) perovskites, based on the XRD results, is shown in Figure 2. Typical perovskite peaks for nonsubstituted samples are well resolved with sharp and intense single peak in the pure LaCoO₃ and LaMnO₃ pattern [20]. Cerium substitution at higher levels cannot be incorporated into the lattice and leads to the formation of a separate CeO₂ phase (observed at 2θ = 28.6° and 56.5°). For = 0, the structures were the rhombohedral LaCoO₃-type (JCPDS-ICDD 25-1060); when = 0.2 and 0.4, the samples exhibited the pattern of cubic LaCoO₃ (JCPD-ICDD 75-0279). The peaks at 32.5° are found to merge single peak which indicates the transformation of structure from rhombohedral to cubic. This characteristic is a drastic indication for Ce substitution influence leading to a space group transformation into cubic lattice symmetry. When is 0.4, characteristic peaks of the Co₃O₄ phase appeared around 37.1° and 56.4°. This conclusion confirms FTIR analysis result. The patterns of LaMnO₃ correspond to the rhombohedral perovskite-type (JCPDS-ICDD 86-1234) and the same lattice geometry observed in Ce-substituted perovskites [21–23]. The substitution , whose peak in the inset is not as sharp as that for the case , and more unreacted CeO₂ aggregates appear.

(a)

(b)

Therefore, its intensity increases with the addition of cerium due to solubility limit mentioned in the introduction section. The peaks between 32 and 33° indicate that augmentation of Ce concentration leads to broadening and peak shifts in the XRD pattern due to changes or distortions of the cell lattice.

Table 2 summarizes the crystallite size, the BET specific surface area, and lattice parameters of all samples. The average crystallite size was determined using Debye-Scherrer’s equation , where is the incident X-ray wavelength , β is full width at half maximum (FWHM) of the peak corresponding to maximum intensity, and θ represents the diffraction angle of the most intense peak in degrees. The crystallite size of the samples was found to be in the range of 13 to 8 nm for La_1−xCe_xCoO₃ and relatively larger 22–14 nm for La_1−xCe_xMnO₃ which decreases with Ce content augmentation. That should be expected since the higher Ce⁴⁺ coordination with their surrounding oxygen atoms (within the same crystal plane) than trivalent La³⁺ tends to inhibit crystal growth, resulting in smaller crystal size [24]. Therefore, the crystallite size was increased in La_0.6Ce_0.4MnO₃ perovskite due to Ce segregation.

3.4. Specific Surface Area

La_1−xCe_xMnO₃ perovskites showed higher specific surface areas (SSA) than La_1−xCe_xCoO₃ which are 19–27 m²/g and 2–5 m²/g, respectively (Table 2). Calcination at high temperature is necessary to obtain the perovskite-type oxides, but such treatment often results in a dramatic decrease in the specific surface area (LaMnO₃ < 30 m²/g and LaCoO₃ < 10 m²/g). Generally, the substituted samples have a larger SSA than pure perovskites and the enhancement was not linear with the substitution. When cerium addition was 0.2, a significant increase in SSA appeared, when = 0.4, with the increased proportion of additional phases (CeO₂) as shown in XRD profiles and their SSA decreased.

The particle size obtained by XRD and BET can be compared to get the information about agglomeration. If defining as a factor to reflect the agglomeration extent of the primary crystalline, φ value of 1.0 indicates no agglomeration [25]. In this work, agglomeration factors were 2.8, 1.57, and 1.38 for La_1−xCe_xCoO₃ with = 0, 0.2 and 0.4, respectively. These results suggest that our synthesized perovskites have agglomeration. However, agglomeration factors of La_1−xCe_xCoO₃ show better results compared to La_1−xCe_xMnO₃ perovskites ( = 1.9, 2.15, and 1.53 for = 0, 0.2, and 0.4, resp.).

3.5. Surface Analysis

The surface compositions of perovskites were studied by XPS. Any charging shift produced in the spectrum by the sample was corrected by taking C 1s position (284.6 eV) as a reference line. Table 3 lists the corresponding binding energies of La 3d, O 1s, Ce 3d, Co 2p, and Mn 2p of La_1−xCe_xCoO₃ and La_1−xCe_xMnO₃ perovskite ( = 0, 0.2, and 0.4) samples. The binding energy values of La 3d were recorded around 834 and 851 eV. The other two peaks at 837 and 854 eV are La 3d satellite peaks. These peak positions are similar to the values recorded from pure La₂O₃ [26], indicating that La ions were in a trivalent state. Moreover, the La 3d_5/2 and La 3d_3/2 peaks shifted to higher energy for of La_0.8Ce_0.2CoO₃, which probably connected with different chemical surroundings (Figure 3(a)). Co 2p peaks were occupied at approximately 779.9 eV which indicates Co³⁺ is dominant in of La_1−xCe_xCoO₃ perovskites. When = 0.2, the peak shifted to 780.3 eV that reveals the increasing of Co²⁺ ion content and besides the peak shifted back at = 0.4. According to the XRD results, no Co₃O₄ phase was observed in = 0.2 perovskite. These findings indicate that Ce⁴⁺ substitution created Co have lower oxidation states as a charge compensation mechanism. The peaks of Mn 2p_3/2 and Mn 2p_1/2 are located at 641.8 and 653.1 eV and assigned to Mn³⁺ ions. Moreover, no peak shift was observed for La_0.8Ce_0.2MnO₃.

(a)

(b)

(c)

(d)

Figure 3(c) shows the O 1s core-level spectra of perovskites. The peak at 530.9 eV for the perovskite is due to O²⁻ ions in the lattice and the peak at 532.7 eV can be attributed to adsorbed oxygen such as OH⁻ whereas adsorbed molecular water was at above 533.2 eV. The adsorbed oxygen decreased at = 0.2 and 0.4 in which proposing cation vacancy defects could be generated as the substitution of Ce ions of perovskites. The ratio of adsorbed and lattice oxygen decreased at = 0.2 of all perovskites which are believed to support reducible oxide structure [27]. XPS spectra of Ce 3d are shown in Figure 3(d). Six different characteristic peaks are indexed to Ce⁴⁺ and presence of Ce³⁺ ions (inset graph of Figure 3(d)) can also be revealed in perovskites. The Ce⁴⁺ and Ce³⁺ atomic percentages have been obtained from the area of the peaks by the CasaXPS fitting program (Table 3).

3.6. Temperature-Programmed Reduction

Temperature-programmed reduction profile of La_1−xCe_xCoO₃ and La_1−xCe_xMnO₃ perovskites are shown in Figure 4. All of the TPR patterns of perovskites including sharp peaks suggest that well-defined crystalline structure is formed. Royer et al. [28] reported two successive steps in TPR profile of LaCoO₃ perovskite. The first reduction step occurs at low temperature (<500°C) which reduces Co³⁺ into Co²⁺. The second reduction step (reduction of the Co²⁺ into Co⁰) starts at the temperature higher than 600°C. Since La³⁺ was nonreducible under the conditions of H₂-TPR, the observed H₂ consumption peaks in the TPR profile of LaCoO₃ were due to the reduction of cation only.

(a)

(b)

As seen from H₂-TPR profile (Figure 4(a)), the H₂ consumption provides evidence for the complete reduction of Co³⁺ to Co⁰ occurring in two steps from Co³⁺ to Co²⁺ with a peak at about 400°C and Co²⁺ to Co⁰ centered at about 600°C in agreement with the literature [28]. For La_0.6Ce_0.4CoO₃ perovskite, three reduction peaks were detected which suggest a multiple-step reduction (Table 4). Compared with the pure LaCoO₃ and LaMnO₃ perovskites, the two reduction peaks of the Ce-substituted samples all shift to lower temperature direction correspondingly. And = 0.2 substitution leads to the highest decrease in the reduction peak temperatures and creates an easier reducibility of the Co³⁺ into Co²⁺. The H₂-TPR results supported XPS analysis by showing that cerium substitution increased the reducibility, especially at the temperature range of 300–550°C, suggesting that Ce⁴⁺ increased the number of cation vacancies within the lattice.

Figure 1(b) shows the H₂-TPR profile of La_1−xCe_xMnO₃ perovskite catalysts. H₂ consumption provides evidence for the reduction of Mn⁴⁺ to Mn²⁺ occurring in two steps from Mn⁴⁺ to Mn³⁺ with a peak maximum at 542°C and reduction of Mn³⁺ to Mn²⁺ at 798°C. For La_0.8Ce_0.2MnO₃ perovskite catalyst, first peak at 571°C and the second peak at 798°C were observed (Table 4). The Ce⁴⁺ insertion decreased the catalyst reducibility of Mn⁴⁺ to Mn³⁺ for LaMnO₃ perovskite, shifting reduction peaks to the higher temperature. But cerium has less influence on the reduction of Mn³⁺ to Mn²⁺. This indicates that the Mn is reduced to +3 during cerium substitution [29]. Based on the H₂-TPR results, it is indicated that the La_1−xCe_xCoO₃ showed higher reducibility (inset of Figure 4) than La_1−xCe_xMnO₃ which was beneficial for catalyst application.

3.7. Catalytic Activity

The values of propane conversion and selectivity as a function of reaction temperature are shown in Figure 5. It can be seen that all Ce-substituted catalysts have better activity than the pure perovskite. Pure LaCoO₃, as prepared in this work as reference, exhibited very low activity (37.7% conversion and 78.8% selectivity at 500°C). In the case of perovskite samples, ever since the addition of cerium, the large enhancement of the activity for dehydrogenation of propane was observed and the maximum activity point moved to the lower temperature.

(a)

(b)

The sample with La_1−xCe_xCoO₃ ( = 0.2) gives the best catalytic performance, about 54.6% conversion and 76.8% selectivity at 500°C. When is 0.4, conversion of 53.7% and selectivity of 75.3% were obtained at the same temperature. At temperatures higher than 500°C, species coexist with metallic Co⁰ (supported by H₂-TPR experiment) which leads to undesirable methanation reaction [15, 30]. Compared with the perovskite-type oxides, the activity of pure Co₃O₄ was low, and CeO₂ did not even show any activity for dehydrogenation of propane which is not mentioned in these figures. Thus, the additional phases would not contribute much to the catalytic activity directly [13]. Consequently, the La_0.8Ce_0.2CoO₃ catalyst shows the highest activity and selectivity to propylene on dehydrogenation of propane.

4. Conclusion

In this work, lanthanum-based perovskite-type oxides La_1−xCe_xBO₃ (B = Mn and Co, = 0, 0.2, and 0.4) were successfully synthesized by sol-gel method and investigated as a catalyst. Structural investigations indicated that La_1−xCe_xMnO₃ had a single perovskite structure of rhombohedral and La_1−xCe_xCoO₃ exhibited a transformation in the phase structure (from rhombohedral to the cubic) with increasing cerium content. The estimated optimal average crystallite size is found to be less than 15 nm for both samples. BET results showed an increase (not linear) in the specific surface area upon Ce content. The catalytic activities in the dehydrogenation of propane were enhanced significantly with Ce substitution and achieved the best when was 0.2 but decreased at 0.4. The cerium substitution when = 0.2 leads to an increase of cation vacancies as charge compensation mechanism and results in enhancement of the catalytic activity, the reducibility, and the selectivity.

Among these catalysts, La_0.8Ce_0.2CoO₃ catalyst shows best performances with high catalytic activity, selectivity, and stability, suggesting that it may a promising candidate for the catalyst applications. La_1−xCe_xMnO₃ ( = 0, 0.2) perovskites with rhombohedral structure show the poorest reducibility as well as the highest specific surface areas. In addition, since we confirmed that the present sol-gel method can be used for perovskite-type oxides with different compositions, it may be useful for SOFC catalyst materials.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors acknowledge the Institute of Physics and Technology, MAS, for the financial support. The authors also thank Professor O. Tegus of Inner Mongolia Normal University, Key Laboratory of Physics and Chemistry, for performing some experiments.

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Copyright © 2017 Nyamdavaa Erdenee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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