Lanthanum-Based Perovskite-Type Oxides La 1 − x Ce x BO 3 ( B = Mn and Co ) as Catalysts : Synthesis and Characterization

La1−xCexCoO3 (x = 0, 0.2, 0.4) and La1−xCexMnO3 (x = 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 2–27m g. 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 (x = 0.2) showed smaller crystallite size, higher SSA, and the highest reducibility and catalytic activity for LaCoO3.


Introduction
Lanthanum-based perovskite-type oxides with general formula ABO 3 (A = La 3+ , Pr 3+ , Nd 3+ , etc.; A = Co 3+ , Mn 3+ , Sr 3+ , etc.) may potentially replace noble metal catalysts due to their high catalytic activity, thermal stability, and low costs [1][2][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.LaBO 3 (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 ABO 3 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., La 3+ by Ce 4+ or Sr 2+ ) 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, Xray photoelectron spectroscopy (XPS), and temperatureprogrammed reduction of H 2 (H 2 -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−x Ce x MnO 3 and La 1−x Ce x CoO 3 ) show promising structural, surface, electronic states and catalytic properties for catalyst application.

Experimental
2.1.Synthesis of the Perovskite.All La 1−x Ce x CoO 3 and La 1−x Ce x MnO 3 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(NO 3 ) 3 ⋅6H 2 O) (Roth, 99.995%), cerium (III) nitrate hexahydrate (Ce(NO 3 ) 3 ⋅6H 2 O) (Aldrich, 99.95%), and cobalt (II) nitrate monohydrate (Co(NO 3 ) 2 ⋅H 2 O) (Roth, 99.995%), and manganese (II) nitrate hexahydrate (Mn(NO 3 ) 2 ⋅6H 2 O) (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.

Characterization.
XRD data were obtained by using Maxima_X, XRD-7000 equipment with CuK  radiation at room temperature.The structural parameters were determined by Rietveld analysis of the diffraction profiles.XAS measurements for Ce L 3 , La L 3 , 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 N 2 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 84.0 ± 0.1 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 H 2 (H 2 -TPR) were carried out on Chem BET TPR/TPD Chemisorption Analyzer from Quantachrome. 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 (Φ = 8 mm).The remaining space of the reactor was filled with quartz sand to minimize possible homogeneous reaction [17].Typical feed gases used were = 2 mL/min, and V(N 2 ) = 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 CH

Results and Discussion
3.1.Elemental Analysis.The elemental compositions of the La 1−x Ce x CoO 3 and La 1−x Ce x MnO 3 ( = 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.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(H 2 O)n and CO 2 , 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.

Structural Properties.
Phase identification of the La 1−x Ce x CoO 3 and La 1−x Ce x MnO 3 ( = 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 LaCoO 3 and LaMnO 3 pattern [20].Cerium substitution at higher levels ( > 0.15) cannot be incorporated into the lattice and leads to the formation of a separate CeO 2 phase (observed at 2 = 28.6 ∘ and 56.5 ∘ ).For  = 0, the structures were the rhombohedral LaCoO 3 -type (JCPDS-ICDD 25-1060); when  = 0.2 and 0.4, the samples exhibited the pattern of cubic LaCoO 3 (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 correspond to the rhombohedral perovskite-type (JCPDS-ICDD 86-1234) and the same lattice geometry observed in Ce-substituted perovskites [21][22][23].The substitution  = 0.4, whose peak in the inset is not as sharp as that for the case  = 0.2, and more unreacted CeO 2 aggregates appear.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  = 0.9/ cos , where  is the incident X-ray wavelength ( Cu = 1.5443Å),  is full width at half maximum (FWHM) of the peak corresponding to maximum intensity, and  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−x Ce x CoO 3 and relatively larger 22-14 nm for La 1−x Ce x MnO 3 which decreases with Ce content augmentation.That should be expected since the higher Ce 4+ coordination with their surrounding oxygen atoms (within the same crystal plane) than trivalent La 3+ tends to inhibit crystal growth, resulting in smaller crystal size [24].Therefore, the crystallite size was increased in La 0.6 Ce 0.4 MnO 3 perovskite due to Ce segregation.

Specific Surface
Area.La 1−x Ce x MnO 3 perovskites showed higher specific surface areas (SSA) than La 1−x Ce x CoO 3 which are 19-27 m 2 /g and 2-5 m 2 /g, respectively (Table 2).Calcination at high temperature is necessary to obtain the perovskitetype oxides, but such treatment often results in a dramatic decrease in the specific surface area (LaMnO 3 < 30 m 2 /g and LaCoO 3 < 10 m 2 /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 (CeO 2 ) 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  =  BET / XRD as a factor to reflect the agglomeration extent of the primary crystalline,  value of 1.0 indicates no agglomeration [25].In this work, agglomeration factors were 2.8, 1.57, and 1.38 for La 1−x Ce x CoO 3 with  = 0, 0.2 and 0.4, respectively.These results suggest that our synthesized perovskites have agglomeration.However, agglomeration factors of La 1−x Ce x CoO 3 show better results compared to La 1−x Ce x MnO 3 perovskites ( = 1.9, 2.15, and 1.53 for  = 0, 0.2, and 0.4, resp.).

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.( = 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 La 2 O 3 [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 0.8 Ce 0.2 CoO 3 , which probably connected with different chemical surroundings (Figure 3(a)).Co 2p peaks were occupied at approximately 779.9 eV which indicates Co 3+ is dominant in of La 1−x Ce x CoO 3 perovskites.When  = 0.2, the peak shifted to 780.3 eV that reveals the increasing of Co 2+ ion content and besides the peak shifted back at  = 0.4.According to the XRD results, no Co 3 O 4 phase was observed in  = 0.2 perovskite.These findings indicate that Ce 4+ 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 Mn 3+ ions.Moreover, no peak shift was observed for La 0.8 Ce 0.2 MnO 3 .
Figure 3(c) shows the O 1s core-level spectra of perovskites.The peak at 530.9 eV for the perovskite is due to O 2− 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 533.2eV.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 Ce 4+ and presence of Ce 3+ ions (inset graph of Figure 3(d)) can also be revealed in perovskites.The Ce 4+ and Ce 3+ atomic percentages have been obtained from the area of the peaks by the CasaXPS fitting program (Table 3).As seen from H 2 -TPR profile (Figure 4(a)), the H 2 consumption provides evidence for the complete reduction of Co 3+ to Co 0 occurring in two steps from Co 3+ to Co 2+ with a peak at about 400 ∘ C and Co 2+ to Co 0 centered at about 600 ∘ C in agreement with the literature [28].For La 0.6 Ce 0.4 CoO 3 perovskite, three reduction peaks were detected which suggest a multiple-step reduction (Table 4).Compared with the pure LaCoO 3 and LaMnO 3 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 Co 3+ into Co 2+ .The H 2 -TPR results supported XPS analysis by showing that cerium substitution increased the reducibility, especially at the temperature range of 300-550 ∘ C, suggesting that Ce 4+ increased the number of cation vacancies within the lattice.
Figure 1(b) shows the H 2 -TPR profile of La 1−x Ce x MnO 3 perovskite catalysts.H 2 consumption provides evidence for the reduction of Mn 4+ to Mn 2+ occurring in two steps from Mn 4+ to Mn 3+ with a peak maximum at 542 ∘ C and reduction of Mn 3+ to Mn 2+ at 798 ∘ C. For La 0.8 Ce 0.2 MnO 3 perovskite catalyst, first peak at 571 ∘ C and the second peak at 798 ∘ C were observed (Table 4).The Ce 4+ insertion decreased the catalyst reducibility of Mn 4+ to Mn 3+ for LaMnO 3 perovskite, shifting reduction peaks to the higher temperature.But cerium has less influence on the reduction of Mn 3+ to Mn 2+ .This indicates that the Mn is reduced to +3 during cerium substitution [29].Based on the H 2 -TPR results, it is indicated that the La 1−x Ce x CoO 3 showed higher reducibility (inset of Figure 4) than La 1−x Ce x MnO 3 which was beneficial for catalyst application.

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 LaCoO 3 , 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.The sample with La 1−x Ce x CoO 3 ( = 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, Co n+ species coexist with metallic Co 0 (supported by H 2 -TPR experiment) which leads to undesirable methanation reaction [15,30].Compared with the perovskite-type oxides, the activity of pure Co 3 O 4 was low, and CeO 2 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 0.8 Ce 0.2 CoO 3 catalyst shows the highest activity and selectivity to propylene on dehydrogenation of propane.

Conclusion
In this work, lanthanum-based perovskite-type oxides La 1−x Ce x BO 3 (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−x Ce x MnO 3 had a single perovskite structure of rhombohedral and La 1−x Ce x CoO 3 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 0.8 Ce 0.2 CoO 3 catalyst shows best performances with high catalytic activity, selectivity, and stability, suggesting that it may a promising candidate for the catalyst applications.La 1−x Ce x MnO 3 ( = 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.

Figure 1 ,
synthesized La 1−x Ce x BO 3 (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 BO 6 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  = 0.2 suggesting the change of oxidation state or oxygen vacancies.For the Ce-substituted LaCoO 3 perovskites, a peak observed at 663 cm −1 which can be related to the existence of cobalt cations into Co 2+ and Co 3+ valence states may be attributed to vibration of Co-O bonds in Co 3 O 4 structure.

Figure 5 :
Figure 5: Conversion of propone (a) and selectivity (b) as a function of the temperature of reaction for catalysts La 1−x Ce x CoO 3 ( = 0, 0.2, and 0.4).

Table 1 :
Weight percentages of the elements present in the samples obtained by EDS analysis.
∘ C. The consumption of H 2

Table 3 :
XPS peak positions and atomic percentages of La1− Ce  CoO 3 and La 1− Ce  MnO 3 perovskites obtained from the fitting of La 3d, Co 3p, Mn 3p, Ce 3d, and O1s XPS spectra.Ce 0.2 CoO 3 La 0.6 Ce 0.4 CoO 3 LaMnO 3 La 0.8 Ce 0.2 MnO 3 1The value in parenthesis is a peak percentage (%) of the Ce 3+ ion.lattice symmetry.When  is 0.4, characteristic peaks of the Co 3 O 4 phase appeared around 37.1 ∘ and 56.4 ∘ .This conclusion confirms FTIR analysis result.The patterns of LaMnO 3 Table 3 lists the corresponding binding energies of La 3d, O 1s, Ce 3d, Co 2p, and Mn 2p of La 1−x Ce x CoO 3 and La 1−x Ce x MnO 3 perovskite

Table 4 :
[28]consumed for the first step and second step of reduction on La 1− Ce  CoO 3 and La 1− Ce  MnO 3 (x = 0, 0.2, and 0.4) perovskite catalysts.Ce x MnO 3 perovskites are shown in Figure4.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 LaCoO 3 perovskite.The first reduction step occurs at low temperature (<500 ∘ C) which reduces Co 3+ into Co 2+ .The second reduction step (reduction of the Co 2+ into Co 0 ) starts at the temperature higher than 600 ∘ C. Since La 3+ was nonreducible under the conditions of H 2 -TPR, the observed H 2 consumption peaks in the TPR profile of LaCoO 3 were due to the reduction of Co n+ cation only.