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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2017.12.006
Open Access
The effects of A/B-site substitution on structural, redox and catalytic properties of lanthanum ferrite nanoparticles
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Ahmad Gholizadeh
School of Physics, Damghan University (DU), Damghan, I.R., Iran
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Tables (5)
Table 1. Catalytic performance (%), CO oxidation temperatures (°C), for La1−xSrxFe1−yCoyO3 catalysts.
Table 2. The structure type and unit cell parameters, crystal volume (V), unit cell volume (V/z) and IR absorption bands (v1, v2) of La1−xSrxFe1−yCoyO3.
Table 3. Rietveld refined structural parameters, Atomic positionsa, bond lengths and bond angles of La1−xSrxFe1−yCoyO3.
Table 4. the values of crystallite size and strain of La1−xSrxFe1−yCoyO3 catalysts obtained from Scherrer and H–W methods.
Table 5. Electric conductivitya, activation energy and band gap energies of La1−xSrxFe1−yCoyO3 catalysts.
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Abstract

In this study, LaFe1−yCoyO3 and La1−xSrxFe0.5Co0.5O3 nano-particles with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60 were prepared using the citrate method. The samples were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and tunneling electron microscopy. Detailed structural analysis of the samples was conducted through refining via the Rietveld method and using the Fullprof program. The structural analysis of LaFe1−yCoyO3 revealed that orthorhombic-toward-rhombohedral phase transition with Co doping and rhombohedral-to-cubic phase transformation took place in La1−xSrxFe0.5Co0.5O3 due to an increase in the Sr content. The electrical conductivity and catalytic activity of La1−xSrxFe1−yCoyO3 were also investigated. The results of the structural analysis of LaFe1−yCoyO3 pointed out to the presence of Co2+, Co3+, Fe3+ and Fe4+ ions that would contribute to the overall oxidation activity of these samples. In addition, substituting the lower ionic radius transition metal would decrease the temperature of the complete CO conversion. The increase in the catalytic activity of the sample y=0.50 in series of LaFe1−yCoyO3 could be mainly attributed to (i) the presence of Fe3+OFe4+ and Co2+OCo3+ couples, (ii) a rhombohedral structure with higher symmetry, and also (iii) the lower value of activation energy and higher value of σOx/σRed. However, substituting Sr for La could increase the temperature of the CO conversion. LaFe0.5Co0.5O3 exhibited 95% CO conversion at 536K.

Keywords:
Nano-particles
Ferrite-cobaltite
X-ray diffraction analysis
Structural phase transition
Electrical conductivity
CO oxidation
Full Text
1Introduction

The catalytic combustion of volatile organic compounds (VOCs) is one of the most important techniques to remove these pollutants from the air stream. This can be achieved at low temperatures by using a large number of supported noble metals [1]; however, due to their high costs and problems related to the sintering of noble metals, base metal oxides are commonly used as an alternative. Of base metal oxides, Perovskite-type oxides (ABO3) have proved themselves as promising catalysts for the complete oxidation of CO [2].

The perovskites can be expressed through the formula of ABO3, where A-site cations are alkaline or rare earth metal elements such as La, Sr or Ba, Sr at while B-site cations are 3d, 4d, or 5d transition metal elements such as Mn, Fe and/or Co. Regardless of rare earth elements at the A-site, the activity patterns of CO oxidation by ABO3 point out to three peaks at Mn, Fe, and Co [1]. Ciambelli et al. [3] studied samples of AFeO3 (A=La, Nd, Sm); their analysis provided evidence on excellent catalytic activity in methane combustion and CO oxidation for LaFeO3 compared to the other samples. In addition, Lima et al. [4] showed that nanocast LaFeO3 was more actively involved in reducing NO than its uncast counterparts, which could be attributed to increase in its specific surface area (SSA). There is also some evidence that LaFeO3 is a weak ferromagnetic element with a Neel temperature of 750K [5]. Of lanthanum-based perovskites, Mn-, Co-, and Fe-containing structures have been reported as the most active and promising catalysts for the complete oxidation of CO. On the other hand, the non-substituted LaCoO3−δ perovskite shows high non-stoichiometry and δ decreases with increase in heat-treatment temperature [6]. In addition, at low temperatures, the diamagnetic ground state of LaCoO3 results from the diamagnetic low-spin (S=0) ground state of Co3+ ions. In this study, a transition from the LS state to the intermediate spin (IS) state, with S=1, was observed at about T=90K. In ABO3 perovskites, the oxidation state of the B3+ ion can be changed by substituting the ions having an oxidation state other than 3 for the A3+ ion. Perovskite LaCoO3 adopts an insulator-antiferromagnetic structure at room temperature due to the absence of Co4+ ions [2,7,8]. In LaCoO3, by substituting the divalent elements of Sr and Ca for La3+, the trivalent Co ions are converted to a mixture of ions Co3+ and Co4+[2,7]. However, since Co4+ is unstable, this substitution increases CO oxidation reactions as a result of an increase in oxygen vacancies [7]. By contrast, insertion of Ce4+ in La-site leads to partial transformation of Co3+ to Co2+, thus enhancing the rate of CO oxidation [8]. Further, lanthanum–strontium cobaltites exhibit large oxygen deficiencies and different spin states [9].

Many researchers believe that the catalytic activity for the complete oxidation of CO is mainly determined by B elements [9–11]. Partial substitution of the B site with other trivalent B′ cations can increase the conversion of CO in perovskites [10,11]. It is worth noting that changes in the catalytic activity of AB1−yB′yO3 perovskites can be classified into two categories: i.e., geometric or electronic [11]. Structural and magnetic analysis of these changes, which are induced by replacement of B cations, can be conducted in order to obtain information about the geometric and electronic factors affecting the catalytic activity for the complete oxidation of CO. For example, Zhong et al. [12] reported that LaFe1−xMxO3 (M=Al, Mn, Co) perovskites would show better performance in methane oxidation due to the simultaneous presence of Fe and M cations. Also, Gholizadeh et al. [9,13] provided evidence that increase of Co substitution in LaMn1−xCoxO3 and La0.7Sr0.3Mn1−xCoxO3 up to 0.50 would increase the conversion temperature of CO oxidation due to the presence of Mn4+Co2+. In a recent study, Gholizadeh et al. [10] investigated samples of La1−xMxMn0.5Co0.5O3 (M=Sr, Ca) catalysts; they claimed that substituting Sr and Ca for La in LaMn0.5Co0.5O3 would result in lower and higher temperature conversion of CO oxidation, respectively. Rousseau et al. [14] too observed that slight substitution of Fe for Co with y=0.2 in La1−xSrxCo1−yFeyO3 would increase the catalytic activity for the complete oxidation of toluene. In another study, Deng et al. [15] synthesized samples of La1−xSrxM1−yFeyO3 (M=Mn, Co; x=0.0, 0.4; y=0.1, 1.0) catalysts; the researchers reported the complete oxidation of toluene at 245°C over La0.6Sr0.4Co0.9Fe0.1O3, ascribing this to the presence of Fe3+OFe4+ couples and a transition of electronic structures. To sum up the above literature review, differences in samples, preparation method, substitutions, and the nature of pollutants can be the origin of discrepancies in catalytic activities.

In the current study, structural, redox, and catalytic properties of LaFe1−yCoyO3 and La1−xSrxFe0.5Co0.5O3 catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60 prepared by the citrate method are investigated. Attempts are made to find out the influence of substituting Co and Sr in Fe-based perovskites on the catalytic activity of La1−xSrxFe1−yCoyO3 for CO oxidation. The findings of the current study are of significant importance for understanding the oxidation process of the perovskite-type CO catalyst.

2Experimental

The La1−xSrxFe1−yCoyO3 nano-catalysts with x=0.00, 0.15, 0.30, 0.45, and 0.60 and y=0.00 and 0.50 were prepared by the citrate method by using metal nitrate precursor in the presence of citric acid and similar to the recipe reported elsewhere [9,10,13]. A solution containing appropriate concentrations of metal nitrates La(NO3)3·6H2O, Fe(NO3)3·9H2O, Co(NO3)2·6H2, and Sr(NO3)2 and citric acid, equal to the total number of moles of nitrate ions, was first evaporated at 60°C overnight. The homogeneous sol-like substance was subsequently dried at 80°C overnight. The resulting spongy and friable material was completely powdered and then kept at 200°C overnight. This precursor was heated at 600°C for 5h to remove any carbonaceous materials. The resulting material was again powdered and subsequently sintered at 900°C for 5h to obtain the product. For convenience, a list of the abbreviations is given in Table 1.

Table 1.

Catalytic performance (%), CO oxidation temperatures (°C), for La1−xSrxFe1−yCoyO3 catalysts.

x  y  Abbreviation sample #  Temperature of CO conversion (K)
      10%  50%  95% 
0.00  0.00  S0C0  494  574  703 
0.00  0.25  S0C25  482  538  616 
0.00  0.50  S0C50  435  473  536 
0.00  0.75  S0C75  475  530  586 
0.00  1.00  S0C100  508  564  635 
0.15  0.50  S15C50  468  538  597 
0.30  0.50  S30C50  445  502  574 
0.45  0.50  S45C50  455  526  596 
0.60  0.50  S60C50  530  586  641 
–  –  La0.7Bi0.3MnO3[18]  438  467  483 
–  –  SmMnO3[19]  482  508  573 
–  –  La0.5Sr0.5Mn0.5Co0.5O3[10]  388  433  448 
–  –  La0.4Ca0.6Mg0.5Co0.5O3[20]  407  465  540 
–  –  LaFe0.8Mg0.2O3[3]  535  606  655 

In this study, the XRD patterns were recorded using a Bruker AXS diffractometer, i.e., D8 ADVANCE (Bruker-AXS, Karlsruhe, Germany), with Cu Kα radiation in the range of 2θ=20–80° at room temperature. The XRD patterns were then analyzed using a commercial X’pert package and the Fullprof program. Following [16], the values of the crystallite size (D) and strain (ɛ) were calculated by using the Scherrer equation and Williamson–Hall (W–H) method.

The FT-IR spectra of the samples were recorded on a Perkin-Elmer FT-IR spectrometer in the wave-number range of 400–1000cm−1. The morphology of the samples was studied through the SEM analysis (Philips XL30) (Philips, Eindhoven, and the Netherlands) and the particle size of the samples was measured by the TEM analysis (LEO Model 912AB) (GmbH, Oberkochen, Germany). The band gap energy of the samples was estimated through the optical absorption spectra of La1−xSrxFe1−yCoyO3 nano-catalysts recorded between 200 and 1100nm wavelengths. The following relation holds between the optical absorption coefficient, α (λ), and the optical band gap energy of a direct band transition [17]:

where B is an energy-independent constant. α(t)=2.303A(λ)/t is the optical absorption coefficient where A(λ) and t represent the absorption spectra and the mean particle size of the sample, respectively. The band gap energy of the samples is estimated by extrapolating the linear part of (αhv)2 versus the hv plot.

A quartz tube with two extra pure-gold wires as parallel electrodes inserted on either side of the inner wall of the cell was used for measuring electrical conductivity. The electrical conductivity of the samples in the air atmosphere (i.e., oxidizing conditions) between the lab temperature to the final temperature was measured by randomly increasing the temperature to 350°C. The reducibility properties of the catalysts were examined through measurement of electrical conductivity of the samples in oxidation (air) and reduction (6%CO in Ar) atmosphere under steady-state conditions at 350°C.

The catalytic tests of the oxidation reactions of CO over the La1−xSrxFe1−yCoyO3 catalysts were undertaken in an experimental set-up using a quartz tube, which was filled with 200mg of the 60–100 mesh-sized catalyst supported on ceramic wool under GHSV of 12,000h−1[18]. In a typical experiment, a model of exhaust gas containing a mixture of 6% CO, in Ar, and air (a stoichiometric ratio with respect to oxygen) was passed through the catalyst bed with a total gas mixture flow rate of 40mL/min at STP. Examination of the catalytic tests was carried out through raising the temperature at random intervals ranging from 50°C to the temperature of complete oxidation. The product stream was analyzed through a GC on an FI detector.

3Results and discussion

Table 1 shows the results of 10%, 50% and 95% of CO oxidation by LaFe1−yCoyO3 and La1−xSrxFe0.5Co0.5O3 nano-catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60. In the Co-substituted samples, an increase in Co replacement for x up to 0.5 in LaFe1−yCoyO3 nano-catalysts contributes to the better transfer of oxygen to the adsorbed CO at the same temperature on the catalyst surface. Also, in the La1−xSrxFe0.5Co0.5O3 samples, the sample S30C50 shows higher catalytic activity for CO conversion at lower temperatures compared to the other samples. By looking at Table 1, it can be observed that, of the samples scrutinized in this study, the sample S0C50 is the best choice for CO conversion and it also showed the best CO catalytic activity among the samples in Table 1. To explain the above catalytic activity, the samples were examined through the techniques of XRD, SEM, FTIR, and TEM and the measurements of electrical conductivity.

The XRD patterns of the LaFe1−yCoyO3 and La1−xSrxFe0.50Co0.50O3 nano-catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60 are shown in Fig. 1. As shown for the XRD pattern of the S0C50, S0C75, S0C100, S15C50, and S30C50 samples, the small splitting of the peaks of the perovskite at about 33°, 41°, 58°, 68°, and 78° is an indication of a rhombohedral lattice. However, the XRD patterns of the S0C0, S0C25, S45C50, and S60C50 samples fit a different structure.

Fig. 1.

XRD patterns of La1−xSrxFe1−yCoyO3.

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To perform Rietveld refinement, good initial values for lattice parameters and the type of the space group obtained from the phase analysis using the X’pert package are needed. The Rietveld analysis of the S0C0, S0C50 and S0C100 samples using the Fullprof program indicated the best fit with the least difference as shown in Fig. 2. The results of the analysis for the S0C0 and S0C25 samples pointed out to the formation of an orthorhombic structure (the space group Pbnm). Also, the Rietveld analysis of the S0C50, S0C75, S0C100, S15C50, and S30C50 samples pointed out to the formation of a rhombohedral structure (the space group R-3c). On the other hand, the XRD patterns of the S60C50 and S60C50 samples fit a cubic structure (the space group Pm-3m). The lattice parameters and structures of the samples examined in this study are given in Table 2. Also, the atomic positions, bond lengths and bond angles of the La1−xSrxFe1−yCoyO3 samples are shown in Table 3.

Fig. 2.

The Rietveld analysis of S0C0, S0C50, S0C100 and S60C50 using Fullprof program.

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Table 2.

The structure type and unit cell parameters, crystal volume (V), unit cell volume (V/z) and IR absorption bands (v1, v2) of La1−xSrxFe1−yCoyO3.

Sample #  Structure (space group)  Lattice parameters  V (Å3V/z (Å3v1 (cm−1v2 (cm−1
S0C0  Orthorhombic (P bnmI)  a=5.5473 (Å),
b=5.5582 (Å),
c=7.8463 (Å) 
242.23  60.56  555  600 
S0C25  Orthorhombic (P bnmI)  a=5.5133 (Å),
b=5.5312 (Å),
c=7.8413 (Å) 
239.12  59.78  565  612 
S0C50  Rhombohedral (R-3c)  a=b=c=5.4299 (Å)
α=β=γ=60.547 (°) 
114.61  57.305  603  525 
S0C75  Rhombohedral (R-3c)  a=b=c=5.39213 (Å)
α=β=γ=60.753 (°) 
112.74  56.37  603  532 
S0C100  Rhombohedral (R-3c)  a=b=c=5.3749 (Å)
α=β=γ=60.720 (°) 
111.58  55.79  598  538 
S15C50  Rhombohedral (R-3c)  a=b=c=5.4350 (Å)
α=β=γ=60.432 (°) 
114.63  57.32  603  525 
S30C50  Rhombohedral (R-3c)  a=b=c=5.4275 (Å)
α=β=γ=60.330 (°) 
113.90  56.95  603  525 
S45C50  Cubic (Pm-3m)  a=b=c=3.8721 (Å)  58.05  58.05  603  508 
S60C50  Cubic (Pm-3m)  a=b=c=3.8657(Å)  57.77  57.77  606  508 
Table 3.

Rietveld refined structural parameters, Atomic positionsa, bond lengths and bond angles of La1−xSrxFe1−yCoyO3.

    S0C0  S0C25  S0C50  S0C75  S0C100  S15C50  S30C50  S45C50  S60C50 
Atomic positions
La/Sr  x  −0.0149  −0.0109               
  y  0.0256  0.0246               
O1  x  −1.0121  −1.010  0.4890  0.5205  0.55136  0.44666  0.4221     
  y  0.4993  0.4454               
O2  x  0.6753  0.6068               
  y  0.3282  0.3382               
  z  −0.0047  −0.0042               
Bond lengths and bond angles
Fe/Co-O1    1.966  1.957  1.94    1.93  1.951  1.950  1.940  1.934 
Fe/Co-O2    2.00  1.92               
La/Sr-O1    2.64  2.63  2.68    2.66  2.730  2.740  2.745  2.740 
La/Sr-O2    2.45  3.40               
Fe-O1-Fe    169.7  170.2  175.4    163.5  176.9  178.2  180  180 
Fe-O2-Fe    157.0  158.0               
a

Atomic positions for Pbnm are 4c(x, y, 0.25) for La/Sr, 4b(0.5, 0.0, 0.0) for Fe/Co, 4c(x, y, 0.25) for O1, and 8d(x, y, z) for O2.

Atomic positions for R-3c are 6a(0.0, 0.0, 0.25) for La/Sr, 6b(0.0, 0.0, 0.0) for Fe/Co, and 18e(x, 0.0, 0.25) for O.

Atomic positions for Pm-3m are 1a(0.0, 0.0, 0.0) for La/Sr, 1b(0.5, 0.5, 0.5) for Fe/Co, and 3c(0.5, 0.5, 0.0) for O. The values of x, y and z for each element in Pbnm and R-3c space groups are written in the table.

The XRD patterns of the La1−xSrxFe1−yCoyO3 catalysts at about ∼33° are shown in Fig. 3 for more comparison. Substituting Co in LaFe1−yCoyO3 resulted in an increase in peaks shifts to a larger 2θ. This shift originated from the decrease in the unit cell volume and it is non-linearly correlated with the Co content. The decrease in the unit cell volume for the values of y<0.50 was higher than for the values of y>0.50. This reduction could not be explained by the Co3+ and Fe3+ ionic radii. In this paper, the Co3+ and Fe3+ ionic radii are 0.545 and 0.645Å, respectively. The ionic radii of all the ions were taken from [21].

Fig. 3.

The XRD patterns of La1−xSrxFe1−yCoyO3 catalysts around 33° about the main peak.

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Troyanchuk et al. [6], observed a weak ferromagnetic behavior in LaCo0.5Fe0.5O3 through magnetic measurements. This weak behavior was attributed to the presence of Co atoms, with Co3+ and Co2+ that strongly differed from those of the end members of LaFeO3 and LaCoO3. Ivanova et al. [22] reported that lattice reduction in LaFe1−yCoyO3 for the values of y0.50 (i.e., Co-rich samples) could be related to the larger ionic radius of high-spin Fe3+ ions compared with low-spin Co3+ ions. Using Mössbauer spectroscopy, Merino et al. [23] provided evidence for the existence of Fe4+ for the values of y<0.50 in Fe-rich samples. Therefore, Co atoms exist in Co-rich samples, as Co3+Co2+ couples but in Fe-rich samples, charge compensation via Fe4+ formation would be produced. According to Tai et al. [24], the Fe4+Fe3+ couple would predominate over the Co3+Co2+ couple in Fe-rich samples. These results point out to the presence of various cations such as Co2+, Co3+, Fe3+, and Fe4+ in LaFe0.5Co0.5O3.

Based on the findings in the literature and in accordance with the results of the structural analysis conducted in this study, a higher decrease in the unit cell volume for the values of y<0.50 compared to the values of y>0.50 would confirm the presence of the ions other than low spin Co3+ and high-spin Fe3+ ions. These findings suggest that, in Fe-rich samples, low spin Co3+ ions substitute for high-spin Fe3+ ions and that the concentration of Fe4+ ions (0.585Å) decrease. However, in Co-rich samples, in addition to substituting low spin Co3+ ions with high-spin Fe3+ ions, the concentration of low spin Co2+ ions (0.745Å) increase. This latter result can explain the lower decrease in the unit cell volume for the values of y>0.50 compared to the values of y<0.50. Consequently, the results of the structural analysis pointed out to the presence of various cations such as Co2+, Co3+, Fe3+, and Fe4+ in LaFe0.5Co0.5O3. Moreover, the results show that the Fe3+Fe4+ and Co3+Co2+ couples play an important role in the catalytic activity of LaFe1−yCoyO3.

Due to the larger ionic radius of Sr2+ ions (rCN: 12=1.44Å) compared to that of La3+(rCN: 12=1.36Å) ions, it is expected that substituting Sr in La1−xSrxFe0.5Co0.5O3 will increase the mean ionic radius of the A cations. However, the results in Tables 2 and 3 shows that the values of the unit cell volume are almost constant with increase in the substitution of Sr up to x=0.30. In order to maintain electrical neutrality, some B ions (here, maybe Co2+) were oxidized to form B4+ (Co4+), which resulted in a net decrease in the average radius of the B ions along with an increase in Sr as shown in Table 3. Since substitution of alkaline-earth cations for La3+ cations in LaFe0.5Co0.5O3 increases the average degree of the oxidation of Fe and Co atoms, increase in Fe4+/Fe3+ and Co4+/Co3+ ratios is observed [21,24]. Consequently, the net effect of the mean ionic radius of the A-site and B-site constitutes a very small change in the lattice volume.

The structural phase transition observed in the Co- and Sr-substituted samples can be explained by the tolerance factor (t=rA+rO/√2(rB+ρO)). Goldschmidt introduced t for estimation of the deviation in a crystal structure from the cubic structure. As a result, in Co-substituted samples, t will increase when 〈rB-O〉 decreases and the orthorhombic structure will distort to the rhombohedral structure, which is characterized by higher symmetry. Substituting Sr in La1−xSrxFe0.5Co0.5O3 for x up to 0.30 resulted in a decrease in 〈rB-O〉 and an increase in 〈rA-O〉, which, consequently, would increase the symmetry of the rhombohedral structure for x up to 0.30. When the angle α of the rhombohedral cell is equal to 60°, then, t will be equal to 1. However, substitution of Sr with values higher than 0.30 resulted in an increase in 〈rA-O〉, which was relatively larger than the decrease in 〈rB-O〉 (see Table 3). Consequently, the tolerance factor decreased and there were transitions in the structural phase from the rhombohedral symmetry to the cubic symmetry. The transition sequence of the structural phase of La1−xSrxFe0.5Co0.5O3 is shown in Fig. 4.

Fig. 4.

Phase transition sequence of La1−xSrxFe1−yCoyO3. LaFe1−yCoyO3 (y=0.0, 0.25) adopts a Pbnm orthorhombic symmetry. For La1−xSrxFe1−yCoyO3 (x=0.00, 0.15, 0.30, y=050 and also x=0.00, y=0.50, 0.75, 1.00), it transforms to the rhombohedral R-3c structure and then to the cubic Pm-3m structure.

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Fig. 5a and b shows the FTIR spectra of the LaFe1−yCoyO3 and La1−xSrxFe0.5Co0.5O3 nano-catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60, respectively. The values of the IR absorption bands of the samples are given in Table 4. The presence of metaloxygen bonds pointed out to the asymmetrical lengthening of the BO bond of the octahedron BO6, which could be mainly resulted from the peaks observed at around 560 and 600cm−1 for the orthorhombic and rhombohedral structures, respectively. However, the widening of the 600cm−1 band and/or the appearance of a shoulder pointed out to the presence of a rhombohedral structure, which had lower symmetry [11]. For instance, the shoulder appeared at about 525cm−1 in the spectra of the S0C50 sample, which would be characteristic of a structure with lower symmetry. In addition, the widening or the appearance of the shoulder at the 560cm−1 band pointed out to the presence of an orthorhombic structure, which had lower symmetry. Therefore, the shoulder observed at around 600cm−1 in the spectra of the S0C0 sample is an indication of a structure with lower symmetry.

Fig. 5.

The FTIR spectra of La1−xSrxFe1−yCoyO3.

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Table 4.

the values of crystallite size and strain of La1−xSrxFe1−yCoyO3 catalysts obtained from Scherrer and H–W methods.

Sample #  Dsch (nm)  DW–H (nm)  ɛW–H×103 (no unit) 
S0C0  24.39  61.33  4.45 
S0C25  32.45  55.55  3.55 
S0C50  29.35  47.90  3.54 
S0C75  23.41  25.12  1.10 
S0C100  35.00  43.10  3.5 
S15C50  22.99  41.60  3.23 
S30C50  22.94  35.66  2.11 
S45C50  18.85  40.27  3.35 
S60C50  19.36  39.74  2.64 

By looking at Table 4, we can conclude that the increase in the unit cell volume is related to decrease in the position of the BO bond. The same also holds true about the relationship of the peak shift with the values of the wave numbers. These findings are in accordance with the results for the XRD patterns and show a shift of the band at 560–603cm−1 toward the larger values of the wave numbers along with an increase in the Co content. This shift of the BO bond can affect both the structural properties and efficiency of the catalyst. It should be noted that, if the absorption band at about 600cm−1 shifts toward the lower values of the wave numbers, the rhombohedral crystal structure distorts to lower symmetry (here, the orthorhombic structure) due to the increase in the eg electrons in the antibonding orbital [10]. In other words, an increase in the number of the eg electrons would decrease the bond order. Further, in this study, the effects of the increase in the eg electrons will be also discussed with reference to the decrease in the catalytic activity.

The values of the crystallite size of the LaFe1−yCoyO3 and La1−xSrxFe0.5Co0.5O3 nano-catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60 are given in Table 5. The results show a minimum value of the crystallite size for the S0C75 sample in the Co-substituted series and a minimum value of the crystallite size for the S30C50 sample in the Sr-substituted series. The decrease in the crystallite size with the increase in the Co content can be associated with the decrease in the unit cell volume while the decrease in the crystallite size with the increase in the Sr content of La1−xSrxFe0.5Co0.5O3 may be due to the lattice microstrain.

Table 5.

Electric conductivitya, activation energy and band gap energies of La1−xSrxFe1−yCoyO3 catalysts.

Sample #  σOx×105−1σRed×105−1)b  σOx/σRed  Ec (kCal/mol)  Eg (eV) 
S0C0  16.40  1.80  9.11  14.04  1.76 
S0C25  41.55  4.07  10.20  10.38  1.55 
S0C50  0.25  0.02  12.50  5.93  1.34 
S0C75  11.03  4.79  2.30  10.50  1.21 
S0C100  4.25  4.11  1.03  18.18  1.15 
S15C50  6.93  4.07  1.70  10.09  1.38 
S30C50  258.30  129.50  1.99  14.64  1.42 
S45C50  169.81  144.93  1.17  13.57  0.95 
S60C50  296.00  275.00  1.08  13.79  0.91 
a

Electric conductivity was measured at 350°C, in which the conductivity under the oxidation atmosphere (σOx), i.e., air, was reached to a maximum.

b

σOx and σRed are the conductivity under the oxidizing (air) and reducing atmosphere (6%CO in Ar), respectively.

The SEM micrographs for the S0C0, S0C50, and S30C50 samples show different morphological structures (Fig. 6). The S0C50 and S30C50 samples show good porosity feature. The presence of citric acid in the Sr-substituted samples and S0C50 sample could contribute to preventing agglomeration of the particles to large extents [9].

Fig. 6.

The SEM micrographs of S0C0, S0C50 and S30C50 samples (scale bar in all the images is 1μm).

(0.36MB).

The TEM micrograph and particle size distribution of the S30C50 sample are shown in Fig. 7. The histogram of the size distribution is fitted using a log-normal function as follows [25]:

where σd is the standard deviation of the diameter and DTEM is the mean diameter obtained from the TEM results (see Fig. 7). The mean diameter for the S30C50 nanoparticles was calculated to be 45.45nm.

Fig. 7.

TEM micrographs and size distribution histograms for S30C50 samples.

(0.21MB).

Fig. 8a and b shows the catalytic performance (%) curves for the nanocatalysts. The results for 10%, 50% and 95% of the CO oxidation are given in Table 1. The effect of the Co and Sr substitutions on the geometric factors could be related to structural changes given in Tables 2–4. It is argued that the increase in the tolerance factor would increase the oxygen vacancies at the surface due to a decrease in the BOB bond strength [9]. Thus, the decrease in BO interactions would result in an increase in the catalytic activity. The tolerance factor of a rhombohedral structure is higher than that of an orthorhombic structure. According to the results of the structural analysis and catalytic activity, it is expected that the higher activity of the CO oxidation of the sample may be related to the higher tolerance factor.

Fig. 8.

(a), (c) CO oxidation as a function of temperature and Arrhenius plots for LaFe1−yCoyO3 nano-particles, respectively; (b), (d) CO oxidation as a function of temperature and Arrhenius plots for La1−xSrxFe0.5Co0.5O3 nano-particles.

(0.22MB).

The activation energy (Ec) of the samples was obtained from the Arrhenius plot [9] as shown in Fig. 8c and d. In addition, the results of Ec and measured electrical conductivity, symbolized as σOx, σRed, and σOx/σRed, are given in Table 5. The values of σOx and σRed represent the electrical conductivity measured under isothermal conditions at 350°C. In this study, the depletion of the charge carriers upon switching from air to 6% CO atmosphere suggests that the conductivity of LaFeO3 is somehow affected by the Co and Sr substitutions. The low band gap energy, presented in Table 5, contributes to their high catalytic activities. A decrease is observed in the values of Eg as the Co substitution increases whereas the values show an increase in the Sr-substituted samples for x up to 0.30 and then decreases as the Sr substitution increases (see Table 5). However, the band gap energy shows that the Co substitutions increase the metallic behavior of the samples whereas the Sr substitutions for x up to 0.30 first decrease the metallic behavior of LaFe0.5Co0.5O3 and then, increase it. In accordance with the results of the structural analysis, the decrease in the band gap energy with the Co substitutions can be attributed to the increase in the presence of Co2+. Indeed, the presence of low spins Co3+ in the Sr-substituted samples would destroy the metallic behavior of LaFe0.5Co0.5O3.

The non-uniformly changes in the values of σOx/σRed, Ec, and Eg point out to the presence of various cations such as Co2+, Co3+, Fe3+, and Fe4+ in the Ca and Sr-substituted samples. The higher σOx/σRed and lower Ec observed with respect to the S0C50 sample are deemed to be suitable for the CO oxidation compared to the other samples studied in this paper.

The presence of the Fe3+OFe4+ and Co2+OCo3+ couples in ABO3 has a crucial role in its catalytic property. Since larger quantities of oxygen are available at lower temperatures in the redox process between Co3+, Co2+ ions and between Fe3+, Fe4+ ions, the overall catalytic activity is enhanced [14]. Based on the results of the previous studies [12,14,15,23,24], it is argued that the reduction of Co3+ to Co2+ and Fe4+ to Fe3+ can be observed at temperatures below ∼500°C, whereas, the reduction of Co2+ to Co0 and Fe3+ to Fe2+, or even further to the metallic Fe0, can be observed at high temperatures above ∼500°C. The reduction of Co3+ to Co2+ and Fe4+ to Fe3+ also takes place in the low-temperature region (i.e., below 500°C), which is accompanied by the formation of an oxygen vacancy. Therefore, it results in the adsorption of CO in the oxygen vacancy and a subsequent reaction of the carbon monoxide with a neighboring oxygen atom. Finally, the diffusion of oxygen proceeds in the bulk and on the surface of the nanoparticles. The oxygen atoms on the surface are first removed before the bulk (subsurface) oxygen starts to react so that the creation of the vacancy lead to the easy diffusion of the lattice oxygen from the bulk to the surface. Following the above line of argument, it can be discussed that, since there are larger numbers of oxygen atoms at low temperatures in samples having higher values of Co3+ and Fe4+ ions, the overall catalytic activity is enhanced [14]. This feature of the S0C50 sample, as obtained from structural analysis, can contribute to the higher catalytic activity when compared to the other samples examined in this study. This finding is consistent with those of other studies on conductivity in the literature.

It is reported that there is volcano-type dependence between the CO oxidation and the electronic configuration of B3+ ions [1,9,10]. In the presence of crystal fields, the octahedral environment of transition metal ions (B) is split up into a higher and a lower energy level, t2g and eg, respectively. The maximum catalytic activity in the volcano curves is obtained in two scenarios; i.e., the occupation of the eg levels of less than one electron when the t2g levels remain either half-filled or completely filled. Finally, based upon the results observed in the absorption bands of the IR spectra, it may be concluded that the rhombohedral structure has lower eg electrons in the antibonding orbital when compared to the orthorhombic structure. Consequently, this behavior of the S0C50 and S30C50 samples can contribute more to the CO adsorption at the surface than the other samples examined in this paper, which results in an increase in the CO oxidation.

Substituting the lower ionic radius transition metal in LaFeO3 decreases the temperature of the complete conversion of CO for x up to 0.50. In addition, the structural analysis and performance of the samples suggested that the better catalytic activity of the S0C50 sample in LaFe1−yCoyO3 could be mainly attributed to the presence of (i) the Fe3+OFe4+ and Co2+OCo3+ couples, (ii) a rhombohedral structure with higher symmetry, and (iii) the lower value of the activation energy and the higher value of σOx/σRed. In addition, substituting Sr+2 ions for La3+ ions in La1−xSrxFe0.5Co0.5O3 increase the temperature of the complete conversion of CO in LaFe0.5Co0.5O3 whereas the S30C50 sample shows the better catalytic activity of the CO oxidation in the Sr-substituted samples. The results suggest that the lower temperature of the conversion of the S30C50 sample in La1−xSrxFe0.5Co0.5O3 can be mainly attributed to (i) the presence of a structure with a higher value of tolerance factor, (ii) a lower value of the crystallite size, and (iii) a higher value of σOx/σRed.

4Conclusion

The results for structural, redox and catalytic analysis of the LaFe1−yCoyO3 and La1−xSrxFe0.50Co0.50O3 nano-catalysts with y=0.00, 0.25, 0.50, 0.75, and 1.00 and x=0.00, 0.15, 0.30, 0.45, and 0.60 summarized as follows:

  • 1.

    The XRD analysis of the Co-substituted samples via X’pert package and Fullprof program provides evidence on the orthorhombic structure (the space group Pnma II) with y=0.00, 0.25 and rhombohedral structure (the space group R-3c) for the samples with y=0.50, 0.75, and 1.00. The results for the Sr-substituted samples indicate that all the peaks of the XRD patterns for the samples with x=0.15 and 0.30 can be well indexed in the rhombohedral structure (the space group R-3c). In addition, the Rietveld refinement of the XRD pattern for the samples with x=0.45 and 0.60 provides evidence on the cubic structure (the space group Pm-3m). The influence of the Sr and Co substitutions on the structure of La1−xSrxFe1−yCoyO3 was examined through the concept of tolerance factor.

  • 2.

    The decrease in the values of the crystallite sizes with the Co and Sr substitutions can be respectively related to the decrease in the unit cell volume and micro-strain.

  • 3.

    In accordance with the results of the structural analysis of LaFe1−yCoyO3, the consistent decrease in the values of Ec and Eg and also the increase in σox/σRed with the Co substitution for x up to 0.50 point out to the presence of different concentrations of various cations such as Co2+, Co3+, Fe3+, and Fe4+. The overall oxidation activity of these samples acidity is associated with Co2+/Co3+ and Fe3+/Fe4+ redox couples.

  • 4.

    Substituting the lower ionic radius for Fe in LaFeO3 and substituting the higher ionic radius for La in LaFe0.5Co0.5O3 decrease the temperature of the CO oxidation. The structural, redox and catalytic results suggest that lower temperatures in the conversion of the CO oxidation for the sample y=0.50 in LaFe1−yCoyO3 can be mainly attributed to the presence of (i) the Fe3+-O-Fe4+ and Co2+-O-Co3+ couples, (ii) a rhombohedral structure with higher symmetry, and (iii) the lower value of the activation energy and the higher value of σOx/σRed. On the other hand, the lower temperature of the conversion of the sample with x=0.30 in La1−xSrxFe0.5Co0.5O3 can be mainly attributed to (i) the presence of a structure with a higher value of the tolerance factor, (ii) a lower value of the crystallite size, and (iii) a higher value of σOx/σRed.

Conflicts of interest

The author declares no conflicts of interest.

Acknowledgment

The author thanks Dr. Malekzadeh at Damghan University for providing the data on the catalytic properties of the samples used in this study.

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