Catalytic Oxidation of CO Over LaMn_1−xB_xO₃ (B = Cu, Fe) Perovskite-type Oxides

Abstract

In this paper, catalytic oxidation of CO over perovskite-type oxides LaMn_1−xB_xO₃ (B = Cu, Fe and x = 0, 0.1, 0.3, 0.5) were investigated. The perovskite catalysts were synthesized by sol–gel method and characterized by XRD, BET, H₂-TPR, XPS and SEM. XRD patterns showed that the samples are single-phase perovskite. By introduction of Cu and Fe in the structure, Specific surface area of LaMnO₃ was decreased, but the reducibility and oxygen vacancy were increased. The synthesized perovskite catalysts show high activity for the CO oxidation. Substitution of Mn by Cu and Fe enhanced the catalytic activity. The cu-containing perovskites showed a higher activity in CO oxidation compared with Fe-containing perovskites. The LaMn_0.7Cu_0.3O₃ perovskite showed the highest activity among the synthesized perovskites (T50 and T90 % of 110 and 142 °C). The excellent activity of LaMn_0.7B_0.3O₃ was associated to reducibility at low temperature, more oxygen vacancies and synergistic effect between Cu and Mn. The apparent activation energies were obtained and LaMn_0.7Cu_0.3O₃ as the most active catalyst, has the least activation energy compared with other synthesized catalysts.

Graphical Abstract

1 Introduction

Carbon monoxide is one of the main gaseous pollutants, which is generally released from the combustion of fossil fuel in diesel engines. There are many methods for removal of CO including adsorption, thermal elimination and catalytic oxidation. Catalytic oxidation of CO is proved to be one of the most efficient techniques to remove this pollutant [1, 2]. Present catalysts are supported noble metal catalysts based on platinum, palladium, and rhodium [3, 4]. High cost, low stability and lack of noble metal limit their applications. Perovskite-type oxides are interesting catalysts for CO oxidation. They have lower cost and higher thermal stability than supported noble catalysts [5, 6].

The general formula ABO₃ is related to perovskite-type oxides which can crystallise in cubic structure [7]. A and B are two cations of very different sizes which must meet the so called tolerance factor tolerance factor (0.8 < t < 1.0) defined by the equation ${\text{t}} = ({\text{r}}_{\text{A}} {\text{ + r}}_{\text{O}} ) /\sqrt 2( {\text{r}}_{\text{B}} {\text{ + r}}_{\text{O}} )$, where r_A, r_B and r_O are the ionic radii for A, B and O, respectively [8].

In mentioned structure coordination number with oxygen anions is 12 and 6 for A and B cation respectively. These catalysts have the capability to incorporate different cation types in their A-site and B-site leading to different compounds with general formula A_1−xA’_xB_1−xB’_xO₃ [9, 10]. Rare earth, alkaline earth and alkali metal cations are usually in A site which are larger in comparison to transition metal cations which are common occupant of B-site [11]. In perovskite-type oxides catalytic properties mainly depends on the nature of A and B ions and on their valence state [12, 13].

Perovskites with lanthanum in A-site exhibit higher activity in comparison to other formulations for catalytic oxidation of CO. First series transition metals are the most common metals for B-site [14–16]. Among the perovskite-type oxides, manganese containing perovskite catalysts has attracted more attention for CO oxidation as they are active in a wide range of reactions for transformation of carbon monoxide [17–19]. Copper and iron were generally regarded as an active cation for CO oxidation [14, 19]. So, by incorporation of these cations into the B-site of the perovskite structure a good performance can be achieved.

In this paper, effects of substitution of Mn by Cu and Fe in B-site of perovskites on chemical–physical properties and activity of catalysts were investigated. For this aim, a series of perovskite-type oxides LaMn_1−xB_xO₃ (B = Cu, Fe and x = 0, 0.1, 0.3, 0.5) were prepared by sol–gel method and characterized by XRD, BET, H₂-TPR, XPS and SEM. The catalytic performances of prepared catalysts were investigated for the CO oxidation at temperature ranges of 50–250 °C.

2 Experimental

2.1 Catalyst Preparation

LaMn_1−xB_xO₃ perovskite catalysts were prepared by the sol–gel method starting from La(NO₃)₃·6H₂O, Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, Mn(NO₃)₂·4H₂O and Citric acid monohydrate. For preparation of 1 g of catalyst, appropriate amount of La, Mn and B nitrates with cation ratios of La:Mn:B, of 1:1−x:x were dissolved in 50 mL of de-ionized water. Citric acid monohydrate is added to the solution of the cations with a molar ratio of 1:0.525 with respect to the total amount of cations. Then the solution was heated up to 80 °C with stirring until a sticky gel was obtained. The gel is heated to 200 °C for 2 h in air to remove the organic ligands and decompose the nitrates and turned into a dark powder and then calcined at 700 °C for 5 h in static air.

2.2 Characterizations

X-ray diffraction (XRD) patterns of perovskite catalysts were recorded on an X-ray diffractometer (D-500, SIEMENS) with a Kα line of copper (λ = 0.154 nm). Measurements of the samples were collected in the 2θ range of 20–80°. The Specific surface areas (m²/g) of the perovskite catalysts was determined by nitrogen adsorption–desorption porosimetry at 77 K using an Autosorb-1 Quantachrome analyzer. Temperature programmed reduction (TPR) experiments were carried out in a Micromeritics Autochem 2900. The H₂-TPR experiments were performed with a 5 % H₂/Ar gas flow at 20 standard cubic centimeters per minute (sccm) and linear heating rate of 10 °C/min at 40–950 °C. X-ray photoelectron spectroscopy (XPS) measurements were performed to evaluate surface composition and oxidation state of perovskite catalysts over a Microlab 310-F scanning microprobe spectrometer with AlKα X-ray source (E = 1486 eV). The morphology of the synthesized particles was observed by scanning electron microscopy (SEM) using a Tescan instrument.

2.3 Catalytic Activity

The activities of perovskite catalysts were evaluated using a quartz tubular reactor with an internal diameter of 9 mm and length of 600 mm under atmospheric pressure and at different temperatures (50–250 °C). The reactant gas (1 % CO, 20 % O₂ and Ar as balance) was passed through 200 mg of catalysts at a rate of 100 mL/min (GHSV = 6000⁻¹). The powder sample were inserted into the middle of a quartz tube. The reactor was heated with an electrical furnace. The catalytic activities were tested under steady state conditions. The gas composition was analyzed before and after the reaction by an online gas chromatograph (Shimadzu 2010) equipped with a HP-Molesieve column (l = 30 m, i.d. = 0.53 mm) and thermal conductivity detector (TCD).

3 Results and Discussion

Figure 1 shows the XRD patterns of LaMn_1−xB_xO₃ perovskite samples. A comparison of XRD patterns with the pattern of LaMnO₃ (ICSD 082315) indicated that catalysts were single-phase perovskite oxides. There were no additional peaks corresponding to secondary phases or starting materials in XRD pattern of perovskites, suggesting that cupper and iron metals are completely dissolved in the perovskite structure. It appears that doping of cupper and iron to the structure doesn’t change in the peak shape or intensity significantly. By introduction of Fe and Mn, no segregation phase was observed in the perovskites. In the section b of Fig. 1 by 2θ comparison of the catalysts main peak, it is revealed that main peak of each catalyst was observed in shifted 2θ which is a result of modifier metal insertion in the LaMnO₃ structure and change of the unit cell size.

Fig. 1

XRD patterns of LaMn_1−xB_xO₃ perovskite samples

BET surface area of perovskites was listed in Table 1. BET surface area values of perovskites are between 27 and 40 m²/g; introduction of B cations causes the decrease in the specific surface area. The same results were observed in the literate [10, 20].

Table 1 Specific surface area, T50 %, T100 % and activation energies of perovskite samples

The reducibility of LaMnO₃ and LaMn_0.7B_0.3O₃ perovskite catalysts was investigated by H₂-TPR. Figure 2 shows the H₂-TPR curves of perovskite catalysts. It can be seen that the LaMnO₃ shows two signal peaks at 417 and 849 °C, which has been associated to the reduction of Mn⁴⁺ to Mn³⁺ and Mn³⁺ to Mn²⁺, respectively [17, 21, 22]. But for the catalysts partially substituted by Fe, they display four signal peaks at 408, 520, 603 and 870 °C [22, 23]. The first and forth signal peaks are related to the reduction of Mn⁴⁺ and Mn³⁺ presented in the perovskite, but the others should be come from the reduction of Fe⁴⁺ to Fe³⁺ and Fe³⁺ to Fe²⁺. Fe²⁺ can be reduced to Fe⁰, but this reduction occurs under temperature higher than 1000 °C. So, for reduction of Iron, two peaks including reduction of Fe⁴⁺ to Fe³⁺ and Fe³⁺ to Fe²⁺ are exist.

Fig. 2

H₂-TPR curves of perovskite samples

In the TPR profile of LaMn_0.7Cu_0.3O₃, The first signal peaks are also related to the reduction of Cu²⁺ to Cu⁰, but the others assigned to the reduction of Mn⁴⁺ and Mn³⁺. When compared the reduction peaks of Mn⁴⁺ of LaMn_0.7B_0.3O₃ with LaMnO₃, this peak shifts to lower temperature especially the reduction peak of LaMn_0.7Cu_0.3O₃ at 380 °C. These results indicate that the Mn–O bond strength can be weakened by the substitution of the B-site elements with copper and iron.

The surface composition of LaMnO₃ and LaMn_0.7B_0.3O₃ perovskites was investigated by XPS. The XPS spectra for Mn 2p_3/2 of LaMnO3 and LaMn_0.7B_0.3O₃ perovskites are shown in Fig. 3. The Mn 2p_3/2 spectra includes two peaks correspond to Mn³⁺ (641.5 eV) and Mn⁴⁺ (644 eV) [24–26]. Mn in all the samples consists of both Mn⁴⁺ and Mn³⁺ ions. By substitution of Mn by copper, the ratio of Mn⁴⁺ to Mn³⁺ was decreased. But, by substitution by iron, this ratio was increased. The XPS spectra for Cu 2p_3/2 and Fe 2p_3/2 of LaMn_0.7B_0.3O₃ are shown in Fig. 4. In Cu 2p_3/2 spectrum, two peaks at the binding energies of 531.2 and 533.4 eV correspond to Cu¹⁺ and Cu²⁺ ions [27–29]. Fe 2p_3/2 spectrum includes three peaks correspond to Fe³⁺ (710.1 eV), Fe⁴⁺ (712.1 eV) and a satellite [14, 24, 30].

Fig. 3

XPS spectra of perovskite samples, Mn 2p_3/2 of (a) LaMnO₃, (b) LaMn_0.7Cu_0.3O₃, (c) LaMn_0.7Fe_0.3O₃, O 1 s of (d) LaMnO₃, (e) LaMn_0.7Cu_0.3O₃, (f) LaMn_0.7Fe_0.3O₃

Fig. 4

XPS spectra of perovskite samples. (a) Cu 2p_3/2 of LaMn_0.7Cu_0.3O₃, (b) Fe 2p_3/2 of LaMn_0.7Fe_0.3O₃

Fig. 5

SEM images of perovskite samples, (a) LaMnO₃, (b) LaMn_0.7Cu_0.3O₃, (c) LaMn_0.7Fe_0.3O₃

The XPS spectra for O 1 s of the samples are also shown in Fig. 3. In O 1 s spectrum, three peaks at the binding energies of 529.5, 531.1, and 533.1 eV can be assigned to lattice oxygen (O_latt), adsorbed oxygen (O_ads), and surface adsorbed water species, respectively [14, 24, 25]. Generally, the adsorb oxygen concentration is related to the oxygen vacancy concentration [31]. Substitution of La and Mn by other cations can result in the formation of oxygen vacancies and thereby causing a decrease in the lattice oxygen concentration. It can be seen from Fig. 3, by substitution of Mn by copper and iron, the ratio of O_ads/O_latt was increased. This result means that more oxygen vacancies were produced in the perovskite structure in the doped samples.

The morphology and particle size of synthesized perovskite catalysts were investigated by SEM. SEM images of perovskites are shown in Fig. 5. Morphology of perovskites was as irregular shaped grains.

Figure 6 shows the temperature profile for CO conversion over LaMn_1−xB_xO₃ perovskite catalysts. At these conditions, all the B-doped perovskite catalysts reach an almost complete conversion at 225 °C. Temperatures for 50 and 90 % conversion of CO (T50 and T90 %) for all catalysts are shown in Table 1. By considering the T50 and T90 % of CO conversion as criteria of activity, Cu-containing perovskite catalysts showed a higher activity than Fe-containing perovskites. Results indicate that the conversion was in order of L-MC-0.3 > L-MF-0.3 > L-MC-0.5 > L-MF-0.5 > L-MC-0.1 > L-MF-0.1 > L-M.

Fig. 6

Catalytic Oxidation of CO over LaMn_1−xB_xO₃ perovskite samples

Catalytic activity of perovskites for CO oxidation depends on reducibility of transition metal cations, oxidation state of ions, oxygen vacancy concentration and specific surface area. Substitution of Mn by other cations in the perovskite reduced the reduction temperature of manganese and increased the reducibility of perovskites [32]. The ratio of Mn⁴⁺/Mn³⁺ was changed by substitution of manganese and this factor can cause structural defects in perovskite structure. Introduction of Cations in B-site increased the ratio of O_ads/O_latt and increased the oxygen vacancy concentration on the surface of perovskite [23, 33]. High reducibility, more structural defects and oxygen vacancies lead to higher catalytic activity and an improvement in perovskite performance. There is no direct relationship was observed between specific surface area and catalytic activity. Therefore, it is concluded that the excellent catalytic activity of LaMn_0.7Cu_0.3O₃ is associated with high reduction ability, oxygen vacancies and more structural defects in its structure.

Reaction rates (rates of CO oxidation) were evaluated from the Eq. 1:

$${\text{R}}_{\text{CO}} = \frac{{{\text{F}}_{\text{CO0}} {\text{X}}_{\text{CO}} }}{{{\text{W}}_{\text{cat}} }}$$

(1)

Where R_CO is the rate of CO oxidation (mol/(s.gr_cat)), F_CO0 is the molar flow rate of CO (mol/s), X_CO is the CO conversion and W_cat is the weight of catalyst by grams. Arrhenius-type plots of the form ln(R_CO) = f(1000/T) for perovskites are shown in Fig. 7, where R is the rate of conversion (mol/(g s)). As observed from these figures, there is a straight-line and from its slope the apparent activation energy can be estimated. Activation energies of synthesized perovskite for CO oxidation were listed in Table 1. Substitution of manganese by copper and iron, reduces the apparent activation energy to 32.2 kJ/mol from 39.2 kJ/mol in LaMnO₃. The lower apparent activation energy coincides with the observed increase in catalytic activity. LaMn_0.7Cu_0.3O₃ which has the lowest activation energy (32.2 kJ/mol), was also found to be the most active catalyst in the synthesized perovskites and LaMnO₃ has the highest activation energy (39.2 kJ/mol). The values of activation energies do not show sensible changes with B cations and x value and remain in the range 32.2 < E_app < 39.2 kJ/kmol. Chan et al. [20] found the apparent activation energy of La_1−xSr_xMnO₃ perovskites between 48.3 and 66 kJ/kmol. Wang et al. [34] calculated activation energy of LaSrNiO₄ equals to 49.3 kJ/mol. Variation of activation energy with introduction of cations in A or B site is small. These values are compared satisfactorily with the values calculated in this work.

Fig. 7

The Arrhenius plots for perovskite samples

4 Conclusions

Catalytic activity of LaMn_1−xB_xO₃ (B = Cu, Fe and x = 0, 0.1, 0.3, 0.5) obtained by sol–gel method evaluated in CO oxidation and characterized by XRD, BET, H₂-TPR, XPS and SEM. Pure perovskite crystal phases are achieved by sol–gel method and no segregation phase was observed in the perovskite structure. Specific surface areas of perovskites were obtained in the range of 27–40 m²/g. LaMn_1−xB_xO₃ perovskites proved quite active for CO oxidation. Substitution of manganese by copper and iron in the perovskite increased the reduction ability and oxygen vacancy concentration of perovskite catalysts. Based on results, Cu-containing perovskite catalysts have a higher activity than Fe-containing perovskite catalysts. LaMn_0.7Cu_0.3O₃ perovskite catalyst showed the highest activity (90 % at 142 °C) among the studied LaMn_1−xB_xO₃ perovskites for CO conversion. Nearly complete elimination of CO was achieved at 150 °C with this catalyst. The excellent catalytic activity of LaMn_0.7Cu_0.3O₃ is associated with the absence of segregation phase, high reduction ability, reduction Mn⁴⁺ in lower temperature, oxygen vacancies and more structural defects in its structure. Apparent activation energies for synthesized perovskites were obtained from Arrhenius-type plots confirmed that the LaMn_0.7Cu_0.3O₃ has the lowest activation energy (32.2 kJ/mol) compared with other synthesized catalysts.