Promotion Effect of Ce Doping on Catalytic Performance of LaMnO₃ for CO Oxidation

Abstract

In this paper, Ce-doped La_1-xCe_xMnO₃ perovskite catalysts are prepared by the sol–gel method, and the promotion effect of Ce doping on LaMnO₃ catalysts for CO oxidation is investigated. The catalysts are characterized by X-ray diffractograms, Raman, N₂ physisorption isotherms, temperature-programed reduction with H₂, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show that the Ce doping greatly improves the catalytic activity of LaMnO₃ for CO oxidation. Among the La_1-xCe_xMnO₃ catalysts, La_0.8Ce_0.2MnO₃ shows the best CO catalytic activity, with 100% CO conversion obtained at 180 °C. The characteristic results show that the LaMnO₃ perovskite phase exists in all Ce-doped catalysts, and the CeO₂ crystalline phase begins to appear at x ≥ 0.1. The high activity of La_0.8Ce_0.2MnO₃ for CO oxidation could be that: (1) it possesses large surface area (25.8 m²/g) to contact with reactants; (2) it has a high surface Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio of 0.27, which means high content of oxygen vacancies used for O₂ adsorption and activation; and (3) it exhibits strong reducibility that is beneficial to CO activation.

1. Introduction

With the continuous improvement of people’s living standards, the number of fuel vehicles is increasing. Carbon monoxide (CO), as a main component of the automobile exhaust, causes serious pollution problems [1,2]. It is hard to react with other substances in the atmosphere, and thus it will directly affect the climate and environment [3,4,5]. Besides, when excessive CO is inhaled into the human body, it will damage the cardiovascular and nervous systems, leading to headache, syncope, and even death [6]. At present, the most common and efficient means of treating CO pollutants is catalytic oxidation, and the core of this technology is a catalyst with high catalytic activity [7].

According to the compositions, the catalysts can be divided into noble metal and non-noble metal catalysts [8]. Because noble metal is scarce and expensive, the non-noble metal catalysts receive broader application prospects in terms of cost and sustainability. Perovskite, as a typical representative of non-noble metal catalysts, is one of the alternative materials to replace noble metal catalysts owing to its simple synthesis, low cost, good catalytic performance, and wide application prospect in various catalytic reactions. In addition, perovskite oxides also have good inclusivity, so that their structural properties can be regulated by doping other elements to achieve the purpose of application [9].

The general chemical formula of perovskite-type oxides can be expressed as ABO₃, where A is a typical lanthanide, alkali metal, or alkaline earth metal cation with large ionic radius; and B is a transition metal cation with small ionic radius, such as Mn, Co, Fe, Ni, Cr, or Ti [10]. The B-site metal plays fundamental role in catalysis, while the A-site metal is mainly to support and stabilize the perovskite structure [11]. The substitution of A- or B-site cation with a foreign one can change the composition and valence state of the B-site metal, while remaining structurally undestroyed [12]. Therefore, perovskite-type metal oxides are widely applied in catalytic reactions, including the CO oxidation. Among the numerous perovskite-type metal oxides, lanthanum-containing manganate (LaMnO₃) attracts increasing attention due to its multifunctional catalytic performances and chemical stability, especially as a low-cost alternative to the noble metal for CO oxidation [13,14].

Tei ji Nakamuya et al. [15] studied the oxidation performance of La_1-xSr_xMnO₃ catalyst for CO removal, showing that with the increase in Sr substitution, the oxidation performance of the catalyst improves, while the re-reduction rate decreases. Peng et al. [16,17] prepared La_0.8K_0.2Mn_1-xCu_xO₃ catalysts and found that the simultaneous substitution of cations at A and B sites could not only convert Mn³⁺ to Mn⁴⁺, but also generate oxygen vacancies, thereby improving the reaction activity. In previous works, it was reported that the Ce doping can increase the surface area, oxygen storage capacity, and, hence, the CO removal efficiency [18]. For example, Gao et al. [19] found that the Ce doping increases the surface area and reduces the grain size of La_1-xCe_xCoO₃, thus improving the CO oxidation activity. Xiang et al. [20] found that Ce⁴⁺ in La_1-xCe_xFeO₃ strengthens the interaction between the catalyst and the adsorbed O₂, facilitating the activation of O–O bonds, thereby accelerating the reaction rate. Mathieu-Deremince et al. [21] conduct CO oxidation on La_1-xCe_xBO₃ (B = Ti, Cr, Mn, Fe, Ni, Co) and found that the introduction of Ce⁴⁺ reduces the oxidation state of Mn ions at the B site, which improves the surface area of the catalyst.

The LaMnO₃ catalyst has remarkable CO catalytic oxidation activity. However, it has not received actual industrial application yet, and its oxygen storage capacity needs to be further optimized. Since the Ce doping can bring more oxygen and facilitate oxygen mobilization, it would be of interest to clarify the influence of Ce doping on the oxygen storage capacity and CO oxidation activity of the LaMnO₃ catalyst. The results of which would then provide new ideas to modify perovskite oxides for catalysis use.

In this work, a series of Ce-doped La_1-xCe_xMnO₃ perovskites were prepared for a CO oxidation reaction. X-ray diffractograms (XRD), Raman, transmission electron microscopy (TEM), N₂ physisorption isotherms, X-ray photoelectron spectroscopy (XPS), and temperature-programed reduction with H₂ (H₂-TPR) were used to study the effects of Ce doping on the catalytic performances of LaMnO₃ for CO oxidation. The results showed that LaMnO₃ with 20% Ce doping at the La site, i.e., La_0.8Ce_0.2MnO₃, has the largest surface area, abundant oxygen vacancies, and strong reducibility among the La_1-xCe_xMnO₃ catalysts (0 ≤ x ≤ 0.25), and, hence, exhibited the highest activity for CO oxidation.

2. Results and Discussion

2.1. Catalytic Performance

Figure 1 shows the catalytic activity of La_1-xCe_xMnO₃ (0 ≤ x ≤ 0.25) for CO oxidation as a function of temperature. The conversion curves of all catalysts are similar, and the activity increases with the reaction temperature. For LaMnO₃, the activity slowly increases in the temperature range of 60−140 °C, and only 9% CO conversion was obtained at 140 °C. After the doping of Ce atoms, the activity of La_1-xCe_xMnO₃ showed a trend of, first, increase, and then decrease, with the best activity obtained from La_0.8Ce_0.2MnO₃, which exhibits 100% CO conversion at 180 °C. The activity La_1-xCe_xMnO₃ is better than that of LaMnO₃. This could be that the Ce doping induces the formation of oxygen vacancies, resulting in charge imbalance on the surface of the catalyst, which makes it easier to adsorb oxygen and promotes the oxidation of CO.

To compare other doping catalysts and indicate the superiority La_0.8Ce_0.2MnO₃ for CO oxidation, a series of LaMnO₃-based catalysts, reported in studies, are compared and shown in

Table 1. Comparison of BET surface area and CO conversion with other reported works.

Catalyst	BET Surface Area (m2/g)	Reaction Conditions	CO Conversion		Reference
			T50 (°C)	T100 (°C)
La0.8Ce0.2MnO3	25.8	0.5% CO and 6.5% O2; WHSV = 30,000 mL/(g·h)	140	180	This work
La0.4Sr0.6MnO3	12.0	1.0% CO and 10.0% O2; WHSV = 90,000 mL/(g·h)	135	163	[22]
LaMn0.8Fe0.2O3	55.7	1.0% CO and 1.25%/O2; GHSV = 12,000 h–1	187	200	[23]
LaAl0.8Mn0.2O3	25.0	1% CO and 20% O2; GHSV = 12,000 h−1	127	227	[24]
2 wt% Au-LaMnO3	17.7	2300 ppm CO and 7% O2; WHSV = 30,000 mL/(g·h)	234	290	[25]

. For substitution of the A-site cation, La_0.4Sr_0.6MnO₃ [22], was shown to have a smaller BET surface area than that of La_0.8Ce_0.2MnO₃. A similar result was obtained for the CO oxidation activity. The substitution of the B-site cation, LaMn_0.8Fe_0.2O₃ [23], obtained 50% CO conversion and 100% CO conversion at 135 °C and 163 °C, respectively, which are slightly lower results than that of La_0.8Ce_0.2MnO₃; LaAl_0.8Mn_0.2O₃ [24] exhibits almost the same BET surface area, showing better low-temperature activity but less high-temperature activity than that of La_0.8Ce_0.2MnO₃. The noble metal loading catalyst, 2 wt% Au-LaMnO₃ [25], shows relatively higher temperature to obtain 50% and 100% CO conversion than that of La_0.8Ce_0.2MnO₃.

2.2. XRD and Raman Results

Figure 2a shows the XRD patterns of La_1-xCe_xMnO₃. Diffraction peaks at 2θ = 22.9°, 32.6°, 40.2°, 46.8°, 52.7°, 58.2°, 68.3°, and 77.8° that are attributed to LaMnO₃ (PDF#75-0440) are observed in all samples, indicating that LaMnO₃ is the main composition of La_1-xCe_xMnO₃. For LaMnO₃ and La_0.95Ce_0.05MnO₃, no diffraction peaks, other than that of LaMnO₃, are detected, suggesting that they have pure phase structure, and all the Ce atoms enter the framework of LaMnO₃ perovskite. For samples at x ≥ 0.1, a new peak at 2θ = 28.5° that is assigned to the characteristic diffraction peak of CeO₂ (PDF#78-0694) appears, indicating that some Ce atoms are presented on the surface of LaMnO₃ as CeO₂ crystallizes. It is noted that the peak intensity at 2θ = 32.6° weakens with the increase in Ce amounts (x > 0.1), which can a result of (1) the entrance of Ce atoms decreases the crystallinity of LaMnO₃ due to its larger ionic radius (relative to that of La atoms) and (2) more CeO₂ is formed and covered on the surface of La_1-xCe_xMnO₃, affecting the diffraction of the materials. Figure 2b shows the Raman patterns of La_1-xCe_xMnO₃. The strong peak of 650 cm⁻¹ appears for all the samples and is assigned to the LaMnO₃ perovskite structure [26]. For the Ce doped samples, La_0.9Ce_0.1MnO₃ and La_0.8Ce_0.2MnO₃, two new peaks, at 465 cm⁻¹ and 600 cm⁻¹, appear, which are attributed to the vibration of the Ce−O bond and the defect introduction pattern caused by Ce³⁺, respectively [27]. This suggests that the Ce atoms entered the framework of LaMnO₃, accompanying the formation of CeO₂ crystals.

2.3. N₂ Physisorption Isotherms

The N₂ physisorption isotherms of La_1-xCe_xMnO₃ catalysts are present in Figure 3, showing that all the samples have a typical IV isotherm with an H₃-type hysteresis loop, which indicates the presence of a mesoporous structure formed by stacking particles. The surface area of the samples, calculated with the BET method, are also present in the picture for convenience, which shows that the doping of Ce atoms can slightly improve the surface area, in accordance with that reported in the literature [28]. As an example, the surface area increases from 21.4 m²/g for LaMnO₃ to 25.8 m²/g for La_0.8Ce_0.2MnO₃. However, the surface area slightly decreases with the further increase in Ce content (e.g., La_0.75Ce_0.25MnO₃), due to the formation of CeO₂ phases on the surface, blocking the pores. Figure 3b shows the pore size distribution of La_1-xCe_xMnO₃, which infers that the pore size distribution is mainly between 0–100 nm for all catalysts. La_0.8Ce_0.2MnO₃, La_0.95Ce_0.05MnO₃, and LaMnO₃ exhibit similar pore size distribution, but quite different CO oxidation activity, hence no direct relationship between them could be correlated. This is possible since the pore size is not the only factor affecting the catalytic activity. Other factors, such as the property of active sites, the oxidation states of metal ions, the synergistic effect between different phases, etc., should also be considered, as discussed below.

2.4. TEM Results

Figure 4 presents the TEM images of LaMnO₃ and La_0.8Ce_0.2MnO₃. Both samples exhibit disordered shape, and the particles are aggregated due to the high temperature applied in the calcination process (Figure 4a,b). From the high-resolution TEM images shown in Figure 4c, d, it is seen that LaMnO₃ mainly exposes the (012) crystal plane with a lattice distance of 0.386 nm, while La_0.8Ce_0.2MnO₃ mainly exposes the (021) crystal planes with a lattice distance of 0.258 nm. Hence, the doping of Ce atoms alters the lattice exposure of LaMnO₃. From the FFT pattern of La_0.8Ce_0.2MnO₃ (Figure 4e), (012), (021), and (122) crystal planes are observed. This indicates that La_0.8Ce_0.2MnO₃ is not a single exposed crystal surface, which is consistent with our previous work [18]. The mapping images of La_0.8Ce_0.2MnO₃ shown in Figure 4f–j show that the La, Ce, Mn, and O atoms are uniformly distributed in the catalyst despite of the formation of CeO₂. This suggests that the CeO₂ phase is finely dispersed around the LaMnO₃ perovskite. This is possible since CeO₂ is formed in situ in the material by a one-pot preparation method.

2.5. XPS Results

XPS measurement is conducted to detect the surface chemistry of the samples, and the spectra are presented in Figure 5. For La 3d XPS spectra, they contain two sets of peaks that are contributed by La 3d_3/2 and 3d_5/2, Figure 5a. The former can be further divided into two peaks at binding energies of 854.8 eV and 850.4 eV, and the latter is divided into two peaks at binding energies of 837.9 eV and 833.5 eV, respectively. The difference between the binding energy (ΔE) of La 3d_3/2 and 3d_5/2 is 16.9 eV, which is similar to that of La₂O₃ (16.8 eV), as reported in the literature [29]. Hence, it can be concluded that the La cations of LaMnO₃ and La_1-xCe_xMnO₃ exist in the +3 oxidation state.

Figure 5b shows the Ce 3d XPS spectra of La_1-xCe_xMnO₃, which are composed of Ce 3d_3/2 and 3d_5/2 peaks, labeled as U and V in the following for convenience. For Ce 3d_3/2, four peaks at binding energies of 900.4, 903.5, 907.0, and 916.0 eV can be fitted and are labeled as U₁, U₂, U₃, and U₄, respectively. Additionally, four peaks are fitted for Ce 3d_5/2 at binding energies of 881.6, 882.8, 888.1, and 897.5 eV and are marked as V₁, V₂, V₃, and V₄, respectively. According to studies [30,31], the V₂ and U₂ peaks belong to the Ce³⁺ species, and V₁, V₃, V₄, U₁, U₃, and U₄ are assigned to Ce⁴⁺ species. From the molar ratio of Ce³⁺/(Ce³⁺+Ce⁴⁺), Table 1, calculated based on the peak area of them, it can be concluded that Ce⁴⁺ is the main Ce species on the surface of La_1-xCe_xMnO₃ catalysts. This can be explained by their good stability at high temperature relative to that of Ce³⁺ [32]. In addition, according to the principle of electro neutrality, it is known that the presence of more low-valence Ce³⁺ (or a higher Ce³⁺ ratio) leads to the generation of more oxygen vacancies [33]. Hence, from the change of the Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio listed in Table 1—La_0.8Ce_0.2MnO₃ > La_0.9Ce_0.1MnO₃ > La_0.75Ce_0.25MnO₃—it is concluded that the surface of La_0.8Ce_0.2MnO₃ contains more oxygen vacancies, and the exposed metal cations have lower coordination numbers that are beneficial to the adsorption of oxygen [34], thereby improving the catalytic activity.

The Mn 2p XPS spectra consist of two asymmetric peaks contributed by Mn 2p_1/2 and Mn 2p_3/2, Figure 5c. Three peaks can be fitted for them, with binding energies at 652.6, 653.6, and 656.5 eV for the Mn 2p_1/2 peak and binding energies at 640.7, 641.7, and 642.6 eV for the Mn 2p_3/2 peak. According to the literature [35,36], the peaks at binding energies of 640.7 and 652.6 eV are assigned to Mn³⁺ ions, and the peaks at 641.7 and 653.6 eV are attributed to Mn⁴⁺ ions, and those at 642.6 and 656.2 eV are satellite peaks of Mn³⁺ ions. The transformation between Mn³⁺ and Mn⁴⁺ species accounts for the activity of La_1-xCe_xMnO₃ for CO oxidation. Hence, the Mn⁴⁺/Mn³⁺ ratio is a crucial factor influencing the reaction rate, as it affects the reaction rate by donating/receiving electrons to/from the reactants, to accomplish a catalytic cycle. From the above results—that La_0.8Ce_0.2MnO₃ exhibits the best activity for the reaction (Figure 1)—it is suggested that the suitable Mn⁴⁺/Mn³⁺ ratio is 0.35. A higher or lower Mn⁴⁺/Mn³⁺ ratio would lead to imbalance in the oxidation and reduction steps.

The O 1s spectra of La_1-xCe_xMnO₃ are also discussed through the deconvolution analysis, as shown in Figure 5d. Three peaks at binding energies of 528.9, 530.9, and 532.7 eV are fitted, which are assigned to lattice oxygen (O_γ), chemically adsorbed oxygen on oxygen vacancy (O_β), and adsorbed oxygen-containing groups (O_α), such as hydroxyl (OH^-) or carbonate (CO₃^2-) [37]. With the increase in Ce doping, the O_β/O_γ ratio of La_1-xCe_xMnO₃ increases first, and then decreases, with the highest value (0.81) obtained at La_0.8Ce_0.2MnO₃,

Table 2. Ratios of surface species of La_1-xCe_xMnO₃ obtained from XPS.

Catalysts	Ce3+/(Ce3+ + Ce4+)	Mn4+/Mn3+	Oβ/Oγ
LaMnO3	-	0.55	0.62
La0.9Ce0.1MnO3	0.23	0.29	0.65
La0.8Ce0.2MnO3	0.27	0.35	0.81
La0.75Ce0.25MnO3	0.20	0.82	0.58

. This indicates that the La_0.8Ce_0.2MnO₃ catalyst possesses the most amounts of oxygen vacancy, which is believed to be the active site of oxygen adsorption and activation.

The change in the oxidation state of surface Mn⁴⁺/Mn³⁺ and its amount of oxygen species clearly demonstrates that the Ce atoms enter the La site of LaMnO₃. However, the effect on different species is diverse. For the Ce species, the Ce³⁺ percentage reaches the highest at La_0.8Ce_0.2MnO₃, which could be attributed to the formation of interfaces between CeO₂ and LaMnO₃. At low Ce doping, the interfaces are less or not well formed, while at high Ce content, the amount of CeO₂ is excess and covers the surface, leading to less surface Ce³⁺ percentage. For the Mn species, the M⁴⁺ percentage abruptly decreases after the Ce doping, due to the presence of Ce⁴⁺, which causes transformation of Mn⁴⁺ to Mn³⁺ according to the principle of electroneutrality. While the Mn⁴⁺ percentage gradually increases with the Ce doping, due to the segregation of CeO₂ and the formation of CeO₂/LaMnO₃ interfaces, exposing more unsaturated surface. For the O species, the Ce doping initially improves, but then decreases, the amount of oxygen vacancy. This suggests that the Ce doping induces the generation of oxygen vacancy on the surface, but the surface would be covered when too much excess CeO₂ is formed.

2.6. H₂-TPR Results

Figure 6 shows the H₂-TPR profiles of La_1-xCe_xMnO₃ catalysts. Overall, the profiles contain two basic reduction peaks: one at the low temperature region of 250–500 °C, and the other at the high temperature region of 600–900 °C. Generally, for LaMnO₃ based perovskite oxides, the first region corresponds to the reduction of Mn⁴⁺ → Mn³⁺, and the second to the reduction of Mn³⁺ → Mn²⁺ [38]. Herein, because the temperature of the second reduction peak largely exceeds that applied in the catalytic reaction (<300 °C), we mainly analyze and discuss the low temperature reduction peak in the following.

In the low temperature reduction region, one small reduction peak is observed for LaMnO₃, which can be attributed to the reduction of Mn⁴⁺ → Mn³⁺. With the addition of Ce atoms, a new peak at higher temperature (363–402 °C) appears for La_1-xCe_xMnO₃, which can be attributed to the reduction of Ce⁴⁺ → Ce³⁺, as suggested in previous work [39]. The increase in reduction temperature and the strengthening of peak intensity support that the Ce atoms are presented and involved in the reduction process. For the sample x = 0.25, i.e., La_0.75Ce_0.25MnO₃, it shows only a broad reduction peak, which could be due to the large amount of CeO₂ formed in the material. Thus, the reduction process of Mn⁴⁺ → Mn³⁺ is overlapped by that of Ce⁴⁺ → Ce³⁺.

2.7. Stability Test of La_0.8Ce_0.2MnO₃

Based on the above discussion, La_0.8Ce_0.2MnO₃ is selected as the model catalyst for stability tests, which are evaluated in two patterns: one is long-term stability tested at a setting temperature (160 °C) to see the activity changes with the reaction time; and the other is cycling stability tested by increasing/decreasing the temperature to complete a cycle to see the activity changes in each cycle. For the long-term stability, we see that the activity remains unchanged (within the uncertainty of experiment) for 30 h, Figure 7a. For the cycling stability, the activity curves are almost the same, except a slight decrease in the activity is observed at 180 °C with the increase in cycling times, Figure 7b. This indicates that the catalyst has good stability in the reaction. The reason for the slight decrease in activity at temperatures below 180 °C, with the increase in cycling time, could be that the surface of the catalyst is refilled with oxygen during the cooling process, occupying the active sites. These oxygens can be released after 180 °C, regenerating the active sites and hence the reaction activity.

To support the stability of La_0.8Ce_0.2MnO₃ in the reaction, we measured the XRD patterns of the fresh and used catalysts. Figure 7c shows that the characteristic diffraction peaks of LaMnO₃ and CeO₂ appear in the XRD patterns of samples before and after the reaction, which confirms that the CO oxidation process does not change the phase structure of La_0.8Ce_0.2MnO₃, and the catalyst has good stability in the reaction.

Hence, it is concluded that the Ce doping improves the surface area, the redox ability, and the catalytic performances of LaMnO₃ for CO oxidation, meanwhile preserving good stability in the reaction. The results suggest that Ce as a promotor can improve catalytic performances of perovskite oxides by inducing the formation of oxygen vacancies, the change of oxidation states of metal ions, and/or the production of synergistic effects, which could be a direction to optimize perovskite oxides for catalysis use.

3. Experimental

3.1. Catalyst’s Preparation

The catalysts were synthesized by a sol–gel method using ethylene glycol and methanol mixture as the complexant and nitrates of La, Ce and Mn as the precursors, as described elsewhere [18]. Briefly, a certain amount of ethylene glycol and methanol mixed solution, with volume ratio of 3:2, was first prepared, to which La(NO₃)₃•nH₂O, Mn(NO₃)₃ and Ce(NO₃)₃•6H₂O, with varied Ce/(La + Ce) molar ratios (0 ≤ x ≤ 0.25), were added. The mixed solution was stirred magnetically in a water bath at 80 °C for 2 h and then dried in an oven at 100 °C for 12 h. The dried solid sample was crashed and calcined at 750 °C for 5 h in an air atmosphere (the heating rate is 2 °C/min). The obtained catalyst was marked as La_1-xCe_xMnO₃ (0 ≤ x ≤ 0.25).

3.2. Catalyst Characterization

XRD patterns were obtained using a Rigaku Ultima IV X-ray instrument with Kα radiation (λ = 1.5418 Å). Raman spectra were tested by LabRAM HR Evolution, using the wavelength of 532 nm. N₂ physisorption was performed on a BEISHIDE 2000PS2 instrument at −196 °C. The sample was degassed in vacuum, at 200 °C, for 5 h, before measurements. The surface area was calculated by the Brunauer−Emmett−Teller (BET) method. TEM images were observed on a JEM-2100Plus instrument. The sample was ultrasonically dispersed in an ethanol solution for several seconds before being deposited on carbon-coated copper grids for observation. XPS spectra were performed on a Thermo Scientific K-Alpha apparatus equipped with a monochromated Al Kα X-ray source. Peak fitting was performed using the Thermo Scientific Advantage software. The binding energy was calibrated with the C1s of adventitious carbon. H₂-TPR profile was plotted on a DAS-7000 apparatus (Huasi instrument Co., Hunan, China). A 30 mg sample was treated in N₂ at 300 °C for 1 h and then cooled to room temperature (RT). Thereafter, a 10 vol% H₂/N₂ mixture was switched, with a flow rate of 20 mL/min. After reaching a stable baseline, the sample was heated from RT to 850 °C, at a heating rate of 10 °C/min, to record the profile.

3.3. Catalytic Test

The CO oxidation reaction was carried out in a fixed bed reactor. The inner diameter of the quartz fixed bed reaction tube is 6 mm, and the length is 400 mm. The simulated flue gas containing 0.5% CO and 6.5% O₂ (balanced with Ar) was passed at a total flow rate of 50 mL/min. The amount of catalyst used was 0.1 g. The catalytic activity in the temperature range of 60−300 °C was detected, and each temperature point was maintained for 30 min. The outlet gas compositions were analyzed by an online gas chromatograph. The CO conversion was calculated as below:

$$\mathrm{CO}\mathrm{conversion}\left(\%\right)=\frac{C_{\left(C{O}_{in}\right)}-C_{\left(C{O}_{out}\right)}}{C_{\left(C{O}_{in}\right)}}\times 100\%$$

where C_(COin) and C_(COout) are the CO concentration at the inlet and outlet, respectively.

4. Conclusions

In summary, La_1-xCe_xMnO₃ catalysts for CO catalytic oxidation are successfully prepared by the sol–gel method, and the effect of Ce doping on the rection is investigated. At low Ce doping (x < 0.1), the Ce atoms enter the LaMnO₃ perovskite structure, but at high Ce doping (x ≥ 0.1), part of Ce stays on the surface of LaMnO₃ as CeO₂ crystalline. The La_0.8Ce_0.2MnO₃ catalyst exhibits disordered shape, and its particles are aggregated. XPS results show that, at x = 0.2, the sample, i.e., La_0.8Ce_0.2MnO₃, possesses the highest surface Ce³⁺/(Ce³⁺ + Ce⁴⁺) and O_β/O_γ molar ratio, implying the generation of more oxygen vacancies on the surface, which is believed to be the active site of O₂ adsorption and activation. As a result, after the doping of Ce atoms, the activity of La_1-xCe_xMnO₃ shows a trend of initial increase and then decrease. La_0.8Ce_0.2MnO₃ shows the highest activity for CO oxidation among the investigated La_1-xCe_xMnO₃ samples, with 100% CO conversion obtained at 180 °C. Moreover, the material possesses good long-term and cycling stability in the reaction, and the structure remain unchanged for the fresh and used La_0.8Ce_0.2MnO₃, as detected by the XRD patterns.