Enhanced the Catalytic Performance of Samarium and Cerium Co-Modified Mn-Based Oxide Catalyst for Soot Oxidation

Abstract

Manganese-based oxides with good redox properties exhibit high soot oxidation activity. To further enhance their catalytic performance, introducing additional metal elements into manganese-based oxides is considered an effective approach. Herein, two rare earth elements (Sm and Ce)-modified MnO_x catalysts were prepared by the co-precipitation method. The synthesized MnO_x catalyst primarily consists of the Mn₃O₄ phase, with trace amounts of Mn₅O₈. The addition of Sm or Ce maintains the predominance of the Mn₃O₄ phase, increases the proportion of Mn₅O₈, and enhances the redox properties, thereby boosting the catalytic activity for NO and soot oxidation. Notably, the coexistence of Sm and Ce achieves optimal soot oxidation activity, with T₁₀ reaching 306 °C. Comprehensive physicochemical characterization elucidates the underlying structure–performance relationships of these catalysts.

1. Introduction

Soot particles are formed from the incomplete combustion of diesel fuel under high-temperature and oxygen-deficient conditions, resulting from the condensation of partially burned carbon elements [1,2,3]. The harmful substances, such as sulfates and polycyclic aromatic hydrocarbons, adsorbed on the surface of soot particles, not only pollute the atmospheric environment but also pose a direct threat to human health [4,5]. Therefore, it is urgent to limit the emission of soot particles in diesel engine exhaust. Currently, the most effective way to eliminate soot particles is to combine catalytic combustion technology with filter capture technology for external purification [6,7]. Developing efficient soot oxidation catalysts is the core of this technology. Pt-based catalysts are widely utilized as commercial catalysts. However, Pt, being a rare noble metal, is very expensive. Therefore, it is essential to develop novel non-Pt-based soot oxidation catalysts with high activity.

Mn-based catalysts used for soot oxidation have received widespread attention due to their high catalytic activity, excellent redox properties, and lower costs [8,9,10,11]. Anantharaman et al. [12] prepared CeO₂, SnO₂, Pr₆O₁₁, and Mn₃O₄ catalysts using the EDTA-citrate complexation method for soot oxidation. They found that Mn₃O₄ had the highest soot oxidation activity because of its higher reducibility. Dai et al. [13] reported several pure MnO_x compounds and evaluated their soot oxidation activity. They believed that the presence of oxygen species over MnO_x could capture and oxidize NO, producing more reactive NO₂, thereby enhancing soot oxidation. Wagloehner et al. [14] evaluated the catalytic activity of various manganese oxide catalysts, such as MnO₂, Mn₂O₃, Mn₃O₄, and nanosized Mn₃O₄, synthesized via flame spray pyrolysis (FSP-Mn₃O₄). They explored the relationship between the physical properties of these catalysts and their effectiveness in oxidizing soot. The findings indicated that FSP-Mn₃O₄ demonstrated the highest catalytic performance, achieving maximal soot oxidation at 305 °C under a tight contact of catalyst and soot, attributed to its superior oxygen mobility. Although single MnO_x demonstrates good catalytic activity in soot oxidation, it has poor thermal stability, leading to agglomeration at high temperatures. To further improve the catalytic performance of MnO_x, various metal oxides, including Fe₂O₃ [15], CeO₂ [16], and Co₃O₄ [17], have been introduced and explored. Among these metal oxides, CeO₂ and Sm₂O₃, considered as environmentally friendly and economically advantageous materials, are quite effective. For example, Liao et al. [18] introduced CeO₂ into MnO₂ using the electrospinning method, which exhibited more active oxygen species and higher oxygen storage capacity. The T₅₀ of soot oxidation was decreased from 439 to 376 °C under tight contact. Sacco et al. [19] found the addition of Ce to MnO_x remarkably improved the soot oxidation activity due to the synergistic effects of Ce and Mn. Yu et al. [20] synthesized CeMnO_x composite oxides via a simple hydrothermal process and investigated their effect on soot oxidation. They found that Ce-doped amorphous Ce₁MnO_x catalysts exhibited higher specific surface area, increased porosity, and enhanced oxygen activation compared to pure MnO_x catalysts, while also demonstrating excellent stability and resistance to sulfur and water during soot oxidation. Additionally, Sm, primarily in its trivalent state, when used to modify reducible MnO_x, enables a large number of oxygen vacancies. This results in excellent catalytic activity in various oxidation reactions, such as the oxidation of NO and CO.

Inspired by the above, in this work, two rare earth metal elements, namely Sm and Ce were introduced into MnO_x by the co-precipitation method for soot oxidation. Adding Sm or Ce into MnO_x improved the soot oxidation activity. The synthesized mixed oxide catalysts were thoroughly characterized by means of XRD, SEM, TEM, BET, XPS, and H₂-TPR to correlate these properties with the catalytic performance. This work provides new references for the use of rare earth elements in the modification of metal oxide catalysts.

2. Results and Discussions

2.1. Characterization of the Catalysts

2.1.1. XRD Analysis

Figure 1 shows the XRD patterns of the catalysts prepared using the precipitation method. The XRD spectrum of MnO_x primarily indicates the presence of the Mn₃O₄ phase. Additionally, a Mn₅O₈ phase is also detected, evidenced by characteristic diffraction peaks at 21.8°, 46.7°, and 47.9°. The introduction of Sm or Ce results in an obvious decrease in the intensity of the XRD patterns, especially for Sm-Ce-MnO_x. This may be attributed to the addition of Sm or Ce, which alters the crystallinity and phase composition of manganese oxide, resulting in smaller crystallite sizes and, consequently, weaker XRD peak intensities. Another effect of adding Sm or Ce is to promote the formation of the Mn₅O₈ phase. No diffraction peaks for Sm₂O₃ or CeO₂ are detected due to their trace amounts. Compared with MnO_x, there is no significant peak shift in the diffraction patterns of Sm-MnO_x, Ce-MnO_x, and Sm-Ce-MnO_x, indicating that the Sm and Ce promoters do not enter the lattice of MnO_x.

2.1.2. SEM and TEM Observations

Figure 2 displays the SEM images of the catalysts. As shown in Figure 2a, MnO_x presents non-uniform aggregated nanoparticles that form distinct clusters. After introducing the Sm element, no significant morphological changes are observed. However, when Ce is introduced, although the catalyst still maintains the aggregated state of nanoparticles, the degree of aggregation is reduced, presenting a more dispersed nanoparticle morphology. This phenomenon is particularly significant when Ce and Sm are simultaneously introduced. This observation is consistent with the findings reported by Yang et al. [21]. The catalyst after the reaction was also characterized by SEM, and the results are shown in Figure S1. It was found that the sintering of nanoparticles after the reaction was more serious. This may be caused by an additional heat treatment experienced by the catalyst during the catalytic soot oxidation process.

Figure 3 displays the TEM and HRTEM images of Sm-Ce-MnO_x. As shown in Figure 3a, the Sm-Ce-MnO_x obtained by the co-precipitation method shows an irregular structure consisting of agglomerated nanoparticles, agreeing with the SEM images. From Figure 3b, clear lattice fringes measuring 0.310 nm and 0.235 nm are evident, corresponding to the (112) plane of Mn₃O₄ and the (−221) plane of Mn₅O₈, respectively [22]. No lattice fringe corresponding to Sm₂O₃ or CeO₂ is detected, which may be attributed to their high dispersion on the surface of Mn₃O₄.

2.1.3. Surface Adsorption and Desorption

The porosity and surface characteristics of the catalysts significantly influence their catalytic performance. The N₂ adsorption–desorption isotherms and pore size distribution of the samples are depicted in Figure 4. The isotherm (Figure 4a) exhibits a typical Type IV sorption behavior with an H3-type hysteresis loop at high P/P₀, indicative of slit-shaped mesopores formed by particle aggregates [23]. Brunauer–Emmett–Teller (BET) analysis was used to measure the specific surface areas of the catalysts, with results summarized in

Table 1. Results summarized from N₂ adsorption and XPS.

Catalyst	SBET a (m2g−1)	Average Pore Size a (nm)	Total Pore Volume a (cm3/g)	Mn4+/Mn b	Oads/Olatt b
MnOx	28.3	23.4	0.167	0.22	0.30
Sm-MnOx	49.8	19.3	0.241	0.28	0.38
Ce-MnOx	55.0	13.6	0.187	0.30	0.41
Sm-Ce-MnOx	88.6	13.6	0.303	0.31	0.53

^a The data obtained from the N₂ adsorption–desorption test. ^b Mn⁴⁺/Mn and O_ads/O_latt ratios obtained from XPS fitting results.

. The surface areas for MnO_x, Sm-MnO_x, Ce-MnO_x, and Sm-Ce-MnO_x are 28.3, 49.8, 55.0, and 88.6 m²g⁻¹, respectively. These data indicate that introducing Sm and Ce significantly increases the specific surface area of the catalysts. This enhancement may be attributed to the changes in the nanoparticle stacking method caused by these elements, leading to finer pore formation, and thereby, affecting the specific surface area. The specific surface area of the catalyst slightly decreases after the reaction. For example, the specific surface area of the Sm-Ce-MnO_x catalyst reduces to 84.96 m²/g post-reaction. The reduction in average pore size, as shown in Table 1, supports this viewpoint. Furthermore, the BJH pore size distribution curves (Figure 4b) reveal peaks primarily between 2 and 50 nm, confirming the presence of mesoporous structures. All BET-BJH parameters are also listed in Table 1. The increased pore volume and surface area provide more active sites for soot oxidation reactions. The soot oxidation reaction is a gas–solid–solid multiphase catalytic process where a high specific surface area facilitates better contact between soot and the catalyst. For these catalysts, Sm- and Ce-modified MnO_x exhibit the highest specific surface area among the catalysts, significantly enhancing the interaction between soot and the catalyst. This could promote soot oxidation.

2.1.4. XPS Analysis

The surface chemical compositions and elemental valence states of the catalysts were characterized by XPS, and the results are shown in Figure 5. Figure 5a shows the Mn 2p XPS spectra. The Mn 2p_3/2 signal exhibits three peaks corresponding to Mn²⁺, Mn³⁺, and Mn⁴⁺ at binding energies of approximately 640.1 eV, 641.5 eV, and 643.1 eV, respectively [24,25]. Similarly, the Mn 2p_1/2 spectra also show three deconvoluted peaks at binding energies of 652.0 eV (Mn²⁺), 653.0 eV (Mn³⁺), and 654.6 eV (Mn⁴⁺). The Mn⁴⁺/(Mn²⁺ + Mn³⁺ + Mn⁴⁺) ratios follow the order of Sm-Ce-MnO_x (0.31) > Ce-MnO_x (0.30) > Ce-MnO_x (0.28) > Ce-MnO_x (0.22). The higher Mn⁴⁺/(Mn²⁺ + Mn³⁺ + Mn⁴⁺) ratio observed on the surface of the Sm-Ce-MnO_x catalyst suggests that more active sites are generated, potentially enhancing their catalytic activity. Figure 5b shows the O 1s spectra of the catalysts. For MnO_x, the binding energy peaks at approximately 529.4 eV and 531.0 eV are attributed to lattice oxygen (O_latt) and surface-adsorbed oxygen species (O_ads), respectively [26,27,28]. The latter encompasses defect-related oxides (O₂²⁻ and O⁻) and hydroxyl groups. Notably, the ratio of O_ads/O_latt for Sm-MnO_x (0.38), Ce-MnO_x (0.41), and Sm-Ce-MnO_x (0.53) is markedly higher compared to that of pristine MnO_x (0.30), suggesting that the introduction of Sm or Ce elements increases the number of active oxygen species on the surface of MnO_x catalysts. The increased proportion of surface-reactive oxygen species, along with a higher concentration of Mn⁴⁺ ions can promote soot oxidation. Additionally, it is observed that the XPS peak positions associated with lattice oxygen (O_latt) in the modified MnO_x shift toward lower binding energies compared to unmodified MnO_x. This shift likely results from a weakened chemical bond between lattice oxygen and Mn, induced by the introduction of other elements [29].

2.1.5. H₂-TPR Analysis

For a soot oxidation reaction, the redox performance of the catalyst is crucial. Enhanced redox properties indicate a more favorable environment for catalyzing soot oxidation, leading to improved catalytic activity. To understand the reduction/oxidation behaviors of the catalysts, the H₂ temperature-programmed reduction (H₂-TPR) profiles of the catalysts are shown in Figure 6. The three peaks detected on the MnO_x catalyst are centered at 194, 244, and 393 °C. The first two low-temperature peaks are attributed to the reduction of Mn⁴⁺ to Mn³⁺. The occurrence of peak splitting may be due to the presence of a non-uniform size distribution of the MnO_x particles. The higher reduction peak at 393 °C is attributed to the reduction of Mn³⁺ to Mn²⁺. With the addition of Sm, the reduction peaks slightly shift toward higher temperatures. However, the area of the reduction peaks has changed. The area of the first reduction peak has significantly increased, while that of the second reduction peak has decreased, indicating that the introduction of Sm is conducive to the presence of high-valence Mn⁴⁺. For the Ce-MnO_x catalyst, the addition of Ce broadens the temperature ranges of both reduction peaks and shoulder peaks appear within each reduction peak. The presence of these shoulder peaks indicates that the reduction in some Mn⁴⁺ and Mn³⁺ is facilitated by introducing Ce, while other Mn⁴⁺ and Mn³⁺ are not affected by Ce. When Sm and Ce are simultaneously introduced in MnO_x, Sm-Ce-MnO_x exhibits similar reduction patterns to those of Sm-MnO_x. Although there is little difference in the temperature of the first reduction peak, the second reduction peak shifts significantly to lower temperatures. Compared to Ce-MnO_x, the two reduction peaks of Sm-Ce-MnO_x do not show shoulder peaks, indicating that the simultaneous introduction of Sm and Ce is more effective in promoting the reduction in bulk MnO_x.

2.2. Catalyst Oxidation Activities

NO₂ is a key constituent of nitrogen oxides (NO_x), predominantly formed by the oxidation of NO. In diesel exhaust, NO_x primarily stems from the combustion-induced reaction between N₂ and O₂ within the engine, initially yielding NO. Due to its instability, NO rapidly reacts with ambient oxygen to produce NO₂, which is unavoidable during exhaust emissions, particularly at elevated temperatures that accelerate NO oxidation, thereby increasing NO₂ levels. NO₂ with a stronger oxidation ability than O₂ is critical to soot catalytic oxidation in the presence of NO [30,31,32]. Therefore, the ability for NO oxidation to NO₂ is an important factor for catalytic soot combustion. To evaluate the ability of NO oxidation to NO₂ of the as-prepared catalysts, the NO oxidation experiments were carried out and the results are shown in Figure 7a. From Figure 7a, the catalysts display the different abilities for NO oxidation to NO₂. Sm-Ce-MnO_x exhibits a lower activation temperature and higher abilities for NO oxidation to NO₂. The NO₂ production of the catalysts during the TPO follows the order of Sm-Ce-MnO_x > Ce-MnO_x > Sm-MnO_x > MnO_x. This finding suggests that the dual-promoter modification of MnO_x with Sm and Ce enhances the NO oxidation capability of MnO_x, thereby significantly boosting the soot oxidation reaction. The synergistic effect of Sm and Ce promoters is effective in boosting the catalytic performance of MnO_x for soot oxidation.

Figure 7b shows the TPO profiles for soot oxidation over the prepared catalysts, with the corresponding T₁₀, T_50, and T₉₀ values listed in

Table 2. Catalytic features of the catalysts for soot oxidation.

Catalyst	T10 (°C)	T50 (°C)	T90 (°C)	SCO2 (%)
MnOx	340	416	461	99.4
Sm-MnOx	332	409	457	99.1
Ce-MnOx	324	396	441	98.9
Sm-Ce-MnOx	306	397	444	99.6

. As shown in Table 2, the T₁₀, T₅₀, and T₉₀ values for soot oxidation over MnO_x are 340 °C, 416 °C, and 461 °C, respectively. In contrast, MnO_x modified with Sm or Ce exhibits higher soot oxidation activity. From Table 2, although the T₅₀ and T₉₀ values for Ce-MnO_x are similar to those of Sm-Ce-MnO_x, the ignition activity of Sm-Ce-MnO_x is much higher than that of other catalysts with the T₁₀ = 306 °C. According to T₁₀, the activity of the catalyst follows a similar order as NO-TPO, indicating that the simultaneous introduction of Sm and Ce into MnO_x increases the number of high-valence Mn ions, thereby promoting NO oxidation and further enhancing the soot oxidation reaction. The catalyst was tested over three cycles, and it was found that its activity remained essentially unchanged with repeated use, indicating excellent stability (Figure S2). Additionally, the CO₂ selectivity of the catalysts during the oxidation of soot was investigated. The S_CO2 value for all three catalysts exceeds 99%, indicating that the production of CO is minimal during the process of soot oxidation, thereby effectively avoiding secondary pollution.

3. Experiment

3.1. Preparation of Catalysts

Sm- or Ce-modified MnOₓ mixed oxides were prepared via a co-precipitation method. Taking Sm and Ce co-modified MnOₓ as an example, Ce(NO₃)₃·6H₂O (purity 99.9%), Mn(NO₃)₂ (50 wt%), and Sm(NO₃)₃·6H₂O (purity 99.9%) were used as precursors. These precursors were dissolved in deionized water and stirred until fully dissolved. NH₃∙H₂O solution was then added dropwise to the mixed solution until its pH value reached 9. The precipitate was subsequently washed several times via centrifugation, dried overnight in an oven at 60 °C, and finally calcined at 500 °C for 4 h. The other catalysts were prepared using the same method while adding different precursors according to the requirements. The resulting catalysts were designated as Sm-MnOₓ, Ce-MnOₓ, and Sm-Ce-MnOₓ, respectively. Importantly, within this series of catalysts, the amounts of Sm and Ce added were in a ratio of 0.1:1 with respect to MnO_x.

3.2. Characterization

The crystalline phases of the catalysts were characterized by powder X-ray diffraction (XRD) using the Kα radiation of Cu (λ = 1.5406 Å) at 40 kV and 40 mA, from 10.0° to 80.0° (PANalytica X’Pert PRO MPD, Almelo, The Netherlands). The morphology of the catalyst was characterized by scanning electron microscopy (SEM) (JEM-6700F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM) (JEM-2010, JEOL, Tokyo, Japan) at a working voltage of 200 KV, respectively. The specific surface area was determined by employing the Brunauer–Emmett–Teller (BET) method on a JW–BK122 F (JWGB, Beijing, China) instrument. Prior to the analysis, the samples were degassed at 220 °C under vacuum for 1 h. The specific surface area of the samples was obtained from the Brunauer–Emmett–Teller (BET) equation.

X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Model VG ESCALAB 250 spectrometer (Thermo Electron, London, UK) using a non-monochromatized Al-Ka X-ray source (hv = 1486.6 eV). The binding energies (BE) were calibrated using the adventitious C 1s peak (284.8 eV) as a standard.

The reducibility of catalysts was investigated through H₂ temperature-programmed reduction (H₂-TPR). For each measurement, a 50 mg sample placed in a U-shaped quartz tube was pretreated in air (20 mL/min) at 500 °C for 1 h. After cooling to 30 °C, the gas flow was switched to 10 vol.% H₂/Ar (50 mL/min), and then the catalyst was heated to 900 °C at a heating rate of 10 °C/min under this atmosphere using a Micromeritics Auto Chem II 2920 (Micromeritics, Norcross, GA, USA) apparatus. During the tests, H₂ consumption was monitored as a TCD signal by the chemisorption analyzer.

3.3. Catalytic Activity Measurements

The catalytic oxidation of NO was evaluated using an infrared spectrometer (Nicolet iS10, Thermo Corp, Madison, WI, USA). A total of 100 mg of catalyst and 300 mg of silica pellets were mixed evenly with a spatula and then loaded into the quartz reactor (i.d. 5 mm). The reactant gas mixture was 500 ppm NO/10%O₂/N₂ with a total flow rate of 500 mL/min. After the gas concentration was stable for 10 min, the temperature was increased from room temperature to 600 °C at a heating rate of 5 °C/min for NO oxidation.

The catalytic performance of soot oxidation was evaluated on the same infrared spectrometer. Prior to the experiment, 10 mg of soot was mixed with 100 mg of catalyst using a spatula for 3 min, followed by the addition of 300 mg of silica to prevent uncontrolled reactions. The reactor system was purged with 500 ppm NO/10% O₂/N₂ with a total flow rate of 500 mL/min and then heated to 600 °C at a rate of 5 °C/min. Each soot-TPO test required approximately 2 h. The temperatures corresponding to the soot conversion of 10%, 50%, and 90% were defined as the T₁₀, T₅₀ and T₉₀, respectively. The selectivity of CO₂ (S_CO2) was calculated by S_CO2 = C_CO2/(C_CO + C_CO2), where C_CO and C_CO2 were defined as the total CO and CO₂ release in the outlet gas during the soot oxidation, respectively, which were obtained by integrating the outlet CO_x concentrations over time.

4. Conclusions

In conclusion, MnO_x catalysts modified with the rare earth element Sm (Sm-MnO_x), Ce (Ce-MnO_x), and co-modified with Sm and Ce (Sm-Ce-MnO_x) were obtained via the co-precipitation method. The introduction of Sm or Ce does not induce significant phase changes and MnO_x predominantly retains its Mn₃O₄ phase. Compared with pure MnO_x, the rare earth element-modified MnO_x catalysts exhibit improved NO and soot oxidation activity, especially for Sm-Ce-MnO_x. The reason for the increased activity of the modified MnO_x catalyst is mainly due to the enhanced redox property, a higher concentration of active oxygen species, increased surface area, and an elevated Mn⁴⁺/Mn³⁺ ratio. These improvements collectively create a more favorable environment for catalytic soot oxidation reactions. This work provides a novel reference for rare earth element-modified transition metal MnO_x in synergistically promoting soot oxidation reaction.