Highly active MnO_x–CeO₂ catalyst for diesel soot combustion

Abstract

A series of MnO_x–CeO₂ composites have been prepared via a facile coprecipitation method and used as catalysts for diesel soot combustion. It is found that the content of manganese oxide has a considerable influence on both the morphology and catalytic activity for diesel soot combustion in the copresence of NO_x and O₂. Superior catalytic activity (T_f = 360 °C) was obtained over the optimized catalyst CM₂₀ [Mn/(Mn + Ce) equals to 20 at%], which is believed to be attributable to the following two aspects: (1) the abundant pore structure, presenting between the loosely-packed homogeneous particles with similar sizes to that of the soot particles, which is beneficial for mass transfer and heat diffusion as well as the sufficient contact between catalyst and soot; (2) high concentration of Mn⁴⁺ and Ce³⁺ cations derived from the charge transfer between Mn and Ce species as active sites can adsorb and activate NO and O₂, and then greatly facilitate NO₂ production.

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1. Introduction

Soot, as one of the main pollutants from diesel emission, has caused severe environmental problems like haze,¹ and several emission control strategies have been adopted to meet the demands of more and more stringent legislation, such as improvements in fuel oil,² modifications of the engine or process^3,4 and exhaust after-treatment systems.⁵ For example, the diesel particulate filter (DPF) used in the after-treatment systems has so far been the most promising technological option. When the diesel exhaust flows through DPF, over 90% of the soot can be trapped and then be continuously or periodically combusted to CO₂.⁶ However, the combustion temperature of soot (around 600 °C)⁷ is far beyond the normal diesel exhaust temperature range (175–400 °C),⁸ making the regeneration of DPF an intractable problem. Aimed at the passive regeneration of DPF, many kinds of catalysts, such as noble metals,⁹ transition metal oxides,¹⁰ perovskite and/or perovskite-like mixed oxides,¹¹ spinel mixed oxides,¹² alkaline metal/alkaline-earth metal oxides,¹³ and the popular ceria-based oxides,^14–19 have been developed to decrease the soot combustion temperature. Additionally, many researches demonstrate that the introduction of macropores or large voids into the catalysts can greatly facilitate the heat and mass transfer as well as improve the contact between soot and the catalysts, eventually to reach a high activity towards soot oxidation.^11,20–25 For example, Wei et al.²³ synthesized a three-dimensionally ordered macroporous (3DOM) LaFeO₃ catalyst loaded with tiny golden nanoparticles (∼2.9 nm), showing the temperature of maximum soot combustion rate (T_m) at 366 °C under loose contact mode between soot and catalyst. Recently, Nascimento et al.²⁵ synthesized Ag/CeO_x/FeO_y catalysts supported on cordierite for the control of diesel soot emission, showing further decreased T_m (334 °C), and T_f (finishing soot combustion temperature, 400 °C), exhibiting great potential in application. However, the high-cost, the sensitivity to sulfur, and easily sintering characteristic of noble metal catalysts,^26,27 as well as the difficulty in the synthesis of 3DOM materials hinder the industrial application of this kind of catalysts.

MnO_x–CeO₂ composites have been widely used in many catalytic reaction processes, such as wet oxidation of toxic organic pollutants²⁸ and oxidation of gaseous²⁹ or even liquid pollutants,³⁰ and proved to be a promising substitute for noble metal catalysts. But for catalytic soot combustion, the previously reported approaches are still hard to achieve a satisfactory performance on MnO_x–CeO₂ composites, i.e., T_m = 393–420 °C, T_f > 400 °C, maybe due to the unsufficient catalyst–soot contact that will restrict the full use of the active sites of the catalysts.^10,31–34

Herein, a series of MnO_x–CeO₂ composites was synthesized through a facile modified coprecipitation method. Especially, the optimized catalyst CM₂₀ [Mn/(Mn + Ce) = 20 at%] shows uniform particle morphology with hierarchically porous structure which is much beneficial to the sufficient contact between catalyst and soot, as well as the high concentration of Mn⁴⁺–Ce³⁺ cations on the surface that favors NO₂ production, thus exhibiting an excellent catalytic activity for the removal of soot in NO_x/O₂ atmosphere, i.e., ignition temperature (T_i) = 280 °C, T_m = 326 °C and T_f = 360 °C. The possible catalytic mechanism for the soot combustion is also proposed.

2. Experimental

2.1 Catalyst preparation

All the chemicals used in this study were of analytical grade, and were used without further purification. MnO_x–CeO₂ composites were prepared by the co-precipitation method reported before³⁵ with some modification. A typical preparation procedure is as follows: firstly, 7.5 mmol nitrate including Ce(NO₃)₃·3H₂O and Mn(NO₃)₂ (50 wt% in aqueous solution) was dissolved in 30 mL deionized water. Then, 30 mL water solution including 30 mmol NaHCO₃ was quickly poured into the nitrate solution and the mixture was stirred for 30 min at room temperature. Next, the precipitates were filtered and washed with deionized water and ethyl alcohol alternately for several times. After drying at 100 °C, the precursors were grinded and then calcined under static air at 500 °C for 2 h with a heating rate of 1 °C min⁻¹. The obtained catalysts were designated CM_y, where y (%) is the Mn/(Mn + Ce) atomic ratio in the recipe. In addition, we prepared two single oxides (CeO₂ and MnO_x) for reference with the same synthesis condition.

2.2 Catalyst characterization

The nitrogen adsorption and desorption curves were obtained using Micromeritics Tristar 3000 at 77 K, and the specific surface area and the pore size distribution were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Powder X-ray diffraction (XRD) patterns of the prepared samples were got using a Rigaku D/Max 2200 PC diffractometer with Cu Kα radiation (40 kV and 40 mA). Field emission scanning electron microscopy (FE-SEM) imaging was performed using Hitachi S-4800. Transmission electron microscopic (TEM) images, selected area electron diffraction (SAED) patterns and element mapping images were obtained using a JEOL-2010F electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) signals were recorded using an ESCAlab 250 instrument. The fitting of the Mn element was carried out using Gaussian fitting parameters. The temperature-programed reduction with hydrogen (H₂-TPR) experiments were performed using a Micromeritics Chemisorb 2750 instrument in a 5% H₂/N₂ flow (25 mL min⁻¹) using 50 mg catalyst with a heating rate of 10 °C min⁻¹. The uptake amount of H₂ was measured using a thermal conductivity detector (TCD).

2.3 Activity testing

Carbon black from Degussa (diameters: 10–50 nm) was used as the model soot particles. Following a well-accepted procedure, soot (10 mg) and catalyst (100 mg) were carefully mixed for 20 min with a spatula to simulate the loose contact mode, then silica pellets (1 g) were added and mixed by the spatula for another 10 min. It is worth noting that silica pellets were added to avoid pressure drop and favor heat transfer.³⁶ A gas mixture of 500 ppm NO, 10% O₂ and balance N₂ was feed with a flow rate of 200 mL min⁻¹. The space velocity was thus calculated to be 120 000 mL (g h)⁻¹. Afterwards, the mixture was heated to 650 °C at a heating rate of 5 °C min⁻¹ in a tube furnace equipped with a thermal couple. To minish the error of the measurement to a least extent, the temperature of the tube furnace was monitored with a standard thermal couple, every gas flow was verified with a mechanical flow meter and the concentration of NO_x was measured by a NO_x analyzer (Thermo Fisher 42i-LS). The analysis of the emissions from the reactor was performed using an online GC-FID analyzer (GC2060 from Shanghai Ruimin Instrument Corp. Ltd.) equipped with a methane converter. The catalytic activities (conversion and selectivity) were calculated as indicated in previous reports in the literature, in terms of T_i, T_m and T_f values, which were respectively defined as the temperatures at 10%, 50% and 90% of soot conversion, and selectivity to CO₂ as well, which was quantified: selectivity (%) = S_CO2(total)/[S_CO(total) + S_CO2(total)], where S_COx(total) represents the area under the CO_x–temperature curve in the range of RT – 650 °C (x = 1 or 2).^37,38

3. Results and discussion

3.1 Catalytic activity measurements

Fig. 1 shows the catalytic activity of the CM_y composites, references MnO_x and CeO₂, and NULL (without catalyst) towards soot oxidation under loose contact mode. It is obvious that compared with NULL, all the CM_y composites and the references MnO_x, CeO₂ show much lower characteristic temperatures and higher selectivity to CO₂ in the soot oxidation. Thereinto, CM₂₀ shows the lowest characteristic temperatures (T_i = 280 °C, T_m = 326 °C, and T_f = 360 °C) as well as the highest selectivity to CO₂ (95.6%), indicating that the interaction between Mn and Ce species can greatly improve the catalytic activity toward the soot oxidation. Even compared with the noble-metal based catalysts supported on 3DOM oxides or cordierite, as well as the MnO_x–CeO₂ catalysts prepared by other methods, CM₂₀ still shows much lower characteristic temperatures (especially T_f) for soot combustion under similar reaction conditions (loose contact mode, shown in Table S1†). It should be noted that even at a higher space velocity [330 000 mL (g_catal h)⁻¹, the highest one among those shown in Table S1†], CM₂₀ still shows a much lower T_m value (350 °C) than that of other reported MnO_x–CeO₂ catalysts (393–420 °C) shown in Table S1.† Fig. S1 and S2† indicates good stability in both catalytic activity and structure for these CM_y composite catalysts (herein, CM₃₃ was chosen as an example). In order to understand the different catalytic performances on these prepared catalysts, XRD analysis, N₂ adsorption/desorption analysis and SEM/TEM observations were conducted to reveal both the phase composition and the morphology/pore structure of different catalysts.

Fig. 1 Catalytic activity (left) and selectivity to CO₂ (right) for soot combustion over different catalysts or NULL (without catalyst) under loose contact mode. Reaction conditions: 500 ppm NO and 10% O₂ in N₂ with a total flow of 200 mL min⁻¹, mass of soot–catalyst–silica is 10–100–1000 mg.

3.2 XRD analyses

Fig. 2 shows the XRD patterns of different CM_y composites and the references MnO_x and CeO₂. It is found that the reference CeO₂ exhibits a single phase of cubic CeO₂ (JCPDS 43-1002), while MnO_x shows three distinguishable manganese oxides of Mn₂O₃ (JCPDS 73-1826), MnO₂ (JCPDS 72-1982) and Mn₅O₈ (JCPDS 39-1218). In addition, compared with reference CeO₂, all the CM_y composites show relatively wide diffraction peaks corresponding to cubic CeO₂ phase, without detectable Mn-containing phases or obvious peak shift, indicating that manganese oxides may exist in the form of highly dispersive and/or amorphous species, thus decreasing the crystallinity of CeO₂. More interestingly, CM₂₀ shows the biggest FWHM (the full width at half maximum) values of the main XRD peaks among all the CM_y composites, suggestive of the smallest crystalline grain according to the Scherrer equation, and the corresponding most abundant surface defect sites, which are beneficial for the improvement of catalytic activity.

Fig. 2 XRD patterns of different CM_y composites and the references MnO_x and CeO₂ (left), (■-CeO₂; ◊-Mn₂O₃; ▲-MnO₂; ▼-Mn₅O₈), and the enlarged region between 25° and 31° of the patterns showing the corresponding FWHM values and peak positions of 2θ (right).

3.3 SEM & TEM measurements for CM₂₀

The typical SEM images and the corresponding EDS spectrum, as well as TEM image and the corresponding element mapping of CM₂₀ shown in Fig. 3 indicate that CM₂₀ is solely composed of uniform particles with loose packing, leaving abundant large pores, as indicated by the arrows in Fig. 3b, which is very different from the morphologies of all the other studied catalysts in Fig. S3† (CeO₂, CM₅, CM₅₀, CM₆₆ and MnO_x). The pore sizes in CM₂₀ are measured to be within 16–103 nm (Fig. 3b), which are consistent with its pore size distribution (10–159 nm) by N₂ adsorption/desorption analysis shown in Fig. S4† and summarized in Table S2.† As can be seen in Table S2,† the specific surface areas of CM_y composites are all larger than that of reference MnO_x and CeO₂. Especially, CM₂₀ shows much enlarged specific surface area and pore volume as well. Besides, its pore size is also broadened. It is believed that the similar particle size of the sample CM₂₀ to that of soot particles (Fig. S3-f†), and the abundant large pores in CM₂₀ are both helpful to its sufficient contact with soot. Also, its high specific surface area and pore volume both favor the dispersion of active sites, thus making a great contribution to catalytic oxidation of soot. The atomic ratio of Mn/(Mn + Ce) is calculated to be 21.9%, according to EDS spectrum in Fig. 3c, similar to that of the starting solution. As can be found in Fig. 3d–f, both the Ce species and Mn species are homogeneously distributed in the prepared composite, indicating that manganese species shows highly dispersive with ceria species. The H₂-TPR results (Fig. S5†) confirm the interaction between Mn and Ce species in CM_y composites. Especially, CM₂₀ shows excellent redox activity, which can be mainly attributed to the interaction between Ce and Mn species, inducing the changes of the local atomic environment for each element, as detected by the following XPS measurements.

Fig. 3 Low-magnification SEM image (a), high-magnification SEM image (b), EDS spectrum (c), typical TEM image (d) and the element mapping (e and f) of CM₂₀.

3.4 XPS measurements

Fig. 4 shows the Mn 2p, Ce 3d and O 1s XPS spectra of the series of CM_y composites, as well as the references MnO_x and CeO₂. The analysis results of different samples are summarized in Table 1, based on the careful peak fitting procedure. The O 1s XPS spectra of the sample CM_y and the references MnO_x and CeO₂ can be deconvoluted into two components after Gaussian fitting, as shown in Fig. 4c, which can be ascribed to lattice oxygen O_latt (529.6 eV) and adsorbed oxygen O_ads (532.4 eV), respectively. Generally, adsorbed oxygen species always correspond to the numbers of oxygen vacancies,³⁹ which are beneficial for the activation of gaseous oxygen molecules in the feed gas. The existence of oxygen vacancies in CM_y can also be evidently confirmed by the results of Raman spectroscopy analysis (Fig. S6†). Compared with reference CeO₂, the main peak in the Raman spectra of CM_y, the F_2g mode of CeO₂ shows a distinct red-shift from 464 to (433–452) cm⁻¹, indicating that the interaction between Mn and Ce disturbed the fluorite-like structure to some extent and resulted in the formation of the oxygen vacancies in the CeO₂ lattice.⁴⁰ It is noting that the peak at 652 cm⁻¹ for MnO_x also shifts down to (605–637) cm⁻¹ for CM_y composites, which could also be related to the formation of oxygen vacancies in CM_y.⁴⁰ Usually, the high ratio of O_ads/O_latt indicates a high activity toward oxidation reaction. But in the present work, CM₂₀, though with the lowest O_ads/O_latt ratio among all the studied catalysts, still shows the highest catalytic activity for soot combustion in NO_x/O₂, which suggests that there exists another key factor besides adsorbed oxygen species determining the catalytic activity.

Fig. 4 XPS spectra of (a) Mn 2p, (b) Ce 3d and (c) O 1s for all CM_y composites and the reference MnO_x and/or CeO₂.

Table 1 XPS surface composition analysis of CM_y composites and the references MnO_x and/or CeO₂

Sample	Oads/(Oads + Olatt) (%)	Mn^2+/Mn^3+/Mn^4+ (%)	Ce^3+/(Ce^3+ + Ce^4+) (%)
MnOx	54.9	27.5/52.1/20.3
CM66	33.3	35.2/48.7/16.1	17.3
CM50	31.1	22.1/56.9/21.1	19.2
CM33	30.5	20.1/58.5/21.3	24.9
CM20	21.2	16.0/54.9/29.0	27.6
CM5	27.7	8.0/56.8/35.1	22.0
CeO2	46.6		18.2

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As shown in the Fig. 4 and Table 1, both Mn and Ce species present various valence states. Thereinto, it is found that the ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) firstly increases and then decreases with the increase of y in CM_y, and the value reaches to a maximum value of 27.6% in CM₂₀, which indicates the charge transfer between Mn and Ce within the interface (eqn (1)), as supported by the previous reports demonstrating negative charge transfer either from Mn to Ce or from Ce to Mn.^41–45 Such a charge transfer will lead to the strong interaction between Ce and Mn species and thus the formation of uniform nanoparticles.⁴⁶ This strong interaction between Mn and Ce can also be reflected in the results of H₂-TPR profiles shown in Fig. S5,† in which, all of the CM_y composites especially CM₂₀ and CM₃₃ show much lower reduction peak temperature than that for either MnO_x or CeO₂. Additionally, it is reported that the Mn⁴⁺ and Ce³⁺ can absorb and activate NO and O₂,^47,48 thus the Mn⁴⁺ and Ce³⁺ cations could serve as active sites for the catalytic soot combustion in NO_x/O₂ atmosphere, by promoting the formation of NO₂.

Ce⁴⁺ + Mn³⁺ ↔ Ce³⁺ + Mn⁴⁺ (1)

To further verify the role of Mn⁴⁺ and Ce³⁺ cations in the catalytic soot oxidation, a parameter “Ω” (defined in eqn (2)) was introduced to quantify the concentration of Mn⁴⁺ and Ce³⁺ cations in the CM_y composites.

Ω = a × b (2)

Where a = (1 − y)[Ce³⁺/(Ce³⁺ + Ce⁴⁺)], b = y[Mn⁴⁺/(Mn²⁺ + Mn³⁺ + Mn⁴⁺)], and y is that in CM_y. Moreover, the relationship between “Ω” value and the catalytic activity over the different catalysts CM_y are shown in Fig. 5. It is found that for the CM_y composites, the larger Ω value is, the higher catalytic activity is, which confirms the role of Mn⁴⁺ and Ce³⁺ cations as active sites for catalytic soot combustion in NO_x/O₂ over CM_y composites. Nevertheless, the sample CM₆₆ seems an exception with a lower Ω value, showing higher catalytic activity than CM₅₀, which can be ascribed to the higher specific surface area and pore volume of CM₆₆ (72 m² g⁻¹ and 0.22 cm³ g⁻¹, Table S2†) than those of CM₅₀ (53 m² g⁻¹ and 0.11 cm³ g⁻¹, Table S2†).

Fig. 5 The different characteristic temperatures (T_m and T_f) and values of “Ω” as a function of y in CM_y. Reaction conditions for catalytic soot combustion: 500 ppm NO and 10% O₂ in N₂ with a total flow of 200 mL min⁻¹, mass of soot–catalyst–silica is 10–100–1000 mg, under loose contact mode.

3.5 SEM and TEM measurements for CM₃₃

It is worth noting that, compared with CM₂₀, CM₃₃ shows a little lower catalytic activity and Ω value, and similar specific surface area but significantly lower pore volume. Therefore, the TEM and SEM analyses for CM₃₃ were further performed, as shown in Fig. 6. It can be found that very different from the homogeneous morphology of CM₂₀ particles shown in Fig. 3, CM₃₃ shows some complex morphology in addition to the highly dispersive particles, such as micro-flakes (1 in Fig. 6c) and microspheres (3 in Fig. 6c). These two kinds of morphologies can also be found in the Ce-rich samples (CeO₂ and CM₅ in Fig. S3†), and the Mn-rich samples (CM₅₀, CM₆₆ and MnO_x in Fig. S3†), respectively. Apparently, either the micro-flakes or the microsphere morphology in CM₃₃ lacks the porous structure, thus leading to the lower pore volume than CM₂₀ (Table S2†), which can strongly affect the efficient contact between catalyst and soot particles and decrease the activity for soot oxidation. Nevertheless, the main component in CM₃₃ still presents similar size and chemical composition (Fig. 6b, 2 in Fig. 6c and S7†) to CM₂₀ particles (Fig. 3), thus it remains higher catalytic activity than other reference samples. Therefore, it is reconfirmed that the homogeneous particles morphology with hierarchically porous structure is one of the key factors in the catalytic process of soot oxidation.

Fig. 6 Low-magnification SEM image (a), high-magnification SEM image (b), TEM image (c) of CM₃₃, and the corresponding SAED patterns to the different areas in (c).

Overall, the catalytic activity for soot oxidation in NO_x/O₂ atmosphere can be ascribed to the following two aspects: (1) abundant Mn⁴⁺ and Ce³⁺ cations (large Ω value) serve as active sites by promoting NO₂ production, e.g., CM₂₀ with the largest Ω value (Ω = 128) shows the best activity toward the soot oxidation. (2) The high surface area and pore volume as well as the typical morphology of the particles with hierarchically porous structure could improve the dispersion of active species and soot diffusion, e.g., CM₆₆ with a lower Ω value (Ω = 62) exhibits a relatively high activity for soot oxidation due to its high specific surface area and pore volume.

3.6 A possible catalytic mechanism for soot oxidation

Based on the above results and analyses, a possible mechanism for the catalytic soot combustion in NO_x/O₂ over the sample CM₂₀ was proposed, as shown in Scheme 1. In this catalytic reaction, the efficient contact between the catalyst and soot, and the abundant active Mn⁴⁺–Ce³⁺ species are two crucial issues determining the excellent catalytic performance. Generally, soot combustion over catalyst in NO_x/O₂ atmospheres is believed to follow two different pathways:⁴⁹ (i) direct soot combustion by surface active oxygen species, (ii) NO-aided soot combustion. In Route (i), oxygen species absorbed and activated by oxygen vacancies (V_O) on the catalyst surface can directly oxidize soot into CO₂ (reaction (3), where O* is the active oxygen species).⁵⁰

O* + C → CO₂ + V_O (3)

Scheme 1 Schematic illustration of (A) the sufficient mixing of CM₂₀ and soot particles, and the possible pathways of soot combustion on CM₂₀ in NO_x/O₂: (B) Route (i): direct soot combustion by surface active oxygen species; (C) Route (ii): NO-aided soot combustion.

While in Route (ii), the NO-aided soot combustion can be expressed as the following reactions:

NO_ads + Mn⁴⁺ → NO⁺–Mn³⁺ (4)

O_ads + Ce³⁺ → O⁻–Ce⁴⁺ (5)

NO⁺–Mn³⁺ + O⁻–Ce⁴⁺ → NO₂ + Mn³⁺ + Ce⁴⁺ (6)

NO₂ + C → CO₂ + NO (7)

As supported by previous reports, it can be rationally deduced that the abundant Mn⁴⁺ and Ce³⁺ cations in CM₂₀ play different roles in the catalytic process. Therein, Mn⁴⁺ could adsorb and activate NO into active species NO⁺, as shown in reaction (4);⁴⁷ while Ce³⁺ could activate the absorbed oxygen O_ads into O⁻ (reaction (5)).⁴⁸ Such resultant NO⁺ and O⁻ species will react with each other to form NO₂ at the interface between Ce and Mn oxides (reaction (6)),⁵¹ meanwhile, the produced Ce⁴⁺–Mn³⁺ will react through reaction (1) to recover the active site Ce³⁺–Mn⁴⁺, thus completing the catalysis cycle. Then in reaction (7),⁵² the produced NO₂, as a stronger oxidant than O₂, can be reduced back to NO by soot, which meanwhile can be oxidized to CO₂ at low temperatures. In order to further confirm the role of NO_x in the catalytic soot oxidation process, the temperature-programed oxidation of NO over CM₃₃ with or without soot was further investigated, as shown in Fig. 7. It was found that with the addition of soot into the catalytic NO oxidation system, the NO₂ proportion in NO_x (= NO + NO₂) decreases in temperature range 200–400 °C, especially in the range of 300–375 °C, where the CO₂ concentration reaches its peak values. This coincidence confirms the reaction between NO₂ and C, which facilitates CO₂ production at the expense of NO₂ (reaction (7)). Moreover, catalysis activity measurement over CM₂₀ without the presence of NO was also conducted for comparison to the case with NO existing in the feed gas, as shown in Fig. S8,† clearly demonstrating the positive effect of NO on the catalytic activity towards soot oxidation.

Fig. 7 The temperature-programed oxidation of NO over CM₃₃ with or without soot under loose contact mode. Reaction conditions: 500 ppm NO and 10% O₂ in N₂ with a total flow of 200 mL min⁻¹, the mass of (soot–)catalyst–silica is (10–)100–1000 mg.

4. Conclusions

In summary, a facile co-precipitation method has been adopted to prepare a series of MnO_x–CeO₂ composites for the catalytic oxidation of soot in the simulated diesel emission. Thereinto, the optimized CM₂₀ shows the highest catalytic activity (T_f = 360 °C), under loose contact mode. The excellent catalytic activity can be attributed to its uniform particle morphology with hierarchically porous structure and the rich active Mn⁴⁺ and Ce³⁺ cations resulting from the suitable doping amount of Mn, which greatly accelerate soot oxidation in NO_x/O₂. A possible mechanism was proposed to interpret the role of Mn⁴⁺–Ce³⁺ species as well as the active oxygen species in the catalytic process of soot combustion in NO_x/O₂. The MnO_x–CeO₂ composites prepared by this low-cost and easily-scalable method could find the promising practical application in the catalytic removal of soot in the diesel exhaust.

Acknowledgements

This research was sponsored by National Key Basic Research Program of China (2013CB933200), China National Funds for Distinguished Young Scientists (51225202) and National Natural Science Foundation of China (51502330).

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Footnote

† Electronic supplementary information (ESI) available: Catalytic performance of samples reported in literatures together with CM₂₀, and NULL as reference; SEM images of MnO_x, CM_y (y = 66, 50 and 5), CeO₂ and soot; N₂ adsorption/desorption isotherms & pore size distributions of CM₂₀ and CM₃₃; structural properties and H₂-TPR profiles of all the catalysts studied, SEM-EDS results of different areas in CM₃₃. See DOI: 10.1039/c6ra25738k

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