Diesel-powered vehicles are generally more popular than gasoline-powered vehicles in the heavy-duty vehicle market because of their higher efficiency,lower operating costs,and higher durability and reliability ^[1]. However,as one of the largest contributors to environmental pollution,the exhaust fumes of diesel engines have been attracted considerable attention from environmental researchers and campaigners working towards renewable energy sources and a cleaner environment. The main pollutants in the exhaust fumes from diesel engines are soot particles and nitrogen oxides (NO_x). Soot particles,in particular,are one of the main sources of urban atmospheric particulate matter,and represent a significant threat to human health and the environment ^{[2, 3]}. The post-treatment of diesel exhaust fumes represents a potential strategy for eliminating the environmental problems associated with the use of diesel-powered vehicles,and the use of catalysts for the treatment of diesel exhaust fumes has been investigated extensively in this regard. The development of novel catalysts for the elimination of the soot particles present in diesel exhaust fumes is therefore highly desired ^{[4, 5]}.

Soot combustion is a deep oxidation reaction involving three phases of gas-solid-solid ^[6]. The contact conditions and intrinsic activities of the catalysts used to affect the combustion of soot play an important role in their defining their activity towards the combustion process. Given that the pore sizes of traditional catalysts (< 10 nm) are generally smaller than the diameters of most soot particles (> 20 nm),it can be difficult for the soot particles to enter the inner pores of these catalysts and react. Three-dimensionally ordered macroporous (3DOM) metal oxides with uniform pore sizes (> 50 nm) and well- defined structures have recently been used in the field of catalysis ^[7]. Given that the pore sizes of 3DOM materials are greater than the average diameter of the soot particles found in diesel fumes,they could not only readily accommodate the soot particles found in diesel fumes in their inner pores,but could also transfer them to their active sites with greater flexibly than nanoparticle samples for soot combustion ^[8]. We recently studied a series of 3DOM mixed metal oxides in terms of their catalytic activity,and showed that these materials exhibited better catalytic performance for the combustion of soot than the corresponding nanoparticle catalysts ^{[9, 10]}. However,3DOM oxide-based catalysts do not show a high level of efficiency towards the oxidation of soot at low temperatures because of the limitations imposed by their poor intrinsic activity. The development of new strategies to enhance the intrinsic activity of 3DOM oxide-based catalysts therefore remains a major challenge. The advantages of supported noble metal nanoparticle (NP) catalysts are well documented in the literature. To date,there have been several studies describing the evaluation of Au,Pt,Pd,and Ag as active components for the combustion of soot ^{[11, 12, 13]}. Synergistic effects can also occur between noble metals and their supports in these cases,which can make a significant contribution to their high catalytic activities. We previously reported the preparation of a series of 3DOM oxide-supported Au and Pt catalysts that exhibited high catalytic activities towards the combustion of soot ^{[14, 15, 16]}. It is noteworthy,however,that noble metal catalysts are a limited recourse and are therefore much more expensive than readily available metal oxide catalysts,making their general use in diesel-powered vehicles cost prohibitive.

A large number of low-cost catalysts have been prepared and studied during the last two decades in terms of their ability to affect the combustion of soot. Among the many different catalysts evaluated in this sense,CeO₂-based oxides have shown particular promise because of their high oxygen storage/release capacity (OSC) ^{[5, 17]}. Several MnO_x-based catalysts have also been reported to show high catalytic activities towards the combustion of soot because of the variable valence states of Mn⁴⁺,Mn³⁺,and Mn²⁺ in the MnO_x ^[18]. Based on the synergistic effect between Mn and Ce for enhancing OSC,mixed MnO_x-CeO₂ systems have been reported as a promising group of cheap and efficient candidate catalysts for the oxidation of soot ^{[19, 20, 21]}. Several K-containing materials have been reported to exhibit higher catalytic activities towards the combustion of soot than other alkali element-modified catalysts ^{[22, 23]}. However,the catalytic activities of K-based catalysts usually degrade after repeated thermal cycles because of the loss of K from the support. The incorporation of K into increasingly stable crystal structures has therefore become a popular approach for the development of recyclable K-based catalysts ^{[24, 25]},as exemplified by K-doped perovskite catalysts ^[26],K-doped spinel catalysts ^[27],and K-doped single metal oxides ^{[28, 29]}.

In this study,we have investigated the use of three commercially inexpensive elements (i.e.,K,Mn,and Ce) as active components for the preparation of novel catalysts for the combustion of soot. A 3DOM structure was formed by Mn and Ce,and KNO₃ was then loaded onto the surface of the resulting 3DOM Mn_0.5Ce_0.5O_δ catalyst. The formation of strong interactions among the K,Mn,and Ce atoms in the resulting catalysts means that the K⁺ ions were stabilized during the soot combustion process. A 3DOM Mn_0.5Ce_0.5O_δ catalyst was successfully synthesized using the colloidal crystal template (CCT) method. A series of 3DOM K-doped Mn_0.5Ce_0.5O_δ catalysts were also successfully prepared using an incipient-wetness impregnation method. The physical and chemical properties of the catalysts were characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),temperature-programmed reduction with H₂ (H₂-TPR) measurements,and Raman spectroscopy. The catalytic performance of these catalysts was also evaluated for the combustion of soot.

The polymethyl methacrylate (PMMA) microspheres were synthesized according to a modified version of the emulsifier-free emulsion polymerization method ^[16]. Briefly,240 mL of deionized water was added to a four-orifice reactor equipped with a water-cooling condenser and mechanical stirrer,which was heated in a hot water bath at 80 °C in Ar atmosphere. Methyl methacrylate monomer (120 mL) was then poured into the reactor,which was subsequently sealed. The resulting mixture was stirred for 20 min and then treated with a preheated (80 °C) solution of potassium persulfate initiator (0.60 g dissolved into 40 mL deionized water). The reaction mixture was then stirred at 80 °C for 2 h,and the resulting emulsion was filtered through a microfiltration membrane. To obtain PMMA as a colloidal crystal template,the filtrate was subjected to centrifugation at 3000 r/min for 10 h,and the resulting supernatant (clear liquid) was decanted to give a solid pellet,which was dried at 30 °C for 24 h to give the highly ordered PMMA arrays.

3DOM Mn_0.5Ce_0.5O_δ was synthesized according to the CCT method using the PMMA arrays as a template ^[16]. Briefly,cerous nitrate and manganese nitrate were dissolved in a mixture of ethylene glycol (65 vol%) and methanol (35 vol%),followed by the PMMA arrays,which were added for impregnation. Upon completion of the impregnation,the PMMA arrays were separated by vacuum filtration and dried at 30 °C for 24 h. The dried sample was then calcined in a tube furnace with an air flow of 80 mL/min to allow for the removal of the CCT. The temperature of the furnace was increased at a rate of 1 °C/min from room temperature to 550 °C. The material was then held at 550 °C for 4 h to give 3DOM Mn_0.5Ce_0.5O_δ composite oxides.

3DOM K-doped Mn_0.5Ce_0.5O_δ (K-MCO) catalysts were prepared by an incipient-wetness impregnation method. Briefly,a certain amount of KNO₃ was dissolved in deionized water,and the resulting solution was added to 3DOM Mn_0.5Ce_0.5O_δ. It is noteworthy that the volume of the KNO₃ solution used during this stage of the process should be equal to the volume of 3DOM Mn_0.5Ce_0.5O_δ. The impregnated samples were then subjected to 10 min of ultrasound irradiation before being calcined in a tube furnace. Different mass ratios of KNO₃ to Mn_0.5Ce_0.5O_δ were used in the current study to achieve different doping dosages of K in the K-MCO catalysts. The resulting catalysts were defined as 5% K-MCO,10% K-MCO,and 20% K-MCO based on the mass ratios of KNO₃ to Mn_0.5Ce_0.5O_δ being 5%,10%,and 20%,respectively. The dosages of the raw materials used to prepare these catalysts are listed in Table 1. Notably,the K-MCO catalysts were also prepared under different conditions to evaluate the impact of the synthetic strategy on their properties,and the corresponding investigative factors are listed in Table 1.

Table 1 Stoichiometric ratio data and synthetic conditions for the catalysts. Catalyst KNO3/ MCO (%) KNO3 (g) MCO (g) Calcination time (h) Calcination temperature (°C) 5%K-MCO 5 0.025 0.5 1 550 10% K-MCO 10 0.050 0.5 1 550 20% K-MCO 20 0.100 0.5 1 550 30% K-MCO 30 0.150 0.5 1 550 40% K-MCO 40 0.200 0.5 1 550 20% K-MCO-450 20 0.100 0.5 1 450 20% K-MCO-650 20 0.100 0.5 1 650 20% K-MCO-750 20 0.100 0.5 1 750 20% K-MCO-850 20 0.100 0.5 1 850 20% K-MCO-2h 20 0.100 0.5 2 550 20% K-MCO-4h 20 0.100 0.5 4 550 20% K-MCO-6h 20 0.100 0.5 6 550 20% K-MCO-8h 20 0.100 0.5 8 550

Table 1 Stoichiometric ratio data and synthetic conditions for the catalysts.

XRD patterns were measured on a powder X-ray diffractometer (Bruker D8 Advance) using Cu K_α (λ = 0.15406 nm) radiation with a nickel filter operating with voltage and current values of 40 kV and 40 mA,respectively. The diffractometer was operated in the 2θ range of 10°-90° with a scanning step of 0.02°. The XRD patterns of the catalysts were compared with those found in the JCPDS reference database for phase identification. The morphological characteristics of the catalysts were observed by field emission scanning electron microscopy (FESEM) on a Quanta 200F instrument using an accelerating voltage of 5 kV. The samples for SEM analysis were dusted with conducting resin and coated with a 10 nm layer of Au prior to being analyzed. Raman spectra were obtained in a backscattering configuration on a Renishaw 1000 confocal laser Raman spectrometer with an 8 mW Ar⁺ laser (532 nm),a 40× long-focus lens and an acquisition time of 20 s. H₂-TPR experiments were conducted on a Quantachrome Autosorb-iQ system. A small sample (100 mg) of the catalyst was loaded into a U-shaped quartz reactor and pre-treated under an atmosphere of Ar at 300 °C for 1 h. After cooling to room temperature,the flow gas was switched to 10% H₂-90% Ar,and the catalyst was heated to 900 °C at a rate of 10 °C/min. The consumption of hydrogen was recorded using a thermal conductivity detector (TCD). The system was calibrated prior to being used with a known amount of CuO.

The catalytic performance of all catalysts was evaluated based on a temperature-programmed oxidation reaction (TPO) in a fixed-bed tubular quartz reactor (φ = 8 mm). Each TPO run was conducted from 150 to 650 °C at a heating rate of 2 °C/min. Printex-U particulates (diameter 25 nm,purchased from Degussa) were used as model soot particles. The catalyst (100 mg) and soot (10 mg) were mixed with a spatula to reproduce the loose contact mode. The reactant gases (50 mL/min) contained 10% O₂ and 0.2% NO balanced with Ar. The outlet gas compositions were analyzed with an on-line gas chromatograph (GC,Sp-3420,Beijing,China) equipped with an FID detector. The catalytic activities of the catalysts were evaluated in terms of their T₁₀,T₅₀,and T₉₀ values,which were defined as the temperatures at soot conversions of 10%,50%,and 90%,respectively. The selectivity to CO₂ formation (S_CO2) was defined as that the CO₂ outlet concentration (C_CO2) divided by the sum of the CO₂ and CO outlet concentration,i.e.,S_CO2= C_CO2/(C_CO + C_CO2). S^m_CO2was defined as the S_CO2value at the maximum temperature corresponding to the highest soot-burnt rate. None of the reactions in the TPO experiments were defined as reaching completion until all of the soot had been completely burnt off.

Fig. 1 shows the XRD patterns of the catalysts. It is clear that a new phase of K₂Mn₄O₈ (2θ = 15.5°,25.2°) appeared when KNO₃ was loaded onto the Mn_0.5Ce_0.5O_δ material,and that the peak intensity increased as the KNO₃ loading increased up to 20%. To further demonstrate that these new peaks correspond to a new phase of K₂Mn₄O₈,we have also included the XRD pattern of calcined pure KNO₃. The key peaks for the calcined pure KNO₃ were seen at 2θ = 23.5°,29.4°,33.8°,and 41.8°,which correspond to the diffraction peaks of KNO₃ ^[30]. Notably,none of the characteristic diffraction peaks of calcined pure KNO₃ were observed in the K-MCO catalysts when the loading of KNO₃ was less than 30%. However,at KNO₃ loadings greater than 30%,the catalysts contained a peak with a 2θ value of 23.5°,which was attributed to the diffraction of KNO₃. This phenomenon therefore suggested that the loading of KNO₃ to form a K₂Mn₄O₈ catalyst with the Mn species was completed when the loading of KNO₃ reached 30%. Fig. 1(b) shows the effect of the calcination temperature on the crystal phase of 3DOM K-MCO catalysts. The new phase of K₂Mn₄O₈ did not appear when the calcination temperature was less than 450 °C. However,increasing the calcination temperature above 450 °C did lead to the formation of a new K₂Mn₄O₈ phase; the peak intensities also increased with increasing temperature but decreased when the calcination temperature was greater than 750 °C,most likely because of the volatilization of the K under these high calcination temperatures. The intensities of the peaks of the cubic fluorite-like structure CeO₂ increased with increasing calcination temperature. As shown in Fig. 1(c),changes in the calcination time had no discernible impact on the XRD patterns of the K-MCO catalysts. The key peaks (2θ = 28.5°,33.1°,47.5°,and 59.1°) could be primarily indexed to the cubic fluorite-like structure of CeO₂ (PDF #43-1002) ^[31]. However,none of samples contained any of the structural features of MnO_x,which indicated that the Mn ions were doped into the crystal lattice of CeO₂ because the ionic radii of Mn³⁺ (0.065 nm) and Mn²⁺ (0.083 nm) are smaller than those of Ce⁴⁺ (0.097 nm) and Ce³⁺ (0.114 nm) ^[21]. These results show that a new K₂Mn₄O₈ phase appeared when K was doped into the 3DOM Mn_0.5Ce_0.5O_δ catalyst. Furthermore,the peak intensities of the new K₂Mn₄O₈ phase were affected by the KNO₃ loading,calcination temperature,and calcination time. The results also suggested that the loading process has a significant influence on the structure of the crystal phase of the Mn_0.5Ce_0.5O_δ material. The appearance of this new phase could make a positive contribution to the catalytic performance of these new catalysts.

Fig. 1. XRD patterns of 3DOM K-MCO catalysts with different KNO3 loadings (a),calcination temperatures (b),and calcination times (c).

Fig. 2 shows the SEM images of 3DOM K-MCO catalysts. Fig. 2(a)-(d) showed that the catalysts were composed of 3DOM structure with an average diameter of approximately 310 ± 20 nm when the KNO₃ loading was less than 30%; the macroporous structure of these catalysts had uniform pore size,windows,and wall thicknesses. Furthermore,the macropores were arranged in a highly ordered array and were interconnected through small windows ^[10]. These SEM images clearly demonstrate that 3DOM K-MCO catalysts with KNO₃ loading of less than 30% have ordered macroporous structures that extend over a long range. As shown in the inserted SEM images,several dark dots were clearly visible in the macropores. These dots were attributed to the windows formed between two spherical pores in the areas not covered by the precursor solutions. However,3DOM structure was destroyed or covered by the excess KNO₃ when the KNO₃ loading was greater than 30%. Furthermore,thin sheets of KNO₃ were formed on the surface of 3DOM Mn_0.5Ce_0.5O_δ when the KNO₃ loading was 40%. Taken together with the XRD results,these thin sheets were attributed to KNO₃ that had not decomposed during the calcination process. Fig. 2(e) and (f) show that the loading of excess KNO₃ led to the complete coverage of the surface and inner pores of the 3DOM Mn_0.5Ce_0.5O_δ material.

Fig. 2. SEM images of 3DOM K-MCO catalysts with different KNO3 loadings. (a) 0%; (b) 5%; (c) 10%; (d) 20%; (e) 30%; (f) 40%.

Fig. 3 shows the SEM images of the 20% K-MCO catalysts prepared with different calcination times. It is clear that 3DOM structures of these catalysts remained stable following calcination time of less than 4 h (Fig. 3(a) and (b)). However,calcination time of more than 4 h led to the fusing of 3DOM structures (Fig. 3(c) and (d)). Compared with the results shown in Fig. 2(c),the SEM images revealed that increasing the calcination time led to some damage to 3DOM structure of the catalysts (calcination time below 4 h). Taken together,the SEM results revealed that 3DOM structure of the catalysts remained intact for calcination time of less than 4 h at calcination temperature of less than 550 °C.

Fig. 3. SEM images of 3DOM 20% K-MCO catalysts with different calcination times. (a) 2 h; (b) 4 h; (c) 6 h; (d) 8 h.

The SEM images of 3DOM 20% K-MCO catalysts with different calcination temperatures are shown in Fig. 4. It is clear that 3DOM structure of the catalysts remained largely unchanged at calcination temperatures of less than 650 °C. However,3DOM structure of 3DOM 20% K-MCO catalysts completely collapsed at calcination temperatures of more than 850 °C to give Mn_0.5Ce_0.5O_δ particles (Fig. 4(c) and (d)). These results therefore indicate that 3DOM 20% K-MCO catalysts have high stability at calcination temperatures of less than 650 °C. For the activity test of these catalysts,the reaction temperature would be less than 500 °C,meaning that their 3DOM structures would remain intact throughout the entire experimental process.

Fig. 4. SEM images of 3DOM 20% K-MCO catalysts with different calcination temperatures. (a) 450 °C; (b) 650 °C; (c) 750 °C; (d) 850 °C.

The N₂ adsorption-desorption isotherms of 3DOM K-MCO catalysts were measured under standard conditions and the results are shown in Fig. 5. The results revealed that the shapes of the adsorption-desorption isotherms varied depending on the KNO₃ loading. For example,the size of the hysteretic loop decreased with increasing KNO₃ loading. The surface area and total pore volume properties of the different catalysts are listed in the Table 2. 3DOM Mn_0.5Ce_0.5O_δ catalyst showed the highest surface area among all of 3DOM catalysts with a value of 45.4 m²/g. The surface area of catalysts decreased from 40.4 to 10.7 m²/g as the KNO₃ loading was increased from 5% to 40%. Taken together with the results of XRD and SEM,this difference in the surface area can be explained in terms of the differences in the crystal form and surface structures of 3DOM K-MCO catalysts. Furthermore,the surface structure of the catalysts was covered during the KNO₃ loading process. The surface area of the catalyst would therefore decrease with increasing KNO₃ loading. The total pore volumes of the catalysts with different KNO₃ loadings would therefore be different.

Fig. 5. N2 adsorption-desorption isotherms of 3DOM K-MCO catalysts with different KNO3 loadings. (1) 0%; (2) 5%; (3) 10%; (4) 20%; (5) 30%; (6) 40%.

Table 2 Textural properties of the catalysts. Catalyst Surface area a (m2/g) Total pore volume b (cm3/g) Mn0.5Ce0.5Oδ 45.4 0.128 5%K-MCO 40.4 0.126 10% K-MCO 34.9 0.127 20% K-MCO 25.1 0.080 30% K-MCO 13.4 0.050 40% K-MCO 10.7 0.040 a Calculated using the BET method. b Calculated by BJH desorption from the cumulative volumes of the pores between 1.7 and 300 nm in diameter.

Table 2 Textural properties of the catalysts.

The redox properties of a catalyst play an important role in controlling its catalytic activity,especially for the deep oxidation reactions encountered in soot combustion. In this work,the redox properties of the catalysts were characterized by H₂-TPR measurements and the results are shown in the Fig. 6. As shown in Fig. 6,the H₂-TPR curves of 3DOM K-MCO catalysts varied with different KNO₃ loadings. Generally speaking,a lower reduction temperature is indicative of a highly active oxygen species,which can lead to high catalytic activity. Furthermore,there is generally a good correlation between low reduction temperature and high activity for soot oxidation,highlighting the importance of the active oxygen species in 3DOM K-MCO catalysts. The H₂-TPR profile of Mn_0.5Ce_0.5O_δ showed a wide reduction peak at 150-600 °C,which was attributed to the reduction of Mn⁴⁺/Mn³⁺,Mn³⁺/Mn²⁺,and Ce⁴⁺/Ce^{3+ [32]}. As the KNO₃ loading increased,the shape and location of the reduction peaks change. For example,two overlapping peaks were observed at 308 and 366 °C in 5% K-MCO. However,a strong reduction peak appeared at 321 °C in 10% K-MCO,and a weak shoulder peak was also observed at 360 °C. Compared with Mn_0.5Ce_0.5O_δ,the reduction peaks of the K-MCO catalysts changed significantly when the KNO₃ loading was greater than 10%. For example,strong reduction peaks appeared at 200-400 °C and systematically shifted to higher temperature as the KNO₃ loading increased,with KNO₃ loadings of 20%,30%,and 40% giving peak temperatures of 322,345,and 360 °C for the K-MCO catalysts,respectively. The different redox properties of 3DOM K-MCO catalysts suggested that the K loading affect the valence states of the Mn and Ce in 3DOM K-MCO catalysts ^[33]. The H₂ consumptions and reduction peak temperatures of 3DOM K-MCO catalysts are listed in the Table 3. These values were calculated by integrating the corresponding peak areas,which were normalized relative to a standard sample (CuO). These results revealed that Mn_0.5Ce_0.5O_δ showed the highest H₂ consumption among all catalysts. There was a decrease in the H₂ consumption following the loading of KNO₃ onto the Mn_0.5Ce_0.5O_δ. However,the H₂ consumption then increased with increasing KNO₃ loading and reached a peak at a loading of 20%. The H₂ consumption then decreased with further increasing the KNO₃ loading. This variation in the H₂ consumption with the KNO₃ loading can be explained as follows. Firstly,the doping of the catalysts with K would have a significant influence on their reduction processes. Secondly,the different crystal forms of the catalysts could also have a significant influence on their H₂ consumption properties. As shown in Table 3,the reduction peak temperatures of the catalysts depended on their KNO₃ loadings.

Fig. 6. H2-TPR profiles of 3DOM K-MCO catalysts with different KNO3 loadings (a),calcination temperatures (b),and calcination times (c).

Table 3 H2 consumptions for the different peak temperatures of 3DOM K-MCO catalysts. Catalyst H2 consumption (μmol)/peak temperature (°C) Total H2 consumption (μmol) Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Mn0.5Ce0.5Oδ — 133.1/305 179.1/399 63.9/504 10.9/761 387.1 5% K-MCO 26.9/240 52.1/308 32.3/359 5.3/553 10.5/802 127.1 10% K-MCO 32.1/274 69.9/321 39.9/349 7.9/645 16.1/805 165.9 20% K-MCO 77.9/277 109.1/321 — 8.1/622 19.9/783 215.0 30% K-MCO 38.2/306 110.5/343 — 4.8/629 14.7/806 168.2 40% K-MCO 45.2/317 104.6/358 — 13.9/623 10.9/814 174.6

Table 3 H2 consumptions for the different peak temperatures of 3DOM K-MCO catalysts.

3DOM K-MCO catalysts were calcined at different temperatures and different times to investigate the effects of parameters on their H₂-TPR profiles,and the results are shown in the Fig. 6(b) and (c). Increasing the calcination temperature led to the appearance of a weak peak at 255 °C,which indicated that high temperature calcination could have a significant influence on the redox properties of 3DOM K-MCO catalysts. Several other strong reduction peaks were observed at 290-400 °C,which were attributed to the reduction of Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn^{2+ [34]}. The results revealed that 3DOM 20% K-MCO-550 catalyst showed the lowest reduction temperature of the catalysts with different KNO₃ loadings,which suggested that 3DOM 20% K-MCO-550 catalyst would show high catalytic activity for soot combustion. As shown in Fig. 6(c),the TPR profiles of the catalysts with different calcination times were similar to those of the catalysts with different calcination temperatures. However,the peak temperatures of the catalysts prepared with longer calcination time were higher than those of the catalysts prepared with different calcination temperatures. H₂-TPR results revealed that variations in the KNO₃ loading led to changes in the redox ability of the catalyst.

Raman spectroscopy was used to investigate the interactions of the K,Mn,and Ce atoms in 3DOM K-MCO catalysts and the results are shown in Fig. 7. As shown in the Fig. 7(a),Mn_0.5Ce_0.5O_δ material showed a strong Raman peak around 440 cm^-1,which was attributed to the strong F_2g mode of the CeO₂ fluorite phase ^[35]. Two new Raman peaks were observed at 634 and 440 cm^-1 when KNO₃ was loaded onto the Mn_0.5Ce_0.5O_δ material. The locations of the Raman peaks remained relatively unchanged as the KNO₃ loading increased. The peak at 440 cm^-1 was attributed to the strong F_2g mode of the CeO₂ fluorite phase. Compared with pure CeO₂,there was an obvious red shift in the Raman spectra of the catalysts,which was attributed to the doping of the CeO₂ lattice with K and Mn. The peak at 634 cm^-1 was assigned to Mn-O vibrations,which were orthogonal to the direction of the MnO₆ octahedral double chains ^{[36, 37]}. A comparison of the Raman spectra of the K-MCO catalysts with pure MnO_x revealed the presence of a red shift (about 10 cm^-1). Compared with the insert of Fig. 7(a),no Raman shifts (712,1050,and 1357 cm^-1) were observed for the K compounds (KNO₃ or K₂CO₃),which indicated that the Mn_0.5Ce_0.5O_δ catalyst had been successfully doped with K ions. As shown in the Fig. 7(b),the calcination temperature of the K-MCO catalysts had very little influence on the peak at 440 cm^-1. However,the intensity of the peak at 634 cm^-1 increased with increasing calcination temperature,and it was attributed to the movement of the Mn atoms to the surface of catalyst because of the high calcination temperature. As shown in Fig. 7(c),the intensity of the Raman peak at 634 cm^-1 increased with increasing calcination time,which indicated that the calcination time also had a significant influence on the surface of the MnO_x.

Fig. 7. Raman spectra of 3DOM K-MCO catalysts with different KNO3 loadings (a),calcination temperatures (b),and calcination times (c).

The catalytic activities of 3DOM K-MCO catalysts were evaluated for the oxidation of soot,and the results are listed in Table 4. For comparison,the combustion reactions of pure soot,3DOM Mn_0.5Ce_0.5O_δ,and pure KNO₃ were also estimated under the same reaction conditions. For the pure soot,the T₁₀,T₅₀,and T₉₀ are 482,564,and 609 °C,respectively. 3DOM Mn_0.5Ce_0.5O_δ catalyst showed higher catalytic activity towards the combustion of soot than pure KNO₃,with T₁₀,T₅₀,and T₉₀ values of 297,358,and 396 °C,respectively. All of 3DOM K-MCO catalysts showed higher catalytic activities towards the combustion of soot than Mn_0.5Ce_0.5O_δ. Furthermore,the activities of the catalysts increased as the mass ratio of KNO₃ to Mn_0.5Ce_0.5O_δ increased up to 20%,and then remained constant with further increases in the KNO₃ loading amount. Consideration of the △T₁₀,△T₅₀,and △T₉₀ values of 3DOM K-MCO catalysts revealed that the values for 3DOM 20% K-MCO catalyst were 191,228,and 241 °C,respectively. These values were much higher than that of the other 3DOM K-MCO catalysts,regardless of the KNO₃ loading,which indicated that 20% K-MCO had the highest catalytic activity among 3DOM K-MCO catalysts. These results therefore demonstrate that the optimum mass ratio of KNO₃ to Mn_0.5Ce_0.5O_δ is 20% for soot combustion. The catalytic activities 3DOM MnO_x and CeO₂ were also tested and the results are listed in Table 4. It revealed that 3DOM MnO_x and CeO₂ showed low catalytic activities,which indicated that the synergistic effects among the K,Mn,and Ce atoms were beneficial in terms of enhancing the catalytic activities of 3DOM K-MCO catalysts.

Table 4 Catalytic activities of 3DOM K-MCO catalysts towards the soot combustion. Catalyst T10 (°C) T50 (°C) T90 (°C) SmCO2(%) △T10 a (°C) △T50 b (°C) △T90 c (°C) No catalyst 482 564 609 71.6 — — — Mn0.5Ce0.5Oδ 297 358 396 94.2 185 206 213 5% K-MCO 300 343 377 95.2 182 221 232 10% K-MCO 294 336 370 95.3 188 228 239 20% K-MCO 291 336 368 95.7 191 228 241 30% K-MCO 293 343 374 95.9 189 221 235 40% K-MCO 294 341 375 95.1 188 223 234 Pure KNO3 353 393 535 90.9 129 171 74 3DOM CeO2 349 393 429 91.9 133 171 180 3DOM MnOx 326 394 431 88.7 156 170 178 20% K-MCO-450 293 337 364 95.8 189 230 245 20% K-MCO-650 289 339 372 94.8 193 225 237 20% K-MCO-750 306 354 386 94.7 176 210 223 20% K-MCO-850 307 353 383 94.6 175 211 226 20% K-MCO-2h 286 332 365 95.5 196 232 244 20% K-MCO-4h 284 331 364 95.3 198 233 245 20% K-MCO-6h 289 333 365 95.2 193 231 244 20% K-MCO-8h 291 343 375 94.9 191 221 234 Mn0.5Ce0.5Oδ d 355 420 500 96.8 127 144 109 20% K-MCO-4h d 297 365 432 97.9 188 199 177 The difference in the T10 values of pure soot and the catalysts. b The difference in the T50 values of pure soot and the catalysts. c The difference in the T90 values of pure soot and the catalysts. d The concentration of NO in the reaction system was 0 ppm.

Table 4 Catalytic activities of 3DOM K-MCO catalysts towards the soot combustion.

As shown in Table 4,it is clear that the calcination temperature also significantly impacted on the catalytic activity. The catalytic activity of the 20% K-MCO catalyst decreased with increasing calcination temperature. Consideration of the XRD and SEM results for the catalysts prepared at different calcination temperatures revealed that there were significant differences in the crystal forms of the catalysts,which could explain the changes in their catalytic activities. As shown in Table 4,the calcination time had very little effect on the catalytic performance of the 20% K-MCO catalyst when the calcination time was not more than 6 h. However,the catalytic performance deteriorated when the calcination time was 8 h. This change was attributed to the collapse of 3DOM structure under the long calcination time.

As shown in Table 4,the T₉₀ values of all the catalysts were less than 400 °C. Compared with Mn_0.5Ce_0.5O_δ,the T₉₀ of 3DOM K-MCO decreased by about 20 ± 5 °C,which indicated that the K had a positive impact on the catalytic activity of this catalyst. Given that the temperature of diesel exhaust fumes is in the range of 175-400 °C,any soot particles covering the K-MCO catalysts would be completely burnt off under the normally operating conditions of a diesel engine (i.e.,the temperature of a diesel exhaust is approximately 400 °C) ^[2]. Furthermore,3DOM K-MCO catalysts showed much higher CO₂ selectivity for soot combustion than they did for pure soot combustion. The CO₂ selectivity remained stable with increasing KNO₃ loading,with the highest CO₂ selectivity of 95.9%. Taken together,these experimental results suggested that 3DOM K-MCO catalysts would be promising catalysts for soot combustion.

In this study,the synergistic effects of the K,Mn,and Ce in the K-MCO catalysts made a significant contribution to increasing the catalytic activity. It is well known that the transfer of active oxygen is an important step in the oxidation of soot. The high activity of these catalysts was attributed to the generation of active oxygen as a consequence of the movement of oxygen between the gas phase and the oxide framework. It is well known that K is an interesting element for soot combustion because it leads to an increase in the amount of chemisorbed oxygen,as well as the formation of eutectic compounds and carbonate intermediates ^{[23, 38]}. The addition of K to the Mn_0.5Ce_0.5O_δ catalyst affected its catalytic activity towards the combustion of soot. To develop a deeper understanding of the mechanism of 3DOM K-MCO-catalyzed combustion of soot,we investigated the catalytic activities of 3DOM Mn_0.5Ce_0.5O_δ and 20% K-MCO-4h under a reaction atmosphere without NO and 10% O₂,and the results are listed in Table 4. It showed that 3DOM Mn_0.5Ce_0.5O_δ and 20% K-MCO-4h catalysts exhibited low catalytic activities when NO was absent in the feed gas.

Based on the above and previously reported results,we have proposed a possible mechanism for 3DOM K-MCO- catalyzed combustion of soot. NO would be readily adsorbed onto the surface of the catalyst because of the basicity of K. Gaseous O₂ would also be adsorbed on the surface of catalyst. Active oxygen would then be readily generated by changing the valence states of the Mn and Ce species on the K-MCO catalyst through the oxygen vacancies on the surface of catalyst. The adsorbed NO would then interact with the active oxygen on the surface of the catalyst to give bidentate/monodentate nitrates. The mass ratio of KNO₃ to Mn_0.5Ce_0.5O_δ would therefore have a significant influence on the oxygen vacancies and NO adsorption at this stage. As shown in H₂-TPR (Fig. 6) and the activity (Table 4) results,it demonstrates that the doping of K is important for determining the catalyst activity. The resulting bidentate/monodentate nitrate species are subsequently desorbed from the surface of the K-MCO catalyst by the formation of NO₂,which can behave as a strong oxidant for soot combustion ^[22]. Several previous studies have reported potential mechanisms for the combustion of soot in the presence of K. For example,Zhang’s group ^[29] reported the function of K in the activation of gaseous O₂ and proposed a possible oxygen spillover mechanism,which explained as an electron transfer process from the soot particles to gaseous oxygen via the formation of active K⁺ sites. In a separate study,Legutko et al. ^[39] showed that a highly active superficial birnessite (KMn₄O₈) phase was formed on the surfaces of Mn₃O₄ particles when they were calcined with K-containing catalysts,which indicated that the core-shell structures of these catalysts effectively favored an increase in the catalytic activity. These results therefore indicate that the presence of K can lead to a significant improvement in the catalyst activity for the combustion of soot.

The stability of a catalyst is one of the most important properties,especially in terms of its practical application. With this in mind,we measured the stability of 3DOM 20% K-MCO-4h catalyst over five consecutive cycles and the results are shown in Fig. 8. 3DOM 20% K-MCO-4h catalyst maintained high catalytic activity in the medium and high temperature ranges (i.e.,T₅₀ and T₉₀) under loose contact conditions between the catalyst and soot particles,with T₅₀ and T₉₀ of 338 ± 7 and 367 ± 5 °C,respectively. However,the T₁₀ values of 20% K-MCO-4h increased significantly after the fifth reaction cycle. This change could be attributed to the volatilization and subsequent loss of some of the K and/or the destruction of part of 3DOM structure over the five reactions. This would limit the ability of the catalyst to activate oxygen at low reaction temperatures,as well as reducing the contact efficiency between the soot and the catalyst compared with the fresh catalyst. It is also clear that the CO₂ selectivity increased slightly after five reaction cycles,with values greater than 95% being recorded for each reaction cycle. This increase could be attributed to the maximum temperature of the highest soot-burnt rate being higher for each consecutive cycle.

Fig. 8. Stability of 3DOM 20% K-MCO-4h catalyst for soot combustion.

We have developed a 3DOM Mn_0.5Ce_0.5O_δ catalyst using a CCT method and loaded different amounts of KNO₃ onto the catalyst using a simple incipient-wetness impregnationmethod. The resulting catalysts contained macropores with an average diameter of 310 ± 20 nm,which were interconnected through open windows of 90-140 nm in diameter,with wall thicknesses of 30-50 nm. The characterization results indicated that the loading of the catalyst with KNO₃ resulted in the formation of a new phase and had a significant impact on the redox and surface properties of the catalysts,while the calcination temperature and calcination time influenced little on the redox properties. 3DOM K-MCO catalysts exhibited high catalytic activity towards the combustion of soot. The calcination temperature had a significant impact on the catalytic activity,with an optimal calcination temperature of 550 °C. In contrast,the calcination time had very little impact on the catalytic activity,with a calcination time of 4 h providing the best results. When 20% K-MCO catalyst was calcined at 550 °C for 4 h,it showed the highest catalytic activity,with T₁₀,T₅₀,and T₉₀ of 284,331,and 364 °C,respectively. Furthermore,3DOM K-MCO catalysts showed high stability operated in the medium and high temperature ranges. The structural and synergistic effects of the K,Mn,and Ce were found to be favorable for enhancing the activities of the catalysts towards the combustion of diesel soot. These catalysts are therefore promising candidates for catalytic oxidation of diesel soot particles because of their easy synthesis,high activity,and low cost.