Synthesis and Catalytic Performance for So - American Chemical

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Three-Dimensionally Ordered Macroporous MnxCe1−xOδ and Pt/ Mn0.5Ce0.5Oδ Catalysts: Synthesis and Catalytic Performance for Soot Oxidation Xuehua Yu,† Jianmei Li,† Yuechang Wei,* Zhen Zhao,* Jian Liu, Baofang Jin, Aijun Duan, and Guiyuan Jiang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, No. 18 Fuxue Road, Chang Ping, Beijing 102249, China S Supporting Information *

ABSTRACT: Three-dimensionally ordered macroporous (3DOM) MnxCe1−xOδ oxides with different ratios of Mn to Ce were successfully synthesized by colloidal crystal template (CCT) method, and 3DOM Pt/Mn0.5Ce0.5Oδ with varied Pt loadings were prepared by in situ ethylene glycol (EG) reduction method. 3DOM MnxCe1−xOδ supports exhibited well-defined 3DOM nanostructure, and Pt nanoparticles (NPs) with 1−2 nm size were evenly dispersed on the inner walls of uniform macropores. Among 3DOM MnxCe1−xOδ catalysts, 3DOM Mn0.5Ce0.5Oδ showed excellent catalytic activity for soot combustion; i.e., T50 is 358 °C and Sm CO2 is 94.2%. 3DOM Pt/Mn0.5Ce0.5Oδ catalysts exhibited higher activity than 3DOM MnxCe1−xOδ and 3 wt % Pt/ Mn0.5Ce0.5Oδ showed the highest catalytic activity for soot combustion (T50 is 342 °C and Sm CO2 is 96.7%). Macropores effect, synergistic effects between Mn and Ce, and synergistic effects between Pt and Mn0.5Ce0.5Oδ support are contributed to high catalytic activities of as-prepared catalysts. MnOx−CeO2 solid solutions have been evidenced as one group of cheap and efficient candidate catalysts for soot oxidation in previous reports.19−21 The catalytic performance for soot combustion is affected by two factors: the contact condition between soot and catalysts, and the intrinsic activity of catalyst. So how to effectively make use of the inner surface for increasing the contact area between catalysts and soot particles is a key factor affecting the catalytic activity for diesel soot combustion of diesel engine exhaust. Recently, three-dimensionally ordered macroporous (3DOM) metal oxides with uniform pore size (>50 nm) and well-defined structure have drawn much attention in the field of heterogeneous catalysis.22,23 Due to ordered macroporous structures, the soot particles not only could enter their inner pores more easily, but also could transfer into inner areas of catalysts and access the active sites more flexibly than disordered macroporous and nanoparticles samples.24 Recently, our group has studied a series of 3DOM metal composite oxides, including La 1 − x K x CoO 3 , 2 5 LaCo x Fe 1 − x O 3 , 2 6 Ce1−xZrxO2,24,27 and so on, 3DOM catalysts showed better catalytic performance than the nanoparticle-type catalyst for soot oxidation. However, because of the limitation of intrinsic activity of 3DOM oxide-based catalysts, the catalytic combustion of diesel soot particles occurs at higher temperatures (ca. 350−500 °C) than the exhaust gas temperatures (ca.175−400 °C).28 Therefore, enhancement of intrinsic

1. INTRODUCTION Because of high efficiency, low operating costs, and high durability and reliability, diesel engines play an important role in power generation, farming, construction, and industrial activities.1 However, the exhaust of diesel engine is one of the largest contributors to environmental pollution problems worldwide.2 Especially, soot particles as a main source of urban atmospheric particulate matter (PM2.5, particle size < 2.5 μm) is directly threatening the environment and people’s health.3 So, the elimination of PM has become a restrictive factor for more extensive application of diesel engines. Nowadays, many techniques have been developed for solving the pollution of diesel engine exhaust. In a number of techniques, a catalyst is one of the determining factors for extensive applications.4 Indeed, a number of catalytic materials have been studied for soot combustion, which include noble metals,5 perovskite-type oxides,6 and CeO2-based oxides,7,8 etc. In the past decade, ceria alone or in combination with other oxides have been carried out on catalyst formulation for soot oxidation. Our group has synthesized numerous CeO2-based oxides for soot catalytic combustion, which include CoOx/ CeO2,9 Ce−Zr−Pr−O solid solutions,10 K−Co−CeO2,11 and so on. The incorporation of manganese cations into the CeO2 lattice greatly improves the oxygen storage capacity of ceria as well as oxygen mobility on the surface of the solid solutions, MnOx−CeO2 solid solutions have been developed as environmentally friendly catalysts for the abatement of contaminants.12−16 In addition, MnOx−CeO2 solid solutions have advantages of high oxidation activity for NO conversion to NO2 and high NOx storage capacity at low temperatures,17,18 which can release abundant NO2 for soot oxidation. Therefore, © 2014 American Chemical Society

Received: Revised: Accepted: Published: 9653

February 18, 2014 May 8, 2014 May 14, 2014 May 14, 2014 dx.doi.org/10.1021/ie500666m | Ind. Eng. Chem. Res. 2014, 53, 9653−9664

Industrial & Engineering Chemistry Research

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2.1.3. Synthesis of 3DOM Pt/MnxCe1−xOδ Catalysts. 3DOM Pt/MnxCe1−xOδ catalysts were prepared by in situ reduction method, which takes EG as both solvent and reducing agent.38 In a typical procedure, 0.5 g of 3DOM MnxCe1−xOδ solid solution was added into a three-necked bottle, and then a certain amount of H2PtCl6 aqueous solution (5 g/L) was added into the reactor. After 10 min, 80 mL of ethylene glycol with PVP (mole ratio, Pt:PVP = 1:20) was added into the mixture. After agitation of the mixture for 20 min, the reaction mixture was then heated to 130 °C in an oil bath and aged at this temperature for 4 h under vigorous agitation to ensure completion of the reaction. The product was filtered and washed several times with absolute alcohol, and then the product was dried at 50 °C for 12 h. Finally, the dried sample was calcined to remove residual PVP and EG under 550 °C for 1 h. In order to get different Pt loading amounts of Pt/ Mn0.5Ce0.5Oδ catalysts, the mass ratios of Pt and Mn0.5Ce0.5Oδ catalysts in theory were changed (0.5, 1, 2, 3, and 5 wt %) by adjusting the volume of H2PtCl6 aqueous solution. 2.2. Physical and Chemical Characterization. X-ray diffraction (XRD) patterns were measured on a powder X-ray diffractometer (Bruker D8 Advance) using Cu Kα (k = 0.15406 nm) radiation with a Nickel filter operating at voltage and current of 40 kV and 40 mA in the 2θ range of 10−90°. The patterns were compared with JCPDS reference data for phase identification. The surface morphology of the catalyst was observed by field emission scanning electron microscopy (FESEM) on a Quanta 200F instruments using accelerating voltages of 5 kV. SEM samples were dusted on conducting resin and coated with 10 nm Au prior to measurement. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEOL JEM2100 transmission electron microscope. A typical TEM sample was prepared by adding several droplets of a nanoparticles/ ethanol mixture onto a carbon-coated copper grid. N2 adsorption−desorption isotherm was measured at 77 K using a Mike instrument ASAP 2420 station automatic, specific surface and porosity analyzer. The samples were degassed at 300 °C for 8 h prior to the measuremenu. The specific surface area was calculated according to the Brunauer−Emmett−Teller (BET) method. The total pore volume depended on the absorbed N2 volume at a relative pressure of approximately 0.99. Temperature-programmed reduction with H2 (H2-TPR) measurements were performed in a conventional flow apparatus. A 100 mg sample was pretreated under Ar by calcination at 300 °C for 1 h and subsequently cooled to 30 °C. Afterward, 10% H2/Ar flow (40 mL min−1) was passed over the catalyst bed while the temperature was ramped from 30 to 800 °C at a heating rate of 10 °C min−1. The hydrogen consumption signal was monitored by a thermal conductivity detector (TCD). O2-TPD measurements were performed on the Quantachrome Instruments of autosorb iQ. A 100 mg amount of catalyst adsorbed O2 at 300 °C for 30 min in the O2 flow (60 mL min−1). Then the catalyst was treated by He flow (60 mL min−1) for 30 min at room temperature and purged residual oxygen. The catalyst was heated to 900 °C at a 10 °C min−1 rate under He flow (60 mL min−1). The desorbed oxygen was monitored by thermal conductivity detector (TCD). X-ray photoelectron spectra (XPS) were recorded on a PerkinElmer PHI-1600 ESCA spectrometer using a Mg Kα Xray source. The catalysts were treated at 150 °C for 48 h in the vacuum drying oven in order to exclude the hydroxyl and

activity of 3DOM oxide-based catalysts remains a major challenge for complete oxidation of soot at low temperature. In order to improve the intrinsic activity of catalysts, the advantages of supported noble metal nanoparticles (NPs) catalysts have been recognized in heterogeneous catalysis for soot combustion. So far, Au, Pt, Pd, and Ag, etc., have been used as the active component for investigating soot combustion.29−31 Generally, three kinds of synergistic effects, which include electronic interactions, surface transport of adsorbates, and so-called “strong metal−support interactions”, were recognized as the main explanations for the high catalytic activity of supported noble metal catalysts.32 In fact, a series of 3DOM Ce-based oxide catalysts have been researched by some groups.33−35 Those catalysts show high activities in the field of heterogeneous catalysis, especially when Ce-based oxides supported noble metal.34 In our previous works, 3DOM oxide-supported Au and Pt catalysts exhibited high catalytic activities for soot combustion.36,37 Among the investigated catalysts, two possible effects are proposed. First, Pt promotes indirectly soot oxidation by catalyzing the oxidation of NO into NO2 by O2, the effective oxidation agent being NO2. Second, the active oxygen species, which increases by Pt NPs, is also indispensable in the soot combustion.32 Based on our previous work, we have demonstrated an effective strategy to enhance the catalytic activities through 3DOM metal composite oxides, which is also an excellent support markedly enhancing the catalytic activity of Au or Pt nanoparticles. In this work, a series of 3DOM MnxCe1−xOδ solid solutions with different x values were successfully synthesized by colloidal crystal template (CCT) method, and 3DOM Pt/Mn0.5Ce0.5Oδ catalysts with different Pt loadings were obtained using ethylene glycol (EG) and poly(vinylpyrrolidone) (PVP) as reducing and protective agents, respectively. The catalytic performance of as-prepared catalysts with varied x values and Pt loadings were evaluated for catalytic combustions of soot particles. Based on the experimental results, the possible mechanisms of catalysts for soot combustion are discussed.

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. 2.1.1. Synthesis of Highly Well Defined Poly(methyl methacrylate) Microspheres. The poly(methyl methacrylate) (PMMA) microspheres were synthesized by a modified emulsifier-free emulsion polymerization method.26 The PMMA colloidal crystal template was assembled by centrifugation (Figure S1 of the Supporting Information). 2.1.2. Synthesis of 3DOM MnxCe1−xOδ Catalysts. 3DOM MnxCe1−xOδ were synthesized by PMMA colloidal crystal template (CCT) method using Ce(NO3)3·6H2O and Mn(NO3)2 aqueous solution (50 wt %) as metal precursors. In a typical procedure, cerous nitrate and manganese nitrate were dissolved into the mixture of ethylene glycol (65 vol %) and methanol (35 vol %), and then the PMMA arrays were added into the above solution for impregnation. After complete impregnation, the PMMA arrays were separated by vacuum filter and dried at 30 °C for 24 h. The dried sample was calcined to remove the CCT in a tube furnace with an air flow (80 mL min−1). The temperature-rising rate was 1 °C min−1 from room temperature to 550 °C, and the calcination at 550 °C was kept for 4 h. Then, 3DOM MnxCe1−xOδ solid solutions were obtained. 9654

dx.doi.org/10.1021/ie500666m | Ind. Eng. Chem. Res. 2014, 53, 9653−9664


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peaks were observed at 2θ of 28.5° (111), 33.1° (200), 47.5° (220), 56.3° (311), 59.1° (222), 69.4° (400), 76.7° (331), 79.1° (420), and 88.4° (422) (as marked by asterisks (*) in Figure 1), indicating the formation of cubic fluorite-like structure of CeO2 (PDF no. 43-1002). With increasing x value, the intensive and sharp diffractions of CeO2 decreased and no MnOx crystal phase was detected until x < 0.9. The absence of manganese oxide phases in Ce-rich materials has been previously reported, and the results are consistent with the recent report concerning the structure features of MnOx−CeO2 solid solutions.12,15,39 The XRD patterns of MnxCe1−xOδ (x < 0.9) suggest that MnOx could easily form solid solutions with CeO2, as these two phases bear certain structural similarities. As shown in Figure 1f, because of more Mnx+ cations than Ce4+ cations in Mn0.9Ce0.1Oδ, the XRD patterns show feature peaks of Mn3O4, and the weak diffraction peaks of CeO2 are observed. When x = 1 (Figure 1g), the pure MnOx was composed of two mixed phases of Mn2O3 (PDF no. 41-1442) and Mn3O4 (PDF no. 24-0734), the feature peaks were detected at 2θ of 23.1° (211), 32.9° (222), 38.2° (400), 55.2° (440), 65.8° (622), and 28.8° (112), 32.3° (103), 36.1° (211), and 59.8° (224) which belonged to Mn2O3, and Mn3O4, respectively (as “▲” and “●” marked in Figure 1). An overview of the XRD patterns of as-prepared 3DOM MnxCe1−xOδ solid solutions indicates the incorporation of Mnx+ cations into the ceria lattice due to the smaller ionic radii of Mn3+ (0.065 nm) and Mn2+ (0.083 nm) than those of Ce4+(0.097 nm) and Ce3+ (0.114 nm), correspondingly.40 Therefore, no crystalline phases of manganite were observed from the XRD patterns (Figure 1d). From the XRD patterns of Pt/Mn0.5Ce0.5Oδ (Figure 1h−l), it can be seen that the crystal phases are similar to those of MnxCe1−xOδ solid solutions (Figure 1d). The results suggest that the loading process has no significant influence on the crystal phase of Mn0.5Ce0.5Oδ. With increasing of Pt loading, no diffraction peaks of Pt NPs in the XRD patterns, even though the loading is 5 wt %, were observed. The crystal structure is also analyzed by Rietveld method using the software of the Fullprof program integrated in the WinPLOTR software (Figure S2 in the Supporting Information). The Rietveld refinement results indicate that Mn and Ce have formed a solid solution in the MnxCe1−xOδ catalysts. There are two possible reasons for this phenomenon. First, the loading amount is so low that no diffraction peak of Pt NPs can be observed by XRD. Second, the particle size of supported Pt NPs is small and uniform. 3.1.2. Results of SEM. Figure 2 shows the scanning electron microscope images of 3DOM MnxCe1−xOδ solid solutions. The SEM images (Figure 2a−g) show that the material has threedimensionally ordered macropores with the average diameter of ca. 310 ± 20 nm, which is smaller than the 440 nm of the PMMA sphere size (Supporting Information Figure S1). The diameter of 3DOM structure indicates that shrinkage (ca. 25− 35%) of the solid solutions takes place during the process of heat treatment. For all 3DOM samples, the macropores have uniform pore sizes, windows, and wall thicknesses, and those macropores are highly periodically arrayed and interconnected through small windows. SEM images clearly demonstrate that all 3DOM samples have long-range ordered macroporous structure. As shown in the inserted SEM images, some dark dots in macropores are visible clearly, and the dark dots are formed at the place between two spheres where the precursor solutions could not cover.24 The macropores were interconnected through open windows, ca. 90−140 nm in diameter, and

carbonate oxygen on the surface of catalysts. The binding energies were calibrated using the C 1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard. 2.3. Activity Measurements. The catalytic performances of all of the catalysts were evaluated with a temperatureprogrammed oxidation reaction (TPO) on a fixed-bed tubular quartz reactor (Φ = 8 mm), and each TPO ran from 150 to 650 °C at a 2 °C min−1 rate. The model soot was Printex-U particulates (diameter, 25 nm; purchased from Degussa). Elemental analysis of Printex-U particulates showed its carbonaceous nature with 92.0% C, 0.7% H, 3.5% O, 0.1% N, 0.2% S, and 3.5% other.37 The catalyst (100 mg) and soot (10 mg) were mixed at a weight ratio of 10:1 with a spatula in order to reproduce the loose contact mode. Reactant gases (50 mL min−1) contain 10% O2 and 0.2% NO balanced with Ar. The outlet gas compositions were analyzed with an online gas chromatograph (GC, Sp-3420, Beijing) by using flame ionization detectors (FIDs). Before entering the FID, CO and CO2 were fully converted to CH4 by a convertor with Ni catalyst at 380 °C. The catalytic activity was evaluated by the values of T10, T50, and T90, which were defined as the temperatures at 10%, 50%, and 90% of soot conversion, respectively. The selectivity to CO2 formation (SCO2) was defined as that of the CO2 outlet concentration (CCO2) divided by the sum of the CO2 and CO outlet concentration, i.e., SCO3 = CCO2/(CCO + CCO2). Sm CO2 was denoted as SCO2 at the maximum temperature corresponding to when the soot-burn rate was the highest. In all TPO experiments, the reaction was not finished until the soot was completely burnt off. In order to investigate the intrinsic activities of the catalysts, the TOF values and activation energies have been calculated in this work. The detailed steps are described in the Supporting Information.25,28

3. RESULTS 3.1. Catalyst Characterization. 3.1.1. Results of XRD. Figure 1 shows the XRD patterns of 3DOM MnxCe1−xOδ solid solutions with different x values. For the pure CeO2, the feature

Figure 1. XRD patterns of MnxCe1−xOδ and x wt % Pt/Mn0.5Ce0.5Oδ ((a) CeO2, (b) Mn0.1Ce0.9Oδ, (c) Mn0.3Ce0.7Oδ, (d) Mn0.5Ce0.5Oδ, (e) Mn0.7Ce0.3Oδ, (f) Mn0.9Ce0.1Oδ, (g) MnOx. Pt loading: (h) 0.5, (i) 1, (j) 2, (k) 3, and (l) 5 wt %). 9655



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Figure 3. TEM and HRTEM images of 3DOM Mn0.5Ce0.5Oδ and x wt % Pt/Mn0.5Ce0.5Oδ catalysts ((a) Mn0.5Ce0.5Oδ. Pt loading: (b) 0.5, (c) 1, (d) 2, (e) 3, and (f) 5 wt %. Insets are size distributions of Pt NPs and HRTEM images).

accumulated by a minicrystal of Mn0.5Ce0.5Oδ solid solution (inset of Figure 3a). The lattice fringe, which is marked as 0.32 nm in the HRTEM images of Mn0.5Ce0.5Oδ, belongs to (111) planes of CeO2. No lattice fringe of manganite was observed in the HRTEM image; it is also demonstrated that solid solution has been formed in 3DOM Mn0.5Ce0.5Oδ, which is consistent with the XRD results of Mn0.5Ce0.5Oδ (Figure 1d). From Figure 3b−f, it indicated that the surface of 3DOM Mn0.5Ce0.5Oδ catalyst was successfully decorated with well-dispersed Pt NPs, and no larger agglomerated particles were observed. The sizes of supported Pt NPs are in the range of 1−3 nm. The average Pt NP sizes are estimated to be 1.51 ± 0.38, 1.48 ± 0.28, 1.68 ± 0.36, 1.87 ± 0.41, and 2.07 ± 0.36 nm for 3DOM Pt/ Mn0.5Ce0.5Oδ catalysts with Pt loadings of 0.5, 1, 2, 3, and 5 wt %, respectively (Figure 3b−f). It shows that the size of Pt NPs slightly increased with increasing Pt loading. The increased Pt NPs size at high Pt loading should be caused by high precursor (H2PtCl6·6H2O) concentration during the reduction process. The lattice fringes of Pt NPs and Mn0.5Ce0.5Oδ solid solution in HRTEM images were clearly observed (Figure 3f); e.g., the lattice fringes were measured to be 0.23 and 0.32 nm indexed as (111) planes of supported Pt NPs and CeO2, respectively. 3.1.4. Results of BET. Nitrogen adsorption−desorption isotherms for MnOx, CeO2, Mn0.5Ce0.5Oδ, and 3 wt % Pt/

Figure 2. SEM images of 3DOM MnxCe1−xOδ and 3 wt % Pt/ Mn0.5Ce0.5Oδ catalysts ((a) CeO2, (b) Mn0.1Ce0.9Oδ, (c) Mn0.3Ce0.7Oδ, (d) Mn0.5Ce0.5Oδ, (e) Mn0.7Ce0.3Oδ, (f) Mn0.9Ce0.1Oδ, (g) MnOx, and (h) 3 wt % Pt/Mn0.5Ce0.5Oδ).

the wall thicknesses were ca. 30−50 nm. According to the results of SEM (Figure 2a−g), the MnxCe1−xOδ samples have open, periodic, and interconnected 3DOM structures. Figure 2h shows the SEM image of 3 wt % Pt/Mn0.5Ce0.5Oδ catalyst. 3DOM structures of Pt/Mn0.5Ce0.5Oδ catalyst are the same as the Mn0.5Ce0.5Oδ solid solution indicating that 3DOM structures are strong and stable even after undergoing the loading process. In addition, from an overview of the whole SEM images, it can be seen that the average size of 3DOM particles is about 5−20 μm, which is not very large and suitable for penetration of soot particles into the inner 3DOM structures. 3.1.3. Results of TEM. TEM and HRTEM images of 3DOM Pt/Mn0.5Ce0.5Oδ catalysts with different Pt loadings are shown in Figure 3. From Figure 3a, it can be seen that the 3DOM structure with overlapped pores can be clearly observed by TEM image. The framework of the 3DOM structure was 9656



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Mn0.5Ce0.5Oδ catalysts are shown in Figure 4. The as-prepared catalysts have a typical type IV adsorption−desorption isotherm

Figure 4. Nitrogen adsorption−desorption isotherms of as-prepared catalysts (a: pure MnOx; b: pure CeO2; c: Mn0.5Ce0.5Oδ; d: 3 wt %Pt/ Mn0.5Ce0.5Oδ).

based on the IUPAC ones.41 The adsorption−desorption isotherm shapes are similar to each other. As shown in Table 1, Table 1. Physicochemical Properties of As-Prepared Catalysts catalysts pure MnOx pure CeO2 Mn0.5Ce0.5Oδ 3 wt % Pt/ Mn0.5Ce0.5Oδ

surface area (m2/g)a

total pore vol (cm3/g)b

pore size (nm)c

58.7 43.8 48.1 54.9

0.224 0.103 0.134 0.139

16.5 9.3 12.8 10.6

Figure 5. H2-TPR of MnxCe1−xOδ and x wt % Pt/Mn0.5Ce0.5Oδ ((a) CeO2, (b) Mn0.1Ce0.9Oδ, (c) Mn0.3Ce0.7Oδ, (d) Mn0.5Ce0.5Oδ, (e) Mn0.7Ce0.3Oδ, (f) Mn0.9Ce0.1Oδ, (g) MnOx. Pt loading: (h) 0.5, (i) 1, (j) 2, (k) 3, and (l) 5 wt %).

a Calculated by BET method. bCalculated by BJH adsorption cumulative volume of pores between 1.7 and 300 nm diameter. c Calculated by BJH adsorption average pore diameter.

are relatively complicated and systematically shifted to lower temperature regions. With increasing x values, some obvious reduction peaks appear in the temperature range of 200−600 °C. In the case of the Mn0.1Ce0.9Oδ catalyst, the H2-TPR profile consists of two overlapping peaks at 350 and 520 °C. The H2TPR profile of the Mn0.3Ce0.7Oδ catalyst shows three overlapping peaks at 279, 368, and 515 °C. However, four reduction peaks at 262, 352, 400, and 511 °C are present in the profile of Mn0.5Ce0.5Oδ catalyst. The shape of the H2-TPR profile of Mn 0.7 Ce 0.3 O δ catalyst is similar to that of Mn0.3Ce0.7Oδ, but the difference is that the reduction peaks of Mn0.7Ce0.3Oδ are shifted to the higher temperature region, whose three overlapping peaks are at 373, 435, and 509 °C. Two reduction peaks at 373 and 492 °C are present in the profile of the Mn0.9Ce0.1Oδ catalyst, whose shape is similar to pure MnOx. The TPR results of MnxCe1−xOδ indicate that Mn and Ce species have interaction with each other in 3DOM Mn x Ce 1−x O δ solid solutions. The interaction may be contributed to good catalytic performance for soot combustion. As shown in Figure 5(h−l), one strong reduction peak appears in the low-temperature range between 100 and 250 °C on the TPR profile of Pt/Mn0.5Ce0.5Oδ catalysts, which correspond to the reduction of oxidic Pt species.43 The reduction peak temperature of Ptn+ decreased with the increase of Pt loading, and the low-temperature peaks range from 164 to 112 °C. Compared with the profile of Mn0.5Ce0.5Oδ catalysts, the Pt/Mn0.5Ce0.5Oδ catalysts show a weak reduction peak at high temperature (450−550 °C) when Mn0.5Ce0.5Oδ catalyst acts as support. An overview of the TPR profile of Pt/ Mn0.5Ce0.5Oδ catalysts indicates that the changed redox ability

pure MnOx has the highest surface area while pure CeO2 shows the lowest surface area; 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/ Mn0.5Ce0.5Oδ exhibit somewhat lower surface areas than pure MnOx. The values of total pore volume and pore size display the same tendency as the order of the surface area. The surface areas of those catalysts are lower than 60 m2/g, which are not higher than other particle catalysts. However, because of macropores in the current catalysts, the efficient surface area may be higher than particle catalysts in the reaction of soot combustion. 3.1.5. H2-TPR Measurement Results. Due to a complicated gas−solid (soot)−solid (catalyst) multiphase reaction for catalyzing soot combustion, the intrinsic redox properties of catalysts play a key role for the combustion of soot. Therefore, TPR by H2 was used to measure these characteristics in the present work. The results of H2-TPR analysis for MnxCe1−xOδ and Pt/Mn0.5Ce0.5Oδ catalysts are shown in Figure 5. The H2TPR profile of pure CeO2 shows a single reduction peak at 522 °C, which is associated with the reduction of surface Ce4+ ions. The H2-TPR profile of pure MnOx shows two overlapped strong reduction peaks at 411 and 547 °C. From different Mn species existing in the pure MnOx, it is possible to propose that two strong broad peaks correspond to the reduction of Mn3+ and Mn2+ cations, respectively.13,42 This result is in agreement with the XRD results, indicating that the crystalline phase of pure MnOx corresponds to mixed phases of Mn2O3 and Mn3O4. Compared with the reduction features of pure MnOx and CeO2, the reduction temperatures of MnxCe1−xOδ catalysts 9657



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450 °C. The crystal transfer of Mn0.5Ce0.5Oδ might result in the large amount of lattice oxygen (O2−) desorption peak when the temperature is located at 550−800 °C.39 The O2-TPD profiles of 3DOM 3 wt % Pt/Mn0.5Ce0.5Oδ catalyst also show three oxygen desorption peaks. The temperature ranges are similar to those of 3DOM Mn0.5Ce0.5Oδ. However, the intensities of desorption peaks are different with the 3DOM Mn0.5Ce0.5Oδ. Compared with 3DOM Mn0.5Ce0.5Oδ, it is clear that the intensity of the desorption peak of 300−450 °C on 3 wt % Pt/ Mn0.5Ce0.5Oδ remarkably increased, indicating that the ability of changing the valence state of Mnn+ and Cen+ is enhanced when Pt nanoparticles are loaded on the 3DOM Mn0.5Ce0.5Oδ. The enhanced ability of oxygen desorption over 3 wt % Pt/ Mn0.5Ce0.5Oδ improves oxidation of NO to NO2 and increases catalytic activity for soot combustion. The results of O2-TPD are in good agreement with the tendency of catalytic activity in Table 3. 3.1.7. XPS Characterization Results. It is well-known that the active species for oxidation reaction is the oxygen species on the surface of the catalyst. To examine the electronic properties of the surface elements, 3DOM Mn0.5Ce0.5Oδ and Pt/Mn0.5Ce0.5Oδ catalysts were studied by XPS, and the results are shown in Figure 7 and Table 2. The Ce 3d spectra are presented in Figure 7A. The curves of Ce 3d XPS spectra were fitted with eight peaks corresponding to four pairs of spin− orbit doublets. The peaks labeled as u‴, u″, u and v‴, v″, v are assigned to Ce4+ 3d3/2 and Ce4+ 3d5/2, respectively. And the u′ and v′ are assigned to Ce3+ 3d3/2 and Ce3+ 3d5/2, respectively.44 It is indicated that the Ce3+ is obtained in the as-prepared catalysts. Because the electron is easy to transform from Ce3+ to oxygen or Mn species during the catalyst preparation, the presence of Ce3+ could create a charge imbalance. The

may be influenced by the catalytic activities of Pt/Mn0.5Ce0.5Oδ catalysts. 3.1.6. Results of O2-TPD. To further investigate the adsorption and activation of oxygen on 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/Mn0.5Ce0.5Oδ, O2-TPD measurements were carried out, and the results are exhibited in Figure 6. As shown

Figure 6. O2-TPD curves of 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/ Mn0.5Ce0.5Oδ catalysts.

in Figure 6, the O2 desorption peaks of 3DOM Mn0.5Ce0.5Oδ can be divided into three parts, for which the temperature ranges are located at 50−250, 250−450, and 550−800 °C. The first desorption peak is relatively weak and belonged to the physical adsorption oxygen species (O2) and chemisorption’s oxygen species (O−2). The second desorption peak is assigned to the desorption of partial lattice oxygen (O−) owing to the changing of valence state Mn (Mn2+/Mn3+/Mn4+) and Ce (Ce3+/Ce4+). In addition, part of chemisorption’s oxygen species is also contributed to the desorption peak of 250−

Figure 7. XPS spectra of Mn0.5Ce0.5Oδ and x wt % Pt/Mn0.5Ce0.5Oδ ((A) Ce 3d, (B) Mn 2p, (C) O 1s, (D) Pt 4f with (a) Mn0.5Ce0.5Oδ, (b) 1 wt % Pt/Mn0.5Ce0.5Oδ, and (c) 3 wt % Pt/Mn0.5Ce0.5Oδ). 9658



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Table 2. Surface Composition and Oxidation State of Ce, Mn, O, and Pt Species Calculated from XPS R (%) catalysts

Ce3+

Ce4+

Mn2+

Mn3+

Mn4+

O2−

O−

O2−

Pt0

Pt2+

Pt4+

Mn0.5Ce0.5Oδ 1 wt % Pt/Mn0.5Ce0.5Oδ 3 wt % Pt/Mn0.5Ce0.5Oδ

14.1 15.5 18.5

85.9 84.5 81.5

43.7 41.6 36.8

34.5 33.1 34.5

21.6 25.2 28.7

8.2 9.6 10.4

23.5 24.7 27.8

68.3 65.7 61.8

22.9 23.6

61.4 52.6

15.7 23.8

Table 3. Catalytic Activities of 3DOM MnxCe1−xOδ and Pt/Mn0.5Ce0.5Oδ for Soot Combustiona catalysts

T10 (°C)

T50 (°C)

T90 (°C)

Sm CO2 (%)

ΔT10 (°C)

ΔT50 (°C)

ΔT90 (°C)

pure soot CeO2 Mn0.1Ce0.9Oδ Mn0.3Ce0.7Oδ Mn0.5Ce0.5Oδ Mn0.7Ce0.3Oδ Mn0.9Ce0.1Oδ MnOx CeO2 and MnOxb 0.5 wt % Pt/Mn0.5Ce0.5Oδ 1 wt % Pt/Mn0.5Ce0.5Oδ 2 wt % Pt/Mn0.5Ce0.5Oδ 3 wt % Pt/Mn0.5Ce0.5Oδ 5 wt % Pt/Mn0.5Ce0.5Oδ

482 349 320 310 297 308 310 326 305 296 295 291 290 294

564 393 361 356 358 370 381 394 372 346 342 343 342 343

609 429 399 392 396 407 421 431 410 380 377 376 373 375

71.6 91.9 96.1 94.6 94.2 93.5 92.1 88.7 94.5 96.3 96.8 96.9 96.7 96.4

133 162 172 185 174 172 156 177 186 187 191 192 188

171 203 208 206 194 183 170 192 218 222 221 222 221

180 210 217 213 202 188 178 199 229 232 233 236 234

ΔT10: T10 of pure soot minus T10 of catalysts. ΔT50: T50 of pure soot minus T50 of catalysts. ΔT90: T90 of pure soot minus T90 of catalysts. bThe mechanical mixture of 3DOM CeO2 and MnOx: the ratio of Mn and Ce is 1:1. a

manganese−ceria solid solutions.47 Second, the surface adsorbed oxygen species of solid solutions have been changed by supported Pt NPs, which leads to the charge balance through the changing valence state of Mn ions. As shown in Figure 7C, the O 1s peaks are fitted into three peaks at 529.5−530.1, 531.0−531.7, and 532.7−533.5 eV. The peak at 529.5−530.1 eV is assigned to the lattice oxygen (O2−), corresponding to lattice oxygen of CeO2 and MnOx phases. The peaks at 531.0−531.7 and 532.7−533.5 eV are assigned to two kinds of chemisorbed oxygen (O− and O2−, respectively). Table 2 shows that the relative intensity of O− and O2− increased with the increasing of Pt loading. The increase in the contents of O− and O2− species indicate that of the supported Pt NPs can enhance the capability of oxygen activation and increase the amount of surface-active oxygen species (O2−, O−), which would give rise to unusual catalytic properties for deep oxidation reaction.32 Figure 7D shows the Pt 4f XPS spectra of Pt/Mn0.5Ce0.5Oδ catalysts. The binding energies of 71.5 and 74.3 eV, 72.7 and 76.1 eV, and 73.6 and 77.8 eV have been assigned to Pt0, Pt2+, and Pt4+ species, respectively. These results indicate that both metallic and ionic platinum species are present on 3DOM Pt/ Mn0.5Ce0.5Oδ catalysts. The XPS spectra were used to estimate the amount of Pt0, Pt2+, and Pt4+ in each sample, and the results are presented in Table 2. The total contents of Pt2+ and Pt4+ of two Pt loaded catalysts are similar to each other, and the contents of ionic states are higher than 75%. However, compared with 1 wt % Pt/Mn0.5Ce0.5Oδ catalyst, the presence of Pt2+ in the 3 wt % Pt/Mn0.5Ce0.5Oδ catalyst has lower content. There are two possible reasons for the high content of Pt ionic states: one is the unsaturated bond; the surface atoms of small size Pt NPs show the strong ability of donating electronics so that O2 molecules are adsorbed on the surface Pt atoms, which will result in the formation of the Pt2+ and Pt4+ being formed. The other is interaction of Pt NPs and

vacancies and unsaturated chemical bond will lead to the increase in chemisorbed oxygen on the catalyst surface. From Table 2, it can be seen that the relative intensity of Ce3+ enhances with the increasing of Pt loading. The increase in Ce3+ content is contributed to the interaction between cerium and the surrounding atoms, which leads to an increase in the oxygen vacancies. The interaction may lead to transference of the lattice oxygen in ceria over the surface of Pt NPs and maintaining cerium at the low-valent state.32 The XPS spectra of Mn 2p are shown in Figure 7B. It should be emphasized that the deconvolution method yields ambiguity in the recognition of the manganese species, especially Mn3+ and Mn4+, due to the small differences in their binding energy values.45 In the Mn 2p XPS spectra, satellites are rather clearly distinguishable and both Mn 2p3/2 and Mn 2p1/2 peaks overlap (shown in Figure 7B). Moreover, the Mn 2p spectra are significantly broadened and show some asymmetry toward both Mn 2p3/2 and Mn 2p1/2 peaks. Due to this feature, it is worth noting that the binding energies of components of as-prepared catalysts are in good agreement with the literature data reported for Mn2+, Mn3+ and Mn4+.46 From Figure 7B, it can be seen that the binding energies of the XPS Mn 2p3/2 peaks are found to be in the range of 640.0−643.0 eV. Three kinds of Mn species, which including Mn2+ (640.3−640.7 eV), Mn3+ (641.4−641.8 eV), and Mn4+ (642.5−643.0 eV), are presented on the surface of as-prepared catalysts. Meanwhile, the Mn 2p1/2 peaks also show three kinds of Mn species in the binding energy (BE) range of 650−655 eV. As shown in Table 2, the relative intensities of Mn2+, Mn3+, and Mn4+ were calculated to be 43.7%, 34.5%, and 21.6% in the Mn0.5Ce0.5Oδ catalyst, respectively. After the Pt NPs supported on the Mn0.5Ce0.5Oδ, the relatively ratio of Mn2+ decreased while that of Mn4+ increased. Two possible reasons for the change the valence state of Mnn+ ions: First, the oxygen mobility of ceria was enhanced in the presence of Mn4+ ions on the surface of 9659



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Table 4. Reaction Rate, Active Oxygen (O*) Density, and TOF Values for Soot Combustion with O2 over the 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/Mn0.5Ce0.5Oδ Catalysts catalysts Mn0.5Ce0.5Oδ Mn0.5Ce0.5Oδ 3 wt % Pt/ Mn0.5Ce0.5Oδ 3 wt % Pt/ Mn0.5Ce0.5Oδ

temp (K)

ν (mol s−1 g−1 × 10−7)a ν* (mol s−1 m−2 × 10−9)b

O* amount (mol g−1 × 10−4)c

O* density (nm−2)d

TOF (s−1 × 10−3)e

558 568 558

0.46 0.64 0.72

0.96 1.33 1.31

0.74 0.88 0.65

0.92 1.1 0.71

0.62 0.73 1.10

568

1.18

2.11

1.01

1.1

1.16

a

Reaction rate. bSpecific reaction rate unit BET surface area. cNumber of active redox sites. dDensity of active redox sites. eRatio of the reaction rate to the active site density.

Mn0.5Ce0.5Oδ catalysts have the higher selectivity of CO2 production and all CO2 selectivity values surpassed 96%. The catalytic activities of 3DOM Pt/Mn0.5Ce0.5Oδ with different Pt loadings follow the order 3 wt % > 2 wt % ≈ 5 wt % > 1 wt % > 0.5 wt %. As shown in Table 3, the values of ΔT10, ΔT50, and ΔT90 of Pt/Mn0.5Ce0.5Oδ catalysts are much larger than those of Mn0.5Ce0.5Oδ, suggesting that Pt NPs play an important role in enhancing catalytic activities of Pt/Mn0.5Ce0.5Oδ, which is attributed to the synergistic effect between Pt and MnxCe1−xOδ. The catalytic activity of supported Pt catalysts is affected by two factors, which include the dosage of the noble metal loading and the ratio (NS/NT) of exposed surface atoms (NS) to total atoms (NT). The relationships of Pt loading and the value of NS/NT are shown in Supporting Information Table S2. In order to compare the catalytic performance of as-prepared 3DOM catalysts, the catalytic activities of other 3DOM catalysts, which were studied by our group previously, are shown in Supporting Information Table S1. Compared with Table 3, it can be seen that the MnxCe1−xOδ solid solutions show higher catalytic activities than other 3DOM metal oxides at relatively high temperature (i.e., activity referred by T50 and T90). 3DOM Pt/Mn0.5Ce0.5Oδ catalysts show similar activities with our previous supported noble metal catalysts when the T50 and T90 were compared as a measure for activity evaluation. However, the activity referred to by T10 is lower than those of other supported noble metal catalysts. By comparing catalytic performance of those 3DOM catalysts, we can see that the asprepared 3DOM catalysts show excellent catalytic activities for soot combustion, especially at relatively high temperatures. 3.3. Calculation of TOF Values and Activation Energy. Table 4 lists the quantified values of the reaction rate, the density of active redox sites, and the TOF for soot combustion under O2 over the 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/ Mn0.5Ce0.5Oδ catalysts at different reaction temperatures. As shown in Table 4, the TOF value of 3DOM Mn0.5Ce0.5Oδ is lower than that of 3 wt % Pt/Mn0.5Ce0.5Oδ, indicating high redox properties of 3 wt % Pt/Mn0.5Ce0.5Oδ catalyst due to synergetic effect between Pt NPs and Mn0.5Ce0.5Oδ. The increased TOF value of 3 wt % Pt/Mn0.5Ce0.5Oδ catalyst indicates that Pt incorporation enhances the redox properties of catalysts. For gas−solid−solid reaction, the activation energy is influenced by many factors.48 In order to calculate the activation energy more accurately, the value of Ea is calculated under the low rate of soot conversion ( Mn0.3Ce0.7Oδ > Mn0.7Ce0.3Oδ > Mn0.1Ce0.9Oδ > MnOx > CeO2. In addition, for the more deeply proved synergistic effect between Mn and Ce, the mechanical mixture composed of CeO2 and manganese oxide was also tested for soot combustion. As shown in Table 3, compared with the pure MnOx or CeO2, the mechanical mixture of CeO2 and MnOx exhibits higher catalytic activity indicating that the synergistic effect between Mn and Ce exists in the mechanical mixture for soot combustion. However, the mechanical mixture of CeO2 and MnOx shows lower catalytic activity than Mn0.5Ce0.5Oδ solid solution. This result indicates that the synergistic effect between Mn and Ce in Mn0.5Ce0.5Oδ solid solution is stronger than that of the mechanical mixture of CeO2 and MnOx. In order to investigate the synergistic effect between noble metal and MnxCe1−xOδ on soot combustion, small size Pt NPs with varied Pt loadings were loaded on 3DOM Mn0.5Ce0.5Oδ support and their catalytic activities are shown in Table 3. The experimental results demonstrated that 3DOM Pt/Mn0.5Ce0.5Oδ catalysts have higher activities than that of MnxCe1−xOδ. With increasing Pt loading, the catalytic activity increased until the Pt loading of 3 wt %; and then it decreased with further increasing of Pt loading. Compared with the CO2 selectivity of MnxCe1−xOδ catalysts, 3DOM Pt/ 9660



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Figure 8. Activation energy of soot combustion over 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/Mn0.5Ce0.5Oδ catalysts.

Mn0.5Ce0.5Oδ. The Ea for Mn0.5Ce0.5Oδ is around 51.2 kJ/mol, which decreased to about 44.2 kJ/mol for 3 wt % Pt/ Mn0.5Ce0.5Oδ. The values of Ea are similar to that previously reported.48 The difference of Ea suggests that the Pt loading can improve the catalytic activity of catalyst.

Figure 9. TEM images of 3DOM Mn0.5Ce0.5Oδ (a, b) and 3 wt % Pt/ Mn0.5Ce0.5Oδ (c,d ) catalysts, and soot.

4. DISCUSSION 4.1. Effect of Macroporous Structure on Soot Combustion. It is well-known that the catalytic soot combustion is a gas−solid−solid reaction, in which the contact between soot and catalysts is one of the most important factors for improving the catalytic activity. In fact, many studies have demonstrated that the tight contact between soot and catalysts shows excellent catalytic performances for soot combustion.49 However, loose contact between soot particles and catalysts is a major way of contact in the process of after-treatment for diesel engine exhaust. Therefore, it is extremely important to study and design the active catalysts, which can improve the contact efficiency between the catalysts and soot particles under loose contact conditions. In order to enhance the contact efficiency, 3DOM materials with uniform pore size and periodic voids interconnected through open windows are designed and synthesized by colloidal crystal template method. As shown in Figure 2, the average diameter of ordered macropores is about 310 nm and the diameter of interconnected open windows is about 100 nm. Soot particles could easily cross those pore sizes and reach the inner area of the 3DOM materials with the help of air flow, with which the 3DOM structures could enhance the contact efficiency between soot particles and catalysts. In this work, in order to demonstrate the contact efficiency, an isothermal reaction was studied under the same conditions of TPO reaction. In this confirmatory experiment, the reaction temperature of soot and 3DOM catalyst was programmed to 290 °C, which means the soot was not ignited. As shown in Figure 9, 3DOM catalysts were still kept intact in the reaction process. From Figure 9a and 9c, it can be seen that the macropores of the 3DOM catalyst are filled with soot particles, indicating that the 3DOM structure is a desirable feature for diesel soot combustion. This is the direct evidence that the perfect macroporous structure provides the ideal reaction place for solid reactants (diesel soot). As shown in the HRTEM images (Figure 9b,d), the soot and 3DOM materials are wellcontacted with each other. Due to this contact efficiency, the number of available active sites of catalysts is increased, especially the inner active sites of the catalysts. More active sites would result in higher catalytic activity. TEM results have intuitionally demonstrated that soot particles can easily enter

the interior of 3DOM catalysts with the help of the air flow in the reaction process under the loose contact conditions and have less resistance to go through the catalyst structure. In fact, our group has synthesized a series of 3DOM materials, including LaFeO3,25 La1−xKxCoO3,26 LaCoxFe1−xO3,27 Ce− Zr−Pr−O,24 Ce1−xZrxO2,28 and so on. 3DOM materials showed higher catalytic activities than the corresponding particle materials. Therefore, it is significantly important to study and design a 3DOM structure for soot combustion. 4.2. Synergistic Effect of As-Prepared Catalysts. 4.2.1. Synergistic Effect of Mn and Ce in Mn0.5Ce0.5Oδ. Due to the nature of the deep oxidation reaction for soot combustion, the redox property of catalysts is another important factor for controlling the catalytic activity except for the structural feature of catalysts. In this work, the redox properties of catalysts were characterized by H2-TPR measurement. The H2-TPR curves of MnxCe1−xOδ show varied reduction temperature with different x values (Figure 5). Generally speaking, the lower reduction temperature means more active oxygen species, which cause high catalytic activity. In fact, a good correlation is established between the low reduction temperature and high activity of soot oxidation, indicating the importance of active oxygen species from the MnxCe1−xOδ solid solutions.21 The strong synergistic effect of manganese and cerium in the solid solutions might be deduced from the strong oxidative properties of manganese in combination with the oxygen storage properties of ceria.19 It is well-known that the transfer of active oxygen is an important step for the soot oxidation process. The high catalytic activity for soot oxidation was attributed to the generation of active oxygen due to the oxygen exchange between gas phase O2 and oxygen into the oxide framework. On the other hand, the influence of the NO presence on the soot combustion is already demonstrated by previous reports.40,49,50 In this investigation, the possible soot combustion mechanism on the MnxCe1−xOδ solid solutions was presumed by the results of characterization, and the possible reaction schematic illustration was shown in Figure 10. The possible mechanism for soot combustion on the MnxCe1−xOδ catalysts can be divided into two steps. The first step is adsorption. The gas phase O2 and NO were adsorbed on the surface of catalysts; due to the changing valence state of Mn 9661



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each element in as-prepared catalysts have been influenced by loading of Pt NPs. In brief, the apparent largest reduction peaks of Pt/Mn0.5Ce0.5Oδ catalysts at lower temperature are corresponding to the reduction of oxidic Pt species; it indicates that 3DOM Pt/Mn0.5Ce0.5Oδ catalyst could show higher redox properties than Mn0.5Ce0.5Oδ catalyst. Preliminary XPS results suggest the relative content of each element in these catalysts could be essential for catalytic performance. The XPS results indicate that the highest active catalyst (3 wt % Pt/ Mn0.5Ce0.5Oδ) exhibits a relatively higher amount of lower valence of Ce, higher valence of Mn, oxidic Pt species, and surface-active oxygen species (O2−, O−) than Mn0.5Ce0.5Oδ and 1 wt % Pt/Mn0.5Ce0.5Oδ. According to the characterization results, the presence of Cen+, Mnn+ (especially Ce3+, Mn4+), Ptn+, and surface active oxygen species on the surface of Pt/ Mn0.5Ce0.5Oδ could create a charge imbalance. And then more NO could be oxidized to NO2 through the vacancies and unsaturated chemical bonds on the catalyst surface, resulting in more absorbed NO3− formation on the surface of catalyst.18 Based on the above discussion, the possible soot combustion mechanism on the Pt/Mn0.5Ce0.5Oδ catalysts was presumed and is shown in Figure 11.

Figure 10. Schematic illustration of MnxCe1−xOδ catalysts for soot combustion.

and Ce species in the MnxCe1−xOδ solid solutions, the oxygen vacancies were formed on the surface of catalysts, and then the active oxygen can be easily generated on the surface vacancies of Mn−Ce from chemisorbed oxygen. Correspondingly, the NO was also interacted with surface active oxygen and formed bidentate/monodentate nitrates on the Mnn+ and Cen+ sites.40 In this section, the ratio of Mn and Ce has significant influence on oxygen vacancies and NO adsorption. As shown in H2-TPR profiles (Figure 5), O2-TPD results (Figure 6), and the activity results (Table 3), the lower reduction temperature and oxygen desorption peak (250−450 °C) of MnxCe1−xOδ solid solutions indicate that synergistic effect between Mn and Ce is one of the important factors for determining the activity of catalysts. The XPS of Mn0.5Ce0.5Oδ also showed that the Mn2+, Mn3+, Mn4+, Ce3+, and Ce4+ were detected. Therefore, the redox reaction of Mn2+/Mn3+ or Mn3+/Mn4+ or Ce3+/Ce4+ and the oxygen defect of Mn−O−Ce bands in the solid solution lattice are expected to contribute to the oxygen vacancies and NO adsorption. The second step is desorption and oxidization. Those bidentate/ monodentate nitrates species were desorbed by formation of NO2 from Mn−Ce solid solutions, which acts as oxidant species for soot combustion via a spillover mechanism when the temperature is increased.51 Several previous works have reported the effect of NO2 for soot combustion under the synergetic effect between Mn and Ce. For example, Tikhomirov et al. demonstrated that NO is oxidized over the catalyst to NO2, and stored as nitrate at low temperatures. At higher temperatures the nitrates decompose, releasing NO2 to the gas phase which acts as the oxidizing agent for soot. The nitrate storage capacity of MnOx-CeO2 is three to five times higher than that of the individual oxides resulting in a major contribution of the released NO2 to the soot oxidation process.19 In the current work, as discussed above, the synergistic effect between Mn and Ce enhanced the catalytic activities of MnxCe1−xOδ solid solutions. 4.2.2. Synergistic Effect of Pt NPs and Mn0.5Ce0.5Oδ. The synergetic effect between Pt NPs and Mn0.5Ce0.5Oδ may also play an essential role in catalytic combustion of soot. As shown in Table 3, compared with Mn0.5Ce0.5Oδ, the Pt/Mn0.5Ce0.5Oδ catalysts gave higher catalytic activities for soot combustion, especially for the activity referred by T50 and T90. Our previous works have reported the synergetic effect between noble metal NPs and supports for soot combustion, for example, the synergetic effect between Au NPs and Ce1−xZrxO2, Au NPs, and LaFeO3, Pt NPs, and Ce1−xZrxO2 and so on.32,36,37 In this work, based on the results of H2-TPR, O2-TPD, and XPS measurements, the redox and the ratio of the valence state of

Figure 11. Schematic illustration of Pt/Mn0.5Ce0.5Oδ catalysts for soot combustion.

As shown in Figure 11, the possible soot combustion mechanism on the Pt/Mn0.5Ce0.5Oδ is similar to that of MnxCe1−xOδ solid solutions. The route of soot combustion on the Pt/Mn0.5Ce0.5Oδ can be denoted as the following pathways. First, because of synergetic effect between Pt NPs and Mn0.5Ce0.5Oδ, a greater numbers of gas phase O2 and NO molecules are adsorbed on the surface of catalyst by forming surface nitrate species than with Mn0.5Ce0.5Oδ. And then NO2 are released from nitrate species. Due to the strong oxidizing property of NO2, the soot particles are directly oxidized to CO2 by NO2. In this process of reaction, the loaded Pt NPs can improve the amounts of active oxygen species on the surface of catalysts, so that they can efficiently enhance catalytic activities for oxidation of NO to NO2. In addition, the ionic states of Ptn+ also can provide the adsorption center for nitrate species. Except the released NO2, the gaseous NO also can form NO2 owing to the strong ability of desorption oxygen (Figure 6). As shown in Figure 8, the difference of Ea values is also proved a higher activity of Pt/Mn0.5Ce0.5Oδ than that of Mn0.5Ce0.5Oδ for soot combustion. From the preceding result characterizations, it can be seen that 3DOM Pt/Mn0.5Ce0.5Oδ catalysts exhibit much higher selectivity of CO2 production for soot oxidation than those MnxCe1−xOδ solid solutions. The possible reason is that large amounts of CO can possibly pass rapidly through 3DOM 9662



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Author Contributions

MnxCe1−xOδ catalysts with the macropores and then gives fewer choices for the contact between CO and catalyst. However, due to the high activity for CO oxidation of supported Pt catalysts,32,52 the major number of CO can be oxidized to CO2 before they release from the macropores of 3DOM structures. Based on the preceding discussion, it can be concluded that 3DOM Pt/Mn0.5Ce0.5Oδ catalysts are the good catalyst system for soot combustion.

†

X. Yu and J. Li contributed equally to this work. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (Grant Nos. 21177160, 21303263, and 21173270), Beijing Nova Program (Grant No. Z141109001814072), and The Science Foundation of China University of Petroleum-Beijing (Grant Nos. YJRC2013-13 and 2462013BJRC003).

5. CONCLUSION In summary, 3DOM MnxCe1−xOδ catalysts with different ratios of Mn to Ce were successfully synthesized by CCT method, and small sizes of Pt NPs were deposited on 3DOM Mn0.5Ce0.5Oδ supports with varied loadings of Pt by in situ EG reduction method. All 3DOM samples show that the macropores with average diameter of ca. 310 ± 20 nm are interconnected through open windows, ca. 90−140 nm in diameter, and the wall thicknesses are ca. 30−50 nm. Pt nanoparticles of 1−2 nm are homogeneously dispersed on the walls of 3DOM structures. The x value in MnxCe1−xOδ and Pt loading in Pt/Mn0.5Ce0.5Oδ have significant influence on redox and surface properties of catalysts. The synthesized 3DOM MnxCe1−xOδ catalysts exhibit high catalytic activity for soot combustion. Compared with pure soot, the T10, T50, and T90 of MnxCe1−xOδ catalysts are remarkably lower. With different x values, the MnxCe1−xOδ catalysts give various catalytic activities and 3DOM Mn0.5Ce0.5Oδ show excellent catalytic activity among MnxCe1−xOδ catalysts; the temperatures of T10, T50, and T90 are 298, 358, and 396 °C, respectively. 3DOM Pt/ Mn0.5Ce0.5Oδ catalysts exhibit higher catalytic activities than MnxCe1−xOδ catalysts. A 3 wt % amount of Pt/Mn0.5Ce0.5Oδ shows the highest catalytic activity, whose T50 and T90 are 342 and 373 °C, respectively. The CO2 selectivity of 3 wt % Pt/ Mn0.5Ce0.5Oδ is also the highest (reach to 96.7%). The structural and synergistic effects of 3DOM Pt/Mn0.5Ce0.5Oδ catalysts are favorable for enhancing the catalytic activity for diesel soot combustion. The as-prepared 3DOM catalysts are promising for practical applications in the catalytic oxidation of diesel soot particles.

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(1) Fino, D.; Specchia, V. Open issues in oxidative catalysis for diesel particulate abatement. Powder Technol. 2008, 180, 64−73. (2) Prasad, R.; Bella, V. R. A review on diesel soot emission, its effect and control bulletin of chemical reaction. Bull. Chem. React. Eng. Catal. 2010, 5, 69−86. (3) Maricq, M. M. Chemical characterization of particulate emissions from diesel engines: A review. Aerosol Sci. 2007, 38, 1079−1118. (4) Twigg, M. V. Progress and future challenges in controlling automotive exhaust gas emissions. Appl. Catal,, B 2007, 70, 2−15. (5) Tschamber, V.; Jeguirim, M.; Villani, K.; Martens, J.; Ehrburger, P. Comparison of the activity of Ru and Pt catalysts for the oxidation of carbon by NO2. Appl. Catal., B 2007, 72, 299−303. (6) Teraoka, Y.; Nakano, K.; Shangguan, W. F.; Kagawa, S. Simultaneous catalytic removal of nitrogen oxides and diesel soot particulate over perovskite-related oxides. Catal. Today 1996, 27, 107− 113. (7) Atribak, I.; Azambre, B.; Bueno-López, A.; García-García, A. Effect of NOx adsorption/desorption over ceria-zirconia catalysts on the catalytic combustion of model soot. Appl. Catal., B 2009, 92, 126− 137. (8) Sullivan, J. A.; Dulgheru, P.; Atribak, I.; Bueno-López, A.; GarcíaGarcía, A. Attempts at an in situ Raman study of ceria/zirconia catalysts in PM combustion. Appl.Catal., B 2011, 108−109, 134−139. (9) Liu, J.; Zhao, Z.; Wang, J. Q.; Xu, C. M.; Duan, A. J.; Jiang, G. Y.; Yang, Q. The highly active catalysts of nanometric CeO2-supported cobalt oxides for soot combustion. Appl. Catal., B 2008, 84, 185−195. (10) Liu, J.; Zhao, Z.; Xu, C. M.; Duan, A. J.; Wang, L.; Zhang, S. J. Synthesis of nanopowder Ce-Zr-Pr oxide solid solutions and their catalytic performances for soot combustion. Catal. Commun. 2007, 8, 220−224. (11) Liu, J.; Zhao, Z.; Xu, J. F.; Xu, C. M.; Duan, A. J.; Jiang, G. Y.; He, H. The highly active catalysts of nanocomposite K-Co-CeO2 for soot combustion. Chem. Commun. (Cambridge, U. K.) 2011, 47, 11119−11121. (12) Tang, X. F.; Li, Y. G.; Huang, X. M.; Xu, Y. D.; Zhu, H. Q.; Wang, J. G.; Shen, W. J. MnOx-CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: Effect of preparation method and calcination temperature. Appl. Catal., B 2006, 62, 265−273. (13) Delimaris, D.; Ioannides, T. VOC oxidation over MnOx-CeO2 catalysts prepared by a combustion method. Appl. Catal., B 2008, 84, 303−312. (14) Drenchev, N.; Spassova, I.; Ivanova, E.; Khristova, M.; Hadjiivanov, K. Cooperative effect of Ce and Mn in MnCe/Al2O3 environmental catalysts. Appl. Catal., B 2013, 138−139, 362−372. (15) Wang, X. Y.; Kang, Q.; Li, D. Low-temperature catalytic combustion of chlorobenzene over MnOx-CeO2 mixed oxide catalysts. Catal. Commun. 2008, 9, 2158−2162. (16) Chen, H. Y.; Sayari, A.; Adnot, A.; Larachi, F. Compositionactivity effects of Mn-Ce-O composites on phenol catalytic wet oxidation. Appl. Catal., B 2001, 32, 195−204. (17) Casapu, M.; Krocher, O.; Elsener, M. Screening of doped MnOx-CeO2 catalysts for low-temperature NO-SCR. Appl. Catal., B 2009, 88, 413−419.

ASSOCIATED CONTENT

S Supporting Information *

Text describing synthesis of well-defined PMMA microspheres, Rietveld refinements, steps for TOF of soot combustion over 3DOM Mn0.5Ce0.5Oδ and 3 wt % Pt/Mn0.5Ce0.5Oδ catalysts, calculation of exposed Pt surface atoms, and accompanying references, figures showing SEM images of PMMA microspheres and PMMA colloidal crystal templates and Rietveld refinement of as-prepared catalysts, and tables listing comparisons of catalytic performance of different 3DOM catalysts for combustion of soot, the number ratio (NS/NT) of exposed surface atoms (NS) to total atoms (NT) of Pt nanoparticles, and the relationship between the soot conversion rate and reaction temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(Z.Z.) Tel.: +86 10 89731586; Fax: +86 10 69724721. E-mail: [email protected]. *(Y.W.) Tel.: +86 10 89731586; Fax: +86 10 69724721. Email: [email protected]. 9663



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