Decorating CeO 2 Nanoparticles on Mn 2 O 3 Nanosheets to Improve

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Decorating CeO Nanoparticles on MnO Nanosheets to Improve Catalytic Soot Combustion Lingli Xing, Yuexi Yang, Chunmei Cao, Dongyue Zhao, Zhongnan Gao, Wei Ren, Ye Tian, Tong Ding, and Xingang Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03645 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Decorating CeO2 Nanoparticles on Mn2O3 Nanosheets to Improve Catalytic Soot Combustion

Lingli Xing†, Yuexi Yang†, Chunmei Cao‡, Dongyue Zhao†, Zhongnan Gao†, Wei Ren†, Ye Tian†, Tong Ding†, Xingang Li*,†

†Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin, 300350, P. R. China ‡Research

Center of Heterogeneous Catalysis and Engineering Sciences, School of

Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, P. R. China

* Corresponding author: Prof. Xingang Li EMAIL: [email protected]

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ABSTRACT Herein, we synthesized CeO2 nanoparticles-decorated Mn2O3 nanosheets on the monolithic Ni foam (Ce/Mn-NF) for catalytic elimination of soot particulates. The macroporous nano-structures created by the Mn2O3 nanosheets improve the soot-catalyst contact efficiency on the external surface of catalysts. The superficial Mn-Ce interaction can produce Mn4+ and oxygen vacancies on the Ce/Mn-NF catalysts through the redox process of Ce4+ + Mn3+  Mn4+ + Ce3+, simultaneously generating surface active oxygen species. Moreover, our results demonstrate that the surface adsorbed oxygen species induced by the Mn-Ce interaction are more active for catalytic soot oxidation than those on the Mn-NF. Thus, the introduction of the CeO2 nanoparticles to the Mn2O3 nanosheets can significantly improve the catalytic activity for soot combustion. For the Ce/Mn-NF-2, the surface atomic ratio of Ce and Mn is close to 1:1, which will create more Mn-Ce interaction sites to generate active oxygen species compared with other catalysts. Accordingly, it exhibits the higher catalytic activity. The tactful design of the MnOx-CeO2 catalysts successfully overcomes the problems of how to construct macroporous nano-structures with mixed metal oxides, and this strategy of material design may be applied in other related catalytic systems.

KEYWORDS: Mn-Ce interaction; Macroporous nano-structures; Soot combustion; Mn4+; Oxygen vacancies; Surface adsorbed oxygen 2


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INTRODUCTION Nowadays, diesel engines have been widely applied in transportation, construction, farming, manufacturing, and electricity generation owing to their low operation costs, good durability, and high efficiency.1,2 However, as one of the hazardous air pollutants, soot particulates in diesel engine emissions cause a serious threat to environment and human health.3,4 At present, the continuously regenerated trap (CRT) is one of the effective after-treatment technologies to eliminate soot particulates, whose catalytic performance highly relies on the screening of catalysts. Therefore, the focus in the CRT system for soot elimination is to develop effective catalysts, which can oxidize soot within the temperature range (250-400 ºC) of diesel engine exhaust. Large number of catalysts for soot combustion has been reported, such as noble metals,5,6 perovskites,7-10 spinel-type

oxides,11,12

hydrotalcites,13,14 alkaline

metal

oxides,15,16

ceria-based

oxides,17-19 and transition metal oxides.20-23 Recently, Fang et al.9 synthesized the perovskite-type La1-xKxFeO3-δ nanotubes, which showed the high material utilization efficiency, and the improved catalytic soot oxidation activity. Hernández et al.10 reported the Ag-modified LaSrAgMn perovskite with the promising catalytic soot oxidation performance under low O2 partial pressure. Xiong et al.12 fabricated spinel-type PdxCo3-xO4 biactive sites by substitution of Co2+ with Pd2+, which greatly enhanced the catalytic soot combustion performance. Generally, soot combustion occurs at the “triple-phase contact sites” of gaseous 3



reactants, soot particulates, and solid catalysts.24 The amount of contact sites and the contact efficiency are determinant in catalytic soot oxidation. Industrially, the powder catalysts are sprayed onto the diesel particulate filter (DPF) carriers for catalytic soot combustion. Nevertheless, soot particulates (> 25 nm) are difficult to enter micropores and mesopores of catalysts, and only limited external surfaces are available for soot combustion. Zhao and coworkers have prepared three dimensionally ordered macroporous composite metal oxides, which can allow soot particulates easily depositing into the inner pores, resulting in the remarkable improvement of catalytic activities for soot combustion.25 Recently, it is reported that the monolithic catalysts with macroporous nano-structures show much higher catalytic performances for soot combustion than conventional powder catalysts because of the greatly improved soot-catalyst contact efficiency.26,27 Thus, the macroporous structure of catalysts is required for diesel soot combustion. It has strong demand to in situ construct macroporous nano-structures on DPF to capture soot and increase its contact chances with active sites. Manganese oxides are commonly employed for oxidizing reactions, such as VOC oxidation, carbon monoxide oxidation, and diesel soot combustion, because of their low cost, excellent redox properties, and environmental compatibility.28 The excellent redox property of manganese oxides is mainly caused by the redox cycle of superficial Mn3+/Mn4+ or Mn2+/Mn3+ correlated with the multivalent Mn species.29-32 Much effort has been made to design efficient MnOx catalysts to substitute of precious metal based catalysts for soot elimination. Wasalathanthri et al.33 found that the manganese oxides 4


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with a high valence showed a high soot oxidation activity. Guo et al.34 revealed that the presence of the high concentration of Mn4+ on the surface of catalysts would enhance the surface oxygen mobility, thus resulting in the excellent NO oxidation activity. So, the enrichment of superficial Mn4+ on manganese oxides is an important factor to improve the redox abilities of the catalysts. CeO2 has been recognized as the excellent oxidation catalyst because of the superior oxygen storage and release capacity.35 The surface of CeO2 is easily enriched with oxygen vacancies via the reduction of adjacent Ce4+ to Ce3+.36 In many oxidation reactions, the concentration of oxygen vacancies is the key factor to determine the catalytic activities.37 Therefore, CeO2 becomes a good choice for modification of MnOx catalysts to improve oxidizability. Interaction between Mn and Ce cations can enhance the oxidizability of MnOx-CeO2 mixed metal oxides,38,39 and forming MnCeOx solid solution is an effective way to amplify this effect. For solid soot reactants, only the Mn-Ce interaction sites on the external surface of catalysts are effective for catalytic soot oxidation. Unfortunately, it is difficult to build MnCeOx solid solution with macroporous nano-structures. Recently, the industrialized nickel foam with three-dimensionally reticular architecture is widely used as catalyst carrier material, due to its low cost, good thermal conductivity, high porosity and superior mass-transfer performance.40-44 For instance, Liu et al.40 in situ synthesized the MnO2@NiCo2O4 nanowires on nickel foam as high-performance monolithic de-NOx catalysts. Peng et al.41 reported the hierarchical 5



CuO-CeO2 composite catalysts supported on macroporous nickel foam for preferential CO oxidation. In this work, the industrialized nickel foam was selected as the DPF carrier material, because its three-dimensionally macroporous structure can facilitate the mass and heat transfer during soot oxidation, and increase the catalyst-soot contact probability. We facilely in situ grew the Mn2O3 crossed nanosheets on monolithic DPF substrate by a hydrothermal method. Then, we deposited the CeO2 nanoparticles on the Mn2O3 nanosheets by an impregnation method. The high dispersion of CeO2 on the Mn2O3 nanosheets generates more interfacial active sites on the external surfaces of the catalysts to promote catalytic soot combustion. To the best of our knowledge, our work first provides a facile preparation method for in situ construction of macroporous nano-structures with MnOx-CeO2 metal oxides on DPF for soot combustion. The tactful design of these nano-structures successfully overcomes the problems for the application of MnOx-CeO2 catalysts in the reaction of catalytic soot oxidation by improving the soot capture efficiency and increasing the interaction chances between the active sites and the captured soot on the external surfaces of the catalysts. Accordingly, the as-prepared monolithic catalysts showed the high catalytic performance under the loose contact mode for soot combustion. Additionally, we detailedly investigated the mechanism of the Mn-Ce interaction, and clearly demonstrated the influence of the Mn-Ce interaction on catalytic soot combustion.

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EXPERIMENTAL SECTION

Scheme 1. Schematic representation describing the preparation process of the Ce/Mn-NF monolithic catalysts.

Catalyst Preparation Scheme 1 illustrates the synthesis procedure of CeO2/Mn2O3 nanosheets and the process for catalytic soot oxidation. All of the chemical reagents were of analytical grade. The substrate nickel foam used in the experiments was provided by Lizhiyuan Battery Materials Co. Ltd. The porosity is more than 98%, the thickness is 1.5 mm, and the mechanical strength is higher than 1 MPa. The substrate material Ni foam with the appropriate size was etched with 2 mol L-1 HCl to remove the surface oxide layers. After 10 min, the nickel foam was cleaned with ethanol and then dried. 7



The catalyst of Mn2O3 nanosheets was prepared by a hydrothermal process. The clean substrate and the homogeneous solution consisting of Mn(NO3)2 (1 mmol), NH4F (2 mmol) and CO(NH2)2 (5 mmol) were simultaneously transferred to the 100 mL autoclave, which was maintained for 5 h at 120 °C. The as-synthesized precursor was rinsed with distilled water, and calcined for 2 h in air at 500 °C to form the Mn2O3 nanosheets (denoted as Mn-NF). The clean Ni foam substrate was also calcined for 2 h at 500 °C, denoted as NF for comparison. The catalyst of the CeO2 nanoparticles-decorated Mn2O3 nanosheets (denoted as Ce/Mn-NF) was synthesized by an impregnation method. The as-prepared Mn-NF precursor was immersed into 0.01, 0.08, or 0.2 M Ce(NO3)3 ethanol solution for 1 h. After dried at 120 °C, the precursor was calcined for 2 h at 500 °C to form the final Ce/Mn-NF catalysts. For simplicity, the Ce/Mn-NF catalysts with the different Ce loading are denoted as Ce/Mn-NF-1, Ce/Mn-NF-2, and Ce/Mn-NF-3, as listed in Table S1. Detailed information on catalyst characterization, activity evaluation and isothermal kinetic measurement is provided in the Supporting Information.

RESULTS AND DISCUSSION Performance for Catalytic Soot Combustion Figure 1A and Table S2 display the catalytic activity results in O2/N2. For comparison, the blank experiment is provided, and its T10, T50, and CO2 selectivity are 8


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502 °C, 560 °C, and 54.5%, respectively. The Soot-TPO results demonstrate that all of the catalysts can improve soot combustion, and increase the CO2 selectivity to 100%. To investigate the catalytic activity of the Ni foam substrate, the NF is used as a catalyst for soot combustion, which decreases the ignition temperature of T10 to 470 °C compared with the blank experiment. Due to the intrinsic redox property of manganese oxide, the catalytic activity of Mn-NF is remarkably enhanced. The catalytic activities of the catalysts with the different Ce loading obey the order of Ce/Mn-NF-2 > Ce/Mn-NF-3 > Ce/Mn-NF-1 > Mn-NF, indicating that there exists an optimal Ce loading for the Ce/Mn-NF catalysts. The introduction of Ce to the Mn-NF can improve the catalytic activity, especially for the Ce/Mn-NF-2, who has the lowest T10 (378 °C) and T50 (446 °C) among the catalysts. Figure 1B and Table S2 display the catalytic activity results in NO/O2/N2. The Ce/Mn-NF-2 catalyst still exhibits the lowest T10 (324 °C) and T50 (385 °C) among the catalysts. Its corresponding T10 and T50 in NO/O2/N2 drop by 54 and 61 °C, respectively, compared with those in O2/N2. Hence, NOx can act as a more efficient mobile oxidizing agent than O2, lowering the light-off temperature for soot combustion.45 We can deduce the existence of two reaction paths during soot combustion, that is, the direct soot oxidation and NOx-assisted soot oxidation.46 Furthermore, we compared the soot combustion activity of the Ce/Mn-NF-2 catalyst with other recently reported catalysts, as listed in Table S3. The as-prepared Ce/Mn-NF-2 catalyst shows the superior catalytic performance. 9



Figure 1. Catalytic soot oxidation activities of the catalysts: (A) in O2/N2, and (B) in NO/O2/N2.

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Structural Properties

Figure 2. XRD full patterns (A) and zoom-in patterns (B) of (a) NF, (b) Mn-NF, (c) Ce/Mn-NF-1, (d) Ce/Mn-NF-2, and (e) Ce/Mn-NF-3. Figure 2 displays the XRD patterns of the NF, Mn-NF, and Ce/Mn-NF. Three diffraction peaks are observed for the reference catalyst NF at 44.5, 51.8 and 76.3 belonging to metallic Ni (JPCDS 04-0850). Meanwhile, there exist five diffraction peaks at 37.3, 43.2, 62.7, 75.3 and 79.3 belonging to NiO phase (JPCDS 47-1049). The Ni and NiO phases exist for all of the catalysts. For the Mn-NF and Ce/Mn-NF catalysts, there are two diffraction peaks at 33.0 and 55.2 belonging to Mn2O3 (JCPDS 41-1442), and three diffraction peaks at 28.5, 47.5, and 56.3 belonging to CeO2 (JCPDS 43-1002). There is little shift of the diffraction peaks belonging to the Mn2O3 and CeO2 phases for the Ce/Mn-NF catalysts. Figure S1 shows the Raman spectra of the catalysts. Two Raman peaks are observed at 480 and 585 cm-1 belonging to the NiO phase for the 11



Mn-NF and Ce/Mn-NF catalysts. For the Ce/Mn-NF catalysts, the Raman peak at 465 cm-1 arises from the F2g vibration mode of fluorite-structured CeO2, whose intensity becomes stronger with the increased Ce loading. Additionally, there exists a broad Raman peak at ~636 cm-1, corresponding to the Mn2O3 phase for the Mn-NF and Ce/Mn-NF catalysts.47-50 The Raman analysis confirms the existence of the Mn2O3 and CeO2 phases, which is consistent with the XRD results. Figure 3, S2 and S3 display the SEM and TEM images of the NF, Mn-NF, and Ce/Mn-NF. From Figure S2a and b, there exists the 3D macroporous architecture for the Ni foam substrate. Figure 3a shows that the NF exhibits uneven wrinkles grown on Ni foam, and porous voids on its surface are too small to capture soot particulates. Figure 3b shows that the crossed Mn2O3 nanosheets are well-distributed and partitioned to form macroporous voids on the substrate for the Mn-NF. Figure 3f shows that the thickness of Mn2O3 nanosheets is 20.3 nm. Figure 3c, d and e show that the macropores created by the crossed nanosheets retain well after loading CeO2. The higher CeO2 loading causes the larger macroporous voids created by the Mn2O3 nanosheets, indicating the decreased number of nanosheets per unit area. It can also be deduced from the decrease of the Mn content in the Ce/Mn-NF catalysts compared with the Mn-NF, as listed in Table S1. Thus, the excessive Ce loading is probably undesirable.

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Figure 3. SEM images of (a) NF, (b) Mn-NF, (c) Ce/Mn-NF-1, (d) Ce/Mn-NF-2, and (e) Ce/Mn-NF-3; and (f) thickness distribution of the Mn2O3 nanosheets in (b). From the HRTEM images in Figure S3, the Mn2O3 phase with the lattice plane (222), and the CeO2 phase with the lattice plane (111) and (220) are observed, which are well correlated with the XRD results. From the TEM images, only few CeO2 nanoparticles can be observed on the Mn2O3 nanosheets for the Ce/Mn-NF-1. With the increased Ce content for the Ce/Mn-NF-2, the more CeO2 nanoparticles are deposited on the Mn2O3 nanosheets. For the Ce/Mn-NF-3, the CeO2 nanoparticles aggregate and almost completely cover the Mn2O3 nanosheets.

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Figure 4. SEM-EDS elemental mapping images of the Ce/Mn-NF-2 (a), and SEM images of the mixture of the Ce/Mn-NF-2 and soot in the mass ratio of 20:1 (b) and 10:1 (c). Basing on the SEM and TEM results, we can deduce that the Ce loading for the Ce/Mn-NF-2 is moderate. The SEM-EDS elemental mapping in Figure 4a indicates that the CeO2 nanoparticles are homogeneously distributed on the Mn2O3 nanosheets for the Ce/Mn-NF-2. Figure S2c and d show the SEM images of the Ce/Mn-NF-2 in the different magnifications. The crossed nanosheets are uniformly grown on the framework of the substrate. Moreover, the actual contact state between the macroporous catalyst and soot was characterized. Figure 4b and c show the SEM images of the mixture of the Ce/Mn-NF-2 and soot at the different mass ratios. Apparently, soot particulates can readily enter the macroporous voids created by the crossed nanosheets, which greatly 14


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increases the interaction chances between the captured soot and the active sites on the external surface of catalysts. Therefore, the excellent catalytic performance for the Ce/Mn-NF-2 may be attributed to both the macroporous nano-structures and the moderate Ce loading on the Mn2O3 nanosheets. Redox Behaviors

Figure 5. H2-TPR profiles of (a) NF, (b) Mn-NF, (c) Ce/Mn-NF-1, (d) Ce/Mn-NF-2, and (e) Ce/Mn-NF-3. Figure 5 shows the H2-TPR curves to examine the effect of the Ce addition on the redox property of the Mn-NF. For the NF, there are two peaks locating at 287 and 341 °C belonging to the reduction of the surface NiO layer and bulk NiO, respectively.51 After the growth of the Mn2O3 nanosheets on Ni foam, the two peaks belonging to NiO 15



apparently shift to the lower temperatures locating at 242 and 311 °C. It indicates that there is an interaction between Mn2O3 and NiO. For the Mn-NF, the broad reduction peak appearing at 457 °C is mainly attributed to the successive reduction of Mn2O3 to Mn3O4, and then Mn3O4 to MnO. After introduction of Ce to the Mn-NF, no single reduction peak belonging to CeO2 is observed for the Ce-containing catalysts. Probably, the presence of Mn cations induces the reduction of Ce4+ cations, and thus the reduction of ceria is accompanied by the reduction of manganese oxide for the Ce/Mn-NF catalysts.52 Additionally, the broad reduction peak belonging to Mn2O3 remarkably shifts to the lower temperature after introducing Ce, suggesting the improved reducibility of the Ce/Mn-NF catalysts. The above results indicate that there is an interaction between Mn2O3 and CeO2 on the Ce/Mn-NF. The introduction of CeO2 improves the reducibility of the Ce/Mn-NF catalysts due to the Mn-Ce interaction. For the Ce/Mn-NF-2, the appropriate Ce loading makes the CeO2 nanoparticles be more evenly distributed on the Mn2O3 nanosheets, thus creating the more Mn-Ce interaction sites. Therefore, the Ce/Mn-NF-2 exhibits the enhanced reducibility among the catalysts, resulting in the excellent catalytic performance for soot combustion. Figure 6 shows the Soot-TPR curves for the catalysts. As soot combustion in nature is a deep oxidation reaction, the chemical property and quantity of surface active oxygen of the catalysts play the important roles. In the Soot-TPR curves, there are three main temperature ranges, which correspond to O2− (200-400 °C), O22− (400-700 °C) and O2− (> 700 °C).53-55 The lattice oxygen (O2−) shows the poor reactivity for soot oxidation, due to 16


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the higher reduction temperature. Considering the reaction temperature range of 200-500 °C in this work, the surface adsorbed oxygen (O2− and O22−) should be the main active oxygen species for catalytic soot oxidation. After loading CeO2, the reduction peaks of the surface adsorbed oxygen shift to lower temperatures, especially for the Ce/Mn-NF-2. It indicates that the superficial Mn-Ce interaction activates the adsorbed oxygen. Furthermore, by integrating and comparing the peaks from 200 to 700 °C in Figure 6, the Ce/Mn-NF-2 has the larger peak area of surface adsorbed oxygen species in Table S4, indicating that it owns the larger amount of surface adsorbed oxygen than other catalysts.

Figure 6. Soot-TPR profiles of (a) Mn-NF, (b) Ce/Mn-NF-1, (c) Ce/Mn-NF-2, and (d) Ce/Mn-NF-3. 17



Superficial Chemical States

Figure 7. (A) XPS Ce 3d spectra of (a) Ce/Mn-NF-1, (b) Ce/Mn-NF-2, and (c) Ce/Mn-NF-3; (B) XPS Mn 2p and (C) O 1s spectra of (a) Mn-NF, (b) Ce/Mn-NF-1, (c) Ce/Mn-NF-2, and (d) Ce/Mn-NF-3. XPS measurement was conducted to probe the superficial chemical states of the Mn, Ce and O elements on the catalysts. Figure 7A displays the complex Ce 3d XPS spectra, 18


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which are divided into eight peaks. The symbols of u and v represent the spin-orbit coupling of Ce 3d3/2 and Ce 3d5/2, respectively. The peaks labeled as u (900.5 eV), u″ (907.2 eV), u‴ (916.2 eV), v (881.8 eV), v″ (888.6 eV), and v‴ (897.8 eV) are assigned to the 3d104f0 electronic state of Ce4+ cations. The peaks labeled as u′ (903.6 eV) and v′ (884.6 eV) are assigned to the 3d104f1 electronic state of Ce3+ cations. The results demonstrate that Ce3+ and Ce4+ coexist on the surface of the Ce/Mn-NF catalysts. As given in Table 1, the Ce3+ content in the Ce/Mn-NF-2 is higher than other Ce/Mn-NF catalysts. The presence of Ce3+ on the catalysts results in a charge imbalance, which will generate surface oxygen vacancies.50,56 Thus, the Ce/Mn-NF-2 probably contains the more surface oxygen vacancies. Figure 7B shows the Mn 2p XPS spectra of the catalysts, which are decomposed into six peaks. For the Mn 2p1/2 spectra, the two peaks at ∼653.3 and ∼654.6 eV correspond to superficial Mn3+ and Mn4+ species, respectively, while the two peaks at ∼641.9 and ∼643.7 eV correspond to superficial Mn3+ and Mn4+ in the Mn 2p3/2 spectra, respectively.57 The other two peaks at ∼646.0 and ∼656.2 eV are attributed to the satellite peaks. The chemical state of the superficial Mn species on the catalysts is the mixture of Mn3+ and Mn4+. The multiplet splitting of Mn 3s peaks in Mn 3s spectra is often utilized to determine chemical state of Mn species.58 Figure S4 and Table S5 show the splitting width of the Mn 3s peaks, which is sensitive to the chemical state of the superficial Mn species with the smaller ΔE values for the higher chemical states.59,60 We observed the coexistence of Mn3+ and Mn4+ on the surface of the catalysts, which is in 19



accordance with the results of the Mn 2p spectra. Table 1 shows that the Mn4+ content of the catalysts with the different Ce loading obeys the order of Ce/Mn-NF-2 > Ce/Mn-NF-3 > Ce/Mn-NF-1 > Mn-NF, which is basically correlated with the results of the activity measurements. After the CeO2 nanoparticles are supported on the Mn-NF, the Mn-Ce interaction leads to the generation of more Mn4+. The Ce/Mn-NF-2 with the optimal Ce loading possesses more Mn4+ than other catalysts, which is also inferred from the variation of the Mn 3s splitting width in Table S5. The manganese species with the higher chemical state are preferable for the oxidation reactions.61 Therefore, the catalytic activities of the Ce/Mn-NF catalysts can be enhanced through the enrichment of superficial Mn4+ on the catalysts. Figure 7C and S5 show the O 1s XPS spectra, which are decomposed into three peaks locating at ~529.0, ~530.8 and ~532.3 eV, which correspond to the surface lattice oxygen (O2-), and surface adsorbed oxygen (O22- and O2-), respectively.62 The relative content of the surface adsorbed oxygen is calculated, as listed in Table 1. It follows the order of Ce/Mn-NF-2 > Ce/Mn-NF-3 > Ce/Mn-NF-1 > Mn-NF > NF, which is correlated with the activity sequence of the catalysts. The above results also prove that the surface adsorbed oxygen is active for catalytic soot combustion, which is consistent with the Soot-TPR results.

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Table 1 Surface compositions and chemical states of the Mn, Ce and O elements determined by XPS in Figure 7 and S5. Catalysts

Ce/(Mn+Ce) (%)

Mn4+/(Mn3++Mn4+) (%)

Ce3+/(Ce4++ Ce3+) (%)

Oads/(Olatt+Oads) (%)

NF

-

-

-

29.6

Mn-NF

0

29.4

-

33.6

Ce/Mn-NF-1

22.4

32.5

13.7

34.5

Ce/Mn-NF-2

44.0

37.3

19.7

39.2

Ce/Mn-NF-3

68.2

33.9

17.6

36.9

According to the above results of XPS, it indicates that there exists the Ce3+/Ce4+ and Mn3+/Mn4+ redox couples on the surface of the Ce/Mn-NF catalysts. With the addition of Ce, the Mn-Ce interaction results in the formation of more Mn4+ and Ce3+ therein, demonstrating the existence of the superficial redox process presented as Ce4+ + Mn3+  Mn4+ + Ce3+. This redox process can induce the generation of the active oxygen species.61 Table 1 gives the surface atomic ratio of the Ce and Mn elements. For the Ce/Mn-NF-2, its surface Ce/Mn atomic ratio is close to 1:1, which indicates the presence of more Mn-Ce interaction sites than other Ce/Mn-NF catalysts. Accordingly, the Ce/Mn-NF-2 can generate more surface active oxygen for catalytic soot combustion.

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Effect of NOx on Soot Combustion

Figure 8. Concentration of NO2 and CO2 for the (A) Mn-NF and (B) Ce/Mn-NF-2 catalysts or the mixture of the catalyst and soot in NO/O2/N2. To illustrate the role of NO2 during soot combustion, the NO-TPO experiments were performed in the presence or absence of soot in NO/O2/N2. Figure 8 shows the NO-TPO profiles of the Mn-NF and Ce/Mn-NF-2 catalysts. The peak temperature of the NO2 concentration profile of the Ce/Mn-NF-2 shifts to the lower temperature by ~30 °C 22


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compared with the Mn-NF. It indicates that the Ce/Mn-NF-2 has the higher activity for NO oxidation to NO2. The difference on the NO2 concentrations upon the catalyst and the mixture of the catalyst and soot is defined as the reactive NO2.54 For the Mn-NF and Ce/Mn-NF-2, the reactive NO2 signals have the similar tendency to the CO2 signals during soot oxidation. The soot oxidation process begins with the increase of the reactive NO2 concentrations. However, when the NO2 concentrations reach the maximum, the rate of soot combustion continuously increases, but does not reach the maximum value. This can be interpreted as follows: (1) in the lower temperature region (< 350 °C), gaseous NO2 derived from NO oxidation can directly react with soot particulates without any obstacles. Meanwhile, the surface adsorbed oxygen is inactive for soot oxidation; (2) the increase of the reaction temperatures can gradually activate surface adsorbed oxygen. Accordingly, in the higher temperature region (> 350 °C), both NO2 and active oxygen can take part in soot oxidation. All of the above findings verify the existence of the two reaction paths for soot combustion, as proposed in the results of Soot-TPO in Figure 1. Kinetic Study Figure S6 illustrates the isothermal anaerobic titration process over the Mn-NF and Ce/Mn-NF catalysts. Table 2 provides the quantified data about the reaction rate, active oxygen amount (O*), and TOF for the Mn-NF and Ce/Mn-NF catalysts during catalytic soot combustion, according to the results in Figure S6.63 Because of the Mn-Ce interaction, the Ce/Mn-NF catalysts can generate more active oxygen species than the Mn-NF, especially for the Ce/Mn-NF-2, as evidenced by the quantity of the active 23



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oxygen in Table 2. The TOF values in Table 2 are in the sequence of Ce/Mn-NF-2 > Ce/Mn-NF-3 > Ce/Mn-NF-1 > Mn-NF, which is correlated with the catalytic activity results. The TOF values indicate that the surface adsorbed oxygen species induced by the Mn-Ce interaction are more active than those on the Mn-NF, which are consistent with the H2-TPR and Soot-TPR results. Therefore, the Ce/Mn-NF-2 with the largest amount of the Mn-Ce interaction sites has the highest intrinsic activity. Table 2 Reaction rates, active oxygen (O*) amounts, and TOF values on the basis of the O* sites over the Mn-NF and Ce/Mn-NF catalysts at 320 °C for soot combustion. Catalysts

Rate (mol g-1 s-110-7)

O* amount (mol g-110-4)

TOF (s-110-3)

Mn-NF

3.2

1.4

2.3

Ce/Mn-NF-1

5.8

2.3

2.5

Ce/Mn-NF-2

9.7

3.0

3.2

Ce/Mn-NF-3

7.8

2.7

2.9

The isothermal kinetic experiments for soot oxidation were conducted at the different reaction temperatures with the conversion of less than 12% for the Mn-NF and Ce/Mn-NF-2 catalysts. The apparent activation energy (Ea) is calculated from Arrhenius plots, as shown in Figure 9. The Ea for the Mn-NF catalyst is about 96.5 kJ mol-1, which decreases to 89.2 kJ mol-1 for the Ce/Mn-NF-2. The kinetic results prove that the introduction of Ce to the Mn-NF can lower the reaction energy barrier for soot oxidation,

24


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which improves the catalytic performance of the Ce/Mn-NF-2 catalyst for soot combustion.

Figure 9. Arrhenius plots of soot combustion over the Mn-NF (▲) and Ce/Mn-NF-2 (■) catalysts. The Catalytic Stability and Water Resistance In order to probe the catalytic stability of the Ce/Mn-NF-2, five consecutive TPO measurements were performed in NO/O2/N2. In Figure 10A, the Ce/Mn-NF-2 shows no apparent deactivation after five cycles of the TPO experiments. In addition, we characterized the spent Ce/Mn-NF-2 catalyst by XRD, SEM and XPS, as shown in Figure S7. The SEM and XRD results of the spent Ce/Mn-NF-2 catalyst have little difference with the fresh one, suggesting that the Ce/Mn-NF-2 has the excellent stability of structure and morphology. Additionally, the spent Ce/Mn-NF-2 catalyst has the similar chemical state of the Ni to the fresh catalyst, as evidenced by the XPS Ni2p spectra in Figure S7. For catalytic soot combustion, the surface adsorbed oxygen species are active to 25



determine the catalytic activity. From the XPS O1s spectra of the fresh and spent Ce/Mn-NF-2 catalysts in Figure S7, they have little difference, as well, demonstrating its high catalytic stability for soot combustion. It is thus evident that the Ce/Mn-NF-2 exhibits the excellent catalytic stability and structural stability, and is potentially to meet the requirement of practical applications.

26


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Figure 10. (A) Catalytic stability of the Ce/Mn-NF-2 catalyst in five Soot-TPO cycles in NO/O2/N2, and (B) Water resistance of the Ce/Mn-NF-2 catalyst during catalytic soot combustion. To probe the water resistance of the catalysts during soot combustion, two Soot-TPO processes over the Ce/Mn-NF-2 catalyst were performed in NO/O2/N2 and 10% H2O/NO/O2/N2. In Figure 10B, the Ce/Mn-NF-2 exhibits the lower T50 and unchanged CO2 selectivity in the presence of 10% H2O. That is, the H2O vapor is beneficial to catalytic soot combustion in conformity with the previous reports.64,65 Zhou et al. suggested that H2O vapor can reduce the activation energy of catalytic soot combustion reaction.64 Burachaloo et al. proposed that OH radicals formed from H2O molecule and O atom could promote the re-oxidization of oxygen vacancies, which greatly favors the catalytic soot combustion.65 Superficial Mn-Ce Interaction As soot combustion in nature is a deep oxidation reaction, the redox ability of the catalysts is the critical factor in determining the catalytic performance besides their structural property. Accordingly, the CeO2 nanoparticles with the excellent oxygen storage/release ability become a good choice to modify the Mn2O3 nanosheets to improve the redox property. After addition of CeO2 in the Mn-NF, the results of H2-TPR show the lowered reduction temperatures of Mn2O3, demonstrating the improved reducibility of the Ce/Mn-NF. It indicates that the interaction between Mn2O3 and CeO2 occurs at their interfaces. Furthermore, based on the analysis of Soot-TPR, the Ce loading promotes the 27



generation of surface adsorbed oxygen and lowers reduction temperatures. It suggests that the Mn-Ce interaction can activate the adsorbed oxygen species on the catalysts and also increase their quantity.

Scheme 2. Schematic diagram describing the Mn-Ce interaction on the surface of the Ce/Mn-NF catalysts. The Mn-Ce interaction on the Ce/Mn-NF catalysts generates a Ce4+ + Mn3+  Mn4+ + Ce3+ balance to simultaneously produce active oxygen species, as evidenced by the XPS results. Scheme 2 illustrates the superficial redox process over the Ce/Mn-NF catalysts. The low valent Mn3+ can give an electron to Ce4+, and thus the Mn4+ and Ce3+ are generated by the interaction between Mn3+ and Ce4+. When the Mn3+ is oxidized to the Mn4+, this reaction process induces the generation of the surface adsorbed oxygen due to the need for balancing the chemical states.66 Thus, the enrichment of superficial Mn4+ on the catalysts is preferable for soot combustion. Meanwhile, the oxygen vacancies are formed via the transformation of Ce4+ to Ce3+. Gaseous oxygen can easily adsorb onto the 28


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oxygen vacancies, enabling the generation of surface adsorbed oxygen, which are active during catalytic soot oxidation. The surface atomic ratio between Ce and Mn on the Ce/Mn-NF-2 is close to 1:1, which can promote the generation of surface active oxygen, inducing the high catalytic soot oxidation performance. CONCLUSIONS In summary, macroporous CeO2/Mn2O3 monolithic catalysts were successfully prepared by combination of the hydrothermal process and wet impregnation method. The synthesized macroporous nano-structures can not only facilitate the mass and heat transfer during soot combustion, but also provide more tangible sites for soot combustion. The Ce/Mn-NF catalysts, particularly the Ce/Mn-NF-2, exhibit the higher intrinsic activity (TOF) and lower activation energy (Ea), resulting in the higher catalytic performance for soot oxidation, than the Mn-NF. Combined with the results of H2-TPR, Soot-TPR and XPS, the excellent catalytic performance of the Ce/Mn-NF catalysts is attributed to the superficial Mn-Ce interaction of Ce4+ + Mn3+  Mn4+ + Ce3+ to form more surface adsorbed oxygen, as key active sites for soot oxidation. Accordingly, the Ce/Mn-NF-2 with the appropriate superficial Ce/Mn couple of Ce:Mn  1:1 exhibits the remarkable catalytic activity with the T10 and T50, as low as 324 and 385 ºC, respectively, for soot oxidation in presence of NO. Furthermore, the Ce/Mn-NF-2 catalyst shows the excellent thermal stability and water resistance in catalytic soot combustion. Our work provides a new insight into designing of highly active nanostructured mixed-metal-oxide catalysts for potentially practical applications. 29



ASSOCIATED CONTENT Supporting Information Catalyst characterization, activity evaluation and isothermal kinetic measurement; ICP-OES results and catalytic activity results; SEM images of the Ni foam and Ce/Mn-NF-2, TEM images and Raman spectra; XPS Mn 3s spectra and Mn 3s splitting width; XPS O 1s spectrum of the NF; isothermal anaerobic titration process; XRD pattern, SEM image and XPS spectrum of the spent or fresh Ce/Mn-NF-2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Xingang Li: 0000-0002-8226-9796 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 30


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This work was supported by the National Natural Science Foundation of China (21878213); the 973 program (2014CB932403); and the Program of Introducing Talents of Disciplines to China Universities (B06006).

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Three-Dimensionally

Ordered

Macroporous

Ce1-xZrxO2-Supported

Gold

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Abstract Graphic:

Synopsis: The high soot-catalyst contact efficiency and plentiful Mn-Ce interaction sites lead to the superior performance of the CeO2 nanoparticles-decorated Mn2O3 nanosheets for soot elimination.

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Decorating CeO 2 Nanoparticles on Mn 2 O 3 Nanosheets to Improve

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