Promoting diesel soot combustion efficiency over hierarchical brush

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Promoting Diesel Soot Combustion Efficiency over Hierarchical Brushlike α‑MnO2 and Co3O4 Nanoarrays by Improving Reaction Sites

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Geng Liu, Jiahuan Yu, Li Chen, Nengjie Feng,* Jie Meng, Fan Fang, Peng Zhao, Lei Wang, Hui Wan, and Guofeng Guan* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, P.R. China S Supporting Information *

ABSTRACT: Improving the accessible reaction sites of catalysts is vital to diesel soot elimination. Herein, hierarchical brushlike α-MnO2 and Co3O4 nanoarrays were in situ grown on AISI304 stainless steel wire-mesh via a two-step hydrothermal method. Morphology investigation displayed that compared with sole α-MnO2 or Co3O4 nanoarrays, α-MnO2 and Co3O4 nanoarrays provided 8-fold reaction sites. XRD, Raman spectroscopy, XPS, H2-TPR, and soot-TPR techniques proved the synergistic effect between cobalt and manganese, namely, weaker Mn−O bonds, more surface active oxygen species, and better redox ability. Kinetic data also showed that the activation energy was decreased, and the pre-exponential factor was increased. αMnO2 and Co3O4 nanoarrays displayed superior catalytic performance (T50 = 354 °C, T90 = 395 °C), durability, and isothermal regeneration activity. In a simulated diesel exhaust at 400 °C, 90% of soot would be eliminated at 12 min, and the regeneration would be finished within 30 min. Finally, the catalyst coating was tightly anchored on the substrate without exfoliation or crazing.

1. INTRODUCTION

both a tight contact and loose contact exist between which thermal regeneration (loose contact) accounts for 80% soot oxidation; yet catalytic regeneration (tight contact) is only 20%.6 Di Sarli and Di Benedetto investigated the soot oxidation pattern of powdered soot−catalyst mixtures using a 3D model.5 But within CDPFs, the contact between soot cake and catalyst coating is close to a 2D condition; namely, soot cake will be oxidized via a redox mechanism at the gas−solid− solid triphase boundary.6 In fact, the poor soot−catalyst contact condition is considered to be one of the main limiting factors for major reported monolithic catalysts, which refers to those monoliths coated with micro meter powdery cata-

Being the footstone of modern transportation business, diesels have been long condemned for releasing carcinogenic soot particulates. Diesel particulate filters (DPFs) are developed to trap soot, and the trapped soot can be eliminated by two methods.1,2 One is active or thermal regeneration needing extra energy since the exhaust temperature (170−500 °C) is too low for soot combustion (>550 °C). However, this regeneration is uncontrolled and will result in a very high temperature rise which may damage the DPFs.3 Another method is passive regeneration using catalyzed-DPFs (CDPFs) which can boost soot oxidation at much lower temperature.4 So, plenty of work on CDPFs has been done, and great progress has been achieved. During passive regeneration, the soot−catalyst contact condition is imperative.5 Di Sarli et al. reported that in the CDPFs coated with highly dispersed ceria, at high soot load, © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

April 20, 2019 June 22, 2019 July 10, 2019 July 10, 2019 DOI: 10.1021/acs.iecr.9b02155 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Scheme 1. Two-Step Hydrothermal Route Describes Preparation of Brushlike α-MnO2 and Co3O4 on AISI304 Wire Mesh

Herein, we in situ developed brushlike hierarchical α-MnO2 and Co3O4 nanoarrays on AISI304 stainless steel wire mesh via a two-step hydrothermal method. Metallic filters, which have been widely used for diesel exhaust purification, were selected because of high thermal conductivity, outstanding mechanical strength, and excellent resistance of alkali corrosion.26,44 The monolith decorated with α-MnO2and Co3O4 coating displayed excellent catalytic activity, hydrothermal stability, adhesion, and isothermal regeneration property. Compared with MnO2 or Co3O4 nanoarrays, brushlike α-MnO2 and Co3O4 nanocomposites provided almost 9-fold reaction sites. The synergistic effect between manganese and cobalt weakened M−O bonds, increased surface oxygen species, and improved oxidation ability. Finally, isothermal regeneration performance was evaluated, and 90% of soot could be abated within 12 min.

lysts.6−8 So, numerous nanocatalysts aimed at improving the contact conditions have been synthesized. Ceria catalysts with many different shapes (fibrous, star, cubic, etc.) have been prepared in Fino’s group.9,10 Reddy and his co-workers synthesized many nanosized transition metal-decorated ceria for efficient soot oxidation.11−13 Numerous three-dimensionally ordered macroporous (3DOM) catalysts with various contents have excellent performance that have been reported,14−18 although it remains a big challenge to facilely coat these powdery objects onto the filters with firm adhesion. Meng19,20 and Li2124 reported a lot of outstanding metallic monoliths and proposed new concepts such as self-capture contact mode and gravitational contact mode. These concepts deepen the understanding of soot−catalyst contacts. In recent work, we synthesized La1‑xKxFeO3‑δ nanotubes via electrospinning, and the contact chance was improved, too.25 Before eliminating soot particulates, the above-mentioned powdered catalysts have to be coated onto the monolithic filters. However, in order to achieve enough adherence against exfoliation, grinding, which may destroy the porous structure of the catalysts, is required.26−28 Moreover, coating routes have several drawbacks including binder usage, fewer accessible sites, coating crack or detachment, low catalysts utilization efficiency, etc.7,29,30 It is urgent to in situ synthesize the catalysts on the substrates. Ma and his co-workers fabricated 3DOM SiOC on cordierite, but the ordered structure was not well kept after coating the LaCoO3 catalyst.31 Tang et al. synthesized 3DOM LaCoO3 on γ-Al2O3-coated cordierite with a low exfoliation rate, yet the route was complicated.32 In contrast, recently, nanoarray-based monoliths have drawn extensive attention on environmental catalysis due to numerous uniform sites, high catalyst utilization efficiency, and better performance tunability.33,34 Mo et al. in situ fabricated metal-doped Co3O4 arrays on Ni foam and achieved outstanding CO oxidation ability.35 Cao et al. grew Co3O4 nanofibers onto 3D Ni foam; the developed macroporous structure of Ni foam and the space of Co3O4 extremely increased the reaction sites.22 Despite Co3O4 is effective for soot deep oxidation, its capability still requires improvement.5 Shang et al. doped Bi2O3 into Co3O4 to improve the oxygen formation ability and lattice oxygen reactivity, while K+ improved the surface oxygen activity.36,37 Jiang et al. found the surface sodium species on mesoporous Co3O4 could remarkably promote NO2-assisted soot oxidation.38 Xiong and co-workers synthesized Pd2+-substituted PdxCo3‑xO4 on 3DOM CeZrO2, where the Pd2+−Co3+ interaction could effectively activate O2 and NO.14 In our previous work, Co3O4 could accelerate the fabrication of α-MnO2 via a redox mechanism, α-MnO2 could catalyze soot oxidation at a much lower temperature.39 Manganese oxides display excellent performance on exhaust purification and soot oxidation.40−42 It will be an effective method to fabricate α-MnO2 and Co3O4 nanoarrays for soot oxidation. Moreover, the synergistic effect may generate lattice distortion, increase surface vacancies, and improve oxidation ability.43

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. 2.1.1. Synthetization of Co3O4 Nanofibers on Stainless Steel Wire Mesh. The first hydrothermal route in Scheme 1 describes in situ preparation of Co3O4 nanofibers on AISI304 stainless steel wire mesh (500 meshes, 9.0% Ni, 19.0% Cr).22 Briefly, a piece of substrate (4.5 × 8 cm2, 5.00 g) was ultrasonically treated in anhydrous ethanol and 2 M HCl for 10 min; then, it was washed in deionized water thoroughly. At the same time, 0.9312 g of Co(NO3)2·6H2O (AR, 3.2 mmol), 0.2368 g of NH4F (AR, 6.4 mmol), and 0.9600 g of CO(NH2)2 (AR, 16 mmol) were dissolved in 160 mL of deionized water. After stirring for 10 min, the solution was transferred into a Teflon-lined stainless steel autoclave with 200 mL capacity, and the cleaned wire mesh was inserted. The hydrothermal reaction was performed at 120 °C for 5 h. Afterward, the pinkish monolith was washed and calcined at 500 °C for 2 h with a heating rate of 1 °C/min. The product was denoted as CSS for brevity. 2.1.2. Synthetization of Brushlike α-MnO2 and Co3O4 Nanocomposites on Stainless Steel Wire Mesh. The second hydrothermal section in Scheme 1 displays decorating CSS with MnO2 nanorods. First, 1.2640 g of KMnO4 (AR, 8 mmol) was dissolved in deionized water (160 mL), and the solution was stirred for 10 min. After ultrasonic treatment for 5 min to remove surface impurities, the weighted CSS was inserted into the solution. This hydrothermal reaction was performed at 160 °C for 24 h. Afterward, the monolith was ultrasonically treated for 10 min and then calcined at 550 °C for 5 h with a heating rate of 1 °C/min. This sample was denoted as MCSS. 2.1.3. Preparation of α-MnO2 Nanorods on Stainless Steel Wire Mesh. α-MnO2 nanorods were grown on stainless steel wire mesh following the procedure of preparing MCSS by replacing CSS with a pure wire mesh substrate. This sample was denoted as MSS. 2.2. Catalysts Characterization. The monolithic structure was identified via X-ray diffraction (XRD) on a Smartlab TM 9 kW (Rigaku, Japan) operating at 40 kV and 100 mA using Cu Kα source of 0.15418 nm. The Debye−Scherrer Kλ equation D = B cos θ was used to calculate the subgrain size.

(

B

)

DOI: 10.1021/acs.iecr.9b02155 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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less than 15%. The evaluation was conducted on a vertical fixed reactor equipped with a cylindrical quartz tube (i.d. = 18 mm, length = 52 cm), where the monolithic mixture was sandwiched in the center using silica wool, and a K-type thermocouple was inserted in the center of the catalyst bed to monitor the temperature. The catalyst bed was pretreated at 150 °C for 30 min in a high-purity N2 flow and then heated at a rate of 6 °C/min until 100% soot conversion in 20% O2/N2 and 500 ppm of NO + 15% O2/N2 gas. The gas rate was controlled at 100 mL/min by a mass flowmeter (space velocity was 8600 h−1). The compositions of the effluent gas were analyzed by an Infralyt 50 instrument. T10, T50, and T90 representing 10%, 50%, and 90% soot conversion, respectively, and Sm CO2 (CO2/(CO2+CO) × 100% at the temperature where soot combustion rate was the maximum) were taken as the performance indicators. 2.3.2. Moisture Effect. In order to study the effect of moisture, soot-TPO was further performed in 6% H2O + 15% O2/N2 and 500 ppm of NO + 6% H2O + 10% O2/N2 conditions. A column filled with spherical anhydrous CaCl2 was set in front of the infrared analyzer to dry the effluent gas. The signal delay on the drying column was 6 °C and was deducted in data processing. 2.3.3. Nitrates Identification. NO-TPO of MSS and MCSS was conducted in 500 ppm of NO + 15% O2/N2 after being treated at 200 °C for 30 min. NO2-TPD of MSS and MCSS was performed after adsorbing NOx at 350 °C until saturation, and then, half the samples were cooled to 100 °C followed by heating to 800 °C in 50 mL of N2, and another half was characterized by FT-IR. In order to directly investigate the activity of nitrates, MCSS was first exposed to 500 ppm of NO + 15% O2/N2 until saturation. Afterward, the sample was loaded with soot and heated in 50 mL of N2 until 600 °C. 2.3.4. Kinetic Study in Soot Combustion. In a more realistic inlet exhaust condition (10% O2/N2 + 500 ppm of NO + 6% H2O), the apparent activation energy Ea and pre-exponential factor A of soot combustion were calculated by the Coats− Redfern (CR) method, which could directly provide kinetic parameters from a single heating rate experiment. Ea and A can be obtained by eq 2)46−48

The coating phase was further characterized by Raman scattering spectroscopy on a microconfocal Raman system (LabRam HR, HORIBA) equipped with an Ar−Kr laser. The exciting source is 514.5 nm, and the spectroscopy is calibrated using a silicon pellet (520 cm−1) in advance. The bond force constant (k, N·m−1) of Mn−O was calculated by Hooke’s law45

ν=

1 2π c

k μ

(1)

where ν is Raman shift (cm−1), c is light velocity, and μ is reduced atomic mass. Fourier transform infrared spectroscopy (FT-IR) was conducted on a WQF-510A instrument (Beijing Beifen-Ruili Analytical Instrument Co., Ltd.). Each active coating was scraped from the monolith and mixed with KBr followed by drying at 110 °C for 30 min. The spectrum was recorded for 32 scans ranging from 400 to 4000 cm−1 at a resolution of 4 cm−1. Morphology of the monoliths was observed on a scanning electron microscope (SEM, Hitachi S-4800, Japan) at an acceleration voltage of 5 kV. Energy dispersive spectroscopy (EDS) was also recorded with a spot size of 100 μm × 100 μm. For each sample, at least two different spots were tested, and the average content was calculated. Furthermore, the coatings were extracted and analyzed on a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30 S-TWIN) with an acceleration voltage of 300 kV. The bulk contents were analyzed by TEM-EDS. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250X (ThermoFisher Scientific, USA) equipped with Al Kα (1486.6 eV) radiation at a pressure below 5 × 10−10 mbar. The pass energy for a full spectrum scan and single element scan is 150.0 and 30.0 eV, respectively. All peaks were calibrated by a C 1s peak (284.80 eV), and the spectrum was fitted using the XPSPEAK41 program by curve fitting with the Lorentz−Gaussian function (L/G approximates 20%) after subtracting Shirley-type backgrounds. H2-TPR was conducted on a chemical adsorption analyzer (Micromeritics Auto Chem II 2920). Then, 50.0 mg of catalyst was pretreated under an Ar atmosphere for 2 h. After cooling to 100 °C, a flow of 10% H2/Ar (50 mL/min) was inlet with the temperature ramped to 700 °C at a rate of 10 °C/min. H2 consumption was recorded online by a TCD and quantified by a standard CuO sample. For comparison, H2-TPR of calcined AISI304 substrate was also recorded. Soot-TPR was performed to investigate the intrinsic oxidation capability of the monoliths directly. Briefly, 5.000 g of soot/catalyst mixture was pretreated at 200 °C for 30 min in a high-purity N2 flow and then heated to 800 °C at a rate of 5 °C/min in a flow of 30 mL/min N2. CO2 and CO in the outlet gas were analyzed by a nondispersive infrared analyzer (Infralyt 50, Germany). 2.3. Activity Evaluations. 2.3.1. Soot TemperatureProgrammed-Oxidation. A loose contact mode representing a real contact condition in DPFs was used in this work. Printex U soot from Degussa was used as a model soot. The mean particulate size is 55 nm (Figure S1a,b). A certain volume of soot/methanol dispersion (2600 ppm) was dropped on the catalysts (about 3.00 g). The amount of soot loaded on the catalysts was quantified by integrating the effluent COx area, and the soot/catalyst ratio was around 1:2.5 with a deviation

ÄÅ É ÅÅ −ln(1 − α) ÑÑÑ E AR ijj 2RT yzz lnÅÅÅ ÑÑÑ = ln[ jj1 − z] − a 2 ÅÅÇ ÑÑÖ β Ea jk Ea zz{ RT T

(2)

where α is soot conversion; T is the corresponding absolute temperature, K; A is pre-exponential factor; Ea is apparent activation energy, J/mol; β is heating rate (6 °C/min), and R is 8.314 J·mol−1·K−1. 2.3.5. Isothermal Catalytic Regeneration of Fresh MCSS. The isothermal regeneration capability of MCSS was evaluated under 500 ppm of NO + 6% H2O + 10% O2/N2 at 400 °C. Though the balance point temperature (BPT) of the inlet gas is used for inspecting applied CDPFs,4 the temperature value here is able to evaluate the regeneration property of the monolith in a short time. The soot−catalyst mixture was heated in a high-purity N2 flow until 400 °C. After being stabilized at 400 °C, the regeneration gas was inlet, and the regeneration was ignited. It should be noted that the temperature rise during the whole process was smaller than 3 °C. The “t90”, which represents 90% of soot conversion, is calculated based on the conversion curve. The regeneration C


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Industrial & Engineering Chemistry Research was continued until soot was totally oxidized, and this evaluation was repeated 10 times. 2.3.6. Isothermal Catalytic Regeneration of Aged MCSS. The hydrothermal stability of MCSS was also investigated. The specimen was treated at 500 °C in 6% H2O + 10% O2/N2 influent gas at the rate of 200 mL/min. Afterward, the aged sample was evaluated by isothermal regeneration 10 times, too. 2.3.7. Adhesion Evaluation. The adhesion between MnO2 and Co3O4 coating and substrate was investigated by an ultrasonic oscillation test in a water bath at the frequency of 40 kHz and power of 150 W for 60 min. After drying and loading soot, isothermal regeneration was also conducted.

3. RESULTS AND DISCUSSION 3.1. Phase Identification. Figure 1 displays XRD patterns of the structured monoliths, where the peaks at 43.58 (111), Figure 2. Raman spectra of AISI304 substrate (a), CSS (b), MSS (c), and MCSS (d).

spinel phase 49. MSS also displays seven characteristic peaks at 183, 281, 391, 466, 512, 584, and 635 cm−1, among which the peaks at 584 and 635 cm−1 indicate well-developed 2 × 2 tunnels of α-MnO2.50 The peak at 584 cm−1 is from the symmetric Mn−O vibration along the c-axis, while the peak at 635 cm−1 is from the symmetric Mn−O vibration perpendicular to the c-axis. Due to partial peak overlapping between MnO2 and Co3O4, the strong peak of Co3O4 at 685 cm−1 is chosen as its existence in MCSS. However, in MCSS, this peak disappears, with only the peaks of α-MnO2 observed. This means that Co3O4 is totally covered by MnO2, and the spectrum is not interfered by Co3O4. In addition, the two main peaks of MnO2 red shift to 574 and 627 cm−1 and become wider. According to eq 1,45 the bond force constant (k), which reflects the strength of Mn−O, is 300.6 (MSS) and 286.7 N· m−1 (MCSS). The synergistic effect between Co and Mn will account for smaller k, namely, a weaker Mn−O bond. Hence, a higher activity of surface oxygen species on MCSS can be deduced. FT-IR spectroscopy is utilized to study the phase compositions and surface species of the coatings. In Figure 3, MSS features the characteristic stretching vibrations at 721 (ν7), 547 (ν6), and 478 cm−1 (ν5), while CSS gives very strong

Figure 1. XRD patterns of MSS, CSS, MCSS, and powdered MnO2 and Co3O4.

50.79 (200), and 74.70° (220) correspond to the AISI304 substrate (JCPDS #33-0397, Fm3̅m). For CSS, the spinel Co3O4 phase (JCPDS #43-1003, Fd3̅m) can be identified with diffraction peaks at 19.00 (111), 31.27 (220), 36.84 (311), and 59.35° (511). α-MnO2 (JCPDS #44-0141, I4/m) with peaks at 12.72 (110), 18.11 (200), 28.82 (310), 37.47 (211), and 49.86° (411) is the coating phase of MSS. Both α-MnO2 and Co3O4 can be seen on MCSS. It should be noticed that the main peaks of α-MnO2 in MCSS shift from 28.84° and 37.47° to 28.61° and 37.39°, implying that cobalt ions (rCo3+ = 61 pm, rCo2+ = 74 pm) have been doped into the α-MnO2 matrix and substituted smaller Mn4+ ions (rMn4+ = 53 pm64 pm),39 in accordance with EDS results. This phenomenon is also observed on powdered α-MnO2 and Co3O4 mixtures, where the two main peaks of MnO2 shift to 28.62° and 37.40°. Moreover, based on two nonoverlapping peaks at 28.84° and 31.27°, the calculated subgrain sizes of MnO2 (35 nm) and Co3O4 (53 nm) of MCSS are smaller than those of MSS (39 nm) and CSS (56 nm), implying better distribution of MnO2 in MCSS. The coating phase was further investigated by surfacesensitive Raman spectroscopy, as Figure 2 displays. The AISI304 substrate is Raman-inactive, and no peak appears. CSS shows five characteristic peaks at 191 (F2g, Co2+-O), 476 (Eg, Co2+-O), 518 (F22g, Co3+-O), 614 (F23g, Co3+-O), and 685 cm−1 (A1g, Co3+-O), in good agreement with the reported Co3O4

Figure 3. FT-IR spectra of the catalyst coatings. D


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Industrial & Engineering Chemistry Research stretching vibrations at 668 (Co2+-O) and 570 cm−1 (Co3+O).49,50 MCSS shows both the absorption bands of MnO2 and Co3O4. On the other hand, the hydrophilic and hydrophobic properties are critical to the stability of potassium-contained catalysts.22,37,51 Moisture will also affect soot oxidation over monoliths.52 Two absorption bands at 3450 and 1643 cm−1 can be ascribed to physisorbed H2O, while the absorption band at 1384 cm−1 corresponds to chemisorbed hydroxyls. OH strongly chemisorbed on MnO2 may occupy oxygen vacancies and inhabit O2 activation.53 On the contrary, physisorbed H2O can attack the soot surface via the following reaction: H2O + C → CO + H2.41 The I1384/I1643 ratio, which can reflect hydrophilicity and hydrophobicity, is calculated. I1384/I1643 in MSS is 3.28 but is only 0.46 and 0.54 in CSS and MCSS, respectively. Improved hydrophobicity will inhibit K+ evaporation and maintain stable activity.42 3.2. Morphology Studies. As Figure S1c reveals that the surface of pure AISI304 wire mesh is quite smooth, which may be difficult to capture soot particulates or to be coated with powdered catalysts. However, after in situ growing catalysts, a uniformly hierarchical architecture can be seen in Figure 4. The minor impurity marked in yellow circles (Figure 4a, e, g) is MnO2 aggregation originating from KMnO4 decomposition.39 4KMNO4 + 2H 2O → 4MnO2 + 4KOH + 3O2

integrated layer of vertical α-MnO2 nanorods can be observed on MSS. The mean width and length of the nanorods are 70 nm and 1.3 μm, respectively. The wide space among the nanorods will provide sites for soot, but the effective sites are limited with a high soot load. The top (Figure 4c, d) and side views (Figure S1d) show that the Co3O4 nanofibers on CSS are 120 nm wide and 11 μm long, respectively. It is noticeable that Co3O4 nanofibers are made of many submicrometer grains, and the cohesive force between these grains is not very strong.22,33 Fluffy Co3O4 nanofibers will surely provide more sites for soot than MSS. After reacting with the KMnO4 solution, the Co3O4 nanofibers are enwrapped by sheetlike birnessite (Figure 4d−f). The average size of the single metal wire increases to 112 μm, and the surface becomes rougher. The size of a birnessite and Co3O4 compound also increases from 120 nm to 1.2 μm. After calcination, the metastable birnessite nanosheets transform to α-MnO2 nanorods. It is noticeable that MnO2 nanorods here are tightly anchored on Co3O4 nanofibers, rather than form an unordered mixture with Co3O4 (Figure S6). The phase transformation is not complete, and minor birnessite nanosheets are left underneath MnO2 nanorods. The nanosheets may wrap Co3O4 and improve the adhesion force between MnO2 nanorods and Co3O4 nanofibers against exfoliation. Figure 4g−h shows that the size of a single metal wire becomes 120 μm, and the surface roughness further increases. Higher roughness is proved to capture soot particulates effectively. α-MnO2 and Co3O4 are upright and brushlike, and the mean size is 2.3 μm. The Co3O4 nanofiber can be considered a trunk, while the α-MnO2 nanorod is a branch. Compared with MSS or CSS, the contact chance between soot and brushlike MCSS is sharply improved (Figure S2). In addition to 500 mesh AISI304 wire mesh, Figure S3 illustrates that brushlike α-MnO2 and Co3O4 composites can be synthesized on various substrates. Moreover, Figure S4 demonstrates that the hydrothermal reaction time will affect the aspect ratio and K/Mn ratio. A higher aspect ratio and K/ Mn ratio will surely accelerate soot oxidation.22 Figure 5 displays TEM images of the coatings peeled off from the monoliths. Co3O4 nanofibers were broken into short nanorods by ultrasonic oscillation, indicating poor cohesion force between the submicrosized particles. Figure 5b illustrates the (220) facets with a lattice distance of 2.85 Å.49 The (220) planes contain more Co2+ ions which are reported to be ineffective for an oxidation reaction but will be the active sites reacting with KMnO4.35,37 MnO2 nanorods in Figure 5c and d expose (200) facets and grow along the [001] direction. It is interesting that Figure 5e and f further confirm the intergrowth relationship between MnO2 nanorods and Co3O4 nanofibers via birnessite transition. The exposed facets of α-MnO2 and birnessite are (200) and (003) with lattice distances of 4.80 and 2.33 Å, respectively. In Figure 5g and j, K uniformly coexists with Mn, and a Co3O4 trunk is also well identified. This distinctly proves that MnO2 nanorods are successfully in situ grown on Co3O4 nanofibers. The red rectangular sections in Figure 5a, c, and e were further analyzed by EDS spectroscopy, with the results listed in Table 1. MnO2 nanorods of MCSS contain both K and Co species, which will promote the catalytic performance.43 3.3. Contact Chance Calculations. In order to quantificationally evaluate the contact chance between soot and catalyst, Meng and co-workers proposed a parameter Cc,19,54 which is defined as

(3)

As discussed below, soot oxidation over the monoliths is not affected by the aggregation remarkably. In Figure 4a and b, an

Figure 4. SEM of MSS (a, b), CSS (c, d), uncalcined MCSS (e, f), and MCSS (g, h). E


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For MSS, d = 70 nm, l = 1.3 μm, N = 56 μm−2, and Savailable = 21 μm2. For MCSS, the vertical height of a MnO2 and Co3O4 is 11 μm. The diameter of MnO2 and Co3O4 is 2.3 μm. The number of MnO2 nanorods anchored on a 1 μm long Co3O4 trunk is 210, and the number density of MnO2 and Co3O4 is 0.30 μm−2. N is calculated as 210 × 11 × 0.3 = 693 μm−2. Also, d = 67 nm, l = 1.0 μm, and Savailable = 190 μm2. The cross-section of Co3O4 is approximately round, Savailable is calculated as following: ij π d2 yz + π dlzzz × N Savailable = jjj k 4 {

(6) −2

where d = 120 nm, l = 10 μm, N = 7 μm , and Savailable = 26 μm2. Therefore, Cc values of MSS, CSS, and MCSS are 21, 26, and 190, respectivly. It can be clearly seen that Cc improves by almost 8-fold via constructing brushlike hierarchical MnO2 and Co3O4 composites. It should be noted that Cc might be not reasonable enough to evaluate the contact states between soot and catalysts because it ignores the important role of the pore structure for soot delivery. Zeolite is a typical example whose micropores are inaccessible for soot.55 Soot usually aggregates into particulates of several hundred nanometers,56,57 so there should be an optimized pore size balancing Cc and real accessible sites. Nevertheless, Cc of MCSS is still convincing because the space among adjacent α-MnO2 and Co3O4 brush is also considered in the calculation. In Figure S2, at a low soot/catalyst ratio (1/10), soot can well enter the wide space of CSS and MCSS, but it mainly deposits on the top of MSS. On the other hand, when the soot/catalyst ratio equals 1/2.5, flat soot cake can be identified on CSS and MSS, but the cake on MCSS is rugged since part of the soot has diffused into the space of adjacent brushlike MnO2 and Co3O4. Scheme 1 vividly describes that soot will transfer into the inner space and load on MnO2 branches. 3.4. Surface and Intrinsic Oxidation Properties. Surface element compositions and valence states of the structured monoliths were characterized by XPS technology. In Figure 6a, specific elements without impurity can be clearly identified. Table 1 shows that the surface concentration of cobalt in MCSS is only 1.0 at. %, which is lower than that of bulk content (2.3 at. %). Cobalt may only be doped into the bottom section of α-MnO2 nanorods. Compared with K2CO3supported catalysts,22,57,58 in Figure 6b, the binding energy (B.E.) of K 2p3/2 (291.9 eV) and 2p1/2 (294.7 eV) shifts to lower B.E. by 0.7 eV, in good agreement with hollandite MnO2.59 This indicates higher electron density around K+, which will be more beneficial for O2 activation.42,58,60 The weak peak at 288.2 eV corresponds to minor CO23 species. The O 1s spectra (Figure 6c) were deconvoluted into three species, namely, hydroxyls (Oh), surface active oxygen species O2− and O− (Os), and lattice oxygen species O2− (Ol).22,43 Due to different coordination environments, the B.E. of O 1s

Figure 5. TEM images of CSS (a, b), MSS (c, d), MCSS (e, f), and element mappings of MCSS (g, j).

Cc =

Savailable Stopview

(4)

where Savailable is the accessible surface area of the catalyst, and Stopview is the area of the substrate from the top view. We take a square area of 1 μm2 for example. Namely, Stopview equals 1 μm2. The number of nanorods per μm2 (N) rather than coverage rate is used here. Since the cross section of a MnO2 nanorod is square, Savailable of MSS is calculated as follows Savailable = (d2 + 4 × d × l) × N

(5)

Table 1. Coating Weights, Bulk (in brackets), Surface Contents, Apparent Ea, and Pre-Exponential Factor A of Monolithic Catalysts Coating weight (%)

K (%)

Mn (%)

Co (%)

O (%)

Ea (kJ·mol−1)

A

0.45 0.64 0.76

(7.8) 8.2  (5.3) 10.6

(22.1) 22.8  (20.2) 26.7

 (37.5) 38.8 (2.3) 1.0

(70.1) 69.0 (62.5) 61.2 (72.2) 61.7

106 109 99

6.7 × 106 2.0 × 106 4.9 × 107

F


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Figure 6. XPS spectra of the monoliths: (a) full spectra and (b) K 2p and C 1s, (c) O 1s, (d) Mn 2p, (e) Mn 3s, and (f) Co 2p spectra.

Table 2. Deconvoluted Binding Energy (eV) and Percentage (in brackets,%) of O 1s, Mn 2p, and Co 2p Spectra Catalyst

Oh

Os

Ol

Mn4+

Mn3+

Mn2+

Co3+/Co2+

CSS MSS MCSS

532.8 (7.0) 532.3 (8.7) 532.6 (1.9)

531.5 (18.1) 531.0 (19.3) 530.8 (31.2)

529.4 (74.9) 530.0 (72.0) 529.6 (67.0)

 643.7 (42.3) 643.5 (44.3)

 642.3 (41.4) 641.9 (51.2)

 641.6 (16.2) 640.4 (4.5)

1.72  

can be further confirmed by the identifiable shape of the top Mn 2p3/2 spectra, and three oxidation states can be observed.42,63 Table 2 shows that both MSS and MCSS possess almost the same Mn4+ of 42.3% and 44.3%. More Mn3+ (51.2%) can be observed on MCSS, while MSS has more Mn2+ (16.2%). Low-valence Mn2+ and Mn3+ will create oxygen vacancies which are vital to O2 activation forming O2− and O− species.43 The deconvoluted Co 2p spectra of CSS are illustrated in Figure 6f, where the spin−orbital splitting (SOS) of 15.1 eV coincides with the Co3O4 phase. The surface Co3+/Co2+ ratio of CSS compiled in Table 2 (1.72) is smaller than the theoretical value 2 in Co3O4. It is accepted that Co3+ is highly active in oxidation reactions, but Co2+ is inert.35,37,49 More Co2+ may lead to two results. First, during the hydrothermal reaction, Co2+ can react with MnO−4 with itself oxidized into

in MSS and MCSS shifts toward lower values compared with that of CSS. In keeping with FT-IR, the quantitative hydroxyl concentrations listed in Table 2 of MSS, CSS, and MCSS are 8.7%, 7.0%, and 1.9%, respectively, which again imply better hydrophobicity and hydrothermal stability of MCSS. In the middle temperature region (300−500 °C), hydroxyl seems to be ineffective for soot oxidation.61,62 Nevertheless, Os species including O− and O2− predominate catalytic performance.22,49 MCSS gives obviously more Os (31.2%) than MSS (19.3%) and CSS (18.1%), and we can infer better activity on MCSS. Figure 6d and e depicts Mn 3s and 2p spectra. Therein, the 3s spectra give the average oxidation states (AOSs), and the 2p spectra provide detailed information on valence states. AOSs are calculated by the Δ value between two splitted Mn 3s peaks following 9.856−1.126Δ.43 The AOSs of MSS and MCSS are 3.93 and 3.87, respectively. Moreover, MnO2 phase G


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Industrial & Engineering Chemistry Research Co3+, thus accelerating the formation of sheetlike birnessite.39 More Co2+ will also lead to worse soot oxidation performance.49 Figure 7 displays the H2-TPR patterns, where the calcined AISI304 substrate only features a weak peak corresponding to

Figure 7. H2-TPR patterns of the catalysts.

minor surface oxides (such as Fe2O3 and NiO) at 426 °C. CSS shows a peak at 330 °C ascribed to the reduction of Co3+ → Co2+ → Co0.64 Fe2O3/NiO will be reduced at higher temperature (465 and 525 °C) only when Co3O4 nanofibers have been reduced. Ranging from 100 to 400 °C, the H2 consumption is 4.08 mmol·g−1 (Table S2), which is very close to the theoretical value (4.14 mmol·g−1). According to our previous work and references,64−66 the two peaks at 250 and 360 °C of MSS are the reduction of α-MnO2 coating, and the peak at 450 °C is substrate reduction. By in situ growing αMnO2 nanorods on Co3O4 nanofibers, the dispersion of MnO2 is improved, so the reduction temperature is reduced to 240 and 315 °C.31 Likewise, only when MnO2 branches are totally reduced, the reduction of Co3O4 trunks will begin and peak at 400 °C. The substrate reduction pattern of MCSS is very like that of CSS, which validates the ascription. In Table S2, from 100 to 400 °C, the H2 consumption value of MCSS (7.81 mmol·g−1) is larger than the sum (5.20 mmol·g−1) of CSS and MSS, implying better oxidation ability of the MnO2 nanorods grown on Co3O4 nanofibers. Cobalt doped into MnO2 will generate lattice distortions and vacancies, thus benefiting lattice oxygen mobility and boosting reduction.43 The intrinsic oxidation ability of the catalysts was assessed by soot-TPR (Figure 8). In the absence of O2, soot can be only oxidized by the oxygen species from the catalysts. First, CSS shows a weak and wide peak centered at 620 °C corresponding to O− and O2− species.22,67 For MSS and MCSS, the generation of CO substituted for CO2 above 650 °C might be attributed to reduction of lattice oxygen species.57,68 The strong CO peak of MCSS clearly indicates increased mobility of lattice oxygen species. CO2 between 200 and 450 °C can be ascribed to surface O2− species, while CO2 between 450 and 620 °C corresponds to surface O−.66,69,70 Considering that the catalytic soot oxidation is finished before 650 °C, O2− and O− might be more important. Especially, between 350 and 500 °C, almost 2-fold CO2 is produced from MCSS than from MSS. These results show that the synergistic effect between Mn and Co may account for abundant surface oxygen species and weaker Mn−O bonds in MCSS, as also revealed by Raman and

Figure 8. CO2 and CO concentrations during Soot-TPR.

XPS analysis. In addition, 9-fold accessible reaction sites of MCSS should be also taken into account for its outstanding performance. 3.5. Catalytic Performance Evaluations. Before discussing the soot oxidation pattern over structured monoliths, we first tested pure soot combustion. As Figure S5 illustrates, both NO and H2O have little promotion effect. Even in 6% H2O + 500 ppm of NO + 10% O2/N2 condition, T10 arrives at 488 °C, and T50 is 561 °C, which is far beyond the temperature region of diesel exhaust (170−500 °C). NO improves CO2 selectivity via reaction: NO + CO → N2 + CO2,22 and H2O advances soot oxidation via the water−gas reaction (C + H2O → CO + H2).41 Based on the above results, soot oxidation over the monoliths was tested in 20% O2/N2 (Figure 9a). After growing catalyst coatings, soot combustion is accelerated with higher CO2 selectivity. In Table 3, T50 of CSS, MSS, and MCSS is 507, 469, and 417 °C. T50 of CSS is higher than the reported values due to a higher soot/catalyst ratio (1:2.5).22,49 Compared with Co3O4 nanofibers, although the MnO2 nanorods on MSS provide fewer reaction sites, the abundant K species will effectively tune the electron distribution states of soot and will activate O2, thus forming surface active oxygen species and achieving soot abatement at much lower temperature.58,60,71,72 Co3O4 has been reported to be active for CO oxidation, and the CO2 selectivity of MCSS is almost 100.0% compared with MSS (85%).35,73 Therefore, soot oxidation over MCSS can be described by eq 7-9 C + O−2 → CO2 −

C + O → CO −

CO + O → CO2 H

(7) (8) (9) DOI: 10.1021/acs.iecr.9b02155 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Soot oxidation patterns of three monolithic catalysts in 20% O2/N2 (a), 6% H2O + 15% O2/N2 (b), 500 ppm of NO + 15% O2/N2 (c), and 500 ppm of NO + 6% H2O + 10% O2/N2 (d).

Table 3. Indicators of Soot Oxidation over MSS, CSS, and MCSS Monoliths in a Series of Inlet Gasesa MSS T10(°C) T50(°C) T90(°C) Tm CO2(%)

CSS

MCSS

A

B

C

D

A

B

C

D

A

B

C

D

410 469 510 86.1

412 478 525 84.7

401 481 532 88.4

376 438 484 88

447 507 559 100

410 480 532 100

406 470 522 100

396 460 514 100

369 417 456 100

308 358 399 99.9

364 414 450 100

302 354 395 99.7

a

A, in 20% O2/N2; B, in 500 ppm of NO+15% O2/N2; C, in 6% H2O+ 10% O2/N2; D, in 500 ppm of NO + 6% H2O + 10% O2/N2.

combustion. NO3−-soot TPR (Figure 10d) shows that NO3− can oxidize soot to CO or CO2 from 280 °C, with itself reduced to NO2− or N2. CO will be reoxidized into CO2 on the catalysts’ surface. Finally, NO-soot TPO results of MSS and MCSS (Figure 10e, f) can well explain the differences between MSS and MCSS in 500 ppm of NO + 15% O2/N2. NO hardly transforms to NO2 or nitrate on MSS, so the temperature cannot be decreased. For MCSS, a small CO2 peak appears at 200 °C due to chemisorbed NOx species.22 Afterward CO2 and NO2 increase simultaneously from 280 °C, in good agreement with Figure 10d. This process can be described by eqs 10−15

After introducing 6% H2O (Figure 9b), T50 of CSS and MCSS decreases to 470 and 414 °C, respectively, but increases to 481 °C for MSS. H2O has been reported to accelerate soot oxidation via tuning the catalyst−soot interface and attacking the soot surface (H2O + C → CO + H2).41 Nevertheless, the chemisorbed hydroxyls on MSS may inhibit O2 activation and depress soot oxidation.53 Figure 9c displays the catalytic soot combustion patterns in 500 ppm of NO + 15% O2/N2. It is eye catching that T50 of CSS and MCSS decreases by 27 and 59 °C, respectively, yet increases by 11 °C for MSS. NO can advance soot combustion via forming highly active NO2 or nitrate/nitrite.22,67,74,75 For CSS, NO2 has been reported to be the active species.49 In order to explore the astonishing effect of NO on MCSS and MSS, NO-TPO was performed, as shown in Figure 10a. A peak of chemisorbed NOx can be observed at 200 °C as Cao et al. reported.22 However, no obvious NO2 is produced from MCSS between 300 and 400 °C where soot combustion is almost finished (T90 = 399 °C). Above 420 °C, NO2 exceeds the thermodynamic equilibrium line (NO + 1/2O2 → NO2) possibly due to thermal decomposition of nitrate/nitrite.76 Although it may conflict with ref 77, this hypothesis can be well evidenced by NO2-TPD (Figure 10b) and FT-IR (Figure 10c) after preadsorbing NO at 350 °C. A strong desorption peak at 470 °C and an asymmetrical stretching vibration of the N−O bond at 1380 cm−1 clearly proves the existence of NO3− on MCSS.69 On the contrary, almost no NO3− adsorbs on MSS. Because NO3− species will only decompose above 400 °C, it is necessary to explore its direct activity toward soot

2NO2 + 2C → N2 +2CO2

(10)

NO + O−2 → NO3‐

(11)

2NO−3 + C → 2NO−2 +CO2

(12)

NO−3 + C → NO−2 + CO

(13)

CO + O− → CO2

(14)

NO−2 +O2 → NO−3

(15)

Furthermore, in a realistic diesel exhaust containing 500 ppm of NO + 6% H2O + 10% O2/N2, T50 of CSS, MSS, and MCSS listed in Table 3 decreases to 460, 438, and 354 °C, respectively, because hydroxyls may advance NO activation, thus promoting soot combustion.78,79 I


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Figure 10. Identification of nitrates: (a) NO oxidation, (b) NO2-TPD, (c) FT-IR of MCSS after adsorbing NOx at 350 °C, (d) NOx-soot TPR, (e) NO-soot TPO of MSS, and (f) NO-soot TPO of MCSS.

In keeping with Raman, XPS, and soot-TPR, the above results prove a significant synergistic effect on MCSS. However, in Figure S6c−f and Table S3, it is difficult to find a synergistic effect on corresponding powdered catalysts.39 T50 of powdered MnO2 and Co3O4 is no better than single powdered MnO2 and Co3O4 except under the 6% H2O + 15% O2/N2 condition. Powdered MnO2 and Co3O4 can be seen as the physical mixture of MnO2 and Co3O4, as Figure S6a and b displays. T50 of the best powdery catalyst MnO2 under 500 ppm of NO + 6% H2O + 15% O2/N2 is 414 °C, which is still higher than that of MCSS. Table S3 lists T50 or Tm values of soot oxidation over reported manganese-based and cobalt-based catalysts. It is obvious that MCSS displays promising performance even under high soot load. For shunning subjectivity of Cc, the kinetic data Ea and preexponential factor A in 500 ppm of the NO + 6% H2O + 10% O2/N2 condition were also calculated by the CR method to deeply clarify the determinate factors for catalytic soot combustion. As Figure 11 displays, the linear correlation coefficient of 0.999 shows reasonable applicability of the CR method. Ea of 162 kJ/mol and A of 2.8 × 108 of pure soot combustion are in agreement with refs 47 and 80. CSS and MSS greatly reduce Ea to 109 and 106 kJ/mol. In contrast to higher Cc, A of CSS (2.0 × 106) is smaller than that of MSS

Figure 11. Kinetic plot of the catalysts using CR method.

(6.7 × 106). This may be due to more inert Co2+ on the (220) facets as TEM and XPS indicate. On the other hand, Ea of MCSS is reduced to 99 kJ/mol due to the improved redox property as Raman, XPS, H2-TPR, and soot-TPR display. According to the references, the decrement on Ea may not totally explain the different activity of MSS and MCSS.73 It is eye catching that A of MCSS (4.9 × 107) is almost 7-fold as J


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Figure 12. Isothermal regenerations of fresh (a, c) and aged (b, d) MCSS.

isothermal regeneration experiment. The catalytic regeneration is carried out at 400 °C, so one test can be finished quickly. Moreover, Figure S9 illustrates the temperature rise with different soot loads. When the soot/catalyst ratio equals 1/2.5 (10,400 ppm), the temperature only rises by 2.9 °C, but it will increase by 14.5 °C with 2-fold soot loading. In Figure S9b, the “t90” values are 12 min, and the total soot can be abatement during 25 min. Since local hot spots are unavoidable with too high a soot load,82 to prevent the catalyst coating from sintering or deactivation, the soot/catalyst ratio is kept at 1:2.5. The decoupling between temperature rise and soot combustion is possible.84 In addition, the thin and hierarchical manufacture of α-MnO2 and Co3O4 nanoarrays will strengthen turbulent flow and heat transfer.85 Ten isothermal regeneration patterns of fresh MCSS are displayed in Figure 12a and c, where the first 3 min with a flat slope is signal delay time, and “t90” should be 12 min. Nevertheless, within 30 min, soot conversions of CSS and MSS (Figure S10) are merely about 60%. The maximum CO2 concentration during initial regeneration is lower (0.76%), and the regeneration time is longer (18 min). Afterward, “t90” stays at 12 min. This might result from the strong interaction between soot and catalyst coating, and soot oxidation over αMnO2 and Co3O4 may induce better activity.43 On the other hand, the hydrothermal stability is crucial for CDPFs.52 So, the hydrothermal aging experiment was conducted in a moisture atmosphere at 500 °C for 5 h (Figure S11a, b), and then, we evaluated the isothermal regeneration property of the aged MCSS. Figure 12b and d shows that “t90” is still 12 min, which again proves that the catalyst has excellent hydrothermal stability as FT-IR indicates. SEM images in Figure S11c−f demonstrate an interesting phenomenon that the short α-MnO2 nanorods grown on

high as that of MSS. Since A is correlated with active sites and Cc of MCSS increases by 8-fold than MSS, therefore, it is Ea together with A, namely, enhanced redox property and more active sites, that promote soot combustion. A similar result has been also reported by Wei et al.15 Furthermore, the reusability of MCSS was investigated by testing its cyclic performance on soot oxidation, as Figure S7a−d displays. Under 20% O2/N2, 6% H2O + 15% O2/N2 and 500 ppm of NO + 6% H2O + 10% O2/N2 conditions, the pattern stays almost unchanged. While under 500 ppm of NO + 15% O2/N2, T10 first decreases from 308 to 270 °C in the first four runs and then increases to 325 °C during the fifth test, but T50 gradually increases from 358 to 386 °C. It is interesting that when 6% H2O was also inlet the pattern recovers stably owing to the promotion effect of hydroxyl.78,79 Moreover, Figure S8 confirms that both the hierarchical structure and K/Mn ratio are well maintained. 3.6. Isothermal Regeneration Properties. For applied CDPFs behind diesels and DOCs, the inlet gas temperature ranges from 170 to 500 °C. BPT, at which temperature the trapped soot can be constantly removed with a stable pressure drop, is utilized to evaluate the catalytic performance.4,81 The lower the BPT is, the more excellent the CDPFs are. BPT can be procured by a bench test, but it is difficult to conduct a bench test at a laboratory. Instead, an isothermal regeneration test is used in numerous references.7,8,82−84 As Di Sarli et al. reported, an isothermal regeneration period contains preheating and isothermal stages, and an isothermal stage is imperative.8 The temperature during isothermal regeneration is greatly affected by soot load since soot combustion is an exothermal reaction (ΔH = −32.8 kJ·g−1). Based on the above TPO results, a more realistic reaction gas condition (500 ppm of NO + 6% H2O + 10% O2/N2) was chosen to conduct the K


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Figure 13. Morphology of CSS (a, b) and MCSS (c, d) after ultrasonic treatment test. (e, f) Isothermal regeneration pattern of MCSS after adherence test.

Co3O4 trunks (Figure 1h) transform into ∼5 μm long MnO2 nanofibers. α-MnO2 nanofibers are still anchored on Co3O4 rather than fall off. Because hydrothermal treatment does not affect the morphology and K/Mn ratio of MCSS, we can deduce that it is the interaction between soot and α-MnO2 and Co3O4 coatings that promotes α-MnO2 nanorods to grow along the [001] direction and form nanofibers. 3.7. Adherence Tests. In traditional coating processes, it remains a challenge to coat powdered catalysts on metallic filters because of weak adherence force between coatings and substrates.7,86 Enough adherence force is vital for CDPFs under harsh serving conditions. SEM images of CSS and MCSS treated by ultrasonic oscillation are shown in Figure 13a−d. Not surprisingly, the upper section of Co 3 O 4 nanofibers drops out (yellow circles) with merely the bottom nanorods left. Since Co3O4 nanofibers are aggregations of small Co3O4 particles, the cohesion between adjacent particles is too weak to combine with each other as TEM shows. On the contrary, MCSS appears to be cleaner because the residual αMnO2 aggregations (Figure 1g) are removed without brushlike α-MnO2 and Co3O4 nanoarrays peeling off. For protecting catalyst coatings from cracking or flaking, the relationships between cohesion, adhesion, and interior stress should be interior stress < cohesion < adhesion. When interior stress is

larger than adhesion, the coating will flake; when interior stress is larger than cohesion but smaller than adhesion, the coating will crack. In the literature, coated catalysts are widely reported to be cracking or flaking.7,29 However, no crack or abscission is observed on MCSS, and we think two factors may take effect. First, unlike the integrated α-MnO2 layer tightly bounded on the AISI304 substrate, MCSS breaks up the whole into parts. Namely, the isolated brushlike α-MnO2 and Co3O4 manufacture can reduce interior stress. When the monoliths are deformed, α-MnO2 and Co3O4 and Co3O4 coatings may not fall off (Figure S12a, b), but MnO2 coating cracks and drops out seriously (Figure S12c). Second, compared with porous Co3O4 nanofibers (weak cohesion), α-MnO2 nanorods covering Co3O4 nanofibers will improve mechanical strength. As Figure 1f and Figure 2f illustrate, the birnessite nanosheets may fill up the pores between adjacent Co3O4 particles, and the interaction between MnO2 branches and Co3O4 trunks also improves the cohesion. In Figure 13e and f, “t90” of the 10 isothermal regenerations is still 12 min, and total soot burns out within 25 min. It distinctly demonstrates excellent adhesion of α-MnO2 and Co3O4 nanoarrays and the stable activity of monolithic MCSS. Moreover, we can also draw a conclusion that the MnO2 aggregations circled in yellow in Figure 1 indeed do not affect L


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the Opening Foundation of Jiangsu Key Laboratory of Vehicle Emissions Control (No. OVEC047), and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (ZK201712).

the activity of monoliths. Finally, XRD and Raman spectra in Figure S13 reveal the stable structure of MCSS.

4. CONCLUSIONS In this paper, we successfully in situ grew hierarchical brushlike α-MnO2 and Co3O4 nanoarrays on metallic filters via a twostep hydrothermal strategy for diesel soot elimination. The accessible reaction sites were greatly increased by nearly an 8fold contact chance compared to MnO2 or Co3O4 nanoarrays. This was in good agreement with the 7-fold pre-exponential factor calculated by the kinetic experiment. The space among isolated brushes benefited for soot mobility, while the brushlike α-MnO2 and Co3O4 nanocomposites provided numerous sites. The synergistic effect between manganese and cobalt resulted in weaker Mn−O bonds, more surface oxygen species, and improved redox property. The catalytic capability was remarkably improved that T50 was reduced to 354 °C, and 90% of the soot load can be eliminated within 12 min at 400 °C. NO was proved to obviously enhance the activity via forming nitrate species. MCSS also displayed excellent longterm durability and tight adhesion. This work paves an effective route to fabricate monolithic nanoarray-type catalysts for soot oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02155. Morphology of Printex U soot, AISI304 substrate, and side view of CSS. Soot load on monoliths with different soot/catalyst ratios. SEM of brushlike MnO2 and Co3O4 grown on various substrates. Effect of hydrothermal duration on morphology and catalytic performance, uncatalyzed soot oxidation. SEM and soot oxidation of powdered catalysts. Reusability evaluation. Morphology and K/Mn ratios of spent catalysts. Isothermal regeneration and temperature rise under various soot loadings. Isothermal regeneration of MSS and CSS. SEM after isothermal regeneration evaluation. Shearing stress evaluation. XRD and Raman results of used sample. Quantitative H2 consumption. Soot oxidation indicators of powdered catalysts. Catalytic performance comparison with literature. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 2583587198. E-mail: [email protected] (N. Feng). *Tel: +86 2583587198. E-mail: [email protected] (G. Guan). ORCID

Nengjie Feng: 0000-0001-7058-2534 Lei Wang: 0000-0003-2476-6206 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their deep thanks to the economic support from the National Key Research and Development Program of China (2018YFC0214106), the Natural Science Fund for Colleges and Universities in Jiangsu Province (18KJB530008), M


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