Effects of Diesel Oxidation Catalyst on Nanostructure and Reactivity of

Article pubs.acs.org/EF

Effects of Diesel Oxidation Catalyst on Nanostructure and Reactivity of Diesel Soot Zhihao Ma,* Lei Li, Ying Chao, Ning Kang, Bin Xu, and Jian Wu College of Vehicle and Communication Engineering, Henan University of Science and Technology, 263 Kaiyuan Road, Luoyang 471003, China S Supporting Information *

ABSTRACT: In order to investigate the nanostructure changes of diesel soot during the oxidation process, two different types of diesel soot were collected, and their nanostructures were studied on the basis of thermogravimetric analysis and highresolution transmission electron microscopy analysis. This work shows that the nanostructure alone does not dictate the reactivity of diesel soot, but rather, the oxidation mechanism has a strong effect on the oxidative reactivity. Soot emitted directly from the engine is oxidized under the surface burning mode, which makes the soot retain the typical core−shell structure. However, the diesel oxidation catalyst (DOC) has an influence on the oxidation mechanism of diesel soot as well as the evolution of nanostructure during the oxidation process. Soot sampled after DOC mainly undergoes an internal burning oxidation process that makes the oxidation more rapid, leading to a hollow capsule-like structure during the early stage of oxidation. However, soot becomes less reactive due to the surface burning mode and the more closed outer shell built by the rearrangement of carbon lamellae during the later stage of oxidation.

1. INTRODUCTION Diesel soot can be effectively removed from the exhaust by means of filtration using diesel particulate filters (DPF); however, regeneration of DPF via soot oxidation can be quite challenging. The oxidative reactivity of diesel soot is an important factor affecting DPF regeneration behavior. Soot nanostructure has an influence on its oxidative reactivity on the basis of the number of potential reaction sites which can be affected by the size and shape of carbon lamella. 1,2 Nanostructure of diesel soot is determined by complex conditions such as fuel properties, composition of the incylinder mixture, combustion temperature, and combustion phasing, which also have an impact on the soot reactivity. Soot nanostructure and reactivity have been studied by several researchers. Vander Wal et al.3 explained the influences that the formation and growth conditions may have on flame soot nanostructure. Zhu et al.4 discussed the differences between morphology, microstructure, and fractal geometry of light-duty diesel engine particulates collected at different engine loads. Lapuerta et al.5,6 investigated the effects of fuel and engine operating conditions on soot nanostructures. Yehliu et al.7,8 carried out a series of studies on the impact of fuel formulation, engine operating mode, and combustion phasing on the reactivity of diesel soot. Raj et al.9 elaborated the effect of the difference in the structures of the PAHs comprising particulates on soot reactivity toward O2. Stanmore et al.10 discussed soot oxidation process particularly including oxidant, catalyst, and their sequent impact on activation energy and oxidation mechanisms. Su et al.11 investigated the differences in microstructure and oxidation behavior of soot from a Euro IV heavy duty (HD) diesel engine and the spark-discharge soot and hexabenzocoronene. Al-Qurashi et al.12,13 reported that exhaust gas recirculation also promotes soot oxidative reactivity as a result of the thermal, dilution, and chemical effects. © 2014 American Chemical Society

In recent years, nanostructure and oxidative reactivity of inDPF soot has attracted increasing attention. In the research of Liati et al.,14 size distributions, fringe length, tortuosity, and separation distance of pre-DOC (diesel oxidation catalyst), post-DOC, post-DPF and in-DPF primary soot were shown in detail. According to the research of Vander Wal et al.,15 the regeneration of DPF can induce densification of primary particles of diesel soot with a hollow interior and highly crystalline outer shell. Similar observations can be found in the research of Song et al.16 They compared the morphology, fringe length, size distribution, and oxidation rate of biodiesel soot with F-T soot during the oxidation process, and the results showed that biodiesel soot is more reactive and underwent a capsule-type oxidation and eventually formed graphene ribbon structures. However, only the morphology of diesel soot based on the original high-resolution transmission electron microscopy images was discussed in ref 15, and the internal nanostructure of primary particles was unclear. Detailed nanostructural characteristics of diesel soot were given in ref 14, but which characteristic weighed more than the others still remains unknown. Although the factors that could influence oxidative reactivity of soot were discussed particularly by Song et al.;16 nevertheless, there are some differences between the physical and chemical properties of biodiesel and diesel.5 Besides, only fringe length was included in their research, and the tortuosity and fringe separation distance, which might also affect oxidative reactivity of soot, were not mentioned. In this work, variations in soot nanostructure during oxidation process were characterized to investigate the Received: February 26, 2014 Revised: May 29, 2014 Published: June 13, 2014 4376

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stack of metal nets that had been fixed in the sample line (point A in Figure 1) before the test. When the engine was operating in the test mode, valve 1 was wide open, thus the exhaust would flow through the sample line. The total exhaust back pressure was controlled in the range of 9−11 kpa (the range when using the DOC-DPF assembly) by adjusting the open angle of valve 2. When the exhaust back pressure became nearly constant, valve 1 would be closed completely while valve 2 wide was left open. The metal nets were replace with new ones, and the aforementioned steps were repeated until sufficient soot was collected. Soot samples were moved to an airtight vial from the metal nets and then placed in a dryer. The sample collected while DOC was removed was named as soot-A, and that collected after DOC was named as soot-B. The engine was fueled with China IV standard diesel whose aromatics content and sulfur content were 24.7% and 0.0021%, respectively, according to the test results from National Fuel Oil Quality Supervision and Inspection Center. During the collection, the temperature of the intercooled air was kept within 50 °C, the outlet water coolant temperature was maintained below 88 °C, and the fuel temperature was kept below 38 °C. 2.2. Thermogravimetric Analysis (TGA). To study soot oxidative reactivity and the nanostructural characteristics at different oxidation stages, soot-A and soot-B were oxidized in a TA Instruments SDT2960 thermogravimetric analyzer. Soot samples were first heated in nitrogen (10 °C/min, 150 mL/ min) at 500 °C to remove the volatile fraction, and when there was no more significant mass loss, the temperature was reduced to 450 °C (in nitrogen). Subsequently, Air (150 mL/min) was introduced into the furnace to start the isothermal oxidation process. When the mass loss took up around 25%, 50%, or 75% of the initial mass (when oxidation occurred), the temperature in the reaction chamber would be reduced to room temperature quickly, and the reaction chamber gas would be switched back to nitrogen in order to obtain soot at different oxidation stages. The TGA experiments were conducted at least two times to assess repeatability. 2.3. High-Resolution Transmission Electron Microscopy. A JEM-2100 HRTEM with specified point-to-point resolution of 0.194 nm operating at 200 keV was used to observe soot morphology and to obtain images with a 600 000× magnification. A drop of soot suspension would be deposited onto the TEM grid after sonication in absolute ethyl alcohol for the HRTEM analysis. Image processing was conducted using MathWorks MATLAB software to obtain the information on fringe length, fringe tortuosity, and fringe separation distance of each soot specimen.17−19 All fringes

differences that DOC may induce in soot nanostructure and oxidation behavior.

2. EXPERIMENTAL SECTION 2.1. Soot Sampling. Soot samples were collected from the exhaust system of a CY4102-CE4B diesel engine (main performance parameters are shown in Table 1). Considering Table 1. Main Parameters of the CY4102-CE4B Diesel Engine item

specifications

cylinder diameter × stroke [mm × mm] displacement [L] compression ratio rated power/speed [kW (r/min)] maximum torque/speed [N·m (r/min)]

in-line four cylinders, fourstroke cycle, forced water cooling, turbocharger, intake inter cooler, cool EGR, high pressure common rail, 16 valves 102 × 118 3.856 17.0:1 100/2800 420/1300−1500

type

the emission load of diesel soot and the temperature range for the DPF’s passive regeneration with the help of DOC, the operating mode was selected at 2050r/min, 75% load (one operating mode in the ESC test with the weight coefficient of 1.0, and the inlet temperature of DOC was about 420 °C). The engine exhaust system, which was altered to collect diesel soot, is shown in Figure 1. Soot was accumulated on a

Figure 1. Schematic of engine exhaust system.

Figure 2. Mass loss (a) and mass loss rate (b) of different soot samples. 4377

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has a concentrated distribution of fringe separation distance in the range of 0.4 to 0.5 nm, and fringe pairs with the separation distance less than 0.4 nm takes up only 27% of the total. The percent of adjacent fringe pairs with separation distance less than 0.4 nm is up to 47% for soot-B. Because the mean separation distance of standard graphite is about 0.335 nm,4,17 soot-B, with the smaller interlayer spacing, seems to be more graphitic. In general, under the influence of DOC, carbon layers in primary particles extend in dimensions, and their separation distances are shortened, leading to a more ordered structure than soot emitted directly from the engine. Although soot-A initially presents a less graphitized structure, the primary particles possess typical shell−core structure at higher burnoff levels, with long-range layers concentrically arranged along the periphery and short layers amorphously arranged inside the core. However, the boundaries between primary particles are hard to distinguish, as shown in Figure 5a−c. It is shown that as oxidation proceeds, soot-A becomes more graphitic. Notably, soot-B might experience a quite different oxidation process. At 25% burnoff, the primary particles possess an interior void, showing a hollow capsulelike structure. At 50% burnoff, the carbon layers of adjacent particles have a trend of jointing together, and the boundaries of particles cannot be distinguished due to rearrangement and coalescence of graphene lamellae. However, no graphite-like structures can be found in the last image in Figure 5f, which reveals the structure of the remaining substances in the expected 75% burnoff specimen (no further oxidation actually occurred). The residual fraction were analyzed by an Oxford INCA Energy X-ray energy dispersive spectrometer in this work, and elements such as Ca, Zn, Fe, S, O, and so forth were detected, indicating that the object in the image may be metallic oxide or sulfate, which are compositions of ash. This observation is also in line with the mass loss curves shown in Figure 2a. Figure 6 plots the average values of fringe length, fringe tortuosity, and fringe separation distance of soot-A and soot-B versus burnoff levels. Because the ash content in soot-B was extremely high, the expected 75% burnoff was not achieved, and no graphite-like structures were found in that specimen; therefore, the nanostructural characteristics of this specimen will not be discussed here. The fringe length of soot-A is initially 1.626 nm, which becomes 1.814 and 1.891 nm at 25% and 50% burnoff; however, there is a slight decrease in fringe length at 75% burnoff, but the proportion of fringes longer than 7 nm is higher in the 75% specimen than the former ones. Hence, in general, lamellae of soot-A become longer upon oxidation. The mean fringe tortuosity values of all the soot-A-derived soot locate in the range of 1.085 to 1.105. The variation in tortuosity is not obvious. Fringe separation distance of soot-A changes from 0.43 to 0.413 nm gradually, presenting a tendency to decrease as oxidation proceeds. However, the variation in nanostructural characteristics of soot-B is entirely different from soot-A. As shown in Figure 6, fringe length decreases obviously, fringe tortuosity increases slightly, and fringe separation distance has a trend of increasing as oxidation deepens. It can be extracted from both Figure 4 and Figure 6 that lamellae of soot-B are initially longer than that of soot-A, although the separation distance is shorter, revealing that initial soot-B has a more graphitic structure, which is consistent with the structure shown in Figure 3. These

whose length was less than 0.4 nm and all fringe pairs that had distances greater than 0.5 nm or less than 0.32 nm were discarded as artifacts.17

3. TEST RESULTS 3.1.1. Soot Reactivity. Figure 2a shows the mass loss due to oxidation of soot-A and soot-B during TGA. It can be seen that almost 50% mass loss of soot-B took place in the first 50 min because oxidation occurred, although it took about 125 min for soot-A, meaning that soot-B has a higher mass loss rate than soot-A during the first half of oxidation. However, oxidation of soot-B stopped after about 65% of mass was lost, indicating extremely high ash content in the sample. As is known, DOC could catalyze the formation of sulfate;20 what is more important was that a part of soot could be oxidized by NO2. This means that there was an extended period to collect sufficient soot, which provided enough time for soot oxidation, but ash survived due to its insensitivity to oxidation, resulting in the high ash content of soot-B. The oxidative reactivity of soot-A and soot-B were evaluated on the basis of their mass loss rate, which is the mass derivative with respect to time. The mass loss rate curves of the two types of soot are compared in Figure 2b. As is shown, both of the mass loss rate curves reach the maximum at the beginning of oxidation and gradually decrease as oxidation proceeds, showing a gradually downward trend in general. The mass loss rate of soot-B exceeds soot-A before 50% burnoff, but it becomes lower than soot-A after 50% burnoff. It is clear that soot-B is more reactive in the early stage of oxidation but less reactive in the later stage of oxidation. 3.1.2. Nanostructure of Primary Particles. Figure 3a reveals the morphology of soot collected without DOC (soot-A). As

Figure 3. HRTEM images of unburnt soot-A (a) and soot-B (b).

seen, a quantity of lamellae is irregularly arranged, and no clear boundary can be found between the inner core and outer shell. Soot-B was collected downstream of DOC. As shown in Figure 3b, extended lamellae are concentrically arranged around the particle interior of the initial soot, exhibiting typical shell−core structure and relatively higher graphitization degree compared with soot-A. Figure 4 plots the fringe length, tortuosity, and separation distance histograms of soot-A and soot-B. More than 91% of fringes in soot-A lies between 0.4 and 3 nm in length, whereas the percent is about 89% for soot-B (i.e., the fringe length distribution for soot-B extends to longer graphene layer plane dimensions). A large proportion of fringes in both soot-A and soot-B has the tortuosity value between 1 and 1.2, and the proportion is about 97% and 98% for soot-A and soot-B respectively, meaning that soot-B is a little more curved. Soot-A 4378

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Figure 4. Histograms of nanostuctural characteristics of unburnt soot: (a−c) reveals the fringe length, tortuosity and separation distance of soot-A; (d−f) shows the fringe length, tortuosity, and separation distance of soot-B.

on soot reactivity, but the mass loss rate of soot-B decreases as oxidation proceeds according to the TGA results, which presents a downward trend of reactivity along with oxidation. Second, although the nanostructure characteristics of soot-B demonstrate a relatively higher resistance toward oxidation, soot-B exhibited higher mass loss rate than soot-A during TGA instead, as shown in Figure 2. It can be presumed that the nanostructure alone cannot dictate soot reactivity, and the difference of oxidation mechanism may be responsible for these two observations. There are two main oxidation modes for soot: the surface burning mode and the internal burning mode.24 The particles are likely to burn from the outside in through surface burning, leading to a slow oxidation rate and inconspicuous changes in the shells and cores. Soot-A experienced surface burning and retained the shell−core structure during oxidation, as shown in Figure 3 and Figure 5. Another feature of surface burning is that progressive occurred ordering during oxidation,25,26 which can also be verified by the structure variation of soot-A shown in Figure 5. Some organic compounds are adsorbed on the particle surface or filled in the gap between adjacent primary particles or in the defects or orifices in particles due to adsorption and condensation during soot formation and transformation.

observations all point out that soot can generate more graphitic structure with the impact of DOC.

4. DISCUSSION The impact of soot nanostructure on its oxidative reactivity can be interpreted as follows according to some previous studies: Carbon atoms in edge sites can form bonds with chemisorbed oxygen due to the availability of unpaired sp2 electrons, whereas carbon atoms in basal planes have only shared π electrons forming chemical bonds.21 Besides, edge positions have greater accessibility for oxygen.22 Thus, the shorter a lamella is the more edge site carbon atoms there will be, which have a positive effect on oxidative reactivity of soot. Curvature derived from five-membered rings within the aromatic framework may impose bond strain as the orbitals overlap, and the electronic resonance stabilization will be lessened. Therein, the C−C bonds are weakened, and individual atoms are more exposed, making them more sensitive to oxidative attack.3,23 The increase in interplanar spacing makes it easier for the combination of oxygen and edge site carbon atoms, consequently promoting oxidative reactivity of soot.22 Nevertheless, the results in the present tests are contrary to the aforementioned theories. First, the variation in nanostructure of soot-B during the oxidation process has a positive effect 4379

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Figure 6. Average fringe length (a), tortuosity (b), and separation distance (c) at different oxidation stages. The error bars indicate the standard error in the measurements.

Figure 5. HRTEM images of soot-A (a−c) and soot-B (d−f) at different oxidation stages.

The combustible mixture in the cylinder of a diesel is always oxygen-rich in general, resulting in the existence of oxygen in the exhaust flow. The inlet temperature of DOC was about 420 °C in our test, and the temperature where soot was collected (point A in Figure 1) was about 400 °C. There was only oxygen in the exhaust flow that can oxidize soot when DOC was removed (soot-A). Oxidation could happen at this temperature level but with a low oxidation rate. So there were no dramatic variations in soot nanostructure through this mild oxidation. Soot underwent a more violent oxidation with NO2 under the action of DOC.32,33 Composition and structure of the periphery of soot changed through the continuous reaction. Volatile fractions kept condensed onto and evaporated off the soot surface, or the fractions were consumed by oxidation as well. Soot deposited on the metal nets might develop porosity through partial oxidation. Meanwhile, organic compounds and other volatile fractions were continuously supplied to the sampling position. These subsequent species might fill the pores; besides, the stacking of particulates could form a barrier between the porous soot and the oxidizers, accordingly preventing the pores stretching inside and restraining the formation of new pores. As a result, no capsule-like hollow particles could be found in the initial specimen of soot-B. In addition, the oxidation reactions in the exhaust might heat the nearby lamellae or adjacent particles, giving rise to the more graphitic structure of initial specimen of soot-B. During TGA, the pores in soot-B exposed by volatilization and surface oxidation then extended and ultimately penetrated the outer shell, making soot-B experienced drastic internal

Volatilization and gasification of these species exposes the available internal surface area in soot.27,28 The exposed defects or orifices will develop into micropores through further oxidation, forming porous soot. The pores are susceptible for oxygen attack because of a large proportion of edge-site atoms. They will stretch inside due to oxidation and finally penetrate the particle shell; consequently, the inner core is available for oxidation. Because the amorphous core is far more reactive than the graphitic shell, the interior will soon burn out, forming a hollow structure.15 The oxidation rate of the internal burning is much higher than the rate of surface burning. One important function of DOC is to turn NO into NO2 with the help of catalyst, and NO2 will then react with soot as an oxidant. RNOx (alkyl nitrite, nitrite esters, and nitro groups) and ROx (anhydrides, lactones, phenols, carbonyls, and ethers) surface complexes may form during the soot-NO2 reaction.29 The RNOx species can decompose into CO or CO2 because of the weak thermo stability, which can promote the soot-NO2 reaction.29 The ROX complexes are vulnerable to oxygen attack, and therefore, the presence of O2 has a positive effect on the soot-NO2 reaction.30,31 However, the soot-NO2 reaction can proceed well in the normal exhaust temperature range, but the soot-O2 reaction rate is extremely low. It is mainly due to the low speed of the formation of the ROx species during the sootO2 reaction in that temperature range.31 Hence, the soot-NO2 reaction has lower activation energy and can proceed at lower temperatures with a higher oxidation rate.30 4380

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(2) Boehman, A. L.; Song, J.; Alam, M. Impact of biodiesel blending on diesel soot and the regeneration of particulate filters. Energy Fuels 2005, 19 (5), 1857−1864. (3) Vander Wal, R. L.; Tomasek, A. J. Soot nanostructure: dependence upon synthesis conditions. Combust. Flame 2004, 136 (1−2), 129−140. (4) Zhu, J. Y.; Lee, K. O.; Yozgatligil, A.; Choi, M. Y. Effects of engine operating conditions on morphology, microstructure, and fractal geometry of light-duty diesel engine particulates. Proc. Combust. Inst. 2005, 30 (2), 2781−2789. (5) Lapuerta, M.; Oliva, F.; Agudelo, J. R.; Boehman, A. L. Effect of fuel on the soot nanostructure and consequences on loading and regeneration of diesel particulate filters. Combust. Flame 2012, 159 (2), 844−853. (6) Lapuerta, M.; Martos, F. J.; Herreros, J. M. Effect of engine operating conditions on the size of primary particles composing diesel soot agglomerates. J. Aerosol Sci. 2007, 38 (4), 455−466. (7) Yehliu, K.; Vander Wal, R. L.; Armas, O.; Boehman, A. L. Impact of fuel formulation on the nanostructure and reactivity of diesel soot. Combust. Flame 2012, 159 (12), 3597−3606. (8) Yehliu, K.; Armas, O.; Vander Wal, R. L.; Boehman, A. L. Impact of engine operating modes and combustion phasing on the reactivity of diesel soot. Combust. Flame 2013, 160 (3), 682−691. (9) Raj, A.; Yang, S. Y.; Cha, D.; Tayouo, R.; Chung, S. H. Structural effects on the oxidation of soot particles by O2: Experimental and theoretical study. Combust. Flame 2013, 160 (9), 1812−1826. (10) Stanmore, B. R.; Brilhac, J. F.; Gilot, P. The oxidation of soot: A review of experiments, mechanisms and models. Carbon 2001, 39 (15), 2247−2268. (11) Su, D. S.; Jentoft, R. E.; Müller, J. O.; Rothe, D.; Jacob, E.; Simpson, C. D.; Tomovic, Z.; Müllen, K.; Messerer, A.; Pöschl, U.; Niessner, R.; Schlögl, R. Microstructure and oxidation behaviour of Euro IV diesel engine soot: A comparative study with synthetic model soot substances. Catal. Today 2004, 90 (1−2), 127−132. (12) Al-Qurashi, K.; Boehman, A. L. Impact of exhaust gas recirculation (EGR) on the oxidative reactivity of diesel engine soot. Combust. Flame 2008, 155 (4), 675−695. (13) Al-Qurashi, K.; Lueking, A. D.; Boehman, A. L. The deconvolution of the thermal, dilution, and chemical effects of exhaust gas recirculation (EGR) on the reactivity of engine and flame soot. Combust. Flame 2011, 158 (9), 1696−1704. (14) Liati, A.; Dimopoulos Eggenschwiler, P.; Schreiber, D.; Zelenay, V.; Ammann, M. Variations in diesel soot reactivity along the exhaust after-treatment system, based on the morphology and nanostructure of primary soot particles. Combust. Flame 2013, 160 (3), 671−681. (15) Vander Wal, R. L.; Yezerets, A.; Currier, N. W.; Kim, D. H.; Wang, C. M. HRTEM study of diesel soot collected from diesel particulate filters. Carbon 2007, 45 (1), 70−77. (16) Song, J.; Alam, M.; Boehman, A. L.; Kim, U. Examination of the oxidation behavior of biodiesel soot. Combust. Flame 2006, 146 (4), 589−604. (17) Vander Wal, R. L. Soot nanostructure: Definition, quantification and implications. SAE Int., [Spec. Publ.] SP; SAE Paper 2005-01-0964. (18) Yehliu, K.; Vander Wal, R. L.; Boehman, A. L. Development of an HRTEM image analysis method to quantify carbon nanostructure. Combust. Flame 2011, 158 (9), 1837−1851. (19) Yehliu, K.; Vander Wal, R. L.; Boehman, A. L. A comparison of soot nanostructure obtained using two high resolution transmission electron microscopy image analysis algorithms. Carbon 2011, 49 (13), 4256−4268. (20) Liati, A.; Dimopoulos Eggenschwiler, P.; Müller Gubler, E.; Schreiber, D.; Aguirre, M. Investigation of diesel ash particulate matter: A scanning electron microscope and transmission electron microscope study. Atmos. Environ. 2012, 49 (0), 391−402. (21) Marsh, H.; Kuo, K. Chapter 4Kinetics and catalysis of carbon gasification. In Introduction to Carbon Science, Marsh, H., Edwards, I. A. S., Menendez, R., Rand, B., West, S., Hosty, A. J., Kuo, K., McEnaney, B., Mays, T., Johnson, D. J., Patrick, J. W., Clarke, D. E., Crelling, J. C.,

oxidation while surface oxidation formed hollow shells. During this stage, internal oxidation was dominant for the mass loss, which had a higher oxidation rate than surface burning on account that the amorphous core was more reactive as the aforementioned. Therefore, soot-B appeared to be more reactive in the early and middle stages of oxidation compared with soot-A, although it was initially more graphitic. The rearrangement of carbon layers during the drastic oxidation shortened fringe length while it enlarged interplanar spacing and slightly improved fringe tortuosity, which is similar to the observation of Song et al.16 However, in the later stage of oxidation, the coalescence of lamellae re-formed a more closed outer shell, and the oxidation mode turned back to surface burning;16 this blocking of tunnels from the soot surface to the interior also reduced the number of edge-site carbon atoms;34 in addition, the ash content of soot-B is large, and would be larger through oxidation, because ash might generate a barrier that could prevent oxygen onto the soot surface,26 causing the reduction of mass loss rate of soot-B in the late oxidation.

5. CONCLUSIONS Nanostructure alone cannot dictate the oxidative reactivity of soot. Soot reactivity through oxidation is largely affected by its oxidation mode. In this present work, nanostructural characteristics of different soot samples during the oxidation process were compared. Soot emitted directly from the engine burned from the outside in with a low oxidation rate and became more graphitic through the surface burning. The particles kept the shell−core structure during the whole oxidation process. Despite the more graphitic initial structure, soot sampled after DOC mainly undergoes an internal burning oxidation process that makes the oxidation more rapid, leading to a hollow capsule-like structure during the early stage of oxidation. However, during the later stage of oxidation, the rearrangement of carbon lamellae re-formed a closed outer shell, leading to the dominance of surface burning and the loss of internal edge sites, resulting in the decrease of soot reactivity. In addition, the coalescence of lamellae also joined the adjacent carbon layers together, making it difficult to distinguish the boundaries between primary particles.

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ASSOCIATED CONTENT

S Supporting Information *

Illustration of image processing. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Science Fund of State Key Laboratory of Automotive Safety and Energy (KF11161), the Ph.D. Startup Research Foundation of Henan University of Science and Technology (09001327), and the Essential Research Project of Henan Province (092300410125).

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