SCIENCE CHINA Technological Sciences •  Review  •

doi: 10.1007/s11431-016-9117-x

A review of fundamental factors affecting diesel PM oxidation behaviors GAO JianBing1,2*, MA ChaoChen1, XING ShiKai3, SUN LiWei1 & HUANG LiYong1 2

1 School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China; Department of Mechanical Engineering Sciences, University of Surrey, Guildford GU2 7XH, UK; 3 School of Vocational and Technical, Hebei Normal University, Shijiazhuang 050024, China

Received October 20, 2016; accepted August 2, 2017; published online September 13, 2017

Diesel particulate matter (DPF) is usually employed to meet the stringent regulations on particulate matter (PM) emissions for diesel engine. To resolve the DPF regeneration problem, comprehensive information about the factors influencing PM oxidation behaviors must be understood. Large amounts of factors related to PM oxidation activity have been investigated, however, some relations are still ambiguous. This paper reviews the factors related to PM oxidation activity that the factors are divided into the engine-correlated and engine-uncorrelated factors. The methods with both advantages and disadvantages to test the oxidation behaviors are introduced. The microstructure and ingredient being fundamental factors affecting PM oxidation behaviors are as the principle line to correlate PM oxidation behaviors and engine-correlated factors. The relations of engine-correlated factors with oxidation behaviors are obtained though advanced technologies that are mutual complementation. The engine-uncorrelated factors are also reviewed that these factors are vital to oxidation activity changes. Multiple-factor analysis rather than single-factor analysis should be developed to make the oxidation behaviors of diesel PM more clear. diesel PM, oxidation behaviors, microstructure, ingredient Citation:

Gao J B, Ma C C, Xing S K, et al. A review of fundamental factors affecting diesel PM oxidation behaviors. Sci China Tech Sci, 2017, 60, doi: 10.1007/s11431-016-9117-x

   Introduction 1   Recent years, diesel particulate matter (PM) has brought about negative effects on climate, biosphere and human health [1–3]. The diesel PM is in smaller diameter and huger number due to the use of advanced technologies, for instance, high pressure common rail and turbocharger. Terribly, smaller particles are more harmful to human health because of their easier access to respiratory tract. Therefore, the legislations on diesel PM mass and number emissions become stricter all over the world [4–6]. Many measures have been done to reduce diesel PM emissions [7–15]. Diesel *Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2017 

particulate matter (DPF) is the most successful diesel PM after-treatment technology that it owns high PM removal efficiency more than 98%. Non-thermal plasma (NTP) is also a potential technology for PM removal with advantages of little back pressure [1,16,17]. However, the filter units of DPF and NTP reactor have to be regenerated periodically to keep high PM removal efficiency [18–20]. The formation mechanism of diesel PM is a complicated progress, also, diesel PM formed at different conditions has different physicochemical properties [21–26]. Due to the complexity in structures and ingredients of diesel PM, and the necessity for DPF and NTP regeneration, a comprehensive understanding of diesel PM oxidation characteristics is needed. The appropriate regeneration temperature and duration can ensure the regeneration efficiency and the fuel economic when active tech.scichina.com   link.springer.com

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regeneration is applied. It is generally known that diesel PM owns complex microstructures, and contains various chemical compositions, such as element carbon (DS), SOF and ash (sulfuric compounds and metal oxides) [27,28]. References [9,29–36] have investigated the influence of factors on diesel PM oxidation, and some regeneration strategies of DPF are raised [37–40]. However, the roles of some factors on PM oxidation characteristics are still ambiguous, also, no paper reviewed the fundamental factors (microstructure and ingredients of diesel PM) affecting the oxidation behaviors of diesel PM. In order to figure out the effect of relevant factors on PM oxidation behaviors, many advanced technologies are employed to test PM physicochemical properties by many researchers. These factors are mutual complementary and correlative that makes it more difficult to restrictive demarcation of their duties on PM oxidation behaviors [41,42]. This paper reviews the fundamental factors associated with diesel PM oxidation behaviors. It is structured using fundamental factors (microstructure and ingredients), rather than listing all the factors (engine design, engine operation conditions and fuel so on) sequentially. Further, discussions are made based on former researches.

   Methods to test PM oxidation behaviors 2   The commonly used method to test PM oxidation behaviors is thermogravimetric analysis (TGA) experiments [27,28,43]. The representative oxidation profiles based on TGA experiments are shown in Figure 1. The TGA profiles show two temperature ranges: 50–400°C, the oxidation and volatilization of volatile organic compounds (VOC); 400–570°C, the oxidation of non-VOC and soot. PM oxidation profiles are distorted because of VOC volatilization that only part of VOC is oxidized. Samples used to perform the TGA experiments are raw PM in some references [44–46], and others are pre-treated in an inert gas atmosphere to remove the VOC contained in raw PM [28,47]. Rodriguez-Fernandez et al. [48] characterized the diesel soot oxidation process through an optimized TGA method. The optimal initial sample mass, air flow rate, ramp rate and the type of crucible were obtained to ensure the accuracy of TGA results. Both the devolatilized and raw PM are tested in reference [48]. Devolatilization before TGA experiments neglects the effect of VOC on PM oxidation. The differential scanning calorimetry (DSC)-based method is put forward to avoid the disadvantages mentioned in the above methods. The hypothesis is made that the heat release in DSC-based method is proportional to oxidation-caused mass loss in TGA method. The heat release profiles are normalized to make it comparable with TGA curves, as shown in Figure 1. The temperature corresponding to maximum oxidation rate and final oxidation of raw PM using TGA and DSC-based method is similar. Due to the VOC contained in raw PM, the

   TGA Figure 1        profiles and normalized DSC profiles of raw PM.

temperature corresponding to the first peak of mass loss rate curves is unequal to it in normalized heat release rate curves. However, the DSC-based method also has some disadvantages that the heat release shows no linear correlations with mass loss during PM oxidation process. Wang-Hansen et al. [49] and Waglöhner et al. [50] tested the formation rate of CO and CO2 to indicate the oxidation process of soot. The oxidation profiles based on the formation rate of CO and CO2 are similar to TGA curves. Darcy et al. [51] established the oxidation models of diesel soot based on much hypothesis. The oxidation profiles obtained using oxidation equations match excellently with those obtained using TGA experiments at specified conditions, however, it differs greatly at other conditions. Simulation method using oxidation models is with limitation to reflect the true oxidation behaviors of diesel PM. For pre-treated PM samples [28,30,48], the initial oxidation temperature is 400–500°C, being greatly different from raw PM. Additionally, the final oxidation temperature has large differences for PM before and after pre-treatment [48,51–53]. As seen in Figure 2, the microstructures change greatly after being pre-treated in a N2 atmosphere that is one of the reasons contributing to the differences of oxidation temperatures as mentioned in reference [27]. Based on the above discussions, a new method should be raised that the VOC can be removed at room temperature and in an inert gas atmosphere. The VOC contained in diesel PM can be removed gradually by periodic vacuumizing in N2 atmosphere and room temperature, then the devolatilized PM is applied to the TGA experiments. This method can precisely test the influence of VOC on soot oxidation behaviors without altering soot microstructures. Further, the oxidation kinetic parameters of diesel PM (activation energy, pre-exponential factor and oxidation rate constant) are calculated based on TGA results [30,45,55–57]. The accuracy of the calculated parameters is greatly dependent on  the  TGA  results  as  indicated  in  eqs. (1)  and  (2) [20,55,56]. Oxidation kinetic parameters vary in the process of PM oxidation that is caused by the changes of physicochemical properties of diesel PM. Appropriate sample mass and gas flow rate are necessary to avoid heat and mass transfer limitations [58]. The heat and  mass  transfer  limitations

 

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   HRTEM Figure 2        images of diesel PM before and after pre-treatment. Specifications of the diesel engine are shown in literature [54]. The diesel PM was sampled at 80% engine load. Pre-treatment procedure: heated to 450°C at 15°C min‒1 from room temperature in N2 atmosphere.

control the oxidation behaviors of diesel PM that leads to the kinetic parameters deviate from the true values. Kinetic parameters calculated based on TGA experiments of diesel PM both before and after pre-treatment should be the “apparent values”, because of the VOC, pre-treatment and the assumption in the derivations of calculation formula [20].

ln( d / dT ) = ln[Af ( )] ln = ln[AE / Rg( )]

E / RT ,

2.315

(1)

0.4567E / RT .

(2)

   Factors influencing diesel PM oxidation 3   PM oxidation behaviors are complex processes that are closely related to PM physicochemical properties. The associative factors are mainly divided into two parts: factors being involved with diesel engines; factors uncorrelated with diesel engines. Most references [59–63] were focused on the engine related factors. For PM sampled in different types of diesel engines, oxidation behaviors differ greatly [51,64]. No matter the engine designs (compression ratio, valve timing,

displacement and the shape of combustion chamber), the effects of design parameters in diesel engines on PM oxidation behaviors are caused by cylinder combustion conditions [65–67]. When diesel PM is emitted from engine cylinders, the after-treatment technologies (diesel particulate filter (DPF), diesel oxidation catalyst (DOC), NTP and particle oxidation catalyst (POC) so on) are applied to reduce PM emissions. Much catalyst is coated on the carrier of POC and DOC that leads to the partial oxidation of hydrocarbon (HC) and soot [2]. Many active ions generate in the plasma zone of NTP reactor, and the active ions own strong oxidizing property [20,54]. PM physicochemical properties change greatly after PM flows through the after-treatment devices that causes the differences in the oxidation behaviors [68,69]. Previous researches concerning the oxidation behaviors of diesel PM are summarized in Table 1. Oxidation temperatures of PM sampled from diesel engines fuelling with biodiesel are low compared with diesel PM. Biodiesel fuel is widely used due to the lack of fossil fuel and to reduce greenhouse gas emissions [60]. The performances and emission characteristics of diesel engines fuelling with biodiesel are investigated [7,9,11–13]. Large amount of oxygen-containing organic compounds contained in biodiesel conduces to fuel combustion in cylinders that leads to the changes of soot microstructures and ingredients. No matter the engine design, engine operation conditions, diesel fuel and after-treatment technologies, the oxidation behaviour differences are considered to be caused by microstructures and ingredients that are visualized by advanced technologies (X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectra so on). Also, the ingredients influence the microstructures of diesel PM, for example, the amorphous carbon is caused by oxygen-containing organic compounds [76,77]. Factors affecting the oxidation behaviors of diesel PM  are  classified  as  engine-

Table 1    Oxidation behaviors of diesel PM Activation energy (kJ mol−1)

T10% (°C)

T90% (°C)

Temperature scope (°C)

Sharma et al. [28]

154–172

541

685

410–695

Stratakis et al. [44]

80–165

400

550

250–580

Collura et al. [70]

−

510

645

245–660

Ruiz et al. [71]

−

460

575

255–585

Meng et al. [45]

143–202

511

634

470–650

Abian et al. [47]

182

595

685

485–745

Chang et al. [72]

170

405

615

100–625

Wang et al. [30]

145

457

555

400–570

Ma et al. [20]

−

491

655

100–673

Lapuerta et al. [73]

−

387

500

327–517

Wang et al. [74]

−

150

650

100–672

Qu et al. [75]

−

160

637

100–651

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correlated and engine-uncorrelated factors. The engine-correlated factors include the engine design, engine operating conditions, fuel and so on, further the effects of the engine-correlated factors on PM oxidation behaviors are showed through microstructures and ingredients of diesel PM. The engine-uncorrelated factors refer to the ones caused by TGA experiments, atmosphere, aging and so on. In the following sections, the detailed analysis of the factors related to PM oxidation behaviors is provided.    Engine-correlated factor 3.1   Engine design, engine operation conditions and fuel influence the cylinder combustion conditions that lead to the differences in microstructures and ingredients in diesel PM. When diesel engine runs at different speed and load, the cylinder combustion conditions differ enormously [78–80]. References [81–88] investigated the effect of biodiesel on the engine combustion characteristics, performance and emissions. Buyukkaya [84] researched the variation of cylinder pressure and heat release rate when the engine fuelled with biodiesel (rapeseed oil). The high viscosity and low volatility of biodiesel fuel have bad effect on the forming of air-fuel mixture. It showed that the ignition delay was shorter for neat biodiesel compared with that of standard diesel. That leads to the cylinder pressure decrease with increasing biodiesel addition in the blends. The conclusions are similar to the results in other references [89–91]. The cylinder combustion leads to the differences in PM compositions and microstructures that show closed relations with PM oxidation behaviors. All the enine-correlated factors can influence PM microstructure and composition being the fundamental factors of different oxidation behaviors. The method that makes the PM ingredients and microstructure as the principle line is used to correlate the engine-related factors with PM oxidation behaviors indirectly. 3.1.1   PM composition-related factor The differences of engine design, engine operation conditions and fuel cause the changes of PM compositions that are one of the fundamental factors leading to the differences in PM oxidation behaviors. This section mainly reviews the effect of PM compositions on oxidation behaviors that the differences in PM compositions are caused by the engine-related factors. The compositions of diesel PM depend on the engine designs and operation conditions when engines fuel with fossil fuels [33,92]. Being an oxygenated, alternative fuel to alleviate energy dilemma, biodiesel is mixtures of mono-alkyl esters derived from vegetable oils and animal fats [93–95]. Generally, biodiesel features higher density, flash point, Cetane number and viscosity that lead to the differences of PM ingredients due to different cylinder combustion conditions [82,83,94,95]. Figure 3 shows the PM compositions of a heavy duty diesel engine in a transient cycle. As

mentioned in reference [92], ash mainly consists of metallic species as Al, Na, Mg, Zn and Ca which stem from engine wear, lubricating oil additives and catalytic converter. Reference [62] pointed that the biodiesel is a significant contribution to ash production. The ash contained in diesel PM is an order of magnitute lower than does conventional GDI soot that the ash mainly comes from lubricating oil additives, such as zinc dialkyldithiophosphates (ZDDP) and calcium sulfonate [92]. The ash compositions that emitted to the atmosphere were also analyzed by scanning electron microscope (SEM)-energy disperse spectroscopy (EDS) methods. Liati et al. [96] observed the diesel PM ash using SEM and transmission electron microscope (TEM) technology. The individual ash is mostly with sizes between 7 and 170 nm, the same order in primary diameter with PM. Figure 4 shows the EDX spectra of PM ash, the metal content can be obtained by the spectra [96,97]. Investigations [92,98–104] have shown the influence of metal and metal oxide on soot oxidation that Ca, Mg and Zn have high catalytic activities on soot oxidation, while the Na shows good catalytic activity on hydroxide. The oxidation activity is usually denoted using oxidation temperature, oxidation duration and activation energy [34,54,105]. Choi and Seong [92] researched the effect of ash on soot oxidation activity. Ash precursors formed in cylinders are homogeneously dispersed in PM which conduces to perform the catalytic activity on PM oxidation. The temperatures at 20%, 50% and 90% mass loss decrease with increasing ash

   PM Figure 3        compositions of a heavy duty diesel engine in a transient cycle [41].

   EDX Figure 4        spectra of ash PM [96].

 

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fraction. Furthermore, the catalytic activity on soot oxidation is related to engine operation conditions that the catalytic action is higher under hot steady-state than others [92]. Bokova et al. [106] researched the catalytic activity of Ce-Al oxides on soot oxidation that it drops the activation energy and temperature corresponding to maximum mass loss rate (Tm) greatly. Mühlbauer et al. [2] indicated that Tm decreases with increasing ash content in PM sampled at different engine conditions. However, the catalytic effect of ash on diesel soot oxidation is less significant than surface chemistry and crystallite structures because of low ash content in diesel PM. The catalytic activity of ash may be more evident for diesel engines equipped with DPF after-treatment devices. With periodical regeneration of PDF, more ash remains on the wall of DPF which leads to huger catalytic activity. The increase of the catalytic activity depends on the content of metallic species to some extent. The increase of the ash content with oxidation process may explain the phenomenon in some references [28,51] that activation energy of diesel PM drops gradually in the final oxidation stage. The fuel additive ash lowers the initial oxidation temperatures of soot emitted by a diesel engine [44,75,107,108], that it is an effective method to achieve DPF “passive” regeneration. Also, the catalytic activity of diesel fuel additive ash is enhanced by VOC. However, the VOC species that can enhance the catalytic activity is still unreported. The sulfate in ash hinders the touch of soot with metallic species. No references indicate that sulfate has influences on PM oxidation behaviors, while it may increase the porosity and specific surface area of soot aggregation. HC content is high compared with ash, especially for diesel PM sampled at low engine loads. However, the influence of HC on soot oxidation is still in debate. Stratakis and Stamatelos [44] argued that VOC has no influence on soot oxidation behaviors without fuel additives. Also, the kinetic parameters are unchanged at different VOC content conditions [44,70,109]. However, for diesel PM sampled at different start of fuel injection, the VOC content increases the apparent rate constants [42]. Oxidation temperatures of VOC are lower than soot, that its oxidation at low temperature may induce slow oxidation of soot. The oxidation and evaporation of soluble organic fraction (SOF) change soot microstructures greatly that shows closely relations with soot oxidation [45]. SOF enhances the oxidation rate, and lowers the energy input for DPF regeneration [27]. However, the amount of SOF is strongly dependent on engine speed and load. Clague et al. [109] indicated that SOF is mainly a product of lubricating oils rather than fuel. During the DPF regeneration process, the regeneration temperature should be based on the engine loads and speeds. SOF contained in PM influences PM ingredients, also, the soot microstructures (specific surface area, porosity, amorphous carbon and so on), it is hard to distinguish whether

the oxidation behaviors are affected by SOF itself or SOF-related microstructures. Collura et al. [33] argued that the influence of SOF on PM oxidation behaviors is caused by SOF-related microstructures other than SOF characteristic itself. Oxidation of SOF at low temperature increases the specific surface area of soot which determines the chemical adsorption with oxygen. SOF contained in PM is detected using gas chromatography-mass spectrometer (GC-MS), FTIR, nuclear magnetic resonance (NMR) spectra and X-ray photoelectron spectroscopy (XPS) technologies [33,71,109,110]. Figure 5 shows the FTIR spectra of PM emitted by diesel engines fuelling with diesel and biodiesel. Huge differences are observed at ~1710 cm−1 position which indicates the COOH content differences in diesel soot and biodiesel soot. Oxygen-containing functional groups lead to the amorphous carbon, also, it provides the active sites for PM oxidation [62]. The biodiesel soot is more easier to oxidised than diesel soot. So that the use of biodiesel fuel not only relieves the energy scarcity and reduces diesel emissions [81–88], but also conduces to DPF regeneration. Yehliu et al. [111] investigated the relations of apparent rate constants with oxygen contents obtained by XPS during soot oxidation. The opinion was emphasized by Agudelo et al. [112] that the oxidation rate constants show positive relations with aliphatic group contents. Song et al. [113] indicated that the oxygen content decreases in the process of PM oxidation for both FT soot and biodiesel soot. The oxygen content drops to zero when nearly 80% mass burnout during soot oxidation. 3.1.2   PM microstructures-related factors The engine-correlated factors influence PM microstructures that are directly correlated with PM oxidation behaviors. This section reviews PM microstructures tested by different technologies, and related the oxidation behaviors with different microstructures caused by the engine-correlated factors. Microstructures of diesel PM are mainly dependent on engine designs, engine operation conditions, fuel and so on. While the factors mentioned above affect the cylinder combustion conditions. The commonly used methods to obtained PM morphology are SEM and TEM [42,105,112,114,115]. TEM figures are intuitional to observe the soot crystallite arrangement while morphology of diesel PM obtained by SEM shows no direct relations with PM oxidation activity. HRTEM images can effectively show the relations of microstructures and oxidation behaviors qualitatively. Microstructures and compositions of diesel PM sampled from diesel engines change tremendously after PM flows through the after-treatments devices (DPF, DOC, POC and NTP) [52,116]. The near edge X-ray absorption fine structure (NEXAFS) spectra show a high absorption at “graphitic peak” position for soot sampled at post-DPF and post-DOC (exhaust flows through DOC and DPF  sequentially).  The  phenomenon  is  caused by partial oxidation of diesel PM. The  soot  crystallite  arranges  more

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   FTIR Figure 5        spectra of diesel and biodiesel PM [110].

orderly with smaller crystallite separation distance that is caused by DOC conducing to the oxidation of hydrocarbon (HC) adhering on PM. The percentage of graphite lamellae smaller than 0.35 nm increases when exhaust flows through DOC and DPF [52]. Soot sampled at post-DPF shows more saturated carbon that indicates high graphitization of soot. The graphitization of soot crystallite means the drop of soot oxidation activity. The reference [52] just described the microstructures changes qualitatively, without any analysis combining the changes with calculated oxidation parameters. Wang et al. [117] investigated the HRTEM images of PM sampled at different engine speeds. Both the PM samples collected at 1200 and 2000 r min‒1 show the core-shell like structures that are similar with other studies [42,118–120]. Su et al. [64] indicated that PM microstructures differed greatly for PM sampled from different engines that was consistent with other results [62,113]. Diesel soot from different origins shows various initial morphologies that depend on soot formation and sampling conditions [42,75,117,121]. Hollow interiors of diesel soot are observed that is caused by partial oxidation [114]. For soot with “onion-like” structures, it is considered to own higher oxidation activity than it with “core-shell” like structures. The “onion-like” structures show highly disordered crystallite, while “core-shell” like structures present void cores and highly ordered crystallite shell. Zhou et al. [122] investigated the impact of intake hydrogen on morphology, nanostructures and oxidation activity of diesel PM. The replacement of diesel fuel by hydrogen affects the combustion process, furtherly decreases the PM emissions. PM samples exhibit “core-shell” structures with hydrogen addition, however, it shows “onion-like” structures without hydrogen addition. The changes of microstructures mean the drop of oxidation activity for the addition of hydrogen. Song et al. [113] put forward a soot model based on HRTEM figures during soot oxidation process. The “onion-like” structures are almost with no changes during the initial oxidation stage, but with smaller diameter. Surface oxidation happens initially due to high oxidation activity of soot surface

with active sites. With oxidation proceeding, soot surface becomes more graphitized, then the oxidation converts to internal oxidation leading to the void cores. The activation energy increases gradually with the oxidation proceeding [28]. The oxidation model is consistent with experimental results in other reference [105]. Nanostructure parameters extracted from HRTEM images describe the relations of oxidation behaviors with PM structures quantitatively [62,111,123]. The commonly researched nanostructure parameters are crystallite fringe separation distance (Sf), crystallite fringe length (Sl) and crystallite fringe tortuosity (ST) that show closely connection with oxidation activity. Sf denotes the tightness of soot crystallite arrangement that small Sf means high orderly arrangement of soot crystallite. Sl is similar to the crystallite size calculated using XRD method, huge Sl is corresponding to high graphitizing degree. ST indicates the wrinkle degree that is related to specific surface area to some extent. Figure 6 shows the Sl and ST distributions for PM sampled from different engines. The distributions of fringe length and fringe tortuosity show shape of unimodal distribution. As can be seen, the distributions of fringe length and tortuosity differ greatly for diesel PM sampled from different engines. Vander Wal et al. [124] compared interplanar spacing calculated from XRD results with mean fringe separation distance extracted from HRTEM figures, as shown in Figure 7. The calculated values based on the two different methods are in consistent generally. Soot crystallite rearranges when soot is treated in high temperature. Heat treatment decreases the fringe separation distance which aggravates the carbon graphitization and enhances crystallite order. Highly ordered arrangement of soot crystallite leads to the decrease of oxidation activity and increase of activation energy. For diesel soot sampled at different start of injection, it shows smaller fringe length and huger fringe tortuosity when diesel injection retards 2° crank angle (CA) than advances. The reaction rate constant of soot for retarding injection is 2.3 times higher than advancing timing [42]. Advanced fuel injection leads to higher cylinder combustion temperature that causes more graphitized soot and lower oxidation activity. The combinations of oxidation behaviors with nanostructure parameters of diesel soot are necessary. Figure 8 shows the average Sl and ST versus activation energy. Huge fringe tortuosity indicates huge contact area of soot with oxygen to some extent that causes the drop of activation energy. Huge fringe length means highly ordered soot crystallite. Much work has been done to investigate diesel soot morphology to explain the oxidation behaviors, while it is ambiguous [31,105,125,126]. Song et al. [113] indicated that the fringe length increases and then drops in the oxidation process, while the tendency is inconsistent with the implication of fringe length. The fringe separation distance and tortuosity should be considered to explain the phenomenon. The  other

 

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   S Figure 6        l and ST distributions. (a) (b) Referred in [42]; (c) (d) referred in [118].

   Average Figure 7        Sf versus temperature [124].

two nanostructure parameters may make the compensation of fringe length. The three nanostructure parameters should be combined together to evaluate the oxidation activity rather than using single parameter. Further, the weigh of the three nanostructure parameters on soot oxidation activity should be researched. Multi-parameters are more accurate to evaluate soot oxidation behaviors than single-parameter evaluation. Raman spectra are non-destructive to diesel PM, and they are complementation for HRTEM. Raman spectra show sensitivity to crystallite structures and molecular structures of

carbon-containing materials [127–129]. The Raman signals of carbon-containing materials result from lattice vibration. Raman spectra contain information about amorphous carbon and graphitization being related to oxidation behaviors, while the discussions are still in debate [34,130–132]. Fringe length obtained from HRTEM images should be comparable to crystallite width calculated using XRD and Raman spectra parameters. Figure 9 compares the crystallite width calculated using XRD and Knight and White method for soot with different origins. The crystallite width calculated based XRD results are similar to the results calculated using Knight and White method. The crystallite size is inversely proportional to intensity ratio of D peak over G peak (ID/IG), as shown La=4.4(ID/IG)−1 (where La is crystallite size) [133]. In comparison, HRTEM images provide visible information about soot crystallite arrangement that the nanostructures of diesel PM can be observed directly. Raman spectra of diesel soot show double peaks, that are G (~1590 cm−1) and D (~1350 cm−1) peaks. G peak is caused by the stretching mode E2g symmetry at sp2 sites, while it is still under debate for D peak. For Raman spectra, curve-fitting methods are used to extract Raman parameters that denote the soot microstructures, and are related to soot oxidation activity. As shown in Figure 10, the Raman spectra are processed,

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   Average Figure 8        Sl and ST versus activation energy [105].

activity. The active surface area is measured by the amount of chemisorbed oxygen in PM. References [2,54,120,131,135] investigated the correlations of Raman parameters (ID/IG, area ratio of D peak over G peak (AD/AG) and full width at half maximum (FWHM)) with soot oxidation activity. Figure 11 shows Raman parameters against oxidation activity of soot from different sources. The oxidation activity increases with ID1/IG, AD1/AG and D1 FWHM increasing while it is inversed for G FWHM. D3 band is related to active sites and active surface area that higher values of D3 FWHM indicate higher oxidation activity. For soot emitted by engines fuelling with biodiesel, more oxygen-containing functional groups are detected that cause it easier to oxidise than diesel soot. Gao et al. [54] also indicated that the apparent oxidation rate constant increased with D1 FWHM, and the activation energy increased with G FWHM. Ruiz et al. [71] pointed that engine loads have negligible impact on Raman parameters (ID1/IG and ID3/IG), while they change greatly for soot emitted by a diesel engine fuelling with diesel and biodiesel fuel. The tendency of Raman parameters during oxidation process is inconsistent [54,121,136]. D1 FWHM changes slightly during initial oxidation stage for soot emitted by a Euro IV diesel engine, however, it decreases greatly for Euro VI diesel soot [121,136]. D3 FWHM decreases with the oxidation proceeding that it is an indicator denoting the content of oxygen-containing functional groups, the oxygen content drops to zero until 80% mass loss [113]. Less oxygen-containing functional groups causing less active sites and smaller active surface area that may lead to the increase of activation energy at the final stage of oxidation. The ID/IG ratio of partially oxidized soot is obtained, that it decreases from 4.72 to 3.45 [131]. The drop of ID/IG ratio means more graphitized and orderly arrangement of soot crystallite. The phenomenon coincides with HRTEM figures obtained during oxidation. Knight and  White  [137]  indicated  that  the  peak  position

   Crystallite Figure 9        width calculated using XRD and Knight and White method [34].

and different peaks are observed at different Raman shifts, D1: ~1350 cm−1, D2: ~1620 cm−1, D3: 1~500 cm−1, D4: ~1200 cm−1, G: ~1590 cm−1 [134]. D1 and G peaks are related to the degree of soot disorder and graphitization (edge sites and basal defects) respectively. D3 peak is caused by oxygen-containing functional groups which provide active sites for soot oxidation, also, it means the amorphous carbon. Reference [135] indicated that the active surface area with active sites adhering was in good agreement with soot oxidation

   Raman Figure 10        spectra [54].

 

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   Raman Figure 11        parameters versus oxidation activity [34]. (a) ID1/IG and AD1/AG versus oxidation activity; (b) G and D1 FWHM versus oxidation activity.

changes slightly during the oxidation process that is caused by the changes of C–H. Raman parameter changes of ID1/IG and ID3/IG with temperature are anomaly in references [138,139] that it increases and then decreases. The tendency of ID/IG during soot oxidation is in agreement with Ferrari’ results [140]. Ferrari and Robertson [140] divided the process of amorphous carbon converting to perfect graphite into three stage, ID/IG increases then drops. It is argued that it is “invisible” to Raman signals for soot crystallite smaller than a certain size. Cylinder temperature of diesel PM formation indicates the soot graphitization to some extent [141,142], and shows negative correlation with VOC percentage and active specific surface area (ASSA). Ruiz et al. [71] determined the ASSA through the amount of oxygen chemisorbed on soot, and ASSA shows positive correlations with VOC content. Microstructures of soot formed at a high temperature atmosphere are with void cores and orderly arranged shells that show low oxidation activity. As referred, the G FWHM, ID3/IG and ID1/IG drop with increasing soot formation temperature that indicates less reactive for soot oxidation [76]. Al-Qurashi and Boehman [131] investigated the impact of exhaust gas recirculation (EGR) on diesel soot oxidation activity that the ID/IG value increases by 10.2% when the EGR ratio increases from 0% to 20%. Diesel soot formed with 20% EGR is more reactive than it with 0% EGR that is in good agreement with HRTEM figures. The cylinder

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temperature is low for diesel soot with 20% EGR that leads to less graphitized and less orderly arrangement of soot crystallite. HRTEM figures of diesel soot with 20% EGR show void cores at initial oxidation stage that also verifies the results of Raman spectra. However, Iwata et at. [143] indicated that soot with higher EGR rate was less reactive comparing to the soot with low EGR rate regardless of the engine conditions. The primary particle sizes of diesel soot increased with increasing EGR rate that was caused by EGR dilution effect [144]. Huger primary particle size means smaller specific surface area of diesel soot that cause less chemisorption on oxygen during oxidation process. Specific surface area is closely related to contact area of soot with oxygen during oxidation process, and the oxidation activity is seriously dependent on ASSA. Agudelo et al. [112] obtained the same results using similar method for soot emitted by a diesel engine fuelling with blends of crude vegetables oils and diesel. Figure 12 shows the ASSA versus oxidation activity of diesel PM sampled at different engine loads when diesel blends with different percentage of n-butanol and hydrous ethanol. As seen, the oxidation activity presents positive correlations with ASSA. The results were obtained by Ruiz et al. [71] that it also includes the influence of soot graphitization and oxygen-containing functional groups rather than the only actions of ASSA. The profiles of pore diameter versus total surface area present double peak shape, and porosity shows inconsistent relations with oxidation activity [112,130,145]. Average diameters of FT soot decrease with oxidation proceeding. The decrease is small at the initial stage, and is aggravated at final stage [113]. Xu et al. [146] argued that smaller particle size results in larger specific surface area favoring soot oxidation, while no experiment is done to verify the opinion. Particle number concentration and diameter distribution differ greatly at different engine operation conditions. Diesel after-treatment technologies (DPF, DOC, POC, NTP and so on) have great influence on number concentration and diameters  distribution   [52,116,147,148].  DPF  effectively

   (Color Figure 12        online) ASSA against oxidation activity [71].

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reduces particle number concentration and diameter, and catalyst adhering on carriers of DOC and POC partially oxidizes diesel PM, also, the active ions formed in the plasma zone of NTP reactor can decrease the diameter. Particle number concentration and diameter distribution have huge influence on specific surface area and porosity that show closed relations with PM oxidation behaviors. While the literature is scare to date the relations of particle number concentration and diameter distribution with PM oxidation behaviors. Experiments should be done to correlate to the number concentration and diameter distribution with oxidation activity.    Engine-uncorrelated factors 3.2   Oxidation activity decreases greatly for partially oxidized PM, the active sites of partially diesel PM reformed after retention in an oxidizing atmosphere. For PM captured on the filter of DPF, it may adhere on DPF for hours or days. Due to the low temperature during aging, no microstructure changes happen. The oxidation activity restored after aging in the oxidative atmosphere. Figure 13 shows the isothermal TGA experiments of partially oxidized PM after aging in different atmospheres. No signs of high activity restoration were observed after being aging in a He atmosphere. The oxidation activity restoration was observed after retaining in air. Before the isothermal experiments, the sample was pre-treated in He atmosphere to exclude the possibility that the restoration was caused by the adsorption of hydrocarbons and other species. Lambe et al. [149] investigated the oxidative aging for laboratory combustion soot in a OH atmosphere, and inorganic secondary coatings was observed. Yezerets et al. [29] also demonstrated the restoration of high initial oxidation activity by exposure of PM in an air atmosphere. This restoration of oxidation activity was tested by temperature-programmed oxidation. The oxidation restoration is enhanced for longer time remained in an air atmosphere, further, restoration is considered to be caused by chemical ingredients, not morphology. However, the speculation is still not verified by experiments. Long time aging in an air atmosphere conduces to the formation of highly reactive groups on soot surface that is good for soot oxidation. In references [29], the oxidation activity restoration was tested, however, the reasons are only the deduction. The oxygen-containing functional groups and active surface area should be tested using FTIR and XPS methods for soot before and after being exposure to the aging atmosphere. Browne et al. [150] observed the oxidation of HC by OH, however, oxidation activity is weakened though condensation and coagulation happen during aging process. As referred in Figure 2, the morphology of diesel PM changes greatly after being pre-treated in a N2 atmosphere. The morphology is the critical factor influencing PM oxidation behaviors, while few work has been done to evaluate the influence of pre-treatment on PM oxidation behaviors. Experimental conditions of TGA also affect PM

oxidation behaviors, such as sample mass, gas flow rate, oxygen content and catalyst [32,58,106,151–153]. References [58,106,152] investigated the heat and mass transfer limitations caused by improper sample mass and gas flow rate. Stanmore and Gilot [153] investigated the influence of sample mass on TGA measurement of carbon black activity. Huge sample mass causes high oxygen consumption rate and oxygen mass flux density. If mass transfer inside the crucible occurs that is predominated by molecular diffusion. For regular TGA curves during oxidation stage, mass loss rate curve (>450 °C) is with one broad and smooth peak. Heat and mass transfer limitations are caused by fast oxidation of soot, as shown in Figure 14. In the temperature zone 450−700 °C, two peaks are observed for differential thermogravimetry (DTG) curves that the first one is deep and sharp, while the second one is smooth and broad. The first peak is caused by the heat transfer limitation, the second peak is the normal one. Inflection points are observed in the corresponding temperature zones for TGA curves as circled by dotted line in Figure 14. Heat generated in soot oxidation process is with limitation to transfer out that induces the soot

   Isothermal Figure 13        experiments after PM aging in different atmospheres. (a) After 3 month aging in air; (b) based on the condition of (a), then, after 1 d under He; (c) based on the condition of (b), then, after 8 d under He; (d) based on the condition of (c), then, after 3 more months in air [29].

   Heat Figure 14        and mass transfer limitations.

 

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to fast burn causing the steep runway peak. Kinetic parameters based on TGA curves with heat and mass transfer limitations are seriously distorted. Heat and mass transfer limitations can be eliminated by decreasing sample mass and oxygen concentration, and increasing gas low rate. Brilhac et al. [32] investigated the oxygen diffusion within the soot packed beds. Soot model is established with simplified one and the oxygen flux density is based on Fick’s law. The diffusion coefficient of oxygen increases with porosity of soot, further, high oxygen diffusion coefficient increases the oxidation activity. In the theory model, the soot is assumed to be regularly placed that is seriously distorted with the true model. Oxygen diffusion within diesel PM influences the PM oxidation behaviors greatly. References studied the effect of oxygen content on PM oxidation behaviors [28,154–156]. The specific rate constant increases with oxygen partial pressure that the increase is evident for oxygen partial pressure smaller than 0.004 bar while it is little when the partial pressure higher than 0.004 bar [154]. The oxidation profiles move towards to low temperature with increasing oxygen content, and higher oxygen content decreases the activation energy [155]. The oxygen transfer limited the soot oxidation when oxygen partial pressure is small [155]. NO2 owns strong oxidizability that conduces to the oxidation of diesel PM [20]. While little reference mentioned the PM oxidation behaviors in NOx atmosphere.

Single-factor analysis of PM physicochemical properties with oxidation activity was unreasonable sometimes as showed in reference [26]. Gao et al. [26] indicated that the activation energy increased during PM oxidation process showing the decrease of oxidation activity. While the three nanostructure parameters (fringe separation distance, length and tortuosity) showed no obvious tendency with activation energy, and the relations of single-parameter with oxidation activity were ambiguous. The authors combined the three parameters with activation energy that the influence of the three parameters on PM oxidation behaviors was evident and consistent with their physical significance. PM oxidation behaviors depend on comprehensive factors including both nanostructures and ingredients [2,3,157]. The factors associated with PM oxidation behaviors mentioned above are summarized in Figure 15. Oxidation behavior of diesel PM is related to DPF regeneration, while it differs greatly for PM sampled from different engines and operation conditions. As can be seen in Figure 15, the effect of factors on PM oxidation behaviors is reflected by microstructures and ingredients. The engine design parameters are constant for a specific diesel engine, the engine operation conditions and fuel lead to different cylinder combustion conditions that cause the diversity in microstructures and ingredients. The design of the regeneration devices should consider the oxidation-related factors, but also the economic, costing  and

   Summary Figure 15        of factors related to diesel PM oxidation behaviors.

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complexity of devices.

   Summary 4   DPF is the most successful technology to decrease diesel PM emissions with the necessary of periodic regeneration to keep excellent performance. PM oxidation is a comprehensive progress that many factors both engine-correlated and engineuncorrelated are closely related to oxidation activity. Review of the factors affecting the PM oxidation behaviors makes the foundation of DPF regeneration. TGA, DSC and numerical modelling methods to test PM oxidation activity are reviewed. The TGA method for both raw and devolatilized PM has some disadvantages to calculate the oxidation kinetic parameters. The devolatilization process of raw PM, the physicochemical properties change greatly that influences the precision of kinetic parameter calculation. The DSC-based method with the hypothesis that the heat release is proportional to oxidation-caused mass loss in the PM oxidation process is put forward to eliminate the disadvantages of TGA method. And the numerical modelling method is less reliable than TGA and DSC methods to reflect the PM oxidation behaviors. Microstructure and ingredient being the fundamental factors affecting PM oxidation behaviors are as the principle line to review the engine-correlated factors. Diesel ash has catalytic actions on soot oxidation, and the catalytic actions are enhanced with the presence of SOF. With the proceeding of the periodic DPF regeneration, the ash adhered on DPF filter walls increases that lead to higher catalytic performance. Oxygen-containing functional groups provide active sites and surface conducing to soot oxidation. Also, the oxygen-containing organic compounds contained in PM cause the amorphous carbon that can be observed by HRTEM figures and Raman spectra. Microstructures are vital factors influencing soot oxidation. Higher porosity and smaller primary diameter cause higher specific surface area contributing to oxygen chemisorption during oxidation process. Soot crystallite arrangement and graphitization tested by XRD, HRTEM and Raman spectra are in good agreement. The TGA experiment conditions have huge influence on PM oxidation behaviors that distortions in TGA curves can be observed if the heat and mass transfer limitations happen. For the partially oxidized PM, the oxidation activity was restored after aging in air atmosphere that was considered to be caused by chemical ingredients rather than morphology. Multiple-factor analysis rather than single-factor analysis should be developed to further figure out oxidation behaviors of diesel PM. Though many experiments have been done to investigate the oxidation behaviors, some result is still ambiguous. Also, the advanced technology testing PM oxidation behaviors with more precision should be developed without destroying PM nanostructure. The specific SOF that en-

hances catalytic actions of ash on PM oxidation should be investigated. The reasons of oxidation activity restoration by aging in an oxidizing atmosphere should be figure out by additional experiments. This work was support by the Science and Technology Planning Project of Hebei Province, China (Grant No. 15273703D). 1

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