Impact of Intake Oxygen Enrichment on Oxidative Reactivity and

Energy Fuels 2011, 25, 602–616 Published on Web 01/18/2011

: DOI:10.1021/ef101108j

Impact of Intake Oxygen Enrichment on Oxidative Reactivity and Properties of Diesel Soot Hee Je Seong and Andr
e L. Boehman* EMS Energy Institute, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802, United States Received February 25, 2010. Revised Manuscript Received December 9, 2010

Oxygen addition to a four-cylinder turbo-charged common rail diesel engine was carried out by intake oxygen enrichment and by fuel oxygenation in order to study the effect of additional oxygen on oxidative reactivity and properties of diesel soot at low and high loads. The analyses of heat release rates and cylinder temperatures indicate that the effect of oxygen enrichment is more appreciable at high load than at low load. Correspondingly, there are more noticeable changes in crystalline structure and oxidative reactivity of soot from high load than of soot from low load. However, the surface O content on the soot does not show a consistent trend at low and high loads with oxygen enrichment. In addition, soot generated from high load with intake oxygen enrichment is observed to contain some metallic species, whereas soot from low load has little or no metallic species, which is attributed to the oxidation of lubricating oil by a synergistic effect of high oxygen concentration and high temperature. There might be an effect of the soot crystalline structure on soot oxidative reactivity, but the major influence on the reactivity is shown to be closely related to the amounts of metallic species present in soot due to the catalytic role of metallic species in soot oxidation.

and increased oxygen concentration in the intake air (intake oxygen enrichment),10-13 also has been investigated for the reduction of diesel emissions. As evidenced by these studies, both oxygen enrichment methods are effective at reducing PM, CO, and HC, but the trends in NOx emissions depend upon the method of oxygen enrichment. The role of increased oxygen in reducing PM has been of great interest. Emissions of engine-out soot are the result of competition between soot formation and soot oxidation during the combustion process. According to many studies of premixed and diffusion flames, oxygenated fuels are found to reduce soot formation by suppressing soot precursors because CO and/or CO2 are directly formed from the fuel, resulting in fewer carbons available for soot formation.8,14 It is unclear whether intake oxygen enrichment leads to higher or lower rates of soot formation, but an increased oxygen concentration in the combustion process is always accompanied by increased flame temperature, which enhances soot oxidation.14 Stricter regulation of diesel particulate matter emissions has required the application of diesel particulate filters (DPFs) and motivated research in the oxidative reactivity of diesel soot.6,15 Because burnoff of the particulate matter, particularly

Introduction Tailpipe emissions have been of great concern for dieselpowered vehicles despite their robust and efficient engine performance. Because of the trade-off in the relationship between particulate matter (PM) and oxides of nitrogen (NOx), it is difficult to minimize emissions of NOx and PM simultaneously. There have been many studies on reducing both emissions by way of changing the engine operating strategy,1-3 fuel modification,4,5 and exhaust aftertreatment systems.6 Oxygen enrichment, which is achieved with oxygenated fuels7-10 *To whom correspondence should be addressed. Telephone: 814-8657839. Fax: 814-863-8892. E-mail: [email protected]. (1) Poola, R. B.; Sekar, R. Reduction of NOx and particulate emissions by using oxygen-enriched combustion air in a locomotive diesel engine. J. Eng. Gas Turbines Power 2003, 125, 524–533. (2) Yun, H.; Reitz, R. D. An experimental investigation on the effect of post-injection strategies on combustion and emissions in the lowtemperature diesel combustion regime. J. Eng. Gas Turbines Power 2007, 129, 279–286. (3) Lilik, G. K.; Herreros, J. M.; Boehman, A. L. Advanced combustion operation in a compression ignition engine. Energy Fuels 2009, 23, 143–150. (4) Kegl, B. NOx and particulate matter (PM) emissions reduction potential by biodiesel usage. Energy Fuels 2007, 21, 3310–3316. (5) Wang, W. G.; Lyons, D. W.; Clark, N. N.; Gautam, M.; Norton, P. M. Emissions from nine heavy trucks fueled by diesel and biodiesel blend without engine modification. Environ. Sci. Technol. 2000, 34, 933– 939. (6) Twigg, M. V. Progress and future challenges in controlling automotive exhaust gas emissions. Appl. Catal., B 2007, 70, 2–15. (7) Song, K. H.; Nag, P.; Litzinger, T. A.; Haworth, D. C. Effects of oxygenated additives on aromatic species in fuel-rich, premixed ethane combustion: A modeling study. Combust. Flame 2003, 135, 341–349. (8) Mueller, C. J.; Martin, G. C. Effects of oxygenated compounds on combustion and soot evolution in a DI diesel engine: Broadband natural luminosity imaging. SAE (Tech. Pap.) 2002, DOI: 10.4271/2002-01-1631. (9) Chen, H.; Shuai, S.-J.; Wang, J.-X. Study on combustion characteristics and PM emission of diesel engines using ester-ethanol-diesel blended fuels. Proc. Combust. Inst. 2007, 31, 2981–2989. r 2011 American Chemical Society

(10) Song, J.; Zello, V.; Boehman, A. L. Comparison of the impact of intake oxygen enrichment and fuel oxygenation on diesel combustion and emissions. Energy Fuels 2004, 18, 1282–1290. (11) Desai, R. R.; Gaynor, E.; Watson, H. C.; Rigby, G. R. Giving standard diesel fuels premium performance using oxygen-enriched air in diesel engines. SAE (Tech. Pap.) 1993, 932806. (12) Karim, G. A.; Ward, G. The examination of the combustion processes in a compression-ignition engine by changing the partial pressure of oxygen in the intake charge. SAE (Tech. Pap.) 1968, 680767. (13) Rakopoulos, C. D.; Hountalas, D. T.; Zannis, T. C.; Levendis, Y. A. Operational and environmental evaluation of diesel engines burning oxygen-enriched intake air or oxygen-enriched fuels: A review. SAE (Tech. Pap.) 2004, DOI: 10.4271/2004-01-2924. (14) Tree, D. R.; Svensson, K. I. Soot processes in compression ignition engines. Prog. Energy Combust. Sci. 2007, 33, 272–309.

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: DOI:10.1021/ef101108j

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diesel soot, is a critical process in DPF systems, the effect of the physical and chemical properties of diesel soot on reactivity has been of interest for many researchers: engine operation methods can be developed in the way that more easily burnable diesel soot is generated unless they have negative effects on engine performance, fuel economy, and other emissions. As evidenced by transmission electron microscopy (TEM),16-20 Raman spectroscopy,21-23 X-ray diffraction (XRD),23,24 near-edge X-ray absorption fine structure (NEXAFS),25,26 X-ray photoelectron spectroscopy (XPS),20,27 and other techniques,25,26 it is well known that the oxidative reactivity of diesel soot is strongly affected by soot properties, which include fringe length and curvature of the graphene layers within the primary soot particles, ordering of the crystalline structure of the primary soot particles, and surface oxygen content of the soot. Vander Wal and co-workers have shown in a series of papers that soot may possess different initial nanostructures in terms of fringe length and curvature distributions depending upon the fuel from which the soot is formed.16-18 Increasing the degree of fuel oxygenation leads to an increasing degree of amorphous nanostructure within the primary soot particles, characterized by narrow fringelength distributions and larger tortuosity.19 Song et al. also examined the oxidative reactivity difference between soots from combustion of biodiesel and Fischer-Tropsch (FT) fuels.28 Although the initial nanostructures of soot samples obtained from the engine exhaust were similar, biodiesel soot

Table 1. Engine Specifications engine

DDC/VM Motori 2.5 L

configuration displacement combustion system compression ratio air induction rated power peak torque injection system valve train

in-line 4 2499 cm3 direct injection 17.5:1 turbo-charged intercooled 103 kW at 4000 rpm 340 N m at 1800 rpm electronically controlled common rail DOHC, 4 valves/cylinder

included more oxygen functional groups, underwent an internal burning, and was observed to be more oxidatively reactive than FT soot, whereas FT soot oxidized through a shrinking core process. According to M€ uller and co-workers, more oxidatively reactive soot contains smaller and more strongly bent graphene layers within the primary particles, more sp3 hybridization, and higher surface oxygen content.20,27 Traditionally, intake oxygen enrichment has been of great interest in reducing soot emissions by enhancing the soot oxidation process. Most studies have focused on engine-out PM emissions, so there is a need to investigate the effect of intake oxygen enrichment on the physical and chemical properties of soot in order to better understand the combustion process under oxygen-enriched conditions in engines. In addition, the understanding of the contribution of soot properties to soot oxidative reactivity is of ongoing interest for DPF applications. Accordingly, in the present work, intake oxygen enrichment was employed and compared to an oxygenated fuel at low and high load conditions in order to probe the combustion process in different ways. From the characterization of soot properties, the impact of intake oxygen enrichment on soot oxidative reactivity was examined with respect to soot properties. Furthermore, soot crystalline structure is discussed, in relation to the soot formation and soot oxidation processes.

(15) van Setten, B. A. A. A.; Makkee, M.; Moulijn, J. A. Science and technology of catalytic diesel particulate filters. Catal. Rev. 2001, 43, 489–564. (16) Vander Wal, R. L.; Tomasek, A. J. Soot oxidation: Dependence upon initial nanostructure. Combust. Flame 2003, 134, 1–9. (17) Vander Wal, R. L.; Tomasek, A. J. Soot nanostructure: Dependence upon synthesis conditions. Combust. Flame 2004, 136, 129–140. (18) Vander Wal, R. L.; Tomasek, A. J.; Pamphlet, M. I.; Taylor, C. D.; Thompson, W. K. Analysis of HRTEM images for carbon nanostructure quantification. J. Nano. Res. 2004, 6, 555–568. (19) Vander Wal, R. L.; Mueller, C. J. Initial investigation of effects of fuel oxygenation on nanostructure of soot from a direct-injection diesel engine. Energy Fuels 2006, 20, 2364–2369. (20) M€ uller, J.-O.; Su, D. S.; Wild, U.; Schl€ ogl, R. Bulk and surface structural investigations of diesel engine soot and carbon black. Phys. Chem. Chem. Phys. 2007, 9, 4018–4025. (21) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; P€ oschl, U. Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information. Carbon 2005, 43, 1731– 1742. (22) Knauer, M. K.; Carrara, M.; Rothe, D.; Niessner, R.; Ivleva, N. P. Changes in structure and reactivity of soot during oxidation and gasification by oxygen, studied by micro-Raman spectroscopy and temperature programmed oxidation. Aerosol. Sci. Technol. 2009, 43, 1–8. (23) Al-Qurashi, K.; Boehman, A. L. Impact of exhaust gas recirculation (EGR) on the oxidative reactivity of diesel engine soot. Combust. Flame 2008, 155, 675–695. (24) Ebert, L. B.; Scanlon, J. C.; Clausen, C. A. Combustion tube soot from a diesel fuel air mixture: Issues in structure and reactivity. Energy Fuels 1988, 2, 438–445. (25) Braun, A.; Shah, N.; Huggins, F. E.; Kelly, K. E.; Sarofim, A.; Jacobsen, C.; Wirick, S.; Francis, H.; Ilavsky, J.; Thomas, G. E.; Huffman, G. P. X-ray scattering and spectroscopy studies on diesel soot from oxygenated fuel under various engine load conditions. Carbon 2005, 43, 2588–2599. (26) Braun, A.; Huggins, F. E.; Shah, N.; Chen, Y.; Wirick, S.; Mun, S. B.; Jacobsen, C.; Huffman, G. P. Advantages of soft x-ray absorption over TEM-EELS for solid carbon studies;A comparative study on diesel soot with EELS and NEXAFS. Carbon 2005, 43, 117–124. (27) M€ uller, J.-O.; Su, D. S.; Jentoft, R. E.; Wild, U.; Schl€ ogl, R. Diesel engine exhaust emission: Oxidative behavior and microstructure of black smoke soot particulate. Environ. Sci. Technol. 2006, 40, 1231– 1236. (28) Song, J.; Alam, M.; Boehman, A. L.; Kim, U. Examination of the oxidation behavior of biodiesel soot. Combust. Flame 2006, 146, 589–604.

Experimental Section Experimental Setup. The test engine was a four-cylinder turbo-charged common rail diesel engine whose specifications are shown in Table 1. It was operated at 30, 75, and 90% loads at 1800 rpm. An ETAS hardware and an INCA software interface were used to access an electronic control unit (ECU) on the engine, which permits adjustment of fuel injection parameters. Injection timing was held fixed and used a dual injection strategy (a pilot injection and a main injection) as the oxygen concentration and fuel composition were varied. The fuel consumption rate was measured gravimetrically by recording the fuel tank weight for a certain amount of time, and the mass flow rate of air into the engine was monitored using a Meriam laminar flow element. The cylinder pressure data were collected per 0.1° crank angle (CA), and 200-cycle averaged cylinder pressure data were used to calculate the apparent heat release rate. Oxygen addition to the combustion process was studied by intake oxygen enrichment and by fuel oxygenation. Intake oxygen enrichment was carried out by increasing the O2 concentration in the intake supply to the engine, which was achieved by supplying additional gaseous O2 from compressed cylinders to a surge tank where O2 was mixed with air, and the O2 concentrations in the intake air and exhaust gases were monitored using an AVL Combustion Emissions Bench II. The oxygenated fuel was a mixture of 30% diethylene glycol dimethyl ether (diglyme, C6H14O3) and 70% ultralow sulfur diesel fuel, whose properties are shown in Table 2. The detailed engine operating conditions and the ratio of oxygen to carbon (O/C) in 603

Energy Fuels 2011, 25, 602–616

: DOI:10.1021/ef101108j

Seong and Boehman

Flame soot and carbon black were also used to study the effect of metallic additives on oxidative reactivity. Flame soot was collected from a laminar diffusion flame burner using ethylene, where the carbon flow rate in the fuel stream is 0.39 g/min and the air flow rate in the oxidizer stream is 6.33 l/min, and carbon black was obtained from Alfa Aesar (lot no. L10M08).

Table 2. Properties of Diesel Fuel and Diglyme properties

diesel

diglyme

cetane number boiling point (°C) oxygen content (%) specific density (g/cm3) heating value (MJ/kg) sulfur content (ppm)

45 184/347 0 0.8466 42.8 9.7

112 162 35.8 0.943 30.3 0

Results and Discussion Apparent Heat Release Rate. Figure 2 displays apparent heat release rates computed from cylinder pressure traces at low and high loads. For 30% load, the start of pilot combustion is advanced with intake oxygen enrichment, and it is the most advanced with the 30% diglyme fuel mixture. Because injection timing is held constant in this study, the physical ignition delay is considered to be similar. Accordingly, the chemical ignition delay, which begins after the air-fuel mixing process, is more responsible for the advanced start of pilot combustion. Because oxygen in the oxygenated fuel is directly incorporated in the fuel, the ignition delay with oxygenated fuels is generally shorter than that with intake oxygen enrichment, despite the greater availability of oxygen with intake oxygen enrichment. However, the advanced start of pilot combustion for the 30% diglyme fuel mixture is due to a cetane number effect, by shifting the start of ignition for the reactant mixture to lower temperature and lower pressure.10 As a consequence of the shorter ignition delay, the peak heat release rate for the pilot combustion was lower because less fuel is available for combustion when ignition occurs. However, the effect of oxygen enrichment on the mixing-controlled burn is not noticeable in this study. Accordingly, oxygen enrichment seems to be limited in its effect on the mixing-controlled burn at low load. Heat release patterns at high load indicate much different trends from those at 30% load in the premixed burn and the mixingcontrolled burn with oxygen enrichment. An advanced start of the pilot combustion is also observed at 75% load with oxygen enrichment (see Table 4). The peak heat release in the pilot combustion is increased and an earlier start of the main combustion is also observed with oxygen enrichment. Because cylinder temperatures are higher at high load than at low load, the peak heat release patterns in the pilot combustion appear increased with oxygen enrichment despite the shorter ignition delay. Figure 2b also indicates that intake oxygen enrichment increased the heat release rate in the mixing-controlled burn compared to that of the 21% O2 condition. Because there is more oxygen available in the mixing-controlled burn phase with intake oxygen enrichment, a more rapid reaction occurred at higher O2 concentration in the intake air. On the other hand, there is no appreciable increase in the heat release rate of the mixingcontrolled burn for the 30% diglyme fuel because available oxygen at this condition, where the O/C ratio is 4.57, is not much higher than at the 21% O2 condition, where the O/C ratio is 4.48. At 90% load, there is a similar peak heat release during pilot combustion to that for 75% load, but the former shows a greater rate of heat release during the main combustion phase due to the increased fuel supply, resulting in a more vigorous mixing-controlled burn. According to Dec’s conceptual model,29 soot particles are generated just after the fuel-rich premixed combustion takes place. Soot volume increases in the diffusion flame along the

the engine for each condition are given in Table 3, and Figure 1 presents an overall schematic of the engine operation. Particulate matter (PM) was collected on 47 mm Teflon filters (Teflo membrane, Pall Corp.) from the engine exhaust using a vacuum pump, and the PM was subsequently scraped from the filters, which was repeated using several different filters at each operating condition to have sufficient amounts of samples for various characterizations. The particulate was then thermally treated at 500 °C for 60 min in a PerkinElmer TGA 7 under nitrogen in order to drive off volatile compounds. To observe the morphology of soot aggregates, thermophoretic sampling was employed at the tailpipe of the engine, as shown in Figure 1. Characterization Tools. After thermophoretic sampling of particles onto TEM grids at different engine operating conditions, particle morphology was analyzed using a Philips EM420T electron microscope and HR-TEM images of soot particles were obtained with a JEOL EM-2010F electron microscope. An SDT Q600 thermogravimetric analyzer from TA Instruments was employed to evaluate soot oxidative reactivity. Each diesel PM sample was placed in an alumina sample cup of 90 μL and was heated under N2 gas with 100 cm3/min to 500 °C and kept for 60 min to drive off volatile compounds. After this thermal treatment at 500 °C, the sample was heated to 550 °C, and N2 gas was replaced by ultra zero air (99.0% purity) at 100 cm3/min for isothermal experiments. XPS spectra were recorded using a Kratos Ultra XPS and postprocessed by using CasaXPS software in order to measure surface O, C, and other elements in the soot. The atomic composition of each element was calculated based upon the spectra with a Shirley background from survey scans, and high-resolution C 1s and O 1s were examined to identify surface oxygen functional groups. XRD patterns of soot samples were collected using a PANalytical X’Pert Pro MPD θ/θ goniometer with Cu KR radiation, fixed slit incidence (0.5° divergence, 1.0° antiscatter, specimen length of 10 mm) and diffracted (0.5° antiscatter, 0.02 mm nickel filter) optics. Resulting patterns were corrected for both 2θ position and instrumental peak broadening using NIST 640c silicon and analyzed with Jadeþ9 software by MDI of Livermore, CA. Using Jadeþ9 software, the full width at half-maximum (fwhm) was manually postprocessed five times for (002) and (10) peaks with a linear background and averaged. The crystallite height (Lc) and crystallite width (La) were calculated from (002) and (10) peaks, respectively, from their fwhms using the Scherrer equation. The Raman spectra of soot samples were obtained on a WITec Confocal Raman Microscope CRM 200. The wavelength was calibrated with a silicon wafer by utilizing the first-order phonon band of Si at 520 cm-1, and a 100 objective lens of the microscope was used to focus and collect the spectra at 10-15 different locations for each sample with 514.5 nm of an Ar ion laser source. First-order Raman spectra were curve-fitted via IGOR Pro 6.10 software (Wavemetrics Inc.), where three Lorentzian-shaped G (∼1580 cm-1), D1 (∼1360 cm-1), and D4 (∼1180 cm-1) and one Gaussian-shaped D3 (∼1500 cm-1) were obtained with a linear baseline, as Sadezky et al. first studied.21 The area of the D3 band (AD3), the ratio of the D1 area to the G area (AD1/AG), and the D1 full width at halfmaximum (D1 fwhm) were used as Raman parameters to determine the order of the soot crystalline structure.

(29) Dec, J. E. A conceptual model of DI diesel combustion based on laser-sheet imaging. SAE (Tech. Pap.) 1997, 970873.

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Table 3. Experimental Conditions and O/C at each Condition engine load

fuel injection timing

experimental condition

O/C in enginea

30%

pilot: 17.7 BTDC main: 2.7 ATDC

75%

pilot: 39.0 BTDC main: 6.6 BTDC

90%

pilot: 42.9 BTDC main: 6.3 BTDC

21% O2 in intake, 100% diesel 24% O2 in intake, 100% diesel 27% O2 in intake, 100% diesel 21% O2 in intake, 30% diglyme þ 70% diesel 21% O2 in intake, 100% diesel 22.5% O2 in intake, 100% diesel 24% O2 in intake, 100% diesel 27% O2 in intake, 100% diesel 21% O2 in intake, 30% diglyme þ 70% diesel 21% O2 in intake, 100% diesel

7.45 8.50 9.64 7.85 4.48 4.88 4.93 5.40 4.57 3.71

a

C15H27.66 is used to calculate.9

Figure 1. Schematic of engine operation.

centerline of the fuel spray, and these soot particles are oxidized in the near-stochiometric diffusion flame around the periphery of the spray flame. Although there has been no detailed investigation about the soot formation and soot oxidation processes with intake oxygen enrichment using spatially integrated natural luminosity (SINL) analysis in engines, the processes at oxygen-enriched conditions can be understood as an extension of EGR studies. With decreasing oxygen concentration, the combustion temperature is reduced, which tends initially to increase soot emissions, and then at sufficiently high EGR, soot emissions in reaction zones would decrease due to the reduced soot formation, although engine-out soot would appear to be high with a reduced role of soot oxidation.30 The decreased rate of oxygen entrainment prolongs the combustion duration and increases the time for soot formation.30,31 Correspondingly, with increased oxygen concentration, the soot volume is expected to increase with higher combustion temperature, and the soot formation process is shortened with reduced combustion duration. Because engine-out soot emissions decrease with increasing oxygen concentration despite increased soot volume, the rate of soot oxidation is higher in the diffusion flame at oxygen-enriched conditions.30 Bulk Cylinder Gas Temperature. To examine the effect of oxygen enrichment on cylinder gas temperature, the measured cylinder pressures were used to calculate the bulk gas

Figure 2. Apparent heat release rate for oxygen enrichment tests: (a) 30% load and (b) 75% and 90% loads.

(30) Cheng, A. S.; Upatnieks, A.; Mueller, C. J. Investigation of fuel effects on dilute, mixing-controlled combustion in an optical directinjection diesel engine. Energy Fuels 2007, 21, 1989–2002. (31) Idicheria, C. A.; Pickett, L. M. Soot formation in diesel combustion under high-EGR conditions. SAE (Tech. Pap.) 2005, DOI: 10.4271/ 2005-01-3834.

temperature for low and high loads at each condition. At 30% load, Figure 3a shows that the 24% O2 condition reached the lowest bulk temperature at 1380 K, whereas the 27% O2 condition reached the highest bulk temperature at 1405 K, which is similar to that of the 21% O2 condition 605

Energy Fuels 2011, 25, 602–616

: DOI:10.1021/ef101108j

Seong and Boehman

Table 4. Ignition Charateristics in Pilot and Main Injection with Oxygen Enrichment for 75% Load

condition

start of combustion in pilot (CAD)

start of combustion in main (CAD)

duration of premixed burn in main (CAD)

21% O2 22.5% O2 24% O2 27% O2 30% diglyme

333.6 335.9 335.1 335.1 333.3

358.8 358.7 358.4 358.1 358.1

2.4 2.2 2.2 2.0 1.7

other studies, soot aggregates are distributed from small size to larger size.34,35 In general, there are many larger aggregates observed for 75% load than for 30% load, which is attributed to the particle agglomeration due to the increased temperature.35 At 30% load, Figure 4a,b shows similar size distributions from small aggregates to large aggregates at 21% O2 and 24% O2. As shown in Figure 4a, because there is no appreciable temperature effect at 30% load with increased oxygen concentration, intake oxygen enrichment seems to have a minor effect on the size distribution of soot aggregates. The wide size distribution of soot aggregates is also observed for 75% load with 21% O2, but there are larger aggregates with scarce small aggregates for 75% load with 24% O2, as indicated in Figure 4c,d. According to Zhu et al.,34 the number of primary particles and the size of soot aggregates increases to some extent with engine load due to increased temperature. As the temperature increases further, the size decreases because soot oxidation becomes a dominant factor over the particle growth through agglomeration. Although there is an enhanced soot oxidation with increased oxygen concentration, it does not seem to be enough to oxidize large aggregates to smaller size during the short oxidation process. Consequently, it is speculated that most of the small soot aggregates are oxidized and the remaining large aggregates are emitted from the cylinder without much oxidation during the soot oxidation process. Because hydrocarbon emissions decrease dramatically by oxidation with increasing oxygen concentration,13 soot precursors as well as small aggregates are expected to be oxidized at higher oxygen concentration. Soot Oxidative Reactivity by TGA. Isothermal oxidation was performed at 550 °C in order to examine soot oxidative reactivity. Figure 5a shows that the soot samples from 30% load have similar weight loss patterns, except for the 27% O2 soot. The three soot samples were completely oxidized in 110 min at this temperature, and the complete oxidation of the 27% O2 soot was observed after 90 min. In comparison to the TGA result from Figure 5a, the soot samples from the high load appear more diverse in their oxidation behavior, as shown in Figure 5b. The 21% O2 soot from 75% load is the least reactive, which takes 150 min for complete oxidation, and the 24% O2 soot from 75% load was shown to be the most reactive among the examined samples. In particular, the two most reactive soot samples, the 22.5% O2 soot and the 24% O2 soot, show that ash remained after the completion of the TGA analysis. Accordingly, this may indicate that there are inorganic compounds present in these soot samples, as observed in others’ investigations of diesel particulate matter.36,37 In addition, the 30% diglyme soot from 75% load is found to be slightly more reactive than the 21% O2 soot from 75% load. Song et al. observed that biodiesel soot

Figure 3. Bulk cylinder gas temperature for oxygen enrichment tests: (a) 30% load and (b) 75% and 90% loads.

and the 30% diglyme condition. These minor temperature changes with oxygen enrichment are consistent with the small changes in the heat release of the mixing-controlled burn at 30% load. For high load, Figure 3b displays that the 27% O2 condition achieved the highest bulk temperature of 1930 K, and the 21% O2 condition had the lowest bulk temperature of 1830 K. The temperatures of the 24% O2 and the 30% diglyme conditions are similar despite the different operating conditions. As expected from the heat release rate, the 90% load condition shows the highest bulk temperature of 2100 K. Accordingly, a strong impact of intake oxygen enrichment on cylinder temperature is clearly observed at the high load condition, as many researchers have observed.32,33 The temperature increase caused by increased oxygen at 75% load is not surprising, as observed in the increased heat release rate of the diffusion burn phase, because more abundant oxidizing gases are involved in the combustion process. TEM Analysis of Aggregated Particles. TEM images at 30% and 75% loads are compared in Figure 4. As shown in

(34) Zhu, J.; 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, 2781–2789. (35) Lee, K.-O.; Megaridis, C. M.; Zelepouga, S.; Saveliev, A. V.; Kennedy, L. A.; Fouad, C.; Charon, O.; Ammouri, F. Soot formation effects of oxygen concentration in the oxidizer stream of laminar coannular nonpremixed methane/air flames. Combust. Flame 2000, 121, 323–333. (36) Berube, K. A.; Jones, T. P.; Williamson, B. J.; Winters, C.; Morgan, A. J.; Richards, R. J. Physicochemical characterization of diesel exhaust particles: Factors assessing biological activity. Atmos. Environ. 1999, 33, 1599–1614. (37) Lim, M. C. H.; Ayoko, G. A.; Morawska, L.; Ristovski, Z. D.; Jayaratne, E. R. The effects of fuel characteristics and engine operating conditions on the elemental composition of emissions from heavy duty diesel buses. Fuel 2007, 86, 1831–1839.

(32) Donahue, R. J.; Foster, D. E. Effects of oxygen enhancement on the emissions from a DI-diesel via manipulation of fuels and combustion chamber gas composition. SAE (Tech. Pap.) 2000, DOI: 10.4271/200001-0512. (33) Zannis, T. C.; Pariotis, E. G.; Hountalas, D. T.; Rakopoulos, D. C.; Levendis, Y. A. Theoretical study of DI diesel engine performance and pollutant emissions using comparable air-side and fuel-side oxygen addition. Energy Convers. Manage. 2007, 48, 2962–2970.

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Figure 4. TEM images of soot aggregate particles collected by thermophoretic sampling: (a) 30% load at 21% O2, (b) 30% load at 24% O2, (c) 75% load at 21% O2, and (d) 75% load at 24% O2.

after the completion of TGA. As evident in Table 5, some metallic species are present only on soot samples from particular conditions, 27% O2 at 30% load, and 22.5% O2 and 24% O2 at 75% load. Although the 27% O2 soot from 30% load and the 22.5% O2 soot from 75% load contain very small amounts of Zn and P, the 24% O2 soot contains N, Ca, and Si as well as more abundant Zn and P. Although ash contents after TGA analyses are not matched with the total amounts of metals using XPS for these soot samples, the presence of ash during TGA is due to the metallic species present in soot samples. It is known that these metals are components of friction reduction additives in the lubricating oil. High cylinder temperatures contribute to volatilization and subsequent oxidation of lubricating oil,38 but the soot

is significantly more reactive than Fischer-Tropsch soot (FT soot), and the former oxidizes through a unique capsuletype oxidation process.28 Also, Vander Wal and Mueller noted that increasing the level of fuel oxygenation generates soot with a less ordered structure.19 Correspondingly, the 30% diglyme soot from 75% load also seems to reflect more reactive soot. As observed for the combustion analyses from heat release rates and the corresponding bulk cylinder gas temperatures, there is a consistent trend in soot reactivity that oxygen enrichment has a greater effect on soot samples from the high load than on soot samples from the low load. Therefore, the physical and chemical properties of the soot samples were extensively investigated to find the main factors affecting soot oxidative behavior with oxygen enrichment. Inorganic Species by XPS and SEM-EDS. At first, inorganic species were investigated using XPS because ash remained

(38) Diaby, M.; Sabliera, M.; Le Negrate, A.; El Fassi, M.; Bocquet, J. Understanding carbonaceous deposit formation resulting from engine oil degradation. Carbon 2009, 47, 355–366.

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from the 90% load condition, which has the highest bulk cylinder gas temperature, does not contain any metals. Because XPS can probe the outer 0-10 nm of the surface of a sample with ∼0.1 at. % detection limits, XPS may not detect all the species in the soot samples. Also, the samples may lose some additional inorganic species during the thermal treatment under N2 flow. Accordingly, SEM-EDS was applied to as-received soot samples in order to detect all the available species present in the soot. Table 5 reveals that sulfur is observed in all the samples with SEM-EDS, whereas there is no sulfur observed with XPS. Although XPS was not employed for the 27% O2 soot from 75% load due to the small quantity of the sample, abundant Zn, Si, P, and Ca were also observed for the 27% O2 soot with the SEM-EDS, as with the 24% O2 soot from the 75% load. In addition,

there are Si, S, and Ca observed for the 90% load soot, which were not detected with XPS. Accordingly, SEM-EDS seems to detect more abundant inorganic species in and on the soot samples. Also, the SEM image also shows that metallic species are inhomogeneously present in the samples. As shown in Figure 6, there are bright particles scattered on the 27% O2 soot. The analysis on a small spot, in Table 5, which is spot #2 in Figure 6, shows much different compositional results from the analyses on larger areas. Accordingly, elemental analysis by SEM-EDS in this work seems to be area-dependent. Although SEM-EDS indicates that metallic species are inhomogeneously scattered in and on the soot samples, it is still difficult to determine if the reason why SEM-EDS provides more abundant amounts of inorganic species than XPS is because SEM-EDS can survey deeper sites within the soot or because the as-received soot samples contain more inorganic species. In the case of sulfur present in the soot samples, N2 may drive off sulfur species with thermal treatment. However, it is questionable if N2 can also remove other metallic species in and on the soot samples, because thermal treatment was performed in TGA, where N2 gas diffuses into the soot particles. Accordingly, the different amounts of metallic species between the two instruments can be ascribed to the difference in detection areas employed by these instruments. Therefore, the results of XPS and SEMEDS should be viewed as qualitative tools in this work. Despite the different amounts of metallic species observed using the two instruments, they both provide valuable information that ash content is from the metallic species, resulting from the vaporized lubricating oil during the combustion process. To investigate whether soot contains metallic species inside the primary soot particles, as-received 27% O2 soot and 21% O2 soot from 75% load were examined using TEMEDS. The results show that there are 0.096 and 0.022 at. % of Si and Ca, respectively, for randomly chosen eight primary particles of the 27% O2 soot, and that the 21% O2 soot does not have any metals inside. Not only are the amounts of Si and Ca from TEM-EDS significantly smaller than those from SEM-EDS, but also other metallic species, such as Zn and P, are not detected using TEM-EDS. Therefore, it is presumed that most of the metallic species are not incorporated within the soot particles during the soot formation process but are deposited on the outermost surface of the soot.

Figure 5. TGA results of soot samples for oxygen enrichment tests: (a) 30% load and (b) 75% and 90% loads.

Table 5. Elemental Analyses of Inorganic Species in the Soot Samples by XPS and SEM-EDS: Samples for XPS are Thermally Treated at 500 °C, and Samples for SEM-EDS are As-Received XPS (at. %) load

condition

Zn

30%

21% O2 24% O2 27% O2 30% diglyme 21% O2

0 0 0.07 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

22.5% O2 24% O2

0.06 0.3

0.1 0.37

0 0.2

0 0.23

75%

a

30% diglyme 21% O2

Ca

Si

Zn

0 0

0 0

0 0

P

Ca

Si

Fe

S

not measured

not measured

27% O2

90%

P

SEM-EDS (at. %)

0 0

Analysis at spot # 2 in Figure 6.

608

0 0

0 0

2.79 1.71 2.05 1.64 3.51a

2.46 2.17 0 1.44 1.88a

0 0

0 0

0 0

0 0 not measured 3.77 1.01 3.52 0.39 3.97 0 3.26 0.13 11.96a 0.38a not measured 2.22 0 1.67 1.23

0 0

1.09 1.14

1.45 1.23 0 0 1.99a

2.11 2.46 3.7 2.04 5.49a

0 0

0.97 0.86

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Figure 6. SEM image of the 27% O2 soot from 75% load.

If these particles are representative of primary soot particles, this TEM-EDS result indicates that the soot formation process occurs before the vaporization of lubricating oil, although there is a chance that a small amount of metallic species are incorporated within the soot particles. When Song et al. investigated primary soot particles after burning diesel fuel containing 200 wt ppm of a metal additive containing Fe and Sr, the existence of Fe and Sr was clearly observed inside the primary particles from HR-TEM.39 Therefore, although it is unknown how much of the metal additives in the lubricating oil was consumed during engine operation, the process of incorporation of metals present in the lubricating oil into soot particles seems to be different from that of the metal additive in fuel during soot formation. From the elemental analyses, it is plausible that higher oxygen concentration in the cylinder, which was not involved in the combustion process, serves to raise peripheral temperatures in the cylinder, which vaporizes the lubricating oil on the cylinder walls and enhances the oxidation of the vaporized oil, from which metals deposited on the soot particles that were formed earlier than the vaporization of the lubricating oil. This may be due to increased flame radiation to the cylinder walls, or greater rates of convection due to thinning of the quench layer with higher oxygen availability, although there has been no research known on this effect. This hypothesis applies only for high temperature, but not for lower temperature conditions, because there is no metal observed on the 24% O2 soot from 30% load, despite the high oxygen concentration in the intake air. Effect of Metallic Species on Soot Oxidation. Figure 5 and Table 5 indicate that soot samples containing metallic species are more oxidatively reactive than those without metallic species. Although the 22.5% O2 soot was not analyzed in the SEM-EDS, there should be an appreciable amount of metals present in it from the fact that Zn and P were observed with the XPS. The SEM-EDS result reveals that there is only Ca present in the 90% load soot, but the 24% O2 soot from 75% load contains various metallic species, such as Zn, Si, P, Fe, and Ca. Previous work shows that Zn, Si, Ca, and Fe appear to be good catalysts in soot oxidation,39-41 but P also has

Figure 7. TGA results of soot and carbon black in the presence of metals: (a) soot mixed with Ca at 500 °C isothermal oxidation and (b) carbon black mixed with Ca and Zn at nonisothermal oxidation.

been observed to inhibit carbon oxidation.42 Therefore, the differences in soot oxidative reactivity seem to be related to the amounts of metallic species present in the soot samples in this study, although it is not clear how much a small amount of P present in soot suppresses soot oxidative reactivity. To examine the effect of Ca on soot oxidative reactivity, Ca was mingled with flame soot in two different ways: one way is where 1 wt % of Ca was impregnated onto flame soot using calcium nitrate tetrahydrate (Ca(NO3)2 3 (H2O)4), and the other way is where 2 wt % of Ca was mixed with flame soot using calcium oxide. As shown in Figure 7a, the oxidative reactivity of the Ca-impregnated soot is greatly increased, but that of the CaO-mixed soot is not apparently different from that of the original soot. According to Neeft et al.,41 the first method (impregnation) is a tight contact mode and the other (physical mixing) is a loose contact mode. Therefore, it seems that Ca plays a catalytic role in soot oxidation, when it is in the tight contact mode with soot. In this light, metallic species may deposit onto soot by tight contact at high temperatures during the combustion process. In addition, carbon black impregnated with Ca and Zn nitrates was further investigated using nonisothermal oxidation. As shown in Figure 7b, 50% burnoff temperatures (T50%) of original carbon black, Ca-impregnated carbon black, and Zn-impregnated carbon black are 733, 560, and 629 °C, respectively. Accordingly, the effect of Ca on soot oxidation is more significant than that of Zn as a single component, which is consistent with Neeft et al.41 Although the elemental content of these metallic species in and on the 22.5% O2 soot, the 24% O2 soot, and the 90% load soot is much smaller than 1 wt %, as a single component, the same enhancing effect by metallic species is expected. Consequently, the amounts of metallic species seem to be a predominant factor influencing

(39) Song, J.; Wang, J.; Boehman, A. L. The role of fuel-borne catalyst in diesel particulate oxidation behavior. Combust. Flame 2006, 146, 73–84. (40) Castoldi, L.; Matarrese, R.; Lietti, L.; Forzatti, P. Intrinsic reactivity of alkaline and alkaline-earth metal oxide catalysts for oxidation of soot. Appl. Cat., B 2009, 90, 278–285. (41) Neeft, J. P. A.; Makkee, M.; Molllijn, J. A. Metal oxides as catalysts for the oxidation of soot. Chem. Eng. J. 1996, 64, 295–302.

(42) Wu, X.; Radovic, L. R. Inhibition of catalytic oxidation of carbon/carbon composites by phosphorus. Carbon 2006, 44, 141–151.

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Carbonaceous materials have been widely used as catalysts and supports by surface modification through gas or liquid oxidation, where surface oxygen functional groups are increased.44-46 As the surface oxygen content of soot appears to increase with oxygen enrichment, especially for soot samples from high load, the impact of oxygen enrichment on surface oxygen functional groups on the soot was also investigated. Figure 8 shows the trends observed in the C 1s and O 1s peaks for all engine load conditions. On the basis of reports in the literature,44-50 the assignment of surface oxygen groups is as follows: (1) ∼286 eV is C-O groups in alcohol groups, (2) ∼287 eV is CdO groups in quinone and/ or carbonyl groups, (3) ∼289 eV is OdC-O in carboxylic acid and ester groups, (4) 530-532 eV is CdO in quinone, carbonyl, and/or carboxylic acid, (5) 531-534 eV is OdC in ester and anhydride and C-O in ether and/or alcohol, and (6) 532-535 eV is O-C in ester, anhydride, and carboxylic acid. Figures 8a and 8c indicate that the peak (3) increases with the increase in the surface oxygen content. Figures 8b and 8d show that the peak width becomes narrower as the surface oxygen content increases. Because the O 1s peak becomes wider at the lower surface oxygen content, the ratio of the peak (6) to the peak (4) becomes larger with the increase in the surface O content. Because the C 1s and O 1s peaks of soot samples for high load are more obvious in their variation, the C 1s peaks of the 75% load 21% O2 soot, the 75% load 24% O2 soot, the 30% diglyme soot, and the 90% load soot were deconvoluted, as shown in Figure 9, and the result is also indicated in Table 7. In comparison to the C 1s peak of the 75% load 21% O2 soot, it is shown that the C 1s peak of the 75% load 24% O2 soot shows a slight increase in (2) and (3), and a decrease in (1). The C 1s peak of the 90% load soot displays a noticeable increase in all the peaks. Among those functional groups, the increase in alcohol groups is observed to be the most significant. In addition, the O 1s peaks were qualitatively compared for the 21% O2 soot and the 24% O2 soot from 75% load, and the 90% load soot. Also, the 90% load soot, which has more O content, has a narrower O 1s peak than that of the 75% load soot. The decreasing trend in (4) may indicate that the contribution in (4) is mainly because of ester and carboxylic anhydride, not because of carboxylic acid, although some researchers assigned (4) to carboxylic acid. Because its increase is relatively small compared with (1) in the C 1s peak, however, it seems to be more reasonable to identify (4) as carboxylic anhydride, ester, and carboxylic acid. This presumption can be also supported by the present sample pretreatment method.

Table 6. Atomic Percentages of Elements in Soot Samples by XPS element (at. %) engine load

condition

O

C

others

30%

21% O2 24% O2 27% O2 30% diglyme 21% O2 22.5% O2 24% O2 30% diglyme 21% O2

7.87 10.76 8.43 8.46 5.75 8.43 9.12 8.43 11.49

92.13 89.24 91.50 91.54 94.25 91.41 90.21 91.57 88.51

refer to Table 5

75%

90%

soot oxidative reactivity, although other physical and chemical properties of soot should also be considered. Therefore, the surface oxygen content and crystalline structure were investigated in the following sections. Surface O Content and Oxygen Functional Groups. The oxygen content of carbonaceous materials is shown to be a good indicator determining carbon oxidative reactivity.20,43 Accordingly, the relation between oxygen content and soot oxidative reactivity was investigated. Because the role of metallic species in soot oxidation is predominant as mentioned earlier, the analysis of surface O content is limited to several soot samples without any metallic species. For the samples from 30% load, the 21% O2 soot, the 24% O2 soot, and the 30% diglyme soot, which show similar soot oxidative reactivities, contain 7.87, 10.76, and 8.46 at. % O content, respectively. Accordingly, the result of the surface O content indicates that the surface O content does not correlate with soot oxidative reactivity for these samples. However, the 21% O2 soot and the 30% diglyme soot from 75% load, which contain 5.75 and 8.43 at. % O content, respectively, display a reactivity difference (see Table 6). Therefore, soot samples from low load and high load have contradictory results regarding the effect of the surface O content on soot oxidative reactivity. Indeed, there is a possibility that the 30% diglyme soot from 75% load is also more oxidatively reactive due to the metallic species present, which were not detected by XPS. Song et al. also acknowledged that the residual impurity of potassium in biodiesel soot might act like an oxidation catalyst during soot oxidation because biodiesel soot is significantly more reactive through a capsuleoxidation process than FT soot.28 However, because its existence was not proved by EELS, they thought that more abundant surface oxygen functional groups in biodiesel soot were the primary factor leading to faster oxidation of biodiesel soot. Although the 30% diglyme soot from 75% load also shows similar aspects to those of biodiesel soot, it is still difficult to determine if surface O content is a key indicator influencing soot oxidative reactivity because of the contradictory results from low load and high load. As mentioned earlier, the combustion process in engines is too complicated to understand where oxygen functional groups are incorporated into soot for different loads. Accordingly, the role of surface O content in soot oxidation is an open question for diesel soot samples.


ao, (45) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orf~ J. J. M. Modification of the surface chemistry of activated carbons. Carbon 1999, 37, 1379–1389. (46) Zhou, J.-H.; Sui, Z.-J.; Zhu, J.; Li, P.; Chen, D.; Dai, Y.-C.; Yuan, W.-K. Characterization of surface oxygen complexes on carbon nanofibers TPD, XPS and FT-IR. Carbon 2007, 45, 785–796. (47) Desimoni, E.; Casella, G. I.; Morone, A.; Salvi, A. M. XPS determination of oxygen-containing functional groups on carbon-fibre surfaces and the cleaning of these surfaces. Surf. Interface. Anal. 1990, 15, 627–634. (48) Zhang, G.; Sun, S.; Yang, D.; Dodelet, J.-P.; Sacher, E. The surface analytical characterization of carbon fibers functionalized by H2SO4/HNO3 treatment. Carbon 2008, 46, 196–205. (49) Lakshminarayanan, P. V.; Toghiani, H.; Pittman, C. U., Jr. Nitric acid oxidation of vapor grown carbon nanofibers. Carbon 2004, 42, 2433–2442. (50) Plomp, A. J.; Su, D. S.; de Jong, K. P.; Bitter, J. H. On the nature of oxygen-containing surface groups on carbon nanofibers and their role for platinum depositions: An XPS and titration study. J. Phys. Chem. C 2009, 113, 9865–9869.

(43) de la Puente, G.; Fuente, E.; Pis, J. J. Reactivity of pyrolysis chars related to precursor coal chemistry. J. Anal. Appl. Pyrolysis 2000, 53, 81–93. (44) Zielke, U.; H€ uttinger, K. J.; Hoffman, W. P. Surface-oxidized carbon fibers: I. Surface structure and chemistry. Carbon 1996, 34, 983– 998.

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Figure 8. XPS peak patterns of C 1s and O 1s: (a) C 1s for 30% load, (b) O 1s for 30% load, (c) C 1s for 75% and 90% loads, and (d) O 1s for 75% and 90% loads.

Figure 9. Comparison of curve-fitted C 1s for high load soot samples: (a) 21% O2 soot of 75% load, (b) 24% O2 soot of 75% load, and (c) 90% load soot. (1) C-O in alcohol and/or ether groups. (2) CdO in quinone and/or carbonyl groups. (3) OdC-O in carboxylic acid, ester, and/or carboxylic anhydride groups.

Because soot samples were thermally pretreated at 500 °C, most of the carboxylic acid groups should be driven off. Because carboxylic anhydride groups will remain even at 600 °C, they could remain on the soot samples after thermal treatment. In comparison to the O 1s peaks of the other samples, the 24% O2 soot from 75% load has a wider O 1s peak, as indicated in Figure 8d. According to the C 1s analyses, the 24% O2 soot may have a similar peak pattern in O 1s to that of the 21% O2 soot. The reason may be that this soot includes more

than 1% of metals, as indicated in Table 5. According to Dimitrov and Komatsu, P2O5, SiO2, CaO, and ZnO have O 1s binding energies in the range of 529.8-533.5 eV.51 Because surface oxygen can exist as metal oxides and independent oxygen atoms adsorbed on metal surfaces as well as oxygen in surface functional groups, a portion of the O 1s may be associated with metal-related surface oxygen. Crystalline Structure of Diesel Soot. As evidenced by the bulk cylinder gas temperatures, intake oxygen enrichment significantly increases the gas temperature at high load, although its effect is minor at low load. In general, the high temperature resulting from high load conditions is a major

(51) Dimitrov, V.; Komatsu, T. Classification of simple oxides: A polarizability approach. J. Solid State Chem. 2002, 163, 100–112.

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Table 7. Relative Ratio of Oxygen Functional Groups in Curve-Fitted C 1s for High Load Soot Samples: (1)-(3) Are Oxygen Functional Groups from Figure 9

Table 8. Crystallite Sizes Calculated from XRD Patterns of Soot Samples

percentage (%) sample

(1)

(2)

(3)

75% load 21% O2 75% load 24% O2 90% load 21% O2

18.25 17.72 28.41

8.10 8.34 11.12

5.92 6.30 7.86

engine load

condition

d002 (nm)

Lc (nm)

La (nm)

30%

21% O2 24% O2 27% O2 30% diglyme 21% O2 22.5% O2 24% O2 30% diglyme 21% O2

0.358 0.356 0.356 0.358 0.356 0.356 0.354 0.355 0.355

1.73 1.67 1.78 1.63 1.59 1.77 1.76 1.67 1.78

3.12 2.76 3.06 2.93 3.00 3.12 1.68 1.87 3.34

75%

90%

crystalline structure of the soot samples, Lc and La were calculated, as shown in Table 8. The interlayer spacing, d002, and Lc show no obvious trends with engine load and oxygen enrichment. However, the 30% diglyme soot and the 24% O2 soot from the high load have significantly reduced La. Because oxygen enrichment induces a higher temperature than the 21% O2 condition, the higher temperature would be attributed to causing the reduced La. However, the 21% O2 soot at 30% load has a similar crystallite size to the 21% O2 soot at 75% load despite the large bulk cylinder gas temperature difference. According to Belenkov, high temperature favors an increased crystallite width due to the layer translation by higher activation energy.54 Consequently, the temperature effect cannot explain the reduced crystallite width with oxygen enrichment at high load. Raman spectroscopy has been widely used to evaluate the soot crystalline structure. Using the D3 area (AD3), AD1/AG, and D1 fwhm, which represent amorphous carbon,22 the density of edge sites,55 and the distribution of crystallite sizes,56 respectively, the soot crystalline structure was evaluated. Figure 11 shows that AD3, AD1/AG, and D1 fwhm for 30% load are similarly distributed for the samples. The AD1/ AG varies in the range of 2.8-3.0, and the D1 fwhm varies in the range of 175-180 cm-1 with oxygen enrichment. In the case of soot for 75% and 90% loads, the AD3 does not vary significantly with oxygen enrichment, as evidenced in Figure 11a, but the AD1/AG and the D1 fwhm vary between 2.3 and 3.0 and between 145 and 180 cm-1, respectively, with oxygen enrichment. Consistent with the other soot characterization results discussed in the previous sections, the variations of AD1/AG and D1 fwhm also appear to be more significant for the soot samples from high load with oxygen enrichment. Figure 12b,c indicates that AD1/AG increases with the increasing O/C, and D1 fwhm also shows a similar trend, except for the 22.5% O2 soot and the 30% diglyme soot. The Raman results are not completely consistent with the XRD results because the 30% diglyme soot from 75% load shows a similar crystalline structure to the 21% O2 soot from 75% load in terms of Raman parameters, despite the smaller crystallite width of the 30% diglyme soot using XRD. However, overall, the soot seems to become less ordered in the crystalline structure for 75% load with an increase in the O2 concentration, whereas there is no noticeable effect on the order of the soot structure from 30% load with an increase in O2 concentration. In general, there is a consensus

Figure 10. XRD patterns of soot samples for oxygen enrichment tests: (a) 30% load and (b) 75% and 90% loads.

factor in producing more ordered soot particles.25,52 To investigate the impact of oxygen enrichment on the crystalline structure of diesel soot, XRD patterns of soot samples were compared, as shown in Figure 10a. All the samples show a (002) peak at 27°, a (10) peak at 42°, and a (110) peak at 77°, which are broad, as has been reported for disorderd carbonaceous materials, such as soot and carbon blacks.23,53 The soot samples from the 30% load condition show no noticeable change in the XRD patterns with oxygen enrichment. However, there is a clear difference observed in the patterns of the samples from the high load condition, as shown in Figure 10b. The (002) peak for the 24% O2 soot became sharp, but the 22.5% O2 soot has a similar pattern to that for the 21% O2. The 30% diglyme soot shows much reduced magnitudes in the (002) and (10) peaks compared with the other samples. The 90% load soot also has a similar pattern to that of the 21% O2 soot, despite a much higher bulk cylinder gas temperature. To better understand the (52) Lee, K.-O.; Cole, R.; Sekar, R.; Choi, M. Y.; Kang, J. S.; Bae, C. S.; Shin, H. D. Morphological investigation of the microstructure, diameters, and fractal geometry of diesel particulates. Proc. Combust. Inst. 2002, 29, 647–653. (53) Darmstadt, H.; Roy, C.; Kaliaguine, S.; Xu, G.; Auger, M.; Tuel, A.; Ramaswamy, V. Solid state C-NMR spectroscopy and XRD studies of commercial and pyrolytic carbon blacks. Carbon 2000, 38, 1279–1287.

(54) Belenkov, E. A. Formation of graphite structure in carbon crystallites. Inorg. Mater. 2001, 37, 928–934. (55) Ferrari., A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 2000, 61, 14095–14107. (56) Jawhari, T.; Roid, A.; Casado, J. Raman spectroscopic characterization of some commercially available carbon black materials. Carbon 1995, 33, 1561–1565.

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Figure 11. Raman analyses of soot samples with standard deviation for 30% load: (a) D3 band area (AD3), (b) AD1/AG (area ratio), and (c) D1 fwhm.

Figure 12. Raman analyses of soot samples with standard deviation for 75% and 90% loads: (a) D3 band area (AD3), (b) AD1/AG (area ratio), and (c) D1 fwhm.

that soot crystallites develop into more ordered structures as engine load increases because of increasing bulk cylinder gas temperature. To investigate the relationship between engine load and the order of the crystalline structure, the D1 fwhm was plotted as a function of maximum bulk cylinder gas temperature in Figure 13. With the increase in the cylinder temperature, the D1 fwhm tends to decrease for the soot samples from different loads at the same 21% O2, although there is no clear correlation observed in the crystallite size by XRD. Therefore, the temperature effect on the soot crystalline structure seems to be more apparent with the Raman analysis. In the same manner, Figure 13 also indicates that soot becomes less ordered with an increase in

the temperature with increasing O2 concentration for the same 75% load. Although higher bulk cylinder gas temperature contributes to increasing crystallite size, resulting in increasing the order of the crystalline structure, Figure 13 shows that high oxygen concentration at high engine load has an opposite effect on the crystalline structure. Therefore, although it was presumed that the increased heat release rate during the diffusion burn reflects the enhanced soot oxidation process with increasing O2 concentration in the intake, the present result may indicate that the increased heat release rate is not relevant to the oxidation process because the increased soot oxidation could induce soot to become more ordered. 613

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Figure 13. D1 fwhm magnitudes of soot samples with standard deviation as a function of maximum bulk cylinder gas temperature.

It is also of interest whether soot particles are structurally influenced by the presence of metallic species. If most of metallic species are surface-bound, as observed in the TEMEDS of the 27% O2 soot from 75% load, the effect of metallic species might be more important during the soot oxidation process than during the soot formation process. Although the amounts of metallic species and types of metallic species have different impacts on soot oxidation, Kim et al. observed that soot emissions decrease due to the enhanced soot oxidation, when iron pentacarbonyl was added into isooctane diffusion flames.57 If the oxidation process is also predominant in this work, primary soot particles experience shrinking in size, which increases the soot crystalline order. Accordingly, the reason why soot became less ordered with increasing bulk cylinder gas temperature cannot be explained by the presence of metallic species deposited on soot particles. Consequently, the impact of metallic species on the soot crystalline order was not considered in this aspect. As observed, transparent precursor particles develop into opaque soot particles by the carbonization process with the growth of polycyclic aromatic hydrocarbons (PAHs).58,59 In the carbonization process, there is a threshold molecular weight influencing a spontaneous disorder/order transition in soot formation.59 Furthermore, Dobbins et al. noted that, because combustion time is too short in engines, precursors would be unlikely to convert into carbonaceous soot depending upon the combustion temperature during the typical time of the combustion process and soot can be oxidized within the cylinder or possibly within the exhaust system under lean conditions.60 Accordingly, it is plausible that soot precursors produced from 75% load may be oxidized by the abundant amounts of oxidizing gases at higher temperature with increased oxygen, which seems to limit the mass growth of soot precursors, resulting in delaying soot inception time. If this is true, the increased heat release rate with increasing O2 concentration at high load is not because of the enhanced soot oxidation, but because of the enhanced soot precursor oxidation, as Flynn et al. proposed from a combination of

Figure 14. Raman analyses of soot samples during soot oxidation: (a) AD1/AG (area ratio) and (b) D1 fwhm.

chemical kinetic models and data analysis of diesel engines that oxygen addition prevents PAH growth by removing soot precursors.61 In the case of precursors from 30% load, however, there are plenty of soot precursors available to be carbonized without oxidation with increased oxygen, but the combustion temperature is not high enough for soot precursors to develop into soot particles or for incipient soot particles to mature during a short period of time. The difference in soot oxidative reactivity can be partly explained using the order of the crystalline structure. The 21% O2 soot from 30% load, which is less ordered in the crystalline structure, is more oxidatively reactive than that from 75% load. Likewise, the small reactivity difference between the 21% O2 soot and the 30% diglyme soot from 75% load would be attributed to the difference in crystallite width of the 21% O2 soot. However, because this difference is not reflected in the Raman parameters, it is not obvious if the difference in crystallite width between these two soots induces the reactivity difference, as observed in the surface O content. According to Song et al., the initial soot structure may not indicate oxidative reactivity differences.28 If this is the same case for the 21% O2 soot and the 30% diglyme soot as they observed, the investigation of the soot crystalline structure during oxidation would show different patterns in the soot crystalline structure. However, still, there is a possibility that the 30% diglyme soot is more reactive than the 21% O2 soot due to the presence of metallic species at concentrations below the detection limit. Soot Oxidation Process. The soot samples containing metals were shown to be appreciably more reactive than the soot samples without metals. To investigate the effect of

(57) Kim, K. B.; Masiello, K. A.; Hahn, D. W. Reduction of soot emissions by iron pentacarbonyl in isooctane diffusion flames. Combust. Flame 2008, 154, 164–180. (58) Dobbins, R. A.; Fletcher, R. A.; Chang, H.-C. The evolution of soot precursor particles in a diffusion flame. Combust. Flame 1998, 115, 285–298. (59) Hurt, R. H.; Crawford, G. P.; Shim, H.-S. Equilibrium nanostructure of primary soot particles. Proc. Combust. Inst. 2000, 28, 2539– 2546. (60) Dobbins, R. A.; Govatzidakis, G. J.; Lu, W.; Schwartzman, A. F.; Fletcher, R. A. Carbonization rate of soot precursor particles. Combust. Sci. Technol. 1996, 121, 103–121.

(61) Flynn, P. F.; Durrett, R. P.; Hunter, G. L.; zur Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C. K. Diesel combustion: An integrated view combining laser diagnostics, chemical kinetics, and empirical validation. SAE (Tech. Pap.) 1999, DOI: 10.4271/1999-010509.

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directly compared in Figure 15. Graphene layers in the centers of primary particles are observed to be less oriented, whereas outer graphene layers are more oriented and much longer in their length. Although it is not easy to find the differences in graphene layers during soot oxidation, Figure 15c clearly shows that the 24% O2 soot has become hollowed out inside the primary particles, whereas there is no hollowing for the 30% diglyme soot. Although the 24% O2 soot was not investigated using TEM-EDS, it would be expected that most metallic species deposit on the outermost surface, such as for the 27% O2 soot. If there is a small amount of metallic species inside the primary soot particles, this raises a question of whether the metallic species can lead to capsule oxidation. These internal hollows are found in biodiesel soot,28 EGR soot,23 and soot oxidized in DPFs.63 Because the inner structure in primary soot particles is less ordered than the outer structure, capsule oxidation is observed when inner pores are open to oxidizing gases during oxidation. Therefore, it is plausible that a small amount of metals inside these particles induces the hollowing-out (“capsule’’) oxidation process, but also it is a possible scenario that the 24% O2 soot, which is structurally disordered compared with the 30% diglyme soot, oxidizes through an internal burning process, as Vander Wal et al. could not find any internal hollowing for printex U during oxidation.63 Conclusions Oxygen enrichment was carried out in order to investigate its effect on soot oxidative reactivity and soot properties at low and high load conditions. From these experiments using a four-cylinder common rail turbo-charged diesel engine, the we conclude the following: (1) Intake oxygen enrichment induces a higher heat release rate with the increase in the oxygen concentration in the engine, resulting in higher cylinder temperature at high load, although its effect on cylinder temperature is not appreciable at low load. (2) Increased oxygen concentration by intake oxygen enrichment leads to increased vaporization of lubricating oil at high cylinder temperature, leading to incorporation of metals into soot particles. Correspondingly, a major contribution to soot oxidative reactivity is from metallic species present in and on soot samples by a tight contact mode, which are from lubricating oil, although the reduced soot crystalline order may also contribute to the reactivity. (3) The surface O content of soot is shown to be an insignificant factor indicating soot oxidative reactivity, which is contributed from metallic oxides as well as surface oxygen functional groups. (4) XRD and Raman results show that soot becomes less ordered in its crystalline structure with intake oxygen enrichment only at high load despite increased peak cylinder temperature, which is opposite to general observations in previous studies. From the current characterizations and many studies in the literature, it is presumed that soot precursors are oxidized at high temperature with abundant oxidizing species, which limits

Figure 15. HR-TEM images during soot oxidation.

metals on the soot oxidation process, the 24% O2 soot and the 30% diglyme soot for 75% load, which showed similar crystallite sizes and surface O content, were examined. For this study, both soot samples were oxidized to 33 and 67 wt % oxidation and were studied using Raman spectroscopy and HR-TEM. Figure 14 displays the changes in the AD1/AG and D1 fwhm with the increase in the degree of oxidation. The AD1/AG values in both soot samples are shown to be invariant upon oxidation. Also, the D1 fwhm of the 24% O2 soot decreases at the first stage of oxidation, and it is observed to be constant at the later stage of oxidation. However, the 30% diglyme soot shows a similar D1 fwhm during oxidation. Accordingly, soot oxidation may not affect significantly the crystalline structure of these soot samples. A more ordered structure upon oxidation has been reported for many soot samples22,23,28,62 because of the thermal annealing effect. Therefore, these soot samples from high load conditions may have different crystalline characteristics during soot oxidation from other known soots. In addition, the HR-TEM images of the two soot samples were (62) Ivleva, N. P.; McKeon, U.; Niessner, R.; P€ oschl, U. Raman microspectroscopic analysis of size-resolved atmospheric aerosol particle samples collected with an ELPI: Soot, humic-like substances, and inorganic compounds. Aerosol. Sci. Technol. 2007, 41, 655–671.

(63) 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, 70–77.

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: DOI:10.1021/ef101108j

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the spontaneous carbonization process from soot precursors to ordered soot particles. (5) HR-TEM images show that there are hollows observed inside primary soot particles in the presence of metallic species when soot particles are oxidized, which may indicate that soot particles, including metallic species, oxidize via the hollowing-out (“capsule’’) oxidation process.

Acknowledgment. The authors thank the General Electric Global Research Center and General Electric Transportation, in particular, David Walker, Omowoleola Akinyemi, Roy Primus, David Watson, Raj Rajiyah, and David Komoroske for their support of this work. This work was a part of the “Clean and Efficient Diesel Locomotive” program sponsored by the U.S. Department of Energy (No. 08NT002788).

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