Effect of Diesel Oxidation Catalysts on the Diesel Particulate Filter

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Effect of Diesel Oxidation Catalysts on the Diesel Particulate Filter Regeneration Process Leonardo Lizarraga,* Stamatios Souentie, Antoinette Boreave, Christian George, Barbara D’Anna, and Philippe Vernoux Universite de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS, Universite Claude Bernard Lyon 1, 2 Avenue A. Einstein, 69626 Villeurbanne, France ABSTRACT: A Diesel Particulate Filter (DPF) regeneration process was investigated during aftertreatment exhaust of a simulated diesel engine under the influence of a Diesel Oxidation Catalyst (DOC). Aerosol mass spectrometry analysis showed that the presence of the DOC decreases the Organic Carbon (OC) fraction adsorbed to soot particles. The activation energy values determined for soot nanoparticles oxidation were 97 ( 5 and 101 ( 8 kJ mol1 with and without the DOC, respectively; suggesting that the DOC does not facilitate elementary carbon oxidation. The minimum temperature necessary for DPF regeneration was strongly affected by the presence of the DOC in the aftertreatment. The conversion of NO to NO2 inside the DOC induced the DPF regeneration process at a lower temperature than O2 (ΔT = 30 K). Also, it was verified that the OC fraction, which decreases in the presence of the DOC, plays an important role to ignite soot combustion.

’ INTRODUCTION Diesel particulate matter (PM) significantly contributes to urban air pollution and has often been associated with adverse health effects.1,2 Furthermore, particles from combustion processes may also influence the radiation budget of the atmosphere and therefore the climate 3 and the hydrogeological cycle.4 In particular, diesel engine emissions constitute a major source of ultrafine particles in urban environments.57 Diesel exhaust PM is a complex mixture of carbonaceous material and hundreds of combustion products. This complex composition highly depends on the engine operation, fuel composition, lubrification oil aftertreatment technology, and exhaust sampling procedure. Exhaust pipes of modern diesel engines present particle size distributions centered at ∼80 nm with a typical number concentration close to 2.0 107 # cm3.8 Total carbon (TC) of diesel PM consists of elemental and organic carbon (EC and OC). EC is formed during fuel pyrolysis, and it is principally graphitic carbon. OC is originated from incomplete fuel combustion and slip of lubrication oil past engine seals; and it primarily consists of polycyclic aromatic hydrocarbons (PAHs) and aliphatic hydrocarbons.6,9 PAHs are harmful substances for human health due to their high carcinogenic, mutagenic, and allergenic potential;2 more than 100 different PAHs have been found in PM. Indeed, soot particles can transport genotoxic compounds across the cellular membrane acting as a Trojan horse.10,11 Diesel particulate filters (DPFs) are considered as the key technology to detoxify the diesel exhaust, reducing more than 95% the number of particles emitted. These filters consist of ceramic monoliths with alternating flow channels, which are closed at the end to force the exhaust flow to go through the porous wall of the honeycomb filter.12,13 Soot accumulation r 2011 American Chemical Society

inside the DPF increases the pressure drop in the aftertreatment. Thus, a periodical regeneration process is required. The optimal way to regenerate DPF is to oxidize the carbonaceous compounds to CO2 and H2O. However, the temperature needed for PM oxidation is elevated, commonly exceeding 773 K.14,15 Diesel oxidation catalysts (DOC) are also used during diesel emission aftertreatment; they consist of honeycomb monoliths impregnated with an active noble metal such as Pt and/or Pd. They are used to optimize the maximum conversion of hydrocarbons and carbon monoxide.16,17 It is well reported that NO2 is more efficient than O2 for soot oxidation, and NO2 can oxidize soot at temperatures as low as 523 K.1823 For the latter, a continuous regenerating trap (CRT) is one of the technologies used for promoting the DPF regeneration,13,23,24 which is composed of two parts, a DOC followed by a DPF. In the DOC, the exhaust temperature is increased by hydrocarbon oxidation, and NO2 is produced by oxidation of NO generated in the engine. The NO2 is then able to oxidize the soot deposited on a DPF to CO2 but with elevated concomitant emission of NO. Recently, secondary effects of including a DOC and/or DPF in the diesel engine afterteatment have been extensively studied.10,14,2533 These works have principally focused on possible reactions occurring in the DPF and/or DOC due to reactant accumulation in the DPF, elevated temperature, and elongated residence time; e.g., formation of nitro-PAHs, which Received: July 27, 2011 Accepted: November 3, 2011 Revised: October 26, 2011 Published: November 03, 2011 10591

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Environmental Science & Technology

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Figure 1. Experimental design to simulate the diesel engine aftertreatment and the analysis system.

have much higher mutagenicity and carcinogenic properties than PAHs.31,32 Furthermore, recent works have reported the emission of nanoparticles with size distribution below 20 nm during DPF regeneration.14,33 The main objective of this work was to investigate the effect of a DOC on the DPF regeneration temperature procedure, using a simulated diesel engine aftertreatment. The novel aspect of this study is the correlation of the chemical composition analysis of the nonrefractory fraction of soot particles, with the oxidizing capacity of NO2 produced in the DOC on the DPF regeneration process.

’ EXPERIMENTAL SECTION Experimental Setup. Figure 1 shows the experimental design used to characterize the exhaust of a simulated diesel engine. The setup consisted of a soot generator; a proxy for diesel vehicles aftertreatment; and a series of instruments to analyze PM and gas phase in the stream. A mini Combustion Aerosol Standard (miniCAST, Jing Ltd. Switzerland) was used as soot generator. Additional gases were injected into the soot generated flow to mimic the diesel engine exhaust. The total flow rate of the stream was 20 L min1 using N2 as carrier gas, and the initial concentrations of the gas phase compounds were 500 ppmv NO, 1000 ppmv propene (C3H6), 1000 ppmv propane (C3H8), 9.0% v/v CO2, and 10% v/v O2 for all experiments unless differently indicated. The hydrocarbon concentration was elevated compared to a diesel exhaust to highlight the impact of DOC performance on soot emission. The total flow was preheated using a heating pipeline, which allowed temperature increase up to 873 K. A stainless steel reactor was used to fix a conventional DOC and a commercial mini DPF, and it was placed in a tubular furnace. The temperatures of the inlet and outlet gas stream (Tinlet and Toutlet) were measured using two thermocouples chromel alumel (type K). These thermocouples were located inside the reactor just before the DOC (Tinlet) and just after the DPF (Toutlet), and both were in contact with the gas stream. The pressure drop values between inlet and the outlet of the reactor were measured using a pressure sensor (Keller). The DOC was composed of a cylindrical cordierite monolith with 2.5 cm length and 2.5 cm diameter. The catalytically active metal phase in the DOC was a mixture of Pt and Pd with a metal ratio equal to 1 and a total metal loading of 3.2 g L1. This type of

DOC is highly effective in hydrocarbons and carbon monoxide oxidation at low temperatures. The mini DPFs used (IBIDEN, Japan) were composed of silicon carbide, and a cell density of 400 cpsi, with 5 cm length and 2.5 cm diameter. To study the effect of a DOC in the aftertreatment on particle characterization, the DPF was replaced by a silicon carbide monolith with open channels to allow PM circulation in the system, which was analyzed by scanning mobility particle sizer (SMPS) and aerosol mass spectrometry (AMS). The monolith was the same dimensions as the DPF to equal its dead volume inside the reactor; avoiding homogeneous or noncatalyzed and/ or nucleation processes. The concentration of the different compounds in the gas stream were analyzed online using a micro-GC (R 3000 SRA) for C3H8, C3H6, O2, CO, and CO2; and a chemiluminescence’s detector for NO, NO2 and NOx (ECO Physics CLD 62). As shown in Figure 1, a DPF was placed before the micro-CG and the chemiluminescence’s detector to filtrate PM. c-TOF-AMS. A compact time-of-flight (c-TOF, Tofwerk) Aerodyne Aerosol Mass Spectrometer (AMS) was used to analyze the non-refractory fraction of combustion PM. The methodology used within the C-TOF AMS is fully described in Drewnick et al.34 The soot particles were sampled through a critical orifice of 100 μm ID at 80 mL min1. An aerodynamic lens focuses particles as a narrow beam into the vacuum chamber, where they are accelerated to a terminal velocity which depends on their aerodynamic size. A mechanical wheel with two radial slits is used for chopping the particle beam. After traveling through the particle-sizing chamber, the particles enter the evaporation and ionization chamber. The nonrefractory aerosol components are flash-vaporized on a hot surface (∼873 K) and ionized by 70 eV electrons emitted from a tungsten filament. Positive charged ions are transferred into the time-of-flight mass spectrometer, collecting the complete mass spectra as a function of particle time-offlight. Mass spectra scans were obtained from m/z 4 to 400. The data were acquired in MS (mass spectrum) and P-TOF (Particle Time-of-Flight) modes. The MS and P-TOF modes were run for cycles of 20 s. Signals from soot particles and from gas phase background were acquired for 10 s in each mode. The MS is the result of the difference between the MS obtained with open and close chopper. Spectra were averaged and saved with a frequency of 3 cycles. The following controls were performed at the beginning of the experiments: baseline check; single ion calibration; 10592

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Table 1. Reactor Temperature, Hydrocarbons Conversions and NO2 Formation with and without the DOC Tinlet (K) Toutlet (K) XC3H8 (%) XC3H6 (%) NO2 (ppmv) without DOC

553

573

0

0

0

with DOC

553

653

13

100

60

m/z calibration for nominal masses 32, 28, 40, and 184; ion efficiency calibration for NH4NO3; and size calibration using polystyrene latex spheres. SQUIRREL data analysis package 34 was used to process mass spectra. Scanning Mobility Particle Sizer. Soot particle size distribution was measured using a SMPS (Model 3080, TSI Inc., St.Paul, MN, USA) with a differential mobility analyzer (DMA) (TSI, Model 3081) column and condensation particle counter (CPC) (TSI, Model 3772). It was operated with a sheath flow of 4 L min1; and an aerosol flow of 0.3 L min1. The particle diameter scanning was from 12.4 to 562 nm, and each scan required 3 min. Soot Generator. The miniCAST soot generator produces soot particles with physical and chemical properties similar to those of diesel engines. It allows size control and chemical composition in a wide range.3537 The generator uses a laminar diffusion flame, and consists of an inner burner gas flow, and an outer sheath air flow. Oxygen is transported by diffusion from the sheath flow, oxidizing the fuel gas. The flame is “cut open” at halfway to its tip, using N2 (Linde Gaz, 99.9995%) for quenching the reaction and stabilizing the formed soot particles. Propane (Air Liquide, 99.9995%) was used as fuel, and filtered compressed air was used as sheath gas flow. The miniCAST was operated at a propane flow of 0.06 L min1, and an air sheath flow of 1.55 L min1. The N2 flow for quenching the flame was 7.5 L min1. After soot particle formation, an air dilution flow of 6.2 L min1 was fixed inside the miniCAST to obtain a total flow of 15 L min1 at the outlet. Under these operating conditions, the miniCAST produces 400 mg of soot per m3 and 2.5 1014 particles number per cm3, and presents a c-TOF-AMS spectrum and a size particle distribution similar to those given by diesel engines.8,38 Thus, soot generated by the miniCAST is a good proxy for diesel PM. By means of AMS, Ferge et al. 35 reported a similar EC/TC ratio (∼0.95) for PM emitted from a diesel engine and from a mini CAST under similar conditions to those used in the present work. In contrast, classical thermal or thermaloptical techniques 39 do not allow estimating low OC concentrations adsorbed on soot particles.

’ RESULTS AND DISCUSSION Chemical Composition of Soot Particles. Mass spectra of soot particles were obtained at two different aftertreatment configurations; i.e., with and without a DOC, to analyze the effect of the DOC on the chemical composition of the PM. As described in the Experimental Section, a nonplugged monolith was fixed in the reactor downstream the DOC to allow soot particles analysis at the SMPS and AMS. The preheating pipeline and the furnace temperatures were set to Tinlet = 553 K in both aftertreatment configurations. Table 1 displays Toutlet, C3H8 and C3H6 conversions (XC3H8 and XC3H6), and NO2 concentration values with and without the DOC. XC3H6 expressed in % is defined as ([C3H6]initial [C3H6]outlet)/ [C3H6]initial 100; an analogue expression was used to calculate XC3H8. As shown in Table 1, in presence of the DOC the hydrocarbons were partially or completely oxidized while NO2 was possibly formed via NO

Figure 2. Mass spectra acquired with the c- TOF-AMS. Blue line mass spectrum with the DOC; red line mass spectrum without the DOC.

oxidation. As well, the Toutlet increase was due to the exothermicity of the hydrocarbons combustion reactions. Indeed, a thermodynamic calculation considering XC3H8 and XC3H6 values and the total flow of 20 L min1 predicts ∼80 K temperature increase with the DOC, in agreement with the temperature increase of Toutlet. The mass spectra obtained with the AMS for various organic species may show slight greater fragmentation than standard electron impact spectra due to the higher internal energy acquired during vaporization of the aerosol.40 However, the PAHs molecular ion signals are observed with significant intensity because aromatic rings are very resistant to fragmentation.40 Thus, PAHs can be used as indicative species of the condensed OC content on soot. Figure 2 shows two c-TOF-AMS mass spectra obtained for soot particles with and without the DOC. Only m/z values between 200 and 400 are shown, which is the range relevant to PAHs molecular ions. Signals with m/z 207, 221, 281, and 295 were due to polydimethylsiloxane contamination by the silicone tubing used in pipeline connections.41 The signals of PAHs molecular ion nominal masses with even carbon number in their structure are signaled with arrows in Figure 2. These ions are more abundant and stable than those with odd number of carbons.40 The signals indicated with the arrows were more intense in the mass spectrum obtained without the DOC than those obtained using the DOC in the reactor. Besides, the estimation of total organic mass loading values were 18 ( 1 and 45 ( 1 μg m3 with and without the DOC, respectively. These results clearly indicate a decrease in the amount of organic compounds in PM due to the DOC inclusion, which is in agreement with previous studies.10,27,29,30 In those studies, however, PM was collected onto filters and then OC and/or PAHs were extracted with different methods and analyzed by chromatographic techniques. Recently, Chirico et al. 42 have reported a decrease of OC concentration on PM from a real diesel engine in the presence of the DOC using AMS. It is worth noting that only the ion signal at m/z 295 (inset Figure 2) was more intense without the DOC; such an ion may be assigned to NO2PAH (e.g., NO2benzo(k)fluranthene or NO2benzopyrene) formation, via reaction between PAHs and NO2. Heeb et al. 10 have found NO2PAHs production in presence of oxidative catalysts in the aftertreatment. Effect of the DOC on Elemental Carbon Oxidation. Experiments on the effect of the DOC on soot oxidation were also performed without the DPF. Soot combustion was followed via variation of particle size evolution with temperature. The particle 10593



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Figure 4. Dependence of ΔP on Toutlet during the DPF regeneration (a) without the DOC and regeneration with air; (b) with the DOC and regeneration with air; and (c) with the DOC and regeneration with reactive mixture.

Figure 3. Kinetic experiments to calculate Ea for soot combustion using the SMPS. (a) with the DOC; (b) without the DOC.

size distribution was measured at different temperatures inside the reactor using a SMPS with and without the DOC upstream the nonplugged honeycomb monolith. The rate of size decrease was modeled using a modified Arrhenius expression, which was introduced by Higgins et al. for the study of soot particles:43,44 T0 Ea ΔDp ¼ Aθ0 1=2 exp ð1Þ RT T where ΔDp is the change of the mode diameter value in the particle size distribution, A is the pre-exponential factor, T0 is 298 K, θ0 is the residence time in the reactor (6.7 s) at T0, Ea is the activation energy, and T is the Tout of the reactor. Figure 3 shows the size distribution of soot particles with (a) and without (b) the DOC for different values of Tout. Shrivastava et al. 22 showed that at 373 K, neither soot oxidation nor evaporation of strongly adsorbed nonvolatile OC occurs. Thus, the initial particle diameter used in this work to calculate ΔDp was the mode diameter of the particles size distribution at 373 K, which was equal to 88 nm. No change of the soot particles size distribution was detected at temperatures below 783 K. However, at higher temperatures, the particle size distribution shifted to lower values, reaching a mode value of 70 nm at 858 K. As the EC/TC ratio in the PM generated from the CAST was near 0.95;

changes in the size distribution due to evaporation and oxidation of OC, occurring between 473 and 673 K,4345 can be assumed negligible. Consequently, the decrease of particle size above 773 K was attributed to EC oxidation. The insets of Figure 3 show the ΔDp values as a function of T, from which Ea can be calculated using eq 1. Moreover, NO2 formation was detected in presence of the DOC and its concentration was practically constant at 60 ppmv in the Tout range 773853 K. The fitting was performed using the nonlinear LevenbergMarquard algorithm. The Ea values were 97 ( 5 and 101 ( 8 kJ mol1 for the soot oxidation reaction with and without the DOC, respectively. These Ea values are in concordance with the reported values for soot oxidation with air,44,47 while they are twice the values reported by Shrivasrava et al. 22 for soot oxidation with NO2. Thus, the use of the DOC did not facilitate the EC oxidation on the soot particles in the gas stream at low temperatures. This can be probably explained by the short residence time of particles in the reactor. Effect of the DOC on the DPF Regeneration. The DPF regeneration process was investigated with and without a DOC. As mentioned in the Experimental Section, the DPF was placed downstream the DOC (Figure 1). Initially, a step of soot charging in the DPF was performed for 1 h under a total flow rate of 20 L min1 (with initial concentrations as described in detail in the Experimental setup section) to accumulate 250 mg of soot, equivalent to 10 g of soot per L of DPF. The latter is in agreement with the soot concentration inside a DPF before the regeneration process is performed in a real diesel aftertreatment.48,49 During this step, no particles were detected with the SMPS at the outlet gas stream, indicating the excellent filtering efficiency of the DPF. The DPF regeneration process was performed after the soot charging step under a total gas flow rate of 10 L min1 using either (i) compressed air or (ii) a reactive mixture of the same concentration as the DPF charging flow (500 ppmv NO, 1000 ppmv C3H6, 1000 ppmv C3H8, 9.0% v/v CO2, 10.0% v/v O2 and N2 as carrier). In the regeneration process, a constant heating rate of 10 °C min1 was used from 573 to 973 K. Figure 4 shows the dependence of the pressure drop (ΔP) on Toutlet during the DPF regeneration process for three different configurations, using compressed air with (a) and without (b) DOC, and using the reactive gas mixture with the DOC (c). 10594



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the kinetic experiments (without the soot accumulation step) shown in Figure 3. Summarizing, the presence of the DOC in the aftertreatment induces two opposite effects on the DPF regeneration: (I) depletion of OC fraction on the soot accumulated inside the DPF, and (II) NO2 formation. According to our knowledge, this is the first study that combines different approaches to investigate the influence of a DOC in the DPF regeneration process.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +33 472 431 054; Fax: +33 472 431 695; E-mail: [email protected]. Figure 5. Dependence of ΔP and the NO2 formation yield (YNO2) on Toutlet during the DPF regeneration with the DOC under reactive mixture feed.

The temperature associated to the maximum of ΔP indicated that the DPF regeneration process was already initiated. This temperature was used to compare the different configurations and it was addressed as critical temperature (Tc). As shown in Figure 4, in the case where only air was fed, a significant increase in Tc was observed from 813 to 873 K in the presence of DOC. This increase may be rationalized by a difference in the composition of the soot particles stored in the DPF during the charging step due to the DOC. This is partly confirmed from a decrease of the organic mass loading and PAHs (Figure 2) observed when the DOC is used upstream the monolith. It is worth noting that the temperature at the reactor outlet, Toutlet, in presence of the DOC during charging was ∼80 K higher than without the DOC. This is possibly due to the exothermicity of the catalytic combustion of the cofeeded hydrocarbons. Furthermore, NO2 formation was detected (see Table 1). Thus, the higher operation temperature and NO2 presence could result in pronounced evaporation or oxidation of the PAHs, or more generally OC, from the soot particles. Hence, the OC content of the soot particles stored in the DPF would be significantly lower, as shown by the AMS data. The OC can facilitate the ignition of the combustion of the black carbon nucleus,46 providing the necessary heat by the exothermicity of their combustion reaction. When the gas mixture was used for the regeneration, the measured Tc (783 K) was lower than without the DOC and feeding only air (813 K). The remarkable Tc difference (783 K vs 883 K) between the two cases where the DOC was utilized (a,c) can suggest a different soot oxidation mechanism. Figure 5 presents the effect of temperature on ΔP in the DPF and the NO2 formation yield during DPF regeneration under feed conditions similar to those during the soot charging step in presence of the DOC. As shown in the figure, at temperatures below 623 K a significant NO2 production yield (∼30%) was observed, which decreased to lower than 5% at higher temperatures. The latter is more likely related to the reduction of NO2 to NO by soot oxidation 18,20,23,50 than to thermodynamic equilibrium limitations. As mentioned above, NO2 is known as a stronger oxidizing agent than O2 for soot oxidation. In this case, the obtained Tc for soot combustion was 783 K, which overbalances the observed negative effect of the DOC in Figure 4, when the regeneration process was performed with air. Soot accumulation inside the DPF allowed EC oxidation by NO2 generated in the DOC, in contrast to the results obtained from

’ ACKNOWLEDGMENT The authors thank Gilles D’Orazio and Frederic Bourgain from the technical service of IRCELYON. The authors acknowledge PSA Peugeot-Citro€en to provide the DOC used in the study. The authors are also grateful to the “ADEME PIREP” project (Contract No. 06-66-C0138 from PREDIT 3 VPE 2006 program) for the funding of this study. ’ REFERENCES (1) WHO:Guidelines for Air Quality. World Heath Organization; 2005. (2) International Agency for Research on Cancer (IARC) Air pollution, part 1, some non-heterocyclic polycyclic aromatic hydrocarbons and some related industrial exposures. In: Monographs on the Evaluation of Carcinogenic Risks to Humans; Lyon, 2008. (3) Jacobson, M. Z. Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols. Nature 2001, 409 (6821), 695–697. (4) Lohmann, U.; Feichter, J. Global indirect aerosol effects: a review. Atmos. Chem. Phys. 2005, 5, 715–737. (5) Morawska, L.; Ristovski, Z.; Jayaratne, E. R.; Keogh, D. U.; Ling, X. Ambient nano and ultrafine particles from motor vehicle emissions: Characteristics, ambient processing and implications on human exposure. Atmos. Environ. 2008, 42 (35), 8113–8138. (6) Kittelson, D. B. Engines and nanoparticles: A review. J. Aerosol. Sci. 1998, 29 (56), 575–588. (7) Querol, X.; Alastuey, A.; Ruiz, C. R.; Artinano, B.; Hansson, H. C.; Harrison, R. M.; Buringh, E.; ten Brink, H. M.; Lutz, M.; Bruckmann, P.; Straehl, P.; Schneider, J. Speciation and origin of PM10 and PM2.5 in selected European cities. Atmos. Environ. 2004, 38 (38), 6547–6555. (8) Barrios, C. C.; Dominguez-Saez, A.; Rubio, J. R.; Pujadas, M. Development and evaluation of on-board measurement system for nanoparticle emissions from diesel engine. Aerosol Sci. Technol. 2011, 45 (5), 570–580. (9) Maricq, M. M. Chemical characterization of particulate emissions from diesel engines: A review. J. Aerosol. Sci. 2007, 38 (11), 1079–1118. (10) Heeb, N. V.; Schmid, P.; Kohler, M.; Gujer, E.; Zennegg, M.; Wenger, D.; Wichser, A.; Ulrich, A.; Gfeller, U.; Honegger, P.; Zeyer, K.; Emmenegger, L.; Petermann, J. L.; Czerwinski, J.; Mosimann, T.; Kasper, M.; Mayer, A. Impact of low- and high-oxidation diesel particulate filters on genotoxic exhaust constituents. Environ. Sci. Technol. 2010, 44 (3), 1078–1084. (11) Rothen-Rutishauser, B. M.; Schurch, S.; Haenni, B.; Kapp, N.; Gehr, P. Interaction of fine particles and nanoparticles with red blood cells visualized with advanced microscopic techniques. Environ. Sci. Technol. 2006, 40 (14), 4353–4359. (12) Liati, A.; Dimopoulos Eggenschwiler, P. Characterization of particulate matter deposited in diesel particulate filters: Visual and analytical approach in macro-, micro-, and nano-scales. Combust. Flame 2010, 157 (9), 1658–1670. 10595


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