Catalytic Filter Systems with Direct and Indirect Soot Oxidation Activity

2005-01-0670SAE TECHNICALPAPER SERIESCatalytic Filter Systems with Direct andIndirect Soot Oxidation ActivityAthanasios G. Konstandopoulos, Eleni Papaioannou, Dimitrios Zarvalis,Sofia Skopa, Penelope Baltzopoulou, Evdoxia Kladopoulou,Margaritis Kostoglou and Souzana LorentzouAerosol & Particle Technology Laboratory, CERTH/CPERIReprinted From: Diesel Exhaust Emission Control 2005(SP-1942)2005 SAE World CongressDetroit, MichiganApril 11-14, 2005400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org

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2005-01-0670Catalytic Filter Systems with Direct and IndirectSoot Oxidation ActivityAthanasios G. Konstandopoulos, Eleni Papaioannou, Dimitrios Zarvalis, Sofia Skopa,Penelope Baltzopoulou, Evdoxia Kladopoulou, Margaritis Kostoglou and Souzana LorentzouAerosol & Particle Technology Laboratory, CERTH/CPERICopyright © 2005 SAE InternationalABSTRACTDiesel Particulate Filters (DPFs) need to be periodicallyregenerated in order to achieve efficient and safe vehicleoperation. Under typical diesel exhaust conditions, thisinvariably requires the raising of the exhaust gastemperature by active means, up to the point thatparticulate (soot) oxidation can be self-sustained in thefilter. In the present work the development path of anadvanced catalytic filter technology is presented. Fullscale optimized Catalytic Diesel Particulate Filters(CDPFs) are tested in the exhaust of a light-duty moderndiesel engine in line with a Diesel Oxidation Catalyst(DOC). The management of the DOC-CDPF emissioncontrol system is facilitated by a virtual soot sensor inorder to ensure energy-efficient operation of theemission control system.INTRODUCTIONAs diesel emission regulations become increasinglystringent the use of Diesel Particulate Filters (DPFs) isever increasing in OEM applications. Modern DPFsexhibit very high filtration efficiencies, however theyneed to be periodically cleaned (regenerated) in order toachieve efficient and safe operation of the vehicle. Astypical diesel exhaust conditions are not hot enough toinitiate and maintain particulate (soot) oxidation, active(engine) means are employed to raise the exhaust gastemperature up to the point that particulate oxidation canbe self-sustained in the filter, at fast enough rates(>650 C). To achieve the oxidation of soot particles atlower temperatures (250 – 550 C) a number of directand indirect catalytic measures can be employedranging from fuel additives, generation of reactivespecies, catalytic combustion of post-injected fuel andfilter coatings promoting soot oxidation.Modern trends [1-4] in passenger car emission controlsystems are now emphasizing on so-called “fit-for-life”solutions overcoming the need for servicing (ashremoval) of the DPF during the vehicle lifetime.Robustness and durability of the engine and emission1control system is also clearly a first priority in heavy-dutysurface transportation. These trends create newopportunities for the application of CDPFs.Catalytic DPF development paths have moved beyondthe “Holy Grail” of passive behavior (largely inheritedfrom 3-way catalytic converters) and now recognize thatrather than focusing solely on regeneration control it ismore advantageous to focus on integrated managementof the DPF over its entire operation profile/modes (e.g.loading, regeneration, ash-induced aging) [5]. Thesemodes are in addition differentiated by their temporalcharacteristics: loading occurs over hours-to-days,regeneration over minutes and ash-induced aging overmonths-to-years of operation. These characteristicscreate new needs for multifunctional catalytic filterswhich:(i)(ii)will exhibit some oxidation activity under moderateexhaust temperature to prolong as much aspossible the intervals between fixed regenerations,exploiting direct (i.e. through oxygen transfer) aswell as indirect (through NO 2 generation) sootoxidation,will exhibit reduced soot ignition temperaturescompared to uncatalyzed filters to allow for energysavings during regeneration(iii) will be tolerant to ash accumulationDuring the last 9 years the APT Laboratory hasaddressed many aspects of Diesel Particulate Emissionscontrol to assist in the systematic and cost-effectiveoptimization of next generation DPF systems. In thepresent paper we present the development path of anadvanced catalytic filter technology. The developmenteffort starts from the small-scale where wet-chemistry aswell as advanced catalyst synthesis techniques viaaerosol routes are employed for the optimization of thecatalyst chemistry and its deposition in the filter. Smallscale catalytic coated filter samples are then evaluatedwith respect to their direct soot oxidation activity bylattice or spill-over oxygen in a side stream of diesel

1. Single metal oxide catalysts, V 2 O 5 , Al 2 O 3 , ZrO 2 ,Nb 2 O 5 , CuO, CeO 2 , TiO 2 , MgO, La 2 O 3 (BakerChemicals) were prepared by milling and sieving thefraction of particles in the range 20-38 µm. Thesecompounds are known to either exhibit soot oxidationactivity or are used as catalyst supports and havebeen used to allow methods development andprovide a reference base.2. Molten catalysts have been advocated as solutionsfor the problem of poor soot-catalyst contact [28-30].The mechanism of action of such “mobile” catalysts,could involve volatile species responsible for theactivity through an evaporation-condensationmechanism on the soot particles or liquid phases thatare formed as the temperature rises and thus theypromote better soot-catalyst contact [31]. Accordinglynumerous eutectic mixtures were prepared: KCO 3 -V 2 O 5 (39.5-60.5% mole), Li 2 CO 3 -K 2 CO 3 (47-53%mole), Li 2 CO 3 -Na 2 CO 3 (58.2-41.8% mole), Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 (31.5-43.5-25% mole), ZnCl 2 -SnCl 2(20-80% mole), Ag 2 O-V 2 O 5 (63-37% mole) by fusinga mixture of precursor compounds having acomposition corresponding to the eutectic point asobtained from published phase diagrams [32].3. A promising Cu/V/K based catalyst formulation wasprepared on different supports (denoted as A, B, C,D) in addition to the original promising formulationsupported on Al 2 O 3 described in [33] This catalystformulation has been the subject of somecontroversy in the literature [24, 27, 34, 35] regardingits thermal stability.To better understand the mechanism of catalyst actionwe undertook a temperature-programmed reaction /desorption (TPR/TPD) study of soot-catalyst mixturesprepared by mixing catalyst and soot in a 2:1 proportion(by weight) inside a vessel placed on a vibrating plate. Astandardized commercial carbon black (ASTM N330,Statex) was used as model soot. BET analysis showedthat N330 soot has a surface area similar to that ofdiesel soot and is consistent with a primary soot particlesize of 40 nm.We studied the soot-catalyst reaction under inert (He)atmosphere, with the objective to investigate whetherthe catalysts could act with a redox mechanism (i.e. inthe absence of gaseous oxygen). The tests wereperformed with soot, catalyst, and soot-catalyst mixturesto allow for a cross-comparison. Samples of 100 mgwere tested in a heated quartz tube reactor with a flowrate of 30 cm 3 /min at temperatures up to 800 C and athermal conductivity detector (TCD) was used to identifychanges in the evolved gas (compared to background).The TCD in principle senses changes in the gas mixturecomposition caused by oxidation of soot towards COand CO 2 as well as by any possible evaporation ofcatalyst components.Based on the comparative study of the TPR/TPD data,XRD analysis of the synthesized catalysts, themelting/decomposition points of the various catalystcompounds and consulting past work [24, 27] weidentified catalyst formulations with differentcharacteristics:(i)With activity for direct soot oxidation (in theabsence of oxygen), (e.g. the E27 vs. E28formulation in Figure 1)(ii) With activity which was enhanced in the presenceof oxygen from the gas phase (e.g. the Ag/V/Oformulation in Figure 2)(iii) From which oxygen was desorbed at differenttemperature ranges e.g. the Cu/V/K formulations(see also [35])(iv) From which evaporation of active compounds wasoccurring (e.g. the E3A formulation Figure 3)(v) With improved thermal stability enhanced by theuse of specific “supports”, (e.g. the E27 formulationin Figure 1).It should be noted that the use of the term “support” isused for description purposes only, since in severalcases it was found that the “support” was formingdifferent compounds with the catalyst precursors. Table1 summarizes the catalyst formulations tested in Figure1 to Figure 3.Table 1. Catalyst formulations.Catalysts3 (Cu/V/K/Al 2 O 3 )7 (Cu/V/K/D)E10 (Ag/V/O)E14 (Cu/V/K/B)E15 (Cu/V/K/C)E27 (Cu/V/K/C)E28 (Cu/V/K/E)E29 (Cu/V/K/B)PrecursorsChloridesChloridesNitratesChloridesChloridesChlorides & NitratesChlorides & NitratesChlorides & Nitrates3

5 0 02 5 01 0 0 005 0 005 0 04 0 03 0 02 0 01 0 04 0 0 03 0 0 02 0 0 01 0 0 0005 0 02 5 01 0 0 05 0 04 0 02 0 0004 0 0 03 0 0 02 0 0 01 0 0 000500CAT + He500CAT + He500CAT + HeE27 (Cu/V/K/C) E28 (Cu/V/K/E) E29 (Cu/V/K/B)2502502500001000CAT + SOOT + He1000CAT + SOOT + He1000CAT + SOOT + He500500500SootSootSootSignal0500400CAT + He/O2(0.95/0.05)0500400CAT + He/O2(0.95/0.05)0400CAT + He/O2(0.95/0.05)3003002002002001001000004000CAT + SOOT + He/O2(0.95/0.05)4000CAT + SOOT + He/O2(0.95/0.05)4000CAT + SOOT + He/O2(0.95/0.05)3000Soot3000Soot3000Soot20002000200010001000100000 200 400 600 80000 200 400 600 80000 200 400 600 800T sample , C T sample , C T sample , CFigure 1. TPD/TPR study of the Cu/V/K catalyst prepared with other precursors than the original formulation, on supports B, C, E.E10 (Ag/V/O)1000CAT + SOOT + He4000CAT + SOOT + He/O2(0.95/0.05)Signal50 0Soot2000Soot0T sample , C00 20 0 40 0 60 0 80 0T sample , CFigure 2. TPD/TPR study of a eutectic Ag/V/O catalyst showing enhanced activity in the presence of oxygen (right) vs. inert gas (left).40 0E3A (Cu/V/K/Al 2 O 3 ) E7 (Cu/V/K/D) E15 (Cu/V/K/C) E14 (Cu/V/K/B)CAT + He100080 01000CAT + HeCAT + HeCAT + He50 020 040 050 00050 0CAT + SOOT + He080 0CAT + SOOT + He0150 0CAT + SOOT + He150 0CAT + SOOT + He25040 0750750Signalg080 0CAT + He/O2(0.95/0.05)Soot080 0CAT + He/O2(0.95/0.05)Sootg01000SootCAT + He/O2(0.95/0.05)0150 0SootCAT + He/O2(0.95/0.05)40 040 050 075000004000CAT + SOOT + He/O2(0.95/0.05)4000CAT + SOOT + He/O2(0.95/0.05)40 0 0CAT + SOOT + He/O2(0.95/0.05)4000CAT + SOOT + He/O2(0.95/0.05)2000Soot2000Soot20 0 0Soot2000Soot00000 200 400 600 8000 200 400 600 8000 200 400 600 8000 200 400 600 800T sample , C T sample , C T sample , C T sample , CFigure 3. TPD/TPR study of the Cu/V/K catalyst prepared on other than the original supports (Al 2 O 3 ) designated as B, C, D.4

In Figure 4 the evolution path of soot ignitiontemperature is summarized based on our preliminarypowder investigations. When the soot ignitiontemperature is plotted in this fashion one may obtain atrade-off between the achieved performance and theresearch effort expended employing the availableresources. The significance of Figure 4 is that itdemonstrated to us that development of more catalystcompositions in powder form would not be expected tolead to substantial improvements (since the evolutionpath slope became less steep) and that we should focusmore on modifications/optimizations of the most activecompositions we were able to develop.On the basis of the experience gained, a new generationof catalysts designated as E35, E36 and E37 derivedfrom the best powder compositions of Figure 4, weresynthesized and tested in a TGA apparatus. The resultsshown in Figure 5 illustrate that the formulation E37represents the most optimized catalyst synthesized sofar in terms of soot ignition activity as well as in oxidationcompletion temperature. This formulation was selectedas the base of our subsequent developments.Temperature of Start Soot Oxidation, C7006005004003002001000soot 11 16 9 6 512 154141317 18 1973101 82CatalystsFigure 4. Evolution path of soot ignition temperature with the developedcatalysts.Soot fraction (-)10.90.80.70.60.50.40.30.2E37 E36 E350.10200 250 300 350 400 450 500Temperature (C)Figure 5. Comparison of E35, E36 and E37 TGA results.5CATALYST DEPOSITION TECHNIQUESWet Chemistry TechniquesCatalyst deposition on the porous filter substrates wasperformed either with a pre-formed catalyst powderslurry filtration procedure or by dipping of the substratesinto catalyst precursor solutions and subsequent firing.The coated filters were thermally treated in a furnace at700 C for 4 hours. Permeability changes on the order of15% were noticed for the filter samples coated by wetchemistry techniques.Aerosol TechniquesThe APT Lab has introduced [36, 37] the Aerosol SprayPyrolysis technique (ASP) which combines catalystsynthesis and deposition on a porous substrate in onestep, as an alternative to wet chemistry multi-steptechniques (powder synthesis, slurry deposition, firing).The ASP procedure also has the advantages of precisecontrol of the catalyst particle composition and of thequantity deposited, as well as its spatial depositionprofile along the porous filter wall. An added benefit isthe fact that the catalyst is deposited with the samemechanism as the soot particles, a feature which isexpected to maximize the contact of the soot particleswith the pre-deposited catalyst sites, during filteroperation.In the Aerosol Spray Pyrolysis technique, catalystnanoparticles of controlled size are synthesized from thethermal decomposition/evaporation of precursor solutiondroplets introduced in a hot-tube aerosol reactor.Subsequently these are in-situ deposited on poroussubstrates introduced in the reactor. Factors affectingthe particle formation are the type of solution precursor,the temperature and the residence time in the reactorprior to deposition on the filter wall.Aerosol/Vapor Phase techniques are expected to lead tonanostructured catalysts with controlled depositionprofiles on the porous filter wall. Accordingly twoadditional coating techniques, namely vapor deposition(VD) in a vacuum sputtering chamber and atmosphericplasma spray (PS) were procured for comparativepurposes.CATALYTIC FILTER ASSESSMENTFilter loading and regeneration with sootCoated Filters were loaded in a dedicated soot reactorunit [13] with diesel soot from a side stream of theexhaust of a modern passenger car diesel enginerunning at a constant point. Filtration velocity wasrepresentative of full scale wall-flow honeycomb filters (4cm/s). After loading without removing the filter from thereactor, the regeneration behavior of the catalyst coateddisks was studied by in-situ TPR and the reaction ratewas followed from the evolution of CO and CO 2 . The

oxygen content of the regeneration gas was 10% O 2 andthe balance was N 2 .Soot-catalyst contactSoot particle – catalyst contact is determined by thedetails of catalyst distribution in the filter (a type of“frozen” randomness) and the details of soot particledeposition and resulting deposit microstructure (a type ofevolving randomness) therefore it is important to study itunder realistic conditions, i.e. depositing fractal sootaggregates from an engine under similar conditions ofPeclet number [18, 38].To illustrate the relevance of realistic depositionexperiments for soot oxidation we have synthesized withASP various binary oxide catalyst formulations andtested them mixed with soot under tight contactconditions to establish their intrinsic catalytic activity. Asseen in Figure 6 the catalysts exhibit a range of catalyticactivities according to their composition.1/m o soot (dm/dt) (s- 1 )0.00120.0010.00080.00060.00040.00022.5X97.5Y5X95Y10X90Y20X80Y60X40Y80X20Y100X100Ysoot0250 350 450 550 650 750Temperature (C)Figure 6. Reaction rate of ASP catalyst powders in tight contact withsoot.Subsequently the various formulations were coated byASP on the surface of filters exhibiting cake filtrationbehavior, and then they were loaded with soot from theengine exhaust. When the loaded catalytic filters wereoxidized, no indication of any catalytic activity wasevident and all samples had identical behavior with theuncatalyzed sample loaded with soot (Figure 7).1/m o soot (dm/dt) (s- 1 )0.0010.00090.00080.00070.00060.00050.00040.00030.00020.000102.5X97.5Y5X95Y10X90Y20X80Y60X40Y100Ysoot250 350 450 550 650 750Temperature (C)Figure 7. Reaction rate of ASP catalyst coated filters in cake contactwith soot.Performing the same experiment with one active catalystformulation (60X40Y) but after depositing it on deep bedfilters the catalytic activity was manifested at differentdegrees depending on the type of deep bed filter contact(designated as contact B and B1), as shown in Figure 8.The results of tight contact for that catalyst formulation(designated as C) are also shown for reference. Thecurve designated as contact A denotes the cake contactresult for this formulations, which as seen from Figure 7,is the same as uncatalyzed soot. These results indicatethat distribution of the catalyst over a porous regionwhere soot can penetrate and deposit achieves morepoints of active soot-catalyst contact than those obtainedby distribution of the catalyst as a layer on a filter.1/m o soot (dm/dt) (s -1 )0.00120.0010.00080.00060.00040.0002contact Ccontact Acontact Bcontact B10250 350 450 550 650 750Temperature (C)Figure 8. Effect of filter type (cake vs. deep-bed) on reaction rate of ASPcatalyst coated soot loaded filters.Comparison of catalyst deposition techniquesA comparison of catalyst coating techniques on filterswas performed employing an open pore, SiC structureexhibiting pronounced deep bed filtration characteristics,as shown in Figure 9.6

Pressure drop (mbar)4504003503002502001501005000 1000 2000 3000 4000 5000 6000 7000 8000Time (s)Figure 9. Pressure drop evolution on an open pore SiC filter disk duringloading with soot. The filter exhibits a long period of deep bed filtrationbehavior.Results from the Aerosol/Vapor phase depositiontechniques for catalyst coating on filters are shown inFigure 10. The catalyst formulation was based on amajor component of the E37 catalyst, since our intentionwas to test the effect of different catalyst depositiontechniques in the specific filter structures and the E37formulation presented originally many challenges inorder to become adaptable to ASP. The deep bedfiltration characteristics are responsible for the twopeaks in the dimensionless reaction rate, since a part ofthe soot is in contact with the catalyst inside the poresand a part is not. The catalyst activity of the 3techniques appears to be in the order ASP=VD>PS.Interestingly enough the ASP technique seems tomaximize the amount of soot in contact with the catalyst(as indicated by the first peak). Given the additionalcomplexity and the cost of the VD and SP techniques(vacuum, plasma management) the ASP techniqueappears as quite promising.1/m o (dm/dt) (s -1 )5.0E-044.5E-04ASP4.0E-043.5E-04VD3.0E-042.5E-042.0E-041.5E-04PS1.0E-045.0E-050.0E+00250 300 350 400 450 500 550 600 650Temperature (C)Figure 10. Comparison of ASP, VD and PS prepared catalytic coatingswith respect to soot oxidation behavior.In Figure 11 we compare TGA results of catalystpowders made with ASP and wet chemistry formulationsusing the same precursors, mixed with soot in tightcontact. The ASP prepared catalyst exhibits a higherreactivity than the wet chemistry formulation.Soot fraction (-)10.90.80.70.6soot0.5WET0.4ASP0.30.20.10200 300 400 500 600 700Temperature (C)Figure 11. Comparison of soot oxidation activity of ASP and WetChemistry prepared catalysts in tight contact.The superiority of the ASP technique over wet chemistryis also observed on the filter samples, depicted in Figure12.1/m o (dm/dt) (s -1 )5.0E-044.5E-044.0E-04ASP3.5E-04WI3.0E-042.5E-042.0E-041.5E-041.0E-045.0E-050.0E+00250 300 350 400 450 500 550 600 650Temperature (C)Figure 12. Comparison of ASP and Wet impregnated (WI) catalyticcoatings with respect to soot oxidation behavior.As the highly porous deep bed filters that we employedfor method development would not become availablecommercially our focus shifted to studying ASP catalystson commercially available media. In addition after somedevelopment effort it became possible to adapt the E37formulation to the ASP technique. The availability ofsintered metal filter media in sheet form made it easy toadapt them in our disk reactor, whereas SiC disks ofnominal structure similar to the commercial DPFs weresourced from a ceramics supplier.7The sintered metal filter was coated with the E37catalyst in the ASP reactor (with a light load of 6 gr/m 2 )and then it was assessed for its catalytic activity in the

engine exhaust. The results shown in Figure 13 implythat probably the catalyst load was not sufficient enougha fact that was confirmed by preparing a similar samplewith a catalyst load of 46.4 gr/m 2 , but with a wetimpregnation technique (shown as WI coated in Figure13), since the ASP reactor was not set up to deliver suchloading levels within a reasonable time scale.1/m o (dm/dt) (s -1 )7.E-046.E-045.E-044.E-043.E-042.E-041.E-04uncoatedASP coatedWI coated0.E+00250 350 450 550 650 750Temperature (C)Figure 13. Comparison of regeneration behavior of sintered metal filters:coated by ASP and coated by Wet Impregnation.As the geometry and structure of the filter was shown toplay a very high role we decided to perform a catalystloading study with the ASP technique on ceramic SiCfilters to have a more direct comparison with the SiCdisks we originally employed. A lower than 46.4 g/m 2catalyst load was studied (25.5 g/m 2 ) and a higher(143.2 g/m 2 ) to provide bounding behavior for the effectof catalyst loading levels. As seen in Figure 14 thecatalytic activity depends on the catalyst load. The lowercatalyst load exhibits a higher activity and we attributethat to the prevention of soot particle interpenetrationinto the filter wall, hence poorer contact with the catalyst.1/m o (dm/dt) (s -1 )6E-045E-044E-043E-042E-041E-04uncoated25 g/m² E-37143 g/m² E-37At this stage it became clear to us that the filter wallmicrostructure is the determining factor for theestablishment of a high catalytic activity and notnecessarily the amount of catalyst, as it is usually thecase for gaseous species catalysis. The reason for thisis as mentioned above the fact that deposition of morecatalyst than a certain amount can actually prevent sootfrom penetrating the filter wall and thus reduce thenumber of soot-catalyst points.Therefore we made efforts to source monolithichoneycombs with deep bed filtration characteristics inour next step towards scale-up. A 25 mm diameter x 50mm long SiC monolith was sourced from a supplier ofsupports for liquid membrane carriers and wasconverted into a wall-flow filter by plugging alternativelyits end channels. As the ASP reactor’s capacity at thetime would not accommodate full size filter samples(which invariably would be available with structuresexhibiting cake filtration characteristics) it was decided tocontinue the catalyst coating study with the E37formulation with a wet impregnation method in order tobe able to coat full-scale samples with this activecatalyst formulation.The small SiC monolith was coated with the E37 catalystand placed in a new reactor, which was built to exposesmall monoliths to the exhaust sidestream. Anuncatalyzed monolith was used for reference. Figure 15shows the regeneration behavior of the two monoliths.The catalyzed monolith starts to regenerate at 350 Cand its peak activity is shifted about 100 C below that ofthe uncatalyzed one.1/m o (dm/dt) (s -1 )8E-047E-046E-045E-044E-043E-042E-041E-04E37 coateduncoated0E+00250 350 450 550 650 750Temperature (C)Figure 15. Comparison of regeneration behavior of an open pore SiCmonolith coated with the E37 catalyst formulation.0E+00250 350 450 550 650 750Temperature (C)Figure 14. Comparison of regeneration behavior of ceramic SiC filters:with cake filtration characteristics at different levels of catalyst coatingwith the ASP method.8

FULL SIZE FILTER TESTINGENGINEFull scale DPFs were tested in the exhaust of a modernEURO III diesel engine (displacement 1.9 L, rated power60 kW), coupled to a servo-controlled dynamometer.The engine was operated under steady state conditionsas well as under transient conditions. Transientoperation was achieved by simulating the speed andtorque profiles of the New European Driving Cycle(NEDC) for the specific engine in the dynamometer, incollaboration with the engine manufacturer.DPF CHARACTERISTICSThe characteristics of the DPFs tested in terms of theircatalyst loading are shown in Table 2, and are denotedas DPF A-D. Overall 4 catalyzed DPFs (at differentloadings of the E37 catalyst in combination with a noblemetal) were tested. An uncatalyzed DPF was also usedas a reference. In Table 2, x is the amount of catalystload (referred as E37) and y is the amount of noblemetal (NM) added in the filter. The noble metal is fromthe Platinum group.Table 2. Characteristics of the DPFs tested.DPFABCDCatalyst LoadingNo catalystE37: 2.7x / NM: 0yE37: 1x / NM: 1yE37: 2x / NM: 1yREGENERATION TECHNIQUEIn actual OEM applications modification of engine tuningin conjunction with fuel post injection through thecommon rail system, EGR rate variation, boost pressurevariation or throttling offers great opportunities toincrease exhaust temperature and it can be achievedwithout adversely affecting fuel penalty and driveabilityconditions. Specific strategies therefore, focusing oncombination of boost pressure, EGR rate increase, andfuel injection modification are been implemented [4, 5].In some cases [39] exhaust temperature is (in addition toengine measures) raised by exhaust-port fuel injection infront of a Diesel Oxidation Catalyst (DOC). The sametechnique is also applied for some retrofit applications[40]. The applicability of the technique relies on havingthe DOC above the hydrocarbon light-off temperature(about 150 C) a condition, which is met with our testengine running over the NEDC, consisting of four citycycles and one Extra Urban (EU). Figure 16 shows theexhaust gas temperature of our engine over the cycle.As our Euro III test diesel engine has a “closed-ECU”9(i.e. a production ECU) we have selected exhaust portinjection in front of a DOC as the means to raise theexhaust gas temperature, and in turn to effectregeneration of the DPF.Engine speed (rpm)300025002000150010005000Engine speedCity cyclesExtra urbancycleExhaust gas temperature00 200 400 600 800 1000 1200Time (s)600500400300200100Exhaust gas temperature(C)Figure 16. Exhaust gas temperature and engine speed over the NEDCcycle.The exhaust port-injection setup is constructed fromstandard automotive spare parts and consists of an airassistednozzle, and auxiliary fuel and air pumps. Fuelflow to the nozzle is controlled by a custom electroniccircuit defining the nozzle opening time and the periodbetween injections. The fuel injection system has beenoptimized with the aid of a Phase-Doppler analyzer interms of droplet size. The fuel injection location wasselected with the aid of CFD calculations so as to allowthe fuel droplets to evaporate and the resulting dieselvapor to reach the DOC inlet with an even distribution.Fuel dosing and calibration of the system has beenperformed at representative engine operating conditionsand its effect on the increase of exhaust temperaturehas been studied providing an operational map given inFigure 17.Exhaust temperature increasedue to fuel injection (C)3503002502001501005002160 rpm, 39 Nm2450 rpm, 59 Nm1730 rpm, 33 Nm2440 rpm, 133 Nm2630 rpm, 83 Nm2950 rpm, 183 Nm0 5 10 15 20 25 30 35 40Fuel injected (g/min)Figure 17. Exhaust temperature increase with port fuel injection atdifferent engine points

REGENERATION CONTROLOur regeneration control algorithm is based on applyinga virtual soot sensor to estimate the soot mass load inthe DPF in real time. The virtual sensor prototype isimplemented in LabView and it has descended fromearlier virtual soot sensors (based on DPF pressuredrop, exhaust flow rate and exhaust gas temperature)that we have described in previous publications [41, 42].Variants of that virtual sensor have been implemented inECUs [5] and have been also adapted in seriesproduction vehicles [4]. Figure 18 shows the virtualsensor interface in a real time application, where theexperimental measurements of the DPF pressure drop,exhaust mass flow rate and exhaust gas temperatureare used to calculate the soot mass load inside the filter.The results of the DPF loading are shown in Figure 19where the soot mass accumulated inside the DPF isdepicted as a function of the challenge mass load, whichis the soot load of a hypothetical absolute filter placedupstream of the test filter. The soot mass accumulated inthe DPF was estimated from the above mentionedvirtual soot sensor. We observe that under the steadystate conditions of the test DPF B exhibits a moderatesoot oxidation activity while DPF C exhibits asignificantly higher one. This increased activity isattributed to the presence of the noble metal, which canassist the oxidation of NO to NO 2 , as well as deliveradditional spill-over oxygen to the catalyst/soot interface.54DPF AMass load (g/m 2 )321DPF BDPF C00 1 2 3 4 5 6Challenge mass load (g/m 2 )7Figure 19. Soot mass load of single DPF A, B and C at steady state.DPF inlet temperature: 370 C, exhaust mass flow rate: 34 g/s, NO x /sootratio: 18.Figure 18. Virtual sensor interface running on a real time application.TESTING OF DOC – DPF IN SERIESProprietary improvements in the virtual soot sensorinclude a detailed account of the effects of soot depositmicrostructure under reactive and non-reactiveconditions [18, 38] and emergency procedures in case ofvirtual sensor failure (caused either by some hardwaremalfunction or by deviations from pre-programmedspecific criteria and error bounds). When this occurs,regeneration would be initiated based on estimating sootmass load from stored emission data. Examples of thevirtual sensor DPF soot mass load estimation are givenin a later section.TESTING OF SINGLE DPFReference tests were conducted without a DOC installedupstream of the DPFs to assess the intrinsic sootoxidation activity of the catalyzed DPF in absence ofNO 2 . These tests were employed with DPFs A, B and Cat 2000 rpm and 5.6 bar, with an exhaust mass flow rateof 34 g/s, exhaust gas temperature of 370 C, NO x /sootratio of 18, low NO 2 content (10-15 ppm). As a DOC wasnot used upstream of the filter, this engine point, whichis representative of the extra-urban part of the NEDC,provides an exhaust gas temperature favorable for sootoxidation.10The DPFs were subsequently tested in series with aDOC (5.66 in x 4 in, 400 cpsi), in the exhaust setupshown in Figure 20, in order to study the influence ofNO 2 in conjunction with the catalytic coating on sootoxidation.DPFDOCFrom engineFigure 20. Schematic of exhaust setup for DOC-DPF in series tests..Initial tests employed DPFs A-D and an engineoperating point where direct NO 2 -soot oxidation wasvery low (due to a NO x /soot ratio of 4) in order to test theability of the catalytic coatings to deliver NO 2 to the sootcollected inside the filter. The results of the DPFloadings at 2400 rpm and 6 bar are shown in Figure 21

while the soot mass load obtained from the virtualsensor is shown in Figure 22.Pressure drop (mbar)140120100806040200DPF BDPF CDPF ADPF D0 1 2 3 4 5 6 7 8Challenge mass load (g/m 2 )Figure 21. Pressure drop of DPFs A, B, C and D as function ofchallenge soot mass at steady state. DPF inlet temperature: 370 C,exhaust mass flow rate: 50 g/s, NO x /soot ratio: 4.Mass load (g/m 2 )876543210DPF ADPF CDPF D0 1 2 3 4 5 6 7 8Challenge mass load (g/m 2 )continuous regeneration state if they were exposed tothe exhaust gas for sufficient long times, as a simple firstorder reaction-deposition dynamic model would indicate[43]. These results demonstrate that the soot oxidationcatalyst content as well as the noble metal presenceaffect DPF soot accumulation rate and need to beoptimized taking into account trade-offs betweenpressure drop, activity and cost.The same DPF samples were also tested under astepped temperature increase procedure, to study theircatalytic activity at higher temperatures. To this endincreasing quantities of engine fuel were injectedupstream of the DOC in order to increase the exhaustgas temperature at the entrance of the DPFs, in steps ofabout 50 C. A representative result for DPF B is shownin Figure 23 in terms of DPF pressure drop and in Figure24 in terms of soot mass load estimated by the virtualsensor.Pressure drop (mbar),Injection profile605040302010Pressure dropInjection profile01000 1000 2000 3000 4000 5000 6000 7000 8000Time (s)T in DPFT out DPFFigure 23. Pressure drop and injection profile at stepped temperaturesoot oxidation for DPF B.700600500400300200Temperature (C)Figure 22. Soot mass load of DPFs A, C and D at steady state. DPF Bsoot mass load is estimated to be close to zero. DPF inlet temperature:370 C, exhaust mass flow rate: 50 g/s, NO x /soot ratio: 4.As seen in Figure 22 the presence of a DOC (henceNO 2 in the DPF inlet) causes an acceleration of sootoxidation with respect to the uncoated filter. Interestinglyenough DPF B despite its zero noble metal contentexhibits continuous regeneration behavior very quicklyand this is maintained up to a challenge mass load of8 g/m 2 . This is attributed to the higher catalyst load(2.7x) it had with respect to the other filters. The thickporous coating creates an effective filter medium withsoot and catalyst in close proximity and at the same timeprovides the extra residence time for increased NO 2turnover. DPFs C and D exhibit similar loading behaviorwith a progressively smaller rate than the uncatalyzedDPF A.Mass load (g/m 2 )5700T in DPF4.54600T out DPF3.550032.540021.530012000.501004.4 5.4 6.4 7.4 8.4Challenge mass load (g/m 2 )Figure 24. Mass load calculated by the virtual soot sensor as a functionof challenge mass load at stepped temperature soot oxidation forDPF B.Temperature (C)The soot loading curves shown in Figure 21 indicate thatthe catalyzed DPFs B, C and D would reach a11

The comparative assessment of the DPFs A-D in termsof the soot mass oxidation rate, and the practicallyimportant rate of pressure drop change is shown inFigure 25. It is evident that all DPFs exhibit significantand similar high temperature catalytic activity withrespect to the uncatalyzed (DPF A).1/mo (dm/dt) (s -1 )5.0E-044.0E-043.0E-042.0E-041.0E-04DPF BDPF CDPF DDPF AMass load (g/m 2 )2.01.81.61.41.21.00.80.60.40.20.0City cyclesMass loadEU cycleT in filterT out filterCity cyclesEnginespeed NO 23770 4270 4770 5270 5770Time (s)EU cycleFigure 26. Mass load variation under transient conditions duringsuccessive NEDCs.7006005004003002001000Temperature (C), NO 2 (ppm),Speed (1:10 rpm)dP/dt (mbar/s)0.0E+00300 350 400 450 500 550 600 650 700Temperature (C)0.010.00-0.01-0.02-0.03DPF DDPF CDPF BDPF A-0.04300 350 400 450 500 550 600 650 700Temperature (C)Figure 25. Comparison of DPFs A-D over stepped temperature sootoxidation. (top): mass oxidation rate, (bottom): Rate of pressure dropchange.Based on these results DPF C was chosen forsubsequent study as an optimum compromise betweencatalyst load (affecting directly the cost and base systempressure drop), direct and indirect reactivity with NO 2and a good catalytic reactivity at higher temperature withsoot.To accelerate the loading process DPF C was initiallyloaded without a DOC placed upstream up to soot massloads of 8–10 g/m 2 . Then the DOC was installedupstream of the DPF C and the system was subjected toexhaust-port injection schedules. By controlling theexhaust-port fuel injection rate and duration it waspossible to reach DPF temperatures in excess of 650-700 C and thus apply varying amounts of externalenergy to the DPF at different points of the transientcycle. Examples of such tests are shown in Figure 27and Figure 28.Figure 27 depicts two successive fuel injections (eachone with a duration of 2 min) performed at different sootmass load levels in the DPF leading to partialregenerations. Figure 28 depicts a completeregeneration achieved by extending the duration of fuelinjection to 33 min. Prolonged operation of the systemwould result in an increase of the “clean” DPF mass loaddue to the ash accumulation. The virtual sensor isprogrammed to take this into account by containinginformation on the flow resistance of ash loaded filtermaterials [43].TRANSIENT TESTING OF OPTIMIZED SYSTEMThe DOC-DPF C system was tested over a number ofNew European Driving Cycles in order to evaluate itsbehavior under transient conditions. With reference toFigure 26 it is evident that the DPF exhibits goodreactivity under NEDC conditions since the soot massdeposited in the DPF is oxidized when the exhausttemperature exceeds about 300 C over the Extra Urban(EU) part of the cycle. The result is a very low rate of netsoot mass loading of the system, which makes studiesof soot accumulation extremely time consuming.12

Mass load (g/m 2 )109876Injection durationMass loadTdownstream DOCTout DPF800700600500400300Temperature (C)load in the DPF by injecting a quantity of fuel upstreamof the DOC at appropriate instants. Since an exponentialtrade-off between soot mass load in the DPF and fuelinjection duration exists as shown in Figure 29, it isevident that maintaining a target (constant) soot load inthe DPF, permits a priori optimization of the DPF sizeand cell density [45], as well as significant fuel penaltysavings.Mass load (g/m 2 )541009400 10400 11400 12400 134008765Injection durationMass loadTdownstream DOCTout DPFTime (s)200410020400 21400 22400 23400 24400Time (s)Figure 27. Controlled regeneration over the NEDC by exhaust-port fuelinjection initiated at different soot loadings of the DPF. Top: at 8 g/m 2 .Bottom: at 6 g/m 2 .Mass load (g/m 2 )80070060050040030020010Injection duration9009Mass load800Tdownstream DOC8Tout DPF700766005500440033002120001004400 5400 6400 7400 8400 9400Time (s)Figure 28. Complete regeneration over the NEDC by exhaust-port fuelinjection initiated at a soot loading of 9 g/m 2 .Using such tests in conjunction with a state-of-the-artreal-time DPF simulator [16, 19] it is possible to calibrateand fully integrate with the soot virtual sensor module,model-based regeneration strategies (see [44]) for anearly description of such strategies). Our currentimplementation relies on maintaining a target soot massTemperature (C)Temperature (C)As an example we estimate based on our data with DPFC, that over the EUDC cycle it is possible to maintain atarget soot load of 5 g/m 2 with a fuel penalty of about0.23% (a model-based controlled regeneration to bringthe soot load whenever it reaches 6 g/m 2 down to 4g/m 2 ). For comparison a regeneration strategy thatwould aim for a periodically completely cleaned DPFwhenever the soot load would reach 10g/m 2 , wouldresult in a fuel penalty of 0.52%.Even though state-of-the art actively managed DPFsystems [4] employing various aspects of the earlyvirtual sensor models [41, 42], achieve impressivelyminimum amounts of fuel penalty (about 0.7%), it isclear that significant gains could be made by bringingnovel and computationally fast simulation algorithms(also known as virtual sensors) into the developments offuture ECUs.Normalized soot mass load10.90.80.70.60.50.40.30.20.100 5 10 15 20 25 30 35Fuel injection duration (min)Figure 29. Normalized soot mass load in the DPF vs. fuel injectionduration.CONCLUSIONIn the last 9 years we have researched extensivelycatalytic DPFs following a systematic bottom-upapproach from powder synthesis and deposition to diskand small monolith samples up to full-scale DPFs withtraditional (wet chemistry) as well novel (aerosol based)techniques, in an effort to learn and understand thegoverning factors and parameters that would lead toimproved designs.DPF microstructure plays a major role in determining theactivity of any soot oxidation catalyst under realistic13

conditions, as it affects directly the extent andeffectiveness of soot-catalyst proximity as well as theextent of NO 2 turnover.Catalytic DPF development is and will likely remain asensitive and highly competitive area. This will beenabled by exploiting the capabilities offered bychemistry (i.e. new catalyst formulations withmultifunctional activity) and geometry (i.e. openstructured filter media) in combination with optimizeddeposition of catalysts and filtration of soot particles topromote soot-catalyst contact as well as increased NO 2turnover.Even with an optimized catalytic filter system however, itis necessary to apply intelligent management of the DPFover its entire regime of operation (loading andregeneration) to minimize the fuel penalty of theemission control system. For this task the role of onboardvirtual sensors/model-based control strategies isvery important. These will become indispensable for thecombined management of future DOC-DPF-NO x trapsystems (i.e. 4-way converters) where each componenthas its own thermal and exhaust compositionrequirements, as well as for the secure regeneration oflow thermal mass DPFs.ACKNOWLEDGMENTSThis work has been supported in part by the EuropeanCommission through the projects IMITEC, COMET aswell as the DEXA cluster projects (ART-DEXA, SYLOC-DEXA, PSICO-DEXA, STYFF-DEXA). We thankMs. I. Papageorgiou, Mr. I. Dolios, Mr. L. Chasapidis andMr. T. Akritidis for their assistance in performing theexperiments presented in the present paper. We arealso thankful to all of our colleagues at the APTLaboratory for their support in carrying out the workpresented. We also thank Mr. Per Stobbe (Stobbe,Denmark) for freely providing porous material samplesfor our research since 1996.REFERENCES1. Ogyu K., Ohno K., Hong S., Komori T., “AshStorage Capacity Enhancement of DieselParticulate Filter”, SAE Tech. Paper No. 2004-01-0949, 2004.2. Young D. M., Hickman D. L., Bhatia G.,Gunasekaran N., “Ash Storage Concept for DieselParticulate Filters”, SAE Tech. Paper No. 2004-01-0948, 2004.3. Bardon S., Bouteiller B., Bonnail N., Girot P., GleizeV., Oxarango L., Higelin P., Michelin J., SchuerholzS., Terres F., “Asymmetrical Channels to IncreaseDPF Lifetime”, SAE Tech. Paper No. 2004-01-0950,2004.4. Boretto G., Imarisio R., Rellecati P., Barucchi E.,Sanguedolce A., “Serial Application of a Catalyzed14Particulate Filter on Common Rail DI DieselEngines For Passenger Cars”, F2004V068, FISITAWorld Automotive Congress, Barcelona, Spain,May 23-27, 2004, 2004.5. Konstandopoulos A. G., Zarvalis D., PapaioannouE., Vlachos N. D., Boretto G., Pidria M. F., FaraldiP., Piacenza O., Prenninger P., Cartus T., SchreierH., Brandstatter W., Wassermayr C., Lepperhof G.,Scholz V., Luers B., Schnitzler J., Claussen M.,Wollmann A., Maly M., Tsotridis G., Vaglieco B. M.,Merola S. S., Webster D., Bergeal D., GorsmannC., Obernosterer H., Fino D., Russo N., Saracco G.,Specchia V., Moral N., D'Anna A., D'Alessio A.,Zahoransky R., Laile E., Schmidt S., Ranalli M.,"The Diesel Exhaust Aftertreatment (DEXA) Cluster:A Systematic Approach to Diesel ParticulateEmission Control in Europe", SAE Tech. Paper No.2004-01-0694 (SP-1861), 2004.6. Swartzmiller S., Li C., Wallin S., Ziebarth R. andMao F., “Properties and Performance of DieselParticulate Filters of an Advanced CeramicMaterial,” SAE Tech. Paper No. 2004-01-0955 (SP-1860), 2004.7. Kasai Y., Kuki T., Miwa S., Senda K., Ogura Y.,“New Cordierite Diesel Particulate Filter Material forthe Diesel Particulate-NO x Reduction System,” SAETech. Paper No. 2004-01-0953 (SP-1860), 2004.8. Mizutani T., Watanabe Y., Ito M., Hashimoto S.,Hamanaka T, “Soot Regeneration Model for SiC-DPF System Design,” SAE Tech. Paper No. 2004-01-0159 (SP-1861), 2004.9. Winkler C., Flörchinger P., Patil M., Gieshoff J.,Spurk P., Pfeifer M., “Modeling of SCR DeNO xCatalyst - Looking at the Impact of SubstrateAttributes,” SAE Tech. Paper No. 2003-01-0845,2003.10. Ohno K., Yamayose K., Sato H., and Ogyu K.,“Further Durability Enhancement of Re-crystallizedSiC-DPF,” SAE Tech. Paper No. 2004-01-0954,2004.11. Konstandopoulos A. G., Kostoglou M., "Theory andPractice of Soot Filter Operation", Proceedings ofGlobal Powertrain Congress 99 (invited paper),Powertrain Int., Stuttgart, October 5-7, 1999.12. Konstandopoulos A. G. and Kostoglou M.,“Periodically Reversed Flow Regeneration of DieselParticulate Traps”, SAE Tech. Paper No. 1999-01-0469, 1999.13. Konstandopoulos A. G., Kostoglou M., SkaperdasE., Papaioannou E., Zarvalis D., and KladopoulouE., “Fundamental Studies of Diesel ParticulateFilters: Transient Loading, Regeneration andAging”, SAE Tech. Paper No. 2000-01-1016 (SP-1497), 2000.

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40. Edgar B., “Emission Control Systems andComponents for Retrofit, and First-fit Applications,”10 th Annual Diesel Emissions Reduction (DEER)Conference, Coronado, California, August 29–September 2, 2004.41. Skaperdas E., Konstandopoulos A. G., "Predictionof Diesel Particulate Filter Loading Behavior forSystem Control Applications", CDAuto01A3018, CDAUTO 01, 3rd International Conference on Controland Diagnostics in Automotive Applications, Sestri-Levante, Italy, 4-6 July, 2001.42. Konstandopoulos A. G., Kladopoulou E., "A VirtualSensor for On-Board Diagnostics and Control ofDiesel Particulate Filters", CDAuto03A2028, CDAUTO 03, 4th International Conference on Controland Diagnostics in Automotive Applications, Sestri-Levante, Italy, 18-20 June, 2003.43. Konstandopoulos A. G., Vlachos N., StavropoulosI., Skopa S., Schumacher U., Woiki D., “Study of aSintered Metal Diesel Particulate Trap”, SAE Tech.Paper Offer No. 05FL-106, (submitted), 2005.44. Kladopoulou E., Yang S. L., Johnson J. H., ParkerG. G., Konstandopoulos A. G., "A Study Describingthe Performance of Diesel Particulate Filters DuringLoading and Regeneration - A Lumped ParameterModel for Control Applications", SAE Tech. PaperNo. 2003-01-0842 (SP-1755), 2003.45. Konstandopoulos A. G., Kladopoulou E., "TheOptimum Cell Density for Wall-Flow MonolithicFilters: Effects of Filter Permeability, Soot CakeStructure and Ash Loading", SAE Tech. Paper No.2004-01-1133 (SP-1861), 2004.DEFINITIONS, ACRONYMS, ABBREVIATIONSASP: Aerosol Spray PyrolysisCDPF: Catalyzed Diesel Particulate FilterDOC: Diesel Oxidation CatalystDPF: Diesel Particulate FilterECU: Engine Control UnitEGR: Exhaust Gas RecirculationEU: European CycleNEDC: New European Driving CyclePS: Plasma SprayVD: Vapor DepositionTCD: Thermal Conductivity DetectorTGA: Thermo-Gravimetric AnalysisTPR: Temperature Programmed ReactionTPD: Temperature Programmed DesorptionWI: Wet ImpregnationXRD: X-Ray DiffractionCONTACTDr. Athanasios G. Konstandopoulos, Head of Aerosol &Particle Technology Laboratory, CERTH/CPERI, P.O.Box 361, Thermi, Thessaloniki 57001, Greece.e-mail: agk@cperi.certh.gr, web: http://apt.cperi.certh.gr16