metal particle emissions in the exhaust stream of diesel engines: an electron microscope study

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  • Metal Particle Emissions in the Exhaust Stream of Diesel Engines: AnElectron Microscope StudyAnthi Liati,*, Daniel Schreiber, Panayotis Dimopoulos Eggenschwiler, and Yadira Arroyo Rojas Dasilva

    Empa Material Science and Technology, Laboratory of Internal Combustion Engines, Ueberlandstrasse 129, CH-8600, Dubendorf,SwitzerlandEmpa, Electron Microscopy Center, Ueberlandstrasse 129, CH-8600, Dubendorf, Switzerland

    ABSTRACT: Scanning electron microscopy and transmission electron micros-copy were applied to investigate the morphology, mode of occurrence andchemical composition of metal particles (diesel ash) in the exhaust stream of asmall truck outfitted with a typical after-treatment system (a diesel oxidationcatalyst (DOC) and a downstream diesel particulate filter (DPF)). Ash consists ofCa-Zn-P-Mg-S-Na-Al-K-phases (lube-oil related), Fe, Cr, Ni, Sn, Pb, Sn (enginewear), and Pd (DOC coating). Soot agglomerates of variable sizes (15 m,exceptionally 13 m), rarely

  • carried out with methods using particle counters (scanningmobility particle sizers (SMPS)) or electrical low-pressureimpactors (ELPI).10,21 Such techniques had a significantcontribution to abatement measures against diesel PMpollution as well as to testing the efficiency of DPFs. Thesedevices are able to detect and measure agglomerates in theexhaust stream with rigorous statistics. However, they are notable to discriminate between soot and ash agglomerates andcannot distinguish any individual constituents of the agglom-erates. A recently developed device, the soot particle aerosolmass spectrometer (SP-AMS), can provide information on theelemental composition of engine-out emissions (Ca, Zn, Mg,S).22

    Within the framework of the present study, we applyscanning electron microscope (SEM) and transmission electronmicroscope (TEM) techniques to investigate the morphology,mode of occurrence, chemical composition, and variations inthe relative amount of ash PM at different sites of the exhauststream of a small truck on a chassis dynamometer. Thesemicroscopy methods apply high magnifications and aim atrevealing important details of the materials down to themicrometer and nanometer scales. Consequently, the numberof examined samples cannot be very high. Studies of ash insidethe DPF are not included in the present work because this wasthe topic of previous research.14,23 The aim of the present workis to highlight the differences in the characteristics and relativeamount of metal-bearing particles (ash) in the exhaust streambefore entering into and after exiting the DPF, with possibleimpacts on metal particles that may escape in the ambient air.The study of metal particles in this paper is bound mainly toatmospheric pollution but is also expected to aid in minimizingthe detrimental effects of metal PM on exhaust after-treatmentsystems.

    2. EXPERIMENTAL SECTION2.1. Experimental Setup and Sampling Procedure.

    The vehicle used for the experiments was an Iveco Daily (2.3-L4-cylinder (F1A) common rail diesel engine with turbocharger;MY 2003; Euro IV emission limits). Sampling was performedduring steady state operation at 2000 rpm engine speed and 13kW output. Commercial fuel (S content

  • 3. The terms agglomerate and aggregate as used in thepresent paper are equivalent to the term particlereported in the majority of articles dealing withmeasurements of the size/mass distribution of sootagglomerates and ash aggregates in the exhaust stream bymeans of, for example, scanning mobility particle sizers(SMPS) or electrical low-pressure impactors (ELPI).10,21

    The term particle as used in the present paper refers tothe individual primary particle constituents of theagglomerates/aggregates. The size of agglomerates/aggregates and their primary particle constituents, asreported in this paper, refer to those physically measuredon the TEM grids.

    3. RESULTS OF SEM IMAGING3.1. Pre-DPF Samples. Pre-DPF samples collected up-

    stream and downstream of the DOC show no differences in theash nature, relative amount and morphological characteristicsand are further considered as a single group. The studiedsamples contain numerous pieces of ash. In one 80 80 mlarge square of the TEM grid, at least 56 pieces of ash 1 min size and several smaller pieces were found in addition toabundant soot agglomerates (Figure 2). The size of the ashaggregates/fragments ranges generally between 0.2 and 2 m(rarely up to 3 and 3.5 m), and their shape is irregular andusually rounded.

    The EDX analyses of several ash aggregates and/orfragments yielded the following elements: Ca, Zn, P, Mg, S,Na, Al, K (attributed to additives in the lube oil), Fe, Cr, Ni, Sn,Pb, Al (engine wear and/or corrosion of engine and after-treatment material).Soot agglomerates are abundant, mostly a few hundreds of

    nanometers in size, while very large ones (ca. 1.53 m and,more rarely, even larger) are common. The SEM-EDX spectraof the soot agglomerates yielded no ash elements implying thatash is not attached to soot at this site of the exhaust stream orthat it occurs in quantities that are not detectable within thesensitivity of the SEM-EDX system.

    3.2. Post-DPF Samples. The samples downstream of theDPF contain notably low amounts of diesel PM (Figure 2).One to two ash pieces (ca. 12 m) and a few smaller ones (afew hundreds of nm) as well as soot agglomerates usually ca.15 m in size and rarely larger were found in numerous of the80 80 m large squares of the TEM grids (Figure 2). In onecase, a 13 3 m soot agglomerate was identified (Figure 2F).Several TEM grid squares were completely free of diesel PM.High magnification images (180300 kx) of isolated ash

    aggregates reveal that they consist of several individual phases(ca. 20400 nm) with rounded outlines sintered together(Figure 3AC). In addition to separate ash aggregates, ash wasalso detected via EDX analyses as attached to soot

    Figure 2. SEM images of samples with soot and ash PM upstream (AC) and downstream of the DPF (DF). The post-DPF samples show adramatic decrease in ash and soot PM compared to the pre-DPF ones. Large and very large soot agglomerates (E, F) are common downstream of theDPF. Rectangles mark sites with ash pieces. Circles in (D) mark sites with soot.

    Environmental Science & Technology Article | Environ. Sci. Technol. 2013, 47, 144951450114497

  • agglomerates. It is reminded that soot agglomerates upstreamof the DPF were free of ash or contained very low ash amounts.Interestingly, small soot agglomerates (1m and even up to 13 m), whereas very small ones (1 m) from escaping intothe ambient air. Quantification of the ash escaping filtration isbeyond the purpose of the present study.


    4.1. Pre-DPF Samples. While SEM imaging provides adetailed analysis of topographic structures, that is, surfacefeatures, (HR)TEM can provide detailed data on the innerstructure of material. Soot agglomerates on the carbon-coatedfilm of the TEM grids are visible at a higher focal planecompared with that of the flat carbon film. For optimumimaging, soot agglomerates trapped on holes of the lacy film arechosen.The TEM imaging of pre-DPF samples allows the

    recognition of several ash aggregates and further reveals aclear distinction of individual phases within the aggregates,which are shown to consist chemically of either the same ordifferent substances. Figure 3D shows such an aggregateconsisting of distinguishable Ca-bearing (lighter gray) and Sn-bearing (darker) phases. It is noted that Figures 3DF arebright-field STEM images, which have the potential to displaydifferent contrasts for the various chemical elements accordingto their atomic number; the lighter elements exhibit brightercontrast than heavier elements (in Figure 3D, the lighter Ca-phase appears brighter than the heavier Sn-phase). Theindividual ash phases observed in the aggregates display a sizerange of 45160 nm. The most frequently found element issulfur either combined with Ca (in the form of CaSO4) or not,in agreement with previous findings.6 Additionally, Fe-, Cr-,Ni-, K-, Na-, Cl-, Sn-, Sb-, Al-, Si-, and Mg-bearing compoundswere sporadically detected.The soot agglomerates are abundantly dispersed over the

    sampling surface of the pre-DPF samples (Figure 3D, G). Inmost cases, either no ash elements or very low amounts of S,

    Figure 3. (AC): SEM images of ash aggregates downstream of the DPF. Primary ash phases with mostly round outlines can be distinguished in (B)and (C). (DI): BF-STEM images; (D, G) upstream of the DPF showing thin soot agglomerates with no or very little ash as well as a multiphaseash aggregate in (D); (E, F, H, I): downstream of the DPF showing multiphase ash aggregates (E, F) and ash-bearing thick soot agglomerates (H, I).The rectangles mark sites of EDX analyses. Elements in parentheses occur in minor amounts.

    Environmental Science & Technology Article | Environ. Sci. Technol. 2013, 47, 144951450114498

  • Cr, Mn, and Fe were detected on the soot agglomerates (Figure3G).4.2. Post-DPF Samples. The post-DPF samples exhibit

    notably low amounts of diesel PM because the highest fractionis retained by the DPF. Ash occurs as separate aggregatesconsisting of primary phases with the same or differentchemical composition. Examples of ash aggregates consistingof Ca-bearing phases as well as S, Zn, Mg, and minor amountsof Cr, Fe, Mn, and Ni are shown in Figure 3E, F. Primary ashparticles 40400 nm in size are distinguished in the aggregates.The most frequently observed ash element is sulfur. Moreover,Zn-, Mg-, Fe-, Mn-, Cr-, and Ni-bearing compounds are alsoidentified.The post-DPF samples show abundant ash attached on soot

    (Figure 3H, I). EDX analyses of several soot agglomeratesclearly reveal the presence of such ash elements as

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