Investigations of Diesel Fine Particulate Matter with Small Angle X-ray Scattering

A. Braun a) , F. E. Huggins a) , J. Ilavsky b) , N. Shah a) , K. Kelly c), A. F. Sarofim c), G. P. Huffman a)

a) University of Kentucky, Consortium for Fossil Fuel Science, Lexington, KY, USAb) Department of Chemical Engineering, Purdue University, West Lafayette, IN , USA

c) University of Utah, Department of Chemical & Fuels Engineering, Salt Lake City, UT , USA

Airborne particulate matter (PM) is of major concern because of its adverse health impacts and its role on global climate changes. Since very fine PM was found to have adverse impact on human health, EPA has issued new standards for PM < 2.5 micron. The CFFS at University of Kentucky, in collaboration with many other U.S. research groups, operates a program to study PM<2.5 and employs a number of analytical tools to characterize PM in terms of molecular structure and morphology in order to identify 1) unique analytical source signatures for PM2.5 derived from the major fossil energy sources, coal and petroleum and 2) structural features that may be

important for human health considerations and 3) achieve, through laboratory experiments and modeling, a basic understanding of the formation mechanisms of the critical PM2.5 structures identified. This poster outlines how USAXS is applied to derive size information about the particulates.

Financial support by the National Science Foundation, grant # CHE-0089133. USAXS was performed at the UNICAT facility at the Advanced Photon Source (APS), which is supported by the Univ. of Illinois at Urbana-Champaign, Materials Research Laboratory (U.S. DOE, the State of Illinois-IBHE-HECA, and the NSF), the Oak Ridge National Laboratory (U.S. DOE under contract with UT-Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce) and UOP LLC. The APS is supported by the U.S. DOE, Basic Energy Sciences, Office of Science under contract No. W-31-109-ENG-38. ).

Objective

Left: Diesel exhaust from heavy duty truck. Right: Correlation between mortality (shown is the survivor rate, blue) and PM concentration (red). With increasing PM concentration, less people survive exposure to an air polluted city. Data taken from the 6 cities study [1].

CFFS

• Direct correlation between mortality rate and PM concentration [1]

• New EPA regulations on soot with particles < 2.5 micron

• Carbon is one dominant constituent of PM

• Need for advanced analytical techniques, source attribution,

environmental forensics: which source causes which PM ?

• Synchrotron radiation: highly intense and brilliant X-ray source

• molecular speciation via X-ray absorption spectroscopy

• fast structural characterization (diffraction, scattering)

• state-of-the-art spectro-microscopical analysis (STXM)

Synchrotrons: X-ray sources with high photon flux, allow for

References[1] D.W. Dockery et.al., New Engl. J. of Medicine (1993):329/24 1753-1759. Diesel Exhaust in the United States, U.S. EPA Publication EPA 420-F-02-048 September 2002.[2] T. Ishiguro, Y. Takatori, K. Akihama, Microstructure of Diesel Soot Particles Probed by Electron Microscopy: First Observation of Inner Core and Outer Shell, Combustion and Flame 1997; 108:231-234.[3] A. Braun, F. E. Huggins, S. Seifert, J. Ilavsky, N. Shah, K. Kelly, A. Sarofim, and G. P. Huffman Size-range analysis of diesel soot with ultra-small angle X-ray scattering, submitted to Combustion and Flame.[4] A. Braun, N. Shah, F. E. Huggins, G. P. Huffman, S. Wirick, C. Jacobsen, K. Kelly, A. F. Sarofim,A study of Diesel PM with X-ray Microspectroscopy, under review at Fuel.[5] A. Braun, N. Shah, F.E. Huggins, S. Seifert, J. Ilavsky, K. Kelly, A. Sarofim, C. Jacobsen, S. Wirick, H. Francis, G.E. Thomas, G.P. Huffman,Assessment of X-ray small-angle scattering, diffraction and spectroscopy as analytical techniques for diesel soot studies, to be submitted to Environmental Science and Technology.

 

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Approach

Kubota2-cylinderZ482B

Land&SeaWater break Dynamometer+ Controller

GC/MS

Gas-phase AnalyzerCO, NO2, NOX, O2

High-volumesampler

Off-line analysis:NMR, TEM, CCSEM,TD-GC/MS, XRF,XAS, XRD,SAXS, isotope analysis

SMPS&OPC PA&PAS

Critical orifice

Diluted exhaust

Compressed, dry, particle-free air

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Scattering vector q [10/nm]

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I = const. N V2 exp (-R2q2/3)

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(110) (112)

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diesel, idle

diesel, load

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Diesel Soot Sample Generation and Periphery

Transmission Electron Microscopy

X-ray Diffraction

Sample 

Lc (002) [Å]

 

La (110) [Å]

 

La (112) [Å]

 

Diesel, idle 11.10 8.67 17.24

Diesel, load 11.78 10.48 16.68

Mix B, idle 10.18 6.96 13.24

Mix B, load 12.86 8.64 14.92

Mix A, idle 8.64 8.93 6.24

Mix A, load 10.78 16.30 11.19

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q [1/A]

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Left-Right: TEM images of fractal diesel PM aggregate of 500 nm size, built up from primary particles. Next: Primary particle of about 40 nm size, with inclusions of onion-like structures of up to 20 nm. Particles are built up from graphene sheets. Particles show complex substructures which are difficult to quantify in structural parameters.

Support results with classical techniques (XRD, TGA, SEM, TEM)

Left: X-ray diffractograms from load/idle soot samples, and reference Bragg peaks of graphite (2H Graphite PDF 26-1079). Center: Comparison of load/idle soot XRD from Diesel and oxygenated Diesel Mix A, Mix B. Right: Table shows crystallite sized as obtained with Scherrer formula.

Quantitative analysis of XRD diffractograms allows determination of the crystallite sizes.

Idle soot particles have smaller crystallites than load soot. Adding oxygenates to the fuel causes bigger differences in the structure between idle and load soot, which is in line with observations from the NEXAFS and TGA.

Left: Closer inspection of log-log plot of small angle scattering curves reveal presence of at least 5 size ranges in Diesel PM, with size L=2/q. See 5 fit curves to a Guinier function (a-e) in scattering plot. Curve with open symbols was obtained after subtraction of Porod- and constant background scattering. Results are in good agreement with quantitative TEM observations [2]. Right: Maxima in Kratky plots of scattering curves (from pellets) provide information about compactness of soot particles and size of agglomerates: L=/q. Sizes are summarized in Table below.

Elementary particles have sizes in 1-2 nm range. They form compact cluster to built subunits of 15-20 nm size. These build up larger structures, the primary particles, of 40-80 nm size, which build the aggregates. Aggregates are found at q-values of 0.001 1/A (fit “a”), though harder to resolve in the scattering curves. Idle soot has generally larger particles than load soot. Exponent of decay allows determination of fractal dimension, and was close to –4 for high q range and thus indicates smooth surfaces of primary particles and sub-units. For low q, exponents of decay are close to –3 for pellets and powders. Acetone immersed soot showed –2.

Samples: Diesel PM from 50/50 Chevron/Phillips reference fuels T22/U15, oxygenated with DEC

and ethanol, operated under idle/load. Oxygenated fuel is called “Mix A” and “Mix B”.

Soot Elementaryunits D[nm]

Sub units D [nm]

Primary Particles D [nm]

high q exponent

Fractal dimension

low q exponent

Fractal dimension

Diesel, idle 1.5 17.4 49.16 3.99 2.01 3.28 2.72

Diesel, load 1.6 14.5 41.50 3.86 2.14 3.12 2.88

Mix A, idle 1.9 21.1 (14.2 78.29 3.97 2.03 3.02 2.98

Mix A, load 1.4 13.8 (12) 36.78 3.96 2.04 3.09 2.91

Mix B, idle 2.0 14.3 (14.5 83.85 3.92 2.08 2.96 2.96

Mix B, load 1.4 22.0 (18.6 48.73 3.98 2.02 2.75 2.75

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Soot particles are aggregates, built up from primary particles which have a complex substructure on nano-sized scale. Agglomerates cannot be destroyed easily into primary particles, as ultra-sonication studies with acetone show. Increasing intensity towards lower q for q<0.05 shows the presence of larger structures. At this q-position, the pellets show intensity plateaus, i.e. absence of larger structures, while powder soot and immersed soot show steep increase of intensity, thus presence of larger structures.

Small Angle X-ray Scattering

Soot powder was pressed to pellets or immersed in acetone and then object to SAXS. The SAXS curve of the unaltered soot powder is the reference curve (green). Large scattering vectors q show an exponent of decay of –4, confirming smooth electron density transition at the primary particle surface. The hump near q=0.01 is indicative to the primary particles with diameters from 40 to 80 nm. The significance of the hump differs depending on whether the sample is pressed as pellet (blue) or kept in solution/acetone (red). Also, the exponent of decay for large q differs. Pellets show most pronounced primary particle hump because fractal aggregation structure is suppressed because of pellet pressure – arms of aggregates get shortened and do not significantly scatter at this length scale anymore.

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Kratky plot of scattering curves (below, Iq2 vs q or Iq3 vs q) allows for better discrimination of the maximum of the hump and thus determination of the particle size. Compact objects show a pronounced intensity maximum in the Kratky plot of Iq2 vs q (powder and pellet). The acetone immersed soot samples show not such a maximum, but a constant intensity plateau for small q. They are plotted therefore on Iq3 vs q (right axis) scale, which shows an intensity maximum at the same q position as for the pellet samples. Powder samples have the maximum at smaller q, thus representing larger structures.

Left: Log-log plot of scattering curves from pellets, powder, and acetone immersed idle soot from reference diesel. Right: Scattering curves of load soot for same sample condition (pellet, powder, acetone).Pellets, powder and immersed samples show differences in the scattering curves.

Left: Schematic sketch of a soot aggregate in powder or solution with dangling branches and primary particles. Branches scatter as elongated objects and overshadow scattering from primary particles. Right: Compressed soot agglomerate, without any fractal superstructure. Primary particle scattering would dominate the scattering curve.

Most morphological and structural studies on soot are based on electron microscopy work.

Conclusion & OutlookThese soot samples have been studied with a variety of other techniques, including thermogravimetric analysis, nuclear magnetic resonance and X-ray absorption spectromicroscopy [3,4,5]. USAXS provides an excellent tool for the structural characterization of particulate matter, such as these soot samples. The preliminary data shown here are promising with respect to deeper analysis and modeling. Additional experiments and analyses are going on in our collaboration.