P1254413601CKYbR - PowerPoint PPT Presentation

Title: P1254413601CKYbR


1
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, USA b) Department of Chemical Engineering,
Purdue University, West Lafayette, IN , USA c)
University of Utah, Department of Chemical
Fuels Engineering, Salt Lake City, UT , USA
CFFS

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 lt 2.5
micron. The CFFS at University of Kentucky, in
collaboration with many other U.S. research
groups, operates a program to study PMlt2.5 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.
Objective
Transmission Electron Microscopy
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
L2?/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.

• Direct correlation between mortality rate and PM
  concentration 1
• New EPA regulations on soot with particles lt 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

Most morphological and structural studies on soot
are based on electron microscopy work.
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.
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.
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 q0.01 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.
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.
X-ray Diffraction
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.
Quantitative analysis of XRD diffractograms
allows determination of the crystallite sizes.
Approach
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 qlt0.05 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.
Synchrotrons X-ray sources with high photon
flux, allow for

• molecular speciation via X-ray absorption
  spectroscopy
• fast structural characterization (diffraction,
  scattering)
• state-of-the-art spectro-microscopical analysis
  (STXM)

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.
Support results with classical techniques (XRD,
TGA, SEM, TEM)
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.
Diesel Soot Sample Generation and Periphery
Conclusion Outlook These 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.
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.
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
108231-234. 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.  
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.
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. ).