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www.usra.edu
Burning Modes and Oxidation Rates of Soot:Relevance to Diesel Particulate Traps
Randy L. Vander Wal(*)(1), Aleksey Yezerets(2), Krishna Kamasamudram(2), Neal W. Currier(2), Do Heui Kim(3),Chong Min Wang(3)
(1) - USRA at the NASA-Glenn Research Center Cleveland, OH44135, USA
(2) - Cummins Inc., Columbus, IN, 47201, USA (3) - Pacific Northwest National Laboratory, Richland, WA, USA
DEER 2007 Conference, Detroit, MI Aug. 13th - 16th
Acknowledgements:Support through contract IND61957 with Cummins Inc.Soot samples supplied by Cummins Inc. and Cabot Corp.
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Outline
I. Current Oxidation Status
II. Alternative Burning Modes via HRTEM
III. Nanostructure Quantification via Image Analyses
IV. Current Efforts
V. Conclusions
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ic
What is Diesel Particulate Matter ?
• Composition:–“Dry carbon”
• turbostratic graphite • initial evidence of fulleren
structures in some cases –Adsorbed HCs–Inorganic materials
• Lube oil ash, H2SO4, HNO3, H2O
• Nanostructure - Amorphous, fullerenic, graphitic
• Morphology: –Primary particles: ~20-40 nm –Agglomerates: 0.1-1 micron
DieselNet
Dieselnet.com
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Reactivity evolution over a life cycle of a soot particle
Printex
A: High reactivity due toambient agingB: Steady-state oxidationC: Steep increase
Diesel Soot
A: High reactivity due to: – Adsorbed HC; ambient aging
0.00E+00
2.00E-04
4.00E-04
6.00E-04
8.00E-04
1.00E-03
1.20E-03
0 20 40 60 80 100
soot oxidized, %
Norm
aliz
ed r
ate
[m
in-1
] Printex,380°C,10%O2
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.010
0 10 20 30 40 50 60 70 80 90 100
soot consumed, %
No
rmal
ized
rat
e, [m
in -1
] Diesel soot 450°C,10%O2
A B
C
B
C A
B: “Steady-state” oxidation C: Increased reactivity at later stages of oxidation
r = A · exp(-Ea/RT) · [C]a · [O2] b · [H2O]c
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Progressive oxidation
Reactivity is increasing with degree of oxidation:
-9
-8
-7
-6
-5
-4
-3
-2
1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55
1000/T (K-1)
Ln
(K,
min
-1)
8.4-19.4%19.4-28.7%28.7-34.6%34.6-42.9%43.3-49.6%49.6-56.5%67.4-72.8%72.8-78.1%88.2-92.2%Linear (8.4-19.4%)Linear (19.4-28.7%)Linear (43.3-49.6%)Linear (28.7-34.6%)Linear (34.6-42.9%)Linear (49.6-56.5%)Linear (67.4-72.8%)Linear (72.8-78.1%)Linear (88.2-92.2%)
IncreasingOxidation
– No measurable changes of Ea, or reaction order in O2* Reaction chemistry appears to be independent of the degree of
carbon oxidation.* Number (density) of reactive sites (A) appears to be near constant!
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Specific Surface Area*
200
300
400
500
600
700
800
0 20 40 60 80Degree of carbon oxidation, %
Sp
ecific
S.A
. m
2/g
Printex
Soot A
Soot B
A. Yezerets, et al. Applied catalysis B: 61 (2005), 134-143.
–BET surface area measured in-situ at different stages of oxidation–samples pre-treated by thermal desorption
* Development of the reactivity does not appear to correlate directly with thespecific surface area
* Need a different parameter which would correlate with the number of active sites *Courtesy: Dr. Do Heui Kim, (PNNL)
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Puzzles (thus far):
* Comparative changes in reactivity
* Comparative evolution of surface areas
Advantages of electron microscopy (HRTEM)
* Direct observation without property assumptions
* Potential to reveal changes in nanostructure (during oxidation)
* Correlate oxidation characteristics (rate) with nanostructure
………What changes in nanostructure?
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Nanostructure and Implications: Reactivity
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Acetylene Derived Soot
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Benzene Derived Soot
0
3
6
9
12
15
18
21
0 0.93 1.9 2.8 3.7 4.7 5.6 6.5
% o
f F
rin
ge
s
Fringe Length (nm)
Ethanol Derived Soot
Amorphous Fullerenic Graphitic
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(g/c
m-s
) x 1
0
Soot Burnout Rates
0
0.5
1
1.5
2
2.5 Bu
rnou
t Rat
e 2
-5
Ethanol Benzene AcetyleneNSC
w
0 5 10 15 20 25 30 35
Percent (%) Excess Oxygen
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ENG-A - Original from Trap
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ENG-A - Post partial Oxidation (TGA, 50%)
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-15 degree tilt
No tilt ENG-A 75% Burnout
Hollow particlesviewed thoughSuccessive tilts
+15 degree tilt
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Eng-A: Oxidized at 450 °C
Eng-A: As-received
“Solid particles” “Hollow particles”
PNNL
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Printex - as supplied
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Printex - Post Partial Oxidation50% Burnout, via TGA
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Summary of Results
Sample
designation
Ash contents,
wt %
SOF contents,
wt%
Observations
ENG(A) 6.5 ± 0.5% 9 ± 1% No shells/capsules
Printex-U™ <0.5% 4 ± 1% No shells
Does Internal burning correlate with either ash or soluble organicfraction (SOF) content??
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A Carbon Black
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Statistical Properties Extracted FromHRTEM Images (of soot nanostructure)
A T = L/A
* Other inputs –Maximum join distance –Minimum fringe length
PositionPosition
LengthLength
FringeFringeSeparationSeparation
OrientationOrientation
TortuosityTortuosity
FringeFringeDensityDensity
Fringe Length (nm)
Tortuosity
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Interpretation(s)
A. Densification - a pseudonym for graphitization
1. Thermally induced densificationCreation of radicals by thermal evolution of volatiles or loss of H-atoms permits lamella growth
2. Oxidation Creation of radical sites by oxidative removal of amorphouscarbon or loss of H-atoms
B. Internal Burning - disordered carbon and/or trapped volatiles preferentially burnout
Trap conditions could promote both densification and/or volatileevolution.
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Phenols
XPS-Characterization of Carbon Nanostructure
C-C sp2
CarbonylsCarboxylic
C-C sp3
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Conclusions * Burning mode dependent upon nanostructure and oxidation conditions
(ash and SOF are not unique predictors)* Diesel soot and Printex U exhibit nearly identical activation energies
and burning rates and even similar active site numbers BUT vastly different surface area evolution! * Measures other than surface area are needed for modeling
burning mode and rate.(A key feature will link the distribution of active sites to the
nanostructure)* Convolved with initial nanostructure are the change(s) enabled by
oxidation.
Implications:* Latter stage burnout will strongly depend upon burnout mode* Soot burning mode(s) could affect regeneration efficiency and models.* DPF regeneration costs fuel and each cycle limits lifetime.
Costs money!