AEM Analysis of Nanoparticles
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Transcript AEM Analysis of Nanoparticles
AEM Analysis
of
Nanoparticles
Charles Lyman
Lehigh University
Bethlehem, PA
PASI - Electron Microscopy - Chile
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Nanoparticles
Exhibit an enormous
surface-to-volume ratio
Courtesy C. J. Kiely
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Size Matters
Surface-to-volume
» The presence of a high proportion of surface and near
surface atoms can greatly affect structural, electronic,
and chemical properties
Reducing the dimensions of a material affects many
properties
» Melting point
» Chemical reactivity
» Optical properties
» Electrical properties
» Magnetic properties
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Melting Temperature of Nanoparticles
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Catalysis: The Oldest Nanotechnology
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Chemical Reactivity
4. Particle Composition - surface composition is most important
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Analysis of Nanoparticles in Electron Microscopes
Nanoparticles
»
»
»
»
»
Bodies of matter < 50-100 nm
May or may not be homogeneous
Must be supported to be analyzed (carbon film)
Weak contrast in TEM, stronger contrast in STEM-ADF
Very small x-ray and EELS signals
Analysis
» 1 nA electron probe current
» Particles < 10 nm analysis require field-emission STEM
» 1 million times magnfication requires high specimen stability
Nanoparticles
Nanoparticle with
core and shell
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Size of the Analysis Region
> 100 µm = 0.1 mm (bulk analysis)
> 100 nm = 0.1 µm (SEM “microanalysis”)
X-ray emission spectrometry (XES)
Electron backscatter patterns (EBSP)
Auger electron spectrometry (AES)
X-ray photoelectron spectrometry (XPS)
< 100 nm = 0.1 µm (TEM “nanoanalysis”)
X-ray emission spectrometry (XES)
Transmission electron diffraction (SAD, CBED)
Electron energy loss spectrometry (EELS)
Atom probe
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STEM Imaging of Nanoparticles
Bright-field STEM
BF
50 nm
Annular dark-field STEM
Best method for
nanoparticle
detection and
analysis
ADF
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ADF
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X-ray Collection Geometry in STEM
Stationary or scanning
electron beam covering
particle
Analyze particles
only on the side of
support shard facing
x-ray detector
Particle stability - a serious issue at 1 Mx
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Bimetallic Nanoparticles
Are these particles all the same?
Supported Metal Catalyst Microstructure
Particle size distribution
Bimetallic particle composition distribution
Surface segregation
Particle shape
Crystallography of surface facets and edges
Support effects
Physical and chemical effects of:
» Gas environment
» Metal-support interactions
» Preparation and processing variables
Catalytic Properties
Correlation of bimetallic
nanoparticle microstructure
with catalytic properties
Activity, selectivity
Stability, poisoning resistance, lifetime
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Measure Particle Size & Particle Composition
X-ray
Detector
Control console
Specimen stage area
VG HB-603 STEM Features
»
»
»
Electron
Beam
Column
300 kV FEG
Optimized for x-ray collection
1 nA in 1.5 nm (FWTM)
Now with aberration-corrector:
»
5 nA in 1.5 nm (FWTM)
– More current in electron probe to
detect smaller amounts of elements
»
50 pA in 0.2 nm (FWTM)
– Determine nanoparticle shape
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30 nm
ADF image showing
Pt-Rh nanoparticles
Analysis of 4-nm particle
56 wt% Pt, 44 wt% Rh
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Quantitative Pt-Rh Measurements
Cliff-Lorimer Method
Find IPt and IRh by subtracting
x-ray background from Pt-M
and Rh-L peaks
Spectrum from 4-nm particle
IPt
CPt
I Pt
k PtRh
1.079
CRh
IRh
IRh
CP t C R h 1
Measured k-factor = 1.079
on known Pt-Rh standard
Use two equations in two
unknowns to find CPt
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Particle composition:
56 wt% Pt
44 wt% Rh
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Bimetallic Nanoparticle Catalysts
gives
Size and Composition Distributions
Good predictor of
catalyst behavior
Pt Content (wt%)
Analytical
Transmission Electron Microscopy (AEM)
Composition-Size Diagram
Composition and size
measured for ~100
indivdual nanoparticles
Particle Diameter (nm)
Bulk
Analysis
Methods
Average Particle Size
and
Average Particle Composition
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Often poor predictors
of catalyst behavior
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Predicted Phase Separation Observed
Two phases observed
Bulk
Pt-Rh Phase Diagram
Pt-rich phase
Rh-rich phase
Dotted miscibility gap was predicted
theoretically from similar systems
C. E. Lyman, et al., Ultramicroscopy, 58 (1995) 25-34
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Applications
Pt-Rh/mordenite
» sulfur-tolerant NO reduction catalyst
Pt-Re/g-Al2O3
» drying alters catalyst microstructure
Pt-Sn/g-Al2O3
» Pt-rich particles aid propane dehydrogenation
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Correlation with NO-Reduction Activity
17Pt/83Rh
60Pt/40Rh
Ptox- Rhred
95Pt/5Rh
75Pt/25Rh
Rhox- Ptred
Most Active
Pt
Rh
Pt-Rh/g-alumina
95/5
60/40
1
NO H2 N2 H2O
2
75/25
17/83
Lakis et al., J. Catal. 154 (1995) 261
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Effect of Adsorbed Gas on PtRh Nanoparticles
Surface energies:
Pt ~ 2.5 J/m2
Pt segregates
to the surface
Gas-Adsorption
Surface Segregation
Pt Content (wt%)
Pt Content (wt%)
Gibbsian Equilibrium
Surface Segregation
Rh ~ 2.7 J/m2
Rh segregates
to the surface
Particle Diameter (nm)
Particle Diameter (nm)
After reduction in H2 at 500˚C
Coimpregnation of Pt and Rh
After reaction in NO + H2 at 300˚C
Sequential Impregnation, Pt first
C. E. Lyman, et al., Ultramicroscopy 34 (1990) 73-80
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C. E. Lyman, et al., Ultramicroscopy, 58 (1995) 25-34
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Line Profile Mode: Rh Segregation to Surface
Line Profile:
14 Analysis points across a 10 nm
Pt-Rh particle
60/40 catalyst
particle ~ 10 nm
Matched to calculated profile
assuming 5.8 wt% Rh core and
monolayers of pure Rh on
surface
Conclusion:
About 1 monolayer of Rh
makes catalyst less active
C. E. Lyman et al., Proc. 2nd Mexican Congress on Electron Microscopy, Cancun, (1994) SSM16
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Modelling X-ray Analysis: Rh Surface Segregation
Simulation
»
Match computer simulation line to
measured composition-size diagram
Result
»
1/2 monolayer of Rh line is close
match to measured data
95Pt/5Rh catalyst
Conclusion:
Both Pt and Rh exposed on
particle surface makes catalyst
more active
Rh adsorbs NO
Pt adsorbs H2
1
NO
H
N2 H2O
Two sites required:
2
2
C. E. Lyman et al., Microchimica Acta 132 (2000) 301
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Sulfur Tolerance in NO Reduction Catalysts
Gas: 400 ppm NO, 0.72% H2, 5 % O2, 13 % CO2 and 8% H2O in N2 balance
Pt/mordenite
Severe loss of activity
when SO2 added
Pt-5%Rh/mordenite
Most activity is retained
when SO2 added
S. Choi, M.S. Thesis, Lehigh University (2001)
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Rh reduces S-Pt association
Sulfur-poisoned Pt/mordenite
Sulfur-poisoned PtRh/mordenite
ADF image
Pt x-ray map
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S x-ray map
X-ray background
S. Choi, M.S. Thesis, Lehigh University (2001)
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Effect of Drying on PtRe Nanoparticles
ADF image
of larger
sintered
particles
Sample
5
1 nm
No drying
Bimetallic
particles
Air 240˚C
Air 550˚C
N2 550˚C
N2 680˚C
Increasing severity of drying
R Prestvik et al., J. Catal. 176 (1998) 246.
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Sintering
of Pt-rich
particles
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Pt-Re/Al2O3 Reforming Catalyst
ADF
image
Spectrum from
alumina support
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Spectrum from
a 1-nm particle
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Pt-Sn Particles on Different Supports
Dispersion vs. Particle Size
Dispersion
CO Chemisorp
Measured
Particle Size
35%
1 nm
9%
1 nm
18%
1 nm
Pt-Sn/g-Al2O3
Pt-Sn/MgO
Pt-Sn/hydrotalcite
Evidence of strong metal
support interaction (SMSI)
After reduction all Pt-Sn
particles ~ 1nm in diameter
Pt-rich particles are most active
for propane dehydrogenation
L. Bednarova et al., J. Catal. 211 (2002) 335
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A Role for EELS
Ultra-high spatial resolution
» Little beam spreading
» Spatial resolution is beam diameter
Other benefits of EELS
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Titania-supported Pt Catalyst
Pt particle
hanging over
edge
Oxygen but no titanium
Ti should
be here
HAADF Image of Pt on TiO2
J. Liu, Microsc. Microanal. 10 (2004) 55-76
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Surface Analysis by EELS: Pd-Ni/TiO2
At the very surface: Pd only as in a grape skin, the “Grape Model”
1: Pd only
3: Pd-Ni
2: Pd only
J. Liu, Microsc. Microanal. 10 (2004) 55-76
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Summary
Nanoparticles often not identical
» Composition-size diagram describes population
» Analyze at least 100 particles
FEG-STEM required for particles < 10 nm
» 1-nA probe current
» Quantitative analysis of 1-nm particles with x-rays
» Better spatial resolution using EELS
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