Transcript AEM Analysis of Nanoparticles

AEM Analysis
of
Nanoparticles
Charles Lyman
Lehigh University
Bethlehem, PA
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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
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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
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Nanoparticles
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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
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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
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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
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Specimen stage area
VG HB-603 STEM Features
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Electron
Beam
Column
300 kV FEG
Optimized for x-ray collection
1 nA in 1.5 nm (FWTM)
Now with aberration-corrector:
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5 nA in 1.5 nm (FWTM)
– More current in electron probe to
detect smaller amounts of elements
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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
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Pt-Rh/mordenite
» sulfur-tolerant NO reduction catalyst
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Pt-Re/g-Al2O3
» drying alters catalyst microstructure
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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
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Simulation
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Match computer simulation line to
measured composition-size diagram
Result
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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
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Ultra-high spatial resolution
» Little beam spreading
» Spatial resolution is beam diameter
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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
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Nanoparticles often not identical
» Composition-size diagram describes population
» Analyze at least 100 particles
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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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