1 PASI - Electron Microscopy - Chile Lyman - Nanoparticles AEM Analysis of Nanoparticles Charles...

1PASI - Electron Microscopy - Chile

Lyman - Nanoparticles

AEM Analysis of

NanoparticlesCharles LymanLehigh UniversityBethlehem, PA



Nanoparticles

Exhibit an enormous surface-to-volume ratio

Courtesy C. J. Kiely



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



Melting Temperature of Nanoparticles



Catalysis: The Oldest Nanotechnology



4. Particle Composition - surface composition is most important

Chemical Reactivity



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 withcore and shell



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



STEM Imaging of Nanoparticles

50 nm

Bright-field STEM

Annular dark-field STEM

Best method for nanoparticle detection and analysis

ADF ADF

BF



X-ray Collection Geometry in STEM

DetectorStationary or scanning electron beam covering particle

Analyze particles only on the side of support shard facing x-ray detector

X-rays

Particle stability - a serious issue at 1 Mx



Bimetallic NanoparticlesAre 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 Activity, selectivity Stability, poisoning resistance, lifetime

Correlation of bimetallic nanoparticle microstructure with catalytic properties



Measure Particle Size & Particle Composition

ColumnControl console

Specimen stage areaV G H B -6 0 3 S T E M F e a tu re s3 0 0 k V F E GO p tim iz e d fo r x -ra y c o lle c tio n 1 n A in 1 .5 n m (F W T M )

N o w w ith a b e rra tio n -c o rre c to r:5 n A in 1 .5 n m (F W T M ) M o re c u rre n t in e le c tro n p ro b e to d e te c t s m a lle r a m o u n ts o f e le m e n ts5 0 p A in 0 .2 n m (F W T M )D e te rm in e n a n o p a rtic le s h a p e

30 nm

Analysis of 4-nm particle56 wt% Pt, 44 wt% Rh

ADF image showing Pt-Rh nanoparticles

X-ray Detector

Electron Beam



Quantitative Pt-Rh Measurements

CPtCRh

kPtRhIPtIRh

1.079IPtIRh

CPt CRh 1

Measured k-factor = 1.079 on known Pt-Rh standard

Use two equations in two unknowns to find CPt

Find IPt and IRh by subtracting x-ray background from Pt-M and Rh-L peaks

Spectrum from 4-nm particle

Particle composition: 56 wt% Pt 44 wt% Rh

Cliff-Lorimer Method



Bimetallic Nanoparticle Catalysts

Average Particle Size and

Average Particle Composition

Often poor predictors of catalyst behavior

Analytical Transmission Electron Microscopy (AEM)Composition-Size Diagram

givesSize and Composition Distributions

Good predictor of catalyst behavior

Particle Diameter (nm)

Pt

Co

nte

nt

(wt%

)

Composition and size measured for ~100 indivdual nanoparticles

BulkAnalysisMethods



Predicted Phase Separation Observed

Dotted miscibility gap was predicted theoretically from similar systems

Two phases observed

Pt-rich phase

Rh-rich phase

Pt-Rh Phase DiagramBulk

C. E. Lyman, et al., Ultramicroscopy, 58 (1995) 25-34



Applications

Pt-Rh/mordenite» sulfur-tolerant NO reduction catalyst

Pt-Re/-Al2O3

» drying alters catalyst microstructure

Pt-Sn/-Al2O3 » Pt-rich particles aid propane dehydrogenation



Correlation with NO-Reduction Activity

Rh60/40

17/83

95/5

17Pt/83Rh 60Pt/40RhPtox- Rhred

75Pt/25RhRhox- Ptred

95Pt/5Rh

75/25

Pt-Rh/-alumina

NO H2 1

2N2 H2O

Lakis et al., J. Catal. 154 (1995) 261

Most Active

Pt



Effect of Adsorbed Gas on PtRh Nanoparticles

Particle Diameter (nm)

Pt

Co

nte

nt

(wt%

)

Pt segregates to the surface

Particle Diameter (nm) P

t C

on

ten

t (w

t%)

After reduction in H2 at 500˚C

Rh segregates to the surface

After reaction in NO + H2 at 300˚C

Gibbsian Equilibrium Surface Segregation

Gas-Adsorption Surface Segregation

Coimpregnation of Pt and Rh Sequential Impregnation, Pt first

C. E. Lyman, et al., Ultramicroscopy 34 (1990) 73-80 C. E. Lyman, et al., Ultramicroscopy, 58 (1995) 25-34

Surface energies: Pt ~ 2.5 J/m2 Rh ~ 2.7 J/m2



Line Profile Mode: Rh Segregation to Surface

Line Profile: 14 Analysis points across a 10 nm Pt-Rh particle

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

60/40 catalyst particle ~ 10 nm

C. E. Lyman et al., Proc. 2nd Mexican Congress on Electron Microscopy, Cancun, (1994) SSM16



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

C. E. Lyman et al., Microchimica Acta 132 (2000) 301

Conclusion:Both Pt and Rh exposed on particle surface makes catalyst more active

Pt adsorbs H2

Rh adsorbs NO

95Pt/5Rh catalyst

NO H2 1

2N2 H2OTwo sites required:



Sulfur Tolerance in NO Reduction CatalystsGas: 400 ppm NO, 0.72% H2, 5 % O2, 13 % CO2 and 8% H2O in N2 balance

Pt/mordenite

Pt-5%Rh/mordenite

S. Choi, M.S. Thesis, Lehigh University (2001)

Most activity is retained when SO2 added

Severe loss of activity when SO2 added



Rh reduces S-Pt association

Sulfur-poisoned Pt/mordenite

Sulfur-poisoned PtRh/mordenite

ADF image Pt x-ray map S x-ray map X-ray background

S. Choi, M.S. Thesis, Lehigh University (2001)



Effect of Drying on PtRe Nanoparticles

No drying Air 240˚C Air 550˚C N2 550˚C N2 680˚CIncreasing severity of drying

R Prestvik et al., J. Catal. 176 (1998) 246.

Sample 5

ADF image of larger sintered particles

1 nm

Bimetallic particles

Sintering of Pt-rich particles



Pt-Re/Al2O3 Reforming Catalyst

Spectrum from a 1-nm particle

Spectrum from alumina support

ADF image



Pt-Sn Particles on Different Supports

After reduction all Pt-Sn particles ~ 1nm in diameter

Dispersion

CO Chemisorp

Measured Particle Size

Pt-Sn/-Al2O3 35% 1 nm

Pt-Sn/MgO 9% 1 nm

Pt-Sn/hydrotalcite 18% 1 nm

Evidence of strong metal support interaction (SMSI)

L. Bednarova et al., J. Catal. 211 (2002) 335

Pt-rich particles are most active for propane dehydrogenation

Dispersion vs. Particle Size



A Role for EELS

Ultra-high spatial resolution» Little beam spreading» Spatial resolution is beam diameter

Other benefits of EELS



Titania-supported Pt Catalyst

HAADF Image of Pt on TiO2

Oxygen but no titaniumPt particle hanging over edge

Ti should be here

J. Liu, Microsc. Microanal. 10 (2004) 55-76



Surface Analysis by EELS: Pd-Ni/TiO2

J. Liu, Microsc. Microanal. 10 (2004) 55-76

At the very surface: Pd only as in a grape skin, the “Grape Model”

1: Pd only

3: Pd-Ni

2: Pd only



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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