1 PASI - Electron Microscopy - Chile Lyman - Nanoparticles AEM Analysis of Nanoparticles Charles Lyman Lehigh University Bethlehem, PA

2 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Nanoparticles Exhibit an enormous surface-to-volume ratio Courtesy C. J. Kiely

3 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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

4 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Melting Temperature of Nanoparticles

5 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Catalysis: The Oldest Nanotechnology

6 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 4. Particle Composition - surface composition is most important Chemical Reactivity

7 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Analysis of Nanoparticles in Electron Microscopes Nanoparticles » Bodies of matter < 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

8 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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

9 PASI - Electron Microscopy - Chile Lyman - Nanoparticles STEM Imaging of Nanoparticles 50 nm Bright-field STEM Annular dark-field STEM Best method for nanoparticle detection and analysis ADF BF

10 PASI - Electron Microscopy - Chile Lyman - Nanoparticles X-ray Collection Geometry in STEM Detector Stationary 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

11 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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 Activity, selectivity Stability, poisoning resistance, lifetime Correlation of bimetallic nanoparticle microstructure with catalytic properties

12 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Measure Particle Size & Particle Composition ColumnControl console Specimen stage area VG HB-603 STEM Features » 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 30 nm Analysis of 4-nm particle 56 wt% Pt, 44 wt% Rh ADF image showing Pt-Rh nanoparticles X-ray Detector Electron Beam

13 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Quantitative Pt-Rh Measurements Measured k-factor = on known Pt-Rh standard Use two equations in two unknowns to find C Pt Find I Pt and I Rh 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

14 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Bimetallic Nanoparticle Catalysts Average Particle Size and Average Particle Composition Often poor predictors of catalyst behavior Analytical Transmission Electron Microscopy (AEM) Composition-Size Diagram gives Size and Composition Distributions Good predictor of catalyst behavior Particle Diameter (nm) Pt Content (wt%) Composition and size measured for ~100 indivdual nanoparticles Bulk Analysis Methods

15 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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 Diagram Bulk C. E. Lyman, et al., Ultramicroscopy, 58 (1995) 25-34

16 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Applications Pt-Rh/mordenite » sulfur-tolerant NO reduction catalyst Pt-Re/  -Al 2 O 3 » drying alters catalyst microstructure Pt-Sn/  -Al 2 O 3 » Pt-rich particles aid propane dehydrogenation

17 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Correlation with NO-Reduction Activity Rh 60/40 17/83 95/5 17Pt/83Rh60Pt/40Rh Pt ox - Rh red 75Pt/25Rh Rh ox - Pt red 95Pt/5Rh 75/25 Pt-Rh/  -alumina Lakis et al., J. Catal. 154 (1995) 261 Most Active Pt

18 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Effect of Adsorbed Gas on PtRh Nanoparticles Particle Diameter (nm) Pt Content (wt%) Pt segregates to the surface Particle Diameter (nm) Pt Content (wt%) After reduction in H 2 at 500˚C Rh segregates to the surface After reaction in NO + H 2 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) C. E. Lyman, et al., Ultramicroscopy, 58 (1995) Surface energies: Pt ~ 2.5 J/m 2 Rh ~ 2.7 J/m 2

19 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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

20 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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 H 2 Rh adsorbs NO 95Pt/5Rh catalyst Two sites required:

21 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Sulfur Tolerance in NO Reduction Catalysts Gas: 400 ppm NO, 0.72% H 2, 5 % O 2, 13 % CO 2 and 8% H 2 O in N 2 balance Pt/mordenite Pt-5%Rh/mordenite S. Choi, M.S. Thesis, Lehigh University (2001) Most activity is retained when SO 2 added Severe loss of activity when SO 2 added

22 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Rh reduces S-Pt association Sulfur-poisoned Pt/mordenite Sulfur-poisoned PtRh/mordenite ADF image Pt x-ray mapS x-ray map X-ray background S. Choi, M.S. Thesis, Lehigh University (2001)

23 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Effect of Drying on PtRe Nanoparticles No dryingAir 240˚CAir 550˚CN 2 550˚CN 2 680˚C Increasing 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

24 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Pt-Re/Al 2 O 3 Reforming Catalyst Spectrum from a 1-nm particle Spectrum from alumina support ADF image

25 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Pt-Sn Particles on Different Supports After reduction all Pt-Sn particles ~ 1nm in diameter Dispersion CO Chemisorp Measured Particle Size Pt-Sn/  -Al 2 O 3 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

26 PASI - Electron Microscopy - Chile Lyman - Nanoparticles A Role for EELS Ultra-high spatial resolution » Little beam spreading » Spatial resolution is beam diameter Other benefits of EELS

27 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Titania-supported Pt Catalyst HAADF Image of Pt on TiO 2 Oxygen but no titanium Pt particle hanging over edge Ti should be here J. Liu, Microsc. Microanal. 10 (2004) 55-76

28 PASI - Electron Microscopy - Chile Lyman - Nanoparticles Surface Analysis by EELS: Pd-Ni/TiO 2 J. Liu, Microsc. Microanal. 10 (2004) At the very surface: Pd only as in a grape skin, the “Grape Model” 1: Pd only 3: Pd-Ni 2: Pd only

29 PASI - Electron Microscopy - Chile Lyman - Nanoparticles 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