Using MEIS to probe segregation effects in bimetallic nanoparticles

Using MEIS to probe segregation effects in bimetallic nanoparticles

Chris BaddeleyEaStCHEM School of Chemistry

University of St Andrews

• Many examples of bimetallic catalysts in industrial use

Importance of bimetallic catalysis

Hydrodechlorination catalysis (CuPd, ICI)

Trans-1,2-dichloroethene



carbon monoxide

hydrogen octane water

8 + 17 + 8

Fischer-Tropsch Catalysis (CoPd, SASOL Technology UK)



Vinyl acetate synthesis (AuPd, BP Chemicals)

ethylene acetic acid vinyl acetate

Vinyl acetate synthesis (AuPd, BP Chemicals)

ethylene acetic acid vinyl acetate

Vinyl Acetate Synthesis

• LEAP process– acetoxylation of ethene over a heterogeneous Pd/Au

catalyst (fluidised bed catalyst)

• Catalyst– Silica supported Pd/Au– Promoted by potassium acetate

Traditional surface science approach

• Detailed characterisation of clean bimetallic surface

• Attempt to correlate reactivity of surfaces with the properties of the clean surface

M.S. Chen, K. Luo, T. Wei, Z. Yan, D. Kumar, C.-W. Yi, D.W. Goodman, Catalysis Today 117 (2006) 37

Problems with traditional approach – structure gap

• Nanoparticles v extended surfaces– Different crystal planes exposed– Role of edges; defects– Differences in electronic properties

• Role of oxide support

• Better to study nanoparticles grown on oxide surfaces

M.S. Chen, K. Luo, T. Wei, Z. Yan, D. Kumar, C.-W. Yi, D.W. Goodman, Catalysis Today 117 (2006) 37

Surface composition of bimetallic particles on oxide supports

• Detailed composition of surface of particles on planar oxide supports from LEIS

K. Luo, T. Wei, C.W. Yi, S. Axnanda, D.W. Goodman, Journal of Physical Chemistry B 109 (2005) 23517

EXPERIMENT THEORY

S.A. Tenney, J.S. Ratliff, C.C. Roberts, W. He, S.C. Ammal, A. Heyden, D.A. Chen, Journal of Physical Chemistry C 114 (2010)

21652

• Segregation phenomena well described by DFT etc

Influence of adsorbate on surface composition of bimetallic surfaces

THEORY

S.A. Tenney, J.S. Ratliff, C.C. Roberts, W. He, S.C. Ammal, A. Heyden, D.A. Chen, Journal of Physical Chemistry C 114 (2010)

21652

K.J. Andersson, F. Calle-Vallejo, J. Rossmeisl, L. Chorkendorff, Journal of the American Chemical Society 131 (2009) 2404

EXPERIMENT

MEIS as a probe of adsorbate induced segregation

• Advantages

• Advantages:– Use of shadowing and blocking to enable selective

illumination of integer numbers of layers– Adsorbate “invisible” in terms of shadowing

underlying atoms

• Extend the use of MEIS to investigate bimetallic particles on oxide surfaces?

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Pre-annealing temperature /K

atom

% A

u

atom % Au in top2 layers beforeAcOHafter AcOH

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85 87 89 91 93 95 97 99 101

Energy /keV

Before AcOH

After AcOH

AuPd

TG Owens, TE Jones, TCQ Noakes, P Bailey and CJ Baddeley; J. Phys. Chem B 110 (2006) 21152

MEIS Analysis of Au/Pd Alloy Nanoparticles

Aims:• To investigate the structural and compositional properties of Au/Pd alloy

nanoparticles supported on planar oxide films• To investigate alloying behaviour and compositional changes as a function

of pre - annealing temperature.• To investigate possible segregation effects caused by adsorption of acetic

acid.

Silicon wafer

Thin silica film

NiAl{110}

Thin Al2O3 film

Experimental Details

• UK MEIS facility (Daresbury)• 100 keV He+ ions• SiO2/Si{100} and Al2O3/NiAl{110} surfaces prepared by

standard methods• Au and Pd deposited by metal vapour deposition

Data Preparation

Scattering Angle

Energy (keV)

Au Surface Feature

Pd Surface Feature

• k2 correction applied to both Au and Pd peaks

• Project data over a relatively wide angular range

• Inverse k2 correction to create spectrum for fitting

Comparison With Single Crystal Data

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84 86 88 90 92 94 96

Ion Count(A.U.)

Energy (keV)

Pd FeatureAu Feature

Pd Feature

Au Feature

TG Owens, TE Jones, TCQ Noakes, P Bailey and CJ Baddeley; J. Phys. Chem B 2006, 110, 21152

Spectrum simulation

Spectra of monometallic systems• Basic line shape of MEIS

spectra is known to be asymmetric

• Used asymmetric Gaussian derived by fitting data from submonolayer Au on Ni{111}

• Incorporate isotopic abundance into each elemental peak

WH Schulte et al; Nuclear Instruments and Methods B 183 (2001) 16

Spectrum simulation – particle shape

• Assume hexagonal, flat-topped particle• For each atom in a particle, take into account stopping power to determine

path-dependent energy loss (SRIM) and include influence of straggling• Shadowing and blocking

– Used values from a psuedo-random geometry for fcc{111}– Needs refinement for bigger particles

Fitting results – Pd60Au40 on SiO2/Si{100}

Homogeneous depth profile

20% top, 60% core, 20% base

Fitted % top/core/base

Particles – not flat surface

J. Gustafson, A.R. Haire, C.J. Baddeley, Surface Science 605 (2011) 220

Pd/Au on silica films – particle size distributions

• Au deposited first in each case

SampleRelative

composition Au loading

(ML)Pd loading

(ML)

Total metal loading (ML)

Measured particle height (layers)

1a (fig. 1a) Pd61Au39 0.96 1.51 2.47 5.0±3.0

1b (fig. 1b) Pd68Au32 0.48 1.03 1.51 4.5±2.0

1c (fig. 1c) Pd68Au32 0.12 0.26 0.38 2.5±2.0

2a (fig. 2a) Pd91Au9 0.10 1.02 1.12 4.0±2.0

2b (fig. 2b) Pd91Au9 0.20 2.05 2.25 4.0±1.5

3 Pd15Au85 1.28 0.23 1.51 10±2.0

SampleRelative

composition Au loading

(ML)Pd loading

(ML)

Total metal loading (ML)

Measured particle height (layers)

1a (fig. 1a) Pd61Au39 0.96 1.51 2.47 5.0±3.0

1b (fig. 1b) Pd68Au32 0.48 1.03 1.51 4.5±2.0

1c (fig. 1c) Pd68Au32 0.12 0.26 0.38 2.5±2.0

2a (fig. 2a) Pd91Au9 0.10 1.02 1.12 4.0±2.0

2b (fig. 2b) Pd91Au9 0.20 2.05 2.25 4.0±1.5

3 Pd15Au85 1.28 0.23 1.51 10±2.0

Results – AuPd composition as a function of annealing temperature

Assume Overbury relationship between surface and bulk comp:xB

s/xAs = xB

b/xAb exp [0.16 (DHsub A – DHsub B)/RT]

DHsub Pd 377 kJmol-1 DHsub Au 368 kJmol-1

S.H. Overbury, P.A. Bertrand, G.A. Somorjai, Chemical Reviews 75 (1975) 547

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Annealing Temperature /K

% A

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sitio

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• Even at room temp, surface composition is Au rich.

• Au enrichment decreases with increasing annealing temp

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predictedmeasured surfacemeasured core

2.4 MLE metal loading; Au39Pd61

A.R. Haire, J. Gustafson, A.G. Trant, T.E. Jones, T.C.Q. Noakes, P. Bailey, C.J. Baddeley, Surface Science 605 (2011) 214

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% A

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sitio

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% A

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• Au surface enrichment not observed for small particles

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• Surfaces enriched in Au despite Au being deposited first

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% A

u co

mpo

sitio

n predictedmeasured surfacemeasured core



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% A

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Base Core Surface Base Core Surface

Pd growing on Au


Pd growing on AuX


Small Pd particles and larger Au particles


Small Pd particles and larger Au particlesX

Pd Au


Au enriched shell, Pd enriched core

Pd Au


Au enriched shell, Pd enriched core

Interpretation of MEIS data

• Au prefers to be at edge sites • [Freund and co-workers; J. Phys. Chem. C 114 (2010) 17099]

• Au highly mobile on oxide surfaces• [I. Beszeda, E.G. Gontier-Moya, A.W. Imre, Applied Physics a-Materials Science & Processing 81 (2005) 673]

Relatively flat Au particles on SiO2

Deposition of Pd onto Au/SiO2

Rapid diffusion of Au to create shell

Intermixing at higher annealing temperature

Influence of acetic acid on surface composition of Pd/Au/Al2O3/NiAl{110}

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573 K anneal after acetic acid flash to 350 K flash to 380 K flash to 430 K

% A

u in

sur

face

laye

r

27% Au12% Au

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27% Au12% Au

0.12 ML Au followed by Pd deposited onto Al2O3/NiAl{110}


Explanation for segregation behaviourAu-rich

bimetallic surface CH3COOH(g)

CH3COO(ads) + H(ads)

2H(ads) H2 (g)

CH3COO(ads) CO2(g)+ 3/2 H2(g) + C (ads)

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% A

u in

sur

face

laye

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27% Au12% Au

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27% Au12% Au


Conclusions / Future Work

• Possible to use MEIS to depth profile Au/Pd alloy nanoparticles supported on planar oxide films– Need user friendly data analysis tool

• Now available from Sortica’s group in Brazil

• Can ‘see through’ the adsorbate layer– MEIS ideal for looking at adsorbate covered catalyst particles

• Better to work with samples enriched in lower Z element.– Many examples (e.g. CoPt) where exactly this type of catalyst is

employed at very low doping levels of heavy element

• Need temperature and pressure dependence of adsorbate induced segregation

Acknowledgements

EaStCHEM School of Chemistry, St AndrewsDr Johan GustafsonDr Andrew Haire

Dr Aoife TrantDr Tim Jones

Dr Tom Owens

MEIS facility, STFC Daresbury LaboratoryDr Tim NoakesDr Paul Bailey

The Knut and Alice Wallenberg Foundation

Spectrum simulation – spectra of bimetallic systems

Spectra of bimetallic systems• Peak intensities normalised to take into account

scattering cross section• Peaks associated with each element are fitted according

to the known overall composition• Assume Gaussian distribution of particle heights• Particles separated into surface/core/base shells• ptop% has composition ctop; pbottom% has composition

cbottom; remainder has composition ccore

• Use an intermediate stopping power between that of pure Au and pure Pd weighted by the total composition

Documents

Using MEIS to probe segregation effects in bimetallic nanoparticles