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Using MEIS to probe segregation effects in bimetallic nanoparticles. Chris Baddeley EaStCHEM School of Chemistry University of St Andrews. Importance of bimetallic catalysis. Many examples of bimetallic catalysts in industrial use. Hydrodechlorination catalysis ( CuPd , ICI).

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using meis to probe segregation effects in bimetallic nanoparticles

Using MEIS to probe segregation effects in bimetallic nanoparticles

Chris Baddeley

EaStCHEM School of Chemistry

University of St Andrews

slide2

Importance of bimetallic catalysis

  • Many examples of bimetallic catalysts in industrial use

Hydrodechlorination catalysis (CuPd, ICI)

Trans-1,2-dichloroethene

slide3

Importance of bimetallic catalysis

  • Many examples of bimetallic catalysts in industrial use

Fischer-Tropsch Catalysis (CoPd, SASOL Technology UK)

+ 17

8

+ 8

carbon

monoxide

hydrogen

water

octane

slide4

Importance of bimetallic catalysis

  • Many examples of bimetallic catalysts in industrial use

Vinyl acetate synthesis (AuPd, BP Chemicals)

vinyl acetate

ethylene

acetic acid

vinyl acetate synthesis
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

Vinyl acetate synthesis (AuPd, BP Chemicals)

vinyl acetate

ethylene

acetic acid

traditional surface science approach
Traditional surface science approach
  • Attempt to correlate reactivity of surfaces with the properties of the clean surface
  • Detailed characterisation of clean bimetallic 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
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
Surface composition of bimetallic particles on oxide supports

THEORY

EXPERIMENT

  • Detailed composition of surface of particles on planar oxide supports from LEIS
  • Segregation phenomena well described by DFT etc

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

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

influence of adsorbate on surface composition of bimetallic surfaces

EXPERIMENT

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

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

meis as a probe of adsorbate induced segregation
MEIS as a probe of adsorbate induced segregation
  • Advantages:
    • Use of shadowing and blocking to enable selective illumination of integer numbers of layers
    • Adsorbate “invisible” in terms of shadowing underlying atoms
  • Advantages

TG Owens, TE Jones, TCQ Noakes, P Bailey and CJ Baddeley; J. Phys. Chem B110 (2006) 21152

  • Extend the use of MEIS to investigate bimetallic particles on oxide surfaces?
meis analysis of au pd alloy nanoparticles
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.
experimental details
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
Data Preparation
  • 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

Au Surface Feature

Energy (keV)

Pd Surface Feature

Scattering Angle

comparison with single crystal data
Comparison With Single Crystal Data

Au Feature

Pd Feature

Ion Count

(A.U.)

Au Feature

Pd Feature

Energy (keV)

TG Owens, TE Jones, TCQ Noakes, P Bailey and CJ Baddeley; J. Phys. Chem B2006, 110, 21152

spectrum simulation
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
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 pd 60 au 40 on sio 2 si 100
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

results aupd composition as a function of annealing temperature
Results – AuPd composition as a function of annealing temperature

Assume Overbury relationship between surface and bulk comp:

xBs/xAs = xBb/xAb exp [0.16 (DHsub A – DHsub B)/RT]

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

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

slide20
Even at room temp, surface composition is Au rich.

Au enrichment decreases with increasing annealing temp

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

slide21

1.5 MLE metal loading; Au38Pd62

0.4 MLE metal loading; Au38Pd62

  • Au surface enrichment not observed for small particles

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

slide22

1.1 MLE metal loading; Au9Pd91

2.3 MLE metal loading; Au9Pd91

  • Surfaces enriched in Au despite Au being deposited first

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

slide23

Pd growing on Au

Base Core Surface

Base Core Surface

slide24

Pd growing on Au

Base Core Surface

Base Core Surface

X

slide25

Small Pd particles and larger Au particles

Base Core Surface

Base Core Surface

slide26

Small Pd particles and larger Au particles

Base Core Surface

Base Core Surface

X

slide27

Pd Au

Au enriched shell, Pd enriched core

Base Core Surface

Base Core Surface

slide28

Pd Au

Au enriched shell, Pd enriched core

Base Core Surface

Base Core Surface

interpretation of meis data
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]

Intermixing at higher annealing temperature

Rapid diffusion of Au to create shell

Deposition of Pd onto Au/SiO2

Relatively flat Au particles on SiO2

influence of acetic acid on surface composition of pd au al 2 o 3 nial 110
Influence of acetic acid on surface composition of Pd/Au/Al2O3/NiAl{110}

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

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

explanation for segregation behaviour
Explanation for segregation behaviour

Au-rich bimetallic surface

2H(ads)  H2 (g)

CH3COOH(g)  CH3COO(ads) + H(ads)

CH3COO(ads)

 CO2(g)+ 3/2 H2(g) + C (ads)

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

conclusions future work
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
Acknowledgements

EaStCHEM School of Chemistry, St Andrews

Dr Johan Gustafson

Dr Andrew Haire

Dr Aoife Trant

Dr Tim Jones

Dr Tom Owens

MEIS facility, STFC Daresbury Laboratory

Dr Tim Noakes

Dr Paul Bailey

The Knut and Alice Wallenberg Foundation

spectrum simulation spectra of bimetallic systems
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