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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. 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

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  1. Using MEIS to probe segregation effects in bimetallic nanoparticles Chris Baddeley EaStCHEM School of Chemistry University of St Andrews

  2. Importance of bimetallic catalysis • Many examples of bimetallic catalysts in industrial use Hydrodechlorination catalysis (CuPd, ICI) Trans-1,2-dichloroethene

  3. 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

  4. Importance of bimetallic catalysis • Many examples of bimetallic catalysts in industrial use Vinyl acetate synthesis (AuPd, BP Chemicals) vinyl acetate ethylene acetic acid

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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?

  11. 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.

  12. 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

  13. 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

  14. 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

  15. 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

  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

  17. 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

  18. Pd/Au on silica films – particle size distributions • Au deposited first in each case

  19. 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

  20. 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

  21. 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

  22. 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

  23. Pd growing on Au Base Core Surface Base Core Surface

  24. Pd growing on Au Base Core Surface Base Core Surface X

  25. Small Pd particles and larger Au particles Base Core Surface Base Core Surface

  26. Small Pd particles and larger Au particles Base Core Surface Base Core Surface X

  27. Pd Au Au enriched shell, Pd enriched core Base Core Surface Base Core Surface

  28. Pd Au Au enriched shell, Pd enriched core Base Core Surface Base Core Surface

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

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