Using MEIS to probe segregation effects in bimetallic nanoparticles Chris Baddeley EaStCHEM 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

Many examples of bimetallic catalysts in industrial use Importance of bimetallic catalysis carbon monoxide hydrogen octane water Fischer-Tropsch Catalysis (CoPd, SASOL Technology UK)

Many examples of bimetallic catalysts in industrial use Importance of bimetallic catalysis 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) 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) 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) 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? 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.

Experimental Details UK MEIS facility (Daresbury) 100 keV He + ions SiO 2 /Si{100} and Al 2 O 3 /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 k 2 correction applied to both Au and Pd peaks Project data over a relatively wide angular range Inverse k 2 correction to create spectrum for fitting

Comparison With Single Crystal Data Ion Count (A.U.) Energy (keV) Pd Feature Au 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 – Pd 60 Au 40 on SiO 2 /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

Results – AuPd composition as a function of annealing temperature Assume Overbury relationship between surface and bulk comp: x B s /x A s = x B b /x A b exp [0.16 (  H sub A –  H sub B )/RT]  H sub Pd 377 kJmol -1  H sub Au 368 kJmol -1 S.H. Overbury, P.A. Bertrand, G.A. Somorjai, Chemical Reviews 75 (1975) 547

Even at room temp, surface composition is Au rich. Au enrichment decreases with increasing annealing temp 2.4 MLE metal loading; Au 39 Pd 61 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

1.5 MLE metal loading; Au 38 Pd MLE metal loading; Au 38 Pd 62 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

Surfaces enriched in Au despite Au being deposited first 1.1 MLE metal loading; Au 9 Pd MLE metal loading; Au 9 Pd 91 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

Base Core Surface Pd growing on Au

Base Core Surface Pd growing on Au X

Base Core Surface Small Pd particles and larger Au particles

Base Core Surface Small Pd particles and larger Au particles X

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

Pd Au Base Core Surface 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 SiO 2 Deposition of Pd onto Au/SiO 2 Rapid diffusion of Au to create shell Intermixing at higher annealing temperature

Influence of acetic acid on surface composition of Pd/Au/Al 2 O 3 /NiAl{110} 0.12 ML Au followed by Pd deposited onto Al 2 O 3 /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 Au-rich bimetallic surface CH 3 COOH(g)  CH 3 COO(ads) + H(ads) 2H(ads)  H 2 (g) CH 3 COO(ads)  CO 2 (g)+ 3 / 2 H 2 (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 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 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 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 p top % has composition c top ; p bottom % has composition c bottom ; remainder has composition c core Use an intermediate stopping power between that of pure Au and pure Pd weighted by the total composition