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fabrication, structure, and chemical properties of surface confined alloys

Andreas Bergbreiter, Albert Engstfeld, Ralf Rötter, Markus Mauksch, Otávio Brandão Alves, Harry E. Hoster, R. Jürgen Behm

motivation

• surface alloys as model system for structural and catalytical investigations

• catalytic properties: strongly related to catalyst structure

• metal-metal interactions: responsible for the atomic arrangement

• knowledge of structure and energetics are important to understand catalytic properties

• linking experiment with theory by comparing

- effective interaction deduced from atomically resolved STM images

- DFT calculations

example: fabrication of a PtRu/Ru(0001) surface alloy:

(a) vapor deposition of Pt onto Ru(0001)

(b) annealing to 1350 K

proposed mechanism of surface alloying for guest metal with negative segregation energy

(i) exchange between adatoms and underlying rest atoms

(ii) guest metal overgrown by islands with little probability due negative segregation energy, i.e. also lower surface energy

conservation of initial foreign metal amount despite changes in surface morphology.

example: conservation of Pt during PtRu/Ru(0001) surface alloy formation due to local equilibrium

generic classes of surface alloys: lateral atom distribution

A. phase separation

B. dispersed intermixing

approach for quantitative description:
2D lattice gas Hamiltonian with pairwise potentials		resulting Hamiltonian z(r): coordination number of the corresponding coordination shell Si: occupation operator at site i (+1 for atom A, -1 for atom B) model only distinguishes between like and unlike pairs
interaction model: Veff > 0: effective attraction between unequal atoms Veff < 0: effective attraction between equal atoms
additive interaction model: suitable for Monte-Carlo simulations
measured	simulated
algorithm scheme to deduce effective cluster interactions - inverse Monte-Carlo (IMC) simulation
EPI calculation from DFT calculated mixing energies of ordered surface alloys

further development: Monte-Carlo simulation of atom distribution and thermal desorption behavior based on DFT calculations

left: simulation; right: experiment

example: PtAu/Pt(111) surface alloys; atom distribution and thermal desorption of CO (Hoster, Eyrich, Bergbreiter, Sakong, Gross, Behm, unpublished results)

Related Literature:

“On the Origin of Ru Bilayer Island Growth on Pt(111)”, A. Bergbreiter, A. Berkó, P.M. Erne, H.E. Hoster, R.J. Behm,
Vacuum, in press.

“Surface alloy formation, short-range order, and deuterium adsorption properties of monolayer PdRu/Ru(0001) surface alloys” H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H.E. Hoster, R.J. Behm, Surf. Sci. 603 (2009) 1439

“Pt_xRu_1-x/Ru(0001) surface alloys — formation and atom distribution”, H. E. Hoster, A. Bergbreiter, P. Erne, T. Hager, H. Rauscher, and R. J. Behm, PCCP 10 (2008) 3812

“Interaction of CO with atomically well-defined PtxRuy/Ru(0001)surface alloys”, H. Rauscher, T. Hager, T. Diemant, H. Hoster, F. Buatier de Mongeot, R.J. Behm, Surface Science 601 (2007) 4608

“Energetics driving the short-range order in CuxPd1-x/Ru(0001) monolayer surface alloys”, A. Bergbreiter, H.E. Hoster, S. Sakong, A. Groß, R.J. Behm, PCCP 9 (2007) 5127

“Formation and short-range order of two-dimensional Cu_xPd_1-x monolayer surface alloys on Ru(0001)”, H.E. Hoster, E. Filonenko, B. Richter, R.J. Behm, Phys. Rev. B 73 (2006) 165413

“Catalytic influence of Pt monolayer islands on the hydrogen electrochemistry of Ru(0001) studied by Ultrahigh Vacuum Scanning Tunneling Microscopy and Cyclic Voltammetry”, H. E. Hoster, B. Richter, R.J. Behm, Journal of Physical Chemistry B 108 (2004) 14780-14788

“Hydrogen adsorption and H₂/CO coadsorption on well-defined bimetallic PtRu surfaces – A model study on the CO tolerance of bimetallic PtRu anode catalysts in low temperature Polymer Electrolyte Fuel Cells”, T. Diemant, T. Hager, H.E. Hoster, H. Rauscher, R.J. Behm,
Surf. Sci. 541(2003) 137-145

“Pt-Ru-model catalysts for anodic methanol oxidation: Influence of structure and composition on the reactivity”, Harry Hoster, Teresa Iwasita, Hermann Baumgärtner, Wolf Vielstich, Physical Chemistry Chemical Physics 3 (2001) 337-346

“CO adsorption and oxidation on bimetallic Pt/Ru(0001) surfaces – a combined STM and TPD/TPR study“, F. Buatier de Mongeot, M. Scherer, B. Gleich, E. Kopatzki and R. J. Behm, Surf. Sci., 1998, 411, 249.

“Correlation between local substrate structure and local chemical properties: CO adsorption on well-defined bimetallic Au/Pd(111) surfaces”, Surf. Sci. 386(1997) 48

S. Müller, J. Phys.: Condens. Matter, 2003, 15, R1429

B. Sadigh, M. Asta, V. Ozolins, A. K. Schmid, N. C. Bartelt, A. A. Quong and R. Q. Hwang, Phys. Rev. Lett., 1999, 83, 1379

A. Christensen, A. V. Ruban, P. Stoltze, K. W. Jacobsen, H. L. Skriver, J. K. Nørskov and F. Besenbacher, Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 56, 5822

A. V. Ruban, H. L. Skriver, and J. K. Nørskov, Phys. Rev. B, 1999, 59, 15990

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