STRUCTURAL AND CATALYTIC STUDIES OF METAL AND ANION ADLAYERS ON ELECTRODE SURFACES

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STRUCTURAL AND CATALYTIC STUDIES OF METAL AND
ANION ADLAYERS ON ELECTRODE SURFACES     J.X.Wa
ng, S.R. Brankovic, R.R.Adzic Brookhaven National
Laboratory, Upton, NY 11973   Berkeley,
2001 
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TOPICS    1.  Structure of coadsorbed
metal-anion adlayers on Au(111).    2. 
Structure and inhibition effects of anions during
O2 reduction 3. Structure of the active phase
of Bi on Au(111) for O2 reduction.   4. Spontaneo
us deposition of Pt on Ru and CO tolerance of a
Ru-supported Pt submonolayer electrocatalyst.
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SURFACE X-RAY SCATTERING FROM A (2 x 2) ADLAYER
RED - ADLAYER BLUE - ADLAYER
SUBSTRATE BLACK - SUBSTRATE
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(No Transcript)
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THALIUM-HALIDE COADSORPTION ON Au(111) IN
0.1MHClO4
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COADSORPTION OF Tl WITH Cl-, AND I- ON Au(111)
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ALLOWED STOCHIOMETRIES FOR METAL-ANION ADLAYERS
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Br- ADSORPTION ON Ag(100) IN 0.1M HClO4 10mM
Br-
c(2 x 2)Br-
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Br- ADSORPTION on Ag(100) in 0.1M NaClO4 50mM
NaBr
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O2 REDUCTION ON Ag(100)-c(2 x 2)Br- 0.1 mM NaClO4
50mM NaBr
O2
N2
Br-
 

625 rpm
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• Molecular-level information from O2 reduction on
  Ag(100)/Br
• Orientation-specific reaction rate on Ag(100)
  can be determined.
• End-on O2 adsorption requires a higher
  overvoltage for reduction than does the bridge
  configuration.
• A 4e- reduction is possible on Ag(100) with the
  end-on O2 adsorption.

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DIAMETERS OF HOLLOW SITES IN THE Br ADLAYERS ON
Ag(100), Au(100) AND Pt(111)
       
Adlayer c( 2 x 2)Br-Ag(100) c(?2 x
2?2)Br-Au(100) (3 x 3)Br-Pt(111) O  
Diameter of 1.88 Å
1.40 Å 0.90 Å
1.32 Å hollow site    
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UPD OF Bi ON Au(111) AND O2 REDUCTION
(2 x 2)Bi
(p x ?3)2Bi
1M HClO4 5mM Bi3 10mV/s
O2
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DROP CELL FOR SXS-ELECTROCHEMICAL MEASUREMENTS
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STABILITY OF THE (2 x 2)Bi PHASE ON Au(111)
DURING O2 REDUCTION
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Pt/Ru ALLOY ELECTROCATALYSTS First
reports Bockris, Wroblowa, J. Electroanal. Chem.,
1964. Petrii, Podlovchenko, Frumkin, Hira Lal et
al. ibid., 1965. New studies Markovic, Ross,
Electrochim. Acta, 2000. Surface modifications
(Pt by Ru ) Wieckowski et al., J. Electroanal.
Chem., 1993. Catalytic action- still
discussed Bifunctional mechanism - Watanabe,
Motoo, J. Electroanal. Chem., 1987. Electronic
effect
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Ru(0001) and Ru(10i0) - Cyclic Voltammetry
Ru(1010)
Ru(0001)
Solution 0.1 M H2SO4, sweep rate 10 mV/s (Wang
et al, J. Phys. Chem. B 2001, 105, 2809)
Solution 0.1 M H2SO4, sweep rate 20 mV/s
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STRUCTURAL MODELS FOR THE OXIDATION OF Ru(0001)
IN H2SO4
Ru-A- H2O RuOH H e-
A-
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Spontaneous Deposition of Pt on Ru(0001)
Solution Containing 10-4 M PtCl62- 0.1 M H2SO4,
immersion time 60 seconds
100 nm x 100 nm
200 nm x 200 nm
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Spontaneous Deposition of Pt on Ru(0001)
Solution Containing 10-2 M PtCl62- 0.1 M H2SO4,
immersion time 60 seconds
27 nm x 27 nm
100 nm x 100 nm
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Model of a cubo-octahedron Ru nanoparticle with a
¼ ML on Pt
Edges 2.5 nm
?Pt 1/4
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• Advantages of the catalysts prepared by
  spontaneous deposition
• EACH Pt ATOM COUNTS !
• Pt is only on the surface of Ru nanoparticles
  (no Pt lost inside
• the particle as in alloy catalysts)
• Controlled design of properties by varying the
  Pt coverage
• (Pt cluster size)
• No problems with components segregation.
• Ultimately low Pt loading

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Oxidation of H2 on a rotating disk thin-film
electrode
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H2/CO Oxidation
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In situ FTIRS of CO adsorption on Pt/Ru(0001)
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• CONCLUSIONS
• Potential-dependent phase behavior and the
  stoichiometries of coadsorbed metal-anion
  adlayers can be explained by symmetry and
  electrostatic considerations, and moderate
  substrate influence.
• Ordered anion adlayers cause a complete
  inhibition of O2 reduction. For some systems, O2
  can be reduced through the hollow sites in the
  low-coverage anion phase.
• End-on O2 adsorption requires a higher
  overvoltage for reduction than does the bridge
  configuration.
• Low-coverage metal adlayers are active phases for
  O2 reduction that support its 4e- reduction on
  Au.
• Spontaneous deposition of Pt on Ru can form
  submonolayer-to-multilayer deposits and can be
  used to make high CO tolerance catalysts with
  ultimately low Pt loadings.

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Acknowledgements     N. Marinkovic (now at
Albert Einstein College of Medicine) K. Tamura
  H. Zajonz B. Ocko J. McBreen   Financial
support DOE