Pt Monometallic and Bimetallic Surfaces

As noted in Section 1.2, accurate determination of adsorbate-induced changes in surface-normal structure, i.e. the Adj2 interplanar spacing between the first and the second atomic layers, can be achieved by measuring the CTRs [1—4, 10, 35]. Previous reviews summarized adsorbate-induced relaxation and reconstruction on well-defined Pt(hkl) and Pt-bimetallic surfaces in aqueous electrolytes at electrode potentials at which a maximum surface coverage of adsorbed species is established [28, 29]. The data revealed that either close to the hydrogen evolu- 

3) The nature of the oxygenated species ad- 4) Details about the vibrational properties of CO sorbed onto Sn is unknown but, for the sake on platinum bimetallic surfaces including the of discussion, this species wdl be collectively PtjSn(lll) system are presented by Korze- 

The potential dependence of the surface relaxation and the stabihty of the Pd film was probed by XRV measurements [32]. Desorption of Hupd caused a slight Pt-Pd contraction, but the surface became expanded again ( 2.5%) after the adsorption of bisulfate anions. At positive potentials, i.e. in the potential region of OH adsorption, there was a significant (but reversible) change in the Pd structure associated with oxide formation. Importantly, if the Pt(lll) monometallic surface had been cycled up to this potential, then a Pt oxide would be formed that would lead to irreversible roughening (which is the precursor to Pt dissolution) of the surface upon reduction. 

Adsorption of CO onto the Pt/Pd surface at 0.05 V also led to no significant changes in the Pt-Pd layer spacing (compared to a 4% expansion of the Pt mono- 

Pd films were also examined at successively higher levels of thickness, ranging from 1 ML to n ML (n 2) regime. It was found that on the top of a pseudo-morphic monolayer film, Pd deposited on Pt(lll) forms three-dimensional is- 

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