Electrochemistry at Well-Characterized Bimetallic Surfaces - Fuel Cell Catalysis: A Surface Science Approach - page 259

always the case, because the values of the d-band center position for sputtered surfaces
were obtained in UHV, whereas, in the electrolyte, there were substantial changes in
surface composition and morphology. Therefore, these values (Fig. 8.12b) are just
virtual, and the results imply that the catalytic activity depends also on the detailed
morphology of the alloy surface. Unfortunately, it was impossible to determine this
morphology in even an approximate manner on polycrystalline samples, and therefore,
in the most recent work [Stamenkovic et al., 2007a], we used well-defined single-
crystal surfaces to elucidate structural versus electronic effects. As for polycrystalline
systems, in UHV, all Pt 3 Ni(hkl) surfaces show a strong enrichment of Pt and the
formation of a Pt-skin layer with different geometry of surface atoms, but also with
significant variations in the density of states (DOS) near the Fermi level. Although
the phenomenon of surface segregation at bimetallic systems has been well studied
under UHV conditions, especially with the progress of the LEIS technique
[Brongersma et al., 2007] there has been no study examining segregation in situ
during reaction conditions.
Great progress has been made, however, in our later study using in situ SXS
[Stamenkovic et al., 2007a], where, by simultaneously fitting the intensity ratio
between two different sets of crystal truncation rod (CTR) data that constrain the fit
to the full CTR data [Robinson, 1986; Warren, 1990], it was possible to reveal the
elemental concentration profile at the surface (Fig. 8.13c). Based on the in situ SXS
results depicted in Fig. 8.13a, the topmost surface layer is confirmed to be 100 at%
Figure 8.13 In situ electrochemical SXS characterization of Pt 3 Ni(111) and Pt(111) surfaces:
(a) XRV measurements for Pt 3 Ni(111) at the (0, 0, 2.7) (filled squares) and Pt(111) at (1, 0, 3.6)
(open triangles); (b) surface coverage by underpotentially deposited hydrogen (H upd ) and
hydroxyl species (OH ad ) calculated from the cyclic voltammograms; (c) segregation profile
ascertained from the SXS measurements. (Reprinted with permission from Stamenkovic et al.
[2007a]. Copyright 2007. American Association for the Advancement in Science.)