Clues for the Molecular-Level Understanding of Electrocatalysis on Single-Crystal Platinum Surfaces Modified by p-Block Adatoms - Fuel Cell Catalysis: A Surface Science Approach - page 228

Figure 7.10 Potential of maximum entropy (PME) of a Pt(111) electrode modified by Bi,
Pb, Se, and S deposition in 1 mM HClO 4 þ 0.1 M KClO 4 solution, as a function of adatom
coverage. The dashed, zero-slope line corresponds to the averaged reference PME value of
unmodified Pt(111). The cartoons show the schematic interpretation for the effect of the
adatoms at high coverage on the potential transients. (Reprinted with permission from
Garcia-Araez et al. [2008].)
and Pb, on the other hand, which clearly decrease the PME. These trends can be easily
explained by considering the differences in work function values and electronegativ-
ities (see Table 7.4) between the adatom and the Pt substrate: more-electropositive
adatoms (Bi and Pb) shift the PME towards lower potential values, while more-
electronegative adatoms (Se and S) displace it towards higher potential values. This
is consistent with the discussion given above to explain the ability of adatoms to
decorate step sites, and reinforces the idea that electropositive adatoms will retain
partial positive charge while electronegative adatoms will remain partially negatively
charged. The consequence of these charge distributions is the formation of surface
dipoles that will affect the electrostatic interaction of the water molecules with the
surface (see the cartoons in Fig. 7.10): water will remain with the oxygen towards
the metal in a broader potential range when the surface is modified with electropositive
adatoms, while the opposite orientation will be favored in the case of electronegative
adatoms.
The formation of surface dipoles by adatom deposition can also be inferred from
work function measurements in UHV. In this regard, Bi and Pb deposition causes a
marked decrease in the work function of a Pt(111) surface [Mazinangokoudi and