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 234

Figure 7.13 Variation of the CO stripping charge formed from formic acid dissociative
adsorption as a function of adatom coverage for a Pt(111) electrode modified with Bi and Se,
as indicated, in 0.5 M H 2 SO 4 solution.
oxidation and the effect of the adatom on this particular path of the overall mechanism
can be studied. Figure 7.13 shows some selected experimental results, plotting the
amount of poison formed as a function of adatom coverage on Pt(111) surfaces.
Two different kinds of behavior are observed. For Se-Pt(111) surfaces, the amount
of poison decreases linearly with the adatom coverage. The same behavior is observed
with S-modified Pt(111) electrodes. This behavior can be explained by arguing a third-
body effect, the amount of poison decreasing merely because there are fewer Pt sites
available for the poisoning reaction. The interesting point here is that the amount of
poison falls to zero at a coverage lower than saturation, indicating that a certain
number of adjacent Pt atoms are necessary for the poisoning reaction. At a coverage
close to 0.25, the probability of finding two adjacent unblocked Pt atoms drops to
zero, and therefore no poison is formed at this coverage, although other reactions
might proceed if the ensemble requirements are less demanding. Similar behavior is
also observed with Sb-, Bi-, and Te-modified Pt(100) electrodes, although in these
cases the linearity is lost at high coverages.
The second behavior observed in the results plotted in Fig. 7.13 is that of
Bi-Pt(111) electrodes. In this case, a very small amount of adatom on the surface
can produce a significant effect on the amount of poison formed. In fact, no poison
can be detected when the Bi coverage is as low as 0.04. It has been calculated that
if this effect were due to an extended electronic modification, it would mean that
one adatom inhibits the poison formation in the surrounding Pt atoms, extending its
influence as far as to 7 neighbors. This is quite unexpected, and contrary to theoretical