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 227

potential owing to the Smoluchowski effect mentioned above [Climent et al., 1999;
G´mez et al., 2000]. The effect of adatom decoration is to quench the local charge associ-
ated with step sites, resulting in a positive shift of the overall PZTC towards that of the
terrace. The N 2 O reduction on the bare stepped surface confirms the existence of two
local PZTCs of terrace and step sites, the local PZTC of the step being lower than that
of the terrace, and the PZTC of the overall surface being a weighted average of the two
local values [Attard et al., 2004; Climent et al., 2002a]. Figure 7.9d shows this result
for Pt(332) in 0.1 M H 2 SO 4 . Curve A in this figure corresponds to the unmodified
surface, showing the two local minima at 0.12 and 0.30 V, due to step and terrace
contributions, respectively. Curve B corresponds to the Bi-decorated surface, showing
only the local minima due to terrace sites at the same potential value.
A novel approach to investigating important interfacial properties of Pt single-
crystal electrodes is the laser-induced temperature jump method [Climent et al.,
2002b, 2004, 2006]. In this technique, a nanosecond laser pulse is used to suddenly
increase the temperature of the interphase. The coulostatic response to this temperature
change allows estimation of the sign and magnitude of the thermal coefficient of the
potential drop at the interphase. Remarkably, since the main contribution to this par-
ameter comes from the effect of the temperature on the dipolar contribution from the sol-
vent, these experiments provide unique information about the interaction of water with
the metallic surface. At low enough potentials, the thermal coefficient is negative,
indicating that interfacial water molecules exhibit a net orientation with their positive
end (hydrogen) towards the metal. As the potential is increased, the oxygen-towards-
the-metal orientation is favored and the thermal coefficient changes to positive values.
The particular potential where the thermal coefficient becomes zero can be identified
with the potential at which the net orientation of water changes. Moreover, it can be
demonstrated from thermodynamic considerations that this particular potential coincides
with the potential of maximum entropy of double-layer formation (PME).
As a result of the electrostatic interaction of the water dipoles with the electric field
at the interphase, the PME is related to the PZC. Then, if the surface is negatively
charged, we can expect the water molecules to be polarized with the positive end
closer to the metal, while the opposite will be true when the surface is positively
charged. The picture is complicated by the existence of a chemical interaction between
the water and the metal surface that tends to orientate the water molecules even in the
absence of any electric field. This natural orientation is with the oxygen closer to the
metal surface in the case of gold and mercury electrodes, as reflected in the fact that
the PME is located slightly negative to the PZC [Benderskii and Velichko, 1982;
Climent et al., 2002c]. The decrease in the work function upon water dosage supports
the same picture for Pt [Villegas and Weaver, 1996]. Consequently, the study of the
shift of the PME of adatom-modified Pt surfaces provides insight on the direction
and magnitude of the surface dipole induced by adatom deposition, and also on the
specific interactions of the water molecules with the metal surface.
Figure 7.10 plots values of the PME as a function of adatom coverage for different
adatoms [Garcia-Araez et al., 2008]. The medium - high coverage region will be
discussed first. In this region, two different behaviors are observed, represented by
S and Se, on the one hand, which displace the PME towards higher values, and Bi