Environmental Engineering Reference
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become positively charged. Now, it is easily understood that electropositive adatoms will
tend to interact more strongly with the lower part of the step, while electronegative ada-
toms will deposit on the terrace, more likely near the upper part of the step. Although it
must be recognized that this explanation should be viewed as a simplified picture, since
other aspects need consideration, such as water adsorption on steps or specific (chemical)
interactions between the adatom and the d-orbitals of surface Pt atoms, it does serve to
rationalize the observed experimental behavior.
7.5 DOUBLE-LAYER EFFECTS OF ADATOM MODIFICATION
The careful analysis of the double-layer effects involved in adatom modification pro-
vides valuable information for the understanding of the consequent electrocatalytic
effects, discussed in the next section. One key parameter for understanding electro-
chemical reactivity is the potential of zero charge (PZC). The notion of electrode
charge and PZC are unambiguously defined only for an ideal polarized interface.
When adsorption phenomena with charge transfer processes are present, as in the
case of Pt electrodes, it is necessary to distinguish the total charge, which includes
the charge involved in the adsorption process, from the true excess electronic
charge on the metal, i.e., the free charge [Frumkin and Petrii, 1975]. It is the latter
that is mainly responsible for the electric field at the interphase and therefore it is a
very important parameter for understanding the electronic effects caused by the
adatom. Unfortunately, none of the methods suitable for the determination of the
PZC can be applied to adatom-modified platinum surfaces.
The CO charge displacement method has been applied to the determination of the
potential of zero total charge (PZTC) of Pt surfaces, and the effect of the surface struc-
ture and the composition of the solution have been investigated in this way [Climent
et al., 1997, 1999, 2000; G ´ mez et al., 2000]. This method involves measurement of
the total charge flow during potentiostatic adsorption of CO. It is assumed that the
charge remaining on the CO-covered surface is negligible, and therefore that the
charge flowing during the adsorption process (displaced charge) equals that initially
present at the interphase at the potential of the experiment:
q
dis
¼
q
f
q
i
q
i
(7
:
22)
where q
dis
is the displaced charge, q
i
is the initial charge on the CO-free interphase, and
q
f
is the final charge at the CO-covered electrode. The experiment can be performed at
different potentials to locate the particular value where the displaced charge equals
zero. Alternatively, one particular value of q
dis
at a given potential is enough to provide
the integration constant to integrate the voltammetric current density to construct the
total charge versus potential curve. From this curve, the value of the PZTC can be read
as the intersection at q ¼ 0. Moreover, from these results and under some assumptions,
it has been possible in some cases to estimate the potential of zero free charge (PZFC)
[Climent et al., 1997; G ´ mez et al., 2000]. However, this method cannot be applied to
adatom-modified surfaces in general, because CO does not adsorb on the adatom, and
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