Environmental Engineering Reference
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has also been reported, for instance, in the case of sulfur [Sung et al., 1997, 1998]. In
this case, oxygen reduction seems the more likely cathodic reaction:
Pt
þ
S
2
þ
2H
þ
þ
2
O
2
!
Pt-S
þ
H
2
O
(7
:
2)
Alternatively, a disproportionation reaction has been proposed for the case of Sn
[Rodes et al., 1988]:
Pt
þ
2Sn
2
þ
!
Pt-Sn
þ
Sn
4
þ
(7
:
3)
Finally, another possibility is an initial physisorption (adsorption without charge
transfer) of the adatom when the electrode is put in contact with the solution, the phy-
sisorbed species remaining on the surface after rinsing, and the reduction taking place
after the first negative scan of the potential.
7.2.2 Voltammetric Characterization of the Modified Electrode
The presence of the adatom on the surface after irreversible deposition is evidenced by
a significant suppression of the hydrogen and anion adsorption processes character-
istic of clean Pt electrodes. In addition, a new redox process takes place, resulting
in two new voltammetric peaks. The voltammetric profile remains stationary over a
wide potential range (typically from 0 to 0.8 - 1.0 V vs. RHE), indicating that the
adatom remains stable on the surface. However, desorption of the adsorbed adatom
takes place if the upper potential limit is increased further, as evidenced from the
decrease of the redox peaks associated with the adatom and the progressive recovery
of the hydrogen and anion adsorption processes on the free Pt sites. Figures 7.1 and 7.2
show some representative cyclic voltammograms corresponding to adatom-modified
Pt(111) and Pt(100) electrodes. It is evident from these figures that the peak potential
and the shape of the voltammograms for the new redox process are characteristic of the
nature of the adatom. Comparison of Figs. 7.1 and 7.2 reveals that the surface process
is also very sensitive to the crystallographic orientation of the electrode surface. The
new redox peak has been attributed to the oxidation/reduction of the deposited ada-
toms according to
Pt
m
-M
!
Pt
m
-M
n
þ
þ
ne
(7
:
4)
From the pH dependence of the peak potential (close to 60 mV/decade), it was
proposed that the oxidation involves adsorption of oxygenated species, leading to
the formation of either the oxide or the hydroxide:
Pt
m
-M
þ
nH
2
O
O
Pt
m
-M(OH)
n
þ
nH
þ
ne
(7
:
5a)
Pt
m
-M
þ
2
H
2
O
O
Pt
m
-MO
n
=
2
þ
nH
þ
ne
(7
:
5b)
where m is the number of Pt sites blocked by the adatom and n is the number of elec-
trons transferred in the oxidation of one adatom. It has been observed that the charge
density associated with this surface process and the blockage of the hydrogen and
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