Environmental Engineering Reference
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shows the evolution of the cyclic voltammogram recorded with this electrode in a sol-
ution containing a small concentration of Bi(III) cations (approximately 5 mM). The
adatoms progressively deposit on the surface, a process that can be monitored by
the simultaneous measurement of cyclic voltammograms [Climent et al., 2001;
Herrero et al., 2000]. In the first stage of the deposition, the voltammetric peak at
0.12 V decreases, while the wave corresponding to anion adsorption on the terraces
remains essentially unaltered, signifying that the adatom is selectively being deposited
on the steps. During this stage, a new peak at 0.25 V grows, linked to the decrease of
the peak of the step, a fact that has been assigned to induced anion adsorption on Pt
sites adjacent to the adatom-covered step site, in view of its marked dependence on
the nature of the anion in solution (e.g., in HClO
4
solution, this peak is not observed).
In a second stage, which starts when the step peak is completely blocked, adatom
deposition on the terrace sites takes place, as evidenced by the progressive blockage
of the hydrogen and anion adsorption processes on the terraces, the appearance of
the redox peak characteristic of Bi oxidation on Pt(111) surfaces at 0.63 V, and the pro-
gressive disappearance of the adatom-induced anion peak at 0.25 V. The redox process
corresponding to adatoms on steps is not observed, because it lies above the potential
range selected in this experiment.
It is noteworthy that the same qualitative behavior is observed with Te deposition.
Similar behavior also occurs with As and Sb, although, in these cases, deposition on
the terrace starts slightly before the step is completely blocked, indicating that the
selectivity for step sites is smaller with these adatoms. Another important difference
observed in the decoration of stepped surfaces with As is that the adatom on the
step features a redox process at 0.57 V. This case is illustrated in Fig. 7.7, where
successive voltammetric cycles recorded with a Pt(775) electrode in 1 mMAs
2
O
3
รพ
0.1 M H
2
SO
4
solution are shown. The selective deposition of As on step sites is evi-
denced by the progressive attenuation of the peak at 0.12 V, while the region for anion
adsorption on the terraces, between 0.40 and 0.50 V, remains unaffected. However, a
new peak grows at 0.57 V, corresponding to the surface oxidation of the adatom. The
possible oxidation of the adatom in this relatively low potential range will mark a
difference for the electrocatalysis of CO oxidation, as will be discussed in Section 7.6.
Completely different behavior is observed with S and Se, as shown in Fig. 7.8. With
these adatoms, deposition on the terrace starts from the very beginning and no selec-
tivity towards the step is observed. Additionally, deposition of the adatom changes the
hydrogen adsorption energy on the (110) step sites, as reflected by the progressive shift
of the peak at 0.12 V towards higher potential values.
The different behavior exhibited by the studied adatoms has been rationalized in
terms of their different work function or electronegativity values, relative to the Pt
substrate (see Table 7.4). Those elements with lower electronegativity tend to create
a surface dipole with excess of positive charge on the adatom. In contrast, more-
electronegative adatoms will retain negative charge. The second basis for the observed
behavior is the particular charge distribution on step sites predicted by the Smoluchowski
effect or the inability of the soft electron cloud to follow the sharp step edge defined by the
atomic nuclei [Smoluchowski, 1941; Zangwill, 1988]. Accordingly, negative charge
will accumulate on the lower part of the step, while the upper part of the step will
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