Environmental Engineering Reference
In-Depth Information
of the surface structure for fundamental studies (see, e.g., Chapter 6), surface modifi-
cation of platinum single-crystal electrodes by irreversible adsorption of adatoms has
been extensively studied in the literature, and it will be the central subject of this chap-
ter. First, methods of preparation and electrochemical characterization will be
reviewed in the next section. Then, the effect of surface modification on electro-
catalysis will be discussed, and some reasonable explanations for the enhancement
of electrocatalytic activity by adatom deposition will be proposed.
7.2 ELECTROCHEMICAL CHARACTERIZATION
OF MODIFIED ELECTRODES
7.2.1 Procedure for Irreversible Adsorption of the Adatom
Many elements of the p-block of the periodic table spontaneously adsorb on the sur-
face of a platinum electrode when this is immersed in a solution containing a soluble
salt of the element, without an external supply of electricity [Clavilier et al., 1988,
1989a, b, 1990a, b; Evans and Attard, 1993; Feliu et al., 1988, 1991, 1993a, b;
G
´
mez et al., 1992; Sung et al., 1997, 1998]. The electrode can then be rinsed and
transferred to an electrochemical cell that does not contain the corresponding ion of
the deposited element, which remains on the surface, irreversibly adsorbed.
The process for this irreversible adsorption has not been investigated in detail. The
mechanism by which the metal is deposited has not been unambiguously elucidated,
and several possibilities have been proposed. One possibility is the formation of local
cells, with the ion of the adatom being reduced and either hydrogen [Szabo and Nagy,
1978] or platinum [Clavilier et al., 1988] being oxidized:
sol
þ
(z
z
0
)e
!
Pt
M
z
0
þ
Pt
þ
M
z
þ
cathode
ads
(7
:
1)
Pt-H
!
Pt
þ
H
þ
þ
e
Pt
þ
H
2
O
!
Pt-OH
þ
H
þ
þ
e
anode
where z
0
, the charge retained by the adsorbed adatom, can be zero in some cases. The
potential of the anodic reaction should be lower than the potential for the reduction of
the adatom, if the local cell is to be spontaneous. For hydrogen oxidation, this con-
dition is plausible, although spontaneous deposition of adatoms also occurs even
when the surface is not expected to be pre-covered by hydrogen. In this regard, it is
worth mentioning that adsorbed Pb and Ge species on Pt electrodes do not remain
stable on the surface unless the rinsing step is avoided or it is done using water in equi-
librium with H
2
/Ar [Clavilier et al., 1990b; G ´ mez et al., 1992]. On the other hand,
the possibility that Pt-OH is formed at low enough potentials is unexpected, especially
in the case of Pt(111) surfaces. In this regard, it has been proposed that the surface
oxidation could take place at defect sites [Clavilier et al., 1988].
In most cases, the adatom deposition is reductive, as sketched in (7.1), because the
adatom precursor in the solution is in an oxidized state. However, oxidative adsorption
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