Environmental Engineering Reference
In-Depth Information
property that has been revealed in the last few decades as a fundamental parameter
with paramount influence on the reactivity of a metal. In some cases, not only can
the surface structure of the substrate material be finely controlled, but so can the
location of the deposited species, achieving a regular distribution of adatoms that
allows the study of structural parameters in great detail.
There are two main approaches to modifying the surface composition of an elec-
trode with foreign metal adatoms. First, underpotential deposition occurs when
there is equilibrium between the interphase and the solution containing the metallic
precursor [Herrero et al., 2001; Szabo, 1991]. Underpotential deposition of a metal
differs from bulk deposition in that the former corresponds to the formation of only
one layer (sometimes two layers) of the metal on a foreign substrate. This process
can take place at potentials more anodic than the Nernst potential for bulk deposition,
owing to the existence of different bonding interactions between the metal and the sub-
strate, stronger than those between the metal atoms in the bulk material. The existence
of equilibrium between the precursor in solution and the adatom on the surface implies
the existence of a relationship between electrode potential and surface coverage. This
can be either an advantage or a disadvantage, depending on the desired experiment.
On the one hand, this relationship allows precise modification of surface composition
simply by selecting the electrode potential. In addition, the existence of equilibrium
allows the use of thermodynamic relationships to characterize the composition of
the adlayer. The clear disadvantage is that the surface composition depends on the
potential, and therefore it is not easy to separate the effect of potential from the
effect of coverage when electrocatalytic activity is investigated in the presence of
the precursor in solution. Moreover, coverage by adatoms will depend on solution
composition, and, although it can be well defined in a solution of simple composition,
characterization of coverage is usually very difficult in the presence of other reductant
or oxidant compounds in solution. This is especially complex when these compounds
adsorb on the electrode, which is the most familiar case with compounds that undergo
electrocatalytic processes.
The second procedure for surface modification of an electrode is irreversible
adsorption of adatoms. Under certain conditions, some adatoms are adsorbed, and
then remain on the surface of the electrode even in the absence of the precursor in sol-
ution [Clavilier et al., 1988, 1989a, 1990a, b; Evans and Attard, 1993; Feliu et al.,
1988, 1991, 1993a, b; G´ mez et al., 1992; Sung et al., 1997, 1998]. In this case,
there is no equilibrium between the species on the surface and the solution—hence
the terminology irreversible. The advantage of this method is that coverage and
potential can be varied independently. This constitutes a distinctive advantage over
underpotential deposition, and therefore most of studies of electrocatalysis with
modified surfaces that have been performed recently are based on this method
[Clavilier et al., 1989b, c; Climent et al., 1998; Fern´ndez-Vega et al., 1989, 1991;
Gomez et al., 1993, 1997; Herrero et al., 1993, 1995a - d, 1996; Leiva et al., 1997;
Llorca et al., 1994; Maci´ et al., 1999, 2001, 2002, 2003; Schmidt et al., 2000a,
2001; Smith and Abru˜ a, 1999a, b; Smith et al., 1999, 2000].
Because of the very high reactivity of platinum in many reactions of technological
interest, such as those in fuel cells, and the well-demonstrated requirement for control
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