Environmental Engineering Reference
In-Depth Information
role that water plays in electrocatalytic processes. For example, water molecules
are co-adsorbed with CO on noble metal surfaces surfaces, and act as the source
of surface oxygen-containing species in the electrochemical oxidation of CO.
Moreover, it has been proposed that co-adsorbed water screens lateral repulsions
within the CO adlayer and stabilizes it [Chang et al., 1990; Zou et al., 1999;
Pronkin and Wandlowski, 2004].
Another type of model electrode uses multilayer electrolytic deposits, which
attracted the interest of electrochemists long before physical methods for their struc-
tural characterization were introduced. These electrodes were usually characterized
by their roughness factors rather than particle size, the former being of the order
of 10
2
-10
3
(for original references, see the review [Petrii and Tsirlina, 2001]).
Multilayer electrolytic deposits have very complex structures [Plyasova et al., 2006]
consisting of nanometer-sized crystallites joined together via grain boundaries, and
hence have very peculiar electrocatalytic properties [Cherstiouk et al., 2008]; they
will not be considered further in this chapter.
Summing up this section, we would like to note that understanding size effects in
electrocatalysis requires the application of appropriate model systems that on the one
hand represent the intrinsic properties of supported metal nanoparticles, such as small
size and interaction with their support, and on the other allow straightforward separ-
ation between kinetic, ohmic, and mass transport (internal and external) losses and
control of readsorption effects. This requirement is met, for example, by metal par-
ticles and nanoparticle arrays on flat nonporous supports. Their investigation
allows unambiguous access to reaction kinetics and control of catalyst structure.
However, in order to understand how catalysts will behave in the fuel cell environ-
ment, these studies must be complemented with GDE and MEA tests to account for
the presence of aqueous electrolyte in model experiments.
15.4 DEPENDENCE OF ADSORPTION PROPERTIES
ON PARTICLE SIZE
The structural sensitivity of chemisorption at the interface between metal electrodes
and liquid electrolytes has been widely documented [Markovic and Ross, 2002].
Among the most thoroughly investigated processes are hydrogen, oxygen, CO and
anion adsorption/desorption on low and high index Pt single-crystalline electrodes.
Owing to the high structural sensitivity of hydrogen (H
UPD
) and coupled hydrogen/
anion adsorption/desorption, it has become common practice to use the potentiody-
namic responses of Pt electrodes in liquid electrolytes to identify their surface crystal-
lography. The validity of this approach has been recently verified by combined
electrochemical (in situ) and transmission electron microscopy (ex situ) characteriz-
ation of preferentially oriented Pt nanoparticles [Solla-Gullon et al., 2006] of size
d
4 nm. For smaller nanoparticles, the applicability of this approach may be ques-
tioned, since decreasing the particle size below about 3 - 4 nm may not only result in
variations in the contributions of facets, edges, and vertices to the surface, but also lead
to changes in adsorption energies.
Search WWH ::
Custom Search