Environmental Engineering Reference
In-Depth Information
3.5 MODEL DETAILS
The results shown above build on a thorough investigation of water interaction and
effects related to the electrical field in the double layer near the surface. While these
details are important when doing the simulations, they are less important for the under-
standing of the results presented above. We have therefore chosen to present them here
at the end of the section.
3.5.1 Water Interaction
As mentioned above, interaction with water may affect the adsorption energy,
especially for species that form hydrogen bonds. The most accurate way of including
the effect of water is to explicitly add water molecules into the simulations. At the
temperatures and pressures relevant for an electrochemical experiment, the water-
containing electrolyte will be liquid. However, since in this context we are mainly
interested in the effect of water on adsorption energies and not so much the actual
structure of liquid water itself, we can probably simplify the problem.
Instead of adding liquid water, we add a water bilayer. The water bilayer on Pt(111)
has been studied extensively in surface science [Thiel and Madey, 1987; Henderson,
2002; Ogasawara et al., 2002]. The water tends to form an ice-like layer structure
oriented in hexagonal planes parallel to the surface, with a coverage roughly corre-
sponding to two-thirds of a monolayer (Fig. 3.13). The vertical hydrogen bonds in
the water layer can either point the hydrogen towards or away from the surface. At
zero potential, the two structures are very close in energy. However, the relative
energy of the two types of water bilayers can change, depending both on the strength
of the electric field and on which other adsorbates are present [Rossmeisl et al., 2006].
From the analysis described above, we now know that a very important molecule
that may be adsorbed together with water is OH. Also, this system has been studied
quite extensively within surface science [Thiel and Madey, 1987; Bedurftig et al.,
1999; Clay et al., 2004; Karlberg and Wahnstrom, 2005]. It appears that a mixed
water22OH system forms a hexagonal structure much like the water structure dis-
cussed above (see Fig. 3.13c, d). Both from DFT calculations and UHV experiments,
the most stable structure appears to be that where every other molecule is water
and every other OH. This is interesting, since it coincides with the electrochemical
observation, discussed above, where the maximum OH coverage was measured to
be about one-third of a monolayer [Stamenkovic et al., 2007a].
With only small variations, water interaction appears to be constant for the different
metals. From Fig. 3.14, which is based on data from [Karlberg, 2006], we can see that
the interaction with water shifts the OH
binding energy down by an approximately
constant amount. Another important point to note is the linear scaling relation between
the binding of O
and the binding of OH
. Interestingly, it appears that the correlation
becomes better in the presence of water. The reason for this improvement is that in the
water/OH layer, OH always binds on top of a surface metal atom. Without water, OH
can sit on top, as a bridge, or in a hollow, depending on the metal, which results in
some scatter in the linear relation in Fig. 3.14. This is an example where trends give
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