66 Trends in activity and stability
In order to better understand this trend in stability DFT calculations were
performed. The stability of the Pt overlayer is quantified in terms of dissolution
potentials vs. RHE: these are plotted in Figure 4.5 for various Pt5Ln alloys. The
surface of the Pt5Ln alloys is modeled as strained Pt slab (Pt overlayer), with a level
of strain induced by the experimental lattice constants of the bulk (see Figure 4.3).
This strained Pt overlayer is represented by periodically repeated slabs separated
by at least 12 Å of vacuum. The unit cell size of Pt was 2 2 atoms large and 4 to
5 layers thick. Two bottom layers were kept fixed, while the remaining atoms can
relax, allowing to assume their minimum energy positions. When the total energies
of the two slabs (with 4 and 5 layers respectively) are subtracted from each other,
the resulting energy corresponds to a single layer of Pt atoms in the bulk. For nonstrained
Pt this energy corresponds to a dissolution potential at standard conditions
of 1.23 V, taken as a benchmark value. The differences in energy in the case of
strained Pt slabs are referred to this benchmark value. It should be noted that the
purpose of this comparison is to illustrate a stability trend and these values should
not be directly interpreted as the experimental dissolution potentials.
Figure 4.5: Stability of the Pt overlayers as a function of lattice strain in Pt5Ln alloys
(percentage compressive strain in the overlayer relative to pure Pt), as predicted by DFT
calculations. The stability is indicated in terms of dissolution potentials vs. RHE. The Pt
overlayers are modeled as a strained Pt slab using experimental values for the lattice parameters.
Calculations and figure made by Vladimir Tripkovi´c.
Figure 4.5 clearly shows that the stability of these overlayers decreases when
higher levels of compression are applied. The more strained the Pt overlayer is, the