Environmental Engineering Reference
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Figure 3.18 Spectrum of free energies of hydrogen adsorption, DG
H
, on binary surface alloys
at T ¼ 298K. The vertical axis shows the number of elements with free energies within 0.1 eV
windows (0.0 - 0.1 eV, 0.1 - 0.2 eV, etc.). The solid vertical line indicates DG
H
¼ 0. The dashed
vertical line gives the hydrogen free energy adsorption for pure Pt. All free energies are refer-
enced to gas phase H
2
. Adapted from [Greeley and Nørskov, 2007]; see this reference for
more details.
overlayers, there are relatively few alloys with free energies close to zero. This result
may be due, in part, to the significant strain effects that are often associated with such
overlayers. These effects lead to significant shifts in the d-band centers [Mavrikakis
et al., 1998], thus resulting in extremes of DG
H
.
For screening purposes, the most important result to emerge from the data in
Fig. 3.17 is that there is a very large number of surface alloys with DG
H
values
roughly equal to zero (and hence a large number of such alloys with near-optimal
values of the HER descriptor). This fact can be seen clearly in Fig. 3.18; the distri-
bution of alloys with particular values of DG
H
is peaked near DG
H
¼ 0.
3.6.7 Stability Considerations
The large number of binary transition metal surface alloys that are found to have near-
optimal values of the HER descriptor suggests, at first glance, that a very large number
of such alloys might have promising HER activities. However, such a large number
immediately raises the question of whether or not it is realistic to expect that our
simple screening procedure could have identified so many novel HER catalysts. In
fact, this is quite unlikely to be the case, since the screening so far has not accounted
for the stability of the surface alloys in the actual reactive environments. In acidic
environments, such stability considerations could be particularly significant.
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