The Effect of Structurally Well-Defined Pt Modification on the Electrochemical and Electrocatalytic Properties of Ru(0001) Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 474

to temperatures T 850K leads to Pt - Ru intermixing [Hoster et al., 2008, 2009b],
and brief annealing to 1300 - 1350K results in the formation of atomically dispersed
monolayer surface alloys with an essentially random distribution of the two com-
ponents in the surface layer (see the atomic-resolution STM images in Fig. 14.3b
and [Buatier de Mongeot et al., 1998; Diemant et al., 2003; Hoster et al., 2008]).
Owing to a highly negative surface segregation energy of Pt impurities in Ru
[Christensen et al., 1997; Ruban et al., 1999] and a barrier for Pt diffusion into
deeper Ru regions, which must be significantly higher than that for Pt exchange
with Ru surface atoms [Hoster et al., 2008], Pt - Ru intermixing is largely confined
to the outermost layer under these conditions. Based on quantitative STM measure-
ments, the loss of Pt into deeper layers is negligible up to Pt surface contents of 0.8
under these conditions, and less than 10% up to 1 ML Pt pre-deposition [Hoster
et al., 2008]. Representative high-resolution STM images illustrating the atom distri-
bution in Pt x Ru 12x /Ru(0001) surface alloys with five different Pt contents and the
base CVs obtained on them are presented in Fig. 14.8. (Note that the surface in
Fig. 14.8e is the same as that used for recording the CV in Fig. 14.4.)
TPD measurements characterizing the desorption of deuterium from Pt x Ru 12x /
Ru(0001) monolayer surface alloys revealed a continuous decrease in the deuterium
desorption barrier with increasing Pt coverage [Diemant et al., 2003, 2008]. This
was attributed to an increasingly weaker binding of D adatoms to Pt n Ru 32n threefold
sites (adsorption ensembles)—or to Pt n Ru 52n pentamers, when considering the
recombinative desorption of adjacent deuterium adatoms—with increasing number
of Pt surface atoms in the respective adsorption ensembles [Diemant et al., 2008].
CO desorption experiments on the same type of surfaces, on the other hand,
showed two dominating adsorption states at high and low temperatures, which were
assigned to atop CO adsorption on Ru and Pt surface atoms, respectively [Rauscher
et al., 2007]. Both states were destabilized with increasing Pt content, which was
rationalized by electronic ligand and strain effects [Rauscher et al., 2007]. This
Pt concentration-dependent destabilization is, however, less pronounced than the
modification introduced by mixed adsorption ensembles for deuterium adsorption.
Hence, for both probe molecules, the surface alloys offer adsorption sites with inter-
mediate stability that exist neither on Ru(0001) nor on pseudomorphic Pt/Ru(0001)
monolayers and that affect the CO and deuterium adsorption behavior in a distinct
way. As will be shown in the following, similar effects are also observed in electroche-
mical experiments.

Electrochemical Properties All CVs are presented on two different scales to show
both the larger and smaller peaks in sufficient detail. At low Pt surface concentrations,
the base CVs are very similar to those of the Pt island-modified Ru(0001) surfaces (see
Fig. 14.5). With increasing Pt surface content, however, the charge in the H upd $
OH ad exchange peaks A and A 0 at about 0.1 V decreases much faster than linearly.
A faster than linear decay of the charge in this peak would be expected if Ru ensembles
with more than one Ru atom were required for OH ad and/or H upd adsorption. Since the
atom distribution in Pt x Ru 12x /Ru(0001) surface alloys is very close to a random
distribution [Hoster et al., 2008], the number of Ru n sites is proportional to x R n or
(1 2 x Pt ) n . As is evident from the plot in Fig. 14.6, the experimental data agree very