Environmental Engineering Reference
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Vogel, 1998; Iwasita et al., 2000; Hoster et al., 2001a, b; Crown et al., 2001, 2002;
Herrero et al., 1999; Brankovic et al., 2001a, 2002b; Markovic and Ross, 2002].
Most of the experimental model studies have been performed on bulk alloy substrates
[Gasteiger et al., 1994a, b; Binder et al., 1972; Hoster et al., 2001a, b; Markovic and
Ross, 2002] or on bimetallic electrode surfaces prepared by electrochemical or electro-
less deposition of Ru on Pt(111) [Stimming and Vogel, 1998; Iwasita et al., 2000;
Crown et al., 2001, 2002; Herrero et al., 1999] or of Pt on Ru(0001) [Brankovic
et al., 2001a, b, 2002a, b] substrates, respectively. These bimetallic electrodes are
characterized by a large number of small deposit islands, which, depending on the
amount of the respective material deposited, are mostly several layers high (multilayer
islands) [Stimming and Vogel, 1998; Herrero et al., 1999; Brankovic et al., 2001a].
This morphology is very different from that of bimetallic (bulk) PtRu nanoparticles,
where, depending on the pretreatment, either Pt and Ru atoms are expected to coexist
on the surface or the bimetallic core of the nanoparticle is covered by a Pt skin layer
[Nashner et al., 1997; Watanabe et al., 1999; Christoffersen et al., 2001; Han et al.,
2006]. Such morphologies can be approximated in planar model systems by depositing
a thin (sub)mono- or multilayer Pt film on a Ru substrate (Pt skin layer) [Brankovic et al.,
2001b; Hoster et al., 2004] or by deposition of controlled amounts of Pt on a Ru
substrate and subsequent controlled annealing to form a surface alloy, where the Pt is
confined to the outermost layer [Hoster et al., 2008].
Aiming at a microscopic, atomic-scale understanding of the chemical properties of
planar, bimetallic PtRu model systems, we recently investigated the interaction of CO,
H 2 and H 2 /CO with such structurally well-defined bimetallic Pt/Ru(0001) and PtRu/
Ru(0001) surfaces under ultrahigh vacuum (UHV) conditions [Buatier de Mongeot
et al., 1998; Rauscher et al., 2007]. The structure of each type of surface was quanti-
tatively characterized by scanning tunneling microscopy (STM) [Hoster et al., 2008].
Results on the interaction of the above adsorbates with Ru(0001) substrates modified
by different amounts of Pt monolayer islands or a Pt monolayer film were reported in
[Buatier de Mongeot et al., 1998; Diemant et al., 2003, in preparation] (see also
Schlapka et al. [2002, 2003]; K¨sberger et al. [2003]; Jakob and Schlapka [2007]).
Similar studies were also performed on the adsorption of CO and deuterium on
PtRu monolayer surface alloys [Diemant et al., 2003, 2008, in preparation;
Rauscher et al., 2007]. Based on atomic-resolution STM images with chemical con-
trast [Buatier de Mongeot et al., 1998; Schmid et al., 1993; Nielsen et al., 1995],
the Pt surface atoms are almost randomly distributed in the surface layer of these
samples [Hoster et al., 2008]. From these spectroscopic and structural data and their
combination with results of theoretical studies [Liu et al., 2003; Christoffersen
et al., 2001; Schlapka et al., 2003; Mavrikakis et al., 1998; Ge et al., 2001; Koper
et al., 2002; Davies et al., 2005; Greeley and Mavrikakis, 2005; Groß, 2006;
Lischka et al., 2007], we have been able to derive clear trends for the chemical prop-
erties of individual local PtRu nanostructures. The Pt-induced changes in the local
chemical properties of these surfaces have been described and discussed in terms of
(i) the geometric ensemble effect, which includes modifications in the adsorption/
reaction properties due to variations in size and composition of the local adsorption
ensemble, (ii) the electronic ligand effect, which describes modifications in the
electronic structure of the adsorption ensemble due to different neighbors, and
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