Environmental Engineering Reference
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et al., 2003; Danielson et al., 1978; Schwarz, 1979; Shimizu et al., 1980; Feulner and
Menzel, 1985; Christmann, 1988; Sun and Weinberg, 1985; Jachimowski et al., 1995;
Lindroos et al., 1987; Held et al., 1992]. Hydrogen adsorbs dissociatively on Ru(0001)
with an initial sticking coefficient of s
0
¼ 0.21 and a maximum coverage of 1 ML,
with the H
ad
atoms adsorbed on the fcc-type threefold-hollow sites [Feulner and
Menzel, 1985; Lindroos et al., 1987; Held et al., 1992]. Comparing hydrogen adsorp-
tion on Ru(0001) and on Pt(111), the hydrogen adsorption energy is significantly
higher on Ru(0001) than on Pt(111). Temperature programmed desorption (TPD)
experiments and density functional theory (DFT) calculations yielded Ru - H
bond energies of 2.86 eV [Feulner and Menzel, 1985; Christmann, 1988] and
2.70 - 2.97 eV [Liu et al., 2003; Greeley and Mavrikakis, 2005], respectively. For
Pt(111), the experimental values are 2.59 eV [Gdowski et al., 1983] and 2.64 eV
[Christmann, 1988], whereas DFT calculations lead to values of 2.5 - 2.56 eV
[Liu et al., 2003; Greeley and Mavrikakis, 2005; Liu and Nørskov, 2001; Hammer
and Nørskov, 1995]. Hence, both theory and experiment predict a higher stability
of H
ad
on Ru(0001) than on Pt(111), with a difference in the range between 150
and 300 meV.
Compared with hydrogen, the adsorption of oxygen on Ru(0001) is significantly
stronger. Increasing exposures to O
2
lead to the sequential formation of (2
2)O
[Madey and Engelhardt, 1975], (2
1)O [Madey and Engelhardt, 1975], (2
2)3O [Kostov et al., 1997], and (1
1)O [Stampfl et al., 1996] phases with coverages
of 0.25, 0.5, 0.75, and 1.0 ML (or larger). Even at u
O
¼ 1, desorption of oxygen was
found to set in only at T . 800K [B ¨ ttcher et al., 1997]. Ru - O bond energies range
from 5.2 eV at u
O
¼ 0.25 to 4.5 eV at u
O
¼ 1 [Stampfl et al., 1999]. Under ambient
conditions, Ru(0001) will be covered by a (1
1)O phase, since the thermo-
dynamically
more
favorable
oxide
formation
(RuO
2
)
is
kinetically
hindered
[Assmann et al., 2003].
Water (D
2
O) formation was shown to occur in UHV even at 80 - 90K upon
exposure of (i) a Ru(0001)-(2
1)O phase to atomic hydrogen [Schick et al., 1996]
or (ii) a Ru(0001)-(1
1)D phase to atomic oxygen [Weiss et al., 1998]. In contrast,
the reaction of a Ru(0001)-(2
1)O phase with molecular H
2
(at 400 - 900K) was
very slow (with a time scale of 10 - 100 minutes), which was attributed to a hindered
dissociative H
2
adsorption [Shi et al., 1981].
H
2
O adsorbed on Ru(0001) forms hydrogen-bonded adlayers [Thiel and Madey,
1987]. According to a recent DFT study, such adlayers should be metastable against
partial dissociation [Feibelman, 2002; Weissenrieder et al., 2004]. Owing to the
presence of kinetic barriers, however, dissociation sets in only at T . 180K, in com-
petition with H
2
O desorption [Andersson et al., 2004]. The formation of O
ad
via
H
2
O
ad
$
OH
ad
þ
H
ad
$
O
ad
þ
2H
ad
is kinetically limited by the simultaneous
formation of site-blocking H
ad
, which, because of the strong Ru - H bond, does not
desorb at low temperatures [Weissenrieder et al., 2004]. Even in the presence of coad-
sorbed oxygen, H
2
desorption ranges from 130K up to 240K [Schiffer et al., 2000].
Thermodynamically, however, H
ad
is metastable against displacement by O
ad
because
of the higher Ru - O bond energy. Similar effects are expected for the formation of
OH
ad
and O
ad
in an acidic electrolyte via H
2
O dissociation, which will likewise be
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