Environmental Engineering Reference
In-Depth Information
description of the coverage-dependent adsorption behavior and an overview of pre-
vious work can be found in [McEwen and Eichler, 2007]. The interaction between
CO and Ru(0001) is characterized by an initial adsorption energy of 165 - 175 kJ
mol
21
p
)R308 CO adlayer [Pfn ¨r et al.,
1983]. The latter adlayer desorbs in a peak at 450 - 480K [Buatier de Mongeot
et al., 1998; Pfn ¨ r et al., 1983; Kostov et al., 1992]. When the CO coverage is increased
to 0.67 ML, a low temperature shoulder develops in the TPD spectra at 400 - 410K
[Buatier de Mongeot et al., 1998; Pfn ¨r et al., 1983; Kostov et al., 1992], reflecting
a reduced adsorption energy of 120 kJ mol
21
at these higher coverages [Pfn ¨ r et al.,
1983]. Coadsorption of CO and hydrogen or CO and oxygen leads to the formation
of mixed adlayers:
up to saturation of a 0.33 ML (
p
(i) a CO
ad
þ
H
ad
adlayer (stable at T , 400K [Peebles et al., 1982])
(ii) a 2CO
ad
þ
O
ad
adlayer (T , 220K [Schiffer et al., 1997])
(iii) a CO
ad
þ
O
ad
adlayer (T , room temperature [Kostov et al., 1992])
(iv) a 2O
ad
þ
CO
ad
adlayer (T , 330K [Kostov et al., 1992; Narloch et al., 1994])
Adlayers containing CO
ad
and O
ad
were found to be ordered and intermixed, whereas
CO
ad
and H
ad
segregate into islands [Peebles et al., 1982; Ciobica et al., 2003;
Riedm ¨ ller et al., 2002]. H
ad
is destabilized by coadsorbed CO
ad
(via compression
of the H adlayer) [Diemant et al., 2003; Peebles et al., 1982], and the same is true
for CO
ad
in the presence of O
ad
(via repulsive CO
ad
-O
ad
interactions) [Schiffer
et al., 1997]. Oxygen desorption sets in only at T . 800K [B ¨ttcher et al., 1997],
where CO
ad
has already left the surface. The strong adsorption bond of O
ad
makes
the Ru(0001) surface essentially inactive for CO oxidation under UHV conditions
[Kostov et al., 1992]. CO oxidation was observed only at high O
2
pressures and
elevated temperatures [Peden and Goodman, 1986], where active, oxygen-rich surface
phases can be formed [Over and Muhler, 2003; Blume et al., 2006; Goodman et al.,
2007; Over et al., 2007].
A similar inhibition was found also for electrochemical CO oxidation. In CO
ad
stripping experiments, numerous potential cycles up to 1 V were necessary to
remove all CO
ad
from a smooth Ru(0001) surface [Zei and Ertl, 2000; Lin et al.,
2000; Wang et al., 2001]. CO bulk oxidation experiments under enforced mass trans-
port conditions on polycrystalline Ru [Gasteiger et al., 1995] and on carbon-supported
Ru nanoparticle catalysts [Jusys et al., 2002] led to similar results. Hence, CO
ad
can
coexist with nonreactive OH
ad
or O
ad
species on Ru(0001) at lower potentials (E ,
0.55 V) [El-Aziz and Kibler, 2002].
Based on electrochemical experiments combined with ex situ analysis by AES,
LEED, and RHEED, Wang et al. (2001) suggested the formation of a (2
2)
(2CO
þ
O) adlayer on Ru(0001) at E ¼ 0.2 V in CO-saturated HClO
4
, similar to
the phase formed in UHV after CO adsorption on a (2
2)O-covered surface
[Schiffer et al., 1997]. From the total charge density transferred after a potential
step to 1.05 V in a CO-free electrolyte, they concluded that only 60% of the CO con-
tent in such an adlayer can be oxidized under these conditions [Wang et al., 2001].
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