Environmental Engineering Reference
In-Depth Information
CO. At E , 0.1 V, the resulting CO adlayer blocks the formation of H upd that would
take place in a CO-free electrolyte. The absence of anodic charges for E , 0.55 V in
the positive-going scan, in contrast to the significant OH ad uptake in this potential
range in a CO-free electrolyte, indicates that CO ad also blocks the re-formation of
the OH/O adlayer. This only sets in at 0.55 V. In a potentiodynamic measurement, how-
ever, about 90% of the OH ad /O ad produced in the potential range up to 0.8 V is directly
consumed for CO ad oxidation. This can be concluded from the very small cathodic
reduction charge visible when the scan is reversed at E ¼ 0.8 V. Only for higher
anodic potential limits (E lim . 0.8 V), does the simultaneous increase of both reduction
and oxidation charge for E , 0.55 V (reduction)/E . 0.55 V (oxidation) point towards
an increasing enrichment of the high potential adlayer in OH ad /O ad . The potentials
required to attain certain OH ad /O ad coverage are much higher than in the absence of
CO. This shift is mainly attributed to kinetic effects, arising from the slow removal of
CO ad . From these data, we conclude that only on scanning the potential to E , 0.1 V
or E . 1.1 V do the respective adlayers become dominated either by CO ad or by
OH ad /O ad , respectively. When the scan direction is reversed, mixed adlayers start to
form at E ¼ 0.55 V in either direction. As a consequence of the strong interaction of
their constituents with Ru(0001), these mixed adlayers show little reactivity for CO 2 for-
mation. This agrees well with results of the CO ad stripping experiments in base electro-
lyte discussed above [Wang et al., 2001]. Based on electrochemical data alone, however,
an unambiguous interpretation of the complex oxidation and reduction peak patterns to
CO 2 and/or OH ad /O ad formation is not possible. For this purpose, DEMS measure-
ments under continuous electrolyte flow are planned for the future.
In conclusion, cyclovoltammetric results in CO-saturated electrolyte demonstrate
that CO oxidation takes place only for E . 0.55 V, and this with very low rates
[Wang et al., 2001; Brankovic et al., 2002a]. Assuming a similar reaction mechanism
for CO oxidation on Ru(0001) as for the extensively studied reaction on Pt surfaces
[Santos et al., 1991; Lebedeva et al., 2000; Shubina et al., 2004], the overall oxidation
of adsorbed CO proceeds via
2H 2 O þ A(Ru) ! OH ad (Ru) þ H 3 O þ þ e
[see (14 : 4a)]
CO ad (Ru) þ OH ad (Ru) rds
COOH ad (Ru) þ A(Ru)
(14 : 9)
COOH ad (Ru) þ H 2 O fast
CO 2 þ A(Ru) þ H 3 O þ þ e
(14 : 10)
A(Ru) þ CO ! CO ad (Ru)
(14 : 11)
where COOH ad may be a stable reaction intermediate or a transition state. The empty
Ru site, A(Ru), must result from fluctuations in the CO adlayer and/or from displace-
ment of CO ad at the high coverage (low adsorption energy), similar to exchange of
CO ad in a CO adlayer at room temperature [Heinen et al., 2006].
If O ad instead of OH ad is the reactive oxygen species for CO oxidation, (14.4a)
would be followed by (14.5), and CO 2 would then be formed directly via
CO ad þ O ad rds
CO 2
(14 : 12)
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