Environmental Engineering Reference
In-Depth Information
In the reverse anodic scan, similar arguments hold true for the removal of the H
upd
adlayer in peak A, which is likely to proceed in a comparable two-step process:
H
upd
þ
H
2
O
!
H
3
O
þ
þ
e
þ
A
(14
:
3)
2A
þ
H
2
O
!
OH
ad
þ
H
upd
(14
:
4)
Also in this case, a heterolytic reaction according to
A
þ
2H
2
O
!
OH
ad
þ
H
3
O
þ
þ
e
(14
:
4a)
would be possible as well, and cannot, on the basis of our data, be excluded for reac-
tion on a pure Ru(0001) electrode.
Because of the high stability of the H
upd
adlayer, the first step (14.3), which is
required for nucleation of a vacancy in the closed H
upd
adlayer, starts only at potentials
considerably above the equilibrium potential for H
upd
!
OH
ad
exchange, at about
0.15 V. With increasing potential, H
upd
is replaced by OH
ad
[Reactions (14.3) and
(14.4) or (14.4a)] and finally OH
ad
by O
ad
according to
OH
ad
þ
H
2
O
!
O
ad
þ
H
3
O
þ
þ
e
(14
:
5)
Only at E ¼ 0.45 - 0.5 V, do the CVs obtained for low (,0.1 V) and higher (0.1 V)
cathodic scan limits match again, indicating that at this point the respective surface
states are identical, independent of the previous treatment. Most simply, at this
point, O
ad
formation via Reactions (14.3), (14.4), and (14.5) is completed, and similar
amounts of O
ad
(0.75 ML; see the next paragraph) are deposited. (About 1.5 ML OH
ad
would be possible as well based on the charge, but this is not consistent with other
observations discussed above.)
In this picture, the kinetic barriers hindering the exchange between the two adlayers
are related to the presence of metastable, but rather strongly bound, adsorbed species
(H
upd
and OH
ad
), which cannot be removed easily, and which block the surface for
adsorption of the respective other species. The nonequilibrium situation is also
reflected in the shape of the corresponding peaks A
0
and A, where the anodic one
(A) is less sharp and extends over a larger potential range.
The symmetric pair of voltammetric peaks in the Ru(0001) base CV in the range
0.1 - 0.3 V ( peaks B and B
0
), which is best seen for a lower potential limit of
E
min
¼ 0.1 V, is tentatively assigned to (14.5), which can run reversibly in both
directions. This assignment is based on the assumption that the surface is covered
by 0.5 ML O
ad
at 0.3 V. Only for more negative potential limits, when OH
ad
is further
reduced to H
2
O and replaced by H
upd
according to (14.1) and (14.2), does the
re-oxidation of the adlayer require H
2
O dissociation according to (14.3) and (14.4).
This provides a simple explanation why the pronounced hysteresis between OH
ad
removal ( peak A
0
) and reformation of OH
ad
/O
ad
(peak A) is only observed when
the potential is scanned to E , 0.1 V.
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