Chemistry Reference
In-Depth Information
where the reaction products are adsorbed at the surface of the electrode.
Owing to its instability, the obtained peroxide radical disintegrates into an
oxygen (O) and a hydroxyl radical (OH), which each further react. In order
to identify at least a partial correspondence between the predicted and the
experimental behaviour, it soon became clear that an atomic oxygen anion
(O
-
) needed to occur in both sub-stage sequences. In literature, it is often
assumed that such an anion is formed by the reaction:
¨
(
)
()
+
OH
∑
+
OH
-
O
-
H O
[4.6]
2
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However, the introduction of the component O
-
in the sub-stage sequences,
which happened exclusively through Equation 4.6, resulted repeatedly in
incompatibility between observation and theoretical prediction. Tens of
mechanism pairs which seemed possible on paper a priori had to be
rejected, and, for a certain time, the feeling prevailed that it would not be
possible to find a combination of two reaction mechanisms with which all
the experimental details would be compatible. The mechanisms verified and
rejected are neither listed nor elaborated here in order to keep the size of
this work within reasonable proportions.
A turning point in the compatibility between theory and experiment was
reached by inserting, in both mechanisms, a sub-stage which directly pro-
duces the O
-
anion:
¨
()
+
(
(
)
)
HO
-
O
-
OH
∑
[4.7]
2
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A stage preceding this reaction is the adsorption of hydrogen peroxide from
the solution. The OH radical formed reacts into a second O
-
, after which
both of them can react into oxygen in two stages, each time by emitting an
electron. The sequence of stages from Equation 4.8 to 4.12 will be referred
to as mechanism 1.
¨
(
)
HO
-
HO
-
k
k
[4.8]
2
2
1
-
1
¨
(
)
()
+
(
)
HO
-
O
-
OH
∑
k
k
[4.9]
2
2
-
2
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¨
(
)
()
+
OH
∑
+
OH
-
O
-
H O
k
k
[4.10]
2
3
-
3
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()
+
()
¨
(
)
+
OO
-
-
Oe
-
-
k
k
[4.11]
2
4
-
4
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¨
(
)
O
-
O
+
e
-
k
k
[4.12]
2
2
5
-
5
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where the global reaction is given in Equation 4.4.
The O
-
ions formed can react not only with each other but also with
hydrogen peroxide. In a competitive relation with mechanism 1, a second
mechanism occurs which will be referred to as mechanism 2 (Equations
4.8-4.10, 4.13 and 4.12, in this order):
