Chemistry Reference
In-Depth Information
Table 4.1
Possible species in solution and adsorbed
at the surface of a glassy-carbon electrode that can
occur and take part in the oxidation of hydrogen
peroxide in alkaline solution
In solution
Adsorbed at the electrode surface
H
2
O
OH
·
HO
2
-
HO
2
-
OH
-
HO
2
·
O
2
O
2
·
-
O
·
O
-
presence in homogeneous and electrochemical reactions of hydrogen per-
oxide in a sufficiently alkaline environment
73-81
.
It is generally accepted that, except for HO
2
-
, none of the particles
represented in the right-hand column of the table occur in the solution; it
generally concerns somewhat reactive or unstable short-living components.
Since they are present only in the vicinity of the electrode-solution inter-
face, they are considered to be adsorbed components.
The following step examines the reactions that are theoretically possible
when making use of the reacting and reaction products presented in Table
4.1, including decomposition reactions of every particle separately and reac-
tions where identical particles react with each other. In this work, a number
of restrictions will be observed. First, it was presumed that no more than
two particles react with each other. Furthermore, it was assumed that no
intermediary reduction steps occur in the global oxidation reaction consid-
ered, and that no more than one electron is transferred in every sub-stage.
Under these conditions and making use of the components from Table
4.1, mechanism pairs were tentatively established, logically starting with the
simplest pair possible. Subsequently, in both mechanisms, every sub-stage
of the postulated reaction sequences is considered as the RDS, and it is
verified if the theoretically predicted transfer coefficient equals 0.5 and if
the reaction orders of hydrogen peroxide and OH
-
are situated in the range
required by the assumed hypothesis. As has already been mentioned, the
value of the transfer coefficient can serve as a criterion only in the poten-
tial range limited by ca.
E
=-0.10 V and
E
= 0.20 V vs. SCE. If the postulated
conditions are met, it is verified if a combination of both mechanisms can
lead to an explanation of the experimental evidence.
Initially, it was assumed that the reaction sequence had to start with the
reaction:
¨
(
)
+
(
)
HO
-
+
O
HO
2
•
O
-
[4.5]
2
2
2
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