Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
A
A -
C
4e -
C +
O 2
4H +
H 2 O
H 2 O
Figure 23: Oxidation catalyst for the multi-electron oxidation of water.
Mn 4 Ca cluster which represents the active site in the OEC to bind two water molecules.
[20, 46]
Figure 24 depicts the successive removal of protons and electrons from the Mn 4 Ca cluster with
coordinated water (S) in ten steps.
H+
2 H 2 O
S + 0
O 2
S n 0
S + 4
e - Mn4Ca
S n 3
H+ S + S + 3
S n 2
2
e - H+
e -
S + 1
H+
S n 1
e -
Figure 24: Extended S-state cycle model. Electrons and protons are removed alternately from the
Mn 4 Ca-cluster. The nine intermediate states of the Mn complex are denoted as S +/n
i
where the
subscript denoted the number of accumulated oxidizing equivalents and the superscript indicates
the relative charge: positive (+) or neutral (n) relative to the dark-stable S 1 -state. [28]
This mechanism has not been completely explained until today but it is known that it involves
the successive oxidation of separate Mn-centers within the cluster by tyr •+ upon illumination and
concerted release of protons from the manganese coordinated water molecules to the liquid phase
(see figure 25-d for the structure of the active site).
In addition to the biocatalytic water oxidation in photosynthesis, different artificial approaches
to water oxidation exist. They include heterogeneous electrocatalysis at electrode surfaces (e.g.
cobalt phosphates referred to as CoP
[47] i ), heterogeneous photocatalysis by oxidic bulk materials in
colloidal form or deposited on electrodes (e.g. metal oxides such as TiO 2 , IrO 2 , Co 3 O 4 , Mn 3 O
[48] 4 )
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