Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
D
C
H 2
D +
2e -
C - H +
H +
Figure 26: Reduction catalyst for the multi-electron reduction of protons (water).
According to Bolton, systems for the reduction half reaction can be subdivided into
photochemical systems, semiconductor systems [48, 59] , photobiological systems [60] , hybrid systems
and thermochemical systems. [61] The different approaches include heterogeneous electrocatalysis
at electrode surfaces (e.g. platinized platinum electrodes). Examples for heterogeneous catalysis by
oxidic bulk materials in colloidal form or deposited on electrodes are metal oxides such as RuO 2 ,
IrO 2
[62]
, PtO 2 , PdO 2 or Fe 2 O 3 and supported materials like RuO 2 + IrO 2 on zeolite. Furthermore,
photocatalysis with colloid metals and metal powders of Ir, Pt, Ni, Au, Ag, Pt, partially in situ
generated from suitable salts like K 2 PtCl 6 as well as core/shell-structured nanoparticles (noble
metal/metal oxide core and Cr 2 O 3 shell) is known.
[29, 48]
The biological example catalysts for water reduction can be found in hydrogenases, enzymes
found in several microorganisms such as bacteria, protozoa or fungi. [63, 60, 64] Three types of
hydrogenases with different functions are known to date: iron-only-hydrogenases ([FeFe]),
nickel-iron-hydrogenases ([NiFe]) and the so called “metal-free”-hydrogenases ([Fe]). Iron-onlyhydrogenases
which mediate the formation of hydrogen from protons and electrons or in other
cases the back reaction with extremely high efficiency as a part of the microorganisms metabolism
are of particular importance for the reversible production and consumption of fuels. From X-
ray crystallographic and FTIR spectroscopic experiments it is known that the reaction center of
the [FeFe]-hydrogenases consists of a [Fe 2 S 2 ]-subcluster with a free coordination site for proton
reduction, linked to one or several heterocubane [Fe 4 S 4 ]-subclusters with intermediate electron
storage capacity (compare figure 27). [63] Impressive is the precisely fine-tuned, structurally and
electronically flexible ligand environment around the dinuclear reaction site which exhibits
mobile and soft carbonyle, cyanide and thiolate ligands, predominantly to stabilize low spin
configuration and low oxidation states of the iron centers and to facilitate the binding and
formation of hydrogen or protons. [60] |38|