Environmental Engineering Reference
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from
Escherichia coli
and CODH I
Ch
are both adsorbed onto conducting graphite
platelets, formed by grinding a piece of pyrolytic graphite. In microorganisms,
CO-reduced CODH I
Ch
produces electrons that are transferred one at a time via a
ferredoxin to a membrane-bound hydrogenase for H
2
evolution [
48
]. In the system of
two enzymes on the graphite platelets in aqueous suspension, CO-reduced CODH
releases electrons to Hyd-2 for H
2
evolution via conduction across the graphite
platelets. The turnover frequency is 2.5 s
1
at 30 ºC, which is comparable to catalysts
operated at high temperatures in industry. Results are shown in Figure
11
.
Figure 11 Water gas shift reaction catalyzed by Hyd-2 hydrogenase from
Escherichia coli
and
CODH I
Ch
co-adsorbed on conducting graphite platelets. Plot shows production of H
2
(blue line) and
depletion of CO (red line) over the course of 55 h, with fresh CO injections of 600
μ
L at the points
indicated. Reprinted with permission from [
59
]; copyright 2009 American Chemical Society.
The development of efficient photo-catalysts for CO
2
reduction plays an
important role in converting CO
2
to carbon-based fuels in a clean and sustainable
way. Experiments have been carried out in which CODH I
Ch
is attached to semicon-
ductor nanoparticles, such as TiO
2
and CdS, to study the visible light-driven
reduction of CO
2
to CO as illustrated in Figure
12
[
61
,
62
]. For TiO
2
(anatase)
the bandgap is 3.1 eV which lies in the UV region, so a Ru-photosensitizer was
co-attached to the nanoparticle along with CODH I
Ch
. Upon illumination, the
turnover frequency for CO production is 0.14 s
1
at pH 6 and 30
C. In the CdS
system, a photosensitizer is not necessary because the band gap lies in the visible
region. For the hybrid CODH-CdS nanorod system, the turnover frequency for CO
production is 1.23 s
1
[
62
].
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