Page 7 - Shimadzu Journal vol.4 Issue1

New Energy

Cutting-Edge Development of Artificial Photosynthesis
That Drives Reduction of Carbon Dioxide

Osamu Ishitani, PhD, Graduate School of Science and Engineering, Tokyo Institute of Technology

Significance of Researching Artificial Photosynthesis If it were possible to use CO2 as a reservoir for solar energy and build a
system that efficiently generates products that could be used as a carbon
The nuclear accident at the Fukushima Daiichi nuclear power station source, then all three of these grave problems could be solved at the
following the Great East Japan Earthquake has shaken the core of same time. Consequently, it would be desirable to build a man-made
Japan's energy policy. Furthermore, increasing our dependence on fossil system that essentially serves the same function as photosynthesis in
resources will only lead to accelerating the depletion of available fossil green plants, which converts light energy into chemical energy. In
resources, which is also the most important raw material for the chemical environments unsuitable for growing plants, creating an artificial
industry. We cannot ignore the fact that, even now, over 20% of photosynthesis system using technology that uses solar energy to
petroleum is consumed as a raw material for chemicals (Fig. 1). In other immobilize CO2 as a resource would provide an ideal solution for the
words, a depletion of our fossil resources will directly lead to a shortage depletion of fossil-based resources and global warming.
in carbon resources.

Furthermore, the increasing concentration of CO2 in the atmosphere has Functions Required for Artificial Photosynthesis
become a major cause of global warming. In response to global pressures
to significantly reduce CO2 emissions, Japan is actively involved in Photosynthesis in plants involves a system combining complicated
proposing countermeasures, but still has not found a fundamental organs that is too difficult to recreate artificially. Therefore, a
solution. technique of isolating the essential functions and then developing a
Nevertheless, it is clear that the three main problems casting a dark system able to perform those functions has been used. Fig. 3 shows a
shadow on the future of mankind, namely a shortage of energy diagram that summarizes the functions necessary for artificial
resources, a shortage of raw materials for carbon compounds, and global photosynthesis. The important functions are summarized as follows:
warming, are all inevitably caused by the energy and chemical industries
1. Capture light with high efficiency: Due to the low photon flux of
being totally dependent on fossil-based resources. Therefore, solutions
sunlight, if a molecular photocatalyst is used, then a system to
for inhibiting CO2 emissions, and developing new energy systems and
efficiently capture and intensify the light (light-harvesting antenna) is
new carbon sources must be proposed in a complementary manner.
required.
As shown in Fig. 2, plant photosynthesis creates a carbon cycle that 2. To use visible light, which has relatively low energy, to drive
contributes to long-term stability of the Earth's atmospheric elements. chemical reactions that require high energy, a Z-scheme electron
However, humans have used large quantities of the fossil resources, transport system must be used. In other words, a system that absorbs
formed from the accumulation of organic matter underground, as raw two photons successively and causes electron transfer is required.
materials for chemicals and fuels. Most of the fossil resources used by
3. An oxidative catalyst that captures four electrons from two water
humans are eventually burned, which has resulted in discharging CO2
molecules is required.
into the atmosphere so fast that it is dramatically changing its
concentration in the atmosphere. Furthermore, we currently have no 4. A catalyst that causes efficient multi-electron reduction of carbon
methods available for immobilizing the CO2 and reusing it as a resource. dioxide is required.

Raw material for
Chemicals
21.0ˋ
Automobile, et al.
40.7ˋ
Household use,
Business use
14.6ˋ
Electric Industry;
Power Agriculture:
city gas Fishery
10.1ˋ
13.6ˋ
Fig. 1 Petroleum Classification by Intended Usage: Fig. 2 Photosynthesis and Artificial Photosynthesis
Prepared from Japanese Oil Federation Data, Oil Industry Today, 2009

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