Page 8 - Shimadzu Journal vol.4 Issue1

New Energy

Light

OER specie OER specie

Fig. 3 Functions Required for Artificial Photosynthesis Fig. 4 Reaction Mechanism of Photocatalytic Reduction of Carbon Dioxide

(3)
CO2 is the most oxidized form of carbon compound, so reduction is by laser spectroscopy . In other words, the excited state of the MLCT
important for its activation. In the photoelectron-transfer reaction, triplet from the rhenium complex generated by light absorption
which is an important step in artificial photosynthesis, absorption of quenches the electron transfer by the reducing agent, so that the
one photon, in principle, drives transfer of only one electron. one-electron reduced (OER) specie of the rhenium complex
I
However, reducing CO2 by one electron is not practical because it [Re(bpy )(CO)3X] is generated. Then the monodentate ligand X -
–•
–
requires a high electric potential of -1.9 V vs. NHE and it also desorbs from that OER specie and is replaced by a CO 2 ligand. In the
generates unstable CO2 compounds. Therefore, there have been case of Re-CN, the OER specie is relatively stable, so that no
–•
-
attempts to build photochemical multi-electron reduction systems. For desorption of CN occurs at all, which is what caused it to show no
example, the two-electron reduction of CO2 can be used to form photocatalytic activity. As mentioned above, reducing CO 2 to CO
industrially useful CO and formic acid. The equilibrium potential of requires two electrons, where the second electron is supplied by the
that reaction is about 1.3 V lower than one-electron reduction. OER specie from another molecule. After desorption of the CO, the
Consequently, achieving practical photoreduction of CO2 requires a photocatalyst is regenerated by the monodentate ligand X, that was
system for causing multi-electron reduction. released into the solution, reattaching to the complex (Fig. 2). Re-SCN
One-electron reduction species of metal complexes, generated in an showed relatively high photocatalytic activity because both desorption
-
excited state achieved when a metal complex absorbs light, or by and reattachment of the SCN ligand from/to the OER specie occur
photoreaction in the presence of a reducing agent, have a strong efficiently. In the case of Re-Cl, on the other hand, the efficiency of
reducibility. For example, in the typical case of a redox photosensitizer the Cl- ligand reattachment is low, which resulted in a lower
[Ru(bpy) 3 ] (bpy = 2,2'-bipyridine) used to drive the electron transfer photocatalytic efficiency .
(2)
2+
by photoexitation, the electric potential of one-electron oxidation at
the ground state is E1/2 = 1.53 V vs. NHE, but -0.59 V if photoexcited. Elucidating the photocatalytic reaction mechanism in detail often
Therefore, obtaining excitation energy significantly increases its provides very valuable information for improving the photocatalyst
reducing power. In addition to this redox photosensitizer for driving function. These results provided the following indicators necessary for
photoelectron transfer, immobilization of CO2 by photoreduction also increasing the efficiency of the CO2 reduction reaction using a
requires a catalyst that enables the multi-electron reduction of CO2. rhenium complex photocatalyst.
(1) The reducing agent must be able to reduce the excited
Photocatalytic Function of Re(I) Complex photocatalyst and generate an OER specie with high efficiency.
J.M. Lehn et al. reported that CO was catalytically generated when a (2) The ligand must desorb from the OER specie and react with CO2
DMF/TEOA (5:1 v/v) solution containing fac-Re(bpy)-(CO) 3 Cl (Re-Cl) very quickly. An OER specie from another molecule must be able to
was exposed to light in a CO 2 atmosphere . With a CO quantum yield reduce the generated CO2 adduct quickly.
(1)
of 0.14, it was the most efficient CO 2 reduction photocatalyst at the
time. Furthermore, this photocatalyst system had the characteristic of (3) After the CO is generated, the original complex must be
regenerated highly efficiently by reattaching the desorbed ligand to
generating only minimal hydrogen or formic acid, even in the
the center of the rhenium.
presence of water. The photocatalytic activity of the rhenium complex
n+
I
fac-[Re(bpy)(CO) 3 X] is highly dependent on the type of monodentate Based on the above photocatalyst design indicators, we have been
ligand X . For example, if the monodentate ligand X is an SCN -
(2)
developing a photocatalyst system that reduces CO2 with high
(Re-SCN) complex, then the photocatalyst has an extremely efficient
-
CO 2 photoreduction quantum yield of 0.3. However, if X is Cl (Re-Cl), efficiency. Several successful examples are described below.
then the photocatalyst efficiency is about half as much, as described
-
above, and if X is CN (Re-CN), then it shows no catalytic activity at all.
Photocatalyst System Combining Two Re(I) Species
Almost all CO 2 photoreduction catalyst systems involve two coexisting
species (a photosensitizer plus catalyst). In contrast, the rhenium To satisfy condition (2) indicated above, the OER specie must be
complex is a unique photocatalyst that functions as both a unstable. However, the OER specie must be stable enough to satisfy
photosensitizer and catalyst. Consequently, it has attracted condition (3). Therefore, to solve this apparent contradiction, we
considerable attention and generated many research projects (Fig. 4). proposed a photocatalyst system that separates the two functions of
The first step of the photocatalytic reaction has been clearly identified the OER specie, specifically by mixing two rhenium complexes. The
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