Literature Seminar (D3 part)

080913 (Sat.)

Wataru Itano (D3)

Molecular Catalyst for H2O oxidation / CO2 reduction~ model for oxidation (reduction) center for Artificial Photosynthesis
0. Introduction@ Photosynthesis the process that converts solar energy to chemical energy
which can be used by biological systems.

@ Solar energy

abundant, totally huge --- but difficult to use whole energy
broad spectral
max ~31%( g = 1.35eV)g< max conversion efficiency using solar light >(光化学系)(吸収端)• solar cell

( silicon ( g = 1.1eV)• photosynthesis

anntena pigment<24%(<690nm ; g = 1.8eV)(<500nm)

O2 + 2H2@ Solar Energy Conversion

2hv / 1e×

4e- / 3ATP
4e- / 2NADPH<42%

chemical

2H2O)

4e / 2H2
4e- / O2

possibility
electrolysis
変換効率
<100%

f uel cell

hv / 1e-×

semiconductor photocatalyst
エネルギー変換効率 <0.1％~low)photosynthesis
エネルギー変換効率 2％~1.0)(easy to store)(2H2 + O2--• water splitting
2H2O

<25%

<25% )

practicality

E

高出力
on demand
エネルギー変換効率 35-60％(熱を含めると80％)(difficult to store)

<10%
<8%

hv

solar cell
低出力
daytime only
エネルギー変換効率
10~20％

Photosynthesis is a good model f or converting solar energy to chemical energy
without producing CO2.
< contents >0. Introduction
1. About Photosynthesis
2. Water Oxidation by Molecular Catalyst
3. CO2 Reduction by Molecular Catalyst
4. Integrated System(p1)(p2-4)(p5-7)(p8-9)(p10-11)

1. About Photosynthesis
1-1. Green plants' photosynthesis@ In chloroplast ()(チラコイド膜,明反応,光化学系)(ストロマ,暗反応, カルビン回路)

photochemicalbiochemical

hv

hv

ATP + NADPH

CO2 + H2O

ADP + NADP+(CH2O)n + O2

chemicalchemicalpage 1

@ In thylakoid membrane(See Lit. Dr. Oisaki (M1))

参考：光合成の科学（東京大学出版）, 電子移動の化学（日本化学会）

page 2

photoenergy transduction is realized by electron transfer (ET) system.
2)

4e-

4)

1)

3)

1) Light is gathred efficiently by LHC (Light Harvesting Complex)• chlorophyll+• pheophytin• carotenoid• phycobilin etc.

P680

< Z scheme >2) In reaction center, photoenergy isstored in a charge-separation (CS)

<10ns
200ps
~1ps (10-12s)> to avoid charge recombination (CR)very fast electron transfer occurr.

~100 s<200ns~100 s<400ps~100 shv(PSII)

OEC
5ms

1ms

~300 s

4e-

~1ms

1ms
~500 s

~20ns

Pheo

3) H2O is 4 electron oxidized by OEC (Oxygen Evolving Complex , Mn-cluster) and O2 is genarated
2H2O

O2 + 4H+ + 4e- (E0 = + 0.815 V) (pH = 5, チラコイド膜内)(H+ gradient is generated
and used for ATP synthesis)

nucleophilic attack of H2O
to oxyl radical(ストロマ)(チラコイド膜内)< OEC in PSII >• Mn3CaO4Mn complex• Rate 50 moleculeO2 / s

(hv)

H+ H+
H+

Catalytic cycle of water splitting
suggested by DFT QM/MM models of the OEC of PSII.
V. S. Batista et al. (J. Am. Chem. Soc. 2008, 130, 3428.)
III

IV

Dashed arrows indicate transformations leading to the following S state in the cycle.
The circles highlight substrate water molecules.

III
III

cf) There are many possibilities.
(Chem. Rev. 2001, 101, 21) (Chem. Rev. 2006, 106, 4455)

4) NADP+ is 2 electron reduced and NADPH is generated at the end(it is used for CO2 reduction in some biochemical cycle...etc)

IVIV(E0 = -0.32 V) (pH = 8, チラコイド膜外)1-2. Toward artificial photosynthesis> omitting further biochemical energy conversions (Of course, they are interesting research areas.)> using H2O as electron donor (H2O oxidation) is reasonable.
> making useful chemical energy (for human) at the reduction center is desirable.

@ simple molecular system f or artif icial photosynthesishv1) Antenna System

-

4e

D

S

2ne-

A

2H2O
3) Oxidation
Catalyst

D

S* A(not yet ?)2H+4) Reduction
Catalyst

O2 + 4H+

CO2 + 6H+
or
CH3OH + H2O

H2
D

S

5) Integarated System

A2) Charge Separation Unit
chemical energy storage< requirement > (See Lit. Dr. Oisaki (M1))1)• possessing high absorption in visible light"Harvesting Device of Visible Light"• stable toward photo-decomposition

2)• properly long lifetime ( )"Photoenergy Storage Device Posessing CS Ability"• sufficiently high energy content• higher quantum yield ( ), higher energy conversion efficiency
concept)

Donor - Sensitizer - Acceptor system

CRCR# pioneering workby T. A. Moore, D. Gust et al(Nature 1984, 307, 630-632)
lifetime= 300ns (CH2Cl2) / 3 s (CH3CN) /( )2.5 s (CH2Cl2 sat. with nBu4NBF4)
quantum~0.04 (CH2Cl2 or CH3CN) /yield ( ) = ~0.25 (CH2Cl2 sat. with nBu4NBF4)# progressive work by S. Fukuzumi et al.

3)

2H2O

*
comparable in CS life time
to photosynthesis centerO2 + 4H+ + 4e-(section 2)

4)(section 3)

5)(section 4)

2H+ + 2eCO2 + 6e- + 6H+*(reaction itself difficult)• multi-electrons donor (acceptor)• stable under highly oxidizing (reducing) conditions

H2
CH3OH + H2O• appropriate oxidation-reduction potential at both site
• minimun energy loss
• higher energy conversion efficiency / lower cost than bio-photosynthesis totally

page 3

@ occuring reaction<oxi.>

3)

2H2O

page 4

4)<red.>+-

0

O2 + 4H + 4e (E = + 1.23 V)(pH = 0)> thermodynamically- E0oxie-(E0 = 0 V)
CH3OH + H2O (E0 = - 0.62 V)(pH = 0)H2

CO2 + 6e- + 6H+> in photosynthesis

E0red

e-

E0red

+

2H+ + 2e-

E0oxi

+

hv

cf) semiconductor photocatalyst (section 4)
simple system for water splitting by hv(2H2O"CS" stateon the surf ace

O2 + H2)• bad responce for visible light• low quantum yield ((low energy conversion efficiency)nmhv@ molecular catalyst
Homogeneous, simple catalyst is hardly applied.
a) multi-electrons donor (acceptor) at one catalytic cycleb) stable under highly oxidizing (reducing) conditions
which causes most chemical functional groups to degradec) high efficincyd) cheap, easily prepared(E0 = 0 V)

(E0 = + 1.23 V)3)are desirable(E0 = - 0.62 V)4)< CO2 red. >< H+ red. >< H2O oxi. > (section 2)• 2e reduction• nature --- Fe• M --- Co, Ni, Rh• 4e oxidation, difficult• nature --- Mn3CaO4Mn cluster• M --- Mn, Ru, Ir, Co
mono or dinuclea, aqua (oxo) complex

M (X+2)

M(X) + H+-+2e

M (X+2)

2+

or

+

H
+H+
H2
H

M(X) + 2H++2H2O

M (X+4)2+• nature --- Fe-Ni sluster• M --- Ru, Re, Cu

H
-

+2e

M (X+2)(section 3)• 1-8e reduction, very difficult

2+

H2< combining oxi. and red. > (section 4)• finding appropriate mediatorcf) mechanistic possibilities of O2 evolving from metal oxos (PNAS, 2006, 103, 15729.)(Scheme 3) Nucleophilic Attack ofHydroxide on High-Valent Metal Oxos

(Scheme 4)
Radical Coupling of Two Oxos

most general(Scheme 5)
Reductive Elimination of bis Hydroxos
or bis -Oxos

unknown

2. Water Oxidation by Molecular Catalyst
input
energyEH2O ~ smaller

hv

S*

S

"photocatalytic / photosensitized / photoinduced"

cat

2H2O

4×e

4e
-

donating efavorable

page 5oxidant"chemical"

4+O2 + 4H

lowering
activationenergycat(+) anode2-1. Water Oxidation
+

-

0

•H

H +e

H2

2H+ + 2e-

(E = - 2.2 V)(E0 = ~ 0 V)

2H2O

O2 + 4H+ + 4e-

2H2O

H2O2 + 2H+ + 2e- (E = + 1.76 V)

H2O•OH + H+ + e-

(E0 = + 1.32 V)"electrochemical / electroclysis"> Water oxidation is inherently difficult.
(a) 4-electron O2-evolving water oxidation
requires least oxidation potential(b) But it demands the loss of 4e- and 4H+ from 2H2O,
all accompanied by the formation of an O-O bond0

Even plants find this task difficult:under ambient sunlight in the chloroplast, the OEC (oxygen-evolving complex)
must be resynthesized every half an hour or soowing to the oxidation damage it suffers from O2 that it has produced.

(E0 = + 2.72 V)(at pH = 0, vs NHE)

2-2. Pioneering work
T. J. Meyer et al. (J. Am. Chem. Soc. 1982, 104, 4029) (J. Am. Chem. Soc. 1985, 107, 3855)

oxo-bridged dinuclea Ru

(III)

-O2

(a few eq.)

TONO2 2~3 (loses its catalytic ef f iciency af ter just a f ew cycles.)catalyst (0.1M)
pH ~0.7 (aq.H2SO4) , CAN as oxidant (Y Ce<30%)

aq.HClO4
cf)

O2 + 4H+ + 4e-

2H2O
Ce4+ + e-

2-3. Recent work

4-H+

(1 eq)(~25 eq)+Ce(IV)-Ce(III)

(III)
(III)
u-O-

(cat. cycle)(III)(E0 = + 1.23 V) (pH = 0, vs NHE)
only capable of water oxidation very slowly

Ce3+(E0 = + 1.72 V)

Review (Angew. Chem. Int. Ed. 2008, 47, 5888)

oxo- or bridged, bi-, and multinuclear transition metal complex> Ligand-Ru2 R. P. Thummel et al. (J. Am. Chem. Soc. 2005, 127, 12802) (Inorg. Chem. 2008, 47, 1835)
two Ru with well-defined geometry(for 2b)(II)

TONO2 572 (48h)catalyst (6.6 10-5M)
pH ~1 (aq. CF 3SO3H)
with Ce(IV) (Y Ce~46%)(II)• The reaction is first order in catalyst
cooperative interaction of the two Ru
centers of a single catalyst molecule(purified by chromatography on alumina)

more active catalyst

electrochemical tuning> Ligand-Ru2 + quinone-ligand (solid state)
K. Tanaka et al. (Inorg. Chem. 2001, 40, 329)
(Inorg. Chem. 2008, 47, 1787)

catalyst is immobilizedon ITO electrodeelectron transferfrom OH to quinone

2+> Ru-containing polyoxometalate
All-inorganic(IV ?)(II)(II)

M. Bonchio et al. (J. Am. Chem. Soc. 2008, 130, 5006)

(II)

(II)

(II)

(II)

(II)

O
tBu

(II)

O

e- mediator

(SQ)

tBu

futher Ce(IV) addition
TON O2 500 (2h)induced an quivalent(RateO2 max 0.125 / s) O2 evolution.
pH ~0.6
robust catalyst
with Ce(IV) (Y Ce <90%)
B. Botar et al. (Angew. Chem. Int. Ed. 2008, 47, 3869)(III)

TON O2 33500 (40h)pH ~4 (aq. H 3PO4 / KOH, 1.0 M)
E = + 1.70V (vs Ag/ AgCl)

(III)

(II)

(II)

• 25H2OTON O2 18 (30~40s)(RateO2 0.6 / s )pH 7 (phospate buf f er)
with [ Ru(III)(byp)3] 3+

continuation ?

> Cyclometalated Ir

S. Bernhard et al. (J. Am. Chem. Soc. 2008, 130, 210) (Nature (News and Views) 2008, 451, 778)

page 6(2.2 eq)

Na3[Ir(III)Cl6]+
Ligand(III)

44-76%
HOMO energy tuning• synthetically accessible, robust, efficient• broad range of oxidation potential
can be obtained by ligand tuning• machanism is not clear(non-1st order dependence in catalyst conc.)

TONO2 2760 (7d)catalyst (3 10-4M)
pH ~0.7
with CAN (Y Ce ~37% (7d))~100% (6h)without noble metal
Next Challenge ~ homogeneous photocatalytic water oxidation

2-4. New amorphous catalyst for water electrolysis(Science 2008, 321, 1072)(Science (News this week) 2008, 321, 620)

Co(II)(NO3)2
pH 7 (potassium phosphate buf f er)
E = +1.29V (@ITO)
continuous O2 evolving froma dark coating on the ITO surface(SEM image of coating)utilize as electrochemical catalyst> easily prepared, earth-abundant catalysts that oxidize water.
(i) forming from earth-abundant metal ions in aqueous solution"elements of(ii) a plausible pathway for self-repair
natural photosynthesis "(iii) a carrier for protons in neutral water(iv) the generation of O2 at low overpotential, neutral pH, 1 atm, and room temperature< other >> O2 (3.0mg, 95 mol) was produced by cat.(~0.2mg) in 10h

TON O2 143 (10h)(RateO2 4 10-3 / s)

> overpotential ( ) ~0.32V (i = 0.1mA / cm2) relatively low
0.41V (i = 1mA / cm2)on the anode (?)(coating)> mechanism ?
OH(簡単のため)-eOH2-e-H+ OCo3+
in solution (?)

Co2+

-H+

(HPO42-)etc.

(HPO42-)etc.

(E = 1.29 V)(触媒をCo2H2KO7Pと仮定)

one or more intermediates preceding O2 formationthat are deprotonated reversibly

Co4+(HPO42-)
etc.

2

-O2

< Requirement for electrode in practical sense >> low overpotential> neutral (pH ~7)> without noble metal

過電圧(

電極電位とその反応の平衡電極電位との差（ある程度の反応を進行させるのに必要）（反応物質の吸着・電子授受・結合の開裂・表面拡散・新たな結合の形成・生成物の脱着などを総合した活性化エネルギーを反映）

so far...
• first-row spinel and perovskite metal oxides
moderate overpotentials (<0.4V)
concentrated basic solutions (pH > 13)• precious metals and precious metal oxides
similar efficiencies
acidic conditions (pH < 1)

< Co2+ >Co2+ + Ru(bpy)33+ in neutal phospate solutions can catalyze water oxidation.
(E0 = + 1.26V)...but oxidized Co species were precipited gradually.

V. Y. Shafirovich et al. (Nouv. J. Chim. 1980, 4, 81)
C, Creutz, N. Sustin et al. (J. Am. Chem. Soc. 1983, 105, 4832)

Can precipitation be exploited f or electrolysis ?

< This paper >page 7> Indium tin oxide (ITO) (a minimal background activity for O2 production)+

E = + 1.29 V+
Co(NO3)2 (0.5 mM)
potassium phosphate (0.1 M, pH 7.0; KPi electrolyte)(w/o Co2+)

a dark coating on the ITO surface
effervescence becomes increasingly vigorous（気泡）

almost no electrolysis(same results with CoSO4, Co(NO3)2, or Co(OTf)2)O2(B) Current density profile for bulk electrolysis at 1.29 V (versus NHE)
in 0.1 M KPi electrolyte at pH 7.0 containing 0.5 mM Co2+.

cf) >40 C are passed over 8 hours(all the Co2+---> Co3+ requires 1.9 C)

in situ f ormation of an oxygen evolving catalyst< What is this coating ? >> morphology

Fig. 2A. SEM image (30° tilt) of the electrodeposited catalyst af ter 30
C/ cm2 were passed in KPi (0.1 M, pH 7.0) containing Co2+ (0.5 mM).
The ITO substrate can be seen through cracks in the dried f ilm.

• The film thickness gradually increases.
At maximum activity, the film is >2 m thick.
• The x-ray powder diffraction pattern shows broad amorphous features
No peaks indicative of crystalline phases(780.7, 798.7 eV)
Co2+ or Co3+ bound to O
> surface> composition(133.1 eV)
phosphate.

• Energy-dispersive x-ray analysis (EDX) spectra
Co, P, K, and O as the principal elemental(2~3 : 1 : 1)
• Microanalytical elemental analysis(Co:P:K) = (2.1 : 1.0 : 0.8)

Fig. S2. X PS spectra of the electrodeposited f ilm

an amorphous Co oxide or hydroxide incorporating a substantial amount of phosphate anion
at a stoichiometric ratio of (Co : P : K) = (~2 : 1 : 1)
< Is O2 really derived from water ? >• prepared catalyst coatings (1.3 cm2, stored under ambient laboratory conditions) were used following experiments.
16,16

O2

( Fig. 3B)
abundance of each isotope
over the course of the experiment.

16,16

O2
16,18

O2

off

on
(1.29 V)

16,18

O2

same as statistical abundances(72.9, 24.9, and 2.1%)

18,18

（拡大）

O2

18,18

O2
(Fig. 3C)O2 production measured by f luorescent sensor (exp.) andby calculation (calc.)(calc.)(Fig. 3A) Mass spectrometric detection of isotopically labeled O2during electrolysis of a catalyst f ilm on ITO in a KPi electrolyte
containing 14.6% 18OH 2.

off

Faradic efficiency ~ 100%.
on(1.29 V)• phospate is stable(31P NMR after stoichiometric(?) electrolysis).

(exp.)
all of the current was caused by 4eoxidation of water to produce O2electrodeposited catalyst cleanly oxidizes H 2O to O2 in neutral KPi solutions.
Ｘ線光電子分光(X-ray photoelectron spectroscopy: XPS)
試料にＸ線を照射した時に起こる光電効果により放出される電子の運動エネルギーを測定することにより内殻電子の結合エネルギーを測定する手法がＸ線光電子分光(X-ray photoelectron spectroscopy: XPS)である。表面に存在する原子種とその量、結合状態を知ることができる。具体的には、SAMなどの超薄膜の膜厚や分子の吸着形態などをＸＰＳを用いて調べている。

3. CO2 Reduction by Molecular Catalyst"photocatalytic / photosensitized/ photoinduced"

S

hv

page 8

S*
cat

CO2

"chemical"

reductant"electrochemical / electroclysis"

ECO2 ~bigger
accepting e-

ne

n×e
catn+

favorable...

(-) cathode(a) might be important for global warming(b) very difficult (multielectron transfer)> more electron reduction, more energetically-favored
But controlling number of electrons is difficult.
(c) Reduction of protons to H2 is competitive(more thermodynamically favored)

3-1. CO2 Reduction( Coord. Chem. Rev. 2002, 226, 211-218)(pH = 7)transition metal catalyst
selective coordination of CO2 to metal before reduction

2H+ + 2e-H2(E0 = - 0.41 V)> d6 metal (Ru(II), Re(I)) or Ni(II) is good for CO2 to CO.
> Re(I) (CO)3 is exceptionally good
(acts as reduction-catalyst and photosensitizer)

3-2. CO2 to CO< In nature >> In Moorella thermoacetica (酢酸生産菌)[Ni-L-Fe4S4] cluster (in carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS))
bif unctional reversible reduction of CO2 to CO

synthesis of acetyl-CoA from CO & CoA-H+(Science 2002, 298, 567)

+H+

CO

+H+

H2O
CO2 + 2e- + 2H+

+e-

-H+

CO + H2O

+e-

CO2
(0)

(II)

-

e

B. Wilson homepage(http://research.uvsc.edu/wilson/CODH.htm)" CODH remains notably the most ef f icient catalyst f or this reduction " (J. Am. Chem. Soc. 2003, 125, 14688)< Early example >> Re(I)(CO)3 acts both as photosensitizer and as reduction catalyst
J-M. Lehn et al. (J. C. S. Chem. Commun. 1983, 536)

Re(I)(bpy)(CO)3Cl (8.7×10-4 M)
1 atm CO2, in DMF-T EOA((HOCH 2CH 2) 3N) (5:1)
250W halogen lamp, >400nm, 4h

TONCO 30

quantum yield ?

• efficient genaration of CO, no dectable amount of H2(H2 was formed in the absence of CO2)
• the activity of the system decreases slowly with time(due to labilization of ligand species)(supposed cycle)
Re(0)

CO2
e

cf)

-

e-

CO
Re(I)
hv

TEOA+
TEOA

*Re(I)TEOA+ + e-

TEOA
* [ Re(I)(bpy)(CO)3X] + e-

cf 2)

[Re(0)(bpy)(CO)3X]+-

CO2 + 2H + 2e

(E0 =ca. +0.8V)[ Re(0)(bpy)(CO)3X]- (E0 = ca. +1.25V)[Re(I)(bpy)(CO)3X] + e-

(E0 = ca. -1.05V)

CO + H2O(E0 = -0.76V)

< Recent study >Re(0)OER (one-electron-reduced species) (a) capturing CO2 after loss of the monodentate ligand (L)page 9(b) donation of the second electron to CO2 by another OER without loss of L.

O. Ishitani et al.
(J. Am. Chem. Soc. 2008, 130, 2023-2031)(supposed mechanism)(= triplet metal-to-ligand charge transfer)(by flush laser photolysis studies)(=(HOCH2CH2)3N)(by UV-vis absorption studies)> So far, the most efficient homogeneous photocatalyst[Re(I)(bpy)(CO)3(CH3CN)]+ / [Re(I){4,4'-(MeO)2bpy}(CO)3{P(OEt)3}]+ = (1 : 25)(as catalyst)• easy L dissociation• by itself
low photocatalytic activity(as photosensitizer)• very high for OER• OER has enough reduction power for [Re(I)-CH3CN]+ (to make 1)= 0.59 at 365nm
in DMF-TEOA (5:1)> For CO2 to CO, visible-light can be utilized
O. Ishitani et al. (Photochem. Photobiol. Sci. 2007, 6, 454)

visible light active
photosensitizer

(II)

(I)

catalyst= 0.12, TNCO = 160= 0.21, TNCO = 232

catalyst (5 10-5 M)
Hg lamp ( >500nm)
DMF-T EOA (5:1)
BNAH (0.1M) as reducing reagent f or Ru< Other report for CO2 to CO >stoichio --- Fe(I)electrochemical --- Ni, di-Pd, Ru, Re
chemical --- (Cu(I)NHC + B-B), (Met + H2) etc...

3-3. Other Reduction> CO2 to HCOOH (2e- reduction) selectively--- a few examples
electrochemical reduction
at carbon electrodes modified by films of [Ru(0)(L)(CO)2]nA. Deronzier et al. (J. Electroanal. Chem. 1998, 444, 253)

(via Ru(I)-H)> CO2 to formaldehyde, methanol, methane selectively --- very rare

next challenge

4. Integrated Systemenergy uptake oxi./red. reaciton by hv
H2O as electron source

page 104-1. semiconductor material> originally repoted by Honda and Fujishima
TiO2 (nm)> many efforts toward improvemnts of semiconductor materials "one step mechanism"> first example of water splitting under visible light (nm) "two step mechanism"

K. Sayama, H. Arakawa et al. (Chem. Comm. 2001, 2416.)"artif icial Z scheme"

Review (2003, 24, 19.)

@ f inding appropriate reversible electron donor (I -) is important(E0 = - 0.41 V) (pH = 7)
(E0 = + 0.67 V) (pH = 7)

< red. >• Pt-loaded SrTiO3(Cr-Ta doped) is best.
(-0.41)(+0.67)(+0.82)

Pt is co-catalyst
no oxidation of H2O (only oxidation of I-)• The initial rate of H2 evolution was 0.8 mol h-1
but the evolution rate decreased gradually and stopped after 20 h (Fig. 1(a)).
Activity was recovered by replacing rxn. solution with a new NaI
H2 formation was significantly suppressed (< 0.1 mol h-1)
when a small amount of NaIO3 (0.1 mmol l-1) was added

accumulation of IO3- inhibits H + reduction(due to competitive IO3- reduction on Pt (?))

< oxi. >(E0 = + 0.67 V) (pH = 7)
(E0 = + 0.82 V) (pH = 7)

• Pt loaded WO3 is best.

no reduction of H2O (only reduction of IO3-)-• I oxidation by h+ in the valence band was very slow.

< mixing both catalyst >• H2 and O2 gases evolved under visible light (>440nm) (Fig.1(B))• stoichiometric water splitting more than 250 h.
• added Pt-WO3 effectively eliminated the IO3- ion by reduction to Iconcentration of I- (100 mmol l-1) did not change
formation of the IO3- was negligible(merit)(drawback)• 2回に分けて励起 ( G• 組み合わせが可能• 4hv / 2e- / H2
• low efficiency
quantum efficiency ~0.1% (420.7 nm)

4-2. Homogeneous Catalyst System

page 11

水を電子源とする光による二酸化炭素還元系の構築をめざし金属ポルフィリンと二酸化炭素還元能を有するレニウム錯体の連結系分子を新たに合成しその光化学挙動について検討した。（図 １８）
金属ポルフィリンへのS2励起により、ポルフィリン
からレニウム錯体への電子移動に続いて二酸化炭素
還元による一酸化炭素の生成に成功した。
quantum yield
T ON ?

reduction

O. Ishitani et al. (CREST H17 report)(E0 - 0.76 V)

from water(E0 ~ +2 V)

oxidaiton

H. Inoue et al.
( J. Am. Chem. Soc. 2003, 125, 5734.)
reduction

homogeneous, integrated system f or H2O oxidation / XXX reduction is very rare.
big challenge in f uture@ Future Challenge ~ from molecular catalyst fieldsemiconductor is more practical (?)* Photoinduced system* New approaches* Low costs catalysts• arificial Z scheme(2 step mechanism)hv
4e-1) Antenna System
2) Charge Separation Unit

D

S

6e-

A

2H2O
3) Oxidation
Catalyst

D

S* A

D

S

4) Reduction
Catalyst

O2 + 4H+

CO2 + 6H+
CH3OH + H2OA* reduce CO2 by 4 or 6 or 8 electrons with high selectivity* New approaches* Low costs catalysts
semiconductor ?
heterogeneous catalyst ?

in gas phase5) Integarated System