第31回無機材料に関する最近の研究成果発表会
表面ナノ構造を制御した半導体 光触媒による水の可視光完全分解
東海大学校友会館 January 27, 2014
前田 和彦
東京工業大学 大学院理工学研究科 化学専攻
JSTさきがけ研究者『光エネルギーと物質変換』領域
Research background
Fossil Fuel (Depletion…)
CO2, SOx, NOx
(Environmental issues!!) Conventional energy-
production system
Combustion
Sun
Photocatalytic H2 production from water and solar energy
O2 O2
2H
2H2O
Combustion
Semiconductor photocatalyst
Basic principle of water splitting on a heterogeneous photocatalyst
H2O H2 + 1 O2 2
hν
ΔG0 = 238 kJ/
Photocatalyst mol
V (vs. NHE)
0 +1.0 +2.0 +3.0
O2/H2O H+/H2 (pH 0)
Valence band (V.B.)
Band gap
O2 H2O
Conduction band (C.B.)
hν
h+ e–
H2 H+
300 400 500 600 700 800 0
0.2 0.4 0.6 0.8 1.0
Normalized F(R∞)
Waveleng th / nm
(Oxy)nitrides as water-splitting photocatalysts
…Production of H2 as a renewable energy carrier H2O H2 + 1 O2
2
Sunlight
(ΔG0 = 238 kJ/mol) Visible
UV
• Wide visible light absorption
• Suitable band structure
• Stable under irradiation
Ta2O5 TaON SrTaO2N
BaNbO2N Ta3N5
LaTiO2N
Maeda & Domen, J. Phys. Chem. C 2007, 111, 7851.
Photocatalyst
GaN–ZnO solid solution…the first “reproducible” example of achieving the visible-light-driven overall water splitting
H2O H2 + 1 O2 2
hν (< 3 eV)
ΔG0=238 kJ/mol
GaN (Ga1–xZnx)(N1–xOx) ZnO (x = 0.42)
Maeda et al., J. Am. Chem. Soc. 2005, 127, 8286.
Maeda et al., Nature 2006, 440, 295.
Maeda & Domen, Chem. Mater. (Review) 2010, 22, 612.
Strategy to develop an efficient photocatalyst
Cocatalyst nanoparticle
(e.g. NiO, RuO2)
GaN:ZnO particle
h ν > E
ge
–h
+Recombination
H2 H+
O2 H2O
・Construction of active sites
・Decreasing activation energy
Development of a new cocatalyst that efficiently promotes the overall water splitting on GaN:ZnO
u Crystallinity u Particle size u Composition u etc.
Development of a new cocatalyst for water splitting
Conventional cocatalysts
NiO, RuO2, IrO2…“single” component metal-oxides
NiO, RuO2, or IrO2
Photocatalyst particle
h ν > E
ge
–h
+Recombination
H2 H+
O2 H2O
Effect of Cr co-loading on water splitting activity
Cocatalyst Activity / µmol h–1 Cr coloading amount
/ wt%
Activity / µmol h–1 Element
(oxide) Loading
amount / wt% H2 O2 H2 O2
None - 0 0
Cr 1 0 0
Fe 1 0 0 1 73 36
Co 1 2.0 0 1 48 24
Ni 1.25 126 57 0.125 685 336
Cu 1 2.0 0 1 585 292
Ru 1 71 27 0.1 181 84
Rh 1 50 1.6 1.5 3835 1988
Pd 1 1.0 0 0.1 205 96
Ag 1 0 0 1 11 2.3
Ir 1.5 9.3 3.1 0.1 41 17
Pt 1 0.9 0.4 1 775 357
Catalyst: 0.3 g, Reactant soln.: distilled water 370~400 mL, Reaction vessel: Inner irradiation-type, Light source: 450 W high-pressure mercury lamp
Maeda et al., J. Catal. 2006, 243, 303. λ > 300 nm
Overall water splitting on Rh2–yCryO3-loaded GaN:ZnO under visible light irradiation
Evac.
▼
H2
O2
Used catalyst: 3.7 mmol Evolved gases: 16.2 mmol
Catalytic cycle!!
Catalyst: 0.3 g, Reactant soln.: H2SO4 aq. 370 mL (pH 4.5), Reaction vessel: Inner
irradiation-type, Light source: 450 W high-pressure mercury lamp with a NaNO2 aq. filter
Maeda et al., Nature, 2006, 440, 295.
0 5 10 15 20 25 30 35
0 0.5 1.0 1.5 2.0
Amount of evolved gases / mmol
Reaction time / h RuO2 data
λ > 400 nm
Rh
Rh/GaN:ZnO
(before Cr2O3deposition)
Photoreduction of Cr6+
Cr2O3/Rh/GaN:ZnO
TEM images of Rh-loaded GaN:ZnO
before and after photodeposition of Cr2O3
Rh (core) Cr2O3 (shell)
Revealed by XAFS and XPS
Maeda et al., Angew. Chem., Int. Ed. 2006, 45, 7806.
Maeda et al., J. Phys. Chem. C 2007, 111, 7554.
Maeda et al., Chem. Eur. J. 2010, 16, 7750.
Time course of overall water splitting
on core/shell-structured Cr2O3/Rh/GaN:ZnO
0 1 2 3 4
0 0.2 0.4 0.6 0.8
Reaction time / h
H2
O2
Cr2O3/Rh
Rh Rh Cr2O3/Rh
+
0 1 2 3 4
0 0.2 0.4 0.6 0.8
Amount of evolved gases / mmol
Reaction time / h
0 1 2 3 4
0 0.2 0.4 0.6 0.8
Reaction time / h
Catalyst: 0.15 g, Reactant soln.: pure H2O 370 mL, Reaction vessel: Pyrex inner irradiation- type, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
λ > 400 nm
水分解光触媒における助触媒研究
H+ H2
助触媒 (RuO2など)
e– 光触媒
従来型
(単一の金属種)
1980
年～
e– 光触媒
H+ H2
Cr2O3シェル コア
異種接合型 (コア/シェル型)
H+ H2
助触媒
e– 光触媒
Cr含有複合酸化物型
2006
年～
(前田、堂免ら
)水分解光触媒研究の分野に助触媒開発という一大研究領域を確立
Major problem in the nanoparticulate core/shell system
5 nm 5 nm
Photodeposition
Aggregated Rh
• Impregnation method
Attempts to have Rh well-dispersed
• New method Adsorption
No positive effect!!
To increase the activity of Cr2O3/Rh/GaN:ZnO by
introduction of Rh nanoparticle core with higher dispersion
TEM images of GaN:ZnO modified with Rh/Cr2O3
(core/shell) nanoparticles by an adsorption method
Rh
Cr2O3
High-dispersion!! Size: 1~3 nm
Sakamoto et al., Nanoscale, 2009, 1, 106.
Maeda et al., Chem. Eur. J., 2010, 16, 7750.
0 1 2 3 4 5 6 0
20 40 60 80
Particle size / nm
Number of Particles / count Average size of
200 particles
1.9 ± 0.6 nm
Size distribution of Rh nanoparticles adsorbed on the surface of GaN:ZnO
Photodeposition method Ave. 7.6 nm (50 particles)
Effect of the amount of Rh on activity
0 0.5 1.0 1.5 2.0 2.5 3.0
0 100 200 300 400 500
0 0.1 0.2 0.3 0.4 0.5
Rate of gas evolution / µmol h-‐1
A m ount of R h added / wt %
Amount of Rh loaded / wt %
H2
O2
Catalyst: 0.15 g, Reactant soln.: pure H2O 400 mL, Reaction vessel: Pyrex inner irradiation- type, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
Increase in H2 evolution site
Saturated
adsorption λ > 400 nm
Comparison of activity
…Rh loading amount: 0.3~0.4 wt%
0 1 2 3 4 5
0 500 1000 1500 2000 2500
Amount of evolved gases / µmol
R eac tion tim e / h Adsorption method
Photodeposition method
H2
O2
H2 O2
Catalyst: 0.15 g, Reactant soln.: pure H2O 400 mL, Reaction vessel: Pyrex inner irradiation- type, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter
λ > 400 nm
High-dispersion of the core component is important!!
Effect of the size of Rh nanoparticles on activity
1.5$nm 3.8$nm 6.6$nm 0
100 200 300 400 500 600
Rate of gas evolution / µmol h -1
Catalyst: 0.15 g, Reactant soln.: H2SO4 aq. 400 mL (pH 4.5), Reaction vessel: Pyrex inner irradiation-type, Light source: 450 W high-pressure Hg lamp with a NaNO2 aq. (2 M) filter Ikeda et al., J. Phys. Chem. C 2013, 117, 2467.
Smaller is better!
λ > 400 nm
Overall water splitting on a particulate photocatalyst promoted by two different types of cocatalysts
H2 evolution cocatalyst
GaN:ZnO particle
h ν > E
ge–
h+ O2
H2O H2
H+
C.B.
V.B.
O2 evolution cocatalyst
Introduction of both H2 and O2 evolution cocatalysts to improve activity!
…But no successful example for constructing such a structure…
Visible light water splitting
…Effect of coloading Mn3O4
0 2 4 6 8 10 12
0 20 40 60 80 100 120 140
160 Mn3O4 + R h/C r2O3 (H2)
Mn3O4 + R h/C r2O3 (O2) R h/C r2O3 (H2)
R h/C r2O3 (O2)
Mn3O4 (H2) Mn3O4 (O2)
Amount of evolved gases / µmol
R eac tion tim e / h
Catalyst: 0.1 g of each, Reactant solution: distilled water 100 mL, Top-irradiation type with a 300 W Xe lamp and a cutoff filter
Mn3O4 0.05 wt %
λ > 420 nm
Summary
n Precise control of Rh core size in Rh/Cr2O3 nanoparticles
ü Successful introduction of size-controlled Rh nanoparticles onto the surface of GaN:ZnO photocatalyst
ü For application in the core component, smaller Rh works better.
ü Loading another oxygen evolution cocatalyst of Mn3O4 nanoparticles further enhances the water-splitting activity.
n Mechanism of H2 evolution on Rh/Cr2O3 nanoparticles
ü The core hosts active sites for H2 formation, while the Cr2O3 shell functions as a selective permeable membrane.
Modification of surface structure in nano-scale is highly important for enhancing water-splitting activity with visible light!
Acknowledgement
u Prof. K. Domen…The Univ. of Tokyo
u Prof. T. Teranishi, T. Ikeda, T. Yoshinaga…Kyoto Univ. & Tsukuba Univ.
u Dr. M. Yoshida, N. Sakamoto, A. Xiong…(former) students of our group
u Dr. D. Lu…Tokyo Institute of Technology
u 日本板硝子材料工学助成会, 日本学術振興会, 科学技術振興機構さきがけ
Collaboration on the core/shell cocatalyst project
TEM observations The boss
Funding support
