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HELIOS
Nanostructured Water Oxidation Photocatalysts
Heinz Frei
February 3, 2010
Goal:

CO2 + H2O
CH3OH + O2
visible light
• Conversion in a single integrated system
(terawatt scale)
hv
O2 H2O
CH3OH
CO2
reduction
CO2
H2O O
2
H2O oxidation
• Inorganic system  robust
Topics today:
Robust inorganic nanoclusters as water oxidation catalysts
All inorganic photocatalytic units in nanoporous silica scaffolds
Turnover frequencies (TOF) for oxygen evolution at Co and Mn oxide
materials reported in the literature
Oxide
TOF
(sec-1)
Overvoltage, η
(mV)
pH
T
(oC)
Quantum
yield
Reference
Co3O4
0.035
325
5
RT
58%
Harriman (1988) [1]
Co3O4
> 0.0025
350
14
30
--
Tamura (1981) [2]
Co3O4
> 0.020
295
14
120
--
Wendt (1994) [3]
Co3O4
> 0.0008
414
14.7
25
--
Tseung (1983) [4]
Co3O4
> 0.006
235
14
25
--
Singh (2007) [5]
Co,P film
> 0.0007
~ 0.1
410
7
7
25
60
---
Nocera (2008) [6]
Nocera (2009) [7]
MnO2
> 0.013
440
7
30
--
Tamura (1977) [8]
Mn2O3
0.055
325
5
RT
35%
Harriman (1988) [1]
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Harriman, A.; Pickering, I.J.; Thomas, J.M.; Christensen, P.A. J. Chem. Soc., Farad. Trans. 1 1988, 84, 2795-2806.
Iwakura, C.; Honji, A.; Tamura, H. Electrochim. Acta 1981, 26, 1319-1326.
Schmidt, T.; Wendt, H. Electrochim. Acta 1994, 39, 1763-1767.
Rasiyah, P.; Tseung, A.C.C. J. Electrochem. Soc. 1983, 130, 365-368.
Singh, R.N.; Mishra, D.; Anindita; Sinha, A.S.K.; Singh, A. Electrochem. Commun. 2007, 9, 1369-1373.
Kanan, M.W.; Nocera, D.G. Science 2008, 321, 1072-1075.
Nocera, D.G. Symposium Solar to Fuels and Back Again, Imperial College, London, 2009.
Morita, M.; Iwakura, C.; Tamura, H. Electrochim. Acta 1977, 22, 325-328.
Nanostructured Co oxide cluster in mesoporous silica
scaffold
Synthesis of Co oxide
clusters in SBA-15 using
wet impregnation method
35 nm bundles
(4 % loading)
65 nm bundles
(8 % loading)
XRD
SBA-15/Co3O4 (8%)
SBA-15/Co3O4 (4%)
Co3O4
EXAFS
free nanorod
bundle
Co3O4 bulk
SBA-15/Co3O4 (8%)
SBA-15/Co3O4 (4%)
• Co oxide clusters are 35 nm bundles of parallel nanorods (8 nm diameter)
interconnected by short bridges
• XRD, Co K-edge EXAFS and reveal spinel structure
Co L-edge XAS spectrum
• Co L-edge absorption spectrum confirms Co3O4 structure
Efficient oxygen evolution at Co3O4 nanoclusters in mesoporous
silica SBA-15 in aqueous suspension
Mass spectroscopic monitoring
O2 evolution
SBA-15/Co3O4
35 nm bundle
TOF 1140 s-1 per cluster
65 nm bundle
O2
Co3O4micron
sized particles
SBA-15/NiO (8%)
F. Jiao, H. Frei, Angew. Chem. Int. Ed. 49, 1841 (2009)
• Visible light water oxidation in aqueous SBA-15/Co3O4 suspension using
Ru2+(bpy)3 + S2O82- method. Mild conditions: 22oC, pH 5.8, overvoltage 350 mV
• High catalytic turnover frequency: 1140 O2 molecules per second per cluster
 TOF of catalyst per projected area = 1 s-1nm-2
 mesoporous silica membrane, 150 μ thick: TOF = 100 s-1nm-2
• O2 yield is 1600 times larger than for 35 nm bundle catalyst compared to
μ-sized Co3O4
• Surface area of nanorod bundle cluster = factor of 100, catalytic efficiency of
surface Co centers = factor of 16
Co K-edge:
No sign of Co oxidation state change
after photolysis
EXAFS:
No sign of structural change
after photolysis
• Co3O4 structure in silica scaffold stable under water oxidation catalysis
• Rate and size of the SBA-15/Co3O4 catalyst driven by visible light are
comparable to Nature’s Photosystem II and are in a range that is adequate
for the keeping up with solar flux (1000 W m-2)
TOF 300 s-1
TOF 1140 s-1
• Abundance of the Co metal oxide, stability of the nanoclusters under use, modest
overpotential and mild pH and temperature make this a promising catalyst for
use in integrated artificial solar fuel systems
Efficient oxygen evolution at nanostructured Mn oxide clusters
supported on mesoporous silica KIT-6
TEM
XAFS
MnO1.51
calcined 600 oC
KIT-6 (3D channels)
MnO2
Mn2O3
Mn3O4
400 oC
64%
36%
-
500 oC
95%
5%
-
600 oC
6%
80%
14%
700 oC
-
81%
19%
800 oC
-
70%
30%
900 oC
-
51%
49%
• Spherical Mn oxide nanoclusters, 70-90 nm diameter, mixed phase (calcination T)
• The phase composition was determined by component analysis of XANES spectra
Efficient oxygen evolution in aqueous solution using Ru2+(bpy)3persulfate visible light sensitization system
Mass Spec
s-1
Mild conditions:
pH 5.8, 22 oC
overvoltage 350 mV
TOF 3,320
per cluster
O2 evolution
TOF 900 s-1
per cluster
F. Jiao, H. Frei, submitted
• Most active catalyst: MnO1.51 with TOF = 3,320 O2 s-1, which corresponds to
0.6 sec-1 nm-2 projected area  200 μm membrane with TOF of 100 s-1nm-2
 meets solar flux
• Very stable upon photochemical use, no leaching of Mn
• Silica scaffold provides:
• high, stable dispersion of nanostructured catalysts
• sustained catalytic activity by protecting the active Mn centers
from deactivation by surface restructuring
Co or Mn oxide/ silica core shell constructs
Mn oxide core/ silica shell construct
silica shell
Co3O4 or MnOx core
Reverse microemulsion method
(Ying, J.Y., Langmuir 24, 5842
(2008))
F. Jiao
Precise matching of redox potentials of the components
in organic molecular systems
Hammarstrom, Chem. Soc. Rev. 30, 36 (2001)
Approach: Well-defined all-inorganic polynuclear photocatalysts
arranged in robust 3-D nanoporous scaffold
nanoporous silica support
200 nm
• Photocatalytic site consists of a hetero-binuclear
unit acting as visible light charge transfer pump
driving a multi-electron transfer catalyst
• 3-D nanoporous support for arranging and
coupling photoactive units
• High surface area required to avoid wasting of
solar photons (one photocatalytic site nm-2
assuming rate of 100 sec-1)
• Nanostructured support for achieving separation
of redox products
MCM-41
SBA-15
Selective assembly of binuclear MMCT units for driving water oxidation catalysts:
TiOCrIII
h
DRS
e- MMCT (visible light)
O
CrIII
1-R
Ti
0.9
OO O O O O
Si Si Si
Al Si Si
0.6
TiIV-O-CrIII  TiIII-O-CrIV
0.3
0.0
CrVI(=O) + TiIII  CrV-O-TiIV
Selective redox coupling
CrV EPR
3
10
Cr-AlMCM-41, cal 630C
as-syn
TiCr-AlMCM-41
1
0
400
600
800
Wavelength (nm)
6000
6040
Energy (eV)
2.00Å(CrIII-O)
6
2.70Å(CrIII-O-Ti)
4
1.59Å(CrVI=O)
TiCr-AlMCM-41
2
Cr-AlMCM-41
0
0
1
2 3 4 5 6
Distance R (Å)
7
Relative intensity
2
TiCr-AlMCM-41
EXAFS
FT Magnitude
Normalized Absorption
X-ray K-edge
III
Cr -AlMCM-41
0.5
0.0
V
Cr
Sp: g=1.977, g//=1.96
TiCrAl-MCM41
VI
Cr Al-MCM41
-0.5
3200 3300 3400 3500
Magnetic field (G)
Han, Frei, J. Phys. Chem. C 112, 8391 (2008)
• Cr EPR, XAFS K-edge, EXAFS, FT-Raman and optical spectroscopy allows step-by-step
monitoring of oxidation state and coordination geometry changes of the Cr center upon TiOCr
formation
Selective assembly of binuclear MMCT units for driving water oxidation catalysts:
TiOCrIII
Cr-O
CrIII
TiOCrIII
Cr-O
B
-4)
|(R)| (Å
|(R)| (Å
-4)
8
4
B
8
4
Cr--Ti
0
0
2
4
R (Å)
6
0
8
0
2
4
R (Å)
6
8
Cr EXAFS curve fitting:
Cr-O
N
DW
Cr-O
N DW
1.97 A
3.8
0.003
2.01 A
3
0.001 3.14
1.72 A
1
0.003
• Second
Cr---Ti N DW
1
0.007
Cr----Si
N DW
2.89
3 0.003
shell peaks confirm oxo bridge structure of MMCT unit
• Cr-O bond of Ti-O-Cr bridge is shorter than for Cr-O-Si, indicating partial
charge transfer character of ground state
Binuclear TiOCrIII pump drives H2O oxidation catalyst under visible light
10 nm
HR-TEM of Ir oxide
nanoclusters in
silica channels
O2 evolution using
Clark electrode
Quantum yield = 14%
(lower limit!)
O2(mg/L)
9
Level of saturated O2 in water
6
Light on
3
IrxOy-TiCr-AlMCM-41
0
-0.5 0.0 0.5 1.0 1.5 2.0
Time (hour)
Han, Frei, J. Phys. Chem. C 112, 16156 (2008)
Nakamura, Frei, J. Am. Chem. Soc. 128, 10689 (2006)
• Efficient visible light water oxidation in aqueous suspension observed
EPR and FT-Raman spectroscopy show formation of TiIV…O2- complex
MMCT
Ti -O-Cr /IrOx
hv
IV
TiIV…O2-
g2 = 2.010
g1 = 2.034
1.0
1.0
simulated spectrum
0.5
after photolysis
superoxide
0.0
g3= 2.005
Raman intensity (a.u.)
3200
before photolysis
3250
3300
Magnetic field (G)
961
0.0005
0.5
0.0
3200
16O18O18O 2
TiIII-O-CrIV/IrOx
TiIII
EPR
Intensity
Relative intensity
10
3
III
III
superimposed EPR spectrum of simulate Ti and Cr
photolysis of IrxOy-TiCr-AlMCM-41+H2O
photolysis of IrxOy-Cr-AlMCM-41+H2O
3300
3400
3500
Magnetic field (G)
FT-Raman
930
18
after photolysis in H2 O
994
O216
after photolysis in H2 O
O2 trapped by transient TiIII
O2- detected in aqueous solution
18O labeling of superoxide when
using H218O
1100
1000
900
-1
Raman shift (cm )
• Electron donation from IrOx catalyst competes successfully with back electron transfer from TiIII
• Flexibility of donor metal selection for matching redox potential of charge-transfer chromophore
and catalyst
V
Elucidation of electron transfer pathways and kinetics of binuclear
charge-transfer chromophore by transient absorption spectroscopy
L-edge X-ray absorption
DRS
Ti
TiMnII-MCM-41
MnII
•
•
Transient absorption spectroscopy of MMCT units using indexmatching liquids (mineral oil, silicone oil, or CHCl3)
5 nanosecond resolution
Transient bleach of MMCT transition observed
Excitation of TiOMn, 400-600 nm
Pump Dependence:
Kinetic and Spectral
(a)
(b)
-3
-2
 OD (10 )(t=0, tavg)
-3
 OD (10 )
0
-4
TiMn-SBA-15
-6
-8
0
2
4
6
Time (s)
Pump Spectral Dependence:
DRS Comparison
Probe: 400nm
Pump
425 nm
445 nm
475 nm
535 nm
8
10
Albery model for dispersive 1st order kinetics:
(Albery et al., J. Am. Chem. Soc. 1985, 107, 1854)
k = k’exp(γx), Gaussian distribution in ln(k)
mean time constant 1/k’ = 1.8 μsec
0
1/k' = 1.8 ± 0.3s
= 2 ± 0.2
-5
-10
t=0 Albery Fits, normalized data
tavg, unnormalized data
DRS Static Spectra
400
450
500
550
Pump (nm)
600
T. Cuk, W. Weare, H. Frei, J. Phys. Chem. C, submitted
• Recovering bleach is due to back electron transfer of excited TiIIIOMnIII → TiIVOMnII
• Spread of first order rate constants  indicates structural heterogeneity of
the silica environment of the binuclear sites
Unusually slow back electron transfer
Ti(III)OMn(III)
e1(Ti)t2g3(Mn)eg1(Mn)
S= 5/2
Ti(IV)OMn(II)
S = 3/2
hv
MMCT
G
e0(Ti)t2g3(Mn)eg2(Mn) S= 5/2
• Substantial structural rearrangement of coordination sphere in excited MMCT state
and polarization of the silica environment imposes barrier to back electron transfer
• Lifetime long → MMCT units suitable for driving MET catalysts with visible light
Selective assembly of binuclear MMCT units for driving water oxidation catalysts:
TiOCoII, TiOCeIII
O
Ti
O O
O O
0.6
O
Si Si Si
Si
O
Si Si
DRS
0.4
1-R
Ti-MCM-41
Ce-MCM-41
0.2
MMCT
300
400
TiIV-O-CeIII  TiIII-O-CeIV
500
600
700
Normalized Absorption
XAFS
TiCe-MCM-41
0.0
Han, Frei, J. Phys. Chem C 112, 8391 (2008);
Microporous Mesoporous Mater. 103, 265 (2007)
Nakamura, J. Am. Chem. Soc. 129, 9596 (2007)
5728 TiCeIII
5727
3
E = +1 eV
CeIII
1
0
Wavelength (nm)
0.4
1-R
Co-MCM-41
0.2
MMCT
0.0

TiIII-O-CoIII
Relative Intensity
3
TiIV-O-CoII
b
a
5730
5760
Energy (eV)
TiCo-MCM-41
Ti-MCM-41 + Co-MCM-41
A
4
2
Normalized Absorption
Si
III
Ce
10
1.5 g = 5.250

CoII
Ce L-edge
2.4
5729 5737
b'
1.6
a'
0.8
CeIV
0.0
5720
5760
Energy (eV)
800
5800
EPR
g// = 2.032
1.0
b
0.5 g = 5.107
g// = 2.034
a
CoII linked to Ti is
high spin, tetrahedral
0.0
400
600
Wavelength (nm)
B
TiCeIV
2000
3000
4000
Magnetic Field (G)
• Selective assembly due to higher acidity of TiOH vs. SiOH
• MMCT excitation by visible light generates donor centers (CeIV, CoIII) of sufficiently positive
potential for driving H2O oxidation catalyst
Coupling polynuclear photocatalysts in nanoporous silica scaffolds
to achieve separation of reduced products from evolving oxygen
Long term goal:

CO2 + H2O
CH3OH + O2
visible light
hν
O2 H2O
CH3OH
(L)
(L = inorg. or C-based conducting
linker)
CO2
CO2
reduction
Two photon system
H2O O2
H2O
oxidation
envisioned integrated system
• Coupling of fuel generating photocatalytic sites (green) with O2 evolving
sites (purple) across nanoscale wall
• Separation of oxygen from methanol
Co or Mn oxide/ silica core shell constructs with nanowires
penetrating SiO2 shell
Mn oxide core/ silica shell construct
silica shell
Co3O4 or MnOx core
Reverse microemulsion method
(Ying, J.Y., Langmuir 24, 5842
(2008))
F. Jiao
HELIOS
Conclusions
•
Development of all-inorganic photocatalytic units on nanoporous silica
supports consisting of heterobinuclear charge-transfer chromophore
coupled to multi-electron catalyst; selective, flexible synthetic methods
(abundant elements, scalable synthetic approach)
•
MMCT chromophores absorb deep in the visible region, possess
donor and acceptor centers with selectable potentials
→ key to thermodynamic efficiency of photocatalyst
•
Long lifetime (microsec) of MMCT states uncovered
•
H2O oxidation to O2 under visible light (TiOCrIII chromophore driving
an IrOx nanocluster catalyst) at > 14 % quantum efficiency,
hydroperoxide intermediate observed
•
Co3O4 and MnO1.51 nanocluster catalysts of abundant materials for
water oxidation, TOF in range suitable for keeping up with solar flux
HELIOS
Acknowledgments
Postdoctoral Fellows:
Feng Jiao
Walter Weare
Hongxian Han
Tania Cuk (Miller fellowship)
N. Sivasankar
Marisa MacNaughtan
Drs. Vittal Yachandra, Junko Yano
Facilities: NCEM-LBNL, SSRL
US Department of Energy, Office of Basic Energy Sciences,
Division of Chemical, Geological and Biosciences
Helios Solar Energy Research Center, funded by DOE-BES
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