nanostructured water oxidation photocatalysts heinz frei february 3, 2010
DESCRIPTION
HELIOS. Nanostructured Water Oxidation Photocatalysts Heinz Frei February 3, 2010. Goal:. CO 2 + H 2 O CH 3 OH + O 2. h v. O 2. H 2 O. CH 3 OH. visible light. Conversion in a single integrated system (terawatt scale) Inorganic system robust. CO 2. CO 2 - PowerPoint PPT PresentationTRANSCRIPT
Nanostructured Water Oxidation Photocatalysts
Heinz Frei
February 3, 2010
HELIOSHELIOS
• Conversion in a single integrated system (terawatt scale)
• Inorganic system robust
CO2 + H2O CH3OH + O2
Goal:
visible light
H2O
H2O
O2
O2
CO2
CH3OH
H2O oxidation
CO2 reduction
hv
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 Overvoltage, η pH T Quantum Reference(sec-1) (mV) (oC) yield
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 410 7 25 -- Nocera (2008) [6]~ 0.1 7 60 -- Nocera (2009) [7]
MnO2 > 0.013 440 7 30 -- Tamura (1977) [8]
Mn2O3 0.055 325 5 RT 35% Harriman (1988) [1]
[1] Harriman, A.; Pickering, I.J.; Thomas, J.M.; Christensen, P.A. J. Chem. Soc., Farad. Trans. 1 1988, 84, 2795-2806.[2] Iwakura, C.; Honji, A.; Tamura, H. Electrochim. Acta 1981, 26, 1319-1326. [3] Schmidt, T.; Wendt, H. Electrochim. Acta 1994, 39, 1763-1767. [4] Rasiyah, P.; Tseung, A.C.C. J. Electrochem. Soc. 1983, 130, 365-368. [5] Singh, R.N.; Mishra, D.; Anindita; Sinha, A.S.K.; Singh, A. Electrochem. Commun. 2007, 9, 1369-1373. [6] Kanan, M.W.; Nocera, D.G. Science 2008, 321, 1072-1075. [7] Nocera, D.G. Symposium Solar to Fuels and Back Again, Imperial College, London, 2009. [8] Morita, M.; Iwakura, C.; Tamura, H. Electrochim. Acta 1977, 22, 325-328.
Nanostructured Co oxide cluster in mesoporous silica scaffold
35 nm bundles 65 nm bundles(4 % loading) (8 % loading)
free nanorodbundle
Synthesis of Co oxideclusters in SBA-15 usingwet impregnation method
• 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
Co3O4 bulkSBA-15/Co3O4 (8%)SBA-15/Co3O4 (4%)
EXAFS
XRD
SBA-15/Co3O4 (4%)
SBA-15/Co3O4 (8%)
Co3O4
Co L-edge XAS spectrum
• Co L-edge absorption spectrum confirms Co3O4 structure
F. Jiao, H. Frei, Angew. Chem. Int. Ed. 49, 1841 (2009)
SBA-15/Co3O4
35 nm bundle
65 nm bundle
O2 evolution
• Visible light water oxidation in aqueous SBA-15/Co3O4 suspension using Ru2+(bpy)3 + S2O8
2- 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
Co3O4micron sized particles
O2
SBA-15/NiO (8%)
Mass spectroscopic monitoring
Efficient oxygen evolution at Co3O4 nanoclusters in mesoporous silica SBA-15 in aqueous suspension
TOF 1140 s-1 per cluster
• Co3O4 structure in silica scaffold stable under water oxidation catalysis
Co K-edge: No sign of Co oxidation state change after photolysis
• 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
EXAFS: No sign of structural change after photolysis
• 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)
• 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
TOF 300 s-1
TOF 1140 s-1
Efficient oxygen evolution at nanostructured Mn oxide clusters supported on mesoporous silica KIT-6
TEM
MnO1.51
KIT-6 (3D channels)
• Spherical Mn oxide nanoclusters, 70-90 nm diameter, mixed phase (calcination T)• The phase composition was determined by component analysis of XANES spectra
XAFS
calcined 600 oC
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%
Efficient oxygen evolution in aqueous solution using Ru2+(bpy)3-persulfate visible light sensitization system
• 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
O2 evolution
TOF 900 s-1
per cluster
Mass Spec
Mild conditions:
pH 5.8, 22 oCovervoltage 350 mV
TOF 3,320 s-1
per cluster
F. Jiao, H. Frei, submitted
Mn oxide core/ silica shell construct
Co3O4 or MnOx core
silica shell
Reverse microemulsion method (Ying, J.Y., Langmuir 24, 5842 (2008))
F. Jiao
Co or Mn oxide/ silica core shell constructs
Hammarstrom, Chem. Soc. Rev. 30, 36 (2001)
Precise matching of redox potentials of the componentsin organic molecular systems
200 nm
nanoporous silica support
Approach: Well-defined all-inorganic polynuclear photocatalysts arranged in robust 3-D nanoporous scaffold
• 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-41SBA-15
Ti
O O O
Si
O CrIII
O O
Al Si
Si Si
MMCT (visible light)
O
Si
h
e-
• 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
CrVI(=O) + TiIII CrV-O-TiIV
Selective redox coupling
Han, Frei, J. Phys. Chem. C 112, 8391 (2008)
CrV EPRX-ray K-edge
3200 3300 3400 3500
-0.5
0.0
0.5
Rel
ativ
e in
ten
sity
Magnetic field (G)
103
TiCrAl-MCM41
CrVIAl-MCM41
CrV
Sp: g=1.977, g//=1.964
6000 60400
1
2
No
rma
lize
d A
bs
orp
tio
n
Energy (eV)
Cr-AlMCM-41, cal 630C
as-syn TiCr-AlMCM-41
400 600 8000.0
0.3
0.6
0.9
1-R
Wavelength (nm)
TiCr-AlMCM-41
CrIII-AlMCM-41
TiIV-O-CrIII TiIII-O-CrIV
DRS
0 1 2 3 4 5 6 70
2
4
62.00Å(CrIII-O)
FT M
agni
tude
Distance R (Å)
1.59Å(CrVI=O)
2.70Å(CrIII-O-Ti)
TiCr-AlMCM-41
Cr-AlMCM-41
0 1 2 3 4 5 6 70
2
4
62.00Å(CrIII-O)
FT M
agni
tude
Distance R (Å)
1.59Å(CrVI=O)
2.70Å(CrIII-O-Ti)
TiCr-AlMCM-41
Cr-AlMCM-41
EXAFS
0 2 4 6 80
4
8
R (Å)
|
(R)|
(Å
-4)
B
Selective assembly of binuclear MMCT units for driving water oxidation catalysts:TiOCrIII
Cr EXAFS curve fitting:
Cr-O N DW
1.97 A 3.8 0.003
0 2 4 6 80
4
8
|(R
)| (
Å-4
)
R (Å)
B
CrIII TiOCrIII
Cr-O
Cr--Ti
• Second 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
Cr-O
Cr-O N DW Cr---Ti N DW Cr----Si N DW
2.01 A 3 0.001 3.14 1 0.007 2.89 3 0.003
1.72 A 1 0.003
Binuclear TiOCrIII pump drives H2O oxidation catalyst under visible light
• Efficient visible light water oxidation in aqueous suspension observed
Han, Frei, J. Phys. Chem. C 112, 16156 (2008)
Nakamura, Frei, J. Am. Chem. Soc. 128, 10689 (2006)
O2 evolution using Clark electrode
Quantum yield = 14% (lower limit!)
-0.5 0.0 0.5 1.0 1.5 2.00
3
6
9
O2(m
g/L
)
Time (hour)
Level of saturated O2 in water
IrxO
y-TiCr-AlMCM-41
Light on
10 nm10 nm
HR-TEM of Ir oxidenanoclusters insilica channels
• Electron donation from IrOx catalyst competes successfully with back electron transfer from Ti III • Flexibility of donor metal selection for matching redox potential of charge-transfer chromophore and catalyst
EPR and FT-Raman spectroscopy show formation of TiIV…O2- complex
3200 3250 3300
0.0
0.5
1.0103
Rel
ativ
e in
ten
sity
Magnetic field (G)
g1 = 2.034
g2 = 2.010
g3= 2.005
superoxide
before photolysis
after photolysis
simulated spectrum
3200 3300 3400 3500
0.0
0.5
1.0
photolysis of IrxO
y-TiCr-AlMCM-41+H
2O
Inte
ns
ity
photolysis of IrxO
y-Cr-AlMCM-41+H
2O
superimposed EPR spectrum of simulate Ti III and CrV
Magnetic field (G)
TiIIITiIV…O2-
TiIV-O-CrIII/IrOx TiIII-O-CrIV/IrOxMMCT
hv
1100 1000 900
after photolysis in H2
18O
after photolysis in H2
16O
Raman shift (cm-1)
Ra
ma
n in
ten
sit
y (
a.u
.)
994
9619300.0005
16O18O-
O2-
18O2-
O2 trapped by transient TiIII
O2- detected in aqueous solution
18O labeling of superoxide when using H2
18O
EPR
FT-Raman
• Transient absorption spectroscopy of MMCT units using index-matching liquids (mineral oil, silicone oil, or CHCl3)
• 5 nanosecond resolution
Elucidation of electron transfer pathways and kinetics of binuclear charge-transfer chromophore by transient absorption spectroscopy
TiMnII-MCM-41
DRS
L-edge X-ray absorption
Ti
MnII
Excitation of TiOMn, 400-600 nm
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
Transient bleach of MMCT transition observed
• Recovering bleach is due to back electron transfer of excited Ti IIIOMnIII → TiIVOMnII
• Spread of first order rate constants indicates structural heterogeneity of the silica environment of the binuclear sites
TiMn-SBA-15
T. Cuk, W. Weare, H. Frei, J. Phys. Chem. C, submitted
Pump Dependence:Kinetic and Spectral
Pump Spectral Dependence: DRS Comparison
-8
-6
-4
-2
0
O
D (
10-3
)
1086420Time (s)
Probe: 400nmPump
425 nm 445 nm 475 nm 535 nm
-10
-5
0
O
D (
10-3
)(t=
0, t
avg)
600550500450400Pump (nm)
t=0 Albery Fits, normalized data tavg, unnormalized data DRS Static Spectra
1/k' = 1.8 ± 0.3s = 2 ± 0.2
(a) (b)
MMCT
Ti(IV)OMn(II)
Ti(III)OMn(III)
e0(Ti)t2g3(Mn)eg
2(Mn) S= 5/2
e1(Ti)t2g3(Mn)eg
1(Mn) S= 5/2
S = 3/2
G
Unusually slow back electron transfer
• 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
hv
Si
O
Si
Ti
O O
O
Si Si
O
Si
Ce
O O
Si Si
III
• 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
TiIV-O-CoII TiIII-O-CoIII
300 400 500 600 7000.0
0.2
0.4
0.6
1-R
Wavelength (nm)
300 400 500 600 700
MMCT
TiCe-MCM-41
Ti-MCM-41
Ce-MCM-41
DRS
TiIV-O-CeIII TiIII-O-CeIV
400 600 8000.0
0.2
0.4
1-R
Wavelength (nm)
TiCo-MCM-41
Ti-MCM-41 + Co-MCM-41
Co-MCM-41
MMCT
5730 57600
1
2
3
4
No
rmal
ized
Ab
sorp
tio
n
Energy (eV)
5727
5728
E = +1 eV
A
a
b
5720 5760 58000.0
0.8
1.6
2.4
No
rmal
ized
Ab
sorp
tio
n
Energy (eV)
5729 5737
a'
b'
B
Ce L-edge
CeIII
TiCeIII
CeIV
TiCeIV
Han, Frei, J. Phys. Chem C 112, 8391 (2008);Microporous Mesoporous Mater. 103, 265 (2007)Nakamura, J. Am. Chem. Soc. 129, 9596 (2007)
XAFS
EPR
Selective assembly of binuclear MMCT units for driving water oxidation catalysts:TiOCoII, TiOCeIII
2000 3000 40000.0
0.5
1.0
1.5
Rel
ativ
e In
ten
sity
Magnetic Field (G)
103
a
b
g = 5.250
g = 5.107g
// = 2.034
g// = 2.032
CoII
CoII linked to Ti is high spin, tetrahedral
• Coupling of fuel generating photocatalytic sites (green) with O2 evolving sites (purple) across nanoscale wall • Separation of oxygen from methanol
CO2 + H2O CH3OH + O2visible light
Coupling polynuclear photocatalysts in nanoporous silica scaffoldsto achieve separation of reduced products from evolving oxygen
Two photon system
envisioned integrated system
(L)
(L = inorg. or C-based conducting linker)
Long term goal:
CO2
CH3OH
H2OO2
H2OO2
CO2
reductionH2O oxidation
hν
Mn oxide core/ silica shell construct
Co3O4 or MnOx core
silica shell
Reverse microemulsion method (Ying, J.Y., Langmuir 24, 5842 (2008))
F. Jiao
Co or Mn oxide/ silica core shell constructs with nanowires penetrating SiO2 shell
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
HELIOSHELIOS
Drs. Vittal Yachandra, Junko YanoFacilities: 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
Postdoctoral Fellows:
Feng JiaoWalter WeareHongxian HanTania Cuk (Miller fellowship)N. SivasankarMarisa MacNaughtan
AcknowledgmentsHELIOSHELIOS