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