Nanostructured Water Oxidation Photocatalysts Heinz Frei February 3, 2010

Conversion in a single integrated system (terawatt scale) Inorganic system  robust CO 2 + H 2 O  CH 3 OH + O 2 Goal: visible light H2OH2O H2OH2O O2O2 O2O2 CO 2 CH 3 OH H 2 O oxidation CO 2 reduction hvhv 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 OxideTOFOvervoltage, ηpHTQuantumReference (sec -1 )(mV)( o C)yield Co 3 O RT58%Harriman (1988) [1] Co 3 O 4 > Tamura (1981) [2] Co 3 O 4 > Wendt (1994) [3] Co 3 O 4 > Tseung (1983) [4] Co 3 O 4 > Singh (2007) [5] Co,P film> Nocera (2008) [6] ~ Nocera (2009) [7] MnO 2 > Tamura (1977) [8] Mn 2 O RT35%Harriman (1988) [1] [1] Harriman, A.; Pickering, I.J.; Thomas, J.M.; Christensen, P.A. J. Chem. Soc., Farad. Trans , 84, [2] Iwakura, C.; Honji, A.; Tamura, H. Electrochim. Acta 1981, 26, [3] Schmidt, T.; Wendt, H. Electrochim. Acta 1994, 39, [4] Rasiyah, P.; Tseung, A.C.C. J. Electrochem. Soc. 1983, 130, [5] Singh, R.N.; Mishra, D.; Anindita; Sinha, A.S.K.; Singh, A. Electrochem. Commun. 2007, 9, [6] Kanan, M.W.; Nocera, D.G. Science 2008, 321, [7] Nocera, D.G. Symposium Solar to Fuels and Back Again, Imperial College, London, [8] Morita, M.; Iwakura, C.; Tamura, H. Electrochim. Acta 1977, 22,

Nanostructured Co oxide cluster in mesoporous silica scaffold 35 nm bundles 65 nm bundles (4 % loading) (8 % loading) free nanorod bundle Synthesis of Co oxide clusters in SBA-15 using wet 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 Co 3 O 4 bulk SBA-15/Co 3 O 4 (8%) SBA-15/Co 3 O 4 (4%) EXAFS XRD SBA-15/Co 3 O 4 (4%) SBA-15/Co 3 O 4 (8%) Co 3 O 4

Co L-edge XAS spectrum Co L-edge absorption spectrum confirms Co 3 O 4 structure

F. Jiao, H. Frei, Angew. Chem. Int. Ed. 49, 1841 (2009) SBA-15/Co 3 O 4 35 nm bundle 65 nm bundle O 2 evolution Visible light water oxidation in aqueous SBA-15/Co 3 O 4 suspension using Ru 2+ (bpy) 3 + S 2 O 8 2- method. Mild conditions: 22 o C, pH 5.8, overvoltage 350 mV High catalytic turnover frequency: 1140 O 2 molecules per second per cluster  TOF of catalyst per projected area = 1 s -1 nm -2  mesoporous silica membrane, 150 μ thick: TOF = 100 s -1 nm -2 Co 3 O 4 micron sized particles O2O2 SBA-15/NiO (8%) Mass spectroscopic monitoring Efficient oxygen evolution at Co 3 O 4 nanoclusters in mesoporous silica SBA-15 in aqueous suspension TOF 1140 s -1 per cluster

Co 3 O 4 structure in silica scaffold stable under water oxidation catalysis Co K-edge: No sign of Co oxidation state change after photolysis O 2 yield is 1600 times larger than for 35 nm bundle catalyst compared to μ-sized Co 3 O 4 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/Co 3 O 4 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 MnO 1.51 KIT-6 (3D channels) Spherical Mn oxide nanoclusters, nm diameter, mixed phase (calcination T) The phase composition was determined by component analysis of XANES spectra XAFS calcined 600 o C MnO 2 Mn 2 O 3 Mn 3 O o C64%36%- 500 o C95%5%- 600 o C6%80%14% 700 o C-81%19% 800 o C-70%30% 900 o C-51%49%

Efficient oxygen evolution in aqueous solution using Ru 2+ (bpy) 3 - persulfate visible light sensitization system Most active catalyst: MnO 1.51 with TOF = 3,320 O 2 s -1, which corresponds to 0.6 sec -1 nm -2 projected area  200 μm membrane with TOF of 100 s -1 nm -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 O 2 evolution TOF 900 s -1 per cluster Mass Spec Mild conditions: pH 5.8, 22 o C overvoltage 350 mV TOF 3,320 s -1 per cluster F. Jiao, H. Frei, submitted

Mn oxide core/ silica shell construct Co 3 O 4 or MnO x 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 components in 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-41 SBA-15

Ti O O O Si O Cr III O O Al Si MMCT (visible light) O Si hh e-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: TiOCr III Cr VI (=O) + Ti III  Cr V -O-Ti IV Selective redox coupling Han, Frei, J. Phys. Chem. C 112, 8391 (2008) Cr V EPRX-ray K-edge Ti IV -O-Cr III  Ti III -O-Cr IV DRS EXAFS

Selective assembly of binuclear MMCT units for driving water oxidation catalysts: TiOCr III Cr EXAFS curve fitting: Cr-ON DW 1.97 A Cr III TiOCr III 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-ON DWCr---Ti N DWCr----SiN DW 2.01 A A

Binuclear TiOCr III pump drives H 2 O oxidation catalyst under visible light Efficient visible light water oxidation in aqueous suspension observed Han, Frei, J. Phys. Chem. C 112, (2008) Nakamura, Frei, J. Am. Chem. Soc. 128, (2006) O 2 evolution using Clark electrode Quantum yield = 14% (lower limit!) HR-TEM of Ir oxide nanoclusters in silica channels

Electron donation from IrO x 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 Ti IV… O 2 - complex Ti III Ti IV… O O 18 O - O2-O2- 18 O 2 - O 2 trapped by transient Ti III O 2 - detected in aqueous solution 18 O labeling of superoxide when using H 2 18 O EPR FT-Raman

Transient absorption spectroscopy of MMCT units using index- matching liquids (mineral oil, silicone oil, or CHCl 3 ) 5 nanosecond resolution Elucidation of electron transfer pathways and kinetics of binuclear charge-transfer chromophore by transient absorption spectroscopy TiMn II -MCM-41 DRS L-edge X-ray absorption Ti Mn II

Excitation of TiOMn, nm Albery model for dispersive 1 st 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 III OMn III → Ti IV OMn II 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

MMCT Ti(IV)OMn(II) Ti(III)OMn(III) e 0 (Ti)t 2g 3 (Mn)e g 2 (Mn) S= 5/2 e 1 (Ti)t 2g 3 (Mn)e g 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 hvhv

Si O Ti O O O Si O Ce O O Si III Selective assembly due to higher acidity of TiOH vs. SiOH MMCT excitation by visible light generates donor centers (Ce IV, Co III ) of sufficiently positive potential for driving H 2 O oxidation catalyst Ti IV -O-Co II  Ti III -O-Co III Ti IV -O-Ce III  Ti III -O-Ce IV Ce L-edge Ce III TiCe III Ce IV TiCe IV 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: TiOCo II, TiOCe III Co II Co II linked to Ti is high spin, tetrahedral

Coupling of fuel generating photocatalytic sites (green) with O 2 evolving sites (purple) across nanoscale wall Separation of oxygen from methanol CO 2 + H 2 O  CH 3 OH + O 2 visible light Coupling polynuclear photocatalysts in nanoporous silica scaffolds to 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: CO 2 CH 3 OH H2OH2O O2O2 H2OH2O O2O2 CO 2 reduction H 2 O oxidation hνhν

Mn oxide core/ silica shell construct Co 3 O 4 or MnO x 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 SiO 2 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 H 2 O oxidation to O 2 under visible light (TiOCr III chromophore driving an IrO x nanocluster catalyst) at > 14 % quantum efficiency, hydroperoxide intermediate observed Co 3 O 4 and MnO 1.51 nanocluster catalysts of abundant materials for water oxidation, TOF in range suitable for keeping up with solar flux

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 Postdoctoral Fellows: Feng Jiao Walter Weare Hongxian Han Tania Cuk (Miller fellowship) N. Sivasankar Marisa MacNaughtan Acknowledgments