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Nanostructured Water Oxidation Photocatalysts Heinz Frei February 3, 2010

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

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Nanostructured Water Oxidation Photocatalysts Heinz Frei February 3, 2010

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  1. HELIOS Nanostructured Water Oxidation PhotocatalystsHeinz FreiFebruary 3, 2010

  2. Goal: CO2 + H2O  CH3OH + O2 hv O2 H2O CH3OH visible light • Conversion in a single integrated system • (terawatt scale) • Inorganic system  robust CO2 CO2 reduction H2O O2 H2O oxidation Topics today: Robust inorganic nanoclusters as water oxidation catalysts All inorganic photocatalytic units in nanoporous silica scaffolds

  3. 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. Acta1981, 26, 1319-1326. [3] Schmidt, T.; Wendt, H. Electrochim. Acta1994, 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. Acta1977, 22, 325-328.

  4. Nanostructured Co oxide cluster in mesoporous silica scaffold 35 nm bundles65 nm bundles (4 % loading) (8 % loading) XRD Synthesis of Co oxide clusters in SBA-15 using wet impregnation method SBA-15/Co3O4 (8%) SBA-15/Co3O4 (4%) Co3O4 EXAFS Co3O4 bulk SBA-15/Co3O4 (8%) SBA-15/Co3O4 (4%) free nanorod bundle • 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

  5. Co L-edge XAS spectrum • Co L-edge absorption spectrum confirms Co3O4 structure

  6. 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

  7. 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

  8. 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

  9. 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) • Spherical Mn oxide nanoclusters, 70-90 nm diameter, mixed phase (calcination T) • The phase composition was determined by component analysis of XANES spectra

  10. Efficient oxygen evolution in aqueous solution using Ru2+(bpy)3- persulfate visible light sensitization system Mass Spec O2 evolution TOF 3,320 s-1 per cluster Mild conditions: pH 5.8, 22 oC overvoltage 350 mV 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

  11. 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., Langmuir24, 5842 (2008)) F. Jiao

  12. Precise matching of redox potentials of the components in organic molecular systems Hammarstrom, Chem. Soc. Rev.30, 36 (2001)

  13. Approach: Well-defined all-inorganic polynuclear photocatalysts arranged in robust 3-D nanoporous scaffold nanoporous silica support MCM-41 SBA-15 200 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

  14. h MMCT (visible light) e- TiIV-O-CrIII TiIII-O-CrIV O Ti CrIII O O O O O O Si Si Si Si Si Al Selective assembly of binuclear MMCT units for driving water oxidation catalysts: TiOCrIII DRS CrVI(=O) + TiIII CrV-O-TiIV Selective redox coupling X-ray K-edge EXAFS CrV EPR Han, Frei, J. Phys. Chem. C112, 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

  15. Selective assembly of binuclear MMCT units for driving water oxidation catalysts: TiOCrIII CrIII TiOCrIII Cr-O Cr-O Cr--Ti Cr EXAFS curve fitting: Cr-O N DW 1.97 A3.80.003 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 • 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

  16. Binuclear TiOCrIII pump drives H2O oxidation catalyst under visible light HR-TEM of Ir oxide nanoclusters in silica channels O2 evolution using Clark electrode Quantum yield = 14% (lower limit!) Han, Frei, J. Phys. Chem. C112, 16156 (2008) Nakamura, Frei, J. Am. Chem. Soc.128, 10689 (2006) • Efficient visible light water oxidation in aqueous suspension observed

  17. EPR and FT-Raman spectroscopy show formation of TiIV…O2- complex TiIV…O2- EPR TiIII FT-Raman 16O18O- 18O2- O2 trapped by transient TiIII O2- detected in aqueous solution 18O labeling of superoxide when using H218O O2- • 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

  18. 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 index-matching liquids (mineral oil, silicone oil, or CHCl3) • 5 nanosecond resolution

  19. Transient bleach of MMCT transition observed Excitation of TiOMn, 400-600 nm TiMn-SBA-15 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 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

  20. 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

  21. Selective assembly of binuclear MMCT units for driving water oxidation catalysts: TiOCoII, TiOCeIII O III Ce Ti O O O O O O Si Si Si Si Si Si Si Han, Frei, J. Phys. Chem C112, 8391 (2008); Microporous Mesoporous Mater.103, 265 (2007) Nakamura, J. Am. Chem. Soc. 129, 9596 (2007) XAFS Ce L-edge TiCeIII TiCeIV TiIV-O-CeIII TiIII-O-CeIV CeIII CeIV CoII EPR TiIV-O-CoII TiIII-O-CoIII CoII linked to Ti is high spin, tetrahedral • 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

  22. 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 H2O CH3OH O2 (L) (L = inorg. or C-based conducting linker) CO2 CO2 reduction Two photon system H2O oxidation O2 H2O envisioned integrated system • Coupling of fuel generating photocatalytic sites (green) with O2 evolving • sites (purple) across nanoscale wall • Separation of oxygen from methanol

  23. 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., Langmuir24, 5842 (2008)) F. Jiao

  24. 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

  25. 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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