Contact info:
Lawrence Berkeley National Laboratory
Physical Biosciences Division
One Cyclotron Road, Calvin Lab
Berkeley, California 94720
USA

Location: Calvin 232
Phone: (510) 486-4325
Fax: (510) 486-6059
Email: hmfrei@lbl.gov

Research Emphasis

The goal of our research is to develop robust artificial systems for the synthesis of fuels and chemicals from carbon dioxide and water using sunlight as energy source. Our approach is to assemble well-defined inorganic polynuclear units in nanoporous silica materials that function as visible light photocatalysts for CO₂ reduction or H₂O oxidation. The very high surface area of the solid support serves to accommodate the photocatalytic sites at sufficient density, while the nanostructured features offer a means for separating reduced from oxidized species on the nanoscale. In current work, binuclear metal-to-metal charge-transfer (MMCT) units such as Ti-O-Co^II, Ti-O-Sn_II or Zr-O-Cu_I that absorb light deep in the visible are anchored on the pore surface. Single photon excitation of the MMCT transition of a ZrOCu site led to two-electron reduction of CO₂ to CO. On the same silica support, a photocatalytic unit consisting of a single Cr^VI embedded in the pore surface coupled to an Ir oxide nanocluster was shown to oxidize water to O₂ under visible light. Reactions are monitored by in situ FT-IR, optical and mass spectroscopy.

Characterization of the polynuclear sites by FT-IR, FT-Raman, EXAFS and optical spectroscopy and high resolution electron microscopy furnishes detailed structural information on the redox units. These examples demonstrate that this is a flexible synthetic method for assembling and efficiently coupling ‘molecular’ visible light-pumped multi-electron transfer catalysts for CO₂ reduction and H₂O oxidation. A long-term goal is to accomplish the reduction of carbon dioxide by water to methanol with visible light. For mechanistic studies, time-resolved step-scan FT-IR spectroscopy is employed in order to unravel elementary redox steps. Transient radicals and surface intermediates are detected on the nanosecond to millisecond time scale, and the dynamics of final products inside the nanopores can be followed. The mechanistic understanding of already working systems combined with the knowledge of electronic and structural details of the polynuclear photocatalytic sites are crucial for designing more efficient systems.

Recent Publications

R. Nakamura and H. Frei, “Visible Light-Driven Water Oxidation by Ir oxide Clusters Coupled to Single Cr Centers in Mesoporous Silica”. J. Am. Chem. Soc. 128, 10668-10669 (2006).

H. Frei, “Selective Hydrocarbon Oxidation in Zeolites”. Science 313, 309-310 (2006).

L.K. Andersen and H. Frei, “Dynamics of CO in Mesoporous Silica Monitored by Time-Resolved Step-Scan and Rapid-Scan FT-IR Spectroscopy”. J. Phys. Chem. B 110, 22601-22607 (2006).

H. Han and H. Frei. “Optical and Infrared Evidence for Binuclear TiOCo^II Charge-Transfer Chromophores in Mesoporous Silica”. Microporous Mesoporous Mater., in press.

W. Lin and H. Frei, “Photochemical CO2 Splitting by Metal-to-Metal Charge-Transfer Excitation in Mesoporous ZrCu^I-MCM-41 Silicate Sieve”. J. Am. Chem. Soc. 127, 1610-1611 (2005).

W. Lin and H. Frei, “Anchored Metal-to-Metal Charge-Transfer Chromophores in Mesoporous Silicate Sieve for Visible Light Activation of Ti Centers”. J. Phys. Chem. B 109, 4929-4935 (2005).

W. Wasylenko and H. Frei, “Direct Observation of Surface Ethyl to Ethane Interconversion upon C₂H₄ Hydrogenation over Pt/Al₂O₃ by Time-Resolved FT-IR Spectroscopy”. J. Phys. Chem. B 109, 16873-16878 (2005).

W. Lin, H. Han, and H. Frei, “CO₂ Splitting by H₂O to CO and O₂ under UV Light in TiMCM-41 Silicate Sieve”. J. Phys. Chem. B 108, 18269-18273 (2004).

Y.H. Yeom and H. Frei, “Time-Resolved Step-Scan and Rapid Scan FT-Infrared Spectroscopy”. In: In-Situ Characterization of Catalysts; Weckhuysen, B. M., Ed; American Scientific Publisher: New York, 2004; p.32-46.

M.K. Ko and H. Frei, ”Millisecond FT-IR Spectroscopy of Surface Intermediates of C₂H₄ Hydrogenation over Pt/Al₂O₃ Catalyst under Reaction Conditions”. J. Phys. Chem. B. 108, 1805-1808 (2004).