Photochemical and electrochemical CO2 reduction using hybrid catalysts

Chao Liu, University of New Hampshire, Durham

Abstract

Carbon dioxide (CO2) is a renewable carbon feedstock for the production of chemicals, materials, and fuels. Although CO2 is often considered as an industrial waste, it should be recycled as a sustainable carbon source. At the highest oxidized state of carbon, CO2 has superior thermodynamic stability, which makes its activation an energy intensive process. Photochemical reduction is a promising approach to achieving sustainable CO2-to-fuel conversion by using natural sunlight as the only energy input. Electrochemical methods have been extensively investigated for efficient CO2 reduction, too. Currently, we still lack efficient and robust catalysts for direct CO2 reduction.

Chapter I provides an introduction to various catalysts known for their activity in mediating CO2 reduction. Heterogenization of molecular catalysts bridges homogeneous and heterogeneous catalysis. Molecular CO2-reduction catalysts, including tricarbonyl Re(I) and macrocylic Co(III) complexes, have been covalently attached onto silica, Kaolin, titania and metal-organic frameworks. Their catalytic performances, surface reactions and stabilities are discussed in Chapter II. The tricarbonyl Re(I) catalysts were immobilized on different supporting materials via covalent linkages. In general, the covalent attachment was achieved by ligand derivatization and further coupling with silane agents for immobilization on silica and Kaolin. Ligand derivatization resulted in significant changes in optical, redox and photocatalytic properties of the tricarbonyl Re(I) complexes. The tricarbonyl Re(I) unit was also incorporated in metal-organic frameworks. A macrocyclic Co(III) catalyst was deposited on TiO2 surfaces for used in photocatalytic CO2 reduction. Infrared studies clearly demonstrated photogenerated electrons in TiO2 were transferred to the surface Co(III) catalyst for CO2 reduction.

Research on TiO2-based nanocomposite materials will be described in Chapter III. Photocatalysis on TiO2 nanomaterials is a very active research direction because of the many advantages of TiO2, including its low cost, non-toxic nature, and biocompatibility. A major limitation of TiO2 nanomaterials is the low efficiency in most photocatalytic processes due to recombination of photogenerated electrons and holes. Different strategies have been studied in order to improve charge separation in TiO2 photocatalysis by modifying surface and/or bulk structures as well as forming nanocomposites. Nanocomposite photocatalysts were prepared by depositing Cu nanoparticles/clusters on TiO2 via different methods. The Cu/TiO2 photocatalysts were then examined in photocatalytic CO2 reduction. Nanosized Au and Ag particles were deposited on surfaces to further improve the catalytic performance of Cu/TiO2 materials. In addition, TiO2 clusters were synthesized in mesoporous silica to achieve highly dispersed Ti sites, which have been reported to possess excellent reactivity in photocatalytic CO2 reduction.

Electrochemical reduction of CO2 using tricarbonyl Re(I) complexes, as well as the metal oxides modified electrodes, are discussed in Chapter IV. To understand the relationship between the catalytic performance and ligand derivatization, several Re(I) complexes with different electron withdrawing/donating groups were synthesized and evaluated by electrochemical methods. The ligand derivatization significantly altered the electrocatalytic properties of tricarbonyl Re(I) catalysts. Conducting glass electrodes coated with oxides of tin and molybdenum were prepared by electrochemical deposition methods and showed promising CO2 reduction activity under electrochemical conditions.