Towards solar fuels : photocatalyst development for water splitting and carbon dioxide reduction.
Date of Issue2013
School of Chemical and Biomedical Engineering
Solar fuels derived from solar energy are envisioned to replace fossil fuels for a sustainable earth and society. Photocatalytic water splitting for hydrogen (H2) production and photocatalytic carbon dioxide (CO2¬) reduction for gaseous or liquid fuels production are two of the most promising but challenging processes. Advancement of these two reactions for solar fuels production is largely hindered by the lack of efficient, stable and low-cost photocatalysts. In this thesis, the work was focused on the development of economical photocatalyst systems toward more efficient and stable solar fuels production via water splitting and carbon dioxide reduction. Firstly, well-dispersed layered double hydroxide (LDH) nanocrystals were used to immobilize the photosensitizer of rose bengal (RB) and Pt nanoparticles to construct a self-assembled RB-LDH-Pt system for more stable H2 generation from water. Such a system offers many advantages in photocatalytic water splitting: (i) immobilizing the organic dye photosensitizer for suppressed self-quenching, (ii) close arrangement between photosensitizer molecules and catalyst nanoparticles for efficient electron transfer, (iii) formation of well-dispersed catalysts nanoparticles on the support surface, and (iv) easy recycle of the expensive catalyst. Secondly, in situ sulfur-doped mesoporous graphitic carbon nitride (mpgCNS) was synthesized from a single precursor of thiourea, by using SiO2 nanoparticles as template. Sulfur group was found to facilitate the condensation of carbon nitride (C3N4) polymer. Higher product yield was obtained from thiourea than that from urea as the starting material. The resultant sulfur-doped mesoporous C3N4 exhibited 30 times higher activity than the native C3N4 for hydrogen evolution. A relatively high apparent quantum efficiency of 5.8% was obtained at 440 nm. The activity enhancement was attributed to 1) stronger and extended light absorption in the visible light region by sulfur doping, 2) more efficient mass and charge transfer in mesoporous structure. Thirdly, noble metal-free C3N4 based photocatalyst was developed for H2 production by loading nickel sulfide (NiS) cocatalyst on C3N4 via a simple hydrothermal method. Both the weight percentage of NiS and the hydrothermal reaction temperature were found to have great influence on the photocatalytic activities. The optimal NiS/C3N4 photocatalyst has 253 times higher activity than the pristine C3N4 and is able to retain over 80% of the activity after 4 runs of 24 h. Fourthly, systematic analysis methods were developed for photocatalytic CO2 reduction by a combination of gas chromatography (GC) and high performance liquid chromatography (HPLC) methods. The developed methods are able to detect and quantify major products in both gas and liquid phases with low detection limits. The effects of several organic additives including commonly used solvents, photosensitizers and sacrificial reagents in photoreaction were investigated. It has been found that alcohol analysis by GC method is more sensitive to organic additives. In comparison, aldehyde and acid analyses by HPLC method are not affected by most of the organics investigated. The importance of carbon source verification is highlighted and techniques for the verification were proposed. Finally, self-assembly of carbon nitride (C3N4) and LDH was constructed by the electrostatic force based on their opposite surface charges. The immobilized nitrate LDH can easily turn into carbonate LDH in situ anion exchange during photocatalytic reduction of CO2. The CO32- anions present in the interlayer space of LDH exhibit much higher reduction efficiency than the free gaseous CO2 molecules. As a result, 2.6 times higher activity for CH4 production can be achieved on 10 wt% LDH/C3N4 than that without the LDH coating. In summary, this thesis was devoted to the development of low-cost, stable and efficient photocatalysts for water splitting and carbon dioxide reduction. Different modification on photocatalytic materials such as sensitization, doping, mesoporous structure formation and cocatalyst loading were studied. Carbon nitride was used as the main photocatalytic materials for both water splitting and carbon dioxide reduction. LDH was introduced as a support to enhance the stability in H2 production and a CO2 capture medium to improve the activity of CO2 reduction. Besides, systematic analytical methods based on GC and HPLC were also developed for the analysing the products of CO2 reduction.