Studies On Photocatalytic Hydrogen Production Using Dye Sensitized POM And TiO₂under Visible Light Irradiation

As a typical photocatalyst, TiO2is most frequently employed owing to its cheapness, nontoxicity, structural stability and intriguing physical and chemical properties etc.. POMs (polyoxometalates) are other kind of photocatalysts with intriguing redox ability as well as photoelectrochemical property. Howerover, both TiO2and POM can only be excited under ultraviolet (UV) light, which occupies only4%～6%in the solar spectra, for visible light, this number reaches to43～46%around. Dye sensitization is a good strategy to broaden the excitation wavelength range of photocatalyst and to utilize visible light.A significant problem in dye sensitization systems is the stability issue, which is also a key factor that limits their pratical applications. POMs are good electron acceptors, they can quickly transfer the electron from photoinduced dye species and suppress the decomposition of the sensitizers. So they are expected to conduct efficient and stable dye-sensitized photocatalysts or systems for hydrogen evolution. How to enhance the activities of dye sensitization systems are the other important challenge, modification of TiO2by metal ions or nonmetal ions could improve its photosensitized activity for hydrogen production.In this dissertation, xanthene dyes, such as EosinY, sensitized POMs and modified TiO2for photocatalytic hydrogen production were studied, and the whole work includes the following five sections:(I). Photosensitization of Keggin POMs (SiW12O404-、PW12O403-etc) with xanthene dyes (EosinY、Erythrosin B、Rose Bengal) were studied. Triethanolamine (TEOA) was used as electron donor in the experiments. The formation of heteropoly blue (POM-) under visible light illumination demonstrated that the electron transfer occurs between the dye and POM. In the presence of Pt as co-catalyst, Eosin Y sensitized PW12O3-or SiW12O4-system displays relatively high activity for hydrogen production with the average rates of53.3μmol h-1,51.8μmol h-1and the apparent quantum yields of9.2%,8.9%during20h irradiation (k>420nm), respectively. (Ⅱ). Eosin Y (EY)-sensitized a-[SiAlW11(H2O)O39]5-(AlSiW11)(an Al3+substituted Keggin POM) for the hydrogen evolution under visible light irradiation (λ>420nm) was investigated. EY can coordinate with AlSiW11. The coordination association between AlSiW11and EY is beneficial to the charge transfer between them and to promote the photochemical stability of EY. Thus, the system displays efficient and stable photocatalytic hydrogen evolution. The research suggests that a chemical association (coordination or covalence) between POM and dye must make contribution to improve stability and activity of dye sensitization system.(Ⅲ). TiO2was modified with a lacunary Keggin-type POM SiW11O398-(SiW11) to obtain SiW11/TiO2composite SiW11is bound at TiO2by a chemical interaction, which was demonstrated by FT-IR (Infrared spectroscopy) and UV (Ultraviolet spectroscopy). SiW11modification promotes TiO2crystallization as well as growth of the particles. EY sensitized SiW11/TiO2displays stable and efficient visible-light H2generation using Pt as a co-catalyst. SiW11, as an excellent electron relay, greatly facilitates the electron transfer from the reduced dye species EY’-H (the protonated form) to conduction band (CB) of TiO2and suppresses effectively the decomposition of EY·-. Thus, the stability of dye-sensitized system is enhanced. The factors which influenced the photocatalytic hydrogen evolution such as concentration of SiW11, concentration of EY and Pt loading content etc. were investigated. Under the optimal conditions, an average apparent quantum yield of11.4%was obtained from the above system during20h irradiation (λ>420nm). The photosensitized electron transfer mechanism was also discussed.(IV). An Al3+coupled and EY-sensitized TiO2photocatalyst was prepared. The coupling of Al3+can markedly improve the visible-light activity of dye-sensitized catalyst. Nevertheless, the stability of the as-prepared catalyst is poor. The effects of different metal ion and water content in photocatalytic system on hydrogen evolution were investigated. The results of hydrogrn evolution combined with IR, XPS (X-ray photoelectron spectroscopy) and UV-Vis specra, suggest that hydrolysis, complexing between Al3+and TEOA as well as degradation of dye result in a decreased activity of the photocatalytic system, the photocatalyst exhibits a enhanced activity and stability when water content is very low (0.5%volume fraction). (V). TiO2was modified by sulfate and phosphate (denoted as S/TiO2and P/TiO2) by simply sulfur or phosphoric acid treatment. The strong chemical coordination occurs between sulfate or phosphate and TiO2. EY sensitized S/TiO2and P/TiO2exhibit enhanced photocatalytic activity for hydrogen evolution upon visible light illumination compared to that of EY sensitized TiO2. The CB edges of S/TiO2and P/TiO2shift toward the negative, and the hydrogen bond interaction between EY·-H and S/TiO2or P/TiO2is enhanced due to the inducing effect of bound sulfate and phosphate, thus, photocatalytic hydrogen evolution is promoted. The factors which influenced the photocatalytic hydrogen evolution such as concentration of sulfate or phosphate, concentration of EY, and Pt loading content were investigated.