Photosynthetic and Photocatalytic Conversion using Dye-sensitization and Surface Modified TiO2

Abstract

This study investigated solar conversion systems with hybrids catalysts based on titanium oxide. In particular, the photocatalytic reduction of CO2 is a promising method to reduce excess CO2 present in the atmosphere, and simultaneously produce solar fuels (e.g. CO, CH4 and CH3OH) by means of an environmental friendly approach (i.e. artificial photosynthesis). However, the efficiency of the process is still very low (<1%) because CO2 one electron reduction to form CO2•- is highly undesirable. An alternative and more favorable pathway is to reduce CO2 though proton-assisted multiple-electron transfer (PMET). Although its detailed mechanism remains unclear, it is proposed to involve intermediates and different side reaction pathways. Thus, Nafion was introduced on Pd-TiO2 surface for enhancing PMET via keeping high proton concentration in the first work. However, it still demends further studies to improve the photocatalytic efficiency and to utilize the visible light which is abundant in the solar light. Therefore, in the 2nd - 4th work, sensitizers were used for visible light activity. In the second work, introducing thin Al2O3 overlayer on dye-sensitized TiO2 markedly enhances the visible photocatalytic activity for hydrogen production and dechlorination of chlorinated organic compound, CCl4. By measuring TDR spectra of bare TiO2 and Al2O3/TiO2 in slurry, the main role of the alumina barrier layer is proven to retard the charge recombination between the electron injected from the excited dye and the oxidized dye. In the third work, water-soluble [SnIV(OH2)2-TPyHP]6+ (s-SnP) and water-insoluble SnIV(OH)2TPP (ins-SnP) were synthesized and immobilized on SiO2 (hetero-SnP) for enrironmental remediation. In the fourth, s-SnP have been also applied for H2 production. This study reports a porphyrin-sensitized TiO2 system for hydrogen production under visible light in which the adsorption of sensitizer on TiO2 is not needed. Although SnP hardly adsorbs on TiO2, hydrogen was successfully produced under visible light. This non-adsorbing sensitizer is liberated from the breaking of anchoring group and dye desorption caused by an extreme conditions during photolysis in aqueous solution. SnP can also improve the price competitiveness comparing with Ru-complexes. 1. Nafion (perfluorinated polymer with sulfonate groups) overlayer on TiO2 was employed for the photoreduction of CO2 in the absence of a sacrificial electron donor. Introducing a thin Nafion overlayer on Pd-TiO2 significantly increased the photosynthetic activity for CO2 reduction under UV and solar light. The main role of the Nafion overlayer is to provide high proton concentration within the layer to facilitate the proton-coupled multielectron transfer reactions and to retard the oxidative degradation of intermediates and products of CO2 reduction. The production of hydrocarbons such as methane, ethane, and propane was clearly higher with Nf/Pd-TiO2 than Pd-TiO2. The photosynthetic activity of Nf/Pd-TiO2 catalyst was maintained through repeated cycles of photoreaction, which confirms the stability of Nafion layer. 2. Dye-sensitized TiO2 nanoparticles that were loaded simultaneously with Pt and Al2O3 overlayer (Al2O3/TiO2/Pt) were synthesized and investigated for the photocatalytic activity under visible light. Introducing thin Al2O3 overlayer (~1 nm thick) on dye-sensitized TiO2 markedly enhanced the visible light activities for the production of hydrogen (in the presence of EDTA as an electron donor) and the dechlorination of CCl4. The Al2O3/TiO2/Pt powder was characterized by HRTEM, EDX, and XPS. In agreement with the photocatalytic activity data, the photocurrent collected via electron shuttles on a Pt electrode immersed in an aqueous photocatalyst suspension under visible light was also enhanced in the presence of Al2O3 overlayer, which indicates an enhanced interfacial electron transfer despite the presence of an insulating surface layer. The initial H2 and chloride generation rate increased from 0.4 and 5 μM min-1 on TiO2/Pt to 0.9 and 7.5 μM min-1 on Al2O3/TiO2/Pt, respectively. The visible light activity of the sensitized photocatalytic reactions highly depended on the thickness of the alumina layer and was optimized at a low level of Al loading (Al/Ti atom ratio ~0.01) above which the activity was markedly reduced with thickening the layer. It is suggested that the alumina layer retards the charge recombination between the electron injected from the excited dye and the oxidized dye. The slower charge recombination in the presence of alumina overlayer was confirmed by time-resolve diffuse reflectance (TDR) spectroscopy.3. Visible light photocatalysis using water-soluble tin porphyrin (s-SnP, [Sn(OH2)2(TPyHP)](NO3)6) and water-insoluble tin porphyrin Sn(OH)2(TPP) (ins-SnP) immobilized on SiO2 (hetero-SnP) was investigated. The visible light photocatalytic activities of s-SnP and hetero-SnP were successfully demonstrated for the degradation of 4-chlorophenol (4-CP) and acid orange 7 (AO7) in water. The visible light activity of hetero-SnP increased with the ins-SnP loading and was saturated above 77 mg/g-SiO2, which corresponded to the homogeneous concentration of [ins-SnP] = 50 μM. It is Q-band of SnP (500-650 nm) that is photocatalytically active under visible light, not Soret band (420-430 nm) whose absorption intensity is much higher. When applied to the degradation of 4-CP and AO7, hetero-SnP was particularly stable and could be used repeatedly without losing the activity whereas the activity of s-SnP was gradually reduced with repeated uses. The photocatalytic degradation reactions of 4-CP, AO7, and other organic substrates were systematically studied to show that the operating mechanisms are very different depending on the kind of substrates. The properties and activities of s-SnP and hetero-SnP as a visible light photocatalyst were investigated in various ways and discussed in detail.4. Hydrogen was successfully produced under visible light irradiation in a tin porphyrin (SnP)-sensitized TiO2 system in the wide pH range (pH 3-11) although SnP hardly adsorbs on TiO2. The number of H2 produced in the SnP/TiO2 system after 9 h irradiation corresponds to the turnover number of 410. The apparent photonic efficiency for H2 evolution was estimated to be 35% with the monochromatic radiation of 550±10 nm. The photochemical production of hydrogen is mediated through the formation of the π-radical anion (SnP•-) that subsequently transfers electron to TiO2. The photogenerated SnP•- was monitored by transient absorption spectroscopy and its lifetime is long enough to survive the slow diffusion from the solution bulk to the TiO2 surface, which makes the adsorption of SnP on TiO2 not required for hydrogen production. This is clearly contrasted with the common ruthenium complex-sensitized TiO2 system where the adsorption of the sensitizer complex is essentially required and the hydrogen production is limited to the acidic condition where the adsorption of the sensitizers on TiO2 is allowed. The photocatalytic activity of SnP was mainly attributed to the Q-band (500-650 nm), not the Soret band (420-430 nm) of which absorption intensity is much higher.