Application of Highly Functional Ti-Oxide-Based Photocatalysts in Clean Technologies
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- Takeuchi, M., Sakai, S., Ebrahimi, A. et al. Top Catal (2009) 52: 1651. doi:10.1007/s11244-009-9300-7
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Various Ti-oxide based photocatalysts such as the highly dispersed Ti-oxide species within zeolite frameworks, TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents as well as visible light responsive TiO2 thin films have been successfully prepared. Characterization studies at the molecular level, such as X-ray absorption fine structure (XAFS) and photoluminescence (PL), revealed that the highly dispersed Ti-oxide species within the nano-spaces of zeolites possess a tetrahedral coordination and that they demonstrate unique and high performance for the photocatalytic decomposition of NOx and the photocatalytic reduction of CO2 with H2O. A high photocatalytic reactivity for the TiO2 semiconducting photocatalysts could be achieved by blending them with hydrophobic siliceous zeolites which was equal to the performance of TiO2 deposited with expensive Pt particles. The role of the siliceous zeolites can be described as a so-called “catch and release effect of organic compounds”, i.e., (i) the condensation of the reactants within the hydrophobic cavities of zeolites and; (ii) the efficient diffusion of the reactant onto the TiO2 photocatalytic sites. Furthermore, a novel photocatalytic system which can convert abundant solar energy into renewable H2 energy by the decomposition of H2O into H2 and O2 can also be achieved by using visible light responsive TiO2 thin film photocatalysts prepared by a RF-magnetron sputtering deposition method. The conversion efficiency of solar energy into H2 energy may be estimated at ca. 0.1% from the initial rate of H2 evolution.
Environmental pollution on a global scale as well as the lack of natural energy resources have drawn much attention to the vital need for ecologically clean chemical technologies—one of the most urgent challenges facing chemical scientists today. Since the photosensitization effect of a TiO2 electrode on water electrolysis was discovered by Honda and Fujishima , pollution-free photocatalysis by TiO2 semiconductors has been widely studied with the final goal of the efficient conversion of clean solar energy into useful chemical energy such as hydrogen [2–9]. The effective use of clean solar energy will lead to many new promising solutions not only for energy issues caused by the exhaustion of fossil fuels but also for the abatement of environmental toxins. Along these lines, photocatalysts which can operate under visible and/or solar light irradiation are strongly desired for applications in the purification and sustenance of our living environment.
In fact, various air-cleaning systems equipped with TiO2 photocatalysts and UV light sources that reduce volatile organic compounds (VOCs) which cause the so-called “sick house syndrome” are now commercially available. However, the removal efficiency of air-cleaning systems still needs to be improved to be as simple and low-cost as possible for widespread use. Although the deposition of small amounts of Pt on TiO2 catalysts is known to enhance their photocatalytic reactivity [25–29], Pt is too costly for common use in home electrical appliances. Meanwhile, the hybridization of adsorbents such as zeolites or mesoporous materials [14–19, 30–34] with TiO2 particles has been reported to show elevated photocatalytic reactivity. In a previous report , TiO2 nano-particles hybridized with siliceous zeolites prepared by impregnation as well as a simple mechanical blending method showed higher photocatalytic reactivity for the complete oxidation of gaseous acetaldehyde than TiO2 catalysts since such siliceous zeolites can efficiently condense acetaldehyde diffused in the gas phase and smoothly supply them onto TiO2 photocatalyst surfaces.
The conversion of solar light energy into renewable clean energy is also one of the most challenging research topics in science. Sunlight including near-infrared, visible and ultraviolet light provide tremendous energy of ca. 87–308 kJ mol−1 so that solar energy should be utilized as efficiently as possible [36–42]. It will, thus, be of great importance to develop effective systems able to convert abundant solar light energy into applicable and sustainable energy resources. At least two systems have been considered for the conversion of sunlight into other renewable energy sources: one is the design of solar cells to convert sunlight into electricity and the other is artificial photosynthesis for the conversion and storage of solar energy into safe and useful chemical energy. Although hydrogen is also the focus of much attention as a renewable clean energy alternative, at the moment, we do not have any highly efficient systems to produce hydrogen in an environmentally harmonious way without producing CO2. From this viewpoint, the photocatalytic or photoelectrochemical decomposition of water to produce hydrogen under solar light irradiation is now of utmost importance.
In this review, the development of highly functional Ti-oxide based photocatalysts, i.e., (i) tetrahedral Ti-oxide species incorporated within the framework of zeolites and mesoporous materials as single-site photocatalysts; (ii) TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents in practical applications for photocatalytic air-cleaning systems; and (iii) the photocatalytic decomposition of H2O into H2 and O2 under solar light irradiation using visible light-responsive TiO2 thin films prepared by a RF-magnetron sputtering deposition method will be summarized.
2 The Design of Highly Dispersed Molecular-Sized Ti-Oxide Species as a Single-Site Photocatalyst Incorporated Within the Framework of Zeolites and Mesoporous Materials
2.1 Photocatalytic Reduction of NOx
The removal of NOx from exhaust emission gases of internal combustion engines or industrial boilers is one of the most urgent issues we face today since NOx is an atmospheric pollutant which causes acid rain and photochemical smog. In fact, a great challenge has been the direct decomposition of NO into harmless N2 and O2. To address such concerns, highly dispersed titanium oxides incorporated within the framework of zeolites or mesoporous materials can be considered as one of the most promising candidates in the design of effective photocatalysts for the decomposition of NOx directly into N2 and O2 [43–48].
These highly dispersed tetrahedral TiO4 species can also be incorporated within the framework of zeolites or mesoporous materials and were observed to show unique photocatalytic performance, especially for the hydrogenolysis reaction of unsaturated hydrocarbons with H2O, the direct decomposition of NO, and the photoreduction of CO2 with H2O. UV light irradiation of these TiO2/SiO2 catalysts in the presence of NO was found to lead to the effective decomposition of NO to produce N2 with high selectivity, while the TiO2 semiconductor photocatalyst decomposed NO into mainly N2O.
2.2 Photocatalytic Reduction of CO2 by H2O
The development of photocatalytic reduction systems of CO2 with H2O into valuable chemicals such as CH3OH or CH4 is a challenging goal in research on environmentally friendly catalysts. We have found that the highly dispersed Ti-oxide species within zeolite frameworks show unique photocatalytic reactivity for the photoreduction of CO2 with H2O as compared with bulk semiconducting TiO2 photocatalysts [49–56]. In fact, UV-irradiation of the Ti/zeolite catalysts in the presence of CO2 and H2O catalyzed the photocatalytic reduction of CO2 to form CH3OH and CH4 as major products as well as CO, O2, C2H4, and C2H6 as minor products.
2.3 Enhancement of the Photocatalytic Reactivity of TiO2 Nano-Particles Hybridized with Hydrophobic Zeolite Adsorbents
The highly dispersed tetrahedral Ti-oxide species incorporated within the framework of zeolite or mesoporous materials was observed to show unique photocatalytic reactivity for the reduction of NOx or CO2 [43–56]. However, such highly dispersed Ti-oxide species are less effective for the complete oxidation of organic compounds into harmless CO2 and H2O. For the purification of polluted air, water and soil, improvement in the photocatalytic reactivity of TiO2 photocatalysts is still strongly desired. In this section, the effective enhancement of the photocatalytic reactivity of TiO2 nano-particles by mixing with hydrophobic zeolite adsorbents has been summarized [30–32, 35, 57].
Figure 8B shows the effects of the TiO2 content on the photocatalytic reactivity of the TiO2/MOR samples for the oxidation of acetaldehyde with O2 under UV light irradiation. TiO2 particles of ca. 5–15 wt% mechanically blended with the hydrophobic zeolite showed almost twice as high photocatalytic reactivity as the commercial TiO2 sample. Since the high-silica zeolite does not have Brönsted acid sites, which strongly adsorb various polar molecules, the acetaldehyde concentrated within the zeolite cavities could smoothly diffuse on the blended TiO2 surfaces. In addition, as seen from the results of UV–Vis absorption measurements, the incident UV light was efficiently irradiated on the entire TiO2 particles of ca. 5–15 wt% blended with the zeolite due to the high transparency of the zeolite powder in UV–Vis light regions. From these results, the major factors for the hydrophobic zeolite in enhancing the photocatalytic reactivity of the TiO2 particles can be concluded as: (i) the condensation effect for acetaldehyde nearby the TiO2 photocatalytic sites; and (ii) the appropriate diluent effect of the TiO2 photocatalysts as an intense absorber of UV light with highly transparent zeolite powders.
3 Separate Evolution of H2O into H2 and O2 Using Visible Light-Responsive TiO2 Thin Film Photocatalysts Prepared by a RF-Magnetron Sputtering Deposition Method [36–42]
The design and preparation of a tetrahedrally coordinated Ti-oxide species incorporated within zeolite frameworks, TiO2 nano-particles hybridized with hydrophobic zeolite adsorbents as well as visible light-responsive TiO2 thin film photocatalysts by a RF-magnetron sputtering deposition method have been summarized. The tetrahedral Ti-oxide species demonstrated unique and high performance for the photocatalytic decomposition of NOx and photocatalytic reduction of CO2 with H2O. The high photocatalytic reactivity of TiO2 semiconducting photocatalysts could be achieved by blending them with hydrophobic siliceous zeolites, its reactivity equal to the high reactivity of TiO2 deposited with Pt particles. Significantly, a more economical method could, thus, be devised with these zeolite photocatalysts. Hydrophobic zeolites can efficiently adsorb and condense the organic compounds within their cavities and provide them onto the hybridized TiO2 surfaces, thus, an important role of such hydrophobic zeolites is the “catch and release” of organic compounds onto the photocatalyst surfaces. In addition, visible light-responsive TiO2 thin film photocatalysts could be prepared by using a RF-MS method as a single-step process. Novel Vis-type TiO2 photocatalysts were also successfully applied for the separate evolution of H2 and O2 from water under solar light irradiation. These Ti-oxide based photocatalytic systems which can effectively operate under solar light irradiation will be one of the most desirable candidates for applications that address environmental and energy issues in the future.