We previously reported on a decomposition system of organic wastes by the use of thermally excited holes in TiO 2 at high temperatures. A good feature of our system is that it makes use of a large number of holes formed, for example, at 350 C for the oxidation of organic wastes. In this investigation, we aim at applying this technology to the complete decomposition of benzene, toluene, and particulate matter (PM) included in the exhaust of diesel engines. Special attention has been paid to the adsorption process of benzene and toluene onto the surface of TiO 2 , as well as to their subsequent decomposition, as studied by thermal analysis, Raman spectroscopy, and ESR in consideration of the specific surface of TiO 2 powders. Both benzene and toluene are found to completely decompose into H 2 O and CO 2 at about 350 C, via fragments of ethylene, butene, propene, and butadiene. On the other hand, PM appears more difficult to decompose due to its powdered form. However, PM was finally confirmed to be decomposed when the mixture of PM and TiO 2 was heated under a pressure of about 280 N/m 2 at about 350 C.
Our decomposition system is similar to a photocatalytic system that makes use of photogenerated holes for the oxidation decomposition of various substances. However, our system differs distinctly in the utilization of thermally excited holes at high temperatures (for example, at about 350°C) in combination with a molten state of PC, as shown in Figure 1. A large number of charge carriers are available in TiO 2 at high temperatures as shown by the product of the Fermi _ Dirac distribution function and the density of states (Kittel, 1986). The number of carriers at room temperature (RT) and 350°C [n RT = n 0 exp(-E g /2kT RT) and n 623K = n 0 exp(-E g /2kT 623), respectively] gives a ratio of n 623K /n RT ≈ 8.8×10 13 , where E g = 3.2 eV. This number approximately was confirmed in our previous experiment based on the single crystals of TiO 2 of the rutile phase (Shinbara et al., 2005). The initial process of the PC decomposition is the formation of radicals in PC caused by thermally generated holes, followed by their propagation throughout the material to break up PC into fragments, resulting in their complete combustion with oxygen to yield H 2 O and CO 2 (Shinbara et al., 2005). Characterization of Various TiO 2 Powders Used for Complete Decomposition of Organic Wastes by Means of Thermally excited Holes at High Temperatures
We are so far involved in complete decomposition of organic wastes as well as volatile organic compounds (VOC) by the use of thermally generated holes in TiO 2 . In view of the practical use of the present system, fixation of TiO 2 powders onto a substrate seems to be the core technology. To realize this, we have tried in the present investigation to fix powdered TiO 2 onto SUS meshes or Ni-Cr wires by means of electrophoretic deposition. We have constructed two kinds of equipment. One is a cartridge system which includes ten pieces of the mesh disk coated with TiO 2 that are arranged vertically. The other is a ''heater-built-in'' system with a triple-coaxial structure equipped with Ni-Cr/Cr 2 O 3 / TiO 2 wires. The former is used in combination with an external furnace; whereas the latter is a stand-alone system. Both systems exhibit excellent performance to completely decompose VOC into H 2 O and CO 2 .
Previously involved in complete decomposition (H 2 O + CO 2) of organic wastes (mainly thermoplastic and thermosetting polymers), as well as exhaust of diesel engines by the use of thermally generated holes in TiO 2 at about 350-500°C under sufficient O 2. Then, we struck on an idea that H 2 can be produced in place of H 2 O under O 2-deficient conditions. Because of this, an attempt has been made in the present investigation to produce hydrogen from methanol or methane. Hydrogen is found to be successfully produced from methanol at 350-400°C under 5-20% O 2 , or from methane at 450-500°C under 50% O 2. The conversion efficiency amounts to about 70-85% for methanol while about 40% for methane.
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