Hisahiro Einaga

Heterogeneous photocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst

Abstract Gas–solid heterogeneous photocatalytic decomposition of benzene, toluene, cyclohexane and cyclohexene over TiO 2 was studied at room temperature, and their reactivities were compared. Catalyst deactivation was ascribed to the formation of the carbon deposits on TiO 2 surface, and the formation and decomposition behavior of the carbon deposits affected the decomposition rate. Deactivated TiO 2 catalysts were photochemically regenerated in the presence of water vapor, and the carbon deposits were decomposed to CO x .

Benzene oxidation with ozone over supported manganese oxide catalysts: effect of catalyst support and reaction conditions.

Catalytic oxidation of gaseous benzene with ozone was carried out over supported manganese oxides to investigate the factors controlling the catalytic activities. The rate for benzene oxidation linearly increased with the surface area of catalyst, regardless of the kinds of catalyst support, whereas the ratio of ozone decomposition rate to benzene oxidation rate was larger for SiO(2)-supported catalyst than Al(2)O(3)-, TiO(2)-, and ZrO(2)-supported catalysts. The rate for benzene oxidation and CO(x) selectivity increased with the reaction temperature (22-100 degrees C) and were improved by the addition of water vapor to reaction gases. Benzene conversion and carbon balance increased with ozone concentration.

Involvement of catalyst materials in nonthermal plasma chemical processing of hazardous air pollutants

Abstract Catalytic effects of metal oxides in nonthermal plasma chemical processing of hazardous air pollutants (HAPs) are discussed, relevant to their activities for the oxidation of HAPs in nonthermal plasma media and their selective control of active oxygen species derived from background O2. In ferroelectric packed-bed reactors, the oxidation power of barium titanate (BaTiO3) is not strong enough to oxidize HAPs and their carbon intermediates to CO2. Only nitrous oxide (N2O) was formed from background N2 and lattice oxygen atoms in BaTiO3. The catalytic effect of BaTiO3 is negligible under aerated conditions. On the other hand, ozone (O3) is formed from background O2 in much higher concentrations in a silent discharge plasma reactor. Manganese dioxide (MnO2)-catalyzed decomposition of O3 promotes decomposition of benzene, which is less reactive than trichloroethylene and tetrachloroethylene. The acceleration of benzene consumption rate is ascribed to the promotion of its oxidative decomposition by the triplet oxygen atom. Catalytic control of in situ active oxygen species could be one of the most effective approaches to increase the energy efficiency of the nonthermal plasma reactor and to achieve the complete oxidation of the carbon atoms in HAPs.

Photocatalytic decomposition of benzene over TiO2 in a humidified airstream

Photocatalytic decomposition of benzene over TiO2 in the gas phase at room temperature was studied with a fixed-bed flow reactor. In a humidified airstream ([H2O]=2.2%), benzene was efficiently decomposed to CO2 and CO with the selectivities of 93 and 7%, respectively. The selectivities were almost independent of the benzene conversion, indicating that CO is not the intermediate of CO2 in the reaction. The selectivity of CO was in the range of 7–10% with varying concentration of O2, H2O, and benzene. The formation of phenol and brownish carbonaceous matter attributable to polymeric products was observed on the catalyst surface. In the absence of O2, benzene oxidation did not proceed at all, showing that O2 is essential for the reaction. The presence of H2O not only suppressed the formation of the carbon deposits on the catalyst surface, but also accelerated the decomposition of them to CO2 and CO. Diffuse reflectance IR study showed that the presence of H2O regenerated the surface hydroxyl groups of TiO2 which were consumed in the photoreaction. With increase in the benzene concentration, the benzene conversion was decreased and the amount of carbon deposits on the catalyst surface was increased.

Effective combination of nonthermal plasma and catalysts for decomposition of benzene in air

Abstract The effective combination of plasma energy and solid surface properties, such as catalysis and adsorption, was investigated using packed-bed type catalyst–hybrid and adsorbent–hybrid reactors that were packed with a mixture of BaTiO 3 pellets and other ceramic pellets (catalyst or adsorbent). The plasma reactor that employed catalysts indicated improvement in CO 2 selectivity and suppression of N 2 O formation compared with the reactor that was packed with BaTiO 3 alone. It was also found that the catalysts and adsorbents in the plasma reactor were useful in enhancing energy efficiency. Furthermore, the catalyst and adsorbent positions in the plasma reactor were very important for induction of surface reactions on the packed materials.

Hydrogen generation from water, methane, and methanol with nonthermal plasma

Hydrogen generation from water, methane, and methanol was investigated with different types of nonthermal plasma reactors under different conditions. With a ferroelectric packed-bed reactor in N/sub 2/, hydrogen gas yield decreased in the order: methanol>methane>water. A similar trend was observed with a silent discharge plasma reactor, but substrate conversions were much lower with the latter reactor. At fixed specific energy densities, higher H/sub 2/ yields were obtained at higher gas flow rates in the reactions of the above substrates. The initial water concentration was optimized at ca. 2.0% to obtain the largest H/sub 2/ amount. Background O/sub 2/ suppressed H/sub 2/ yield. The ferroelectric packed-bed reactor could be operated continuously for 10 h without any decrease in H/sub 2/ yield.

Performance evaluation of a hybrid system comprising silent discharge plasma and manganese oxide catalysts for benzene decomposition

A hybrid system comprising a silent discharge plasma reactor (SDR) and manganese oxide (MnO/sub 2/) catalyst was used for the decomposition of benzene in air. The benzene conversion was greatly enhanced by combining MnO/sub 2/ with the SDR in the latter part. The MnO/sub 2/ catalyst decomposed benzene by using ozone (O/sub 3/) that was formed in the SDR as the oxidant precursor. With an increase in the amount of water vapor in air, the benzene conversion was decreased, due to the deactivation of high-energy electrons, the diminished formation of O/sub 3/ in SDR, and decreased activity of MnO/sub 2/ for the benzene oxidation with O/sub 3/. The only products of the reaction were CO/sub 2/ and CO. The carbon mass balance was not perfect due to the deposition of intermediates on MnO/sub 2/ during the reaction. The intermediates were subsequently decomposed to CO/sub 2/ and CO by MnO/sub 2/ in the presence of O/sub 3/. In dry air, the selectivities to CO/sub 2/ and CO were 70% and 30%, respectively, and were almost independent of specific energy density. The CO/sub 2/ selectivity was improved to 90% by humidifying the background air.

The stabilization of active oxygen species by Pt supported on TiO2

Abstract Pt/TiO2 was active for the oxidation of CO to CO2 in the dark at ambient temperature after UV irradiation. Pure TiO2 was inactive and the presence of Pt supported on TiO2 was indispensable for the reaction. Based on the ESR study, it was found that Pt deposited on TiO2 stabilized the O− and O3− species photoformed on the TiO2, and they were responsible for the CO oxidation.

Catalytic Oxidation of Benzene in the Gas Phase over Alumina-Supported Silver Catalysts

Catalytic properties of Ag/Al(2)O(3) for complete oxidation of benzene with ozone at 295-373 K were studied and compared with those of Mn/Al(2)O(3). At the reaction temperature of 295 K, the Ag/Al(2)O(3) catalysts showed selectivity to CO(x) (ca. 80%) higher than that of the oxide of metals in the first transition series (Fe, Mn, Co, Ni, Cu) supported on Al(2)O(3), which had selectivities of 28-62%. The catalyst showed gradual deactivation from accumulation of byproduct compounds on the catalyst surface. FTIR studies revealed that the byproduct compounds consisted of easily decomposable species and hardly decomposable species. The rate for benzene oxidation linearly increased with Ag loadings (approximately 15 wt %) and was not improved at higher loading levels. The ratios of ozone decomposition to benzene oxidation and ozone decomposition to CO(x) selectivity were evaluated to be 7.5 and 80%, respectively, and they were independent of benzene conversion. The Ag/Al(2)O(3) catalyst showed steady-state activities at a reaction temperature of 313-373 K, and the conversion increased with the increase in the reaction temperature. The presence of water vapor in the reaction gas inhibited the catalyst deactivation, and steady-state activity was obtained at a reaction temperature of 295 K, while it did not affect the activities for benzene oxidation but improved the CO(2) selectivity.

Photocatalytic oxidation of benzene in air

Photocatalytic oxidation of benzene in air at room temperature was studied in order to obtain the information on its reactivity on the photoirradiated TiO 2 catalyst. The objective of this paper is to describe in detail the dependence of the rate for benzene photooxidation on humidity, initial benzene concentration, and incident light intensity, since they are important factors for construction of VOC control system utilizing solar energy. The reaction mechanism is also discussed to understand the decomposition behavior of benzene.