Norman N. Lichtin

Oxidative photocatalytic degradation of benzene vapor over TiO2

The variables included in a parametric study of the photocatalytic oxidative degradation (PCO) of benzene in a closed reactor over films of Degussa P-25 TiO2 under fluorescent black-light radiation centered at 367 nm were: concentrations of benzene, O2 and water vapor; photons/sec incident on the catalyst; and temperature. Concentrations of benzene and CO2 were measured. All measurements of photocatalytic behavior were performed with freshly deposited films of P-25 which had been preconditioned by irradiating for 15 h while flowing 80 ml min−1 of dry air through the reactor. It was found that water vapor promoted the dark-evolution of CO2 and inhibited the dark-adsorption of benzene by the preconditioned catalyst. Under irradiation, water vapor promoted the formation of CO2 and the initial removal of benzene. Dark-adsorption of benzene increased linearly with concentration of benzene vapor. The initial rate of photocatalytic removal of benzene was first order in concentration of benzene vapor but the higher the initial concentration of benzene the more the rate of its removal diminished as reaction proceeded. Photocatalytic production of CO2 lagged far behind removal of benzene. The initial rates of photocatalytic removal of benzene vapor and generation of CO2 were, respectively, independent of concentration of O2 from 800 ppmv to 100% and approximately one-third order in O2. The initial kinetic orders in number of photons/sec entering the reactor of removal of vaporized benzene and generation of CO, over the range (7–42) × 10−8 Einstein s−1 were, respectively, 1.85 ± .03 and 1.60 ± .07 in air into which 0.44 mmol of water had been injected and 1.59 ± .07 and 1.41 ± .05 in dry air. Over the range 15–70 °C, the rate of removal of benzene did not vary significantly with temperature in the presence or absence of water vapor while the rate of generation of CO2 varied in a way corresponding, respectively, to EA = 3.9 ± 0.4 and 9.4 ± 0.9 kJ mol −1 in the presence and absence of water vapor. Mechanistic implications of the data are discussed.

TiO2-photocatalyzed oxidative degradation of binary mixtures of vaporized organic compounds

Abstract Rates of photocatalytic removal of the components for 14 binary mixtures of vaporized organic components over Degussa P-25 TiO2 at 360 nm in the presence of air were measured. In some cases, intermediate products were identified and their rates of removal measured. Seven binary mixtures were reacted in a static reactor: CH3OH strongly inhibited removal of CH2Cl2 while CH2Cl2 somewhat inhibited removal of CH3OH. CH3OH and trichloroethylene (TCE) strongly inhibited each others removal. Strong memory effects were observed in inhibition of removal of CH2Cl2 from its mixture with CH3OH and of CH3OH from its mixture with TCE. CCl4 significantly promoted removal of CH3OH while CH3OH had little effect on the rate of removal of CCl4. Either 2-propanol or t-butanol and CCl4 had little effect on each others rates of removal. CH2Cl2 somewhat diminished only the initial rate of removal of C2Cl3F3 (Freon 113). The Freon somewhat inhibited the much faster removal of CH2Cl2. Methanol somewhat inhibited removal of acetone while acetone had a negligible effect on the removal of methanol. Seven binary mixtures were reacted in flow reactors: TCE and perchloroethylene (PCE) somewhat promoted each others removal. TCE strongly promoted removal of iso-octane, CH2Cl2 and CHCl3. A strong memory effect occurred with iso-octane and probably with methylene chloride and chloroform. TCE substantially inhibited removal of acetone. The onset of inhibition of removal of acetonitrile by TCE was gradual and a memory effect occurred. Iso-octane somewhat reduced the rate of removal of benzene. Benzene had no effect on the rate of removal of iso-octane but greatly diminished the yield of acetone produced as an intermediate. Rationalizations of some of the data in terms of adsorption effects and chemical mechanism are presented.

A Multilayer Iron‐Thionine Photogalvanic Cell

The iron--thionine photogalvanic system is being studied for the direct conversion of solar energy to electricity. This communication reports the development of such a cell with two transparent semiconductor electrodes, which allows the construction of multilayer devices for increased conversion efficiency. Each layer contains a SnO/sub 2/ anode and an indium tin oxide cathode. The thionine was contained in the solution. The output of a four-layer cell was measured, and the efficiency of the multilayer device is estimated. Monochromatic quantum efficiencies were measured for single-layer cells as a function of wavelength and cell thickness; results indicate that 25 ..mu..m solution layers would be more efficient than 80 ..mu..m layers. Several thiazine dyes other than thionine can be used; results are reported for a cell containing one layer of thionine solution and one layer of methylene blue solution. (DLC)

ANION AND SOLVENT EFFECTS ON THE RATE OF REDUCTION OF TRIPLET EXCITED THIAZINE DYES BY FERROUS IONS

Abstract— The lifetimes of the triplet excited states of thionine and methylene blue were measured in aqueous and 50 v/v% aqueous acetonitrile solutions acidified with 0.01 N sulfuric or trifluoromethyl‐sulfonic acid. The rate constants for reaction of the triplet excited dyes with ferrous ions were measured in the same solutions. The triplet lifetimes in the absence of added quenchers were insensitive to a change in acid from trifluoromethylsulfonic to sulfuric or to a change in solvent from water to 50v/v% aqueous acetonitrile (τ for triplet thionine ˜7.5 μs, τ for triplet methylene blue ˜4.5 μs). In contrast, the rate constant for reaction of the triplet dyes with ferrous ions increased by nearly a factor of 10 with a change in acid from trifluoromethylsulfonic to sulfuric. In solutions containing sulfate ions this reaction rate constant increased with increasing sulfate concentration and with a change in solvent from water to 50 v/v% aqueous acetonitrile. The results are discussed in terms of the possibility of association of the positively charged reactive ions with sulfate anions. Quenching of the triplet excited dyes by ferric ions or by ground state dye molecules was shown to be negligible at the concentration used for the ferrous ion quenching study.

A PREVIOUSLY UNREPORTED INTENSE ABSORPTION BAND AND THE pK, OF PROTONATED TRIPLET METHYLENE BLUE

Abstract— Excitation by a Q‐switched giant ruby laser (1.2 J output at 694 nm ˜50 ns flash) of 2–10 µM solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water‐CH3CN at pH values in the range 2.0–9.3 converted the dye essentially completely to its T1 state. The absorption spectrum of T1 dye was measured in different media at pH 2.0 and 8.2 by kinetic spectrophotometry. Previously reported T‐T absorption in the violet in acidic and alkaline solutions and in the near infrared in alkaline solution was confirmed. Values found for these absorptions in the present work with 30% ethanol in water as solvent are λmax ‐ 370nm, εmax, ‐ 13,200 M‐1 cm‐1 at pH 2 and λmax,˜420nm, εmax 9000 M‐1 cm‐1, λmax, ‐ 840 nm, εmax ‐ 20,000 m‐1 cm‐1 at pH 8.2. Long‐wavelength T‐T absorption in acidic solution is reported here for the first time: λmax, ˜ 680 nm, emax˜ 19,000 M cm‐1 in 30% ethanol in water at pH 2. Observation of a pH‐independent isobestic point ˜ 720 nm confirms that the long‐wavelength absorptions are due to different protonated states of the same species, MB+(T1) and MBH2+(T1). The pKa of MBH2+(T1) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.14± 0.1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB+(T1) by H2PO4 in water at pH 4.4 was found to be 4.5 ± 0.4 times 108M‐1s‐1.

Photocatalytic oxidative Degradation of vapors of some organic compounds over TiO2

Rates and diffuse reflectance FTIR spectra have been measured for the photocatalytic reactions of a number of vaporized organic compounds with O2 over Degussa P-25 TiO2 irradiated at wavelengths in the vicinity of 360 nm. Trichloroethylene (TCE) has been investigated extensively. Other compounds investigated in some depth include perchloroethylene (PCE), i-octane, methanol, acetone and acetonitrile. Rates of consumption of components and intermediates have been determined for several binary mixtures. Photoefficiencies (Ep) of their consumption have been evaluated for 17 compounds. Principal findings include the following: Ep of consumption significantly greater than unity has been observed for TCE, PCE methanol and ethanol, with values up to 90 observed with TCE. For the 17 compounds, Ep varies over a factor of more than 108. TCE substantially promotes the consumption of i-octane, CH2Cl2 and CHCl3 and inhibits the consumption of acetone and acetonitrile. These effects last after the TCE is completely consumed. Promotion and inhibition by added Cl2 has been observed. Mechanistic rationalizations are proposed.

Sensitization of an iron-thiazine photogalvanic cell to the blue: An improved match to the insolation spectrum

Abstract Sensitization of a totally illuminated thin layer (TI-TL) photogalvanic cell based on the iron-thiazine photoredox reaction has been demonstrated by both current action spectra and enhanced white light output. Singlet sensitization of thionine ( λ max = 601 nm) and of methylene blue ( λ max = 660 nm) to wavelengths below 400 nm has been achieved. Photogalvanic output has been obtained upon illumination of a single solution containing two photoredex dyes and three sensitizers with monochromatic light throughout the wavelength range ∼375–700 nm. Sensitization with rhodamine 6 G considerably enhances cell output under illumination with white light. With some sensitizers, however, white light output is inhibited.

Reactions of Phenylalanine Derivatives, Methionine Derivatives, and their Mixtures in Aqueous Solution with Pulse-Radiolytically Generated H Atoms

Specific rates of reaction of H atoms with phenylalanine, acetylphenylalanine, acetylphenylalanylamide and acetylmethionine have been determined. The decay of cyclohexadienyl radicals followed strictly second order kinetics in the absence or presence of methionyl compounds and the radicals formed by the action of H atoms upon them. No reaction of cyclohexadienyl radicals with parent methionyl compounds was detectable. The bimolecular radical-radical reactions observed with monomeric radicals take place on a time scale over which only intramolecular transformations occur in the radicals produced by the addition of H atoms to the ribonuclease-S-peptide molecule. The virtually identical reactivity of monomeric phenylalanyl and methionyl compounds to reaction with H atoms can be compared to evidence that the reaction of H atoms with ribonuclease-S-peptide takes place about five times as often at the methionyl as at the phenylalanyl residue.

Amorphous silicon, germanium, and silicon-germanium alloy thin-film transistor performance and evaluation

The first hydrogenated amorphous germanium and hydrogenated amorphous silicon‐germanium alloy field‐effect transistors with appreciable field‐effect response in n‐ and p‐channel modes were developed by reducing the dihydride content in the films. Field‐effect mobilities were derived from transistor characteristics. Hole mobilities are superior to those in pure hydrogenated amorphous silicon which offers the opportunity for improved thin‐film devices.

Electrodic Phenomena at the Anode of the Totally Illuminated, Thin Layer Iron‐Thionine Photogalvanic Cell

Properties of Corning thin‐film electrodes in contact with 0.005–0.5Msolutions of sulfuric acid in aqueous solvents containing were characterized. Schottky‐Mott plots gave flatband potentials in the range 0–0.2V vs. SCE and charge carrier densities in the range in 0.01M sulfuric acid, independent of solvent composition. Standard reduction potentials vs. SCE in 0.01M acid at room temperature were calculated from equilibrium compositions to be −0.04V for , 0.33V for and 0.14V for in water and −0.06V for , 0.28V for and 0.11V for in 50 v/v/o aq. . Voltammetric data show that establishment of protonic equilibrium is rapid compared with cathodic reduction of TH+ at in water and in 50 v/o aq. . In contrast, protonic equilibration between the two electron‐transfer steps is slow for thionine‐l‐sulfonic acid. Voltammetric data show that reduction of TH+ and oxidation of are kinetically controlled at both and Pt electrodes with reversibility greater at platinum than at. Reversibility is slightly reduced by addition of to the solvent. Rectification (inhibition of oxidation of ) is not severe enough to prevent the use of as a selective anode but may reduce efficiency of photogalvanic conversion. Weak but persistent adsorption of TH+ on activates photogalvanic conversion. Adsorption of varied considerably from sample to sample of and was strong in some cases. Both oxidation and reduction of the Fe+3/Fe+2 couple are much less reversible at than at Pt. Rectification (inhibition of oxidation of Fe2+) is pronounced. Both oxidation and reduction on become more reversible with increasing fraction of ; the major effect is enhancement of oxidation of Fe2+ but even with the couple is much less reversible than at Pt. Implications of the data with respect to efficiency of totally illuminated thin‐layer iron‐thionine photogalvanic cells are discussed.