Joseph Rabani

The Measure of TiO2 Photocatalytic Efficiency and the Comparison of Different Photocatalytic Titania

The dependence of hydroxyl radical yield on the substrate concentration, pH, oxygen concentration, and light intensity, using different TiO2 preparations, was investigated. The quantum yields of formaldehyde and carbon dioxide, obtained with the aid of an integrating sphere in the methanol and formate systems, respectively, were used to derive the primary yield of hydroxyl radicals. The limiting yield of •OHads, achieved at high scavenger concentrations, is independent of the nature of the scavenger (methanol or formate) and is nearly constant in the range 1 < pH < 12. While the presence of air induces a 15-fold increase in product yield, compared to air-free systems, further increase of oxygen concentration has only a small effect. The effect of changing the •OH scavenger concentration, pH, and light intensity corresponds to simple competition between pseudo first- and second-order reactions. Under the conditions of the present work the scavengers CH3OH, HCO2- and HCO2H react only with the hydroxyl radic...

Quantum yields of hydroxyl radicals in illuminated TiO2 nanocrystallite layers

The yield of hydroxyl radicals has been determined by illumination of TiO 2 layers immersed in air saturated aqueous methanol solutions. This yield is equal to half the measured formaldehyde yield in the pH range 7-13. A detailed mechanism is proposed, accounting for the lack of accumulation of hydrogen peroxide. The effect of changing methanol concentration, pH and light intensity (the latter by three orders of magnitude) is in agreement with a very simple mechanism. In contrast to hydroxyl radicals, which react via hydrogen abstraction, leading to formation of HCHO, there is no sign for reaction of methanol with mobile holes. Thus, the limiting quantum yield observed at high methanol concentration is related to the maximum yield of ○OH ads under air saturated conditions at the given pH and light intensity. The effect of light intensity shows the expected inverted square root dependency. The yield of ○OH ads is nearly constant in the range 7 < pH < 12. This system may be useful for comparative tests of different TiO 2 preparations.

Conversion of Solvated Electrons into Hydrogen Atoms in the Photo‐ and Radiation Chemistry of Aqueous Solutions

Radiation chemical and photochemical experiments are reported which show that the conversion of solvated electrons to H atoms is not specific to the H3O+ ion. Proton donors in general may react in the conversion eaq—→H. The relative reaction rates correlate with the pK values of these acids as implied by Bronsted general acid catalysis law. The implications of this result for the nature of the solvated electron in water are briefly considered.

Unusually efficient photosensitization of nanocrystalline TiO2 films by pomegranate pigments in aqueous medium

Unusually high photocurrents, with quantum yields up to Φ = 0.74, are obtained using TiO2 photosensitized by adsorbed natural pigment(s) from pomegranate liquid, with aqueous iodide/iodine acting as charge carriers. An open circuit photovoltage up to 0.45 V is observed. The quantum yield of the photocurrent, the photovoltage and the stability are unusually high for aqueous solutions. In contrast to the intensively investigated Ru(dcbpy)2(SCN)2 system, increasing the TiO2 nanocrystallites diameter from 5 to 18 nm considerably decreases the photocurrent. Apparently, the TiO2 porous layer functions as a nano-filter for the pigment mixture. The effects of changing charge carrier concentrations and the photocurrent spectrum and yields under various conditions are reported. Ethanol used as solvent for the charge carriers yields comparable results (tested at pH 2.8). Our main observations suggest that it may be possible to construct Graetzel-type cells with no special care to eliminate traces of water.

Yield and Reactivity of Electrons and H Atoms in Irradiated Aqueous Solutions

Using specific scavengers of solvated electrons eaq—, and of atomic hydrogen H in aqueous solutions irradiated with x rays, evidence is obtained that the hydrogen yield Ghydrogen=3.7±0.1 between pH 2–4. It is shown that this yield corresponds to the sum of the yields of molecular hydrogen GH2=0.5, the primary yield of atomic hydrogen GH=0.55 and yield of solvated electrons Geaq—=2.65. Evidence is obtained showing that H atoms formed as such (GH=0.55) appear in the bulk of the solution even at neutral pH. The velocity constants of the reactions of H atoms with acetate, isopropanol, glucose, glycerol, and nitrite were obtained. The reactivity of eaq— with Haq+, acetone and ferricyanide was studied and relative velocity constants obtained. An upper limit, keaq—+H2O≤103 liter mole—1 sec—1 is found for the reaction of eaq— with water to yield hydrogen.

The Radiation Chemistry of Aqueous Solutions of Cytochrome c

The mechanism of the action of ionizing radiations on aqueous solutions at or near neutral pH-from the point of view of radiation biology, the most significant range-has been clarified only recently. The experimental results are best understood by assuming that after primary ionization H20 --m--+ H20 ++ e (1) in aqueous solution the electron is stabilized (1-3) by interaction with the dipolar dielectric solvent, e + aq -eaq (2) The solvated electron, eaq, thus formed reacts with suitable acceptors in the bulk (1-7): e, + S -Saq (3) A specific acceptor of interest is H+, (4), which, on reacting with eq, yields H atoms.

On some fundamental reactions in radiation chemistry: Nanosecond pulse radiolysis

Abstract Several fundamental reactions have been investigated using nanosecond pulse radiolysis. The following reaction rate constants were measured: k ( N2O − →N2+O − )>10 8 s −1 , k ( H + +O2 − )= (5 ± 1) × 10 10 dm 3 mol −1 s −1 . k ( HO2→H + +O2 − ) = (7 ± 2) × 10 5 s −1 , k ( CO2 − +O2 ) = (4.2 ± 0.4) × 10 9 dm 3 mol −1 s −1 .

Superoxide dismuting activity of an iron porphyrin

Abstract The efficiency of tetrakis-(4-N-methylpyridyl)porphineiron(III) as a superoxide dismuting catalyst has been determined using a pulse radiolysis technique.

Photosensitization of nanocrystalline TiO2 films by pomegranate pigments with unusually high efficiency in aqueous medium

An aqueous system based on a TiO2 layer photosensitized by natural pigment(s) from pomegranate, shows relatively high stability under illumination with a quantum yield of 0.74 and a photovoltage of 0.46 V.

One electron reduction of metmyoglobin and methemoglobin and the reaction of the reduced molecule with oxygen.

We have used the pulse radiolysis technique to reduce with solvated electrons (see article) a single Fe(III) site in methemoglobin and metmyoglobin. The reduction process was followed spectrophotometrically and the reactions rate constants were measured: (see article) =6.5 +/- 1-10(10) M-1-S-1. (see article)=2.5 +/- 0.3-10(10) M-1-S-1. Approx. 60% of the (see article) have reacted with the hemin group, and the rest of the (see article) have probably reacted with the globin moiety. We followed the reaction of the reduced proteins to yield the oxyderivatives and measured the rate constants of the oxygenation process k reduced methemoglobin + O2 = 2.6 +/- 0.6-10(7) M-1-S-1 and k myoglobin + O2 = 1.8 +/- 0.2-10(7) M-1-S-1, all the rate constants were measured at pH = 6.8, I = 0.004, T = 22 +/- 2 degrees C. The high rate constant for reduced methemoglobin indicates that one-site-reduced methemoglobin is probably in the R state, as predicted for methemoglobin from X-ray analysis. The spectra of the reduced and oxygenated species were measured under similar conditions at gamma = 450-650 nm. We were able to follow slight changes in the micro-second time scale, these changes were attributed to conformational changes. We were not able to detect any reaction between the radical (see article) and the hemin group (which would result in a complex such as heme O-2). This may be due to kinetic reasons.