The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2–), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption–desorption equilibrium at the TiO2–solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity...

Generation and Detection of Reactive Oxygen Species in Photocatalysis

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

Highly active oxide photocathode for photoelectrochemical water reduction

https://chemical.eng.usm.my/bassim/PDF/photocatalytic%20degradation.pdf

Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: a review.

TiO2 nanoparticles when subjected to UV irradiation exhibit blue coloration as electrons are stored within the particles. Upon contact with gold nanoparticles, a partial disappearance of the blue color is seen as the stored electrons are transferred from TiO2 to Au nanoparticles. The charge distribution between the semiconductor and metal nanoparticles causes the Fermi level to shift to more negative potentials. By employing C60/C60•- as a probe−redox couple, we were able to estimate the apparent Fermi levels of TiO2 and TiO2/Au nanoparticles. A Fermi level shift of −22 mV observed for the Au−TiO2 nanocomposite is indicative of improved charge separation in semiconductor−metal systems and demonstrates its usefulness for improving the efficiency of photocatalytic reactions.

Charge Distribution between UV-Irradiated TiO2 and Gold Nanoparticles: Determination of Shift in the Fermi Level

Electron spin resonance measurements were carried out at 77 K under irradiation for anatase TiO 2 powders treated by heating at various temperatures in the air. For the untreated powder photoproduced holes were trapped at the surface forming Ti 4+ O − Ti 4+ OH − radicals, while, for the heated powder, they were trapped as Ti 4+ O 2− Ti 4+ O − radicals at the surface. Photoproduced electrons were trapped as Ti 3+ at the desorption of surface hydroxyl groups and the change in the surface accompanied by the crystalline growth. No photoproduced radicals were detected for rutile TiO 2 powder, which may explain the low photocatalytic activity of this crystalline structure.

ESR investigation into the effects of heat treatment and crystal structure on radicals produced over irradiated TiO2 powder

Electron spin resonance studies of photocatalytic interface reactions of suspended M/TiO2 (M = Pt, Pd, Ir, Rh, Os, or Ru) with alcohol and acetic acid in aqueous media

Fully halogenated chlorofluorocarbons (CFCs) cause depletion of the stratospheric ozone layer as well as global warming. Partially fluorinated alcohols are important in relevance with potential substitutes of CFCs. The OH reaction rate constants have been measured for CF{sub 3}CH{sub 2}OH, CF{sub 3}CF{sub 2}CH{sub 2}OH, and CF{sub 3}CH(OH)CF{sub 3} over the temperature range 250--430K. Kinetic measurements have been carried out using the discharge flow, laser photolysis, and flash photolysis methods, combined respectively with the laser-induced fluorescence technique to monitor the OH radical concentrations. The influence of impurities contained in the sample of CF{sub 3}CF{sub 2}CH{sub 2}OH has been investigated by means of sample purification using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these three fluorinated alcohols. The Arrhenius rate constants have been determined from the respective kinetic data as {kappa}(CF{sub 3}CH{sub 2}OH) = (2.00 {+-} 0.37) {times} 10{sup {minus}12} exp[{minus}(890 {+-} 60)/T], {kappa}(CF{sub 3}CF{sub 2}CH{sub 2}OH) = (1.40 {+-} 0.27) {times} 10{sup {minus}12} exp[{minus}(780 {+-} 60)/T], and {kappa}(CF{sub 3}CH(OH)CF{sub 3}) = (6.99 {+-} 1.56) {times} 10{sup {minus}13} exp[{minus}(990 {+-} 70)/T] cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. A method of predicting the OH reaction rate constants for fluorinated alcohols, hydrofluorocarbons, alkanes, and alcohols hasmore » been proposed.« less

Measurement of the OH Reaction Rate Constants for CF3CH2OH, CF3CF2CH2OH, and CF3CH(OH)CF3

In order to find the optimum condition for measuring the explosion limits of flammable gases by using metal wire fusing as the ignition source, the explosion limits of the methane/air system have been measured under various conditions in a bell jar type vessel of volume 3 dm 3 . Metal wires used were of copper, Nichrome, platinum, molybdenum and tungsten, with diameters ranging from 0.3 to 1.0 mm. Fusing was made by 12, 24, 36 and 48 V of direct current (DC), and also by 100 V of alternating current (AC). With these experiments, the effect of ignition abilities on explosion limits measurement has been made clear. As a result, it has been found that the fusing of metals of relatively high melting point, such as molybdenum, with a relatively high electric voltage, e.g., DC 36–48 V, is suitable for the explosion limit measurement. The explosion limits of methane in air have been determined to be 4.7–16.3%.

Fusing ignition of various metal wires for explosion limits measurement of methane/air mixture

Spontaneous ignition limits of silane and phosphine

This Note describes the first rotationally resolved detection of NO2 in the C 2A2 state by optical–optical double resonance (OODR) spectroscopy. The C 2A2–X 2A1 transition is symmetry forbidden. Direct experimental evidence of the C 2A2 state had not been provided until the recent vibrationally resolved detection using photoelectron spectroscopy of NO−2 by Weaver et al. [J. Chem. Phys. 90, 2027 (1989)]. We have measured hundreds of the OODR spectra in the 500 ∼600 nm range and found that 11 rovibronic levels in the spectral region of 513.8∼516.2 nm belong to the Ka = 1 stack of a vibrational level of C 2A2.

Hidekazu Nagai

Papers

Electron spin resonance studies of photocatalytic interface reactions of suspended M/TiO2 (M = Pt, Pd, Ir, Rh, Os, or Ru) with alcohol and acetic acid in aqueous media

Measurement of the OH Reaction Rate Constants for CF3CH2OH, CF3CF2CH2OH, and CF3CH(OH)CF3

Fusing ignition of various metal wires for explosion limits measurement of methane/air mixture

Spontaneous ignition limits of silane and phosphine

Possible rotationally resolved detection of NO2 in the C̃ 2A2 state by optical–optical double resonance spectroscopy