2-2-dimethyl-5-hydroxy-1-pyrrolidinyloxy and titanium-dioxide

When hexavalent chromium (Cr(VI)) is added to a TiO2 photocatalytic reaction, the decolorization and mineralization efficiencies of azo dyes Acid Orange 7 (AO7) are enhanced even though the mechanism is unclear. This study used 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) as the scavenger and the analysis of Electron Spin Resonance (ESR) to investigate this enhancement effect by observing the hydroxyl radical (OH) generation of the Cr(VI)/TiO2 system under UV and visible light (Vis) irradiation. With Cr(VI), the decolorization efficiencies were approximately 95% and 62% under UV and Vis, and those efficiencies were 25% less in the absence of Cr(VI). The phenomena of the DMPO-OH signals during the ESR analysis under Vis 405 and 550 nm irradiation were obviously the enhancement effects of Cr(VI) in aerobic conditions. In anoxic conditions, the catalytic effects of Cr(VI) could not be achieved due to the lack of a redox reaction between Cr(VI) and the adsorbed oxygen at the oxygen vacancy sites on the TiO2 surfaces. The results suggest that by introducing the agents of redox reactions such as chromate ions, we could lower the photoenergy of TiO2 needed and allow Vis irradiation to activate photocatalysis.

Topics: Azo Compounds; Benzenesulfonates; Catalysis; Chromium; Coloring Agents; Cyclic N-Oxides; Electron Spin Resonance Spectroscopy; Hydroxyl Radical; Light; Photolysis; Titanium; Water Pollutants, Chemical; Water Purification

We previously reported that irradiation of titanium dioxide (TiO2) in ethanol generates both singlet oxygen (1O2) and superoxide anion (O2*-) as measured by EPR spectroscopy. The present study describes the production of reactive oxygen species upon irradiation of TiO2 in aqueous suspension as determined by EPR spectroscopy using 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and 5,5-dimethyl-pyrroline-N-oxide (DMPO). Photoproduction of 1O2 by suspended TiO2, detected as 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (4-oxo-TEMPO), was measured in water and deuterium oxide (D2O) in the presence or absence of sodium azide (NaN3) and under air or argon atmospheres. Production of a DMPO-OH adduct was examined in 4-oxo-TMP containing medium in the presence or absence of dimethyl sulfoxide (DMSO). The signal for the DMPO spin adduct of superoxide anion was not observed in aqueous conditions. Kinetic analysis revealed that 1O2 was produced at the surface of irradiated TiO2 in aqueous suspension as was observed in ethanol. Kinetic analysis revealed that the formation of DMPO-OH adduct reflects oxidation of DMPO by 1O2 rather than the trapping of the hydroxyl radical produced by the reaction of photo-exited TiO2 and water. The production of large amounts of 1O2 by TiO2 in aqueous suspension as compared to those in ethanol and possible formation of hydroxyl radical in aqueous suspension but not in alcohol, suggest that irradiation of TiO2 in aqueous environments is biologically more important than that in non-aqueous media.

Topics: Antioxidants; Cyclic N-Oxides; Electron Spin Resonance Spectroscopy; Hydrogen Peroxide; Photosensitizing Agents; Piperidones; Reactive Oxygen Species; Spin Labels; Spin Trapping; Superoxides; Titanium; Ultraviolet Rays