Natsuko Watanabe

Photocatalytic degradation mechanism for heterocyclic derivatives of triazolidine and triazole

In an attempt to improve the understanding of the basic mechanisms on the degradation of pollutants in water by TiO2 photocatalysis, we discussed the primary degradation mechanism of three triazolidine derivatives, such as 1,2,4-triazolidine-3,5-dione (TRIANE), 4-hydroxy-1,2,4-triazolidine-3,5-dione (OH-TRIANE) and 4-phenyl-1,2,4-triazolidine-3,5-dione (Ph-TRIANE) and one triazole derivative of the 4-phenyl-1,2,4-triazole-2,5-dione (Ph-TRIOLE), on the basis of the experimental results together with molecular orbital (MO) calculation of frontier electron density and partial charge. The above four heterocycles were selected as molecular probe, principally because the highest frontier electron density was situated at different places of the molecule, while their structures were very similar, two major pathways have been revealed by LC/MS analysis for each heterocyclic compound. The pathway (a) corresponded to the hydroxylation of the atom bearing the highest electron density, via oxidation on nitrogen atom of azo group with respect to the photodegradation of TRIANE, OH-TRIANE and Ph-TRIOLE and opening of aromatic ring when Ph-TRIANE was used. This initial attack occurred with the preferential electrophilic attack of OH radicals. The pathway (b) was caused by the attack of active species on to carbon atom of carbonyl group. Considering the calculation of the relative electrophilic density of this carbon atom for the four heterocyclics and the evolution of large amount of N2 gas at the initial degradation step, it has been suggested that this step (b) was favored by adsorption of carbonyl group on TiO2 surface as theoretically determined by partial charge and confirmed by IR analysis. The participation of hole (h+) to form R–CO+ was envisaged in this step. The presence of 1,2,4,5-tetrazixane-3,6-dione also agrees with the participation of TiO2 surface. The nitrogen inorganic analysis (N2, NH4+ and NO3−) determined by gas chromatography and ion liquid chromatography, show that the hydrazo group were photoconverted mainly into N2 gas and partially to NH4+ ions. No nitrate ions were observed at the beginning of the degradation even when a OH substituent was present on one nitrogen atom (case of OH-TRIANE). However, the presence of –OH group increased the NO3−/NH4+ ratio observed after a few hours of irradiation. The presence of OH also improved the total mineralization of carbon atom into CO2, which was attributed to mesomeric effect of –OH group.

Mechanistic examination of the titania photocatalyzed oxidation of ethanolamines

In this study we focus on elucidating the mechanism of the photocatalyzed transformation of the primary, secondary and tertiary amines found in ethanolamine, diethanolamine and triethanolamine when present in illuminated aqueous titania dispersions. Photodecomposition of these ethanolamines leads to the evolution of CO2 through prior formation of various intermediate species. Ammonium (NH4+) and nitrate ions (NO3−) are the ultimate products formed in the photoconversion of the amine nitrogen atoms, with NH4+ cations produced in greater quantity than NO3− anions for all three ethanolamines. Photooxidation of triethanolamine yields various intermediates, including a 3-pyrrolidone derivative, diethanolamine, and then ethanolamine, before complete mineralization occurs. The nature of the initial steps in the photodegradation was predicted by computer-aided molecular orbital (MO) calculations of point charges, and by frontier electron densities of all atoms in the ethanolamine structures.

Near-quantitative mineralization of two refractory triazines under hydrothermal-supercritical aqueous conditions assisted by ozone and UV/ozone

Refractory atrazine and cyanuric acid were degraded under hydrothermal and supercritical aqueous media (HY-SC) conditions, as well as in the presence of ozone (HY-SC/O3) and UV-illuminated ozone (HY-SC/UV/O3) to assess whether the efficacy of the decomposition process could be enhanced using a single-pass flow-through treatment device under a constant pressure of 23 MPa. The progress of the degradation was evidenced by the extent of removal of total organic carbon (TOC) in solution, by UV absorption spectroscopy (opening of the atrazine and cyanuric acid heterorings), and by the extent of deamination (formation of NH4+) and dechlorination (release of Cl− ions) of the two compounds. Loss of atrazine was confirmed by LC-MSD techniques in the positive ion mode. Formation of various intermediates from the degradation of atrazine was substantiated by positive and negative ion mode MSD analyses. Dechlorination of atrazine occurred around 100 °C under hydrothermal conditions for the HY-SC and HY-SC/O3 processes but not for HY-SC/UV/O3; it did increase rapidly at higher temperatures (beginning at ca. 220–230 °C) for all three methods: HY-SC through HY-SC/UV/O3. Deamination, removal of TOC, and loss of atrazine mass spectral features began around 260–280 °C in hydrothermal aqueous media. Degradation of cyanuric acid showed a similar behavior. For the treated effluent solution in the collector reservoir, ozonation enhanced somewhat both dechlorination and mineralization, but had no significant effect on the deamination of either atrazine or cyanuric acid.

Photocurrent enhancement from an active hybrid TiO2 film electrode fabricated by a sol–gel method: photocurrent generation during the photooxidation of 4-nonylphenol and 4-nonylphenol polyethoxylate on TiO2/OTE electrodes

Photocurrent is generated during the photooxidation of nonylphenol polyethoxylate surfactants (NPE-n; n = 1, 5, 7, 9 and 10), precursors of 4-nonylphenol (NP), on illuminated TiO2/OTE electrodes on which TiO2 semiconductor particles were immobilized by a pasting (PA), a sol–gel (SG), or both techniques (HY). The photooxidative process of NP and NPEs was examined by absorption spectroscopic methods monitoring the cleavage of the benzene ring moiety, and by the evolution of CO2 gas. The kinetics of photooxidation and the magnitude of the generated photocurrent scaled with the length of the ethoxyl side-chain for NPEs where n = 0, 1, 5, 7 and 10. The relative order of adsorption of NPEs on the positive TiO2 surface was predicted from calculated partial negative point charges of all the C and O atoms in the NPEs using molecular orbital methods. Both photodegradation and photocurrent generation were enhanced for the NPE-9 substrate using the HY TiO2/OTE electrode compared to the PA and SG electrodes. Although, biodegradation of NPEs yields NP in rivers, no formation of NP was observed during the photooxidation of NPE-9 because of predominant opening of the benzene ring. Photooxidation of NPEs is facilitated under an applied bias of +0.3 V and scales with the number of ethoxylate groups.

Mechanistic inferences of the photocatalyzed oxidation of chlorinated phenoxyacetic acids by electrospray mass spectral techniques and from calculated point charges and electron densities on all atoms

The photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) taking place in UV irradiated aqueous TiO2 dispersions was revisited to obtain mechanistic details on the basis of similar degradation of the related 2,3-dichlorophenoxyacetic acid (2,3-D), 4-chlorophenoxyacetic acid (4-M) and phenoxyacetic acid (PhA). Mechanistic changes were inferred from the different positions of the chlorine substituents. As well, the compounds were compared for differences in degradation rate and initial adsorption on the TiO2 surface as a function of the number of chlorines. The initial mechanistic sequence(s) in the TiO2-photocatalyzed oxidation of each substrate was predicted theoretically by molecular orbital (MO) calculations of frontier electron densities and point charges of all the atoms in the phenoxyacetic acid structures.