Elizabeth R. Carraway

Photocatalytic Production of H2O2 and Organic Peroxides on Quantum-Sized Semiconductor Colloids

Illuminated (320 ≤ X ≤ 370 nm), aqueous suspensions of transparent quantum-sized (Q-sized) ZnO semiconductor colloids in the presence of carboxylic acids and oxygen are shown to produce steady-state concentrations of H_2O_2 as high as 2 mM. Maximum H_2O_2 concentrations are observed only with added electron donors (i.e., hole scavengers). The order of efficiency of hole scavengers is as follows: formate > oxalate > acetate > citrate. Isotopic labeling experiments with ^(18)O_2 are consistent with the hypothesis that hydrogen peroxide is produced directly by the reduction of adsorbed oxygen by conduction band electrons. Quantum yields for H_2O_2 production are near 30% at low photon fluxes. However, the quantum yield is shown to vary with the inverse square root of absorbed light intensity [Φ ∝ ((I_(abs))^(-1)½)], with the wavelength of excitation, and with the diameter of the Q-sized colloids. The initial rate of H_2O_2 production is 100-1000 times faster with Q-sized ZnO particles (D_p = 4-5 nm) than with bulk-sized ZnO particles (D_p = 0.1 µm).

Photocatalytic oxidation of organic acids on quantum-sized semiconductor colloids.

A detailed analysis of the reaction products and mechanisms of the photocatalytic oxidation of acetate in the presence of quantum-sized ZnO colloids (Dp ≈ 40 A) is presented. The principal oxidation products and reaction intermediates are determined to be CO_2, HCO_2^-, CHOCO_2^-, HCHO, CH_3OOH, CH_3COOOH, and H_2O_2. Formate and glyoxylate, which are found as intermediates in the photooxidation of acetate, also serve as effective electron donors on illuminated ZnO surfaces. The proposed relative reactivity of electron donors toward photooxidation is in the following order: CHOCO_2- > HCO_2^- > HCHO > CH_3CO_2^- ≥ H_2O_2 CH_3COOOH > CH_3OOH. Observed product distributions are discussed in terms of pathways involving direct oxidation of surface-bound acetate by valence band holes (or trapped holes) and the indirect oxidation of acetate by surface-bound hydroxyl radicals. The product distribution observed at low photon fluxes is not consistent with oxidation primarily by free hydroxyl radicals. A mechanism involving the reaction of an intermediate carbon-centered radical with > ZnOH surface sites is proposed. When electron donors are strongly adsorbed to semiconductor surfaces, surface-mediated reactions appear to play a dominant role in the determination of the time-dependent product distributions.

Solubilization of polycyclic aromatic hydrocarbons by perfluorinated surfactant micelles

Due to their chemical and thermal stability, perfluorinated surfactants (PFSs) are promising materials for the development of novel environmental remediation applications. This stability also leads to the persistence of PFS in the environment; therefore, their properties and behavior should be well understood. This study focused on polycyclic aromatic hydrocarbon (PAH) and PFS interactions, particularly the solubilization of PAHs by PFS micelles. Naphthalene. phenanthrene, and pyrene were selected as representative PAHs and an anionic PFS, ammonium perfluorooctanoate (APFO) was used. Critical micelle concentration (CMC) values of APFO measured by surface tension, fluorescence probe, and solubility enhancement methods fell in the range of 20-30 mM at 22 +/- 1 degrees C. Apparent solubilities of molecular oxygen and PAHs in APFO micellar solutions depended linearly on the APFO concentration. Molar solubilization ratio (MSR) values were determined to be 9.50 x 10(-4), 4.17 x 10(-3), 2.31 x 10(-4), and 4.09 x 10(-5) and mole fraction micellar partition coefficient (Kmic) values were found to be 1.89 x 10(2), 9.50 x 10(2), 2.12 x 10(3), and 3.79 x 10(3) for oxygen, naphthalene, phenanthrene, and pyrene, respectively at 22 +/- 1 degrees C. log Kmic values for three PAHs were shown to be linearly correlated with the log values of octanol-water partition coefficients (log Kow).

PAH degradation by UV/H2O2 in perfluorinated surfactant solutions

Polycyclic aromatic hydrocarbons (PAHs) solubilized in perfluorinated surfactant (PFS) solutions were degraded by direct photolysis and UV/H2O2 process. The subsequent recovery and reuse of these surfactant solutions were also demonstrated. Phenanthrene and pyrene were selected as representative PAHs and an anionic PFS: ammonium perfluorooctanoate (APFO) was used. In our experiments, micellar APFO solutions retarded the phenanthrene photolysis and enhanced the pyrene photolysis. The results indicate that the photochemical reactivity of compound in micelles is strongly dependent on specific properties of the solubilizate, possibly due to the different excited state behaviors of compound. UV/H2O2 process exhibits a greatly enhanced rate of PAH photolysis in both water and APFO compared to direct photolysis. indicating that hydroxyl radicals may be generated or penetrated at the sites of PAHs solubilized in the micelles. Additionally. a smaller rate enhancement by UV/H2O2 in micelles than in water suggests that micelles provide some degree of protection from hydroxyl radical attack. The possibility of recovery and reuse of PFS has been demonstrated by measuring the solubilizing capacity of APFO after direct photolysis and UV/H2O2 process. Overall, this study demonstrates UV/H2O2 process can be an effective treatment method for not only PAH degradation but also surfactant recovery and reuse.

Micellar effect on the photolysis of hydrogen peroxide

Photolysis experiments were performed to quantify the effect of three anionic surfactants on the photolysis of hydrogen peroxide (H2O2) at the ambient laboratory temperature of 22+/-1 degrees C. H2O2 photolysis in water, methanol, and surfactant monomeric solution was also conducted to compare the photochemical reactivity of H2O2 in different media. Photolysis rates were highest for water, followed by micellar solutions, and lowest for methanol. The results show that the photochemical reactivity of H2O2 is less favorable in organic solvent than in water and surfactant micelles affect H2O2 photolysis. Retarded photolysis of H2O2 in micellar solutions implies that a fraction of H2O2 dissolved in water partitions into micellar pseudophase of surfactant. H2O2 partitioned into micelles has less photochemical reactivity and thus photolysis rate was retarded in the presence of micelles. Photolysis inhibitory level by micelles was shown to be dependent on the kinds of surfactants used in this study. In addition, the inhibitory effect by surfactant monomers was negligible due to the absence of micelles.