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

Abstract

Illuminated (320 < < 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 H2O2 as high as 2 mM. Maximum H2O2 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 18C>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 H2O2 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 [ < ((/abs)"1)1/2], with the wavelength of excitation, and with the diameter of the Q-sized colloids. The initial rate of H2O2 production is 100-1000 times faster with Q-sized ZnO particles (Dp = 4-5 nm) than with bulksized ZnO particles (Dp = 0.1 µ ).

Photoacoustic spectroscopic analysis of photoinduced change in absorption of titanium(IV) oxide photocatalyst powders: A novel feasible technique for measurement of defect density

“…H 2 O 2 can be produced by reaction of photoexcited electrons with molecular oxygen via superoxide anion (O 2 -) [23,24], and EPR studies have suggested the liberation of peroxide species in the presence of oxygen [6], though we have no information on the origin. Anatase TiO 2 samples of large specific surface area have been shown to have large amounts of hydroxyl groups and physisorbed water through studies of infrared spectroscopy [6,25], thermogravimetry [5,26,27] and 1 H NMR [28], and these species on Type 2 samples might participate in the hole trapping and/or peroxy species productions.…”

The combination of heterogeneous photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance

“…On the basis of the degradation rate results, the authors suggest that the synergy should not be attributed to water-semiconductor interface phenomena but should rather involve active species in the aqueous phase. Hydrogen peroxide is known to be produced by both photocatalysis and sonolysis in water: under photocatalysis H 2 O 2 is formed mainly by the reduction of adsorbed dioxygen by conduction band electrons [33], while sonolysis of water produces H 2 O 2 as a result of recombination between hydroxyl radicals produced by the implosion of cavitation bubbles [34]. By monitoring H 2 O 2 formation during the degradation runs, the authors [18] conclude that the sonophotocatalytic synergistic effect is strictly related to the accumulation of hydrogen peroxide in the aqueous phase.…”

Photocatalytic Degradation of 2,4-dichlorophenol in Irradiated Aqueous ZnO Suspension

“…12. Even though hole and hydrogen peroxide may aid the degradation of chlorophenols in ZnO dispersion, experimental evidences have proved the main oxidant to be hydroxyl radical (Minero et al, 2000;Hoffmann et al, 1994). Tentatively therefore, we propose the mechanistic pathways in Fig.…”