Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer Reactions of Phenol on a Titanium Dioxide−Fluoride System

Abstract

The effect of fluoride ions on the photocatalytic degradation of phenol in an aqueous suspension of TiO2 has been investigated. Fluoride ions displace surficial hydroxyl groups and coordinate surface-bound titanium atoms directly. For 0.01 M fluoride concentration and 0.10 g L -1 of TiO2 in the range pH 2-6, the degradation rate of phenol is up to 3 times that in the absence of fluoride ions. This behavior has been correlated with the computed surface speciation. The decrease in the degradation rate of phenol as a function of the substrate concentration observed in naked TiO2 at a high concentration of phenol (over 0.01 M) is largely diminished in the presence of fluoride ions. A photocatalytic model which takes into account the primary events and recombination reactions is able to account for these experimental results. The competition between OH-radical-mediated reaction versus direct electron transfer is discussed. Finally, under a helium atmosphere and in the presence of fluoride ions, phenol is slowly but significantly degraded, although total organic carbon does not decrease, suggesting the occurrence of a photocatalytically induced hydrolysis.

Mechanisms of catalyst action in the TiO2-mediated photocatalytic degradation and cis–trans isomerization of maleic and fumaric acid

“…The poor mass balance again implies that further degradation of the early intermediates to CO 2 was probably faster than the initial step. While TOC experiments were not run to check this, the result is consistent with Minero's report of much greater mineralization with O 2 than without [56,57]. Again, the implication is that solution phase chemistry is not responsible for the isomerization reaction.…”

“…The pH of the slurry was adjusted to 3 to maximize surface coverage by fluoride ions [56,57]. As noted in the text, experiments were carried out both under conditions of constant O 2 purging and Ar purging.…”

“…The addition of fluoride anion to TiO 2 suspensions at low pH has been shown to produce homogenous hydroxyl radicals and dramatically reduce the surface-bound chemistry of phenol, a poorly adsorbing substrate [56,57]. Broadly speaking, this effect is attributed to displacement of the surface hydroxyls by fluoride (maximized at pH 3-4) and the concomitant change in surface properties.…”

Photocatalytic oxidation of gaseous ammonia over fluorinated TiO2 with exposed (001) facets

“…The C element is ascribed to residual carbon. The fluoride peak at binding energy 684.5 eV (F 1s) only appears in the F-TiO 2 sample, indicating the formation of surface fluoride ( Ti-F) by a ligand exchange reaction between F − and the surface hydroxyl group on the TiO 2 surface [40][41][42][43][44]. No peak for F − ions in the lattice at 688.5 eV was observed in the spectra, because the hydrothermal process could prevent the substitution of F − for O 2− in the lattice of TiO 2 [21,39,40,45].…”

Graphene oxide–epoxy hybrid material as innovative photocatalyst

“…It is well known that irradiation of TiO 2 with an energy source higher than its band gap produces electrons and holes in the conduction band and valence band, respectively. These photogenerated holes and electrons can combine with the surfaceadsorbed species (e.g., water and oxygen) to form highly reactive radical species such as hydroxyl radical and superoxide radical anion and can completely mineralize most organic compounds [1][2][3][4][5][6][7].…”