Pathway of Oxygen Incorporation from O₂ in TiO₂ Photocatalytic Hydroxylation of Aromatics: Oxygen Isotope Labeling Studies

Abstract

The hydroxylation process is the primary, and even the rate-determining step of the photocatalytic degradation of aromatic compounds. To make clear the hydroxylation pathway of aromatics, the TiO(2) photocatalytic hydroxylation of several model substrates, such as benzoic acid, benzene, nitrobenzene, and benzonitrile, has been studied by an oxygen-isotope-labeling method, which can definitively assign the origin of the O atoms (from oxidant O(2) or solvent H(2)O) in the hydroxyl groups of the hydroxylated products. It is found that the oxygen source of the hydroxylated products depends markedly on the reaction conditions. The percentage of the products with O(2)-derived hydroxyl O atoms increases with the irradiation time, while it decreases with the increase of substrate concentration. More intriguingly, when photogenerated valence-band holes (h(vb)(+)) are removed, nearly all the O atoms (>97 %) in the hydroxyl groups of the hydroxylated products of benzoic acid come from O(2), whereas the scavenging of conduction-band electrons (e(cb)(-)) makes almost all the hydroxyl O atoms (>95 %) originate from solvent H(2)O. In the photocatalytic oxidation system with benzoic acid and benzene coexisting in the same dispersion, the percentage of O(2)-derived hydroxyl O atoms in the hydroxylated products of strongly adsorbed benzoic acid (ca. 30 %) is much less than in that of weakly adsorbed benzene (phenol) (>60 %). Such dependences provide unique clues to uncover the photocatalytic hydroxylation pathway. Our experiments show that the main O(2)-incorporation pathway involves the reduction of O(2) by e(cb)(-) and the subsequent formation of free (⋅)OH via H(2)O(2), which was usually overlooked in the past photocatalytic studies. Moreover, in the hydroxylation initiated by h(vb)(+), unlike the conventional mechanism, the O atom in O(2) cannot incorporate into the product through the direct coupling between molecular O(2) and the substrate-based radicals.

Computational consideration on advanced oxidation degradation of phenolic preservative, methylparaben, in water: mechanisms, kinetics, and toxicity assessments

“…The latter route was easier to occur than the former in the presence of O 2 , and the degradation products were HOO • and MPB-OH, in line with the experimental results [17,19]. This theoretical finding can further explain the experimental results as to why the O atom of O 2 cannot be incorporated into the degradation product through direct coupling between molecular O 2 and substrate-based radicals during AOPs [42]. As for H-abstraction intermediate ( • MPB(-H)␣) (Fig.…”

Exploring the Reactivity of Multicomponent Photocatalysts: Insight into the Complex Valence Band of BiOBr

“…Indeed, when nitrobenzene ( E 0 =2.9 V vs. NHE) was used as the substrate (Table 2, entries 7 and 8), neither visible nor UV irradiation induced the oxidation of the substrate. In contrast, even though direct oxidation of nitrobenzene by the h vb + of TiO 2 is also not viable ( E vb =2.7 V),6b nitrobenzene can be oxidized in the TiO 2 /UV system, forming hydroxylated products with approximately 50 % 18 O. This can be attributed to the oxidation of H 2 O by h O 2p + (cannot relax as in the case of BiOBr) to produce .…”

“…(1)];6a–c 2) direct oxidation of substrate by h vb + to form a cationic radical, which undergoes hydrolysis to yield the products [Eq. (2)];6b,c and 3) reaction of molecular O 2 with the cationic radical of the substrate to form an O 2 adduct, which further decomposes to the hydroxylated product [Eq. (3)] 6a,d…”

Hydrothermal synthesis of Ag2O/Bi2O3 microspheres for efficient photocatalytic degradation of Rhodamine B under visible light irradiation

“…[35]. Therefore, it is possible that holes (h + ) and hydroxyl radicals (•OH) were served as the main active species involved in the photodegradation of organic compounds.…”