Determining the TiO₂-Photocatalytic Aryl-Ring-Opening Mechanism in Aqueous Solution Using Oxygen-18 Labeled O₂ and H₂O

Abstract

The molecules O2 and H2O dominate the cleavage of aromatic sp(2) C-C bonds, a crucial step in the degradation of aromatic pollutants in aqueous TiO2 photocatalysis, but their precise roles in this process have remained elusive. This can be attributed to the complex oxidative species involved and to a lack of available models for reactions with a high yield of direct products. Here, we used oxygen-18 isotope labeled O2 and H2O to observe the aromatic ring-opening reaction of the model compound 3,5-di-tert-butylcatechol (DTBC), which was mediated by TiO2 photocatalysis in an aqueous acetonitrile solution. By analyzing the primary intermediate products (∼75% yield), especially the seven-membered ring anhydrides that were formed, we obtained direct evidence for the oxygen atom of dioxygen insertion into a C-C bond of the aromatic ring. This indicates that molecular oxygen is the ultimate ring-opening agent in TiO2 photocatalysis and that it undergoes single O atom incorporation rather than the previously proposed molecular oxygen 1,2-addition processes. The ratio of intradiol to extradiol products depends on the particle size of TiO2 catalysts used, which suggests that the O2 activation is correlated with the available coordination sites on the TiO2 surface in the photocatalytic cleavage of the aromatic ring.

An effective heterogeneous iron-based catalyst to activate peroxymonosulfate for organic contaminants removal

“…The kinetics of decoloration of the dyes may be described by a pseudo-first-order equation (Eqs. (1) and (2)):…”

Facile fabrication of g‐C3N4/MIL‐53(Al) composite with enhanced photocatalytic activities under visible‐light irradiation

“…Among these photocatalysts, TiO 2 is one of the best because of its nontoxicity, acceptable stability and high photocatalytic activity. However, TiO 2 is responsive only to UV light, which accounts for no more than 4% of the solar spectrum, greatly limiting its photocatalytic efficiency and practical applications . Developing a novel photocatalyst with efficient visible‐light absorption and excellent stability remains a great challenge.…”

In Situ ATR-FTIR Investigation of Photodegradation of 3,4-Dihydroxybenzoic Acid on TiO2

“…It was reported that molecular oxygen is the ultimate ring-opening agent of 3,5-di-tert-butylcatechol in TiO 2 photocatalysis using water/acetonitrile mixed solvent [6]. However, the photodegradation of 3,4-dihydroxybenzoic acid observed in the absence of oxygen as can be seen from .…”

“…Catechol and its derivatives are important intermediates in the photo-degradation of benzene and its derivatives because OH radicals attack the aromatic ring before the latter is cleaved [4,5]. The two hydroxyl groups of catechol and its derivatives lead to strong adsorption on TiO 2 , which may be a crucial factor for the cleavage of the aromatic ring [6]. Aliphatic carboxylic acids are subsequently formed after the opening of aromatic ring, which also bind to TiO 2 [7].…”