Pathways of the photocatalytic reaction of acetate in H2O and D2O: A combined EPR and ATR-FTIR study

Abstract

The adsorption and photocatalytic degradation of acetate on TiO2 surfaces was investigated in H2O and D2O by ATR-FTIR and EPR Spectroscopy respectively. These studies were carried out in the dark and under UV(A) illumination to gain additional insights into the adsorption behaviour with the identification of paramagnetic species formed during the oxidation of acetate. Isotopic exchange during the adsorption of D2O on TiO2 surface led to different interactions between the adsorbate and OD groups. At different pH levels, several surface complexes of acetate can be formed such as monodentate, or bidentates. Under UV(A) irradiation of TiO2 aqueous suspensions, the formation of hydroxyl and methoxy radicals evidenced as the corresponding spin-adducts, were found to dominate in alkaline and acidic suspensions respectively. Two possible pathways for the oxidation of acetate have been suggested at different pH levels in solution in terms of the source of the spin adduct formed.These proposed pathways were found to be in good agreement with ATR-FTIR and EPR results.

Application of EPR Spectroscopy in TiO2 and Nb2O5 Photocatalysis

“…Recently, the spin trapping technique is widely employed in the photocatalytic reaction as a valuable tool to study the reaction mechanism [21,87]. Among several spin traps agents, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline-1-oxide (FDMPO), α-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN), and N-tert-butyl-a-phenylnitron (PBN), 5,5-Dimethyl-1-pyrrolin-N-oxid (DMPO) is the most used spin trap due to the well resolved and characteristic EPR spectra with the superoxide and hydroxyl radicals [10,[86][87][88]. Figure 14 shows the EPR spectra for the DMPO-OH and DMPO-O 2 adducts after the reaction of the hydroxyl radicals and superoxide with DMPO under irradiation.…”

“…Several research groups have been explored the photocatalytic activity, as well as the photocatalytic mechanism involved in the presence of different photocatalysts employing the EPR spin trapping techniques [19,21,[86][87][88]91]. Fu et al [92] investigated the photocatalytic degradation of 4-chlorophenol (4-CP) using N-doped TiO 2 prepared by the high-temperature nitridation of commercial P25 (Degussa) and undoped P25-TiO 2 annealed with N 2 in the same process as a reference sample, under UV and visible light (λ > 420 nm).…”

“…To better understand the interfacial interactions between the acetate and the TiO 2 at different pH, Belhadj et al [88] investigated the adsorption and the photocatalytic degradation of acetate on anatase surface (UV100) by combining the EPR and attenuated total reflection Fourier transform infrared (ATR-FTIR) techniques. In this study, the authors employed DMPO as a spin trap to probe the formed reactive oxygen species during the photocatalytic degradation of acetate in H 2 O and D 2 O under aerobic conditions at different pH.…”

“…According to the EPR results, the authors considering that the formation of DMPO-OH/OOH adducts give direct evidence that ( • OH and O2 •− ) are the main active species responsible for the photodegradation of 4-CP, strongly suggesting that the photocatalytic reaction of organic compounds over N-doped TiO2 proceed via surface intermediates of oxygen reduction or water oxidation (indirect path), not via direct reactions with holes trapped at the N-induced midgap level. To better understand the interfacial interactions between the acetate and the TiO2 at different pH, Belhadj et al [88] investigated the adsorption and the photocatalytic degradation of acetate on anatase surface (UV100) by combining the EPR and attenuated total reflection Fourier transform infrared (ATR-FTIR) techniques. In this study, the authors employed DMPO as a spin trap to probe the formed reactive oxygen species during the photocatalytic degradation of acetate in H2O and D2O under aerobic conditions at different pH.…”

In situ attenuated total reflection infrared spectroscopy study of the photocatalytic steam reforming of methanol on Pt/TiO2

“…Infrared spectroscopy has been widely used to study the interactions of methanol and water with the TiO2 surface [6][7][8][9][10] and to provide information on the photoreaction mechanism [10][11][12][13][14][15]. In addition to the detection of surface species formed during the photoreaction, time resolved FT-IR spectroscopy proved to be a powerful tool to distinguish between photopromoted free CB electrons and shallow trapped (ST) electrons and to evaluate their lifetime [8,[16][17][18].…”