Highly selective phenol production from benzene on a platinum-loaded tungsten oxide photocatalyst with water and molecular oxygen: selective oxidation of water by holes for generating hydroxyl radical as the predominant source of the hydroxyl group

Abstract

Particles of tungsten oxide loaded with nanoparticulate platinum (Pt/WO 3 ) photocatalytically produced phenol from benzene with high selectivity (e.g., 74% at 69% of benzene conversion) in water containing molecular O 2 ; the selectivity for phenol were much higher than those on conventional titanium oxide 10 (TiO 2 ) photocatalysts (both the unmodified and Pt-loaded) that generated CO 2 as a main product. Results confirmed that photoexcited electrons in the Pt/WO 3 photocatalysts mainly generated H 2 O 2 from molecular O 2 through a two-electron reduction; the H 2 O 2 generated did not significantly contribute to the undesirable peroxidation of phenol produced. In contrast, the oxygen radical species, such as O 2 or HO 2 , generated on TiO 2 photocatalysts partially contributed to the successive oxidation of phenol and 15 other intermediates to reduce the selectivity toward phenol. More importantly, the reactions using 18 Olabeled O 2 and H 2 O clearly revealed that the holes generated on Pt/WO 3 react primarily with H 2 O molecules, even in the presence of benzene in aqueous solution, selectively generating OH radicals that subsequently react with benzene to produce phenol. In contrast, a portion of holes generated on TiO 2 photocatalysts reacts directly with benzene molecules, which are adsorbed on the surface of TiO 2 by 20 strong interaction with surface hydroxyl groups. This direct oxidation of substances by holes undoubtedly enhanced non-selective oxidation, consequently lowing selectivity for phenol by TiO 2 . The two unique features of Pt/WO 3 , the absence of reactive oxygen radical species from O 2 and the ability to selectively oxidize water to form OH, are the most likely reasons for the highly selective phenol production. 65 benzene to phenols using Pt/TiO 2 suspended in water containing a high concentration of substances such as benzene (e.g., 1:1 v/v),

Multielectron reduction of molecular oxygen in photocatalytic decomposition of organic compounds by bismuth tungstate particles without cocatalyst loading

“…( Fig. 4) Small particles in a size range of a few or several nanometers that were not observed in nonplatinized samples are seen in all of the images, and any form of BWO could be platinized by photocatalytic deposition using hexachloroplatinic acid and methanol as a precursor of Pt and an electron donor, respectively [28]. Immediately after the commencement of photoirradiation, the color of the suspensions changed from light pale yellow to dark grey or black, indicating the formation of metallic Pt particles, but little evolution of hydrogen was detected in either the one-step or two-step deposition method, as is usually observed for deposition of Pt on titania particles.…”

Tungsten-based nanomaterials (WO 3 & Bi 2 WO 6 ): Modifications related to charge carrier transfer mechanisms and photocatalytic applications

“…Unlike TiO 2 -noble metal heterojunction, wherein noble metal induces singleelectron based oxygen reduction process, metal islands at the surface of WO 3 acts as co-catalyst to trigger multielectron based oxygen reduction pathways. The latter is more useful for the partial oxidation of organic compounds leading to selective synthesis (like benzene hydroxylation to phenol, partial oxidation of cyclcohexane to cyclohexanol and cyclohexanone), while former always results in complete mineralization [107,108]. The photoaction spectrum analysis concluded that the photoexcited electron reacts with oxygen to initiate multielectron reduction, rather being trapped to generate W 5+ upon metal loading [109].…”

Application of metal oxide semiconductors in light-driven organic transformations

“…It was hypothesized that the high selectivity is due to the presence of the platinum co-catalyst, which favors the twoelectron reduction of O 2 . 152 This results in the formation of mainly H 2 O 2 and thus avoids the formation of stronger oxidizing radical species, such as˙O 2 − and˙HO 2 . The formed H 2 O 2 can be rapidly decomposed in water and oxygen, consequently, having an insignificant effect on the oxidation pathway (Scheme 55).…”