Heteropolyacid‐Based Materials as Heterogeneous Photocatalysts

Abstract

Heteropolyacids (HPAs) that are often used as heteropolyanions are cheap and stable compounds that have been extensively used as acid and oxidation catalysts as a result of their strong Brønsted acidity and ability to undergo multielectron‐transfer reactions. HPAs, which are very soluble in water and polar solvents, have been also used as homogeneous photocatalysts for the oxidation of organic substrates in the presence of oxygen, but their use in heterogeneous systems is by far desirable. Dispersing HPAs onto solid supports with high surface area is useful to increase their specific surface area and hence (photo)catalytic activity. Moreover, owing to the high energy gap between the HOMO and LUMO positions of the HPAs, these compounds are activated only by UV light. Consequently, only less than 5 % of the solar light can be used in photocatalytic reactions, which restricts the practical application of HPAs. This microreview is oriented to describe the reported literature on the use of HPA‐based materials as heterogeneous photocatalysts for environmental purposes, that is, for the complete or partial oxidation or reduction of organic molecules.

Photocatalytic conversion of glucose in aqueous suspensions of heteropolyacid–TiO2 composites

“…On the other hand, the oxidation of Table 2 Photoreactivity assessment of bare TiO 2 powders: glucose conversion, X, selectivity to the identified reaction products, S, and carbon mass balance, B, after 360 minutes of irradiation glucose to gluconic acid (as rst step) and to glucaric acid (as second step) were observed only during the runs carried out by using PW 12 containing composite photocatalysts. On the contrary, the presence of PW 11 , that does not show any acid function, induces an increase of the oxidizing ability of TiO 2 , probably acting through the formation of the heteropoly-blue species 26 and consequently reducing the electron-hole recombination on TiO 2 . However, it is worth noting that the sample PW 11 -0.125/TiO 2 solv is more oxidizing than the sample PW 11 -0.250/TiO 2 solv.…”

“…This process can occur both with the plenary HPA Keggin species (H 3 PW 12 O 40 ) and with the lacunary Keggin cluster (K 7 PW 11 O 39 ). 26 HPA photoreduction has been proved to be synergistically enhanced in HPA/TiO 2 composites where photo-generated electrons can be transferred from the conduction band of TiO 2 to HPA. In this way the charge-pair recombination in the TiO 2 is delayed.…”

Organo-functionalized metal–oxide clusters: synthesis and characterization of the reduced cationic species [NaVIV6O6{(OCH2CH2)2NH}6]+

“…Its structure consists of a fully reduced cyclic {NaV 6 N 6 O 18 } framework incorporating six diethanolamine ligands, chloride and hydroxide anion balance charge. The {NaV 6 N 6 O 18 } core adopts an Anderson-like structure 16 comprising of six edge-sharing distorted {VO 5 N} octahedra linked to the central {NaO 6 } unit. The octahedral geometry around each vanadium atom of the cyclic core is defined by five atoms from the diethanolamine ligands and one terminal oxygen atom (VvO t ).…”

“…The octahedral geometry around each vanadium atom of the cyclic core is defined by five atoms from the diethanolamine ligands and one terminal oxygen atom (VvO t ). The {V 6 O 12 } ring in C3 may be regarded as a hexadentate ligand to the metal Na, forming a {NaV 6 …”

“…Broadly considered, these are the ability to accept multiple electrons in a chemically reversible manner, 2 and the presence of characteristic near-UV oxygen-to-metal (MvO t ) charge-transfer bands. 2 Their practical application is found, for example, in oxidative chemical [3][4][5] and photochemical [6][7][8] catalysis. Reduced POMs support unpaired electrons and are thus attractive as potential magnetic centers.…”