Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
1.6.1 Photocatalysis
POMs, especially polyoxotungstates, are effective homogeneous photocatalysts for the
mineralization of organic pollutants in the aquatic environment (i.e. degradation of organic
pollutants to CO 2 , H 2 O and the corresponding inorganic anions). 22 POMs (in a particular
[PW 12 O 40 ] 3− and [SiW 12 O 40 ] 4− ) are at least as effective as the well-studied TiO 2 (metal oxide
particulates) in their photocatalytic activity. Furthermore, POMs as photocatalysts have
advantages relative to TiO 2 in terms of their selective reduction precipitation of metal ions
and their unique to form metal nanoparticles in which POMs act as both reducing reagents
and stabilizers. The redox chemistry of POMs renders them to act as multielectron relays. OH·
radicals generated by the reaction of the photo-excited polyoxotungstates with H 2 O seem to
play a key role in the process. The photocatalytic cycle is closed by reoxidation (regeneration)
of the reduced POMs by either dioxygen O 2 which is the mainly process or by metal ions, in
the latter process metal ions are reduced and precipitated and removed from the solution.
Hence, POMs as photocatalysts can be used for decontamination of waste waters from both
organic and inorganic pollutants:
POM + S
hυ
POM red + S ox
POM red + M n+
POM + M red
It is noteworthy that in the absence of O 2 , the regeneration of the catalyst will not take place,
at least at the first stages of the photolysis. Therefore, the blue color of the one-electron
reduced POMs, [XW 12 O 40 ] (n+1)− , will be accumulated.
The two previous simplified equations (where POM = [XW 12 O 40 ] n− , S = organic substrate,
and M n+ = metal ion) can be clarified as the followings: POM can efficiently absorb light in
the near UV-vis region and subsequently excite at the O−M CT band (Ligand−Metal Charge
22