Tungstophosphoric acid (TPA) in 50% v/v ethanol–water solutions was used to impregnate carbon, TiO2, SiO2 and γ-Al2O3 (Spheralite and Akzo) at 20°C. The solutions were characterized by UV-visible and nuclear magnetic resonance spectroscopies and the results indicated that the main species present was the undegraded anion of the acid both before and after the contact with carbon, TiO2, SiO2 and γ-Al2O3 Akzo. However, the nuclear magnetic resonance spectrum of the solution in contact with γ-Al2O3 Spheralite allowed to observe a tungstodiphosphate anion. X-ray diffraction of the impregnated solids dried at 70°C showed the same diffraction patterns as the supports, possibly owing to a high dispersion of non-crystalline species. The Fourier transform infrared and nuclear magnetic resonance spectroscopies of carbon, TiO2 and SiO2 impregnated with TPA showed that the species present was the undegraded [PW12O40]3− anion. When using carbon and TiO2, the supported acid was thermally stable up to 425°C, similarly to the bulk acid, whereas on SiO2 support, the anion underwent partial degradation from 365°C onwards. In TPA-impregnated γ-Al2O3 samples dried at 70°C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed us to observe a partial degradation of the anion, the extent of which was not appreciably increased by the subsequent calcination. The specific activity of the catalysts in the isopropanol dehydration reaction showed that TPA supported on carbon, TiO2 and SiO2 present higher acidity than the bulk acid, unlike in alumina where the partially degraded species gave a lower acidity.

Acid catalysts prepared by impregnation of tungstophosphoric acid solutions on different supports

The degradation of 4-chlorophenol with near-UV light by silica-immobilized polyoxometalate (POM-in-SiO2) catalysts has been studied. The silica-immobilized Na6W7O24 (SW7), H4W1032 (SW10), H3PW12O40 (SPW12), and H6P2W18O62 (SP2W18) were prepared by means of the sol-gel hydrothermal technique through the hydrolysis of tetraethoxysilane in aqueous solution of the corresponding polyoxometalate, respectively. The degradation of 4-chlorophenol was monitored by measuring Cl- and CO2 concentrations and analyzing reaction intermediates by GC/MS analysis. During irradiation, 4-chlorophenol first dechlorinated to form hydroquinone and p-benzoquinone, and then these intermediates further mineralized to form CO2 and H2O. The degree to which 4-chlorophenol was mineralized by photocatalytic oxidation was investigated. Results indicate less than 15% for SW7 but nearly complete mineralization for SW10 after 60 min of photoirradiation. The present studies suggest that POM-in-SiO2 catalysts may be a novel type of photocatalyts for the purification of the environmentally chlorophenol-contaminated water.

Photocatalytic degradation of aqueous 4-chlorophenol by silica-immobilized polyoxometalates

Polyoxometalates (H(3)PMo(12)O(40), H(3)PW(12)O(40), H(4)PMo(11)VO(40)) supported on oxygen- and alkyl-functionalized graphene sheets were investigated. Discrete molecular species were directly observed by electron microscopy at loadings below 20 wt.%. The interaction between the polyoxometalates and the graphene surface was found to significantly impact their vibrational spectra and a linear correlation between the frequency of the M-O(c)-M vibration and the dispersion was evidenced by FTIR. While bulk-like electronic properties were observed for small aggregates (2-5 nm), UV-vis spectroscopy and cyclic voltammetry revealed changes in the electronic structure of isolated molecular species as a result of their interaction with graphene. Because of the ability to disperse alkyl-functionalized graphene in a variety of polar and nonpolar solvents, the materials synthesized in this work provide an opportunity to disperse polyoxometalates in media in which they would not dissolve if unsupported.

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Structure, Stability, and Electronic Interactions of Polyoxometalates on Functionalized Graphene Sheets

In this paper, the decomposition of H3PW12O40 in aqueous solution or in mixed solutions of waterethanol or wateracetone is investigated by potentiometric titration and 31P NMR. Identification of ...

A study of the decomposition behaviour of 12- tungstophosphate heteropolyacid in solution

Previous studies have put into evidence the key role of a charge transfer excited state, noted X, in the primary steps of laser photolysis of sodium decatungstate solutions. In this work, we report a thorough study of the reactivity
 of the X transient towards different organic and inorganic substrates in acetonitrile and in water.
 The bimolecular reaction between X and the substrates is shown to occur on the nanosecond time-scale, either
 by H atom or electron transfer, and leads to the formation of the reduced form Y of decatungstate.We find
 that H atom transfer reactions are ruled by Z–H (Z=C, N, O) binding energies and lead to Y formation quantum yields that may reach the X formation quantum yield, whereas electron transfer reactions are ruled by the substrates' oxidation potential (Marcus reorganisation energy λ=1.0–1.3 eV for
 aromatics
 in acetonitrile)
 and lead to Y formation
 quantum yields similar to that obtained in the pure solvent.

Koichi Amimoto

Papers

Charge-transfer absorption spectra of some tungsten(VI) and molybdenum(VI) polyoxoanions