Keggin-type Polyoxotungstates Research Articles

Palladium-Substituted Keggin Type Polyoxotungstate Chemically Modified Electrode and Its Catalytic Activity Toward Hydrogen Peroxide Reduction

Keggin type heteropolytungstate, in which one of the positions normally occupied by a tungsten cation is occupied by a palladium cation instead, is used to prepare a chemically modified electrode (CME). The electrochemical and electrocatalytic properties of such a CME are investigated. The electron transfer between the palladium-substituted heteropolytungstate and the glassy carbon electrode is fast. The palladium-substituted heteropolyanion CME, with a surface apparent coverage about 1.94 x 10−9 mol.cm−2, is shown to be an excellent catalyst for the electrochemical reduction of hydrogen peroxide, while the perfect and lacunary heteropolyanions show no catalytic activity. The effect of solution pH on the stability of the CME, as well as on the electrocatalytic activity toward H2O2, was also investigated. The catalytic current increases linearly with increasing concentration of H2O2 up to 150mmol.L−1, and the detection limit is 4.0 x 10−6mol.L−1. Due to its sensitivity and durability, this CME could be used as analytical sensor for detecting H2O2 in real samples.

Alkene epoxidation by hydrogen peroxide in the presence of titanium-substituted Keggin-type polyoxotungstates [PTi xW 12 − xO 40] (3 + 2 x)− and [PTi xW 12 − xO 40 − x(O 2) x] (3 + 2 x)− ( x = 1 and 2)

The Ti-substituted Keggin type polyoxotungstates [PTiW 11O 40] 5−, [PTi 2W 10O 40] 7−, [PTiW 11O 39(O 2)] 5−, and [PTi 2W 10O 38(O 2) 2] 7− have been used as catalysts for the epoxidation of alkene with H 2O 2. The alkene epoxidation results from the synergistic interaction between a tungsten-peroxo site with an adjacent Ti-peroxo ( η 2) site which acts as an electrophilic center for the alkene on the catalyst. The kinetic feature for the alkene oxidation implies the involvement of ·OH radicals with the competition with H 2O 2 dismutation.

Maternal factors related to “AC” insecuke attachment in the strange situation

Some tetra-n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW11O39]7−, α-[PW11M(H2O)O39]5−, MII=Co, Ni, α-[PW11CuO39]5−, α-[PW11Fe(H2O)O39]4− and α-[PW11Fe(OH)O39]5−, with the general formula [(C4H9)4N]4Hx[anion]·nH2O, were studied by powder X-ray diffraction. The salts α-[(C4H9)4N]4H3[PW11O39] and α-[(C4H9)4N]4H[PW11CuO39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell (Im3̄ symmetry) and the other has a body-centered tetragonal unit cell (I41 or I4122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) Å, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C4H9)4N]4Hx[PW11M(H2O)O39]·nH2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C4H9)4N]4H[PW11Fe(OH)O39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.