Genesis of V4+ in heteropoly compounds Cs(x)H(4-x)PVMo11O40 during thermal treatment, rehydration and oxidation of methanol studied by EPR spectroscopy

J. K. Lee, V. Russo, J. Melsheimer, K. Köhler, R. Schlögl

Publikation: Beitrag in FachzeitschriftArtikelBegutachtung

30 Zitate (Scopus)

Abstract

The development and structural changes of V4+ and Mo5+ centers in caesium salts and the acid form of molybdovanadophosphorus heteropoly compounds Cs(x)H(4-x)PVMo11O40·nH2O (x = 0, 3, 4) of Keggin structure were investigated by electron paramagnetic resonance (EPR) after preparation, thermal treatment in oxygen, reduction by methanol and catalytic oxidation of methanol. The various types of paramagnetic centers are found to characterize the different states and structural transitions of the heteropoly anion. In addition, the EPR spectra reflect well the different reducibility, thermal stability and mobility of vanadium and Keggin ('lattice') oxygen of the Cs4, the Cs3H and the H4 compounds. The V4+ centers in the Cs4 salt are different in structure, lower in concentration and thermally more stable than those in the acid form. The reduction of vanadium during thermal treatment due to the loss and oxidation of coordinated ('lattice') oxygen to O2 is observed in the acid and the Cs3H salt only, but not in the Cs4 compound. As deduced from quantitative EPR measurements and manganometric redox titrations, between 0.5 and 8% of the vanadium is present as V4+ after preparation and thermal treatment in oxygen, whereas the majority of vanadium is reduced by reaction with methanol. V4+ substitutes Mo atoms in the Keggin unit in the fully exchanged Cs salt (Cs4). The presence of the bulky caesium counter-cations prevents the removal of vanadium from the Keggin unit to the secondary structure and thus the formation of vanadyl bridged polymers.

OriginalspracheEnglisch
Seiten (von - bis)2977-2983
Seitenumfang7
FachzeitschriftPhysical Chemistry Chemical Physics
Jahrgang2
Ausgabenummer13
DOIs
PublikationsstatusVeröffentlicht - 1 Juli 2000

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