Electron spin resonance studies of free and supported 12-heteropoly acids. Part 3.—The stability of unsupported H3+n(PVnMo12–nO40)·xH2O in air and in a vacuum

Abstract

Dehydration up to 773 K of solid H_3+n(PV_nMo_12–nO₄₀)·xH₂O heteropoly acids (HPA) has been studied by e.s.r. spectroscopy. Several V⁴⁺ signals observed during this process have been simulated and fitted by means of a computer program. These signals reflect the state of the heteropoly acid after thermal treatment in air and vacuum. Loss of water of crystallization and constitution as well as destruction of the heteropoly acid are well documented by appropriate V⁴⁺ signals. Regeneration of a destroyed Keggin anion by adsorption of water is observed. Monocrystal samples show angular dependence in the hydrated state; they appear polycrystalline after dehydration. The influence of vanadium content is indicated by a temperature-dependent signal of exchange-coupled V⁴⁺ ions obtained from the PV₃Mo₉ HPA.