Infrared study of the thermal decomposition of heteropolyacids of the series H3+xPMo12–xVxO40

Abstract

F.t.i.r. spectroscopy has been used for the study of dehydration of heteropolyacids (HPA) of the series H_3+xPMo_12–xV_xO₄₀ carried out in vacuo at 298–673 K and also for the study of ammonia sorption on the same samples. Pumping at room temperature results in the departure of water of crystallization; the protons, which form H₅O⁺₂ ions in the hydrated samples, are involved in the formation of hydrogen bonds between the HPA anions (Keggin units) characterized by a band at 3250 cm^–1. The intensity of this band progressively decreases with increasing temperature of outgassing, indicating the departure of the ‘water of constitution’. No bands were present for OH groups that were not involved in the formation of hydrogen bonds. Within the range 400–1100 cm^–1 the bands characteristic of Keggin unit vibrations were observed. The substitution of Mo by V atoms results in the shift of most of these to lower frequencies. The formation of hydrogen bonds influences most strongly the vibrations of MoO groups, hence these terminal oxygen atoms are involved in the formation of such bonds. Ammonia is sorbed in the form of NH⁺₄ ions, thus confirming the presence of Brønsted-acid sites. No coordinatively bound ammonia was observed. Formation of NH⁺₄ ions influences most strongly the vibrations of MoO and Mo—O_b—Mo groups, but the Mo—O_c—Mo groups (O_b is an oxygen atom at the common apex of two [MoO₆] octahedra, O_c is an oxygen atom at the common edge of two [MoO₆] octahedra) indicating the position of NH⁺₄ ion in the immediate vicinity of three terminal and three O_b atoms.