Electron spin resonance studies of free and supported 12-heteropoly acids. Part 2.—Reduction of H3[PMo12O40]·xH2O and H4[SiMo12O40]·xH2O and oxygen adsorption

Abstract

Hydrogen and carbon monoxide reductions of SiMo₁₂ and PMo₁₂ heteropoly acids (HPA) have been studied, the various Mo⁵⁺ signals detected being identified as arising from stages in the dehydration and destruction of the HPA in a similar manner to vacuum heat treatment. Oxygen adsorption studies revealed the formation of O^–₂ radicals on the supported HPA, prereduced at high temperatures, a phase in which the Keggin structure is already destroyed. In contrast, HPA in which the Keggin structure is preserved proved unable to stabilise O^–₂ species. The stability of the supported HPA was shown to be markedly dependent on the nature of the support.