FT-IR and microbalance studies of diammonium ions formation in heteropolyacids

The sorption of gaseous ammonia into solid polyoxometalates such as the Keggin (H₄SiMo₁₂O₄₀, H₄SiW₁₂O₄₀ and H₃PW₁₂O₄₀) and Dawson type (H₆P₂W₁₈O₆₂) heteropolyacids was studied. The combination of microbalance measurements and FT-IR spectroscopy confirmed the formation of diammonium ion N₂H₇⁺ in the bulk of heteropolyacids of both types, which was found to depend on both the ammonia equilibrium pressure and sorption temperature. The diffusion coefficients of the ammonia molecules penetrating into the bulk of heteropolyacids were calculated.