Interaction of NO and NO2 on 12-tungstophosphoric acid

On exposure of the solid 12-tungstophosphoric acid (H₃PW₁₂O₄₀· nH₂O (HPW)) to NO₂ at 150°C NO₂ is sorbed, while HNO₃ is produced. NO₂ is found to penetrate into the bulk structure whose reaction occurs with the water hydrogen-bonded to the protons to produce HNO₃. The NO₂ retained on the catalyst is stoichiometrically equivalent to the total number of protons on and in the solid. This, together with infrared spectra, suggests that NO₂ is associating (1/1) with the protons, producing a nitronium salt of the solid acid. Although insignificant quantities of NO are taken up by HPW, after exposure to NO₂, approximately stoichiometric quantities of NO are sorbed and the infrared spectra show the presence of N₂O₃.