The interaction of nitrogen oxides with metal-oxygen cluster compounds (heteropoly oxometalates)

Nitrogen dioxide is sorbed by the solid heteropoly acids, the first sorbates producing HNO3 while those subsequently taken up remain strongly bound on the acid, up to three NO2 per heteropoly anion. The sorption of NO2 is shown to be dependent on the elemental composition of the anions of the solid acids and hence on the acid strengths of these materials. The sorbed NO2 associates with both the surface and bulk protons to form HNO2+ thus demonstrating that NO2 is capable of penetrating into the crystallographic structure of the solid acid. Prior sorption of NO2 facilitates the takeup of NO which forms N2O3. 12-tungstophosphoric acid (HPW) supported on silica produces quantitatively different but qualitatively similar results. The microporous ammonium salt of HPW produces markedly different results when exposed to NO2 with the predominant product being N-2 evidently resulting from the ammonium cation functioning as a source of the reductant ammonia.