Calorimetric study of ammonia sorption on H4SiW12O40

Abstract

Using a Microscal flow microcalorimeter, integral sorption and desorption enthalpies of ammonia on silicotungstic acid H₄SiW₁₂O₄₀ dehydrated at 433 K and 483 K were determined. The quantitative coverage determination enabled us to estimate the amounts of the irreversibly as well as (much smaller) reversibly bonded ammonia at its equilibrium state in the gas phase (p_NH3 = 1.01 kPa). It has been shown that irreversible sorption at 373 K results in the formation of NH₄⁺ ions while at 298 K two NH₃ molecules are adsorbed per one proton in heteropolyacids. This suggests the formation of an NH₄⁺·NH₃ adduct which is also present in N₂H₇I, the only known solid salt of a diammonium ion.