Ammonia sorption by Dawson acid studied by IR spectroscopy and microbalance

Abstract

Using in-situ microbalance and infrared spectroscopy techniques the double ammonia proton complexation was traced. The results confirm the formation of N 2H 7 + dimer in solid Dawson acid H 6P 2W 18O 62, previously reported only for N 2H 7I and for (N 2H 7) 4SiW 12O 40. The formation of such dimers was evidenced by the microbalance results, the molar ratio of ammonia to proton was measured as 2:1 at 10.7 kPa and 298 K. The formation of NH 4 + monomer (band at 1410 cm −1) and N 2H 7 + dimer (1460 cm −1) was revealed by IR spectroscopy. Enthalpy of ammonia sorption on Dawson structure was calculated to be −127.9 kJ mol −1, indicating the lower acid strength of Dawson-type compared to that of the Keggin-type heteropolyacids, like H 4SiW 12O 40.