Investigating the solid–gas phase reaction between WO3 powder, NH3 and H2O vapors to prepare ammonium paratungstate

Abstract

The solid–gas phase heterogeneous reaction between tungsten oxide powder, ammonia and water vapors was studied with the aim of preparing ammonium paratungstate (APT, (NH4)10[H2W12O42]·xH2O (X=4, 10)). The effects of the composition, crystal structure and particle size of the WO3 powder were investigated along with the effect of the partial pressure of ammonia and water vapor on the products. The as-prepared APT was characterized with powder XRD, FTIR, Raman, SEM, TEM and TG/DTA-MS measurements. At 43.40kPa and 12.23kPa ammonia partial pressures after one day two partially reduced intermediates, i.e. W5O14 and (NH4)2W2O7·0.5H2O, were identified, which transformed into APT·4H2O in 30days with a yield of almost 100%. (NH4)2W2O7·0.5H2O was produced for the first time ever, while for W5O14 this was a new preparation route, as it was only produced in great vacuum at high temperature before. At 1.56kPa ammonia partial pressure APT·10H2O was also a product of the reaction. At lower ammonia partial pressures the reaction was very slow, and only small changes were detected in the structure after 30days. The results showed that this novel synthesis of APT is not sensitive to the reaction conditions, in contrast to the previously only available wet chemical crystallization process. According to the measurements, the as-produced APT is equivalent to the commercial ones. As an additional feature, our method is capable to yield APT nanoparticles for the first time, which is important for both research and industry due to the greater surface area.