Vacuum pyrolysis of ammonium paratungstate: Study on reaction mechanism and morphology changes of product

Abstract

At present, tungsten oxide is mainly produced by atmospheric decomposition of ammonium paratungstate (APT), which usually causes uneven particle size and morphology. In this paper, non-agglomerated tungsten oxide was prepared by vacuum pyrolysis (vacuum pressure is about 20 Pa) of APT. The vacuumpyrolysis mechanism and characterization changes of APT were investigated. The results showed that the pyrolysis of APT in a vacuum could be divided into five main stages based on thermal effect, crystallization water removing (25−170 °C), ammonia removing (170−250 °C), (NH4)6.84H3.16[H2W12O42] dissociation (250−390 °C), hexagonal ammonium tungsten bronze and WO3 part reduction (390−500 °C), h-WO3 crystal transformation (500−600 °C). The effects of pyrolysis temperature, heating rate, and holding time on the morphology and particle size of products were further studied. With increasing temperature, the particle size of the products decreased, and there was no agglomeration. Coarse tungsten oxide (62.43 μm) was obtained at a temperature increase rate of 1 °C/min. The shorter heat preservation time (10 min) was more conducive to the preparation of fine tungsten oxide (32.41 μm). The crystal morphology during APT decomposition into tungsten oxide was "hereditary". Compared with atmospheric decomposition, the vacuum environment is conducive to the preparation of narrow tungsten oxide products.