Intermediates from Decomposition of Ammonium Paratungstate

Ammonium paratungstate (APT) is a white crystalline salt with the chemical formula (NH₄)₁₀(H₂W₁₂O₄₂)·4H₂O. It is considered as the most important raw material for the tungsten industry. The majority of tungsten-based catalysts are derived from the reduction of APT.

The thermal decomposition course of APT in air and nitrogen, to the production of WO₃, was a field that attracts a lot attention. The other intermediate, ammonium tungston bronze has attracted considerable attention over the years because of its unique crystalline structure. Ammonium tungsten bronze can be prepared by the hydrothermal reduction of APT, where a time and a higher pressure are necessary. It is also reported that ammonium tungsten bronze can be prepared by reduction of APT with hydrogen (H₂).

To clarify reaction intermediates from the nonisothermal reduction of APT in hydrogen, a experiment was conducted.

APT with a purity over 99% was used as the starting raw material. It was calcined using a quartz reactor in a flow of H₂ (99%) at a rate of 50 ml min^-1  at temperatures 200 °C, 350 °C and 600 °C for 2 hours. Later on, thermal decomposition course of ammonium paratungstate (APT) in hydrogen was studied using thermogravimetric and differential thermal analyses. X-ray diffractometry, infrared spectroscopy and diffuse reectance spectroscopy were used to characterize and identify the intermediate solid products. Thermogravimetric (TG) and differential thermal analysis (DTA) curves were recorded on heating up to 800 °C in a flow of H₂ (30 ml min^-1).

In conclusion, it was discovered that APT decomposes to tungsten metal W⁰ by different tungsten intermediate compounds. Ammonium tungsten bronze, which precedes the formation of WO₃, is relatively the most stable intermediate through reduction of APT to tungsten metal W⁰. From the results, we can concluded the order of intermediates forming through decomposition of ammonium paratungstate: W⁰ >> WO₂ >> (NH4)_0.33 WO₃ >> WO3.

• < Prev
• Next >