Nano violet tungsten oxide may be prepared by using ammonium paratungstate 5(NH4)2O•12WO3•5H2O as raw material.
1. Ammonium paratungstate heated above 400°C, the reaction in formula (1) occurs:
5(NH4)2O•12WO3•5H2O=12WO3+10NH3+10H2O (1)
The reaction product NH3 in the formula (1) or NH3 introduced in other ways reacts in the formula (2) under the catalytic action of tungsten oxide WOx (2≤x≤3) to generate reducing gas hydrogen H2.
2NH3=N2+3H2 (2)
2. When the reaction temperature rises above 500°C, the reaction product WO3 in formula (1) reacts with the reaction product H2 in formula (2) and/or H2 introduced in other ways to form the reduction reaction in formula (3):
WO3+0.1H2=WO2.9+0.1H2O (3)
3. When the reaction temperature continues to rise above 600°C, the reaction product WO2.9 in formula (3) and the reaction product H2 in formula (1) and/or H2 introduced in other ways occur in formula (4). Reduction reaction:
WO2.9+0.18H2=WO2.72+0.18H2O (4)
4. In formula (1), formula (3) and formula (4), there is water vapor H2O generated. At high temperature:
WOx+(4-x)H2O⇌WO2(OH)2+(3-x)H2 (5)
Hydrated tungsten oxide WO2(OH)2 is a gas at high temperature. The WO2.72 crystal nucleus formed in the formula (4) can be grown into needle-shaped violet tungsten oxide crystals by gas phase migration of WO2(OH)2.
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