Tungsten has four stable oxides: yellow oxide (WO3), blue oxide (WO2.90), purple oxygen
compound (WO2.72) and brown oxide (WO2).
Tungsten trioxide is a lemon yellow crystalline powder with a density of 7.2~7.4g/cm3, a melting point of about 1470°C and a boiling point of 1700~2000°C. When it is higher than 800°C, it sublimates significantly, and the heat of formation of tungsten trioxide is 202.8 calories/ mol, tungstic anhydride is slightly soluble in water (0.2g/L), insoluble in all inorganic acids except hydrofluoric acid. Tungstic acid is easily soluble in caustic solution (NaOH or KOH) and ammonia water to form tungstate [NaWO4, K2WO4 and (NH4)2WO4]. Tungsten trioxide dissolves slowly in ammonia solution, and dissolves more slowly when heated to high temperature. Tungsten trioxide is easily reduced by various reducing agents. At normal temperature, even a small amount of organic matter can restore it and change its color. But it returns to its original color when heated in air. At 700~900℃, tungsten trioxide is easily reduced to metal tungsten by hydrogen, carbon monoxide and carbon.
Tungsten dioxide is a brown (chocolate) powder with a density of 10.9~11.1g/cm3, a boiling point of about 1700°C, and a heat of formation of 134 kcal/mol. At 575~600°C, hydrogen reduces tungsten trioxide to form tungsten dioxide . Tungsten dioxide is insoluble in water, alkaline solution, hydrochloric acid and dilute sulfuric acid. Nitric acid can oxidize tungsten dioxide into high-valent oxides. Tungsten dioxide is quickly oxidized to tungsten trioxide in air, and turns into blue oxide when heated to 500°C in nitrogen oxide. At 1020°C, tungsten dioxide can be reduced to metal tungsten by carbon.
At 250~300°C, powdery purple tungsten oxide (WO2.72) can be obtained by reducing tungsten trioxide with hydrogen or carbon monoxide and heating tungsten trioxide to 200~250°C in vacuum. Nano-WO2 has some special properties, that is, low-dimensional nano-WO2 wires have semiconductor properties.
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