Low-temperature Hydrothermal Reduction of Ammonium Paratungstate to Ammonium Tungsten Bronze

Hydrothermal synthesis has become a popular and important technique in solid state chemistry. It allows for the formation of novel compounds that cannot be formed by traditional high-temperature solid state reactions. With this technique, ammonium paratungstate can be reduced to ammonium tungsten bronze, at a lower temperature of 200 °C in nonaqueous solvents.

The tungsten products have attracted considerable attention over the due to the many crystalline structures they can transfer and because of their electrochemical and electronic properties, which can be applied in electrochromic displays and as electrocatalysts.

Here’s the procedures of low-temperature hydrothermal reduction of ammonium paratungstate:

The reaction was carried out in a 45-ml Parr acid digestion bomb filled to 70% of capacity with solvent and 3 g of ammonium paratungstate, (NH4)10W12041 • 5H20, 0btained from Alfa Chemical. The solvents used were glacial acetic acid, ethylene glycol, and a 2:1 ethylene glycol/acetic acid mixture. The reaction took place at 2000C for 5 days at autogenous pressure. Shorter reaction times did not lead to complete reduction of the product, and longer times did not improve the reduction. The dark blue product was filtered and washed with acetone, then stored in a glovebox under nitrogen. The compound rapidly oxidizes in air. The pressure of the acetic acid reaction is approximately 8 atm, whereas for ethylene glycol the pressure is slightly above I atm.

Pressure affects the degree of reduction and as stated earlier glacial acetic acid heated at 2000C produces a vapor pressure of about 8 atm, whereas ethylene glycol produces no vapor pressure. The reaction in acetic acid reduced the tungsten to the greatest degree, followed by the ethylene glycol/ acetic acid mixture, whereas ethylene glycol alone produced very little reduction of the tungsten. The final product has been found to have the hexagonal bronze structure with lattice parameters similar to the high-temperature preparations.

We have shown that ammonium paratungstate can be reduced in nonaqueous solvents to ammonium tungsten bronze at low temperature of 200 °C, which has a less energy consumption and not sensitive to experiment conditions.

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