A study of preparation of tungsten nitride catalysts with high surface area

摘要

Tungsten oxides, to be used as precursors for tungsten nitride catalysts, were prepared by a sol-gel method. The preparation conditions were as follows: the solution molar ratio of 1:1 (methanol and water), temperature of 80 deg C , and the range of R(methanol/H_2WO_4 mole ratio) between 15 and 30. The XRD results showed that the prepared gels were amporphous and suggested they were tungsten oxide hydrate. However, this result disagreed with that of Raman spectroscopy and XANES. The highest surface area was 28.57m~2/g, which was obtained from the tungsten oxide with the R ration of 25. After the gel was heated at 500 degC, it was crystallized tungsten oxide(WO_3). The results of Raman spectroscopy and XANES showed that the unheated gel had a considerable amount of structural water in its structure. Such structural water may have an effect on nitriding the sample. Tungsten nitride catalysts were prepared in NH_3 flow with the flow rate of 100ml/min, at 670-700degC with the heating rate of 1.0degC/min by the temperature programmed reaction(TPR) method. The prepared nitride catalysts were found to be mostly #beta#-W_2N from the XRD results. The surface area of the nitride from unheated gel was 79.53m~2/g, which was higher than that of the nitride from a commercial tungsten trioxide. The temperature for nitriding the prepared gels of this present study was lower than that of the commercial tungsten trioxide.