텡스텐 나이트라이드 촉매의 제조

김재형, 김경림

연세대학교 화학공학과

A Study of Preparation of Tungsten Nitride Catalysts with High Surface Areas

Jaehyung Kim and Kyunglim Kim

Department of Chemical Engineering, Yonsei University

1. INTRODUCTION

The nitrides or carbides that are prepared by introduction of nitrogen or carbon into transition metals (tungsten or molybdenum) were reported to similar to transition metal sulfides and novel metal catalysts by several research groups.  The nitrides and carbides generally possess high thermal stability, mechanical strength, and unique catalytic behavior, but their usage in catalysis industry has been limited due to their low surface areas.  So, they had tried to prepare nitrides and carbides with high surface areas by TPR (temperature programmed reaction) with topotactic synthesis method.

In this study, we think the nitrides with high surface areas can be prepared from oxide precursors with high surface areas by TPR method.  So, for preparing oxides with high surface areas sol-gel method was used, because It is well known that porous materials and catalysts are prepared by sol-gel method recently.  Sol-gel method may give prepared materials not only to have high surface areas, but also to be calcinated at relative low temperature.  So the tungsten oxides, precursors of nitrides, were prepared from tungstic acid that was dissolved in solution of methanol and distilled water by sol-gel method in this study, because we would better prepare bulk tungsten oxide powder for preparing tungsten nitride catalysts.  The tungsten oxide that had largest surface area was chosen among them and nitrided with NH3 by TPR method, and characterized by several surface analyzers.

2. EXPERIMENTAL METHODS

Tungsten oxide, to be used as precursors for the preparation of tungsten nitride catalysts, was prepared by sol-gel method.  The precursor of tungsten oxide, tungstic acid (H2WO4,, was mixed with the solution that was the mixture of methanol and water (1:1).  The mole ratio (R = methanol/H2WO4) ranges of the mixture solution were varied 15∼30.  After these solutions were aged for 24 hrs and dried at 110℃, uncrystallized materials were obtained.  And noncrystallized material had to be analyzed by TGA-DTA for heating point of crystallized tungsten oxides.  The unheated materail was heated at 500℃ according to TG-DTA results.  For bulk properties of prepared tungsten oxides from sol-gel method X-ray diffraction and surface area of N2 adsorption were analyzed.

Finally precursors of tungsten nitrides were chosen commercial tungsten trioxide (Adrich, 99.99%,  2.24□/g) and heated and unheated tungsten oxides of R ratio 25 by sol-gel method.  They were treated with NH3 to high temperature by TPR of topotactic synthesis method.

For nitriding of tungsten oxides, oxide precursors (1g) was loaded into 6 □ quartz tubular reactor, and heated to 400℃ for 1 hr and also to final temperature (670∼700℃) at 1.0℃/min on flowing NH3.  Then the samples were cooled down to room temperature, and passivated for 2 hrs in flowing mixture of 1% O2/He.

3. RESULTS

In this study, tungsten oxides from sol-gel method were amorphous.  They had main peaks at 25.6°and 16.5°as the XRD results in □ of Fig. 1, and coincided with tungsten oxide hydrate(WO3·H2O).  And the surface areas of these materials were shown in Table 1.  The material with highest surface areas among the prepared materials was the gel of R ratio 25. It's considerable higher than the surface areas of commercial tungsten trioxide.  So, the tungsten oxide of R ratio 25 was chosen as a precursor of tungsten nitride.  Also for preparing crystallized tungsten oxide, the chosen gel need be heated at 500℃ where the endothermic region of tungsten oxide was finished as the TGA-DTA results, and the heated sample at this temperature was crystallized tungsten trioxides as compared with □ and □ of Fig. 1.  They had main peaks at 23.55°and 24.6°in □ of Fig. 1, and coincided with tungsten trioxide.  And the surface areas of crystallized tungsten oxide was 13.30□/g, and still higher than commercial tungsten trioxide.

Fig. 4 shows the Raman spectra of the tungsten oxides.  As the results of Raman spectroscopy, the spectra of tungsten oxides are almost the same.  The bands are 717 □-1 and 807 □-1 are assigned to the W―O stretching vibrations, and the bands at 273 □-1 and 326 □-1 are assigned to W―O bending vibrations in the spectrum of commercial tungsten trioxide.  The bands ascribed to lattice vibrations below 200 □-1.  The hydrates involving the structure water generally present a distinct structural feature.  The basic structural element of the hydrates consists of a layer of octahedra and molecular structural environments of W and O similar to each other.  Accordingly, the bands ascribed to W―O stretching modes are similar in frequencies and are observed at around 700 □-1.  Each hydrate involves a variety of numbers of structural water.  In all hydrates, also the Raman band ascribed to the W〓O stretching mode has been observed at about 950 □-1.

The gel had the bands at 707 □-1, 804 □-1, 262 □-1, and 326 □-1.  And it was not that of tungstic hydrate, but closer that of tungsten trioxide, because the bands at 950 □-1 and around 700 □-1 was not shown clearly.  Therefore this result was not coincided with the XRD results of prepared materials.  But the band at 717 □-1 of W―O stretching vibrations and others in tungsten oxides was shifted lower and broader than that of commercial tungsten trioxide.  The gel had a little structure water, although the gel didnt have structure water as much as tungstic hydrate.  So this shift of Raman bands was effected a little structure water in structure of the gel.

The heated gel at 500℃ had the bands at 713 □-1, 806 □-1, 268 □-1, and 326 □-1., and it was closer that of tungsten trioxide than unheated gel.  But the spectrum of this material was shifted to lower slightly.

The results of XRD showed the changes of precursors during nitriding with NH3 to given temperature in Fig.3∼Fig.4.

The unheated material wa converted to tungsten trioxide as increased temperature during nitriding.  The peaks of □-W2N were appeared clearly, and increased to 670℃.  These peaks were not appeared at the temperature in case of nitriding commercial tungsten trioxide.  After treated the unheated sample with NH3 to 675℃, this sample had a small peak of tungsten metal at 40.45°in Fig. 3.  This temperature is very low as compared with final temperature of other nitrided samples.

And unheated materials possessed some structure water as the results of XRD and Raman spectroscopy.  The fact was important to nitride the unheated material with NH3 by TPR method.  Most of structure water were vaporized when the unheated sample was heated over 500℃, and the sample was converted to crystallized tungsten trioxide.  Also the structure water was vaporized over endothermic region at 225℃, during nitriding the unheated sample.  Its easy that the nitrogen of decomposed NH3 is replaced the oxygen of the sample, too.  Therefore the nitride was easy to produce from unheated sample by TPR method.

REFERENCES

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2. Volpe, L. and Boudart, M., J. Solid State Chem., 59, 332 (1985).

3. Takase, A. and Miyakawa, K., Jpn. J. Appl. Phys., 30(8B), L1508 (1991).

4. Hilbrig, F., Herbert, E. G., Knozinger, H., Schmelz, H., and Lengeler, B., J.

Phys. Chem., 95, 6973 (1991)

Table. 1  The surface areas of prepared materials and their nitride

Sample	Unheated tungsten oxide	Heated tungsten oxide	Tungsten trioxide	Nitrided unheated tungsten oxide	Nitrided heated tungsten oxide	Nitrided tungsten trioxide
Surface area (□/g)	31.88	13.3	2.24	79.53	68.92	60.83