Influence of phase changes, nucleation and particle growth on the activity of tungsten as a catalyst for the ammonia-decomposition reaction

The activity of tungsten as a catalyst for the NH3 decomposition reaction has been investigated using an initial NH3 pressure of 300 Torr and a catalyst temperature of 1250 K. The nitriding of the tungsten was followed by thermal desorption of the nitrogen taken up by the catalyst. In experiments where the nitride was decomposed in this manner the activity for NH3 decomposition was found to increase by a factor of two in an autoaccelerating manner while the nitriding rate increased even faster. The autoaccelerating growth in activity was found to vary as the cube of the total reaction time, behaviour consistent with nucleation as being rate-limiting in particle growth during a phase change. There are two phase changes involved in the experiment, viz. tungsten to tungsten nitride and tungsten nitride reverting to tungsten. It was shown that the nucleation and growth of tungsten in tungsten nitride was responsible for the increase in activity. The morphology and composition of the samples were also investigated by scanning electron microscopy and Auger electron spectroscopy with argon-ion sputtering to profile the nitride film formed. The scanning electron micrographs confirmed that there was an increase in surface area by particle growth. The particle growth is rapid at grain boundaries and different grain orientations have quite different growth rates. The thickness of the nitride film determined by an Auger depth profile was ca. 1 µm, in good agreement with that estimated from the nitrogen gas pressure produced on decomposition (ca. 1.5 µm) and with the size of the nitride particles observed in the scanning electron microscope.