Plasma-Assisted Catalysis of Ammonia Using Tungsten at Low Pressures: A Parametric Study

The production of ammonia (NH3) with low-pressure, radiofrequency plasmas is studied in this paper in a wide range of experimental conditions using tungsten as a catalyst. The relative position of the tungsten foil in the pyrex tube was observed to dramatically impact ammonia formation. By positioning the catalyst in the middle of the tube, the concentration of NH3 peaked at 120 W with ≈20 mol %, while it decreased by more than a factor of 2 at 300 W. When the foil was placed close to the end of the tube, the production of NH3 was rather stable beyond 120 W. These results provide clear evidence of the surface’s role in the local enhancement of the NH3 formation rates. In the plasma volume, at some distance from the foil, the decomposition of NH3 is the major occurring process and the decomposition rate increases with the power primarily due to a higher electron density. The optimum production of NH3 was found to be at 45 mol % N2 and 120 W, and the position of the maximum was observed to slightly decrease to <40 mol % N2 with an RF power of 60 W, highlighting that not only the material characteristics play a role but also the discharge conditions. The measured NH3 decreased by increasing the pressure from 3 to 5 Pa, which is associated with a decrease in the electron temperature. The temperature of the discharge was found to have a negligible effect on NH3 formation up to 673 K, demonstrating one of the key features of plasma catalysis in respect to thermal catalysis. The largest energy yield of 0.075 g-NH3 kW h–1 was obtained with an equimolar mixture of N2–H2 at 30 W and 3 Pa. Overall, our results show that the changes in the electron density, electron temperature, and gas composition allow a more effective tuning of the catalytic properties of tungsten than varying the bulk gas temperature.