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

Abstract

The production of ammonia (NH 3 ) with lowpressure, 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 NH 3 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 NH 3 was rather stable beyond 120 W. These results provide clear evidence of the surface's role in the local enhancement of the NH 3 formation rates. In the plasma volume, at some distance from the foil, the decomposition of NH 3 is the major occurring process and the decomposition rate increases with the power primarily due to a higher electron density. The optimum production of NH 3 was found to be at 45 mol % N 2 and 120 W, and the position of the maximum was observed to slightly decrease to <40 mol % N 2 with an RF power of 60 W, highlighting that not only the material characteristics play a role but also the discharge conditions. The measured NH 3 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 NH 3 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-NH 3 kW h −1 was obtained with an equimolar mixture of N 2 −H 2 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.

Plasma-assisted catalysis for ammonia synthesis in a dielectric barrier discharge reactor: key surface reaction steps and potential causes of low energy yield

“…We will address this point further in section 6.4. Note that experimental studies using power at high levels (up to 300 W) have shown that the energy yield decreased as power increased [64,65].…”

Plasma Radicals as Kinetics-Controlling Species during Plasma-Assisted Catalytic NH3 Formation: Support from Microkinetic Modeling

“…Partly motivated by the idea that the catalyst surface could get “oversaturated” due to the adsorption of atomic plasma radicals, some works in plasma catalysis have discussed the potential impact of the dissolution of atomic species into the catalyst subsurface. − Along these lines, for plasma-catalytic NH 3 synthesis, we and other authors , have brought up the possibility of a “hydrogen sink” effect, where the catalyst subsurface may act as a reservoir of atomic hydrogen for subsequent hydrogenation reactions while also inhibiting the loss of catalyst-bound, atomic hydrogen via ER “recombination” reactions ( r54 ). Thus, to examine (i) whether dissolution effectsquantified by changes in TOF NH3 when dissolution reactions occurare significant in plasma-assisted NH 3 synthesis and/or (ii) the conditions necessary for dissolution effects to be significant, we added the dissolution reactions below: N * + ◊ → normalN ◊ + * H * + ◊ → normalH ◊ + * …”

Recent Advances in Plasma Catalysis

“…Mehta et al have demonstrated that this plasma–catalyst synergy is able to circumvent equilibrium constraints prevalent in thermal-only catalytic transformation . Parametric evaluations by Antunes et al of a tungsten catalyst show that electron temperature, electron density, and gas composition were critical in tuning the catalytic properties of tungsten, interestingly indicating that the catalyst position in the reactor also plays a significant role in the yield of NH 3 . Plasma kinetics models performed by van’t Veer et al further predict that synthesized ammonia is decomposed by microdischarges, but subsequent plasma catalytic reactions cause a net ammonia gain in the afterglows of microdischarges in a packed DBD reactor .…”