Plasma-activated catalytic formation of ammonia from N₂–H₂: influence of temperature and noble gas addition

Abstract

In the ITER tokamak, injection of nitrogen is foreseen to decrease the heat loads on the divertor surfaces. However, once dissociated, nitrogen atoms react with hydrogen isotopes to form ammonia isotopologues. The formation of tritiated ammonia may pose some issues with regards to tritium inventory and operation duty cycle. In this paper, we report a study of the eect of three parameters of relevance for the fusion environment on the ammonia production, including the presence of a catalytic surface, sample temperature and noble gas addition. Results of ammonia formation from N 2 /H 2 RF plasma (both with and without tungsten or stainless steel surface)show the importance of the presence of a catalyst in the ammonia formation process.By increasing the temperature of the W samples up to 1270 K, ammonia formation demonstrated a continuous decrease due to two major factors. For high temperatures above 650 K and 830 K, for stainless steel and W, respectively, the reduction results from the thermal decomposition of ammonia. For the lower temperature range, the temperature rise leads to the formation of more stable nitrides that do not tend to react further with hydrogen to form NH 2 and NH 3 . Interestingly, the addition of helium or argon to the N 2 /H 2 plasma show opposite eects on the ammonia production.He eectively decreases the percentage of NH 3 by acting as a barrier for the surface processes. On the other hand, argon impacts more the plasma processes probably by increasing the active nitrogen species in the plasma and as a consequence the percentage of formed ammonia.

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

“…Over the past decades, research on plasma-catalytic ammonia synthesis has transitioned from exploratory and trial-and-error approaches to more fundamental understanding based on experimental and modelling work, as described in detail in this review paper. Operando plasma and catalyst diagnostics, such as surface techniques such as XPS and FTIR, 193,195 pic labelling 326 and spectroscopy 194,327 will aid in further improving our understanding of how plasma and catalyst interact at the molecular level, rather than on the macroscopic level.…”

“…Nowadays, atmospheric pressure DBDs are studied most, although some papers also describe low pressure MW and RF discharges. [181][182][183]187,[190][191][192][194][195][196] A summary of quantified plasma-driven conversions to ammonia in various plasma reactors under a wide variety of conditions is shown in Fig. 9.…”

“…Reactors operating under radiofrequency (RF) plasma excitation have been researched to a lesser extent, as compared to DBD reactors, 261 but have recently gained substantial attention in several research groups. [190][191][192][193][194][195][196]298 In the late 1980s and early 1990s, Uyama et al and Tanaka et al used zeolite 182,187 as well as Fe 183,188 and Mo 183 wires as catalyst in the downstream of their low pressure (650 Pa) RF plasma apparatus. Fe catalysts resulted in the highest product yield.…”

“…They are the only authors to report the formation of both ammonia and hydrazine 182,183,188 and speculated that the H and NH x radicals formed in the discharge are the main adsorbates. 183 Recently, Ben Yaala et al 193,195 placed W and stainless steel catalysts in their low pressure (2 Pa) RF plasma reactor. They reported the thermal decomposition of ammonia at high temperature (above 830 K on W) and the creation of stables nitrides (starting at 650 K on stainless steel), inhibiting ammonia formation.…”

“…They reported the thermal decomposition of ammonia at high temperature (above 830 K on W) and the creation of stables nitrides (starting at 650 K on stainless steel), inhibiting ammonia formation. 195 Shah et al [190][191][192]194 placed a catalyst bed very close to the plasma electrodes in a low pressure (40 Pa) RF plasma setup. They studied various transition metal based catalysts, 191,194 Ga alloys 192,194 and a Ni-MOF (metal organic framework).…”