Ammonia Synthesis by Radio Frequency Plasma Catalysis: Revealing the Underlying Mechanisms

Abstract

Nonthermal plasma is a promising alternative for ammonia synthesis at gentle conditions. Metal meshes of Fe, Cu, Pd, Ag, and Au were employed as catalysts in radio frequency plasma for ammonia synthesis. The energy yield for all these transition metal catalysts ranged between 0.12 and 0.19 g-NH 3 /kWh at 300 W and, thus, needs further improvement. In addition, a semimetal, pure gallium, was used for the first time as catalyst for ammonia synthesis, with energy yield of 0.22 g-NH 3 /kWh and with a maximum yield of ∼10% at 150 W. The emission spectra, as well as computer simulations, revealed hydrogen recombination as a primary governing parameter, which depends on the concentration or flux of H atoms in the plasma and on the catalyst surface. The simulations helped to elucidate the underlying mechanism, implicating the dominance of surface reactions and surface adsorbed species. The rate limiting step appears to be NH 2 formation on the surface of the reactor wall and on the catalyst surface, which is different from classical catalysis.

Enhancement of the Yield of Ammonia by Hydrogen‐Sink Effect during Plasma Catalysis

“…Table S4 summarizes the binding energies for atomic N, atomic H, NH, NH 2 and NH 3 species on different metals. Some of these metals have also been experimentally tested in an earlier work . The standard deviation for the reported yields was less than 5 % for all cases.…”

“…Several hypotheses have been proposed in relation to ammonia formation mechanisms in a plasma‐catalyst system, suggesting key differences with thermal catalysis and providing clues on what factors could be key to optimize the catalyst under plasma conditions. For example, Schneider and coworkers postulated in their work with atmospheric DBD plasma and supported transition metals that the kinetically relevant nitrogen source was vibrationally excited N 2 .…”

“…However, the potential role of atomic plasma species on the reaction was not yet considered in the above work. On the other hand, under different conditions, e. g. low‐pressure RF plasma, modeling have suggested the prominence of atomic species in the plasma over vibrationally excited ones . One emerging understanding, common to the investigated plasma scenarios, is the diminished importance of N 2 dissociation to determine ammonia formation rates compared to thermal catalysis.…”

Sustainable Non‐Thermal Plasma‐Assisted Nitrogen Fixation—Synergistic Catalysis

“…[67,79] Based on this, Shah further demonstrated that the electron temperature is the main factor controlling N 2 dissociation in the gas phase. Although in their studies the metal-woolc atalysts wereu sed, they came to the same conclusions as Shah et al [77] that Au was the most effective catalyst under both atmospheric and low pressures ( Figure 5). under atmospheric dielectric barrier discharge conditions.…”

“…[76][77][78] In addition to exploring the effects of the above-mentionedc atalysts, the authors also discussed the mechanisms of the fundamentaln onthermalp lasma-assisted ammonia synthesis based on ap roposal by Carrasco et al, [79] who stated that the plasma-assisted ammonia synthesis under vacuum occurs mainly through the ER and Langmuir-Hinshelwood (LH) mechanisms (they described the interactions between surface-adsorbed and gasphase speciesa nd the interactions betweent wo surface-adsorbed species, respectively). [77] Iwamoto et al [80] and Aihara et al [81] also compared the catalytic activities of different single-metal catalysts (Fe, Cu, Pd, Ag, etc.) [77] Iwamoto et al [80] and Aihara et al [81] also compared the catalytic activities of different single-metal catalysts (Fe, Cu, Pd, Ag, etc.)…”