Plasma-enhanced catalytic
ammonia synthesis has been proposed as
an alternative pathway for green nitrogen fixation in the case of
medium- and small-scale operation. Recently, Mehta et al. [Nat. Catal.20181269275] postulated that plasma-induced vibrational
excitations of N2 decrease the dissociation barrier, without
influencing the subsequent hydrogenation reactions and ammonia desorption
at atmospheric conditions. In this paper, this postulation is substantiated
with experimental data of unpromoted and promoted, alumina-supported
ruthenium ammonia synthesis catalysts. Within the temperature regime
for plasma-enhanced catalytic ammonia synthesis over ruthenium-based
catalysts (>200 °C), synergy is experimentally observed between
the catalyst and the plasma by a lowered apparent activation energy.
While the apparent activation energy for thermal-catalytic ammonia
synthesis typically ranges from ∼60 to ∼115 kJ mol–1 depending on the promoters, the apparent activation
energy for plasma-enhanced catalytic ammonia synthesis ranges from
∼20 to ∼40 kJ mol–1, consistent with
the hypothesis that ammonia synthesis is enhanced via plasma-induced
vibrational excitations of N2. Further support follows
from the observation that the effects of promoters and supports on
activity are similar for thermal catalysis and plasma-enhanced catalysis.
As promoter and support influence activity via enhancing dissociation
of N2, it follows that breaking the N–N bond is
still relevant in plasma-enhanced catalytic ammonia synthesis.
Ammonia is one of the most produced chemicals, mainly synthesized from fossil fuels for fertilizer applications. Furthermore, ammonia may be one of the energy carriers of the future, when it...
Plasma-based NOX synthesis via the Birkeland-Eyde process was one of the first industrial nitrogen fixation methods. However, this technology never played a dominant role for nitrogen fixation, due to the...
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