(506b) Revealing Plasma Catalysis Mechanisms of N2 Reduction Using an in-Situ FTIR Dielectric Barrier Discharge Reactor

No new innovation has
succeeded in displacing the ammonia synthesis paradigm for over 100 years.  The Haber-Bosch process involves
energy-intensive conditions of 700 K and 100 atm, consuming 1-2% of annual
world energy output and emitting 1.9 metric tons of CO₂ per metric
ton of NH₃.  High temperatures are required for NH species
desorption, but the production of NH₃ should be more
thermodynamically favorable at low temperatures [1].  Another
major challenge is the seemingly unbreakable scaling relation where catalysts
must bind N strongly enough to break the N₂ triple bond, but weakly
enough to allow desorption of NH₃[2].  A new
approach utilizes non-thermal plasma to provide the activation energy for NH₃
synthesis at low temperatures.  Plasma activation eliminates the thermal
equilibrium reaction condition, enabling the reaction to occur under lower
temperature and pressure for NH₃ production.  N₂
excitation in the plasma reduces the N binding strength required of catalyst
surfaces, suggesting the possibility of breaking the linear scaling relation [3].
Laboratory-scale NH₃ synthesis using plasma at low temperatures and
pressures has been demonstrated, and several studies have reported enhancements
with the addition of catalysts. However, low NH₃ yields have been
obtained with accompanying energy efficiency tradeoffs: plasma-catalysis has
achieved 20-30 gNH₃/kWh compared to 500 gNH₃/kWh using
Haber-Bosch synthesis [4].  The
trial-and-error approach that has characterized most plasma-catalysis studies
has left a dearth of fundamental scientific understanding [1]. Therefore,
we present the first in-situ observations of catalyst surface intermediates for
plasma-assisted NH₃synthesis.

N₂ activation
at low temperature and pressure was achieved using non-thermal plasma for the
production of NH₃.  A
near-ambient pressure dielectric barrier discharge plasma batch reactor
equipped with in-situ FTIR spectroscopy was used for experiments with Ni/γ-Al₂O₃
and Fe/γ-Al₂O₃ catalysts (referred to as Ni and Fe
hereafter).  Thermal experiments without plasma under otherwise identical
conditions produced no gas phase or surface-adsorbed species.  With plasma,
peaks were observed in the N-H stretching region in Fig. 1a for Fe and Ni
catalysts.  Overall, a higher concentration of gas phase products was detected
using the Fe catalyst than using Ni. The surface spectra in Fig. 1b reveal
significant differences in the adsorbed intermediates between the two
catalysts.  The more intense peaks over Ni suggest a higher concentration of
adsorbed surface intermediates on Ni than on Fe, which could imply stronger
binding of N-H species on Ni than on Fe under plasma conditions.  The higher
concentration of adsorbates on Ni than on Fe, and specifically the detection of
more NH_{x (ad)} and NH_{3 (ad)} than N₂H_4
(ad) on Ni with respect to Fe, suggests that the relative N binding
strength on these metals may be changed by plasma.  Elucidation of these
effects is paramount to rational catalyst design; for example, theoretical
calculations may need to take into account charging effects from the plasma
that excite the metals above ground state energy.  These insights should
provide guidelines for improvements that could lead to a more efficient and
sustainable NH₃ synthesis process.

Figure 1.  FTIR spectra of the gas phase (a) and of
surface intermediates (b) after 2 h of exposing Fe/γ-Al₂O₃
and Ni/γ-Al₂O₃ to 8 N₂ : 4 H₂
: 3 He plasma at 598 K.  The spectra for the same conditions in the absence of
plasma for the Fe/γ-Al₂O₃ catalyst are included for
reference.

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12.0pt">References

none">1.        J.G. Chen et al., Science 360(6391), 150%">eaar6611 (2018).

none">2.        A.J. Medford et al., J.
Catal. 328, 36–42 (2015).

none">3.        P. Mehta et al., Nat. Catal.
1, 269–275 (2018).

none">4.        J. Hong et al., ACS Sustain.
Chem. Eng. 6, 15–31 (2018).

none">