7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Warm air plasma for nitrogen fixation coupled with heterogeneous catalysis
Xuekai Pei*
* School of Electrical Engineering and Automation, Wuhan University, Wuhan, Hubei, China
Email: peixuekai@gmail.com / peixuekai@whu.edu.cn
Fixation of nitrogen due to naturally occurring
electrical discharges (i.e., lightning) is well known.
Based on the same principle, reactive nitrogen produced
from N2 and O2 by the electrically-powered non-thermal
air plasma has been recently proposed as an alternativetechnology for nitrogen fixation. The main barrier to thistechnology is the relative energy inefficiency of theplasma that converts air and water to nitric acid using
electrical power.
This report has provided considerable insights into
the potential of catalyst to enhance the energy efficiency
of NOx production in pin-pin DC glow discharge and air
plasma jet systems [1-5]. A significant reduction in energy
costs, up to 45%, is realized when the γ-Al2O3 catalyst isemployed in the DC glow discharge system at lower gas
flow rates. Despite being located away from the plasmazone, the catalyst sees a notable increase in NOx
production, with neutral species like N, O, andelectronically excited N2* likely contributing to thisenhancement. Additionally, a significant boost in NO x
production correlates with an increase in oxygen content
in the nitrogen gas mixture, suggesting that O2 or O
atoms play a vital role in NOx formation on the catalyst
surface.
The results highlight the importance ofunderstanding gas flow and catalyst positioning within
plasma-based NOx production systems. An innovative
approach was also explored in the study - the use of an
air plasma jet infused with floating γ-Al2O3 powder for
NOx production as shown in Figure 1. Although thefloating catalyst did sometimes destabilize the plasma
discharge, this was effectively mitigated by employing aneodymium magnet ring, which enhanced the uniformity
and stability of the plasma jet. The floating catalytic
particle approach did significantly increase energy
efficiency at higher discharge currents. The lowest
recorded energy cost for NOx production using thisapproach was 2.9 MJ/mol, the lowest value observedacross our discharge types and heterogeneous catalyst
coupling methods studied.
The study demonstrates the remarkable potential of
floating γ-Al2O3 catalysts in magnet-stabilized air plasma
jet systems for energy-efficient NOx production. The
research underscores the need to maximize theinteraction between plasma and catalysts for efficient
NOx production, suggesting that future studies should
investigate the positioning of catalysts as close to theplasma zone as possible and explore the use of floating
catalytic particles.

Figure 1. Experimental setup of the air plasma jet powered byfloating γ-Al2O3 within the gas stream. (b) Overhead view ofthe plasma jet nozzle without magnetic stabilization. (c)Overhead view of the plasma jet nozzle with magneticstabilization applied.

References[1] Pei X K, Gidon D and Graves D B. Propeller arc: design and basic
characteristics, Plasma Sources Science and Technology, 2018, 27(12):125007.
[2] Pei X K, Gidon D, Yang Y J, et al. Reducing energy cost of NOx
production in air plasmas, Chemical Engineering Journal, 2019, 362:
217-228.
[3] Pei X K, Gidon D and Graves D B. Specific energy cost for
nitrogen fixation as NOx using DC glow discharge in air, Journal ofPhysics D: Applied Physics, 2020, 53(4): 044002.
[4] Shao K T, Pei X K, Graves D B, Mesbah A. Active learning-guided
exploration of parameter space of air plasmas to enhance the energy
efficiency of NOx production, Plasma Sources Science and Technology,
2022, 31(5): 055018.
[5] Pei X K, Li Y H, Luo Y, et al. Nitrogen Fixation as NOx Using Air
Plasma Coupled with Heterogeneous Catalysis at Atmospheric Pressure,Plasma Processes and Polymers, (submitted) 2023.