AVS 72 Session PS2-MoA: Plasma Catalysis and Synthesis

Plasma-assisted methane reforming provides an electrified pathway for producing hydrogen and value-added hydrocarbons without direct CO₂ emissions while potentially enabling co-production of solid carbon. Here, we systematically investigate how the timescale of energy deposition governs methane plasma chemistry by examining atmospheric-pressure pulsed CH₄ discharges spanning six orders of magnitude in pulse duration, from nanoseconds to milliseconds, while maintaining comparable pulse energies of 10–50 mJ/pulse. Experiments were conducted in a coaxial flow reactor operated at low repetition frequencies (50–300 Hz) to specifically minimize pulse-to-pulse interactions. Plasma characteristics (gas temperature, electron density, presence of radicals, etc.) were quantified using electrical diagnostics and high-resolution OES, and reaction products were analyzed via online mass spectrometry.

Substantial changes in discharge physics and reaction products were observed across the ns-to-ms pulse-width range. Electron density varied from ~10¹⁴–10¹⁸ cm⁻³ and gas temperature ranged from ~2500–5200 K, depending on pulse duration and discharge current. Short-pulse discharges (<1 µs) generated high electron densities (10¹⁷–10¹⁸ cm⁻³) and highly non-equilibrium conditions, where methane conversion proceeded through both electron-impact and thermal dissociation pathways. In contrast, longer pulses (>10 µs) operated at lower electron densities (10¹⁴–10¹⁷ cm⁻³) and were dominated primarily by thermal chemistry. Nanosecond discharges exhibited the highest total C₂ selectivity (>70%), producing approximately 17% C₂H₆, 18–20% C₂H₄, and 35% C₂H₂. Increasing pulse duration shifted product distributions toward sequential dehydrogenation pathways consistent with Kassel-type chemistry, promoting acetylene formation at the expense of ethylene and ethane.

Energy deposition timescale produced a distinct tradeoff between selectivity and conversion. Rapid energy deposition and quenching in short-pulse plasmas favored partially dehydrogenated intermediates, whereas longer pulses improved methane conversion and process efficiency through increased reaction volume and residence time. The best overall performance was achieved near 100 µs pulse duration, yielding methane conversion energy cost as low as ~370 kJ/mol CH₄ and specific energy requirements of ~15 kWh/kg for C₂H₂ and ~40 kWh/kg for H₂, comparable to thermal plasma processes. These results demonstrate that pulse duration serves as a powerful control parameter for tuning plasma reaction pathways and optimizing selective methane conversion to hydrogen and value-added hydrocarbons.

Nitrogen-containing compounds, such as ammonia and nitrate, are critical to applications in agriculture, polymer synthesis, and biomanufacturing. Industrially, ammonia is primarily produced by the Haber-Bosch process, in which nitrogen reacts with fossil-fuel-derived hydrogen over a catalyst bed, and nitrate or nitric acid is produced by subsequent oxidation of ammonia via the Ostwald process. Both the Haber-Bosch and Ostwald processes are carried out in large, centralized manufacturing facilities with substantial capital expenditure and supporting infrastructure. Such production schemes create significant logistic and economic challenges, including transportation, safety and handling, and supply chain vulnerabilities. In addition, both processes require elevated temperatures and pressures making them incompatible with intermittent renewable energy. Recently, there has been a growing interest in sustainable and decentralized alternatives for nitrogen fixation. Electrochemical approaches have been challenged by the activation of molecular nitrogen but can efficiently reduce nitrate to ammonia. In comparison, nonthermal plasmas can react air to efficiently produce nitrate. These complementary features have been exploited in combined strategies, but only as separate reactors.

In this work, we investigated an asymmetric plasma-electrocatalyst process that directly converts air and water into tunable nitrogen products within a single reactor. Specifically, the plasma and electrocatalytic electrodes share the same electrical circuit and electrolyte, with a plasma serving as the anode and a cobalt oxide-based electrocatalyst acting as the cathode. We first evaluated the performance of the system in a single-supply configuration, in which the plasma and electrocatalyst operating conditions are coupled. We then introduced an independent cathodic bias to decouple the plasma and electrocatalyst operating conditions. With the latter configuration, we were able to demonstrate significant control over product selectivity, with final compositions ranging from nitrate-dominant to ammonia-dominant and solution pH ranging from acidic to alkaline.

Coupling nonthermal N₂ plasmas with heterogeneous catalysts offers a promising route to sustainable ammonia synthesis under mild conditions. Here, we demonstrate a plasma-catalytic pathway that utilizes H₂O as a carbon-free hydrogen source and leverages trace CO₂/CO_x impurities as catalytic promoters rather than undesirable contaminants. Using a multi-modal operando spectroscopy platform capable of simultaneously probing plasma-phase species and surface-bound intermediates, we identify a distinct vibrational feature at ~2200 cm^-1. Isotopic labeling confirms this band arises from surface-bound isocyanate (NCO) species formed via reactions between plasma-activated nitrogen and trace CO_x present at ppm levels in commercial N₂ feeds.

We show that these NCO intermediates react readily with water vapor to produce ammonia at ambient temperature and pressure. Notably, NCO formation is observed across a broad range of metal catalysts, including Pd, Ni, Ag, Au, and Cu, with metal-dependent vibrational signatures, binding energetics, and reaction kinetics for NCO + H₂O conversion. These results indicate that ammonia synthesis via NCO intermediates is a generalizable pathway and that both catalyst identity and gas composition can be tuned to control activity and selectivity. This work highlights the potential of plasma-catalyst coupling to access new reaction mechanisms enabled by trace species and metastable intermediates under non-equilibrium conditions.

1. Introduction

   The disposal of plastic waste has become an increasing global challenge as single use consumer plastics continues to grow. Catalytic deconstruction is a potential method for upcycling this plastic waste, with plasma being added at different steps to improve the yields or vary/modify the products.

   This work investigates the effects of a plasma pre-treatment using atmospheric pressure air plasma to pre-treat polymers prior to catalytic deconstruction. The structural changes of the polymers and the effect on the product distribution under different plasma conditions is investigated.

2. Methods

   Plasma treatment is performed in a sealed vial with a rubber septum cap. A steel hypodermic needle is used to both deliver synthetic air and apply a high voltage in a pin-to-cup configuration. The grounding cup is made up of a copper pipe soldered to a copper plate. The vial is placed the copper “cup” with the whole assembly surrounded with a 3D printed holder to prevent arcing from the high voltage to ground. The treatment voltage, frequency, flow rate, and power are varied and the modification of polypropylene and polyethylene are investigated using XPS, FTIR, NMR, and MALDI-TOF measurements.

3. Results and Discussion

   Plasma pre-treatment has been shown to improve yields and modify product distribution depending on the plasma parameters. Previous work has shown this trend with polypropylene and ZSM-5/SBA-15 composite catalyst [1]. The method has been shown to apply with polyethylene and a FeRu-HM catalyst to improve the yield of jet fuel (C₈-C₁₆) products. Further investigation of the structural changes of the plasma treated polymers is therefore necessary to better understand this process.

4. Conclusion

Through variations in plasma pre-treatment, catalytic deconstruction can be tuned to produce different product distributions. This method can be applied to other polymer based catalytic deconstruction applications but requires more understanding of the changes in the pr-treatment step.

Acknowledgement

This material is based upon work supported by U.S. Department of Energy (DOE) no. DE-EE0009945.

[1] A. Goodin et al., J. Vac. Sci. Technol. A 44(3), (2026).

[2] G. Pandey et al., ACS Sustain. Chem. Eng. 14(16), (2026).

[3] S. Chakraborty et al., J. Environ. Chem. Eng. 13(2), (2025).