Non-thermal plasma cuts operating heat for methane chemical looping hydrogen production

A research team has detailed a non-thermal plasma-assisted chemical looping steam reforming of methane (CL-SRM) workflow that delivers effective methane conversion and high-purity hydrogen at mild furnace temperatures, published in Engineering. Conventional CL-SRM splits partial oxidation of methane (POM) and water splitting (WS) into separate redox stages to generate syngas and pure hydrogen streams without dedicated downstream separation units, yet its real-world rollout is constrained by high thermal input required to break methane’s C–H bonds and limited accessible lattice oxygen on oxygen carrier materials. The study targets these two bottlenecks by integrating dielectric barrier discharge (DBD) non-thermal plasma with lanthanum-based perovskite oxygen carriers, including reference LaFeO₃ and cerium-modified La₀.₅Ce₀.₅FeO₃ synthesized via sol–gel processing.

Experimental testing in a packed-bed DBD setup confirmed stark performance gaps between plasma-aided and purely thermal reaction runs at identical furnace settings. Under 600 °C furnace temperature with plasma discharge at approximately 25 W, La₀.₅Ce₀.₅FeO₃ supported 53.5% methane conversion and a hydrogen yield of 0.57 mmol·g⁻¹ with 92% hydrogen purity, while nearly no measurable methane transformation occurred when plasma power was switched off at the same thermal condition. The plasma-driven reaction performance outmatched data collected from pure thermal operation at 800 °C. The DBD discharge generates around 100 °C local bed temperature elevation during the POM phase, measured via infrared thermal imaging, and lattice oxygen consumption across redox cycles rises noticeably with rising bed temperature for both perovskite materials under plasma activation, whereas thermal-only trials show minimal lattice oxygen utilization below 700 °C. Subsequent water splitting steps following plasma-enabled POM generate far greater hydrogen output than thermal POM pre-treatment, as plasma reactions consume more bulk lattice oxygen and form abundant surface oxygen vacancies to facilitate water dissociation. Ten-cycle durability assessments at 600 °C verify stable methane conversion and hydrogen production over LaFeO₃, with post-cycle X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) characterization confirming retained perovskite crystal frameworks and intact chemical composition of oxygen carriers after repeated redox cycling.

Mechanistic analysis combining BOLSIG+ electron collision modeling, density functional theory (DFT), and in-situ plasma diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) identifies vibrationally excited CH₄ᵛ species as the primary driver of improved reactivity. Calculated electron–molecule collision rate coefficients show vibrational excitation pathways dominate radical and ionization routes within the experimental reduced electric field range of 120–180 Td, and linear correlation exists between CH₄ᵛ generation rate and overall methane conversion rate. DFT energy profiles reveal ground-state methane dissociative chemisorption carries a 1.07 eV reaction barrier; symmetric stretching vibrational excitation adds 0.37 eV intrinsic molecular energy to methane molecules, lowering this barrier to 0.70 eV and easing C–H bond cleavage. In-situ DRIFTS spectra capture abundant CHₓ hydrocarbon fragments and intensified surface hydroxyl consumption under plasma exposure, while XPS depth profiling demonstrates plasma preferentially depletes surface lattice oxygen to form nanoscale oxygen-deficient layers with elevated adsorbed oxygen ratios, expanding accessible reactive oxygen sites for redox turnover.

Energy balance and exergy analysis further distinguish the plasma-mediated route from conventional thermal reactors. The plasma workflow requires 26.3 kJ sensible furnace heat at 600 °C, compared to 35.4 kJ for thermal-only reforming at 800 °C. Unlike combustion-fired or resistance-heated reactors that rely on heat conduction across reactor walls with large temperature gradients and thermal resistance, the DBD system delivers reaction energy directly via voltage-driven electron transfer, opening the possibility of adiabatic reactor designs that eliminate external heating demand. The maximum energy efficiency of the plasma-assisted hydrogen production system reaches 25.5% for the La₀.₅Ce₀.₅FeO₃ carrier, with efficiency scaling alongside operating temperature due to strengthened plasma-perovskite surface synergies. The work establishes an electrified low-temperature pathway for activating stable methane molecules over metal oxide redox materials, offering a framework to reduce thermal energy demand for endothermic methane reforming processes.

The paper “Chemical Looping Steam Reforming of Methane under Mild Conditions via Non-Thermal Plasma,” is authored by Zunrong Sheng, Donglong Fu, Tingting Yang, Xianhua Zhang, Zheyuan Ding, Chunlei Pei, Sai Chen, Zhi-Jian Zhao, Jinlong Gong. Full text of the open access paper: https://doi.org/10.1016/j.eng.2026.01.001. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.