Optimization of nonthermal plasma (NTP) catalytic CO₂ methanation: effect of the excitation waveform, pellet size and residence time

Nonthermal plasma (NTP) catalysis is a promising electrified catalytic technology for many sustainable applications, such as energy storage and decarbonization reactions, powered by renewable energy (via green electricity). Dielectric barrier discharge (DBD) plasmas, commonly driven by a sinusoidal waveform, are widely employed in NTP catalysis research. However, the energy efficiency of such DBD is relatively low owing to various reasons, such as excessive capacitive currents and poor selectivity. This study investigates the impact of plasma excitation waveforms, catalyst pellet sizes and residence time on the performance of NTP catalytic CO₂ methanation over a Ni/MgAlO_x catalyst. In particular, the critical role of the discharge waveform in optimizing the DBD NTP catalytic system was systematically explored. Findings demonstrate the advantages of the multi-pulse waveform in minimizing capacitive losses, delivering higher energy per discharge event and sustaining energy interactions over extended durations, which mitigate the effect incurred by changing the pellet size and residence time (across the conditions investigated here). As a result, our DBD system (by multi-pulse wave excitation, with 710-900 μm catalyst pellets and a ∼25 mm bed length) achieved a very high CH₄ yield (∼72.3%) at significantly lower volumetric power densities (∼7 W cm⁻³).