Delayed dissociation in CO₂ nanosecond pulsed discharges

Plasma-assisted CO₂ conversion into value-added chemicals is an active topic of research, exploring different plasma sources and operation regimes. Atmospheric pressure nanosecond repetitively pulsed discharges (NRP), in particular, have shown remarkable non-equilibrium properties and high efficiencies in converting CO₂. The pathways to CO₂ dissociation are long debated in the literature. In particular, vibrationally induced dissociation and thermolysis have been suggested to explain the dissociation degrees observed in MW and RF discharges. A recent study evinced a delayed dissociation mechanism in atmospheric pressure NRP [1], that is likely a signature of these additional dissociation mechanisms. In this work we adapt the 0D Lisbon Kinetics (LoKI-B+C) simulation tool [2] to nanosecond pulsed discharges, to interpret the measurements reported in [1]. We consider an axisymmetric volume with varying radius, that expands/contracts at the sound speed as the pressure increases/decreases in the plasma channel. The chemical kinetics is based on [3], with additional reactions accounting for formation and relaxation of electronically-excited states. We show that the observed delayed dissociation is due to thermally-driven dissociation in the afterglow via CO₂+CO₂→O+CO+CO₂, following gas heating due to the quenching of electronic and vibrational states, while CO subsequently recombines as the gas cools down until the values measured in the effluent.

[1] C. Montesano et al, J. Phys. Chem. C 127 (2023) 10045

[2] https://nprime.tecnico.ulisboa.pt/loki/

[3] L. Vialetto et al, Plasma Sources Sci. Technol. 31 (2022) 055005.