Non-equilibrium excitation of CO₂ in an atmospheric pressure helium plasma jet

The energy efficient excitation of CO₂ in atmospheric pressure plasmas may be a method to generate solar fuels from renewable energies. This energy efficiency can be very high, if only specific states of the molecules in the plasma are populated creating a strong non-equilibrium. This requires a specific design of the plasma source, method of plasma excitation and choice of gases and admixtures. In this paper, non-equilibrium excitation and dissociation of CO₂ in an atmospheric pressure helium RF plasma jet is analysed for varying absorbed plasma power and admixture levels of CO₂. The concentrations of CO₂ and of CO, as well as the vibrational and rotational temperatures of the possible degrees of freedom of the molecules are evaluated by Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational spectra are modelled based on Maxwell–Boltzmann state populations using individual temperatures for each degree of freedom. A strong non-equilibrium excitation of CO₂ and CO has been found. Whereas the rotational temperatures are 400 K or below, the vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric vibration of CO₂ of 700 K. The dependence of these excitation temperatures on plasma power and admixture level is rather weak. The mass balance, the energy and conversion efficiency are consistent with a very simple chemistry model that is dominated by CO₂ dissociation via Penning collisions with helium metastables. A conversion efficiency up to 30% and an energy efficiency up to 10% is observed in the parameter range of the experiment. (paper)