Time-Resolved Optical Emission Spectroscopy Reveals Nonequilibrium Conditions for CO₂ Splitting in Atmospheric Plasma Sustained with Ultrafast Microwave Pulsation

Among the pool of Power-to-X technologies, plasmas show high potential for the efficient use of intermittent renewable energies. High efficiencies of CO2 conversion have been reported while using microwave plasmas at vacuum conditions which are, however, not suitable for CO2 mitigation at industrial scales. Here we show that ultrafast pulsation of microwaves allow significant improvements of energy efficiencies during CO2 splitting at atmospheric pressure as compared to continuous wave operation of the microwave source. Moreover, by the interrogation of the plasma with time-resolved optical emission spectroscopy we can, for the first time, observe the evolution of the vibrational and rotational temperatures and define a time window where nonequilibrium can be expected at the beginning of the pulse of an atmospheric CO2 microwave plasma. In spite of the evidence of nonequilibrium in our system, thermal mechanism appears to dominate the CO2 dissociation. It is shown that a fine control of the energy deposition in the plasma is possible with ultrafast pulsation of the microwave energy supply.