Efficient CO2 reduction in microwave plasma via vibrational excitation

Sustainable energy generation by means of wind or from solar radiation through photovoltaics or concentrated solar power will be a significant part of the energy mix in 2025. Intermittency (due to e.g. day/night cycle) as well as regional variation of these energy sources requires means to store and transport energy on a large scale. A promising option is creating artificial solar fuels (or CO2 neutral fuels) with sustainable energy, which can easily be deployed within the present infrastructure for conventional fossil fuels. A candidate raw material would be CO2 itself (fitting in carbon capture and utilization, CCU, strategies). Presently, no efficient schemes are yet available for the conversion of CO2 into fuels. A plasma chemical approach potentially offers high energy efficiency (up to 90%) due to selectivity in the reaction processes that can be tailored via its inherently strong out-of-equilibrium processing conditions. At the same time, it is characterized by efficient and fast power switching, low investment costs, no scarce materials required, and high power density, which are all advantageous for addressing intermittency. In this presentation, the plasma chemical approach will be introduced and examples will be discussed of research carried out at the DIFFER to ultimately enable a scale up to industrial applications. In particular, a common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (up to ~3000 K) and conversion degrees (up to 30%), respectively. The results are interpreted on basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry still to dominate the initial experiments. Pulsing the power is shown to decrease gas temperatures and improve efficiency. Novel reactor approaches are proposed to tailor the plasma dynamics to achieve the non-equilibrium in which vibrational excitation is dominant.