PPPS-2013: CO2 conversion in non-thermal plasma processes

Summary form only given. Intermittent sources of renewable energy that are increasingly becoming available along with forthcoming depletion of fossil fuels has recently stimulated the research interest in CO₂ neutral fuels. More precisely, CO₂ capture and utilisation in a closed loop carbon cycle are envisaged. Renewable energy is thereby stored through CO₂ activation, dissociation and further hydrogenation in chemical fuels. Such hydrocarbon fuels of high energy density would fit into the existing transport and energy infrastructure. Plasma-processing of CO₂ in the gas phase under non-equilibrium conditions is thereby considered as promising substitute to conventional routes to specifically tackle the rate-limiting dissociation into CO. To become an economically viable alternative to conventional fuel processing routes, the energy efficiency of the CO₂ processing step has to be maximised. This in turn requires a better understanding of CO₂ activation channels and reaction mechanisms in plasma-assisted processes. For this purpose two model systems have been studied: (i) a microwave (MW) driven plasma at sub-atmospheric pressures and relevant flow-rates of tens of liters per minute CO₂, and (ii) a mid-frequency (kHz range) dielectric barrier discharge (DBD) operated at atmospheric pressure. Energy efficiencies as high as 60 % were established for the MW plasma using mass spectrometry. These experiments confirmed the importance of low specific injected energies (around 1 eV/molecule CO₂) as established earlier. However, plasmadiagnostic studies on the DBD system revealed that the reduced electric field as plasma parameter is as essential as the injected energy. Typically, the energy efficiency of DBD processes in CO₂ fall short of 10 %. The densities of CO and byproducts, among them O₃, were established by FT-IR absorption spectroscopy. Time-resolved optical - mission and infrared laser absorption spectroscopy were used to deduce (electronic) excitation processes as well as to distinguish potential gas phase and surface processes. It transpires that the conversion process in DBDs is significantly determined by electronic excitation and ionisation processes. Moreover slow formation and depletion rates of CO observed in pulsed DBDs suggest a nonnegligible contribution of surface processes.