Enhancement of CO2 splitting in a coaxial dielectric barrier discharge by pressure increase, packed bed and catalyst addition

CO2 splitting in a coaxial dielectric barrier discharge (DBD) is demonstrated. Empty as well as packed bed reactors with and without cerium oxide as a catalyst are investigated, but despite most other studies, the DBDs are operated also above atmospheric pressure, up to 2 bar. Argon is admixed since it reduces the breakdown voltage and thus, enables plasma operation at the elevated pressure with still moderate high voltage amplitudes of up to 13.5 kVpp. A detailed electrical characterisation of the discharge is correlated with the plasma-chemical CO2 dissociation. In particular, the discharge power, discharge voltage, minimum sustaining voltage as well as the cell and effective barrier capacites are determined as a function of pressure and gas composition. In combination with an equivalent circuit model the electrode area covered by the plasma is elucidated. Increasing the pressure requires a higher minimum sustaining voltage amplitude, but the optimum of power input for each given voltage amplitude depends on pressure and gas composion. Increasing the pressure from 1 to 2 bar enhances the CO2 conversion from 3.4 % to 8.9 % for nearly the same specific energy input (0.35 and 0.31 eV/particle) in the empty reactor and to 12.7 % with a packed bed in the discharge space. The energy efficiency increase from 6 % to 20 % is explained by a higher number of collisions between CO2 molecules and active plasma species leading to dissociation. Moreover, reactor performance can be effectively increased by adding glass spheres coated with ceria (CeO2).