CO₂ conversion in a non-thermal, barium titanate packed bed plasma reactor: The effect of dilution by Ar and N₂

Carbon dioxide conversion upon dilution by argon and nitrogen at atmospheric pressure has been studied using a barium titanate packed bed, non-thermal plasma reactor. The results show that the packed bed reactor with the BaTiO₃ ferroelectric packing material directly contacting to electrodes provided a higher CO₂ conversion and energy efficiency than a dielectric barrier discharge reactor with and without packed materials using electrodes covered by dielectric layers. In the packed bed reactor, the CO₂ conversion increased from 19% in pure CO₂ to 36% upon diluting the CO₂ with 80% argon and to 35% with 80% nitrogen at a specific energy input (SIE) of 36 kJ L⁻¹. The energy efficiency was approximately constant at ca. 0.24 mmol kJ⁻¹ for pure CO₂ dissociation upon increasing SIE, but decreases upon dilution by Ar and is more pronounced by N₂. In addition to the major products of CO and O₂, up to 100 ppm of ozone was produced in a CO₂/Ar plasma, but not in a CO₂/N₂ plasma where some nitrogen oxides (N₂O, NO and NO₂), were observed with a total maximum concentration of 3120 ppm. Possible mechanisms are presented for the effect of Ar and N₂ on the dissociation of CO₂ and for the formation of O₃, N₂O and NO_x based on discharge processes in the plasma and the subsequent chemistry. Electrical characterisation of the pure CO₂, CO₂/Ar and CO₂/N₂ plasmas was also undertaken. The results suggest the formation of by-products can be controlled by varying the dilution gas fraction and operating conditions.