Effect of temperature on the CO2 splitting rate in a DBD microreactor

Abstract

A novel plate-to-plate dielectric barrier discharge microreactor (micro DBD) has been demonstrated in CO₂ splitting. In this design, the ground electrode has a cooling microchannel to maintain the electrode temperature in the 263-298 K range during plasma operation. A small gap size between the electrodes of 0.50 mm allowed efficient heat transfer from the surrounding plasma to the ground electrode surface to compensate for heat released in the reaction zone and maintain a constant temperature. The effect of temperature on CO₂ conversion and energy efficiency was studied at a voltage of 6-9 kV, a frequency of 60 kHz and a constant CO₂ flow rate of 20 ml min^-1. The CO₂ decomposition rate first increased and then decreased as the electrode temperature decreased from 298 to 263 K with a maximum rate observed at 273 K. Operation at lower temperatures enhanced the vibrational dissociation of the CO₂ molecule as opposed to electronic excitation which is the main mechanism at room temperature in conventional DBD reactors, however it also reduced the rate of elementary reaction steps. The counterplay between these two effects leads to a maximum in the reaction rate. The power consumption monotonously increased as the temperature decreased. The effective capacitance of the reactor increased by 1.5 times at 263 K as compared to that at 298 K changing the electric field distribution inside the plasma zone.

Fig. 1. Schematic diagram of the experimental set-up.

Fig. 2. (a) Voltage in a DBD reactor and (b) charge as a function of time at five temperatures. Discharge gap: 0.50 mm, CO₂ flow rate: 20 ml min⁻¹, voltage: 8.7 kV, frequency: 60 kHz.

Fig. 3. Current waveforms at a temperature of (a) 263 K and (b) 293 K. Other input parameters are the same as those in Fig. 2.

Fig. 4. (a) Lissajous figure at 8.7 kV at several electrode temperatures. (b) Power dissipated at several electrode temperatures. Experimental conditions are the same as those in Fig. 2.

Fig. 5. Lissajous figure at a voltage of 8.3 kV (peak to peak) at 263 and 293 K. Lines AB and CD represent the discharge-off phase when there is only a displacement current and their slopes correspond to the cell (C_cell) in the plasma-off period. Lines BC and DA represent the discharge-on phase when the gas breakdown occurs in the gap and the plasma is ignited. The slope of these lines is effective capacitance (C_d). Experimental conditions are the same as those in Fig. 2.

Fig. 6. Theoretical CO₂ breakdown voltage calculated by eqn (9) and experimental breakdown voltage based on the electrode surface temperature (V_E) and gas temperature (V_g)_.

Fig. 7. OES spectra of CO₂ plasma at a power of 15 W and an electrode temperature of (a) 273 and (c) 293 K. The corresponding Boltzmann plots of the selected lines of the Q branch of the Angstrom band of CO (0–1) at (b) 273 K (d) 293 K.

Fig. 8. CO₂ conversion as a function of (a) power at different temperatures and (b) temperature at different input powers.

Fig. 9. Schematic diagram of vibrational and electronic excitation levels of the CO₂ molecule (reproduced from ref. with permission from the Royal Society of Chemistry).

Fig. 10. Energy efficiency as a function of input power.