气体添加对水电极同轴介质阻挡放电直接分解 CO₂ 的影响

CO₂ conversion and utilization can convert the greenhouse gas into valuable fuels and chemicals, rather than regarding it as a waste. Due to the high stability of CO₂ molecules, the traditional methods for CO₂ conversion have limitations in terms of operating conditions, conversion and selectivity, catalyst preparation and activity maintenance. The rapid development of atmospheric pressure non-thermal plasma technology has provided new approaches for CO₂ conversion. Dielectric barrier discharge (DBD), as a main form of non-thermal plasma, has received extensive attention in the field of plasma CO₂ conversion. The performance of this process is affected by various factors, and it has been found that using the DBD reactor with water electrode and adding auxiliary gas (e.g., He, Ar and N₂) can improve the reaction performance to a certain extent. However, the research on the discharge characteristics and the reaction performance of CO₂ conversion in a DBD reactor with water electrode and auxiliary gas addition is limited. It is still unclear how to optimize the reaction process and performance. To address these issues, the direct decomposition of CO₂ has been performed in a DBD reactor with water electrode. The auxiliary gases He, Ar and N₂ are added into reactant stream with different concentrations to evaluate their influence on electrical characteristics and reaction performance of direct CO₂ decomposition process. The DBD reactor is self-designed using quartz tube and stainless-steel rod. A casing tube is formed with two quartz tubes and the water at 0°C circulates inside the casing tube, acting as the low voltage electrode. The stainless-steel rod is set coaxially with the inner quartz tube and acts as the high voltage electrode. The low voltage electrode is grounded after connecting to a reference capacitor. The DBD reactor is powered by custom-built AC power source. A Tektronix high-voltage probe and a Pearson current coil monitor are used to collect the applied voltage and the total current, while a Pintech differential probe is used to sample the voltage across the reference capacitor. All of these signals have been saved using a Tektronix digital oscilloscope. A Techcomp gas chromatography (GC) is used to analyze the gaseous products. The electrical characteristics has been obtained by analyzing the voltage-current wave forms and the Lissajous figure. Based on the voltage-current wave forms, the number of current spikes and the electron density are determined. While the discharge power and the breakdown voltage are calculated according to the Lissajous figure. The open-source software BOLSIG+ has been used to determine the average electron energy and the rate constant for electron impact excitation. The reaction rate for electron impact excitation is obtained by the electron density and the rate constant. The reaction performance is evaluated by the parameters of CO₂ conversion, CO yield and energy efficiency. The following conclusions can be drawn: ① adding He, Ar and N₂ increases the discharge channels, and this effect is enhanced by increasing the concentrations of these gases. In addition, adding these gases increases the discharge power while reducing the breakdown voltage, so that more energy is used for the excitation of the gas molecules. ② Adding Ar and He increases the average electron energy and rate constant for electron impact excitation, but adding N₂ reduces these two parameters. Meanwhile, adding N₂ and Ar increases the electron density, which was reduced by adding He. Combing the electron density and the rate constant, the reaction rate for electron impact excitation is increased by increasing the concentration of these auxiliary gases, and the increasing trend follows the order of Ar>He>N₂. ③ For the reaction performance, the effect of these auxiliary gases also follows the order of Ar>He>N₂. The highest CO₂ conversion and CO yield is 15.2% and 9.3%, respectively, in the presence of 80% Ar. However, adding these gases reduces the process energy efficiency. Optimizing the reactor structure and operating parameters, and exploring a suitable high-performance catalyst are the feasible approaches to improve the process energy efficiency.