FIGURE 7 Discharge power, pH, and liquid conductivity after discharge, as well as H 2 O 2 concentration for both gas and liquid scavengers. The liquid and gas flow rates were 0.75 ml min −1 and 0.4 L min −1 . For the pure water cases the discharge power, pH, liquid conductivity after discharge, and H 2 O 2 concentration were 0.6 W, 5.2, 12 µS cm −1 , 9.1 × 10 −8 mol s −1 for gas, and 9.8 × 10 −8 mol s −1 for liquid HSIEH ET AL.
Optical emission spectroscopy was used to characterize an electrical discharge plasma reactor with a liquid H 2 O film contacting different carrier gases. The plasma gas temperatures for Ar, He, and 1% N 2 in Ar were 1000-1200 K and did not vary significantly with liquid flow rate. Increasing solution conductivity by adding KCl to deionized water in the Ar case lowered the temperature by 13%, increased the discharge power and lowered the H 2 O 2 formation rates. The temperature was highest in the case of air (2400 K), and in the case of Ar the temperature increased with the addition of O 2 . The temperatures for this reactor are comparable to previous studies with discharges in humid gases, while the effects of liquid conductivity were similar to those reported with direct discharge in the liquid phase.
A pulsed electrical discharge plasma formed in a tubular reactor with flowing argon carrier gas and a liquid water film was analyzed using methylene blue as a liquid phase hydroxyl radical scavenger and simultaneous measurements of hydrogen peroxide formation. The effects of liquid flow rate, liquid conductivity, concentration of dye, and the addition of ferrous ion on dye decoloration and degradation were determined. Higher liquid flow rates and concentrations of dye resulted in less decoloration percentages and hydrogen peroxide formation due to initial liquid conductivity effects and lower residence times in the reactor. The highest decoloration energy yield of dye found in these studies was 5.2g/kWh when using the higher liquid flow rate and adding the catalyst. The non-homogeneous nature of the plasma discharge favors the production of hydrogen peroxide in the plasma-liquid interface over the chemical oxidation of the organic in the bulk liquid phase and post-plasma reactions with the Fenton catalyst lead to complete utilization of the plasma-formed hydrogen peroxide.
Electrical discharges in gas-liquid environments and in liquids (including water and a range of organic compounds) have been studied for applications in electrical transmission, chemical destruction in pollution control, chemical synthesis, polymerization and polymer surface treatment, biological inactivation and biomedical treatments, and materials and nano-particle synthesis, and combustion. It has been found that for the purposes of chemical synthesis plasma generated by moderate frequency low energy pulses in a flowing carrier gas with the addition of a spray of fine aerosol particles or a thin film of liquid leads to the highest energy yield for the production of hydrogen peroxide from pure water 1 . Previous work supports the hypothesis that, in the case of hydrogen peroxide generation by plasma with water droplets, the water droplets function to collect the gas phase generated hydrogen peroxide as well as to provide a large source of water molecules to the plasma. At low plasma power the water droplets are not fully vaporized, and due to the high solubility of hydrogen peroxide in water, the hydrogen peroxide formed in the gas phase is captured by the liquid water droplets where the plasma generated radicals do not cause significant degradation. In addition, large temporal and spatial gradients in the plasma reactor lead to more efficient utilization of the plasma to synthesize hydrogen peroxide. Using inspiration and techniques developed in the previous work with water, the main objective of the present work is to develop and explore the synthesis of organic compounds from organic liquid droplets injected into the plasma. The focus is on hydrocarbons that are liquids at room temperature (e.g., hexane). Plasma processes with liquid organic droplets and thin liquid films may provide the means to introduce reactive species (e.g., OH and H radicals) into organic compounds through spatial and temporal control of the plasma.
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