ISSN 1203-8407 © 2012 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 15, No. 1, 2012 197 Hydrogen Peroxide Generation by DC and Pulsed Underwater Discharge in Air Bubbles Ranhua Xiong*, Anton Nikiforov, Patrick Vanraes, and Christophe Leys Research Unit Plasma Technology (RUPT), Department of Applied Physics, Ghent University, Jozef Plateaustraat 22, B-900, Ghent, Belgium Abstract: The generation of H 2 O 2 in underwater discharge in air bubbles is studied with consideration of the influence of electrodes polarity, input power, solution conductivity and the inter-electrode distance. The efficiency of hydrogen peroxide generation strongly depends on the polarity, input power and the inter-electrode distance. Discharges in air bubbles with water as a cathode have significantly higher energy yield of hydrogen peroxide in comparison with negative DC or pulsed discharges. The generation of hydrogen peroxide by DC discharge increases with decrease in the inter-electrode distance, but it is opposite for pulsed discharges. Different efficiency of H 2 O 2 production is explained based on physical processes which result to formation of OH radicals. Keywords: Hydrogen peroxide generation; Underwater discharge; Atmospheric plasma Introduction Over the past years, underwater electrical dis- charges have received a lot of attention in view of possible applications in different fields of science and technologies such as advanced oxidation of water pollutions, sterilization, organic synthesis (1-3). Under- water discharges can be used as a source of active species (O 3 , OH, O 2 * ,H 2 O 2 ), ions (H 3 O + ,O + ,H - ,O - , OH - ), UV radiation and shock wave (4-5, 39). It is clear that these high reactive species produced in liquid medium can be used to degrade many organic compounds, for sterilization, water purification, etc. (6, 7, 40). There are a lot of methods by which under- water plasma can be generated and sustained. In general, underwater discharges can be divided into three main groups based on the physics of underwater breakdown (8). Streamer/spark discharges in a gas and liquid medium are always generated by high voltage pulsed with duration from a few nanoseconds to microseconds with the current up to some kA (43). The second group of underwater discharges is diaphragm or capillary discharge where two reservoirs filled with a conductive liquid are separated by a dielectric barrier in which a current pathway (diaphragm) is made between two reservoirs or water streamers (16, 37). The plasma is formed in the pathway by AC or DC high voltage across the reservoirs. The third group is electrical discharges in gas or vapor bubbles or on the water surface (18, 41, 42). Different kinds of experi- mental setups developed for discharge in gas or vapor bubbles were summarized in a recently published *Corresponding author; E-mail: Ranhua.Xiong@UGent.be review article (9). One of the types is a discharge in bubbles which are produced by external gas flow through a glass tube in which a metal high voltage electrode is inserted. This type of reactor with artifici- ally produced bubbles has an advantage of significantly reducing of the input power because of absence of Joule heating of liquid medium. Additionally, this discharge can be initiated by DC voltage with much lower voltage compared to direct liquid phase discharge (10-11). Reducing the rate of erosion of electrode is another advantage of this type of reactor. Presence of plasma/liquid interface allows initiating of a wide range of chemical reactions in gas phase as well as in a liquid medium. Most of the researcher revealed direct correlation of plasma-liquid treatment efficiency (purification, sterilization, etc.) with amount of H 2 O 2 produced by plasma. Hydrogen peroxide formation by means of plasma is useful indicator for commercial applications in chemical, environmental and dis- infection processes (14-15). A recent review (13) on hydrogen peroxide formation in electrical discharge plasma with liquid water shows major factors affecting hydrogen peroxide formation in gas and liquid. Among of them are: UV radiation, temperature, ozone, pH, solution conductivity, salt type and electrode materials. The present study is focused on the hydrogen peroxide formation in DC and pulsed discharge and on the effect of input power, solution conductivity, inter-electrode distance and water polarity on hydrogen peroxide formation. In order to estimate the efficiency of hydrogen peroxide formation by the reactor, the energy yield and generation rate of hydrogen peroxide are compared with literature.