Influence of capillary geometry and applied voltage on hydrogen peroxide and OH radical formation in ac underwater electrical discharges

The generation of hydroxyl (OH) radicals and hydrogen peroxide (H2O2) in underwater capillary discharges with different geometries is studied. It is found that the efficiency of the production of these active species depends on the input power and on the length of the capillary. A maximal rate of H2O2 generation of 3.6 x 10(-3) mmol 1s(-1) has been measured in a 1 mm capillary at a discharge power level of 77W. The hydrogen peroxide yield is lower for the 1 mm capillary (h(H2O2) = 4.1 g kW(-1) h(-1)) than for the 5 mm capillary (h(H2O2) = 7.8 g kW(-1)h(-1)). The kinetics of OH formation is investigated by recording emission spectra in the UV range (280-400 nm). It is shown that the rate of OH formation sharply decreases with increasing capillary length, for capillaries longer than 3 mm. A comparison of the efficiency of different plasma-solution reactors for the generation of active species has been carried out. In the investigated capillary discharge the hydrogen peroxide yield is higher than the values reported for a diaphragm discharge and for a glow discharge with the electrolyte cathode at atmospheric pressure.