This paper presents a review of the literature dealing with the formation of hydrogen peroxide from plasma processes. Energy yields for hydrogen peroxide generation by plasma from water span approximately three orders of magnitude from 4 × 10 −2 to 80 g kWh −1 . A wide range of plasma processes from rf to pulsed, ac, and dc discharges directly in the liquid phase have similar energy yields and may thus be limited by radical quenching processes at the plasma-liquid interface. Reactor modification using discharges in bubbles and discharges over the liquid phase can provide modest improvements in energy yield over direct discharge in the liquid, but the interpretation is complicated by additional chemical reactions of gas phase components such as ozone and nitrogen oxides. The highest efficiency plasma process utilizes liquid water droplets that may enhance efficiency by sequestering hydrogen peroxide in the liquid and by suppressing decomposition reactions by radicals from the gas and at the interface. Kinetic simulations of water vapor reported in the literature suggest that plasma generation of hydrogen peroxide should approach 45% of the thermodynamics limit, and this fact coupled with experimental studies demonstrating improvements with the presence of the condensed liquid phase suggest that further improvements in energy yield may be possible. Plasma generation of hydrogen peroxide directly from water compares favorably with a number of other methods including electron beam, ultrasound, electrochemical and photochemical methods, and other chemical processes.
The formation rates and energy yields of H2 and H2O2 from pure water exposed to a nonthermal pulsed plasma-gliding arc reactor equipped with a spray nozzle were determined. Both H2 and H2O2 formation rates were the highest with the argon carrier where the maximum energy yields were 13 and 81 g/kWh, respectively. Both H2O2 and H2 were suppressed with carrier gases (air and nitrogen) where significant amounts of nitrates are formed. The energy yields of H2 and H2O2 with the argon carrier gas in the present work are higher than previously reported microwave plasma, corona, and AC gliding arc. The results for H2 are very close to an ideal quenching limit of 45% of thermodynamic limitations, thereby suggesting that the presence of water droplets in the plasma enhance energy yield through enhanced quenching of radicals and reactions that destroy the desired molecular products.
Low temperature atmospheric pressure plasma (produced by a 250 mW pulsed gliding arc discharge) with water spray was utilized to inactivate bacteria colonies of Escherichia coli grown on the surface of an agar substrate. The pH, solution conductivity, H2O2, and nitrate concentrations were determined for air and argon carrier gases and different water flow rates. Control experiments conducted by spraying solutions of H2O2 in the absence of the discharge demonstrated that this chemical and its delivery by spraying account for approximately two to three orders of magnitude (depending upon bacterial loading) of the bacterial colony decontamination process for both carrier gases when bacteria are allowed to grow on the agar plate to form a biofilm. Reactive species or other factors arising from the gas flow from the plasma with the water spray caused bacteria inactivation of one to two orders of magnitude beyond those of spraying H2O2 alone.magnified image
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