Effect of heating on dissociation of water vapor in high-frequency plasmas and formation of hydrogen peroxide in a cold trap downstream of the plasma

Abstract

At low flow rates (0.7–2.8 mmol hr−1) and long residence times (2.3–8.5 s) nearly 60% of the input water vapor was decomposed by a 13.56-MHz rf discharge. Downstream of the discharge a trap cooled by liquid nitrogen collected nearly constant yields of H2O2. The decomposition is representable by the equation 2H2O=H2O2+H2. The overall rate of decomposition was found to depend on the absorbed power density. Heating the rf plasma and its spatial afterglow from 25 to 600°C did not significantly change the percent decomposition of H2O and the formation of H2O2. Above 600°C, however, a continuous decrease in H2O2 yield was observed with increasing temperature, and this was associated with the increasing formation of H2O from the dissociated products such as highly excited OH radicals which otherwise produce the precursors of H2O2. The same heating effects were observed in the case of the spatial afterglow of a 2.45-GHz microwave cavity discharge in water vapor under essentially similar conditions. It appears that at the high temperatures the reaction OH+OH→H2O+O is favored over the reaction O+OH→O2+H. This limits the formation of O2 and consequently decreases the H2O2 yield.