The formation of H_2O_2 was studied with ultrasound irradiation in the presence of TiO_2 or Al_2O_3 particles. The effect of varying the temperature of cavitation bubbles in aqueous solutions of two rare gases (Ar, Xe or mixture of Xe and Ne (Xe:Ne=3:7)) on the formation of H_2O_2 were estimated. The very high temperatures (several thousand K) of collapsing gas bubbles lead to the thermal dissociation of water molecules into OH radicals, and the reaction of two OH radicals causes the formation of H_2O_2 consequently. After 60 min sonication with 25〜36kHz ultrasound the highest concentration of H_2O_2 (612μM) was observed when Xe was saturated in the solution. While the increased concentration of H_2O_2 after 60 min sonication was lower (135μM) in the solution saturated with Ar or mixture of Xe and Ne. The difference in the increased concentration of H_2O_2 was not significant between Ar and mixture of Xe and Ne. These results suggest that the threshold for the formation of cavitation bubbles seems to be the lowest in the solution saturated with Xe and to be the important factor for the formation of H_2O_2 during the sonication in the presence of TiO_2 or Al_2O_3, respectively.
