Non-equilibrium sonochemistry – NEQSON

The NEQSON project is focused on the fundamental research of sonoluminescence and sonochemistry to reach the highest yield of excited and/or ionized species during the cavitating event. The UV/Vis/NIR emission spectroscopy will be applied to investigate the excited neutral (noble gases, singlet oxygen O2, C2*) and ionized (Xe+, Kr+, Ar+) species in the gas phase of the cavitating bubbles created with multifrequency ultrasound. The important challenge of this project is to find out the optimal conditions for plasma formation during multibubble cavitation. It is suggested that plasma can be formed during multibubble cavitation under non-equilibrium conditions. Basically, such conditions can be achieved more easily with high frequency ultrasound (>200 kHz), due to the much shorter time of bubble implosion (<<1 µsec) than that at low frequency. The presence of gases with relatively low ionisation potentials, like Kr or Xe, should also facilitate plasma formation. Non-equilibrium plasma is characterized by multiple temperatures related to different particle degrees of freedom. In non-equilibrium plasma the temperatures of electron and vibrational excitations of species are higher than the translational gas temperature. Electronically and vibrationally excited molecules generated inside the cavitating bubble could play a central role in mode-selective sonocatalytic reactions. Furthermore, the sonophotochemical excitation of non-volatile species, like uranyl ions or functional nanosized catalysts, will be studied to enhance the sonocatalytic advanced oxidation process of wastewater treatment. The innovative approach of this project consists in the use of the light emitted by cavitating bubbles not only as a tool to study the mechanisms of the processes inside the bubbles but also as an initiator for chemical reactions in solutions. Cavitation bubbles can be considered as in situ generated micrometric internal sources of light and/or strong transient heat. Due to this reason ultrasound, which is not attenuated by suspensions and is operated with a very easy handling, has a great advantage over conventional photocatalysis. The employment of sonochemistry will drastically simplify the wastewater management. Several functional nanocatalysts with high specific activity in sonocatalysis will be developed during NEQSON project. Potentially, these catalysts can be used not only in sonochemistry but also in other fields, like fuel cells or solid-state electrolytes. The efficiency of sonocatalytical systems will be tested with model solutions of real wastewaters, including radioactive wastes.