This work is focused on the kinetics of hydrogen

This work is focused on the kinetics of hydrogen formation during sonolysis of water in the presence of powdered metal oxides. In cantharidin to H2O2, using H2 as a probe of sonochemical activity allows to minimize the secondary processes related for instance to reactivity and typical for H2O2. Several nanometric and micrometric metal dioxides (TiO2, ZrO2 and ThO2) were studied in argon-saturated water submitted to low- (20kHz) and high-frequency (362kHz) ultrasound in order to evaluate the effect of chemical nature as well as the particle size of these catalytically active metal oxides towards the sonochemical reactivity at different ultrasonic frequencies.

Materials and methods

Results and discussion

In summary, a chemical dosimeter based upon hydrogen evolution during water sonolysis has been applied to study the influence of solid particles towards the sonochemical activity in aqueous suspensions of metal oxides. In contrast to the conventionally used determination of H2O2 formation rate, H2-probe is much less insensitive to the secondary catalytic degradation process at oxide surface and typical for H2O2. In general, hydrogen probe confirmed previous results dealing with the enhancement of bubble nucleation in the presence of solid particles in sonicated solutions. On the other hand, the study of H2 emission brings new insights about the mechanisms of interaction of solid particles with cavitation bubbles. The striking difference between low- and high-frequency ultrasound has been observed in our experiments. At 20kHz, solid particles improve water sonolysis whatever the size and the chemical nature of the studied metal oxides. However, the highest H2 yield normalized to the specific surface area of metal oxide is observed for micrometric particles where sonolysis is accompanied by particles fragmentation indicating the contribution of mechanochemical water molecule splitting in addition to the overall sonochemical process. At high-frequency ultrasound, the effect of solid particles towards H2 yield could be positive or negative depending on particle size. The micrometric particles inhibit the sonolysis of water due to strong ultrasound attenuation. On the other hand, the relatively small and various directional effects of nanoparticles results in a combination of nucleation enhancement and ultrasound attenuation. The latter process is much weaker for nanoparticles compared to micrometric ones.

V. Morosini acknowledges CEA – France/DEN/DRCP/MAR/PAREC for the support of his post-doctoral study.

The high temperatures and pressures generated within cavitation bubbles gave birth to the phenomenon of sonochemistry, a unique class of high-energy chemistry . Unlike the traditional high-energy chemistry such as radiation and laser chemistry, sonochemistry is capable of stimulating chemical and biochemical processes by generating high-energy conditions localised on a microscopic scale . The most desired effect of ultrasound is to increase efficiency by enhancing cavitation activity for sonochemical reactions (e.g., degradation of organic pollutants) and sonoprocesses (e.g., emulsification and extraction) without increasing power input . One method of concentrating energy is to use a cantharidin high-intensity focused ultrasound (HIFU) which allows acoustic energy to be focused, and even without cavitation the absorption of the acoustic energy generates tremendous heating at the focal point. This has allowed HIFU to find success in medical and therapeutic applications such as non-invasive device for localised necrosis of diseased tissue , hemostasis , drug delivery and imaging .
Although cavitation can be both beneficial and detrimental , it is generally undesired in some medical applications. For this, HIFU research is mainly focused on restricting or suppressing cavitation at the focal region by methods of dual frequency and over-pressurisation to alter cavitation thresholds; and wave form manipulation such as pseudorandom signals and frequency sweeps to disrupt the standing wave patterns. However, Bailey et al. hypothesised that the attenuation in cavitation activity by linear sweeps (from 250kHz to 290kHz) is caused by bubbles not being able to oscillate in resonance during the sweep. Others have probed the resonance response of bubbles by monitoring the radial oscillation of shelled microbubbles that has been subjected to either a pulse of “upward” or “downward” frequency sweep for a given frequency range. The focus of these studies is to increase signal to noise ratio to improve contrast agent imaging and the results indicated that “downward” frequency sweep is more efficient when the transmitted frequency is chosen above the resonance frequency of the interrogated microbubbles. In these studiesa much lower acoustic power, below the threshold of acoustic cavitation, was used.