Sonocatalysis

Sonocatalysis is a field of sonochemistry which is based on the use of ultrasound to change the reactivity of a catalyst in homogenous or heterogenous catalysis. It's generally used to support the catalysis. This way of catalysis is known since the creation of sonochemistry in 1927 by (1887–1975) and Robert Williams Wood (1868–1955). Sonocatalysis (and even sonochemistry at all) depends on ultrasounds which were discovered in 1794 by the Italian biologist Lazarro Spallanzani (1729–1799)

General concept
Sonocatalysis isn't a self-sufficient catalysis technique, due to supporting the catalysis in reaction. Sonocatalysis and sonochemistry both come from a phenomenon called “acoustic cavitation”, which happens when a liquid is irradiated by ultrasounds. Ultrasounds will create huge local variations of pressure and temperature, affecting liquid's relative density and creating cavitation bubbles when liquid pressure decreases under its vapor pressure. When these bubbles blow up, some energy is released, which comes from the transformation of kinetic energy into heat. Sonocatalysis may happens as in homogenous phase as in heterogenous phase. This depends on the phase in which the catalyst is compared to the reaction The blowing of cavitation bubbles can cause intense local pressure and temperature conditions, going to a 1000 atm pressure and a 5000 K temperature. This may provoke the creation of highly energetic radicals. Bubbles' blowing causes the formation of hydroxyl radical and hydrogen radical in a water-based environment. Next, these radical may match to produces different molecules as water, hydroperoxyl , hydrogen peroxide  and dioxygen

Radical formation reactions due to the decomposition of water by ultrasound can be described this way:

Energy from ultrasonic irradiation ultrasoniques differ from heat energy or electromagnetic radiation energy. It differs in time and in pressure and energy received by a molecule.. . For example, a 20 kHz ultrasound make an 8,34 x 10-11 eV energy, while a 300 nm laser makes an 4.13 eV energy. This ultrasound cause a shorter reaction time and a better yield.

Direct and indirect irradiation
There are two types of irradiation in sonocatalysis and sonochemistry: direct irradiation and indirect radiation. In direct irradiation, the solution is in touch with sound waves emitter (generally a transducer), while these two elements are separated by an irradiated bath in indirect irradiation. The bath transmits the radiations to the solution due to convection. While indirect irradiation is the most used irradiation technique, direct irradiation is possible too, especially when the irradiated bath may be the container for the solution too.

Homogenous catalysts
Metal carbonyls, such as Fe(CO)5, Fe3(CO)12, Cr(CO)6, Mo(CO)6 and W(CO)6, are very often used in homogenous catalysis, because these are stable species at standard temperature and pressure, due to their structures. Furthermore, their catalytic capacities are well-known and efficient. .

Heterogenous catalysts
Carbon-based species like carbon nanotube, graphene, graphene oxide, activated carbon, biochar, g-C3N4, carbon-doped materials, Buckminsterfullerene (C60), and mesoporous carbons, are very used in heterogeneous sonocatalysis. These species are great sonocatalysts because they favour the degradation process during the sonocatalysis. Furthermore, they have a huge activity and stability for sonocatalysis, and they show the nucleation effect. These properties come from features like optic activities, electrical resistivities and conductivities, chemical stabilities, forces, and their porous structures. These species are today on the technologic rise and they are more and more used.

Transducers
Sonocatalysis needs other equipments more than catalysts to generate ultrasound, like ultrasound that create ultrasound by the transformation from electrical energy to mechanical energy. There are two types of transducers: piezoelectric transducers, and magnetostrictic transducers. The most used transducer is the piezoelectric one because it's cheaper, lighter and less bulky. This transducer is constituted of single crystals or ceramic and two electrodes fixed on the side of the precedent materials. These electrodes receive a voltage which equals at the most to the transducer's resonance frequency. Then, single crystals may be compressed or dilated, and that creates a wave

Some examples of transducers

 * The ultrasonic cleaner is a bath full of liquid. The liquid can transmit acoustic energy from the bottom of the bath to the solution in the container. This cleaner generates often ultrasound with low frequencies ( from 20 to 60 kHz ) and is cheap. However, it has some inconveniences, like the difficulty to control the liquid temperature in the bath, and the fact that irradiation isn't equal everywhere in the bath


 * The cup-horn reactor seems like the ultrasonic cleaning, but it may irradiate using both direct and indirect irradiation. While ultrasonic cleaning only generates ultrasound with low frequencies, the cup-horn reactor can generate ultrasound with high frequencies too, and with a higher intensity. However, this equipment is very expensive due to its conception
 * The « whistle » reactor is a reactor in which the reaction mix is continuously pumped through an adjustable-width opening, in a delimited area where cavitation happens. Ultrasonic waves are generated in this area by the vibration of blades during the passing of the pumped solution. This reactor is often used for homogenous reaction mixes, as the solid part of heterogenous mixes can’t pass through the whistle. About its use, this type of reactor is less used than the two precedent others

Applications
The use of sonocatalysis has risen. Today, sonocatalysis is used in lots of fields, like medicine, pharmacology, metallurgy, environment, nanotechnology, and wastewater treatment.

The example of pyrazole
Several studies showed that sonocatalysis could favour pyrazole synthesis yield, compounds that has antimicrobial, antihypertensive, anti-inflammatory and anticonvulsant activities.

A study developed a new way of synthsis for this molecule, using ecological and economical reactants while keeping a high yield and using sonocatalysis. The following table contains is an example for the 3-methyl-5-phenyl-4,5-dihydro-1H-pyrazole-1-carbothioamide synthesis: (*) synthesis conditions are described on the picture above

Pollutants degradation
An example of the sonocatalysis use is to degrade pollutants. Indeed, ultrasound can generate the radical from a water molecule. his radical is a strong oxydizing agent, which can degrade persistent organic pollutent. However, reaction speed for hydrophobic compounds is low. That's why ultrasound are often paired with a solid catalyst. The add of this catalyst means lthe add of atomic nucleus that amplify the cavity phenomenon, and so the ultrasonic efficiency too. Near the solid-liquid contact surface is applied an other pressure on one of the sides of the bubble, that causes a more violent blowing of the bubble.

46 cationic red bleaching
This principle can apply to the oxidated bleaching of 46 cationic red by zinc oxide held by bentonite. Actually, more than 10% to 20% of organic dyes are lost and released in nature. Finding new ways to improve dyes’ bleaching is an actual topic, as these dyes may be toxic and carcinogenous. L’oxydation comes from the radical, whose oxidant capacities are known. Indeed, we can observe that a higher concentration of the radical provokes a better 46 red cationic bleaching, as the bleaching of cationic red is of 17,8% before using ultrasound and of 81,6% after using ultrasound. However, sonocatalysis’ efficiency mainly comes from the combination of both catalyst and ultrasound. For example, we observe a cationic red bleaching of only 25,4% by applying only ultrasound.

Tetracycline elimination
Another example of pollutant degradation is the elimination of tetracycline, an antibiotic which is frequently found as pollutant in wastewater. When tetracycline is dissolved in aqueous solution, using only ultrasound is inefficient to degrade tetracycline, because it’s kinetically unfavourable. The addition of catalysts like titanium dioxide or hydrogen peroxide  to ultrasound may degrade faster tetracycline: thirty minutes are enough when ultrasound and both catalysts are used

Rhodamine B degradation
Sonocatalysis use is found in rhodamine B degradation too. Rhodamine B is a synthetic dye which may be harmful for aquatic plant when released in wastewater.

The Fenton's reaction
Sonocatalysis can be applied for reactions like Fenton's reaction. By associating sonocatalysis (at a 20 kHz frequency) and Fenton's reaction, with a 5.0 mg/L iron chloride mass concentration and a PH of 4, degradation efficiency is about 80% after 12 minutes.