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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 is generally used to support catalysis. This method of catalysis has been known since the creation of sonochemistry in 1927 by Alfred Lee Loomis (1887–1975) and Robert Williams Wood (1868–1955). [1] Sonocatalysis depends on ultrasounds, which were discovered in 1794 by the Italian biologist Lazarro Spallanzani (1729–1799). [2]
Sonocatalysis is not a self-sufficient catalysis technique but instead supports a catalyst in the 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 the 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 happen in the homogenous phase or the heterogenous phase. This depends on the phase in which the catalyst is compared to the reaction. [1]
The blowing of cavitation bubbles can cause intense local pressure and temperature conditions, going to a 1000 atm pressure and a 5000 K temperature. [1] 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 radicals may combine to produce different molecules, such as water , hydroperoxyl , hydrogen peroxide and dioxygen [3]
Radical formation reactions due to the decomposition of water by ultrasound can be described this way:
Energy from ultrasonic irradiation differs from heat energy or electromagnetic radiation energy in time, pressure, and energy received by a molecule... [1] For example, a 20 kHz ultrasound creates an 8.34 x 10−11 eV energy, while a 300 nm laser creates a 4.13 eV energy. This ultrasound causes a shorter reaction time and a better yield.
There are two types of irradiation in sonocatalysis and sonochemistry: direct irradiation and indirect radiation. In direct irradiation, the solution is in touch with a sound wave 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. [2]
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. [4] Furthermore, their catalytic capacities are well-known and efficient. [5]
Carbon-based species like carbon nanotubes, graphene, graphene oxide, activated carbon, biochar, g-C3N4, carbon-doped materials, Buckminsterfullerene (C60), and mesoporous carbons, are very often used in heterogeneous sonocatalysis. These species are great sonocatalysts because they favour the degradation process during 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 becoming more frequently used. [3]
Sonocatalysis needs equipment other than catalysts to generate ultrasound, like transducers that create ultrasound by the transformation from electrical energy to mechanical energy. There are two types of transducers: piezoelectric transducers and magnetostrictic transducers. Piezoelectric transducers are used more often because they are cheaper, lighter, and less bulky. These transducers are 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, creating a wave. [2]
The use of sonocatalysis has risen. [6] Today, sonocatalysis is used in lots of fields, like medicine, pharmacology, metallurgy, environment, nanotechnology, and wastewater treatment.
Several studies showed that sonocatalysis could increase pyrazole synthesis yield, compounds that has antimicrobial, antihypertensive, anti-inflammatory and anticonvulsant activities.
A study developed a new way of synthesis for this molecule, using ecological and economical reactants while keeping a high yield and using sonocatalysis. [7]
The following table contains is an example for the 3-methyl-5-phenyl-4,5-dihydro-1H-pyrazole-1-carbothioamide synthesis:
Duration (min) | Yield (%) | |
---|---|---|
Reaction under sonocatalysis (*) | 20 | 76 |
Reaction without sonocatalysis (*) | 20 | 16 |
Literature [8] | 120 | 66 |
(*) synthesis conditions are described on the picture above
An example of the use of sonocatalysis is to degrade pollutants. Ultrasound can generate the radical from a water molecule. This radical is a strong oxidizing agent, which can degrade persistent organic pollutant. However, the reaction speed for hydrophobic compounds is low, so ultrasound is often paired with a solid catalyst. The addition of this catalyst means the addition of atomic nuclei that amplifies the cavity phenomenon, and so does the ultrasonic efficiency. Near the solid-liquid contact surface, pressure is applied on one of the sides of the bubble, causing a more violent blowing of the bubble. [3]
This principle can apply to the oxidated bleaching of 46 cationic red [9] by zinc oxide held by bentonite. More than 10% to 20% of organic dyes are lost and released in nature. Finding new ways to improve dyes’ bleaching is an important topic, as these dyes may be toxic and carcinogenic. The oxidation 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 yield for bleaching of cationic red is 17.8% before using ultrasound and 81.6% after using ultrasound. [9] 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. [9]
Another example of pollutant degradation is the elimination of tetracycline, an antibiotic that is frequently found as a pollutant in wastewater. When tetracycline is dissolved in aqueous solution, using only ultrasound is inefficient to degrade tetracycline, because it is kinetically unfavourable.
The addition of catalysts like titanium dioxide or hydrogen peroxide to ultrasound may speed up degradation: thirty minutes are enough when ultrasound and both catalysts are used. [10]
Sonocatalysis is used in rhodamine B degradation too. Rhodamine B is a synthetic dye that may be harmful for aquatic plant when released in wastewater. [11]
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. [12]
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.
Cavitation in fluid mechanics and engineering normally refers to the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial cavitation and non-inertial cavitation.
Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.
Ethylene oxide is an organic compound with the formula C2H4O. It is a cyclic ether and the simplest epoxide: a three-membered ring consisting of one oxygen atom and two carbon atoms. Ethylene oxide is a colorless and flammable gas with a faintly sweet odor. Because it is a strained ring, ethylene oxide easily participates in a number of addition reactions that result in ring-opening. Ethylene oxide is isomeric with acetaldehyde and with vinyl alcohol. Ethylene oxide is industrially produced by oxidation of ethylene in the presence of a silver catalyst.
A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions." In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts. The use of each catalysts depends on the preferred application and required catalysis reaction.
In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reaction and is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction with effective collision and correct orientation. Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular.
Sonication is the act of applying sound energy to agitate particles in a sample, for various purposes such as the extraction of multiple compounds from plants, microalgae and seaweeds. Ultrasonic frequencies (> 20 kHz) are usually used, leading to the process also being known as ultrasonication or ultra-sonication.
Ultrasonic cleaning is a process that uses ultrasound to agitate a fluid, with a cleaning effect. Ultrasonic cleaners come in a variety of sizes, from small desktop units with an internal volume of less than 0.5 litres (0.13 US gal), to large industrial units with volumes approaching 1,000 litres.
In chemistry, the study of sonochemistry is concerned with understanding the effect of ultrasound in forming acoustic cavitation in liquids, resulting in the initiation or enhancement of the chemical activity in the solution. Therefore, the chemical effects of ultrasound do not come from a direct interaction of the ultrasonic sound wave with the molecules in the solution.
High-intensity focused ultrasound (HIFU), or MR-guided focused ultrasound surgery, is an incisionless therapeutic technique that uses non-ionizing ultrasonic waves to heat or ablate tissue. HIFU can be used to increase the flow of blood or lymph or to destroy tissue, such as tumors, via thermal and mechanical mechanisms. Given the prevalence and relatively low cost of ultrasound generation mechanisms, the premise of HIFU is that it is expected to be a non-invasive and low-cost therapy that can at least outperform care in the operating room.
A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.
Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.
Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH). In real-world applications of wastewater treatment, however, this term usually refers more specifically to a subset of such chemical processes that employ ozone (O3), hydrogen peroxide (H2O2) and UV light or a combination of the few processes.
An ultrasonic horn is a tapering metal bar commonly used for augmenting the oscillation displacement amplitude provided by an ultrasonic transducer operating at the low end of the ultrasonic frequency spectrum. The device is necessary because the amplitudes provided by the transducers themselves are insufficient for most practical applications of power ultrasound. Another function of the ultrasonic horn is to efficiently transfer the acoustic energy from the ultrasonic transducer into the treated media, which may be solid or liquid. Ultrasonic processing of liquids relies of intense shear forces and extreme local conditions generated by acoustic cavitation.
Elephant's toothpaste is a foamy substance caused by the quick decomposition of hydrogen peroxide using potassium iodide (KI) or yeast and warm water as a catalyst. How rapidly the reaction proceeds will depend on the concentration of hydrogen peroxide.
Ultrasonic antifouling is a technology that uses high frequency sound (ultrasound) to prevent or reduce biofouling on underwater structures, surfaces, and medium. Ultrasound is just high frequency sound. Ultrasound has the same physical properties as human-audible sound. The method has two primary forms: sub-cavitation intensity and cavitation intensity. Sub-cavitation methods create high frequency vibrations, whilst cavitation methods cause more destructive microscopic pressure changes. Both methods inhibit or prevent biofouling by algae and other single-celled organisms.
Sonoelectrochemistry is the application of ultrasound in electrochemistry. Like sonochemistry, sonoelectrochemistry was discovered in the early 20th century. The effects of power ultrasound on electrochemical systems and important electrochemical parameters were originally demonstrated by Moriguchi and then by Schmid and Ehert when the researchers investigated the influence of ultrasound on concentration polarisation, metal passivation and the production of electrolytic gases in aqueous solutions. In the late 1950s, Kolb and Nyborg showed that the electrochemical solution hydrodynamics in an electrochemical cell was greatly increased in the presence of ultrasound and described this phenomenon as acoustic streaming. In 1959, Penn et al. demonstrated that sonication had a great effect on the electrode surface activity and electroanalyte species concentration profile throughout the solution. In the early 1960s, the electrochemist Allen J. Bard showed in controlled potential coulometry experiments that ultrasound significantly enhances mass transport of electrochemical species from the bulk solution to the electroactive surface. In the range of ultrasonic frequencies [20 kHz – 2 MHz], ultrasound has been applied to many electrochemical systems, processes and areas of electrochemistry both in academia and industry, as this technology offers several benefits over traditional technologies. The advantages are as follows: significant thinning of the diffusion layer thickness (δ) at the electrode surface; increase in electrodeposit/electroplating thickness; increase in electrochemical rates, yields and efficiencies; increase in electrodeposit porosity and hardness; increase in gas removal from electrochemical solutions; increase in electrode cleanliness and hence electrode surface activation; lowering in electrode overpotentials ; and suppression in electrode fouling.
In chemistry, the oxygen reduction reaction refers to the reduction half reaction whereby O2 is reduced to water or hydrogen peroxide. In fuel cells, the reduction to water is preferred because the current is higher. The oxygen reduction reaction is well demonstrated and highly efficient in nature.
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