1 3
Topics in Current Chemistry (2020) 378:2
positively promote the reaction rates by affecting the mass (mixing) and heat transfer phenomena or resulting in some structural alterations of the solids, such as erosion, exfoliation, fragmentation, or deformation [6, 8, 22, 32, 33].
The utilization of US irradiation is a complex aspect, since the formation of cavitation in liquids can be affected by numerous parameters [2, 22, 23]. Some of them
are described in most of the articles, but some were not reported. The frequency
and the power of the irradiated US waves can be considered the most fundamental
parameters [16]. Increase of the US frequency leads to shortening of the expansion
and compression pressure cycle, and, as a result, to a negative impact on the effectiveness. The formed bubbles/cavitation at higher frequencies have a smaller size
and less violent implosion effects, although they have a better size distribution and
rate formation. At lower frequency, the cavitation phenomena is more violent and
intense with a consequent of higher localized pressure and temperature, as well as
higher concentration of free radical formation. However, there are many cases where
the high frequencies have desired impact on reaction rates and material synthesis.
Other important parameters that should be taken into consideration for the effective US utilization in synthesis of catalysts and catalytic reactions are the solvent,
the presence/concentration of dissolved gases, temperature, and pressure [34]. The
physicochemical parameters of the solvent, for example, the solubility of air or oxygen, viscosity, surface tension, or vapor pressure, play key roles in the cavitation
threshold. The increment of the latter parameter has a negative impact on the cycle
formation, while, on the contrary, increment of the rest has a positive effect. Initiation of cavitation is facilitated by the presence of dissolved gases. However, the
extent of the assistance upon cavitation is related to the physical properties of the
gas. Contrary to the chemical processes, the increase of the temperature (until a specific range) has a negative impact on the sonochemical reaction due to increase of
the vapor pressure and to the decrement of the gaseous solubility. However, there are
many circumstances revealing that the temperature increase has positive and desired
effects. An increase of the reactor pressure could cause a decrease of the solvent’s
vapor pressure.
1.4 Ball Milling
1.4.1 A Brief History
The earliest recorded mechanochemical process, according to Takacs [35], dates
to the fourth century BC, in which Theophastus of Eresos noted the synthesis of
elemental mercury by grinding cinnabar (HgS) with acetic acid in a Cu vessel, the
first documented separation of an elemental metal [3, 7, 35]. Since a solvent was
needed even in a minimal amount, this process is regarded nowadays as liquidassisted grinding (LAG). From this point and afterward, mechanochemistry-based
approaches were applied widely in metallurgy and mining, and more details can be
found elsewhere [35–37]. By the use of a pestle and mortar and without a liquid (dry
grinding), it was the great experimental physicist, Michael Faraday (discovered the
laws of electrolysis, electromagnetic induction, and the rotation of polarized light by
33
Reprinted from the journal
Topics in Current Chemistry (2020) 378:2
positively promote the reaction rates by affecting the mass (mixing) and heat transfer phenomena or resulting in some structural alterations of the solids, such as erosion, exfoliation, fragmentation, or deformation [6, 8, 22, 32, 33].
The utilization of US irradiation is a complex aspect, since the formation of cavitation in liquids can be affected by numerous parameters [2, 22, 23]. Some of them
are described in most of the articles, but some were not reported. The frequency
and the power of the irradiated US waves can be considered the most fundamental
parameters [16]. Increase of the US frequency leads to shortening of the expansion
and compression pressure cycle, and, as a result, to a negative impact on the effectiveness. The formed bubbles/cavitation at higher frequencies have a smaller size
and less violent implosion effects, although they have a better size distribution and
rate formation. At lower frequency, the cavitation phenomena is more violent and
intense with a consequent of higher localized pressure and temperature, as well as
higher concentration of free radical formation. However, there are many cases where
the high frequencies have desired impact on reaction rates and material synthesis.
Other important parameters that should be taken into consideration for the effective US utilization in synthesis of catalysts and catalytic reactions are the solvent,
the presence/concentration of dissolved gases, temperature, and pressure [34]. The
physicochemical parameters of the solvent, for example, the solubility of air or oxygen, viscosity, surface tension, or vapor pressure, play key roles in the cavitation
threshold. The increment of the latter parameter has a negative impact on the cycle
formation, while, on the contrary, increment of the rest has a positive effect. Initiation of cavitation is facilitated by the presence of dissolved gases. However, the
extent of the assistance upon cavitation is related to the physical properties of the
gas. Contrary to the chemical processes, the increase of the temperature (until a specific range) has a negative impact on the sonochemical reaction due to increase of
the vapor pressure and to the decrement of the gaseous solubility. However, there are
many circumstances revealing that the temperature increase has positive and desired
effects. An increase of the reactor pressure could cause a decrease of the solvent’s
vapor pressure.
1.4 Ball Milling
1.4.1 A Brief History
The earliest recorded mechanochemical process, according to Takacs [35], dates
to the fourth century BC, in which Theophastus of Eresos noted the synthesis of
elemental mercury by grinding cinnabar (HgS) with acetic acid in a Cu vessel, the
first documented separation of an elemental metal [3, 7, 35]. Since a solvent was
needed even in a minimal amount, this process is regarded nowadays as liquidassisted grinding (LAG). From this point and afterward, mechanochemistry-based
approaches were applied widely in metallurgy and mining, and more details can be
found elsewhere [35–37]. By the use of a pestle and mortar and without a liquid (dry
grinding), it was the great experimental physicist, Michael Faraday (discovered the
laws of electrolysis, electromagnetic induction, and the rotation of polarized light by
33
Reprinted from the journal