Ultrasonics reduction

Reed Justin A, Andrew C, Halaas HJ, Paul P, Alex R, Thomas MJ, Grieser F (2003) The effects of microgravity on nanoparticle size distributions generated by the ultrasonic reduction of an aqueous gold-chloride solution. Ultrason Sonochem 10(4—5) 285-289... 

The boron substructure was eliminated from the imidazo[4,5-i/][l,3,2]diazaborolidine framework 56 following ultrasonic reduction with metallic lithium in THE and anions of the imidazoles 90 formed. The observed reactivity mled out the possibility of isolating the intermediates from these single-electron-transfer reactions <2004CC1860>. 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

The transfer efficiencies for ultrasonic nebulizers (USN) are about 20% at a sample uptake of about 1 ml/min. Almost 100% transfer efficiency can be attained at lower sample uptakes of about 5-20 pl/min. With ultrasonic nebulizers, carrier gas flows to the plasma flame can be lower than for pneumatic nebulizers because they transfer sample at a much higher rate. Furthermore, reduction in the carrier-gas flow means that the sample remains in the mass measurement system for a longer period of time which provides much better detection limits. 

The second class of grinding equipment is used to prepare dispersions. Typical of this class are baU and pebble mills, ultrasonic mills, and attrition mills. SoHds, eg, sulfur, antioxidants, accelerators, and zinc oxide, are generaUy ground on this equipment (see Size reduction). BaU mill action is assisted in some mills by a combination of dispersion circulation by an external pump and mechanical osciUation of an otherwise fixed nonrotary mill chamber. Where baU mill chambers are rotated it is necessary to experimentally estabHsh an optimum speed of rotation, the size and weight of the baU charge, and ensure the mills do not overheat during the grinding period. 

Preparation of nickel powders from nickel salt by ultrasonic chemical reduction method... 

Suhtnicion nickel powders luive been synthesized successfully from aqueous NiCh at various tempmatuTKi and times with ethanol-water solvent by using the conventional and ultrasonic chemical reduction method. The reductive condition was prepared by flie dissolution of hydrazine hydrate into basic solution. The samples synthesized in various conditions weae claractsiz by the m ins of an X-ray diffractometry (XRD), a scanning electron microscopy (SEM), a thermo-gravimetry (TG) and an X-ray photoelectron spectroscopy (XPS). It was found that the samples obtained by the ultrasonic method were more smoothly spherical in shape, smaller in size and narrower in particle size distribution, compared to the conventional one. 

To find the effect of reaction temperature and ultrasoimd for the preparation of nickel powders, hydrothermal reductions were performed at 60 °C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution s color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. 

Fig. 1. XRD patterns of the sample prepared at 80 °C for (a) 10 min and (b) 40 min by the conventional and ultrasonic hydrothermal reduction method...

The spherical fine nickel powders have been prepared fiom aqueous NiCU and hydrazine hydrate at various temperatures wife ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased wife inrareasing fee reaction temperature in both fee method, but was relatively shorter in fee ultrasonic mefeod. Compared to the conventional one, the surface morphology and particle size of fee sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by fee ultrasonic mefeod was relatively higher than feat obtained by fee conventional one. 

Recently, Somorjai, Yang et al. [143] examined this reaction over lwt.% Pt/SBA-15 utilizing an elaborate preparation protocol. Preformed Pt nanoparticle sols of five different mean sizes, obtained by alcohol reduction in the presence of a protecting polymer (PVP) were combined with SBA-15 silica exhibiting 9nm pores. After 3h low-power ultrasonic treatment, the Pt particles were evenly distributed throughout the pores of the support (Figure 12 (a)-(e)). 

Atobe, M., Sasahira, M. and Nonaka, T., 2000, Ultrasonic effects on Electro-organic Processes part 42. Product selectivity control in reductive homo and cross couplng of acrylonitrile . Paper presented on the 197" meeting of the Electrochemical Society May 2000, Toronto Camab Abstract no. 108. 

When the conditions are controlled properly, Zn can mediate the reduction of the C-C double bond of a, (3-unsaturated carbonyl compounds in the presence of a nickel catalyst in aqueous ammonium chloride (Eq. 10.7). The use of ultrasonication enhances the rate of the reaction.15 Sodium hydrogen telluride, (NaTeH), prepared in situ from the reaction of... 

Moholkar et al. [11] studied the effect of operating parameters, viz. recovery pressure and time of recovery in the case of hydrodynamic cavitation reactors and the frequency and intensity of irradiation in the case of acoustic cavitation reactors, on the cavity behavior. From their study, it can be seen that the increase in the frequency of irradiation and reduction in the time of the pressure recovery result in an increment in the lifetime of the cavity, whereas amplitude of cavity oscillations increases with an increase in the intensity of ultrasonic irradiation and the recovery pressure and the rate of pressure recovery. Thus, it can be said that the intensity of ultrasound in the case of acoustic cavitation and the recovery pressure in the case of hydrodynamic cavitation are analogous to each other. Similarly, the frequency of the ultrasound and the time or rate of pressure recovery, are analogous to each other. Thus, it is clear that hydrodynamic cavitation can also be used for carrying out so called sonochemical transformations and the desired/sufficient cavitation intensities can be obtained using proper geometric and operating conditions. 

Gonzalez-Garcia J, Banks CE, Sljukic B et al (2007) Electrosynthesis of hydrogen peroxide via the reduction of oxygen assisted by power ultrasound. Ultrason Sonochem 14 405 112... 

Reduction Mechanism of Metal Ions in Aqueous Solution Under Ultrasonic Irradiation... 

To synthesize metal nanoparticles in an aqueous solution, the reduction reactions of the corresponding metal ions are generally performed. Gutierrez et al. [21] reported the reduction of A11CI4 and Ag+ ions in an aqueous solution by ultrasonic irradiation under H2-Ar mixed atmosphere. They found that the optimum condition of these reductions was under the 20 vol% H2 and 80 vol% Ar atmosphere. Following this study, many papers reported the sonochemical reduction of noble metal ions under pure Ar atmosphere to produce the corresponding metal nanoparticles [22-28],... 

It has been reported that the sonochemical reduction of Au(III) reduction in an aqueous solution is strongly affected by the types and concentration of organic additives. Nagata et al. reported that organic additives with an appropriate hydro-phobic property enhance the rate of Au(III) reduction. For example, alcohols, ketones, surfactants and water-soluble polymers act as accelerators for the reduction of Au(III) under ultrasonic irradiation [24]. Grieser and coworkers [25] also reported the effects of alcohol additives on the reduction of Au(III). They suggested that the rate of the sonochemical reduction of Au(III) is related to the Gibbs surface excess concentration of the alcohol additives. 

It is also observed in Fig. 5.3 that Pd(II) ions are partly adsorbed on AI2O3 before ultrasonic irradiation the concentration of Pd(II) just before irradiation becomes ca. 0.8 mM, although 1 mM Pd(II) was added in the sample solution. From a preliminary adsorption experiment, the rate of Pd(II) adsorption on A1203 was found to be slow compared with those of Pd(II) reduction in the presence of alcohols. Therefore, it is suggested that the sonochemical reduction of Pd(II) in the presence of alcohols mainly proceeds in the bulk solution. The mechanism of the Pd/Al203 formation is also described in the section of sonochemical synthesis of supported metal nanoparticles. 

Figure 5.5 shows the changes in the concentration of Au(III) at different ultrasound intensities [29], where the intensities are determined by the calorimetric method. It can be seen that the concentration of Au(HI) decreases with increasing irradiation time and the reduction behavior is clearly dependent on the ultrasound intensities. At more than 1.20 W cm-2, the reduction of Au(III) was completely finished within the 20 min irradiation. On the other hand, it was also observed that no reduction occurred in a conventional ultrasonic cleaning bath (Honda Electric Co., W-113, 28 kHz, 100 W, bath-volume ca. 2 L) [29]. 

It is widely considered that the physical properties of dissolved gases affect the sonochemical efficiency. The ratio of specific heats, y = Cp/Cv, the thermal conductivity, and the solubility in water are the important parameters. The effects of dissolved gas on the reduction of Au(III) under ultrasonic irradiation are shown in Fig. 5.6 [29]. It can be seen that the changes in the concentration of Au(III) are strongly dependent on the types of dissolved gas. 

Table 5.2 shows the average size of the formed Pd nanoparticles. As described in the previous section, the added alcohol acts as a precursor for the reduction of Pd(II) under ultrasonic irradiation and affects the rate of the reduction. Therefore, it is considered that smaller Pd particles are formed when the rate of Pd nucleation (corresponding to the rate of Pd(II) reduction) is faster. In addition, it can be seen from Table 5.2 that, in the presence of 1-propanol, the size of the formed Pd particles and its standard deviation become smaller with increasing amount of... 

Table 5.2 Average size and standard deviation of Pd particles formed on the A1203 surface and rate of Pd(II) reduction by ultrasonic irradiation [30, 31] ...

It has been reported that bimetallic nanoparticles with core/shell structure can be prepared by ultrasonic irradiation. Mizukoshi et al. reported the formation of bimetallic nanoparticles of Au core/Pd shell structure [42,43] from the sonochemical reduction of Au(III) and Pd(II), where the stepwise reduction of metal ions was observed to proceed during ultrasonic irradiation. That is, the reduction of Pd(II) started after the reduction of Au(III) finished. Vinodgopal et al. reported... 

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