Hydrothermal Reduction

Fig. 13. Digital photograph showing various stages of the hydrothermal reduction of reaction of CU2O in a sealed tube.

In this study, the chemical reduction in aqueous solution using conventional and ultTMonic hydrothermal reduction method were conducted for the preparation of fine nickel powders from the aqueous solution of nickel salt by reducing with hydrazine. The differences in the reaction parametera and final product properties resulting from two methods were identified to find the effects of ultrasound. 

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. 

Li et al. reported first on the decoration of hydrothermal carbon spheres obtained from glucose with noble metal nanoparticles [19]. They used the reactivity of as-prepared carbon microspheres to load silver and palladium nanoparticles onto then-surfaces, both via surface binding and room-temperature surface reduction. Furthermore, it was also demonstrated that these carbon spheres can encapsulate nanoparticles in their cores with retention of the surface functional groups. Nanoparticles of gold and silver could be encapsulated deep in the carbon by in situ hydrothermal reduction of noble-metal ions with glucose (the Tollens reaction), or by using silver nanoparticles as nuclei for subsequent formation of carbon spheres. Some TEM images of such hybrid materials are shown in Fig. 7.4. 

Several high-temperature procedures have been described in the literature for the preparation of the transition-metal dioxides. Direct oxidation of the metals, lower oxides, chlorides, or nitrate precursors provides a convenient route to the dioxides of several metals Ti, Mn, Ru, Rh, Os, Ir, and Pt.1,3-5 (Syntheses of the rutile forms of rhodium and platinum dioxides by direct oxidation requires application of high pressures.5) Reduction of higher oxides is the most common method of synthesis for these dioxides V02, Nb02, Mo02, W02, and /3-Re02.4,6-8 Stoichiometry in these reactions is most readily controlled by use of the respective metal or a lower oxide as reductant. Chromium dioxide is normally synthesized by hydrothermal reduction of the trioxide.9... 

Chromium(iv) oxide, Cr02, is normally synthesized by hydrothermal reduction of Cr03. It has an undistorted rutile structure (i.e., no M—M bonds as in MoOz). It is ferromagnetic and has metallic conductance presumably due to delocalization of electrons into energy bands formed by overlap of metal d and oxygen pn orbitals. 

A consideration of the electronic structures of V3O7 and V4O9 has led to the conclusion that the former cannot contain V ions and that the real formula of the latter is probably close to V408(0H). The hydrothermal reduction of VjOg by SO2 yields a series of oxides in which the oxidation state of vanadium... 

SELECTIVE PRODUCTION OF Co AND Ni POWDERS THROUGH HYDROTHERMAL REDUCTION OF LEACH SOLUTIONS OF A SYNTHETIC MATTE CONTAINING Cu-Ni-Co-Fe-S... 

Key Words Hydrothermal Reduction, Nickel Powder, Cobalt Powder. 

Variety of methods have been reported in literature for producing nickel and cobalt powders from different starting materials these include spray pyrolysis [5], ultrasonic spray pyrolysis under reduced atmosphere [6-10], wet chemical (reduction) methods [3,11], electrolysis [12,13] and controlled hydrothermal reduction of aqueous salt solutions of nickel and cobalt [14,15. Among these reported laboratory scale methods, aqueous processing routes such as hydrothermal, wet chemical and electrolysis processes seem economically and technically attractive because of their low temperature operation and control over particle characteristics through easy manipulation of process parameters including use of additives and/or surfactants [16,17]. 

Figure 1. PARR IL capacity autoclave used for hydrothermal reduction studies.

After each hydrothermal reduction experiments, the autoclave was allowed to cool to room temperature and the solution and metal powders were taken out. The powders were filtered, properly washed with distilled water followed by washing with dilute sodium carbonate (Na2COs) solution to neutralize any free acid and finally with distilled water. Percentage precipitation (reduction) of metal was calculated from the analysis of the filtrate by an AAS mentioned above. The metal powders were treated with sodium potassium tartrate (to protect... 

As mentioned in the previous section, the raffinate containing nickel were taken for hydrothermal reduction, after adjusting its pH to -12.0 in presence of H2 gas for precipitation of Ni powder. It may be mentioned that, precipitation parameters were initially optimized by using pure nickel sulfate solutions (results not presented here), and nickel precipitation from the raffinate were... 

E.L. Abdel-Aal, et al., Effect of synthesis conditions on preprartion of nickel metal nano powders via hydrothermal reduction technique , Powder Technology, 171 (2007), 63-68. 

Selective Production of Co and Ni Powders Through Hydrothermal Reduction of Leach Solutions of a Synthetic Matte Containing Cu-Ni-Co-Fe-S.425... 

B. Deng, A. W. Xu, G. Y. Chen, R. Q. Song, and L. P. Chen, Synthesis of copper-core/carbon-sheath nanocables by a surfactant-assisted hydrothermal reduction/carbonization process. J. Phys. Chem. B, 110, 11711-11716,2006. 

Accurately as for silver-based nanostructured crystals, MW-assisted solution phase growth methods have been diffusively used to produce Au NCs with different sizes and shapes. Spherical particles with average diameters below 10 nm were differently achieved, for instance upon PVP-assisted hydrothermal reduction of... 

Deng, B Xu, A.W., Chen, G.Y., Song, R.Q. and Chen, L. (2006) Synthesis of copper surfactant-assisted hydrothermal reduction/carbonization process./oumol of Physical Chemistry B, 110,11711 16. 

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