Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei
Graphical abstract
Introduction
Molybdenum (Mo) is a representative refractory metal and owns many excellent properties, including superior thermal stability, low expansion coefficient, high thermal and electrical conductivity, high creep and excellent corrosion resistance. Thus, it has been widely used as a heat-resistant material or an alloy element of steel in various fields including electronics, metallurgy, aerospace and electrical industries [[1], [2], [3], [4], [5], [6], [7]]. In addition, Mo alloys (Ni–Mo) and other compounds such as Mo2C and MoO3 are very attractive for other fields such as hydrogen evolution reaction [8], electrocatalytic N2 fixation to NH3, respectively [[9], [10], [11]]. However, due to its high melting point of 2610 °C, it is difficult to prepare Mo alloy by the traditional melting-casting method. Thus, powder metallurgy with Mo powders as the raw materials has become the main method to product Mo alloys. The fabrication of molybdenum powders with suitable morphology, size, and purity has been extensively studied.
In literature, many methods have been proposed to prepare Mo powder, including hydrogen reduction [[12], [13], [14]], molten salt synthesis [[15], [16], [17]], and self-propagating high-temperature synthesis [18], ball milling [19], carbothermal pre-reduction and hydrogen deep reduction [20]. However, many efforts are still needed before these methods could be widely used for industrial application. Hydrogen plays an important role in today's chemical industry, which has been widely used as a reducing agent in reduction of many metal oxides [1,12,15,21]. At present, hydrogen reduction of molybdenum oxide is still the main method to obtain high purity metallic-Mo powder in industrial production, which mainly includes two stages: hydrogen reduction of MoO3 to MoO2 at 600–700 °C and then reduction of MoO2 to Mo at 900–1400 °C [[21], [22], [23], [24]]. However, during the hydrogen reduction process, the formation of molybdenum bearing gaseous intermediate phase MoO2(OH)2 is inevitable. When the concentration of MoO2(OH)2 is high, owing to its generation, transportation, reduction and deposition, the produced Mo powders always have fairly large particle sizes of several micrometers. Zhang et al. pointed out that the most crucial issues for size-controlled synthesis of Mo particles are controlled nucleation and growth [2,15,22,25]. Therefore, how to control nucleation and growth during the preparation process of Mo powder has been an important problem. The supply of sufficient quantity of well-dispersed nuclei and the controlled growth of them are of great importance.
In the present study, it was found that by adding a small number of Mo nuclei, the dispersed nucleation and controlled growth can be achieved. The effects of the sizes of nuclei, additive amount and reaction temperature on the morphology evolution and particle size of the products are investigated. Meanwhile, the hydrogen reduction mechanism with or without the addition of Mo nuclei is analyzed in detail.
Section snippets
Materials
MoO2 powders and commercial Mo powders (99.9% purity) used in current study were purchased from Jinduicheng Molybdenum Co., Ltd., Xi'an, China. The ultrafine Mo powders were prepared by our newly proposed process of “carbothermic pre-reduction of MoO3 by carbon black + deep reduction by hydrogen” [20,22]. Fig. 1(a) shows FE-SEM images of the MoO2 powders, from which it can be seen that the MoO2 has the platelet morphology with the particle size of about 5 μm. As shown in Fig. 1(b), the
X-ray diffraction analyses
The phase compositions of the Mo products were characterized and the corresponding XRD results are shown in Fig. 4, and it can be seen that the product was only Mo in all samples prepared at 900 °C as shown in Fig. 4 (a) and 4 (b). Fig. 4 (c) shows the XRD patterns of products after reducing the mixture of MoO2 and 10 mass% Mo at 1000 °C and 1100 °C, and it can be concluded that the samples have been also completely reduced. Therefore, based on the results of XRD patterns in Fig. 4, all the
Mechanisms analyses of hydrogen reduction of MoO2
As many investigations have shown, there are two reaction mechanisms including pseudomorphic transformation and chemical vapor transport (CVT) during the hydrogen reduction of molybdenum oxide [4,24,26,27]. Generally, under an extremely low partial pressure of water vapor, the reduction reaction obeys pseudomorphic transformation mechanism, and the produced Mo products retains the morphology of original MoO2; while under an extremely high partial pressure of water vapor, the reduction reaction
Conclusions
Mo powders with controllable particle sizes were successfully prepared via hydrogen reduction of MoO2 powders with the addition of Mo nuclei in the range of 900 °C–1100 °C. It was concluded that when pure MoO2 was reduced by hydrogen, Mo powder with large particle sizes will be generated. After the addition of Mo nuclei to MoO2, the dispersed nucleation and controlled growth can be achieved. It is found that the particle size of Mo nuclei and its addition amount, as well as the reaction
Acknowledgements
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51725401).
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