- Email: [email protected]
Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei Yong Zhang, Shuqiang Jiao, Kuo-Chih Chou, Guo-Hua Zhang* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China
highlights
graphical abstract
Influence of Mo nuclei addition on hydrogen reduction process of MoO2 was studied. Mo
powders
with
controllable
sizes could be prepared by adding Mo nuclei to MoO2. Without addition of nuclei, Mo particles with large particle sizes were obtained.
article info
abstract
Article history:
The size-controlled preparation of Mo powders is always a challenge and important task in
Received 19 August 2019
the molybdenum metallurgy. In the current study, Mo powders with controllable sizes are
Received in revised form
synthesized by hydrogen reduction of MoO2 powders with the assistance of Mo nuclei in
23 October 2019
the range of 900e1100 C. The influences of the particle sizes of Mo nuclei, the additive
Accepted 2 November 2019
amount as well as reaction temperature on the morphology and particle sizes of the final
Available online xxx
products are studied. For the hydrogen reduction of MoO2 without any additive, the obtained Mo powders always have large particle sizes. However, the addition of small
Keywords:
amounts of nuclei in MoO2 can help Mo nucleate dispersedly, and the growth of Mo could
Hydrogen reduction
be also controlled by adjusting the sizes of added nuclei, amount of addition and the re-
Molybdenum
action temperature. With the addition of Mo nuclei, the different sizes of Mo powders with
Size-controlled
the good dispersity can be prepared. As adding commercial Mo powders with the particle
Nucleation and growth
size of about 2.03 mm, the micron-sized Mo powders ranged from 2.11 mm to 3.25 mm could be prepared. While for the case of adding ultrafine Mo nuclei of about 170 nm, Mo powders from 0.28 mm to 0.88 mm can be obtained. Moreover, the more the amounts of nuclei added and the lower the reaction temperature (in the range of 900e1100 C) is, the smaller the particle size of the prepared Mo powder will be. The current method is a facile and feasible
* Corresponding author. E-mail address: [email protected] (G.-H. Zhang). https://doi.org/10.1016/j.ijhydene.2019.11.008 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
2
international journal of hydrogen energy xxx (xxxx) xxx
method, and is potential to be used for industrial production of Mo powder with controllable particle sizes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 [1e7]. In addition, Mo alloys (NieMo) 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 [9e11]. 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 [12e14], molten salt synthesis [15e17], and self-propagating hightemperature 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 600e700 C and then reduction of MoO2 to Mo at 900e1400 C [21e24]. 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 sizecontrolled 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.
Experimental 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 þ 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 mm. As shown in Fig. 1(b), the commercial Mo has an average particle size of 2.03 mm, but has a poor uniformity. From Fig. 2, it can be seen that the ultrafine Mo powders have been prepared, with the mean particle size of about 170 nm.
Experimental procedure In our previous paper [26], the schematic diagram of the experimental apparatus has been described in detail. The samples of pure MoO2 or the mixture of MoO2 with commercial Mo or ultrafine Mo powder (with the additive amount of 10, 20 or 40 mass%) were mixed uniformly in agate mortar for 40 min. The FE-SEM image of the mixture of MoO2 with 20 mass% Mo nuclei was shown in Fig. 3, from which it can be seen that platelet MoO2 had been homogeneously mixed with the Mo particles, meanwhile, the ultrafine Mo particles had a better dispersity around MoO2 than commercial Mo particles. The mixture was put into the alumina crucibles with the length, width and height of 45 mm, 15 mm and 20 mm, respectively. The thickness of layer was about 16 mm. The crucible with the sample was put into the quartz tube which was pushed into the constant temperature zone of furnace with the heating elements of SiC. Before heating, the high purity argon gas was introduced into the tube to drive the air out. Then the temperature of furnace was raised to the targeted value with a constant heating rate (5 C/min) in the protection of flowing argon (400 ml/min). As the temperature increased to the desired temperature (900 C, 1000 C, 1100 C), the argon gas was switched to the reducing gas H2 (200 ml/ min) to start the reduction reaction. Subsequently, after the experiment was finished, H2 was switched to argon gas again and the samples were cooled down to the room temperature. The phase composition of sample was examined via X-ray diffraction (XRD; TTR III, Rigaku Corporation, Japan) using Cu Ka radiation in the range of 2q ¼ 10e90 with a scanning rate of 10deg./min. The morphology was characterized by field-
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e FE-SEM images of (a) commercial Mo, (b) commercial MoO2.
Fig. 2 e FE-SEM images of ultrafine Mo powders.
emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany) and transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan).
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 samples in the current study could be reduced to metal Mo powders under the current experimental conditions.
Results FE-SEM analyses X-ray diffraction analyses Reduction of MoO2 without the additive 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
Fig. 5 presented the FE-SEM images of product Mo obtained by reducing pure MoO2 with H2 at different temperatures without additive. It can be obviously seen that, at three different temperatures (900 C, 1000 C and 1100 C), all the produced Mo powders have fairly large particle sizes of several micron meters. Meanwhile, with the increase of the temperature from
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e FE-SEM images of the mixture of MoO2 with (a) 20 mass% ultrafine Mo powders, (b) 20 mass% commercial Mo powders. 900 C to 1000 C and 1100 C, the particle sizes of the Mo powders significantly increase from 2.06 mm to 3.54 mm and 5.63 mm, respectively.
Reduction of MoO2 with the addition of commercial Mo powder Fig. 6 presents the FE-SEM images of Mo products prepared by hydrogen reduction of MoO2 with the assistance of different amounts of commercial Mo in the temperature range from 900 C to 1100 C. It can be clearly seen that the particle sizes of the Mo products are much smaller compared to the Mo powders prepared without the additive. The particles sizes were measured rigorously by the statistics of >300 particles from six different fields in the FE-SEM images, and the results are shown in Table 1. Fig. 6(aec) shows the FE-SEM images of Mo powders obtained by completely reducing MoO2 with different additive amounts at 900 C. It can be seen from Fig. 6 (c) that Mo grains with the average particle size of 460 nm were prepared with 40 mass% addition of commercial Mo nuclei at 900 C, but the overall morphology of produced Mo kept the morphology of the raw MoO2, even if they were composed of a lot of small Mo grains with the sizes of about 300 nm. However, as the mass ratio of additive was decreased to 10 mass% and 20 mass%, from Fig. 6 (a) and 6 (b), there was abrupt change in particle size of the prepared Mo. Many small grains were generated and adhered to the surface of larger grains, meanwhile, it can be seen the average particle size increased to 1.45 mm and 1.78 mm, respectively. Therefore, with the increase of the addition amount of Mo nuclei, the particle sizes of Mo product decreased at 900 C. Meanwhile, at 1000 C, it could be seen from Fig. 6 (d) that Mo with an average particle size about 2.55 mm were obtained with 10 mass% addition of
nuclei. With the increase of additive amount from 10 mass% to 20 mass% and 40 mass%, the average particle size decreased to about 2.37 mm and 2.11 mm, as shown in Fig. 6(e) and (f). When the temperature was increased to 1100 C, the FE-SEM images of the products were shown in Fig. 6(gei). The average particle sizes were 3.25 mm, 2.51 mm, 2.17 mm, corresponding to the additive amount of 10 mass%, 20 mass% and 40 mass%, respectively. From the above analyses, it can be concluded that the grain sizes of products increase with the increase of reaction temperature and the decrease of additive amount of Mo nuclei.
Reduction of MoO2 with the addition of ultrafine Mo powder Fig. 7 shows the FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the assistance of different addition amounts of ultrafine Mo powder from 900 C to 1100 C. It can be seen that by adding a small amount of ultrafine Mo nuclei, ultrafine Mo powder could be prepared successfully. By comparing the reduction of pure MoO2 via H2 as shown in Fig. 5, the grain sizes of products decreased significantly. The particle sizes were measured, and the results are shown in Table 2. At 900 C, as shown in Fig. 7(aec), the average particle sizes were 0.6 mm, 0.58 mm and 0.28 mm, corresponding to the addition amount of 10%, 20% and 40%, respectively. Whereas as the temperature was increased to 1000 C, the particle sizes of produced Mo were 0.69 mm, 0.64 mm and 0.46 mm, respectively. When the temperature was further increased to 1100 C, as shown in Fig. 7(gei), it could be seen that the particle size was about 0.88 mm, 0.82 mm and 0.66 mm, respectively. From the above results, it could be concluded that with the increase of the addition amount of Mo
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e XRD patterns of products by hydrogen reduction of the mixture of (a) MoO2 and commercial Mo at 900 C, (b) MoO2 and ultrafine Mo at 900 C, (c) MoO2 and 10 mass% commercial Mo or ultrafine Mo at 1000 C and 1100 C.
Fig. 5 e FE-SEM images of Mo product obtained by reducing MoO2 without additive at: (a) 900 C; (b) 1000 C; (c) 1100 C.
nuclei and the decrease of reaction temperature, the particle size of Mo became smaller, which was similar to the case of adding commercial Mo powder.
demonstrate that the single-crystalline Mo grains have been prepared.
TEM analyses
Discussion
Fig. 8 shows the typical TEM images of Mo powder obtained by hydrogen reduction of MoO2 at 1000 C with the addition of 20 mass% ultrafine Mo powder. As shown in Fig. 8(a), it can be seen that the Mo grains have a uniform size of about 0.6 mm, which is consistent with the FE-SEM image (Fig. 7). Besides, the selected area electron diffraction (SAED) patterns
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
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 6 e FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the addition of different amounts of commercial Mo at different temperatures: (a) 10 mass%, 900 C; (b) 20 mass%, 900 C; (c) 40 mass%, 900 C; (d) 10 mass%, 1000 C; (e) 20 mass%, 1000 C; (f) 40 mass%, 1000 C; (g) 10 mass%, 1100 C; (h) 20 mass%, 1100 C; (i) 40 mass%, 1100 C.
Table 1 e Particle sizes of the products prepared by hydrogen reduction of MoO2 at different temperatures with the addition of different amounts of commercial Mo powder (mm). mass ratio
0%
10%
20%
40%
900 C 1000 C 1100 C
2.06 3.54 5.63
1.78 2.55 3.25
1.45 2.37 2.51
0.46 2.11 2.17
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 obeys the chemical vapor transformation (CVT) mechanism and the morphology of the Mo products changes significantly relative to that of the raw molybdenum oxide. These two mechanisms have been widely accepted and used for the reduction of molybdenum oxide by hydrogen. MoO2 ðsÞ þ 2H2 OðgÞ ¼ MoO2 ðOHÞ2 ðgÞ þ H2 ðgÞ
(1)
MoO2 ðsÞ þ 2H2 ðgÞ ¼ MoðsÞ þ 2H2 OðgÞ
(2)
MoO2 ðOHÞ2 ðgÞ þ 3H2 ðgÞ ¼ MoðsÞ þ 4H2 OðgÞ
(3)
These two kinds of mechanisms are mainly determined by the concentration of MoO2(OH)2 gaseous transport phase which is generated from the reaction between molybdenum dioxide and water, as showed by reaction (1). In the reduction process by hydrogen, Mo is mainly produced by the reduction reactions of MoO2 (reactions (2)) and the volatile molybdenum oxide (reactions (3)), and the produced Mo atoms will be used for nucleation and grain growth of Mo. Specifically, when the concentration of MoO2(OH)2 is high enough, such as at a high reaction temperature, the migration rate of Mo through gaseous phase could be promoted. Therefore, Mo atoms can be carried away from the reaction interface to nucleate independently. However, a high concentration of MoO2(OH)2 can also promote the growth of Mo grains, and leads to a large size. Whereas, if the concentration of MoO2(OH)2 is low, corresponding to the case of a low reaction temperature or a low concentration of MoO2 which could result in a low reaction rate and a low concentration of water vapor. In this case, the transport of Mo would be difficult and the produced Mo can only be deposited on the nearby unreacted molybdenum dioxide to nucleate and grow. With the reaction interface
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the addition of different amounts of ultrafine Mo at different temperatures: (a) 10 mass%, 900 C; (b) 20 mass%, 900 C; (c) 40 mass%, 900 C; (d) 10 mass%, 1000 C; (e) 20 mass%, 1000 C; (f) 40 mass%, 1000 C; (g) 10 mass%, 1100 C; (h) 20 mass%, 1100 C; (i) 40 mass%, 1100 C.
Table 2 e Particle sizes of the products prepared by hydrogen reduction of MoO2 at different temperatures with the addition of different amounts of ultrafine Mo powder (mm). mass ratio
0%
10%
20%
40%
900 C 1000 C 1100 C
2.06 3.54 5.63
0.6 0.69 0.88
0.58 0.64 0.82
0.28 0.46 0.66
moving from the particle surface towards the core of the particle, the Mo powders with the morphology to the raw material MoO2 can be produced, which could be from the morphology of products by reducing MoO2 with the addition of 40 mass% commercial Mo at 900 C (Fig. 6 (c)). Of course, the particle size and number of nuclei also affect the morphology and particle sizes of products, which will be illustrated in the following section.
The nucleation and growth mechanisms of Mo in hydrogen reduction of MoO2 with the addition of Mo nuclei For the preparation of Mo powders with controllable particle size, the dispersed nucleation and controllable growth are the crucial issues. In the traditional preparation of Mo powders by
reducing pure molybdenum dioxide with H2, dispersed nucleation and its controlled growth are difficult to be achieved. At a low temperature, dispersed nucleation is difficult due to the low concentration of MoO2(OH)2; while at a high temperature, the growth of Mo particles by CVT mechanism is hard to control due to the high concentration of MoO2(OH)2. However, with the assistance of a small amount of Mo nuclei, which dispersedly distribute around the MoO2 particles, the dispersed nucleation and controlled growth of Mo can be achieved. The Mo nuclei will grow by depositing Mo atoms generated by reducing MoO2(OH)2, and the finally prepared Mo will be larger than the added Mo nuclei. Even if Mo nuclei were added to the system, as shown in Fig. 6 (b), the particle sizes of some Mo are smaller than that of the added Mo nuclei at 900 C, which indicates that besides the added nuclei, new nuclei were also formed in this reaction. Therefore, the reaction process may have two main steps: the nucleation and growth of new Mo nuclei, as well as the growth of added Mo nuclei [28e30]. In detail, in the hydrogen reduction process, part of the generated Mo atoms by reducing MoO2(OH)2 will be used to dispersed nucleation, and another part of the Mo atom will deposit on the added nuclei and the newly formed nuclei, resulting in the growth of Mo nuclei. When the Mo nuclei are added to the reaction, which distributed around the MoO2 evenly, the produced Mo atom
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e TEM and SAED patterns of Mo powders obtained by hydrogen reduction of MoO2 at 1000 C with the addition of 20 mass% ultrafine Mo powder.
will deposit on the nearby Mo nuclei as well as the outer surface of the MoO2 to nucleate and grow. Even though the Mo atoms generated by reducing MoO2(OH)2 will make grains grow, but the influence of the particle growth is greatly weakened due to the separation and dilution of MoO2 by the added Mo nuclei which leads to the decrease of local concentrations of water vapor and MoO2(OH)2. Especially, when the additive amount of Mo nuclei is large, the relative content of MoO2 will be greatly decreased, and fewer Mo atoms will be generated but distributed to more nuclei, which will lead to the slight growth of each Mo grain. Therefore, at the same reaction temperature, for the MoO2 with 10 mass% additive, there were a fewer number of nuclei but a higher content of MoO2, which are beneficial for the growth of Mo nuclei to be a relatively larger size; while for the MoO2 with 40 mass% additive, there were more nuclei but a lower content of MoO2, and a much smaller particle size of products will be generated. Consequently, by adding Mo nuclei, the Mo powders with sizecontrolled can be successfully prepared in the hydrogen reduction process. As for the influences of particle size of nuclei, the morphologies of the corresponding products were shown in Figs. 6 and 7. For the addition of commercial Mo powders with the particle size of about 2.03 mm, the micron-sized Mo powders ranged from 2.11 mm to 3.25 mm could be prepared. In comparison, for the addition of ultrafine Mo powders with the particle size of 170 nm, Mo powders from 0.28 mm to 0.88 mm can be obtained. The reason could be that, the particle sizes of ultrafine Mo powders are only one tenth of the commercial Mo, thus the number of ultrafine Mo nuclei will be 1000 times of the commercial Mo nuclei at a constant additive amount. Thus, the smaller the size of nuclei added and the more the addition amounts of nuclei are, the smaller the size of the prepared Mo powders will be. It should be pointed out that temperature is an important factor affecting the morphology and particle size of products [25,29]. The Mo particles can grow not only by atom-by-atom addition, but also by the assembly of smaller particles into larger particles. Specifically, when the temperature is low, the
aggregation degree of the small particles Mo powders to large particles is not high, so the particle size of Mo is relatively small. With the increase of temperature, the degree of aggregation improves and a large number of small particles are aggregated to form large particles. Therefore, there is an increase in particle size of Mo with the increase of temperature, as shown in Tables 1 and 2
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 Ce1100 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 temperature were crucial factors for preparing Mo powders with different particle sizes. As temperature increased, the particle size of the produced Mo powder became large. However, with the increase of addition amount of Mo nuclei, the particle size of product will become fine. As adding the commercial micron sized Mo powder (10 mass %-40 mass%) with the particle size of about 2.03 mm as nuclei, Mo powder ranged from 2.11 mm to 3.25 mm can be prepared in the temperature range of 1000e1100 C. Whereas, by adding ultrafine Mo nuclei with the particle size about 170 nm, Mo powder ranged from 0.28 mm to 0.88 mm can be obtained.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51725401).
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
references [16] [1] Ku JG, Oh JM, Kwon H, Lim JW. High-temperature hydrogenreduction process for the preparation of low-oxygen Mo powder from MoO3. Int J Hydrogen Energy 2017;42:2139e43. [2] Sun GD, Zhang GH, Jiao SQ, Chou KC. Shape-controlled synthesis of ultrafine molybdenum crystals via salt-assisted reduction of MoO2 with H2. J Phys Chem C 2018;122:10231e9. [3] Gu S, Qin M, Zhang H, Ma J, Qu X. Preparation of Mo nanopowders through hydrogen reduction of a combustion synthesized foam-like MoO2 precursor. Int J Refract Metals Hard Mater 2018;76:90e8. [4] Wang L, Zhang GH, Wang JS, Chou KC. Study on hydrogen reduction of ultrafine MoO2 to produce ultrafine Mo. J Phys Chem C 2016;120:4097e103. [5] Manukyan K, Davtyan D, Bossert J, Kharatyan S. Direct reduction of ammonium molybdate to elemental molybdenum by combustion reaction. Chem Eng J 2011;168:925e30. [6] Hoseinpur A, Bafghi MS, Khaki JV, Jalaly M, Sakaki M. A mechanistic study on the production of nanosized Mo in microwave assisted combustive reduction of MoO3 by Zn. Int J Refract Metals Hard Mater 2015;50:191e6. [7] Oh JM, Kim W, Lee BK, Suh CY, Kim HS, Lim JW. New method for preparation of low oxygen Mo powder by reduction of MoO3 using Ca. Powder Metall 2014;57:291e4. NV, Jovic VD, Gajic -Krstajic L, Jovic BM, Antozzi AL, [8] Krstajic Martelli GN. Electrodeposition of NieMo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution. Int J Hydrogen Energy 2008;33(14):3676e87. [9] Han JR, Ji XQ, Ren X, Sun XP. MoO3 nanosheets for efficient electrocatalytic N2 fixation to NH3. J Mater Chem 2018;6:12974e7. [10] Jia J, Zhou WJ, Li GX, Yang LJ, Wei ZQ, Cao LD, Wu YS, Zhou K, Chen SW. Regulated synthesis of Mo sheets and their derivative MoX sheets (X: P, S, or C) as efficient electrocatalysts for hydrogen evolution reactions. ACS Appl Mater Interfaces 2017;9:8041e6. [11] Ren X, Zhao JX, Wei Q, Ma YJ, Li BH, Tang B, Sun XP. Highperformance N2-to-NH3 conversion electrocatalyzed by Mo2C nanorod. ACS Cent Sci 2019;5:116e21. [12] Wang L, Zhang GH, Chou KC. Synthesis of nanocrystalline molybdenum powder by hydrogen reduction of industrial grade MoO3. Int J Refract Metals Hard Mater 2016;59:100e4. [13] Liu B, Gu H, Chen Q. Preparation of nanosized Mo powder by microwave plasma chemical vapor deposition method. Mater Chem Phys 1999;59(3):204e9. [14] Kim GS, Lee YJ, Kim DG, Do Kim Y. Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogenreduction. J Alloy Comp 2008;454(1e2):327e30. [15] Sun GD, Zhang GH, Chou KC. Preparation of Mo nanoparticles through hydrogen reduction of commercial
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
9
MoO2 with the assistance of molten salt. Int J Refract Metals Hard Mater 2019;78:68e75. Huang Z, Liu J, Deng X, Zhang H, Lu L, Hou Z, Zhang S. Low temperature molten salt preparation of molybdenum nanoparticles. Int J Refract Metals Hard Mater 2016;54:315e21. Won CW, Nersisyan HH, Won HI, Lee JH. Refractory metal nanopowders: synthesis and characterization. Curr Opin Solid State Mater Sci 2010;14:53e68. Nersisyan HH, Lee JH, Won CW. The synthesis of nanostructured molybdenum under self-propagating hightemperature synthesis mode. Mater Chem Phys 2005;89:283e8. Heidarpour A, Karimzadeh F, Enayati MH. In situ synthesis mechanism of Al2O3eMo nanocomposite by ball milling process. J Alloy Comp 2009;477:692e5. Wang DH, Sun GD, Zhang GH. Preparation of ultrafine Mo powders via carbothermic pre-reduction of molybdenum oxide and deep reduction by hydrogen. Int J Refract Metals Hard Mater 2018;75:70e7. Martı´nez M, Toledo M, Cabrera M. Molybdenum oxide reduction using syngas and heat from an inert porous media reactor. Int J Hydrogen Energy 2016;41:12747e61. Sun GD, Zhang GH, Ji XP, Liu JK, Zhang H, Chou KC. Sizecontrolled synthesis of nano Mo powders via reduction of commercial MoO3 with carbon black and hydrogen. Int J Refract Metals Hard Mater 2019;80:11e22. Bolitschek J, Luidold S, O’Sullivan M. A study of the impact of reduction conditions on molybdenum morphology. Int J Refract Metals Hard Mater 2018;71:325e9. Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Metals Hard Mater 2002;20:261e9. Sun GD, Wang KF, Ji XP, Liu JK, Zhang H, Zhang GH. Preparation of ultrafine/nano Mo particles via NaCl-assisted hydrogen reduction of different-sized MoO2 powders. Int J Refract Metals Hard Mater 2019;80:243e52. Wang KF, Sun GD, Wu YD, Zhang GH. Fabrication of ultrafine and high-purity tungsten carbide powders via a carbothermic reductionecarburization process. J Alloy Comp 2019;784:362e9. Majumdar S, Sharma I, Samajdar I, Bhargava P. Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metall Mater Trans B 2008;39:431e8. Lassner E, Schubert WD. Properties, chemistry, technology of the element, alloys, and chemical compounds. Vienna University of Technology. Vienna, Austria: Kluwer; 1999. p. 124e5. Wang Y, He J, Liu C, Chong WH, Chen H. Thermodynamics versus kinetics in nanosynthesis. Angew Chem Int Ed 2015;54:2022e51. Swihart MT. Vapor-phase synthesis of nanoparticles. Curr Opin Colloid Interface Sci 2003;8:127e33.
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei Yong Zhang, Shuqiang Jiao, Kuo-Chih Chou, Guo-Hua Zhang* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, China
highlights
graphical abstract
Influence of Mo nuclei addition on hydrogen reduction process of MoO2 was studied. Mo
powders
with
controllable
sizes could be prepared by adding Mo nuclei to MoO2. Without addition of nuclei, Mo particles with large particle sizes were obtained.
article info
abstract
Article history:
The size-controlled preparation of Mo powders is always a challenge and important task in
Received 19 August 2019
the molybdenum metallurgy. In the current study, Mo powders with controllable sizes are
Received in revised form
synthesized by hydrogen reduction of MoO2 powders with the assistance of Mo nuclei in
23 October 2019
the range of 900e1100 C. The influences of the particle sizes of Mo nuclei, the additive
Accepted 2 November 2019
amount as well as reaction temperature on the morphology and particle sizes of the final
Available online xxx
products are studied. For the hydrogen reduction of MoO2 without any additive, the obtained Mo powders always have large particle sizes. However, the addition of small
Keywords:
amounts of nuclei in MoO2 can help Mo nucleate dispersedly, and the growth of Mo could
Hydrogen reduction
be also controlled by adjusting the sizes of added nuclei, amount of addition and the re-
Molybdenum
action temperature. With the addition of Mo nuclei, the different sizes of Mo powders with
Size-controlled
the good dispersity can be prepared. As adding commercial Mo powders with the particle
Nucleation and growth
size of about 2.03 mm, the micron-sized Mo powders ranged from 2.11 mm to 3.25 mm could be prepared. While for the case of adding ultrafine Mo nuclei of about 170 nm, Mo powders from 0.28 mm to 0.88 mm can be obtained. Moreover, the more the amounts of nuclei added and the lower the reaction temperature (in the range of 900e1100 C) is, the smaller the particle size of the prepared Mo powder will be. The current method is a facile and feasible
* Corresponding author. E-mail address: [email protected] (G.-H. Zhang). https://doi.org/10.1016/j.ijhydene.2019.11.008 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
2
international journal of hydrogen energy xxx (xxxx) xxx
method, and is potential to be used for industrial production of Mo powder with controllable particle sizes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 [1e7]. In addition, Mo alloys (NieMo) 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 [9e11]. 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 [12e14], molten salt synthesis [15e17], and self-propagating hightemperature 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 600e700 C and then reduction of MoO2 to Mo at 900e1400 C [21e24]. 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 sizecontrolled 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.
Experimental 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 þ 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 mm. As shown in Fig. 1(b), the commercial Mo has an average particle size of 2.03 mm, but has a poor uniformity. From Fig. 2, it can be seen that the ultrafine Mo powders have been prepared, with the mean particle size of about 170 nm.
Experimental procedure In our previous paper [26], the schematic diagram of the experimental apparatus has been described in detail. The samples of pure MoO2 or the mixture of MoO2 with commercial Mo or ultrafine Mo powder (with the additive amount of 10, 20 or 40 mass%) were mixed uniformly in agate mortar for 40 min. The FE-SEM image of the mixture of MoO2 with 20 mass% Mo nuclei was shown in Fig. 3, from which it can be seen that platelet MoO2 had been homogeneously mixed with the Mo particles, meanwhile, the ultrafine Mo particles had a better dispersity around MoO2 than commercial Mo particles. The mixture was put into the alumina crucibles with the length, width and height of 45 mm, 15 mm and 20 mm, respectively. The thickness of layer was about 16 mm. The crucible with the sample was put into the quartz tube which was pushed into the constant temperature zone of furnace with the heating elements of SiC. Before heating, the high purity argon gas was introduced into the tube to drive the air out. Then the temperature of furnace was raised to the targeted value with a constant heating rate (5 C/min) in the protection of flowing argon (400 ml/min). As the temperature increased to the desired temperature (900 C, 1000 C, 1100 C), the argon gas was switched to the reducing gas H2 (200 ml/ min) to start the reduction reaction. Subsequently, after the experiment was finished, H2 was switched to argon gas again and the samples were cooled down to the room temperature. The phase composition of sample was examined via X-ray diffraction (XRD; TTR III, Rigaku Corporation, Japan) using Cu Ka radiation in the range of 2q ¼ 10e90 with a scanning rate of 10deg./min. The morphology was characterized by field-
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e FE-SEM images of (a) commercial Mo, (b) commercial MoO2.
Fig. 2 e FE-SEM images of ultrafine Mo powders.
emission scanning electron microscopy (FE-SEM; ZEISS SUPRA 55, Oberkochen, Germany) and transmission electron microscopy (TEM, JEM-2100F, JEOL Ltd., Tokyo, Japan).
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 samples in the current study could be reduced to metal Mo powders under the current experimental conditions.
Results FE-SEM analyses X-ray diffraction analyses Reduction of MoO2 without the additive 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
Fig. 5 presented the FE-SEM images of product Mo obtained by reducing pure MoO2 with H2 at different temperatures without additive. It can be obviously seen that, at three different temperatures (900 C, 1000 C and 1100 C), all the produced Mo powders have fairly large particle sizes of several micron meters. Meanwhile, with the increase of the temperature from
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e FE-SEM images of the mixture of MoO2 with (a) 20 mass% ultrafine Mo powders, (b) 20 mass% commercial Mo powders. 900 C to 1000 C and 1100 C, the particle sizes of the Mo powders significantly increase from 2.06 mm to 3.54 mm and 5.63 mm, respectively.
Reduction of MoO2 with the addition of commercial Mo powder Fig. 6 presents the FE-SEM images of Mo products prepared by hydrogen reduction of MoO2 with the assistance of different amounts of commercial Mo in the temperature range from 900 C to 1100 C. It can be clearly seen that the particle sizes of the Mo products are much smaller compared to the Mo powders prepared without the additive. The particles sizes were measured rigorously by the statistics of >300 particles from six different fields in the FE-SEM images, and the results are shown in Table 1. Fig. 6(aec) shows the FE-SEM images of Mo powders obtained by completely reducing MoO2 with different additive amounts at 900 C. It can be seen from Fig. 6 (c) that Mo grains with the average particle size of 460 nm were prepared with 40 mass% addition of commercial Mo nuclei at 900 C, but the overall morphology of produced Mo kept the morphology of the raw MoO2, even if they were composed of a lot of small Mo grains with the sizes of about 300 nm. However, as the mass ratio of additive was decreased to 10 mass% and 20 mass%, from Fig. 6 (a) and 6 (b), there was abrupt change in particle size of the prepared Mo. Many small grains were generated and adhered to the surface of larger grains, meanwhile, it can be seen the average particle size increased to 1.45 mm and 1.78 mm, respectively. Therefore, with the increase of the addition amount of Mo nuclei, the particle sizes of Mo product decreased at 900 C. Meanwhile, at 1000 C, it could be seen from Fig. 6 (d) that Mo with an average particle size about 2.55 mm were obtained with 10 mass% addition of
nuclei. With the increase of additive amount from 10 mass% to 20 mass% and 40 mass%, the average particle size decreased to about 2.37 mm and 2.11 mm, as shown in Fig. 6(e) and (f). When the temperature was increased to 1100 C, the FE-SEM images of the products were shown in Fig. 6(gei). The average particle sizes were 3.25 mm, 2.51 mm, 2.17 mm, corresponding to the additive amount of 10 mass%, 20 mass% and 40 mass%, respectively. From the above analyses, it can be concluded that the grain sizes of products increase with the increase of reaction temperature and the decrease of additive amount of Mo nuclei.
Reduction of MoO2 with the addition of ultrafine Mo powder Fig. 7 shows the FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the assistance of different addition amounts of ultrafine Mo powder from 900 C to 1100 C. It can be seen that by adding a small amount of ultrafine Mo nuclei, ultrafine Mo powder could be prepared successfully. By comparing the reduction of pure MoO2 via H2 as shown in Fig. 5, the grain sizes of products decreased significantly. The particle sizes were measured, and the results are shown in Table 2. At 900 C, as shown in Fig. 7(aec), the average particle sizes were 0.6 mm, 0.58 mm and 0.28 mm, corresponding to the addition amount of 10%, 20% and 40%, respectively. Whereas as the temperature was increased to 1000 C, the particle sizes of produced Mo were 0.69 mm, 0.64 mm and 0.46 mm, respectively. When the temperature was further increased to 1100 C, as shown in Fig. 7(gei), it could be seen that the particle size was about 0.88 mm, 0.82 mm and 0.66 mm, respectively. From the above results, it could be concluded that with the increase of the addition amount of Mo
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e XRD patterns of products by hydrogen reduction of the mixture of (a) MoO2 and commercial Mo at 900 C, (b) MoO2 and ultrafine Mo at 900 C, (c) MoO2 and 10 mass% commercial Mo or ultrafine Mo at 1000 C and 1100 C.
Fig. 5 e FE-SEM images of Mo product obtained by reducing MoO2 without additive at: (a) 900 C; (b) 1000 C; (c) 1100 C.
nuclei and the decrease of reaction temperature, the particle size of Mo became smaller, which was similar to the case of adding commercial Mo powder.
demonstrate that the single-crystalline Mo grains have been prepared.
TEM analyses
Discussion
Fig. 8 shows the typical TEM images of Mo powder obtained by hydrogen reduction of MoO2 at 1000 C with the addition of 20 mass% ultrafine Mo powder. As shown in Fig. 8(a), it can be seen that the Mo grains have a uniform size of about 0.6 mm, which is consistent with the FE-SEM image (Fig. 7). Besides, the selected area electron diffraction (SAED) patterns
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
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 6 e FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the addition of different amounts of commercial Mo at different temperatures: (a) 10 mass%, 900 C; (b) 20 mass%, 900 C; (c) 40 mass%, 900 C; (d) 10 mass%, 1000 C; (e) 20 mass%, 1000 C; (f) 40 mass%, 1000 C; (g) 10 mass%, 1100 C; (h) 20 mass%, 1100 C; (i) 40 mass%, 1100 C.
Table 1 e Particle sizes of the products prepared by hydrogen reduction of MoO2 at different temperatures with the addition of different amounts of commercial Mo powder (mm). mass ratio
0%
10%
20%
40%
900 C 1000 C 1100 C
2.06 3.54 5.63
1.78 2.55 3.25
1.45 2.37 2.51
0.46 2.11 2.17
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 obeys the chemical vapor transformation (CVT) mechanism and the morphology of the Mo products changes significantly relative to that of the raw molybdenum oxide. These two mechanisms have been widely accepted and used for the reduction of molybdenum oxide by hydrogen. MoO2 ðsÞ þ 2H2 OðgÞ ¼ MoO2 ðOHÞ2 ðgÞ þ H2 ðgÞ
(1)
MoO2 ðsÞ þ 2H2 ðgÞ ¼ MoðsÞ þ 2H2 OðgÞ
(2)
MoO2 ðOHÞ2 ðgÞ þ 3H2 ðgÞ ¼ MoðsÞ þ 4H2 OðgÞ
(3)
These two kinds of mechanisms are mainly determined by the concentration of MoO2(OH)2 gaseous transport phase which is generated from the reaction between molybdenum dioxide and water, as showed by reaction (1). In the reduction process by hydrogen, Mo is mainly produced by the reduction reactions of MoO2 (reactions (2)) and the volatile molybdenum oxide (reactions (3)), and the produced Mo atoms will be used for nucleation and grain growth of Mo. Specifically, when the concentration of MoO2(OH)2 is high enough, such as at a high reaction temperature, the migration rate of Mo through gaseous phase could be promoted. Therefore, Mo atoms can be carried away from the reaction interface to nucleate independently. However, a high concentration of MoO2(OH)2 can also promote the growth of Mo grains, and leads to a large size. Whereas, if the concentration of MoO2(OH)2 is low, corresponding to the case of a low reaction temperature or a low concentration of MoO2 which could result in a low reaction rate and a low concentration of water vapor. In this case, the transport of Mo would be difficult and the produced Mo can only be deposited on the nearby unreacted molybdenum dioxide to nucleate and grow. With the reaction interface
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 7 e FE-SEM images of Mo product obtained by hydrogen reduction of MoO2 with the addition of different amounts of ultrafine Mo at different temperatures: (a) 10 mass%, 900 C; (b) 20 mass%, 900 C; (c) 40 mass%, 900 C; (d) 10 mass%, 1000 C; (e) 20 mass%, 1000 C; (f) 40 mass%, 1000 C; (g) 10 mass%, 1100 C; (h) 20 mass%, 1100 C; (i) 40 mass%, 1100 C.
Table 2 e Particle sizes of the products prepared by hydrogen reduction of MoO2 at different temperatures with the addition of different amounts of ultrafine Mo powder (mm). mass ratio
0%
10%
20%
40%
900 C 1000 C 1100 C
2.06 3.54 5.63
0.6 0.69 0.88
0.58 0.64 0.82
0.28 0.46 0.66
moving from the particle surface towards the core of the particle, the Mo powders with the morphology to the raw material MoO2 can be produced, which could be from the morphology of products by reducing MoO2 with the addition of 40 mass% commercial Mo at 900 C (Fig. 6 (c)). Of course, the particle size and number of nuclei also affect the morphology and particle sizes of products, which will be illustrated in the following section.
The nucleation and growth mechanisms of Mo in hydrogen reduction of MoO2 with the addition of Mo nuclei For the preparation of Mo powders with controllable particle size, the dispersed nucleation and controllable growth are the crucial issues. In the traditional preparation of Mo powders by
reducing pure molybdenum dioxide with H2, dispersed nucleation and its controlled growth are difficult to be achieved. At a low temperature, dispersed nucleation is difficult due to the low concentration of MoO2(OH)2; while at a high temperature, the growth of Mo particles by CVT mechanism is hard to control due to the high concentration of MoO2(OH)2. However, with the assistance of a small amount of Mo nuclei, which dispersedly distribute around the MoO2 particles, the dispersed nucleation and controlled growth of Mo can be achieved. The Mo nuclei will grow by depositing Mo atoms generated by reducing MoO2(OH)2, and the finally prepared Mo will be larger than the added Mo nuclei. Even if Mo nuclei were added to the system, as shown in Fig. 6 (b), the particle sizes of some Mo are smaller than that of the added Mo nuclei at 900 C, which indicates that besides the added nuclei, new nuclei were also formed in this reaction. Therefore, the reaction process may have two main steps: the nucleation and growth of new Mo nuclei, as well as the growth of added Mo nuclei [28e30]. In detail, in the hydrogen reduction process, part of the generated Mo atoms by reducing MoO2(OH)2 will be used to dispersed nucleation, and another part of the Mo atom will deposit on the added nuclei and the newly formed nuclei, resulting in the growth of Mo nuclei. When the Mo nuclei are added to the reaction, which distributed around the MoO2 evenly, the produced Mo atom
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 8 e TEM and SAED patterns of Mo powders obtained by hydrogen reduction of MoO2 at 1000 C with the addition of 20 mass% ultrafine Mo powder.
will deposit on the nearby Mo nuclei as well as the outer surface of the MoO2 to nucleate and grow. Even though the Mo atoms generated by reducing MoO2(OH)2 will make grains grow, but the influence of the particle growth is greatly weakened due to the separation and dilution of MoO2 by the added Mo nuclei which leads to the decrease of local concentrations of water vapor and MoO2(OH)2. Especially, when the additive amount of Mo nuclei is large, the relative content of MoO2 will be greatly decreased, and fewer Mo atoms will be generated but distributed to more nuclei, which will lead to the slight growth of each Mo grain. Therefore, at the same reaction temperature, for the MoO2 with 10 mass% additive, there were a fewer number of nuclei but a higher content of MoO2, which are beneficial for the growth of Mo nuclei to be a relatively larger size; while for the MoO2 with 40 mass% additive, there were more nuclei but a lower content of MoO2, and a much smaller particle size of products will be generated. Consequently, by adding Mo nuclei, the Mo powders with sizecontrolled can be successfully prepared in the hydrogen reduction process. As for the influences of particle size of nuclei, the morphologies of the corresponding products were shown in Figs. 6 and 7. For the addition of commercial Mo powders with the particle size of about 2.03 mm, the micron-sized Mo powders ranged from 2.11 mm to 3.25 mm could be prepared. In comparison, for the addition of ultrafine Mo powders with the particle size of 170 nm, Mo powders from 0.28 mm to 0.88 mm can be obtained. The reason could be that, the particle sizes of ultrafine Mo powders are only one tenth of the commercial Mo, thus the number of ultrafine Mo nuclei will be 1000 times of the commercial Mo nuclei at a constant additive amount. Thus, the smaller the size of nuclei added and the more the addition amounts of nuclei are, the smaller the size of the prepared Mo powders will be. It should be pointed out that temperature is an important factor affecting the morphology and particle size of products [25,29]. The Mo particles can grow not only by atom-by-atom addition, but also by the assembly of smaller particles into larger particles. Specifically, when the temperature is low, the
aggregation degree of the small particles Mo powders to large particles is not high, so the particle size of Mo is relatively small. With the increase of temperature, the degree of aggregation improves and a large number of small particles are aggregated to form large particles. Therefore, there is an increase in particle size of Mo with the increase of temperature, as shown in Tables 1 and 2
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 Ce1100 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 temperature were crucial factors for preparing Mo powders with different particle sizes. As temperature increased, the particle size of the produced Mo powder became large. However, with the increase of addition amount of Mo nuclei, the particle size of product will become fine. As adding the commercial micron sized Mo powder (10 mass %-40 mass%) with the particle size of about 2.03 mm as nuclei, Mo powder ranged from 2.11 mm to 3.25 mm can be prepared in the temperature range of 1000e1100 C. Whereas, by adding ultrafine Mo nuclei with the particle size about 170 nm, Mo powder ranged from 0.28 mm to 0.88 mm can be obtained.
Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51725401).
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008
international journal of hydrogen energy xxx (xxxx) xxx
references [16] [1] Ku JG, Oh JM, Kwon H, Lim JW. High-temperature hydrogenreduction process for the preparation of low-oxygen Mo powder from MoO3. Int J Hydrogen Energy 2017;42:2139e43. [2] Sun GD, Zhang GH, Jiao SQ, Chou KC. Shape-controlled synthesis of ultrafine molybdenum crystals via salt-assisted reduction of MoO2 with H2. J Phys Chem C 2018;122:10231e9. [3] Gu S, Qin M, Zhang H, Ma J, Qu X. Preparation of Mo nanopowders through hydrogen reduction of a combustion synthesized foam-like MoO2 precursor. Int J Refract Metals Hard Mater 2018;76:90e8. [4] Wang L, Zhang GH, Wang JS, Chou KC. Study on hydrogen reduction of ultrafine MoO2 to produce ultrafine Mo. J Phys Chem C 2016;120:4097e103. [5] Manukyan K, Davtyan D, Bossert J, Kharatyan S. Direct reduction of ammonium molybdate to elemental molybdenum by combustion reaction. Chem Eng J 2011;168:925e30. [6] Hoseinpur A, Bafghi MS, Khaki JV, Jalaly M, Sakaki M. A mechanistic study on the production of nanosized Mo in microwave assisted combustive reduction of MoO3 by Zn. Int J Refract Metals Hard Mater 2015;50:191e6. [7] Oh JM, Kim W, Lee BK, Suh CY, Kim HS, Lim JW. New method for preparation of low oxygen Mo powder by reduction of MoO3 using Ca. Powder Metall 2014;57:291e4. NV, Jovic VD, Gajic -Krstajic L, Jovic BM, Antozzi AL, [8] Krstajic Martelli GN. Electrodeposition of NieMo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution. Int J Hydrogen Energy 2008;33(14):3676e87. [9] Han JR, Ji XQ, Ren X, Sun XP. MoO3 nanosheets for efficient electrocatalytic N2 fixation to NH3. J Mater Chem 2018;6:12974e7. [10] Jia J, Zhou WJ, Li GX, Yang LJ, Wei ZQ, Cao LD, Wu YS, Zhou K, Chen SW. Regulated synthesis of Mo sheets and their derivative MoX sheets (X: P, S, or C) as efficient electrocatalysts for hydrogen evolution reactions. ACS Appl Mater Interfaces 2017;9:8041e6. [11] Ren X, Zhao JX, Wei Q, Ma YJ, Li BH, Tang B, Sun XP. Highperformance N2-to-NH3 conversion electrocatalyzed by Mo2C nanorod. ACS Cent Sci 2019;5:116e21. [12] Wang L, Zhang GH, Chou KC. Synthesis of nanocrystalline molybdenum powder by hydrogen reduction of industrial grade MoO3. Int J Refract Metals Hard Mater 2016;59:100e4. [13] Liu B, Gu H, Chen Q. Preparation of nanosized Mo powder by microwave plasma chemical vapor deposition method. Mater Chem Phys 1999;59(3):204e9. [14] Kim GS, Lee YJ, Kim DG, Do Kim Y. Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogenreduction. J Alloy Comp 2008;454(1e2):327e30. [15] Sun GD, Zhang GH, Chou KC. Preparation of Mo nanoparticles through hydrogen reduction of commercial
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
9
MoO2 with the assistance of molten salt. Int J Refract Metals Hard Mater 2019;78:68e75. Huang Z, Liu J, Deng X, Zhang H, Lu L, Hou Z, Zhang S. Low temperature molten salt preparation of molybdenum nanoparticles. Int J Refract Metals Hard Mater 2016;54:315e21. Won CW, Nersisyan HH, Won HI, Lee JH. Refractory metal nanopowders: synthesis and characterization. Curr Opin Solid State Mater Sci 2010;14:53e68. Nersisyan HH, Lee JH, Won CW. The synthesis of nanostructured molybdenum under self-propagating hightemperature synthesis mode. Mater Chem Phys 2005;89:283e8. Heidarpour A, Karimzadeh F, Enayati MH. In situ synthesis mechanism of Al2O3eMo nanocomposite by ball milling process. J Alloy Comp 2009;477:692e5. Wang DH, Sun GD, Zhang GH. Preparation of ultrafine Mo powders via carbothermic pre-reduction of molybdenum oxide and deep reduction by hydrogen. Int J Refract Metals Hard Mater 2018;75:70e7. Martı´nez M, Toledo M, Cabrera M. Molybdenum oxide reduction using syngas and heat from an inert porous media reactor. Int J Hydrogen Energy 2016;41:12747e61. Sun GD, Zhang GH, Ji XP, Liu JK, Zhang H, Chou KC. Sizecontrolled synthesis of nano Mo powders via reduction of commercial MoO3 with carbon black and hydrogen. Int J Refract Metals Hard Mater 2019;80:11e22. Bolitschek J, Luidold S, O’Sullivan M. A study of the impact of reduction conditions on molybdenum morphology. Int J Refract Metals Hard Mater 2018;71:325e9. Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Metals Hard Mater 2002;20:261e9. Sun GD, Wang KF, Ji XP, Liu JK, Zhang H, Zhang GH. Preparation of ultrafine/nano Mo particles via NaCl-assisted hydrogen reduction of different-sized MoO2 powders. Int J Refract Metals Hard Mater 2019;80:243e52. Wang KF, Sun GD, Wu YD, Zhang GH. Fabrication of ultrafine and high-purity tungsten carbide powders via a carbothermic reductionecarburization process. J Alloy Comp 2019;784:362e9. Majumdar S, Sharma I, Samajdar I, Bhargava P. Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metall Mater Trans B 2008;39:431e8. Lassner E, Schubert WD. Properties, chemistry, technology of the element, alloys, and chemical compounds. Vienna University of Technology. Vienna, Austria: Kluwer; 1999. p. 124e5. Wang Y, He J, Liu C, Chong WH, Chen H. Thermodynamics versus kinetics in nanosynthesis. Angew Chem Int Ed 2015;54:2022e51. Swihart MT. Vapor-phase synthesis of nanoparticles. Curr Opin Colloid Interface Sci 2003;8:127e33.
Please cite this article as: Zhang Y et al., Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.008