Study on Hydrogen Reduction of Ultrafine MoO2 To Produce Ultrafine

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Study on Hydrogen Reduction of Ultrafine MoO2 To Produce Ultrafine Mo Lu Wang, Guo-Hua Zhang,* Jing-Song Wang, and Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT: In the present study, the hydrogen reduction of ultrafine MoO2 to produce ultrafine Mo powders has been carried out. It is found that ultrafine spherical Mo powders can be obtained when the reaction temperature is above 923 K, and the reduction reaction obeys the chemical vapor transport (CVT) mechanism. Whereas when the reduction temperature is below 883 K, the final products of Mo powders appear to have the same morphology as the initial materials MoO2. In this case, the reduction reaction obeys the pseudomorphic transport mechanism. It was found that when the temperatures are in the range of 923−1023 K, the rate-controlling step for the reduction reaction is interfacial chemical reaction. While in the range of 863−883 K, the rate-controlling step changes with the reduction extent: when the reduction extent is in the range of 0−0.8, it obeys the nucleation and growth model; when the reduction extent is in the range of 0.8−1, the diffusion model is obeyed.

1. INTRODUCTION Molybdenum metal powders with suitable morphology, size, and purity are of immense importance for alloy preparation by powder metallurgy (PW).1 Because of the good high-temperature strength, creep resistance, low coefficient of thermal expansion, and high thermal conductivity, metallic Mo is widely used as an alloying agent for manufacturing steel, cast irons, and superalloys.2 The industrial production of metallic molybdenum powders is a stepwise process that begins with the reduction of MoO3 to MoO2 by hydrogen (723−923 K) and then MoO2 is further reduced to metallic Mo (1123−1223 K).3 The mechanism and kinetics of hydrogen reduction of MoO2 to Mo have been investigated by many researchers.4−14 Von Destinon-Forstmann4 carried out the investigation of hydrogen reduction of MoO2 to Mo over the temperature range of 873− 1123 K and reported that the kinetics follows a linear rate equation with the activation energy extracted to be 114.64 kJ/ mol. Kennedy and Bevan5 conducted the reduction of MoO2 to metal Mo in the range of 923−1073 K and reported that the reaction kinetics followed the more usual deceleratory path characteristic of the contracting sphere model between the limits α = 0.01 and α = 0.75 with the activation energy of 99 kJ/ mol. Sichen and Seetharaman7 reported that the reduction of MoO2 to Mo appeared to be influenced by the transport of H2 and H2O through the voids of the samples; the activation energy for this reaction was calculated to be 85.2 kJ/mol for the isothermal studies. Schulmeyer and Ortner10 studied the mechanisms of hydrogen reduction of MoO2 to Mo at two extreme local dew points and found that two different reaction paths can occur: Pseudomorphic transformation at low dew points and transformation via chemical vapor transport (CVT) at high dew points. Majumdar et al.12 conducted the isothermal reduction of MoO2 to Mo by hydrogen in the temperature © 2016 American Chemical Society

range of 898−1173 K and reported that the kinetic equation obeyed Johnson-Mehl-Avrami-Komolgorow (JMAK) model with the corresponding activation energy to be 136 kJ/mol. More recently, Dang et al.13 did the studies of hydrogen reduction of MoO2 to Mo under both isothermal and nonisothermal conditions, proposed a new model to analyze the reaction kinetics, and reported the hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface; the activation energy was extracted to be 90.6−92.5 kJ/mol. As mentioned above, although many related studies have been carried out, the mechanism and kinetics behaviors of hydrogen reduction of MoO2 to Mo is a long debated question. In the authors’ previous study,15 ultrafine MoO3 powders were used to investigate the reduction process from MoO3 to MoO2 and ultrafine MoO2 powders were prepared. In the present work, the as-prepared ultrafine MoO2 powders previously were used to illuminate the reduction mechanism and kinetics from MoO2 to Mo to obtain the ultrafine Mo powders. The influences of reaction temperature and time on the morphology of Mo powders were considered.

2. MATERIALS AND EXPERIMENTAL PROCEDURES 2.1. Materials. Ultrafine MoO2 powders obtained by reducing ultrafine spherical MoO3 at 813 K were used as the experimental raw materials.15 Figure 1 shows the X-ray diffraction patterns of the studied MoO2 powders. As can be seen, the raw materials used are pure MoO2 without any Received: November 21, 2015 Revised: January 12, 2016 Published: January 26, 2016 4097

DOI: 10.1021/acs.jpcc.5b11394 J. Phys. Chem. C 2016, 120, 4097−4103

Article

The Journal of Physical Chemistry C

introduced into the system to drive the air out. Then the furnace was heated from room temperature to the desired reduction temperatures (863 K, 873 K, 883 K, 923 K, 973 K, 1023 and 1073 K) with the heating rate of 10 K/min. When the temperature was stable and the air was driven out completely, Ar gas was switched to the reducing gas H2 to start the reduction reaction. After the experiment was finished, H2 was switched to Ar again and the samples were cooled down to the room temperature. In order to establish the isothermal experimental temperature, a serial of nonisothermal thermo-gravimetry experiments were carried out first. After samples were positioned well, H2 was introduced into the furnace to get the air out first and then the furnace was heated from room temperature to 1273 K at the heating rate of 5, 10, 15, and 20 K/min, respectively. In all the TG experimental runs, a constant gas flow rate of 60 mL/min of H2 ( 293 K), the reduction reaction obeyed chemical vapor transformation mechanism (CVT), while when the dew point of H2 is extremely low (τ(H2) > 233 K), the reduction reaction obeyed pseudomorphic transformation mechanism. It is found that, during the reduction process of MoO2 to Mo at 1023 K, the morphology of final products Mo experienced a large change compared to that of the raw materials MoO2 (from particle shape to small spherical shape), as shown in Figure 8, which may be due to the reaction process obeying the chemical vapor transport (CVT) mechanism shown in eq 3. At a higher temperature, the reaction rate is larger, so more H2O and gaseous intermediate phase TP will be produced (as in the case of high dew points10) than at a lower temperature to make the reaction follow the CVT mechanism.

Figure 8. FE-SEM micrographs of ultrafine MoO2 samples reduced by pure H2 at 1023 K. (a) Reduced for 5 min (α = 0.2162); (b) reduced for 10 min (α = 0.5677); (c) reduced for 30 min (α = 1).

Figure 9 presents the FE-SEM micrographs of ultrafine MoO2 samples reduced by pure H2 at 873 K for various times. It can be seen from Figure 9a, when the reduction time is 50 min (α = 0.3726), the products keep the skeleton or dentritic structure as the raw MoO2 powders, as shown in Figure 2. In addition, it can be easily seen that there are many small grains positioned on the large platelet-shaped particles, which may be the nucleus of metal Mo powders. As the reaction proceeds forward (100 min, α = 0.8697), the small nucleus became compact and many cracks or fissures can also be observed; in the meantime, several small whiskers can be seen obviously. When the reaction products were all metal Mo powders, as shown in Figure 9c (150 min, α = 1), the products presented the same particles shaped as raw materials MoO2. One of the significant differences is that the as-prepared metal Mo powders have many cracks and fissures which may result from the tensile stress generated due to the volume decrease during the oxygen

H2

H2

MoO2 (particle) → TP(g) → Mo (spherical grain)

(3)

However, when the reaction temperature is 873 K, the morphology of final products Mo almost keep the same shape as that of raw materials MoO2 except for the high porosity which resulted from the stress due to the removal of oxygen.12,13 In addition, because of the slow reaction rate and low saturation vapor pressure of gaseous transport species TP at 873 K, chemical vapor transport mechanism may not be dominant, but the pseudomorphic transformation mechanism would be. Consequently, the morphology of raw materials MoO2 was conserved, and the corresponding transformation route can be described as follows: MoO2 + 2H 2 = Mo + 2H 2O 4100

(4) DOI: 10.1021/acs.jpcc.5b11394 J. Phys. Chem. C 2016, 120, 4097−4103

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The Journal of Physical Chemistry C

dependence of the rate constant is described by the Arrhenius equation,

In a word, when the reduction of MoO2 to Mo happens at a higher temperature (1023 K), the reduction mechanism obeys the CVT mechanism, which is beneficial for grain refining of final product Mo. However, when the reduction temperature is lower (873 K), the reduction mechanism obeys the pseudomorphic transport mechanism, which is beneficial for keeping the same morphology as the initial reactant MoO2 except for the existed porosities. The reduction mechanism may be described as shown in Figure 10.

⎛ ΔE ⎞ ⎟ k = A exp⎜ − ⎝ RT ⎠

(5)

where A is the pre-exponential factor (frequency factor) (min−1), ΔE is the activation energy (J/mol), R is the gas constant (8.314 J/mol/K), and T is the absolute temperature (K). 4.2.1. 923−1023 K. As can be seen from Figure 8, the nucleation and growth of the product Mo grains are separated from the MoO2 particles because a lot of gaseous intermediate phases TP are generated, so the diffusion of hydrogen should not be the rate-controlling step. In addition, the nucleation and growth process may also not be the rate-controlling step because of the linear kinetics curves. Therefore, the reduction rate may be controlled by the interfacial chemical reaction. When using all the models in Table 1 to fit the experimental data, it is found that model 3 can give the best fitting, and the results are shown in Figure 11. It is observed that the data

Figure 10. Schematic diagram of the proposed possible reduction mechanism.

4.2. Reduction Kinetics. As mentioned above, the shape of reduction curves at high temperatures (923−1023 K) is very different from that at low temperatures (863−883 K), which illustrates that the reduction mechanisms are different. Here, the model fitting method is used to analyze the reduction kinetics, and the rate expressions of different gas−solid reaction models are listed in Table 1.15−18 The expressions are generally applied for the kinetics analysis of gas−solid reaction and encompass the most common mechanism. In Table 1, k is the rate constant as a function of temperature, and the temperature Table 1. List of Rate Expressions of Different Gas−Solid Reaction Kinetics Models15−18 Figure 11. Plot of g(α) α vs reaction time t. model Geometrical Contraction Models contracting area (R2) contracting volume (R3) Reaction-Order Models zero-order (F0/R1) first-order (F1) second-order (F2) third-order (F3) Diffusion Models diffusion(D1) diffusion(D2) Jander equation (2D, n = 1/2) Jander equation (2D, n = 2) Jander equation (3D, n = 1/2) Jander equation (3D, n = 2) Nucleation and Growth Models power law (P2) power law (P3) Avrami-Erofe’ev (A1.5) Avrami-Erofe’ev (A2) Avrami-Erofe’ev (A3) Avrami-Erofe’ev (A4)

integral form g(α) = kt

no.

1 − (1 − α)1/2 1 − (1 − α)1/3

1 2

α −ln(1 − α) (1 − α)−1 − 1 0.5[(1 − α)−2 − 1]

3 4 5 6

α [(1 − α) ln(1 − α)] + α [1 − (1 − α)1/2]1/2 [1 − (1 − α)1/2]2 [1 − (1 − α)1/3]1/2 [1 − (1 − α)1/3]2

7 8 9 10 11 12

α1/2 α1/3 [−ln(1 [−ln(1 [−ln(1 [−ln(1

13 14 15 16 17 18

2

− − − −

α)]2/3 α)]1/2 α)]1/3 α)]1/4

display a good linear relation with respect to model 3; the slopes of each straight line determine the rate of reaction k which increase as the temperature increases. According to eq 5, one can easily obtain ln k = −

ΔE 1 + ln A R T

(6)

Substitution of the different reduction rate constants k obtained in Figure 11 into eq 6, and the corresponding calculated kinetic curves, are shown in Figure 12. From Figure 12 it can be seen that the experimental data agree well with the Arrhenius equation, and the extracted activation energy is 104.36 kJ/mol, which is almost the same as the results obtained by previous investigators.4,5,13 Therefore, the reduction kinetics obeys the chemical reaction model in the temperature range of 923−1023 K and the rate constant k can be expressed by the following equation: ⎛ 104360 ⎞ ⎟ k = 2.4 × 104 exp⎜ − ⎝ RT ⎠ 4101

(7) DOI: 10.1021/acs.jpcc.5b11394 J. Phys. Chem. C 2016, 120, 4097−4103

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The Journal of Physical Chemistry C

Figure 13. Plot of g(α) = [−ln(1 − α)]1/2 vs reaction time t (α is in the range of 0−0.80).

Figure 12. Arrhenius plot for the reduction of ultrafine MoO2 to Mo.

4.2.2. 863−883 K. With use of the models listed in Table 1 to fit the experimental data, it was found that none of them could satisfy the requirement. So it can be deduced that a single reduction mechanism could not explain the experimental results. In the meantime, as mentioned above, the reduction curves have a long induction period at lower temperature and a lot of nuclei are formed on the initial MoO2 as shown in Figure 9, so it can be considered that reduction of MoO2 to Mo in the initial period obeys the nucleation and growth model; as the reaction proceeds, a large number of Mo grains could have been formed and grown gradually, which could form a dense Mo shell around the original MoO2 particles and subsequently inhibit the diffusion of the product gas H2O and reactant gas H2. Therefore, the diffusion of gas through the product layer in the later period may be the rate-controlling step. According to the best fit for the experimental results, it is found that model 16 (g(α) = [−ln(1 − α)]1/2, the number 2 is called the Avrami exponent) can describe the reduction mechanism best at the reaction extent between 0 and 0.8. However, when the reaction extent is above 0.8, it is found that the diffusion model 8 can fit the experimental results best. The limit of the reaction extent 0.8 is in agreement with 0.75 obtained by Kennedy and Bevan5 and 0.93 obtained by Orehotsky and Kaczenski.9 The corresponding fitting curves are presented in Figure 13 and Figure 14, respectively.

Figure 14. Plot of g(α) = [(1 − α) ln(1 − α)] + α vs reaction time t (α is in the range of 0.8−1).

are obeyed, while when the reduction extent is from 0.8 to 1, the diffusion model is obeyed.

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5. CONCLUSIONS In the present study, the hydrogen reduction of ultrafine MoO2 to Mo in the temperature range of 863−1023 K was investigated. The following conclusions can be drawn. (1) It is found that one can obtain ultrafine spherical Mo powders by reducing ultrafine MoO2 by hydrogen at the temperature above 923 K, and the reduction reaction obeys the chemical vapor transport (CVT) mechanism. When the reduction temperature is below 883 K, the original morphology of MoO2 was conserved, and the reduction reaction obeys the pseudomorphic transformation mechanism. (2) When the temperatures are in the range of 923−1023 K, the rate-controlling step for the reduction reaction of ultrafine MoO2 to Mo is the interfacial chemical reaction, while in the range of 863−883 K, the ratecontrolling step changes with the reduction extent. When the reduction extent is from 0 to 0.8, nucleation and growth model

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: 86-1082377750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51304018 and 51474141).

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DOI: 10.1021/acs.jpcc.5b11394 J. Phys. Chem. C 2016, 120, 4097−4103