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Study on kinetics of hydrogen reduction of MoO2
Int. Journal of Refractory Metals and Hard Materials 41 (2013) 356–362
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Study on kinetics of hydrogen reduction of MoO2 Jie Dang, Guo-Hua Zhang ⁎, Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China Department of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 9 April 2013 Accepted 19 May 2013 Keywords: Thermal gravimetric (TG) analysis Isothermal reduction Non-isothermal reduction Model Hydrogen Kinetics
a b s t r a c t The reduction mechanisms of MoO2 under both isothermal and non-isothermal conditions have been investigated in this work. The results showed that the product Mo was kept the same platelet shape as the initial MoO2. The reduction kinetics of MoO2 was analyzed by using a new model, which was in an explicit form and incorporated different variables, such as time, temperature, hydrogen content, etc. It was found that the model predicted curves agreed well with the experimental data. The results indicated that hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface, and the corresponding activation energy was 90.6–92.5 kJ/mol. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Metallic molybdenum, having a body-centered cubic crystal structure with a melting point of 2610 °C and a density of 10.22 g/cm3, offers excellent mechanical, thermal and electric properties and good corrosion resistance, and is widely used as an alloying agent for manufacturing steels, cast irons, and super-alloys to increase their mechanical strength, hardness, swiftness and resistance to corrosion and wearing [1,2]. Hydrogen plays an important role in today's chemical industry [3], and can be used as a reducing gas in reduction of metal oxides [4,5]. Reduction of MoO3 powders by hydrogen is one of the methods used to obtain metallic molybdenum of high purity. In general, the two-stage flow scheme is employed [6]. The first step is to reduce MoO3 powders to MoO2 powders and the second step is to produce molybdenum powders from MoO2 powders [7]. Many excellent papers [8–20] of studying on reduction of MoO3 to MoO2 with hydrogen have been published. However, only several studies [1,7,14,19,21,22] have been focused on the reduction of MoO2 to Mo. Orehotsky and Kaczenski [19] reported a linear dependence of weight loss with reduction time for the reduction of MoO2 to Mo in a static bed during the initial period when the interface (MoO2/Mo) is within 1 cm of the top of the bed. Von Destinon–Forstmann [21] studied the reduction of MoO2 in the temperature range of 873 K to 1123 K, and found that the reduction of the dioxide to the metal followed a linear rate equation and had an activation energy of 114.7 kJ/mol. Du and
⁎ Corresponding author at: Department of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 1062333703. E-mail address: [email protected] (G.-H. Zhang). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.05.009
Seetharaman [22] studied the isothermal and non-isothermal reduction of MoO3 to Mo and reported that the reduction rate of MoO2 to Mo appeared to be influenced by gas diffusion through voids, and the activation energy under isothermal reduction condition was 85.2 kJ/mol. Majumdar et al. [14] reported that the kinetic equation for the MoO2–Mo reaction between 898 K and 1173 K obeyed Johnson–Mehl–Avrami– Kolmogorov (JMAK) type behavior, and the activation energy was calculated as 136 kJ/mol. Kim et al. [1] found that the reduction of MoO2 powder by hydrogen obeyed nucleation and growth model with the activation energy of 50.2–65.9 kJ/mol. Schulmeyer and Ortner [7] studied the reduction of MoO2 under two extreme local dew points and found that two different reaction paths occurred: pseudomorphic transformation at low dew points and transformation via CVT at high dew points. Though these investigators investigated the stage of hydrogen reduction of MoO2 to Mo, studies on the mechanism and kinetics behavior of hydrogen reduction of MoO2 to Mo are still limited and sometimes contradictory. Therefore, more detailed researches should be done to understand the reduction mechanism of MoO2 to Mo. In the present work, the reduction of MoO2 was investigated under both isothermal and non-isothermal conditions using thermal gravimetric (TG) analysis. The influence of hydrogen content on reduction was also considered. Meanwhile, a new model describing the reduction of MoO2 with hydrogen was developed. 2. Materials and experimental procedure 2.1. Material Commercially available MoO2 powders from Jinduicheng Molybdenum Co., Ltd. were used for experiments, which were reduced from
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MoO3 powders with the highest reduction temperature of around 873 K. Fig. 1 shows the SEM image of raw MoO2, from which it can be seen that the powder is composed of many small platelet-shaped MoO2 grains and is fairly loose. The powder size distribution of MoO2 was measured by the laser interferometer (SEISHIN LMS-30), and the density function is shown in Fig. 2, in which the vertical coordinate of right upper black dot of every small zone represents the total volume fraction of the powders in this zone. The average powders size was found to be 14.377 μm. 2.2. Experimental procedure The weight change of MoO2 powders during reduction was monitored by using a HCT-2 thermal analysis system, which included a TG microbalance with a precision of ±0.1 μg. Fig. 3 shows the schematic diagram of the apparatus. In each experimental run the sample of about 40 mg in weight was used and filled into an alumina crucible. After the crucible with the sample was placed in the heating furnace, argon was introduced into the system to get air out of the furnace. In the isothermal experiment, the furnace was firstly heated up from room temperature to the desired reduction temperature with a heating rate of 20 K/min in argon atmosphere. When the thermal balance was stabilized, the argon gas was switched to pure hydrogen, and the weight decrease due to the reduction was then monitored continuously. After the experiment was finished, hydrogen was changed to argon again, and the sample was cooled to the room temperature. In the non-isothermal experiment, the reducing gas (pure hydrogen or its mixtures with Ar) was firstly introduced into the furnace to get the air out then the furnace was heated from room temperature (around 303 K) to 1273 K at a heating rate of 10, 15 or 20 K/min, respectively. In all experimental runs, a constant flow rate of 60 ml/min was kept during the reduction. This level was found to be sufficient for diminishing the diffusion resistance of gas in the gas-boundary layer. Hydrogen and argon used in the experiments were in high purity (b 5 ppm O2). The dew point of H2 used was in the range of −8.5 to −7.6 °C. The flow rate of gas was controlled by gas flow controllers (Alicant, Model MC-500SCCM-D). XRD (Model, TTRIII, Japan) measurement was carried out for sample. The morphologies of these samples were observed using SEM (Model S250MK3, CAMBRIDGE) technique. 3. Results The extent of reduction was calculated as a mass fraction of oxygen removed during reduction as shown in Eq. (1), R ¼ wt =w
ð1Þ
where wt is the lost mass of sample after time t and w is the theoretical total lost mass of the sample. If MoO2 powders were reduced completely, w is calculated to be 0.2501w0, where w0 is initial mass of MoO2 sample.
Fig. 2. Powder size distribution of MoO2.
3.1. Isothermal reduction Isothermal reduction of MoO2 powders by hydrogen was studied using pure H2 in the temperature range of 923 to 1126 K. The time dependences of the reduction extent at six temperatures between 923 and 1126 K are presented in Fig. 4, from which it is apparent that the reduction extent is close to a linear function of reduction time. 3.2. Non-isothermal reduction 3.2.1. Effect of heating rate Fig. 5 shows the reduction curves of MoO2 by pure hydrogen at 3 different heating rates, viz. 10, 15 and 20 K/min. It is shown in the figure that the reduction became apparent at around 880 K, and a higher heating rate led to a faster reaction. The maximum rate of the reduction appeared at 1005 K, 1036 K, and 1056 K at the heating rate of 10, 15 and 20 K/min (Fig. 5b), respectively. The slow reaction rates at low temperatures and high temperatures are due to the relatively small reduction rate constant and the exhaustion of sample, respectively. The highest reduction rate had occurred at the temperature when both reduction rate constant and oxide concentration were at the high levels. 3.2.2. Effect of hydrogen content Effect of hydrogen content on the non-isothermal reduction of MoO2 powders was studied by reaction of MoO2 powders with H2–Ar gas mixtures with hydrogen content varying from 70 to 100 vol.%. The reduction curves are presented in Fig. 6. It is clearly shown that the increase in hydrogen content causes a visible increase in both reduction extent and rate.
Fig. 1. Morphologies of MoO2 powders: (a) amplified by 5000 times; (b) amplified by 550 times.
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Fig. 3. Schematic diagram of the experimental apparatus for TG analysis. 1. HCT-2 thermo-gravimetric analyzer, 2. Alumina crucible, 3. Data collector, 4. Exhaust gases, 5. Gas flow controller.
3.3. SEM studies for morphological analysis Fig. 7 represents the SEM micrographs of samples completely reduced at different temperatures in pure hydrogen, which is significant for understanding the reduction mechanism. It can be seen that the product Mo reduced at different temperatures are all kept the same platelet shape as the initial MoO2. Fig. 7f shows that there are many pore channels at the surface of Mo grains and these cracks and craters result from stress by tension due to a volume decrease during oxygen removing process [7]. These results are close to the results of reduction of MoO2 by H2 at extremely low dew point in reference [7]. 4. Discussion 4.1. Kinetics analysis Due to the fact that the MoO2 powder is fairly loose, which is beneficial to hydrogen diffusion, and taking the results of SEM analyses in Fig. 7 into consideration, a mechanism model of the MoO2 reduction with hydrogen can be described as shown in Fig. 8. The reduction mechanism can be simply divided into the following steps. i) Hydrogen in the bulk gas phase transfers to the surface of MoO2 platelets. ii) Hydrogen diffuses through the gas boundary layer. iii) Hydrogen diffuses through the product layer to the reaction interface. iv) Physisorption of hydrogen molecules. v) Dissociation of hydrogen molecules. 1=2H2 →H
Fig. 5. Non-isothermal reduction of MoO2 powders with pure hydrogen: (a) reduction extent versus temperature; (b) DTG versus temperature.
vi) Chemisorption of H. vii) Chemical reaction in the MoO2/Mo interface. 2H þ 1=2MoO2 ⇄Mo þ H2 O
ð3Þ
ð2Þ
Fig. 4. Reduction curves of MoO2 powders with pure H2 at different temperatures.
Fig. 6. Non-isothermal reduction of MoO2 powders with different hydrogen contents.
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Fig. 7. Morphologies of samples completely reduced by pure hydrogen at different temperatures.
viii) H2O diffuses through Mo layer to the surface of the platelets. ix) H2O diffuses through the gas/particle boundary to the gas phase. In most cases, step (iii), (vii) and (viii) will likely be the rate controlling steps. However, it is clearly shown in Figs. 1 and 7 that MoO2 grains are platelet-shaped and the product layer has many cracks and craters; therefore, the diffusions of H2 and H2O through the product layer may not be the controlling step. Therefore, the chemical reaction at the MoO2/Mo interface is the most likely controlling step. Chou et al. [23–25] have developed a new model to describe the gas-solid reaction, and all the formulae are analytic with a form of explicit function, which are not only easy to use but also enable the easy performance of a theoretical analysis. According to this model, a formula describing the hydrogen reduction of MoO2 can be developed by assuming the chemical reaction at the MoO2/Mo interface to be the controlling step.
4.1.1. Reduction kinetics under isothermal condition In Fig. 9, x is the thickness of the product layer Mo and H0 is the thickness of the MoO2 platelet. The reduction extent ξ can be calculated in the light of the following equation ξ ¼ 2x=H 0
ð4Þ
Differentiating Eq. (4) with respect to time t, one obtains dξ 2 dx ¼ : dt H0 dt
ð5Þ
The concentration of hydrogen at the interface of MoO2/Mo, CH, is in equilibrium with hydrogen partial pressure, which can be expressed as CH ¼ K 1
qffiffiffiffiffiffiffiffiffi PH 2
ð6Þ
where K1 is the equilibrium constant of Eq. (2). For the reaction, the rates of reactions on both sides of the Eq. (3) can be expressed as f
f
b
b
2
vr ¼ K r C H ðMoO2 =MoÞ vr ¼ K r C H2 O ðMoO2 =MoÞ Fig. 8. Process for the reduction of the MoO2 platelet.
where Kfr and Krb are the reaction rate constants.
ð7Þ ð8Þ
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Substituting Eq. (16) into Eq. (15), the relation of the reduction extent with temperature will be expressed as ξ¼
4.2.1. Isothermal reduction Define
The overall reaction rate of Eq. (3) is b
f
2
BT ¼
b
vr ¼ v r −vr ¼ K r C H ðMoO2 =MoÞ−K r C H2 O ðMoO2 =MoÞ ¼
f b 2 K r K 1 P H2 ðMoO2 =MoÞ−K r P H2 O ðMoO2 =MoÞ
ð9Þ
Since the controlling step is assumed to be the interfacial chemical reaction, the resistances of diffusion in product layer and other steps are negligible, therefore P H2 ðMoO2 =MoÞ ¼ P H2
ð10Þ
P H2 O ðMoO2 =MoÞ ¼ P H2 O ≈0
ð11Þ
So, Eq. (9) becomes f
2
ð12Þ
The growth rate of x should be proportional to the overall reaction rate, vr, namely dx v ¼ r dt V m
ð13Þ
where Vm is a coefficient that depends on the substance and reaction. Combining Eqs. (5), (12) and (13) yields dξ ¼ dt
2K fr K 1 2 P H2 H0 V m
ð14Þ
Since the hydrogen pressure is constant and not a function of time, integrating the above equation with the initial condition of ξ = 0 when t = 0, one can obtain 2 2K fr K 21 P H2 2 K 0r K 0H P H2 2ΔH þ Δε t¼ exp − t ξ¼ H0 V m Vm H0 RT ΔEapp 2 K 0 P H2 t exp − ¼ RT H0 V m
ð15Þ
f Δε 0 0 0 02 where K 1 ¼ K H exp − ΔH RT , K r ¼ K r exp − RT , K0 = Kr KH , ΔEapp = 0 0 2ΔH + Δε; KH, Kr are constants independent of temperature; ΔEapp is the apparent activation energy. 4.1.2. Reduction kinetics under non-isothermal condition In fact, the temperature of MoO2 reduction is not always constant. The simple case of increasing temperature with a constant rate η is considered in the present study. The relation of temperature T with time t should be T ¼ T 0 þ ηt where T0 is the starting temperature of the reaction.
ð16Þ
H0 V m 2K 0 P H2
ð18Þ
where BT is a function of P H2 and H0. BT will be constant as the hydrogen partial pressure and the thickness of the platelet are fixed. Combining Eqs. (15) and (18), one obtains ξ¼
v r ¼ K r K 1 P H2
ð17Þ
4.2. Application of the new model
Fig. 9. Schematic diagram for the reduction of a MoO2 platelet by H2.
f
ΔEapp T−T 0 2 K 0 P H2 exp − RT η H0 V m
ΔEapp 1 t exp − RT BT
ð19Þ
Eq. (19) describes the influence of temperature on the reduction extent, with which the apparent activation energy can be calculated under the condition of chemical reaction being the controlling step. Furthermore, it can be obtained that the higher the temperature, the larger the reduction extent for the same reaction time. However, the increase rate will decrease as temperature increases. The isothermal reduction curves are fitted by Eq. (19) and the results are shown in Fig. 10, from which it can be seen that the model fits the experimental data fairly well. The extracted activation energy for the hydrogen reduction of MoO2 is 92.5 kJ/mol. So, the corresponding kinetics formula is given as Eq. (20). ξ¼
1 92526 exp − t 0:0002634 RT
ð20Þ
4.2.2. Non-isothermal reduction — effect of heating rate Substituting Eq. (18) into Eq. (17), the relation of the reduction extent with temperature under non-isothermal condition will be expressed as ξ¼
ΔEapp T−T 0 1 exp − RT η BT
ð21Þ
This is the formula which can describe the non-isothermal reduction of MoO2 platelet by hydrogen. Using this model to fit the curves of non-isothermal reduction of MoO2 with pure hydrogen, the fitted results are shown in Fig. 11 and the corresponding kinetics formulae are given as follows: ξ¼
1 90563 T−831 exp − 0:0004745 RT 10
ð22Þ
ξ¼
1 90563 T−849 exp − 0:0004745 RT 15
ð23Þ
ξ¼
1 90563 T−852 exp − 0:0004745 RT 20
ð24Þ
Eqs. (22), (23) and (24) represent the hydrogen reduction of MoO2 at a heating rate of 10, 15 and 20 K/min, respectively. It is found that the extracted activation energy at non-isothermal condition agrees well with that obtained in the isothermal condition (90.6 kJ/mol vs. 92.5 kJ/mol).
J. Dang et al. / Int. Journal of Refractory Metals and Hard Materials 41 (2013) 356–362
Fig. 10. Comparisons of experimental data with model calculated results for hydrogen reduction of MoO2 at different temperatures (r is the correlation coefficient).
In the “Introduction” section, it can be concluded that the mechanism and kinetics behavior of hydrogen reduction of MoO2 to Mo reported by different authors [1,14,21,22] are inconsistent, which may be resulted from different raw materials (MoO2) and experimental conditions. In the present study, MoO2 powder is composed of many small platelet-shaped MoO2 grains and is fairly loose, while MoO2 grains used in references are not platelet-shaped [1] or not reported [14,21]. Some researchers also used the cylindrical pellet for study [22]. On the other hand, the dew point of hydrogen [7] and the size of powders will also influence the reduction mechanism and kinetics behavior of MoO2. However, the activation energy obtained in this paper is in the range of 50.2 kJ/mol [1] to136 kJ/mol [14].
361
Fig. 12. Comparisons of experimental data with predicted result by the new model for non-isothermal reduction of MoO2 with different hydrogen contents.
Then,
ξ¼
P H2 BP
ΔEapp T−T 0 exp − RT η
ð27Þ
When PH2 = 1, BP = BT. Therefore, the curves of Fig. 6 can be calculated by Eq. (27) with the parameters of Eq. (22). The comparisons are shown in Fig. 12 and it can be seen that the calculated results agree well with the experimental data, validating the reasonability of the new model. 5. Conclusion
4.2.3. Non-isothermal reduction — effect of hydrogen content Define BP ¼
H0 V m 2K 0
ð25Þ
where BP is a function of H0. BP will be constant if the thickness of the platelet is fixed. Combining Eqs. (18) and (25), one obtains BP ¼ BT P H2
ð26Þ
In this paper, the reduction of MoO2 by hydrogen was studied under both isothermal condition and non-isothermal condition. The results showed that the products Mo reduced at different temperatures were all kept the same platelet shape as the initial MoO2. The reduction kinetics model has been proposed, which incorporates various factors such as time, temperature, partial hydrogen content, etc. Good agreements have been achieved between the experimental measured and theoretical calculated results. It was found that the hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface with the apparent active energy 90.6–92.5 kJ/mol. Acknowledgment The authors gratefully acknowledge support by Jinduicheng Molybdenum Co., Ltd. in raw material and financial support from the National Natural Science Foundation of China (No. 11220158). References
Fig. 11. Comparisons of experimental data with model calculated results for non-isothermal reduction of MoO2 by pure hydrogen.
[1] Kim BS, Kim E, Jeon HS, Lee HI, Lee JC. Study on the reduction of molybdenum dioxide by hydrogen. Mater Trans 2008;49:2147–52. [2] Manukyan K, Davtyan D, Bossert J, Kharatyan S. Direct reduction of ammonium molybdate to elemental molybdenum by combustion reaction. Chem Eng J 2011;168: 925–30. [3] Heidebrecht P, Sundmacher K. Thermodynamic analysis of a cyclic water gas-shift reactor (CWGSR) for hydrogen production. Chem Eng Sci 2009;64:5057–65. [4] Plascencia G, Utigard T. The reduction of Tokyo and Sinter 75 nickel oxides with hydrogen. Chem Eng Sci 2009;64:3879–88. [5] Janković B, Adnađević B, Mentus S. The kinetic study of temperature-programmed reduction of nickel oxide in hydrogen atmosphere. Chem Eng Sci 2008;63:567–75. [6] Martins GP, Kangsadan T, Scott G, Wagner C, Van Hoose J. A 21st century perspective on molybdenum powder production by hydrogen reduction. Mater Sci Forum 2007;561–565:447–52. [7] Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Metals Hard Mater 2002;20:261–9.
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[8] Arnoldy P, De Jonge J, Moulijn JA. Temperature-programmed reduction of molybdenum (VI) oxide and molybdenum (IV) oxide. J Phys Chem 1985;21:4517–26. [9] Enneti RK. Rate control mechanism for the hydrogen reduction of MoO3 to MoO2. Int J Refract Metals Hard Mater 2012;33:122–3. [10] Enneti RK, Wolfe TA. Agglomeration during reduction of MoO3. Int J Refract Metals Hard Mater 2012;31:47–50. [11] Lalik E. Kinetic analysis of reduction of MoO3 to MoO2. Catal Today 2011;169:85–92. [12] Lalik E, David WIF, Barnes P, Turner JFC. Mechanisms of reduction of MoO3 to MoO2 reconciled? J Phys Chem B 2001;105:9153–6. [13] Leisegang T, Levin AA, Walter J, Meyer DC. In situ X-ray analysis of MoO3 reduction. Cryst Res Technol 2005;40:95–105. [14] Majumdar S, Sharma IG, Samajdar I, Bhargava P. Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metall Mater Trans B 2008;39:431–8. [15] Ressler T, Jentoft RE, Wienold J, Gunter MM, Timpe O. In situ XAS and XRD studies on the formation of Mo suboxides during reduction of MoO3. J Phys Chem B 2000;104:6360–70. [16] Słoczynski J. Kinetics and mechanism of MoO3 reduction. Comments on: in situ XAS and XRD studies on the formation of Mo suboxides during reduction of MoO3. J Phys Chem B 2002;106:7718–20.
[17] Słoczyński J. Kinetics and mechanism of molybdenum (VI) oxide reduction. J Solid State Chem 1995;118:84–92. [18] Spevack PA, McIntyre NS. Thermal reduction of molybdenum trioxide. J Phys Chem 1992;96:9029–35. [19] Orehotsky J, Kaczenski M. The kinetics of the hydrogen reduction of MoO2 powder. Mater Sci Eng 1979;40:245–50. [20] Orehotsky J, Jamiolkowski L, Gerboc J. The hydrogen reduction of MoO3 powder to MoO2. Mater Sci Eng 1979;41:237–41. [21] von Destinon-Forstmann J. The hydrogen reduction kinetics of molybdenum oxides. Can Metall Q 1965;4:1–12. [22] Sichen D, Seetharaman S. Application of a nonisothermal thermogravimetric method to the kinetic study of the reduction of metallic oxides: part I. A general treatment and its application to the reduction of the oxides of molybdenum by hydrogen. Metall Mater Trans B 1992;23:317–24. [23] Chou KC. A kinetic model for oxidation of Si–Al–O–N materials. J Am Ceram Soc 2006;89:1568–76. [24] Hou XM, Chou KC. Model of oxidation of SiC microparticles at high temperature. Corros Sci 2008;50:2367–71. [25] Dang J, Chou KC, Hu XJ, Zhang GH. Reduction kinetics of metal oxides by hydrogen. Steel Res Int 2012, http://dx.doi.org/10.1002/srin.201200242.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Study on kinetics of hydrogen reduction of MoO2 Jie Dang, Guo-Hua Zhang ⁎, Kuo-Chih Chou State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China Department of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 9 April 2013 Accepted 19 May 2013 Keywords: Thermal gravimetric (TG) analysis Isothermal reduction Non-isothermal reduction Model Hydrogen Kinetics
a b s t r a c t The reduction mechanisms of MoO2 under both isothermal and non-isothermal conditions have been investigated in this work. The results showed that the product Mo was kept the same platelet shape as the initial MoO2. The reduction kinetics of MoO2 was analyzed by using a new model, which was in an explicit form and incorporated different variables, such as time, temperature, hydrogen content, etc. It was found that the model predicted curves agreed well with the experimental data. The results indicated that hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface, and the corresponding activation energy was 90.6–92.5 kJ/mol. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Metallic molybdenum, having a body-centered cubic crystal structure with a melting point of 2610 °C and a density of 10.22 g/cm3, offers excellent mechanical, thermal and electric properties and good corrosion resistance, and is widely used as an alloying agent for manufacturing steels, cast irons, and super-alloys to increase their mechanical strength, hardness, swiftness and resistance to corrosion and wearing [1,2]. Hydrogen plays an important role in today's chemical industry [3], and can be used as a reducing gas in reduction of metal oxides [4,5]. Reduction of MoO3 powders by hydrogen is one of the methods used to obtain metallic molybdenum of high purity. In general, the two-stage flow scheme is employed [6]. The first step is to reduce MoO3 powders to MoO2 powders and the second step is to produce molybdenum powders from MoO2 powders [7]. Many excellent papers [8–20] of studying on reduction of MoO3 to MoO2 with hydrogen have been published. However, only several studies [1,7,14,19,21,22] have been focused on the reduction of MoO2 to Mo. Orehotsky and Kaczenski [19] reported a linear dependence of weight loss with reduction time for the reduction of MoO2 to Mo in a static bed during the initial period when the interface (MoO2/Mo) is within 1 cm of the top of the bed. Von Destinon–Forstmann [21] studied the reduction of MoO2 in the temperature range of 873 K to 1123 K, and found that the reduction of the dioxide to the metal followed a linear rate equation and had an activation energy of 114.7 kJ/mol. Du and
⁎ Corresponding author at: Department of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 1062333703. E-mail address: [email protected] (G.-H. Zhang). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.05.009
Seetharaman [22] studied the isothermal and non-isothermal reduction of MoO3 to Mo and reported that the reduction rate of MoO2 to Mo appeared to be influenced by gas diffusion through voids, and the activation energy under isothermal reduction condition was 85.2 kJ/mol. Majumdar et al. [14] reported that the kinetic equation for the MoO2–Mo reaction between 898 K and 1173 K obeyed Johnson–Mehl–Avrami– Kolmogorov (JMAK) type behavior, and the activation energy was calculated as 136 kJ/mol. Kim et al. [1] found that the reduction of MoO2 powder by hydrogen obeyed nucleation and growth model with the activation energy of 50.2–65.9 kJ/mol. Schulmeyer and Ortner [7] studied the reduction of MoO2 under two extreme local dew points and found that two different reaction paths occurred: pseudomorphic transformation at low dew points and transformation via CVT at high dew points. Though these investigators investigated the stage of hydrogen reduction of MoO2 to Mo, studies on the mechanism and kinetics behavior of hydrogen reduction of MoO2 to Mo are still limited and sometimes contradictory. Therefore, more detailed researches should be done to understand the reduction mechanism of MoO2 to Mo. In the present work, the reduction of MoO2 was investigated under both isothermal and non-isothermal conditions using thermal gravimetric (TG) analysis. The influence of hydrogen content on reduction was also considered. Meanwhile, a new model describing the reduction of MoO2 with hydrogen was developed. 2. Materials and experimental procedure 2.1. Material Commercially available MoO2 powders from Jinduicheng Molybdenum Co., Ltd. were used for experiments, which were reduced from
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MoO3 powders with the highest reduction temperature of around 873 K. Fig. 1 shows the SEM image of raw MoO2, from which it can be seen that the powder is composed of many small platelet-shaped MoO2 grains and is fairly loose. The powder size distribution of MoO2 was measured by the laser interferometer (SEISHIN LMS-30), and the density function is shown in Fig. 2, in which the vertical coordinate of right upper black dot of every small zone represents the total volume fraction of the powders in this zone. The average powders size was found to be 14.377 μm. 2.2. Experimental procedure The weight change of MoO2 powders during reduction was monitored by using a HCT-2 thermal analysis system, which included a TG microbalance with a precision of ±0.1 μg. Fig. 3 shows the schematic diagram of the apparatus. In each experimental run the sample of about 40 mg in weight was used and filled into an alumina crucible. After the crucible with the sample was placed in the heating furnace, argon was introduced into the system to get air out of the furnace. In the isothermal experiment, the furnace was firstly heated up from room temperature to the desired reduction temperature with a heating rate of 20 K/min in argon atmosphere. When the thermal balance was stabilized, the argon gas was switched to pure hydrogen, and the weight decrease due to the reduction was then monitored continuously. After the experiment was finished, hydrogen was changed to argon again, and the sample was cooled to the room temperature. In the non-isothermal experiment, the reducing gas (pure hydrogen or its mixtures with Ar) was firstly introduced into the furnace to get the air out then the furnace was heated from room temperature (around 303 K) to 1273 K at a heating rate of 10, 15 or 20 K/min, respectively. In all experimental runs, a constant flow rate of 60 ml/min was kept during the reduction. This level was found to be sufficient for diminishing the diffusion resistance of gas in the gas-boundary layer. Hydrogen and argon used in the experiments were in high purity (b 5 ppm O2). The dew point of H2 used was in the range of −8.5 to −7.6 °C. The flow rate of gas was controlled by gas flow controllers (Alicant, Model MC-500SCCM-D). XRD (Model, TTRIII, Japan) measurement was carried out for sample. The morphologies of these samples were observed using SEM (Model S250MK3, CAMBRIDGE) technique. 3. Results The extent of reduction was calculated as a mass fraction of oxygen removed during reduction as shown in Eq. (1), R ¼ wt =w
ð1Þ
where wt is the lost mass of sample after time t and w is the theoretical total lost mass of the sample. If MoO2 powders were reduced completely, w is calculated to be 0.2501w0, where w0 is initial mass of MoO2 sample.
Fig. 2. Powder size distribution of MoO2.
3.1. Isothermal reduction Isothermal reduction of MoO2 powders by hydrogen was studied using pure H2 in the temperature range of 923 to 1126 K. The time dependences of the reduction extent at six temperatures between 923 and 1126 K are presented in Fig. 4, from which it is apparent that the reduction extent is close to a linear function of reduction time. 3.2. Non-isothermal reduction 3.2.1. Effect of heating rate Fig. 5 shows the reduction curves of MoO2 by pure hydrogen at 3 different heating rates, viz. 10, 15 and 20 K/min. It is shown in the figure that the reduction became apparent at around 880 K, and a higher heating rate led to a faster reaction. The maximum rate of the reduction appeared at 1005 K, 1036 K, and 1056 K at the heating rate of 10, 15 and 20 K/min (Fig. 5b), respectively. The slow reaction rates at low temperatures and high temperatures are due to the relatively small reduction rate constant and the exhaustion of sample, respectively. The highest reduction rate had occurred at the temperature when both reduction rate constant and oxide concentration were at the high levels. 3.2.2. Effect of hydrogen content Effect of hydrogen content on the non-isothermal reduction of MoO2 powders was studied by reaction of MoO2 powders with H2–Ar gas mixtures with hydrogen content varying from 70 to 100 vol.%. The reduction curves are presented in Fig. 6. It is clearly shown that the increase in hydrogen content causes a visible increase in both reduction extent and rate.
Fig. 1. Morphologies of MoO2 powders: (a) amplified by 5000 times; (b) amplified by 550 times.
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Fig. 3. Schematic diagram of the experimental apparatus for TG analysis. 1. HCT-2 thermo-gravimetric analyzer, 2. Alumina crucible, 3. Data collector, 4. Exhaust gases, 5. Gas flow controller.
3.3. SEM studies for morphological analysis Fig. 7 represents the SEM micrographs of samples completely reduced at different temperatures in pure hydrogen, which is significant for understanding the reduction mechanism. It can be seen that the product Mo reduced at different temperatures are all kept the same platelet shape as the initial MoO2. Fig. 7f shows that there are many pore channels at the surface of Mo grains and these cracks and craters result from stress by tension due to a volume decrease during oxygen removing process [7]. These results are close to the results of reduction of MoO2 by H2 at extremely low dew point in reference [7]. 4. Discussion 4.1. Kinetics analysis Due to the fact that the MoO2 powder is fairly loose, which is beneficial to hydrogen diffusion, and taking the results of SEM analyses in Fig. 7 into consideration, a mechanism model of the MoO2 reduction with hydrogen can be described as shown in Fig. 8. The reduction mechanism can be simply divided into the following steps. i) Hydrogen in the bulk gas phase transfers to the surface of MoO2 platelets. ii) Hydrogen diffuses through the gas boundary layer. iii) Hydrogen diffuses through the product layer to the reaction interface. iv) Physisorption of hydrogen molecules. v) Dissociation of hydrogen molecules. 1=2H2 →H
Fig. 5. Non-isothermal reduction of MoO2 powders with pure hydrogen: (a) reduction extent versus temperature; (b) DTG versus temperature.
vi) Chemisorption of H. vii) Chemical reaction in the MoO2/Mo interface. 2H þ 1=2MoO2 ⇄Mo þ H2 O
ð3Þ
ð2Þ
Fig. 4. Reduction curves of MoO2 powders with pure H2 at different temperatures.
Fig. 6. Non-isothermal reduction of MoO2 powders with different hydrogen contents.
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Fig. 7. Morphologies of samples completely reduced by pure hydrogen at different temperatures.
viii) H2O diffuses through Mo layer to the surface of the platelets. ix) H2O diffuses through the gas/particle boundary to the gas phase. In most cases, step (iii), (vii) and (viii) will likely be the rate controlling steps. However, it is clearly shown in Figs. 1 and 7 that MoO2 grains are platelet-shaped and the product layer has many cracks and craters; therefore, the diffusions of H2 and H2O through the product layer may not be the controlling step. Therefore, the chemical reaction at the MoO2/Mo interface is the most likely controlling step. Chou et al. [23–25] have developed a new model to describe the gas-solid reaction, and all the formulae are analytic with a form of explicit function, which are not only easy to use but also enable the easy performance of a theoretical analysis. According to this model, a formula describing the hydrogen reduction of MoO2 can be developed by assuming the chemical reaction at the MoO2/Mo interface to be the controlling step.
4.1.1. Reduction kinetics under isothermal condition In Fig. 9, x is the thickness of the product layer Mo and H0 is the thickness of the MoO2 platelet. The reduction extent ξ can be calculated in the light of the following equation ξ ¼ 2x=H 0
ð4Þ
Differentiating Eq. (4) with respect to time t, one obtains dξ 2 dx ¼ : dt H0 dt
ð5Þ
The concentration of hydrogen at the interface of MoO2/Mo, CH, is in equilibrium with hydrogen partial pressure, which can be expressed as CH ¼ K 1
qffiffiffiffiffiffiffiffiffi PH 2
ð6Þ
where K1 is the equilibrium constant of Eq. (2). For the reaction, the rates of reactions on both sides of the Eq. (3) can be expressed as f
f
b
b
2
vr ¼ K r C H ðMoO2 =MoÞ vr ¼ K r C H2 O ðMoO2 =MoÞ Fig. 8. Process for the reduction of the MoO2 platelet.
where Kfr and Krb are the reaction rate constants.
ð7Þ ð8Þ
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Substituting Eq. (16) into Eq. (15), the relation of the reduction extent with temperature will be expressed as ξ¼
4.2.1. Isothermal reduction Define
The overall reaction rate of Eq. (3) is b
f
2
BT ¼
b
vr ¼ v r −vr ¼ K r C H ðMoO2 =MoÞ−K r C H2 O ðMoO2 =MoÞ ¼
f b 2 K r K 1 P H2 ðMoO2 =MoÞ−K r P H2 O ðMoO2 =MoÞ
ð9Þ
Since the controlling step is assumed to be the interfacial chemical reaction, the resistances of diffusion in product layer and other steps are negligible, therefore P H2 ðMoO2 =MoÞ ¼ P H2
ð10Þ
P H2 O ðMoO2 =MoÞ ¼ P H2 O ≈0
ð11Þ
So, Eq. (9) becomes f
2
ð12Þ
The growth rate of x should be proportional to the overall reaction rate, vr, namely dx v ¼ r dt V m
ð13Þ
where Vm is a coefficient that depends on the substance and reaction. Combining Eqs. (5), (12) and (13) yields dξ ¼ dt
2K fr K 1 2 P H2 H0 V m
ð14Þ
Since the hydrogen pressure is constant and not a function of time, integrating the above equation with the initial condition of ξ = 0 when t = 0, one can obtain 2 2K fr K 21 P H2 2 K 0r K 0H P H2 2ΔH þ Δε t¼ exp − t ξ¼ H0 V m Vm H0 RT ΔEapp 2 K 0 P H2 t exp − ¼ RT H0 V m
ð15Þ
f Δε 0 0 0 02 where K 1 ¼ K H exp − ΔH RT , K r ¼ K r exp − RT , K0 = Kr KH , ΔEapp = 0 0 2ΔH + Δε; KH, Kr are constants independent of temperature; ΔEapp is the apparent activation energy. 4.1.2. Reduction kinetics under non-isothermal condition In fact, the temperature of MoO2 reduction is not always constant. The simple case of increasing temperature with a constant rate η is considered in the present study. The relation of temperature T with time t should be T ¼ T 0 þ ηt where T0 is the starting temperature of the reaction.
ð16Þ
H0 V m 2K 0 P H2
ð18Þ
where BT is a function of P H2 and H0. BT will be constant as the hydrogen partial pressure and the thickness of the platelet are fixed. Combining Eqs. (15) and (18), one obtains ξ¼
v r ¼ K r K 1 P H2
ð17Þ
4.2. Application of the new model
Fig. 9. Schematic diagram for the reduction of a MoO2 platelet by H2.
f
ΔEapp T−T 0 2 K 0 P H2 exp − RT η H0 V m
ΔEapp 1 t exp − RT BT
ð19Þ
Eq. (19) describes the influence of temperature on the reduction extent, with which the apparent activation energy can be calculated under the condition of chemical reaction being the controlling step. Furthermore, it can be obtained that the higher the temperature, the larger the reduction extent for the same reaction time. However, the increase rate will decrease as temperature increases. The isothermal reduction curves are fitted by Eq. (19) and the results are shown in Fig. 10, from which it can be seen that the model fits the experimental data fairly well. The extracted activation energy for the hydrogen reduction of MoO2 is 92.5 kJ/mol. So, the corresponding kinetics formula is given as Eq. (20). ξ¼
1 92526 exp − t 0:0002634 RT
ð20Þ
4.2.2. Non-isothermal reduction — effect of heating rate Substituting Eq. (18) into Eq. (17), the relation of the reduction extent with temperature under non-isothermal condition will be expressed as ξ¼
ΔEapp T−T 0 1 exp − RT η BT
ð21Þ
This is the formula which can describe the non-isothermal reduction of MoO2 platelet by hydrogen. Using this model to fit the curves of non-isothermal reduction of MoO2 with pure hydrogen, the fitted results are shown in Fig. 11 and the corresponding kinetics formulae are given as follows: ξ¼
1 90563 T−831 exp − 0:0004745 RT 10
ð22Þ
ξ¼
1 90563 T−849 exp − 0:0004745 RT 15
ð23Þ
ξ¼
1 90563 T−852 exp − 0:0004745 RT 20
ð24Þ
Eqs. (22), (23) and (24) represent the hydrogen reduction of MoO2 at a heating rate of 10, 15 and 20 K/min, respectively. It is found that the extracted activation energy at non-isothermal condition agrees well with that obtained in the isothermal condition (90.6 kJ/mol vs. 92.5 kJ/mol).
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Fig. 10. Comparisons of experimental data with model calculated results for hydrogen reduction of MoO2 at different temperatures (r is the correlation coefficient).
In the “Introduction” section, it can be concluded that the mechanism and kinetics behavior of hydrogen reduction of MoO2 to Mo reported by different authors [1,14,21,22] are inconsistent, which may be resulted from different raw materials (MoO2) and experimental conditions. In the present study, MoO2 powder is composed of many small platelet-shaped MoO2 grains and is fairly loose, while MoO2 grains used in references are not platelet-shaped [1] or not reported [14,21]. Some researchers also used the cylindrical pellet for study [22]. On the other hand, the dew point of hydrogen [7] and the size of powders will also influence the reduction mechanism and kinetics behavior of MoO2. However, the activation energy obtained in this paper is in the range of 50.2 kJ/mol [1] to136 kJ/mol [14].
361
Fig. 12. Comparisons of experimental data with predicted result by the new model for non-isothermal reduction of MoO2 with different hydrogen contents.
Then,
ξ¼
P H2 BP
ΔEapp T−T 0 exp − RT η
ð27Þ
When PH2 = 1, BP = BT. Therefore, the curves of Fig. 6 can be calculated by Eq. (27) with the parameters of Eq. (22). The comparisons are shown in Fig. 12 and it can be seen that the calculated results agree well with the experimental data, validating the reasonability of the new model. 5. Conclusion
4.2.3. Non-isothermal reduction — effect of hydrogen content Define BP ¼
H0 V m 2K 0
ð25Þ
where BP is a function of H0. BP will be constant if the thickness of the platelet is fixed. Combining Eqs. (18) and (25), one obtains BP ¼ BT P H2
ð26Þ
In this paper, the reduction of MoO2 by hydrogen was studied under both isothermal condition and non-isothermal condition. The results showed that the products Mo reduced at different temperatures were all kept the same platelet shape as the initial MoO2. The reduction kinetics model has been proposed, which incorporates various factors such as time, temperature, partial hydrogen content, etc. Good agreements have been achieved between the experimental measured and theoretical calculated results. It was found that the hydrogen reduction of MoO2 was controlled by the chemical reaction at the reaction interface with the apparent active energy 90.6–92.5 kJ/mol. Acknowledgment The authors gratefully acknowledge support by Jinduicheng Molybdenum Co., Ltd. in raw material and financial support from the National Natural Science Foundation of China (No. 11220158). References
Fig. 11. Comparisons of experimental data with model calculated results for non-isothermal reduction of MoO2 by pure hydrogen.
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