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Rate control mechanism for the hydrogen reduction of MoO3 to MoO2
Int. Journal of Refractory Metals and Hard Materials 33 (2012) 122–123
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
Rate control mechanism for the hydrogen reduction of MoO3 to MoO2 Ravi K. Enneti Research and Development, Global Tungsten and Powders Corp, Towanda, PA 18848, United States
a r t i c l e
i n f o
Article history: Received 15 February 2012 Accepted 12 March 2012
Schulmeyer et al. carried out experiments to understand the mechanisms involved in the reduction of MoO3 to Mo [1]. The findings of the study indicated that reduction of the oxides takes place by chemical vapor transport (CVT) and reactions follow the shrinking core or cracking core model (CCM). The research study showed that the reduction of MoO3 to Mo occurs in two stages. The first stage occurs at lower temperatures (450–650 °C) and involves reduction of MoO3 to MoO2. The MoO2 is further reduced to Mo powder during the second stage at 1000–1100 °C. The study also provided data regarding the progress of isothermal reduction of MoO3 at 550 °C. The data reported in the study is shown in Table 1 [1]. In the present report, this data for reduction of MoO3 to MoO2 (first stage) was further analyzed to identify the rate control mechanism. The rate control mechanism for the reduction of MoO3 to MoO2 is estimated based on shrinking core model (SCM). Fig. 1 shows the schematic of progress in reduction of MoO3 based on SCM. The rate controlling step for reduction is either the diffusion of reducing gas (through the outer gas film or through product formed on the outer surface of the oxide) or the actual chemical reaction between reducing gas and metal oxide. For SCM, the time required for completion of reaction based on different control mechanisms are listed below [2]: Diffusion of reactant gas through outer gas film :
t ¼X τ
Diffusion of reactant gas through outer product :
t τ
2=3
¼ 1−3ð1−X Þ
þ 2ð1−X Þ
Chemical reaction :
1 t = ¼ 1−ð1−X Þ 3 τ
ð1Þ
ð2Þ ð3Þ
where t is the time of reduction, τ is the total time for reduction; X is the fraction of reactant product. The left hand side (LHS) of the
E-mail address: [email protected]. 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.03.006
equation will be equal to right hand side (RHS) for the appropriate rate controlling mechanism of the reduction. The values for the estimates based on Eqs. (1), (2) and (3) for the data reported by Schulmeyer et al. are summarized in Table 2. In the analysis, the time for complete reduction of MoO3 was considered as 98 min. The results in Table 2 clearly show that the LHS and RHS values for Eq. (1) are nearly identical suggesting the gas transport or diffusion of the reduction gas through the outer gas film is the rate control mechanism for reduction of MoO3. The plot showing the experimental
Table 1 Experimental data obtained from the research study carried out by Schulmeyer et al. [1]. The data is for isothermal reduction of MoO3 at 550 °C in hydrogen atmosphere (dew point of hydrogen is 10 °C). Reduction time
MoO3 (wt.%)
Mo4O11 (wt.%)
MoO2 (wt.%)
Mo (wt.%)
5 min 30 min 51 min 72 min 98 min 24 h
85 25 – – – –
7 42 50 19 1
8 33 50 81 99 99
– – – – – 1
Gas film Surface of shrinking un reacted core
Surface of the particle
Product (Ash) layer Un reduced MoO3 particle
Fig. 1. Schematic showing progress of MoO3 to MoO2 reduction based on SCM.
Table 2 Data for estimation of rate controlling mechanism during reduction of MoO3. 1
Reduction time MoO2 (wt.%)
t τ
X
1−3ð1−X Þ2=3 þ2ð1−X Þ 1−ð1−X Þ
5 min 30 min 51 min 72 min 98 min
0.05 0.31 0.52 0.73 1.00
0.08 0.33 0.5 0.81 0.99
0.00 0.05 0.11 0.39 0.88
8 33 50 81 99
0.03 0.12 0.20 0.42 0.78
=3
R.K. Enneti / Int. Journal of Refractory Metals and Hard Materials 33 (2012) 122–123
data and progress of reduction for three rate controlling mechanisms with relative reaction time is shown in Fig. 2. The experimental data follows the trend for reduction mechanism controlled by diffusion of reactant gas through outer gas film. The finding is in very good agreement with general understanding of the reduction mechanism of MoO3. It is widely accepted that the reduction of MoO3 is controlled by chemical vapor transport and progresses from outside to the inside of the MoO3 particles. It is concluded for the analysis that the reduction of MoO3 follows by a shrinking core model with the reduction gas transport being the rate limiting mechanism.
1.2 Diffusion of reactant gas through outer gas film Diffusion of reactant gas through outer product
Relative reaction time
1
123
Chemical reaction
0.8
0.6
0.4
References 0.2
0
0
0.2
0.4
0.6
0.8
1
Amount of reduction Fig. 2. Plot showing the experimental data and progress of reduction reaction with relative reaction time for the three rate controlling mechanisms.
[1] Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Metals Hard Mater 2002;20:261–9. [2] Gavhane KA. Chemical reaction engineering — II. Arihant Printers; 2009. p. 4-4–4-52. 5.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Short communication
Rate control mechanism for the hydrogen reduction of MoO3 to MoO2 Ravi K. Enneti Research and Development, Global Tungsten and Powders Corp, Towanda, PA 18848, United States
a r t i c l e
i n f o
Article history: Received 15 February 2012 Accepted 12 March 2012
Schulmeyer et al. carried out experiments to understand the mechanisms involved in the reduction of MoO3 to Mo [1]. The findings of the study indicated that reduction of the oxides takes place by chemical vapor transport (CVT) and reactions follow the shrinking core or cracking core model (CCM). The research study showed that the reduction of MoO3 to Mo occurs in two stages. The first stage occurs at lower temperatures (450–650 °C) and involves reduction of MoO3 to MoO2. The MoO2 is further reduced to Mo powder during the second stage at 1000–1100 °C. The study also provided data regarding the progress of isothermal reduction of MoO3 at 550 °C. The data reported in the study is shown in Table 1 [1]. In the present report, this data for reduction of MoO3 to MoO2 (first stage) was further analyzed to identify the rate control mechanism. The rate control mechanism for the reduction of MoO3 to MoO2 is estimated based on shrinking core model (SCM). Fig. 1 shows the schematic of progress in reduction of MoO3 based on SCM. The rate controlling step for reduction is either the diffusion of reducing gas (through the outer gas film or through product formed on the outer surface of the oxide) or the actual chemical reaction between reducing gas and metal oxide. For SCM, the time required for completion of reaction based on different control mechanisms are listed below [2]: Diffusion of reactant gas through outer gas film :
t ¼X τ
Diffusion of reactant gas through outer product :
t τ
2=3
¼ 1−3ð1−X Þ
þ 2ð1−X Þ
Chemical reaction :
1 t = ¼ 1−ð1−X Þ 3 τ
ð1Þ
ð2Þ ð3Þ
where t is the time of reduction, τ is the total time for reduction; X is the fraction of reactant product. The left hand side (LHS) of the
E-mail address: [email protected]. 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.03.006
equation will be equal to right hand side (RHS) for the appropriate rate controlling mechanism of the reduction. The values for the estimates based on Eqs. (1), (2) and (3) for the data reported by Schulmeyer et al. are summarized in Table 2. In the analysis, the time for complete reduction of MoO3 was considered as 98 min. The results in Table 2 clearly show that the LHS and RHS values for Eq. (1) are nearly identical suggesting the gas transport or diffusion of the reduction gas through the outer gas film is the rate control mechanism for reduction of MoO3. The plot showing the experimental
Table 1 Experimental data obtained from the research study carried out by Schulmeyer et al. [1]. The data is for isothermal reduction of MoO3 at 550 °C in hydrogen atmosphere (dew point of hydrogen is 10 °C). Reduction time
MoO3 (wt.%)
Mo4O11 (wt.%)
MoO2 (wt.%)
Mo (wt.%)
5 min 30 min 51 min 72 min 98 min 24 h
85 25 – – – –
7 42 50 19 1
8 33 50 81 99 99
– – – – – 1
Gas film Surface of shrinking un reacted core
Surface of the particle
Product (Ash) layer Un reduced MoO3 particle
Fig. 1. Schematic showing progress of MoO3 to MoO2 reduction based on SCM.
Table 2 Data for estimation of rate controlling mechanism during reduction of MoO3. 1
Reduction time MoO2 (wt.%)
t τ
X
1−3ð1−X Þ2=3 þ2ð1−X Þ 1−ð1−X Þ
5 min 30 min 51 min 72 min 98 min
0.05 0.31 0.52 0.73 1.00
0.08 0.33 0.5 0.81 0.99
0.00 0.05 0.11 0.39 0.88
8 33 50 81 99
0.03 0.12 0.20 0.42 0.78
=3
R.K. Enneti / Int. Journal of Refractory Metals and Hard Materials 33 (2012) 122–123
data and progress of reduction for three rate controlling mechanisms with relative reaction time is shown in Fig. 2. The experimental data follows the trend for reduction mechanism controlled by diffusion of reactant gas through outer gas film. The finding is in very good agreement with general understanding of the reduction mechanism of MoO3. It is widely accepted that the reduction of MoO3 is controlled by chemical vapor transport and progresses from outside to the inside of the MoO3 particles. It is concluded for the analysis that the reduction of MoO3 follows by a shrinking core model with the reduction gas transport being the rate limiting mechanism.
1.2 Diffusion of reactant gas through outer gas film Diffusion of reactant gas through outer product
Relative reaction time
1
123
Chemical reaction
0.8
0.6
0.4
References 0.2
0
0
0.2
0.4
0.6
0.8
1
Amount of reduction Fig. 2. Plot showing the experimental data and progress of reduction reaction with relative reaction time for the three rate controlling mechanisms.
[1] Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Metals Hard Mater 2002;20:261–9. [2] Gavhane KA. Chemical reaction engineering — II. Arihant Printers; 2009. p. 4-4–4-52. 5.