CO–15%CO2混合氣體還原碳化MoO2制備Mo2C的動力學機理分析

王璐; 李紅肖; 闕標華; 薛正良

doi:10.13374/j.issn2095-9389.2021.12.27.006

CO–15%CO₂混合氣體還原碳化MoO₂制備Mo₂C的動力學機理分析

doi: 10.13374/j.issn2095-9389.2021.12.27.006

王璐^{1, 2, ,},
李紅肖³,
闕標華¹,
薛正良³

1.
武漢科技大學鋼鐵冶金及資源利用省部共建教育部重點實驗室，武漢 430081
2.
佛山（華南）新材料研究院，佛山 528200
3.
武漢科技大學省部共建耐火材料與冶金國家重點實驗室，武漢 430081

基金項目: 國家自然科學基金資助項目（52104310）; 廣東省基礎與應用基礎研究基金資助項目（2019A1515110361）

詳細信息

通訊作者:
E-mail: wanglu@wust.edu.cn

中圖分類號: TF841.2
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- 被引次數: 0
出版歷程
- 收稿日期: 2021-12-27
- 網絡出版日期: 2022-03-22
- 刊出日期: 2023-04-01

Kinetics and mechanism of the reduction–carburization processes of MoO₂ to Mo₂C with CO–15% CO₂ mixed gases

1.
Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China
2.
Foshan (South China) Institute for New Materials, Foshan 528200, China
3.
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China

More Information

Corresponding author: E-mail: wanglu@wust.edu.cn

摘要

摘要: 對CO–15%CO₂混合氣體還原碳化MoO₂制備Mo₂C的反應機理及其動力學展開研究并采用熱力學軟件FactSage 7.3、場發射掃描電子顯微鏡 (FE-SEM)、X射線衍射(XRD)、熱重(TG)、比表面積測試(BET)和模型擬合等手段和方法對實驗數據進行分析。結果表明：變溫實驗中，升溫速率越快，MoO₂的開始反應溫度和完全還原溫度越高；恒溫實驗中，溫度越高，MoO₂的還原碳化速率越快；反應前后物相組成表明MoO₂是經一步反應直接生成Mo₂C，沒有中間產物金屬Mo的生成，并且還發現所得Mo₂C基本與MoO₂具有一致的片狀形貌，但是由于氣體的進入與逸出、產物摩爾體積的縮小以及沉積碳的減少，Mo₂C顆粒表面會產生微孔和裂紋導致比表面積增長近20倍；動力學分析結果表明該還原碳化過程由形核長大與界面化學反應共同控制，其中形核長大過程占比68.9%，表觀活化能為80.651 kJ·mol^–1；界面化學反應占比31.1%，表觀活化能為121.002 kJ·mol^–1。
- CO–15%CO₂混合氣體 /
- 二氧化鉬 /
- 碳化鉬 /
- 反應機理 /
- 動力學
Abstract: Molybdenum carbide (Mo₂C), as an alternative to platinum group metals, has been widely used in the hydrocarbon and hydrogen evolution reactions due to its excellent catalytic performance. The exploitation of its preparation method with high efficiency and low cost, therefore, received increasing attention in recent decades. In the current work, the preparation method of Mo₂C by reducing MoO₂ with CO–15%CO₂ mixed gases was proposed, in which the main focus was laid in the reaction kinetics and reduction mechanism studies of the reduction-carburization processes. To determine the isothermal reaction temperature, the nonisothermal reactions of MoO₂ in CO–15%CO₂ mixed gases under different heating rates (2, 5, 10, and 15 K·min^–1) were conducted first. After that, the isothermal reactions in the temperature range from 993 to 1153 K were carried out. Different analytical technologies, such as the thermodynamic calculation, Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Thermogravimetric (TG), Brunauer-Emmett-Teller (BET), and model fitting methods were adopted to analyze the experimental data. The results revealed that both the beginning (953 to 997, 1015, and 1031 K) and ending reaction temperatures (1100 to 1201, 1318, and 1383 K) were gradually increased with the increase of the heating rate (2 to 5, 10, and 15 K·min^–1); besides, the reaction rate increased with increasing the temperature was also obtained. Phase transformation process of MoO₂ to Mo₂C was found to proceed by a one-step reaction (MoO₂→Mo₂C) without the formation of intermediate product Mo. The study also discovered that both Mo₂C and MoO₂ maintained the similar platelet-shaped morphology during the reaction process, but partial micro-pores and cracks were formed on the product surface because of the entry of reaction gases and escape of the product gases as well as the shrinking of the molar volume, increasing the specific surface area of the as-obtained Mo₂C by nearly 20 times when compared to that of the raw material. Kinetics analysis revealed that the reduction-carburization process of MoO₂ to Mo₂C were not controlled by a one-step reaction mechanism but by the co-action of nucleation growth and interfacial chemical reactions. It was also discovered that the nucleation growth accounted for 68.9% and the chemical reaction accounted for 31.1%, with the extracted activation energies of 80.651 and 121.002 kJ·mol^–1, respectively. The work would make a better understanding of the reaction processes of MoO₂ to Mo₂C in CO–15%CO₂ mixed gases.
- CO–15%CO₂ mixed gases /
- MoO₂ /
- Mo₂C /
- reaction mechanism /
- kinetics

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圖 1 MoO₂粉末.(a) XRD圖；(b) FE-SEM形貌圖

Figure 1. MoO₂ powder: (a) XRD pattern; (b) FE-SEM morphology

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圖 2 CO–15%CO₂混合氣體下程序升溫還原MoO₂的變溫動力學曲線

Figure 2. Nonisothermal kinetic curves for the temperature-programmed reduction of MoO₂ by CO–15%CO₂ mixed gases

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圖 3 CO–15%CO₂混合氣體下恒溫還原MoO₂的動力學曲線

Figure 3. Isothermal kinetic curves for the reduction of MoO₂ by CO–15%CO₂ mixed gases

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圖 4 不同升溫速率下CO–15%CO₂混合氣體中程序升溫還原MoO₂所得最終產物XRD結果

Figure 4. XRD patterns of the samples obtained by the temperature-programmed reduction of MoO₂ by CO–15%CO₂ mixed gases at different heating rates

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圖 5 CO–15%CO₂混合氣體恒溫還原MoO₂所得產物結果圖. (a) 物相分析; (b) 晶粒尺寸

Figure 5. Results of the product obtained by reducing MoO₂ with CO–15%CO₂ mixed gases at different temperatures: (a) XRD pattern; (b) grain size

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圖 6 (a)CO–15%CO₂混合氣體在993 K下還原MoO₂的失重曲線；(b)不同反應進度下對應產物的XRD結果

Figure 6. (a) kinetic curves of the reduction of MoO₂ by CO–15%CO₂ mixed gases at 993 K; (b) XRD patterns of the samples obtained at different reaction extents

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圖 7 CO–15%CO₂混合氣體在不同升溫速率下還原MoO₂所得產物形貌. (a) 2 K·min^–1; (b) 5 K·min^–1; (c) 10 K·min^–1; (d) 15 K·min^–1

Figure 7. Morphologies of the samples obtained by the temperature-programmed reduction of MoO₂ by CO–15%CO₂ mixed gases at different heating rates: (a) 2 K·min^–1; (b) 5 K·min^–1; (c) 10 K·min^–1; (d) 15 K·min^–1

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圖 8 不同溫度下CO–15%CO₂混合氣體恒溫還原MoO₂所得Mo₂C形貌圖. (a) 993 K; (b) 1025 K; (c) 1058 K; (d) 1093 K; (e) 1123 K; (f) 1153 K

Figure 8. Morphologies of the samples obtained by the isothermal reduction of MoO₂ to Mo₂C by CO–15%CO₂ mixed gases at different temperatures: (a) 993 K; (b) 1025 K; (c) 1058 K; (d) 1093 K; (e) 1123 K; (f) 1153 K

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圖 9 993 K下CO–15%CO₂混合氣體恒溫還原MoO₂不同反應進度所得產物形貌. (a1～a3) α = 0.263; (b1～b3) α = 0.503; (c1～c3) α = 0.744; (d1～d3) α = 1

Figure 9. Morphologies of the samples obtained by reducing MoO₂ with CO–15%CO₂ mixed gases at 993 K for different reaction extents: (a1–a3) α = 0.263; (b1–b3) α = 0.503; (c1–c3) α = 0.744; (d1–d3) α = 1

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圖 10 CO和MoO₂二元相圖

Figure 10. Binary phase diagram between CO and MoO₂

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圖 11 CO–15%CO₂混合氣體還原MoO₂為Mo₂C的反應機理圖

Figure 11. Mechanism for the reduction of MoO₂ to Mo₂C by CO–15%CO₂ mixed gases

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圖 12 不同溫度下制備的Mo₂C樣品的線性掃描伏安曲線

Figure 12. Linear sweep voltammetry curves of the samples obtained at different temperatures

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圖 13 不同模型擬合1093 K下的動力學數據. (a) 形核與核心長大模型; (b) 化學反應模型

Figure 13. Kinetic-fitting results for the experimental data obtained at 1093 K with different gas-solid reaction models: (a) nucleation-growth model; (b) chemical reaction model

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圖 14 1093、1123和1153 K下恒溫動力學曲線和擬合結果比較

Figure 14. Comparisons of fitting and measured reaction extent vs reaction time curves for the experimental data obtained at 1093, 1123, and 1153 K

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表 1 常見的氣固反應動力學模型方程^[31–33]

Table 1. List of the common gas-solid reaction kinetic models^[31–33]

No.	Model	Integral form g(α) = kt	Explicit form
1	Power law (P2)	α^1/2	α= (kt)²
2	Power law (P3)	α^1/3	α = (kt)³
3	Avarami-Erofeev (A1.5)	[?ln(1 ? α)]^2/3	α = 1 ? exp[? (kt)^3/2]
4	Avarami-Erofeev (A2)	[?ln(1 ? α)]^1/2	α = 1 ? exp[? (kt)²]
5	Avarami-Erofeev (A3)	[?ln(1 ? α)]^1/3	α = 1 ? exp[? (kt)³]
6	Avarami-Erofeev (A4)	[?ln(1 ? α)]^1/4	α = 1 ? exp[? (kt)⁴]
7	First-order (F1)	?ln(1 ? α)	α = 1 ? exp(? kt)
8	Second-order (F2)	(1 ? α)^–1 ? 1	α = 1 ? (kt +1)^–1
9	Third-order (F3)	1/2[(1 ? α)^–2 ? 1]	α = 1 ? (2kt + 1)^–1/2
10	Contracting area (R2)	1 ? (1 ? α)^1/2	α = 1 ? (1 ? kt)²
11	Contracting volume (R3)	1 ? (1 ? α)^1/3	α = 1 ? (1 ? kt)³

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