Reduction Behavior and Characteristics of Metal Oxides in the Nanoscale
Abstract
:1. Introduction
2. Experimental Procedure
2.1. Materials
2.2. Reduction Techniques
3. Results
3.1. Characterization of Nanosized Metal Oxides
3.2. Reduction Behavior of Metal Oxides
3.2.1. Non-Isothermal Reduction of Nanosized Metal Oxide Powder
3.2.2. Isothermal Reduction of Metal Oxide Compacts
3.3. Reduction Kinetics and Mechanism
- In the initial stages (5–10% extent), the Ea values obtained from the reduction of NiO and Fe2O3 are 9.6 and 12.43 kJ/mole, indicating that the reduction is controlled by a gaseous diffusion mechanism. The Ea value for WO3 is relatively high (23.68 kJ/mole) but is still in the range of the gas diffusion mechanism. The considerable difference in the Ea values between NiO, Fe2O3, and WO3 is in accordance with the slower reduction rate of WO3 compared to other oxides, as was previously shown in Figure 4a–c. This finding is in agreement with the principles of gas–solid reactions in which the rate-determining step is considered the slowest step in the whole reactions [11,12,13,14,15]. In the present study, nanosized metal oxides were reduced in a H2 atmosphere, and the chemical reaction easily proceeded at the grain surface. The produced metallic phase at this stage could not be able to surround the whole oxide grain surface. Under these conditions, the gas diffusion inside the oxide grains is relatively the slowest process in the gas–solid reaction and is considered the determining step.
- In the later stages (80–85 % extent), the Ea values for the reduction of NiO and Fe2O3 are 13.24 and 21.65 kJ/mole, indicating that the reduction reactions are still also controlled by the gaseous diffusion mechanism. On the other hand, the Ea value for the reduction of WO3 could not be calculated due to the stopping of reduction at 700–900 °C at lower extents depending on the applied reduction temperature, and the reduction is completed only at 1000 °C.
4. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Ea Values (kJ/mol) and the Corresponding Rate-Controlling Step | Oxide | Ea Initial (kJ/mol) | Ea Initial (kJ/mol) | |
---|---|---|---|---|
Ea Values | Rate-Controlling Step | |||
8–16 | Gas diffusion | NiO | 9.6 | 13.24 |
29–42 | Combined gas diffusion and interfacial chemical reaction | Fe2O3 | 12.43 | 21.65 |
>90 | Solid-state diffusion | WO3 | 23.68 | - |
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Halim, K.S.A.; El-Geassy, A.A.; Ramadan, M.; Nasr, M.I.; Hussein, A.; Fathy, N.; Alghamdi, A.S. Reduction Behavior and Characteristics of Metal Oxides in the Nanoscale. Metals 2022, 12, 2182. https://doi.org/10.3390/met12122182
Halim KSA, El-Geassy AA, Ramadan M, Nasr MI, Hussein A, Fathy N, Alghamdi AS. Reduction Behavior and Characteristics of Metal Oxides in the Nanoscale. Metals. 2022; 12(12):2182. https://doi.org/10.3390/met12122182
Chicago/Turabian StyleHalim, K. S. Abdel, A. A. El-Geassy, Mohamed Ramadan, M. I. Nasr, A. Hussein, Naglaa Fathy, and Abdulaziz S. Alghamdi. 2022. "Reduction Behavior and Characteristics of Metal Oxides in the Nanoscale" Metals 12, no. 12: 2182. https://doi.org/10.3390/met12122182