Reduction Kinetics of Co₃O₄ Powders by Hydrogen Plasma and Hydrogen Gas at Low Temperatures

The reduction of cobalt oxide (Co₃O₄) powders was studied using hydrogen plasma (HP) and hydrogen gas at low temperatures. The reduction experiments were performed in a pulsed plasma reactor at 540 V, 532 Pa, under atmospheres of 100% hydrogen at a flow rate of 300 cm³/min, with varying times up to 60 min, and temperature range of 523–653 K. The results showed that the reduction of Co₃O₄ powders occurs in two steps: Co₃O₄ → CoO → Co, when using either HP or hydrogen gas. At lower temperatures, hydrogen plasma provided a significant increase in reduction kinetics compared to gas, since atomic hydrogen is a more powerful reductant than molecular hydrogen due to the increase in the density of crystalline defects provoked by the impact of ions incidents contained in plasma.The values for the activation energy were determined to be 35.4 kJ/mol for plasma and 90.8 kJ/mol for gas, and the kinetic equation that best fits to both experimental data was [1 – (1 – α)^1/3 ]² = k.t.

Keywords:
Reduction; cobalt oxide; hydrogen gas; hydrogen plasma