Kinetics of the reduction of Fe sub 2 O sub 3 with hydrogen

The kinetics of the heterogeneous reduction of Fe{sub 2}O{sub 3} with gaseous hydrogen was studied over the temperature range of 300-400C. As Fe{sub 2}O{sub 3} is reduced, the resulting oxygen vacancies act as anion dopants of the remaining Fe{sub 2}O{sub 3}, thus increasing its conductivity. The final reaction product is FeO. It reoxidizes quickly upon exposure to air forming Fe{sub 3}O{sub 4}, as revealed by X-ray diffraction analysis. The progress of the reaction was followed by measuring the variation of the electrical conductivity with the time of reduction was found to be quite complex, and can be divided into three regions: (a) region 1, at short times, where the process is controlled by the chemical reaction at the outer surface between hydrogen and ferric oxide. This is an activated reaction characterized by an activation energy of 56.5 kJ mole{sup {minus}1}; (b) region 2, which signals the onset of diffusion control on the overall rate of the process, where the reaction starts to penetrate through the solid phase. Within this region, it was found that the conductivity varied linearly with the square root of the time; and (c) region 3 where the process is controlled by diffusion through the solid phase. The behavior of the system in regions 2 and 3 was found to be in agreement with the predictions obtained from solving Fick's second law of diffusion.