Study the structure stability of NiFe_2−xCr_xO₄ (x = 0, 0.08) during H₂/CO₂ cycle reaction

Abstract

H₂/CO₂ cycle reaction activities of spinel structure NiFe_2−xCr_xO₄ (x = 0, 0.08) prepared by co-precipitation were determined. The results showed that pure NiFe₂O₄ had almost lost its CO₂ decomposition activities after 15 cycles, while Cr³⁺ doped NiFe₂O₄ still had about 40% of the initial reaction activity value after 50 reaction cycles. The magnetic properties of samples annealed at 350 °C indicated that M _s and M _r decreased from 32.49 to 26.04 emu/g and 9.39 to 7.31 emu/g, respectively, but H _c increased from 230 to 1800 Oe with the increasing of Cr³⁺content. XRD Rietveld analysis showed that there appeared 23.15% Fe_yNi_1−y (0<y < 1) and no Fe₃C in pure NiFe₂O₄ system after the first H₂/CO₂ cycle reaction. With the increasing of cycle times, the phase abundance of NiFe₂O₄ decreased rapidly. At the same time, Fe₃C appeared and its content increased fast. After 15 cycles, the phase abundance of NiFe₂O₄ is less than 5%wt, but those of Fe_yNi_1−y (0 < y < 1) and Fe₃C enhanced to 48.15 %wt and 46.92 %wt, respectively. However, the cycle reaction life of NiFe_2−xCr_xO₄ (x = 0.08) was much longer than NiFe₂O₄. The spinel structure stability was improved dramatically because of the existing of Cr³⁺ in the cell of NiFe₂O₄. After 50 cycles, the phase abundance of NiFe₂O₄ still had 20 %wt.