Intermediate formation in the reduction of Ni-ferrite with irradiation of high-flux infrared beam up to 1823 K

Ni-ferrite (NiFe2O4) has a lower reaction temperature for the O2-releasing reaction when irradiation by a high-flux solar beam than that predicted by the estimation based on the thermodynamic data. The reaction mechanism of Ni-ferrite in the O2-releasing reaction of a two-step water-splitting process at high temperatures (1,273–1,823 K) was clarified by means of X-ray diffractometry (XRD), extended X-ray absorption fine structure (EXAFS) analysis, Mössbauer spectroscopy and Magnetization measurement. The analysis of the EXAFS and Mössbauer spectra for Ni-ferrite before and after the O2-releasing reaction shows that a lattice defect (Fe3+ (B-site)⇒Fe3+ (interstitial A-site)) intermediate with a spinel-type structure was formed in the early stage of the O2-releasing reaction (up to 1,723 K). It is suggested that irradiation by a high-flux infrared beam resulted in the formation of the lattice defect intermediate with a Frenkel defect. The formation of the lattice defect intermediate with a Frenkel defect was due to the high reactivity of Ni-ferrite in the O2-releasing reaction, as compared with other ferrites.

Graphical AbstractMössbauer spectra of Ni-ferrite samples obtained after heating at different temperatures .Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► O2-releasing reaction mechanism of Ni-ferrite at high temperatures was clarified. ► XRD, EXAFS, Mössbauer and magnetization measurement reveal an intermediate formation. ► Lattice defect was formed in Ni-ferrite spinel without O2 release up to 1723 K. ► Intermediate with defect causes the enhancement of O2 gas evolution above 1773 K.