Morphological properties of ultra-fine (Ni,Zn)-ferrites and their ability to decompose CO₂

Ultra-fine oxygen-deficient ferrites can decompose CO₂ gas, which causes the greenhouse effect, into C and O₂ at a low temperature of about 300°C. In the present study, two ultra-fine powders of ternary ferrites of composition (Ni,Zn)Fe₂O₄, as potential catalysts for CO₂ decomposition, were prepared by hydrothermal synthesis or a coprecipitation method, and their abilities to decompose CO₂ investigated. X-Ray diffraction measurements identified the crystal structure of the ferrites as spinel-type. The Brunauer-Emmett-Teller (BET) surface area of the ferrite prepared by hydrothermal synthesis was above 110 m² g^-1 and larger than for the sample obtained by the coprecipitation method. Particle sizes were very small, about 5-10 nm, in the both specimens. The CO₂ decomposition efficiency of the reduced oxygen-deficient ternary ferrite prepared by hydrothermal synthesis was better than that of the coprecipitation sample. The difference of the CO₂ decomposition efficiency is discussed in terms of the morphology and crystallinity based on TEM observations.