Decomposition of carbon dioxide over the putative cubic spinel nanophase cobalt, nickel, and zinc ferrites

Nanophase (4-6 nm) cobalt, nickel, and zinc spinet ferrites were synthesized by coprecipitation, and the decomposition of carbon dioxide at 300 degrees C was studied. Oxygen deficiency was obtained by reducing the materials in hydrogen. The divalent metals strongly influence the extent of the reactions; oxygen-deficient nickel ferrite was found to reduce three and nine times more carbon dioxide than the zinc and cobalt analogues, respectively. Carbon monoxide rather than carbon is produced for all of these ferrites. The reducibility in hydrogen and the degrees of oxygen-deficiencies were studied by X-ray absorption spectroscopy. The X-ray absorption near edge structure reveals that removing oxygen from the spinels leads to the concomitant reduction of iron(Ill) to iron(II), while the valence state of the divalent metal is unchanged. Of the three ferrites, nickel ferrite exhibits the highest degree of oxygen-deficiency. Extended X-ray absorption fine structure of the nickel ferrite shows that, although minor amounts of metallic nickel are expelled through over-reduction, the material is oxygen-deficient in a putative spinet structure.