Morphological and compositional changes of MFe₂O₄@Co₃O₄ (M = Ni, Zn) core-shell nanoparticles after mild reduction

NiFe₂O₄@Co₃O₄ and ZnFe₂O₄@Co₃O₄ core-shell nanoparticles were synthesized and reduced to be used as catalysts for the Fischer-Tropsch synthesis. The as-synthesized materials were determined to have an incomplete Co₃O₄ shell around the ferrite core with a maximum thickness of 3 nm. Using in-situ TEM, it was shown that reduction of these core-shell nanoparticles in pure hydrogen at 230 °C and 250 °C, respectively, resulted in the formation of small cobalt islands on the ferrite surface. Catalytic testing of the core-shell materials, NiFe₂O₄@Co₃O₄ and ZnFe₂O₄@Co₃O₄, after reduction showed a cobalt-time yield of 13.64 μmol_CO·g_Co ⁻¹·s⁻¹ and 4.27 μmol_CO·g_Co ⁻¹·s⁻¹ and a C₅₊ selectivity of 47 C-% and 68 C-%, respectively. The observed difference in cobalt-time yield and selectivity between NiFe₂O₄@Co₃O₄ and ZnFe₂O₄@Co₃O₄ core-shell nanoparticles was ascribed to a combination of effects that arose from the morphological and compositional changes that occurred after reduction and under Fischer-Tropsch synthesis conditions.