Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis

Abstract

Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe₂ O₄ , can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe²⁺ , Co²⁺ , and Fe³⁺ during co-precipitation, a mixture of LDH, (Fe^II Co^II )_2/3 Fe^III_1/3 (OH)₂ (CO₃ )_1/6 ⋅m H₂ O, and the target spinel CoFe₂ O₄ can be obtained in the precursor. During calcination, the remaining Fe^II fraction of the LDH is oxidized to Fe^III leading to an overall Co²⁺ :Fe³⁺ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]_Spinel ∥[001]_LDH . Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material.