Decomposition of Carbon Dioxide at 500 °C over Reduced Iron, Cobalt, Nickel, and Zinc Ferrites: A Combined XANES-XRD Study

A series of ferrite spinels [AFe(2)O(4), where A = Fe(II), Co(II), Ni(II), or Zn(II)] synthesized by coprecipitation were reduced in hydrogen and subsequently studied in the decomposition of carbon dioxide at 500 degrees C by a combination of in situ XANES, XRD, and mass spectrometry methods. The reducibilities of the ferrites in hydrogen depend on the A metal. In the reduction to the metallic states, XANES data reveal that iron(II) oxide is formed as in intermediate product with A(II) being expelled to form A(II) oxides as the spinel structure breaks down. Carbon monoxide is produced in the decomposition of carbon dioxide through the oxidation of iron(II) or metallic iron. The highest activities in the reaction were observed for nickel and zinc ferrite. However, as opposed to cobalt ferrite and magnetite, the regeneration to the original ferrite structure in carbon dioxide was not complete for these two materials. For nickel ferrite, which exhibits high reducibility and carbon dioxide reactivity, magnetite is formed as the major reoxidation product.