In recent years, several works have been addressed to decrease carbon dioxide emission or to capture, to storage and to use it. An attractive option is its use as feedstock of chemical industry, especially in dehydrogenation reactions (such as ethylbenzene dehydrogenation to produce styrene), providing an exothermic process which can be operated at lower temperatures, making negligible the cracking products. Aiming to find alternative catalysts for this reaction, magnesia-supported iron oxides were studied, being prepared by two different methods. The classical impregnation produced a spinel (MgFe2O4) coexisting with magnesia containing Fe3+ species, this catalyst showing higher specific surface area and being more active and selective than magnesia. Moreover, the deposition of iron nanoparticles through a magnetic fluid on magnesia produced magnesia-supported hematite nanoparticles co-existing with magnesia containing Fe3+ species. In this case, the specific surface area and the activity were even higher and the solid is much more reducible than the other sample. These findings were associated to hematite nanoparticles and to the lower tendency of iron species to diffuse into magnesia lattice. They show that the Fe3+ species are more active and selective to styrene as hematite nanoparticles than when they are in the environment of magnesium ferrite. (c) 2014 Elsevier B.V. All rights reserved.