Vol.:(0123456789) 1 3 Topics in Catalysis https://doi.org/10.1007/s11244-018-1031-1 ORIGINAL PAPER Characterization of CeO 2 –Fe 2 O 3 Mixed Oxides: Infuence of the Dopant on the Structure Rodrigo Brackmann 1,2  · Fabio Souza Toniolo 1  · Edivaldo dos Santos Filho 3  · Odivaldo Cambraia Alves 4  · Ângelo Marcio de Souza Gomes 5  · Carla Brandão Woyames 6  · Martin Schmal 1 © Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract CeFex (x = 0, 1, 3, 5, 10, 15 and 20 at.%) mixed oxides synthesized by an adapted Pechini method were characterized by Raman spectroscopy, high-resolution transmission electron microscopy, electron paramagnetic resonance, magnetization and 57 Fe Mössbauer spectroscopy ( 57 Fe-MS) measurements in order to evaluate the oxygen vacancies formation and the chemi- cal environment of Fe +3 inserted into the CeO 2 crystalline lattice. Fe +3 introduction into the CeO 2 structure resulted in an increase of the oxygen vacancies concentration, which indicates that this is the predominant charge compensation mechanism in the formation of CeFex solid solutions by the Pechini method. Fe +3 insertion in CeO 2 led to the formation of substitutional solid solutions, in which Fe +3 replaced octahedral Ce +4 sites in the crystalline lattice. Fe +3 could be found in the form of isolated sites with orthorhombic distortion or Fe +3 species in pairs or clusters coupled by strong spin–spin interactions. No evidence of Fe +3 insertion into tetrahedral interstitial sites was found. Isolated Fe +3 species showed a less distorted chemi- cal environment and greater ionic character of the Fe–O bonds than the clusters, being the concentration of both type sites approximately equal for all the Fe +3 doped contents. It was found that pure CeO 2 and all the CeFex mixed oxides presented ferromagnetic properties even at room temperature possibly due to their small crystallite size and the presence of oxygen vacancies. At high Fe +3 concentrations (above 10 at.%), probably super-exchange interactions (Fe +3 –O −2 –Fe +3 ), with an antiferromagnetic character, also took place, reducing the ferromagnetism of the CeFex mixed oxides. Keywords CeO 2  · Mixed oxides · Oxygen vacancies · Ferromagnetism · Heterogeneous catalysts 1 Introduction Ceria (CeO 2 ) has a fuorite type structure (Fm3m space group), a face-centered cubic structure in which each Ce +4 is coordinated to eight oxygen anions (coordination num- ber 8) and each O −2 , in turn, coordinates tetrahedrally to four Ce +4 cations. Defects on CeO 2 structure can be classi- fed as intrinsic or extrinsic. Defects related to the thermal disorder of the material or which can be created by the reaction between the solid and the surrounding atmosphere (redox processes) are understood as intrinsic. Extrinsic * Martin Schmal schmal@peq.coppe.ufrj.br 1 Chemical Engineering Program, Laboratory Nucleus of Catalysis COPPE/UFRJ, Federal University of Rio de Janeiro, Ilha do Fundão, C.P. 68502, Rio de Janeiro CEP 21941-914, Brazil 2 Present Address: Department of Chemistry, Federal University of Technology - Paraná (UTFPR), Via do Conhecimento, Km 01, Fraron, C.P. 571, Pato Branco, Paraná CEP 85503-390, Brazil 3 Institute of Science and Technology, Federal University of the Valleys of Jequitinhonha and Mucuri (UFVJM), Rodovia MGT 367, km 583, n° 5000, Diamantina, MG 39.100-000, Brazil 4 Department of Physicochemistry, Chemistry Institute, Fluminense Federal University (UFF), R. Outeiro de S. J. Batista s/n, Niterói, RJ CEP 24020-150, Brazil 5 Institute of Physics, Federal University of Rio de Janeiro, Ilha do Fundão, C.P. 68528, Rio de Janeiro CEP 21941-972, Brazil 6 Metallurgical and Materials Engineering Program - COPPE1, Federal University of Rio de Janeiro, Ilha do Fundão, C.P. 68505, Rio de Janeiro CEP 21941-972, Brazil