Structural, physical and chemical properties of nanostructured nickel-substituted ceria oxides under reducing and oxidizing conditions


This work reports the synthesis of nanostructured Ce1-xNixO2-delta (x = 0.05, 0.1, 0.15 and 0.2) oxides prepared by a cation complexation route and with the main objective of studying their redox properties using a combination of electron microscopy, synchrotron radiation X-ray diffraction (SR-XRD) and X-ray absorption near-edge spectroscopy (XANES). The Ce1-xNixO2-delta series of nanopowders maintain the cubic crystal structure (Fm3m space group) of pure ceria, with an average crystallite size of 5-7 nm indicated by XRD patterns and confirmed by transmission electron microscopy. In situ SR-XRD and XANES carried out under reducing (5% H-2/He; 5% CO/He) and oxidizing (21% O-2/N-2) atmospheres at temperatures up to 500 degrees C show a Ni solubility limit close to 15 at% in air at room temperature, decreasing to about 10 at% after exposure to 5% H-2/He atmosphere at 500 degrees C. At room temperature in air, the effect of Ni on the lattice parameter of Ce1-xNixO2-delta is negligible, whereas a marked expansion of the lattice is observed at 500 degrees C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce4+ cations to the much larger Ce3+, possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce4+/Ce3+ couple in nanocrystalline Ni-substituted ceria is greatly enhanced in comparison to pure ceria or achieved by using other dopants (e.g. Gd, Tb or Pr), where it is limited to less than 5% of Ce cations.



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Fuentes, RO; Acuna, LM; Albornoz, CA; Leyva, AG; Sousa, N; Figueiredo, FM

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This work has been supported by: Science and Technological cooperation agreement MINCyT (Argentine) - MEC (Portugal), Project PO/14/04; the Brazilian Synchrotron Light Laboratory (LNLS, Brazil), under proposals D04B-XAFS1-17893 and D12A-XRD1-17860; Agencia Nacional de Promocion Cientifica y Tecnologica (Argentina, PICT 2012-1506); and by the Portuguese Foundation for Science and Technology (FCT) through projects CICECO-Aveiro Institute of Materials (FCTUID/CTM/50011/2013), financed by national funds through the FCT/MEC, and when applicable, co-financed by FEDER under the PT2020 Partnership Agreement, and Investigador FCT 2013 contract number IF/01174/2013. The authors are grateful to Anna Paula da Silva Sotero Levinsky, Cristiane Rodella, Fabio Zambello, Tamiris Boucas Piva and Simone Bau Betim for their invaluable experimental assistance at the LNLS. Dr L. M. Acuna and Dr R. O. Fuentes are members of CIC-CONICET, Argentina.

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