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

• Published in: RSC advances 2016-01, Vol.6 (69), p.64861-6487
• Main Authors: Fuentes, R. O, Acuña, L. M, Albornoz, C. A, Leyva, A. G, Sousa, N, Figueiredo, F. M
• Format: Article
• Language: English
• Subjects: Cations; Cerium oxide; Nanostructure; Nickel; Oxidation; Oxides; Temperature; X-rays
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c6ra14853k

This work reports the synthesis of nanostructured Ce 1− x Ni x O 2− δ ( 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 Ce 1− x Ni x O 2− δ series of nanopowders maintain the cubic crystal structure ( Fm 3 m 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 °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 °C. At room temperature in air, the effect of Ni on the lattice parameter of Ce 1− x Ni x O 2− δ is negligible, whereas a marked expansion of the lattice is observed at 500 °C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce 4+ cations to the much larger Ce 3+ , possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce 4+ /Ce 3+ 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. Nanocrystalline Ni-substituted ceria is highly reversible and it has an extraordinary redox cyclicality associated to a high thermochemical stability of the material at relatively high temperature.