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Abstract: CeO2 doped with praseodymium, neodymium and/or zirconium atoms were prepared by coprecipitation and by the sol–gel method. Structural properties were investigated by in situ XRD and Raman spectroscopy while oxygen storage capacity (OSC) was measured by transient CO oxidation. All the compounds, except pure Nd2O3, have a fluorite-type structure as well as a Raman band at 560cm−1 characteristic of the oxygen vacancies involving non-stoichiometric oxides. The lattice parameter under hydrogen, being dependent on the temperature, revealed two reduction mechanisms: one at a low temperature at the surface and another at a high temperature in the bulk. Ce–Nd binary oxides show a strong tendency towards crystallite aggregation, which reduces accessibility to gases and OSC properties. Zirconium improves the thermal resistance to sintering of both Ce–Nd and Ce–Pr oxides. The Zr–Ce–Pr–O followed by Zr–Ce–Nd–O compounds displaying high oxygen mobility at a low temperature, appear to be very promising for practical applications such as OSC materials. [Copyright &y& Elsevier]

Abstract: Both concentrated and diluted sodium silicate solutions have been investigated by combining 29Si NMR spectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified. For the most concentrated solution ([Si]=7mol/l; pH=11.56; Si/Na atomic ratio=1.71), the results evidence the preponderant presence of neutral Si7O18H4Na4 complexes, which behave like colloids of about 0.6–0.8nm able to form very small aggregates with an average size lower than 3nm. Addition of distilled water to this initial concentrated solution leads, on one hand, to a doubling of the colloid size, i.e. 1.2–1.5nm, and, on the other hand, to a progressive decrease of the aggregate size until their total disappearance. Such a behavior could be explained by considering, first, the dissociation of the neutral Si7O18H4Na4 complexes present in the concentrated solution into Na+ ions and charged (Si7O18H4Na4− n ) n − complexes (with 1⩽ n ⩽4) and, second, the condensation of these siliceous charged species in order to form larger (Si7 y O18 y − z H4 y −2 z Na(4− n )y) n y− colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si7O18H4Na4− n ) n − groups. [ABSTRACT FROM AUTHOR]