title: Reduction Kinetics of La2Mo2O9 and Phase Evolution during Reduction and Reoxidation.
abstract: An amorphous reduced form of oxide ion conductor La2Mo2O9 had been proposed as sulfur-tolerant anode material for solid oxide fuel cell, but its oxygen content was not known. In this paper, we investigate the reduction kinetics by diluted hydrogen of La2Mo2O9 to amorphous, and the oxygen range of the amorphous form. The reduction kinetics is studied as a function of the powder specific surface area and of the temperature, on powders synthesized by solid state reaction and by polyol process using two different solvents. The reduction process was carried out by TGA under 10% H2 diluted in argon, and its kinetics is analyzed and modeled. As expected, small particles and high temperature lead to higher reduction rates. Several reduction steps were identified by XRD during the process. At 700 °C La2Mo2O9 is directly reduced into the amorphous phase La2Mo2O7-y, whereas at 760 °C reduction occurs through an intermediate crystallized La7Mo7O30 (≅ La2Mo2O8.57) phase before amorphization. In both cases, further reduction of La2Mo2O6.2 amorphous phase leads to an exsolution of metallic molybdenum and a molybdenum deficiency in the amorphous phase. Reoxidation of amorphous La2Mo2O7-y was studied by TGA, DTA and XRD. At low temperature in air, the reduced compounds are reoxidized while remaining amorphous. The annealing for 60 h at 350 °C in air of reduced La2Mo2O6.66, obtained beforehand by solid state reaction, gives an amorphous phase with composition La2Mo2O8.85. The existence domain of the reduced amorphous phase in terms of oxygen content therefore ranges at least from O6.2 to O8.85, thus including the composition La2Mo2O8.50 of the amorphous surface layer at the origin of a huge increase of ionic conductivity recently reported in nanowires of La2Mo2O9.
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.|Institut des Matériaux Jean Rouxel (IMN), Univ. Bretagne-Loire, UMR CNRS 6502, Univ. Nantes, 2 rue de la Houssinière, BP32229, F-44322 Nantes cedex 3, France.
authors.forename: Gaëtan
authors.initials: G
authors.lastname: Buvat
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.|Unité de Recherche Synthèse et Structure de Nanomatériaux UR11ES30, Faculté des Sciences de Bizerte, Université de Carthage , 7021 Jarzouna, Tunisie.
authors.forename: Houssem
authors.initials: H
authors.lastname: Sellemi
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.
authors.forename: Uday K
authors.initials: UK
authors.lastname: Ravella
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.
authors.forename: Maud
authors.initials: M
authors.lastname: Barré
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.
authors.forename: Sandrine
authors.initials: S
authors.lastname: Coste
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.
authors.forename: Gwenaël
authors.initials: G
authors.lastname: Corbel
authors.affiliation: Institut des Molécules et Matériaux du Mans (IMMM), Univ. Bretagne-Loire, UMR CNRS 6283, Univ. Maine, avenue Olivier Messiaen, F-72085 Le Mans cedex 9, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Lacorre