Study of mixed Pr/La molybdates, La5.4-xPrxMo1-yNbyO12-δ, for hydrogen separation membranes Abraham Sánchez Caballero 1 , José M. Porras-Vázquez 1 , Lucía dos Santos-Gómez 1 , Javier Zamudio-García 1 , Antonia Infantes-Molina 1 , David Marrero-López 2 , Enrique R. Losilla 1 1 Universidad de Málaga, Dpto. de Química Inorgánica, Cristalografía y Mineralogía 29071-Málaga, Spain, 2 Universidad de Málaga, Dpto. de Física Aplicada I, 29071-Málaga, Spain Lanthanide molybdates, with general formula Ln 5.4 MoO 11.1-d , are materials with mixed proton-ionic conductivity that can be used as hydrogen separation membranes. Their symmetry (cubic or rhombohedral) depends on the synthetic conditions and composition. 1-3 In this work, it has been studied the influence of praseodymium and niobium doping on the La 5.4-x Pr x Mo 1-y Nb y O 12-d (x= 1.35, 2.7, 4.05, 5.4; y= 0, 0.1) series, with two main objectives: i) increase the electronic conductivity (praseodymium doping) and ii) increase the amount of oxide ion vacancies and enhance the ionic conductivity (niobium doping). The materials have been prepared by the freeze-drying method and characterised by X-ray diffraction (Rietveld method), X-ray photoelectron spectroscopy, scanning electron microscopy and complex impedance spectroscopy. The compositions with a higher amount of praseodymium crystalize with a cubic fluorite-type structure under any sintering condition; however, for low praseodymium contents, a R1 rhombohedral polymorph is stabilised at low cooling rates. The best conductivity values are obtained in a N 2 atmosphere for compositions with a high praseodymium content, due to the existence of the Pr 4+ /Pr 3+ pair (confirmed by X-ray photoelectron spectroscopy), which confers a high electronic conductivity. In a reducing atmosphere, wet 5%H 2 -Ar, the conductivity values drastically decrease due to the reduction of Pr 4+ to Pr 3+ . References 1. Shlyakhtina, A. V.; Savvin, S. N.; Lyskov, N. V.; Kolbanev, I. V.; Karyagina, O. K.; Chernyak, S. A.; Shcherbakova, L. G.; Nú nez, P. Polymorphism in the Family of Ln 6‑x MoO 12‑δ (Ln = La, Gd - Lu; x = 0, 0.5) Oxygen Ion- and Proton-Conducting Materials. J. Mater. Chem. A 2017, 5, 7618−7630. 2. Lopez-Vergara, A.; Porras-Vazquez, J. M.; Infantes-Molina, A.; Canales-Vazquez, J.; Cabeza, A.; Losilla, E. R.; Marrero- Lopez, D. Effect of Preparation Conditions on the Polymorphism and Transport Properties of La 6‑x MoO 12‑δ (0 ≤ x ≤ 0.8). Chem. Mater. 2017, 29, 6966−6975. 3. López-Vergara, A.; Vizcaíno-Anaya, L.; Porras-Vázquez, J. M.; Baldinozzi, G.; dos Santos-Gómez. L.; Canales-Vazquez, J.; Marrero-López, D.; Losilla, E .R. Unravelling crystal superstructures and transformations in the La 6-x MoO 12-δ (0.6 ≤ x ≤ 3.0) series: a system with tailored ionic/electronic conductivity, Chem. Mater. 2020, 32,7052–7062.
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