Fe valence fluctuations and magnetoelastic coupling in Pb-based multiferroics perovskites

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Lead-based multiferroics perovskites with nominal compositions Pb(Fe1/2Nb1/2)O-3 and Pb(Fe2/3W1/3)O-3 were synthesized following a two-stage method. Magnetic proprieties were investigated and correlated to anelastic proprieties, measured by the conventional pulse-echo method. The discussions are focused in the region around 250 K, where magnetoelectroe-lastic instabilities have been observed. X-ray absorption near-edge structure (XANES) study further indicates that the edge position varies with temperature revealing a fluctuation on the valence of iron ions with the temperature, which can be related to a variation in anelastic and magnetic proprieties. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim2102386390Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [Proc. 2008/04025-0, 2010/11187-6

Cu-Modified SrTiO3 Perovskites Toward Enhanced Water-Gas Shift Catalysis: A Combined Experimental and Computational Study

A water-gas shift reaction (WGS) is important and widely applied in the production of H2. Cu-modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu-modified SrTiO3 (nominally SrTi1-xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into a stable metallic phase under conditions of WGS reactions for all compositions. Among the characterized materials, the x = 0.20 composition showed the highest turnover frequency, lowest activation energy, and the highest WGS rate at 300 °C. According to density functional calculations, the formation of CuO is energetically less favorable compared with SrTiO3, explaining why the segregated CuO phase on the SrTiO3 surface is reduced to Cu during the catalytic reaction, while SrTiO3 remains. For x = 0.20, the size of the segregated CuO phase is optimum for facilitating the catalytic reaction. In contrast, a higher Cu content (x = 0.3) results in an aggregation of smaller CuO particles, resulting in fewer surface active sites and a net decrease in catalytic performance