Palladium local structure of La1−xSrxCo1−yFey−0.03Pd0.03O3−δ perovskites synthesized using a one pot citrate method

Abstract

Nanostructured La_1−xSr_xCo_1–yFe_y−0.03Pd_0.03O_3−δ (LSCF-Pd) perovskites with fixed La–Sr composition (x = 0.4) and two different Fe contents (y = 0.2 and 0.8) were successfully synthesized using a one pot citrate method starting from nitrates of the metal cations. Pd-free La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF) systems were prepared for comparison. LSCF powders were calcined at 1300 °C and characterized by XRD and Rietveld refinement, EXAFS, XPS, TPR analyses. Promotion of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ and of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ by incorporation of palladium was evidenced by solving the local environment of Pd using EXAFS spectroscopy. XPS analyses, in agreement with TPR measurements, demonstrated an increase of superficial oxygen vacancies, the variation being much more pronounced in the La_0.6Sr_0.4Co_0.8Fe_0.17Pd_0.03O_3−δ sample. It is argued that this increase is associated with the introduction of Pd in the B site of LSCF. Moreover, ionic Pd⁴⁺ was detected as the only palladium species on the perovskite surface, whereas Pd metal clusters of about 2 nm, not detectable in the surface, are embedded in the matrix and strongly interact with the bulk. This result has strong consequences in the synthesis and in the design of new perovskite materials that can be used as cathodes in fuel cell application at intermediate operating temperatures.