Journal
SOLID STATE IONICS
Volume 131, Issue 3-4, Pages 237-248Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0167-2738(00)00664-0
Keywords
perovskites; Sr-substitution; Rietveld refinement; hole-doping; oxygen vacancies; CO oxidation
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The effect of Sr substitution in LnNiO(3) (Ln = Pr, Sm, Eu) Perovskites on the structural and catalytic properties was studied by means of textural measurements, X-ray and neutron powder diffraction (XRD, NPD), thermogravimetric analysis, temperature-programmed reduction and catalytic activity in CO oxidation. All the samples were prepared using soft-chemistry procedures, decomposing citrate precursors at 1073 K, and subsequently oxygenating them at 1273 K under a very high oxygen pressure (200 bar). This treatment, necessary for the stabilization of Ni3+-Ni4+ mixed valences, resulted in very low specific areas (1-2 m(2) g(-1)). The X-ray diffraction profiles indicated pure Perovskite phases with orthorhombic distortion, which increased in the order Pr < Sm < Eu. The Rietveld refinement of NPD data for a selected Pr0.95Sr0.05NiO3 sample showed a significant reduction in the Ni-O distances with respect to the undoped compound, concomitant with the hole-doping of the d-bands of the solid. Temperature-programmed reduction profiles revealed that the reduction of Ni3+ occurs through two consecutive reduction steps (Ni (3+/4+) --> Ni2+ --> Ni-0). Both unsubstituted and Sr-substituted samples were tested in CO oxidation and it was found that the intrinsic activity is enhanced with Sr-substitution particularly for the Pr0.95Sr0.05NiO3 sample. This higher activity in CO oxidation appears to be related to the relative ease of oxygen removal from the hole-doped Perovskites and to the presence of oxygen vacancies. (C) 2000 Elsevier Science B.V. All rights reserved.
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