H. Falcon et al., Large enhancement of the catalytic activity for CO oxidation on hole doped(Ln,Sr)NiO3 (Ln = Pr, Sm, Eu) Perovskites, SOL ST ION, 131(3-4), 2000, pp. 237-248
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 me
asurements, X-ray and neutron powder diffraction (XRD, NPD), thermogravimet
ric 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, nec
essary 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 indicat
ed 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 distance
s with respect to the undoped compound, concomitant with the hole-doping of
the d-bands of the solid. Temperature-programmed reduction profiles reveal
ed that the reduction of Ni3+ occurs through two consecutive reduction step
s (Ni (3+/4+) --> Ni2+ --> Ni-0). Both unsubstituted and Sr-substituted sam
ples were tested in CO oxidation and it was found that the intrinsic activi
ty is enhanced with Sr-substitution particularly for the Pr0.95Sr0.05NiO3 s
ample. This higher activity in CO oxidation appears to be related to the re
lative ease of oxygen removal from the hole-doped Perovskites and to the pr
esence of oxygen vacancies. (C) 2000 Elsevier Science B.V. All rights reser
ved.